The heat transfer enhancement mechanism for composite phase change material based on variable heating power and pore density

Abstract

Adding foam metal materials is one of the key methods to enhance the thermal conductivity of phase change materials (PCMs). To investigate the heat transfer mechanism of PCMs under different heat source powers and foam metal pore densities, a visualized thermal energy storage device was designed and constructed in this study. The effects of heat source power and the pore density of copper foam metal on the thermal properties of the composite PCMs, including internal temperature differences, equivalent thermal conductivity, comprehensive heat transfer coefficient, and heat storage performance during the melting process, were thoroughly analyzed. An empirical correlation for calculating the Nusselt number (Nu) of the composite PCMs was also derived. The research results indicate that the rate of temperature rise in the internal temperature difference of the composite PCMs increased with the rise in heat source power, while the melting time shortened accordingly. When the heat source power was increased from 30 W to 90 W, for composite PCMs with copper foam, the pore density of copper foam increased from 5PPI to 25PPI, and the rate of temperature rise in the internal temperature difference during the melting process enhanced from 176.9 % to 183.5 %, while the reduction in melting time enhanced from 61.9 % to 62.5 %. The comprehensive heat transfer coefficient of the composite PCMs is directly proportional to the heating power and inversely proportional to the pore density. When the heating power is increased from 30 W to 90 W, the comprehensive heat transfer coefficient of the composite PCM with a pore density of 5PPI is increased by 61 %, while that of 25PPI is increased by 59 %. Additionally, based on the structural parameters of copper foam metal, an empirical correlation for the Nu during the melting process of the composite PCM was derived, the error between the experimental value and the calculated value is ±5 %. In addition, the dimensionless heat-storage rate and dimensionless heat-storage density decrease with the increase of heating power. For 5PPI foam metal copper composite paraffin PCM, when the heating power increases from 30 W to 90 W, the dimensionless heat-storage rate and dimensionless heat-storage density decrease from 1.14 to 1.04 and from 0.98 to 0.93, respectively. For the foam metal copper composite paraffin PCM with different pore density, the dimensionless heat-storage rate decreases with the increase of pore density. When the heating power is 30 W, the values are 1.14, 1.11 and 1.08, respectively. On the other hand, the dimensionless heat-storage density increased with the increase of pore density, and the values of 0.984, 0.988, and 0.995, respectively.