Study On Extreme Height Of Chimney Effect Of Tunnel Shaft And Influencing Factors

Due to the complex structure,strong sealing and narrow space of shaft tunnel engineering,it is difficult to control the fire and it is difficult to put out the fire when there is a fire,which often causes serious casualties and property losses.Therefore,it is of great significance to study the tunnel shaft stack effect.Combined with "Liaoning Natural Science Foundation Project"(Project Number: 20180550336),FDS fire simulation software is used in the paper to simulate the smoke movement of tunnel fire under natural ventilation.The relationship between the limit height of shaft stack effect and its influencing factors is obtained by continuously changing the shaft section size,section shape,placement mode and fire source power.The main research contents and results of this paper are as follows:(1)There is a critical value for the stack effect of shaft height.Within a specific height range,the flue gas movement speed increases continuously with the increase of the shaft height,and the stack effect increases with the rise in the shaft height.The flue gas movement speed reaches the maximum value when the shaft height reaches a particular critical value.The flue gas movement speed starts to slow down gradually when the shaft height continues to rise.The corresponding shaft height is the limit height of the shaft stack effect when the flue gas movement speed reaches the maximum value.(2)Under the condition that the tunnel is naturally ventilated,the fire source power is 5MW,the inner wall of the shaft is made of concrete,and the cross-section of the shaft is square,there shall be a simulation of the tunnel fire situation under different cross-section sizes of the shaft,so as to get the stack effect limit heights of the shaft under different cross-section sizes of the shaft.Through analysis,it is concluded that the limit height of the shaft stack effect increases with the increase of the cross-section of the shaft when the tunnel is naturally ventilated,the power of fire source is 5MW,the inner wall of the shaft is made of concrete,and the cross-section of the shaft is square.It could be concluded according to the data analysis and fitting that y=160.32ln(x)+265.88(x represents the side length of the shaft section square,and y represents the limit height.0.4≤x≤9.92),which could be used to estimate the ultimate height of the tunnel shaft stack effect.(3)Under the condition that the tunnel is naturally ventilated,the fire source power is 5MW,the inner wall of the shaft is made of concrete,and the cross-sectional area of the shaft is 1m2,there shall be simulation on tunnel fire situation under different shaft shapes and placement modes,so as to get the stack effect limit heights of the shaft under different shaft cross-sectional shapes and placement modes.Through analysis,it is concluded that when the tunnel is naturally ventilated,the fire source power is 5MW,the inner wall of the shaft is made of concrete,and the cross-sectional area of the shaft is 1m2,the stack effect limit height when the cross-sectional shape of the shaft is rectangular is higher than the stack effect limit height when the cross-sectional shape of the shaft is square.The limit height of vertical shafts with the same shape and size is greater than that of horizontal shafts.(4)Under the condition that the tunnel is naturally ventilated,the inner wall of the shaft is made of concrete,the cross-sectional dimension of the shaft is 1m×1m,and the location of the fire source is unchanged,there shall be a simulation on tunnel fire situation under different fire source powers,so as to get the ultimate height of the shaft stack effect under different fire source powers.Through analysis,it is concluded that when the tunnel is naturally ventilated,the inner wall of the shaft is made of concrete,the cross-sectional dimension of the shaft is 1m×1m,and the location of the fire source is unchanged,the limit height of the stack effect of the shaft also increases with the increase of the power of the fire source.When the fire source power is constant,the increase of shaft height has little effect on the highest temperature in the shaft,but the temperature at the top outlet of the shaft decreases with the increase of shaft height.When the height of the shaft is constant,the highest temperature in the shaft also increases with the increase of the fire source power,and the temperature at the top outlet of the shaft also increases.