Research On Convection Heat Transfer Of Supercritical Pressure CO₂ In Rock Fracture And Porous Media

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 CO₂enhanced 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.CO₂buoyancy 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 CO₂in 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 CO₂flows 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.