The impact of intense turbulence on reaction zones of premixed combustion

For intensely turbulent premixed combustion at extremely large nominal Karlovitz numbers, the common notion is that extremely small scales of turbulence penetrate and broaden the reaction zone (distributed reaction zone regime). However, experimental or computational observations of this phenomenon are lacking. In this work, the fundamental mechanistic understanding of intensely turbulent premixed combustion is reexamined leveraging a manifold-based model for premixed combustion. According to the model, the premixed flame structure is governed by the progress variable dissipation rate. As this dissipation rate increases, coinciding with the presumably very fast mixing rates in intensely turbulent premixed combustion, reactivity is suppressed compared to laminar(-like) premixed flames, and an apparent reaction zone disappears. To evaluate this mechanistic notion, turbulent statistically planar premixed flames are first simulated. Despite an unburned Karlovitz number that would indicate mixing rates fast enough to suppress reactivity, no suppression of reactivity is observed. This is not due to failure of the model but rather the substantial reduction of the progress variable dissipation rate due to the increasing molecular viscosity and increasing turbulence length scales across the flame. Within the flame, the local progress variable dissipation rate is small enough to still support a laminar-like flame structure with a strong, distinct reaction zone. To combat the decrease of the progress variable dissipation rate in the planar configuration, a recirculating configuration is then simulated; the recirculation generates shear and a consistent turbulent production mechanism within the flame. As the Karlovitz number and progress variable dissipation rate increase, the reactivity is suppressed, validating the manifold-based model. With the increasing progress variable dissipation rate, the reaction zone does not broaden but rather thins, weakens, and ultimately vanishes. A simple scaling argument is then used to explain the relationship between the reaction zone length scale and small-scale turbulence length scale as the turbulence becomes more intense. This analysis indicates that the reaction zone thins at the same rate as the turbulence length scales shrink. Therefore, the reaction zone may never broaden due to reaction zone straining. Novelty and significance statement This work reexamines turbulent premixed flame structure at extreme Karlovitz numbers where the smallest scales of the turbulence are nominally thinner than the reaction zone and provides new mechanistic insights into the interaction of intense turbulence with premixed flames. Traditional understanding has hypothesized a broadened, distributed reaction zone, but this work provides a contrarian view that the reaction zone may not be broadened by turbulence and instead be thinned and weakened by intense turbulent mixing. This work further shows that a manifold-based model can in fact quantitatively describe intensely turbulent premixed combustion regimes.