High-Speed Shadowgraph Visualization Studies of the Effectiveness of Ventilating a V-Gutter Flame Holder to Mitigate Screech Combustion Instability in an Aero-Gas Turbine Afterburner

Abstract

Screech combustion instabilities are high frequency (>1000 Hz) transverse periodic oscillations driven by combustion and which are then manifested as large amplitude oscillations in the afterburner duct pressure, accompanied by the characteristic high-pitched audible tones. These screech instabilities which are detrimental to the engine are conventionally suppressed by embedding Helmholtz resonator arrays in the afterburner liner. This method has been found inadequate when mixed mode combustion instability oscillations occur and also when the frequencies of oscillation were lower. The design of practical Helmholtz resonator arrays is classified and so is not available in the open domain. Hence, it was necessary to evolve a robust design solution to mitigate screech combustion instabilities in an afterburner. In an afterburner, V-gutters are used as flame stabilizers. The high Reynolds number flow past a V-gutter array is dominated by the presence of vortices characterized by the Kelvin–Helmholtz instability, which is a convective flow instability related to the shear layers separating from the V-gutter lips and the Benard–von Karman instability which is related to the asymmetric vortex shedding of the flow in the flame holder wake. The shedding of von Karman vortices at non-reacting and near the blowout conditions, and the transition from a Kelvin–Helmholtz instability to that of a Bernard–von Karman instability during near flame blowout create conditions for the frequency to get locked-on to the duct transverse acoustic mode frequency; screech is triggered. Hence, a smart flame stabilization method which has the intrinsic property of preventing the lock-on between the frequency of the vortex shedding from the V-gutter and the duct transverse acoustic frequency was developed. The test rig with optically accessible critical zones around the V-gutter flame stabilizer had the capability to operate the afterburner model under simulated inlet conditions of pressure and temperatures. A FastCam SA-4 Photron high-speed camera was used in this experimental investigation and high-speed shadowgraph flow visualization studies were carried out to develop a comprehensive method of introducing an aerodynamic splitter plate concept through a ventilated V-gutter; mitigation of screech combustion instability has been demonstrated.