近年来日益增长的能源需求和温室效应问题促使二氧化碳在可再生能源开发利用领域的应用受到广泛关注，如以超临界CO2为工质的增强型地热系统和高温气冷堆等。因此本文分别对超临界压力CO2在岩石裂隙和多孔介质内的对流换热规律进行了实验和数值模拟研究。为完善增强型地热系统场地规模数值模型，本文利用搭建的高温高压实验系统对超临界压力CO2在开度为0.2 mm的花岗岩水平光滑裂隙内的层流对流换热开展实验研究，分析了CO2质量流量、岩石初始温度对采热率和局部对流换热特性的影响规律，提出了考虑CO2物性变化的水平裂隙内超临界压力CO2层流流动的局部对流换热准则关联式。对超临界压力CO2在花岗岩压裂裂隙内的对流换热开展实验研究，结果表明在相同质量流量和岩石初始温度的条件下，超临界CO2流经本文研究对象压裂裂隙的采热率低于具有相同等效水力开度的平滑裂隙。基于裂隙CT扫描结果重构裂隙形貌并建立三维耦合传热数值模型，模拟结果表明本文研究的压裂裂隙开度不均匀，局部存在流动滞留区，导致裂隙内平均换热性能减弱。在平滑裂隙内单层填充一定数量的球形支撑剂，并通过实验研究超临界压力CO2在此种裂隙内的对流换热性能，分析了裂隙内支撑剂孔隙率和CO2物性变化对裂隙内平均对流换热性能的影响规律。研究表明裂隙内的支撑剂增强了裂隙内流体间的掺混，强化了裂隙内对流换热性能，裂隙内支撑剂孔隙率越小，平均对流换热性能越强，从而CO2流过岩石裂隙的采热率越大。采用实验和数值模拟相结合的方法，对超临界压力CO2在竖直光滑花岗岩裂隙内的流动换热进行研究，探讨了浮升力和变物性对竖直裂隙内的局部对流换热性能和平均对流换热性能的影响，提出了高温储层竖直裂隙内浮升力影响的无量纲参数判断依据。对超临界压力CO2在多孔介质中的内部对流换热规律开展实验研究，分析了CO2质量流量、入口压力、CO2温度和流动方向对内部对流换热系数的影响规律，提出了考虑物性修正的超临界压力CO2在多孔内部对流换热系数的准则关联式。研究了浮升力对内部对流换热系数的影响，研究表明向下流动时多孔结构内的浮升力强化了弥散作用，导致向下流动时的内部换热性能强于向上流动。对以水和CO2为工质的增强型地热系统热储建立局部非热平衡的热-流-力耦合数值模型。对氦气和CO2为工质的气冷堆球床内耦合传热过程建立局部非热平衡的体积平均模型，获得球床的等效导热系数，为气冷堆堆芯事故设计提供重要参数。
In recent years, the growing energy demand and the greenhouse effect lead to the utilization of carbon dioxide in the renewable energy development, such as CO2 enhanced geothermal systems, supercritical CO2 cooled reactor, supercritical CO2 solar thermal system, etc. This paper investigates convection heat transfer characteristics of supercritical pressure CO2 in rock fracture and porous media by experimentally and numerically.The laboratory apparatus was designed to operate at temperatures up to 280 °C, fluid pressures up to 14 MPa, and confining pressure up to 28 MPa. To improve enhanced geothermal systems simulation model, this paper presents experimental investigations of the laminar convection heat transfer of supercritical pressure CO2 in a smooth parallel-plate horizontal granite fracture with an aperture of 0.2 mm. The effect of mass flow rate and rock temperature on the heat extraction rate and local heat transfer characterisitics are analyzed. Consideration of the variations of thermophysical properties, an empirical correlation is developed by fitting the local Nusselt number and the Prandtl number and viscosity with the property-ratio method. Transient heat extraction process of supercritical pressure CO2 in a fracturing fracture by Brazilian technique is investigated experimentally. Compared with the smooth parallel-plate fracture, CO2 flowing through the fracturing fracture with an equivalent hydraulic aperture extractes less heat from the hot rock. Numerical simulations of the reconstructed fracture based on micro-computed tomography scan data are carried out. The results show that the less efficient heat exchange in a fracturing fracture is caused by nonuniform aperture distribution, which leads to the channeling effect. The permeability of a rock fracture will be improved by filling the fracture with proppants. Therefore, the average heat transfer characteristics of supercritical pressure CO2 in a fracture with one-layer ceramic proppants are studied experimentally. The effect of proppants packing porosity and thermophysical propertis of CO2 on the heat transfer coefficients are analyzed. The results show that proppants reinforce the fluid mixing in the fracture and enhance convection heat transfer of supercritical pressure CO2 in the rock fracture. Supercritical CO2 can extract more heat from hot rock with smaller proppants packing porosity. Convection heat transfer characteristics of supercritical pressure CO2 in a vertical smooth parallel-plate fracture are investigated experimentally and numerically. The effect of buoyancy and thermophysical properties variations on local heat transfer coefficients and average heat transfer coefficients are presented. CO2 buoyancy weakens the convection heat transfer characteristics and heat extraction rates from vertical fracture are smaller than that from horizontal fracture. The effect of buoyancy on the fluid flow and heat transfer are described by a non-dimensional parameter.To improve the local thermal non-equilibrium model for supercritical pressure CO2 in porous media, this paper presents experimental investigations of internal heat transfer coefficients of supercritical pressure CO2 in porous media. The effect of mass flow rate, fluid pressure, fluid temperature and flow direction on the internal heat transfer coefficient are analyzed. Based on Xu correction, an empirical correlation for correcting the influence of the thermophysical properties variations is established based on the experimental data. The results show that the internal heat transfer for downward flow is larger than that for upward flow because that the buoyancy force enhances thermal dispersion when CO2 flows downward. A thermal-hydraulic-mechanical model is established based on local thermal non-equilibrium model to describe water or CO2 flows in an EGS fracturing reservoir. Local thermal non-equilibrium model with the revised internal heat transfer coefficients and radiative heat flux is applied and the effective thermal conductivities of the helium-saturated or CO2-saturated pebble bed core are obtained, which are importance references for thermal hydraulic designs of high temperature gas cooled reactor core.
