Research On Groundwater Flow And Heat Transfer Characteristics Of Single Well Cycling Ground Heat Exchanger System

Currently, as a new kind of groundwater heat pump system, single well cycling ground heat exchanger system (SWCGHES) contents three forms of thermal well, e.i. standing column well (SCW), pumping and recharging well (PRW), and pumping and recharging well filled with gravel (PRWFG). Their pumping and injection pipes are placed in a same well, which is pumping water in the low part and recharging water in the top part. The SCW needs to drill hole in the bedrock directly, and then most of the water circulates in the well bore and the heat exchange take place in the well wall, while small part of the water goes out of the borehole and exchanges the heat with aquifer raw water. There are some clapboards in PRW that the thermal well is divided into three parts, i.e. injection zone (in the top part), seals zone (in the middle part), and production zone (in the low part). The PRWFG has similar form with PRW. The difference between them is that the diameter of borehole in PRWFG is longer than the one in PRW. Moreover, the gap of borehole in PRWFG is filled with sorted gravel. In this paper, relative theoretical and experimental researches on these three thermal wells have been carried out. The main research work and results are as follows:A physical simulation experiment table of SWCGHES has been designed and set up, while the experimental testing system and data collecting system have been selected. Additionally, the error of this experiment has been calculated. The results show that the measurement error of the experiment table is within an acceptable range, which can accurately reflect the actual physical phenomena.Different characteristics of three thermal wells have been simulated by replacing the prefabricated inserts. In SWCGHES relative experimental research has been carried out, such as distances between pumping and injection screens (DPIS) variations, initial aquifer temperature variations, and load variations. Additionally, the bleed experiment has been developed in SCW and PRWFG. The results show that with increasing the DPIS, the outlet water temperature and heat transfer quantities of these three thermal wells can be improved significantly. Among these three thermal wells, SCW’s load capacity is the least and temperature variations between inlet and outlet water is the maximum, while its thermal influence scope is the smallest. Moreover, in heating condition the bleed can improve the outlet water temperature and heat absorption quantities of SCW and PRWFG. Smaller bleed rate can improve the heat load capacity of the thermal well significantly. However, with increasing the bleed rate, this kind of improvement effect is weaker.The research on PRW system in different operating modes has been carried out. Time of operating condition has been distributed by heating season and air-conditioning season of three representative regions, e.i. Beijing, Shenyang, and Shanghai, respectively. These experiments include four conditions, e.i. continuous heating condition (CH), continuous cooling condition (CC), summer and winter condition (SW), and winter and summer condition (WS). The results show that in the operation of PRW in cold region the system can not restore in recovery period under the experimental condition. Additionally, the underground aquifer can not make their own return to the original state. Auxiliary equipments must be used for the heat supply of the aquifer, in order to ensure the system in a long-term reliable operation. In the cooling and heating load considerable area, using the SW operation mode can improve the stability of the system. However, in cold area SW operation mode should be used, so that it can keep the outlet water temperature at a higher level. On the contrary, in warm area WS operation mode should be used, while it can maintain the outlet water temperature at a lower level.The multi-zone coupling CFD simulation model has been established. The motion equation in porous media has been analyzed and derived by N-S equation and Bernoulli equation, while the flow model in porous media has been developed. In addition, the experience factors in the derived equation have been determined by experiments. Moreover, the experimental results of SWCGHES are compared with the simulation results, while the accuracy of the simulation model has been validated.Three aspects of simulation study have been carried out that include porosity variations, initial aquifer temperature variations, and outlet water flow rate variations. In these three thermal wells, the aquifer flow field and aquifer temperature field with outlet water flow rate variations have been analyzed. The results show that the effect of porosity on characteristics of these three thermal wells is weaker; however, this effect on PRW and PRWFG is stronger than SCW. The effect of outlet water flow rate on SWCGHES is greater. With the outlet water flow rate increasing, the outlet water temperature of these three thermal wells changes, while the thermal influence scope improves significantly. The initial aquifer temperature variation can not affect the trend of the aquifer temperature variation, while the trend of outlet water temperature is almost the same except some temperature difference among them. However, improving the initial aquifer temperature can increase the mean temperature of outlet water and accumulative heat absorption quantities, which show that the initial aquifer temperature is a key factor in SWCGHES.This paper is a part of national natural science foundation "Study on the Coupling Mechanisms of Multi-Pattern Flow and Heat Transfer in SWCGHES "(41002085). The research in this paper provides a comprehensive grasp of operating characteristics of SWCGHES, while an important theoretical basis and technical reserves are provided for design and application of these systems.