| With the rapid development of the construction industry,China’s building energy consumption is also increasing,reducing the energy consumption of HVAC system is an important measure of building energy saving.The ground source heat pump system is the ground source heat pump system which uses the energy in the underground rock and soil to carry on the heat exchange.It is called the air conditioning technology which has the development prospect and the energy saving potential in the 21 st century.The heat transfer process between the buried pipe heat exchanger and the surrounding soil has always been an important concern in the research of soil source heat pump technology.At present,the research is mainly based on the pure heat conduction or groundwater seepage theory to establish the heat transfer model of buried pipe heat exchanger,including linear heat source and cylindrical heat source model,the research content is usually established under the condition of constant heat flow,and the form of analytical solution is mostly the double integral of time and space,the form of complex calculation time;For the study of time-varying heat flow in the form of general functions,scholars introduce the idea of step heat flow and give an approximate analytical solution,but the calculation accuracy is not high.In this paper,according to the dynamic characteristics of building load,the analytical and numerical simulation research on the heat transfer between the buried pipe heat exchanger and soil under the condition of variable heat flow is established,which is of great significance to the rationality and economy of the design and operation of the ground source heat pump system.Specific research contents are as follows:(1)The general solution of one-dimensional heat conduction equation in spherical coordinate system is established by using the similarity transformation method(STM),which is expressed in the form of Kummer function;Some identities related to the properties of the general solution are derived,which lays a foundation for the application of the general solution in various heat conduction problems.Then,the general solution is applied to two kinds of heat conduction problems: one is the singlephase Stefan problem in which the latent heat of phase transition varies with space position;The other is the nonlinear heat conduction caused by a point heat source.These applications show that the general solution of similarity transformation can not only solve the linear heat conduction problem,but also is an important tool for solving the nonlinear heat conduction problem.(2)For the problem of infinite and finite long heat sources whose intensity varies with time in the form of power function,the analytical solution of temperature field is given by using the general solution of similarity transformation and its properties.Firstly,the analytical solution of the continuous point heat source is given by the general solution and its properties.Then,by integrating the length of the heat source along the continuous point,the analytical solution of the infinite long line heat source model is obtained.Finally,the analytical solutions of the vertical finite long heat source model and the inclined finite long heat source model are obtained by using the virtual heat sink method and the linear superposition principle.By comparing the temperature responses of finite(vertical)and infinite heat sources,the calculation error caused by neglecting the axial heat transfer under the condition of variable heat flow is studied.The results show that the infinite long line heat source can only describe the temperature field of the finite long line heat source in the 0 < < ? region;With the increase of ,the scope of application becomes smaller.The error mainly comes from the additional heat source action at the top and bottom of the infinite long line heat source,and the top effect is stronger than the bottom effect.(3)According to the intensity function is changing with time limited(vertical)long line heat source problem,put forward the piecewise constant approximation(two parameters)and the piecewise linear approximation to the original general function change of heat source,the method of using the linear superposition principle and(2)the answer gives the approximate analytical solution of the problem.The results and errors of different approximate methods are analyzed qualitatively and quantitatively by calculating examples.The results show that the more the number of sections,the more accurate the results are.Under the same number of segments,piecewise linear approximation is more accurate than segment constant approximation.Moreover,the advantages of piecewise-linear approximation are more obvious when the location near the heat source,the running time is long and the heat source intensity varies greatly.(4)In view of the simplified conditions in the process of derifying the analytical solution of the finite long wire and cylinder heat source model under the condition of variable heat flow,this paper uses the finite element software COMSOL to establish the numerical model of the heat transfer process of the buried pipe heat exchanger and carries out a comparative analysis.Firstly,the correctness of the numerical model was verified by comparing the simulation results of the numerical model with the computational process of the analytical solution under the same conditions.Then respectively established in the process of analytical solution is derived to ignore heat source radial dimensions,backfill material and soil differences in thermal and physical properties and material permeability simulation simplified conditions,such as drilling process,the results show that when considering backfill material thermal physical properties and soil is consistent with the temperature field of small,material,the greater the temperature conductivity,the higher the temperature,The temperature when water flow can penetrate the buried pipe is higher than that when water flow can not penetrate the buried pipe,and the higher the flow velocity is,the higher the temperature is.With the increase of heat source intensity and running time,the relative errors caused by these two simplified conditions will increase,but they will decrease with the increase of soil radius.The linear heat source model neglecting the radial size of the heat source leads to a lower temperature.The larger the radius of the cylindrical heat source model,the higher the temperature will be.With the increase of soil radius and running time,the errors of the two models become smaller,but increase with the increase of heat source intensity.There are 85 pictures,8 tables and 98 references in this paper. |
