Numerical Simulation Of Heat Transfer In A Vertical Ground Heat Exchanger

In a ground source heat pump system, one of the most important components is theground-coupled heat exchanger, through which thermal energy is exchanged betweenheat carrier fluid (i.e., water or water-antifreeze fluid) and soil. The heat transferperformance of ground-coupled heat exchanger plays an important role to determinewhether the ground source heat pump system could efficiently and economically workfor a long period of time. Since the heat transfer performance of ground-coupled heatexchanger depends on the parameters such as the effective thermal conductivity of soil,the thermal conductivity of grout material, the geometric structure of borehole and theoperation conditions of ground-coupled heat exchanger, it is important to analyze howthese parameters to influence the heat transfer performance of heat exchanger. In thispaper, a3D Computational Fluid Dynamic numerical model for ground-coupled heatexchanger is constructed and analyzed. All these factors to influence the ThermalResponse Test of ground-coupled heat exchanger are thoroughly studied, and a newfour-thermal-resistance heat transfer model of ground-coupled heat exchanger isproposed and studied. All the working conditions of heat exchanger are optimized basedon this proposed four-thermal-resistance heat transfer model of ground-coupled heatexchanger.Based on the study of ground-coupled heat exchanger in this paper, someconclusions could be drawn as follows:1.The environment parameters of Thermal Response Test, such as fluctuation of airtemperature and flow velocity of ground water, have a certain influence to change thethermal properties of ground soil. Some error could be introduced to evaluate thethermal conductivity of ground soil once the fluctuation of air temperature, flowvelocity of ground water and speed of air are beyond some certain values. Therefore, toreduce the error of testing environment on the TRT test, a thermal insulation material issupposed to be used to reduce the heat transfer between the tubes above the groundsurface and the testing environment, and the heat transfer surface of tubes exposed toambient should be minimized as small as possible. If possible, the flow velocity ofground water should be measured and the testing results of TRT could be correctedbased on the seepage velocity of ground water.2.Based the four-thermal-resistance heat transfer model of ground-coupled heat exchanger, the heat transfer performance of ground-coupled heat exchanger isthoroughly studied, and the corresponding correlations of thermal resistance in thismodel are obtained through a large amount of numerical simulation. It is found that thisanalytical heat transfer model of ground-coupled heat exchanger has the same accuracyas the3D CFD numerical model. Based on this heat transfer model, all the designparameters and working conditions are comprehensively studied for the ground-coupledheat exchanger, and the influence of these parameters on the heat transfer of heatexchanger are obtained. Some constructive comments of the design and operation ofground-coupled heat exchanger are concluded.