Investigation of Thermoacoustic Instability for Swirling Flames in a Dual Annular Stratified Burner

Stratified flames have gained prominence in recent years due to the improved flame stability with reduced NOx and CO emissions and resilience to high turbulence. In the present study, the thermoacoustic instability in a stratified swirl dual annular burner was investigated for swirling flames varying the equivalence ratio at constant Reynolds number. The stratification ratio was varied from premixed condition (SR=1) to rich inner stream (SR=3) to study the effect of stratification on the suppression of thermoacoustic instabilities. The premixed swirling flames indicated the onset, coupling and amplification of thermoacoustic instability in the range of equivalence ratios from 0.8 to stoichiometry (1.0). This was followed by the subsequent decoupling of the oscillations at rich equivalence ratio conditions (Φg = 1.1). Under stratified conditions (SR=2 and SR=3), thermoacoustic instability was triggered across the same range of equivalence ratio as well. However, stratification helped to reduce the peak amplitudes in the sound pressure by 72% at SR=2 and 64% at SR=3. The heat release amplitude was also damped by approximately 70% in both cases. This shows that stratification can be deployed, within certain range, to suppress and control the thermoacoustic instabilities. The absence of thermoacoustic instability under very lean conditions near blowoff also allows the stratified burner for lean operation thus reducing the CO emissions.