The Heat Transfer Models And Experimental Study On The Coil Ground Heat Exchanger Inside Foundation Piles

With the increasing awareness to save energy and national policy support, ground source heat pump technology earns more and more attention. As a new form of ground source heat pump exchanger, the coil ground heat exchanger inside piles makes full use of ground floor area of the building to solve the problem of inadequate outdoor drilling area, and to cut the high cost of borehole and underground heat exchanger, which greatly improve the heat transfer effectiveness and construction efficiency. Because of these advantages, the coil ground heat exchanger inside piles is bound to be a trend for future development.The diameter of the coil ground heat exchanger inside piles is much larger than that of the borehole, while the depth of the pile is usually less than that of the borehole. First of all, in view of its special geometry and structure conditions, this paper takes the radical size and heat capacity of the pile into consideration so that a solid cylindrical heat source model is proposed to describe the heat transfer process of the spiral coil ground heat exchangers. By means of the Green's function method and the virtual heat source approach analytical solutions have been derived, applicable for temperature distribution of the one-or-two-dimensional cylindrical heat source models with infinite or finite length. The geometrical and physical parameters which impact on this heat transfer process have been analyzed and discussed accordingly. Besides, two representative temperatures, i.e. integral average temperature and mid-point temperature on cylindrical surface, are also calculated discussed.Secondly, taking into account the layout of the spiral coil inside the foundation piles, another heat transfer model of the coil ground heat exchanger inside foundation piles, referred to as the coil model, is proposed and optimized. The longitudinal temperature distribution of the infinite coil model changes periodically with the cycle of T=B. While considering the effect of heat transfer and the security of piles, a principle for reasonable choice of the coil pitch is proposed in the engineering applications. In view of the temperature rise caused by thermal resistance of the tube wall and the convective resistance inside the tube, we get the temperature response of the fluid in the tube.Thirdly, through the setting up a laboratory device, experimental study was carried out on the heat transfer around coil ground heat exchangers inside piles. Comparing the experimental data and model predictions, we have verified that the coil model is basically consistent with the actual situation. The experiment proves the influence of the heating power, rate of flow, and the thread pitch on the heat transfer of the coil ground heat exchanger inside piles, and obtains a coincident conclusion with the theoretical model. Besides, according to the situation observed in the experiments, this paper points out the shortage of the model, and puts forward the measures for improvement.Finally, the detailed building load of a real project is calculated by using DEST-C software. Based on the infinite coil model, the coil ground heat exchanger inside foundation piles is designed with the impact of the peak load taken into consideration.