针对中深层地热单井循环系统井内热贯通导致的换热功率低的问题,提出一种内管末端变径的井下传热强化方法,并建立数值模型,利用FLUENT进行为期30 d的模拟计算。结果表明,采用内管末端变径的方式能有效增强地下水“互动”,充分利用含水层的高温来提高单井换热功率。将井下换热分为导热区和采灌区两部分,随着封堵比例的增加,抽水中的含水层补给占比增加,且采灌区换热功率在系统换热功率中的占比逐渐增加。当封堵比例增大到100%时,采灌区换热功率达到导热区的1.76倍,井口出水温度可基本稳定在58 ℃,系统换热功率稳定在约995.46 kW,相较于内管等径系统,换热功率可提高84.71%。同时,单井循环系统仅导热区的延米换热量就可达到154.23~216.89 W/m,超过了闭式同轴套管换热系统稳定运行的最高延米换热功率,而系统换热功率可达到闭式系统的3.57~6.60倍,在单井换热系统中具有显著优势。
Abstract
To solve the problem of low heat transfer power caused by thermal penetration in medium-deep single geothermal well circulation（SGWC） systems, a heat transfer enhancement method is proposed in this paper by changing the diameter of inner pipe at the end of the downhole heat exchanger. A numerical model is established by the FLUENT software and a numerical study is carried out with the calculation time of 30 days. The results show that the method of diameter enlargement at the end of the inner pipe can effectively enhance the "interaction" of groundwater and thus more water with high temperature can be pumped from the pumping aquifer to increase the heat transfer power. In this analysis, the downhole heat transfer zone is divided into two parts, which are the conduction zone（CZ） and the pumping and recharging zone（PRZ）. As the blockage ratio increases, the proportion of the aquifer recharge in the pumping water increases, meanwhile, the heat transfer power of the PRZ are gradually increasing in the system. If the blockage ratio increases to 100%, the outlet temperature and heat transfer power can gradually stabilize at 58 ℃ and 995.46 kW respectively, with the heat transfer rate ratio between PRZ and CZ reaching 1.76. Compared to the single geothermal well circulation without enlarged-diameter inner pipes, the heat transfer rates can be increased by 84.71%. Furthermore, under the stable operation, the heat transfer amount flux per meter of SGWC system in the CZ is 154.23-216.89 W/m, which exceeds the highest value of the downhole coaxial deep well heat exchanger, and the ratio of the heat transfer power in the mentioned two systems varies from 3.57 to 6.60 under different operation conditions. The numerical study shows that SGWC system has great advantages.
关键词
地热供暖 /
废弃井 /
地下水回灌 /
数值模拟 /
传热性能
Key words
geothermal heating /
abandoned wells /
recharging（underground waters） /
numerical simulation /
heat transfer performance
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基金
天津市科技计划（20YFYSGX00020）; 中国科学院科技服务网络计划（STS）（KFJ-STS-QYZD-2021-02-006）
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