Shear-Flow Driven Combustion Instability

Vortical structures are reported to cause flame/heat release oscillations in reacting flows or pressure oscillations in nonreacting flows [1-8]. In these cases, hydrodynamic instabilities leading to the formation of the vortical structures define the mode frequency, perturb the heat release, and feed energy into the acoustic held, which, under the circumstances, acts as an amplifier. During the past years, the authors have developed an approach to capture flame-acoustic shear-layer coupling. 

A theoretical framework for the investigation of shear-flow dynamics is found in modern linear stability analysis in which unstable modes have been characterized as  

The latter evolves into a self-sustained resonator that, once perturbed, continues to oscillate indefinitely due to the transfer of energy from the mean flow to the fluctuation. Details of how to conduct this analysis can be found in [9]. 

To understand what flow characteristics give rise to this mode, the study is extended to a family of velocity profiles defined as 

In premixed reacting flows, the recirculation zone is made up of hot products separated from cold reactants by the flame front. To capture some of the impact of combustion on the unstable mode, the stability analysis was repeated while assuming the existence of a density or temperature profile superimposed on the velocity profile. 

---

Progress has been made in both reduced-order modeling and model-based control of combustion dynamics. Advances in modeling were obtained by investigating shear-flow driven combustion instability. The authors of this chapter determined that shear-layer instability occurs when an absolutely unstable mode is present. This mode can be predicted and matched to experimental data. Also, it is shown that the temperature profile determines a transition to absolutely unstable operation. Parameters of the shear-flow modes together with a POD-based approach led to the derivation of a new reduced-order model that sheds light on the interactions between hydrodynamics, acoustics, and heat release. A RePOD... 

---