The nature of coherent deflagrations

The nature of coherent deflagrations in an enclosure, vented into the atmosphere, is analysed. Experimental observations in an empty 547-m3 vented enclosure of the SOLVEX programme are analysed by means of large eddy simulations (LES). The LES model is based on the renormalization group theory and the dilution of the methane-air mixture, as it flows out of the vent, is taken into account. A comparison between simulated and experimental pressure transients and, the dynamics of the propagating flame front has given an insight into the nature of the complex simultaneous interactions between flow, turbulence and combustion inside the enclosure and in expelled gases. LES processing of experimental data unveiled that the substantial intensification of premixed combustion occurs only outside the enclosure , leading to a steep coherent pressure rise in both the internal and the external deflagrations. The external explosion does not affect burning rate inside the enclosure. The LES model shows excellent agreement with experimental pressures measured at different locations within and outside the enclosure, up to the point where the flame reaches the shear layers at the edge of the external jet. The modelling of the subsequent combustion required the use of one additional ad hoc parameter. It is suggested that this quantity is necessary to account for the unresolved subgrid scale increase of flame surface density in these highly turbulent layers. The mechanism of combustion intensification in this region is discussed.