Flame-induced pressure gradients in turbulent premixed natural gas-air and hydrogen-air jet flames

This study focuses on flame-induced pressure gradients in turbulent premixed jet flames and its potential role in the occurrence of flame flashback. A new procedure is proposed to determine these pressure gradients experimentally from the Favre-averaged momentum equations. The procedure involves a novel experimental method to determine Favre-averaged quantities from particle image velocimetry data. The resulting pressure distributions are compared for two fuel-air mixtures with identical unstretched laminar flame speed (a stoichiometric natural gas-air mixture and a lean ($\varphi =0.49$) hydrogen-air mixture) for stable and near-flashback conditions. In all four cases the flame-induced pressure gradients are closely related to the intermittent behavior of the flame. Furthermore, the pressure gradients for the stable and near-flashback flames show only small differences indicating that the mean pressure distribution is not a suitable indicator for the occurrence of flame flashback. Detailed analysis shows a mild, but systematic shift in the orientation of the instantaneous flame fronts, which tend to align more perpendicular to the flow for the flames closer to flashback. This change in orientation results in local deceleration of the flow, thus increasing the probability of flashback.

Novelty and significance

This work presents original results of experiments in premixed hydrogen-air and natural gas-air turbulent jet flames. A new methodology is introduced to calculate Favre-averaged quantities and the pressure field in a flame from a combination of PIV and Mie scattering measurements. The focus of the experiments and follow up analyses is on the flame characteristics near flashback, since flame flashback is one of the phenomena that hampers the transition from the use of natural gas to hydrogen in, for example, gas turbines.