Lagrangian coherent structures during combustion instability in a premixed-flame backward-step combustor.

Area of Science:

• Fluid Dynamics
• Combustion Science
• Acoustics

Background:

• Combustion instability is a critical phenomenon in various engineering applications, often linked to complex flow dynamics.
• Understanding the interplay between flow structures, flame behavior, and acoustic oscillations is crucial for controlling combustion processes.
• Lagrangian coherent structures (LCS) offer a powerful framework for analyzing fluid transport and identifying dynamically significant flow features.

Purpose of the Study:

• To quantitatively compare large-scale coherent structures in unstable versus stable combustion flow fields.
• To investigate the role of Lagrangian coherent structures in characterizing flow dynamics and flame behavior during combustion instability.
• To determine if LCS can serve as surrogate boundaries for flame structures.

Main Methods:

• Experimental investigation of a confined flow past a backward-facing step at a Reynolds number of 18600 and equivalence ratio of 0.9.
• Utilized time-resolved particle image velocimetry (PIV) for velocity field measurements and CH* chemiluminescence for flame visualization.
• Calculated finite-time Lyapunov exponent (FTLE) fields to extract LCS ridges and analyze flow structures.

Main Results:

• FTLE ridges effectively delineated large-scale vortical structures and their modulation in the unstable (long combustor) case, contrasting with small-scale vortices in the stable (short combustor) case.
• Saddle-type flow features, identified by FTLE, separated distinct flow structures in both stable and unstable configurations.
• FTLE ridges aligned with flame boundaries upstream, acting as surrogate flame boundaries, and captured flame curl-up phenomena.

Conclusions:

• Lagrangian coherent structures, particularly FTLE ridges, are effective in identifying and characterizing large-scale flow structures associated with combustion instability.
• The study demonstrates that FTLE ridges can serve as dynamic surrogate boundaries for flames, encompassing flame curl-up.
• Flame propagation was observed to follow backward-time FTLE ridges, linking successive vortex roll-up events.