Shock Tube Experiments Research Articles

On Richtmyer–Meshkov finger collisions in a light fluid layer under reshock conditions

The fingers known as bubbles (spikes) resulting from the penetration of light (heavy) fluids into heavy (light) fluids are significant large-scale features of Richtmyer–Meshkov instability (RMI). Through shock-tube experiments, we study finger collisions in light fluid layers under reshock conditions. Four unperturbed fluid layers with varying thicknesses are created to analyse the motion of waves and interfaces during finger collisions. The wave dynamics, sensitive to initial layer thicknesses, are characterized by a one-dimensional theory. Eight perturbed fluid layers, with four thicknesses and two interface phase combinations, are generated to explore the finger collision mechanism. It is shown that after reshock, the initial in-phase and anti-phase cases undergo spike–bubble rear-end collisions (SBCs) and spike–spike head-on collisions (SSCs), respectively. Compared with SBCs, SSCs significantly suppress spike growth, leading to the attenuation of perturbation growth, especially for larger thicknesses. As the initial thickness decreases, an SSC impedes the downstream interface from reversing its phase, resulting in abnormal RMI, thereby reducing the SSC's effectiveness in attenuating growth. The effects of rarefaction waves enhance both interfaces’ amplitudes and the whole layer's thickness, diminishing the intensity of finger collisions, while the second reshock exerts an opposing influence. Linear and nonlinear models, incorporating the influence of reshocks and rarefaction waves, are developed to predict the interface perturbation growth before and after finger collisions.

Influence of flow nonuniformities and real gas effects on cylindrical shock wave convergence

Convergence of cylindrical shock in argon is studied both experimentally and numerically. Shock tube experiments are conducted, where a planar shock is first transformed to a cylindrical shape and then converged to its focal axis. Numerical simulations of the converging shock using equations of state for an ideal gas and a real gas (SESAME 5173 model) are conducted and compared. High temporal resolution data of cylindrical shock convergence is presented. When comparing the trajectories of the converging shock of initial shock Mach number (MS) of 4.63, the convergence exponent (α) in experiments is found to be 0.833. This α value in experiments is higher than the value obtained from computations with argon treated as an ideal gas but agrees well with the real gas computations. It is revealed that the form of convergence varies with different MS. An asymptotic approach of α toward the self-similar solution for high MS is attributed to an earlier transition of shock motion to self-similarity, while a significantly higher α observed at lower MS is attributed to the negative influence of upstream nonuniformities and weaker initiation of the shock. It is found that even before the shock reflection, real gas effects are significant enough to affect the convergence of the shock and limit the extreme conditions predicted by the ideal gas computations. For an MS of 4.63, the maximum temperature reached is 9250 K before reflection, leading to 0.12% of the argon gas undergoing the first stage of ionization.

Shock Response of Sandwich Panels with Additively Manufactured Polymer Gyroid Lattice Cores

This work evaluates the shock response of sandwich structures with gyroid lattice cores made from Polylactic Acid (PLA), Acrylonitrile Butadiene Styrene (ABS), and Thermoplastic Polyurethane (TPU) using shock tube experiments coupled with high-speed photography and digital image correlation (DIC). The study found that gyroid lattice structures exhibit high energy absorption capacity and good structural integrity under shock loading, making them suitable for high strength-to-weight ratio and energy absorption applications in aerospace and defense industries. The sandwich panel with a TPU-core structure exhibited the highest deformation under shock loading, with a maximum deflection of approximately 6mm, followed by ABS at 0.9mm and PLA at 0.82mm. Under shock loading, the sandwich panel with TPU gyroid core exhibited the highest specific deformation energy (0.146J/g), which is consistent with its flexibility, though it remained in the same general order as ABS (0.104J/g) and PLA (0.070J/g). Interestingly, the specific total energy of the TPU gyroid core sandwich, while the lowest at 39.310J/g, was still relatively close to ABS (54.098J/g) and PLA (52.620J/g), despite the substantial difference in inherent stiffness of TPU compared to ABS and PLA. This suggests that while TPU's stiffness is much lower compared to PLA and ABS, its total energy absorption capability in gyroid form is not as drastically reduced, indicating the importance of geometry and mass distribution within the gyroid structure. Overall, the results of this study highlight the importance of careful design and optimization of these structures to utilize their unique properties fully.

Interface evolution induced by two successive shocks under diverse reshock conditions

The effects of reshock conditions, including the interface evolution state before reshock and the second shock intensity, on interface instability induced by two successive shocks propagating in the same direction are investigated via shock-tube experiments. It is observed that the reshock promotes the interface instability, and the post-reshock perturbation evolution relates to both the pre-reshock interface evolution state and second shock intensity. For the linear evolution of the twice-shocked interface, existing models perform poorly when either the pre-reshock interface shape effect or the secondary compression effect is pronounced, as current reduction factors fail to accurately describe these effects. Besides, the reshock-induced linear amplitude growth rate shows a non-monotonic dependence on the scaled pre-reshock amplitude, primarily due to the shape effect of the pre-reshock interface. For the post-reshock nonlinear evolution, the model proposed by Zhang & Guo (J. Fluid Mech., vol. 786, 2016, pp. 47–61) offers reasonable predictions when the second shock is weak. However, when the second shock is moderately strong, the model overestimates the bubble growth and underestimates the spike evolution under the influence of the significant secondary compression effect. Furthermore, empirical linear and nonlinear models capable of describing the dependence of the post-reshock evolution on reshock conditions are proposed based on the present experimental results and existing models.

Coupled Richtmyer–Meshkov and Kelvin–Helmholtz instability on a shock-accelerated inclined single-mode interface

The coupling of Richtmyer–Meshkov instability (RMI) and Kelvin–Helmholtz instability (KHI), referred to as RM-KHI, on a shock-accelerated inclined single-mode air–SF $_6$ interface is studied through shock-tube experiments, focusing on the evolution of the perturbation distributed along the inclined interface. To clearly capture the linear (overall linear to nonlinear) evolution of RM-KHI, a series of experiments with a weak (relatively strong) incident shock is conducted. For each series of experiments, various $\theta _{i}$ (angle between incident shock and equilibrium position of the initial interface) are considered. The nonlinear flow features manifest earlier and develop faster when $\theta _{i}$ is larger and/or shock is stronger. In addition, the interface with $\theta _{i}>0^{\circ }$ evolves obliquely along its equilibrium position under the effect of KHI. RMI dominates the early-time amplitude evolution regardless of $\theta _{i}$ and shock intensity, which arises from the discrepancy in the evolution laws between RMI and KHI. KHI promotes the post-early-stage amplitude growth and its contribution is related positively to $\theta _{i}$ . An evident exponential-like amplitude evolution behaviour emerges in RM-KHI with a relatively strong shock and large $\theta _{i}$ . The linear model proposed by Mikaelian (Phys. Fluids, vol. 6, 1994, pp. 1943–1945) is valid for RM-KHI within the linear period. In contrast, the adaptive vortex model (Sohn et al., Phys. Rev. E, vol. 82, 2010, p. 046711) can effectively predict both the interface morphology and overall amplitude evolutions from the linear to nonlinear regimes.

Decomposition of CH3NH2${\rm CH}_3{\rm NH}_2$: Implications for CHx/NHy${\rm CH}_{\rm {\it x}}/{\rm NH}_{\rm {\it y}}$ radical–radical reactions

AbstractExperiments on methylamine () decomposition in shock tubes, flow reactors, and batch reactors have been re‐examined to improve the understanding of hydrocarbon/amine interactions and constrain rate constants for + reactions. In high‐temperature shock tube experiments, the rapid thermal dissociation of provides a fairly clean source of and radicals, allowing an assessment of reactions of with and NH. At the lower temperatures in batch and flow reactors, is mostly consumed by reaction with H to form + ; these results are useful in determining the fate of the radical. Interpretation of these data, along with flow reactor data for the /H system at lower temperature, indicates that at temperatures up to about 1400 K at atmospheric pressure and above 2000 K at 100 atm, the + reaction forms mainly methylamine. At sufficiently high temperature, H‐abstraction to form + NH and addition–elimination to form + H become competitive. The + NH reaction, with a rate constant close to collision frequency, forms + H, also leading into the hydrocarbon amine pool. Thus, methylamine can be expected to be an important intermediate in co‐combustion of natural gas and ammonia, and more work on the chemistry of is desirable.

Kinetic insights into double-branched acyclic ether: Methyl tert-butyl ether and 2,2-dimethoxypropane

This study presents the first kinetic mechanism for methyl tert-butyl ether (MTBE) containing low-temperature chemistry, as well as the first mechanism for 2,2-dimethoxypropane (DMP). The mechanisms have been validated against ignition delay time experiments conducted in a high-pressure shock tube and rapid compression machine. The rapid compression machine conditions were set to 20 and 40 bar for MTBE, and 10 and 20 bar for DMP in a temperature range of 584 to 946 K. Shock tube experiments have been performed for DMP at 20 and 40 bar for stoichiometric and fuel-lean conditions in air at temperatures ranging from 913 to 1173 K. A pronounced negative temperature coefficient regime with two-stage ignition has been observed for both fuels. The developed mechanism consists of reaction rate constants that were primarily modeled in analogy to iso-octane and dimethoxymethane, and calculated thermodata on the G4//B3LYP-D3BJ/def2-TZVP level of theory. Simulations have been performed to analyze the fuel oxidation at different temperatures. Over the full temperature range, H-atom abstraction occurs mainly on the α-side for DMP and on the β-side for MTBE. At low temperatures, both fuels isomerize to the peroxy radical. The dominant MTBE radicals then tend to produce cyclic ether, while the DMP radicals react with O2, enabling significant chain branching and explaining the higher reactivity of DMP. With rising temperature, β-scission of the fuel radicals and unimolecular elimination reactions start to dominate the oxidation process.Novelty and Significance StatementThe novelty of this research is the first observation of a two-stage ignition of methyl-tert butyl ether (MTBE) in an RCM and a discussion of its fundamental ignition chemistry, which is based on a developed detailed kinetic mechanism. In addition, 2,2-dimethoxypropane (DMP) has been investigated experimentally and theoretically to explain the difference in reactivity to MTBE despite their strong molecular similarity. The experiments include RCM and shock tube experiments. The kinetic model is based on rate constant analogies and newly calculated thermo data on the G4//B3LYP-D3BJ/def2-TZVP level of theory. This work is significant, as MTBE is still a widely used octane booster and the developed model could help to improve engine simulations. Furthermore, the findings of DMP provide insights into future fuel design.

Quasi-one-dimensional non-equilibrium method for shock tube and stagnation line flows

Shock tube experiments are used to investigate non-equilibrium thermochemistry and radiative processes in reacting gas hypersonic flows. Boundary layer and shock structure are known to influence the spatial variation of the test slug state properties. This work derives and validates a novel method for viscous, quasi-one-dimensional, non-equilibrium flow in a shock tube assuming constant shock speed. The proposed method fully resolves the shock structure. Mirels' estimate for boundary layer growth around the test slug determines the dilation rate at the centerline. This, along with relevant boundary conditions, appropriately models core flow in a shock tube. The flow equations are discretized by finite differences on a staggered grid. The resulting highly non-linear set of algebraic equations is solved by Newton iterations. The Jacobian matrix is block tridiagonal with a Schur complement, allowing efficient inversion. This culminates in a unique and computationally efficient quasi-one-dimensional method offering improved modeling of the physical characteristics of shock tube experiments. Results of a 3 km/s, 66.6 Pa argon test case solved by a viscous, axisymmetric Navier–Stokes solution had agreement with the proposed method in temperature and pressure profiles to within 2% and post-shock velocity to within 15%. Reacting gas shock tube experiments in synthetic air and synthetic Titan atmospheres were analyzed. Radiance values in the non-equilibrium and equilibrium regions were compared under various assumptions for the shock structure and radial velocity distribution. These results highlight the necessity of a dedicated shock tube solver when analyzing shock tube thermochemistry, particularly when determining reaction rates and relaxation parameters.

A comprehensive experimental and theoretical study of thermal response mechanisms of TKX-50 and HMX

TKX-50 and HMX, as two highly promising energetic materials, are widely used in rocket propellants. TKX-50/HMX cocrystal can also improve the defects of TKX-50 and HMX and vastly expand their application scope. Their response mechanisms to the same thermal stimulation, including reactivity and decomposition and combustion behavior, are important in understanding their combustion characteristics and engine performance. This study systematically investigates the thermal response mechanisms of TKX-50 and HMX under the same thermal stimulations based on experimental measurements and theoretical analysis. Ignition delay times and burn times of TKX-50 and HMX with different particle sizes at various temperatures and pressures are measured in shock tube experiments. ReaxFF MD simulations are performed to analyze the thermal decomposition of TKX-50 and HMX in terms of potential energy evolution, intermediate and final products, and possible reaction pathways. Ab initio kinetics of hydrogen atom abstraction reactions of TKX-50 and HMX with three highly active species, Ḣ, ȮH, and NO2, are studied. The barrier heights, reaction enthalpies and rate coefficients for all reactions are further analyzed. The results show that: (1) the ignition delay time and burn time of TKX-50 (128, 138, and 55 µm) are measured to be 200–600 µs and 40–400 µs respectively, which are almost the same as those of HMX (6 and 0.8 µm) in the same temperature range; (2) the burn time of TKX-50 is less affected by pressure but more affected by particle size, whereas both pressure and particle size have a significant effect on the burn time of HMX; (3) TKX-50 is shown to be readily decomposed and less affected by temperature changes than HMX under the same conditions; (4) ȮH is generated more rapidly in the TKX-50 system, but more persistently in the HMX system; (5) H atom abstraction reactions from TKX-50 by all species are less endothermic but more exothermic than those from HMX.

Fulvenallenyl Radical (C7H5·)-Mediated Gas-Phase Synthesis of Bicyclic Aromatic C10H8 Isomers: Can Fulvenallenyl Efficiently React with Closed-Shell Hydrocarbons?

To understand the reactivity of resonantly stabilized radicals, often found in relevant concentrations in gaseous environments, it is important to determine their main reaction pathways. Here, it is investigated whether the fulvenallenyl radical (C7H5·) reacts preferentially with closed-shell molecules or radicals. Electronic structure calculations on the C10H9 potential energy surface accessed by the reactions of C7H5· with methylacetylene (CH3CCH) and allene (H2CCCH2) were combined with RRKM-ME calculations of temperature- and pressure-dependent rate constants using the automated EStokTP software suite and kinetic modeling to assess the reactivity of C7H5· with closed-shell unsaturated hydrocarbons. Experimentally, the reactions were attempted in a chemical microreactor heated to 998 ± 10 K by preparing fulvenallenyl radicals via pyrolysis of trichloromethylbenzene (C7H5Cl3) and seeding the radicals in methylacetylene or allene carrier gas, with product identification by means of photoionization mass spectrometry. The measured photoionization efficiency curve of m/z = 128 was assigned to a linear combination of the reference curves of two C10H8 isomers, azulene (minor) and naphthalene (major), presumably resulting from the C7H5· plus C3H4 reactions. However, the calculations demonstrated that these reactions are too slow, and kinetic modeling of processes in the reactor allowed us to conclude that the observation of naphthalene and azulene is due to the C7H5· plus C3H3· reaction, where propargyl is produced by direct hydrogen atom abstraction by chlorine (Cl) atoms from allene or methylacetylene and Cl stem from the pyrolysis of C7H5Cl3. Modeling results under the copyrolysis conditions of toluene and methylacetylene in high-temperature shock tube experiments confirmed the prevalence of the fulvenallenyl reaction with propargyl over its reactions with C3H4 even when the concentrations of allene and methylacetylene largely exceed that of propargyl. Overall, the reactions of fulvenallenyl with both allene and methylacetylene were found to be noncompetitive in the formation of naphthalene and azulene thus attesting the inefficiency of the fulvenallenyl radical reactions with the prototype closed-shell hydrocarbon species. In the meantime, the new reaction pathways revealed, including H-assisted isomerizations between C10H8 isomers and decomposition reactions of various C10H9 isomers, emerge as relevant and are recommended for inclusion in combustion kinetic models for naphthalene formation.

Vibrational energy relaxation in shock-heated CO/N2/Ar mixtures.

Experimental and numerical studies were performed on the vibrational energy relaxation in shock-heated CO/N2/Ar mixtures. A laser absorption technique was applied to the time-dependent rovibrational temperature time-history measurements. The vibrational relaxation data of reflected-shock-heated CO were summarized at 1720-3230K. In shock-tube experiments, the rotational temperature of CO quickly reached equilibrium, whereas a relaxation process was found in the time-dependent vibrational temperature. For the mixture with 1.0% CO and 10.0% N2, the vibrational excitation caused a decrease in the macroscopic thermodynamic temperature of the test gas. In the simulations, the state-to-state (StS) approach was employed, where the vibrational energy levels of CO and N2 are treated as pseudo-species. The vibrational state-specific inelastic rate coefficients of N2-Ar collisions were calculated using the mixed quantum-classical method based on a newly developed three-dimensional potential energy surface. The StS predictions agreed well with the measurements, whereas deviations were found between the Schwartz-Slawsky-Herzfeld formula predictions and the measurements. The Millikan-White vibrational relaxation data of the N2-Ar system were found to have the most significant impact on the model predictions via sensitivity analysis. The vibrational relaxation data of the N2-Ar system were then modified according to the experimental data and StS results, providing an indirect way to optimize the vibrational relaxation data of a specific system. Moreover, the vibrational distribution functions of CO and N2 and the effects of the vibration-vibration-translation energy transfer path on the thermal nonequilibrium behaviors were highlighted.

Insights into shock velocity variation in double-diaphragm shock tubes

A comprehensive understanding of shock formation and propagation in shock tubes is crucial for their diverse applications. The shock velocity in single-diaphragm shock tubes, characterized by initial acceleration and subsequent attenuation due to viscous effects, has been extensively investigated. However, limited studies exist on the double-diaphragm mode of operation. In this study, shock tube experiments were conducted using helium at pressures of 10–60 bar as driver gas and argon at pressures of 100–600 Torr as driven gas. The shock velocity profiles in the double-diaphragm mode show a sequence of acceleration and deceleration stages of the shock front, strongly influenced by the driver-to-driven pressure ratios (P41) and the pressure in the intermediate section (Pmid). Particularly, at high values of P41, peak shock velocities can exceed those measured near the end wall by about 12%. Large axial temperature gradients arise in the driven gas due to the accelerating and decelerating shock. Selecting appropriate diaphragms to maintain the intermediate section's pressure close to the value of the driver pressure can reduce peak shock velocities and post-shock temperatures. An in-house one-dimensional (1D) weighted essentially non-oscillatory scheme-based code was utilized to analyze wave interactions in the shock formation region, revealing that the post-shock gas behind the secondary diaphragm and inhibition of the primary diaphragm's opening and subsequent reopening can lead to unique shock profiles in double-diaphragm shock tubes. These insights deepen our understanding of wave propagation in shock tubes and suggest ways to mitigate undesirable effects in double-diaphragm shock tubes.

Inversion of electron densities in plasma wakes of hypersonic targets

Focusing on the plasma wakes of hypersonic targets (PWHT), this work provides a method for the inversion of electron densities in the PWHT by integrating electromagnetic radiation theory and three-wave coupling theory (TWCT). Utilizing the Computational Fluid Dynamics (CFD) method, taking a blunt cone as an example, the spatial distribution characteristics of electron densities in the plasma wake are simulated and analyzed in detail. The Zakharov system of equations (ZSE) is applied to the PWHT for the discussion about the electromagnetic radiation generated by plasma parametric instability. Combining the wavenumber spectrum of electromagnetic radiation and the TWCT, an inverse method of the electron densities of plasma wakes is proposed and applied to a blunt cone model. The results indicate that the inversion error rate is effectively held below 1.37 %. Furthermore, reasons for errors are elucidated through the utilization of the wavenumber spectrum structure. The proposed method may contribute to the development of hypersonic wake diagnostic techniques, and offer new insights and directions for wind tunnel experiments, shock tube experiments, as well as research on hypersonic target detection.

On the power-law exponent of multimode Richtmyer–Meshkov turbulent mixing width

Turbulent mixing induced by the Richtmyer–Meshkov (RM) instability occurs extensively in natural phenomena and engineering applications. Among the physical quantities characterizing the RM turbulent mixing, the mixing width has prominent importance. The total mixing width h can be divided into the spike mixing zone width hs and the bubble mixing zone width hb. For multimode perturbed RM problems that commonly occur in engineering practice, early instability develops rapidly into the self-similar regime. In this regime, it is widely accepted that hs,bt∼tθs,b, where t is the time and θs,b is the power-law exponent. However, this scaling law is associated with two open questions. (1) How should a reasonable reference interface be selected to segment h into hs and hb? (2) Are the resulting θs and θb equal to each other or not? To answer these two questions, in this study, we propose a general definition of reference interface based on the position corresponding to any fixed value of either the mass fraction, volume fraction, or density. Under this definition, the invariance of fraction and density profiles by self-similar transformation leads to hs,bt∼tθs,b with θs=θb. The general definition covers those provided in linear electronic motor experiment [Dimonte and Schneider, “Density ratio dependence of Rayleigh–Taylor mixing for sustained and impulsive acceleration histories,” Phys. Fluids 12, 304–312 (2000)] and shock tube experiment [Krivets et al., “Turbulent mixing induced by Richtmyer-Meshkov instability,” AIP Conf. Proc. 1793, 150003 (2017)]. Moreover, these two definitions are proved to be, respectively, special cases of newly proposed general definition. Finally, it is deduced that θs≠θb observed in high-density ratio experiments is possibly because the turbulent mixing has not entered a self-similar regime. Compared to the low-density ratio cases, mixing of high-density ratio is much more difficult to enter the self-similar regime.

A detailed kinetic submechanism for OH[formula omitted] chemiluminescence in hydrogen combustion revisited. Part 1

In this series of two papers, we present a novel physically sound kinetic submechanism aimed at modeling the formation and decay of chemiluminescent (electronically excited) OH∗ molecules in the ignition and combustion of hydrogen and hydrogen-based mixtures over a wide range of thermodynamic parameters and mixture compositions. The present part of this series describes the preparatory stages of the work, such as the aggregation of the data set for the OH∗ chemiluminescent emission (including the OH∗ emission profiles obtained in our shock-tube experiments), the choice of the basic kinetic model of hydrogen oxidation, and, importantly, quantum chemical calculations and theoretical estimates for the rate constant of the nonadiabatic preassociation reaction H + O + M ⇆ OH∗ + M, recognized as a major source of excited OH∗ molecules in hydrogen-air flames. These rate constant estimates are carried out in a wide range of temperatures (200–4000 K) and pressures (10−3−100 bar). It is shown that the rate constant of this key reaction exhibits distinct non-Arrhenius temperature behavior and, moreover, has a complex fall-off pressure dependence, with the transition from a third to a second-order, high-pressure regime occurring at pressures about 10 atm. The obtained dependence on temperature and pressure is in excellent agreement with the known experimental data and can be recommended (as part of the appropriate reaction mechanisms) for further kinetic modeling of OH∗ chemiluminescence accompanying the high-temperature oxidation of hydrogen and other fuels.Novelty and Significance StatementAlthough ultraviolet OH∗ chemiluminescence as an optical signature of the combustion process and the underlying chemistry is known to provide unique capabilities for combustion diagnostics, the development of the detailed reaction mechanisms intended for quantitative interpretation of the OH∗ emission measurements has now almost stagnated. Indeed, existing kinetic models contain a discouragingly small number of elementary processes and, apparently, do not account for the full variety of possible reaction pathways of OH∗ formation and depletion. Therefore, in this serial work, we have revised the current ideas about the kinetics of OH∗ production and consumption during ignition and combustion and focused first on the OH∗ kinetics in a reacting H2/O2/Ar/N2 mixture. The fundamentally significant and novel results of this Part of the present series are (i) the rate constant of the nonadiabatic preassociation reaction H + O + M ⇆ OH∗ + M (the primary source of OH∗ in hydrogen combustion), theoretically obtained for the first time as a function of temperature and pressure, and (ii) the OH∗ emission profiles obtained in our shock-tube experiments. The other aspects described here, such as the aggregation of the literature-based data set for the post-shock OH∗ chemiluminescent emission and the choice of the basic kinetic model of hydrogen oxidation, are also of interest and relevance.

Analysis of single-mode Richtmyer–Meshkov instability using high-order incompressible vorticity—streamfunction and shock-capturing simulations

Two- and three-dimensional simulation results obtained using a new high-order incompressible, variable-density vorticity–streamfunction (VS) method and data from previous ninth-order weighted essentially nonoscillatory (WENO) shock-capturing simulations [M. Latini and O. Schilling, “A comparison of two- and three-dimensional single-mode reshocked Richtmyer-Meshkov instability growth,” Physica D 401, 132201 (2020)] are used to investigate the nonlinear dynamics of single-mode Richtmyer–Meshkov instability using a model of a Mach 1.3 air(acetone)/SF6 shock tube experiment [J. W. Jacobs and V. V. Krivets, “Experiments on the late-time development of single-mode Richtmyer–Meshkov instability,” Phys. Fluids 17, 034105 (2005)]. A comparison of the density fields from both simulations with the experimental images demonstrates very good agreement in the large-scale structure with both methods but differences in the small-scale structure. The WENO method captures the small-scale disordered structure observed in the experiment, while the VS method partially captures such structure and yields a strong rotating core. The perturbation amplitude growth from the simulations generally agrees well with the experiment. The simulation bubble and spike amplitudes agree well at early times. At later times, the WENO bubble amplitude is smaller than the VS amplitude and vice versa for the spike amplitude. The predictions of nonlinear single-mode instability growth models are shown to agree with the simulation amplitudes at early-to-intermediate times but underpredict the amplitudes at later times in the nonlinear regime. Visualizations of the mass fraction and enstrophy isosurfaces, velocity and vorticity fields, and baroclinic vorticity production and vortex stretching terms from the three-dimensional simulations indicate that, with the exception of the small-scale structure within the rollups, the VS and WENO results are in good agreement.

Ignition Delay and Reaction Time Measurements of Hydrogen–Air Mixtures at High Temperatures

Induction and reaction times of hydrogen–air mixtures (ϕ = 0.5–2) have been measured behind reflected shock waves at temperatures of 1000–1600 K, pressures of 0.1, 0.3, 0.6 MPa in the domain of the extended second explosion limit. The measurements were performed in the shock tube with a completely transparent test section of 0.5 m long, which provides pressure, ion current, OH and high-speed chemiluminescence observations. The experimental induction time plots demonstrate a clear increasing of the global activation energy from high- to low temperature post-shock conditions. This trend is strongly pronounced at higher post-shock pressures. For a high-temperature range of T > 1200 K, induction time measurements show an activation energy for the global reaction rate of hydrogen oxidation of 64–83 kJ/mole. Detected reaction times exhibit a big scatter and a weak temperature dependence. The minimum reaction time value was nearly 2 µs. Obtained induction time data were compared with calculations carried out in accordance with the known kinetic mechanisms. For current and former shock-tube experiments within a pressure range of 0.1–2 MPa, critical temperatures required for strong (1000–1100 K), transient and weak auto-ignition modes behind reflected shock waves were identified by means of the pressure and ion-probe measurements in stoichiometric hydrogen-air mixture. The transfer from the strong volumetric self-ignition near the reflecting wall to the hot spot ignition (transient) was established and visualized below <1200 K with a post-shock temperature decreasing.

Ab initio kinetic study on the abstraction reactions of methylcyclohexane and implications for high‐temperature ignition simulations from shock tube experiment

AbstractMethylcyclohexane (MCH) is the simplest alkylated cyclohexane, and has been widely employed in surrogate models to represent the cycloalkanes in real fuels. Thus, extensive experimental and kinetic modeling studies have been performed to understanding the combustion chemistry of MCH. However, through a detailed literature analysis, there still lack a systematic theoretical study on the abstraction reactions of MCH, which are the main initial oxidation pathway of MCH. Herein, this work reports a systematic ab initio chemical kinetic study on the abstraction reactions of MCH with different radicals/species. Specifically, reaction rate constants of 30 abstraction reactions of MCH with H/O/OH/O2/HO2/CH3 at different sites are computed using transition state theory (TST) by using quantum chemistry calculation results at DLPNO‐CCSD(T)/CBS//M06‐2X/cc‐pVTZ level. The computed results are incorporated into a detailed mechanism to simulate newly measured ignition delay times (IDTs) of MCH in this work at equivalence ratios of 0.5, 1.0, and 2.0, pressures of 2 and 5 bar, temperatures ranging from 1140 to 1640 K. The updated detailed mechanism demonstrates improvement in the prediction of IDTs, especially at fuel‐rich conditions. The fuel concentration and dilution effect on the IDTs are discussed, and a general Arrhenius expression is adopted to fit the IDTs from both this work and literature work. This work should be valuable for further optimization of detailed kinetic mechanisms and also for gaining insight into the combustion chemistry of MCH.

Chemical Kinetic Analysis of High-Pressure Hydrogen Ignition and Combustion toward Green Aviation

In the framework of the “Multidisciplinary Optimization and Regulations for Low-boom and Environmentally Sustainable Supersonic aviation” project, pursued by a consortium of European government and academic institutions, coordinated by Politecnico di Torino under the European Commission Horizon 2020 financial support, the Italian Aerospace Research Centre is computationally investigating the high-pressure hydrogen/air kinetic combustion in the operative conditions typically encountered in supersonic aeronautic ramjet engines. This task is being carried out starting from the zero-dimensional and one-dimensional chemical kinetic assessment of the complex and strongly pressure-sensitive ignition behavior and flame propagation characteristics of hydrogen combustion through the validation against experimental shock tube and laminar flame speed measurements. The 0D results indicate that the kinetic mechanism by Politecnico di Milano and the scheme formulated by Kéromnès et al. provide the best matching with the experimental ignition delay time measurements carried out in high-pressure shock tube strongly argon-diluted reaction conditions. Otherwise, the best behavior in terms of laminar flame propagation is achieved by the Mueller scheme, while the other investigated kinetic mechanisms fail to predict the flame speeds at elevated pressures. This confirms the non-linear and intensive pressure-sensitive behavior of hydrogen combustion especially in the critical high-pressure and low-temperature region which is hard to be described by a single all-encompassing chemical model.

Kinetics and thermodynamics of unimolecular dissociation of n-C3H7I

Abstract The present work expands previous studies on the kinetics of the n-C3H7I unimolecular decomposition and the thermodynamic properties of n-C3H7I and i-C3H7I molecules, by providing combined experimental and theoretical data on the rate constant for reaction of n-C3H7I + Ar ⇌ n-C3H7 + I + Ar, as well as thermodynamic data for iodopropane isomers, calculated based on the density functional theory. The n-C3H7I dissociation rate constant has been precisely determined in shock-tube experiments by applying atomic resonance absorption spectrometry (ARAS) at the resonance transition wavelength of atomic iodine (183.0 nm) in a temperature range from 830 to 1230 K at a pressure of 3–4 bar. The resulting expression is presented in the Arrhenius form: k 1st = 1.17 × 1013exp(−191.4 kJ mol−1/RT) (s−1). Theoretical RRKM/ME calculation of the temperature- and pressure-dependent rate constant and channel branching ratio have been based on quantum chemical calculations and were performed over a wide range of thermodynamic conditions (T = 300–2000 K, p = 10−4 to 102 bar). Additionally, the thermochemistry of the reactions of n-C3H7I dissociation and isomerization has been calculated on B3LYP/cc-pVTZ-PP level of theory. Thermodynamic data, which are provided in NASA polynomial format, are in a better agreement with the available experimental data and previous theoretical estimates.