Abstract
This paper investigates how targeted interference between two well characterized sources of hydrodynamic disturbances can modify the response of premixed bluff body stabilised H2/CH4 flames with and without swirl. We introduce modulations into the Flame Transfer Function (FTF) through hydrodynamic interference between the shedding of vortices/wakes from different shaped bodies upstream of the flame and the vortex roll-up at the flame base caused by acoustic forcing. By placing a set of small diameter cylinders, a streamlined body, or a swirler upstream of the bluff body and varying the distance from the dump plane, the gain and phase of the FTFs could be modulated at targeted frequencies providing a method to suppress thermoacoustic instabilities. We further investigate the flame response which shows that modulations in the fluctuating global heat release rate are caused by linear superposition along the flame front. At frequencies leading to destructive interference, large-scale wrinkling of the flame front occurs which increases the flame surface area but is offset by the simultaneous pinch-off of the flame tip which decreases flame surface area. Their combined effect reduces the amplitude of the fluctuating global heat release rate. At frequencies of constructive interference, large-scale wrinkling of the flame occurs before the flame tip pinches off, leading to an overall increase in the flame surface and amplitude of the fluctuating global heat release rate.
Highlights
Gas turbines burning hydrogen can potentially help deliver large-scale zero carbon power generation and facilitate rapid decarbonisation over the short to medium term
This paper presented a method which can be used to passively modulate the gain and phase of Flame Transfer Function (FTF) of bluff-body stabilized CH4/H2 flames by controlling the interference between convective and acoustic disturbances through careful scaling of relative time delays
Modulations in the gain were produced for all geometries demonstrating that their physical origin is the vortex/wake shedding which scales with Stdg = f dg/up where dg is the characteristic length of the upstream geometry and up the approach velocity
Summary
Gas turbines burning hydrogen can potentially help deliver large-scale zero carbon power generation and facilitate rapid decarbonisation over the short to medium term. Further insight into the origin of the modulations was recently presented in a set of papers by Gatti et al [18,24] They demonstrated that specific changes to the burner geometry could generate a large dip in the gain which coincided with the suppression of vortical structures (measured by PIV) at frequencies where the dips occurred showing that the modulations may be driven by disturbances produced by the upstream cold flow. We investigate the effect of flow disturbances generated upstream by three different geometries; a set of small diameter cylinders, a streamlined body, and a swirler Their upstream location is varied to adjust the relative time-delay between the acoustic forcing and when the flow disturbances reach the flame base and elucidate how they govern acoustic-convective interference.
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