Heat Transfer Research On Two-phase Flow In Subterranean Heat Exchangers Of Direct Expansion Ground-coupled Heat Pump System

Ground-coupled Heat Pump (GCHP) system is one of the most critical forms in renewableenergy using in construction field,and it is known as one of air conditioning techniques whichhave the greatest developmental future in the21th century. Compared with Second LoopGround-coupled Heat Pump(SL-GCHP),Direct Expansion Ground-coupled Heat Pump(DX-GCHP) has advantages of more efficient and energy saving and is the development trend ofGCHP. The fluid flow and heat transfer in Ground Heat Exchanger System(GHES) is complextwo-phase flow and phase change heat transfer proeess whose mechanism is not clear sofar.Classieal studies mostly employ experimental methods to investigate such a process whilepure theoretic alanalys is less reported.The theoretical investigation of GHES based on fluid dy-namics and heat transfer still remains to be further studied. Therefore,the following aspects ofresearch work have been done in the present thesis.According to the refrigerant’s flow and heat transfer process, introduced multiphase Mix-ture model,utilized user defined functions to add the quality and heat transfer source betweenphase interfaces,and established modified two-phase flow mixture model in vertical ground heatexchanger.With this model, three-dimensional calculations of flow and heat transfer of backfill materi-al,U-tube and refrigerant were done. Summarized several influence factors of flow developmentand the effect of heat transfer by analysising the flow evolution mechanism and heat transfercharacteristics.For heat transfer occurring during import tube section,selected import a pipeline section asthe research object, discussed phase change characteristics parameters, heat transfer characteris-tics and the effect of flow characteristics in different import working parameters，such as drydegree,mass flow,evaporation temperature.Based on SPSS, via analyses variances and multi-factor examed influence of outlet temper-ature, total pressure recovery coefficient, outlet volume fraction of gas with import working pa-rameters,and proposed model between inlet and outlet parameters by regression method; sug-gested reducing the design depth of heat exchanger system,and proposed working parameterscorresponding best heat transfer performance of evaporation full development section by mul-ti-objective programming model.In this paper,two-phase Mixture model is firstly applied in boiling heat transfer performancecalculation in vertical ground heat exchanger,and its computational accuracy accord with the en-gineering need.The findings will be helpful to the comprehension of flow evolution mechanismand heat transfer characteristics inside ground heat exchangers as well as provide a reference for the design and structure optimization of underground heat transfer systems.