Study On Phase Change Heat Transfer Mechanism In Solid-Liquid Phase-Change Energy Storage And Thermal Control Devices

In recent years,solid-liquid phase change has been widely used in various fields such as daily life,industrial production and military actions due to its various advantages like dense energy storage,small temperature variation range during phase change,simple equipment for application,etc.Thermal storage and thermal control are the two major application of solid-liquid phase change.It is worth noting that there are an abundance of fluid flow and heat transfer problems during solid-liquid phase change process,such as the coupled transport of sensible heat and latent heat,unsteady evolution of solid-liquid interface and natural convection caused by temperature gradient induced heterogeneous density.These factors interact with each other leading to complex mechanism,which affects the energy transport performance of the solid-liquid phase change process.Hence,in-depth study on the mechanism of fluid and phase-change heat transfer in latent heat thermal storage units and thermal control applications not only has important academic significance for further improving the basic theory of fluid flow and heat transfer in the solid-liquid phase change process,but also provides key technical support for the design and optimization for thermal storage units and control strategies.At present,solid-liquid phase change mechanism as well as the enhancing approaches of heat transfer have been studied extensively,but the effect of natural convection in the liquid phase during phase change process have not been fully understood.The enhancing performance of fins with different parameters in the solid-liquid phase change process also needs to be further examined quantitatively.In addition,the strengthening approaches of heat transfer need to be further enriched.Therefore,this paper aims at improving the engineering application of solid-liquid phase change process.Using numerical simulation and experimental observation combined methods,in-depth research on the phase change heat transfer mechanism and enhancement techniques on the thermal response of the PCM is earied out.The mechanism of natural convection on the heat transfer process is discussed in particular.The influence of fin parameters is clarified quantitatively.According the enhancement method of solid-liquid phase heat transfer is proposed.The main conclusions are as follows:(1)An ice storage model considering natural convection process was established based on the enthalpy-porosity method.An orthogonal experiment scheme was designed by using the Taguchi method.Based on the numerical model,the heat transfer characteristics of solidification process under different initial water temperatures and different fin parameters were studied.The mechanism of the influence of natural convection on the phase change process during ice storage was revealed.Especially,the cross-impact law of fin structural parameters was clarified.The results showed that natural convection plays an important role in the solidification process that the mixing of the working fluid in the chamber is enhanced,making the temperature distribution in the liquid area more uniform.When the initial temperature of the water is higher than 4 ? density flip can be observed in the liquid zone.The effect of fin thickness and fin number is more obvious in the early stage of solidification.However,the enhancement effect of fin height becomes more important as the process continues.Hence,in the late stage,compared to increasing the fin thickness or the fin number,increasing the height of fins can enhance the ice storage volume.The primary and secondary factors of the fin geometry that affects solidification depends on the initial temperature.As the value of Ste increases,the thickness of the fins is more important in the early stage of solidification.While in the late stage,the influence of the Ste number is negligible.(2)Flat-fin heat sinks filled with phase change material(PCM)were designed.An experimental test platform for heat transfer characteristics of heat sinks was bulit.Effects of different flat fin geometry parameters,heat flux,volume fraction of PCM and other working conditions were examined.The strengthening effect and mechanism of coupling fins into the heat sink is discussed emphatically.The results indicated that,temperature rise of heat sinks is effectively controlled by the filling of PCMS under all heat fluxes conditions.The lower the heat flux,the longer the temperature platform period caused by phase changing.Under the criterion of using the same amount of metal,temperature of the heat sink base decreases with fin number under the same working conditions.And the time required for the heat sink base to reach the set point temperature(SPT)increases.The enhancement ratio of the fins,thermal capacity and thermal conductance of heat sinks increases as well.The higher the heat flux,the more obvious the fins number increment in improving the performance of the heat sinks.In addition,increasing the volume fraction of the PCM in the heat sink can effectively enhance the heat performance of the heat sink.(3)The transient performance of flat-fin heat sinks filled with copper foam was experimentally studied.The effect of porosity and pore density of copper foam were also examined parametrically.The results showed that the heat sinks filled with copper foam inhibit the temperature rise effectively and prolong the time for base to reach the same temperature.Thus the thermal conductance of heat sinks increases after filling copper foam.The lower the porosity of copper foam,the lower the temperature of the heat sink base is.When the pore density is fixed,lower porosity of the foamed metal means thick metal skeleton,which enhances the thermal conductivity of the phase change material.Reducing the porosity and increasing the pore density have great differences for the enhancement ratio in the early stage of Melting.In the late stage of Melting,the effect of porosity and the change of the pore density have little difference for the heat sink.