Numerical Simulation Of Three Dimensional Unsteady Heat Transfer Of The Ground Heat Exchanger

With the gradual consumption of traditional energy, the development and use of new energy is paid more and more attention over the world. The geothermal energy is a renewable and clean energy resource, the ground source heat pump technology has a broad development space. The geological structure has differences, which lead to the. differences of geothermal resources and soil thermal properties in different region. Therefore, it is essential to study thoroughly the effect of material properties, structure dimensions and operating conditions on the heat transfer performance of the ground heat exchanger with a view of predicting accurately the heat transfer performance.The thermal response test is basis of reasonable design of heat exchanger, and now internationally accepted test method is a constant heat flow method for soil thermal properties. In the Thermal Response Test, the data processing is based on the line heat source model. However, the existing line source model regarded the all materials around the tube as an en entity, which ignored the effects of thermal properties of backfill soil, structure parameters and operating conditions on the equivalent thermal conductivity of the soil.In the paper the physical and mathematical model of three dimensional unsteady coupled heat transfer in the ground heat exchanger was built on gambit and fluent for a typical ground heat exchanger, and the coupled heat transfer process of ground heat exchanger is simulated. The numerical simulation results and test results were compared; the computed results shown well consistency with the experimental ones. On this basis, the effects of the thermal properties of the backfill material and structure parameters as well as operating conditions on the heat transfer performance of the ground heat exchanger were systematically investigated and analyzed. The results showed that the thermal properties of backfill material have greater impact on the equivalent thermal conductivity of soil. It was concluded that the thermal conductivity of backfill material should be not lower than that of soil and the specific heat of backfill material is as lower as possible in the design or construction process of the ground heat exchanger. The thermal properties of pipe, flow rate of fluid and borehole depth have less impact on the test results. It was suggested that the ordinary polyethylene pipe is suitably used in the ground heat exchanger, the reasonable flow rate range is between 0.5 m/s to 1.1m/s, and the reasonable borehole depth range is between 60m to 80m. At the same time, the equivalent thermal conductivity can be accurately measured when the tube spacing of U-tube pipe is increased. However, the gradient is very small, the tube spacing of U-tube pipe has smaller impact on the equivalent thermal conductivity of soil.