Suk Ho Chung

An integral analysis of the structure and propagation of stretched premixed flames

Abstract The structure and propagation of stretched premixed flames with preferential diffusion are analyzed using an approximate integral approach which yields algebraic relations for the characteristic scales, overall conservation requirements, and balances and continuity between the various scalar properties and their transport rates. Specific flame configurations studied include the one-dimensional steadily propagating planar flame, the steady curved flame in nonuniform flow, and the outwardly/inwardly propagating spherical flame. Linearized results agree with those obtained from asymptotic analysis assuming small stretch. The present work yields enhanced insight into the dominant and coupled physical processes governing these complex flame phenomena, and demonstrates the utility of the integral analysis in theoretical combustion studies.

Lifted flames in laminar jets of propane in coflow air

Characteristics of lifted flames in laminar axisymmetric jets of propane with coflowing air have been investigated experimentally. Approximate solutions for velocity and concentration accounting virtual origins have been proposed for coflow jets to analyze the behavior of liftoff height. From the measurement of Rayleigh scattering to probe the concentration field of propane, the validity of the approximate solutions was substantiated. From the images of OH PLIF and CH chemiluminescence and the Rayleigh concentration measurement, it has been shown that the positions of maximum luminosity in direct photographs of lifted tribrachial flames can reasonably be located along the stoichiometric contour. The liftoff height in coflow jets was found to increase highly nonlinearly with jet velocity and was sensitive to coflow velocity. The blowout and reattachment velocities decreased linearly with the increase in coflow velocity. These behaviors of liftoff height and the conditions at reattachment and blowout can be successfully predicted from the approximate solutions.

On the Structure and Extinction Dynamics of Partially-Premixed Flames: Theory and Experiment

Abstract The interaction and extinction dynamics of a partially-premixed flame ensemble, consisting of a premixed flame and a nonpremixed flame, are theoretically and experimentally studied for the counter-flow configuration generated by a premixed fuel/oxidizer/inert stream and a fuel/inert stream. Separate asymptotic analyses are performed for the structure and extinction of a single merged flame, a binary premixed-nonpremixed flame and a binary premixed-premixed flame; in the last case it is recognized that under certain situations a nonpremixed flame can exhibit the characteristics of a premixed flame. Theoretical results show that the extinction of a binary flame always occurs in a single stage, with the individual flames separated. The extent of separateness can be further modified by the preferential diffusion nature of the mixture. These predictions are completely substantiated by experimental results obtained by using mixtures consisting of methane, ethane or propane as the fuel and O2/N2 as the ...

Theory of laminar flame propagation in off-stoichiometric dilute sprays

Abstract The structure and propagation of a steady, one-dimensional planar, low-speed flame in a dilute, monodisperse, sufficiently off-stoichiometric and weakly-heterogeneous spray, with bulk gas-phase burning, upstream droplet vaporization and downstream droplet vaporization/combustion, is analyzed using activation energy asymptotics. A prevaporized mode and a partially prevaporized mode of flame propagation are identified. Results show that lean and rich sprays exhibit qualitatively opposite behavior in response to the extent of mixture heterogeneity; specifically, the burning intensities of lean and rich sprays are respectively reduced and enhanced with increasing liquid fuel loading and increasing initial droplet size. Classification of all possible spray burning modes as a function of the mixture stoichiometry and initial droplet size is also presented.

Characteristics of Laminar Lifted Flames in a Partially Premixed Jet

Abstract The liftoff and blowout characteristics of laminar flames in a partially premixed jet are studied for propane and n-butane fuels mixed with air. As the flow rate increases, flame lifts off from a nozzle attached flame and liftoff height increases highly nonlinearly with flow rate, and then blowout occurs. The jet velocities at liftoff and at blowout decrease linearly with the increase in air dilution and are independent of nozzle diameter. The liftoff height at blowout is proportional to the square of nozzle diameter and to the square of fuel mass fraction. Correlations for the liftoff heights and blowout conditions are derived by using a cold jet theory for velocity and species concentrations, based on the tribrachial nature of lifted flames. That is, at the flame anchoring point, the flame has the characteristics of a stoichiometric flame and its propagation speed balances axial flow velocity. The experimental findings of liftoff and blowout are successfully predicted.

Effect of electric fields on the liftoff of nonpremixed turbulent jet flames

The effect of electric fields on the liftoff of nonpremixed turbulent jet flames has been investigated by applying high-voltage alternate current (ac) to the nozzle of propane fuel. Flame liftoff velocities and liftoff heights were measured as functions of applied voltage and frequency. The fuel jet velocity at flame liftoff increased and flame liftoff height decreased with increasing voltage, implying that the range of flame stability can be extended with the ac charging. Meanwhile, the effect was minimal when applying direct current (dc). This stabilization effect with ac charging was also influenced by the frequency. As the applied voltage increased, a streamer corona was generated between the flame edge and the nozzle. When the jet velocity and, thus, the liftoff height becomes large, then the ac charging effect disappeared in such a way that the flame liftoff height became comparable to that of a free jet without applying voltage. The liftoff velocity was correlated linearly with voltage in the corona-free electric field enhanced regime.

Characteristics of lifted flames in nonpremixed turbulent confined jets

Characteristics of lifted flames in nonpremixed jets were studied experimentally with emphasis on the effects of the entrained flow field which was varied by placing a plate near the nozzle and by confining the jet. Results show that lifted flame behavior in a confined jet is drastically different from that of a free jet. In the confined jet, the liftoff height is linearly proportional to the nozzle diameter and the flow velocity, while the liftoff height is independent of the nozzle diameter in the free jet. The ratio of the liftoff height at blowout to the nozzle diameter maintains a near-constant value of 50 for both the free and confined jets. The blowout velocity is linearly proportional to the nozzle diameter in the free jet, whereas it is independent of the nozzle diameter in the confined jet. The jet velocity at liftoff maintains a near-constant value for the free jet, while the liftoff velocity decreases with the increase in the nozzle diameter for the confined jet. The blockage effect of the plate near the nozzle exit systematically reduces the liftoff height, and a criterion is proposed to include such an effect in interpreting liftoff behavior.

Propagation speed of tribrachial (triple) flame of propane in laminar jets under normal and micro gravity conditions

The propagation speed of tribrachial (triple) flames in laminar propane jets has been investigated experimentally under normal and micro gravity conditions. We found in the present experiment that the displacement speed varied nonlinearly with axial distance because the flow velocity along the stoichiometric contour was comparable to the propagation speed of tribrachial flame. Approximate solutions for the velocity and concentration accounting density difference and virtual origins have been used in determining the propagation speed of tribrachial flame and the concentration field was validated from the measurement of Raman scattering. Under the microgravity condition, the results showed that the propagation speed of tribrachial flame decreased with the mixture fraction gradient, in agreement with previous studies. The limiting maximum propagation speed under the microgravity condition is in good agreement with the theoretical prediction, ie, the ratio of maximum propagation speed to the stoichiometric laminar burning velocity is proportional to the square root of the density ratio of unburned to burnt mixture.

An experiment on the breakup of impinging droplets on a hot surface

Breakup characteristics of liquid droplets impinging on a hot surface are investigated experimentally with the wall temperatures in the Leidenfrost temperature range of 220–330°C for n-decane fuel. Factors influencing droplet breakup are wall temperature, impinging velocity, droplet diameter and impinging angle. The 50% breakup probability shows that the impinging velocity decreases linearly with the droplet diameter increase and there exists an optimum impinging angle near 80° having the minimum value in the impinging velocity for given wall temperature and droplet size. Near the wall temperature of 250°C corresponding to the Leidenfrost temperature, a peculiar nonlinear behavior in the breakup probability is observed.

NORMAL AND MICROGRAVITY EXPERIMENT OF OSCILLATING LIFTED FLAMES IN COFLOW

Characteristics of oscillating lifted flames in coflow have been investigated experimentally with highly diluted propane in both normal gravities and microgravities to elucidate the oscillation mechanism. In normal gravity, oscillating lifted flames with the frequency of 2–5 Hz were observed in a certain range of fuel mole fraction and jet velocity for the jet Reynolds number smaller than 100. The oscillation frequency in terms of the Strouhal number, defined in terms of the stoichiometric laminar burning velocity, correlated well with jet velocity. This demonstrated the importance of the propagation characteristics of tribrachial flames at the base of lifted flames in the flame oscillation Microgravity experiments have been conducted after the oscillating lifted flames were stabilized in normal gravity. During the change in gravitational acceleration, the flames rapidly stabilized as nozzle attached flames and became stationary. This behavior confirmed that the lifted flame oscillation in normal gravity is due to the buoyancy driven instability. Numerical simulation has also been performed accounting the change in gravitational acceleration. The results successfully predicted the transition behavior from oscillating lifted flame in normal gravity to stationary nozzle attached flame in microgravity and the variation of flame shapes during the transition.