I have been struggling with the idea of “pushing fire.” For the record I don’t think that you can push fire around with water but I know many people who argue this saying that in their experience water streams applied from the exterior have worsened conditions on the interior where they were operating. So, I keep looking. Well the other day I came across a passage in Svensson’s dissertation that may hold an answer, just not sure. I included the longer passage simply for context but what I am really interested in is the passage in red.
Please comment if you understand what I am saying or if you believe that the passage in red describes your experience.
“Manually applied water sprays have two extinction effects that can be identified and observed visually through experiments:
- - They generate a decrease in the rate of pyrolysis mainly induced by a decrease in radiation from flames and hot gases back, i.e. they reduce the externally applied heat flux.
- - They penetrate through the flames, hit the burning surface and cause a decrease in the rate of pyrolysis, induced by a cooling of the surface, i.e. they increase the rate of heat loss from the surface.Practical consequences of this are that it is very hard to reach a high degree of evaporation (and consequently suppression) when applying water on surfaces, especially when using manually applied water sprays. The purpose of the jet or the spray is, amongst others, in addition to the strictly practical reasons to increase the surface of the extinguishing media. According to Svensson et.al. (1999) (paper I), extinguishing mechanisms, how water is applied to fires, and how the jet or the spray affects the fire can be summarized as:
- - The momentum in the spray is transferred to a stream of air, which increases turbulence and stirring. This will usually increase the rate of heat release in the initial stage.
- - The effect of the spray on the gases generates a decrease in the rate of heat release and radiation from the flames.
- - The jet or the spray penetrates through the flames, hits the burning surface and causes a decrease in the rate of pyrolysis.All of these effects can be observed during real firefighting, although they are very hard to physically model. In addition, adding human aspects of fighting fires, variations in application rates, water spray cone angles and drop diameters (depending on e.g. settings and wear of the nozzle), the situation becomes very complex. “
What the dissertation does not speak to is how long this “initial stage” lasts. What I don’t understand is how the HRR is increased because of the increased turbulence. HELP??? From the experiments I have seen the introduction of water streams has led to an almost immediate decrease in temperature and HRR because of surface cooling.