Studies On The Development Of Tunnel Fire Under The Function Of Stack Effect

With the increasing of the demand for convient transportation, the tunnelbecomes the most effective solution in huge citys. However, the tunnel fire occursfrequently in recent years, and cause heavy casualties and property losses. Due to therestriction of tunnel wall, the toxic gas with high temperature is hard to be dischargedduring a fire, and will accumulate under the ceiling. The harmful combustionproduction in tunnel will threat personnel safety in the tunnel, and delay rescueoperation. Natural smoke extraction which uses natural smoke vents or smoking shaftis a common way to exhaust fire smoke. Therefore, it is important to study thedevelopment characteristics of tunnel fire and smoke exhaust effect.This paper is based on the similarity principle of small size 1:10 experimentalplatform, studying on the characteristics of the tunnel fire development which mainlyincludes the dimensionless heat release rate, the dimensionless maximum temperatureunder the ceiling and the longitudinal temperature distribution. The prediction modelwere established, including the dimensionless HRR and the dimensionless maximumtemperature under the ceiling. The results show that the dimensionless HRR and thedimensionless maximum temperature exponential attenuation with the increase of firedistance from shaft. The influence of natural ventilation on HRR can be divided intotwo zones:zoneⅠis near shaft area, while zoneⅡis far shaft area. The lenghth ofzone Ⅰ is proportional to shaft height, and can be expressed by formula* *S =3.03+1.5H. The HRR and the maximum temperature increases linearly withshaft height, when S* =1, the HRR is 1.35 times as big as no shaft situation. In farshaft area, the HRR and the maximum temperature has little change with the increaseof shaft height. The temperature distuibution results show that the natural ventilationcan cool the downstream smoke, the decrease of smoke temperature is exponentialwith the S*.By changing the shaft height and fire diance from shaft, to study the effect ofsmoke exhaust, including the change laws of temperature with the vertical shaft, shaftexit velocity and mass flow rate. The results show that with the distance from the firesource becoming further, the near shaft smoke by the horizontal inertia force getssmaller, resulting in shaft discharge smoke mass flow rate decreases. When the shaftis low, stack effect is relatively small, mainly in horizontal inertial force under theaction of smoke from shaft mouth downstream dischare, leading to poor exhaustefficiency. When the shaft at a certain height, the shaft height is greater than thereflux area, and the stronger stack effect weakened the large scale vortex due to theboundary layer on the effects of the smoke exhaust smoke increases linearly, leadingto exhaust efficiency enhancement.