We present an experimental and numerical study of turbulent flames stabilized in the flow undergoing   
vortex breakdown produced by a swirl burner with a central bluff body. The intention behind introducing a  bluff body is  
to stabilize the  Central Recirculation Zone (CRZ) due to the vortex breakdown phenomenon and consequently  stabilize combustion. A bi-stable  flame behaviour  is however still experimentally  observed from flame visualizations, pressure drop and flame transfer function measurements.  
In case of fully premixed conditions, below a critical value of the equivalence ratio, the flame is lifted from the tip of the bluff body  sharply spinning at a given frequency due to a precessing vortex core. As soon as this critical value is exceeded, the flame suddenly jumps  to the tip of the bluff body and precession of the vortex core disappears. This flame behaviour  has implications on emissions, pressure drop and flame dynamics (Flame Transfer Function).   
We show here that  Large Eddy Simulation  can correctly  capture  the transient characteristics of the velocity field  experimentally  identified in a water lab facility and also the  experimental flame jump. We also show a good agreement with experimentally measured transfer function in support of the validation exercise.  We therefore analyse the LES results in order to identify the physical mechanisms controlling  the problem. Specifically, we phase average the LES results at the frequency of precession in case of the  lifted flame, which allows to study  the problem in the frame of reference moving with the spinning vortex. The phase average data  show that the Central Recirculazion zone extends completely without any interruption from the  tip of the bluff body to the flame. The flame however is not pushed to the bluff body but stay lifted with apex positioned approximately  an half bluff  body diameter downstream the bluff body tip. In line with experimental flame visualization from the literature, the apex of the flame is not  positioned in the center of the vortex where it would be intuitively  pushed upstream but appears instead off center, close to the mean zero phase  averaged  axial velocity iso-surface, on the side of positive values.   
This peculiar flame anchoring behaviour is explained with a simple one-dimensional model which emphasizes the role of turbulent radial flux of reactants into the flame brush.   The analysis of lean premixed flame is completed by a detailed monitoring of the fast flame jump as an attempt to shed light on the mechanisms that pushes the flame upstream at the critical conditions.


Experimental and Numerical Analysis of Gas Premix Turbulent Flames Stabilized in a Swirl Burner With Central Bluff Body

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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