Heat transfer in vertically aligned phase change energy storage systems

Convection effects on heat transfer are analyzed in low temperature and ver tically aligned phase change energy storage systems. This is performed by d etailed temperature measurements in the phase change material (PCM) in eigh teen locations forming a grid of six radial and three axial positions. The system constitutes a double pipe configuration, where commercial grade para ffin wax is stored in the annular space between the two pipes and water flo ws inside the inner pipe. Vertical alignment of the system allowed for reve rse of the pow direction of the heat transfer fluid (HTF), which is water. Therefore, the PCM is heated from the bottom for HTF flow from bottom to to p and from the top as the HTF flow direction is reversed. For the former ca se, natural convection affects the melting process. Collected data are used to study variations in the transient temperature distribution at axial and radial positions as well as for the two-dimensional temperature field The data is used to calculate the PCM hear transfer coefficient and to develop correlations for the melting Fourier number. Results indicate that the PCM heat transfer coefficient is higher for the case of PCM heating from bottom to top. Nusselt number correlations are del eloped as a function of Raylei gh, Stefan, and Fourier numbers for the HTF flow from bottom to top and as a function of Stefan and Fourier numbers for HTF flow from top to bottom. T he enhancement ratio for heat transfer caused by natural convection increas es and then levels off as the inlet temperature of the HTF is increased.