58794 - Impact of Thermoacoustic Instability on Precessing Vortex Core Dynamics in a Ch4/h2/air Technically Premixed Combustor 

The precessing vortex core (PVC) is a self-excited helical instability occurring in swirl nozzles and is driven by the coherent precession of the vortex breakdown bubble (VBB) around the flow axis. Several prior studies have shown that the PVC oscillation has a significant impact on fuel-air mixing, emissions and thermoacoustic characteristics in swirl-stabilised combustors. In this paper, we study the onset of thermoacoustic modes and the dynamics of precessing vortex core oscillations in a single nozzle swirl stabilised model combustor (PRECCINSTA), that is operated in two configurations; premixed and technically premixed. In the former configuration, perfectly premixed fuel air mixture is supplied through the plenum while in technically premixed cases, fuel is injected through 12 orifices in the radial swirler passages and mixing occurs within the nozzle. The experiments were performed at atmospheric conditions with H₂/CH₄ fuel at a global equivalence ratio of 0.65 and 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 the configurations. The non-reacting flow cases and the premixed case with 0% H₂ in fuel show strong coherent PVC oscillations which appear to be suppressed in the corresponding technically premixed case. The coupling between heat release oscillations and local fuel-air ratio fluctuations due to unsteady mixing in the technically premixed cases, give rise to an additional axisymmetric thermoacoustic oscillation in the combustor. 

A new multi resolution modal decomposition method, using a combination of wavelet transforms and POD (WPOD) of the experimental time resolved high speed stereoscopic PIV(sPIV) measurements is performed. WPOD results for the technically premixed case with 0% H₂ reveals intermittent helical PVC oscillations along with axial oscillations associated with the thermoacoustic oscillation. The results suggest that axial pulsation of the flow causes intermittent merger between central wake recirculation zone (CWRZ) and vortex breakdown bubble (VBB) and therefore intermittent PVC oscillations. Given that the thermoacoustic (m= 0) mode frequency is less than half the PVC (m= 1) mode frequency, we get approximately two cycles of PVC within a thermoacoustic oscillation time period. Increasing the volume percentage of H₂ in the fuel in both cases, causes a net upstream shift in the mean position of the VBB, resulting in a sustained merger of the VBB and centerbody wake. This leads to suppression of the PVC as suggested by our recent results from computational work which show that VBB-CWRZ merger results in PVC suppression.