Study Of The Effect Of Smoke Shaft Inclination On The Thermal Stratification And Movement Of Smoke From A Shaft Tunnel Fire

With accelerated urbanisation,rapid population growth and the rapid increase in the number of tunnels built to meet people’s travel needs,tunnel fires are becoming more frequent,causing huge casualties and economic losses.Therefore,controlling the development of tunnel fires and responding quickly to fire disasters is the key to solving these problems.In practice,due to geological engineering,topography and other factors,vertical smoke extraction tunnels are constructed as inclined type.However,less research has been conducted on the smoke evacuation capacity and smoke flow characteristics of inclined shafts.Therefore,this paper investigates the smoke stratification and smoke propagation characteristics of inclined shaft tunnels by theoretical analysis,size reduction experiments and numerical simulation.Based on Froude’s similarity criterion,a reduced scale 1:10 experimental model was constructed to investigate the stability of the flue gas at different angles of the inclined shaft in the absence of longitudinal wind.Firstly,the variation of the thickness of the smoke layer in the shaft area（location B,2.7 m from the centre of the fire source）was analysed,and the smoke movement pattern of the inclined shaft and the vertical shaft were compared and analysed.The research results show that the inclined shaft can inhibit boundary layer separation and flue gas suction penetration and can maintain the stability of the flue gas layer well;however,the vertical shaft is affected by the shaft height in the smoke extraction process,the shaft height h=0.3 m is the critical height for boundary layer separation and flue gas suction,through which the stability of the flue gas layer is affected.Secondly,the relationship between the stratification curves at different locations and the shaft inclination was investigated.The results show that the stratification curve at position A（1 m from the centre of the fire source）is highly overlapping and independent of shaft height or shaft inclination,and that the stronger thermal flotation keeps the flue gas layer stable.With an increase in the smoke extraction area of the shaft and the height of the vertical shaft,the laminarisation curve at the rear of the shaft at position C（3.7 m from the centre of the fire source）increases in a clockwise direction and the smoke layer gradually destabilises.The intersection of T/T_aver=1 with the stratification curve moves to the left as the angle decreases,indicating that the thermal buoyancy of the flue gas layer behind the inclined shaft is high and its stratification intensity value is large enough to maintain the stability of the flue gas layer.The relationship between the smoke and the longitudinal wind speed when the inclined shaft is located on one side of the tunnel has been studied by numerical simulation.Firstly,a comparative analysis of the flue gas suction through and boundary layer separation in the lateral position of the shaft under natural ventilation conditions and the reasons for this was carried out.The results show that smoke absorption through the shaft is less likely to occur when it is located on the centreline of the tunnel.Secondly,the phenomenon of longitudinal wind speed and flue gas bifurcation is revealed,giving the feasibility of setting the shaft on the tunnel side,and then carrying out the temperature distribution,flow field distribution,gas mass flow rate and CO concentration variation law of different inclined shafts under longitudinal wind.The results of the study show that the longitudinal air velocity inhibits smoke suction penetration and improves the performance of vertical shaft smoke extraction.The gas mass flow rate shows an increase followed by a gradual decrease with increasing longitudinal wind speed.At the same time the ability of the inclined shaft to inhibit boundary layer separation decreases with increasing longitudinal wind speed at lower heights.the higher the shaft height,the gas mass flow rate decreases slightly with increasing velocity of the longitudinal wind（2 m/s-3 m/s）.Longitudinal air velocity has a diluting effect on CO in tunnel fires.The higher the longitudinal air velocity,the lower the concentration of CO discharged from the shaft and the lower its smoke extraction efficiency.Finally,taking into account the gas mass flow and the CO concentration in the inclined shaft,the longitudinal air velocity is set at 1.5 m/s.The tunnel is able to maintain a stable flue gas layer and optimise the smoke extraction effect.