Investigation on tunnel fire phenomenon under different emergency ventilation systems

Abstract

In this research study, the ventilating effect on tunnel fire development has been investigated under 4 ventilation mode throughout the tunnel: natural ventilation, fully-transverse ventilation, semi-transverse exhaust and longitudinal ventilation system. The investigation is performed by two means - full-scale fire test and numerical study. To investigate the first three ventilation modes, a full-scale experiment was conducted with two different fire intensities of 0.7 5MW and 1. 5MW as stipulated in AS4391 (1999) and the spatial temperature was recorded. The data was analyzed and compared case by case. The experimental results have shown that the ventilation system plays an important role on both fire development and heat removal. Interestingly, semi-transverse exhaust ventilation system gave the lowest temperature profile along the tunnel. To complement the experimental study, Computational Fluid Dynamics (CFD) techniques were utilized to validate against the measurements and give more in-depth analysis on the phenomenon. In general, CFD can successfully capture the trend of fire development and the predicted temperature profiles were reasonably well-matched with measurements. The mass loss rate and stability of pressure boundaries at two ends of tunnel were found deterministic in the accuracy of numerical results. In the experiment setup, longitudinal ventilation system was not installed, hence, the flow field under this ventilation mode was investigated by numerical approach. Attention is paid on the impact of the location of active fan group on the critical velocity because assuming an evenly distributed airflow at one end of scaled tunnel is unrealistic. The air flow field in a generic 3-lane D-shape tunnel with a fire size of 5MW was analyzed under activation of different fan groups. Highest upstream velocity was found when the nearest fan group was activated. Moreover, the upstream velocity exhibits second order polynomial relationship with the distance of the active fan group from the fire source. On the other hand, sensitivity study on changes of dimensions, orientations and nature of fire source are also performed. Solid fire source was found to have significant effect on the upstream velocity; while the dimensions and orientation of fire were found with minor influence. In solid fire, the upstream velocity decreased and asymptotes at the distance 200m away from the fire source. Such levelling-off characteristic of upstream velocity was also found correlating with the heat release rate. In order to compensate the unexpected decrement in upstream velocity, fan group located closer to the fire source shall be activated to overcome backlayering. In summary, this sensitivity test has shown that the upstream velocity was affected by some commonly neglected parameters which could contribute to an undesirable ventilation performance. Further investigation on other parameters in estimating critical velocity is essential in response to any tunnel fire event in the future.