  This paper presents an experimental study on the emergence of the central
recirculation zone (CRZ) in a V-shaped premixed swirling flame, using
simultaneous measurement of particle image velocimetry (PIV) and CH*
chemiluminescence. The results show that either increasing the Reynolds number
(Re) or decreasing the equivalence ratio ({\Phi}) would facilitate the
emergence of CRZ. Further analysis demonstrates that the CRZ characteristics
and its emergence are strongly influenced by the inner shear layer (ISL)
surrounding the CRZ, while the swirl intensity remains unchanged. Dimensional
analysis is performed to understand the underlying mechanism, suggesting the
CRZ emergence is controlled by a non-dimensional parameter, Re_s=|{\gamma}|_max
D/{\nu}_s, defined based on the maximum ISL intensity (|{\gamma}|_max), the
exit diameter (D), and the kinematic viscosity ({\nu}_s) of the burnt gas. By
estimating the temperature and viscosity with a simple heat-loss model, we show
in the |{\gamma}|_max D-{\nu}_s regime diagram that the cases with and without
CRZ are separated by a single boundary line, corresponding to a critical Re_s
of about 424. This verifies the applicability of the proposed Re_s criterion to
lean-premixed V-shaped swirling flames under various conditions. Unlike most
previous works that attribute the CRZ of swirling flames to vortex breakdown,
the present work reveals the non-negligible effect of the ISL, especially the
CRZ suppression when the ISL is weakened by flame heating.
