Begell House Inc. Heat Pipe Science and Technology, An International Journal HPST 2151-7975 3 2-4 2012
Editorial on the post-16th IHPC special issue of Heat Pipe Science and Technology, An International Journal vii-viii Jocelyn Bonjour University Lyon, CNRS, INSA-Lyon, CETHIL UMR5008, F-69621, Villeurbanne, France
THERMAL TRANSPORT BEHAVIOR OF A LIQUID PLUG MOVING INSIDE A DRY CAPILLARY TUBE 97-124 Ashish Kumar Bajpai Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, India Sameer Khandekar Department of Mechanical Engineering, Indian Institute of Technology Kanpur, Kanpur (UP) 208016, India Computational simulation of flow and heat -transfer characteristics of a single isolated liquid plug (of glycerin and water, respectively) flowing in a round capillary tube (internal diameter, D = 2.0 mm), so as to understand its local thermo-hydrodynamic transport behavior, is reported. Both, constant heat flux and constant-temperature boundary conditions have been considered at the tube wall. When an isolated liquid plug moves in a capillary tube, there is a difference in the advancing and receding dynamic contact angles of the two menisci, respectively. This has been considered in the simulations. The linearized simplification of Hoffman-Tanner's law is used to model the variation in the two respective apparent dynamic contact angles with the velocity of the liquid plug (i.e., Ca = µU/&#963;). Simulations are carried out for a range of Capillary numbers and lengths of liquid plugs (L/D ratio). It has been found that variation in dynamic contact angle leads to an enhanced local and average heat-transfer coefficient in the moving liquid plug; the local fluid circulation being affected by menisci deformation. In addition, as the length of the liquid plug is increased, the heat-transfer coefficient decreases and finally shows the asymptotic transport behavior of Poiseuille flow. Other than the flow Capillary number, the fluid Prandtl number and the L/D ratio of the liquid plug, strongly affect the local thermo hydrodynamics.
ANALYSIS OF THE ONSET OF DRYING OF A POROUS CERAMIC WICK APPLIED TO A CAPILLARY PUMPED LOOP AND A LOOP HEAT PIPE 125-151 Paulo H. D. Santos Federal University of Technology - Parana, Department of Mechanical Engineering, 84016-210, Ponta Grossa, PR, Brazil Amir A. M. Oliveira University of Michigan Ann Arbor, MI 48109-2125; Federal University of Santa Catarina, Department of Mechanical Engineering, LabCET -Laboratory of Combustion and Thermal Systems Engineering, 88040-900, Floriano'polis, SC, Brazil Edson Bazzo Federal University of Santa Catarina, Department of Mechanical Engineering, LabCET -Laboratory of Combustion and Thermal Systems Engineering, 88040-900, Floriano'polis, SC, Brazil In this work, a theoretical model for the prediction of the heat flux at the onset of drying of porous wicks used in capillary evaporators of capillary pumped loops and loop heat pipes is developed. The model is based on the solution of the macroscopic conservation equations for mass and energy applied to the liquid-saturated porous wick. The onset of drying is the heat flux that causes the invasion of the vapor phase, before the failure of the capillary evaporator by the capillary limit. One loop heat pipe and one capillary pumping loop with a sintered ceramic wick made of 80% mullite and 20% alumina particles, both using water as working fluid, are tested for operation in steady state at progressively higher heat loads. The predictions are compared to the measurements and good qualitatively agreement is found. The model is used to assess the effects of the wick pore size and the effective thermal conductivity on the onset of drying.
MODELING TRANSIENT ACCELERATION EFFECTS ON HEAT PIPE PERFORMANCE 153-167 Jay Ambrose Lockheed Martin Space Systems Company, 1111 Lockheed Martin Way, Sunnyvale, California 94089, USA Acceleration forces are an important factor in the design of heat pipes for space systems. Since typical aerospace heat pipes have a low capillary pumping head, they can be affected by relatively small accelerations. In some spacecraft acceleration forces are negligible once on orbit. Manned spacecraft tend to have higher thrust propulsion systems and therefore higher acceleration loads. This coupled with the stringent operational requirements associated with manned systems has limited the application of heat pipes to such systems. In the current work, the application of heat pipes in a spacecraft with a known acceleration environment will be addressed. The generic acceleration problem will be discussed. An updated approach for thermohydraulic modeling will be presented. Specific methods used for treating acceleration deprime and recovery will be presented, along with example simulations and comparisons with available data. The current work is intended to describe on-orbit acceleration forces on heat pipes and how they may be addressed in typical spacecraft systems.
RELATIVE THERMAL PERFORMANCE OF SUPERCRITICAL CO2, H2O, N2, AND He CHARGED CLOSED-LOOP THERMOSYPHON-TYPE HEAT PIPES 169-185 Robert Dobson Department of Mechanical Engineering, University of Stellenbosch, Private Bag X1, Matieland 7602, South Africa Compared with water, CO2's relatively high density at high temperatures and pressures makes it significantly more efficient as a working fluid for trans-critical power generation, and also, much smaller physically sized pipes and turbo machinery can be used. As such this paper considers the use of CO2 as the working fluid for closed and closed-loop two-phase thermosyphon-type heat pipes. It also compares the relative performance of CO2 compared with superheated and supercritical water, helium, nitrogen, and sodium. A time-dependent (transient) computer simulation program of a typical closed-loop thermosyphon has been developed. The simulation model requires the disctretization of the loop into a series of control volumes, the application of the equations of change (conservation of mass, momentum, and energy) to each control volume, and the solution of the set of nonlinear partial differential equations so generated using an explicit numerical formulation technique and appropriate properties functions and equations of state, for closure. The assumptions made include one-dimensional and quasi-equilibrium flow conditions and upwind differencing. The results of this analysis are collated in a tabular and graphical form thereby allowing the relative performance and scaling characteristics of the various working fluids to be readily established. It is also envisaged that such information will be useful in the design of high performance heat transfer systems, especially in the light of a greater awareness of and the everincreasing imperative for sustainable and pollution-free energy and heat transfer solutions.
EVAPORATION FROM MICROPOROUS SURFACES IN A MECHANICALLY PUMPED TWO-PHASE LOOP 187-201 Matthias Winter Institute for Technical Thermodynamics, Center of Smart Interfaces, Technische Universitat Darmstadt, Petersenstrasse 17, 64287 Darmstadt, Germany Peter Stephan Institute for Technical Thermodynamics, Technische Universität Darmstadt, Alarich-Weiss-Str. 10, 64287 Darmstadt, Germany The increasing application of high-power-density electronic components, for example, in space applications or data processing centers, causes the need of heat-transfer devices that can handle the high heat fluxes dissipated by these components. A prototype of a mechanically pumped two-phase loop has been developed at the Institute for Technical Thermodynamics in cooperation with OHB System AG and ESA/ESTEC. The loop incorporates miniature evaporators that can be mounted directly on the heat-dissipating device, minimizing temperature differences between the heat-dissipating part and the cooling device. Three evaporators are arranged in parallel to investigate the performance of the loop during multievaporator operation at different heat loads at the evaporators. In this study the heat-transfer coefficients of the new sintered surface could be increased up to 20 W/(cm2 K). The fouling of the surface slows down due to a higher porosity (bigger grain size, here 60 µm). The critical heat flux of the porous surfaces measured is limited to approx. 90 W/cm2 and does not increase with the volume flux (above 30 mL/min) as it happens with the smooth reference and a new microstructured surface. The microstructured surface reaches a critical heat flux of 152.6 W/cm2 and a heat-transfer coefficient of 6.5 W/(cm2 K) which is the best overall performance of all tested structures inside this evaporator type until now. System tests with three evaporators at different heat loads show stable system behavior. The saturation pressure inside the evaporators depends on the system heat load.
THE INFLUENCE OF GROOVE SHAPE ON LOOP HEAT PIPE PERFORMANCE 203-222 Masakazu Kuroi Department of Aerospace Engineering, Nagoya University, Furo-cho Chikusa-ku Nagoya, Aichi 464-8603 Japan Hosei Nagano Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan The influence of groove shape on loop heat pipe (LHP) performance has been investigated by calculation and experiment. In the calculation, the effects of the width, depth, and the number of axial grooves on steady-state performance were examined. Calculation results indicated the existence of optimum width and depth to lower the evaporator temperature. The reduction of the width and depth advances the growth of the pressure loss through the grooves. By contrast, the expansion of the width and depth advances the reduction of contact area between the evaporator casing and the wick, and the increase of the heat leak from the grooves to the core. Experimental investigations were conducted with a LHP, which is able to exchange wicks. Polytetrafluoroethylene (PTFE) wicks were fabricated as the parameters of the number of axial grooves and the existence or nonexistence of circumferential grooves. The experimental results showed that there is the optimum number of axial grooves. The circumferential grooves lower the evaporator temperature with a small heat input and the distribution of the evaporator temperature by their thermal uniformity effect. However, with a large heat input, the operating temperature of the wick whose grooves are only in the axial direction is the lowest. About start-up time, the larger the axial groove passage area becomes, the shorter the start-up time becomes.
OSCILLATING HEAT PIPES WITH TEMPERATURE CONTROL 223-231 Naoko Iwata Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Chuo-ku, Sagamihara-shi, Kanagawa, Kanagawa, 252-5210, Japan Hiroyuki Ogawa Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Chuo-ku, Sagamihara-shi, Kanagawa, Kanagawa, 252-5210, Japan Yoshiro Miyazaki Fukui University of Technology, 3-6-1, Gakuen, Fukui-shi, Fukui, 910-8505, Japan Experiments are conducted to verify that an oscillating heat pipe (OHP) with liquid reservoir works as a temperature-controllable heat pipe, because the saturated vapor pressure of the liquid reservoir controls OHP inner pressure. We orient the liquid reservoir vertically, horizontally, and vertically inverted, and measure the temperature and pressure of the OHP with and without the liquid reservoir. Without the liquid reservoir, heating section temperature and OHP pressure increase with heat input, but with the liquid reservoir an almost constant heating section temperature is maintained by regulating the liquid reservoir temperature. OHP inner pressure and liquid reservoir inner pressure are equivalent even with varying heat input and liquid reservoir orientation. We conclude that the liquid reservoir temperature controls the OHP heating section temperature, and that the liquid reservoir pressure controls the OHP inner pressure.
AUTOMATED CHARGING OF EMBEDDED HEAT PIPE SYSTEMS 233-245 Wessel W. Wits Faculty of Engineering Technology, University of Twente, P.O. Box 217, 7500 AE, Enschede, The Netherlands Johannes van Es Dutch National Aerospace Laboratory, Marknesse, The Netherlands Gert Jan te Riele Thales Nederland B.V., Hengelo, The Netherlands High-performance electronics cooling moves from utilizing modular heat pipes to fully integrated heat pipes. Filling and sealing of these nontubular embedded heat pipes is a challenge, since traditional methods such as evacuate, fill, and pinch do not suffice. This paper presents a charging station that is able to fill and seal embedded heat pipes in a reliable manner. A small, low-cost charge component is connected to the heat pipe. This allows the fill head of the charging station to mount the heat pipe in an automated manner. The charging station uses specially designed zero-dead-volume valves to minimize internal fluid volumes and thus increase the fill accuracy. Inside the fill head, the internal fluid line for filling and the plunger performing a pressed plug seal alternate position during the charging process. Prototype testing shows that the charging station is able to charge heat pipes in around 5 min with a final fill accuracy of 1.5 µL for a total fluid volume of 300 µL.
TWO-PHASE COOLING SYSTEM FOR AIRCRAFT ACTUATORS 247-262 Olivier Berder Euro Heat Pipes SA, Rue de I'Industrie 24, B1400 Nivelles, Belgium V. Dupont Euro Heat Pipes SA, Rue de I'Industrie 24, B1400 Nivelles, Belgium L. Barremaecker Euro Heat Pipes SA, Rue de I'Industrie 24, B1400 Nivelles, Belgium C. Guichard Euro Heat Pipes SA, Rue de I'Industrie 24, B1400 Nivelles, Belgium The current developments of future aircrafts are promoting electrical energy in replacement of conventional hydraulic and pneumatic systems. Electricity contributes to economical and mass savings while meeting major specifications: safety, reliability, and respect of environment. On the other hand, the "all-electric aircraft" development trend induces new difficulties in the field of thermal management. Electronic designers are faced with growing thermal challenges with regard to withdrawing heat dissipation, and they need to constantly optimize dependability, mass, and compaction of their new equipment in order to be compliant with the ever-more-stringent technical requirements from avionics equipment manufacturers. Euro Heat Pipes is currently working hand in hand with major primary flight control actuators and airplane manufacturers so as to propose an adequate two-phase cooling solution for A350 actuators and next-actuator generations as well.
SET-POINT ACTIVE CONTROL OF VAPOR-MODULATED LOOP HEAT PIPE 263-280 Alejandro Torres IberEspacio, Calle Magallanes 3, 4th floor, 28015, Madrid, Spain Donatas Mishkinis IberEspacio, Calle Magallanes 3, 4th floor, 28015, Madrid, Spain Francisco Romera IberEspacio. S. A.C/Magallanes, 3, 4th floor, 28015 Madrid, Spain Javier Corrochano IberEspacio S.A., Avda. Premios Nobel, 53, 28850 Torrejón de Ardoz, Madrid, Spain Tarik Kaya Department of Mechanical and Aerospace Engineering, Carleton University, Ottawa, Ontario, K1S 5B6, Canada A novel advanced active temperature control method, based on a variable-conductance vapor-modulated loop heat pipe (VMLHP) technology, is presented and discussed. The pressure-regulating valve (PRV) is a key element of the VMLHP. The back-pressure reservoir of the PRV has been charged by actuating fluid to provide the system thermal regulation by means of pressure control inside of the VMLHP. Argon and ammonia (single- and two-phase fluids) have been investigated as PRV actuating agents. The active control of the VMLHP is achieved by the PRV actuating fluid pressure adjustment provided by the PRV temperature conditioning. A miniature heater and Peltier element have been attached to the PRV body for this purpose. For argon, the results show that the temperature set point can be modified up to 5°C without exceeding the hardware temperature limitations with very little heater/Peltier element power. A ratio of about 1°C set point increase per 10°C argon temperature increase is obtained. Regarding ammonia, the test goal was to keep evaporator temperature constant, whatever the power applied to VMLHP (up to 170 W), by means of a PID controller. In this case, again very little control power was needed (less than 4 W) to achieve a precise temperature control. A possible application of this active control is to counterbalance the heat fluxes which can reach the PRV back-pressure reservoir from a hot/cold environment or from the superheated vapor of the working fluid. This is very important when high-temperature control precision (less than 0.1°C) is required.