Impact of Hydrogen Addition on the Thermoacoustic Instability and Precessing Vortex Core Dynamics in a CH4/H2/Air Technically Premixed Combustor

Abstract

We study the impact of H2 enrichment on the unsteady flow dynamics and thermoacoustic instability in PRECCINSTA swirl combustor. The experiments were performed at atmospheric conditions with H2/CH4 fuel mixtures at a global equivalence ratio of 0.65 and a constant thermal power of 20 kW. We analyze data with three fuel compositions: 0%, 20% and 50% H2 in two operating modes, premixed (PM) and technically premixed (TPM). A new multi-resolution modal decomposition method, using a combination of wavelet transforms and proper orthogonal decomposition (WPOD) is performed on time resolved flow velocity and OHPLIF measurements. Thermoacoustic oscillations are observed in the TPM operating mode alone, indicating that the primary heat release driving mechanism is due to fuel-air ratio oscillations. WPOD results for the 0% H2 TPM case reveals intermittent helical PVC oscillations along with axisymmetric hydrodynamic flow oscillations due to the thermoacoustic oscillations. These oscillations cause local flame extinction near the nozzle centrebody resulting in liftoff. A precessing vortex core (PVC) then develops in the flow and enables intermittent flame reattachment. In the 0% H2 premixed case, the flame remains lifted off the centrebody despite the presence of PVC oscillations. H2 enrichment results in the suppression of flame lift-off and the PVC in both operating modes. We show from flow strain rate statistics and extinction strain rate calculations that the increase of the latter with H2 addition, allows the flame to stabilize in the region near the centrebody where the pure CH4 cases show lift off.

Numerical Investigation of Combustion Instabilities in Swirling Flames with Hydrogen Enrichment

“…This can be attributed to the fact that the higher equivalence ratio and hydrogen addition increases the extinction stretch rate of the lean premixed flame (Taamallah et al 2015b), which determines the localised flame extinction in the turbulent combustion (Sarli and Benedetto 2007). A recent experimental study (Datta et al 2021) on hydrogen addition and PVC in TP PRECCINSTA flames shows H 2 enrichment results in the suppression of flame lift-off and the PVC, which is consistent with this analysis. Stochastic extinction near the flame base reduces the radial density gradient, which was found to be a favorable condition for the formation of PVC (Oberleithner et al 2015).…”