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 the single nozzle PRECCINSTA swirl combustor. We analyze data from two operating modes, premixed (PM) and technically premixed (TPM). The experiments were performed at atmospheric conditions with H2/CH4 fuel mixtures at a global equivalence ratio of 0.65 while maintaining a constant thermal power of 20 kW. We examine the effect of H2 addition on the flow dynamics by analyzing cases with three fuel compositions: 0% H2, 20% H2 and 50% H2 in both operating modes. A new multi resolution modal decomposition method, using a combination of wavelet transforms and proper orthogonal decomposition (WPOD) of the experimental time resolved high speed flow velocity and OH-PLIF measurements is performed. Thermoacoustic oscillations are observed in the TPM operating mode alone. WPOD results for the 0% H2 TPM operating mode case reveals intermittent helical PVC oscillations along with axi-symmetric hydrodynamic flow oscillations due to the thermoacoustic oscillation. These oscillations cause local flame extinction near the nozzle centrebody resulting in liftoff. A precessing vortex core (PVC) oscillation develops in the flow that enables intermittent flame reattachment and results in intermittent thermoacoustic oscillations in this case. In the 0% H2 PM case, the flame remains lifted off of the centrebody despite the presence of PVC oscillations in this case as well. 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. The lack of thermoacoustic oscillations in the PM operating mode shows that the primary heat release driving mechanism is due to fuel-air ratio oscillation that the thermoacoustic oscillation generates. The time averaged flow fields and the emergence of the PVC when the flame is lifted off, together suggest that PVC oscillations are caused by the separation between the vortex breakdown bubble and the wake behind the centrebody, as suggested by prior computational studies.

Impact of precessing vortex core dynamics on the thermoacoustic instabilities in a swirl-stabilized combustor

“…Therefore, if the non-reacting flow in this configuration has a PVC, so would the reacting flow when the flame is clear of the wavemaker region. This observation is consistent with the emergence of the PVC in a reacting flow reported by Oberleithner et al (2015) and Datta et al (2021) when flames are lifted and stabilized downstream of the wavemaker region at the upstream end of the bubble.…”

“…The mode also shows helical rollup along the inner shear layer. This spatial SPOD mode structure is characteristic of a PVC oscillation seen in past studies of this instability (Tammisola & Juniper 2016;Manoharan et al 2020;Datta et al 2021;Gupta et al 2021). The coherent rollup along the shear layer is due to the helical forcing imposed on the shear layers, as has been argued in recent studies (Oberleithner et al 2013;Manoharan et al 2020).…”

“…2015; Taamallah, Shanbhogue & Ghoniem 2016; Datta et al. 2021). It has also been shown that PVCs can enhance fuel/air mixing (Freitag et al.…”

Dynamics and bifurcations of laminar annular swirling and non-swirling jets

“…Indeed, the complex nature of predicting when and where such aerodynamically stabilised flames exist was a key motivator for our interest in this geometry. This paper will show that the number and location of stagnation points in the steady flow is a strong function of the swirl ratio, centrebody diameter and Reynolds number, thereby demonstrating the significant parameter sensitivities that underlie the complex and very different observations of mean flame configurations in various combustion experiments and simulations (Wang et al 2005;Zhang et al 2011;Datta et al 2021). Furthermore, this paper will explore the birth of non-axisymmetric and unsteady flow solutions such as centrebody wake vortices and PVCs, which govern the asymmetries often observed in combustion system flames (Chterev et al 2014).…”

“…2011; Datta et al. 2021). Furthermore, this paper will explore the birth of non-axisymmetric and unsteady flow solutions such as centrebody wake vortices and PVCs, which govern the asymmetries often observed in combustion system flames (Chterev et al.…”