Cellular Flame Structure Research Articles

Electrical perturbation of cellular premixed propane / air flames

The phenomenon originally called polyhedral flame structure was first reported 100 years ago. Subsequent investigations showed that polyhedral structure was only one example of a more general phenomenon known now as cellular flame structure, and the range of combustion mixtures that produce them has been broadened to include lean mixtures of H[sub 2]/air, lean H[sub 2]/Br[sub 2], and rich mixtures of hydrocarbons from ethylene to octane with air. Of particular interest to the authors is the role of charged species in flames, and especially in flames that exhibit cellular structure. The electrical aspects of combustion has a long and distinguished history and this subject has been the subject of a classic monograph by Lawton and Weinberg. Electrical perturbation has been reported to affect the temperature of flames, to stabilize them at high flow rates and, in the absence of gravity, to change the speed of flame propagation, and to affect the amount of soot produced. The authors report here that premixed propane/air flames exhibiting cellular structure are quite susceptible to perturbation by electric fields. Since only charged species in the flame would be affected by the potential, and a small current would not modify transport properties of neutral species appreciably,more » this observation suggests that studies of this type may be useful in helping to further elucidate the role of charged species in flames.« less

Vibratory instability of cellular flames propagating in tubes

In this paper, we study the vibratory instability of a cellular flame, propagating downwards in a tube, which results from the coupling between the longitudinal acoustic modes of the tube and the modification of the cellular flame structure by the acceleration of the acoustic field. We assume that the wrinkling of the flame is of small amplitude a0, which is the case when the flame burning velocity is just above the critical velocity characterizing the Darrieus–Landau instability threshold. We demonstrate that, in this case, the growth rate of the corresponding thermoacoustic instability, non-dimensionalized with the acoustic frequency, is proportional to (kca0)2, where kc is the critical wavenumber of the cellular instability. If one extends the result up to amplitudes of the same order as the wavelength, then one obtains a relative growth rate of order unity which is much larger than the one obtained from the study of the vibratory instability of the planar flame. As is observed in experiments, the theory predicts that the primary sound is generated when the amplitude of the cells is sufficiently large that the fundamental tone becomes unstable first and that the vibratory instability for the fundamental tone occurs in the lower half of the tube. This suggests that the coupling between cellular flame and acoustic field studied here is the mechanism for primary sound generation.

Gravitational effects on cellular flame structure

An experimental investigation has been conducted of the effect of gravity on the structure of downwardly propagating, cellular premixed propane-oxygen-nitrogen flames anchored on a water-cooled porous-plug burner. The flame is subjected to microgravity conditions in the NASA Lewis 2.2-second drop tower, and flame characteristics are recorded on high-speed film. These are compared to flames at normal gravity conditions with the same equivalence ratio, dilution index, mixture flow rate, and ambient pressure. The results show that the cellular instability band, which is located in the rich mixture region, changes little under the absence of gravity. Lifted normal-gravity flames near the cellular/lifted limits, however, are observed to become cellular when gravity is reduced. Observations of a transient cell growth period following ignition point to heat loss as being an important mechanism in the overall flame stability, dominating the stabilizing effect of buoyancy for these downwardly-propagating burner-anchored flames. The pulsations that are observed in the plume and diffusion flame generated downstream of the premixed flame in the fuel rich cases disappear in microgravity, verifying that these fluctuations are gravity related.

On the hexagonal structure of cellular flames

The effect of hexagonal pattern formation in steady spherical flames is studied both analytically and numerically. It is shown that the hexagonal cells emerge as a subcritical bifurcation from the basic state provided the flame radius exceeds a certain critical value.

Chemical Kinetic and Thermal Aspects of Cellular Premixed Flames

Abstract The behavior of stationary and rotating polyhedral Bunsen flames of butane/air under systematic thermal and chemical modification of the reactive mixture is experimentally investigated. As the concentrations of additives CF3Br, N2 and O2 are gradually increased in the combustible mixture, the variation of the temperature in the trough and crest regions of the polyhedral flames is examined. It is found that addition of 0.36 (5.0) molar percentage of CF2 Br (N2) completely removes the cellular structure of the polyhedral flame. Also, in butane/air mixtures at different velocity, the critical temperatures corresponding to the onset of cell formation are found to be nearly identical. The results suggest that in conjunction to the hydrodynamics and thermodiffusive flame instability mechanisms, chemical-kinetic effects also play some role in the formation, amplification or sustenance of cellular flame structures. The observations are consistent with the recent flame stability theories which are based o...

Electron-beam-induced acoustic-wave enhancement of gaseous combustion

The combustion rate of premixed gases in a closed vessel was increased by injecting a high-current electron beam into the gas mixture within about 20 ms of spark ignition. This effect was observed with the fuels ethylene, methane, ethane, propane, and n-butane. Experimental results provide strong evidence that e-beam excitation of the fundamental longitudinal-acoustic mode of the cylindrical chamber is the mechanism of combustion enhancement. An observable combustion enhancement required that the amplitude of the fluid velocity oscillation in this acoustic mode be greater than or approximately equal to the flame propagation speed and was associated with a wrinkled or cellular flame structure with dimensions on the order of 1/2 cm. These results are in good agreement with values for the threshold acoustic velocity amplitude and dimension of cellular structure predicted for a periodically accelerated flame.

Cellular flame structure and turbulent combustion

Knowledge of flame interaction with perturbation of the surrounding medium is insufficient. This study investigates various parameters and methods for clarifying the combustion mechanism. The conditions for the appearance of a cellular structure were formed by analysis of diffusion phenomenon. Because such a spontaneous transition is characteristic of laminar regimes, photographs of laminar flames of hydrogen 2H2 plus (alpha) 2 plus nN2 are shown. The mixture becomes leaner as the flame surface breaks into pieces. It is determined that the more strongly the laminar flame is broken up, the steeper is the slope K /SUB p/ on the initial segment of the curve of turbulent burnup velocity as a function of pulsation velocity. Other examples are also given. The change in laminar flame surface with fixed excess oxidizer coefficient as inert gas concentration varies is studied. In conclusion, an approximate expression is derived which corresponds to burnup velocities which can be predicted for various operational regimes by this function.

On self-turbulization of a laminar flame

In the framework of the diffusional-thermal flame model, an asymptotic nonlinear differential equation is derived for the evolution of a disturbed spherical flame front. A quantitative description is presented of the formation of cellular flame structure and the subsequent self-turbulization of the flame. A cellular flame may form only when the combustible mixture contains a sufficiently light reactant in small concentration. If there is an excess of the light reactant, the laminar flame front remains smooth. An explanation is given for the anomalous stability of a spherical flame in comparison with a plane flame. The existence of a regular hexagonal cellular structure is shown.

The mechanism of interaction between a combustion region and acoustic resonator

This paper is intended to explain the mechanism of vibrations in a combustion chamber, appearing during the interaction of the turbulent flame and the acoustic resonator. The combustion chamber together with the resonator forms a self-sustained vibrating system, including a source of energy located within the flame, a mechanism controlling the energy release connected with the change of flame structure during vibrations, and a feedback consisting in the influence of acoustic vibrations on the change of the flame structure. The experimental results obtained for a fully developed turbulent flame showed that the combustion instability was closely related to the type of fuel and the composition of the fuel-air mixture. Though the combustion process proceeds according to the distributed volume reaction model, the combustion instability occurs exactly for fuel-air mixture characterized by cellular flame structure of laminar flames.

Cellular flame structure under conditions of a constant-volume bomb and its relationship with vibratory combustion

Cellular flame structure under conditions of a constant-volume bomb and its relationship with vibratory combustion

Cellular flame structure and vibratory flame movement in N-butane-methane mixtures

Cellular flame structure and vibratory flame movement in N-butane-methane mixtures