Numerical simulation of fire in a tunnel: Comparative study of CFAST and CFX predictions

Abstract

The mathematical modeling of fire growth and smoke movement in any enclosure is a formidable task. Two types of deterministic models are in vogue, zone models and field models (popularly known as CFD technique). CFAST is a popular zone model used for modeling of fires in enclosures. Likewise, CFX is a general purpose CFD code used for various purposes including modeling of fires. In the present paper, a tunnel of length 150 m having a rectangular cross-section of 80 m 2 has been considered for analyzing the temperature and velocity profiles generated by fire, placed at a distance of 20 m from one end of portal, by both CFAST and CFX. The simulation by CFAST has been carried out by dividing the tunnel into 1, 2, 5, 8, 10, 12 and 15 compartments of equal size, where these compartments are joined by openings or vents having same cross-section as that of the tunnel. In case of tunnel divided into 15 compartments the fire source position lies at the position of vent; CFAST predicted very high temperatures. The simulations have also been carried out by dividing tunnel into unequal sized compartments such that position of fire was at the center of the compartment. It was found that for accuracy of results, location of fire source inside compartment is an important factor. Computational difficulty was experienced when tunnel was divided into more than 15 compartments. In this paper, a comparative study of temperatures predicted by CFAST and CFX has been done. The CFX and CFAST predictions show that smoke temperature changes with a pattern roughly similar to that of heat release rate. The temperature profiles at selected positions cannot be predicted by CFAST unlike CFX. The detailed features like flame tilt, flow field can only be observed from CFX predictions.