Cone-Angles, Droplets, Velocities & Flow-rates for 3-D Fog Applications -
A review of current research - June 2000
There is a wealth of international research related to 'ideal' cone (spatial) angles, water droplet sizes, discharge velocities and flow-rates associated with 'new-wave' gas-phase cooling and gaseous suppression techniques. Although a large proportion of the literature is directly concerned with fixed Water Mist Fire Suppression Systems (WMFSS), this research is often closely associated and relative to tactical firefighting applications. Whilst it may be useful for the firefighter to have a basic understanding of such concepts it should be emphasised that these scientific principles generally evolve from theoretical computer models although subsequent practical experience often supports the data. Any suggestion that application parameters may rely heavily on great precision is incorrect and firefighters should be fully aware that their application technique and tactical approach is far more influential on the outcome than is droplet sizing and spatial angles etc. It is also worthy of note that such parameters are constantly dynamic in direct proportion to compartmental size, geometry, fuel load and of course the fire's rate of heat release. The 'ideal' application in a small room may not be equally as effective in a larger compartment or in situations where the fire load is abnormally high and adjustments in flow-rate and technique may be necessary.
MECHANISMS OF EXTINCTION BY WATER OF CLASS 'A' FIRES
The suppressive effects water may have on Class 'A' fires are - (Grant & Drysdale FRDG 1/97)
Fuel Cooling - Cooling of the combustible solid fuel surface, which reduces the rate of Pyrolysis and thus the supply rate of fuel to the flame zone. This reduces the rate of heat release by the fire; consequently the thermal feedback from the flame is also reduced and this augments the primary cooling effect of the suppression agent. The application of a water spray to the fuel bed is typical of this method;
Flame Cooling - Cooling of the flame zone directly; this reduces the concentration of free radicals (in particular the chain-branching initiators of the combustion reaction). Some proportion of the heat of reaction is taken up by heating an inert substance (such as water) and therefore less thermal energy is available to continue the chemical break-up of compounds in the vicinity of the reaction zone. One function of the new water mist technology is to act in this manner, the fine droplets providing a very large surface area per unit mass of spray in order to increase the rate of heat transfer;
Flame Inerting - Inerting the air feeding the flame by reducing the oxygen partial pressure by the addition of an inert gas (eg N2, CO2, H2O vapour). This is equivalent to the removal of the oxidiser supply to the flame by the production of water vapour. This is the dominant mechanism by which water mists can suppress large confined fires.
In a discussion of water-mist fire extinction mechanisms Mawhinney added to the above the possibilities of thermal radiation attenuation, dilution of the flammable vapour/air mixture and chemical inhibition.
INTERACTION OF WATER SPRAYS WITH FLAMES AND GASES
The use of fine water droplets for gaseous phase fire suppression has been studied for at least 50 years. Herterich identified a need for consistent terminology when discussing firefighting sprays, especially when considering the characteristic 'size' of the droplets. Average sizes of droplets that appear of most interest in firefighting terms fall within the range of 100-1000 microns (0.1-1.0 mm). A spectrum of drop sizes classes them into five categories - (1) Colloidal (Below 1 micron - appears as smoke); (2) Dust (between 1-10 microns) appears as oil or sea fog; (3) Fine (between 10-100 microns - appears as clouds or mist); (4) Average (between 100-1000 microns - appears as drizzle or rain); (5) Coarse (1000-10000 microns - appears as coarse heavy droplets). The cut off between sprays and mists appears somewhat arbitrary and the US NFPA has recently suggested a practical definition of 'water-mist' as a spray in which 90 percent of the water volume is contained in droplets less than 1000 microns (1.0 mm) in diameter. An alternative definition of 'water-mist' has been advanced by Ramsden who suggests the NFPA definition may be too 'loose', recommending a finer droplet range of 80-200 microns diameter is more suited to water-mist systems.
In firefighting terms the size of an individual droplet, or some mean drop size within a spray, is of great importance when discussing other attributes of the spray as the resistance offered by the surrounding air to the forward motion of the droplets is proportional to the droplet diameter. Therefore the carrying power, or penetration, of the spray is strongly dependant upon the drop size distribution. The efficiency of heat transfer to water droplets, which is fundamental to their use in firefighting applications, is also dependant on droplet geometry and in particular the ratio of the total surface area of the spray to its volume; maximising this ratio is beneficial in promoting rapid absorption of heat from the environment and subsequent evaporation of the droplet. The practical penetration achieved by a particular spray is governed by the relative magnitudes of the kinetic energy of the initial liquid and the degree of aerodynamic resistance offered by the surrounding gas. All other things being equal, the penetration of a spray is much greater than for an individual drop, since the leading droplets impart forward momentum to the surrounding gas, reducing the air drag on the following drops and thus creating a 'pathway' for them, resulting in better overall penetration. There is a growing body of contemporary research concerned with the interaction between water droplets and buoyant fire plumes. This literature suggests there may exist a critical heat release rate above which a given drop size would not contribute to fire extinguishment due to its failure in reaching the relevant 'cooling' zone. With this in mind it has been noted in numerous studies that the 'ideal' water droplets for gas-phase cooling and gaseous suppression applications by firefighters fall with the 200-400 (0.2-0.4mm) micron range.
The Annual BFRL Conference on Fire Research in 1998 produced an interesting (NIST) paper from Alageel, Ewan and Swithenbank - University of Sheffield UK - that investigated the Mitigation of Compartment Jet Fires Using Water Sprays. The main objective of the study was to investigate the interaction of water-sprays with a ceiling jet fire in a ventilation controlled state and close attention was paid to the effectiveness of different spray angles, droplet diameters, stream velocities and water flow-rates. It was generally observed that water applications into the gas layers utilising different spray angles of 30, 60, 75, 90, 120, 135 and 150 degrees produced varying reductions in compartmental temperatures but spray cones within the 60-75 degree range were found to be most effective in reducing the overall temperature. For these angles the limiting behaviour due to the effectiveness in penetrating the flame indicated that spray velocities in excess of 18 metres/second (40 mph) should be used. The mean droplet diameters of 100 to 600 microns were analysed and it was further noted that droplets within the 300 micron (0.3mm) range maximised any cooling effects within the compartment. In terms of flow-rate it was reported that, for these compartmental dimensions (which were the same as a standard container being 35 Cu.m), the optimum flow-rate was between 120-180 lpm (32-48 gpm). Where this flow rate was exceeded the compartmental temperatures were not reduced any quicker and much water was observed as 'run-off' whereas at flow rates below 120 lpm the overall cooling of the gases was seen to be much less effective.
As an extinguishing medium it has been stated that water has a theoretical cooling capability of 2.6 MW per litre per second although in practical terms, its capability is more likely to be around 0.84 MW l/s. It is prudent to try and match your flows with the likely heat release rates that may be encountered on initial entry in structures sited within your locality. The average one roomed residential fire is likely to reach intensities in excess of 7MW at flashover and a minimum flow of 500 lpm (132 gpm) will be required to handle this situation safely and effectively. However, such a flow-rate is too high for an optimised gas-cooling application and a flow of around 100-150 lpm (26-40 gpm) will be more suited to the same fire during it's pre-flashover stage where gas cooling/inerting is relevant. To avoid bring in larger streams and playing 'catch-up' as the fire escalates, the firefighter should ideally be equipped with an initial attack hoseline that provides this flow range of 100-500 lpm with a selectable flow option at the nozzle.
Rasbash attempted to estimate the heat transfer between flames and water sprays and produced a plot of convective heat transfer rate against drop velocity for drop sizes ranging from 50 microns to 2mm whilst assuming a flame temperature of 1,000 deg C. In general, higher velocities and smaller droplet diameters were found to increase the heat transfer rates. For example, a 2mm drop at 0.07m/s (terminal velocity in still air) produced a heat transfer rate of 167 kW Sq.m while the same drop travelling at 2 m/s achieved a value of 293 kW Sq.m. For a 50 micron drop at velocities of 0.01 m/s and 0.5 m/s the corresponding heat transfer rates were 1.7 MW Sq.m and 2.5 MW Sq.m respectively. An estimation of droplet penetration was also presented and it was noted that drops of larger initial size were able to penetrate further into the flame before complete evaporation occurred. More recent implementations of this type of model have been developed where input data include details of the hot gas layer and emperical drop size data from a range of commercial sprinklers and water mist nozzles where individual droplet behaviour may be studied within an overall simulation of spray/fire interaction. Of interest, the IFEX 3000 one litre impulse gun http://www.ifex3000.de discharges its 'burst' of 2-200 micron droplets at 120 m/s with a maximum throw of 16 metres but having tested it during container simulations firefighters have reported it may lack deep penetration into the superheated gas layers within the confines of a structure fire. Whilst its cooling capability appeared to be effective at close range its interaction with the buoyant fire plume seemed to affect the tiny droplets ability to penetrate gases in the overhead.
In terms of droplet penetration the influence of nozzle exit pressure is disputed by some and the use of high-pressure systems as a means of increasing the throw of fine sprays appears to be questionable. The effect on drop size may also be contrary to expectations where an increase in pressure may result in a larger droplet rather than a smaller range. However, further research in this area is suggested.
Most references taken from The Suppression and Extinction of Class 'A' Fires Using Water Sprays - Grant & Drysdale - FRDG 1/97 - University of Edinburgh
COMPARTMENT FIREFIGHTING - FLOW-RATES
Paul Grimwood Fogattack@USA.com