Research On Mechanism Of Heat Transfer Enhancement In The Multiphase Flow Of Microencapsulated Phase Change Materials (MEPCM)

Phase change material is incapable of storing and releasing industrial waste heat in time for its poor heat transfer performance. This leads to a technical bottleneck of mismatching in time and intensity between energy storage and release in the waste heat recovery unit. In view of this current international problem, the phase change micro nanocapsules suspending fluid high efficiency microchannel waste heat recovery unit and phase-change microcapsule suspension reciprocating motion high efficiency waste heat recovery unit are put forward in this thesis. Based on the multifield synergy principle of heat transfer enhancement, the formation mechanism of micro-convection near the wall of MEPCM, the heat transfer enhancement mechanism and industrial application prospect of waste heat recovery of micro-convection and latent heat are analyzed theoretically and numerically, and the key control parameters influencing the heat transfer enhancement are clarified. Then, the effects induced by these parameters on the process of waste heat recovery are analyzed numerically. It lays a scientific theoretical foundation for the industrial application of the two new waste heat recovery units. The results are as follows:A phase change micro nanocapsules suspending fluid high efficiency microchannel waste heat recovery unit and phase-change microcapsule suspension reciprocating motion high efficiency waste heat recovery unit are put forward;Based on the multifield synergy principle of heat transfer enhancement, a theoretical model describing heat transfer enhancement process of the phase change micro nanocapsules suspending fluid high efficiency microchannel waste heat recovery unit is built. Aiming at the deficiency of current widely used numerical simulation methods like DPM model and the equivalent specific heat method, the jet multiphase flow numerical simulation methods which based on VOF method and melting/solidification model is established. Not only can this method reflect real condition in the process of solidification with energy release and melting with energy storage of MEPCM, but also accurately predict the formation process of micro-convection between MEPCM and carrier fluid, and the heat transfer enhancement process of micro-convection and latent heat, which lay a scientific theoretical foundation for the heat transfer enhancement of MEPCM suspension waste heat unit;Comparing with the numerical simulation of three kinds of working conditions such as static common micro-capsules with no phase change, static MEPCM phase change particles without relative movement between micro-capsules and carrier fluid,and static MEPCM phase change particles with relative movement between micro-capsules and carrier fluid, The research results show that the micro-convection of micro-encapsulated particle back flow side increases with the increase of Reynolds number Re, and the process of melting and solidification of MEPCM induces micro-convection in back flow side. The direct driving force of MEPCM heat transfer enhancement comes from this micro-convection effect, and the effect of heat transfer enhancement increase with the increase of micro-convection intensity.The best heat transfer enhancement effect is under the condition of relative movement between micro-capsules and carrier fluid, and the surface convection heat transfer coefficient of MEPCM is maximum, which is approximately equal to 20 times than the static micro-capsule with no phase change. Latent heat and relative movement of MEPCM can play a substantial role of heat transfer enhancement. And in the condition of relative movement of MEPCM and carrier fluid, heat transfer enhancement induced by latent heat can achieve the best effect;Through the waste heat recovery efficiency comparative analysis of high efficiency microchannel waste heat recovery unit in which the pure carrier fluid, common microcapsules and microencapsulated phase change microfluidic respectively flow,the research results show that microencapsulated phase change microfluidic can play a significant effect on heat transfer enhancement, which make The maximum heat transfer enhancement factor of industrial waste heat storage up to 70.91, and the maximum heat transfer enhancement factor of industrial waste heat release up to 64.32. Not only can the capacity of waste heat recovery be substantially improved, but also the speed of solidification with energy release and melting with energy storage can be increased significantly, and in less than one second of time, the waste heat recovery heated fluid can be heated about 4.46?, which can effectively solve the current international technical bottleneck of mismatching in time and intensity between energy storage and release in the waste heat recovery unit for the phase change material being incapable of storing and releasing industrial waste heat in time for its poor heat transfer performance,and can provide technical support in which China's industrial waste heat is efficiently recycled and has a broad industrial application prospect.