EXPERIMENTAL RESEARCH ON VERY HIGH-INTENSITY COMBUSTION IN HIGH-SPEED PREMIXED GAS FLOW

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Very high-speed combustion or very high-intensity combustion would be possible in two types, which are the very intense turbulent flames and the detonations. Both types in premixed gases are generated from propagating flames. In the presence of obstacles in the combustible gas, the turbulence is produced by the induced flow. The turbulence will increase the transport of mass and energy and the area of flame front, leading to the increase in the rate of combustion reactions and, hence, to the enhancement of the pressure waves. All these effects to increase the burning rate depend on the induced flow velocity, which in turn depends on the burning rate itself. This relation is able to form a positive feedback system. Based on this idea, we found that the propagating flame can easily be accelerated even in fully unconfined gas mixtures when comb-shaped grids are placed repeatedly in the flame path.In the present research, we investigate the structures of the very high-speed deflagration an … More d the mechanisms of the transition to detonation. They are observed by schlieren framing or streak photographs, using a high-speed camera. The brightness of the flames is also observed at a point where the light emissions are measured by photocell system. The transient pressure waves are measured by piezoelectric pressure transducers. The combustible gases we used are the stoichiometric mixtures of methane, ethane or hydrogen with oxygen slightly diluted by nitrogen, argon or helium. Deflagration of any mixtures can be accelerated to any velocity by repeated obstacles. The noticeable accelerations of the flames are observed just after disappearing of the large eddies formed behind obstacles, or grid wires. The accelerating flames seen by schlieren framing photographs, look as fine grain which are much smaller than the scales of turbulence in the other burning regions. The fine and intense turbulent flames generate blast waves, as observed by pressure measurements. The mechanisms of the flame acceleration do not depend on the kind of mixtures we used. For any mixture the transition of deflagration to detonation does occur, but the onset of detonation of methane mixtures delays up to over 1000 m/s of flame speed. The very fast deflagration forms "shock + flame" structure. Less