Experimental study of flame morphological behaviors and wall heat flux profiles from an adjacent fire with different separation distances in a longitudinally ventilated tunnel

An understanding of sidewall incident heat flux exposed to an adjacent fire source is important for evaluating flame wall interactions and the performance of structural systems in longitudinally ventilated tunnels. This work experimentally investigated the total heat flux profiles from an adjacent fire to a tunnel wall surface under the action of longitudinal flow velocity ranging from 0 to 2.5 m/s. A total of 192 tests were conducted, in which the separation distances between the fire and sidewall were also varied (5∼15 cm). Square sand-filled burners with dimensions of 5 and 10 cm were placed near the wall at four heat release rates (11.67, 17.43, 23.33, and 29.16 kW) to generate buoyant-controlled diffusion flames, and gaseous propane was employed as the fuel. Flame morphological behavior was recorded by two digital cameras from both the side view and downstream view. The heat flux profiles on the sidewall surface were measured by a total of 48 heat flux gauges. The flame transition behavior from persistent touching-wall flame to non-touching conditions, was discussed based on the analysis of the interactions among the longitudinal flow inertia, buoyancy and pressure thrust. The heat flux mappings of the wall surface were obtained for different burner-sidewall distances and heat release rates, which were consistent with the flame morphological behavior. The location of the peak heat flux migrated downstream at lower heights with increasing longitudinal flow velocity. In still air, the peak total heat fluxes upon the wall surface decreased with increasing separation distance, and raised with increasing heat release rate. Then, a correlation of the peak heat flux in still air was developed to describe the data well in the current work. While peak total heat fluxes presented an earlier increasing and later decreasing trend with a modified Froude number u²/gS under the effect of longitudinal flow velocity, which was well interpreted based on the change in radiative and convective heating effects on the wall surface, together with the cooling effect caused by forced flow.