The hot and dry rock geothermal resources have huge potential, green renewable, and it is a strategic energy source. It is increasingly highly valued by major countries in the world. The deep dry hot rock geothermal development is usually injecting fluid working medium (such as water and carbon dioxide) into the underground fissure channel. The thermal energy is circulated to the ground system. The flow and heat transfer characteristics of the hot working fluid in the fracture are one of the important scientific issues. It is also the basis for the optimization design of the mining program and the development of an efficient heat recovery technology. To address this issue, the Chinese Academy of Sciences Researcher Bai Bing and researcher Li Xiaochun of Wuhan Institute of Rock and Soil Mechanics made new progress on the basis of previous work.
The research team has developed a new type of coupling system for the flow and heat transfer mechanics of high-temperature fractured rock masses. Systematic experimental studies have been conducted on the flow and heat transfer of water in single fractures of granite in different confining pressures, different ambient temperatures, and different flow conditions. The current formula for the overall heat transfer coefficient of cracks was verified and screened. It was found that some commonly used formulas showed unreasonable conditions such as numerical oscillations or even negative values under high flow conditions. A simplified formula for the overall heat transfer coefficient (OHTC) was proposed to be valid. Solve this problem. In addition, some heat transfer coefficient formulas in the derivation for ease of analysis assume that the temperature of the inner wall surface is constant along the radial direction is also unreasonable, which will lead to the presence of temperature discontinuities at (±R, 0). Problem. The researchers proposed a new set of analytical methods for the overall heat transfer coefficient based on the Green's function method, which can effectively avoid these problems. Specific analytical solutions are given when the radial temperature is a 0-3 polynomial function distribution. The fracture flow heat transfer numerical analysis model successfully achieved the numerical modeling problem when the fracture and rock differ by 2 orders of magnitude. The experimental data agree well. The local heat transfer coefficient (LHTC) is used to characterize the local heat transfer characteristics of the hot hot rock fractures. It is found that when the crack opening is small, the local heat transfer coefficient is usually large, the local undulation of the fracture (representing roughness) and local The heat transfer coefficient is negatively correlated, ie the LHTC is larger at the crevice of the crevice and smaller at the protuberance. The LHTC is smaller at the smooth fissure and larger at the roughness, and the fitting of the LHTC to the undulation degree is given. Relationships. Based on this model, a simulation study of the flow and heat transfer characteristics of water and carbon dioxide was conducted. It was found that under the same conditions, the overall heat transfer coefficient and local heat transfer coefficient of gaseous carbon dioxide and gaseous water increase with the increase of flow. The OHTC and LHTC of gaseous carbon dioxide are always smaller than those of gaseous water, and the supercritical water heat transfer coefficient is better than that of supercritical carbon dioxide. As far as carbon dioxide is concerned, the tight heat exchange capacity is better than that of the carbon dioxide.
The research work was funded by the National Natural Science Foundation of China (Grant 41672252, 41272263). Research results were published in Applied Thermal Engineering, Rock Mechanics and Rock Engineering, Computers and Geotechnics, Environmental Earth Sciences, and Geothermics.
Crack flow heat transfer experimental principle
Temperature Distribution in Granite Blocks and Fractures (left injection flow 30mL/min, fracture opening 125.5 microns, time 900s)
Comparison of Local Heat Transfer Coefficients of Carbon Dioxide under Different Conditions
