Flame morphology boundaries and fundamental combustion properties in unobstructed channels

We investigated the effect of fundamental combustion properties (FCP) on the 3D morphology and dynamics of flames and shocks during acceleration and transition to detonation in unobstructed channels. To achieve this, an extensive experimental campaign was conducted using a simultaneous schlieren visualization setup. The effect of selected FCP was assessed by evaluating nine different mixtures of hydrogen, methane, and hydrogen/methane blends, using oxygen with and without dilution by nitrogen, helium, or argon. The experimental results revealed two characteristic flame evolution behaviors during flame acceleration (FA), depending on the mixtures: (i) a symmetric flame inversion (tulip flame) during the early stages of FA, followed by a short, symmetric flame in the later stages, with the formation of a precursor compression wave located relatively far from the flame, and (ii) an asymmetric, wrinkled flame during the early stages, which develops into a longer flame with the tip inclined toward a corner of the channel, accompanied by the formation of multiple precursor compression waves ahead of the flame in the later stages of FA. For a more robust statistical analysis, a morphology database was compiled from literature sources reporting similar flame morphologies to those observed in our experiments. This database was analyzed using the Feature Elimination Technique in conjunction with the Logistic Regression Model, which enabled the identification of FCP boundaries between the observed flame morphologies. The analysis showed that the pairs of properties most influencing flame morphology are the expansion ratio and the ratio of the laminar flame speed to the sound speed in the combustion products, i.e., ( σ , σ s L / c b ) , as well as the latter ratio with the heat capacity ratio, i.e., ( σ s L / c b , γ ) . Additionally, this methodology helped to identify experimental conditions where little or no data is available in the literature, such as for mixtures with Lewis numbers smaller than unity, which are expected to be affected by thermodiffusive instabilities. These boundaries can, therefore, serve as guidelines for selecting experimental conditions that develop specific flame and shock morphologies and dynamics. Novelty and significance statement This study establishes, for the first time, a direct link between fundamental combustion properties (FCP) and the observed flame and shock morphologies during flame acceleration in unobstructed channels from ignition to detonation onset. The results offer predictive insights into flame morphology behavior (i.e., morphology boundaries as a function of FCP), identify previously unexplored regimes, and serve as a dataset for validation of numerical simulations and provide guidance to target specific regimes in experiments, thereby advancing the current understanding of flame acceleration and the deflagration-to-detonation transition.