Experimental and numerical study of flame acceleration and transition to detonation in narrow channels

From a scientific point of view, Deflagration to Detonation Transition (DDT) continues to draw significant interest in the research community as an outstanding, physics-​rich fundamental problem in combustion science. From a practical perspective, it is important to study and understand DDT in order to develop engineering correlations and simulation tools that can be applied to the prevention and mitigation of explosions. In the current study, flame acceleration and transition to detonation of stoichiometric H₂​/O₂ mixts. in narrow channels was investigated using a combined exptl. and numerical approach. The exptl. setup included direct-​, schlieren- and shadowgraph visualization of a 6 mm x 6 mm square channel of 1 m in length. The channel was closed in the region where the mixt. was ignited, and open at the other end. Exptl. x-​t diagrams using shadowgraph, revealed that transition to detonation regularly occurred around the first two-​thirds of the channel (~ 25 - 55 cm)​; close-​ups to the ignition location using schlieren and shadowgraph visualization showed important details of the ignition kernel. Three-​dimensional numerical simulations using quarter symmetry with a simplified chem.-​diffusive model are in reasonable agreement with the exptl. results, and shed light into the DDT mechanism in this type of configuration

Origins of turbulent mixing behind detonation propagation into reactive-inert gas interfaces

International audienc

Chemistry Modeling Effects on the Interaction of a Gaseous Detonation with an Inert Layer

International audienc

Quenching limits and dynamics of multidimensional detonation waves confined by an inert layer

International audienceTwo-dimensional inviscid simulations are conducted to assess the effect of chemistry modeling on the detonation structure and quenching dynamics of detonations propagating into a semiconfined medium. Two different simplified kinetic schemes are used to model the chemistry of stoichiometric H 2-O 2 mixtures: single-step and three-step chain-branching chemistry. Results show that the macroscopic characteristics of this type of detonations (e.g. detonation velocity and cell size irregularity) are very similar for both models tested. However, their instantaneous structures are very different before and upon interaction with an inert layer. Specifically, the minimum reactive layer height, h crit , capable of sustaining detonation propagation is larger when a more realistic description of the chemistry is used. This outcome suggests that the quenching limits predicted numerically are dependent on the choice of chemical modeling used

Influence of the chemical modeling on the quenching limits of detonation waves confined by an inert layer

The effect of chemistry modeling on the flow structure and quenching limits of detonations propagating into reactive layers bounded by an inert gas is investigated numerically. Three different kinetic schemes of increasing complexity are used to model a stoichiometric H2-O2 mixture: single-step, three-step chain-branching and detailed chemistry. Results show that while the macroscopic characteristics of this type of detonations (e.g. velocities, cell-size irregularity and leading shock dynamics) are similar among the models tested, their instantaneous structures are significantly different before and upon interaction with the inert layer when compared using a fixed height. When compared at their respective critical heights, hcrit (i.e. the minimum reactive layer height capable of sustaining detonation propagation), similarities in their structures become apparent. The numerically predicted critical heights increase as hcrit,Detailed << hcrit,1-Step < hcrit,3-Step. Notably, hcrit,Detailed was found to be in agreement with experimentally reported values. The physical mechanisms present in detailed chemistry and neglected in simplified kinetics, anticipated to be responsible for the discrepancies obtained, are discussed in detail

Hot surface ignition dynamics in premixed hydrogen–air near the lean flammability limit

The dynamics of ignition of premixed hydrogen–air from a hot glow plug were investigated in a combined experimental and numerical study. Surface temperatures during heating and at ignition were obtained from 2-color pyrometry, gas temperatures were measured by high-speed Mach–Zehnder interferometry, and far-field effects were captured by high-speed schlieren imaging. Numerical simulations considered detailed chemical kinetics and differential diffusion effects. In addition to the known cyclic (puffing) combustion phenomenon, singular ignition events (single puff) were observed near the lean flammability limit. Detailed analysis of the results of our numerical simulations reveal the existence of multiple combustion transients within the thermal boundary layer following the initial ignition event and, at late times, sustained chemical reaction within a thermal plume above the glow plug. The results have significant implications for ignition from hot surfaces within near-flammability limit mixtures, at the edge of plumes resulting from accidental release of hydrogen, or within the containments of nuclear power plants during severe accidents

Heat and momentum losses in H2–O2–N2/Ar detonations: on the existence of set-valued solutions with detailed thermochemistry

The effect of heat and momentum losses on the steady solutions admitted by the reactive Euler equations with sink/source terms is examined for stoichiometric hydrogen–oxygen mixtures. Varying degrees of nitrogen and argon dilution are considered in order to access a wide range of effective activation energies, Ea,eff/RuT0, when using detailed thermochemistry. The main results of the study are discussed via detonation velocity-friction coefficient (D–cf) curves. The influence of the mixture composition is assessed, and classical scaling for the prediction of the velocity deficits, D(cf,crit)/DCJ, as a function of the effective activation energy, Ea,eff/RuT0, is revisited. Notably, a map outlining the regions where set-valued solutions exist in the Ea,eff/RuT0–α space is provided, with α denoting the momentum–heat loss similarity factor, a free parameter in the current study.Agence Nationale de la Recherche | Ref. FASTD ANR-20-CE05-0011-01Universidade de Vigo/CISU

Dynamics of ignition of stoichiometric hydrogen-air mixtures by moving heated particles

Studying thermal ignition mechanisms is a key step for evaluating many ignition hazards. In the present work, two-dimensional simulations with detailed chemistry are used to study the reaction pathways of the transient flow and ignition of a stoichiometric hydrogen/air mixture by moving hot spheres. For temperatures above the ignition threshold, ignition takes place after a short time between the front stagnation point and separation location depending upon the sphere's surface temperature. Closer to the threshold, the volume of gas adjacent to the separation region ignites homogeneously after a longer time. These results demonstrate the importance of boundary layer development and flow separation in the ignition process

Effects of differential diffusion on ignition of stoichiometric hydrogen-air by moving hot spheres

Studying thermal ignition mechanisms is a key step for evaluating many ignition hazards. In the present work, two-dimensional simulations with detailed chemistry are used to study the effect of differential diffusion on the prediction of ignition thresholds of a stoichiometric hydrogen-air mixture by moving hot spheres. Numerical experiments showed an increase of 40 K in the minimum ignition temperature required for ignition when diffusion of species at different rates is taken into account. Detailed analysis of the species profiles at the ignition location and a sensitivity study of the system to the diffusivity of H_2 and H revealed the key role played by the diffusion of H atoms in preventing ignition to take place at temperatures below 1000 K