The Experimental Platform Research Of Ground Heat Exchangers In Ground Source Heat Pump Systerm Based On Similar Theory

In recent years,ground-coupled heat pump(GCHP)system has gained rapid development and wide attention in China due to its characteristics of high efficiency,energy saving,environmental-friendliness and multiple application.There are many factors influencing the heat transfer performance of GCHP system,and the influence of groundwater seepage and geological stratification can not be ignored.In the actual project,the seepage is difficult to detect and the geological conditions are complex,so it is more advantageous to study the performance of the GCHP system by using the model experimental platform considering seepage and geological stratification than field tests.However,in a large number of experimental platforms of ground heat exchangers(GHEs)in GCHP systerm,the geological stratification and local seepage are often lacking,and most of them are the study of single GHE.Meanwhile,few studies have researched on the similarity between the model experimental platform and the prototype based on the similarity theory.In this paper,a similar model experimental platform for the GHEs of the GCHP system,which has considered the phenomenon of groundwater seepage and geological stratification,is established based on a prototype using the similarity theory and Darcy's theorem.Besides,the influence factors of the similarity between the model experimental platform and the prototype have been studied.Based on the differential equations of heat transfer considering seepage and geological stratification in GCHP system and soil,four similar criteria number between the model experimental platform and the prototype are derived using similarity theory,and the similarity constants of each parameter are determined.Taking a project as a prototype,a double GHEs model experimental platform with double layer geology and local seepage and satisfying the similar conditions was established,and the similarity between the prototype and the experimental platform was verified.In this process,a numerical model of stratified heat transfer for double GHEs was established without consideration of seepage.According to the test data of the experimental platform,the accuracy and reliability of the established layered heat transfer model of GHEs was verified.Due to the lack of prototype data,the prototype was simulated by the layered heat transfer model of the GHEs,and the calculated results are compared with the measured results of the experimental platform in the water temperature,the pipe wall temperature and the soil temperature.In addition,the application of similarity model in GCHP systerm numerical model is verified.At last,due to the four similar criteria number between the prototype and the experimental platform can not be fully satisfied because of the limitation of the actual situation,the factors that affect the similarity between the experimental platform and the prototype are analyzed by the numerical model one by one.The results show that,the condition of forming the similar model of GHEs of GCPH systerm under the condition of groundwater seepage and geological stratification is the four similarity criteria(H_o?_oF?_eR?N_u)between the prototype and the model are equal.The premise of the condition is the soil,the buried pipe and the fluid in the tube of the in the experimental platform are the same as prototype.Comparing the results of the prototype numerical calculation with the experimental results under the constant inlet temperature condition and the intermittent operation with the ratio of operation to stop is 2:1,the average relative error between the measured and calculated values of the water temperature of the GHEs is 1%and 1.7%,respectively,and the average relative error of the soil temperature at the contrast is 1.3%and 1.43%,respectively.All the relative errors are small,which proves that the experimental platform is similar to the prototype and has a high similarity.When the same performance computer was used to calculate the prototype numerical model and the small model that fully satisfying the similar conditions,it shows the calculation time of the small model is only 23.88%of the time-consuming of the prototype calculation.Meanwhile,the average relative error of the water temperature and the soil temperature between the prototype and the small model is 0.20%and 0.08%.The similarity between the small model and the prototype shows that the similar model established in this study can be effectively and accurately applied to the numerical model calculation of GHEs of GCHP system.The study of the influence factors of the similarity between the prototype and the experimental platform shows that the average relative error of the water temperature and the soil temperature between the prototype and the experimental platform will be more than 5%when the velocity ratio of the prototype and the experimental platform changes from 1/8 to 1/1 or the initial soil temperature difference between them is above 1?.The the average relative error of the outlet temperature and the soil temperature of the GHEs is 0.73%and 0.78%,respectively when the copper tubes are used instead of the PE pipes in the experimental platform.It is shown that the flow velocity in the GHEs and the initial temperature of the soil have great influence on the accuracy of the results obtained by using the similar model between the prototype and the experimental platform,and the material of the buried pipe has little effect on it.