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Laminar flame speeds at elevated pressure for methane-based fuel blends are important for refining the chemical kinetics that are relevant at engine conditions. The present paper builds on earlier measurements and modeling by the authors by extending the validity of a chemical kinetics mechanism to laminar flame speed measurements obtained in mixtures containing significant levels of helium. Such mixtures increase the stability of the experimental flames at elevated pressures and extend the range of laminar flame speeds. Two experimental techniques were utilized, namely a Bunsen burner method and an expanding spherical flame method. Pressures up to 10 atm were studied, and the mixtures ranged from pure methane to binary blends of CH4/C2H6 and CH4/C3H8. In the Bunsen flames, the data include elevated initial temperatures up to 650 K. There is generally good agreement between model and experiment, although some discrepancies still exist with respect to equivalence ratio for certain cases. A significant result of the present study is that the effect of mixture composition on flame speed is well captured by the mechanism over the extreme ranges of initial pressure and temperature covered herein. Similarly, the mechanism does an excellent job at modeling the effect of initial temperature for methane-based mixtures up to at least 650 K.

The fluorescence and phosphorescence from liquid acetone was studied to aid the development of quantitative mixing measurements in liquid-fueled systems. The timeresolved spectrum of the acetone fluorescence and phosphorescence was captured under typical operating conditions using two excitation wavelengths (266 and 285 nm). A chamber of liquid acetone was purged and pressurized with nitrogen gas for various durations before the measurements. The lifetime for phosphorescence from liquid acetone was found to vary from <175 ns to 3.5 µs, depending on the duration of the N 2 pressurization, the detection wavelength and the excitation wavelength. The N 2 pressurization presumably removes dissolved O 2 and shows little enhancement in the overall lifetime beyond 24 hours in the current setup. The measured phosphorescence lifetimes increased with detection wavelength, which is related to the observed red shift in the phosphorescence spectra with delay time after the laser pulse. The phosphorescence to fluorescence ratio was measured to be 0.035 for 266 nm excitation and 0.080 for 285 nm excitation. The overall short lifetimes and low phosphorescence signal are related to the vibrational distribution established in the triplet state for the different excitation wavelengths.

There is currently a lack of experimental data required for kinetic model validation of the effect of oxidizer vitiation on laminar flame speeds of aviation fuels. This study examines the role of vitiation through the introduction of CO 2 and H 2 O to the oxidizer stream at varying pressures (0.5-5.0 atm) at 450 K, conditions relevant to vitiated combustion devices, using n-decane as the model fuel. The experimental portion of this effort has acquired laminar flame speed data of n-decane in vitiated air using two separate techniques. A well-validated Bunsen Flame Technique was used to primarily examine the effect of total dilution and vitiation over a range of equivalence ratios and the Combustion Bomb Technique was used to investigate vitiation effects at various pressures and equivalence ratios. Overlap between measurement techniques has been performed as well as comparison to an analytical model to better understand the thermodynamic and chemical kinetic effects that vitiation has on hydrocarbon fuel combustion and flame structure. Experimental data shows that CO 2 has the largest effect in reducing the flame speed over the range of equivalence ratios and pressures studied. Based on a kinetic analysis, chemical kinetic effects play a major role in reducing the flame speed when CO 2 is present. The impact of chemical kinetic effects due to the diluent species on flame speed was found to have the following trend: CO 2 > H 2 O > N 2. Nomenclature X n = mole fraction concentration of species n given [mole or vol %] Φ = fuel:air equivalence ratio: (fuel/air)/(fuel/air) stoich T Ad = adiabatic flame temperature [K] C P = molar heat capacity [J/mol-K] α = thermal diffusivity [m 2 /s] BFT = Bunsen Flame Technique

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Laminar flame speeds of lean H2/CO/CO2 (syngas) fuel mixtures have been measured for a range of H2 levels (20–90% of the fuel) at pressures and reactant preheat temperatures relevant to gas turbine combustors (up to 15 atm and 600 K). A conical flame stabilized on a contoured nozzle is used for the flame speed measurement, which is based on the reaction zone area calculated from chemiluminescence imaging of the flame. A O2:He mixture (1:9 by volume) is used as the oxidizer, rather than standard air, in order to suppress the hydrodynamic and thermo-diffusive instabilities that become prominent at elevated pressure conditions for lean H2/CO fuel mixtures. All the measurements are compared with numerical predictions based on two leading kinetic mechanisms: the H2/CO mechanism of Davis et al. and the C1 mechanism of Li et al. The results generally agree with the findings of an earlier study at atmospheric pressure: 1) for low H2 content (<40%) fuels, the model predictions are in good agreement with measurements at both 300 K and 600 K preheat temperature; but 2) the models tend to over predict the temperature dependence of the flame speed for medium (∼40–60%) and high (> 60%) H2 content fuels, especially at very lean conditions. The elevated pressure (∼15 atm) results, however, reveal that the effect is less pronounced than at atmospheric pressure. The exaggerated temperature dependence of the current models may be due to errors in the temperature dependence used for so-called “low temperature” reactions that become more important as the preheat temperature is increased. The radiation effects associated with CO2 addition to the fuel (up to 40%) is found to be less important for medium and high H2 content syngas fuels at elevated pressure and preheat temperature.

The photophysics of vapor and liquid acetone are experimentally examined from subcritical to supercritical conditions with 266 nm excitation, motivated by an interest in using acetone to study transcritical fuel injection and mixing. The fluorescence quantum yield for acetone vapor is measured, and the values are compared to the models of Thurber and Braeuer. The measured fluorescence yields are found to be similar to the model predictions. In addition, the two models are found to converge as the excitation wavelength or pressure increases. For 266 nm excitation, the difference between the model predictions is approximately 2% for pressures above 30 atm. Liquid acetone fluorescence and phosphorescence data are obtained at various pressures and temperatures. The liquid fluorescence signals do not change with pressure and temperature. In addition, liquid fluorescence is not affected by oxygen quenching, as compared to vapor fluorescence. It was found that the oxygen saturation concentration in liquid acetone is extremely small compared to the concentration needed to affect the fluorescence signal. The liquid phosphorescence is also found to be invariant with pressure, but it decreases exponentially as the temperature of the liquid is raised. A corresponding decrease in the measured phosphorescence lifetimes indicates that the phosphorescence yield is reduced asymptotically with temperature, with little change above ~375 K. This is likely due to acetone-acetone quenching of the excited triplet state. Most of the liquid acetone photophysics can be explained as a high density limit of the vapor photophysics models.