Robert J. Cattolica

OH radical nonequilibrium in methaneair flat flames

Abstract The OH radical concentration and rotational temperature were measured in atmospheric pressure methaneair flat flames using absorption spectra obtained with a tunable dye laser. The OH radical nonequilibrium and the associated recombination in the post flame gases were observed for fuel-air equivalence ratios of 0.8, 0.9, 1.0, 1.1, 1.2. The predicted peak OH concentrations from one-dimensional laminar flame models were found to be in good agreement with the measured values. The OH decay profiles were consistent with a second-order dependence of the decay on the OH concentration. Although this dependence is associated with the recombination chemistry for the fuel-rich flames, it appears that for the lean flames diffusional transport is responsible. The OH decay rate constant was nearly the same for the stoichiometric and lean conditions, but was found to increase substantially for the fuel-rich conditions.

Laser-induced fluorescence of the CH molecule in a low-pressure flame

Abstract The laser-induced fluorescence from [ A 2 Δ ( υ ′ = 0)→ X 2 Π ( υ ″ = 0)] band of the CH radical was studied in a low-pressure (20 torr) methane-oxygen flame (φ = 1.06). A time-resolved fluorescence technique was used to measure the relative CH concentration profile and the quenching of the A 2 Δ excited state through the flame. The pressure dependence of the quenching was also measured and used to determine an effective quenching cross section of 6 A 2 in the CH 4 -O 2 flame. Analysis of the fluorescence spectra scanned at different delays after the laser excitation, according to a pseudo-three-level model, yields a rotational energy transfer (RET) rate in the A 2 Δ(υ′ = 0) electronic state which is a factor of four faster than the electronic quenching rate of 1.57 × 10 7 sec -1 in the flame at 2000 K.

Combustion-torch ignition: Fluorescence imaging of OH concentration

Abstract The temporal and spatial development of the OH concentration during the ignition of a lean methaneair mixture (φ = 0.65) by a combustion torch has been studied. In the experiment the combustion torch was formed by a jet of high temperature combustion product gases that exit a thin-plate circular orifice connecting a small cylindrical prechamber with the main combustion chamber. This starting jet was driven by a spark-ignited flame which propagates through the prechamber. By decreasing the diameter of the prechamber orifice the initial gas velocity of the combustion torch was systematically increased. With this variation in velocity the flow field of the combustion torch, determined from high-speed schlieren videography, was altered significantly. At the lowest velocity a laminar vortex-ring structure was formed. As the velocity was increased the combustion-torch flow field develops the features of a highly turbulent jet. The fluid physics of the combustion torch has a significant influence on its chemical structure and the development of the subsequent ignition process in the main combustion chamber. Unique observations of the chemical structure of the combustion-torch ignition process were obtained by quantitative imaging of the OH concentration using laser-induced fluorescence.

Rotational-level-dependent quenching of OH A 2Σ(v′ = 1) by collisions with H2O in a low-pressure flame

Abstract The collisional quenching of OH A 2Σ(v′ = 1) by H2O was determined from laser-induced fluorescence measurements in the burned-gas region of stoichiometric H2/O2/Ar flames at low pressure (19 and 38 Torr). The collisional quenching was measured as a function of the initial laser-populated rotational level for N′ =4, 5, 8, 9, 10 and 11). The quenching cross section after laser excitation to N′ = 4 is 32±2.5 A2 at a temperature of 1048 K, approximately 40% of its value at room temperature. The quenching cross section decreases by 17% as the initial laser-excited rotational level is increases from N′ = 4 to N′ = 11. This variation in quenching rate has a significant effect on the determination of OH rotational temperature from laser-excitation spectra. Accounting for the variation of quenching with rotational level reduces the rotational temperature by 10% at 1100 K. The collisional quenching cross section of OH A 2Σ(v′ = 1) by H atoms was calculated to be 10±3 A2 for N′ = 5 at a temperature of 1048 K.

Electronic quenching and vibrational relaxation of no A2Σ(υ′=1 and υ′=0) by collisions with H2O

Abstract Temporally-resolved measurements of laser-induced flourescence from the A2Σ state of nitric oxide in a series of low-pressure (19, 38, and 76 torr), stoichiometric H2/O2/Ar flames were used to obtain electronic quenching and vibrational energy-transfer rates. Using a three-level rate-equation system to model the evolution of the flourescence signal from the directly excited A2Σ(υ′=1) state and the collisionally-populated A2Σ(υ′=0) state, we have determined the electronic quenching cross sections for the υ′=1 (36±2 a2) and υ′=0 (35 ± 3 a2) levels, as well as the vibrational energy transfer (υ′=1–0) cross section (1.8 ± 0.4 a2) for NO due to collisions with H2O at 1340 K. With the temperature lowered from 1340 to 1050 K, the electronic quenching cross sections remain nearly constant while the vibrational energy transfer cross section increased to 3.2±0.5 a.

Linear imaging of gas velocity using the photothermal deflection effect.

A new technique for measuring a single spatial component of gas velocity at many points simultaneously along a line using the laser-induced photothermal deflection effect is presented. In this technique a pump beam from a CO2 laser is focused and crossed with a planar probe beam from an argon laser above a low velocity gas jet. When the CO2 laser is pulsed, an ethylene/nitrogen mixture flowing out of the jet absorbs it; the resulting temperature and index of refraction gradient deflects the probe beam plane. The velocity distribution across the jet strongly affects the shape and amplitude of the probe beam deflection. The deflection is recorded with a high-speed 2-D array camera and the images are processed to give the velocity distribution across the jet. Velocities were measured for four different flow rates and also for a shear layer. The velocity distributions have the expected parabolic shape and the peak velocities agree with those calculated from the flow rate. A previously derived expression for the deflection angle was used to calculate the deflection of the planar probe beam as a function of time. The calculated images appear very similar to the measured ones.

Laser-fluorescence measurements of nitric oxide in low-pressure H2/O2/NO flames

The concentration profiles of NO in low-pressure (76 Torr) H 2 /O 2 /Ar flames to which nitric oxide was added (0.94% and 1.74% in argon), were measured by pulsed, laser-induced fluorescence. Laser excitation of NO in the (1,0) band of the A 2 Σ→X 2 Π transition at 214.34 nm was followed by detection, either time-integrated or temporally resolved, of the fluorescence in the (1,4) band at 252.2 nm. The temporally resolved fluorescence measurements were used to determine the collisional de-excitation rates needed to convert time-integrated fluorescence signal into nitric oxide concentration. Five flames were studied with H 2 /O 2 equivalence ratios of 0.88, 0.98, 1.22, 1.37, and 1.50. In these flames the collisional deexcitation rate decreases rapidly above the burner surface as the density decreases with increasing temperature. A 20% decrease was observed for the lean flames, and a 30% decrease for the rich flames. For the near stoichiometric flame, the measured de-excitation rate constant was used to determine a collision cross section of 38 2 at 1414 K for collisional deexcitation of the A 2 Σ ( v ′=1) state (including both electronic quenching and vibrational relaxation) by water. Within the precision of the measurement technique (±10%), no significant removal of nitric oxide was observed in these flames.

Linear imaging of gas velocity using the photothermal deflection effect

The velocity profile across a cold laminar jet of a mixture of ethylene and nitrogen gas was measured with the photo‐thermal velocimetry technique for a series of volumetric flow rates. (AIP)

OH radical nonequilibrium in methane-air flat flames*†

The OH radical concentration and rotational temperature were measured in atmospheric pressure methane-air flat flames using absorption spectra obtained with a tunable dye laser. The OH radical nonequilibrium and the associated recombination in the post flame gases were observed for flue-air equivalence ratios of .8, .9, 1.0, 1.1, 1.2. Predicted peak OH concentrations from one-dimensional laminar flame models1–3 were found to be in good agreement with those observed in the experiment. Both the measured and laminar flame model4 predicted OH decay profiles were consistent with a second order dependence of the decay on the OH concentration, i.e., d[OH]/dt=−α[OH]2[M]. For the fuel-rich flames this second order dependence can be derived analytically from the recombination chemistry and partial exquilibrium arguments5. An integration of the chemical rate equations6 governing the chemistry in the recombination dominated post flame region of the fuel-rich flames also leads to this second order dependence. For the fuel-lean flames, the integration of the chemical rate equations yields OH decay profiles which are not second order in [OH]. For these flames, it appears that diffusional transport effects are responsible for the observed second order behavior of the OH decay profiles. The decay rate constant, α, characterizing the measured OH decay was found to remain relatively constant for the lean and stoichiometric flames and to increase substantially for fuel-rich conditions. This result agrees, with the laminar flame models, and is consistent with the integration of the chemical rate equations and an analytic derivation for the fuel-rich flames. Values of the decay rate constant, α, measured in this experiment are comparable to those obtained from other experimental observations7 at similar temperatures. Decay rate constants obtained at somewhat lower temperature8 are two orders of magnitude larger than those found in the present work. This increase in the decay rate constant cannot be attributed to the temperature dependence of the recombination rate constants. Recent measurements9 duplicating the conditions of ref. 8 indicate that the (OH) is a factor of 100 larger than was reported. This error accounts for the unexpectedly large decay rate constants which were reported.