地埋管换热器热短路及其对热物性测试影响分析

为了更准确地建立垂直地埋管换热器钻孔内传热模型,采用模拟分析的方法探讨了不同钻孔、支管和布置形式的冷热支管热量回流情况,并拟合得到了热短路热阻的表达式。建立了三维数值模型,分析了管内流体流速、埋深对热短路的影响,流速越小,热短路损失率越大和单位管长换热量越小,然而流速过大,热短路损失率减少并不明显且能耗加大;增加埋深可以增大埋管进出口的温差,但冷热支管间的热损失也大大增加。对比了流体积分平均温度与几何平均温度的差别,由于忽略了热短路的影响,往往线性几何平均温度值偏大。借助于试验平台,分析了流速变化和埋深变化对土壤平均传热系数测试的影响,结果表明:流速越小,热短路损失率越大,土壤平均传热系数越小,即埋管的换热能力也越低;埋管的深度应综合换热量要求、热短路损失、投资而确定。

Thermal short-circuiting and its influence on thermal response in borehole heat exchangers

The fluid extracts or rejects heat from subsurface by downward leg of pipe(DLP)and upward leg of pipe(ULP)in vertical borehole heat exchangers(BHEs).As the borehole diameter is only 0.11 m to 0.2 m,the temperature difference between DLP and ULP inevitably leads to thermal short-circuiting.In order to build accurate heat transfer model for the BHEs,the vertical underground heat exchangers with different boreholes,pipe diameters and arrangements were investigated numerically,and a best-fit correlation for the thermal short-circuiting resistance was presented in dimensionless form.A 3-D equivalent rectangular numerical model for BHEs was established and the influences of velocity and borehole depth on the short-circuiting loss rate were analyzed.Smaller velocity leads to larger thermal short-circuiting loss rate and lower average heat flux per unit length,but too large velocity has no obvious effect on reducing short-circuiting loss rate.The temperature difference between inlet and outlet increases with the borehole depth,while the heat transfer between DLP and ULP also increases greatly.The relationship between the thermal short-circuiting and the fluid temperature change is explored. The geometric mean fluid temperature ignoring the short-circuiting is always higher than the integral mean fluid temperature.With an in situ thermal conductivity test rig established in Nanjing,the results indicate that lower velocity results in lower effective subsurface conductivity and smaller heat transfer capacity in the BHEs.The measured subsurface thermal conductivity with 50 m borehole is 2.50 W·(m·K)^-1,31.8% higher than that with 100 m borehole,which is 1.896 W·(m·K)^-1.The heat exchange amount,short-circuiting and investment should be taken into account while determining the borehole depth.