**2. Problem Statement and Boundary Conditions**

Figure 1 schematically shows the tube with twisted tape. The height of the proposed twisted tape (*H*) is 95% of the tube diameter (*D* = 20 mm) with a thickness (*t*) of 0.4 mm. Four different twisted tape pitches (*P*) are investigated, as shown in Figure 1a. To study the position of truncated twisted tape, three different locations, i.e., entrance, center and exit, are examined, as shown in Figure 1b. Note that the length of the tube is 400 mm. Water enters the tube at 300 K with uniform velocity, and a symmetrical constant heat flux of 5000 W/m2 is applied on the walls, while the twisted tape walls are thermally insulated. A pressure outlet condition is also employed for the tube outlet [44–46].

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**Figure 1.** Schematic of the proposed system for (**a**) different twisted tape pitches and (**b**) different twisted tape positions. **Figure 1.** Schematic of the proposed system for (**a**) different twisted tape pitches and (**b**) different twisted tape positions.

**Figure 1.** Schematic of the proposed system for (**a**) different twisted tape pitches and (**b**) different

**Figure 1.** Schematic of the proposed system for (**a**) different twisted tape pitches and (**b**) different

**Figure 1.** Schematic of the proposed system for (**a**) different twisted tape pitches and (**b**) different

#### **Figure 1.** Schematic of the proposed system for (**a**) different twisted tape pitches and (**b**) different twisted tape positions. **3. Governing Equations 3. Governing Equations**  twisted tape positions. twisted tape positions. twisted tape positions.

**3. Governing Equations**

**3. Governing Equations**

**3. Governing Equations**

**3. Governing Equations**  This research investigates a steady laminar flow of incompressible nanofluid, neglecting the effects of radiation and viscosity losses. The governing equations are defined as follows: This research investigates a steady laminar flow of incompressible nanofluid, neglecting the effects of radiation and viscosity losses. The governing equations are defined as follows: **3. Governing Equations 3. Governing Equations 3. Governing Equations Figure 1.** Schematic of the proposed system for (**a**) different twisted tape pitches and (**b**) different twisted tape positions. **Figure 1.** Schematic of the proposed system for (**a**) different twisted tape pitches and (**b**) different twisted tape positions. **Figure 1.** Schematic of the proposed system for (**a**) different twisted tape pitches and (**b**) different twisted tape positions.

This research investigates a steady laminar flow of incompressible nanofluid, neglecting the effects of radiation and viscosity losses. The governing equations are defined as follows: . ൫ሬ⃗൯=0 (1) . (ρ → *V*) = 0 (1) This research investigates a steady laminar flow of incompressible nanofluid, neglecting the effects of radiation and viscosity losses. The governing equations are defined as follows: This research investigates a steady laminar flow of incompressible nanofluid, neglecting the effects of radiation and viscosity losses. The governing equations are defined as follows: This research investigates a steady laminar flow of incompressible nanofluid, neglecting the effects of radiation and viscosity losses. The governing equations are defined as follows: This research investigates a steady laminar flow of incompressible nanofluid, neglecting the This research investigates a steady laminar flow of incompressible nanofluid, neglecting the This research investigates a steady laminar flow of incompressible nanofluid, neglecting the

$$\nabla.(\rho \overrightarrow{VV}) = -\nabla p + \mu \overrightarrow{\mathcal{V}}^2 \overrightarrow{V} \tag{2}$$

$$\nabla \cdot (\rho \overrightarrow{\mathbf{V} \mathbf{C}\_p T}) = \nabla \cdot (k \nabla T) \tag{3}$$

. ൫ሬ⃗൯ = . () (3) Different parameters calculated in this study, including the average and local heat transfer coefficient and Nusselt number, friction factor and PEC are defined as follows [36,37]: . ൫ሬ⃗൯ = . () (3) Different parameters calculated in this study, including the average and local heat transfer . ൫ሬ⃗൯ = . () (3) Different parameters calculated in this study, including the average and local heat transfer . ൫ሬ⃗൯ = . () (3) Different parameters calculated in this study, including the average and local heat transfer . ൫ሬ⃗ሬ⃗൯ = − + ଶሬ⃗ (2) . ൫ሬ⃗൯ = . () (3) . ൫ሬ⃗ሬ⃗൯ = − + ଶሬ⃗ (2) . ൫ሬ⃗൯ = . () (3) . ൫ሬ⃗ሬ⃗൯ = − + ଶሬ⃗ (2) . ൫ሬ⃗൯ = . () (3) Different parameters calculated in this study, including the average and local heat transfer coefficient and Nusselt number, friction factor and PEC are defined as follows [36,37]:

coefficient and Nusselt number, friction factor and PEC are defined as follows [36,37]: = 4 (4) coefficient and Nusselt number, friction factor and PEC are defined as follows [36,37]: coefficient and Nusselt number, friction factor and PEC are defined as follows [36,37]: coefficient and Nusselt number, friction factor and PEC are defined as follows [36,37]: Different parameters calculated in this study, including the average and local heat transfer coefficient and Nusselt number, friction factor and PEC are defined as follows [36,37]: Different parameters calculated in this study, including the average and local heat transfer coefficient and Nusselt number, friction factor and PEC are defined as follows [36,37]: Different parameters calculated in this study, including the average and local heat transfer coefficient and Nusselt number, friction factor and PEC are defined as follows [36,37]: *Dh* = 4*A P* (4)

$$f = \frac{2\Delta P}{\rho V^2} \frac{D\_h}{L} \tag{5}$$

$$h\_{\mathbf{x}} = \frac{q^{\prime\prime}}{T\_w - T\_b} \tag{6}$$

$$h\_{\mathbf{x}}$$

$$\mathbf{M}\_{\mathbf{U}} = \mathbf{I}\_{\mathbf{b}}$$

$$\mathbf{N}u\_{\mathbf{x}} = \frac{h\_{\mathbf{x}\mathbf{D}\_{\mathbf{h}}}}{k} \tag{7}$$

$$\mathbf{1} \quad \mathbf{C}^{\mathbf{l}}$$

(6)

(6)

(8)

(**b**)

(**b**)

(6)

(6)

(8)

(8)

(8)

$$Nu\_{avg} = \frac{1}{l} \int\_{0}^{l} Nu\_{x} dx \tag{8}$$

$$Nu\_{x} / Nu\_{x}$$

$$\text{PEC} = \frac{\text{Nu} / \text{Nu}\_0}{\left(f / f\_0\right)^{1/3}} \tag{9}$$

(/)ଵ/ଷ (9)

(/)ଵ/ଷ (9)

(/)ଵ/ଷ (9)

= /

= /

= /

### **4. Numerical Procedure 4. Numerical Procedure**

The commercial ANSYS Fluent computational fluid dynamics (CFD) code is employed to perform the simulation and solve the equations [47–49]. The velocity–pressure coupling is resolved using the coupled algorithm, and the convection terms are discretized using the second-order upwind scheme. The convergence criteria of 10−6 are also selected. The commercial ANSYS Fluent computational fluid dynamics (CFD) code is employed to perform the simulation and solve the equations [47–49]. The velocity–pressure coupling is resolved using the coupled algorithm, and the convection terms are discretized using the second-order upwind scheme. The convergence criteria of 10−6 are also selected.

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### *4.1. Grid Study 4.1. Grid Study*

Figure 2 illustrates the computational mesh with finer mesh generated in the near-wall region due to velocity and temperature boundary layers and high gradients of variables. A fully structured mesh is created as shown using ANSYS meshing to have a high degree of quality and faster convergence in CFD FLUENT code, which also results in less computational time. Figure 2 illustrates the computational mesh with finer mesh generated in the near-wall region due to velocity and temperature boundary layers and high gradients of variables. A fully structured mesh is created as shown using ANSYS meshing to have a high degree of quality and faster convergence in CFD FLUENT code, which also results in less computational time.

**Figure 2.** The meshing of the computational domain. **Figure 2.** The meshing of the computational domain.

Different grid numbers are examined for grid independency analysis in the case of fully filled twisted tape with a pitch of L/4 and a Reynolds number of 1000. The average Nusselt number is selected as the selection criterion [50] presented in Table 1. As listed, case 3 is chosen for all the simulations, since using finer meshing leads to relative errors of less than one percent. Different grid numbers are examined for grid independency analysis in the case of fully filled twisted tape with a pitch of L/4 and a Reynolds number of 1000. The average Nusselt number is selected as the selection criterion [50] presented in Table 1. As listed, case 3 is chosen for all thesimulations, since using finer meshing leads to relative errors of less than one percent.

**Table 1.** Grid independence analysis. **Case Number of Elements Nusselt Number Error (%)**

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**Table 1.** Grid independence analysis.

#### *4.2. Validation* To obtain a reliable result, the experimental results of Qi et al. [42] for the average Nusselt

To obtain a reliable result, the experimental results of Qi et al. [42] for the average Nusselt number for laminar fluid flow in a heat exchanger using stationary twisted tape are used. They experimentally examined pure water and nanofluid flow for different Reynolds numbers for a twisted tape length of 1600 mm and a pitch size of 100 mm with a width and thickness of 16 and 2 mm, respectively. Figure 3 displays the average Nusselt number for the cases of pure water and stationary twisted tapes for different Reynolds numbers. As shown, the results are in excellent agreement with the experimental data of Qi et al. [42], where the maximum difference is less than 2%. number for laminar fluid flow in a heat exchanger using stationary twisted tape are used. They experimentally examined pure water and nanofluid flow for different Reynolds numbers for a twisted tape length of 1600 mm and a pitch size of 100 mm with a width and thickness of 16 and 2 mm, respectively. Figure 3 displays the average Nusselt number for the cases of pure water and stationary twisted tapes for different Reynolds numbers. As shown, the results are in excellent agreement with the experimental data of Qi et al. [42], where the maximum difference is less than 2%.

**Figure 3.** Validation of the present numerical results with the experimental data of Qi et al. [42]. **Figure 3.** Validation of the present numerical results with the experimental data of Qi et al. [42].

#### **5. Results and Discussion 5. Results and Discussion**

Numerical simulations are performed at four pitch values () of L, L/2, L/3 and L/4, four Reynolds numbers () of 250, 500, 750 and 1000, three twisted tape truncation (λ) percentages of 25, 50 and 75% and three positions of twisted tape at the entrance, center and exit of the tube, which are investigated in the following: Numerical simulations are performed at four pitch values (*P*) of L, L/2, L/3 and L/4, four Reynolds numbers (*Re*) of 250, 500, 750 and 1000, three twisted tape truncation (λ) percentages of 25, 50 and 75% and three positions of twisted tape at the entrance, center and exit of the tube, which are investigated in the following:

### *5.1. Effect of Twisted Tape Pitch 5.1. E*ff*ect of Twisted Tape Pitch*

tube where cooling is required.

In the first step, the effect of twisted tape pitch on the hydrothermal characteristics of the tube is examined and analyzed. Figure 4 represents the local Nusselt number () throughout the tube length for the plain tube (PT) and four twisted tape pitch values at = 250 while the twisted tape is fully fitted in the tube (no truncation). Using the twisted tape and decreasing its pitch magnitude noticeably augments the local along the tube length. The reason is that the twisted tape inserts create secondary flow as a result of flow swirling, which consequently improves the flow mixing, disturbs the thermal boundary layer and increases the heat transfer rate [51]. In other words, the twisted tape redirects the colder core fluid with a better cooling capacity to the heated walls of the In the first step, the effect of twisted tape pitch on the hydrothermal characteristics of the tube is examined and analyzed. Figure 4 represents the local Nusselt number (*Nu*) throughout the tube length for the plain tube (PT) and four twisted tape pitch values at *Re* = 250 while the twisted tape is fully fitted in the tube (no truncation). Using the twisted tape and decreasing its pitch magnitude noticeably augments the local *Nu* along the tube length. The reason is that the twisted tape inserts create secondary flow as a result of flow swirling, which consequently improves the flow mixing, disturbs the thermal boundary layer and increases the heat transfer rate [51]. In other words, the twisted tape redirects the colder core fluid with a better cooling capacity to the heated walls of the tube where cooling is required.

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**Figure 4.** Local along the tube length for plain tube (PT) and twisted tape inserts with P = L, L/2, L/3, L/4 at = 250. **Figure 4.** Local *Nu* along the tube length for plain tube (PT) and twisted tape inserts with P = L, L/2, L/3, L/4 at *Re* = 250. **Figure 4.** Local along the tube length for plain tube (PT) and twisted tape inserts with P = L, L/2, L/3, L/4 at = 250.

The cause of the heat transfer enhancement in Figure 4 can be seen in Figure 5, in which the streamlines colored by velocity magnitude are illustrated for PT and four twisted tape pitch values. It is visible that as the twisted tape pitch value decreases, the flow path undergoes more changes. This is because more swirl flow fronts can be seen in lower pitch values with higher radial velocity, implying stronger secondary and mixing flow. As a result, it leads to a more effective redirection of core colder fluid towards the heated wall and, consequently, more heat is transferred between the fluid and heated wall. The cause of the heat transfer enhancement in Figure 4 can be seen in Figure 5, in which the streamlines colored by velocity magnitude are illustrated for PT and four twisted tape pitch values. It is visible that as the twisted tape pitch value decreases, the flow path undergoes more changes. This is because more swirl flow fronts can be seen in lower pitch values with higher radial velocity, implying stronger secondary and mixing flow. As a result, it leads to a more effective redirection of core colder fluid towards the heated wall and, consequently, more heat is transferred between the fluid and heated wall. The cause of the heat transfer enhancement in Figure 4 can be seen in Figure 5, in which the streamlines colored by velocity magnitude are illustrated for PT and four twisted tape pitch values. It is visible that as the twisted tape pitch value decreases, the flow path undergoes more changes. This is because more swirl flow fronts can be seen in lower pitch values with higher radial velocity, implying stronger secondary and flow. As a result, it leads to a more effective redirection of core colder fluid towards the heated wall and, consequently, more heat is transferred between the fluid and heated wall.

**Figure 5.** Streamlines colored by velocity magnitude for PT and twisted tape inserts with P = L, L/2, L/3, L/4 at = 250. **Figure 5.** Streamlines colored by velocity magnitude for PT and twisted tape inserts with P = L, L/2, L/3, L/4 at = 250. **Figure 5.** Streamlines colored by velocity magnitude for PT and twisted tape inserts with P = L, L/2, L/3, L/4 at *Re* = 250.

Figure 6 shows how decreasing the twisted tape pitch magnitude affects the cooling of the heated wall at = 250. The temperature distribution on the heated wall implies that the secondary and mixing flow intensity affect the heated wall. In PT, the uniform enhancement of temperature along the tube length is visible, showing the thermal boundary layer development without any disturbance. On the contrary, as the twisted tape is inserted in the tube, the change in temperature distribution on the heated wall is visible. The temperature value on the heated wall decreases along the tube length and strengthens as the pitch value reduces. In higher pitch values, some hotspot regions are visible on the heated wall temperature contour; however, these regions decline at lower pitch values, resulting in better cooling performance of the system. Figure 6 shows how decreasing the twisted tape pitch magnitude affects the cooling of the heated wall at = 250. The temperature distribution on the heated wall implies that the secondary and mixing flow intensity affect the heated wall. In PT, the uniform enhancement of temperature along the tube length is visible, showing the thermal boundary layer development without any disturbance. On the contrary, as the twisted tape is inserted in the tube, the change in temperature distribution on the heated wall is visible. The temperature value on the heated wall decreases along the tube length and strengthens as the pitch value reduces. In higher pitch values, some hotspot regions are visible on the heated wall temperature contour; however, these regions decline at lower pitch values, resulting in better cooling performance of the system. Figure 6 shows how decreasing the twisted tape pitch magnitude affects the cooling of the heated wall at *Re* = 250. The temperature distribution on the heated wall implies that the secondary and mixing flow intensity affect the heated wall. In PT, the uniform enhancement of temperature along the tube length is visible, showing the thermal boundary layer development without any disturbance. On the contrary, as the twisted tape is inserted in the tube, the change in temperature distribution on the heated wall is visible. The temperature value on the heated wall decreases along the tube length and strengthens as the pitch value reduces. In higher pitch values, some hotspot regions are visible on the heated wall temperature contour; however, these regions decline at lower pitch values, resulting in better cooling performance of the system.

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**Figure 6.** Temperature contours on the heated wall for PT and twisted tape inserts with P = L, L/2, L/3, L/4 at = 250. **Figure 6.** Temperature contours on the heated wall for PT and twisted tape inserts with P = L, L/2, L/3, L/4 at *Re* = 250. **Figure 6.** Temperature contours on the heated wall for PT and twisted tape inserts with P = L, L/2, L/3, L/4 at = 250.

To better understand the temperature distribution, four different cross-sections are defined along the tube length and are displayed in Figure 7. In the following, different parameters, such as temperature and velocity, are illustrated in them. To better understand the temperature distribution, four different cross-sections are defined along the tube length and are displayed in Figure 7. In the following, different parameters, such as temperature and velocity, are illustrated in them. To better understand the temperature distribution, four different cross-sections are defined along the tube length and are displayed in Figure 7. In the following, different parameters, such as temperature and velocity, are illustrated in them.

**Figure 7.** Generated cross-sectional surfaces throughout the tube length for post-processing purposes. **Figure 7.** Generated cross-sectional surfaces throughout the tube length for post-processing purposes. **Figure 7.** Generated cross-sectional surfaces throughout the tube length for post-processing purposes.

Figure 8 demonstrates the cross-sectional temperature contours on the surfaces shown in Figure 7 for PT and four twisted tape pitch values at = 250. The PT temperature contours show the normal development of the thermal boundary layer throughout the tube, leading to a great drop in heat transfer along the tube. On the other hand, the thermal boundary layer disturbance is intensified and gets thinner as the twisted tape is inserted in the tube, which is more effective for a lower pitch in heat transfer between the fluid and heated wall. Besides, fewer hotspot regions causing a reduction in heat transfer enhancement is visible at lower pitch values of the twisted tape, proving the cooling process improvement. Another point that can be noticed in this figure is that the presence of twisted tape and lowering its pitch value redirects the core colder fluid to the vicinity of the hot wall. The twisted tape causes more efficient heat dissipation from the wall and, consequently, more heat is transferred from the wall to the fluid. It should be noted that the contours are almost symmetrical, related to the center of the tube in all cases. Figure 8 demonstrates the cross-sectional temperature contours on the surfaces shown in Figure 7 for PT and four twisted tape pitch values at = 250. The PT temperature contours show the normal development of the thermal boundary layer throughout the tube, leading to a great drop in heat transfer along the tube. On the other hand, the thermal boundary layer disturbance is intensified and gets thinner as the twisted tape is inserted in the tube, which is more effective for a lower pitch in heat transfer between the fluid and heated wall. Besides, fewer hotspot regions causing a reduction in heat transfer enhancement is visible at lower pitch values of the twisted tape, proving the cooling process improvement. Another point that can be noticed in this figure is that the presence of twisted tape and lowering its pitch value redirects the core colder fluid to the vicinity of the hot wall. The twisted tape causes more efficient heat dissipation from the wall and, consequently, more heat is transferred from the wall to the fluid. It should be noted that the contours are almost symmetrical, related to the center of the tube in all cases. Figure 8 demonstrates the cross-sectional temperature contours on the surfaces shown in Figure 7 for PT and four twisted tape pitch values at *Re* = 250. The PT temperature contours show the normal development of the thermal boundary layer throughout the tube, leading to a great drop in heat transfer along the tube. On the other hand, the thermal boundary layer disturbance is intensified and gets thinner as the twisted tape is inserted in the tube, which is more effective for a lower pitch in heat transfer between the fluid and heated wall. Besides, fewer hotspot regions causing a reduction in heat transfer enhancement is visible at lower pitch values of the twisted tape, proving the cooling process improvement. Another point that can be noticed in this figure is that the presence of twisted tape and lowering its pitch value redirects the core colder fluid to the vicinity of the hot wall. The twisted tape causes more efficient heat dissipation from the wall and, consequently, more heat is transferred from the wall to the fluid. It should be noted that the contours are almost symmetrical, related to the center of the tube in all cases.

tape plane.

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**Figure 8.** Cross-sectional temperature contours for PT and twisted tape inserts with P = L, L/2, L/3, L/4 at = 250. **Figure 8.** Cross-sectional temperature contours for PT and twisted tape inserts with P = L, L/2, L/3, L/4 at *Re* = 250.

Figure 9 displays the cross-sectional velocity contours on the surfaces shown in Figure 7 for PT and four twisted tape pitch values at = 250 . The velocity contours imply the intensity of secondary and mixing flow in the presence of twisted tape inserts. In other words, the higher velocity of the fluid near the heated wall shows a stronger secondary flow, and, as a result, a higher fluid momentum near the wall and a better cooling process could be achieved. It is visible in this figure that inserting twisted tape with a pitch value of L results in a high-velocity region of the fluid, which is strengthened as the twisted tape pitch value decreases, resulting in the stronger secondary flow observed in Figure 5. It should be noted that the contours are almost symmetrical, related to the center of the tube in all cases; however, for the case with P = L, it is almost symmetrical related to the twisted Figure 9 displays the cross-sectional velocity contours on the surfaces shown in Figure 7 for PT and four twisted tape pitch values at *Re* = 250. The velocity contours imply the intensity of secondary and mixing flow in the presence of twisted tape inserts. In other words, the higher velocity of the fluid near the heated wall shows a stronger secondary flow, and, as a result, a higher fluid momentum near the wall and a better cooling process could be achieved. It is visible in this figure that inserting twisted tape with a pitch value of L results in a high-velocity region of the fluid, which is strengthened as the twisted tape pitch value decreases, resulting in the stronger secondary flow observed in Figure 5. It should be noted that the contours are almost symmetrical, related to the center of the tube in all cases; however, for the case with P = L, it is almost symmetrical related to the twisted tape plane.

**Figure 9.** Cross-sectional velocity contours for PT and twisted tape inserts with P = L, L/2, L/3, L/4 at = 250. **Figure 9.** Cross-sectional velocity contours for PT and twisted tape inserts with P = L, L/2, L/3, L/4 at *Re* = 250.

To better quantify the heat transfer modification in the presence of twisted tape inserts with different pitch values, Figure 10 is provided for various . increases as changes due to the more effective advection phenomenon and fluid momentum in higher fluid velocities. As seen in this figure, as an example, using twisted tape with pitch values of L, L/2, L/3 and L/4 increases the average by about 26.87, 55.03, 86.59 and 151.42% at = 1000 compared with the PT, respectively. To better quantify the heat transfer modification in the presence of twisted tape inserts with different pitch values, Figure 10 is provided for various *Re*. *Nu* increases as *Re* changes due to the more effective advection phenomenon and fluid momentum in higher fluid velocities. As seen in this figure, as an example, using twisted tape with pitch values of L, L/2, L/3 and L/4 increases the average *Nu* by about 26.87, 55.03, 86.59 and 151.42% at *Re* = 1000 compared with the PT, respectively.

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**Figure 10.** Variations of average ratio with respect to PT at various pitch values and . **Figure 10.** Variations of average *Nu* ratio with respect to PT at various pitch values and *Re*. **Figure 10.** Variations of average ratio with respect to PT at various pitch values and .

Figure 11 shows the variations of the friction coefficient ratio with respect to PT for different pitch values and . This figure shows that applying twisted tape enhances the friction coefficient ratio due to the added surface area and flow blockage [52]. Furthermore, higher values for the friction coefficient ratio are observed in lower pitch values due to the creation of the more intense secondary flow shown in Figure 5; however, when the twisted tape pitch is equal to L, its enhancement is insignificant due to the creation of very weak secondary flow in the tube. Moreover, this figure reveals that the friction coefficient ratio increases as changes because more intense swirling flow. Figure 11 shows the variations of the friction coefficient ratio with respect to PT for different pitch values and *Re*. This figure shows that applying twisted tape enhances the friction coefficient ratio due to the added surface area and flow blockage [52]. Furthermore, higher values for the friction coefficient ratio are observed in lower pitch values due to the creation of the more intense secondary flow shown in Figure 5; however, when the twisted tape pitch is equal to L, its enhancement is insignificant due to the creation of very weak secondary flow in the tube. Moreover, this figure reveals that the friction coefficient ratio increases as *Re* changes because more intense swirling flow. Figure 11 shows the variations of the friction coefficient ratio with respect to PT for different pitch values and . This figure shows that applying twisted tape enhances the friction coefficient ratio due to the added surface area and flow blockage [52]. Furthermore, higher values for the friction coefficient ratio are observed in lower pitch values due to the creation of the more intense secondary flow shown in Figure 5; however, when the twisted tape pitch is equal to L, its enhancement is insignificant due to the creation of very weak secondary flow in the tube. Moreover, this figure reveals that the friction coefficient ratio increases as changes because more intense swirling flow.

**Figure 11.** Variations of friction coefficient ratio with respect to PT at various pitch values and . **Figure 11.** Variations of friction coefficient ratio with respect to PT at various pitch values and . **Figure 11.** Variations of friction coefficient ratio with respect to PT at various pitch values and *Re*.

#### *5.2. Effect of Truncated Twisted Tape Position and Percentage 5.2. Effect of Truncated Twisted Tape Position and Percentage 5.2. E*ff*ect of Truncated Twisted Tape Position and Percentage*

So far, it has been shown that the twisted tape with a pitch of L/4 results in the best thermal performance of the system. Therefore, the following simulations are performed for this pitch value. To show the effect of twisted tape truncation percentage on local throughout the tube length for different twisted tape positions, Figure 12 is provided at = 250. In Figure 12a, the twisted tape is embedded at the entrance of the tube with different values for λ. It is visible that as the flow enters the twisted tape at the tube inlet, the thermal boundary layer is disturbed and, due to the creation of secondary flow, the local increases; however, as the flow passes the twisted tape, the thermal boundary layer starts to develop normally, and as a result, it tends to develop towards the local So far, it has been shown that the twisted tape with a pitch of L/4 results in the best thermal performance of the system. Therefore, the following simulations are performed for this pitch value. To show the effect of twisted tape truncation percentage on local throughout the tube length for different twisted tape positions, Figure 12 is provided at = 250. In Figure 12a, the twisted tape is embedded at the entrance of the tube with different values for λ. It is visible that as the flow enters the twisted tape at the tube inlet, the thermal boundary layer is disturbed and, due to the creation of secondary flow, the local increases; however, as the flow passes the twisted tape, the thermal boundary layer starts to develop normally, and as a result, it tends to develop towards the local So far, it has been shown that the twisted tape with a pitch of L/4 results in the best thermal performance of the system. Therefore, the following simulations are performed for this pitch value. To show the effect of twisted tape truncation percentage on local *Nu* throughout the tube length for different twisted tape positions, Figure 12 is provided at *Re* = 250. In Figure 12a, the twisted tape is embedded at the entrance of the tube with different values for λ. It is visible that as the flow enters the twisted tape at the tube inlet, the thermal boundary layer is disturbed and, due to the creation of secondary flow, the local *Nu* increases; however, as the flow passes the twisted tape, the thermal boundary layer starts to develop normally, and as a result, it tends to develop towards the local *Nu*

tape is inserted at the tube entrance.

curve of PT. Consequently, lower values of λ cause a higher heat transfer rate when the twisted tape is inserted at the tube entrance. in at the twisted tape entrance, the local decreases, and its curve tends to reach the value of the fully fitted twisted tape.

thermal boundary layer disturbance and stronger mixing flow. In this case, after a sudden increase

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curve of PT. Consequently, lower values of λ cause a higher heat transfer rate when the twisted

In Figure 12b, the twisted tape is embedded at the center of the tube with different values for λ. In this figure, the local curve starts to grow as the fluid reaches the twisted tape due to the thermal boundary layer disturbance. It is visible that the local at the truncated cases surpasses the fully fitted twisted tape, and a higher maximum value for is visible for higher values of λ. As the flow passes the twisted tape, the thermal boundary layer starts to grow and becomes fully developed.

**Figure 12.** Local along the tube length for different λ values at three twisted tape positions for (**a**) entrance, (**b**) center, (**c**) exit and P = L/4. **Figure 12.** Local *Nu* along the tube length for different λ values at three twisted tape positions for (**a**) entrance, (**b**) center, (**c**) exit and P = L/4.

Figure 13 displays the streamlines colored by velocity magnitude in different twisted tape truncation percentage values inserted at the tube entrance for = 250. In the truncated cases, the flow path swirls to the end of tube length, but the secondary flow intensity is reduced as the fluid In Figure 12b, the twisted tape is embedded at the center of the tube with different values for λ. In this figure, the local *Nu* curve starts to grow as the fluid reaches the twisted tape due to the thermal boundary layer disturbance. It is visible that the local *Nu* at the truncated cases surpasses the fully fitted twisted tape, and a higher maximum value for *Nu* is visible for higher values of λ. As the flow passes the twisted tape, the thermal boundary layer starts to grow and becomes fully developed.

In Figure 12c, the twisted tape is embedded at the exit of the tube with different values for λ. This figure also shows the local *Nu* enhancement as the fluid enters the twisted tape as a result of thermal boundary layer disturbance and stronger mixing flow. In this case, after a sudden increase in *Nu* at the twisted tape entrance, the local *Nu* decreases, and its curve tends to reach the value of the fully fitted twisted tape.

Figure 13 displays the streamlines colored by velocity magnitude in different twisted tape truncation percentage values inserted at the tube entrance for *Re* = 250. In the truncated cases, the flow path swirls to the end of tube length, but the secondary flow intensity is reduced as the fluid passes the twisted tape. As a result, truncating the twisted tape results in a reduction in heat transfer rate but less material is used, causing fewer production expenses, and also the pressure loss penalty reduces due to less flow lockage and contact area between the fluid and solid. *Symmetry* **2020**, *12*, x FOR PEER REVIEW 13 of 23 passes the twisted tape. As a result, truncating the twisted tape results in a reduction in heat transfer rate but less material is used, causing fewer production expenses, and also the pressure loss penalty reduces due to less flow lockage and contact area between the fluid and solid. *Symmetry* **2020**, *12*, x FOR PEER REVIEW 13 of 23 passes the twisted tape. As a result, truncating the twisted tape results in a reduction in heat transfer rate but less material is used, causing fewer production expenses, and also the pressure loss penalty

**Figure 13.** Streamline colored by velocity magnitude for different λ values at Re = 250 and P = L/4. **Figure 13.** Streamline colored by velocity magnitude for different λ values at Re = 250 and P = L/4. **Figure 13.** Streamline colored by velocity magnitude for different λ values at Re = 250 and P = L/4.

To show the effect of truncated twisted tape inserts and changes in the flow patch shown in Figure 13 on the temperature distribution of the heated wall, Figure 14 illustrates the temperature contours at the wall. It is visible that although there are some hotspots after the truncated twisted tape, the number of hotspots is still lower than for the PT, showing the effective cooling process even after the fluid passes the twisted tape. This figure proves the presence of secondary flow (not as intense as it is in the fully twisted tape insert) after the fluid passes the truncated twisted tape. To show the effect of truncated twisted tape inserts and changes in the flow patch shown in Figure 13 on the temperature distribution of the heated wall, Figure 14 illustrates the temperature contours at the wall. It is visible that although there are some hotspots after the truncated twisted tape, the number of hotspots is still lower than for the PT, showing the effective cooling process even after the fluid passes the twisted tape. This figure proves the presence of secondary flow (not as intense as it is in the fully twisted tape insert) after the fluid passes the truncated twisted tape. To show the effect of truncated twisted tape inserts and changes in the flow patch shown in Figure 13 on the temperature distribution of the heated wall, Figure 14 illustrates the temperature contours at the wall. It is visible that although there are some hotspots after the truncated twisted tape, the number of hotspots is still lower than for the PT, showing the effective cooling process even after the fluid passes the twisted tape. This figure proves the presence of secondary flow (not as intense as it is in the fully twisted tape insert) after the fluid passes the truncated twisted tape.

**Figure 14.** Temperature contours on the heated wall for different λ values at Re = 250 and P = L/4. **Figure 14.** Temperature contours on the heated wall for different λ values at Re = 250 and P = L/4. **Figure 14.** Temperature contours on the heated wall for different λ values at Re = 250 and P = L/4.

To clearly show the effect of twisted tape position on the local throughout the tube length at different twisted tape truncation percentage values, Figure 15 illustrates the local for three different twisted tape truncation percentage values of 25, 50 and 75% at = 250 for different positions, as shown in Figure 1b. This figure shows that when the twisted tape is at the tube entrance, no sudden increase in is visible; in contrast, there is a maximum value for along the tube length when the twisted tape is inserted at the center and exit of the tube. For all values of λ, the highest maximum throughout the tube length corresponds to the layouts where twisted tape is inserted at the tube exit, which is higher as λ changes. To clearly show the effect of twisted tape position on the local throughout the tube length at different twisted tape truncation percentage values, Figure 15 illustrates the local for three different twisted tape truncation percentage values of 25, 50 and 75% at = 250 for different positions, as shown in Figure 1b. This figure shows that when the twisted tape is at the tube entrance, no sudden increase in is visible; in contrast, there is a maximum value for along the tube length when the twisted tape is inserted at the center and exit of the tube. For all values of λ, the highest maximum throughout the tube length corresponds to the layouts where twisted tape is inserted at the tube exit, which is higher as λ changes. To clearly show the effect of twisted tape position on the local *Nu* throughout the tube length at different twisted tape truncation percentage values, Figure 15 illustrates the local *Nu* for three different twisted tape truncation percentage values of 25, 50 and 75% at *Re* = 250 for different positions, as shown in Figure 1b. This figure shows that when the twisted tape is at the tube entrance, no sudden increase in *Nu* is visible; in contrast, there is a maximum value for *Nu* along the tube length when the twisted tape is inserted at the center and exit of the tube. For all values of λ, the highest maximum *Nu* throughout the tube length corresponds to the layouts where twisted tape is inserted at the tube exit, which is higher as λ changes.

*Symmetry* **2020**, *12*, x FOR PEER REVIEW 14 of 23

**Figure 15.** Local along the tube length for different twisted tape positions at three λ values of (**a**) 25%, (**b**) 50%, (**c**) 75% and P = L/4. **Figure 15.** Local *Nu* along the tube length for different twisted tape positions at three λ values of (**a**) 25%, (**b**) 50%, (**c**) 75% and P = L/4.

Figure 16 illustrates the streamlines colored by velocity magnitude at different positions of the twisted tape in the tube at a twisted tape truncation percentage of 75% and = 250. This figure shows that when the twisted tape is embedded at the tube entrance, the whole flow patch is affected. When the twisted tape is at the tube center, half of the tube length is affected, and for the case in which twisted tape is at the tube exit, the flow path before reaching the twisted tape is similar to that Figure 16 illustrates the streamlines colored by velocity magnitude at different positions of the twisted tape in the tube at a twisted tape truncation percentage of 75% and *Re* = 250. This figure shows that when the twisted tape is embedded at the tube entrance, the whole flow patch is affected. When the twisted tape is at the tube center, half of the tube length is affected, and for the case in which twisted tape is at the tube exit, the flow path before reaching the twisted tape is similar to that of PT.

of PT. The effect of the position of the truncated twisted tape inserts on the temperature distribution of the heated wall is depicted in Figure 17. The same temperature distribution before the flow reaches the twisted tape as that of PT for the heated wall temperature distribution is also visible in this figure. Moreover, flow mixing as the fluid passes the twisted tape and the strong secondary flow in the twisted tape regions are observable in this figure.

*Symmetry* **2020**, *12*, x FOR PEER REVIEW 15 of 23

*Symmetry* **2020**, *12*, x FOR PEER REVIEW 15 of 23

**Figure 16.** Streamline colored by velocity magnitude for different twisted tape positions at Re = 250, P = L/4 and λ = 0.75. **Figure 16.** Streamline colored by velocity magnitude for different twisted tape positions at Re = 250, P = L/4 and λ = 0.75. the twisted tape as that of PT for the heated wall temperature distribution is also visible in this figure. Moreover, flow mixing as the fluid passes the twisted tape and the strong secondary flow in the

twisted tape regions are observable in this figure.

**Figure 17.** Temperature contours on the heated wall for different twisted tape positions at Re = 250, P = L/4 and λ = 0.75. **Figure 17.** Temperature contours on the heated wall for different twisted tape positions at Re = 250, P = L/4 and λ = 0.75.

**Figure 17.** Temperature contours on the heated wall for different twisted tape positions at Re = 250, P = L/4 and λ = 0.75. To quantify the heat transfer rate for different truncation values and positions of the twisted tape, Figure 18 is provided to show the variations of the ratio compared with PT at various λ and Re values, different twisted tape positions and a pitch of L/4. For all twisted tape positions and , higher values of λ result in lower and heat transfer due to the fact that the secondary and mixing To quantify the heat transfer rate for different truncation values and positions of the twisted tape, Figure 18 is provided to show the variations of the ratio compared with PT at various λ and Re values, different twisted tape positions and a pitch of L/4. For all twisted tape positions and , higher values of λ result in lower and heat transfer due to the fact that the secondary and mixing flows in a tube fully fitted with twisted tape are much stronger than for a tube equipped with a truncated twisted tape, as shown in Figure 14. As seen in this figure, using twisted tape with a pitch value of L/4 at the entrance of the tube and λ values of 0, 25, 50 and 75% increase the average by about 71.26, 68.50, 57.59 and 37.34% at = 250 and 151.42, 133.99, 109.52 and 71.43% at = 1000 in comparison with the PT, respectively. To quantify the heat transfer rate for different truncation values and positions of the twisted tape, Figure 18 is provided to show the variations of the *Nu* ratio compared with PT at various λ and Re values, different twisted tape positions and a pitch of L/4. For all twisted tape positions and *Re*, higher values of λ result in lower *Nu* and heat transfer due to the fact that the secondary and mixing flows in a tube fully fitted with twisted tape are much stronger than for a tube equipped with a truncated twisted tape, as shown in Figure 14. As seen in this figure, using twisted tape with a pitch value of L/4 at the entrance of the tube and λ values of 0, 25, 50 and 75% increase the average *Nu* by about 71.26, 68.50, 57.59 and 37.34% at *Re* = 250 and 151.42, 133.99, 109.52 and 71.43% at *Re* = 1000 in comparison with the PT, respectively.

flows in a tube fully fitted with twisted tape are much stronger than for a tube equipped with a truncated twisted tape, as shown in Figure 14. As seen in this figure, using twisted tape with a pitch value of L/4 at the entrance of the tube and λ values of 0, 25, 50 and 75% increase the average by about 71.26, 68.50, 57.59 and 37.34% at = 250 and 151.42, 133.99, 109.52 and 71.43% at = 1000 in comparison with the PT, respectively. Considering the position of the twisted tape, for all values of λ at Re of 1000, when the truncated twisted tape is placed at the tube entrance, a higher *Nu* is obtained, followed by the cases with twisted tape at the tube center, and the lowest *Nu* correspond to the cases with twisted tape at the tube exit. This is due to the fact that as the fluid passes the twisted tape, the flow is still affected by the twisted tape and swirl flow is visible to the end of the tube, as shown in Figure 16. Thus, for the case of twisted tape inserts in the tube entrance, more of the tube length experiences swirl flow, and as a result, the cooling process improves.

**Figure 18.** Variations of average ratio with respect to PT at different twisted tape positions and λ values for Re = 1000. **Figure 18.** Variations of average *Nu* ratio with respect to PT at different twisted tape positions and λ values for Re = 1000.

Considering the position of the twisted tape, for all values of λ at Re of 1000, when the truncated twisted tape is placed at the tube entrance, a higher is obtained, followed by the cases with twisted tape at the tube center, and the lowest correspond to the cases with twisted tape at the tube exit. This is due to the fact that as the fluid passes the twisted tape, the flow is still affected by the twisted tape and swirl flow is visible to the end of the tube, as shown in Figure 16. Thus, for the case of twisted tape inserts in the tube entrance, more of the tube length experiences swirl flow, and as a result, the cooling process improves. Figure 19 represents the variations of friction coefficient ratio with respect to PT at various λ for *Re* of 1000, different twisted tape positions and a pitch of L/4. It is visible that for all twisted tape positions, as λ increases, the friction coefficient ratio is reduced due to the lower level of solid material Figure 19 represents the variations of friction coefficient ratio with respect to PT at various λ for *Re* of 1000, different twisted tape positions and a pitch of L/4. It is visible that for all twisted tape positions, as λ increases, the friction coefficient ratio is reduced due to the lower level of solid material and, as a result, less flow blockage and smaller contact area between the fluid flow and solid. Furthermore, for all values of λ, there are almost similar values for the friction coefficient ratio for the cases where twisted tape is placed at the tube entrance and center; however, lower values for the friction coefficient ratio are observable when the twisted tape is embedded at the tube exit. The cause of this scenario can be attributed to the fact that when the twisted tape is at the tube exit, no swirl flow is generated before the fluid reaches the twisted tape. *Symmetry* **2020**, *12*, x FOR PEER REVIEW 17 of 23

**Figure 19.** Variations of friction coefficient ratio with respect to PT at different twisted tape positions and λ values for Re = 1000. **Figure 19.** Variations of friction coefficient ratio with respect to PT at different twisted tape positions and λ values for Re = 1000.

As discussed above, the application of twisted tape inserts in the captured cases increases both the heat transfer rate as a desirable outcome and the friction coefficient as an undesirable result. Therefore, there is an interplay between the benefits of using twisted tape in heat transfer enhancement and their side effects in forcing more pumping power into the system. To analyze this issue, the dimensionless PEC number introduced in Equation (9) is discussed here. In fact, this

increases, the sensitivity of PEC to the λ value is reduced.

λ (%) 0 25 50 75

Entrance Center Exit

PEC 0.94 0.96 0.98 1.00 1.02 1.04 1.06 1.08 1.10 1.12

viewpoint of energy-saving potential. Generally, higher values of PEC imply superior energy saving. Figure 20 illustrates the PEC parameter for all cases investigated in this study for Re of 1000. It is visible that decreasing the twisted tape pitch value from Figure 20a–d enhances the PEC number, showing the fact that applying twisted tape with a lower pitch value is efficient from the viewpoint of both heat transfer enhancement and energy saving. It is also visible that as the twisted tape pitch

(**a**) (**b**)

PEC 0.90 0.95 1.00 1.05 1.10 1.15 1.20 1.25 1.30

> λ (%) 0 25 50 75

Entrance Center Exit

**6. Conclusions**

reduction.

Canada.

and λ values for Re = 1000.

1.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

3.0

f/f0

As discussed above, the application of twisted tape inserts in the captured cases increases both the heat transfer rate as a desirable outcome and the friction coefficient as an undesirable result. Therefore, there is an interplay between the benefits of using twisted tape in heat transfer enhancement and their side effects in forcing more pumping power into the system. To analyze this issue, the dimensionless PEC number introduced in Equation (9) is discussed here. In fact, this number is used to evaluate the practical use of any modified heat transfer technique from the viewpoint of energy-saving potential. Generally, higher values of PEC imply superior energy saving. Figure 20 illustrates the PEC parameter for all cases investigated in this study for Re of 1000. It is visible that decreasing the twisted tape pitch value from Figure 20a–d enhances the PEC number, showing the fact that applying twisted tape with a lower pitch value is efficient from the viewpoint of both heat transfer enhancement and energy saving. It is also visible that as the twisted tape pitch increases, the sensitivity of PEC to the λ value is reduced. the heat transfer rate as a desirable outcome and the friction coefficient as an undesirable result. Therefore, there is an interplay between the benefits of using twisted tape in heat transfer enhancement and their side effects in forcing more pumping power into the system. To analyze this issue, the dimensionless PEC number introduced in Equation (9) is discussed here. In fact, this number is used to evaluate the practical use of any modified heat transfer technique from the viewpoint of energy-saving potential. Generally, higher values of PEC imply superior energy saving. Figure 20 illustrates the PEC parameter for all cases investigated in this study for Re of 1000. It is visible that decreasing the twisted tape pitch value from Figure 20a–d enhances the PEC number, showing the fact that applying twisted tape with a lower pitch value is efficient from the viewpoint of both heat transfer enhancement and energy saving. It is also visible that as the twisted tape pitch increases, the sensitivity of PEC to the λ value is reduced.

**Figure 19.** Variations of friction coefficient ratio with respect to PT at different twisted tape positions

λ (%) 0 25 50 75

Entrance Center Exit

As discussed above, the application of twisted tape inserts in the captured cases increases both

*Symmetry* **2020**, *12*, x FOR PEER REVIEW 17 of 23

**Figure 20.** Variations of PEC number at different twisted tape positions, λ and P values. (**a**) P = L, (**b**) P = L/2, (**c**) P = L/3 and (**d**) P = L/4 for Re = 1000. **Figure 20.** Variations of PEC number at different twisted tape positions, λ and P values. (**a**) P = L, (**b**) P = L/2, (**c**) P = L/3 and (**d**) P = L/4 for Re = 1000.

Furthermore, for all λ values, placing the twisted tape at the tube entrance leads to higher PEC magnitudes. Therefore, for P = L, L/2, L/3 and L/4, the optimum cases from the viewpoint of energy saving are twisted tapes with λ = 75, 50, 50 and 0%, for which the related PEC numbers at = 1000 are almost equal to 1.08, 1.24, 1.4 and 1.76, respectively. In addition, PEC numbers in all cases are tabulated in Appendix A of this paper. Furthermore, for all λ values, placing the twisted tape at the tube entrance leads to higher PEC magnitudes. Therefore, for P = L, L/2, L/3 and L/4, the optimum cases from the viewpoint of energy saving are twisted tapes with λ = 75, 50, 50 and 0%, for which the related PEC numbers at *Re* = 1000 are almost equal to 1.08, 1.24, 1.4 and 1.76, respectively. In addition, PEC numbers in all cases are tabulated in Appendix A of this paper.

pressure drop penalty, the PEC number was calculated. The obtained results indicated that using the twisted tape and reducing its pitch value increases the Nusselt number, friction coefficient and PEC number due to the generation of efficient secondary and mixing flow. The average Nusselt number increased by about 151.42 for a Reynolds number of 1000 in the case of fully fitted twisted tape at a pitch value of L/4. It was also found that increasing the twisted tape truncation percentage reduced both heat transfer and pressure drop in comparison with the fully fitted twisted tape case. Moreover, the best position for the truncated twisted tapes was at the tube entrance to reach the highest thermal performance. Ultimately, it was concluded that for P = L, L/2, L/3 and L/4, the optimum cases from the viewpoint of energy saving are twisted tapes with λ = 75, 50, 50 and 0%, for which the related PEC numbers at a Reynolds number of 1000 are almost equal to 1.08, 1.24, 1.4 and 1.76, respectively. The finding of this research provides a framework for researchers working in this area toward higher performance based on energy saving according to both heat transfer enhancement and pressure drop

**Author Contributions:** R.M. and P.T. performed and designed the numerical simulations; M.G., H.A., R.M., H.M.A. and W.Y. analyzed the data and performed the discussion; M.G., H.A., R.M., P.T., H.M.A. and W.Y.

**Funding:** The APC was funded by Dr Wahiba Yaïci from CanmetENERGY Research Centre, Natural Resources

wrote the paper. All authors have read and agreed to the published version of the manuscript.

**Conflicts of Interest**: The authors declare no conflict of interest.

This paper investigated the laminar convection heat transfer in a tube equipped with twisted tape inserts. A parametric study was conducted to evaluate the impact of key design variables, including the twisted tape truncation percentage, pitch value, position in the tube and Reynolds


**4. Conclusions**

To investigate the influences of process parameters and powder formulation on microstructure, element distribution, and wear behavior, a series of laser cladding experimental runs was conducted on the AISI 1045 mild carbon steel with the cladding powder of Ni35A/TiC based on full factorial design. The correlation between process parameters and the wear resistance was then established after ANOVA of experimental results. Based on the results, the following conclusions can be obtained:

• The microstructure at the top region of the cladding layer can be controlled by scanning speed. At lower speed (5 mm/s), the dendritic crystal of TiC transforms into near sphere fine grains for the top area. This refines the microstructure to reduce stress concentration at grain tips and improves the wear properties.

**Author Contributions:** Methodology, X.H. and C.L.; experiment, C.L.; analysis, X.H., C.L., H.Z., C.C., G.L., J.J., M.F., and M.Z.; writing—original draft preparation, X.H. and C.L.; writing—review and editing, C.C. and H.Z.; supervision, X.H. and C.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Fujian University of Technology, Grant No. GY-Z18163; Fujian Innovation Center of Additive Manufacturing, Grant No. ZCZZ20-04; and Fujian Provincial Department of Human Resources and Social A ffairs, Grant No. GY-Z19045.

**Acknowledgments:** The authors gratefully acknowledge the support from the Public Service Platform for Technical Innovation of Machine Tool Industry in Fujian Province at the Fujian University of Technology.

**Conflicts of Interest:** The authors declare no conflict of interest.


*Edited by Ahmad Naqiyuddin Bakar and Mohd Nazip Suratman*

This book aims to examine the context and practice of national parks regarding a countries' obligations to safeguard biodiversity through the protection and management of forest-protected areas. The book examines the wider impacts of national parks within the scope of an integrated environmental hub at the global and regional level and eventually delves into the country case. Three areas are covered: theoretical underpinnings and concepts related to national parks, exploring their various modalities and integrated concerns for the environment; an empirical review in lieu of effective management of protected areas as defined by the World Conservation Union IUCN, addressing the efficient use of human and material resources, including national/agency-protected area regulations and legislation, policies, international conventions and designations, management plans, and/or agreements associated with those areas; and evaluation of challenges underlying a country's intention to gauge the potential of a national park and pinpoint adequate attention on exploiting new strategies for national park management.

Published in London, UK © 2020 IntechOpen © Jack Over / unsplash

Protected Areas, National Parks and Sustainable Future

Protected Areas, National

Parks and Sustainable Future

*Edited by Ahmad Naqiyuddin Bakar*

*and Mohd Nazip Suratman*


*2.1. Electrical Installation Methods*

First, the different installation possibilities that can be used for the electrical installation of the BIPV system are briefly introduced. They are shown in Figure 1. In the string inverter approach (Figure 1a), several PV modules are series connected to form a string and are then coupled to the string inverter. Multiple strings are possible per inverter. The Maximum Power Point Tracking (MPPT) is done per string. A second option is the use of micro-inverters (Figure 1b) where one inverter is coupled with a single PV module. The MPPT is done per module, and the output is coupled directly to the AC grid. A third option is the use of series power optimizers (Figure 1c). There is again one converter per module, but the output is now DC instead of AC. The output ports of multiple converters are then series connected and coupled to a central inverter that regulates the DC bus voltage. The last option is the use of parallel power optimizers (Figure 1d). As with the series power optimizers, their output is DC. However, the DC voltage level is higher. They are parallel connected to the same DC bus, and the MPPT is also done per module. The different installation possibilities will be further evaluated for BIPV applications in Section 2.8.


*2.1. Chemistry*

The main approach to producing 4-arylcoumarins unsubstituted at the hydroxy group is acid-catalyzed Pechmann condensation between resorcinol **4** and ester of β-keto-carboxylic acids **5** [63] (Scheme 1). Using this approach, we synthesized 7-hydroxy-4-arylcoumarins **6a–d** with yields of 63-81% by interaction of resorcinol **4** with esters **5a–d** (Scheme 1). Ester **5a** is commercially available, while compounds **5b–d** were obtained by the reaction of substituted acetophenones **5b–d** with diethyl carbonate in the presence of sodium hydride.

**Scheme 1.** Synthesis of 7-hydroxy-4-arylcoumarins **6a–d.**

Monoterpenoid bromides **8a–d** were obtained from the corresponding alcohols (geraniol, (−)-nopol, (−)- and (+)-myrtenols) by interaction with PBr3 according to the procedure [46] (Scheme 2). Geraniol and (−)-nopol were purchased from commercial sources, while (−)- and (+)-myrtenols were synthesized from (−)-myrtenal and (+)-α-pinene in accordance with the methods [46]. The choice of monoterpenoids was based on the results we obtained previously, i.e., when both the absolute configuration of the pinane cycle and the length of the bridge played important roles [46]. A further consideration was the desire to compare the activity data obtained with bulky bicyclic substituents with corresponding data for products with acyclic monoterpene fragments. Note that coumarin-containing derivatives of geraniol can be considered as analogues of the natural coumarin auraptene, having the same monoterpenoid fragment (Figure 1).

**Scheme 2.** Synthesis of monoterpenoid bromides **8a–d**.

The target monoterpenoid-arylcoumarin hybrids **3** were synthesized by the reaction of 7-hydroxycoumarins **6a–d** with monoterpenoid bromides **8a–d** using DBU in DMF (Scheme 3). To compare and identify the importance of the monoterpenoid fragment, compounds **10a, c, d** containing a benzyl substituent were also synthesized. The products were purified by recrystallization or column chromatography, and obtained with yields of 12–65%. In the case of the synthesis of nopol derivatives, a low conversion was observed, and double purification on SiO2 was required, for example, for compounds **3ab** or **3cb** with yields less than 20%).

**Scheme 3.** Synthesis of monoterpenoid-arylcoumarin hybrids.


**Seasonal Succession of Phytoplankton Functional Groups and Driving Factors of Cyanobacterial Blooms in a Subtropical Reservoir in South China**

**Lingai Yao 1,2, Xuemin Zhao 2,\*, Guang-Jie Zhou 3, Rongchang Liang 2, Ting Gou 2, Beicheng Xia 1,\*, Siyang Li 2 and Chang Liu 2**

Received: 12 March 2020; Accepted: 16 April 2020; Published: 19 April 2020

**Abstract:** Freshwater phytoplankton communities can be classified into a variety of functional groups that are based on physiological, morphological, and ecological characteristics. This classification method was used to study the temporal and spatial changes in the phytoplankton communities of Gaozhou Reservoir, which is a large municipal water source in South China. Between January 2015 and December 2017, a total of 155 taxa of phytoplankton that belong to seven phyla were identified. The phytoplankton communities were classified into 28 functional groups, nine of which were considered to be representative functional groups (relative biomass > 10%). Phytoplankton species richness was greater in the summer and autumn than in the winter and spring; cyanobacterial blooms occurred in the spring. The seasonal succession of phytoplankton functional groups was characterized by the occurrence of functional groups P (*Staurastrum* sp. and *Closterium acerosum*) and Y (*Cryptomonas ovata* and *Cryptomonas erosa*) in the winter and spring, and functional groups NA (*Cosmarium* sp. and *Staurodesmus* sp.) and P (*Staurastrum* sp. and *Closterium acerosum*) in the summer and autumn. The temperature, nitrogen, and phosphorus levels were the main factors driving seasonal changes in the phytoplankton communities of Gaozhou Reservoir. The functional group M (*Microcystis aeruginosa*) dominated the community during the cyanobacterial blooms in spring 2016, with the maximum algal cell density of 3.12 × 108 cells L−1. Relatively low temperature (20.8 ◦C), high concentrations of phosphorus (0.080–0.110 mg L−1), suitable hydrological and hydrodynamic conditions (e.g., relatively long retention time), and relatively closed geographic location in the reservoir were the key factors that stimulated the cyanobacterial blooms during the early stages.

**Keywords:** subtropical reservoir; functional groups; phytoplankton; seasonal succession; environmental factors; cyanobacterial bloom

#### **1. Introduction**

Phytoplankton, as primary producers in aquatic ecosystems, are important for the maintenance of a stable aquatic environment [1,2], and they strongly affect the total productivity of aquatic ecosystems [3]. Phytoplankton are particularly sensitive to physical, chemical, and biological changes in the aquatic environment [4], and increases in nutrient availability often lead to severe phytoplankton proliferation [5]. Once blooms form, aquatic ecosystem services, such as drinking water quality, fisheries, and landscape, become damaged [6]. The process of bloom formation has a destructive effect on phytoplankton community structure, due to the dominance of certain phytoplankton species [7]. Changes in the composition and structure of the phytoplankton community (e.g., species composition, diversity index, community structure, and quantitative distribution) can be used to evaluate the eutrophication status of reservoirs [8]. Thus, these metrics represent important indicators of the environmental quality of aquatic ecosystems [9].

Reynolds et al. [10] and Padisák et al. 2009 [11] proposed functional groups to describe both phytoplankton community structure and the changes to that structure. The species of phytoplankton from specific habitats with similar sensitivities are classified into the same functional group; in total 31 functional groups were devised for freshwater phytoplankton [12]. Phytoplankton functional groups have been used to study the effects of changes in aquatic ecology on the physiological, morphological, and ecological characteristics of phytoplankton in various rivers, lakes, and reservoirs worldwide [13–16]. Studies have shown that phytoplankton functional groups H, X1, LO, and S1 are the most representative during the rainy and dry seasons in semi-arid reservoirs in Brazil [17], while phytoplankton functional groups LM, P, T, and Y exhibit strong seasonal variation in Erhai Lake, Yunnan, China [18]. The application of phytoplankton functional group classification methods in aquatic ecosystems provides key data that reflect the dynamics of phytoplankton communities [19,20].

There are many methods for assessing the nutritional status of aquatic environments. The assessment of eutrophication in aquatic ecosystems is essentially a multivariate comprehensive decision-making process for aquatic ecosystems [21–23]. Based on the EU Water Framework Directive [19], Padisák et al. developed the Q index method, based on the taxonomy of the phytoplankton functional groups, to evaluate the ecological status of different types of water bodies [24]. Phytoplankton functional groups and Q index evaluation methods have been widely used in phytoplankton related research studies [17,25]. In addition, the comprehensive trophic level index (TLI (Σ)) has been widely used for evaluating eutrophication in reservoirs [26,27]. TLI (Σ) index, which is based on water quality parameters, provides a continuous value that represents the trophic state of a reservoir. Continuous numerical changes provide a basis that can be used to investigate the mechanisms underlying eutrophication [26]. In this study, two evaluation methods, Q index and TLI (Σ) index, were used to study phytoplankton community succession and the factors driving succession in a reservoir in southern China (i.e., Gaozhou Reservoir).

Gaozhou Reservoir is a large artificial reservoir in western Guangdong, China, with an annual water supply of 50 billion cubic meters. This reservoir is located in the monsoon region of China, between the northern tropics and southern subtropics. The climate in this region is warm, with abundant sunshine and rainfall. However, rainfall is unevenly distributed and it changes substantially with season. The rainfall in the spring and summer is usually twice that in autumn and winter [28,29]. In recent years, pollution originating from domestic sewage and agricultural non-point sources has increased in the catchment areas of Gaozhou Reservoir, resulting in serious eutrophication and cyanobacterial blooms [30,31]. Cyanobacterial blooms have been recorded four times since 2009 (in 2010, 2011, 2013, and 2016) [32,33], threatening the water quality security of the reservoir. Although many studies have investigated the seasonal succession of phytoplankton functional groups in rivers and lakes, the seasonal succession of phytoplankton functional groups in drinking water reservoirs, as well as the associated cyanobacterial blooms, remain largely unstudied.

The objectives of this study were (1) to identify the phytoplankton composition in the reservoir; (2) to determine the phytoplankton functional groups and their seasonal succession; (3) to analyze the driving factors for the occurrence of cyanobacterial blooms; and, (4) to understand and assess the status of the aquatic environment in the reservoir, while using the phytoplankton functional group classification method, as well as the Q index and the TLI (Σ) index. The study provides basic and useful data regarding seasonal succession of phytoplankton communities and environmental determinants of algal species and abundance, which can be effectively used to develop strategies for the management of cyanobacterial blooms in the reservoir.

#### **2. Materials and Methods**

#### *2.1. Study Period and Site Description*

The Gaozhou Reservoir is located in Maoming City, Guangdong Province, China. This region has a subtropical climate. The reservoir has a canyon shape, with a storage capacity of 1.15 × 109 cubic meters and a catchment area of 1022 km2 [34]. The water retention time (WRT) in the Gaozhou reservoir was about 451 days [34]. Three rivers (the Shenzhen River, the Pengqing River, and the Guding River) feed into the rain collection area of the two sub-areas of the reservoir: the Liangde and Shigu areas (Figure 1).

**Figure 1.** of the Gaozhou Reservoir and of the sampling sites in the reservoir.

The water samples were collected at seven sites in the Gaozhou Reservoir (Figure 1) in January, April, July, and November of 2015, 2016 and 2017 (12 sampling events with 84 samples in total). Sites S1 and S2 were located in the Liangde reservoir district; sites S3 and S4 were located in the Shigu reservoir district; and, sites S5, S6, and S7 were located at the entrances of the Shenzhen River, the Guding River, and the Pengqing River, respectively.

#### *2.2. Sample Analysis*

A portable Professional Plus Multiparameter Instrument (Pro Plus, YSI, USA) was used to measure the water temperature (WT), pH, dissolved oxygen (DO), and electrical conductivity (EC). Samples (5 L each) were taken at 0.5 m water depth at each site. The measurements of water quality parameters were performed according to the Chinese National Standard Method [35] (similar to those of the American Public Health Association [36]). The total nitrogen (TN), total phosphorus (TP), permanganate index (CODMn), and suspended solids (SS) were measured with unfiltered water, while nitrate (NO3-N) and ammonium (NH4-N) were measured with filtered water. The chlorophyll-a concentrations were determined after extraction with acetone [35,37].

At each site, a phytoplankton net (No.25, Beijing Purity Instrument) was dragged at 0.5 m water depth for five minutes. Subsequently, a 25-mL filtered water sample was collected, fixed by adding 1 mL of formalin solution, and then inspected using the eyepiece visual field counting method under a microscope. At each site, an additional 1 L water sample was collected at 0.5 m water depth and 10 mL of Lugol's solution was added on site in order to fix the sample. After sedimentation for 24 h, the water sample was concentrated to 30 mL, and then examined under a microscope to quantify cell densities [38]. Phytoplankton biomass was calculated according to the method that was described by Hillebrand et al. [39], while assuming that 1 mm3 equals 1 mg [40].

#### *2.3. Data Analysis*

The European Water Framework Directive (European Parliament and Council, 2000) developed a Q index evaluation method that is based on the interactions between functional groups and environmental characteristics [41]. The Q index primarily uses the composition of functional groups and phytoplankton biomass to characterize the eutrophic state of the water body, and it reflects aquatic ecological health status [11]. The Q index is calculated, as follows:

$$\mathbf{Q} = \sum\_{i=1}^{n} \left( \frac{\mathbf{n}\_i}{\mathbf{N}} \times \mathbf{F}\_i \right),\tag{1}$$

where N is the total biomass; n is the number of functional groups; ni is the biomass of the i-th functional group; and, Fi is the value of the i-th functional group [24].

We assigned an F value that ranges from 0 to 5, with higher values for the more-pristine assemblages in the reservoir, and lower values for assemblages that are typical of less-pristine conditions. Following Reynolds et al. [10], species representing > 10% of the total biomass were considered to be representative functional groups (i.e., the functional groups that made a substantial contribution to total biomass).

The Q index was divided into five levels: the values 0–1, 1–2, 2–3, 3–4, and 4–5 correspond to bad, tolerable, medium, good, and excellent, respectively [24]. Smaller Q values indicated a higher degree of eutrophication.

We used chlorophyll a (Chla), TP, TN, transparency (SD), and the CODMn to calculate the Comprehensive Trophic Level Index (TLI (Σ)), which was used to evaluate the eutrophication status of Gaozhou Reservoir [27,42]:

$$\text{TLI}\ (\Sigma) = \sum\_{\mathbf{j}=1}^{\mathbf{m}} \mathcal{W}\_{\mathbf{j}} \times \text{TLI}\ (\mathbf{j})\_{\prime} \tag{2}$$

where TLI (Σ) is the comprehensive trophic level index; TLI (j) is the nutrition status index representing the *j*th parameter; and, W*j* is the relevant weight of the nutrition status index of the jth parameter.

In order to calculate the nutritional status index of each individual item, we used the following formulas:

$$\text{TLL (Chl a)} = 10 \times \text{(2.5 + 1.086 \ln (Chl-a))},\tag{3}$$

$$\text{TLL (TP)} = 10 \times (9.436 + 1.624 \ln \text{(TP)}),\tag{4}$$

$$\text{TLI (TN)} = 10 \times (5.453 + 1.694 \ln \text{ (TN)}),\tag{5}$$

$$\text{TLI (SD)} = 10 \times (5.118 - 1.940 \text{ ln (SD)}),\tag{6}$$

$$\text{TLI (COD}\_{\text{Mn}} = 10 \times \text{(0.109 + 2.661 \ln \text{(COD}\_{\text{Mn}}))}.\tag{7}$$

Here, Chla is expressed in mg m-3; SD is expressed in m; and, all other variables are expressed in mg L−1.

The values of the TLI (Σ) index were divided into six levels: oligotrophic (0–30), mesotrophic (30–50), light-eutrophic (50–60), mid-eutrophic (60–70), high-eutrophic (70–80), and hyper-eutrophic (>80) [23,43].

One-way analysis of variance (one-way ANOVA) was used to compare the difference of environmental factors among seasons. We took three samplings of each sampling site as three replicates for each season. Redundancy analysis (RDA) was used to reveal the contribution of 13 environmental factors to variations of nine representative functional groups of phytoplankton for each season. Statistical analysis was performed while using the SPSS 19.0 statistical package software (IBM, Armonk, NY, USA), and RDA analysis was analyzed using the Canoco 5.04 software.

#### **3. Results**

#### *3.1. Phytoplankton Dynamics*

In total, 155 phytoplankton taxa were identified across all of the samples collected from the Gaozhou Reservoir between January 2015 and December 2017. These taxa included cyanobacteria, diatoms, chlorophytes, dinoflagellates, cryptophytes, euglenophytes, and chrysophytes. There were 103, 109, 126, and 120 taxa in samples collected in the winter, spring, summer, and autumn, respectively; species richness in the summer and autumn was greater than that in the winter and spring (Table 1).

Chrysophytes 1 0.97% 1 0.92% 1 0.79% 1 0.83% 1 0.65% Diatoms 26 25.24% 26 23.85% 29 23.02% 28 23.33% 30 19.35% Euglenophytes 2 1.94% 3 2.75% 3 2.38% 3 2.50% 3 1.94% Chlorophytes 56 54.37% 55 50.46% 65 51.59% 64 53.33% 92 59.35% Total taxa 103 100% 109 100% 126 100% 120 100% 155 100%

**Table 1.** Species of phytoplankton and their respective proportion (%) in different seasons in the Gaozhou Reservoir.

The phytoplankton taxa in the reservoir fell into 28 of the 31 groups previously described: M, S1, SN, H1, H2, X1, X2, X3, Y, LM, LO, K, TC, E, D, C, B, A, MP, W1, W2, WO, G, J, F, T, NA, and P (Table 2). Here, nine groups were considered to be representative functional groups (marked with "*a*" in Table 2).

During the study period, the average densities of algal cells in the winter, spring, summer, and autumn were 5.42 × 106 cells L−1, 29.7 × 106 cells L−1, 13.2 × 106 cells L−1, and 11.3 × 106 cells L−1, respectively. The density of algal cells was highest in the spring, followed by the summer, autumn, and winter (Figure 2). In the spring of 2016, cyanobacterial blooms occurred at S3 and S4, where S3 is near the water supply outlet of the reservoir. The algal cell densities at S3 and S4 reached a maximum of 3.12 × 108 cells L−1 and 1.51 × 108 cells L−1, respectively.

**Table 2.** Main phytoplankton taxa, functional groups, and respective F factors in the samples collected from the Gaozhou Reservoir from 2015 to 2017.

a Descriptor species (>10% of the total biomass).

In total, 18 representative functional groups were identified in the winter and spring, and 20 were identified in the summer and autumn (Figure 2). The functional groups P (*Staurastrum* sp. and *Closterium acerosum*) and Y (*Cryptomonas ovata* and *Cryptomonas erosa*) were the dominant phytoplankton taxa in the winter and spring. Functional group X2 (*Chroomonas acuta*) was more abundant in the winter, while functional group M (*Microcystis aeruginosa*) grew faster in the spring. Functional groups NA (*Cosmarium* sp. and *Staurodesmus* sp.) and P (*Staurastrum* sp. and *Closterium acerosum*) dominated the communities in the summer and autumn. In spring 2016, the community at site S3 was dominated by functional group M (*Microcystis aeruginosa*), comprising 98.5% of the total biomass, while the community at site S4 was dominated by both functional group M (*Microcystis aeruginosa*) and functional group P (*Melosira varians*), comprising, respectively, 86.4% and 12.1% of the total biomass.

**Figure 2.** Composition and abundance of the representative functional groups of phytoplankton in Gaozhou Reservoir from 2015 to 2017. (**a**) abundance in winter; (**b**) relative biomass in winter; (**c**) abundance in spring; (**d**) relative biomass in spring; (**e**) abundance in summer; (**f**) relative biomass in summer; (**g**) abundance in autumn; and (**h**) relative biomass in autumn.

#### *3.2. Variations in Environmental Factors*

The average WTs in the winter, spring, summer, and autumn were 20.8 ◦C, 28.0 ◦C, 31.2 ◦C, and 26.5 ◦C, respectively. The pH was 8.63 in the summer, which was significantly higher than that in

all other seasons (p < 0.05). However, there were no significant differences in pH among the samples that were collected during the other seasons. DO and EC were highest in the spring, measuring 10.03 mg L−1 and 66.1 μs cm−1, respectively. These values were significantly higher than those that were measured during the other three seasons (p < 0.05). Across all samples, the TN concentrations ranged from 0.20 mg L−1 to 2.60 mg L−1, with an average of 0.66 mg L−1. The TN concentrations of the spring samples (average: 0.94 mg L−1; maximum: 1.15 mg L−1) were significantly higher than those of the other three seasons (p < 0.05). Across all of the samples, the TP concentrations ranged from 0.008 mg L−1 to 0.110 mg L−1, with an average of 0.025 mg L−1. The average TP concentration was the highest in the spring of 2016 (0.056 mg L−1), but there was no significant difference in TP concentration between spring and each of the other three seasons. Throughout the survey period, the ratio of nitrogen concentration to phosphorus concentration at each sampling site was greater than 16, and this value did not differ significantly among seasons. Seasonal rainfall was 50–1116 mm, with the most rain falling in the summer, followed by the spring, autumn, and winter (Table 3).

**Table 3.** Mean and range (Min–Max) of environmental factors in the Gaozhou Reservoir for each season from 2015 to 2017.

WT: water temperature; DO: dissolved oxygen; EC: electrical conductivity; SS: suspended solid; TN: total nitrogen; NH4-N: ammonium nitrogen; NO3-N: nitrate nitrogen; TP: total phosphorus; CODMn: permanganate index; Chla: chlorophyll a concentration; WL: water level.

Among all of the environmental factors, measured during the survey period, TP was most strongly affected by season. The average TP in spring of 2016 was 0.056 mg L−1. TP values at S3 and S4 were higher (0.080 mg L−1 and 0.110 mg L−1) than those at other sites, and these values were 4–5 times greater than those in spring 2015 and 2017 (Figure 3).

#### *3.3. Changes in Q and TLI (*Σ*)*

The average Q index across all of samples from 2015 to 2017 was 1.87–3.48 (Figure 4), indicating that the water quality of the reservoir was primarily good and medium. However, water quality in spring of 2016 was obviously bad. The average TLI (Σ) index for all samples from 2015 to 2017 was 34.4–52.6 (Figure 4), implying that water quality was lightly eutrophic. The average TLI (Σ) index was substantially lower in the samples that were collected in the spring of 2016, as compared to those collected in other seasons.

The Q index and the TLI (Σ) index were especially abnormal in the spring of 2016, coinciding with local cyanobacterial blooms. The TLI (Σ) indexes of the samples from sites S3 and S4 were 63.7 and 71.6, respectively, while the Q indexes were 0.21 and 0.24, respectively. The degree of eutrophication in the water of the reservoir was abnormally high.

**Figure 3.** Seasonal changes in total phosphorus (TP) at different sampling sites in Gaozhou Reservoir from 2015 to 2017.

**Figure 4.** Evaluation of the ecological status of Gaozhou Reservoir from 2015 to 2017 based on the Q and TLI(Σ) indexes.

#### *3.4. Redundancy Analysis*

In this study, the functional groups with greater than 10% relative biomass were defined as the dominant functional groups. Nine groups were identified in Gaozhou Reservoir: M, X2, Y, LO, J, F, T, NA, and P (Table 2). Figure 5 shows the results of the RDA, based on dominant functional groups and major environmental factors.

**Figure 5.** Redundancy analysis of the representative functional groups of phytoplankton and the environmental factors in Gaozhou Reservoir. (**a**) winter; (**b**) spring; (**c**) summer; and, (**d**) autumn.

WT, nitrogen, and phosphorus were the most important environmental factors affecting the phytoplankton functional groups (Figure 5). However, the influencing factors were different in different seasons. In the winter, WT and CODMn were positively associated with groups X2 and Y, but they were negatively associated with group M (Figure 5a). In the spring, WT was the main environmental factor affecting group M (Figure 5b). In the summer, TP, TN, and NH4-N were the main environmental factors affecting groups Y and NA (Figure 5c). In the autumn, WT, pH, and TP were the main environmental factors affecting groups X2 and NA (Figure 5d). The redundant environmental factors differed among seasons. The redundant environmental factors were TP and rainfall in the winter; CODMn, DO, and TN in the spring; WT, CODMn, and EC in the summer; and, TN, rainfall, and EC in the autumn.

#### **4. Discussion**

#### *4.1. Phytoplankton Seasonal Dynamics*

We found that phytoplankton species richness in Gaozhou Reservoir was greater in the summer and autumn than in the winter and spring. Generally, the mechanisms regulating the changes in phytoplankton communities are complex [44,45]. For example, the nutrient resources of aquatic ecosystems strongly affect ecosystem diversity [46]. Species are season-specific, which is a basic manifestation of phytoplankton adaptations to natural environments [10,47].

Functional groups P (*Staurastrum* sp. and *Closterium acerosum*) and Y (*Cryptomonas ovata* and *Cryptomonas erosa*) prefer weaker light intensity [10,11]. Light intensity is greater in the summer and autumn, but weaker in the winter and spring, as the Gaozhou Reservoir is located in the subtropics. Therefore, functional groups P and Y dominated the communities during the winter and spring [16,48,49]. Similarly, functional group X2 (*Chroomonas acuta*), which tolerates low temperatures [50], grew faster in the winter and spring.

The functional groups NA and P were dominant in the summer and autumn, when the nitrogen concentrations in Gaozhou Reservoir were lower than in the winter and spring. *Cosmarium* sp. and *Staurodesmus* sp. were the dominant taxa in the aquatic environment when the nutrient levels were low to moderate [11].

Functional group M had clear advantages at sites S3 and S4 in spring 2016, when cyanobacterial blooms were occurring at these two sites. Functional group P had strong advantages at most of the sampling sites in the winter and autumn of 2015, and functional group H2 dominated the community in the summer and autumn of 2016.

#### *4.2. Environmental Driving Factors*

Our results were consistent with previous studies that reported that temperature and nutrient availability strongly influenced phytoplankton species and biomass in aquatic ecosystems [51,52]. For example, the RDA results showed that WT was highly correlated with different functional groups in the winter as compared to the spring. WT was correlated with functional groups Y and X2 in the winter; with functional group M in the spring; and, with functional groups F, LO, X2, and NA in the autumn. In the winter and spring, NO3-N was highly correlated with the functional group F, and NH4-N was highly correlated with the functional groups T and NA. In the summer, NH4-N and TP affected additional functional groups, including Y, NA, X2, P, and J. TP affected the functional groups NA and X2 in the summer and autumn.

In winter, environmental parameters, including TP, rainfall, and SS, were located in the first quadrant (Figure 5a), but no functional groups were located in this quadrant, which indicated that these parameters had no direct impact on the functional groups. In the spring, quadrant II had no functional groups (Figure 5b), but several parameters, including TP, TN, and DO, were observed in this quadrant. There were no functional groups in quadrant III in the summer, and the environmental factors in this quadrant were WT, CODMn, and EC (Figure 5c). There were no functional groups in quadrant IV in the autumn, and the environmental factors in this quadrant were TN, DO, rainfall, and EC (Figure 5d). This seasonal difference in environmental factors among quadrants indicated that the redundancy of environmental factors differed among seasons, and the relationship between environmental factors and functional groups also fluctuated seasonally. For example, the environmental factors that were positively affected in the winter were redundant variables, which included TP. This suggested that the constraints associated with other factors (such as WT) would limit the occurrence and development of functional groups, even if TP were suitable. In the summer, the negatively affected environmental factors were also redundant variables, including WT. Obviously, water temperature is the highest in the

summer and, thus, WT no longer acts as a restrictive environmental condition to limit the development of functional groups. Under these conditions, the environmental factors affecting the phytoplankton community structure are nitrogen and phosphorus nutrient levels.

#### *4.3. Analysis of Cyanobacterial Blooms*

Phytoplankton blooms are a comprehensive manifestation of the long-term aquatic ecological risks that are associated with a regional water environment [5,53]. The cyanobacterium *Microcystis aeruginosa* is induced at low temperatures and is dormant at high temperatures [54,55]. Guo et al. (2016) found that *M. aeruginosa* was induced at 15 ◦C, grew well at 19 ◦C, and entered dormancy at 31 ◦C in Dianchi Lake, China [55]. In Gaozhou Reservoir, the *M. aeruginosa* bloom occurred after the WT increased from 20.8 ◦C in winter 2016 to 28.0 ◦C in spring 2016; *M. aeruginosa* entered dormancy in the summer of 2016 at 31.2 ◦C. In addition, the reservoir also experienced blooms in the spring seasons of 2009, 2010, and 2013. The environmental conditions before the blooms had several similarities, such as low temperatures in the winter and increasing temperatures in the spring [32]. The Gaozhou Reservoir is rainy in the spring and, thus, nutrient concentrations in the reservoir increase due to the input of rainwater runoff. Previous studies recorded high concentrations of phosphorus at sampling sites S3 and S4 in the spring of 2016; these high levels of phosphorus were associated with the rainfall and the hydrodynamic conditions at these two sites. Sampling sites S3 and S4 are located in the bay of the Reservoir. The Gaozhou reservoir has a relatively long hydraulic retention time (approximately 451 days). In this study, S3 and S4 were two sites with serious cyanobacterial bloom. S4 site was around by the shoreside of the reservoir and closer to the dam, while S3 site might be affected by S4 site, due to the short distance between the two sites. Hence, the areas with cyanobacterial bloom were relatively closed, with lower flow velocity and less water exchange when compared to other areas of the reservoir [34]. These conditions may result in higher concentrations of phosphorus at these sites, which may induce cyanobacterial blooms. In addition, the prevailing winds blew many phytoplankton cells into the reservoir bay, resulting in a higher density of phytoplankton cells in this area when compared to surrounding locations. Therefore, environmental conditions and changes increased the aggregation of cyanobacterial cells at sites S3 and S4.

#### *4.4. Environmental Implications*

Here, data and analysis suggested that the low temperatures in Gaozhou Reservoir in the winter and spring, coupled with the dramatic difference in temperature between the winter and spring, resulted in the rapid growth of *Microcystis aeruginosa*. In addition, the high concentrations of phosphorus in the reservoir in the spring might have provided sufficient nutrients in order support the explosive proliferation of cyanobacteria. Finally, the specific hydrodynamic conditions of the water body might have inhibited dispersion, allowing for the growing cyanobacterial cells to further aggregate.

#### **5. Conclusions**

This study demonstrated that there was seasonal succession of phytoplankton species and functional groups in the Gaozhou Reservoir. The phytoplankton diversity was higher in the summer and autumn than in the winter and spring. Functional groups P (*Staurastrum* sp. and *Closterium acerosum*) and Y (*Cryptomonas ovata* and *Cryptomonas erosa*) were dominant in the winter and spring, while functional groups NA (*Cosmarium* sp. and *Staurodesmus* sp.) and P (*Staurastrum* sp. and *Closterium acerosum*) were dominant in summer and autumn. The temperature, nitrogen, and phosphorus levels were the important factors that were associated with the seasonal changes in phytoplankton community structure in the reservoir. The key factors leading to the blooms of *Microcystis aeruginosa* were temperature and phosphorus levels. The low temperatures in the winter of 2016 and the high concentrations of phosphorus in the spring of 2016, together with the favorable rainfall and hydrodynamic conditions in the reservoir, were the major factors triggering the phytoplankton blooms. **Author Contributions:** Conceptualization, L.Y., X.Z. and B.X.; data curation, L.Y., R.L. and T.G.; investigation, L.Y., R.L. and T.G.; methodology, L.Y., X.Z. and T.G.; validation, B.X., G.-J.Z., S.L. and C.L.; writing—Original draft, L.Y., X.Z., G.-J.Z., B.X. and S.L.; writing—Review and editing, L.Y., B.X., G.-J.Z. and X.Z. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was funded by the Key-Area Research and Development Program of Guangdong Province (2019B110205004), National Natural Science Foundation of China (41977353, 41401115), the International Science & Technology Cooperation Program of Guangzhou<PHONE_NUMBER>10) and the Special Fund of Chinese Central Government for Basic Scientific Research Operations in Commonweal Research Institutes (PM-zx703-201602-048, PM-zx097-201904-131).

**Acknowledgments:** We thank Kunying Li and Yanhui Feng for water sample collection and analysis.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**

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# **Cartilage Repair and Regeneration Focus on Multi-Disciplinary Strategies**

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