Thermal performance evaluation of PCM-MF composite heat sinks under varying ambient conditions

This study focuses on the effect of ambient conditions and choice of phase change material (PCM) on the thermal performance of a metal foam(MF)-PCM composite heat sink. The PCMs considered are paraffin wax (PF), docosane (DOC) and eicosane (EIC) and their melting temperature ranges are 44.5–60.1 °C, 40.3–44.5 °C and 36.3–38.1 °C, respectively, as obtained from differential scanning calorimetry measurements carried out at a heating/cooling rate of 0.5 °C/min. Experimental investigations are carried out at different ambient conditions (temperature varying from 25 to 45 °C) for three different heat input levels of 5, 10 and 15 W (1.39, 2.78, 4.14 kW/m2). Thermal performance of two types of metal foams (copper (Cu) and aluminum (Al)) are evaluated to arrive at the proposed composite heat sink configuration. Due to the introduction of metal foam into the PCM a maximum reduction of 15.1% in the peak temperature is observed (at heat flux of 2.78 kW/m2 at an ambient temperature of 25 °C) and the time taken to reach a set point temperature of 50 °C is enhanced by 1.8 times (at heat flux of 4.14 kW/m2 at an ambient temperature of 25 °C) for the considered fixed duration experiments. Furthermore, rise in ambient temperature is found to significantly increase the maximum temperature attained in the heat sink coupled with reduction in the duration of latent heat phase of PCM. Effect of ambient temperature on the thermal performance of the PCM-MF composite heat sink is demonstrated by evaluating the variation of Nusselt number as a function of modified Stefan and Fourier number during the PCM-charging (heating) process. Low melting point PCM-MF composite (EIC-MF-Cu) is recommended for normal (25 °C) and intermediate (35 °C) ambient conditions at low heat input load (≤ 1.39 kW/m2); while DOC-MF-Cu exhibits better performance if the conditions involve high heat input (2.78 to 4.14 kW/m2) and higher ambient temperature (≤ 45 °C). Also, high melting point PCM based composite heat sink (PF-MF-Cu) showed better readiness for subsequent charge-discharge cycles at elevated ambient temperature (35 and 45 °C) due to its higher melting/crystallization point.