Developing fire suppression systems that meet the requirements of specific firefighting applications is an important goal. For certain fires, applying water in a fine spray or mist can be particularly effective, reducing water usage and minimizing equipment damage. In situations requiring fuel isolation, the use of foams is particularly advantageous. A system combining the benefits offered by water mists with those of fuel isolation is one of the aims of current research. Such a system targets difficult environments such as engine compartment fires, which are traditionally extinguished using harmful gaseous suppressants.
Particle size characterization plays an important role in the analysis of atomized sprays, with advances in modern particle sizing technology allowing more in-depth understanding of the mechanisms underlying spray behaviors, and enabling determination of the impact of surfactants on effectiveness of water mists. The Spraytec particle analyzer from Malvern Instruments aided researchers in their work.
Water Mists
Water mists are defined by the U.S. National Fire Protection Association code NFPA 750 as sprays with a Dv99 of 1,000 µm or less, measured at the coarsest part of the spray in a plane 1 m from a nozzle operating at its minimum design pressure. When a water mist is applied, the water droplets evaporate rapidly, drawing heat of vaporization from the fire and surrounding area. Simultaneous cooling and oxygen displacement by the resulting vapor suppresses the flames. The particle size of the water droplets in the mist is a critical parameter as this directly influences the effectiveness of the spray. Smaller droplets have a relatively high specific surface area, resulting in better heat transfer and more rapid vaporization.
Water mist fire suppression systems are attractive, as they generally require less than a tenth of the water used by conventional sprinkler systems. Their low water delivery rate also minimizes damage to sensitive equipment and eliminates the splatter of liquid fuel associated with sprinklers. Economic and environmental pressures, and the development of new technology, are driving current interest.
Enhancing Performance
Although water mists are highly effective, the reduced energy available to drive the suppression mechanisms observed as a fire is reduced in size can result in insufficient oxygen displacement, preventing complete extinction. Even where extinction is achieved, reignition from hot surrounding surfaces (burnback) may be a problem. A potential solution is the use of surfactant-enhanced water mists.
Surfactants, the principal components of foam concentrates, facilitate the formation and spread of air-water foams. Foams suppress a fire by spreading over the liquid’s surface, preventing fuel evaporation and reducing heat transfer to the fuel source. They also impede ignition or reignition. As water mists are commonly used to protect machinery spaces containing flammable liquids, a system incorporating the advantages of fuel isolation would have considerable benefits.
Measuring Particle Size of Fire-Retardant Sprays
The high concentrations, wide plume widths, and high exit velocities of fire-suppressing mists make accurate measurement of their droplet size characteristics challenging.
Laser diffraction calculates droplet size distributions by measuring the intensity of light scattered by particles, and can operate in the size range from 0.1 to 2,000 microns. An intense laser light source enables measurements over a wide range of spray concentrations. The laser output is expanded and passed through the spray measurement zone. Scattered light is collected using a Fourier lens system and focused onto a silicon diode detector array, which measures the intensity of scattered light as a function of angle. Modern instrument lenses can achieve working ranges of more than one meter, allowing the characterization of wide spray plumes within a single measurement. Rapid detection rates -- up to one measurement every 100 microseconds -- allow atomization dynamics to be followed in real time.
Developed specifically for the characterization of sprays, the Spraytec from Malvern Instruments uses patented algorithms to automatically correct potential inaccuracies caused by multiple scattering between particles -- a common difficulty, where particle concentration is high.
Impact of Surfactants on Water Mist Effectiveness
To combine the benefits of surfactants and water mists, a system must behave both as a vaporizing spray when it leaves the injector, and as a spreading foam at the fuel surface. Researchers at Maryland University carried out experiments using water and various concentrations of proprietary surfactant solutions containing fluorinated compounds (Forafac™). Fire suppression and burnback experiments were conducted using a Tyco AM4 intermediate pressure nozzle in a burn room, using different experimental configurations representative of conditions used in machinery spaces. The particle size of the water mist was monitored using the Spraytec.
General Nozzle Performance
The nozzle system’s general performance was predetermined using water. Changes in particle size over time were measured at a distance of 0.5 m from the nozzle, at a pressure of 3 bar. The transmission value reported during the measurements relates to the concentration of spray, with lower values correlating with high spray concentrations. In this case, the water reservoir was exhausted in 40 seconds. Nozzle output was relatively stable in terms of the median droplet size (Dv50), although at the end of the spraying process the droplet size became larger because of unstable flow of liquid through the nozzle. There was also a tendency toward formation of some larger droplets at higher liquid flow rates, as evidenced by the gradual increase in the Dv90 as the transmission decreased.
Fire Suppression Performance
Droplet sizes for different surfactant formulations were tested successfully, using the Spraytec system, at 0.5 m below the nozzle and 5 points extending out 0.6 m from the centerline. In this case an atomization pressure of 12 bar was used in order to ensure more complete atomization of the liquid.
The droplet size was shown to increase when surfactant is used – indeed increasing the surfactant concentration to 0.2% leads to a further increase in size. This may be considered counterintuitive as surfactants are designed to reduce surface tension and should therefore facilitate atomization. However, the rate of surfactant diffusion to the surface of the liquid was slow compared to the speed of surface formation during atomization. For this reason a low equilibrium surface tension is not achieved. Droplet coalescence and shear induced foaming are also observed as the droplets travel through the air, shifting droplet size distribution to larger sizes.
During suppression experiments, target surface temperature in the compartment was monitored and three stages were observed: free burn before spray release, followed by cooling and suppression in response to spray. The compartment is quickly cooled and flame height limited by the use of pure water, but the fire continues to burn. The results show extinction is achieved only when using a surfactant-enhanced water mist. Video recordings of suppression tests showed that when using the surfactant solutions, islands of foam form on the fuel surface and connect. After one minute these produce an almost continuous layer. This limits accessibility and results in complete isolation of the fuel surface, and flame extinction. Data show that the surfactant solutions also protect against re-ignition for at least nine minutes after extinction.
Conclusion
Research confirms that surfactant-enhanced water mists maintain cooling behavior, improve extinction, and increase protection against reignition when compared with pure water analogs. Analytical instrumentation that allows the measurement of key variables such as particle size, under such challenging conditions, underpins studies critical to the development of optimized solutions for use in challenging fire suppression applications.
Authors:
Dr Paul Kippax, Product Manager – Diffraction Systems, Malvern Instruments Ltd.
Dr. André W. Marshall, Associate Professor, Dept. of Fire Protection Engineering, Dept. of Aerospace Engineering (Adjunct) University of Maryland
Martial Pabon, DuPont Chemical Solutions Enterprise Chantereine, Mantes la Jolie, France