| Ground coupled heat pump system has been recognized as a new energy efficient system for space cooling and heating in commercial buildings and residential. Many factors affect the development of ground source heat pump. Heat exchanger heat transfer is one of keys. Establishing a practical mathematical model to prediction of underground heat exchanger performance has important significance is of guiding the ground-source heat pump system design.This thesis studies vertical U-tube ground heat exchanger, establishing a two-dimensional numerical heat transfer model and algorithm programmed in MATLAB environment.In this method,the borehole heat exchanger is divided into two parts:the borehole region and the soil region.In the soil region,heat conduction in the radial and vertical direction is considered.Applying energy conservation principle,an explicit transient two-dimensional numerical model is built to describe the heat conduction in the soil region.In the PE pipe,only considered is the temperature variation along the axis.An implicit one-dimensional numerical convection-diffusion model is established.Two legs of pipe are connected with each other using a resistance.and pipe are connected with borehole wall using another resistance.The model is validated by using three places of testing data in Mianyang,Dujiangyan and Deyang.The maximum deviations of outlet temperature are0.6℃,0.8℃and0.7℃, respectively.Using the established two-dimensional numerical model,the influence of heat circuit is analyzed. Two boreholes with depth of40m and80m were calculated. The temperature rising ratio of outlet leg to inlet leg of the former is1.84. And the ratio of the latter is2.40.it is conclusion that borehole depth aggravates heat short circuit phenomenon between the2-legs of the U-tube.In a word, this model reflects the main characteristics of the U-tube ground heat transfer and accurate, it is valuable value to guide the ground-source heat pump system design. |
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