Experimental study on the effect of longitudinal ventilation on the ignition characteristics of fires with multiple ignition sources in tunnels

Tunnels are long, narrow and confined spaces, and in the event of a fire, multiple sources of ignition are usually involved under the action of flames and hot smoke. However, unlike the common assumption of simultaneous ignition in many existing studies, real tunnel fires typically feature sequential ignition, where a primary fire source ignites surrounding combustibles over time. Moreover, longitudinal ventilation, as the most common ventilation method in tunnels, has many complex and contradictory effects on the ignition characteristics of tunnel fires. Therefore, this study addresses this critical by investigating the sequential ignition behavior of multiple oil pool fires under longitudinal ventilation conditions. Data on temperature, gas concentration and radiant heat flux were obtained. The results show that there are two typical ignition conditions for multi-source fires in tunnels under longitudinal ventilation conditions. The ignition time intervals of adjacent oil pans show different patterns with the change of wind speed, but in general, the ignition time intervals of the third oil pan are shorter than those of the second oil pan; and the combustion durations of adjacent oil pans become shorter and shorter with the sequence of oil pan ignition. In addition, when v = 0 m/s, the peak temperature of the ceiling flue gas is symmetrically distributed, and the concentration of the upstream ceiling gas varies with the ignition process of the oil pan. When v > 0 m/s, the peak temperature near the upstream ceiling remains close to ambient levels, and the gas concentration of the upstream ceiling basically has no obvious change. Considering the multiple ignition sources as a whole, it can be found that the temperature rise of the ceiling gas downstream of the multiple ignition sources is exponentially decaying. Therefore, for each critical ignition state, a prediction formula for the temperature rise of the smoke gas downstream of the tunnel multi-fire source is proposed. The results of this study help people to have a comprehensive understanding of the ignition process of tunnel multi-source fires.