Numerically investigating fire suppression mechanisms for the water mist with various droplet sizes through FDS code

Numerically investigating fire suppression mechanisms for the water mist with various droplet sizes through FDS code

Nuclear Engineering and Design 241 (2011) 3142–3148 Contents lists available at ScienceDirect Nuclear Engineering and Design journal homepage: www.e...

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Nuclear Engineering and Design 241 (2011) 3142–3148

Contents lists available at ScienceDirect

Nuclear Engineering and Design journal homepage: www.elsevier.com/locate/nucengdes

Numerically investigating fire suppression mechanisms for the water mist with various droplet sizes through FDS code Yuh-Ming Ferng ∗ , Cheng-Hong Liu Department of Engineering and System Science, Institute of Nuclear Engineering and Science, National Tsing Hua University, 101, Sec. 2, Kuang-Fu Rd., Hsingchu 30013, Taiwan, ROC

a r t i c l e

i n f o

Article history: Received 16 August 2010 Received in revised form 26 May 2011 Accepted 1 June 2011

a b s t r a c t With rapid progress in computer capability recently, it becomes feasible to investigate the sophisticated phenomena related to the fire, especially for the interaction of fire and water spray, by way of the computational fluid dynamics (CFD). In this paper, a fire simulation CFD code FDS is used to numerically investigate the different droplet sizes on the fire suppression/extinguishment mechanisms. The CFD models adopted in the FDS are first assessed against the previous experimental work of Kim and Ryou. The droplet size interested is varied from 100 ␮m to 1000 ␮m that is located within the droplet size range for a water mist. Based on the sensitivity simulations with different droplet sizes, the dependency of fire extinguishing time on the discharged droplet size can be obtained. The fire extinguishing time decreases with the decreasing droplet size for a mist with relatively fine droplet size since both the evaporation cooling and the oxygen displacement are the dominant mechanisms of fire suppression. However, this trend is reverse for a mist with larger-size droplets for the sake that the direct cooling of flame is the major suppression mechanism. These conclusions are also confirmed by comparing the simulation distributions of gas temperature, oxygen concentration, and steam concentration after the mist actuation and just before the fire extinguishment. © 2011 Elsevier B.V. All rights reserved.

1. Introduction In the nuclear power industry, fire protection programs had been fundamentally changed due to the occurrence of a fire in the Browns Ferry nuclear power plant (NPP) unit 1 on March 22, 1975. Based on 10 CFR 50.48 requirements, the U.S. Nuclear Regulatory Commission (NRC) permitted existing reactor licensees to voluntarily adopt the fire protection requirements contained in NFPA 805 (NFPA 805, 2001). The computational fluid dynamics (CFD) fire model is one of the key tools needed to implement NFPA 805 in NPPs. The phenomena related to the fire, especially fire extinguishing process by the spray system, are one of the most sophisticated topics for CFD simulations. With economical merits and extinguishing capability, water-based systems such as the sprinkler and the water mist, have been still one of the most effective and reliable fire fighting tools in the industry. Especially for a water mist (the size for which 99% of droplet volume (Dv 0.99) < 1000 ␮m, i.e. coarser sprayer (NFPA 750, 2010; Mawhinney, 1994)), it is very suitable and used to suppress the fires in electrical appliances that might be damaged by water wetting. Fire suppression mechanisms for a

∗ Corresponding author. E-mail addresses: [email protected], [email protected] (Y.-M. Ferng). 0029-5493/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nucengdes.2011.06.002

water-based system mainly include the direct cooling of fuel and environmental gases, the displacement of oxygen (O2 ), the attenuation of thermal radiation, etc. However, dominant suppression mechanisms for a sprayer system with fine droplets are different from those with relatively larger droplets. For a spray system with larger droplets, more droplets would penetrate the fire plume and subsequently cool the burning surface and the environmental gases. On the other hand, smaller-size droplets, ejected from a sprayer, tend to be evaporated before contacting a fire and then evaporated steam would enclose its flame, causing the O2 displacement and the fire suppression. Mawhinney (1994) had discussed relative importance of various fire suppression mechanisms such as the heat extraction, the oxygen displacement and the radiant heat attenuation for a water mist. Downie et al. (1995) had studied the suppression of a large methane fire by a water mist. Their results showed a significant reduction in the O2 concentration and an increase in the CO concentration inside a flame as a mist was activated. Using a prescribed heat source, Nam (1996, 1999a) simulated the interaction of fire plume and water spray to investigate the penetration capability of droplets. Novozhilov et al. (1995, 1997a, 1999) combined the water spray model with the fire extinction model to simulate fire extinguishment with a water spray and to investigate the fire plume/water spray interaction. Parasad et al. (1998, 2002) had also conducted a lot of research works in understanding the interaction of watermist with the fire and its flame.

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The interaction of water mist on polymethylmethacrylate (PMMA) fires in a confined space under different external radiant heat fluxes had been studied by Qin and Chow (2005). Their results revealed that a water mist would suppress diffusion flames essentially by the O2 displacement, the evaporation cooling and the radiant heat attenuation. Using 1-D two-phase model, Chelliah (2007) numerically investigated the variation of optimal droplet size on the flame inhibition/suppression. Neophytou and Mastorakos (2009) used one dimensional (1-D) code with the detailed chemistry and transport equations to study the effects of droplet diameter, overall equivalence ratio and droplet residence time on the flame propagation. Nmira et al. (2009) also developed a two-phase model to investigate the efficiency of water mist systems in mitigating thermoplastic fires in a tunnel. The majority of this paper numerically investigates the mechanisms of fire suppression for the water mist with various droplet sizes through the FDS 5.2 code (McGrattan et al., 2007). The FDS is one of five fire models that had been verified and validated by NRC (NUREG-1824, 2007), which is suitable to be analytical tool for the fire protection programs of a nuclear power plant. An experimental work (Kim and Ryou, 2003) previously conducted in an enclosed compartment of 4.0 m × 4.0 m × 2.3 m with the water mist is used to validate the present CFD fire model. In addition, under the similar solution domain and the mist with various droplet-sizes, the relationship of fire extinguishing time and the discharged droplet sizes can be also simulated. The simulation range of droplet sizes chosen in this paper is varied from 100 ␮m to 1000 ␮m. 2. Fire suppression models There are two main approaches to treat the gas-droplet twophase phenomena occurred in the fire suppression by a water spray, which include the Eulerian–Eulerian approach (Hoffmann and Galea, 1993a,b; Hassan, 1996; Parasad et al., 1998, 2002; Consalvi et al., 2004; Nmira et al., 2009) and the Eulerian–Lagrangian approach (Chow and Fong, 1991; Nam, 1996, 1999b; Novozhilov et al., 1995, 1997b,c, 1999; Chelliah, 2007; Collin et al., 2007, 2008; Neophytou and Mastorakos, 2009), respectively. In the FDS, the Eulerian–Lagrangian method is adopted to treat the gas phase as a continuum and the water droplets as the individual dispersed particles. The momentum, heat and mass transport between both phases are considered through the various interfacial transfer models. The governing equations adopted in the FDS code for modeling gas and droplet thermal-hydraulic behavior, droplet and flame interaction, and fire suppression characteristics consist of the Eulerian equations for the continuous gas phase and the Lagrangian equations for droplet dispersed phase, respectively. The Eulerian equations include the continuity, momentum, energy and state equations, and large eddy simulation (LES) turbulence model (McGrattan et al., 1998; Zhang and Chen, 2000; Wang et al., 2002). The Lagrangian equations are composed of position, mass, momentum, and energy. The thermal radiation model (Tuntomo et al., 1992) for the attenuation of liquid droplets is also included. Detailed descriptions of above equations can be referred to the FDS manual and the author’s previous works (Lin et al., 2009; Ferng and Lin, 2010; Ferng and Liu, 2011). As the aforementioned description, the main task of this paper is to investigate the effects of droplet sizes on the fire suppression mechanisms. In order to focus droplet size effects, the present simulations assume that all droplets with a constant size are discharged from a mist. In addition, the physical phenomena associated with fire suppression by a water spray are so complicated for CFD simulations. In addition to cooling the fuel and the surrounding gas, water droplets from a spray also change the pyrolysis rate of fuel. Therefore, a simplified fire suppression model is adopted in the FDS. Based on the experiments of Hamins and McGrattan (2002),

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the exponential nature of fire suppression by water is observed. The local burning rate of fuel can be assumed to follow this trend. That is ˙ f (t) = m ˙ f,0 (t) e m





k(t) dt

(1)

˙ f (t) m

˙ f,0 (t) m

where is the burning rate per unit area of fuel and is that when no water is applied. ˙ w ) k(t) is a linear function of the local water mass per unit area (m and can be expressed as k(t) = amw (t)

(2)

a is a proportional constant and is set to be −1.0 (Hamins and McGrattan, 2002). Then, as water droplets encounter with a fuel, its corresponding heat release rate (q˙  ) is reduced and followed by f ˙  q˙  =m (t) Hf f f

(3)

where Hf is the reaction heat per unit mass. 3. Results and discussion In order to validate the fire suppression models adopted in the FDS code, an experiment from the previous work (Kim and Ryou, 2003) is first simulated. As schematically shown in Fig. 1, the test domain is an enclosed compartment of 4.0 m × 4.0 m × 2.3 m with a square hole of 0.38 m on the top. A rectangular methanol pan with the dimension of 30 cm × 30 cm × 5 cm is set on the center of room floor and five water mist nozzles are installed at 1.8 m above the floor. The Ceiling gas temperature is measured with four thermocouples located at 1.8 m above the floor. These thermocouples are the aluminum sheathed K-type ones. Simulation conditions and nozzle specifications are indicated in Table 1.

Fig. 1. Schematic of simulation compartment.

Table 1 Simulation conditions and nozzle specifications. Compartment dimension (m) Fuel pan size (m) Heat release rate of fuel (kW) Vent area (m) Vent flow velocity (m/s) Nozzle specifications Sauter mean diameter (␮m) Orifice diameter (mm) Operating pressure (bar) Flowrate (l/min) K-factor Spray pattern Spray angle

4 × 4 × 2.3 0.3 × 0.3 26.64 0.38 × 0.38 5 121 3 13 6 1.66 Hollow cone 70–90◦

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Fig. 3. Time histories of mean ceiling gas temperature for experimental measurement and model prediction.

gradually. The former is named as the sudden cooling regime and the latter is the gradual cooling regime. The predicted results can reproduce these characteristics and correspond well with the measured ones. Defined as the critical cooling time (Kim and Ryou, 2003), a duration of sudden cooling regime is considered as a fire extinguishing time herein and is a key parameter to determine the performance of a water mist. The fire extinguishing time predicted by the present CFD fire model is near 24.5 s, which is close to the measured one of 27 s. Under the same domain and conditions, sensitivity simulations are conducted to investigate the fire extinguishing mechanisms for a water mist with different sizes of water droplets. The droplet sizes chosen in this paper are ranged from 100 ␮m to 1000 ␮m. The relationship between the fire extinguishing time and the droplet sizes of a water mist is shown in Fig. 4. The shorter the extinguishing time is, the better the performance of a water mist is. Three distinct

Fig. 2. Calculated fire suppression phenomenon at the different times after the actuation of water mist.

In the experiment, a water mist is actuated at 70 s after the fire ignition. Discharged from the nozzle, mists would suppress the fire by the direct cooling, the evaporation cooling, or the O2 displacement. As shown in Fig. 2, fire suppression characteristics at the different times after the actuation of a water mist are simulated. It can be clearly demonstrated in this figure that the fire is gradually suppressed by the continuous discharge of mists. In addition, as the time exceeds 24 s, the fire is predicted to be completely extinguished by the mists. Fig. 3 compares the time history of mean ceiling gas temperature between experimental measurement (green line) and model prediction (red dash line). The temperature of hot gas layer is drastically cooled down after the activation of a water mist due to the evaporation heat removal from the mists. After the fire is extinguished by the mists, the ceiling temperature would decrease

Fig. 4. Relationship between fire extinguishing time and discharged droplet size.

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Fig. 5. Shaded contours of steam fraction for the water mist with droplet size of 100 ␮m at (a) time = 1 s after mist activation and (b) time = 23 s before fire extinguishment (b).

regions are clearly observed in this figure. As the discharged droplet size of a water mist is less than 500 ␮m (Region I), the extinguishing time would increase (i.e. the mist performance decreases) with the increasing droplet size. Travelling into the flame, smaller droplets discharged from a water mist would be evaporated. This phenomenon causes the cooling of surrounding gas and the reduction of O2 concentration due to the large amount of steam generation. Lack of sufficient O2 and high temperature, the fire would be suppressed. With the relatively higher specific surface, smaller droplets would result in better heat transfer and more rapid vaporization. Therefore, the water mist with finer droplet sizes has the better performance of fire suppression. However, this trend is reversed as the discharged droplet size from a water mist is between 500 ␮m and 750 ␮m (Region II). In this region, as the droplet size increases, the fire extinguishing time would decrease, in other words, the mist performance would increase. Before being completely evaporated, the droplets with larger sizes would have enough mass and momentum to directly impinge into a fire. This phenomenon directly cools the burning fire surface, which is named as the direct cooling of fire suppression. The larger-size droplets have higher mass and momentum, which can enhance the direct cooling effect and subsequently elevate the performance of fire suppression. Therefore, the extinguishing performance of a water mist increases as the increase in the droplet size, which is clearly revealed in Region II of Fig. 4. It is also shown in this figure that the performance of fire suppression

slightly decreases with the increasing the droplet size, as its size is larger than 750 ␮m (Region III). This calculated result is mainly obtained under the simulation assumption of same water injection flowrate. The number of droplets would be substantially reduced as their sizes increase under the same flowrate condition, which may decrease the capability of fire suppression. Without interference of other parameters, the present work is focused on the sensitivity study of one parameter only, i.e. droplet size. However, in the real application, the injection rate would be raised as the relatively larger-size droplets are used in a water mist. This predicted Region III may not be occurred. As the aforementioned description, the O2 displacement is the main mechanism to suppress a fire for a water mist with smallersize droplets. The direct cooling mechanism is responsible for the fire extinguishment for that with larger-size droplets. These can be also confirmed by the simulation results shown in Figs. 5–10. Fig. 5(a) and (b) shows the shaded contours of steam fraction for a water mist with the droplet size of 100 ␮m at the time = 1 s after the spray activation and the time just before the fire extinguishment (23 s), respectively. These 2-D plots are presented by cutting the central plane of solution domain. As a water mist discharges the droplets of 100 ␮m, most of droplets are quickly evaporated due to high temperature from the fire, consequently causing the sudden increase in the steam fraction along the trajectory of droplet flow. This phenomenon is clearly revealed in Fig. 5(a). As shown in Fig. 5(b), enough steam generated from the droplet evaporation

Fig. 6. Shaded contours of oxygen concentration for the water mist with droplet size of 100 ␮m at (a) time = 1 s after mist activation and (b) time = 23 s before fire extinguishment (b).

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Fig. 7. Shaded contours of oxygen concentration for the water mist with droplet size of 1000 ␮m at (a) time = 1 s after mist activation and (b) time = 26 s before fire extinguishment (b).

surrounds the fire. In the mean time, the generated steam would displace the rest of gases including O2 , causing a great reduction in the O2 concentration. This phenomenon of oxygen displacement can be clearly revealed by comparing the distribution of O2 concentration around the fire between at 1 s after the spray activation (Fig. 6(a)) and just before the fire extinguishment (Fig. 6(b)). It is also revealed in Fig. 6(b) that the O2 concentration is greatly displaced by the steam generated from the droplet evaporation just before the fire extinguishment. On the contrary, the direct cooling of fire itself is the main mechanism of fire suppression for a water mist with relatively large droplet size. In the present simulation conditions, the direct cooling is dominant for the fire suppression as the droplet size is larger than 500 ␮m. This suppression mechanism is essentially to reduce the flame temperature as well as it surrounding gas temperature, which is clearly confirmed in Fig. 7. This figure shows the 2-D shaded contours of temperature for the water mist with the droplet size of 1000 ␮m at 1 s after the spray activation (a) and the time just before the fire extinguishment (26 s) (b). It is revealed in Fig. 7 that the

flame temperature is directly cooled down by larger-size droplets from about 800 K at the mist activation (plot (a)) to about 600 K just before the fire extinguishment (plot (b)). The corresponding gas temperature is decreased from about 600 K to about 400 K. In order to clearly demonstrate the different fire suppression mechanisms by various droplet sizes discharged from a water mist, the transient line curves for the gas temperature, the steam fraction, and the O2 concentration are presented, respectively. The observed point is selected at the hottest point near the flame. Fig. 8 shows the gas temperature histories for a water mist with the droplet sizes of 100 ␮m (blue) and 1000 ␮m (red), respectively, at the observed point from 10 s before the fire extinguishment. For the droplet size of 1000 ␮m, the calculated peak gas temperature near the flame is continuously decreased and the temperature is not high enough to support the fire burning, revealing that the direct cooling of flame is the main mechanism of fire suppression. As discussed above, the fire is essentially extinguished by the droplet evaporation and the O2 displacement for a water mist with smaller-size droplets. There-

Fig. 8. Time histories of gas temperature for the mist with droplet sizes of 100 ␮m and 1000 ␮m at the observed point from 10 s before fire extinguishment.

Fig. 9. Time histories of steam fraction for the mist with droplet sizes of 100 ␮m and 1000 ␮m at the observed point from 10 s before fire extinguishment.

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mechanisms are so sophisticated that the CFD models included in the FDS 5.2 cannot completely simulate these phenomena, such as the influences of temperature and O2 concentration changes on the radiative transfer, surface cooling, pyrolysis modification, etc. However, the FDS is one of the NRC licensing fire codes. The present simulation results can provide the useful information to assist the research in performing the fire suppression simulation for the nuclear reactor safety (NRS) using the FDS code. References

Fig. 10. Time histories of oxygen concentration for the mist with droplet sizes of 100 ␮m and 1000 ␮m at the observed point from 10 s before fire extinguishment.

fore, just before the fire extinguishment, the gas temperature is close to the saturated temperature (373 K) due to the droplet evaporation for 100 ␮m droplets, as clearly shown in the blue line of Fig. 8. The corresponding time histories (Fig. 9) of steam fraction for both droplet sizes also confirm it. As clearly shown in Fig. 10, the O2 concentration for a water mist with the smaller-size droplet (100 ␮m) is reduced to ∼14.5%, which cannot sustain a fire. However, the corresponding O2 concentration for the larger-size droplet (1000 ␮m) is still kept to ∼16%. These calculated results indicate that the droplet evaporation is essentially the dominant mechanism of fire suppression for a water mist with relatively small-size droplets. 4. Conclusions The main objective of this paper is to investigate the fire suppression mechanisms for the water mist with various droplet sizes through the FDS 5.2 code. The CFD fire models adopted in the FDS have been validated against the experimental work of Kim and Ryou. The relationship between the fire extinguishment time and the discharged droplet size would be obtained from the sensitivity results of the present simulation work. A large amount of steam generated from the evaporation of finer droplets would displace the O2 content, causing the reduction in its concentration. According to the present simulations, as the droplet size is less than 500 ␮m, the O2 displacement mechanism dominates the fire suppression, causing that the performance of fire extinguishment increases with the decreasing droplet size. However, relatively larger-size droplets are capable of penetrating fire plume and directly impinge on the fire flame. This direct cooling mechanism is obviously enhanced, as the droplet size is larger than 500 ␮m. That is, the better the fire suppression is, the larger the droplet size is. This trend would reverse if the discharged droplet size is larger than 750 ␮m. This conclusion is essentially resulted from the simulation assumption of same water injection rate from a mist with various droplet sizes. It should be noted that above conclusions are based on the simulation results of the FDS 5.2. The threshold value of droplet sizes for governing different fire suppression mechanisms is just qualitatively, but not quantitatively, predicted since the fire suppression

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