Delin Zhu

Morphology and burning rates of expanding spherical flames in H2/O2/inert mixtures up to 60 atmospheres

Recognizing that previous experimental studies on constant-pressure, outwardly propagating, Spherical flames with imaging capability were limited to pressures less than about 5 atm, and that pressures within internal combustion engines are substantially higher, a novel experimental apparatus was designed, to extend the environmental pressure to 60 atm. Results substantiate previous observations of the propensity of cell formation over the flame surface due to hydrodynamic and diffusive-thermal instabilities and provide convincing evidence that wrinkled flame is the preferred mode of propagation in hydrogen/air mixtures in environments with pressures above only a few atmospheres. It is further shown that, by using helium as the diluent, and by reducing the oxygen concentration of the combustible, diffusional-thermal instability can be mostly suppressed and the hydrodynamic instability delayed. Stretch-free laminar flame speeds were subsequently determined for such smooth flames up to 20 atm and were compared with the calculated values, allowing for detailed chemistry and transport.

Outward propagation, burning velocities, and chemical effects of methane flames up to 60 ATM

Using a specially designed high- and constant-pressure combustion chamber, the propagation and morphology of spark-ignited expanding spherical methane flames were imaged using schlieren cinematography and a high-speed digital camera. Stretch-free laminar burning velocities were subsequently determined for methane/air flames up to 20 atm and methane/oxygen/helium flames up to 60 atm. Computational simulation using GRI-MECH 3.0 showed satisfactory agreement with the experimental data up to 20 atm, and moderate deviation for pressures above 40 atm. Markstein lengths, global activation energies, and overall reaction orders were also determined as functions of pressure, with the latter two parameters exhibiting non-monotonic behavior caused by the changeover from H-O2 to HO2 chemistry similar to that of the explosion limits of homogeneous hydrogen/oxygen mixtures.

Experimental and numerical determination of laminar flame speeds: Mixtures of C2-hydrocarbons with oxygen and nitrogen

Using the counterflow flame technique, laminar flame speeds of mixtures of ethane, ethylene, acetylene and propane with oxygen and nitrogen have been accurately determined over extensive lean-to-rich fuel concentration ranges and over the pressure range of 0.25 to 3 atm. These data are then compared with the numerically calculated values obtained by using the various kinetic schemes in the literature as well as one compiled in the present study. The present scheme yields close agrrement with all of the experimental flame speeds except for diluted, rich acetylene flames, for which the calculated values are higher. The relative importance and influence of the individual reactions on the flame speed and reaction mechanism are assessed and discussed with the aid of sensitivity analysis. The study also demonstrates that C2 schemes validated through comparisons based on methane flame speeds may not be accurate enough for flame speed predictions of the C2 fuels, and that the C2 schemes developed through comparisons with the flame speeds of the C2 fuels are rather insensitive to the details of the C3 sub-mechanism. The importance of having accurate values of the thermophysical properties of radicals for flame simulation is also emphasized.

An Experimental Study of Ignition in Nonpremixed Counterflowing Hydrogen versus Heated Air

ABSTRACT A variable pressure, counterflow combustion chamber has been built for the experimental investigation of ignition in a convective-diffusive system. In this paper we present our results on ignition of nonpremixed, counterflowing jets of 20% H2 in N2 versus heated air, within a wide range of pressures and flow strain rates. The system was brought to ignition by increasing gradually the temperature of the air stream. Each steady-state situation just prior to ignition was characterized by measuring detailed centerline axial flow velocity and temperature distributions, for ambient pressures between 0.1-6.0 atm and pressure-weighted strain rates between 50-400 s−1. The ignition temperature, defined as the maximum temperature of the air jet just prior to ignition, was found to increase with increasing flow strain rate at all pressures. Furthermore, its sensitivity to strain-rate variations was found to be much higher at elevated and reduced pressures (above ∼ 2 atm and below ∼ 0.5 atm) than it was at at...

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 ...

Optically accessible high-pressure combustion apparatus

The design and operation of a novel optically accessible high-pressure combustion apparatus is presented. The apparatus provides optical access for the direct observation of the morphology and development of premixed reaction fronts at elevated pressures. A chamber-in-chamber design with an innovative connecting system allows for safe, constant-pressure measurements, alleviating the extreme overpressures encountered in high-pressure combustion processes within closed bombs. Auxiliary design features include gap-adjustable electrodes for spark ignition and ports for jet stirring. As a result, the apparatus is well suited for the study of laminar premixed flames, flame instabilities, turbulent flames, and detonations. Results from the study of centrally ignited hydrogen and methane fuels in oxygen-inert mixtures up to 60 atm initial pressure demonstrate the suitability of the apparatus for high-pressure combustion experiments.

Microgravity Burner-Generated Spherical Diffusion Flames: Experiment and Computation

Abstract Microgravity experiments were conducted in the 2.2-s drop-tower facility at the NASA Glenn Research Center to study the transient response of the burner-generated spherical diffusion flame caused by its initial displacement from the steady-state position. The experiment involved issuing H2/CH4/inert mixtures of constant fuel mass flow rates from a bronze, porous, 1.27-cm-diameter, spherical burner into atmospheric air. The experimental results on the flame trajectory were found to agree well with those obtained through fully transient computational simulation with detailed chemistry and transport, and appropriate initial conditions. Furthermore, although steady-state behavior should exist for such flames, the experimental and computational results indicated that it cannot be reached within the 2.2-s microgravity duration for the fuels and mass-flow rates tested. To assess the role of radiation on the flame dynamics and extinction, computations were performed without radiation, with radiation employing the optically thin approximation, and with radiation utilizing a detailed emission/absorption statistical narrow band (SNB) model. The computation showed that while the influence of radiative heat loss on the position of the flame is small, proper consideration of radiative effects is crucial in assessing the state of flame extinction. Specifically, while all simulations of the experimental cases studied incorporating radiative heat loss revealed that the flame extinguishes well before the attainment of steady state, simulations accounting for gaseous reabsorption of radiative emissions were required to adequately represent the experiments in terms of extinction time, with the optically thin simulations predicting premature extinction during the flame expansion process. Effects of heat loss to the porous burner were also examined, and the lack of correspondence between the visible flame luminosity and flame strength, as related to flame temperature and heat release rate, was noted.

Aerothermochemical studies of energetic liquid materials: 1. Combustion of HAN-based liquid gun propellants under atmospheric pressure☆

The gasification and microexplosion characteristics of droplets of liquid gun propellants under atmospheric pressure have been experimentally investigated. Results show that the propellant explosion temperature is around 200°C and is substantially in excess of previously reported values. The droplet surface regression rate prior to the onset of microexplosion is found to be close to that of water and therefore is insensitive to the water content in the propellant as well as the oxygen concentration in the hot environment; these results demonstrate the dominance of water in the gasification process. It has also been determined that the propellant density attains a critical value of about 1.5 g/cm3 at the state of microexplosion.

Experiments and numerical simulation on the laminar flame speeds of dichloromethane and trichloromethane

The laminar flame speeds of blends of dichloromethane and trichloromethane with methane in air at room temperature and atmospheric pressure were experimentally determined using the counterflow twin-flame technique, varying both the amount of chlorinated compound in the fuel and the equivalence ratio of the unburned mixture. A detailed kinetic model previously employed for simulation of chloromethane combustion was expanded to include the oxidation kinetics of dichloromethane and trichloromethane. Numerical simulation shows that the expanded kinetic model predicted the flame speeds to within 3 cm/s of the measured values. Carbon flux and sensitivity analyses indicate that the reaction kinetics of the methane flame doped with chlorinated methanes are qualitatively similar, despite the variation in the chlorinated methane fuel structure.

Laminar Burning Velocities of Trifluoromethane-Methane Mixtures--Experiment and Numerical Simulation

Abstract The laminar burning velocities of trifluoromethane(CHF3)–methane(CH4)–oxygen(O2)–diluent mixtures were determined over extensive fuel concentration ranges using the counterflow flame technique. Numerical simulation was performed by employing a detailed kinetic model compiled on the bases of the GRI-Mech for methane combustion and a recent CHF3 reaction kinetic model. Comparisons between the experimental data and numerical results indicate that while the qualitative experimental trends are well predicted by the model, there exist significant numerical disagreements between model and experiment. Through sensitivity and flux analyses, we identified several rate parameters which are influential to burning velocity predictions, and proposed reasonable adjustments to these parameters either based on new experimental measurements or by considering their associated uncertainties. The effect of CHF3 addition on the reduction of CH4 burning velocities was also experimentally and numerically examined. By substituting the inert gas in the unburned CH4–O2–inert mixture with CHF3 while maintaining a constant adiabatic flame temperature, both burning velocities and mass burning rates decrease with an increase in CHF3 substitution, thus demonstrating positively the combined kinetic and transport effects of CHF3 on burning velocity reduction and in flame inhibition.