Propagation of Laminar Flames in Wet Premixed Natural Gas-Air Mixtures

The present work investigates the effect of adding small amounts of humidity on the inhibition of natural gas-air flames. The inhibition is quantified by measuring and calculating the laminar burning velocities (Su) of premixed ames from a C1-C2 mechanism. The experimental apparatus consists of a Mache-Hebra burner, equipped with flow controllers and air purification system. Steam is generated by injecting water into a preheated natural gas-air stream, by means of a syringe pump. The burning velocities are determined experimentally from the schlieren photography using the total flame area.The results indicate decreasing burning velocities with increasing steam concentration, demonstrating the importance of thermal capacity of water vapour on slowing down the flame propagation. There is no indication of flame acceleration due to kinetic considerations, even when the flames are doped with minute moisture loadings. It is shown in the calculations that the laminar burning velocity depends strongly on the number of grid points, and so a scaling relationship is developed for adjusting the computed values of Su. The kinetic model predicts closely the experimental results, but the agreement between the experimental and numerical data is better at lower temperatures. The relationship between Su and the concentration of the added water vapour, as calculated from the model, is linear. For the natural gas considered in this work, the laminar burning velocity at the atmospheric pressure decreases by 1.81cm s-1 at 150°C for each percentage point of humidity present in the gas mixture, and by 1.18 cm s−1 at 20°C

Chemical kinetic modeling of high pressure propane oxidation and comparison to experimental results

A pressure dependent kinetic mechanism for propane oxidation is developed and compared to experimental data from a high pressure flow reactor. The experiment conditions range from 10--15 atm, 650--800 K, and were performed at a residence time of 200 {micro}s for propane-air mixtures at an equivalence ratio of 0.4. The experimental results include data on negative temperature coefficient (NTC) behavior, where the chemistry describing this phenomena is considered critical in understanding automotive engine knock and cool flame oscillations. Results of the numerical model are compared to a spectrum of stable species profiles sampled from the flow reactor. Rate constants and product channels for the reaction of propyl radicals, hydroperoxy-propyl radicals and important isomers with O{sub 2} were estimated using thermodynamic properties, with multifrequency quantum Kassel Theory for k(E) coupled with modified strong collision analysis for fall-off. Results of the chemical kinetic model show an NTC region over nearly the same temperature regime as observed in the experiments. The model simulates properly the production of many of the major and minor species observed in the experiments. Numerical simulations show many of the key reactions involving propylperoxy radicals are in partial equilibrium at 10--15 atm. This indicates that their relative concentrations are controlled by a combination of thermochemistry and rate of minor reaction channels (bleed reactions) rather than primary reaction rates. This suggests that thermodynamic parameters of the oxygenated species, which govern equilibrium concentrations, are important. The modeling results show propyl radical and hydroperoxy-propyl radicals reaction with O{sub 2} proceeds, primarily, through thermalized adducts, not chemically activated channels

Chemical kinetic modeling of high pressure propane oxidation and comparison to experimental results. Revision 1

A pressure dependent kinetic mechanism for propane oxidation is developed and compared to experimental data from a high pressure flow reactor. Experimental conditions range from 10--15 atm, 650--800 K, and a residence time of 198 ms for propane-air mixtures at an equivalence ratio of 0.4. The experimental results clearly indicate a negative temperature coefficient (NTC) behavior. The chemistry describing this phenomena is critical in understanding automotive engine knock and cool flame oscillations. Results of the numerical model are compared to a spectrum of stable species profiles sampled from the flow reactor. Rate constants and product channels for the reaction of propyl radicals, hydroperoxy-propyl radicals and important isomers (radicals) with O{sub 2} were estimated using thermodynamic properties, with multifrequency quantum Kassel Theory for k(E) coupled with modified strong collision analysis for fall-off. Results of the chemical kinetic model show an NTC region over nearly the same temperature regime as observed in the experiments. Sensitivity analysis identified the key reaction steps that control the rate of oxidation in the NTC region. The model reasonably simulates the profiles for many of the major and minor species observed in the experiments

Pathway and kinetic analysis on the propyl radical + 02 reaction system

In this study of the reaction of alkyl radicals with molecular oxygen, we analyze the propyl + 02 reaction system using thermochemical kinetics, Transition State Theory (TST), molecular thermodynamic properties, quantum Kassel analysis (quantum RRK) for k(E) and modified strong collision analysis for fall off. Cyclic transition states for both hydrogen transfer and the H02 concerted elimination from propylperoxy are calculated using semi-empirical (MOPAC PM3) calculations [8] in addition to transition states for H02 elimination and epoxide formation from hydroperoxy-isopropyl. Computed rate constants for propyl + 02 are compared to the values of Gulati and Walker who measured the rate constants at 50 torr and over a temperature range of 653 to 773 K. Computed rate constants are also used in a detailed chemical kinetic mechanism and compared to the n- propyl + 02 data of Slagle. They measured the rate of disappearance of n-propyl by reaction with 02 over a temperature range of 297 to 635 K and a pressure range of 0.4 to 7 Torr, as well as the fall off data of the Kaiser and Wallington