Effect of Volumetric Flow Rate on Heat Transfer Characteristics of Single-Fractured Rock with Different Surface Morphology and External Temperature

Abstract

The primary aim of this study is to explore how varying flow rates impact the heat transfer in single fractures, taking into account the effects of surface roughness, aperture and external temperature of the rock. Utilizing COMSOL Multiphysics, the fluid flow and heat transfer through 3D fracture models characterized by different roughness and apertures were simulated with volumetric flow rates ranging from 1 × 10−6 m3/s to 1 × 10−5 m3/s. The combined effects of these factors on key metrics, including outlet temperature, thermal breakthrough time, energy extraction efficiency, and heat transfer coefficients were systematically analyzed. The results indicate that water flow rate dominantly influences heat transfer, followed by fracture surface morphology and rock external temperature. Higher flow rates enhance both heat transfer and total heat extraction, while also increasing temperature non-uniformity, which improves overall heat extraction efficiency. Surface roughness significantly affects temperature distribution, leading to heterogeneous thermal profiles, especially in narrower fractures. Additionally, higher external temperatures and flow rates facilitate faster thermal breakthroughs by reducing thermal resistance. The interplay between surface roughness and thermal breakthrough time is intricate, with increased roughness prolonging breakthrough times in smaller apertures but potentially reducing them in larger ones. At smaller apertures, increasing the JRC from 2.29 to 17.33 results in a 1.01 to 1.20 times increase in thermal breakthrough time, whereas at larger apertures, thermal breakthrough time decreases by a factor of 1.01 to 1.29. This highlights the importance of carefully selecting fluid parameter in the design of geothermal projects to optimize heat extraction efficiency.