Two-Scale Study On Heat Transfer And Flow Characteristics Of Microencapsulated Phase Change Material Suspensions In Pipes

With the increasing requirement of indoor air quality,the energy consumption of HVAC(heating,ventilating and air conditioning)system increases year by year.The proportion of transport energy consumption accounts for a large proportion of the HVAC system consumption.As a latent heat functional thermal fluid,the microencapsulated phase change material suspension(MPCMs)has both good fluidity and high energy density,and can be used to replace the coolant carrier in the traditional air-conditioning system.It is an effective way to realize the "energy saving and emission reduction" of the HVAC system.However,the phenomena,such as the heterogeneous flow and the latent heat release of the suspension,greatly increases the difficulty in understanding the behavior of flow and heat transfer of suspension flow.Therefore,from the system design point of view,it has a practical and theoretical significance in exploring multi-scale characteristics of MPCMs,depicting phase change process of microcapsules and analyzing macroscopic slurry flow and heat transfer phenomena.Furthermore,it is useful to describe the heat transfer characteristics of microcapsule suspension.Based on the foregoing background,a two-scale model of MPCMs was proposed by coupling phase change model of microcapsules and flow and heat transfer model of suspensions.Based on associative multi-scale method,this model can depict the heat exchange process between capsules and the carrier fluid by using an information transmission and the bidirectional exchange process.Then,three sub models(i.e.phase change model of microcapsules,fluid-flow model of MPCMs,and scale correlation model)were analyzed and verified with the experimental data.The phase transition process of phase change material(PCM)inside capsules was studied under different particle size and the selection principle of the macro-scale flow model was proposed.In addition,this paper proposed three different schemes,including the analytical method,the multiple regression analysis method and the RBF(radial basis function)neural network,to simplify and extract the phase change information at the micro-scale.All these schemes could decrease the computation significantly,which raise the efficiency of the two-scale model that presented in this paper.Finally,a straight pipe sytem drived by pump and a closed loop thermosiphon system were simulated and experimented,which validated the accuracy of two-scale model for thermal transfer system with and without additional pump power.The strengthened effect of microcapsules on heat transfer of carrier fluid was analyzed.The heat transfer enhancement of MPCMs was studied under different systems.The effects of various controllable parameters on the heat transport capability of different systems were discussed quantitatively.The results show that the two-scale model is capable to predict the performance of for both the straight pipe segment and the closed loop system.The maximum deviation of wall temperature of the pipe between the simulation result and the experimental data is less than 7.43%and 19.80%,respectively.Compared with the traditional computational fluid dynamics(CFD)method,the lattice Boltzmann method(LBM)using the immersed moving boundary scheme to deal with the moving interface,can reproduce the phase change process of PCM inside capsules more accurately and efficiently.Furthermorer,compared with LBM model,a reasonable selection of traditional CFD flow model can greatly improve the computational efficiency when study the flow phenomenon of MPCMs.For two-scale correlation scheme,the prediction error of analytical method is less than 2.42%when phase change process is simple.The average prediction error of RBF neural network method is only 1.77%,while maximum errors of several points are 53.62%.The performance of the multiple regression analysis method is in between,and its average and maximum error are 2.88%and 9.09%,respectively.According to experiment and simulation results in the pump-driven direct pipe segment system,the average heat transfer coefficient of the pipeline can be effectively improved by increasing the wall heat flux,raising the suspension flow rate,reducing the inlet temperature and appropriately selecting microcapsules with small particle size and high potential.In the thermosiphon loop,the heat transfer efficiency of the loop is closely related to the utilization rate of latent heat of microcapsule.With an increase in heating power and cooling temperature,the Nu number at the inlet of heating section increases while the corresponding heat transfer effectiveness ?h,in and the thermal resistances of the system Rsus decrease.The research in this paper is helpful to explore the energy saving potential of replacing the traditional refrigerant carrier with MPCMs in the air-conditioning system,and to provide guidance for the equipment selection and optimization of the new cooling system.