Temperature decay and heat exhaust efficiency under ceiling multi-point centralized smoke exhaust in a ventilated tunnel

Tunnel fire is one of the major challenges that cannot be ignored in the safe operation of tunnels. Considering the structure characteristics and bigger heat release rates of tunnel fires, multi-point centralized smoke exhaust coupling with longitudinal ventilation mode is practical in tunnel engineering project, which is significantly important to study the ceiling longitudinal temperature decay and heat exhaust efficiency of such tunnels. Experimental tests are conducted. The experimental variables include heat release rate, number of smoke vents and smoke exhaust volume, while maintaining the longitudinal wind speed at the critical velocity during the experiment. The results show that the tunnel ceiling smoke temperature significantly decreases after passing through the smoke exhaust vent, and the temperature decreases rapidly with the increase of smoke exhaust volume. When there are multiple smoke exhaust vents, the tunnel ceiling temperature between the smoke exhaust vents shows a significant segmented decay trend. The sudden decrease in temperature caused by smoke exhaust only affects the overall change in ceiling temperature and does not affect the decay trend of ceiling temperature. The empirical coefficient $\beta$ was proposed to characterize the reduction effect of smoke exhaust on the ceiling longitudinal temperature, the smoke exhaust efficiency $E$ was calculated. The correlation between empirical coefficient $\beta$, heat exhaust efficiency $E$, dimensionless smoke exhaust volume $V^{\ast }$, and dimensionless heat release rate $Q^{\ast }$ was proposed. The correlation between the longitudinal profile difference of CO concentration and smoke temperature and smoke exhaust volume was obtained. The correlation between coefficient $\beta$ and heat exhaust efficiency has been verified. The research results can provide support for tunnel fire safety and tunnel fire hazard rescue.