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• The mathematical model can accurately predict the melting heat transfer process of PCMs. • The Rayleigh number increased with the heat flux, and the higher the heat flux, the larger the amplitude. • Increasing the heat flux improved the integrated heat transfer coefficient and thermal storage performance of the CPCMs. • Conduction was the primary heat transfer mechanism; however, the proportion of natural convection increased with the heat flux. To investigate the effect of heat flux on the enhanced heat transfer in phase change materials (PCMs) composed of copper metal foam (CMF) and paraffin wax (PX), a semi-cylindrical visualized heat storage device with a varied heat flux of 7.3 kW/m2, 8.5 kW/m2, 9.7 kW/m2, and 10.9 kW/m2 was established in this paper. Moreover, a 3-D mathematical model of composite phase change materials (CPCMs) was established using hybrid grid to study the temperature distribution, imbalance effect, flow, heat transfer mechanisms, and heat storage performance during the melting process. The results showed that the melting time of the CPCMs decreased as the heat flux increased. Compared with a heat flux of 7.3 kW/m2, the melting time was shortened by 10.05 %, 20.71 %, and 27.21 %, respectively. In addition, the Rayleigh number (Ra) increased with the increase in heat flux, and the amplitudes of Ra were 1.45×108,1.54×108,1.61×108, and 1.74×108, respectively, which indicated that the higher the heat flux, the larger the amplitudes. Moreover, the heat transfer mechanism during the melting process was dominated by conduction; however, with the increase in heat flux, the maximum flow velocity of the liquid paraffin increased, which caused the proportion of natural convection to increase from 27.80 % to 31.30 %. Hence, the integrated heat transfer coefficient of CPCMs increased from 5.69 W/(m·K) to 5.81 W/(m·K). However, the temperature imbalance effect of the CPCMs was exacerbated, as evidenced by the increased maximum temperature difference in the vertical direction of the center of the CPCMs from 24.7 K to 35.6 K. Besides, the heat storage performance was enhanced. Compared with a heat flux of 7.3 kW/m2, the heat storage capacities of the CPCMs increased by 4.21%, 9.46%, and 12.66 % with increasing heat flux, and the heat storage rates of the CPCMs increased by 15.3 %, 34.9 %, and 52.3 %, respectively. Furthermore, the relative error of the overall melting time was 3.4 %, indicating that the model provided reasonable predictions. [ABSTRACT FROM AUTHOR]

A visual experimental system was built to explore the effect of low-pore-density copper metal foam (CMF) on the heat transfer during paraffin wax (PX) melting, and the effective thermal conductivities of composite phase change materials (PCMs) were obtained. A three-dimensional mathematical heat transfer model was established, the microstructure of the CMF was characterized using a high-definition camera, and a physical model of the CMF ball was determined. The melting heat transfer mechanism of the composite PCMs was investigated through the experimental and simulation results, including the temperature distribution, velocity vector, solid-liquid interface, volume fraction, heat transfer mechanism, and heat storage performance. The results show that as the CMF filling ratio increased from 0% to 2.13%, the temperature gradients of the composite PCMs were 86.1, 74.6, 29.2, and 12.6 K, indicating that the filling of CMF effectively alleviates the thermal stratification phenomenon during PX melting. Incorporation of the CMF increased the thermal conductivity of the composite PCMs, but suppressed their natural convection. When the CMF filling ratio was less than 1.28%, natural convection dominated the heat transfer type during PX melting; meanwhile, when the CMF filling ratio was greater than 1.28%, heat conduction was the major heat transfer mechanism. The composite PCMs exhibited great heat storage performance when the CMF filling ratio was 1.68%, and the heat storage rate was 13.25% higher than that of pure PX with the same heat storage capacity. In addition, a theoretical calculation model for predicting the heat transfer mechanism of the composite PCMs was established, and the results show that as the CMF filling ratio increased from 0.43% to 2.15%, the natural convection proportion of heat transfer performance decreased from 82.74% to 13.99% during the melting process. • The mathematical model can well predict the melting heat transfer process of PCMs. • The filling of CMF leads the paraffin temperature distribution more uniform. • Natural convection is dominated as the CMF filling ratio is less than 1.28%. • The composite PCM has better heat storage performance as CMF filling rate is 1.68%. [ABSTRACT FROM AUTHOR]

• A greater proportion of copper metal foam (CMF) can enhance the heat transfer of paraffin wax and reduce the temperature gradient. • A theoretical model was built to calculate the comprehensive heat transfer coefficient of composite PCMs and to determine the intensity change of heat conduction and natural convection of PCMs with different CMF proportions. • Melting time and heat storage capacity of CMF composite PCMs first increased and then decreased with increasing CMF proportion. • Heat storage rate and comprehensive heat transfer coefficient first decreased and then increased with increasing CMF proportion. • Composite PCMs with a CMF proportion of 0.86% exhibit the best heat storage performance. Phase-change materials (PCMs) can overcome the low energy density and utilization efficiency of solar energy. However, the low thermal conductivity of PCMs significantly affects the thermal efficiency of solar devices; thus, improving the thermal conductivity of PCMs has been the focus of many recent studies. In this study, copper metal foam (CMF) composite PCMs were prepared using paraffin wax and high-pore-density CMF. The effect of the CMF proportion on heat transfer enhancement in the PCM melting process was analyzed using experimental equipment for heat storage visualization. Comprehensive heat transfer coefficients of CMF composite PCMs were obtained. The experimental results showed that as the CMF proportion increased from 0 to 2.15%, the melting time of the CMF composite PCMs and their heat storage capacity first increased and then decreased, whereas the heat storage rate and comprehensive heat transfer coefficient first decreased and then increased. The minimum temperature gradient of 8.09 K was reached when the CMF proportion was 2.15%. More importantly, as the proportion of CMF increased from 0.43 to 2.15%, the proportion of natural convection decreased from 87.90 to 17.69%, and natural convection was the major heat transfer mechanism in the melting process of composite PCMs with a low CMF proportion. The composite PCMs with a CMF proportion of 0.86% has the best heat storage performance when the heat storage capacity and heat storage rate were both considered. [ABSTRACT FROM AUTHOR]