On the principle of positive pressure air supply of safety channel in shield tunnel
Yueming Yuan · Applied Mechanics and Materials · 2013
The paper is engaged in a study of the principle of positive pressure air supply of safety channel in the so-called shield tunnels,which refers to a special surplus space under the driveway in the tunnel designated for man to escape in a safe way.As is known,once a fire takes place in the driveway,the ventilation air should be sent into the safety channel to keep a certain moving speed or pressure in the evacuation outlet so as to ensure the safety channel from the smoke invasion.The minimum wind speed in the evacuation outlet is 0.7 m/s.Based on its typical structural features and the air flow characteristics in the safety channel,there should be four assumptions to be taken into account.The first assumption was that the mean wind speed and the mean static pressure can be used instead of the flow velocity and static pressure respectively.The second assumption is that the flow coefficient,the coefficient of partial resistance and the coefficient of the frictional resistance can be taken as the corresponding constants.Whereas the third assumption could be taken as the air flow through the evacuation exit,the only result of the static pressure in safety channel,the fourth one,was supposed that the potential energy in the air flow direction should be identical for the horizontal structures of the safety channel without the effect of the natural wind pressure.Thus,the safety channel can be simplified to be an air supply duct with approximately uniformly distributed wind velocities.And,since the air supply duct is a uniformed cross-sectional evacuation exit,the evacuation exit should also be uniformly spaced,too,which can be described by the fluid mechanics.The calculation formula can thus be presented for assessing the air-current rate and the total pressure of the fans in the tunnel air shaft.The formula can in turn be used in an actual project to make certain the flowing rate and total pressure of the fan in tunnel air shaft.And,in so doing,we can arrive at the optimum combination: that is to say,the minimum air volume of the fan is 9.55 m3/s,the air pressure at the inlet of the opening evacuation section should be 1 057 Pa.So the formula can be used to check the design of the engineering and to guide the safety channel ventilation.