Research On Heat Transfer Characteristic Of Vertical Ground Heat Exchangers In Ground Source Heat Pump Systems

For utilizing heat exchangers buried in the ground to transfer heat with the soil and upgrade the tenor of the renewable geothermal energy in shallow layer, a ground source heat pump (GSHP) system is known as one of air conditioning techniques which have the greatest developmental future in the 21th century. It is a matter of common observation that the development of GSHP technique is restricted by a lot of factors, in which the heat transfer of a ground heat exchanger (GHE) is the key of research on GSHP techniques at all times. Due to lacking in an comprehensive acquaintance with the heat transfer process, the arbitrary design for the constructure and size of GHEs would lead to the serious drop in performance of heat exchangers, and the increase in energy consumption of the whole air conditioning system, to top it all, going so far as do the system paralysis, all which should bring great barriers of the promotion and popularization of the new technology.The desertation first launched on the soil trait and the commutative load rejected to or extracted from the soil which were tightly related with the heat transfer of geothermal heat exchangers. The soil thermal property and ground water seepage flow behavior as well as the load distribution in various regions and buildings were analyzed intensely. Two primary heat transfer manners describing the heat quantity transport in the soil and three important characteristic representing the thermal load property were put forward. These traits directly determined the heat transfer performance of GHEs and whether or not the heat exchangers could keep the good performance in the long run in the limited buried places, which in turn decided the years of the sustained and effective operation of GSHP systems.Through theoretical analysis, the mathematical and physical model of a vertical U-tube GHE under two kinds of heat transfer manners was established in which the thermal load characteristic of GHEs can be incarnated with the help of boundary conditions and initialization conditions. For such kind of coupled heat transfer problem, the methodology of a straightforward discretization and an overall solution utilizing the finite-volume method in all mediums zones, was adopted in this paper. In view of the inlet and outlet temperature and heat transfer continuable feature of GHEs focused in the design and operation of GHSP systems, the energy efficiency coefficient and heat transfer endurance were introduced to quantify the energy efficiency performance and the endurance period according to maximal available temperature difference between the inlet and outlet in the U-tube heat exchanger. The performance of GHE in soil with and without groundwater flow was analyzed firstly through dynamic numerical simulation of heat transfer process between the soil and the working fluid by the use of the optimized infinite boundary size. Basing on the model, the varied regularity of energy efficiency performance and heat transfer endurance with the conditions including the different configuration and dimension of the U-tube heat exchangers, the soil thermal behavior, stratified feature in the soil, thermal load characteristic and operational mode were discussed. Regional heat transfer theory and migratory property under the condition of dynamic heat transfer capacity in U-tube heat exchangers were brought forward in the thesis. It supplies theory support and technology accumulation for the optimal design of the GHE and the parametric matching of air conditioning systems.According to the heat transfer model of GHE, an experimental facility in situ for measuring the performance of exchangers was upbuilt. Through being compared with experiments in testing boreholes, the accuracy and reliability combined with its implementation in the mathematic model of GHE was validated. From the point of view of the experiments, the influences rules of the inlet water temperature, the flow velocity and operational mode on the temperature field in the surrounding soil and heat transfer characteristic of GHE were verified and analyzed. Thus all above can make contribution the basic data for engineering practice in GSHP systems.In the light of the simulative and experimental testing value in the soil surrounding the GHE, the minor temperature of the soil in the axial direction should be found during the heat transfer between the circulating fluid in the U-tube and the soil. So the soil heat transmission outside the borehole can be analyzed by two dimensional heat transfer model. On the basis of that, a new heat transfer analytical model considering the axial flowing based upon a multipole theory model combined with the thermal resistance network balance principle was developed. The heat transfer region was divided into two parts at the borehole wall. Both steady and transient heat transfer methods were used to analyze the transmission of heat inside and outside borehole respectively. As for inside borehole, multipole theory model in the U-tube was used to solve the heat transfer problems. As for outside borehole, constant heat flux cylindrical source model was used to calculate the borehole wall temperature. To a certain heat transfer duration, adapting this model to analyze the heat transfer performance can not only afford high reliability but also need a minor calculating work capacity.The research in this desertation can make the design and engineering practice of GSHP more reasonable. Furthermore, it can also provide theoretical base and technique support for the popularization and development of applications in the GSHP system utilizing shallow statum geothermal energy which have great energy saving and environmental protection advantaged potentiality.