Effect of confinement ratio on flame structure and blow-off characteristics of swirl flames

Abstract

Blow-off of swirl flames as well as its mechanism are fundamental issues and attract continuous attention due to its wide application in modern combustors. This study experimentally investigated the blow-off and flame macrostructure transition behavior of unconfined and confined premixed CH4/air flames stabilized on a swirl and bluff-body burner. PIV and OH-PLIF techniques were used to measure the flow field and instantaneous OH profile, respectively. It was observed that the combustor confinement could significantly broaden the blow-off limits, which was further widened with the increase of confinement ratio (CR) of the confined combustor. The influence of CR on flame stabilization was revealed by analyzing flame structures and flow fields. For the unconfined combustor, when the flames are far away and close to the blow-off, the shear layer and inner recirculation zone (IRZ) play the critical role in stabilizing the flame front, respectively. For the confined combustor, when the flame was far from blow-off, both the shear layer and the outer recirculation zone (ORZ) contributed to the stabilization. With the decrease of equivalence ratio, the stabilization effect of ORZ started to decrease and gradually weakened. When near lean blow-off limit, for CR = 2 (Φ= 0.48), the flame was mainly stabilized by the IRZ, while for CR = 3 (Φ = 0.46), the flame stabilization depended on the upstream IRZ/inner shear layer (ISL) and the downstream IRZ/ISL and ORZ/outer shear layer (OSL). Furthermore, the ISL vortexes would further accelerate the blow-off near the lean limit for confined and unconfined flames. The transition from stable to unstable was extended by increasing the CR in the confined combustor.