Study on the improvement mechanism of the premixed turbulent combustion by preferential diffusion effect

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This study has been carried out the verification and determination qualitatively of the preferential diffusion effect, which becomes important particularly for the lean mixtures, on the premixed turbulent combustion. And, an attempt has been made to be proposed a new technique to improve the premixed turbulent combustion by application of the molecular preferential diffusion effect. In order to examine it, fuel/oxygen/inert-gas mixtures with nearly identical laminar burning velocities were used. The main conclusions are as follows.From the instantaneous tomograms of turbulent flame, the turbulent flame areas at the same turbulence intensity are nearly equal, depending slightly on the kind of fuel and inert gas. This indicates that the local burning velocity is an important factor as governing the turbulent burning velocity.From the continuous tomograms, the local flame displacement velocities of propagating turbulent flame S_F as local burning velocity are not a constant equal to the laminar burning velocity and distributed over a wide range. This fact seems to contradict the concept of the conventional models. And, S_F of hydrogen mixture tends to distribute more extended than one of hydrocarbon mixtures.For leaner methane and hydrogen mixtures and richer propane mixtures, S_F on the convex part toward the unburned mixture tends to be larger than that toward the burned gas. This phenomenon can be explained qualitatively by the concept of the preferential diffusion. This indicates that the quantitative value of preferential diffusion effect can be determined by using S_F.Judging from the results, a new technique to improve the turbulent combustion can be proposed : The turbulent combustion characteristics of fuel/oxygen/inert-gas mixtures can be improved by using the smaller molecule with higher diffusion coefficient than oxygen as fuel and the heavier molecule with lower diffusion coefficient than oxygen as inert gas.