CHARACTERISTICS OF FIRE-INDUCED AIR MOVEMENT IN A VENTILATED TUNNEL

Abstract

This project is a study of the behavior of smoke movement in tunnel fire by experiment and simulation. The experiment is carried out in a scaled tunnel and simulation is conducted by using a commercial CFD software package, called FLUENT. The patterns of fire-induced airflow in tunnel are visualized. Different cases involving varying fire intensities and ventilation velocities are studied to reveal the sensitivities of upstream back flow and thermal stratification to ventilation velocity, heat release rate and tunnel inclination. Special attention is placed on the wave-shaped smoke layer which has not been covered in existing literature. This layer is very sensitive to overall ventilation velocity and least affected by heat release rate. Based on the experimental measurements, a new exponent of Reynolds number n = 3 is proposed in mixed convection parameter. Quantitative study on the stratification parameter S is also given. Critical velocity is introduced. When ventilation velocity exceeds the critical velocity, upstream back flow will disappear and thermal stability may collapse. The optimum ventilation velocity should be equal and slightly smaller than the critical velocity. The simulation incorporates the modified k-ε model for turbulence and combustion models for fire including stochastic and deterministic methods. Some characteristic features in tunnel fire such as upstream back flow and stratification are reproduced. The simulated results have a good agreement with the experiment. The simulation is extended to enclosure fires including shopping mall fire, room fire and tunnel fire to further establish the consistency and effectiveness of the three combustion models. Emphasis is placed on the comparison of their performances in enclosure fires simulation. It is subsequently established that Mixture Fraction/PDF combustion modelling approach provides a similar prediction at low heat release rate as and a more realistic simulation at high heat release rate than Finite Rate Reaction Approach, they both provide more accuracy and versatility than the Volumetric Heat Source Approach by taking into consideration of the process of the combustion reaction. PDF model is recommended for diffusion flame while FRR is suitable for low intensity fire.