Low-pressure Hydrocarbon Flames Research Articles

Direct quantification of O-atom in low-pressure methane flames by using two-photon LIF

Quantification of the O-atom concentration is particularly important for testing the detailed kinetic mechanisms in flames. In this work, relative concentrations of O-atoms are measured by using the two-photon Laser-Induced Fluorescence method (TPLIF). The direct calibration for determining the absolute O-atom mole fraction is achieved by comparing alternately the integrated TPLIF signals from the atomic oxygen and from a known quantity of a noble gas (Xe). Both species have close spectral features with a two-photon absorption around 225 nm, while the fluorescence signals are collected around 844 and 834 nm for O-atom and Xe, respectively. Application of this method previously adopted in plasma environments is demonstrated here for the first time in low-pressure hydrocarbon flames. One major achievement in this work is that the O and Xe fluorescence signals are measured in the flame, thus allowing to properly correcting for the quenching rate of both species in this high temperature environment. Due to the fast quenching rates of O-atom and of Xe in the N2 diluted flames, a specific flame diluted in Ar was stabilized at 2.8 kPa for the calibration procedure. From Stern-Volmer plots obtained in a large pressure range of flames, the quenching rate determination could be extended to nitrogen-diluted flames. Then, the O-atom profiles were measured in absolute mole fraction in three low-pressure (5.3 kPa) premixed flames of CH4/O2/N2 with equivalence ratios equal to 0.8, 1.0 and 1.25. The experimental profiles O-atom mole fraction profiles are compared to the simulated ones. Their gradient locations and the absolute mole fractions at 25 mm are found in good agreement. However, the simulated profiles do not capture the experimental continuous decrease of O in the burnt gases.

Investigation of NCN and prompt-NO formation in low-pressure C1–C4 alkane flames

Temperature, CH, NCN, and NO profiles were measured for eight low-pressure hydrocarbon flames fueled by methane, ethane, propane, and butane using laser-induced fluorescence (LIF) diagnostics. These measurements were used (1) to assess NCN and prompt-NO formation chemistry across a series of fuels of increasing number of carbons at different equivalence ratios ( ϕ = 1.07 and 1.28); (2) to examine the predictive capabilities of current C1–C4 hydrocarbon and NCN formation/consumption combustion mechanisms on properly capturing prompt-NO formation and (3) to examine the postulation that additional prompt-NO precursors (other than CH) exist for fuels larger than methane. For a given equivalence ratio, the measured peak CH concentration is fairly constant across all four fuels, while both the peak NCN and post-flame NO concentrations steadily increase. Furthermore, it is found that as the fuels increase in number of carbons, i.e., methane to butane, the correlation between the peak NCN and post-flame NO remains high, while the correlation between peak CH and peak NCN and peak CH and post-flame NO becomes increasingly lower. This is especially evident for rich flame cases. The experimental profiles are compared to numerical calculations using two comprehensive kinetic mechanisms suitable for C4 chemistry, where the CH + N 2 → NCN + H reaction is assumed as the only prompt-NO initiation reaction. For the ϕ = 1.28 flame cases, CH is over-predicted using both mechanisms for all four fuels and by as much as 60%, while for the ϕ = 1.07 cases, CH is predicted to within 15% of the experimentally-derived results, although there is some discrepancy concerning the spatial locations of the CH profiles. For both NCN and NO, there is an increasing under-prediction for the ϕ = 1.28 cases as the fuel increases in number of carbons, while for the ϕ = 1.07 cases there is a systematic under-prediction of NCN and NO with a weaker (although evident) fuel dependence. From the experimental results and the comparison to modeling predictions, it is apparent that additional work concerning CH formation and consumption kinetics is necessary to accurately capture the CH concentration profiles across a broad range of conditions. Furthermore the comparisons to the modeling predictions using only a single prompt-NO precursor, CH, indicate a reasonable plausibility that (an) additional prompt-NO precursor (s) exist and become important when considering fuels larger than methane, especially under rich flame conditions. Possible precursors in addition to CH are discussed.

06/01133 C 2 Swan band laser-induced fluorescence and chemiluminescence in low-pressure hydrocarbon flames : Smith, G. P. et al. Combustion and Flame, 2005, 141, (1–2), 66–77

06/01133 C 2 Swan band laser-induced fluorescence and chemiluminescence in low-pressure hydrocarbon flames : Smith, G. P. et al. Combustion and Flame, 2005, 141, (1–2), 66–77

Fiber laser intracavity absorption spectroscopy of ammonia and hydrogen cyanide in low pressure hydrocarbon flames

Fiber laser intracavity spectroscopy (FLICAS) with Er-doped fiber laser has been applied for the first in situ detection of ammonia and hydrogen cyanide in a low pressure (30 Torr) stoichiometric methane/nitrogen/oxygen flame doped with 0.13–1.36% of NH 3. High resolution spectra of NH 3 and HCN have been recorded in the range of 1.5 μm (6440–6540 cm −1). The minimum detectable concentration of HCN and NH 3 at a laser pulse duration of 8 μs and room temperature is 1 × 10 13 molecule/cm 3. The method can be used for in situ detection of many other molecules in the hostile combustion environment.

C 2 Swan band laser-induced fluorescence and chemiluminescence in low-pressure hydrocarbon flames

Laser-induced fluorescence from the excitation of the C 2( d- a) 2 , 0 band at 438 nm is used to determine spatial profiles and relative lower state concentrations in several low-pressure hydrocarbon–air premixed flames, to determine the fuel-rich flame kinetics. Quenching loss rate constants are derived from the measured fluorescence decay rates. Quantitative Swan band chemiluminescence intensities, coupled with computer modeling of the flame kinetics and excited state loss rates, lead to recommended values of rate constants for the excited C 2 ( d ) -state chemiluminescence production kinetics.

Intracavity laser absorption spectroscopy measurements of CN using red system A–X. Simultaneous observation of CN, NH 2, HNO and [formula omitted] in low pressure hydrocarbon flames doped with nitrogen oxides

Absorption spectra of the CN radical have been measured by Intracavity Laser Absorption Spectroscopy in hydrocarbon flames doped with minor amount of nitrogen oxide. Spectra of the CN red band system ( A 2Π← X 2Σ + ) have been observed in the spectral range of 642 nm along with lines of other species from flame: NH 2, HNO, 1 CH 2 , and NO 2. The sensitivity of about 2.3×10 10 cm −3 at 1700 or ∼100 ppb at 30 Torr at 1700 K has been demonstrated.

Laser absorption spectroscopy diagnostics of nitrogen-containing radicals in low-pressure hydrocarbon flames doped with nitrogen oxides.

Absolute concentration profiles of NH2 and HNO have been measured in low-pressure methane/air flat flames doped with small amounts of NO and N2O. Addition of a small amount of nitrogen oxides does not alter significantly the flame speeds, temperature profiles and other parameters of the relatively well-understood methane/air flames. Intracavity laser absorption spectroscopy (ICLAS) and cavity ring-down spectroscopy (CRDS) are high-sensitivity techniques used to measure absolute concentrations of minor species in flames. In this work ICLAS is used to monitor NH2 and HNO, whereas CRDS is used for temperature measurements using OH spectra in the UV range. The (090)-(000) and (080)-(000) bands of the A2A1-X2B1 electronic transition of NH2 and (100)-(000) and (011)-(000) bands of the A1A"-X1A' transition of HNO are used. Methane flames of different equivalence ratios are used. NH2 and HNO are observed in the flame as well as in the zone surrounding the flame, closer to the walls of the low-pressure chamber where the burner is located. An absorption originating from the species in this zone can affect substantially the results of line-of-sight experiments. A slow flow of nitrogen through the optical window holders was added in order to separate the spectra of HNO originating from the central flame zone. Calculations based on the commonly used GRI-Mech chemical mechanism predict two maxima in the HNO concentration profile in the NO doped flames. The first is located in the vicinity of the burner, and the second is closer to the luminescence flame zone. We were able to observe the first maximum, and its measured location agrees well with prediction. On the other hand, GRI-Mech strongly underpredicts the observed absolute concentration of HNO in this maximum. The measured absolute concentrations of NH2 are in reasonable agreement with the GRI-Mech predictions.

Temperature measurement in fuel-rich non-sooting low-pressure hydrocarbon flames

Several temperature measurement techniques were compared for the investigation of fuel-rich, premixed, flat, low-pressure flames of propene, acetylene, and cyclopentene with maximum temperatures around 2400 K. Vibrational Stokes/anti-Stokes Raman measurements with a KrF excimer laser using CO, H2, and H2O as temperature indicators were examined at 50 and 200 mbar for different flame stoichiometries. Also, laser-induced fluorescence (LIF) of OH in the A-X band was used, and complementary lifetime measurements were performed to account for a variation of fluorescence quantum yield with rotational quantum number. LIF using seeded NO (0.2–1.0%) was applied both in point-wise and 1D temperature measurement in spite of the anticipated interaction of NO with the fuel-rich flame chemistry. Furthermore, thermocouple measurements were performed in the preheat zone. These techniques were compared with respect to accuracy and the potential for routine applications under fuel-rich low-pressure conditions.

Collisional Quenching of CH(A), OH(A), and NO(A) in Low Pressure Hydrocarbon Flames

Excited state lifetimes have been measured for the A-states of CH, OH, and NO in a number of low-pressure, premixed, laminar flow methane flames. From these lifetimes, collisional quenching rates were determined as a function of height above the burner and thus as a function of flame temperature and composition. The results were compared with values calculated using a model of the flame chemistry to predict collider mole fractions, together with parameterizations of quenching rate coefficients for each collider. Measured OH and NO quenching rates agree well with those calculated from these quenching rate coefficients and modeled flame composition data. This indicates that collisional quenching corrections for laser-induced fluorescence measurements can be calculated from knowledge of major species mole fractions and gas temperature. Predicted quenching rates for CH range from agreement with measured values to 27% higher than measured values. This discrepancies suggest insufficient knowledge of high temperature quenching by H 2O and N 2.

C2 creation, emission, and laser-induced fluorescence in flames and cold gases

We describe photochemical production of C2 in the upper (d3Πg) and the lower (a3Πu) levels of the Swan-band transitions by 266 and 292-nm laser irradiation of flames and room-temperature flows of acetylene and ethylene. Topics treated include the spectroscopy of the Swan bands, lifetimes and quenching of the Swan-band emission, intensity dependences of the Swan-band emission in several environments, profiles of C2 in low-pressure hydrocarbon flames, and the affect of Swan-band emission on three-photon-excited fluorescence detection of atomic hydrogen in hydrocarbon flames.

Time-resolved CH (A^2Δ and B^2∑^−) laser-induced fluorescence in low pressure hydrocarbon flames

Total collisional removal rate constants for the CH A(2)Delta and B(2)Sigma(-) electronic states are obtained in low pressure (<20-Torr) hydrocarbon flames. The B state is consistently removed ~70% faster than the A state. Variations of +/-50% are observed for different rotational levels and positions in the flame. For these flames, A-state emission following excitation of the B state indicates a rapid electronic-to-electronic energy transfer pathway that is insensitive to collision environment. Upper limits to the collider specific cross sections are obtained for H(2)O, N(2), and CO(2). The CH concentration and temperature profiles are measured and parametrized using a unique method.

Relaxation effects in electronically excited CH and C2 in low pressure hydrocarbon flames

Relaxation effects in electronically excited CH and C₂ in low pressure hydrocarbon flames

Energy distribution in electronically excited CH and C2 in low pressure hydrocarbon flames

A study has been made of the rotational energy distribution of electronically excited CH and of the vibrational energy distribution of electronically excited C2 radicals in low pressure (0.2 to 35 Torr) hydrocarbon flames. The rotational energy of the CH radical in the A2Δ (ν = 0) state has been found to be statistically distributed in the levels with quantum numbers 14 to 21. This distribution can be described by a temperature which has, however, no thermodynamic significance. In the methane–oxygen flame this temperature has been found to be independent of both the composition of the gas mixture and the pressure, whereas in the ethylene–oxygen system it varies with these parameters. Hydrogen–methane–oxygen flames behave as ethylene flames. Inert diluents such as nitrogen and carbon dioxide do not affect the temperature at very low pressures. This indicates that the energy distribution is not perturbed measurably by collisions before emission takes place. In order to explain these observations we have to accept that at least two reactions produce CH* in ethylene flames.The cross section for energy transfer out of the high rotational levels of CH(A2Δ) to flame gases is found to be smaller than 0.6 Å2 and that for transfer to CO2 is about 2 Å2.A statistical distribution also has been observed in the vibrational energy distribution of electronically excited C2. The corresponding temperature is about 6 500 °K and is independent of the composition of the gas mixture and of the fuel. It decreases very slowly with increasing pressure above 3 Torr. Inert diluents added to the gas mixture do not alter this temperature.The cross section for de-excitation of C2(A3πg) is found to be smaller than 2.5 Å2.