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NanoMist, the ultra-fine fine water mist system (< 10 µm), exhibits extremely high-energy absorption behavior due to the huge droplet surface area combined with the high vaporization rate of nearly micron size droplets. Further, 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 beyond conventional water mist system. The main thrust of recent computational fluid dynamics (CFD) simulations was to evaluate the properties of ultrafine mist in both local and total flooding fire suppression modes utilizing different mist delivery methods. Once an understanding of the ultrafine mist behavior was achieved, the behavior was compared with other water spray or mist technologies in terms of extinguishment time, wetting nature of the mist, total water needed and mass flow requirement. CFD modeling made the new technology development and evaluation program quick and affordable. The results of the simulations yielded improved understanding of generating, scaling, and delivering such a fine mist cloud into a fire location while addressing 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 decided potential for water mist in many applications. Throughout the years, the concept of using an ultra-fine water mist (with droplet diameters < 10 µm) for fire suppression has been generally ignored, based on the notion that such extremely small droplets would not have enough momentum to reach the fire, and would vaporize before contact. This opinion is changing, however, as a patented ultra-fine water mist technology with the trade name NanoMist has recently attracted a great deal of attention. Using droplets less than 10 µm in diameter, the technology has demonstrated efficient fire suppression capability in various configurations. With the help of CFD and laboratory tests, scientists from NanoMist Systems have demonstrated that this ultra-fine mist may be a potential alternative to clean gaseous fire suppression agents such as HFC-227ea, for meeting both 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, while addressing concerns about the premature loss of liquid droplets. The efficiency of fire suppression by water mist depends critically on the mist size, mist stability, the transport behavior of mist in an obstructed space, and the efficiency and rate of droplet vaporization.
Simulation speeds process development
Using conventional experimental methods, it would have taken at least five years and associated cost to optimize the process technology for the delivery of ultra fine water mist. The use of CFD reduced the cost and time required for the development and evaluation program. The CFD simulations provide a relatively low-cost design option that provides physical insight and reduces the probability of design failure. A CFD simulation provides fluid velocity, pressure, temperature, and other variables, as appropriate, throughout the solution domain for problems with complex geometries and boundary conditions. As part of the analysis, an engineer may change the geometry of the system or the boundary conditions, and observe the effect of the changes on fluid flow patterns or distributions of other variables. CFD also can provide detailed parametric studies that can significantly reduce the amount of experimentation necessary to develop new or modified equipment. CFD was the main engine for NanoMist Systems, LLC to explore new areas and pursue out-of-the-box ideas before investing significant resources in laboratory testing.
CFD model details
The CFD simulations were carried out using a commercial CFD program, FLUENT, supplied by Fluent Inc., Lebanon, New Hampshire. A key advantage of this software package is its discrete phase model (DPM), which provides an excellent tool to predict 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 ê-å 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 volumetric heat release rate was adjusted to provide a maximum plume temperature of approximately 1800-1900 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 µm), and NanoMist™ (1 µm).
The temperature contours of simulated fire are shown in Figure 1. The peak temperature was monitored upon starting the mist injection from the floor.
Figure 1: Temperature (K) contours of simulated pool fire.
DPM stochastic droplet trajectories of water mist droplets surrounding the firebase are shown for droplets discharged from the floor is shown in figure 2. The mist is entrained into the firebase due to the fire’s entrainment field
Figure 2: Entrainment of the mist into the firebase as predicted by the DPM model of Fluent CFD.
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 800K for a mass flow rate of 0.05 kg/s.
Figure 3: Centerline temperature with increasing mass flow rates for different mist classes.
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% 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 testing confirms simulation results
Figure 4 shows flooding of a heptane firebase by NanoMist (mist droplets < 10 micron). The mist surrounding the fire is entrained into firebase. The fire goes out within 10 seconds
Figure 4: NanoMist flooding test on heptane pool fires showing the fire extinction process.
The remarkable fire suppression efficiency of the NanoMist predicted by CFD simulations was very well reproduced by tests that surrounded the heptane pool fires with mist. These fires go out in 10 seconds or less depending on the mist loading (or number density of droplets) and mist deployment momentum. Based on experiments, the water required to suppress the 0.3m diameter-heptane pool fire using the ultra fine mist was less than 100 ml. The tests validated the results of the simulation - 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 attractive features of ultrafine mist fire suppression technology, including 1) nearly self entrainment of mist into the firebase, 2) a remarkably reduced amount of water compared to other water mist technologies, and 3) the relatively non-wetting nature on surfaces because of fast vaporization. These properties make ultrafine mist is an excellent alternative to the existing clean gas fire suppression agents such as HFC-227ea in many fire applications pending full-scale evaluation. Potential uses include data centers, electronic cabinets, restaurant kitchens, and others.
Finally, the CFD simulations have created a productive path that allows NanoMist Systems to pursue its goal of successfully commercializing ultrafine mist technology into various industrial applications. In particular, 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. Preliminary results are very positive. NanoMist Systems, LLC is continuing field-testing of various fire scenarios, technology evaluation, and commercialization efforts on particularly electronic space fire protection technology.
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