Evolution Behavior of Heat Release Rate of Twin Fire Sources in a Tunnel Under Low Pressure Environment

Abstract In the low-pressure environment at high altitudes, highway tunnels, as important transportation carriers, can experience confined space fires due to accidental incidents such as vehicle collisions and rear-end collisions. 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 relationships among Q˙, oxygen, boiling point, fire spacing, and pressure were subsequently scrutinized and disclosed. The findings revealed 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, heat transfer efficiency increased and pyrolysis products diffused more rapidly, leading to a relatively higher fuel pyrolysis rate. Meanwhile, the fuel boiling point decreased due to the lower pressure, allowing liquid fuel to vaporize at lower temperatures and in less time, which reduced the sensible heat consumption required for preheating the liquid phase and accelerated the formation of gaseous fuel. Additionally, the effect of surface tension caused liquid droplets to break into smaller particles more easily under low pressure, increasing the evaporation surface area and thus accelerating the evaporation rate. Under the combined effect of these factors, the fuel could more quickly reach the flammable mixture concentration and heat feedback equilibrium state required for quasi-steady combustion under low-pressure environments. During the quasi-steady combustion stage, the ratio of Q˙ was approximately equal to the ratio of pressure. At the lowest pressure, Q˙ experienced 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 was proposed based on energy balance considerations.