Experimental Study On Solid-Liquid Phase Change Enhanced Heat Transfer In Phase Change Heat Storage/Release Device

In order to assist the energy-saving transformation of air source heat pump,this paper takes the energy saving as the main goal and takes into account the functions of defrosting and defrosting suppression,proposes a simple storage/heat release device,and carries out experimental and numerical research on the structure optimization and design of the device.In order to clarify the melting and solidification characteristics of phase change materials,the heat storage method was selected to carry out the paraffin-melting and solidification heat transfer characteristics experiment in the circular heat storage unit.In the melting experiment: when the temperature is low,the melting time increases obviously;When the temperature is higher,the melting time decreases not obviously.The temperature rise curve is separated in order from high to low in the system.The vertical height difference affects the temperature difference at the moment of complete melting.The vertical contour of the liquid region is approximately isotherm.In the solidification experiment,the slope of temperature drop curve is the same before entering the solid-liquid variable temperature region and after leaving the solid-liquid variable temperature region.The comparison experiment of copper tube heat storage and outer wall heat storage was carried out.The area of heat storage of the latter was 5.8 times higher than that of the former,but the average latent heat heat storage power was only 2.5 times higher.In order to determine the unfavorable position of phase change heat transfer in the system and optimize the basic shape of heat storage unit,the heat storage of copper tube and heat release of copper tube were used to carry out experimental research on heat transfer characteristics in circular,rectangular and triangular cavities with paraffin as phase change medium.The circular cavity has the best performance in the comparative experiment: in shape,it has the center symmetry,which is easy to operate;The complete melting time is the shortest;Solid fraction changes evenly;The heat transfer in the lower part of the system is relatively good.Heat transfer to the bottom of the system is more advantageous;In the solidification experiment,the heat convection in the circular cavity is the most intense and the heat transfer in the system is the strongest.The heat transfer characteristics of circular cavity heat storage process are studied.The heat in the upper part of the system is large and evenly distributed,while the heat in the lower part of the system is small and unevenly distributed.In order to determine the best position of heat storage tube and heat release tube,a numerical study was carried out on the design of heat storage and heat release device.Compared with single tube,the constant volume method can greatly increase the heat transfer area on the premise of keeping the total amount of material unchanged.According to the change of horizontal spacing and vertical height,the strengthening is divided into four stages,and the best position of heat storage tube and heat release tube is optimized.For the purpose of enhancing heat transfer,experimental research on storage/exothermic device was carried out.Two methods,precise strengthening and integral strengthening,were adopted: foam copper for precise strengthening and expanded graphite for integral strengthening.Four groups of heat storage experiments were carried out: basic experiment,precise reinforcement,integral reinforcement and double-effect reinforcement.The results show that precise reinforcement can reduce the melting time by 12.82 %.Exothermic experiments were carried out for precise reinforcement experiments.The energy saving reconstruction scheme of the combination of storage/heat release unit and air source heat pump is given,and the method of predicting the volume of storage/heat release unit is also given.The research of this paper can provide a new idea for saving heat transfer cost,designing simple storage/heat release device and simplifying the energy saving scheme of air source heat pump.