Preliminary study on extinguishing shielded fire with water mist

https://doi.org/10.1016/j.psep.2020.05.043Get rights and content

Highlights

  • Sheltered fire can be successfully extinguished by water mist as Kp <1.60.

  • Developed empirical correlation Kp=-0.54αc+2.36 can predict fire extinction limit.

  • Water mist-flame interaction is visualized experimentally and numerically.

  • Simulation underestimates water mist fire suppression ability under higher Kp and Q˙.

Abstract

Fires in commercial and industrial areas, such as large warehouses containing goods on shelves, are inevitably shielded by nearby objects that act as obstacles, making such fires difficult to extinguish. Water mists, as an alternative to the halon fire-extinguishing agent, are capable of bypassing obstacles owing to the small size of the water particles. Therefore, by varying the distance between a plate obstacle and the nozzlefire source, half-scale experiments were performed under different working pressures to determine the critical condition of shielded sand-burner fire extinguishment. The flame temperature and radiant heat flux were measured using thermocouples and a radiometer. The interaction between a water mist spray and a shielded fire was visualized via laser light sheet illumination. The fire-extinguishing capability was analyzed based on the plate obstacle block ratio and the plume–spray thrust ratio. The results indicate that an empirical linear correlation can be adopted to predict the critical plume–spray thrust ratio required for fire extinguishment under different block ratios. In addition, Fire Dynamics Simulator was used to simulate the spray–plume interaction under the shielding conditions employed. The experimental and numerical results show a similar suppression tendency in cases with a low block ratio and fire size. This preliminary study may provide some raw data on shielded fire suppression with a water mist, in addition to serving as a reference for the optimal design of water mist systems.

Introduction

With the increased awareness of environmental protection and sustainable development in the international communities, the once widely used halon fire-extinguishing agents (mainly Halon 1301 and 1211) have been gradually phased out owing to their strong ozone depletion potential since the signing of the Montreal Protocol in 1987 (Montreal Protocol, 2020). The Significant New Alternatives Policy (SNAP) in the United States lists some recommended fire-extinguishing agents including in-kind alternatives (e.g., halocarbons, inert gas, carbon dioxide), and not-in-kind alternatives (e.g., powdered aerosols, foam, water mist) (Significant New Alternatives Policy (SNAP), 2020). Moreover, water mists have been considered to be a viable alternative by the United Nations Environment Programme because they do not contribute to stratospheric ozone depletion or the greenhouse effect (Halons Technical Options Committee, 2018).

Therefore, researchers in the fire-suppression communities have increasingly focused on fundamental and engineering studies of water mist fire suppression. Grant et al. (2000) and Wang (2019) have performed significant studies and state-of-the-art reviews on research and applications of water mist, including development trends and challenges and its use in explosion mitigation, fire smoke control, and hazardous decontamination. Suppression of obstacle-shielded fires with water mist is a typical challenge and would be applicable in certain commercial and industrial locations such as warehouses where fire occurs under a package or under/within a multilayer storage rack. Fire survey data from the U.S. Fire Administration’s (USFA’s) National Fire Incident Reporting System (NFIRS) and the National Fire Protection Association (NFPA) reveal an estimated average of 1210 fires in warehouse properties per year (excluding refrigerated or cold storage) during the five-year period of 2009–2013 (Structure Fires in Warehouse Properties, 2016). Water mist droplets have been proved to have a better ability to extinguish flames particularly around obstructions owing to their longer suspension time of closely following the flow field of combustion gases (Fisher et al., 2007; Sakurai et al., 2013). Water mist with additives has shown good ability in terms of suppression effectiveness and temperature control in high-hazard storages fires (Santangelo and Tartarini, 2012). However, owing to the complexity of fire extinguishing processes, the relationship between a fire scenario and the characteristics of a water mist system remains poorly understood, especially in cases where flames are shielded by obstacles. Thus, a combination of laboratory and computational studies with proper validation is needed (Wang, 2019).

Research on shielded fire suppression started with the use of halon fire-extinguishing agents and then gradually shifted to water mist owing to the Montreal Protocol and International Maritime Organization regulations. Hirst et al. (1977) and Dyer et al. (1977) systematically conducted wind tunnel experiments for pool fire suppression behind an obstacle in terms of the impact of step height, airflow, pressure, and agent mass requirements on extinguishing pool fires by using Halon 1001, Halon 1301, and Halon 1211. Takahashi et al. (2000) studied suppression of a nonpremixed methane flame stabilized by a backward-facing step using Halon 1301 and revealed two distinct flame stabilization and suppression regimes by means of schlieren photography. Grosshandler et al. (2000) conducted further studies experimentally and numerically but used two gaseous fire-extinguishing agents (i.e., Halon 1301 and  N2). Presser et al. (2006) conducted a series of water mist transportation experiments in a homogeneous turbulent flow over unheated and heated cylinders and a body-centered cube arrangement of spheres by using particle image velocimetry. The data, including the airflow and droplet entrainment around the obstacle, were measured. The conclusion was that larger droplets move ballistically, and smaller droplets are entrained into the surrounding aerodynamic flow field. It should be noted that the aforementioned study considered not only a heated obstacle without fire but also confirmed the ability of a water mist to bypass obstacles. Thus, the key is to determine whether and to what extent water mist spray is effective against shielded fires. Chiu and Li (2015) conducted a series of full-scale fire suppression experiments using self-made mist spray nozzles in a wind generator scenario, in which the sheltering conditions of only two fire source locations under containers were considered. A shielded fire situation was addressed in the work of Yu (2012), Yu et al. (2017), who focused on a Froude-based scaling model for fire extinguishment with water sprays and mists. However, there remains no specific conclusion on how to pre-evaluate or optimize the design of a water mist system for shielded fire suppression. Nevertheless, further studies, both experimental and numerical, on extinguishment of shielded fires under different situations with water mist must be conducted.

Therefore, in this study, half-scale fire extinguishing experiments with a water mist that followed Froude-based scaling laws were conducted by varying the nozzle working pressure and the vertical distance between a 10-cm-diameter plate obstacle and the water mist nozzle. To determine the critical cases of fire suppression under different sheltering conditions, various nozzle working pressures and fire sheltering conditions (defined by block ratio) were tested and analyzed. The bypassing motion of the mist droplet and its effect on fire extinguishment were revealed by analyzing the flame temperature, radiant heat flux, and visualization of the fire extinguishing process. Fire Dynamics Simulator (FDS) v. 6.3.2 was employed to simulate the interaction process of water mist and shielded fire. The results of this work may be useful for a theoretical understanding of shielded sand-burner fire suppression mechanisms with water mist and the optimal design of a water mist system in practical application in related areas.

Section snippets

Experiment setup

Fig. 1 shows the small-scale experimental apparatus tested in this work. The entire experimental platform was placed in a 10 m × 6 m × 4 m (length × width × height) enclosure room comprising a gas burner, a water mist system, a baffle plate adjustment system, and a temperature and radiation collection system. Half-scale experiments were conducted using a Froude-based scaling model. Table 1 lists some important scaling parameters. In this study, for scaling fire suppression using a water mist, the

Typical characteristics of water mist

A LaVision ParticleMaster shadow system was used to measure the droplet size. Droplet size measurements performed using a shadowgraphy technique have been widely used and have been verified in terms of reliability in water-based sprinklers such as impinging jet nozzles (Ren and Marshall, 2014) and water mist nozzles (Wang et al., 2018). The spray angle θs was determined based on laser-sheet illuminated spray patterns. The average K-factor of the nozzle was 0.231 L/(min·MPa1/2) at 1.0 MPa to 4.0 

Numerical simulations

Numerical simulation is an effective and low-cost tool that can be used for predicting the fire suppression ability of water mist, because large-scale fire tests are difficult to repeat in large numbers to obtain data for different cases owing to their high cost and potential security risk. Moreover, the fire suppression model must be validated under various shielding situations through a comparison of the experimental results. FDS v. 6.3.2, developed by NIST and based on a large-eddy

Conclusions

A series of half-scale experiments was conducted to determine the critical nozzle working pressure for sand-burner fire extinguishment using a water mist under various shielding conditions. Based on block ratio and plume–spray thrust ratio analysis, critical fire extinguishing conditions were discussed with the visualization of the fire extinguishing process. The interaction between shielded fire and water mist was simulated by using FDS. The main conclusions drawn are as follows:

The empirical

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.

Acknowledgments

The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (No. 51874265), the Key national R&D program (No. 2018YFC0809502), the University Synergy Innovation Program of Anhui Province (No. GXXT-2019-027) and the Fundamental Research Funds for the Central Universities (No. WK2320000046).

References (28)

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