Influence of longitudinal ventilation on the mass flow rate distribution of fire smoke flow in tunnels

•Effects of the longitudinal ventilation on the upstream spreading smoke is studied.•A theoretical model for calculating the mass flow rate of upstream smoke is proposed.•The ration of mass flow rates is deduced from ceiling temperatures distribution.•The influences of tunnel length on the ratio of the mass flow rates are considered. Experimental investigation on the fire plume mass flow rate distribution in the upstream and downstream direction of fire is performed. Series of tests were conducted in a reduced-scale tunnel model with longitudinal ventilation. The ceiling temperatures along the tunnel were measured under different heat release rate of the fire source and longitudinal velocity. Based on the ceiling temperature decay coefficients, the ratios of the upstream fire plume mass flow rate to the total fire plume mass flow rate released by the fire, δ=ṁupṁ0, were calculated. The results show that the value of δ is significantly affect by the longitudinal velocity (Va). When Va is smaller than 0.50Vc(Vc is the velocity at which the fire smoke is fully confined at the impinging point.), that is Va≤0.50Vc, δ would attain constant value, being 0.5. When Va>0.50Vc, δ would decrease with longitudinal velocity. Once Va excesses Vc, δ will be zero in terms of no fire smoke presence in the upstream side of the fire. Thus a correlation between δ and Va is obtained. A verification of the correlation is also presented at last by comparing to other experimental data.