Research On Flow And Heat Transfer Characteristics Of High Temperature Gas In Porous Media

As an advanced technology that can realize the low-carbon development of high-carbon resources,the process of underground coal gasification can produce raw gas that can be converted into high value-added products such as natural gas,syngas and methanol by controlling coal combustion.However,in addition to being mined along the exhaust channel,some of the gas will escape along the porous media of coal and rock,which will not only cause waste of resources,but also cause potential safety hazards.Therefore,it is of great significance to study the energy and mass transfer of raw gas produced in underground coal gasification system in coal and rock for the utilization of coal resources and environmental protection.In this paper,the fluid transport process in porous media is reviewed through literature research,and a physical model is established to describe the fluid flow and heat transfer problems in the microscopic scale of coal-rock porous media.According to the local non-thermal equilibrium effect of porous media,the flow and heat transfer characteristics of raw gas in dual coal-rock porous media under different processes and structures are numerically simulated to reveal the seepage law and mechanism of raw gas in coal-rock pores.Firstly,a two-dimensional coal-rock porous medium model with micron-sized Y-shaped fractures is established.By changing the geometric parameters such as deflection angle,length and width of fractures,the flow and heat transfer characteristics of raw gas in fractures-pores are explored.The results show that the flow of raw gas in each fracture is relatively independent,and the velocity gradient and temperature gradient of raw gas near the fracture wall are large.When the fracture characteristic size becomes smaller,the raw gas is easy to produce high flow rate and low temperature drop.In asymmetric fractures,this phenomenon mainly exists in fractures with smaller deflection angle and shorter length.In addition,the average seepage velocity of raw gas in pores is only one thousandth of that in fractures.Secondly,by establishing a millimeter-scale throat-shaped coal-rock porous medium model,the influence of the width,length and position of the elliptical fine neck and necking throat on the energy quality change law of raw gas in coal rock is explored.It is found that the narrower the throat,the greater the temperature gradient of the raw gas and the smaller the average flow velocity along the way.The longer the thin neck throat,the greater the flow rate ratio of the throat section to the raw gas in the straight fracture.In the elliptical necking throat,the throat effect will make the difference between the actual flow space and the fracture geometry.Near the fracture wall,the airflow velocity of the raw gas will be in a dynamic equilibrium state.In addition,the alternation of throat position will lead to the increase of heat exchange intensity between fluid and wall.Finally,a three-dimensional Y-shaped tortuous fractured coal-rock porous medium model was established to study the flow and heat transfer characteristics of raw gas in coal-rock by changing the porosity of coal-rock porous medium,the pressure difference between inlet and outlet,and the roughness element shape of fracture wall.It is found that the porosity of coal-rock porous media is positively correlated with the average velocity of raw gas in pores and fractures,and negatively correlated with the average temperature of raw gas.The increase of inlet and outlet pressure difference will increase the flow rate of raw gas in coal-rock porous media and improve its heat transfer efficiency.The roughness element will increase the friction and heat transfer area during the flow of the raw gas,resulting in eddy current in the raw gas at the "dead end" of the crack,thereby increasing the local heat transfer between the fluid and the solid.In this paper,the flow and heat transfer law of high temperature raw gas in dual coal-rock porous media has been deeply understood,which can provide basic theoretical guidance for the efficient and economic development of raw gas in the process of underground coal gasification.