Numerical Programming Study On Phase Change Heat Transfer Performance Of Vertical Buried Heat Exchanger Coupling Inside And Outside Of Tube

Faced with the increasing shortage of fossil energy,renewable energy utilization has occupied an important position in urban construction.Geothermal resources are abundant,clean and renewable.Ground source heat pump(GSHP)technology can efficiently utilize shallow geothermal resources,and it realizes cooling supply in summer and heating supply in winter,Moreover,it plays an important role in energy saving,emission reduction and optimization of energy structure.Meanwhile,the research of this technology has attracted great attention and promoted its maturity.Ground source heat pump system releases or absorbs heat to the soil around the ground heat exchanger in summer and winter.In turn,the change of soil temperature affects the heat transfer capacity of the heat pump system.When buried tube heat exchangers are used to store cold in underground at summer night or to collect heat in cold areas with high intensity in winter,the soil temperature around the buried tube heat exchangers might drop below freezing point.At this time,the water in the soil pore will freeze,which affects the heat transfer performance of buried tube.The heat transfer performance of buried tube heat exchanger is related to the initial investment and operation cost of the whole system.Therefore,it is very important to simulate the heat transfer performance of buried tube which is more consistent with the actual situation for the research and development of ground source heat pump technology.Due to the coupled heat transfer between the fluid and the surrounding soil and coupled with the phase transformation of the porous liquid in the soil,the heat transfer mechanism is very complex.At present,there are few studies on the effect of soil freezing on the heat transfer performance of the buried pipe heat exchanger;there is no mature CFD simulation software for the coupled heat transfer in and out of the pipe under the condensation of porous media in the soil around the buried pipe.A self-compiled CFD software is proposed to simulate the related problems in this study.In this paper,the vertical single U-tube heat exchanger is equivalent to a square double-tube heat exchanger.The solid phase increment method is used to deal with the phase change heat transfer in soil.A mathematical model of the coupled heat transfer between turbulent flow field in pipe and surrounding soil considering the phase change of frozen soil is established.The three-dimensional dynamic value calculation programis compiled by Fortran language to simulate heat transfer inside and outside the coupled pipe.The influence of soil freezing on the outlet water temperature and soil temperature of buried pipes,and the influence of various factors on the heat transfer performance of buried pipes during soil freezing around buried pipes were simulated and analyzed.Because the latent heat of phase change will be released when the liquid water in the soil pore condenses,which delays the decreasing speed of soil temperature.The thermal conductivity will change when the liquid water becomes solid.The mechanism and process will affect the heat transfer performance of the underground tube heat exchanger.If the influence of soil freezing is neglected,the design error will occur.In the same scenario of this study,the soil temperature and effluent temperature under the two conditions are simulated and calculated by using the models considering and not considering the soil phase change heat transfer.By changing the level of several important influencing factors,the variation of heat transfer performance of buried pipes and the error of heat transfer of delayed meters between the two conditions without considering the frozen heat transfer are obtained.The results show that in the case of soil freezing,the heat transfer of extended rice calculated without considering the effect of soil freezing is on the low side,resulting in design errors;and varying the level of several influencing factors result in different degrees of calculation deviation.In the design of buried pipes,the effect of soil freezing on the heat transfer performance of buried pipes should be considered according to the specific conditions.The main task of this study is to provide a programming method for studying the effect of soil freezing on the heat transfer performance of buried pipe heat exchanger,and to study the law of the influence of soil freezing around buried pipe on the heat transfer of buried pipe and the surrounding soil temperature,so as to provide theoretical help for the reliable design of buried pipe heat exchanger.