Transitions and blowoff of unconfined non-premixed swirling flame

Santhosh, R and Basu, Saptarshi (2016) Transitions and blowoff of unconfined non-premixed swirling flame. In: COMBUSTION AND FLAME, 164 . pp. 35-52.

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Abstract

The present experimental work reports the first observations of primary and secondary transitions in the time-averaged flame topology in a non-premixed swirling flame as the geometric swirl number S-G (a non dimensional number used to quantify the intensity of imparted swirl) is varied from a magnitude of zero till flame blowout. First observations of two transition types viz. primary and secondary transitions are reported. The primary transition represents a transformation from yellow straight jet flame (at S-G = 0) to lifted flame with blue base and finally to swirling seated (burner attached) yellow flame. Time-averaged streamline plot obtained from 2D PIV in mid-longitudinal plane shows a recirculation zone (RZ) at the immediate vicinity of burner exit. The lifted flame is stabilized along the vortex core of this RZ. Further, when S-G similar to 1.4-3, the first occurrence of vortex breakdown (VB) induced internal recirculation zone (IRZ) is witnessed. The flame now stabilizes at the upstream stagnation point of the VB-IRZ, which is attached to the burner lip. The secondary transition represents a transformation from a swirling seated flame to swirling flame with a conical tailpiece and finally to a highly-swirled near blowout oxidizer-rich flame. This transition is understood to be the result of transition in vortex breakdown modes of the swirling flow field from dual-ring VB bubble to central toroidal recirculation zone (CTRZ). The physics of transition is described on the basis of modified Rossby number (Ro(m)). Finally, when the swirl intensity is very high i.e. SG similar to 10, the flame blows out due to excessive straining and due to entrainment of large amount of oxidizer due to partial premixing. The present investigation involving changes in flame topology is immensely important because any change in global flame structure causes oscillatory heat release that can couple with dynamic pressure and velocity fluctuations leading to unsteady combustion. In this light, understanding mechanisms of flame stabilization is essential to tackle the problem of thermo-acoustic instability. (C) 2015 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Item Type:	Journal Article
Publication:	COMBUSTION AND FLAME
Publisher:	ELSEVIER SCIENCE INC
Additional Information:	Copy right for this article belongs to the ELSEVIER SCIENCE INC, 360 PARK AVE SOUTH, NEW YORK, NY 10010-1710 USA
Keywords:	Non-premixed; Swirling flame; Blowoff; Vortex breakdown bubble
Department/Centre:	Division of Mechanical Sciences > Mechanical Engineering
Date Deposited:	06 Apr 2016 05:31
Last Modified:	06 Apr 2016 05:31
URI:	http://eprints.iisc.ac.in/id/eprint/53483

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