Research On Heat Transfer Performance Of Double U-tube Vertical Ground Heat Exchanger In Ground Source Heat Pump

With the rapid development of global economy,energy crisis has been a problem and attracting more and more attation.With the increasing of energy consumption in the worldwide,environmental pollution is becoming more and more serious.Energy and environment issues have been one of the most concerns.Ground source heat pump(GSHP)is a new technology of energy conservation and environmental protection and it is in a rapidly development stage.Study of enhancing heat transfer performance of ground heat exchanger(GHE)is of great significance.It is important in the GSHP research field.However,heat transfer performance varies with different buried pipe laying.Therefore,the main research of this thesis are the heat transfer performance of the vertical single U-tube and double U-tube GHE and their comparative analysis.Firstly,the composition of the soil is introduced in this thesis.Based on the modelling and computation,the heat transfer mechanism and porous medium theory are analyzed in detail.Then,the three-dimensional numerical heat-transfer models of the single U-tube and double U-tube of GHE are established using Gambit software as the same size as the actual project.Finally,the models are meshed and imported into Fluent software for three-dimensional simulation of heat transfer process.Due the simulation and analysis in winter operating condition is similar to those in summer,the simulation and analysis is only studied in summer operating condition in this thesis.The influences of inlet velocity of water,soil thermal conductivity and inlet temperature of water on the heat transfer performance of the vertical single U-tube and double U-tube GHE are studied with the non-seepage and seepage conditions,respectively.Results show that the heat transfer performance of single U-tube and double U-tube are enhanced with the increase of the inlet flow velocity.When the flow velocity is less than 0.53m/s(V<0.53m/s),the heat transfer performance of the double U-tube is less than the single U-tube at the same flow rate.When the flow velocity is more than 0.53 m/s(V>0.53m/s),the result is the opposite..Along with the increase of soil thermal conductivity,the average quantity of single U-tube heat per unit depth and that of double U-tube are increscent.Furthermore,the growth rate of double U-tube is higher than that of single U-tube.In addition,the groundwater seepage weakens the influence of soil thermal conductivity on single U-tube and double U-tube heat transfer of GHE.When the groundwater seepage velocity is 0.53 meter per year,compared with the non-seepage conditions,the outlet temperature of water in single U-tube is only reduced by 0.05 K,the average quantity of single U-tube heat per meter isincreased by only 0.59W/m,but the outlet temperature of water in double U-tube is reduced by 0.25 K,the average quantity of double U-tube heat per meter is increased by 5.93W/m.The groundwater seepage can reduce the temperature of the circulating water and improve the heat transfer effect of GHE.And the effect of double U-tube GHE is more effective than single U-tube GHE.With the increase of seepage velocity from 0 m/a to 60 m/a to 120 m/a,the average quantity of single U-tube heat per unit depth increases by7.36% and 27.56%,respectively,and the average quantity of double U-tube heat per unit depth increases by25.88% and 61.55%,respectively.The average quantities of single and double U-tube heat per unit depth are increscent along with the increase of inlet temperature of water and the growth rate of double U-tube GHE is slightly larger than single U-tube GHE.The higher inlet temperature of water is,the more remarkable the result of the groundwater seepage improves the heat transfer effect of U-tube GHE is.The researches of this paper are aimed at providing theoretical basis for practical engineering application to some extent.