Experimental study on thermal smoke backlayering length with an impinging flame under the tunnel ceiling

Model scale tests were performed with varying tunnel width and height in order to study the effect of tunnel cross section and ventilation velocity on smoke backlayering length in case of large heat release rate. The results show that in tests with the same fuel and ventilation velocity, the backlayering length trends to increase with increasing tunnel cross-sectional area after the tunnel cross-sectional area reaches a certain value, under which the critical velocity is obtained. The reason should be as the tunnel height or width increases, the blockage effect of flame inside the tunnel is reduced, thus leading to a smaller local ventilation velocity under the ceiling to confine the backlayering and ultimately a larger backlayering length is formed. In order to establish an integral model of backlayering length considering the effect of both tunnel height and width, new dimensionless forms of backlayering length and ventilation velocity, including hydraulic diameter of tunnel, are introduced. The proposed equation can correlate well with all the data from the model scale tests with different fuels, ventilation velocities and tunnel cross sections, and also Runehamar full scale tunnel tests.