Temperature Characteristic And Longitudinal Ventilation Control Of Fire Smoke In Tunnels

The occurrence of several fatal tunnel fires in recent years warns people of the severe situation of tunnel fire safety while they enjoy the convenience brought by tunnels. Tunnel fire would cause disastrous consequences if it is not well controlled. The first fatal factor that causes casualties in fires is the toxic smoke produced by the combustion. The study of fire characteristic and smoke control in tunnels is an important issue in the area of tunnel fire safety, knowledge of which could provide meaningful reference to tunnel fire protection engineering.Theoretical analysis, experimental comparisons and CFD numerical simulations are conducted on the fire characteristic and smoke control in tunnels. The main work and conclusions in this study are as follows:A simple correlation is theoretically presented and numerically studied for the temperature distribution of fire-induced flow along tunnel ceiling, which could correlate the flow temperature for various tunnel aspect ratios and fire intensities. The aspect ratio of the tunnel cross-section in the numerical models varies from0.5-2.0, and the heat release rate of the fire source varies from4.4-22MW. The numerical results indicate that the correlation could be utilized for all tunnels but with separate fitting constants for aspect ratios less than unity and greater than or equal to unity, including all fire intensities considered.Afterwards, the above correlation is evaluated to correlate the temperature distribution of fire-induced flow under mechanically ventilated tunnels. The effects of the longitudinal ventilation velocity and the fire heat release rate are introduced into this correlation by replacing the reference flow excess temperature and its position with the maximum flow excess temperature and its position, respectively. Experimental data ranging from reduced-scale to laboratory-scale tunnels from two past studies are employed. Supplementary data are obtained from numerical simulations. Several past empirical correlations for the maximum flow excess temperature and its position are validated with adjusted coefficients by these data. The evolved correlation for the temperature distribution of fire-induced flow along mechanically ventilated tunnels is evaluated and modified. Results indicate that this correlation could predict the temperature distribution with engineering accuracy. Tunnel blockage ratio,φ, is defined as the ratio of the cross-sectional area of the fire source to that of the tunnel. These fire sources in reality correspond to those that have considerable cross-sectional area such as trains or heavy goods vehicles, which is very common. The effect of tunnel blockage ratio on the maximum temperature beneath the ceiling in tunnel fires is experimentally analyzed. Results indicate that the presence of blockage changes the local ventilation condition, which makes the maximum temperature decreases with the increasing blockage ratio for small fires. However, for large tunnel fires, the maximum temperature does not vary with blockage ratio. Previous models have been modified based on the above analysis by introducing a factor that accounts for the blockage effect, which are more generally applicable.The effect of tunnel blockage ratio on critical velocity in tunnel fires is numerically studied. A proposed empirical consideration that accounts for the blockage ratio effect has been verified. A new formula for predicting the critical velocity in blocked tunnels has been presented based on the above consideration. Data from present numerical simulations and previous full-scale and reduced-scale tunnel fire experiments that have considerable tunnel blockage ratios are employed to examine this formula. The formula shows good agreements with these data, which can be used appropriately to calculate the critical velocity for tunnels with blockage.The effect of the forced longitudinal ventilation on the heat release rate of tunnel fires is quantitatively analyzed and experimentally examined. Results indicate that the relative increment of tunnel fire heat release rates varies proportionally with the quantity uS/(g1/2r5/2) that represents the relative value of the longitudinal air volumetric flow rate to the mass loss rate of the fire source.