Study On Heat Transfer Characteristics Of The Middle-deep Coaxial Casing Ground Heat Exchanger

The middle-deep geothermal source heat pump system is a new heating technology.Compared with the traditional shallow buried tube ground source heat pump system,the underground heat exchange can not only save the floor space,but also reduce the energy consumption of the system,which is beneficial to clean heating.Downhole heat exchange device as the core of underground heat exchange system.To ensure the stability and efficiency of downhole heat exchange,coaxial casing heat exchanger are usually selected.This paper adopts a numerical heat transfer model of a downhole coaxial casing heat exchanger for middle-deep geothermal.The numerical model considers the geothermal gradient of the underground rock-soil,the backfill material and the heat capacity of the pipe wall and fluid.Calculate the heat transfer equation of geothermal downhole coaxial casing heat exchanger under the condition of fixed inlet temperature and actual engineering data,this condition is more realistic and more reliable.The heat transfer characteristics of downhole coaxial casing heat exchanger is mainly studied.The specific research content is as follows:First,based on middle-deep geothermal well in the project.Extensive collection and arrangement of exploration geological characteristics and underground geotechnical distribution data,obtain the physical parameter values to which the numerical simulation analysis is applied.Explain the engineering project and geological overview,the overall structure of the heat exchange system,the design of the heat exchange test plan and the results of the heat exchange test.Through the collation of data and corresponding mathematical calculations,the change trend of underground rock-soil temperature distribution with increasing borehole depth,structural characteristics of each layer,geothermal gradient value and geothermal resource storage of a single-hole geothermal well are obtained.Experimental design schemes for middle-deep geothermal source heat pump system.Field test under 3 kinds of working conditions,through the heat transfer test,the water temperature at the outlet of the downhole heat exchanger and the corresponding heat transfer power are obtained under each working condition.Secondly,for the complex heat transfer process of the underground heat exchange system in the middle-deep layer,based on the finite difference method,the numerical model is solved and simulated.Comparing the simulation calculations under the three working conditions with the field heat transfer test under steady state at the outlet water temperature values of the two,the results show that the two are in good agreement.The distribution of the surrounding temperature field after the heating operation of the downhole heat exchange system under three conditions for one heating season(120 days)was simulated.Then,the influence of various factors on the water temperature,heat exchange and heat short circuit value at the outlet of the middle-deep coaxial casing heat exchanger was simulated and analyzed by using the control variable method and numerical model.The factors include circulating fluid flow,inlet temperature of circulating fluid,tube diameter of heat exchanger,thermal conductivity of inner tube,thermal conductivity of backfill material,thermal conductivity of rock-soil,ground temperature gradient and depth of borehole.Finally,the principles of energy-saving analysis and performance evaluation indexes of middle-deep geothermal source heat pump system is described.The energy consumption of the system is analyzed and the main links of energy saving are proposed.The technical economy of this system was evaluated.The results obtained through the research can provide a theoretical basis for the analysis of the heat transfer performance of coaxial casing heat exchanger in the middle-deep geothermal well.To provide a research basis for the use of middle-deep geothermal source heat pump system for heating in northern areas where heating is mainly used.It has important guiding significance and practical value for the promotion of middle-deep geothermal source heat pump projects.