R.D. Cook

A Shock Tube Study of OH + H2O2 → H2O + HO2 and H2O2 + M → 2OH + M using Laser Absorption of H2O and OH

The rate constants of the reactions: (1) H2O2+M-->2OH+M, (2) OH+H2O2-->H2O+HO2 were measured in shock-heated H(2)O(2)/Ar mixtures using laser absorption diagnostics for H(2)O and OH. Time-histories of H(2)O were monitored using tunable diode laser absorption at 2550.96 nm, and time-histories of OH were achieved using ring dye laser absorption at 306 nm. Initial H(2)O(2) concentrations were also determined utilizing the H(2)O diagnostic. On the basis of simultaneous time-history measurements of OH and H(2)O, k(2) was found to be 4.6 x 10(13) exp(-2630 K/T) [cm(3) mol(-1) s(-1)] over the temperature range 1020-1460 K at 1.8 atm; additional measurements of k(2) near 1 atm showed no significant pressure dependence. Similarly, k(1) was found to be 9.5 x 10(15) exp(-21 250 K/T) [cm(3) mol(-1) s(-1)] over the same temperature and pressure range.

High-temperature shock tube measurements of dimethyl ether decomposition and the reaction of dimethyl ether with OH.

We measured the first high-temperature rate measurements of two dimethyl ether (DME) reactions, (1) DME + Ar --> CH3O + CH3 + Ar and (2) DME + OH --> CH3OCH2 + H2O, in a shock tube by monitoring OH radicals. OH was measured with a narrow-line width laser absorption diagnostic using the well-known R1(5) line of the A-X(0,0) transition at 306.7 nm. The rate k1 is in the falloff regime at high temperatures, so it was measured at several pressures from 0.6 to 11.5 atm and temperatures from 1349 to 1790 K. OH radicals were formed by shock-heating mixtures of DME and O2 in Ar. These mixtures take advantage of the rapid decomposition of the product CH3O, forming H-atoms, which react with O2 to form OH. In carefully chosen mixtures, OH concentration is primarily sensitive to k1 and the well-known rate of H + O2 --> OH + O. Uncertainty in the k1 measurements was estimated to be +/-35%. The rate measurements were then modeled using RRKM theory, which describes the data quite well. Both the rate measurements and the RRKM model were fit from 1000 to 1800 K using the Troe falloff form: k(1,infinity)(T) = (4.38 x 10(21))T(-1.57) exp(-42,220 K/T) s(-1), k(1,o) = 7.52 x 10(15) exp(-21,537 K/T) cm3 mol(-1) s(-1), and F(cent) = 0.454 exp(-T/2510). The rate of k2 was measured at pressures near 1.6 atm and temperatures from 923 to 1423 K. OH radicals were generated by the thermal decomposition of the OH precursor tert-butyl hydroperoxide (TBHP), and k2 was inferred from the observed decay of OH with an estimated uncertainty of +/-40%. The high-temperature measurements were compared with several rate evaluations and previous low-temperature measurements. The rate evaluation by Curran et al. of k2 = (6.32 x 10(6))T2 exp(328 K/T) (cm3 mol(-1) s(-1)) was found to be an excellent fit to both the previous low-temperature measurements and this work.

Mulit-Species Measurements Behind Reflected Shock Waves in Hydrocarbons Using Laser Absorption

There is a critical need for accurate kinetic targets to test and refine large reaction mechanisms for jet fuel, and other practical fuels and surrogates mixtures. To this end, concentration time-histories for four species: C2H4, OH, CO2, and H2O, were measured behind reflected shock waves during n-the oxidation of n-heptane, an important normal alkane fuel surrogate component. Experiments were conducted at temperatures of 1300 to 1600 K and a pressure of 2 atm using a mixture of 300 ppm n-heptane in stoichiometric oxygen (φ=1) in argon. Ethylene was monitored using IR gas laser absorption at 10.53 microns; OH was monitored using UV laser absorption at 306.5 nm; and CO2 and H2O were monitored using tunable IR diode laser absorption at 2.7 and 2.5 microns, respectively. These time-histories provide kinetic targets to test and refine reaction mechanisms for nheptane and also serve to demonstrate the potential of this type of data for validation of reaction mechanisms. Comparisons are made with the predictions of the Sirjean et al./JetSurF 1.0 (2009) reaction mechanism for n-alkanes.

A diode laser absorption sensor for rapid measurements of temperature and water vapor in a shock tube

Time-resolved measurements of gas temperature and water vapor concentration are made in a shock tube using a novel diode laser absorption sensor (100 kHz bandwidth). Gas temperature is determined from the ratio of fixed-wavelength laser absorption of two water vapor rovibrational transitions near 1.4 μ m, and H2O concentration is determined from the inferred temperature and the absorption for one of the transitions. Wavelength modulation spectroscopy is employed with second-harmonic detection to improve the sensor sensitivity. The sensor is validated in a static cell and shock tests with H

Shock tube measurements of ignition delay times and OH time-histories in dimethyl ether oxidation

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Multi-species time-history measurements during n-hexadecane oxidation behind reflected shock waves

O-Ar mixtures, yielding an overall accuracy of better than 1.9 % for temperature and 1.4 % for H2O concentration measurements over the range of 500-1700 K. The sensor is then demonstrated in a preliminary study of combustion in H2/O2/Ar and heptane/O2/Ar mixtures in the shock tube.