Central recirculation zone in a V-shaped premixed swirling flame

TL;DR

This study experimentally investigates the formation of the central recirculation zone in V-shaped premixed swirling flames, revealing the influence of flow parameters and shear layer dynamics, and proposing a new non-dimensional criterion for CRZ emergence.

Contribution

The paper introduces a new Re_s criterion based on shear layer intensity to predict CRZ formation, emphasizing the role of the inner shear layer over vortex breakdown.

Findings

CRZ emergence is facilitated by higher Re or lower {\Phi}

CRZ characteristics are strongly influenced by the inner shear layer

A critical Re_s of about 424 separates cases with and without CRZ

Abstract

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.