The Effect Of Enclosed Air Interlayer On Heat Transfer Between Surrounding Rock And Airflow For Roadway

With the rapid development of economy and science,the pace of underground mining and underground space construction is accelerating,but the thermal hazard problem has become more serious.High rock temperature is a problem that must be faced in future deep resource exploitation and underground space development and utilization.Aiming at the thermal hazards of underground space and geothermal high-temperature mines,a closed air interlayer heat insulation and cooling method is proposed.In order to study the effect of this insulation and cooling method,the heat transfer characteristics of the surrounding rock and airflow of the air interlayer roadway are studied.These results provide guidance for the effective application of this heat insulation and cooling method in practical engineering.The paper uses a combination method of experiment and numerical simulation to study the influence of air interlayer on the heat transfer of surrounding rock and airflow.First,a physical model of heat transfer between surrounding rock and airflow in air interlayer roadway is established,and its heat transfer process is analyzed.The two-dimensional differential equation of heat conduction of surrounding rock of roadway and two-dimensional natural convection flow and heat transfer equation of annular interlayer are derived.The definite solution conditions for solving the equations are given.Then according to the similarity theory,the relationship between the criteria in the heat transfer process is analyzed and the experimental model accordingly is designed.A similar experimental system is built specifically for the study of the temperature field of the surrounding rock of a high-temperature roadway with insulated air interlayer.The temperature distribution characteristics and dynamic changes along the radial direction of the upper,middle and lower parts of the tunnel are studied by experiments,and the influence of ventilation parameters and the temperature of the original rock on the temperature field of the tunnel is analyzed.Finally,the influence of air interlayer on the internal temperature distribution of surrounding rock and the dynamic change rule of surrounding rock temperature field are studied by numerical simulation.The influence of air interlayer on the unstable heat transfer of tunnel with the changes of Fo and Bi is studied.The main conclusions of the research are as follows:1.For a roadway with air interlayer,after the roadway is ventilated,the temperature of the inner wall of the air interlayer drops rapidly,and soon approaches the air temperature.The dimensionless temperature of the middle and lower parts of the inner wall of the air interlayer is reduced to within 0.1 under Fo=1,and the difference between the temperature of the inner wall of the air interlayer and the air flow temperature is 11.5%.After that,the temperature of the inner wall of the air interlayer gradually decreases,and the dimensionless temperature approaches 0.05.The relationship between ? and Fo conforms to the Logistic function.For a roadway without air interlayers,after the roadway is ventilated,the temperature of the surrounding rock wall also drops rapidly.After that,the temperature of the surrounding rock wall decreases gradually,and the dimensionless temperature gradually becomes 0.1.Under the same conditions,the wall temperature of the air interlayer roadway is lower than that of the traditional roadway.Under the same ventilation parameters,the surrounding rock transfers lower heat to the ventilation air flow through convective heat transfer,which delays the heat dissipation of the surrounding rock.2.After the tunnel is ventilated,affected by the natural convection in the air interlayer,the temperature of the tunnel in the middle and lower part of the wall is first reduced,while the temperature of the upper part of the tunnel is similar to the initial temperature of tunnel.The temperature distribution in the tunnel is approximately elliptical.With the extension of the ventilation time,the temperature distribution of tunnel gradually changes from an ellipse to a circular shape,and continuously spreads to the deep part of tunnel in a circular symmetrical distribution.The farther away from the tunnel wall,the smaller the influence of natural convection in the air interlayer on the temperature distribution of the upper,middle and lower parts of the tunnel.At any time of ventilation,from the wall of the tunnel to the depth of the surrounding rock,the temperature of the surrounding rock gradually increases in the form of an exponential function until it approaches initial temperature of tunnel.However,the temperature distribution of tunnel without air interlayer presents a circumferential symmetrical distribution,and the temperature distribution is the same as the heat conduction of a single-layer cylinder.3.The range of tunnel disturbance is closely related to the natural convection in the interlayer.The intensity of disturbance in the upper middle and lower part of tunnel temperature field varies with the heat conduction at the top of the air interlayer,the heat conduction instability in the middle and the vortex flow at the bottom.The bottom of the surrounding rock is most disturbed and the disturbance radius is the largest.From bottom to top,the perturbation radius decreases.The temperature disturbance range of the upper,middle and lower parts of tunnel has a power function relationship with the Fourier number,while the intensity of the disturbance is the same in all areas in the surrounding rock of the traditional roadway.4.The changes of airflow temperature and velocity will not change the trend of the temperature in the surrounding rock with the dimensionless radius and ventilation time.The higher the airflow velocity,the lower the temperature of the surrounding rock,and at the same time,the faster the perturbed range of the surrounding rock will spread.The lower the airflow temperature,the more obvious the rate of decrease of the surrounding rock temperature.The original rock temperature has little effect on the roadway temperature field.5.There is a close relationship between the number of unstable heat transfer criteria of surrounding rock and the Bi and Fo.The relationship of three dimensionless numbers is obtained by simulation data fitting.This formula has a large error when Fo<0.25,and the maximum error and the average error are 15.89% and 3.76%.In engineering,this formula can be used to estimate the unstable heat transfer criterion number of the air interlayer roadway,so as to calculate the unstable heat transfer of the surrounding rock.6.When Bi<21.05,the stable value of the unstable heat transfer criterion number of the air interlayer roadway increases with the increase of the Bi.When Bi>21.05,the stable value of the unstable heat transfer criterion of surrounding rock all tends to be 0.264.For traditional roadways,the stable value of the unstable heat transfer criterion number of surrounding rock has been increasing with the increase of Bi.With the increase of the Bi,the larger the difference between the stable value of the unstable heat transfer criterion number of the air interlayer roadway and the stable value of the traditional roadway unstable heat transfer criterion number,which shows that the cooling effect of the air interlayer roadway is more significant in the case of a larger Bi.