Impact of chamber back pressure on the ignition dynamics of hydrogen enriched premixed flames

The impact of chamber back pressure and H 2 -enrichment on flashback of premixed CH 4 - H 2 -Air flames triggered by ignition dynamics is investigated. Different back pressures are imposed by varying the exhaust flow blockage with perforated plates of different porosities. Experiments measuring the ignition transient, from kernel formation to stable flames and flashback, with increasing levels of H 2 -enrichment covering a wide range of operating conditions are conducted. Time-series of pressure, velocity and heat release rate are examined to characterise the ignition process. It is shown that the final state of the flame is highly modified by the over pressure caused by the volumetric expansion of the hot gases. Three different scenarios are identified: Soft ignition with direct stabilisation, transient flashback and permanent flashback. The magnitude of the over pressure generated at ignition is significantly amplified by small increases in mean back pressure which, in turn, modifies the flowrate through the injector thereby promoting flashback. In addition to its role as a pressure amplifier, increasing the combustor back pressure also affects the time lag between the over pressure and the peak in heat release rate just after ignition. The impact of H 2 -enrichment on flashback is investigated by keeping both the bulk injection velocity U b and laminar burning velocity S l0 constant for flames with 0% to 100% H 2 -content. It is shown that the magnitude of the over pressure approximately scales with U b /S 0 and is independent of the total thermal power and H 2 enrichment. Although the mean flow conditions and over pressures are kept constant, the stabilisation mode directly depends on H 2 -content. This shows that flashback triggered by the ignition dynamics cannot be fully described by classical kinematic arguments and a new mechanism is presented.& COPY; 2022 The Author(s). Published by Elsevier Inc. on behalf of The Combustion Institute. This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ )