Features of the Interaction of the Combustion Front of Diluted Methane–Oxygen Mixtures with Hollow Cylindrical and Conical Obstacles at Low Pressures

Abstract

It is shown that the front of the flame of a thoroughly mixed diluted methane-oxygen mixture at 298 K and 100–300 Torr propagating to the ends of hollow cylindrical and conical obstacles does not form a von Kármán path (vortex shedding) behind them; however, this instability occurs under the same conditions in the flow of hot products after obstacles. The reason that vortex shedding is not observed behind an obstacle during flame propagation but appears in the course of propagation of a reflected stream of hot products is that thermal conductivity reduces the curvature of the flame and leads to its stabilization. Indeed, the convex areas of the chemical reaction zone in a combustible mixture give off more heat in relation to cold ones than in a flat flame: the heat from them is not only transmitted forward in the direction of flame propagation but also in the lateral directions. The resulting cooling of the reaction zone causes the flame regions that burst forward to lag behind. The opposite situation is observed in concave areas, where the temperature rises for the same reasons. The rate of reactions increases and they spread forward faster as the flame spreads. Thus, the surface of the curved front of the flame is evened out. In other words, thermal conductivity has a stabilizing effect on a curved flame. This effect is missing in non-reactive gas. This effect is absent in a nonreacting gas. Calculations based on the acoustic approximation of the Navier–Stokes equations for a compressible reacting medium make it possible to take into account the main observed feature of the flame front approaching an obstacle in the form of a cylinder: vortex shedding is not observed behind the obstacle during flame propagation. Thus, a qualitative model allows obtaining both the mode of the emergence of a von Kármán instability in a chemically inert gas and its absence during flame propagation.