Effect of High-Amplitude Forcing on Turbulent Combustion Intensity and Vortex Core Precession in a Strongly Swirling Lifted Propane/Air Flame

Abstract

The present work reports stereoscopic particle image velocimetry (PIV) measurements in a strongly swirling nonreacting jet and partially premixed lifted flame. The spatial distributions of the average velocity and components of turbulent kinetic energy were calculated from the measured ensembles of the instantaneous velocity fields. A pronounced bubble-type vortex breakdown was observed for the studied flows. Based on proper orthogonal decomposition (POD) of the PIV data and on estimates of velocity fluctuation spectra by a laser Doppler velocimetry (LDV) probe, it was concluded that the combustion did not fundamentally affect the type of coherent structures in the strongly swirling flow: a pair of secondary helical vortices was induced by a precessing vortex core in both cases. Because a strongly swirling jet flow is usually insensitive to weak forcing, strong perturbations were superimposed on the flow bulk velocity to force the formation of ring-like vortices in the flow and to investigate the possible outcomes on the turbulent combustion process. The forcing frequency was below that of the precession. Based on the CH Ã chemiluminescence signal, it was observed that the forcing provided an increase in the turbulent combustion rate near the flame onset, as the entrainment of ambient air to the rich mixture must have increased. Moreover, for a forcing amplitude typically above the magnitude of reverse flow inside the bubble-type recirculation zone, a dramatic suppression of the vortex core precession took place in the reacting case. This effect was accompanied by a quasi-periodical vanishing of the recirculation zone due to interaction of the forced ring-like vortices with the lifted flame.

Experimental Study of the Interactions Between Air Flow Rate Modulations and PVC in a Swirl-Stabilised Liquid Fuel Burner

“…DMD is a mathematical tool which enables to decompose experimental or numerical data into a sum of mono-frequential modes. Although it seems quite close to the Proper Orthogonal Decomposition (POD) used in some PVC studies [12,19,20], DMD provides interesting properties for the present case. First, since both the siren-induced modulations and the PVC are spectrally narrow phenomena without strong harmonics, they appear in two separate modes.…”

“…However, strongly swirling flows can often exhibit a large scale helical structure, known as a Precessing Vortex Core (PVC) which can disturb the combustion process [7][8][9][10]. Studies show that the PVC can be destroyed when the flow is acoustically forced upstream of the swirler [11,12]. A similar scenario is envisaged in Providakis et al [13] to explain the disappearance of the PVC when the operating point presents a strong longitudinal thermo-acoustic instability.…”

Helical modes in low- and high-swirl jets measured by tomographic PIV

“…Thus, both structures represent counter-winding mode. Similar 3D reconstructions were obtained earlier from the 1D LDV [10] and 2D PIV measurements [6,24] in swirling jets with VB. In this study, the winding and rotation directions of the outer helix are determined without any additional assumptions and support hypothesis behind the reconstruction of 2D PIV by Alekseenko et al [24].…”

“…The structure detected by Oberleithner et al [6] in a high-swirl jet consisted of a helical vortex core and a secondary helical vortex in the outer mixing layer of the jet. Alekseenko et al [24] found similar vortex structures for both a high-swirl air jet and a lifted flame of fuel-rich propane/air mixture. It was concluded that the modification of the outer helical vortex (OHV) (by a high-amplitude forcing of the axisymmetric mode |m| = 0) can be used to improve combustion by entraining more surrounding air upstream of the flame base.…”

Experimental Study of Thermo-Acoustic Instability Triggering in a Staged Liquid Fuel Combustor Using High-Speed OH-PLIF

“…What is first striking is the disappearance of the peak around 184 Hz which was attributed to rotational motions of the flame. Previous studies have observed the disappearance of convective rotational structures when the combustor is submitted to high-amplitude pressure oscillations [22][23][24]. A similar phenomenon might be at stake here.…”