Session Z12: Reacting Flows: Instabilities (12:15pm - 1:00pm CST)

On the Development of Reduced Order Models for the Prediction of Acoustically Forced Laminar Diffusion Methane Flame Lift Off

Recent experiments involving laminar diffusion methane flames in a triple jet burner have observed periodic flame oscillations and eventual lift off when exposed to acoustical forcing at a high enough amplitude [Vargas, et al., APS DFD 2020]. POD coefficients obtained from high speed visible images of the oscillating flame first accumulate more than 90$\%$ of the energy within the first three terms at moderate amplitude excitation, before lift off, but that changes to approximately the first 10 coefficients after periodic lift off initiates. This transition is clearly observed through the strong deformation of the POD phase-portrait trajectories beyond the critical pressure amplitude. In the present work, the SINDy approach [Brunton, et al., PNAES, 2016] is employed to identify a ROM that governs the behavior of these POD coefficients. For the ROM to replicate the low amplitude flame dynamics appropriately, a special experimental data folding procedure is employed to overcome under sampling issues caused by image acquisition limitations. The quality of the ROM generated is confirmed by the fact that its solutions remain within the experimental phase-portrait trajectories over hundreds of periods. Current work is using this ROM to predict the onset of periodic flame lift off.   

Laminar Flame Dynamics of Multi-Port Fuel Jets Under Acoustic Forcing

The present experiments investigate the response of multi-port gaseous jet diffusion flames to applied transverse acoustic forcing corresponding to a standing wave at a resonant frequency of 332 Hz. Microjet flames with jet Reynolds numbers in the range of 20 to 100 were explored and high speed visible imaging enabled time-resolved quantification of intensity-based flame response. The oscillatory flames were analyzed via proper orthogonal decomposition (POD) to extract spatial modes and their corresponding phase trajectories via POD coefficient plots. Different regimes of flame response were identified based on the degree of forcing, including weakly oscillatory combustion, transition to multi-mode periodic liftoff, and highly perturbed periodic liftoff and reattachment of the flame preceding extinction. Sparse mode distribution and symmetries in the phase portraits at low amplitude excitation suggested that a reduced order model could be used to predict the observed transitions in the combustion process, and this will be described in a separate study [Alves, et al., APS DFD 2020].   

Not Participating

The study presents a G-equation based reduced-order model for predicting the flame transfer function (FTF) with transverse acoustic modulation. A novel two-step approach is employed in this model: first, a steady-flame profile, which is obtained via detailed simulation, is employed to predict the linear mode-shape of the flow field; and then the simulation based on the G-equation is carried out to capture the dynamic behavior of the flame accurately. The main advantage of this method is that the flame profile in the nontrivial aerodynamic environment can be precisely replicated, and the flame dynamic is predicted under the physically-consistent flow modulation mode. In the present work, we demonstrate the efficacy of our model with the consideration of a premixed Bunsen flame, and the comparison of our predictions with the DNS simulation results will be discussed in detail.   

A Lagrangian Analysis of Extinction and Reignition in Bluff-Body Stabilized Flames

The blow-off dynamics of flames stabilized on a meso-scale bluff-body are investigated using direct numerical simulations (DNS) data with a Lagrangian particle tracking analysis. Two dimensional DNS are performed using lean premixed hydrogen-air flames in the presence of hydrodynamic instabilities. The sequence of events that lead to local extinction and final flame blow-off are investigated by following the Lagrangian particles by examining their local characteristics in terms of explosive dynamics by employing Computational Singular Perturbation (CSP) and Tangential Stretching Rate (TSR) techniques. In particular, we investigate the effects of strain rate, time scales (flow and chemical), and CSP and TSR metrics on the development of localized extinction spots and their growth leading to final blow-off. The critical flame extinction that could lead to the ultimate blow-off is discussed.   

Drag-induced coiling of extruded polymerizing tubes

When a thin stream of aqueous sodium alginate is extruded into a reacting calcium chloride bath, it polymerizes into a soft elastic tube that spontaneously coils helically under the influence of the ambient fluid drag. We quantify the radius and frequency of this drag-induced coiling instability using experiments, and explain the results using scaling, theory and simulations, as a function of the scaled extrusion rate. We find that the rate of extrusion determines the natural curvature of the tube, which is naturally straight at low extrusion rates, helical at intermediate rates and random at high extrusion rates. By independently controlling the calcium chloride concentration and the extrusion rate, this allows us to `print' rough features on the tubes. By further co-extruding a second liquid, we can control the relative buoyancy of the tube, allowing us to print complex three-dimensional filamentous structures in the ambient fluid.