Turbulent flame stabilization modes in premixed swirl combustion: Physical mechanism and Karlovitz number-based criterion

Modern low-emission, lean premixed gas turbine combustion systems often rely on confined swirling flows associated with sudden expansions to enhance flame anchoring. Through the establishment of multiple recirculation zones and shear layers, such complex reacting flows give rise to several possible average flame shapes or macrostructures. Among these, the single conical flame stabilized along the inner shear layer (ISL) and the double conical flame stabilized along both the inner as well as the outer shear layers (OSL) and the outer recirculation zone (ORZ) are of special interest. One of the reasons is that the transition between these two flames has been previously linked to the onset of thermo-acoustic instabilities under acoustically coupled conditions. In this study we investigate the mechanism underlying the flame transition to the ORZ/OSL and propose a criterion for its occurrence, in an acoustically uncoupled combustion system. To reach this goal, the effects of the fuel composition (CH4-H2), Reynolds number, swirler blade angle and heat loss were experimentally analyzed. We find evidence that the transition starts with an intermittent inflammation of the ORZ caused by a flame kernel detaching from the ISL. Above a critical equivalence ratio, the flame kernel expands and spins along with the ORZ flow. Next, we explore the effect of the operating conditions on the onset of an ORZ flame. We propose a Strouhal number to describe the ORZ flame spinning frequency (fORZ) also shown to be a predominately hydrodynamic quantity. Finally, we show that the flame transition to the ORZ is governed by a balance between a flame time to a flow time that can be expressed in a form of a Karlovitz number (KaORZ) defined as the ratio of the ORZ spinning frequency and extinction strain rate; the former is a surrogate for the bulk azimuthal strain rate in the ORZ.