Numerical Study of the Effects of the Jet Fan Speed, Heat Release Rate and Aspect Ratio on Smoke Movement in Tunnel Fires

Abstract

In this study, the effects of the jet fan speed, heat release rate and aspect ratio on smoke movement in tunnel fires have been investigated. The jet fan speed was changed from 6.25 (25%) to 12.5 m/s (50%), 18.75 m/s (75%), and 25 m/s (100%). The heat release rate was set up from 3.9 to 6 MW and 16 MW, the aspect ratio was changed from 0.6 to 1 and 1.5, respectively. The lower the jet fan speed is, the longer the smoke back-layering length is. With a higher velocity, the smoke tends to move out of the tunnel quickly; however, smoke stratification also occurs, and this reduces visibility. This could make it difficult for people to evacuate. With a higher heat release rate, the smoke tends to move far away from the fires quickly when compared with other cases. Additionally, the higher the heat release is, the longer the smoke back-layering is. Finally, with a higher aspect ratio, the smoke back layering length in the tunnel is also longer. The smoke layer thickness is also larger than in other cases. The correlation of velocity, heat release rate and aspect ratio has been investigated to avoid the smoke back layer length in tunnel fires.

Application of the Bernoulli Effect for Improving Smoke Exhaust Efficiency in Tunnel Fires

“…Zhou et al measured the effective wind speed and ambient temperature according to the distance between the portable fan and a fire and developed a theoretical model based on their findings [9]. Thien Khieu et al analyzed the effects of the heat release rate and aspect ratio of the jet fans on smoke movement during tunnel fires [10]. In addition, studies related to tunnel fire analyses have been conducted, such as "Analysis of the Importance of Oxygen Concentration in Tunnel Fires [11]," "Interpretation of Summer Ventilation Flow in Tunnels using CFD [12]," and "Comparison Analysis between Experimental Data and CFD on Longitudinal Ventilation [13]."…”

“…CFD is based on theoretical principles of mass, momentum, and energy conservation. The equations for each are listed below as follows: (10) for mass conservation, (11) for momentum conservation, and (12) for energy conservation. Equation (10) states that the rate of increase in the fluid mass within a control volume is equal to the mass of the fluid flowing in through a control surface with the same flow rate [39].…”

“…The equations for each are listed below as follows: (10) for mass conservation, (11) for momentum conservation, and (12) for energy conservation. Equation (10) states that the rate of increase in the fluid mass within a control volume is equal to the mass of the fluid flowing in through a control surface with the same flow rate [39]. This principle is based on the continuity equation and indicates that the rate of change of mass over time is zero.…”

Some Tips on Numerical Modeling of Airflow and Fires in Road Tunnels

“…One should have in mind that modeling inaccuracies concern especially regions close to walls and ceiling. Therefore they may impact on the obtained picture of such phenomena as backlayering [12,13] and plug-holing [11], which are essential for assessing the efficiency of ventilation systems.…”