Experimental investigation on influence of different transverse fire locations on maximum smoke temperature under the tunnel ceiling

https://doi.org/10.1016/j.ijheatmasstransfer.2012.04.052Get rights and content

Abstract

In former studies, fires were always assumed to occur at the longitudinal centerline of tunnels. In fact, fires will occur at any locations in tunnels, with different distances to the sidewall. A set of model scale experiments were carried out, to investigate the influence of different transverse fire locations on maximum smoke temperature under the tunnel ceiling. Results show that the restriction effect of the sidewalls of tunnels cause the maximum smoke temperature rise under the ceiling to increase compared with the unconfined space, even fires occurs at the longitudinal centerline. The maximum smoke temperature rises above the fire keep almost unchanged with the fire moving closer to the sidewall at the beginning and then increase significantly after the distance between the fire and the sidewall decreases to a certain value. For small pools of wall fire, the “mirror” effect is reasonable, and for large pools, will bring a relatively large error without considering the influence of the equivalent diameter of a wall fire, resulting in underestimating the mass flow rate of fire plume and then overestimating the smoke temperature. Under all fires, the maximum smoke temperature rise under the ceiling decreases exponentially as the longitudinal distance from fire increases. Correlations for related parameters are proposed.

Introduction

In recent years, disastrous tunnel fires have accidentally occurred, such as the Korea Daegu Fire in 2003, killing 189 people and the Viamala Tunnel Fire in Switzerland in 2006, killing nine people [1]. The fire-induced smoke will influence the safe evacuation of occupants and affect the firefighters from extinguishing the fire. In fact, 85% of the deaths in building fires were caused by the toxic smoke according to statistics [2]. During fires, the accumulation of heat generated by fires can cause the temperature of the confined space to be raised significantly. The hot smoke, even the fire flame, will directly contact the tunnel structure. The strength of the steel bars in the concrete is reduced once the bars are exposed to flame and hot smoke, which eventually leads to sinking or collapse of the tunnel ceiling [3]. Other than the structural issue, sprinklers or detectors installed in tunnels are quite likely to be activated by the smoke travelling along the ceiling. Therefore, it is in practice worth studying the maximum smoke temperature under the tunnel ceiling so as to enhance the safety level in tunnels.

Alpert [4] developed a set of equations to predict the temperature profile under the ceiling, where the distances between the fire and the vertical walls are greater than 1.8 times of the ceiling height. However, according to a wide survey, for fires in tunnels, the requirement of the Alpert equations could not be met, resulting from the special aspect ratio (the ratio of width over height) of tunnel section, even though the fire occurs at the longitudinal centerline of tunnels. Therefore, further validation should be conducted for the Alpert equations when the fire is relatively close to the sidewall. Kurioka [5] proposed a model to predict the maximum smoke temperature under the tunnel ceiling based on model scale experiments and Hu [6] and Wang [7] justified the model by fire experiments in actual tunnels respectively. In these former studies, the fire sources were always positioned at the longitudinal centerline of tunnels, as in large numbers of studies on tunnel fires [8], [9], [10], [11], [12]. However, in fact, fires will occur at any location in tunnels, with different distances to the sidewall. When the fire occurs near the sidewall, the air entrainment of the fire-induced thermal plume will be influenced by the sidewall and the heat feedback obtained by the fire plume from the tremendously heated sidewall will increase. Therefore, the maximum smoke temperature under the tunnel ceiling will be different from that of a fire at the longitudinal centerline.

To develop empirical equations for determining the maximum smoke temperature under the tunnel ceiling, applicable for fires occurring at any position, a set of reduced-scale experiments were carried out in this study.

Section snippets

Theories

Kurioka [5] derived one empirical equation to predict the maximum smoke temperature rise under the ceiling based on model scale experiments, as follows:ΔTmaxTa=γQ2/3Fr1/3ε,Q2/3/Fr1/3<1.35,γ=1.77,ε=6/5,Q2/3/Fr1/31.35,γ=2.54,ε=0,whereQ=Q˙ρacpTag1/2H5/2Fr=V2gHQ˙, Ta, ρa, cp, H and V are the heat release rate (HRR), the ambient air temperature, the ambient air density, specific heat capacity, the height from the fire surface to the ceiling and the longitudinal wind speed in the tunnel

Experiments

The experiments were conducted in a small-scale urban road tunnel model as shown in Fig. 1. A scale ratio of 1:6 is applied in current cases. The tunnel is 6 m long, 2 m wide and 0.88 m high. Its aspect ratio is determined based on a survey mentioned above.

The Froude modeling was applied to build up the physical scale model. The dimensional relationships between the fluid dynamics variables were derived from first principles by Morgan et al. [18] and also mentioned in NFPA 92B [19]. By holding the

Fires at the longitudinal centerline of model tunnel

The values predicted by the Alpert equation are calculated by substituting both the experimental HRR and the distance from the fire to the ceiling into Eq. (8). The experimental and the predicted results are compared in Fig. 4a, which shows that the measured maximum smoke temperature rise is slightly more than what the Alpert equation predicts as a whole. As mentioned above, the Alpert equation was developed for the ceiling jet being not confined by the vertical sidewall largely. In our

Conclusions

With regard to maximum smoke temperature under the ceiling in a tunnel fire, all the models built by former studies are arbitrarily assuming that the fire happens at the longitudinal centerline. In fact, fires may occur at any locations in the tunnel, with different distances to the sidewall. A theoretical analysis and a set of reduced-scale experiments were carried out in this study, to investigate the influence of different transverse fire locations on maximum smoke temperature under the

Acknowledgement

This work was supported by National Natural Science Foundation of China (NSFC) under Grant No. 50904055, the Chinese Universities Scientific Fund (CUSF) under Grant No. WK2320000005 and the Anhui Provincial Natural Science Foundation under Grant No. 1208085QE81.

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