Abstract:

To understand the influence of groundwater advection on the performance of borehole heat exchanger (BHE), an unsteady state coupled heat transfer model for a BHE with single U-tube was established by combining the transient moving finite line heat source model in porous medium outside the borehole and the steady state model for the heat transfer inside the borehole. The coupled model was validated by the data obtained in a soil thermal response test. The influence of groundwater advection on outlet water temperature and dynamic soil temperature responses was discussed. The heat transfer efficiency of BHE and the heat transfer rate per unit borehole depth were taken as two indices to assess the heat transfer performance of BHE. The heat transfer performances of BHE varied significantly with the type of soil in which it was buried. Ignoring the influence of groundwater advection in sand gravel would lead to a deviation of heat transfer rate per unit borehole depth up to 41% in the ground loop design; groundwater advection improved the heat transfer of BHE and an increase in advection velocity might help to shorten the time for the surrounding soil to reach a stable temperature. It was recommended that the mass flow rate of the inlet fluid to the BHE be more than 0.4 kg·s^-1, but within a certain upper limit; the influence of the rise in inlet fluid temperature on the heat transfer efficiency of BHE could be ignored. In addition, the effects of groundwater advection in different types of soil under typical hydrological conditions on the heat transfer efficiency of multiple BHEs connected in series were compared. Heat transfer efficiency was jointly determined by such factors as soil physical properties, ground water advection velocity and number of BHEs connected in series.

Key words: borehole heat exchanger, groundwater advection, physical properties of soil, heat transfer efficiency of boreholes heat exchanger

摘要：

为了研究渗流对地埋管换热器性能的影响,综合多孔介质中移动有限长线热源与钻孔内准三维传热模型建立了地埋管换热器钻孔内、外非稳态耦合传热的解析模型,并通过热响应试验数据验证了耦合模型的正确性。探讨了渗流作用下埋管出口水温及其周围土壤温度动态响应的变化规律,利用埋管换热能效系数和单位井深换热量两个指标的变化评估了渗流对埋管换热器传热性能的影响。结果表明:不同类型的土壤中埋管传热性差别较大, 若忽略渗流速度较大的砂砾层中渗流的影响将导致其中埋管的单位井深换热量设计偏差高达41%;渗流对埋管散热起到促进作用且散热达到稳定所需的时间随渗流速度增大而缩短;推荐采用埋管的进口质量流量流速大于0.4 kg·s^-1,但不宜过大;埋管进口温度对换热能效系数的影响可忽略。并对典型水文条件下各土壤中渗流对串联管群的换热能效系数的影响进行了对比,指出地下管群环路的换热能效系数由土壤物性、渗流速度及串联埋管的钻孔数量共同决定的。

关键词: 地埋管换热器, 地下水渗流, 土壤物性, 管群换热能效系数