Numerical Simulation And Optimization Of Phase Change Heat Storage Devices

Energy conservation is a topic of common concern in today’s world and it is also the key elements of China’s"12th Five-Year Plan". Energy-saving technology has been to a fixed direction in academia. Phase change heat storage technology is one of the energy-saving methods. It is not only used for the recovery of steam such as waste heat, but also for achieveing the time-sharing utilization of heat. Phase change heat storage technology has wide applications in areas such as industrial waste heat recovery and utilization of solar energy. This technology can effectively improve the enengy efficiency and produce significant energy saving effect. So the study of phase change heat storage technology has parctical significance.The phase change material (PCM) used in heat storage systems usually has low thermal conductivity, so the systems have low charging-discharging rates. So it is very important to find out the influencing factors of charging-discharging rates and the way to strengthen the phase change heat transfer. Also, the process of phase change is associated with conduction and convection phenomena, in addition to the problem of a moving boundary, so we usually use numerical simulation approach to solve the problem of multi-dimensional phase transition. In this paper, the software Fluent was employed to simulate the impact factors and the ways to enhance heat transfer in the process of phase change. The main research content and conclusions are as follows:(1)Establish a shell and tube phase change thermal storage unit for the paraffin phase change heat transfer process. According to the experimental data, set the size of500X100mm, set the boundary conditions. Comparing the average temperature of export and the temperature curve of the monitoring point1,results show that the calculation data coincide with experimental data.(2) Compare the calculation results of two conditions, we can find that the liquid paraffin region has natural convection phenomenon and velocity field. When the heat transfer fluid is water,60℃、0.88m/s, and paraffin liquid fraction of about50%, natural convection begin to promote the melting of paraffin, and continue to the end of melting. Improve the rate of liquid phase change materials can increase the role of natural convection and shorten the melting time.(3)The heat transfer fluid temperature is the main factor to affect the phase change thermal storage process. When the temperature increases from60℃to80℃,the melting completion time of paraffin reduces29.97s. We can see that the increase in temperature can significantly speed up the melting process of paraffin. The heat transfer fluid flow rate is also the factor to affect the phase change thermal storage process, the increase in flow rate can speed up the melting process too, but the increase is not obvious. When the flow rate increases from0.88m/s to1.15m/s, the time reduces8.45s.(4) Add an aluminum fin in the phase transition region. Near the fin, we can see isotherms parallel to the fin and the heat transfer in the axial direction through the fin. Comparing the calculation results between smooth tube and fin tube, we find that the phase change thermal resistance decreases, the thermal conductivity of paraffin increases and the completely melting time reduces24.8s.(5) The fin height is set to2mm,3mm,3.5mm and4mm, thickness is set to0.4mm,0.6mm,0.8mm and1.0mm, and the fin number is set to1,3,5and7. Comparing the calculation results, we can find that the paraffin melting rate is accelerated with the increase of the fin height, thickness and number, but the trend has slow down. The increase of the fin height, thickness and number will reduce the volume of PCM and the heat storage capacity, so five aluminum fins,0.6mm X3mm, are fit for10X50mm model.