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This paper proposes novel microencapsulated phase change material slurries (MPCSs) as both the energy storage media and heat transfer fluids. The flow and heat transfer characteristics of MPCSs have been experimentally investigated. A series of experiments were conducted in laminar, transition and turbulent flow conditions for MPCSs in a circular tube under constant heat flux, respectively. The results of pressure drop measurements showed that transportation costs of slurries were close to pure water. The heat transfer experiments demonstrated that proposed MPCSs could enhance the heat transfer performance as the heat transfer fluids for thermal system applications in comparison with pure water. The average enhancement percentages of the Nusselt number were 23.9%, 20.5% and 9.1% for MPCS of 5 wt%, and enhancement of the Nusselt number was achieved when phase change material in the microcapsules were in solid, solid/liquid and liquid states, respectively. However, heat transfer enhancement of MPCS depends on the following combination factors: the slurry concentration, the flow rate, the pumping power and the heating rate. Importantly, the phase change process must be carefully controlled in the heat transfer test section with above combination factors in order to take advantages of MPCS over pure water. [ABSTRACT FROM AUTHOR]

• 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]

Three nanoscale metal oxides composed of nano TiO 2 , nano Al 2 O 3 , and nano MgO were added to the phase change microcapsule slurry for optimising the heat transfer behaviour. The thermal and rheological properties of the resulting microencapsulated phase change materials (MPCMs) and microencapsulated phase change material slurries (MPCSs) were characterized to determine the effects of added metal oxides in optimising the performance of MPCSs. The impacts of factors like metal oxides, concentrations, and flow rates on the heat transfer behaviour of MPCSs were investigated by forced convective heat transfer experiment with various working conditions. Compared to 6 wt% MPCSs, the heat transfer coefficients (h x) of 6 wt% slurries containing 1 wt% TiO 2 , 1 wt% Al 2 O 3 , and 1 wt% MgO increased by 4.0 %, 2.5 %, and 7.13 %, respectively. Nano-MgO showed the most significant heat transfer enhancement effect on MPCSs. Moreover, the heat transfer performance gradually enhanced as a function of the concentration for nano MgO. Overall, the addition of metal oxides enhanced heat transfer by increasing the thermal conductivity of the slurry, as well as improved the micro-convection effect. The proposed MPCSs are promising for applications in solar photovoltaic/thermal (PV/T) systems, chip cooling, thermal management, buildings, and automotive cooling. [Display omitted] • Forced convection heat transfer experiment were conducted with bonded triangular tubes. • Heat transfer enhancement achieved by optimising heat transfer medium and tube structure. • Metal oxide type and concentration have significant effect on heat transfer of slurries. • Nano-MgO showed the most significant heat transfer enhancement effect on slurries. [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]

A novel nano phase change material emulsion (NPCE) with low supercooling and high thermal conductivity was prepared by sonication method. N -octadecane was employed as phase change material, multi-walled carbon nanotubes (MWCNTs) were utilised as high thermal conductivity material, and octadecanol was utilised as nucleating agent. The characterization and thermal properties of the nanoemulsions prepared with various concentrations of MWCNTs and octadecanol were measured and analysed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), particle size analyser, differential scanning calorimeter (DSC) and thermal conductivity meter. The results indicated that the nanoemulsions prepared had great stability, low supercooling and enhanced thermal conductivity. The thermal conductivity was enhanced by 4.32% for 10 wt% nanoemulsion with addition of 1 wt% MWCNTs. The supercooling degree of 20 wt% nanoemulsion was decreased by 36.4% from 17.3 °C to 11.0 °C with addition of 1 wt% octadecanol. It can be concluded that the nanoemulsions prepared were able to be utilised as heat transfer and energy storage fluids, with great potential in thermal system applications. • The enhancement ratio of thermal conductivity was 4.32% for 10 wt% nanoemulsion. • The supercooling of 20 wt% nanoemulsion was decreased by 36.4% with addition of 1 wt% octadecanol. • Fabricated nanoemulsions presented good stability and thermo-physical properties. • Fabricated nanoemulsions have great potential application in the thermal systems. [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]

To explore the effect of metal foam(MF)pore density on the melting and heat transfer of phase-change materials(PCMs), a semi-cylindrical visualization experimental setup was established to experimentally study copper MFs with different pore densities coupled with paraffin wax. The results showed that as pore density increases, the melting time of the composite PCMs decreases compared to that of pure paraffin wax by 11.5%, 16.2% and 18.1% for pore densities of 5, 20, and 30 PPI, respectively. Composite PCMs with a pore density of 30 PPI exhibited fastest melting and lowest maximum temperature difference during melting. Considering the relative thermal energy storage(TES) capacity and relative TES rate, composite PCMs with a pore density of 30 PPI exhibited optimal overall thermal storage performance. Furthermore, heat conduction was the primary means of thermal transmission during the melting of the bottom paraffin. However, the melting of the upper paraffin was nevertheless influenced by natural convection, Hence, as pore size decreased, the influence on the weakening of natural convection became more significant. The results of the study provide guidance for the optimal design of latent heat storage devices for semi-cylinders. • The metal foam's pores are smaller the more effectively it transfers heat. • The copper foam with a 30 PPI pore density exhibited the best thermal storage performance. • The natural convection effect was less noticeable as pore size decreased. • The rate of heat storage increased as pore density continued to rise. [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]