| Energy and environmental issues have become increasingly prominent nowadays.Phase-change heat transfer technology is an effective way to alleviate energy problems.It is a technology that utilizes latent heat absorption and release energy to reduce energy waste.In this paper,the numerical simulation method is used to improve the heat transfer efficiency of composite phase change materials(CPCM)and to design optimized phase change heat devices.Firstly,a model of the shell-and-tube heat exchange unit is established,and the independence is verified.Determine the appropriate number of grids and time steps.Paraffin wax is used as the filling material.The foaming aluminum,foamed nickel and foamed copper are used as the base.The liquid fractions of the three composites are studied over time.The results shows that the melting rate of the composite phase change material is significantly improved.The complete melting time of the copper-paraffin CPCM is 68.2 % shorter than that of paraffin wax,which is the highest heat exchange efficiency among the three PCMs.The influence of the porosity of the foam copper,the inlet flow rate and inlet temperature of the hot fluid,the heat exchanger unit diameters on phase transformation process is comprehensively analyzed.The increase of porosity will increase the heat storage capacity of the heat exchanger and enhance the natural convection of the phase change material,but the thermal conductivity is reduced.In practical applications,it should be considered according to the situation.The effect of fluid inlet flow rate on heat transfer efficiency weakens with the increasing flow rate.The complete liquid phase time at a flow rate of 20 m/s is only 7.9 % shorter than that of 1 m/s.Taking the flow rate and pipe pressure into account,an inlet flow rate of 2 m/s is preferred.The fluid inlet temperature significantly affects heat transfer efficiency.For every 5 K increase on inlet temperature,the complete melting time is reduced by about 25 % compared with time on the previous temperature.The increase of the inner pipe diameter has little effect o n the heat transfer so the pipe diameter is selected mainly considering the actual flow rate and the pipe materials consumption.The ribs are added to optimize the heat exchange devices and their effects are in comparison.The influence of the parameters and structures of the ribs on the heat transfer efficiency of the CPCM are investigated.The liquid phase cloud diagram shows that the liquid phase distribution is more uniform after adding ribs and the heat transfer efficiency is obviously improved.The height of ribs is positively correlated with the heat transfer efficiency.The heat exchanger unit with the height of 16 mm of a rib is about 50 % time-saving compared to finless unit.The smaller width of ribs causes obvious wavy temperature distribution of the PCM but the effect on heat transfer is not significant.When the rib width is 3 mm or 9 mm,the complete phase transition time of the latter one is reduced by 11.9 % compared with the former.The ribs with T-shaped,cross-shaped and Y-shaped branches can reduce the temperature difference between the end and the root themselves.The heat storage efficiency of the straight ribs is improved by 20.4 %,and mentioned three kinds of ribs with branches increase by 27.3 %,34.1 % and 38.9 %,respectively.In summary,the application of foamed copper/paraffin CPCM and changing the fluid inlet temperature is an effective method to improve the phase transformation heat transfer efficiency.At the same time,parameters and structures optimized ribs can significantly promote the heat transfer efficiency. |
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