Experimental Investigation of the Horizontal Double Pipe Heat Exchanger Utilized Phase Change Material

The melting of a phase change material (PCM) inside a horizontal concentric two-pipe heat exchanger is experimentally investigated. The PCM RT-42 (Rubitherm) is contained in the annular space between the inner tube of diameter 25 mm and outer insulated shell of diameter 75mm. The hot water as a heat transfer fluid (HTF) flows inside the inner tube at different temperatures, 60, 70 and 80 °C. The experimental setup involves twelve thermocouples to record the transient temperatures of PCM at different locations. Also, the progress of the solid-liquid front during the melting process is tracked photographically by a digital camera. The experimental findings revealed that the heat conduction, accompanied by the same melting rate around the inner tube of the heat storage unit, is predominant at the beginning of the melting process. As time progresses, the natural convection role is strengthened and causes a higher melting at the top part than is shown at the lower part, which is still dominated by conduction. Also, the melting rate is directly affected by the temperature of the HTF. The melting time is reduced by about 27% and 46% when the HTF temperature varies from 60 to 70°C and 80°C, respectively. The variation of the Nusselt number during the melting process is evaluated. There is a positive influence of the HTF temperature on the energy stored by the PCM.