Analysis of Combustion Induced Vortex Breakdown Driven Flame Flashback in a Premix Burner With Cylindrical Mixing Zone

Abstract

In earlier experimental studies of the authors a previously unknown mechanism leading to flame flashback—combustion induced vortex breakdown (CIVB)—was discovered in premixed swirl burners. It exhibits the sudden formation of a recirculation bubble in vortical flows, which propagates upstream into the mixing zone after the equivalence ratio has exceeded a critical value. This bubble then stabilizes the chemical reaction and causes overheat with subsequent damage to the combustion system. Although it was shown earlier that the sudden change of the macroscopic character of the vortex flow leading to flashback can be qualitatively computed with three-dimensional as well as axisymmetric two-dimensional URANS-codes, the proper prediction of the flashback limits could not be achieved with this approach. For the first time, the paper shows quantitative predictions using a modified code with a combustion model, which covers the interaction of chemistry with vortex dynamics properly. Since the root cause for the macroscopic breakdown of the flow could not be explained on the basis of experiments or CFD results in the past, the vorticity transport equation is employed in the paper for the analysis of the source terms of the azimuthal component using the data delivered by the URANS-model. The analysis reveals that CIVB is initiated by the baroclinic torque in the flame and it is shown that CIVB is essentially a two-dimensional effect. As the most critical zone, the upstream part of the bubble was identified. The location and distribution of the heat release in this zone governs whether or not a flow field is prone to CIVB.

Experimental and Theoretical Investigation of the Flashback of a Swirling, Bluff-Body Stabilised, Premixed Flame

“…On the 13 other hand, heat and chemical species diffusion towards the wall tend to quench the flame [12][13][14]. It is argued 14 that flame flashback is the ultimate result of a competition between these two effects [5].…”

New Perspectives on Turbulent Combustion: Multi-Parameter High-Speed Planar Laser Diagnostics

“…For the first time, these measurements were extended to confined swirling flows. Hence, theory validation for CIVB-driven flashback, developed by [71] on the basis of a numerical study, became feasible [72].…”

“…Baroclinic push theory, firstly proposed by Ashurst [70], explained flame propagation in vortex tubes and could be used to explain the origin and onset of CIVB-driven flame propagation [71]. However, the fundamental mechanism of the flow transitions is not yet fully understood.…”

Experimental Investigation of the Effects of Central Fuel Injectors on Premixed Swirling Flames

“…Swirl stabilized premixed combustion is the dominant technology used to get control stabilized combustion in gas turbines and other combustion systems. Nevertheless, swirl burners with lean premixed combustion are prone to the following flashback mechanisms 1 , -Flame propagation in the wall boundary layer -Turbulent flame propagation in the core flow -Combustion instabilities, and -Combustion induced vortex breakdown (CIVB) Combustion induced vortex breakdown 2 has been identified as one source of sudden flame transition and flashback in swirl combustors where this sudden transition can cause severe system failure due to the risk of overheating of upstream burner positioned components.There have been considerable efforts and research focused on the understanding of this phenomenon [2][3][4][5][6][7][8] . However, there are still areas of improvement that need to be assessed.…”

“…The group also used the vorticity transport equation 4 to analyse the source terms of the azimuthal components of the flow. They found that the CIVB is initiated by the baroclinic torque which produces considerable levels of negative axial velocity in the vortex core.…”