Ignition of fuel–air mixtures from a hot circular cylinder

Ignition of hydrogen–air, ethylene–air and n-hexane–air mixtures from a horizontally and vertically oriented heated circular cylinder was studied experimentally in a wide range of equivalence ratio. Initial pressure and temperature were 101.3 kPa and 296 K, respectively. The cylinder with outer diameter 10 mm and heated length 10 mm was designed for high temperature uniformity. Two-color pyrometry measured the surface temperature; Time-resolved Mach–Zehnder interferometry acquired ignition dynamics, gas temperature fields and heat transfer characteristics. Ignition from the horizontal cylinder occurred at temperatures between 960 K and 1100 K for hydrogen, between 1060 K and 1110 K for ethylene, and between 1150 K and 1190 K for n-hexane. Vertical cylinder orientation increased ignition thresholds by 50–110 K for ethylene and n-hexane, whereas only little variation was observed for hydrogen. Infinite-fringe interferograms visualized the ignition dynamics and identified the most favorable ignition locations, which coincided with locations of lowest wall heat flux (largest thermal boundary layer thickness) and long residence time. Gas temperature fields were obtained by post-processing the interferograms, resolving the temporal and spatial development of thermal boundary layers and enabling local heat transfer analysis. The convective pattern around a horizontal cylinder features distinctly shallow temperature gradients, i.e., low heat flux, at the cylinder top due to thermal plume formation, which promotes ignition compared to the vertical cylinder. An analytical scaling model for ignition from hot surfaces was evaluated to determine the sensitivity of ignition threshold to heat transfer variations, and to reveal the influence of chemical mixture properties. This analysis predicts a particularly low sensitivity for hydrogen–air mixtures at temperatures near the extended second explosion limit, and a larger sensitivity of ethylene–air and n-hexane–air mixtures, which is in accordance with the experiments

Hot Surface Ignition of n-Hexane Mixtures Using Simplified Kinetics

Hot surface ignition is relevant in the context of industrial safety. In the present work, two-dimensional simulations using simplified kinetics of the buoyancy-driven flow and ignition of a slightly lean n-hexane–air mixture by a rapidly heated surface (glowplug) are reported. Experimentally, ignition is most often observed to occur at the top of the glowplug; numerical results reproduce this trend and shed light on this behavior. The numerical predictions of the flow field and hot surface temperature at ignition are in quantitative agreement with experiments. The simulations suggest that flow separation plays a crucial role in creating zones where convective losses are minimized and heat diffusion is maximized, resulting in the critical conditions for ignition to take place

Detonation in hydrogen–nitrous oxide–diluent mixtures: An experimental and numerical study

Knowledge of H_2–N_2O mixtures explosive properties is important to the safety of nuclear waste storage and semi-conductor manufacturing processes. The present study provides new experimental data on H_2–N_2O detonations, and proposes a thermochemical model which is used to numerically simulate detonation propagation. Detonation cell size has been measured in a variety of H_2–N_2O–Ar mixtures. Even at low initial pressure, these mixtures are very sensitive to detonation with cell size of few millimeters. Using a reduced version of a detailed reaction scheme, 2-D Euler simulations have been used to examine the features of detonation in H_2–N_2O–Diluent mixtures. A PLIF model has been applied to allow for direct comparison with experimental results. Statistical analysis of the cellular cycle dynamics has been performed

Experimental and numerical study on moving hot particle ignition

Ignition thresholds for n-hexane-air were experimentally and numerically determined using a moving hot sphere of 6 mm in diameter. The novel experimental setup built for this purpose was described in detail. Two-color pyrometry was used for surface temperature measurements, and shearing interferometry flow field visualization was used to observe the onset of an ignition kernel, and subsequent flame formation and propagation. The probability of ignition was found to be 90% at a sphere surface temperature of 1224 K. Analysis of the interferograms at the ignition threshold indicated that ignition occurs near the region of flow separation. Numerical simulations of the transient development of the 2-D axisymmetric motion and ignition were performed. Four reduced chemical mechanisms, including high and low temperature chemistry, and two diffusion models were used to determine their impact on the numerical prediction of ignition thresholds. The simulation results were unaffected by the choice of diffusion model but were found to be sensitive to the chemical kinetic mechanism used. The predicted ignition threshold temperatures were within 6–12% of the experimentally determined values. The numerical fields of the energy source term and a wall heat flux analysis confirmed the experimental observation that ignition occurs near the region of flow separation at the ignition threshold. Detailed analysis of the species temporal evolution at the ignition location revealed that n-hexane is present in small amounts, demonstrating the importance of accounting for fuel decomposition within the thermal boundary layer when developing simple chemical reaction models

Experimental and numerical study on moving hot particle ignition

Ignition thresholds for n-hexane-air were experimentally and numerically determined using a moving hot sphere of 6 mm in diameter. The novel experimental setup built for this purpose was described in detail. Two-color pyrometry was used for surface temperature measurements, and shearing interferometry flow field visualization was used to observe the onset of an ignition kernel, and subsequent flame formation and propagation. The probability of ignition was found to be 90% at a sphere surface temperature of 1224 K. Analysis of the interferograms at the ignition threshold indicated that ignition occurs near the region of flow separation. Numerical simulations of the transient development of the 2-D axisymmetric motion and ignition were performed. Four reduced chemical mechanisms, including high and low temperature chemistry, and two diffusion models were used to determine their impact on the numerical prediction of ignition thresholds. The simulation results were unaffected by the choice of diffusion model but were found to be sensitive to the chemical kinetic mechanism used. The predicted ignition threshold temperatures were within 6–12% of the experimentally determined values. The numerical fields of the energy source term and a wall heat flux analysis confirmed the experimental observation that ignition occurs near the region of flow separation at the ignition threshold. Detailed analysis of the species temporal evolution at the ignition location revealed that n-hexane is present in small amounts, demonstrating the importance of accounting for fuel decomposition within the thermal boundary layer when developing simple chemical reaction models

Ignition characteristics of dual-fuel methane-n-hexane-oxygen-diluent mixtures in a rapid compression machine and a shock tube

Ignition delay times of methane-n-hexane-oxygen-dulent mixtures were studied experimentally and numerically in a wide temperature range (640–2335 K) using both a rapid compression machine (RCM) and a shock tube (ST). The RCM results demonstrated a two-stage ignition and negative temperature coefficient (NTC) behavior. Increasing n-hexane concentration, pressure and equivalence ratio shortened the ignition delay time. For the ST experiments, the addition of 10% n-hexane (relative to methane) can reduce the ignition delay time dramatically. However, no further reduction effect can be achieved with increasing addition of n-hexane from 10% to 20%. In addition, increasing equivalence ratio reduces the effect of n-hexane addition on ignition delay time. Three detailed chemical mechanisms, CaltechMech, GalwayMech and LLNLMech, were evaluated based on a quantitative error analysis. LLNLMech and CaltechMech demonstrated the best performance in the RCM and ST temperature ranges, respectively. Chemical kinetic analyses showed that the addition of n-hexane to methane provides some chemical pathways not available for methane oxidation which result in the production of active radicals and eventually accelerate the ignition of the methane-oxygen mixtures. The crucial intermediate species for the ignition process are H_2O_2 and H under RCM and ST conditions, respectively

Study of kinetic mechanisms and explosives properties of the hydrogen-nitrous oxide and silane-nitrous oxide systems : application to the industrial safety

La présente étude s’inscrit dans le cadre d’une évaluation des risques liés d’une part au stockage des déchets nucléaires et d’autre part à la production des semi-conducteurs. L’ojectif est d’obtenir des paramètres fondamentaux sur les propriétés explosives des mélanges hydrogène-protoxyde d’azote et silane-protoxyde d’azote. Pour le système hydrogène-protoxyde d’azote, les temps caractéristiques de réaction derrière une onde de choc réfléchie, les vitesses fondamentales de flamme et les largeurs des cellules de détonation ont été mesurées expérimentalement sur une large gamme de composition et de condition. Un mécanisme cinétique détaillé a été développé et validé sur les données de la présente étude et de la littérature. Des mécanismes cinétiques réduits ont été obtenus par une méthode de réduction automatique et inclus dans un code de simulation numérique bi-dimensionnelle d’onde de détonation. Pour le système silane-protoxyde d’azote, l’évolution temporelle des atomes d’oxygène derrière une onde de choc réfléchie et les vitesses fondamentales de flammes ont été étudiées expérimentalement. Une étude préliminaire d’analyse des produits solides de combustion formés en bombe sphérique a également été réalisée. Un mécanisme cinétique réduit de la littérature a été modifié afin de reproduire les profils des atomes d’oxygène.The present study is part of a risk assessment related, on one hand, to the storage of nuclear wastes, and on the other hand, to the production of semi-conductors. The aim is to obtain fondamental parameters on the explosive properties of hydrogen-nitrous oxide and silane-nitrous oxide mixtures. For the hydrogen-nitrous oxide system, caracteristic times of reaction behind reflected shock waves, laminar flame speeds and detonation cell widths were measured experimentally over a wide range of composition and condition. A detailed kinetic mechanism has been developed and validated against the data of the present study and of literature. Reduced kinetic mechanisms have been obtained using an automatic method of reduction and included in a two-dimensional numerical simulation code of detonation wave. For the silane-nitrous oxide system, the time profiles of oxygen atoms behind reflected shock waves and laminar flame speeds were studied experimentally. A preliminary analytical study of solid combustion products formed in a spherical bomb was also performed. A reduced kinetic mechanism of the literature was modified to reproduce the profiles of oxygen atoms