Analysis on the fire safety aspects for tunnels in Hong Kong with mathematical models

Abstract

This thesis is aimed to study the fire and ventilation aspects related to tunnels, as the chance of having an accidental fire in vehicular tunnels might not be so low as expected. Consequent to a fire incident resulting from a truck carrying illegal diesel outside a tunnel in Lion Rock Tunnel in 1999; and a vehicle fire that occurred in Cross Harbour Tunnel in 2000, people in Hong Kong are quite concerned about the safety aspects of vehicular tunnels. The current regulations, codes, design guides, practices and safety requirements on fire and ventilation in tunnels are reviewed. A preliminary hazard assessment on spill fires with empirical expressions available in the literature and fire zone model was reported. The results and cooling jet expressions were applied to assess the probable tunnel environment and it was found that hot smoke would be generated rapidly and the smoke front would travel with high speed. A better understanding of tunnel fires would help the tunnel authority to provide better fire safety management. Computational Fluid Dynamics (CFD) is a practical tool for engineers in studying the fire environment in a tunnel and pursuing an optimum design due to the high cost, complexity, and limited information obtained from experimental methods. Air flow induced by an accidental vehicular fire in a tunnel was simulated with a fire field model using the CFD package PHOENICS. Sensitivity analyses on the grid size and the number of iterations on the required computing time and accuracy of the results were carried out. Based on the results, simulations on different tunnel fire scenarios were presented. The parameters varied were the fire size, ventilation system and its capacity. Systems considered are longitudinal, semi-transverse, transverse, partial transverse, and combined longitudinal and semi-transverse ventilation systems. By applying the software, comparisons and engineering judgment were made between different types of tunnel ventilation systems on the basis of temperature distribution contours. It was found that grid resolution and iteration number are two of the important considerations with respect to capturing the details of the smoke and heat distribution in the tunnel. The computing time required for CFD simulations is still a concern in the engineering profession. Moreover, whether CFD models are good enough to predict the temperature distribution patterns and flow in a confined enclosure was investigated. Transient results predicted should be converged at every time step. The simulation results of room fire and tunnel fire were compared with previously published data on fire tests. According to the room fire verification results, a larger number of iterations and finer grid configuration can help acquiring more accurate simulation results. Also, the results show that the effect of cell refinement at the fire source is significant for simulation when the fire was located adjacent to the wall. Based on the comparisons of vertical temperature profile along the tunnel axis, the CFD model usually predicted reasonable agreement with the measured fire data in the tunnel pool fire experiment. The important roles of longitudinal wind effect, radiation losses, heat release rate estimates and judgment in the selection of input data as well as the evaluation of the results of model runs have been demonstrated. Hence, the present findings on validating the fire field model have implication for identifying the modeling strategy that best represents the physical phenomenon. Fire safety management in the incidents is discussed and recommendations are made to the Authority for better fire safety management. For improving the outdoor air quality, the Government is trying to introduce Liquefied Petroleum Gas (LPG) vehicles in Hong Kong which has brought about new safety impact and challenges.