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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]

• Condensation heat transfer and pressure drop were investigated experimentally with R410A and R32 in four different tubes. • The surface structures inside tubes were smooth, hydrophobic, herringbone and hydrophobic/herringbone, respectively. • The performance enhancement factor and specific sensitivity factor were used to evaluate the influence of overall heat transfer performance. An experimental investigation was performed to evaluate the condensation heat transfer characteristics for R32 and R410A inside four horizontal round tubes with 12.7 mm outer diameter. The saturation temperatures were 35 °C and 45 °C, the vapor qualities ranged from 0.2 to 0.8, and the mass flux ranged from 50 kg m−2 s−1 to 200 kg m−2 s−1. Different inside surface structures of the four tubes were smooth (ST), hydrophobic (HYD), herringbone (HB) and hydrophobic/herringbone (HYD/HB), respectively. Effects of mass flux, vapor quality, refrigerant and saturation temperature on condensation heat transfer in different tubes were discussed with the aid of flow pattern visualizations. Experimental results show that the heat transfer coefficient of R32 was higher than that of R410A due to different physical properties of the refrigerant. HB tube had the highest heat transfer coefficient at low vapor quality and HYD tube was the best at high vapor quality. The performance enhancement factors (PEF) of HB tube and HYD tube were better, ranging from 1.07 to 1.4 and from 1.02 to 1.31, respectively. However, as the composite of HB and HYD, the HYD/HB tube had the lowest overall performance, with the PEF ranged from 0.94 to 1.12. Additionally, the specific sensitivity factor (SSF) was used to evaluate the influence of various parameters on heat transfer and pressure drop. [ABSTRACT FROM AUTHOR]

• The condensation heat transfer characteristics of R410A on the outside of three enhanced tubes were experimentally investigated. • Effect of surface structure, mass flux, average vapor quality, and saturation temperature on HTC and frictional pressure drop. • The six correlations are evaluated and the correlation with the highest accuracy is selected and modified to obtain a predictive correlation suitable for the enhanced tubes. Condensation heat transfer characteristics on the outer annular walls of double-sided horizontal tubes having three different enhancement geometries are investigated experimentally. The enhancement geometries are (1) a fine fin array, (2) a rectangular convex platform array, and (3) a T-shaped column array. The inner surface geometries are threads of various pitches. The experiments are with various mass fluxes, average vapor qualities, and saturation temperatures. They were conducted with refrigerant R410A. The results show the effects of enhancement geometry. Six correlations are used to predict the heat transfer coefficients of smooth tube, and the correlation with the highest accuracy is selected and corrected to obtain a new correlation suitable for the enhanced tubes. The new correlation can predict 92% of the data points within a prediction error of +22% to -30%. Frictional pressure drops are correlated with mass flux, average vapor quality, and saturation temperature. The heat transfer performances of the three experimental tubes are compared using a performance evaluation factor and the enhancement geometries with better condensation heat transfer performances are identified. [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]