Session: 04-21: Combustion Dynamics - Flame Transfer Functions

Paper Number: 83014

83014 - The Effect of Rectangular Confinement Aspect Ratio on the Flame Transfer Function of a Turbulent Swirling Flame 

The Flame Transfer Function(FTF) has been experimentally investigated for a premixed, turbulent and highly swirled flame in nonuniform enclosures with varying wall spacing. A range of equivalence ratios and inlet bulk velocities are considered to vary the height of the flame. The flame is shown through mean imaging to display strong flame-wall interaction that increases with confinement as a function of increasing height and decreasing wall spacing. As a consequence of the decreasing wall-spacing the outer recirculation zone is observed to increasingly affect the flame geometry, seemingly making the flame more compact. Additionally the more confined flames become increasingly asymmetric, with significant changes to the mean flame shape.

Characteristic peaks and dips are observed in the amplitude of the FTFs in several of the flame configurations, caused by constructive and destructive interference between different sources of flow perturbations. It is shown that changes in the geometrical confinement distinctly affects these interactions, in a way that is not captured by changes in relative confinement due to changing flame size within a constant geometry. Specifically, the FTFs in the narrower enclosure contain more pronounced peaks and dips due to the interactions between convective perturbations from the swirler and bluff body. 

Analysis of the phase-averaged dynamics has been conducted for two critical operational conditions in the least and most confined enclosures: a short flame given by a bulk velocity U_b = 10ms^-1 and equivalence ratio Φ = 1.0; and a longer flame where U_b = 18ms^-1 and Φ = 0.6. The analysis of the shorter flames shows a similar behaviour in both enclosures, both in terms of the global response and the local structure of the heat release rate oscillations. On the contrary, the taller flame case has shown a suppression of the response for higher frequencies that increases with the narrowing of the combustor, showing that a combination of flame-wall interactions, flame asymmetry and elongation has a significant impact on the structure and response of taller confined flames.

Presenting Author: Aksel Ånestad Norwegian University of Science and Technology

Presenting Author Biography: I recently started as a PhD student working under the guidance of Prof. Nicholas A. Worth at the Norwegian University of Science and Technology. My main project is concerned with the effect of axial staging on the stability and emissions performance of swirl flame combustors.

Authors:

Aksel Ånestad Norwegian University of Science and Technology
Byeonguk Ahn Norwegian University of Science and Technology
Håkon T. Nygård Norwegian University of Science and Technology
Nicholas A. Worth Norwegian University of Science and Technology