arXiv subjects

Matei Radulescu

Publications and source records attributed to Matei Radulescu.

8 recordsLinked to original sources

arXiv2024

Detonation attenuation and quenching in hydrogen mixtures after the interaction with cylinders

The attenuation and quenching of H$_2$/O$_2$ detonations transmitted across a column of cylinders were studied experimentally and analytically at sub-atmospheric pressures. Two distinct transmission regimes were observed: successful transmission and complete quenching. The transition between the two regimes was found to correlate with the ratio of inter-cylinder separation distance (b) to a characteristic detonation scale for large blockage ratios (BRs), with critical limits comparable with those previously reported for detonation diffraction from slots. Based on available cell size measurements, the critical transmission limit was $b/λ=4.5\pm3$. The proposed theoretical model based on Whitham's geometric shock dynamics confirmed the equivalence between the detonation diffraction at abrupt area changes and around cylinders with large BRs. Complete quenching observed experimentally was accounted for by the weak transmitted shock strength upon detonation failure. For the tested BRs, the transmitted shock speed ranged between 50% and 60% of the Chapman-Jouguet detonation speed. As a result, the post-shock temperatures fell below the cross-over regime for hydrogen ignition, leading to very long ignition delay times ($t_i$) despite the temperature rise induced by Mach reflection. The long $t_i$ suppressed auto-ignition, while the high isentropic exponent prevented further convective mixing required for re-initiation. This explained the fundamental difference between arresting hydrogen and hydrocarbon detonations, for which transmitted fast flames punctuated by auto-ignition events were always observed. The transmitted shock strength was found to be well-predicted by our previous self-similar multiple discontinuity gas-dynamic model. Over a very narrow range near the critical conditions, both hot spot re-ignition and detonation re-initiation from Mach shock reflections were observed.

physics.flu-dyn↗

arXiv2023

The critical conditions for the re-ignition and detonation formation from Mach reflections of curved decaying shocks

The objective of this study is to determine the critical conditions for a detonation wave formation following a Mach reflection of two incident shocks. This problem is central to the propagation mechanism of cellular detonations, where such periodic reflections occur continuously. A new experimental technique is introduced that permits the isolation of this Mach reflection. The technique is inspired by that of White (1963) and uses a detonation passing through a bifurcated converging-diverging nozzle to reproducibly generate the desired shock reflection at its exit, where the two transmitted mis-aligned incident shocks interact. The collision process was monitored with high-speed Schlieren videos. The experiments were performed in mixtures of methane-oxygen and hydrogen-oxygen-argon. Three distinct regimes of transmission were identified: detonation formation, decaying Mach shock followed by a flame, and inert Mach shock with no re-ignition. The condition separating the re-ignition and no re-ignition was very well predicted by the critical decay rate ignition theory of Eckett et al. On the other hand, the transition between detonation formation and ignition was found to correlate much better with the critical curvature concept of quasi-steady curved detonations of Kasimov and Stewart. Both mixtures were found in excellent agreement with this criterion, established on the basis of experimental velocity-curvature data available in the literature for these mixtures. For the hydrogen detonations, this criterion was also in agreement with the critical curvature predicted by ZND theory. For the methane detonations, order of magnitude disagreement was observed with the ZND model prediction, further confirming the propensity of irregular cellular detonations to propagate under higher curvatures than predicted by laminar detonation theory.

physics.flu-dyn↗

arXiv2022

Critical diffraction of irregular structure detonations and their predictability from experimentally obtained D-K data

The present work reports new experiments of detonation diffraction in a 2D channel configuration in stoichiometric mixtures of ethylene, ethane, and methane with oxygen as oxidizer. The flow field details are obtained using high-speed schlieren near the critical conditions of diffraction. The critical initial pressure for successful diffraction is reported for the ethylene, ethane and methane mixtures. The flow field details revealed that the lateral portion of the wave results in a zone of quenched ignition. The dynamics of the laterally diffracting shock front are found in good agreement with the recent model developed by Radulescu et al. (Physics of Fluids 2021). The model provides noticeable improvement over the local models using Whitham's characteristic rule and Wescott, Bdzil and Stewart's model for weakly curved reactive shocks. These models provide a link between the critical channel height and the critical wave curvature. The critical channel heights and global curvatures are found in very good agreement with the critical curvatures measured independently by Xiao and Radulescu (Combust. Flame 2020) in quasi-steady experiments in exponential horns for three mixtures tested. Furthermore, critical curvature data obtained by others in the literature was found to provide a good prediction of critical diffraction in 2D. These findings suggest that the critical diffraction of unstable detonations may be well predicted by a model based on the maximum curvature of the detonation front, where the latter is to be measured experimentally and account for the role of the cellular structure in the burning mechanism. This finding provides support to the view that models for unstable detonations at a meso-scale larger than the cell size, i.e., hydrodynamic average models, are meaningful.

physics.flu-dyn↗

arXiv2022

Ignition Behind Decaying Shock Waves : Detonation Cells

We first address the problem of initiation behind decaying shock waves analytically and numerically. The ignition along a particle path crossing the shock is analysed in terms of its volumetric expansion, evaluated as a function of the shock strength, decay rate, and curvature using the shock change equations. Closed form solutions are derived for 1-step and 2-step chemistry models. The analytical results are found in excellent agreement with numerical simulations using a detailed thermo-chemical model. The newly developed model is used to study the ignition behind the lead shock in detonation cells. New experiments are reported for cellular dynamics in a very regular H$_2$-O$_2$-Ar system. It is found that the ignition of approximately half of the gases crossing the shock is quenched. In the experiments, this gas reacts behind the transverse waves. Previous experiments in a very irregular system of methane-oxygen are also analysed. In these experiments, more than 70\% of the gases crossing the shock are quenched due to shock non-steadiness. These gases are shed as non-reacted pockets, which react much slower and burn out due to turbulent diffusion. The main conclusion of the study is that both irregular and regular cellular detonations experience ignition quenching due to shock non-steadiness.

physics.flu-dyn↗

arXiv2017

Investigation of Turbulent Mixing and Local Reaction Rates on Deflagration to Detonation Transition in Methane-Oxygen

In the current study, the influence of turbulent mixing and local reaction rates on deflagration to detonation transition (DDT) was investigated using a state-of-the-art large eddy simulation (LES) strategy. Specifically, detonation attenuation by a porous medium, and the subsequent re-initiation for methane-oxygen, a moderately unstable mixture, was considered. The purpose of the investigation was to validate the numerical strategy with previous experimental observations, and to determine what specific roles turbulent mixing and shock compression have on flame acceleration during the final stages of DDT. The modelling procedure adopted was a grid-within-a-grid approach: The compressible linear eddy model for large eddy simulation (CLEM-LES). It was found that average turbulent fluctuations greater than the laminar flame speed were required in order to maintain wave velocities above the Chapman-Jouguet (CJ)-deflagration velocity threshold, a precursor requirement for DDT to occur. It was also found that sufficient turbulent burning on the flame surface was required in order to drive pressure waves to sufficiently strengthen the leading shock wave, locally, in order to trigger auto-ignition hot spots in the wave front. These local explosion events, which were found to burn out through turbulent surface reactions, drive transverse pressure waves outward. Upon subsequent shock reflections or interactions of the transverse waves, new local explosion events occurred, which further strengthened the adjacent leading shock wave above the CJ-detonation speed. Eventually, through this process, the wave sustained the CJ-detonation speed, on average, through the cyclic mechanism of local explosion events followed by turbulent surface reactions. Finally, combustion of the flame acceleration process was found to lie predominantly within the thin-reaction zones regime.

physics.flu-dyn↗

arXiv2014

Evolution of shock instability in granular gases with viscoelastic collisions

Shocks in granular media have been shown to develop instabilities. We address the role that early stages of shock development have on this type of instability. We look at the evolution of shock waves driven by a piston in a dilute system of smooth inelastic disks, using both discrete-particle and continuum modelling. To mimic a realistic granular gas, viscoelastic collisions are approximated with an impact velocity threshold $u^*$ needed for inelastic collisions to occur. We show that behaviour of the shock evolution is dependent on the ratio of piston velocity to impact velocity threshold $u_p/u^*$, and the coefficient of restitution $\varepsilon$. For $u_p/u^*=2.0$, we recover shock evolution behaving similar to that observed in purely inelastic media. This is characterized by a short period where the shock front pulls towards the piston before attaining a developed structure. No pullback is seen for $u_p/u^*=1.0$. Results show the onset of instability for these stronger shocks during this evolving stage. These results suggest that the early stages of shock evolution play an important role in the shock instability.

cond-mat.soft↗

arXiv2013

Instability in Shocked Granular Gases

Shocks in granular media, such as vertically oscillated beds, have been shown to develop instabilities. Similar jet formation has been observed in explosively dispersed granular media. Our previous work addressed this instability by performing discrete-particle simulations of inelastic media undergoing shock compression. By allowing finite dissipation within the shock wave, instability manifests itself as distinctive high density non-uniformities and convective rolls within the shock structure. In the present study we have extended this work to investigate this instability at the continuum level. We modeled the Euler equations for granular gases with a modified cooling rate to include an impact velocity threshold necessary for inelastic collisions. Our results showed a fair agreement between the continuum and discrete-particle models. Discrepancies, such as higher frequency instabilities in our continuum results may be attributed to the absence of higher order effects.

physics.flu-dyn↗

arXiv2010

Detonation waves via molecular dynamics simulations of reactive hard disks

Videos of the molecular dynamics simulations of a colliding hard disk reactive medium. A simple event-driven algorithm is used to simulate these colliding hard disks allowing for the study of detonations at the microscopic level. This video was submitted as party of the Gallery of Fluid Motion for the American Physical Society 63rd Annual DFD Meeting.

physics.flu-dyn↗