Research On Performance Of The Heat Transfer Of Vertical Ground Heat Exchanger In Ground Source Heat Pump System

As an environmental protection and energy-saving technology, ground source heat pump(GSHP) is in a highly development stage of current environment and energy crisis. The study of the heat transfer performance of ground heat exchanger(GHE) is of great significant, for that it is the core in the GSHP research field. With the advantages like economical use of land, vertical U-tube GHE is more suitable for China’s basic national condition of the low per capita share of land. Therefore, the heat transfer performance of the vertical U-tube GHE is mainly researched in this article.GSHP utilizes soil, surface water, groundwater, even municipal sewage as the heat source or heat sink. The theory of the heat transfer performance between soil(or surface water, groundwater, municipal sewage) and the fluid in U-tube is studied in this paper. The theory of GSHP system is described and the heat transfer performance of vertical U-tube GHE is simulated intents to strengthen the heat transfer between soil and fluid in U-tube. The three-dimensional numerical model of the heat transfer established in Gambit software has the same size with the actual project. After meshing, the model was imported into Fluent software for simulated calculation. The numerical simulation in steady and unsteady state operating condition is based on soil thermal properties in Xi’an region. The simulation analysis in unsteady state operating condition can be started when the heat transfer effect is ideal in steady state. It plays the role of validation for the model.The simulation in steady operating condition in summer is mainly researched in this paper, for the simulation analysis in summer operating condition is similar to that in winter. The influence of groundwater seepage velocity and soil porosity on heat transfer performance of GHE is researched in this article. It can be drawn that groundwater seepage promotes the heat transfer of GHE. The heat transfer between GHE and soil is weakened with the increase in soil porosity at the groundwater seepage velocity of 30m/a(9.5129×10-7m/s). Then, the influence of inlet velocity of water in U-tube, soil thermal conductivity and specific heat on heat transfer performance of GHE is studied with the groundwater seepage and without groundwater seepage, respectively. It turns out that the heat transfer capacity of GHE is enhanced with the increase in flow rate. The inlet velocity of water in U-tube of the vertically buried GHE is suggested to choose from 0.6m/s to 1.2m/s. Soil of larger thermal conductivity benefits the operation of GSHP, while the rise in heat-exchange rate per depth decreases with the increase in soil thermal conductivity. The influence of soil specific heat on heat transfer is similar but not a circumstance to thermal conductivity. The result also shows that the groundwater seepage strengthens the influence of inlet velocity on heat transfer of GHE while weakens that of soil thermal conductivity and specific heat. Multi-layer soil is also researched in this article, and it shows that the heat transfer effect of GHE in homogeneous soil plays the best role. Finally, the circulate situation of GSHP system is researched, and it shows that the intermittent operation mode promotes the heat transfer efficiency of GHE. The researches of this paper are aimed at providing theoretical basis for practical engineering application to some extent.