A study of the effect of tunnel aspect ratio on control of smoke flow in tunnel fires.

In the event of a tunnel fire, the emergency ventilation system is often brought in action to create a safe route upstream clear of smoke for evacuation and fire fighting. The "critical ventilation velocity" is used to represent the value of the ventilation velocity which is just able to force the smoke moving in one direction. This value has become one of the important criteria for the design of the tunnel ventilation systems.
This study reviewed current knowledge on the critical ventilation velocity and studies of tunnel fires. The literature review showed that the critical ventilation data are limited in number. The influence of fire power on the critical ventilation velocity remains uncertain and in addition, the most important issue which is the effect of tunnel geometry on the critical ventilation velocity has not been studied yet.
To establish better model prediction of the critical ventilation velocity, the present work systematically investigated the effect of tunnel geometries on the critical ventilation velocity on five small scale model tunnels which have approximately the same height but different widths. Three dimensional Computer Fluid Dynamics (CFD) simulations were also carried out to investigate the flow behaviour and compare the modelling results with the experimental results.
The present work found that cross-sectional geometry did affect the critical ventilation velocity. The critical ventilation velocity has been related to the distribution of the fire plume inside the tunnel at critical ventilation conditions. The present work also explored the new dimensionless of the critical ventilation velocity and the heat release rate and suggests that the mean hydraulic tunnel height (H) should be used as the characteristic length for the buoyant forces in the dimensionless analysis instead of tunnel height (H). A simple one dimensional relationship has been derived based on the new dimensionless analysis for predicting the critical ventilation velocity for large scale tunnels in any cross-sectional geometry.
Finally, the scaling problem was resolved by comparing the present results with the large scale results obtained in the literature review, expressed in the new dimensionless analysis. The results showed that the present results agreed with most of the large scale experimental results. This suggests that the present results can be used with high degree of confidence to predict the critical ventilation velocity for larger scale tunnels in any cross-sectional geometry.