The combined effect of a water mist system and longitudinal ventilation on the fire and smoke dynamics in a tunnel

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Abstract

The individual and combined effect of a water mist system (WMS) and longitudinal ventilation (LV) on the fire HRR, smoke temperature and back-layering (BL) degree were explored in a 3 m (width)×2.2 m(height)×30 m (length) tunnel model. A series of 39 tests has been conducted in 6 configurations, varying the velocity, water volume flow rate and nozzles arrangement. When the WMS or LV is imposed individually, the reduction effect on the HRR gets stronger with larger water volume flow rates or higher velocities. Yet, under the combined effect, the effect of the LV velocity on the HRR is non-monotonic: the HRR first reduces more, and then rises again with higher LV velocities for given WMS settings. The reduction in temperature monotonically increases with higher LV velocities, in contrast to the HRR behavior. The above phenomenon is explained to be due to the relatively ‘low’ flame height in the present paper, as a consequence of which the interaction between the flame and the water droplets mainly occurs in the lower part of the tunnel, while the interaction between the hot smoke and the water droplets mainly happens in the upper part. The fire HRR behavior mainly relates to the interaction in the lower part, while the temperature depends on both regions. Besides, for a certain BL distance, the required ventilation velocities reduce by 13%–55% with activated WMS, compared to the same conditions without WMS. The effect of covering the fire with a shield is also discussed. For relatively small velocities, the fire HRR, smoke temperature and BL distance are larger with the shield in place.

Introduction

In order to increase safety and prevent catastrophic damage in tunnel fires, Fixed Fire Fighting Systems (FFFS), such as Water Mist Systems (WMS), are considered in most long urban tunnels in China as a means of fire control and suppression. FFFS is nevertheless not a traditional fire protection system for tunnels in most countries due to the lack of design guidelines. In WMS, about 90% the droplets have a diameter of less than 500 μm [1]. The current paper will focus on water droplets in this range.

Carvel and Ingason stated in Ref. [2] that the main function of the WMS in tunnel fires is ‘protection’, rather than ‘suppression’. The latter is defined in terms of ability to ‘arrest the rate of fire growth and significantly reduce the energy output of the fire shortly after operation’, which requires a substantial amount of water and has yet to be studied in details. Fire suppression [[3], [4], [5], [6]] is beyond the scope of the current paper.

The aim of the present paper is to understand the impact on the fire and smoke dynamics of the WMS and LV individually and combined. Limited previous studies have focused on this means of smoke control in tunnels. Mid-scale tests and CFD simulations were conducted by Blanchard et al. [7] to understand the interaction between water mist drops and fire smoke in a longitudinally ventilated tunnel. Four major mechanisms of fire heat loss were identified and each of them was quantified, showing that the cooling effect of water drops accounts for the largest share, approximately 43%. Sun et al. [8] and Li et al. [9] treated the WMS as a water curtain and explored its ability to prevent smoke and heat spread with a reduced-scale model tunnel. The ratio of the water mist momentum to the smoke momentum is calculated for both tests in Ref. [9], with the obviously higher values in Ref. [8] than [9], therefore different results in terms of blocking efficiency were obtained. Sun et al. [10] reproduced the experiments in Ref. [8] and visualized the entrainment mechanism of the smoke by the water mists with the aid of FDS. It could be observed that due to the spray-induced impinging jets onto the floor, there appeared a clear upward flow motion between the nozzles, acting as a ‘barrier’ to block the smoke spread. The water mist curtain was also studied in Refs. [[11], [12], [13]], to explore its ability to stop smoke spread, improve visibility and attenuate thermal radiation. However, it is noted that the effect of LV were not discussed and the water mist failed to block the smoke flow in Refs. [[11], [12], [13]]. Tang et al. [14] built a 1/14 small-scale model and placed two mist nozzles upstream the fire source. The results showed that lower LV velocities would be required to control smoke if the water mist was imposed to the tunnel near the fire, especially with higher pressure or more nozzles. The cooling effect of the water drops, along with the increase in inertial force of the longitudinal airflow induced by the water mists were supposed as the two main mechanisms for the smoke temperature reduction.

There are also some studies based on the water spray systems with relatively larger droplets (larger than 500 μm), which could be referential. In order to improve the basic understanding of water spray in longitudinal tunnel flow, two series of well-defined small-scale tests were conducted by Ingason [15] and Li et al. [16], with water spray system and automatic sprinklers, respectively. Ingason [15] used two wood cribs to simulate the HGV fires and investigated the possible fire spread between them. Besides, the effect of the water spray on the smoke temperature near the ceiling was studied by Ko et al. [17] and Wang et al. [18], with a full-scale and small-scale model tunnel, respectively. The results of both studies showed that the temperature decay along the longitudinal direction would be more significant with larger water volume flow rate. A method to predict the maximum temperature and temperature distribution near the ceiling was proposed, considering the water flow density and the water volume flow rates, respectively. The critical velocity was also studied by Ko et al. [17] and Wang et al. [19], showing that for a certain fire size, the required critical velocity would decrease with a larger water flow density (or water volume flow rate).

Then let us look at the combined effect of WMS and LV on the fire control. It is worth noting that in most of the studies mentioned above on smoke control, be it experiments or CFD simulations, the fire HRRs (Heat Release Rates) are set to be unchanged after the activation of the WMS (or the water spray system). The propane fire is adopted in some of the studies to achieve the constant HRR [14,17,18,20], as the gas flow rate could be easily maintained during the tests. In other tests using pool fires (heptane or methanol) [8,9,11], the WMS (or the water spray system) is placed at a certain longitudinal distance from the fire source. Therefore, the effect of the water droplets on the HRR is ignored. However, the HRR is not often the same before and after the WMS activation in real tunnel fire incidents, as the direct interaction between the water droplets and fire flame is common if the WMS is used to control (or reduce) the fire size. Several studies considered this aspect. In the mid-scale tests of Blanchard et al. [7], the WMS is set above the fire source (heptane pool fire) and the HRR reduces after the WMS activation. However, only one specific set of conditions in terms of LV velocity and WMS settings is considered in this study. Ingason [15] mentioned how the HRR performs after the activation of the water spray system, placed above a wood crib fire source. The results show that a larger water volume flow rate and a lower LV velocity leads to lower HRR. It is noted that only two LV velocity values (0.62 m/s and 0.21 m/s) are adopted. It can be seen in general that there is still a strong need to investigate this aspect thoroughly.

Therefore, in this paper, a series of 39 fire tests are carefully designed in a mid-scale tunnel to study the impact on the fire and smoke dynamics under the combined effect of WMS and LV. The WMS is placed right above the fire source, varying the water volume flow rate and layout. More importantly, the LV velocity is varied in a relatively wide range to take a closer look at its effect on the WMS and the smoke. Finally, the behavior of a fire source shielded at its top will be discussed.

Section snippets

Tunnel model and fire source

All the experiments were conducted in a 3 m (width)×2.2 m(height)×30 m (length) model tunnel, as shown in Fig. 1 (a) and (b). The ground is made of reinforced concrete and the side walls are made of brick and concrete. For convenience of preparation and observation work before and during the experiments, two 8-mm-thick fire-resistance windows were set on one side, which measured 1.92 m (length) × 1.47 m (height) each.

Industrial methanol (99% pure) was used as fuel and poured into a

The identification of the quasi-steady state

The pool fire was not extinguished by the WMS in any of the tests. After activating the WMS, quasi-steady state conditions pertain, until the fuel is burnt out. Let us consider Configuration 1 as an example to introduce the quasi-steady state. Fig. 4 gives the evolution of the fuel mass loss rate, mf˙, and temperature rise for upstream thermocouples, starting from ignition, up to activation of nozzles. The nominal LV velocity values of 0.6 m/s, 0.5 m/s and 0.4 m/s (Tests 14, 15 and 16) were set

Effect of longitudinal ventilation (LV) only

The results of Configuration 0 (no WMS) are briefly discussed here, as they will serve as reference results to quantify the effect of the WMS.

Conclusions

Experiments have been conducted to study the fire and smoke dynamics under the individual and combined effect of WMS and LV. The maximum HRR was around 234 kW and the fire flame height remained below 1.1 m (half the tunnel height), implying that only relatively small fires have been addressed in the present paper. The main conclusions are as follows:

  • (1)

    Due to the relatively low flame height, the interaction between the flame and the water droplets mainly occurs near the floor (‘lower region’ of

Author statement

Yuanjun Liu: Writing- Original Draft preparation, Formal analysis, Investigation, Software, Conceptualization, Methodology. Zheng Fang: Writing- Reviewing and Editing, Methodology, Resources, Supervision, Funding acquisition. Zhi Tang: Writing- Reviewing and Editing, Conceptualization, Resources, Methodology, Project administration, Funding acquisition. Tarek Beji: Writing- Reviewing and Editing. Bart Merci: Writing- Reviewing and Editing, Supervision.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This research was funded by National Natural Science Foundation of China (NSFC) under [grants Nos. 51508426, 51576144, 51508426, 51711530232], and China Scholarship Council [grant No. 201606270109].

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