| Underground heat hazard,which features high rock temperature,high geothermal gradient,high terrestrial heat flow and hot humid airflow,is becoming increasingly prominent in deep mines or geothermal anomaly mines.In addition,the thermal environment of some mines is affected by geothermal water upwelling.Groundwater,as the carrier of heat,transfers and accumulates heat in the form of heat conduction and heat convection,which will considerably alter the characteristics of the temperature field of fractured surrounding rock.Moreover,if geothermal water flows into the roadway,its contact with air will aggravate the deterioration of the thermal environment of mines.Hence,research on the mechanism of water-heat transfer in fractured rock mass of geothermal anomaly mines is crucial for thermal environment control of these mines.In this dissertation,tests of fluid flow in rough-walled fractures and water-rock convective heat transfer were conducted under different roughnesses,apertures,flow velocities and water-rock temperatures,and the fluid flow-heat transfer process in the fracture network under the dynamic assignment of heat transfer coefficient was simulated.Accordingly,the heat transfer and heat accumulation mechanism of deep circulating groundwater in fractured rock mass was revealed.Besides,the influence of deep circulating thermal water upwelling(DCTWU)on the thermal environment of high geo-temperature(HGT)mines was studied.Based on the mechanism of water-heat migration in fractured rock mass,the thermal environment control strategy for geothermal anomaly mines was put forward.The main results are as follows:1)The formation mechanism of mine heat hazard was revealed by analyzing geothermal geology and hydrogeology.In addition,similar simulation tests were performed on surrounding rock airflow heat transfer and water transfer under high temperature and water injection conditions to investigate the influence of DCTWU on the thermal environment of HGT roadways.The main results are as follows:(1)According to the heat transfer mode of the temperature field,the formation mechanisms of mine heat hazard are classified into three categories,i.e.,deep-source heat hazard,rock temperature geothermal anomaly heat hazard and hot water geothermal anomaly heat hazard.(2)The temperature and flow rate of thermal water trickling(TWT)directly influence heat transfer of the airflow system.(3)Affected by the high temperature of TWT,the growth rates of sensible heat and latent heat at each point differ in different periods.2)A test system for fluid flow in fractured rock mass was developed.Based on the system,the flow characteristics of fluid under different fracture surface roughnesses,normal effective stresses and flow rates were explored.In this way,the mechanisms of the influences of roughness,aperture and other factors on fluid flow in fractures were revealed.The main results are as follows:(1)The deformation process of fractures conforms to the Barton-bandis equation.(2)Due to the roughness of fractures,the fluid is subject to additional pressure loss as a result of the inertial effect.With the decrease of aperture or the increase of surface roughness,the fluid is more prone to overpressure drop so that the water permeation capacity of fractures weakens.For undulating fractures,the greater the JRC,the greater the pressure drop.For planar fractures,the pressure drop increases with the increase of profile peak angle,but an inflection point exists as the profile peak height changes.(3)The classification of flow regime in the range of laminar flow was clarified,and the judgment method of fluid flow regime in fractures was proposed.Furthermore,reliability of the above classification and judgment methods was verified theoretically and experimentally.(4)The criterion for judging the critical point between Darcy and post-Darcy flows was improved.(5)The law of different-regime fluid flow behaviors in rough fractures was studied.In the pre-Darcy regime,the fluid flow behavior satisfies the Izbash equation;In the Darcy regime,the cubic law was modified with relative roughness;In the post-Darcy regime,the fluid flow behavior satisfies the nondimensionalized Forchheimer formula where the linear and nonlinear terms were characterized by relative roughness.Moreover,the physical significance of the nondimensionalized Forchheimer formula under different conditions was disclosed.3)A fluid flow-heat transfer test system of fractured rock mass was developed.Based on the system,tests were carried out on water-rock convective heat transfer in rough-walled fractures under different inlet water temperatures,apertures,flow velocities,tortuosities and rock temperatures.The main results are as follows:(1)The calculation method of heat transfer coefficient of the water-rock interface was deduced.(2)The transient variation law and steady-state distribution law of water-rock temperature along the fracture were studied.It is found that fluids with a high flow rate can take away massive heat in a short time,but they will then enter the attenuation stage and the stabilization stage.On the contrary,fluids with a low flow rate take away a relatively small amount of heat,but the heat transfer rate will remain stable for a longer time.After reaching the steady state,the water temperature along the fracture meets the linear distribution at a high velocity and the nonlinear distribution at a low velocity.(3)When the inlet water temperature decreases or the aperture rises,the heat transfer rate and the heat transfer coefficient both grow linearly.The curves of heat transfer rate and heat transfer coefficient variations with flow rate are divided into the heat transfer surge area and the heat transfer stable area in accordance with the change threshold.This threshold does not vary with tortuosity or rock temperature,and the curve shapes and slopes of the areas on both sides of the threshold differ significantly.In addition,the interaction between variables in the heat transfer surge area and the heat transfer stable area causes the strengthening or weakening of water-rock heat transfer.(4)The characteristic equation of water-rock heat transfer applicable to the engineering field was determined in the light of the boundary layer theory as a bridge.4)The model of fluid flow and heat transfer in the fracture network under the dynamic assignment of heat transfer coefficient was established on the basis of the Darcy’s law modified with roughness,the dimensionless Forchheimer formula and the characteristic equation of water-rock heat transfer.Taking pressure drop and heat transfer coefficient as a bridge,the numerical experiments on fluid flow and heat transfer in the fracture network under different pressure gradients,heat transfer coefficients and rock mass physical parameters(including initial rock temperature,fracture density and roughness)were performed.In this way,the heat transfer and accumulation mechanism of DCTWU in fractured rock mass was concluded.The main results are as follows:(1)Under the same pressure gradient,the Forchheimer flow in the fracture network corresponds to a greater pressure loss than the Darcy flow,and the regional average flow velocity is lower.(2)With the increase of pressure gradient,the cataphalanx of the temperature field of fractured rock mass moves at an accelerated rate.(3)The physical parameters of rock mass influence fluid flow and heat transfer in the fracture network notably.To be more specific:(1)The initial rock temperature of the thermal reservoir determines the temperature rise of DCTWU;(2)As the fracture density increases,the effective area of water-rock heat transfer expands,and the total heat transfer rate and total heat quantity increase accordingly.However,potential risk of heat transfer “short circuit” also exists;(3)Roughness has both positive and negative effects on water-rock heat transfer.As roughness increases,the fluid flow path of fluid becomes longer and the heat transfer coefficient grows.Nevertheless,in a highly rough fracture network,the Forchheimer flow corresponds to a greater pressure loss,and the regional average velocity and the water-rock heat transfer intensity both decrease.5)The PCA-LSTM-GA model for predicting airflow temperature and humidity was constructed.Based on the mechanisms of fluid flow and heat transfer in fractured rock mass and the formation mechanism of mine heat hazard,the thermal environment control strategy for geothermal anomaly mines was proposed.Meanwhile,the “composite heat-insulation zone” structure for sealing water and insulating heat in fractured rock mass is designed,and its water sealing and heat insulation mechanism and cooling effect are disclosed through numerical simulation.The main results are as follows:(1)The PCA-LSTM-GA model performs well in predicting airflow temperature and humidity at the end of the TWT roadway.(2)The thermal environment control strategy for geothermal anomaly mines was proposed,and the “composite heat-insulation zone” structure was arranged in a geothermal anomaly roadway.The structure succeeds in controlling the heat dissipation of surrounding rock while sealing water.(3)The airflow temperature fields under DCTWU and non-DCTWU conditions were compared by means of numerical simulation.At the same time,the airflow temperature fields before and after the change of process parameters and material parameters of the grouting-shotcreting layer were compared to conclude the water seal and heat insulation mechanisms of the “composite heat-insulation zone” structure.(4)The steady-state airflow temperature at the roadway outlet under different working conditions was calculated.According to the calculation results,the outlet airflow temperature in the presence of the “composite heat-insulation zone” structure is 56% lower than that in the case of DCTWU,which verifies the cooling effect of the “composite heat-insulation zone” structure. |
