Research On Natural Convection Heat Transfer Characteristics Of Non-ventilated Coal Mining Face

With the maturity of coal mine mechanization and intelligent technology,intelligent green mining without ventilation in coal mines becomes possible.Based on the research background of natural convection flow and heat transfer in non-ventilated coal mining face,this paper studies the characteristics of natural convection heat transfer in a closed cavity through a combination of experiments and numerical simulations.This paper firstly builds an experimental platform based on the similarity criterion,and conducts an experimental study on the natural convection heat transfer of a closed cavity under the simultaneous action of the internal heat source and the wall temperature difference.Secondly,a numerical model consistent with the similar experimental platform is established.The temperature field and velocity field of the cavity are numerically simulated,and the experimental results and numerical simulation results are compared to verify the correctness of the turbulence model,and the effects of different gases under different environmental pressures and different surrounding rock materials on natural convection are studied;Finally,the physical models of turbulent natural convection in a closed cavity driven by an internal heat source and natural convection in a composite cavity with porous media are established respectively.The influence of various factors on the natural convection heat transfer in the closed cavity was analyzed from different angles such as height,number,cavity inclination angle and aspect ratio,Ra number and porous medium parameters.The research results are helpful to further reveal the natural convection flow and heat transfer law of the ventilated coal mining face,and have great effects on the control of the thermal environment of the non-ventilated coal mining face,the avoidance of the mechanical heat damage of the coal mining face,and the elimination of mine gas explosion and spontaneous combustion fire.realistic meaning.In order to study the phenomenon of natural convection heat transfer in the non-ventilated coal mining face,an experimental platform with an internal heat source was built based on the similarity criterion.and the velocity field are experimentally measured.The experimental results show that due to the coupling effect of natural convection flow and heat transfer,the temperature field and velocity field both appear stratified;the wall temperature remains unchanged,and the higher the heat source temperature,the stronger the natural convection heat transfer;on the other hand,the temperature and heat stratification will The fluid is inhibited from flowing upwards and tends to flow horizontally.On the basis of the above research,a physical model consistent with the experimental platform was established,and the effects of different gases under different environmental pressures and different surrounding rock thermal conductivity on the natural convection heat transfer of the closed cavity were studied by numerical simulation.The results show that under the same environmental pressure,the performance of air and nitrogen in promoting heat dissipation of heat source is similar and better than that of methane;low pressure has little effect on natural convection heat transfer;when the environmental pressure increases from 0.1MPa to 0.15MPa,natural convection heat transfer is enhanced.,the temperature distribution in the space is more uniform,and the average heat transfer coefficient on the surface of the heat source increases by 25.6%;the thermal conductivity of the surrounding rock is positively correlated with the natural convection heat transfer intensity in the closed cavity.The maximum average heat transfer coefficient of natural convection on the surface does not exceed 10.58W·（m²·K）^-1,indicating that forced convection or air cooling must be used to further improve the heat transfer capacity of the heat source surface.In order to study the effect of natural convection formed by the heat dissipation of mechanical and electrical equipment on the temperature field and velocity field of the unventilated working face,a physical model of turbulent natural convection in a closed cavity driven by an internal heat source was established.The mathematical model and simulation method established in this paper are used to compare with the experimental results of Larson D W.The maximum error is 8.5%,which verifies the correctness of the research method.By studying the temperature field,flow field,local Nu number,average Nu number,dimensionless temperature and velocity of the vertical centerline,etc.,analyze and compare the position,height,number of heat sources,as well as the inclination and aspect ratio of the closed cavity.Influence of temperature and velocity distribution.The heat source is located at the bottom of the center of the cavity,which is conducive to convective heat transfer.There is an optimal heat source size,so that the natural convection heat transfer efficiency of the heat source is maximized.Within the research range,the larger the cavity aspect ratio and the inclination angle,the weaker the natural convection heat transfer to a certain extent.heat intensity.In order to study the effect of the goaf on the natural convection heat transfer of the non-ventilated working face,a natural convection model of the composite cavity containing porous media was established,and the effects of the dimensionless parameters Rayleigh number Ra,the width of the porous media region and the porosity on the composite cavity were studied.Influence of cavity flow and heat transfer.The Rayleigh number Ra and the width of the porous medium have a great influence on the natural convection heat transfer in the composite cavity,while the porosity of the porous medium has little effect on the natural convection heat transfer in the composite cavity.The larger the Ra number,the stronger the natural convective flow and heat transfer in the composite cavity;and the increase of the thickness of the porous medium in the composite cavity will weaken the strength of flow heat transfer on the fluid side.The increase of porosity increases the flow and heat transfer in the porous media area of the composite cavity,but it has a certain weakening effect on the flow and heat transfer in the fluid domain.