| Abstract:For the applications of low-temperature solar heat utilization technology, Paraffin is chosen as phase change materials (PCM) for study, because of its advantages of high phase change latent heat, no sub-cooling, no apparent phase separation, non-corrosive and low cost. However, the deficiency of its low thermal conductivity restricts its wide application in the field of energy storage to a certain extent, which is particularly critical to solve. Shell and tube heat exchanger is one of the most widely used in industrial production of heat exchange equipment. But in recent years, numerical simulation on melting heat transfer of the paraffin usually ignores the volume expansion caused by the changing density during the phase change process, and the phase change heat transfer problem considering the convection inside the tube-and-shell device is rarely reported.In this paper, the mathematical model for phase change heat transfer of a horizontal circular tube is established, and the thermal behaviors of paraffin used as PCM are studied by numerical simulation methods. And the melting and solidification process of the typical tube and shell heat exchanger is further analyzed to obtain the heat charge and discharge characteristic of the device. The effects of the device structure parameters and operating conditions on the melting process are discussed, and an improved tube and shell unit is obtained by optimize the structure. By improving the efficiency of heat exchanger, the shortage low thermal conductivity of the paraffin is made up. Therefore, the main research content and conclusions are as follows:(1) The mathematical model of a horizontal circular tube is established firstly in this paper to verify the validity of the model by comparison with the literature results. The density changes and nature convection are taken into account during phase change process simulation. And the liquid fractions, exergy efficiency and the total heat transferred to the cylinder trends over time are given to analyze the heat-storage characteristics with PCM. And the performance characterization parameters are determined:liquid fraction and exergy efficiency.(2) To fully use the storage heat of paraffin wax, this paper presents numerical investigations on melting of PCM using a paraffin blend, RT27, inside a tube-and-shell unit. The position and diameter of inner tube, sub-cooling degree and the heating-surface temperature are considered as variables for study. Through the evaluation and comparison of melting time and exergy efficiency, the optimal parameters of the horizontal annulus were obtained. The results show the heat exchange structure is changed by the different position and diameter of the tube, and temperature distribution and the region of nature convection are also influenced, which result in the different melting rate of the paraffin.(3) In this paper, an innovative tube and shell design with hierarchical tube array in place of the uniform tubes is proposed and studied. Two structural parameters are explored numerically:the number of lower hierarchy pipes n, the ratio of higher hierarchy tube diameter to lower hierarchy tube diameter r. The outcomes for changing the parameters are analyzed in terms of temperature distribution and paraffin melting time. The results show that the different positions and diameters of inner tube determine the tube interior temperature distribution, which in turn determines the strength and positions of the resulting natural convection, which in turn determines the local melting rate; The paraffin melting time is shortest for combination n=2, r=2; PCM melts faster with smaller outer layer heat sink diameter. |
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