A Comparison of Sensitivity Metrics for Two-Stage Ignition Behavior in Rapid Compression Machines

A rapid compression machine (RCM) multi-zone model is used to simulate the ignition of primary reference fuel (PRF) mixtures that exhibit two-stage ignition behavior. Sensitivity coefficients for each reaction in the PRF mechanism are calculated from four different metrics: (1) first-stage energy release, (2) first-stage pressure rise, (3) first-stage ignition delay time, and (4) total ignition delay time. The sensitivity coefficients are used to provide four unique rankings, and the rankings are compared using Spearman’s rank correlation coefficient. Special emphasis is given to comparing the rankings based on first-stage energy release and total ignition delay time. The level of agreement between these two rankings is shown to depend on the reaction conditions. Simulation cases with high peak heat release rates during the first stage of ignition tend to exhibit disagreement in the rankings, indicating that new kinetic information can be obtained by studying first stage energy release in addition to total ignition delay time. Simulations show that the high peak heat release rates are associated with energy release across a broad range of temperatures (range can be in excess of 100 K even for lean conditions). This distribution leads to a discrepancy between sensitivity coefficients calculated for the total ignition delay time and the first-stage energy release. Sensitivity coefficients for the total ignition delay time are characterized by reactivity at the highest temperatures in the RCM, while sensitivity coefficients for the first-stage energy release are characterized by reactivity across the full range of temperatures in the RCM

Ignition of mixtures of SiH sub 4, CH sub 4, O sub 2, and Ar or N sub 2 behind reflected shock waves

Ignition delay times in mixtures of methane, silane, and oxygen diluted with argon and nitrogen were measured behind reflected shock waves generated in the chemical kinetic shock tube at Langley Research Center. The delay times were inferred from the rapid increase in pressure that occurs at ignition, and the ignition of methane was verified from the emission of infrared radiation from carbon dioxide. Pressures of 1.25 atm and temperatures from 1100 K to 1300 K were generated behind the reflected shocks; these levels are representative of those occurring within a supersonic Ramjet combustor. Expressions for the ignition delay time as a function of temperature were obtained from least squares curve fits to the data for overall equivalence ratios of 0.7 and 1.0. The ignition delay times with argon as the diluent were longer than those with nitrogen as the diluent. The infrared wavelength observations at 4.38 microns for carbon dioxide indicated that silane and methane ignited simultaneously (i.e., within the time resolution of the measurement). A combined chemical kinetic mechanism for mixtures of silane, methane, oxygen, and argon or nitrogen was assembled from one mechanism that accurately predicted the ignition of methane and a second mechanism that accurately predicted silane hydrogen ignition. Comparisons between this combined mechanism and experiment indicated that additional reactions, possibly between silyl and methyl fragments, are needed to develop a good silane methane mechanism

Spontaneous Ignition Characteristics of Hydrocarbon Fuel-air Mixtures

Although the subject of spontaneous ignition of liquid fuels has received considerable attention in the past, the role of fuel evaporation in the overall spontaneous ignition process is still unclear. A main purpose of this research is to carry out measurements of ignition delay times, using fuels of current and anticipated future aeronautical interest, at test conditions that are representative of those encountered in modern gas turbine engines. Attention is focused on the fuel injection process, in particlar the measurement and control of man fuel drop size and fuel-air spatial distribution. The experiments are designed to provide accurate information on the role of fuel evaporation processes in determining the overall ignition delay time. The second objective is to examine in detail the theoretical aspects of spontaneous ignition in order to improve upon current knowledge and understanding of the basic processes involved, so that the results of the investigation can find general and widespead application

Ignition and combustion of lunar propellants

The ignition and combustion of Al, Mg, and Al/Mg alloy particles in 99 percent O2/1 percent N2 mixtures is investigated at high temperatures and pressures for rocket engine applications. The 20 micron particles contain 0, 5, 10, 20, 40, 60, 80, and 100 weight percent Mg alloyed with Al, and are ignited in oxygen using the reflected shock in a shock tube near the endwall. Using this technique, the ignition delay and combustion times of the particles are measured at temperatures up to 3250 K as a function of Mg content for oxygen pressures of 8.5, 17, and 34 atm. An ignition model is developed which employs a simple lumped capacitance energy equation and temperature and pressure dependent particle and gas properties. Good agreement is achieved between the measured and predicted trends in the ignition delay times. For the particles investigated, the contribution of heterogeneous reaction to the heating of the particle is found to be significant at lower temperatures, but may be neglected as gas temperatures above 3000 K. As little as 10 percent Mg reduces the ignition delay time substantially at all pressures tested. The particle ignition delay times decrease with increasing Mg content, and this reduction becomes less pronounced as oxidizer temperature and pressure are increased

A Chemical Kinetic Mechanism for the Ignition of Silane/Hydrogen Mixtures

A chemical kinetic reaction mechanism for the oxidation of silane/hydrogen mixtures is presented and discussed. Shock-tube ignition delay time data were used to evaluate and refine the mechanism. Good agreement between experimental results and the results predicted by the mechanism was obtained by adjusting the rate coefficient for the reaction SiH3 + O2 yields SiH2O + OH. The reaction mechanism was used to theoretically investigate the ignition characteristics of silane/hydrogen mixtures. The results revealed that over the entire range of temperature examined (800 K to 1200 K), substantial reduction in ignition delay times is obtained when silane is added to hydrogen

Mathematical modelling of synthetic fuels ignition by local heat sources

The numerical study is executed for polymethylmethacrylate ignition (typical model fuel for hybridrocket motors) by single particles in the shape of parallelepiped, polyhedron, and disk. The initial temperature ofheat source is varied in the range of 950-1150 K, the size of hot particle is varied in the range of 2-6 mm. It isfound that the change of these parameters significantly affect on the basic characteristics of the process -ignition delay time at conditions close to the critical ignition conditions. For considered particles with differentshapes ignition delay time are arranged in ascending order: parallelepiped, polyhedron, and disk. Three ignitionmode of polymer is studied. They are characterized by an initial temperature of the heat source, the ignitiondelay time and the position of the ignition zone in the vicinity of a hot particle. It is shown that taking intoaccount the dependence of thermophysical characteristics on the temperature of polymer increase the ignition delay time due to the increasing of accumulated energy by near-surface layer

Global kinetics for n-heptane ignition at high pressures

A kinetic mechanism of 1011 elementary reactions with 171 chemical species for n-heptane ignition is analysed and reduced to 4 global steps with adjusted rate coefficients to describe ignition at pressures around 40 atm. Two of these steps account for the high temperature branch and the other two for the low temperature branch of the ignition mechanism. The ignition delay time passes through a negative temperature dependence during the transition between the two branches. This is accounted for by the reversible third reaction step, which models the first and second 02-addition in the degenerated chain branching mechanism at low temperatures. Ignition delay times calculated with the adjusted 4-step model are compared to those from the detailed kinetics and experimental data. Finally the 4-step mechanism is analysed by asymptotic methods and explicit ignition delay time formulas are derived

Influence of the thickness absorbing film on the PETN ignition threshold by a laser pulse

Numerical simulation of the PETN ignition by a film, which is heated by a laser pulse was conducted. There are shown that dependence of threshold energy of ignition of PETN by a laser pulse has a linear dependence from the thicknesses of the absorbing film. Calculations shown that critical the temperature on the boundary of two materials by the end of a laser pulse with threshold density doesn't depend from the thickness of the absorbing film. The ignition delay time of PETN by the thick film less than the ignition delay time of PETN by the thin film. The reason is that the thicker contain more heat then in the thinner one

Influence of the thickness absorbing film on the PETN ignition threshold by a laser pulse

Numerical simulation of the PETN ignition by a film, which is heated by a laser pulse was conducted. There are shown that dependence of threshold energy of ignition of PETN by a laser pulse has a linear dependence from the thicknesses of the absorbing film. Calculations shown that critical the temperature on the boundary of two materials by the end of a laser pulse with threshold density doesn't depend from the thickness of the absorbing film. The ignition delay time of PETN by the thick film less than the ignition delay time of PETN by the thin film. The reason is that the thicker contain more heat then in the thinner one

Theoretical kinetic computations in complex reacting systems

Nasa Lewis' studies of complex reacting systems at high temperature are discussed. The changes which occur are the result of many different chemical reactions occurring at the same time. Both an experimental and a theoretical approach are needed to fully understand what happens in these systems. The latter approach is discussed. The differential equations which describe the chemical and thermodynamic changes are given. Their solution by numerical techniques using a detailed chemical mechanism is described. Several different comparisons of computed results with experimental measurements are also given. These include the computation of (1) species concentration profiles in batch and flow reactions, (2) rocket performance in nozzle expansions, and (3) pressure versus time profiles in hydrocarbon ignition processes. The examples illustrate the use of detailed kinetic computations to elucidate a chemical mechanism and to compute practical quantities such as rocket performance, ignition delay times, and ignition lengths in flow processes