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Fire Technology, 45, 341–354, 2009 Ó 2008 Springer Science+Business Media, LLC. Manufactured in The United States DOI: 10.1007/s10694-008-0068-2

Evaluation of Surfactant Enhanced Water Mist Performance Georges LeFort, Ecole Nationale Supe´rieure de Me´canique et d’ Aerote´chnique (ENSMA), Futuroscope Chasseneuil Cedex 86960, France Andre´ W. Marshall*, Department of Fire Protection Engineering, University of Maryland, 0151 Glenn L. Martin Hall, College Park, MD 20742, USA Martial Pabon, DuPont Chemical Solutions Enterprise, Chantereine, Mantes la Jolie, France Received: 22 March 2006/Accepted: 17 September 2008

Abstract. Water-mist technology provides efficient fire suppression for compartments while minimizing water usage. Even with the many advantages of water mist systems, there is still room for improvement. Water mist systems have demonstrated effectiveness at suppressing flammable liquids (Class B) fires in compartments. However, an especially challenging fire suppression scenario for water mist systems is the small Class B fire. This scenario is often realized after a large fire has been reduced in size or ‘controlled’ by water mist. The small fire scenario is challenging because a small fire may not be able to generate enough vaporized water to displace sufficient oxygen for complete extinction. It should also be noted that even if the Class B fire is extinguished with a water mist system, re-ignition from the hot surrounding surfaces may occur at any time. In the present work, an additive is introduced into the water supply and its effect on the water mist suppression performance is studied. This ForafacTM additive is a specific formulation, which includes fluorinated surfactants for creating a robust fire suppression foam. The enhanced suppressant exiting the mist nozzle is dispersed in the form of small droplets (not as a continuous foam) similar to a pure water mist spray. However, these droplets create a foam blanket on the surface of the fire, which acts to isolate the fuel from the air. With this formulation, the efficiency of the water mist system is improved even on small fires and most importantly the re-ignition of class B fires is prevented. Keywords: surfactant, water mist, suppression, class B fires

1. Introduction The potential benefits of surfactant enhanced water mist are evaluated in this study. The focus application for surfactant enhanced water mist is the protection of machinery spaces. Water mists are often used to suppress fires in confined spaces as an alternative to gaseous agents. Foams are commonly used to extinguish Class B liquid fires such as those created by leaks or spills. The mist foam

* Correspondence should be addressed to: Andre´ W. Marshall, E-mail: [email protected]

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combination is a logical choice for machinery spaces because these spaces are often confined and full of flammable liquids.

1.1. Water Mists Water mists have been studied for at least 50 years [1]. During the past decade environmental, economical issues, and technological breakthroughs have brought this technology to the forefront. Water mists are defined by NFPA 750 as a water spray for which the DV99 (99% volume diameter) as measured at the coarsest part of the spray in a plane 1 m from the nozzle, at its minimum operating design pressure, is less than 1000 lm [2]. A variety of water mist technologies are available for creating these small drops [3]. In this study, an intermediate pressure (12.1–34.5 bar) water mist nozzle is used. The small drops in a water mist create a large surface area (for a given volume of water) to enhance vaporization. The vaporization of the mist provides both oxygen displacement and cooling. These effective suppression mechanisms allow water mists to use less than one tenth of the water necessary for standard sprinklers, providing a distinct advantage over sprinklers in terms of water supply. The decreased water delivery requirements also reduce the potential for water damage to sensitive equipment. One of the most attractive advantages of water mist is that it does not splatter liquid fuel like conventional sprinklers during suppression of Class B fires. The efficacy of water mist fire suppression has been demonstrated in numerous studies and in a wide range of applications including Class B pool fires, spray fires, fires in aircraft cabins, shipboard machinery, engine room spaces, shipboard accommodation spaces and electronics applications [4]. For machinery spaces on ships and turbine enclosures, water mists are helpful to extinguish fires and moreover to cool hot surfaces to prevent possible reignition.

1.2. Foams Foams are made of foam concentrate, water, and air (typically >80%) [5]. Surfactants are the principle components of the foam concentrate. A variety of surfactants are used in the foam concentrate to not only facilitate the formation of foam, but also to enhance its spreading characteristics. Surfactants are long molecules consisting of a hydrophilic head and a hydrophobic tail. These surfactants are suspended in liquid solutions at low concentrations. At a free surface, the surfactants adopt a preferred orientation where the hydrophobic head is attracted toward the water and the hydrophobic tail is repelled away from the water toward the gas. This behavior is illustrated in Figure 1. The polar separation forces created by the surfactant at the liquid–gas interface facilitates organization of the two fluids into foam. Typically, foam is formed by entraining gas into liquid. This can be accomplished through direct injection or agitation. The forces created by the surfactants near liquid–gas interfaces in this mixture act to trap liquid in thin layers or ‘lamella’ between volumes of gas producing foam [6]. The surfactants also reduce the surface tension of the liquid. In fire suppression applications, this effect is exploited to help spread a protective film of water and foam over liquid fuel sur-

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Figure 1. [6].

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Surfactant molecules stabilizing a thin liquid film in foam

faces as shown in Figure 2. Surfactants can be fluorinated or hydrocarbon based, but fluorinated surfactants have a lower surface tension and associated exceptional filming characteristics. Hydrocarbon surfactants, however, have superior foaming characteristics. Fluorinated and hydrocarbon surfactants are typically used together in fire suppression applications to take advantage of their respective performance benefits. Fire fighting foams are commonly used for extinguishing spill fires involving flammable liquids such as polar solvents (i.e. methanol) and non-polar solvents

Foam

Spread Film

Fuel

Figure 2. surface.

Aqueous film forming foam (AFFF) spreading over fuel

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(i.e. heptane). To extinguish liquid fires, foam must be quickly spread at the surface of the fuel to prevent fuel gasification and reduce heat loading to the fuel surface [5]. Foams are also used to maintain a protective barrier at the fuel surface in order to prevent these substances from igniting or reigniting. The protective barrier formed by the foam is only temporary. Over time, the high radiant flux from the fire or surrounding hot surfaces acts to destroy the protective barrier established by the foam [7].

1.3. Surfactant Enhanced Water Mist Addition of surfactants into the water mist supply has the potential of providing the cooling and oxygen displacement benefits of water mist combined with the fuel isolation advantages of foam. The author is only aware of one other study, which has tested the effect of surfactant additives on the suppression performance of water mists [8]. In this study, Kim et al. performed suppression experiments using swirl-type nozzles and standard pendant sprinklers with both standard foaming additives and aqueous film forming foam (AFFF) additives. These suppression tests were conducted in a variety of nozzle orientations on Class B and wood crib fires in a well-ventilated enclosure. The characteristic drop sizes in the water sprays were measured in this investigation for both nozzle types; however, no measurements were reported for the sprays with additives. The suppression tests revealed that the sprinkler had little effect on the Class B fires. In contrast, the pure water mist was able to control, but not extinguish the Class B fires. When surfactants were added, the Class B fires were easily extinguished. However, Kim noted that much higher concentrations of AFFF additives (1–3%) were required to achieve suppression performance similar to standard foaming agents used at a much lower concentration (0.3%). A thin foam layer was reported to grow on the surface of the liquid fuel fires for all of the tests with additives. Kim’s study focused primarily on evaluating suppression performance. The burnback characteristics of the post-extinction foam layer were not considered. In order to realize the benefits of surfactant enhanced water mist, the mist must behave like a vaporizing spray after leaving the injector and behave like a spreading foam upon arrival at the fuel surface. In the current investigation, the behavior of the surfactant enhanced water mist is evaluated with respect to these features from the injector exit to the fuel surface. Ultimately, this behavior will govern the extinction and reignition delay (burnback) performance. The specific objectives of this investigation are to characterize the surfactant effects on spray properties, extinction times, and burnback times in Class B fires. These objectives are achieved through spray experiments, fire suppression experiments, and re-ignition delay (burnback) experiments.

1.4. Surfactants and the Environment A proprietary surfactant (Forafac WMTM) was used in this study. Forafac WMTM is a recipe containing various chemicals including a partially fluorinated surfactant. In 2002 the primary manufacturer of fluorinated surfactants for fire fighting foams stopped their manufacturing activity largely because of a potential

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persistent bioaccumlative and toxic (PBT) chemical issue. This supplier was using the electrochemical fluorination (ECF) process to manufacture their fluorosurfactants for fighting foams. This ECF process made PFOS-derived products and the resulting fire fighting foams contained various residual levels of PFOS (Perfluorooctane sulfonate). An Organization for Economic Co-operation and Development (OECD) hazard assessment has been performed on the basis of information that was available in 2002. This assessment concluded that perfluorooctane sulfonates are persistent, bioaccumulative and toxic to mammalian species and therefore may indicate cause for concern.1 The fluorinated surfactant present in Forafac WMTM is manufactured by DuPont using the telomerization process. The process used to make the fluorinated surfactant present in Forafac WMTM does not use or contain PFOS either as active ingredient, as a byproduct, or as a degradation product. Furthermore, the surfactant Forafac WMTM has been designed to provide optimal fire suppression performance while using a minimum amount of additive in water. Knowing that the best results in terms of fire suppression are obtained on Class B fires, the use of Forafac WMTM is recommended on this specific type of fire. It should also be noted that water mist systems are installed in closed rooms. Hence, after a fire event, aqueous effluents and wastes can be recovered and treated as appropriate [5]. A recent toxicological study [9] demonstrated that Forafac WMTM is considered to have very low toxicity by inhalation. In fact, at design concentration, it is not classifiable as a dangerous substance (LC50 > 5 mg/l) according to the Official Journal of the European Communities EEC Directive 93/21.

2. Approach Spray characteristics, suppression performance, and burnback performance were evaluated at various concentrations of Forafac WMTM. The surfactant was thoroughly mixed with water in the water supply reservoir prior to testing. Spray characterization and suppression experiments were also conducted using pure water to establish a performance baseline. The spray characterization was conducted using a spray chamber facility. High speed photography was used to evaluate changes in the atomization process resulting from surfactant addition. Drop sizes were also measured with a Malvern particle size analyzer to determine the impact of surfactant addition on the drop size distribution of the spray. The suppression and burnback experiments were conducted in a burn room using a test configuration relevant to machinery spaces where Class B fires are a hazard. The type of fuel, fire size, and nozzle type were selected to reflect major fire fighting foam and water mist standards and typical fire scenarios for machinery spaces.

1

European parliament legislative resolution on the proposal for a directive of the European Parliament and of the council relating to restrictions on the marketing and use of perflurooctane sulfonates (amendment of council directive 76/769/EEC).

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2.1. Spray Characterization Spray characteristics of a Tyco-Grinnell intermediate pressure AquaMist AM4 nozzle are evaluated in this investigation. The AM4 nozzle is illustrated in Figure 3. The spray behavior is described through photographs taken near the injector and drop size measurements performed in a plane well below the injector. Short time exposure photographs were taken with a Canon D30 digital SLR camera and a 550EX flash. The flash setting was fixed at its minimum setting of about 1/6400 s. Even with this short exposure time, clear images of the spray could only be obtained at operating pressures below 1.72 bar which is much lower than the recommended operating pressure range. A Malvern Spraytec particle size analyzer was used to measure the drop sizes at 0.5 m below the injector. Drop sizes were measured at five radial locations extending from the centerline to 0.6 m. An exposed probe volume of approximately 12.3 cm3 was used for these measurements. An average planar drop size distribution is calculated from this profile. The average is weighted based on the drop concentration and flux area assuming axisymmetric flow.

2.2. Suppression Experiments The experiments were designed to produce understandable and easy to compare results for evaluation of combined water mist and foam performance under realistic conditions. Many fire tests for water mist have already been conducted [10–13]. In this study, an available small-scale burn room was used having a total volume of 9.1 m3 having two opposing vents of 1.6 and 0.093 m2 and one overhead vent of 0.067 m2. An illustration of the suppression experiment configuration is provided in Figure 4. A typical intermediate pressure water mist nozzle was selected for this investigation as described previously. Only one nozzle was used because of the small burn room volume. A fire size of 210 kW was selected to adequately challenge the nozzle. The fire size is based on quasi-steady measurements during free burn experiments. This fire was created with a 0.53 m diameter pan of heptane. The centerline of the pan was placed 0.56 m from the centerline of the nozzle. An

Figure 3. Medium pressure Tyco-Grinnell AquaMist AM4 nozzle used in this study.

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Figure 4. Burn room configuration for suppression and burnback experiments, V = 9.1 m3.

off-axis fire location was chosen to improve the repeatability of the extinction time measurements. The primary focus of the suppression experiments was to quantify the extinction time. Detailed measurements were also conducted to characterize the suppression behavior. Thermocouple measurements were taken at ten equally spaced locations above the fire at elevations ranging from 0.28 to 1.9 m above the floor. Type K, 0.010’’ wire exposed junction thermocouples were used for these measurements. A target was also placed 0.53 m (one pool diameter) from the centerline of the pool fire to evaluate heat loading to objects in the vicinity of the pool fire during suppression. The target is instrumented with a Gardon heat flux gauge and five Type K surface thermocouples. Digital video was also acquired during the suppression experiments for analysis of the fire size, visualization of the foam formation, and visualization of the flame behavior near the fuel surface. The surfactant concentrations are expressed in terms of mass percentage of active ingredient. Multiple tests were conducted for each suppressant to ensure that the data is repeatable. For each test, a free burn was conducted for 60 s to allow the pool fire to reach a quasi-steady burning rate. The nozzle was activated at the end of the free burn period and suppressant was applied until extinction or until all of the fuel is consumed. The nozzle was operated at an injection pressure of 12.1 bar for all tests delivering approximately 0.73 m3 h-1 of water. Without the fire, the application rate to the pan was measured to be 3.0 kg min-1 m-2. This experimental approach provides extinction times as well as detailed suppression data to facilitate analysis and comparison of this extinction time performance for each of the suppressants.

2.3. Burnback Experiments After extinction, the nozzle was allowed to operate for an additional 60 s to completely coat the fuel surface with suppressant. After the nozzle was shut-off, a

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small pan (0.15 m diameter) filled with heptane fuel was placed in the center of the large pool. This small pan was ignited exactly 180 s after completion of the coating period. The time to reignite 50% of the large pool was determined. This burnback time is defined as the reignition time. Digital video of the foam degradation and reignition was also recorded and evaluated during these experiments.

3. Results 3.1. Spray Characterization Atomization behavior is compared using pure water and a surfactant–water mixture with the AM4 intermediate pressure nozzle. Figure 5 shows the short exposure time photographs of the atomization process. The general atomization mechanisms are clearly observed for both pure water and the 0.05% Forafac WMTM mixture. The jet is first deflected to form a sheet. Aerodynamic waves grow on the sheet and break into ligaments. These ligaments ultimately break to form drops. The Forafac WMTM image shows numerous discrete drops being formed near the injector just as in the pure water case. It can be concluded that the spray is dispersed in the form of discrete drops even when surfactant is added to the water. However, the Forafac WMTM image shows a more opaque spray with larger fragments being created near the frame arms. This may indicate that some foam is generated at the injector and that the drop size is altered by the addition of surfactant. Drop size measurements taken 0.5 m below the nozzle reveal that the drop sizes increases with the addition of surfactant. The measured drop size distributions for

Figure 5. a Atomization of pure H2O. b Atomization of a surfactant– H2O mixture (0.05% ForafacTM) using a medium pressure AquaMist AM4 mist nozzle. Nozzle operated at a low pressure 25 psi to image details of the initial spray.

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pure water and 0.20% Forafac WMTM are provided in Figure 6. It is clear from the measured distributions that the drop size increases with the addition of surfactant. Characteristic drop sizes were calculated from these distributions and are presentedP in Table P 1. The Sauter Mean Diameter (SMD) is given by SMD ¼ ni D3i = ni D2i summed over all possible drop sizes in the measured spray. The SMD represents a characteristic drop size having the same volume to diameter ratio as the measured spray. Alternatively, the DV50 provides a volume based characteristic drop size: 50% of the spray volume is contained in drop sizes smaller than DV50. Both characteristic drop sizes increases by more than 40% with the addition of 0.20% ForafacTM.

Figure 6. Measured volume distributions for pure H2O and 0.2% ForafacTM in a plane 0.5 m below the AM4 nozzle; pure H2O and 0.2% FORAFAC volume distributions overlap (indicated with dark gray). Table 1

Drop Size Measurements of Pure H2O and Surfactant– H2O Mixtures. Drop Size Measurements Represent Planar Averages 0.5 m Below the Nozzle Concentration of active matter SMD (lm) Dv50% (lm)

Pure H2O 133 217

0.05% ForafacTM 167 265

0.2% ForafacTM 187 306

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This drop size increase is surprising because surfactant reduces the surface tension, which should facilitate atomization. The conventional effect of surface tension on atomization can be understood through the Weber number, We ¼ qU 2 L=r, of an arbitrary volume of liquid having characteristic length L where the density, q, and relative velocity, U, are best thought of as representing gas quantities. The We describes the ratio of inertial forces to surface tension forces. At critically high We, the inertial forces and associated shear overcome the surface tension forces causing the liquid volume to deform and fragment. If the surface tension is reduced, the We becomes supercritical at smaller characteristic lengths and corresponding smaller drop sizes. However, the drop size measurements do not support this argument. The previous We argument does not strictly apply to surfactant enhanced flows. Although surfactants reduce surface tension, it does so slowly. The time scales for surface tension reduction by surfactant diffusion and re-orientation on ‘fresh’ fluid surface is on the order of seconds. However, the entire spray surface is generated via atomization in milliseconds. Therefore, the surfactant does not have sufficient time to reduce the surface tension before atomization is completed. During the atomization process, the surface tension of the surfactant–water mixture is essentially the same as that for pure water. Recognizing the disparity in time scales between surface creation and surface tension reduction explains why the drop size should not decrease with surfactant addition, but does not shed light on why the drop size increases. The increase in drop size may be explained by agglomeration or shear induced foaming as the drops travel through the air. These possibilities continue to be explored, but as of yet no precise explanation has been determined for this phenomenon.

3.2. Suppression Experiments The suppression experiments provide insight into the qualitative and quantitative behavior of surfactant enhanced water mist. Figure 7 shows the transient average surface temperature of the target placed inside of the compartment. Different stages in the test are clearly observed in this figure. A free burn stage is observed, followed by a cooling stage, and finally a suppression stage. It should be noted that this suppression stage is completed by extinction in the cases with surfactant; however, the pure water is unable to extinguish the fire. Heat flux measurements to the target also show similar trends. The suppression behavior can be seen more clearly in the thermocouple measurements taken above the pool. It is important to note that these thermocouple measurements should not be interpreted as gas temperatures. These thermocouples are cooled by impinging water drops making it difficult to relate the measured temperature to the gas temperature. However, these measurements provide useful qualitative information related to the flame height. Figure 8 shows the transient temperature distribution for pure water and for 0.20% ForafacTM. It is clear that the pure water mist quickly cools the compartment and limits the flame height; however, the flame persists at its suppressed condition. The 0.20% ForafacTM mixture also quickly cools the compartment but continues to reduce the size of the flame until it is completely extinguished. The

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Figure 7. Average target surface temperatures illustrating path to extinction for surfactant-H2O mixtures.

path to extinction can also be observed in the fire suppression video images. Figure 9 shows snapshots of the fire at various times after activation of the nozzle for the 0.20% ForafacTM case. After 20 s of application, islands of foam are clearly seen on the surface of the pool. By 60 s, these islands of foam have connected forming a nearly continuous layer of foam. This foam layer limits the fuel surface available for burning and drastically reduces the fire size. The exposed fuel surface continues to decrease until the fire is completely detached from the surface (blown out) resulting in extinction. The impact of the surfactant addition is clearly seen in Table 2. A fire that could not be extinguished with pure water is easily extinguished with 0.05% ForafacTM in about 214 s and about 90 s with 0.20% ForafacTM.

3.3. Burnback Experiments After the pool fire is extinguished the nozzles remains on for 60 s. During this period of time the foam completely coats the surface of the fuel. The mass flux during this period is approximately 3.0 kg m-2 min-1. Table 3 shows the burnback times describing how long the pool is protected from reignition. With 0.05% ForafacTM, the pool is protected for nearly 9 min and with 0.20% Forfac the pool is protected for over 10 min. This data indicates that burnback resistance may be improved further by increasing the surfactant concentration.

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Figure 8. Thermocouple measurements above the pool during a pure H2O suppression experiments showing a persistent fire. b 0.20% Forafac suppression experiments showing fast and complete extinction.

Figure 9. Photograph of foam spreading and flame size reduction during 0.20% Forafac suppression experiments.

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Table 2

Extinction Time Measurements Concentration of active matter

Pure H2O

Test 1 extinction time (s) Test 2 extinction time (s) Average extinction time (s)

NA* (460) NA* (429) NA* (435)

0.05% Forafac

0.2% Forafac

196 231 214

70 110 90

*Extinction times in parenthesis result from depletion of fuel in the pan.

Table 3

Re-Ignition (Burnback) Time Measurements Concentration of active matter Test 1 burnback time (s) Test 2 burnback time (s) Average burnback time (s)

0.05% Forafac 540 511 526

0.2% Forafac 639 647 643

4. Conclusions This study evaluates the potential benefits of surfactant enhanced water mist through characterization of spray properties, suppression behavior, and re-ignition behavior. High-speed photographs reveal that the behavior of the spray, the breakup length, and even the spray pattern are not fundamentally modified with the addition of surfactant; although drop size measurements show that the drop size unexpected increases for surfactant–water mixtures. A possible explanation of the measured drop size increase could be an agglomeration or shear foaming phenomenon, but as of yet no precise explanation has been determined. Visualization experiments are currently being refined to gain insight into this surprising behavior. Suppression mechanisms are maintained or improved when surfactant–water mixtures are used with medium pressure water mist nozzles. The cooling stage remains virtually the same as that of pure water for surfactant–water mixtures. However, the overall extinction behavior is improved with surfactant addition since foam is formed on the fuel surface and gradually reduces the fuel surface exposed to oxidizer. This research clearly demonstrated that a water mist generated with pure water is unable to extinguish a pool fire that is easily extinguished when surfactant is added. Moreover surfactant provides a burnback or re-ignition resistance by creating a foam layer at the fuel surface. The re-ignition is significantly delayed with surfactant whereas no protection is provided by pure water. The burnback resistance is shown to increase with increasing concentration.

Acknowledgements This work is supported by DuPont de Nemours (France). Special appreciation is given to Ms. Wu Di and Messrs. Yaniv Yankovich and Andy Blum for their help in establishing facilities and conducting the experiments used in this investigation.

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