Impact of hole geometry on quenching and flashback of laminar premixed hydrogen-air flames

Abstract

The ability of an aperture to prevent flame propagation, i.e., to quench it, is essential to the design of flashback-resistant premixed burners, making the determination of hydrogen-air flame quenching distances crucial. However, current experimental methods for estimating quenching distances are not readily applicable to multi-perforated burners and do not account for the aperture geometry. This study presents a novel method for determining quenching widths, here applied to lean hydrogen-air flames stabilized over narrow oblong slits with aspect ratios varying from 1, i.e. circular holes, to values exceeding 20, representing elongated slits over 10 mm in length. Quenching widths $W_Q$ are determined for hydrogen-air mixtures with equivalence ratios $0.45\le \varphi \le 0.75$ at $T_0=300$ K. Results show that transitioning from circular holes to elongated slits reduces the quenching width $W_Q$ by a factor of two, consistently across the range of equivalence ratios, with a smaller reduction only observed for the leanest flames at $\varphi =0.45$. A physics-based model is developed to generalize these findings and predict quenching widths across different aperture geometries. Predictions are compared with previous measurements. A quenching Peclet number ${\text{Pe}}_{D_h}=D_h/{\delta }_f$, defined as the ratio of the hydraulic diameter $D_h$ of the aperture (with a width $W_Q$) to the flame thickness ${\delta }_f=\alpha /S_L$, is shown to effectively collapse the experimental data for all apertures and underscores the influence of slit geometry on flame quenching by a cold wall. These insights could inform the design of safer premixed hydrogen-air burners by optimizing aperture geometry.

Novelty and significance statement

This work advances the understanding of hydrogen-air flame quenching by multi perforated plates, offering essential insights for designing safer, flashback-resistant hydrogen burners. A key contribution is the novel experimental approach developed to determine quenching distances of hydrogen-air flames across apertures of varying geometries. This method uniquely enables the evaluation of the impact of a slit length on its ability to prevent flame propagation, crucial for perforated burner configurations. It reveals a significant reduction in quenching width when transitioning from circular holes to elongated slits. Alongside the new experimental method and reference data, the study introduces a geometry-independent quenching Peclet number, validated experimentally, establishing a valuable predictive criterion.