Ab Initio Calculations of Phase Stabilities in the Fe-Al-C System and CALPHAD-Type Assessment of the Iron-Rich Corner

The goal of this article is to improve the description of the Fe-Al-C phase diagram through the density-functional theory (DFT) calculation of the Fe3AlC compound. Several binary stoichiometric compounds in the iron-rich part of the ternary Fe-Al-C system and the ternary stoichiometric carbide Fe3AlC are studied ab initio. The results in terms of lattice parameter, bulk modulus, magnetization, and enthalpy of formation are compared with the calculated values available in the literature. Via a fitted entropy of formation of the ternary carbide, the Gibbs energy of formation of this compound is evaluated as a function of temperature. A CALPHAD-type procedure is then used to calculate ternary Gibbs isotherms at 800 °C, 1000 °C, and 1200 °C. The isotherms are compared with the experimental data of the literature

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