Analysis of heat transfer and fluid flow during the melting process of PCM within tubular thermal energy storage unit

In the present paper, a numerical and experimental study is implemented to analyze the melting behavior of a Latent Heat Thermal Energy Storage System (LHTES) in a horizontal cylinder with using paraffin 27 as the Phase Change Materials (PCM). By varying the internal diameter and wall temperature of the horizontal cylinder, the liquid fraction and mean heat transfer coefficient are adopted to evaluate the effect of Stefan number (Ste) and Grashof number (Gr) on the phase change process of PCM. The results indicate that both the effect of wall temperature and internal diameter on the melting process is significant. Furthermore., the dimensionless analysis results show that the heat flux decreases monotonically during the whole melting period when Ste number less than 0.093, this means that the natural convection cannot significantly promote the melting process of PCM. It is found that increasing the Grashof number from 2.28 x 104 to 1.54 x 105 with a constant Stefan number enhances the heat transfer, also for a constant Grashof number the PCM melts at a faster melting rate when the Stefan number increases from 0.046 to 0.313. Besides, with the Gr number increasing by 97 % and 238 % while the Ste number stays constant, the melting rate with respect to the dimensionless time (Ste center dot Fo) is increased by 19 % and 31 %; by contrast, with the Ste number increasing by 73 % and 239 % while the Gr number stays constant, the melting period is decreased by 50 % and 76 %. Finally, correlations for the melt fraction and Nusselt number (Nu) with a combination of the Fourier, Grashof and Stefan numbers are developed for designing the LHTES appropriately.