The Simplified Layered Numerical Model Of Ground Heat Exchanger And Its Application

Due to characteristics of environmental protection and energy saving,ground source heat pump(GSHP)system has been widely used at home and abroad currently,its key part is the Ground Heat Exchanger(GHE).System realizes heat transfer by GHE,injecting heat into the ground in summer and extracting in winter.Due to the large initial investment of GHE,it is particularly important to design GHE size reasonably for reducing economic investment.Analysis of underground heat transfer by calculation model is the key of GHE design and operation.Traditional calculation model in general will deal with soil as a homogeneous matter.In recent years,an increasing number of researchers pay attention to layered soil structure,and establishing a reasonable layered model is an important direction of GHE research.At present,the layered model has no accurate analytical solution,and adopts the three-dimensional layered numerical model mostly.Becase of modeling complexly,difficulty in change,large number of grid and more time-consuming calculation,the three-dimensional layered numerical model is not suitable for engineering application and heat transfer optimization analysis,and it is more important to establish a simplified layered numerical model.This article,considering the vertical layered soil condition,established a two-dimensional layered numerical model for the vertical U type GHE,and verified the accuracy of the model with the actual data and three-dimensional layered numerical model.Then,using the precise three-dimensional layered numerical model as a comparative object,this paper discussed the accuracy of two-dimensional layered numerical model in the cases of different number of soil layers and layering unevenly.Results showed that the two cases have little effect on accuracy of two-dimensional layered numerical model.Under different layered soil condition,the two-dimensional layered numerical model can replace the three-dimensional layered numerical model.Using the two-dimensional layered numerical model,we compared the heat transfer of traditional homogeneous model and layered soil model.Results showed that the borehole wall temperature and heat flux of layered soil structure have obvious layered characteristics.homogeneous model calculation results have much error and the error increases with time,layered model can more accurately describe the actual heat transfer properties of layered soil.Basing on the vertical layered simplification method,we further put forward the horizontal radial layered simplification method,and applied it to pile foundation GHE.Becase of more fully use of land area,reducing the initial investment and enhancing the advantages of the heat exchanger heat transfer performance compared with vertical buried pipe,pile foundation GHE already is a hot spot in current research.Pile foundation diameter size is large and the difference of thermal physical properties between inside and outside of pile is obvious.It is necessary to consider layered structure of pile foundation.The traditional calculation model of pile foundation GHE is mostly three dimensional numerical model which has disadvantages of modeling complexly and slow calculation.This paper,using the horizontal radial layered simplification method,established a annular layered numerical model for the pile foundation rotational symmetric multiple U-tube GHE,and verified the accuracy of the model with the actual data.Meanwhile,we compared the annular layered numerical model with the classical heat transfer model,such as the line-heat-resource model and the cylindrical source model.Results showed that the annular layered numerical model has high accuracy.Finally,basing on the annular layered numerical model and Fluent-Matlab coupling operation method,we realized the inverse calculation of pile foundation GHE thermal response test(TRT)with the actural TRT data.This provided a new way for the inverse calculation of pile foundation GHE TRT.