Françoise Baillot

The role of secondary instabilities in the stabilization of a nonpremixed lifted jet flame

A nonpremixed lifted jet flame is studied dynamically in the hysteresis zone. High-speed laser tomography images show clearly that, in the case of an organized jet, the flame is located on streamwise counter-rotating vortex filaments generated to secondary instabilities and ejected towards ambient air. Particle image velocimetry is used to evaluate the amplitude of the translational and rotational velocity of these filaments. The use of an acoustic field to force jet instabilities shows that the flame, following large filament ejections, moves back upstream very close to the nozzle without anchoring at it. The role of streamwise vortices in the stabilization mechanism of the lifted flame is confirmed by measurements obtained with a disordered jet, from a straight tube burner. From these results, it is proposed that secondary vortices at the flame base are sufficiently strong to create a premixed zone and to oppose flame propagation.

Physical mechanisms of a lifted nonpremixed flame stabilized in an acoustic field

Main acoustic effects on a nonpremixed lifted flame in the hysteresis zone are analyzed by means of high speed tomography and LDA techniques. A burner with a convergent profiled tube is designed to obtain a jet with organized eddy structures, even with no excitation. A chart summarizes the different flame responses to a sine-shaped forcing. Depending on ranges of amplitudes and frequencies, acoustics either can prevent reattachment to the burner and enhance combustion, or weaken the flame stability (large fluctuations of the liftoff height and/or reattachment). In all the cases, the flame behavior is driven by the organization of jet vortices, which differs from one case to another: Without acoustics or with low amplitudes of excitation, the streamwise vortices due to secondary instabilities are of paramount importance in the stabilization of the flame; for high amplitudes, it is rather the oscillating jet column that acts on the flame.

Behaviour of an air-assisted jet submitted to a transverse high-frequency acoustic field

Acoustic instabilities with frequencies roughly higher than 1 kHz remain among the most harmful instabilities, able to drastically affect the operation of engines and even leading to the destruction of the combustion chamber. By coupling with resonant transverse modes of the chamber, these pressure fluctuations can lead to a large increase of heat transfer fluctuations, as soon as fluctuations are in phase. To control engine stability, the mechanisms leading to the modulation of the local instantaneous rate of heat release must be understood. The commonly developed global approaches cannot identify the dominant mechanism(s) through which the acoustic oscillation modulates the local instantaneous rate of heat release. Local approaches are being developed based on processes that could be affected by acoustic perturbations. Liquid atomization is one of these processes. In the present paper, the effect of transverse acoustic perturbations on a coaxial air-assisted jet is studied experimentally. Here, five breakup regimes have been identified according to the flow conditions, in the absence of acoustics. The liquid jet is placed either at a pressure anti-node or at a velocity anti-node of an acoustic field. Acoustic levels up to 165 dB are produced. At a pressure anti-node, breakup of the liquid jet is affected by acoustics only if it is assisted by the coaxial gas flow. Effects on the liquid core are mainly due to the unsteady modulation of the annular gas flow induced by the acoustic waves when the mean dynamic pressure of the gas flow is lower than the acoustic pressure amplitude. At a velocity anti-node, local nonlinear radiation pressure effects lead to the flattening of the jet into a liquid sheet. A new criterion, based on an acoustic radiation Bond number, is proposed to predict jet flattening. Once the sheet is formed, it is rapidly atomized by three main phenomena: intrinsic sheet instabilities, Faraday instability and membrane breakup. Globally, this process promotes atomization. The spray is also spatially organized under these conditions: large liquid clusters and droplets with a low ejection velocity can be brought back to the velocity anti-node plane, under the action of the resulting radiation force. These results suggest that in rocket engines, because of the large number of injectors, a spatial redistribution of the spray could occur and lead to inhomogeneous combustion producing high-frequency combustion instabilities.

A Numerical Study on the Effects of CO2/N2/Ar Addition to Air on Liftoff of a Laminar CH4/Air Diffusion Flame

The addition of exhaust gas to a combustor may cause liftoff of a diffusion flame due to several possible mechanisms. Understanding the relative influence of these mechanisms is of importance for the further development of exhaust gas recirculation combustion technology. The authors present a numerical study on the effects of CO2, N2 (two of primary exhaust gas components) and Ar addition on the liftoff of a laminar CH4/air diffusion flame. A gradual switch-off approach was used to identify the relative importance of the different mechanisms. A detailed reaction scheme and complex thermal and transport properties were employed. The simulation results were validated by comparing the calculated and previously measured critical ratios of the 3 additives for liftoff. The results show that the numerical simulation successfully reproduced the previously measured critical ratios of liftoff for all 3 studied additives. Detailed analysis of the numerical results suggests that the addition of N2 affects flame liftoff due to the sole effect of dilution. On the other hand, the addition of CO2 causes flame liftoff due to the dilution, thermal and chemical effects, with the dilution effect being the most significant one, followed by the thermal and chemical effects. All 3 effects tend to reduce combustion intensity and cause flame liftoff, leading to the smaller critical ratio of CO2 than that of N2. The radiation and transport property effects are negligible for CO2 addition. Ar addition affects flame liftoff due to dilution, thermal, and transport property effects. However, whereas the dilution effect tends to reduce combustion intensity and cause flame liftoff, the thermal and transport property effects tend to increase combustion intensity and resist flame liftoff for Ar addition, which results in the greater critical ratio of Ar than that of N2. Therefore, for the 3 studied additives in this paper, CO2 has the minimum critical ratio, whereas Ar has the maximum for liftoff.

Impact of CO2/N2/Ar Addition on the Internal Structure and Stability of Nonpremixed CH4/Air Flames at Lifting

The authors focused on how adding CO2 to the air influences the transition from an attached flame to a lifted flame issued from a coaxial nonpremixed methane-air jet. To discriminate between effects due to a diluent (dilution, thermal, or chemical impacts), chemically and thermally inert N2 and chemically inert Ar were also investigated. Flame lifting always occurs, essentially controlled by the critical flow-rate ratio, (Qdiluent/Qair)lifting. CO2 has the strongest ability to break flame stability, followed by N2, then by Ar. A unique attachment height and OH thickness characterize lifting for all the diluents; lifting is attained once the same critical flame edge propagation speed is reached. (Qdiluent/Qair)/(Qdiluent/Qair)lifting is the affine parameter of similarity laws describing Ha and EpOH evolutions with dilution. Aerodynamics competes with dilution to impose lifting and boundary effects cannot be ignored in a fine analysis. The flame behaves differently according to whether lifting results from aerodynamics or dilution.

Behavior of cylindrical liquid jets evolving in a transverse acoustic field

This paper presents a theoretical and an experimental investigation of low-velocity cylindrical liquid jets submitted to transverse planar acoustic waves. For this purpose, the behavior of a liquid jet traversing the section of a Kundt tube was examined. Experiments reported that the liquid jet could be either deviated from its trajectory or deformed as a succession of lobes oriented in space and whose length and width depend on the jet acoustic environment. Furthermore, for a sufficient acoustic velocity, the jet deformation increases in such proportion that a premature and vivid atomization mechanism disintegrates the liquid flow. Theoretical models are proposed to understand these behaviors. The first one calls out for acoustic radiation pressure to explain the jet deviation. The second one consists in a modal analysis of the vibrations of a jet when submitted to a transverse stationary acoustic field. As a first approach, a simplified two-dimensional model is proposed. This model reports that a sudden...

Experiments on imploding spherical flames: Effects of curvature

An experimental study of imploding combustion is carried out by creating spherical pockets of unburned methane/air or propane/air mixtures for several equivalence ratios. The aim is to quantify the influence of the isotherm contour upon the determination of the flame displacement speed as a function only of the curvature effects of the stretch. Three optically visualized isotherm contours are followed during pocket consumption: the schlieren, shadowgraph, and luminous contours. To quantify the curvature effects, both the asymptotic law and its extended formulation are used to study the flame speed theoretically. Despite the presence of large curvatures, using the extended law to calculate the flame speed relative to the unburned gases is not convincing. Only the asymptotic formulation is relevant in determining both the speed relative to the unburned gases and that relative to the burned gases. Markstein lengths in the different cases, as a function of the equivalence ratio, are obtained with respect to both the unburned and burned gases. It is shown that the length, and consequently the flame speed, depends on the choice of the isotherm contour on which they are evaluated. Using the determination of the flame speed relative to the burned gases, it is suggested that the luminous contour gives the best estimation of the pocket dynamics, compared with the schlieren and shadowgraph contours.

Non-Premixed Lifted Flame Stabilization Coupled with Vortex Structures in a Coaxial Jet

An experiment concerning the non-premixed lifted flame stabilization studies the coupling between the flame behavior and the vortices developed in the inner mixing layer of a coaxial air/methane jet. It completes a previous global approach in which we show that “macro parameters” (potential cores, inner jet spreading) were efficient to describe the overall lifted flame behavior. Here, the jet dynamics is locally determined in vertical and transverse planes by means of particle image velocimetry and time resolved laser tomography. A systematic identification of vortices, obtained from PIV fields, is developed from a criterion proposed by Graftieaux et al. The analysis relies on the quantification of the Kelvin-Helmholtz vortices and the secondary streamwise vortices. Local parameters characterizing the vortices are quantified as functions of the air flow rate velocity, Uo. They contribute to formulate a scenario explaining how a small change of Uo produces a great change in the flame behavior.

Atomization and deviation of cylindrical water jets in a transverse acoustic field

This work deals with a particular breakup mode experienced by cylindrical liquid jets when submitted to an intense transverse acoustic wave. Experiments on low speed water jets (< 1 m/s) of diameters 3 mm and 6 mm show that sound waves with a frequency ranging from 500 Hz to 1800 Hz can produce bulges along the jet. When the sound level is high enough, these bulges can trigger an effective atomization mechanism where the jet flattens as a liquid sheet before disintegrating. Sound field can also induce steady deviation of the jet. Both phenomena are theoretically studied. A first model, which treats bulges as outward marks of one particular mode of vibration of the liquid column, is proposed. This model leads to a criterion for the onset of atomization that satisfactorily agrees with experimental observations of the present work. A second analysis identifies deviations as radiation pressure effects. It predicts the direction of experimental deviations with success.