Research on characteristics of thermal smoke movements of twin fires in a tunnel under reduced pressure

In this study, numerical simulations were conducted to investigate the behavior of thermal smoke movement in tunnels under reduced pressure, with the aim of providing scientific guidance for controlling smoke and heat in the event of tunnel fires. The variables examined included fire spacing (0-6 m), atmospheric pressure (55-100 kPa), and overall heat release rate (6-18 MW). The analysis revealed that the temperature method, which considers relative temperature deviations along tunnel centerlines and on tunnel sidewalls, is effective in identifying the initial position of one-dimensional smoke movement (TY=0 - TY=4.9) / TY=0 <= 5 %. It was found that under reduced pressure, the mass of air entrained into the smoke gas per unit time decreased, resulting in a lower mass-flow rate. Specifically, at 100 kPa, the mass-flow rate was approximately 1.4 times higher than at 55 kPa for the same simulation scenario. The effect of fire spacing on the mass-flow rate was found to be weaker compared to pressure, particularly during the noninteraction stage of the fire. The air entrainment coefficient (beta) displayed a negative correlation with environmental pressure and positively correlated with fire-heat release rate. In all cases, the beta value for dual fire sources in a tunnel ranged from 0.0008 to 0.0022. Finally, a segmented function was developed to forecast the mass-flow rate of thermal smoke, taking into account various factors such as fire source parameters, environmental parameters, and tunnel restrictions.