Influence of inner shear layer on the emergence of central recirculation zone in a V-shaped premixed swirling flame

This paper presents an experimental study on the emergence of the central recirculation zone (CRZ) in a basic V-shaped premixed swirling flame, using simultaneous measurement of particle image velocimetry and CH* chemiluminescence. Unlike most previous works that explored CRZ formation controlled by vortex breakdown (VB), the present experiment maintains a constant swirl intensity to rule out the influence of VB. We find that the CRZ characteristics and its emergence are strongly affected by the strength of the inner shear layer (ISL) surrounding the CRZ. However, the critical ISL intensity corresponding to CRZ emergence varies with the Reynolds number (Re) and the equivalence ratio (Φ). Upon employing dimensional analysis, the underlying mechanism can be interpreted with a non-dimensional parameter, Res=γmaxD/νs, defined based on the maximum ISL intensity (γmax), the exit diameter (D), and the kinematic viscosity (νs) of the burnt gas. The resultant γmaxD−νs regime diagram demonstrates the collapse of the critical Res value for various cases with and without CRZ. The Res mechanism explains the dependence of the critical ISL intensity on the equivalence ratio, further emphasizing the non-negligible roles of both the ISL intensity and the viscosity of the burned gas, in addition to VB, in determining the emergence of CRZ for V-shaped swirling flames.