John D. Mertens

Ignition and Oxidation of Lean CO/H2 Fuel Blends in Air

Ignition and oxidation characteristics of CO/H 2 fuel blends were studied using both experimental and computer simulation methods. Shock-tube experiments were conducted behind reflected shock waves at intermediate temperatures (890 < T < 1300 K) for three pressure regimes of approximately 1, 2.5, and 15 atm. Results of this study provide the first undiluted fuel-air ignition-delay-time experiments to cover such a wide range of CO/H 2 composition (5-80% H 2 ) over the stated temperature range. Emission in the form of chemiluminescence from the hydroxyl radical (OH*)A 2 Σ + → X 2 Π transition near 307 nm was used to monitor the reaction progress from which ignition delay times were determined. In addition to the experimental analysis, chemical kinetics calculations were completed to compare several chemical kinetics mechanisms with the new experimental results. The models were in excellent agreement with the shock-tube data, especially at higher temperatures and lower pressures, yet there were some differences between the models at the higher pressures and lowest temperatures, in some cases by as much as a factor of 5. Ignition-delay-time and reaction-rate sensitivity analyses were completed at higher and lower temperatures and higher and lower pressures to identify the key reactions responsible for ignition. The results of the sensitivity analysis indicate that the ignition-enhancing reaction H + O 2 = O + OH and hydrogen oxidation kinetics in general were most important, regardless of mixture composition, temperature, or pressure. However, lower-temperature, higher-pressure ignition-delay-time results indicate additional influence from HO 2 - and CO-containing reactions, particularly, the well-known H + O + M = HO 2 + M reaction and the CO + O + M = CO 2 + M and CO + HO 2 = CO 2 + OH reactions. Differences in the rates of the CO-related reactions are shown to be the cause of discrepancies among the various models at elevated pressures. Additional calculations were performed to show that the mixtures used are insensitive to small levels of water vapor, and the disagreement between experiment and model at the lowest temperatures and higher H 2 concentrations cannot be explained by possible impurities.

Design and teamwork: a must for freshmen

This paper presents the philosophy, content, and operation of a freshman-level course designed to introduce the basic principles of problem solving and decision making while the students are engaged in a semester-long design project. One of the major components of this course is the incorporation of teamwork into every facet of student activity. >

A shock tube study of the reactions of CN and NCO with NO2

The total rate coefficients for the reactions C N + N O 2 → p r o d u c t s (1) N C O + N O 2 → p r o d u c t s (2)were measured at temperatures between 1000 and 1600 K and pressures near 1.4 atm in a shock tube. Narrow-line laser absorption was utilized to record time histories of CN and NCO radicals in C 2 N 2 /NO 2 /Ar laser photolysis experiments. In the experiment, CN production is nearly instantaneous via photolysis of C 2 N 2 ; production of NCO radicals follows from the main product channel of reaction (1). A least-squares fit to the results for reaction (1) is given by the expression k 1 = 1.59 × 10 13 exp ⁡ ( 570 / T [ K ] ) c m 3 m o l − 1 s − 1 with corresponding uncertainty factors of f =0.91 and F =1.09 at the low-temperature extreme; f =0.84 and F =1.17 result at the high-temperature extreme. The uncertainty factors give the limiting values of the rate coefficient: k min = fk best fit , {itk max = Fk best fit . The rate coefficient of reaction (2) was measured as k 2 =4.5×10 12 cm 3 mol −1 s −1 ( f =0.75, F =1.27) for temperatures near 1250 K. Incorporating the data from the present study with the results from lower-temperature studies gives the following empirical expressions for the total rate coefficients valid for temperatures ranging from 300 to 1600 K: k 1 = 6.16 × 10 15 T [ K ] − 0.752 exp ⁡ ( − 173 / T [ K ] c m 3 m o l − 1 s − 1 k 2 = 3.25 × 10 12 exp ⁡ ( 356 / T [ K ] ) c m 3 m o l − 1 s − 1 .

A shock tube study of reactions of NCO with O and NO using NCO laser absorption

The absorption coefficient of NCO at 440.4790 nm (vac) was determined using a cw ring dye laser and shock-heated HNCO/N2O/Ar mixtures at temperatures and pressures ranging from 2180 K to 3070 K and 0.9 to 1.1 atmospheres, respectively. The source of NCO in these experiments was the reaction O+HNCO→OH+NCO (2a) where the O-atoms were produced by the thermal dissociation of N2O. This determination of the NCO absorption coefficient allows quantitative measurements in shock tube studies of elementary reactions involving the NCO radical. This new NCO diagnostic was used to study the reactions of NCO with O and NO in shock-heated HNCO/N2O/Ar and HNCO/NO/Ar mixtures, respectively. The second-order rate coefficient of the reaction NCO+O→products (3) was determined to be: k3=4.7×1013 cm3 mole−1 s−1 (f=0.74, F=1.30) T=2180–3070 K, where f and F define the lower and upper uncertainty limits, respectively. The second-order rate coefficient of the reaction NCO+NO→products (8) was determined over the temperature range of 2380 K to 2660 K. These results were combined with the lower-temperature results for k8 from three previous studies, resulting in the expression: k8=1.4×1018T−1.73exp(−384/T, K) cm3 mole−1 s−1 T=294–2660 K.

Ignition of Lean CO/H2/Air Mixtures at Elevated Pressures

Shock-tube ignition delay time experiments and chemical kinetics model calculations were performed for several fuel blends of carbon monoxide and hydrogen in air at elevated pressures. Due to the interest in coal-derived fuels, namely syngas, these data are important for characterizing the ignition and oxidation of possible fuel blends used in gas turbines and for the validation of chemical kinetics models. Three lean, CO/H2 (80/20%, 90/10%, and 95/5% by volume) fuel blends in air were studied behind reflected shock waves at temperatures between 929 and 1304 K and pressures ranging from 1.7 to 15 atm. Ignition delay times were monitored using chemiluminescence emission from excited hydroxyl radicals. Results exhibit the second-explosion limit behavior from hydrogen oxidation kinetics at low temperatures and high pressures for all mixtures. In addition, comparisons of modeling results and experimental data show good agreement for the entire temperature range at high pressure and poor agreement with the data at low temperatures in the intermediate pressure regimes. Ignition and reaction sensitivity analyses indicate that the H + O2 + M = HO2 + M termination reaction is important at all conditions herein, and the early formation of HO2 suppresses the growth of the ignition-enhancing radicals H and OH.Copyright

A shock tube study of H+HNCO→H2+NCO and the thermal decomposition of NCO

The reaction of atomic hydrogen with isocyanic acid (HNCO) to produce molecular hydrogen and the NCO radical, H+HNCO→H2+NCO (3a) and the thermal dissociation reaction NCO+Ar→N+CO+Ar (4) have been studied in shock-heated mixtures of HNCO dilute in argon. Quantitative time histories of the NCO radical were measured behind the shock waves using cw, narrow-linewidth laser absorption at 440 nm. The second-order rate coefficients of reactions (3a) and (4) were determined to be k3a=5.5×1014 exp(−13.700/T, K) (f=0.50, F=1.5) T=2260−3250 K k4=2.2×1014 exp(−27,200/T, K) (f=0.40, F=1.8) T=2370−3050 K cm3 mol−1 s−1, where f and F define the lower and upper uncertainty limits, respectively. The results for reaction (3a) were combined with the lower-temperature results from two previous studies for the reverse reaction (−3a), resulting in the expression: k3a=9.0×107 T1.66 exp(−7000/T, K) cm3 mol−1 s−1, T=591−3250 K The experimental results for k4 were used to calculate values of the collision efficiency (βx) and threshold energy (E0) for reaction (4) using unimolecular rate constant theory: βx=0.020 (−65%.+100%) E0=61.4±1.5 kcal/mol

Measurement of H2O2 Broadening Parameters near 7.8 μm with a Shock Tube

Hydrogen peroxide is an important intermediate species in the combustion of hydrogen and hydrocarbon-based fuels at low temperatures (850-1200K) and elevated pressures. Part of the reason for the importance of H2O2 is that the molecule produces a considerable amount of hydroxyl radicals prior to the ignition event, so it is important to have a good understanding of the kinetic reactions involving this species. In the past, a few groups–including the authors of this work–have investigated hydrogen peroxide at these elevated temperatures by using shock tubes [1]-[3]. The shock tube is an ideal experiment for investigating combustion chemistry at elevated pressures and temperatures of interest to this study. Measurements have also been made at temperatures below 900 K within static cells [4]-[6]. It is important to know how this species behaves experimentally in a combustion environment to develop and validate chemical kinetics models.

A Laser Photolysis Shock Tube Study of the Reaction of OH with NH3

The reaction of OH with NH3 has been studied in reflected shock wave experiments using laser photolysis of NH3/N2O/Ar mixtures. Quantitative time-histories of the OH(X 2Πi) radical were measured behind the shock waves using cw, narrow-line width laser absorption at 307 nm. OH was generated using post-shock laser photolysis of ammonia followed by the reaction of atomic hydrogen with N2O:

A shock tube study of the reactions of NH with NO, O2, and O

N{H_3} + \hbar \nu (193nm) \to N{H_2} + H{N_2}O + H \to {N_2} + OH