Heat Transfer Enhancement Of Phase Change Materials In Heat Storage System Using V-fins And Metal Foam

Thermal energy storage technology can recover waste heat,reduce energy waste,and thus alleviate the energy crisis.However,the thermal conductivities of most phase change materials(PCMs)are relatively small,which limit the thermal efficiency of latent heat storage systems.Focusing on the subject of heat transfer enhancement of PCMs,this paper adopts numerical calculation methods to study the heat transfer and flow properties during the melting process of PCMs in shell-and-tube heat storage units with the heat transfer enhancement means of adding V-shaped fins and foam metal.The effects of the number,thickness,length,angle,distribution of fins,and the arrangement of porosity,pore density and gradient porosity of foam metal on the thermal storage properties of the thermal storage units are analyzed,and the comprehensive heat storage properties of the heat storage units are evaluated by the melting temperature distribution and variation curve with time,liquid fraction distribution,variation of liquid fraction with time,actual heat storage per unit length and phase change heat storage rate density of PCMs.The main research contents and conclusions are as follows:(1)The increment of number,thickness and length of fins can improve the heat storage properties of the shell-and-tube phase change heat storage units.With the increase of the number,thickness and length of fins,the improvement of heat storage properties become weaker and the energy storage decreases.When the fins thickness increases from 0.75 mm to 2 mm,the energy storage per unit length decreased by 86.7 k J/m.The increasing of fins number and length bring more significant enhancement of PCMs melting compared to that of fins thickness.In the case of the same heat storage,the comprehensive properties of the V-shaped angle that are too large or too small are poor,and the comprehensive property is the best when the V-shaped included angle is45°.In addition,it is found that the inverted three-fins shell-and-tube heat storage unit has better heat storage property than the heat storage unit that is not inverted.With the increase of the number of fins,the heat storage properties of the inverted arrangement become worse.When the number of fins is five or seven,the comprehensive property of the V-fins inverted arrangement system is better than that of the V-fins not inverted system.In the case of the same number of fins,an appropriate increase in the number of lower fins can effectively improve the overall melting rate.(2)The addition of copper foam greatly improves the heat storage property of the heat storage unit.The melting rate of the PCMs in the V-fins metal foam composite heat storage unit with a porosity of 0.9 is4.71 times and 1.62 times faster than that of the seven-fins V-fins heat storage unit in the fins-number comparison group and of the 45 mm V-fins heat storage unit in fins-length comparison group,respectively.Low porosity is more effective than high porosity,but also loses more heat storage.Increasing the pore density can improve the melting rate of PCMs,but the improvement weakens as the pore density increases.(3)Choosing the appropriate two-layers and three-layers gradient copper foam porosity arrangements can improve the heat storage property of the heat storage unit,and the improvement of the three-layers is better than that of the two-layers.The combination of arranging high porosity in the inner layer and low porosity in the outer layer cannot promote the effect.By comparing all the cases,the gradient porosity metal foam heat storage unit with the inner layer porosity of 0.87,the middle layer of 0.90,and the outer layer of 0.93 has the best energy storage effect,and its melting rate is higher than that of the single-layer porosity PCMs 10.5%.