Effect of Fire Heat Release Rate on Temperature, Velocity, Visibility and CO Concentration Distributions in Under Ground Tunnel

Abstract

A Numerical study has been undertaken in underground tunnel using Fire Dynamic Simulator (FDS) version 5.5.3. The tunnel is horizontal and completely below ground. It is 300 m long, with a partial circle cross-section around 4 in surface and with 2.16 m hydraulic diameter. Ventilation devices are mounted by groups of jet fans under tunnel ceiling with 100m distant apart. Emergency doors with 100 m apart are connected to a walkway (cross-passage) for motorists escaping. In the case of longitudinal or transverse ventilation systems, one of the main aspects deals with the preservation of the smoke’s natural stratification. It must be preserved in a clean air layer in the lower part of the tunnel, which is crucial in order to facilitate the motorists' evacuation. after motorists evacuating, jet fans start to work at highest speed that is cause to sweep all smoke upstream the fire to downstream to allow fire fighters enter from traffic direction In the present work Three case studies are simulated for 400 seconds in which the smoke reaches to steady state condition with different HRR of heptane pool with 23 Mw ,50Mw and 100Mw .Results illustrate the temperature contours at different cross sections upstream and downstream fire location ,temperature contours at human level , visibility contours at human level , velocity contours at mid plane and the simulation for motorists escaping from different exits and calculating evacuation time required. This study shows that evacuation time depends on different factors and it decreases with increasing of heat release rate of the fire. The local temperature at human occupancy level doesn't exceed the safety limit of 60 C according to National Fire Protection Association (NFPA 502 ) and the visibility at human level before operating the jet fans decreases with fire HRR due to the decrease of the ceiling jet smoke momentum , which was defined as the flow of smoke under tunnel ceiling, extending from the point of fire plume impingement on ceiling ,that results in smoke dropping and penetration to reach the human occupancy level .