Computer simulation has greatly shortened the path toward the development of ultrafine water mist fire suppression technology for NanoMist Systems, LLC. NanoMist is the trade name for the proprietary process technology of producing, scaling, transporting, and delivering the ultrafine fine water mist system.
The fire suppression chemical Halon 1301 was banned after it was identified as an ozone-depleting substance, and the search for newer and safer fire suppression agents continued. Among several other clean agents, the non ozone-depleting chemical HFC-227ea is the most popular agent in use today. It has some limitations, however, because of toxic by-products that may often reach harmful levels, and it also has a global-warming potential. From an environmental and toxicity point of view, ultrafine water mist is an ideal compound. Potentially, it may be a permanent solution for fire suppression technology.
 (1-3) NanoMist flooding test on heptane pool fires showing the fire extinction process. (Photos courtesy of NanoMist Systems, LLC.) |
NanoMist, the ultrafine (< 10 microns) fine water mist system, exhibits extremely high-energy absorption behavior because of the huge droplet surface area combined with the high vaporization rate of nearly micron-sized droplets. Also, the gas-like dispersion behavior adds to its ability to act as a total flooding agent. These features make it a powerful fire suppression system compared to the conventional water mist system. The main thrust of recent computational fluid dynamics (CFD) simulations was to evaluate the properties of ultrafine mist in local and total flooding fire suppression modes using different mist delivery methods. Once the behavior of ultrafine mist was understood, it was compared with other water spray or mist technologies in terms of extinguishment time, the wetting nature of the mist, the total water needed, and the mass flow requirement. CFD modeling made the new technology development and evaluation program quick and affordable. The simulation results yielded an improved understanding of generating, scaling, and delivering such a fine mist cloud into a fire location and at the same time addressed concerns about the premature loss of droplets. Relatively simple fire tests were then conducted to determine the accuracy of the overall trend predicted by the CFD model.
Government and private industry researchers have recognized a potential for water mist in many applications. Over the years, the concept of using an ultrafine water mist for fire suppression has been generally ignored, based on the belief that such extremely small droplets would not have enough momentum to reach the fire and would vaporize before contact. This opinion is changing, however. NanoMist’s patented ultrafine water mist technology has demonstrated efficient fire suppression capability in various configurations. With the help of CFD and laboratory tests, Nano-Mist Systems scientists have demonstrated that this ultrafine mist may be a potential alternative to clean gaseous fire suppression agents such as HFC-227ea for meeting government and industrial fire protection needs, pending full-scale testing. The CFD results have yielded an improved understanding of how to generate, scale, and deliver fine mist clouds into a fire location and address concerns about the premature loss of liquid droplets. The efficiency of water mist fire suppression depends critically on mist size, mist stability, the transport behavior of mist in an obstructed space, and the efficiency and rate of droplet vaporization.
CFD MODEL
The CFD simulations used a commercial CFD program, FLUENT, supplied by Fluent Inc., of Lebanon, New Hampshire. A key advantage of this software package is its discrete phase model (DPM), which predicts the rate of vaporization of water droplets subjected to a fire field. The Navier-Stokes equations along with energy and species conservation equations were solved using suitable boundary and initial conditions. In this study, a medium-scale fire was generated using a volumetric heat-generation source term within a specified region located at the center of the room. Combustion chemistry and radiation models were not activated in this study. A standard κ–ε model was used for the turbulent flow. A Lagrangian discrete-phase model was solved with stochastic particle trajectories (influenced by turbulent fluctuations) for inert droplet vaporization in the presence of a hot-gas environment. The model essentially handles the vaporization of droplets exposed to the fire field and the subsequent cooling of the local gas field.
Specified boundary conditions included pressure inlets surrounding the fire (relevant to an open pool fire, as in the experiments), a pressure outlet at the top, and a wall boundary for the bottom floor. The volume-tric heat release rate was adjusted to provide a maximum plume temperature of approximately 1,800 to 1,900°K. Three water mist mass flow rates were studied: 0.01, 0.03, and 0.05 kg/s. The mist was introduced at the floor level with an injection momentum sufficient to surround the firebase. The various mist classes included sprinkler droplets (0.5 mm), commercial water mist (100 microns), and NanoMist (1 micron).
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The temperature contours of simulated fire are shown in Figure 1. The peak temperature was monitored on starting the mist injection from the floor.
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Figure 2 depicts DPM stochastic droplet trajectories of water mist droplets surrounding the firebase for droplets discharged from the floor. The mist is entrained into the firebase because of the fire’s entrainment field.
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In the CFD study, the fire cooling behavior of the mist is determined from the predicted centerline peak temperatures. The mist puts out the fire because of the severe cooling brought by the energy absorption by the latent heat of water combined with other factors such as water vapor dilution of oxygen and radiation blockage. The centerline profiles in Figure 3 show the ability of all the sprays to cool and put out the fire. Micron-sized droplets are more sensitive to increased mass flow rates than are the larger droplets, as seen from the steep decrease in peak temperatures. For the smallest droplet sizes, the temperature reaches about 800°K for a mass flow rate of 0.05 kg/s.
Within 10 seconds, the fire centerline temperature is reduced to so low a value that the combustion cannot continue. The total available water surface area increases rapidly for such extremely small droplets at a fixed mass flow. However, for sprinkler- or commercial-type mists, the cooling behavior was nominal; a marginal increase of 21 percent in the extent of cooling was observed with a factor of five increase in water mass. Under these conditions, the additional water is ineffective since the droplet vaporization time is greater than the residence time of the droplet within the fire domain.
PHYSICAL TESTS
Photos 1-3 show flooding of a heptane firebase by NanoMist (mist droplets < 10 microns). The mist surrounding the fire is entrained into the firebase. The fire is extinguished within 10 seconds.
The fire suppression efficiency of the NanoMist system predicted by CFD simulations was reproduced in tests in which heptane pool fires were surrounded with mist. These fires were extinguished in 10 seconds or less, depending on the mist loading (or number density of droplets) and mist deployment momentum. Based on experiments, less than 100 milliliters of the water was required to suppress the 0.3-meter-diameter heptane pool fire using the ultrafine mist. The tests validated the simulation results: that the mist is self-entrained into the fire and no additional momentum is necessary for delivery. In addition to the validation, the CFD predictions combined with laboratory tests indicated some features of ultrafine mist fire suppression technology, including (1) near self-entrainment of mist into the firebase, (2) a reduced amount of water used compared to other water mist technologies, and (3) relatively reduced wetting of surfaces because of fast vaporization. These properties make ultrafine mist a potential alternative (pending full-scale evaluation) to the existing clean gas fire suppression agents such as HFC-227ea in many fire applications. Potential uses include data centers, electronic cabinets, restaurant kitchens, and others.
Finally, the CFD simulations have guided research efforts to evaluate a possible alternative to clean gas fire suppression chemical agents such as HFC-227ea. NanoMist Systems is now continuing to use CFD to explore other, more complicated fire threats in advance of expensive, full-scale testing. For example, the company is collaborating with the U.S. Naval Research Laboratory and Hughes Associates, Inc. in exploring both CFD design model testing of various scenarios and subsequent model confirmation through real-scale fire test evaluations in their sophisticated fire test facilities. NanoMist Systems, LLC is continuing field-testing of various fire scenarios, technology evaluation, and commercialization efforts on particular electronic space fire protection technology. ■
