Fire suppression with water mist is widely considered to be an alternative to gaseous fire suppression agents. Much commercial activity has occurredin the last 5 years to develop technology for fire suppression systems based on water mist. Although researchers in the 1950's recognized the dominant mechanisms of extinguishment, recent experimental work has improved the understanding of the extinguishing properties of water mist. This paper proposes a classification terminology for water mists to facilitate discussion of water mist systems. It describes the primary mechanisms of extinguishment, namely heat extraction, oxygen displacement, and radiation attenuation, and invokes theoretical considerations of vapourlair mixture dilution and kinetic effects at the molecular level. Evidence of an effect of invigoration of combustion caused by the introduction of water mist is presented. Not all fires are extinguished rapidly by water mist: the time to extinguish a fire is a function of fuel type, geometry and mist characteristics. An understanding of the suppression properties of water mist allows a designer to make reasonable choices in designing a suppression system for a particular fire scenario, and is essential for the development of algorithms for computer models of water mist suppression systems.
A series of full-scale fire suppression tests was conducted at the San Pedro de Anes test tunnel facility near Gı´jon, Asturias, Spain in February 2006. The fuel was wooden pallets or a mixed load of wood and high density polyethylene pallets. Fire protection was provided by water mist systems in different configurations. Because of facility restrictions, some scenarios of great interest, such as a free burn fire, could not be investigated. However, in order to complement the experimental results, a number of computational fluid dynamics simulations were conducted on a 140 m section of the tunnel facility. The Fire Dynamics Simulator, version 4, was used for the numerical investigation. An algorithm was developed to allow the fire to spread along the top of a series of pallet loads in such a way that the measured heat release rate was reproduced. Verification and validation studies confirmed that the model predicted the measured ventilation speeds and peak temperatures. The agreement between the simulations and the field measurements was very good prior to activation of the water mist. Back-layering was modeled well. After activation of the mist, the simulations predicted a large drop in gas temperatures, and retreat of the back-layer, but under-predicted the thermal cooling by the water mist downstream of the fire. With the suppression system, high temperatures and heat fluxes were limited to the immediate vicinity of the burning pallets. The model was then used to simulate a free burn fire in the tunnel. The simulation demonstrated the catastrophic conditions created by an unsuppressed fire in a tunnel when compared against the thermally managed conditions under suppressed conditions.
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The high cost of conducting large, full-scale fire tests for the evaluation of suppression systems in tunnels tends to limit both the extent of the instrumentation provided and the number of tests that are conducted. Because of the variability of the large fires, performance criteria based on single point measurements derived from experience with smaller test fires were not reliable indicators of performance. Yet decisions about the acceptability of suppression systems must be based on the limited amount of performance information available. A means was sought to reduce the reliance on single point instrumentation readings, and to augment the value of the limited amount of test data by integrating the field testing with CFD modeling. In this study a computational fluid dynamic (CFD) model was used to simulate a series of full-scale fire tests of water mist systems conducted in 2006 in a highway test tunnel. The NIST Fire Dynamics Simulator version 4 (FDS4) was used to simulate five of the tunnel fire tests. The task was to confirm that the simulations could achieve a reasonable degree of agreement with the conditions measured in the tests. The model could then be used to evaluate the performance of the water mist system over a broader range of performance indicators than were measured. This paper illustrates what is unique about very large fire tests and presents highlights of the modeling. The level of agreement between simulation and test results is demonstrated for one test. Agreement was deemed to be good enough to establish confidence in applying the model to examine the conditions that would occur with an unsuppressed fire, which had not been tested. CFD modeling can be used to improve the understanding of the performance of the suppression system, and to augment the value of the test results. A second, complementary paper has been submitted to the SFPE Journal of Fire Protection Engineering to provide more detailed information about the FDS4 modeling than can be covered in this paper.
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