Turbulence-flame interactions in high-Karlovitz-number lean premixed hydrogen piloted jet flames

Turbulence is known to exaggerate the thermodiffusive response in lean premixed hydrogen flames. The mean local flame speed has been demonstrated to increase with Karlovitz number, but the range of validity has not been established; furthermore, several studies observed a decrease in turbulent/local flame speed at moderately-high Karlovitz numbers. All of the aforementioned DNS studies were conducted in the canonical flame-in-a-box (FIAB) configuration. To investigate these observations further and to establish whether similar behaviour is observed in realistic configurations, six DNS have been conducted to reproduce a laboratory piloted jet flame. The jet internal diameter was 7.04 mm, with bulk inflow velocities from 6 m/s to 192 m/s (increasing in factors of 2), corresponding to jet Reynolds numbers of 2,500 to 80,000; the reactants had an equivalence ratio of φ = 0.32 at atmospheric conditions. Complementary turbulent FIAB simulations are used to compare the realistic configuration with the canonical abstraction. The mean local flame speed is found to peak in the 48 m/s case, after which a decrease is observed; the transition is found to occur at slightly higher Karlovitz number in the FIAB configuration. A key difference between the two configurations is shown to be that the jet flames are effectively unconfined, whereas the turbulent FIAB area confined by the periodic boundary conditions (so there is a limit on how much flame surface area can be generated in a given volume). Finally, at the highest inflow speed, the jet flame appears to become lifted (there is a significant region near the inlet with little-to-no heat release), and turbulence-flame interaction is shown to be distinctly different to that at lower Karlovitz numbers; the thermodiffusive response is suppressed without the flame becoming fully distributed.