Evolution behavior of heat release rate of twin fire sources in a tunnel under low pressure environment

A profound comprehension of the heat release rate (Q˙) in a tunnel is imperative for estimating the intensity of thermal disasters and forecasting the potential heat transfer from fires to both the surrounding environment and individuals trapped in underground buildings. In this research, simulations utilizing a liquid pyrolysis model were conducted to investigate the Q˙ of two heptane fires in a tunnel, considering variations in fire spacings (0-6 m) and pressures (52-100 kPa). Initially, the numerical model underwent validation through the application of classical prediction correlations and the examination of full-scale experimental data, ensuring its accuracy and reliability. The correlation among Q˙, oxygen, boiling point, fire spacing, and pressure is subsequently scrutinized and disclosed. The findings reveal that the fuel heat release rate profiles undergo two stages over time: an initial fire growth stage and a medium quasi-steady combustion stage. In low-pressure environments, the fuel pyrolysis rate is relatively high, and the stable combustion stage is reached faster due to the combined effects of boiling point, fuel preheating, and surface tension. During the quasi-steady combustion stage, the ratio of Q˙ is approximately equal to the ratio of pressure. At the lowest pressure, Q˙ experiences a decline of approximately 50% compared to standard atmospheric pressure. To assess Q˙ throughout the entire burning process, a composite parameter incorporating boiling point, pressure, and fire spacing is proposed based on energy balance considerations.