Back-layering Length Research Articles

Numerical and theoretical evaluations of the propagation of smoke and fire in a full-scale tunnel

Most CFD tunnel fire simulations have so far focused on the thermal field and the critical velocity for suppression of the hot backlayering flow. However, there is a great need in understanding the characteristics of a real-scale tunnel fire in terms of the flame propagation and the toxic gas generation. In this study, an extension of the eddy dissipation concept incorporating two chemical reaction steps is integrated into an internationally recognised CFD fire simulation code. A full-scale over-ventilated tunnel fire is simulated with the model and the measured temperature profiles are correctly reproduced. The model is then used to investigate the characteristics of a tunnel fire in three aspects: the length of backlayering, the flame length and the effects of an object within the tunnel. An inverse dependence of the backlayering length with the wind velocity, and the levelling-off of a maximum of the backlayering length for a ventilation velocity of 2m/s are predicted. Both the CFD prediction and an analytical correlation indicate that an increase of the heat generation rate enhances the flame length, and while, a growth of the ventilation velocity shortens the flame length. The effects of blockage in a tunnel on the propagation of smoke and fire are numerically investigated with the model. The results of this study have shown that the model is promising in qualitatively examining the correlation of the propagation of smoke, the production and the transport of carbon monoxide and soot with the ventilation rate and the fire heat release rate.

Model scale tunnel fire tests with longitudinal ventilation

Results from a series of tests in a model tunnel (1:23) are presented. Tests were carried out with longitudinal ventilation under different fire conditions. Wood cribs were used to simulate the fire source, which was designed to correspond to a scaled-down HGV (Heavy Goods Vehicle) fire load. The parameters tested were: the number of wood cribs, type of wood cribs, the longitudinal ventilation rate and the ceiling height. The heat release rate, fire growth rate, maximum gas temperature beneath the ceiling, temperature distribution, total heat flux at floor level, flame length, and back-layering length were investigated. Correlations for these parameters were investigated and proposed for longitudinal flow in tunnels.

Study of critical velocity and backlayering length in longitudinally ventilated tunnel fires

Experimental tests and theoretical analyses were conducted to investigate the critical velocity together with the backlayering length in tunnel fires. The experiments were performed in two longitudinally ventilated model tunnels. The proposed correlations for critical velocity are found to comply well with experimental data in both tunnels. The critical Froude number and the critical Richardson number were analyzed using the experimental data. The backlayering length was related to the ratio of longitudinal ventilation velocity to critical velocity. Experimental data show that the relation between the ratio of ventilation velocity to critical velocity and the dimensionless backlayering length follows an exponential relation. A correlation based on experimental data to predict the backlayering length is proposed. Further, comparison of experimental data of critical velocity and backlayering length with results from large-scale tests shows that there is a good agreement in both scales. The effect of accident vehicle obstruction on critical velocity and backlayering length was also analyzed. Experimental data show that the decrease in rate of critical velocity due to obstruction is slightly greater than the ratio of cross-sectional area of the model vehicle to tunnel cross-sectional area, and the backlayering length with an accident vehicle set inside the tunnel gets smaller.

Studies on buoyancy-driven back-layering flow in tunnel fires

The back-layering length and critical longitudinal ventilation velocity in tunnel fires will be studied in this paper. A semi-empirical model was formulated and compared with former expressions appearing in the literature predicting the back-layering length. An equation on predicting the critical longitudinal ventilation velocity was further derived by setting the back-layering length to be zero. Field tests were carried out in a new tunnel with pool fires up to 3.2 MW. The results from the full-scale burning tests were applied to examine the equation. In addition, some scenarios were simulated using Computational Fluid Dynamics (CFD). Critical velocities predicted in this study were similar to those observed in the field tests, CFD simulations and estimation by a simple model by Thomas. As the plume configuration for a bigger tunnel fire is not the same as that for a small fire, other empirical expressions for large tunnel fires gave some lower critical velocity in comparing with the result for small tunnel fire in this paper.

Experimental and Numerical Studies on Longitudinal Smoke Temperature Distribution Upstream and Downstream from the Fire in a Road Tunnel

Four full-scale tests are conducted in a real road tunnel, with ceiling jet temperature distributions measured 200 m upstream and downstream from the fire. Two sizes of pool fires, 1.8 and 3.2 MW, with two different fire surface heights, 0.2 and 1.7 m from the floor level, are considered. Longitudinal ventilation velocities are also varied. The experimental data obtained in these four tunnel fire tests are used for validation of FDS 4.0 parallel simulation on tunnel fires. The ceiling jet temperature distributions upstream and downstream, and thus the back-layering length are compared. Results show that the temperatures predicted by FDS 4.0 are near to the measured data. In near fire regions, for instance, not more than 40–80 m away from the fire, the temperature predicted is very close to the full-scale data. There is a deviation between the predicted value and the measured one, shown to be mainly less than 4–5 C, at positions further away. The back-layering length predicted by FDS 4.0 also seems to agree fairly well with that deduced from the full-scale experiments.

Influence of Stationary Vehicles on Backlayering Characteristics of Fire Plume in a Large Cross Section Tunnel

This study is a part of an investigation project to examine emergencies in a large cross sectional tunnel, to be constructed on the New Toumei Expressway in Japan. Backlayering characteristics, the most important behavior in tunnel fires, are investigated through large-eddy simulation for various longitudinal air flows and fire sizes. The accuracy of this simulator has been previously confirmed by comparing with experimental results. In this study, specifically, stationary vehicles in a tunnel are considered in the simulation for the first time. Their presence induces turbulence in the longitudinal air flow and greatly affects the backlayering characteristics. However, earlier studies have not yet considered these effects. The results show that the presence of stationary vehicles reduces the backlayering length. Furthermore, the ratio of large-sized vehicles to total traffic is found to have little effect on fire behavior.