Study Of Heat Transfer Model Outside The Borehole Of Vertical U-tube Of Ground-coupled Heat Pump

Ground-coupled Heat Pump (GCHP) is a method to save energy and protect environment. Heat transfer outside the hole wall is an unsteady process and very complex, then it becomes a key question in the design of GCHP, so it's very significant to study the model of heat flow outside the borehole of GCHP. Heat transfer outside the hole wall was studied, and some models were put forward, including analytical model of a single hole with or without seepage, the model of heat flow in tube group system and analytical model of GCHP under dynamic load, and there is a great reference value for the design of GCHP. Main works of this thesis are as follows:First, by the method of Green's function and virtual heat sink, finite long cylindrical surface and cylinder source model without seepage were put forward considering the distribution of heat flow in radical direction. The two models are demonstrated more precise by comparing with experimental results. After analysis of error, the difference among finite long cylindrical surface, cylinder source model and line source model is large relatively in the case that dimensionless quantity In (F0) is smaller than a certain value, while the difference among the three models is small when In (F0) is larger than the value.Second, finite long line source model without seepage was put forward considering the boundary conditions of the ground surface and the depth of the bore, then effect factors were analyzed. The temperature rise in the downstream region of seepage is slightly higher than that in the upstream region. Compared with infinite long line source model, the temperature rise close to ground surface and the bottom of borehole calculated by model in this paper decreases gradually, and is more precise.Third, on the basis of finite long cylinder source model of one single borehole, a concept of critical radius was proposed, and the range to integral is approximated, then finite long body source model of tube group system was built. Analysis shows that cube source model is more precise if tubes-are arranged in a rectangular form, while cylinder source model is more precise if tubes are arranged in a cinquefoil form. Body source model of tube group system is recommended as it's precise and concise.At last, for heat transfer of GCHP under dynamic load, analytical model is put forward on the basis of Duhamel's integral. The model is precise and concise on condition that the function of dynamic load is known, so it's very easy in practice use. Analysis shows that temperature rise at the midpoint of depth of the borehole is significantly different with integral mean temperature rise along the depth, and the latter is more appropriate to be taken as representative temperature rise.