To clarify the characteristics of the sidewall flame quenching layer, experiments were performed using a combustion vessel. The results indicate that the thickness of the quenching layer is approximately constant until the flames propagate a certain distance from the leading edge downstream of the quenching wall: however, after that, the thickness rapidly increases with flame propagation. When the flame propagates further, the thickness of the quenching layer again remains constant, maintaining a high value. Regarding this phenomenon, it is hypothesized that when a flame propagates in the velocity boundary layer formed on the wall surface, the flames are stretched by the velocity gradient in the velocity boundary layer, which affects flame quenching: this sidewall quenching phenomenon was analyzed. The results indicate that the burning velocity of the flame propagating in the velocity boundary layer decreases due to flame stretch. Namely, the equation for the quenching Peclet number, P e Q =S·d q 3 /α =constant, holds. Here, S is the burning velocity under the influence of flame stretch, d q ′ is the thickness of the quenching layer, and α is the thermal diffusivity of the mixture. Further, when the flames suffer the cooling effect of the wall, as the flame thickness is greater than that of adiabatic flames, it is estimated that the thickness of the preheat zone also increases. Therefore, the influence of the flame stretch is greater than for an adiabatic flame. In this way, the wall surface affects flame quenching not only by exerting a quenching effect on the flame due to its cooling effect but also by stretching the flame by the velocity gradient in the velocity boundary layer formed on the surface of the wall.
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