基于准三维模型的垂直U型埋管换热特性影响因素的分析

Analysis on Influence Factors of Heat Exchange Characteristics of Vertical U-bend Ground Heat Exchanger Based on Quasi-three Dimensional Model

为了探讨各因素对地源热泵地下埋管换热特性的影响,建立了同时考虑流体温度沿程变化与U型两支管热干扰的垂直U型埋管准三维传热解析解模型。基于模型求解,分析了回填物与管材导热性、管脚间距、循环流体流量及钻孔深度等对埋管换热特性的影响。结果表明:回填物与管材的导热性、管脚间距及循环流体流量的加大均可以强化地下换热器的换热效果。但回填物导热性不可无限制增大,其大小要考虑对增大管脚热干扰的影响及其与管脚间距的相互关联性,应对两者进行优化设计。同时,流体流量的加大会增加单位埋管换热量,但其增加幅度越来越小,应以流动阻力增加作为限制条件,采用变流量调节来进行优化。此外,单个钻孔不宜太深,以免降低单位埋管换热能力。试验验证表明,所建模型具有一定的预测精度,其最大预测相对误差在3%以内,可为地埋管换热器传热特性分析及其优化设计提供参考。

In order to investigate the influences of various factors on heat exchange characteristics of ground heat exchangers（GHE）,a quasi-three dimensional heat transfer analytical model was developed for the vertical U-bend GHE.The variation of fluid temperature along the U-tube and the thermal interference between two adjacent legs of U-tube were considered in this model.Based on the solution of the model,the influences of various factors,which include the thermal conductivity of grout and pipe,spacing between the two legs of U-tube,flow rate of circulated fluid and borehole depth,on the heat exchange characteristics of GHE were discussed.The results indictate that the increase in the thermal conductivity of grout and pipe,spacing between the two legs and fluid flow rate can all improve heat exchange effectiveness of GHE.But the thermal conductivity of grout should not be increased unrestrictedly,the decision of its value has to take into account the influence of increased thermal interference between the two-legs and its correlation with the spacing of two legs,and should be further optimized for them.At the same time,the increase of fluid flow rate can lead to the increase of heat flow per unit buried coil length,but the increasing degree become more and more small,a restriction in flow resistance increase must be taken into account to optimize it by a variable flow rate design method.Addionally,the depth of a borehole should not be too large in case that the heat exchange rate per unit coil length should decrease.The experimental validation shows that the predicted relative error is less than 3%.This means that the model developed can provide references for the analysis of heat exchange characteristic and optimization design of GHE.