our calculations refer to the volume of the fireball at chemical freeze-out. A lower value from our estimation is thus expected. Figure 6 demonstrates the transverse mass spectra of positive pions for the most central collisions at $\sqrt{s_{NN}}$ = 2.7 GeV. We compare our calculations with flow, represented by the solid line, with the experimental data [@Ivanov:2013yla]. We notice a good agreement between experimental data and model calculations. After comparison, we get the value of the surface velocity, $\beta_{s}$ which gives the value of the radial flow at this energy.

![The transverse mass spectra of $\pi^{+}$ at $\sqrt{s_{NN}}$ = 2.7 GeV. Symbols are experimental data [@Ivanov:2013yla] while line is our result.](mT_spectra_pi.pdf){height="18em"}

![The transverse momentum spectra of $\pi^{-}$ at various energies. Symbols are experimental data while lines are our model results with flow.](allpT.pdf){height="18em"}

In figure 7, we show the $p_{T}$ spectra of $\pi^{-}$ for the most central collisions at various $\sqrt{s_{NN}}$ from 7.7 GeV to 62.4 GeV. We use our model with the effect of flow to calculate $p_{T}$-spectra at all energies where we take only the surface velocity, $\beta_{s}$ as a free parameter. We compare our results with the experimental data [@Kumar:2014tca; @Abelev:2008ab; @Afanasiev:2002mx; @Alt:2006dk] and find that our model 