Experimental study on multiple fire hazards both inside and outside a naturally ventilated tunnel: Burning rate and flame characteristics of unequal fires

Abstract

This study investigated the burning rate and flame characteristics of unequal fires separated inside and outside a naturally ventilated tunnel. Outside fire size and fire separating distance were varied. In this scenario, the asymmetric fire behaviors were formed by asymmetric airflow distribution induced by the unequal inside and outside fires. The variations of their fire behaviors were grouped into Regions Ⅰ and Ⅱ with the boundary of S/d_in = 1 (S is fire separating distance and d_in is inside fire size), but the inside and outside ones differ from each other. The outside flames were always tilted upstream with similar angles and heights, while the tilt direction of the inside flames changed with the increasing outside fire size in Region Ⅰ and the flame heights were lowered when S/d_in > 3 (in Region Ⅱ-b). Moreover, the burning rate of the outside fire was lowered in Region Ⅰ, while that of the inside fire was lowered in Region Ⅱ-b. The discrepancy was due to the different influence ranges of fire interaction and heat feedback enhancement. The results and interaction mechanism were compared with those of equal inside and outside fires for analysis. The evolution of the flame tilt angles was theoretically analyzed and the correlations of the dimensionless flame heights and heat release rates were established.