Research On Heat Transfer Performance And Parameter Influence In Closed Loop Medium-deep Downhole Heat Exchange

The closed loop medium-deep downhole heat exchange technology,which is hereinafter referred to as “closed loop deep downhole heat exchange” or “deep downhole heat exchange”,is a kind of technology in line with the development direction of heat exchange on the side of geothermal heating source.However,the application of this technology in China is still in the stage of exploration and demonstration.The design and operation experience is insufficient and the research is also relatively scarce.Besides,the deep downhole heat exchanger(DDHE)is the key component of this technology for heat exchange,and its optimal design method is a research hotspot which has gotten widespread concern.In this paper,the heat transfer problem in closed deep downhole heat exchange was taken as the research object,the influence law of parameters on closed deep downhole heat exchange was studied by numerical simulation combined with experimental method,and reasonable suggestions for the optimal design of DDHE were put forward.Firstly,theoretical analysis and simplification of deep downhole heat transfer were performed.Based on FLUENT software,a numerical model of deep downhole heat exchange was constructed.By comparing with experimental data,it was verified that the model can accurately simulate the heat transfer process.Secondly,a closed loop downhole heat exchange experimental platform was built,and the results of experiment and numerical simulation are compared.The result shows that the experimental and numerical simulation method of closed loop downhole heat exchange put forward in this paper are both feasible to analyze such problems.Then,the basic heat transfer model of deep downhole heat exchange was constructed by combining the characteristics of typical geothermal fields in northern China with the actual engineering cases,and the coupling effects of well depth with other parameters on heat exchange were studied.The effects of flow rate,pipe diameter ratio,inner tube thermal conductivity,geotechnical thermal conductivity and temperature gradient of the geothermal reservoir on heat exchange under different well depth conditions were obtained and sensitivity analysis of parameters was carried out.It is found that with the increase of well depth,the outlet temperature of deep downhole heat exchange increases linearly,and the heat transfer rate increases rapidly at first and then slows down.The outlet temperature and heat transfer rate provided by deep downhole heat exchange are obviously higher than that of shallow buried tubes,which has potential for direct heating.With the increase of pipe diameter ratio,the outlet temperature decreases.Under the same pipe diameter ratio,the outlet temperature is higher when the inner and outer pipe diameters are larger.Well depth plays a decisive role in outlet temperature and diameters of inner and outer pipes have different effects on outlet temperature under different well depths.Geotechnical temperature gradient has the greatest effect on deep downhole heat exchange while the inner diameter has the smallest effect.When the well depth is small,the geotechnical thermal conductivity,outer diameter and flow rate have a greater impact.When the well depth is large,the effect of inner tube insulation is significant.Finally,based on the influence law of parameters on deep downhole heat exchange and sensitivity analysis results,the optimization design method of DDHE was discussed.It was proposed that the design of geometric parameters can be determined according to the sequence of well depth,maximum outer diameter and minimum inner diameter.For the design of adiabatic inner tube,from the point of view of technical feasibility and economic rationality,the sectional adiabatic setting can effectively reduce the length of adiabatic section of inner tube while ensuring good heat exchange so that the initial investment of system is reduced.It was found that the rational value of adiabatic ratio is 0.875.In addition,when the well depth is small,much more attention should be paid to the design of the outer diameter and flow rate.When the well depth is large,it is particularly important to ensure the insulation of the inner tube.