Combustion efficiency during fires in tunnels with natural ventilation by vitiated air including descending smoke

Combustion efficiency in tunnel fires is measured using a 1:20 scale model tunnel. A series of model experiments with propane and heptane fires is conducted to confirm whether combustion efficiency is affected by tunnel length. The results show that combustion efficiency is little affected by tunnel length, and its average value is 89% for propane fires and 80% for heptane fires. In the case of a long tunnel, descending smoke and backflow of air vitiated by mixing fresh air with descended smoke occurs, and the fire is completely surrounded by vitiated air. In this vitiated fire condition, a ghosting flame is observed close to the fire, and ultimately the fire self-extinguishes due to lack of oxygen. A calorimetry method of estimating the heat release rate (HRR) under the vitiated fire condition based on the mass reaction rate is developed as a traditional calorimetry method is difficult to apply to the vitiated fire condition. Using the method, the chemical HRR and combustion efficiency are estimated under the vitiated fire condition. In the vitiated fire, the chemical HRR of the fire decreases to half or less that in the normal fire, but combustion efficiency is 94% for heptane fires.