Investigation of transient PVC dynamics in a strongly swirled spray flame using high speed planar laser imaging of SnO2 microparticles

The Precessing Vortex Core (PVC) dynamics is investigated in a laboratory scale swirl stabilized spray combustor. This helical structure commonly found in swirling flows used to stabilize flames in many combustion systems has a notable impact on the structure of the flow, spray and flame formed by the injector unit. The dynamics of the PVC is examined by combining laser sheet illumination of flame front tracers filmed with a high speed camera, Proper Orthogonal mode Decomposition (POD) and wavelet analysis. In distinction with most applications of laser sheet illumination, in which the flow is seeded with oil droplets that vanish at a relatively low temperature when crossing the flame front, the present implementation relies on micronic tin oxide (SnO2) particles that vanish at much higher temperatures. This is well suited to flows where fresh reactants are already at high temperature or where hot gases are recirculated as is the case in most swirling injection configurations. A first set of experiments carried out with stagnation point premixed flames is used to determine the temperature (T-v approximate to 1770 K ) at which the tin dioxide particles vanish and to identify the chemical reaction that controls this process. Because the light scattered by the solid particles is quite intense, the method can be used at very high repetition rates. It is then applied at a 100 kHz frame rate to a swirling injection configuration in a situation where the flame is well stabilized and the combustor does not exhibit thermo-acoustic instabilities. It is found that the PVC takes the form of a double helix with brief intermittent switching to a single, followed by a triple helicoidal geometry. The POD-wavelet analysis indicates that these moments correspond to flashback events that randomly occur and during which the PVC characteristics change for a few milliseconds at a time. In a second set of experiments the chamber length is augmented giving rise to a combustion instability coupled by a longitudinal acoustic mode at a frequency that is an order of magnitude lower than the PVC frequency. The planar slices of the flame reveal a large cyclic motion of the lower edge of the flame in the inner recirculation zone. It is found that the PVC is modulated both in amplitude and frequency, the modulating frequency corresponding to that of the thermoacoustic oscillation. The double helix PVC is also observed to vanish in a random manner. (C) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Automated summary

The dynamics of the Precessing Vortex Core (PVC) in flame formation were investigated using laser sheet illumination and analysis methods, revealing characteristics of double helix and triple helix geometries. By increasing the chamber length, cyclic motion of flames in the inner recirculation zone was observed, with the PVC being modulated in amplitude and frequency by combustion instability and longitudinal acoustic modes. The PVC was also found to vanish in a random manner, indicating complex interactions in the combustion process.