Session R34: Reacting Flows: Instabilities

Propane-air Markstein numbers from wavenumber measurements of self-excited thermoacoustic parametric instability

Self-excited thermoacoustic parametric instabilities in downward propagating laminar premixed propane-air flames in open-closed tubes are studied. The spatial and temporal evolution of the flame in transition to parametric instability is investigated by pressure fluctuation measurements and synchonized high speed imaging. Different tube lengths are used to modulate the resonant frequency, and the parametric flame response to varying acoustic frequencies for the same mixture is studied in detail. The Markstein number, characterizing the influence on flame stretch/curvature on flame stability, is retroactively determined from wavelength measurements of cellular flames at the onset of parametric instabilities. The Markstein number is estimated by employing a one-step chemistry laminar flame model under acoustic-flow excitation whose analytical functions are reduced into a Mathieu equation. The Markstein number obtained in the present study are compared with Markstein numbers in the literature from direct, indirect and computational measurements.   

Adaptive control of thermoacoustic instabilities using a reinforcement learning approach: an experimental demonstration on a methane/hydrogen turbulent flame

Combustion instability poses a significant challenge in propulsion systems, leading to undesirable pressure oscillations that can compromise the system performance and safety. In this work, we implement an adaptive control policy based on reinforcement learning (RL) to suppress combustion instability in a lab-scale bluff body burner powered with a mixture of hydrogen and methane. We first characterize the combustion instability in the burner by examining its response to changes in equivalence ratio, hydrogen enrichment, and combustor length. In addition, we study the flame shape evolution under self-excited instability and stable conditions. Second, we implement a control system using a microphone, a loudspeaker, and a FPGA chip. We train a RL model to perform active control over a wide range of operating condition and benchmark its performance against a conventional phase-shift controller. Lastly, we discuss RL as an adaptive control scheme to mitigate combustion oscillation across diverse scenarios encountered in real-world applications.   

Effect of swirler-induced perturbations on the thermoacoustic source term

The contributions of salient flow structures to the thermoacoustic source term are investigated experimentally via open-loop control of swirler vane angle. Experiments are performed for methane-air, turbulent, premixed, and swirling flames stabilized inside a gas turbine model combustor. The mixture fuel-air equivalence ratio and the mean bulk flow velocity are 0.7 and 9.0 m/s, respectively. Two swirler vane angle control schemes are examined: an actuation from 30॰ to 60॰ and back to 30॰ as well as an actuation from 30॰ to 0॰ and back to 30॰. Simultaneous pressure and flame chemiluminescence along with stereoscopic particle image velocimetry are employed. The results show that the above actuation mechanisms change the pressure amplitude by about -44% and +22%, respectively. Facilitated via the above open-loop control, the role of salient flow structures on the dynamics of the thermoacoustic source term is studied. It is obtained that using the swirler vane actuation to increase the sizes of the precessing vortex core, shear layers, and outer recirculation zones reduces the thermoacoustic source term, while enlarging high velocity zones tends to increase the source term. The findings underline the importance of active flow control for mitigating the thermoacoustic oscillations.   

Acoustically Coupled Single and Coaxial Fuel Jet Combustion at a Pressure Antinode

These experiments explore the combustion dynamics of single and coaxial methane laminar jet diffusion flames in the presence of a standing acoustic wave, conducted in the vicinity of its pressure antinode. As with earlier flame studies near a pressure node [Sim, et al., CST 2020; Vargas, et al., JFM 2023], flames are studied inside a closed cylindrical waveguide via high-speed visible imaging and analyzed using proper orthogonal decomposition (POD). A wide parameter space is explored here, including different burner geometries, annular-to-jet velocity ratios, and fuel jet Reynolds numbers (below 100), in addition to varying applied acoustic frequencies and pressure amplitudes. Flame-acoustic coupling processes differ significantly based on different operating conditions, producing sustained oscillatory combustion (SOC), multi-frequency periodic lift-off and reattachment (PLOR), permanent flame lift-off (PFLO) with low-level oscillations, and eventual flame blowoff (BO). Trends in instability transitions are identified, and spectra and phase portraits extracted from POD mode coefficients capture distinct signatures associated with these transitions. Comparable flame dynamics are observed in experiments at the Air Force Research Laboratory involving much higher pressures at the PAN and fuel jet Reynolds numbers over 5,000 [Plascencia, et al., AIAA Journal, July2024].   

Enstrophy Transport During Longitudinal Combustion Instability in a High-Pressure Rocket Combustor

Enstrophy transport involves both large- and small-scale turbulence dynamics. Therefore, understanding the various physical phenomena contributing to its production and destruction is critical. This understanding is essential for comprehending flame-turbulence interactions.  This study utilizes a highly resolved adaptive mesh refinement (AMR) based large-eddy simulation (LES) dataset to examine enstrophy transport in a high-pressure rocket combustor experiencing longitudinal combustion instability. Specifically, the four terms of the enstrophy transport equation—vortex stretching, dilatation, baroclinic torque, and viscous dissipation are analyzed both in physical and state-space. Results indicate that vortex stretching and baroclinic torque predominantly produce enstrophy in fuel-rich regions, while dilatation and viscous dissipation contribute to its destruction. Notably, baroclinic torque, arising from the non-alignment of density and pressure gradients, exhibits the highest enstrophy production. This study also reveals a quasi-cyclic behavior of enstrophy dynamics, with production phases interspersed with dissipation phases, correlating with flame weakening and re-intensification. These nonlinear enstrophy fluctuations impact mixing and unsteady heat release, amplifying acoustic waves and sustaining combustion instabilities.   

Dynamics of planar flames within closed channels

The dynamics of premixed planar flames propagating within closed rectangular channels is numerically studied within the context of a hydrodynamic theory. The flame is asymptotically modelled as a surface propagating at a speed derived by considering the transport and chemical processes occurring within the thin flame zone. Unlike freely propagating flames that evolve in near-isobaric conditions, flame dynamics in closed vessels is influenced by gas compression resulting in a notable rise in pressure and temperature. In addition to the flame stretch, flame speed depends on the pressure buildup which results in a reduction of the flame thickness. A hybrid Navier-Stokes/embedded-manifold numerical methodology has been developed and is used to simulate the evolution of planar flames. This methodology is validated against exact analytical solutions and is used to study the influence of confinement, compression and increases in burning rate on intrinsic flame instabilities such as the Darrieus–Landau instability prevalent in premixed combustion. The long-time non-linear evolution of flame surface morphology, instability growth and propagation speed have been examined, quantifying the effects of heat release, Markstein number and channel length.   

Rayleigh-Taylor Unstable Flames: A Transition to Distributed Burning?

Distributed burning is an extreme regime of turbulent, premixed combustion in which the flame's reaction zone is thickened when it burns through turbulent reactants. But what happens when the reactants are laminar and the turbulence is generated by an instability of the flame front itself? Will the flame still thicken? In this presentation, we will use a DNS parameter study of Boussinesq model flames to show that Rayleigh-Taylor unstable flames do undergo a thickening transition, but that its character is different from the transition to distributed burning. In particular, thickening starts on the products side of the flame instead of in the preheat region. Lowering the Prandtl number expands the thickened region towards the reactants side of the flame, but the front of the flame remains substantially thinner than laminar. We explore the behavior of these unusual 'thick and thin' transitional flames using detailed measurements of their internal structure.   

Existence of Invariant Region to Damköhler number in a Reactive Viscous Fingering

Chemical reactions can have a profound impact on hydrodynamic instability by modifying the viscosity and density at the interface between two fluids [1]. A prominent example of this phenomenon is viscous fingering (VF), which occurs within a porous medium when a less viscous fluid displaces a more viscous fluid [2]. It is prevalent in various transport phenomena, such as secondary oil recovery, chromatography separation, and CO₂ sequestration [2]. We study a reactive displacement that involves the chemical reaction A + B → C with miscible fluids undergoing radial flow. The flow dynamics are modeled using Darcy’s law coupled with convection-reaction-diffusion equations. The viscosity depends on the concentrations of

the reactants and product and is characterized by two dimensionless parameters, R_b and R_c [3]. We solve this system using a hybrid scheme comprising the pseudospectral method and the compact finite difference method [4]. Numerical and experimental results have shown that chemical reactions can induce instability and radial flow requires a minimum viscosity contrast to trigger the instability [4, 5]. Further, through linear stability analysis and nonlinear simulations, we illustrate the impact of the Damköhler number (Da) on viscous fingering for various values of (R_b, R_c). Interestingly, we found an invariant region in the (R_b, R_c) phase plane that remains unaffected by changes in Da [6].