Long-term heat transfer performance of underground coaxial heat exchanger for medium-deep geothermal
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摘要: 中深层地热井下同轴换热器具有取热量大、出口水温高的特点，近年来受到供热行业的高度关注。目前的研究主要是对单个供热周期内的中深层地热井下同轴换热器的换热性能进行分析，缺乏对其长期换热性能的相关研究。本文根据中深层地热井下同轴换热器供暖季供热、非供暖季停歇运行的特点，基于能量守恒方程建立换热器的数值换热模型，并采用有限体积法对模型进行离散，通过Matlab平台数值分析其在长期运行过程中换热性能的变化规律。结果表明：换热器的换热性能随着运行年份逐渐下降，且下降程度逐年减小，最终达到准稳态。其中，次年平均换热量的下降比例最大，且换热器埋深越大，换热量的下降比例越小。埋深为2 000、2 500、3 000 m换热器的次年平均换热量的下降比例依次为4.00%，3.78%和3.56%，第30年的平均换热量较第1年分别下降13.7%、13.1%、12.4%。岩体温度逐年下降，其受干扰半径逐年增加。在30 a运行期间，埋深2 000 m的换热器在每年供热季结束时的岩体温度受干扰半径从13 m增加至105 m。此外，换热器深度越大，其周围岩体温度受干扰半径越大。本研究结果阐明了中深层地热井下同轴换热器在长期取热过程中换热性能的变化规律，对换热器长期取热的设计具有指导意义。Abstract: Underground coaxial heat exchanger(UCHE) for medium-deep geothermal has the characteristic of the high heat transfer capacity and outlet water temperature, so it has attracted great attention in the heating industry. However, the current research mainly focus on the heat transfer performance of the UCHE during a single heating period, with insufficient analysis on the long-term heat transfer performance. In this paper, considering the operation characteristics of heat supply in heating season and stopping working in non-heating season for underground coaxial heat exchanger for medium-deep geothermal, a numerical heat transfer model is proposed based on the energy conservation equation. The numerical model is solved by the finite volume method and calculated in Matlab platform, which is used to analyze the heat transfer performance of UCHE during the long-term operation. The results show that the heat transfer performance of the heat exchanger declines with the operating years and the degree of decline decreases year by year, and finally reaches the quasi-steady state. Among them, the decrease proportion of the average heat transfer in the next year is the largest, and the greater the buried depth of the heat exchanger, the smaller the decrease proportion of the heat exchanger. The decline ratios of HTC in the second year are 4.00%, 3.78% and 3.56% for the heat exchangers with lengths of 2 000, 2 500, and 3 000 m, respectively, and the corresponding decline ratios of 30-year operations are 13.7%, 13.1% and 12.4%, respectively. Rock-soil temperature declines and its disturbed radius increase with operating years. During 30-year operations, the disturbed radius increases from 13 m to 105 m for heat exchanger with 2 000 m depth. Furthermore, the disturbed radius is larger for the deeper heat exchanger. The research results demonstrate the evolution of the heat transfer performance of UCHE during the long-term operation, which has the guiding significances to the UCHE design.
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Keywords:
- geothermal wells /
- medium-deep /
- coaxial /
- heat exchanger /
- long-term heat transfer
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