Water-rock heat exchange in granite fractures

doi:10.3785/j.issn.1008-973X.2023.11.012

Journal of ZheJiang University (Engineering Science)

2023, Vol. 57

Issue (11): 2244-2253 DOI: 10.3785/j.issn.1008-973X.2023.11.012

Water-rock heat exchange in granite fractures

Ruo-tao LIU1(

),Guan RONG1,2,*(

),Yan-di WU1,Bo-wen LI1

1. State Key Laboratory of Water Resources and Hydropower Engineering Science, Wuhan University, Wuhan 430072, China
2. Key Laboratory of Hydraulic Rock Mechanics, Ministry of Education, Wuhan University, Wuhan 430072, China

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Abstract

A rock fracture water-rock heat exchange test system was developed to study the seepage heat transfer in fractured rock masses. Convective heat transfer tests were carried out on granite fissure specimens at different temperatures, volume flow rates and hydraulic openings using the controlled variable method. The fracture seepage flow pattern was analyzed based on the Forchheimer equation, and the effects of temperature, volume flow rate, and hydraulic opening were quantitatively studied by heat transfer coefficient calculation. Sensitivity analysis of each influencing factor was conducted. Results showed that, under the condition of constant volume flow rate and hydraulic opening, the outlet flow temperature and heat transfer coefficient increased as the temperature of the fractured sample increased from 70 ℃ to 100 ℃. Under the condition that the temperature and hydraulic opening of the crack sample remained unchanged, when the volume flow rate changed from 10 mL/min to 80 mL/min, the outlet flow temperature decreased linearly, and the heat transfer coefficient had a power function relationship with the volume flow rate, and its growth rate slowed down with the increase of the volume flow rate. When the temperature and volume flow rate of the crack sample remained unchanged, the heat transfer coefficient decreased linearly when the hydraulic opening of the fissure increased. The heat transfer coefficient increased with the fissure roughness increasing. Sensitivity analysis of influencing factors on the heat transfer coefficient was conducted by the sensitivity function, and results show that the heat transfer coefficient is most affected by the temperature of the crack sample, followed by the volume flow rate of the crack, and least affected by the hydraulic opening.

Key words： geothermal development granite fissure heat transfer coefficient seepage heat transfer

Received: 23 August 2022 Published: 11 December 2023

CLC:

TU 458

Fund: 国家自然科学基金资助项目( 41772305，51579189)

Corresponding Authors: Guan RONG E-mail: liuruotao@whu.edu.cn;rg_mail@163.com

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Cite this article:

Ruo-tao LIU,Guan RONG,Yan-di WU,Bo-wen LI. Water-rock heat exchange in granite fractures. Journal of ZheJiang University (Engineering Science), 2023, 57(11): 2244-2253.

URL:

https://www.zjujournals.com/eng/10.3785/j.issn.1008-973X.2023.11.012 OR https://www.zjujournals.com/eng/Y2023/V57/I11/2244

花岗岩裂隙中的水-岩换热

为了研究裂隙岩体中的渗流传热问题，研发岩石裂隙水-岩换热试验系统，采用控制变量法对花岗岩裂隙试样开展不同温度、体积流量、水力开度下的对流换热试验. 基于Forchheimer方程分析裂隙渗流流态，计算换热系数，定量研究温度、体积流量、水力开度的影响，并进行各影响因素的敏感性分析. 结果表明，在体积流量和水力开度不变的情况下，随着裂隙试样温度由70 ℃升高到100 ℃，出口水流温度和换热系数均提高；在裂隙试样温度与水力开度不变的情况下，体积流量从10 mL/min 增大到80 mL/min，出口水流温度线性降低，换热系数与体积流量增长关系为幂函数关系，且随着体积流量增大其增长速度减缓；裂隙试样温度与体积流量保持不变，裂隙水力开度增大，换热系数线性减小，裂隙粗糙度增大，换热系数增大. 利用敏感性函数对换热系数进行影响因素的敏感性分析，换热系数受裂隙试样温度的影响最大，其次是裂隙体积流量，影响最小的是水力开度.

关键词： 地热开发, 花岗岩, 裂隙, 换热系数, 渗流传热

Fig.1 Physical diagram of fracture water-rock heat transfer test system

Fig.2 Flow chart of fractured water-rock heat exchange test

Fig.3 Variation of temperature of water flow at outlet of fissure with volume flow rate

Fig.4 Variation of fracture pressure gradient with volumet flow rate

Tab.1 Variation of heat transfer coefficient with volume flow rate for different fissure specimen temperatures

Fig.5 Variation of heat transfer coefficient with temperature of fissure specimen at different volume flow rates

Fig.6 Variation of heat transfer coefficient difference with volume flow rate at different fracture specimen temperatures

Fig.7 Relationship between normalized heat transfer coefficient and normalized flow velocity for slit specimens

Fig.8 Nonscale analysis of Nusselt, Prandtl and Reynolds numbers

Fig.9 Variation of heat transfer coefficient with fracture hydraulic opening at different volume flow rates

Fig.10 Variation of heat transfer coefficient with fracture roughness at different volume flow rates

Fig.11 Heat transfer coefficient as a function of volume flow rate

Fig.12 Heat transfer coefficient as a function of hydraulic opening of fissure

Tab.2 Sensitivity factors of heat transfer coefficients with respect to volume flow rate and fissure hydraulic opening at different fissure specimen temperatures

Tab.3 Sensitivity factors of heat transfer coefficients of different rock samples with respect to volume flow rate and temperature of fissure specimens


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