Study on the micro conjugate heat transfer characteristics of coal reconstructed from CT images under the influence of temperature

Abstract

Heat injection in coal seams, a new method to enhance gas drainage, is faced with problems such as low heat conduction efficiency and considerable heat dissipation. Based on experiments, this study reconstructed a micro heat transfer model through CT images to discuss the interaction between coal temperature rise-induced fracturing and the heat transfer performance of coal, which can provide a new insight into efficient heat injection and mining in coal seams. First, coal samples were modified by being heated at four temperatures and then scanned in situ by an industrial micro CT. Then, fractures were reconstructed by the interactive threshold segmentation and interactive top hat method to generate a 3D mesh model. Meanwhile, a conjugate heat transfer model including fracture fluid and coal matrix was established. Finally, the simulation result of thermal conductivity was compared with the results of the hot wire method and the laser flash method, and the influence of thermal fracturing structure evolution on coal heat transfer was analyzed. The following findings were obtained: The conjugate heat transfer model is applicable to fractured coal. The average relative error of its thermal conductivity is 5.85%, and it boasts high accuracy in characterizing heterogeneity and anisotropy. The fracture propagation and water evaporation of coal block heat transfer, while the adhesion and crystal contact surface expansion of heated coal are conducive to heat transfer. These two mechanisms compete, ultimately affecting the thermal conductivity. After the coal sample is heated within the temperature range of 40–100 °C, the fracture damage coalescence enhances the anisotropy of heat transfer in coal. The maximum difference in thermal conductivity between the bedding direction and the direction perpendicular to bedding is 0.181 W/(m·K), about 4.6 times larger than the initial difference. The research results lay a theoretical foundation for improving the heat transfer efficiency of heat injection in coal seams.