Shock Tube Research Articles

Probing the effect of fuel components on the auto-ignition behavior of ammonia/natural gas blends: A case study of ethane addition

Ammonia (NH3) is carbon-free and is known as a promising renewable fuel to achieve carbon reduction and carbon neutrality. However, ammonia presents much lower fuel reactivity compared to hydrocarbon fuels. Thus, ammonia is often blended with high reactivity fuels in practical combustion facilities, and natural gas is a good candidate due to its low carbon emissions and massive reserves. Methane is often used to represent natural gas in previous experimental and simulation studies, while the larger alkanes in natural gas can affect the fuel reactivity of ammonia/natural gas blends. This study aims to explore the effects of fuel components on the auto-ignition behaviors of ammonia/natural gas blends. As methane and ethane are the most important species in natural gas, a blend of 90 % methane and 10 % ethane by volume is used to represent natural gas in this study. A low-pressure shock tube was used to measure the ignition delay times (IDTs) of ammonia/methane, ammonia/ethane, and ammonia/natural gas blends in ‘air’ mixtures, with different ammonia blending ratios, at stoichiometric condition, at 1 atm and over the temperature range 1200 – 1800 K. The experimental results show that the IDTs of fuel blends increase with ammonia blending ratios. Meanwhile, 10 % ethane in natural gas can influence the fuel reactivity of ammonia/natural gas blends. A detailed kinetic model proposed in our previous study is used to simulate these new IDTs in addition to experimental data available in the literature. Overall, the kinetic model can predict well the auto-ignition behavior of ammonia/methane, ammonia/ethane, and ammonia/natural gas blends over a wide range of experimental conditions. The simulation and sensitivity analyses results show that the interaction reaction between ammonia and ethane, C2H6 + ṄH2 〈=〉 Ċ2H5 + NH3, is important for predicting the fuel reactivity of ammonia/ethane blends below 1600 K at the studied conditions.

Effects of unburnt reaction progress on stretch flame dynamics under elevated temperatures

Flames in practical combustors and engines are inevitably subject to the effects of upstream chemical reaction progress and stretch, induced by elevated thermodynamic conditions and flow non-uniformities, respectively. Recent shock tube experiments and simulation studies on flame propagation have shown that flame speed under engine-relevant conditions can be enhanced with non-negligible upstream chemical reaction progress, especially when low-temperature heat release is involved in the unburnt mixture. On the other hand, depending on the mixture equivalence ratio and diluents, nonequidiffusion (including the non-unity Lewis number effect and the preferential diffusion effect) can couple with the flame stretch to fundamentally affect the flame propagation, which can manifest as either facilitation or suppression. Depending on the transport property of a reacting mixture, there hence can be either inhibition or promotion from unburnt reaction progress and stretch effects on flame propagation. In the current work, through one-dimensional numerical simulations of transient planar and spherical flames of n-heptane/air under elevated thermodynamic conditions, the combined effects of upstream chemical reaction progress and stretch on flame propagation are investigated. Results show that for both lean and rich n-heptane/air mixtures, flame speed can be substantially promoted with reaction progress, while the rich mixtures can exhibit opposite stretch dependence after first-stage ignition. Different definitions of effective Lewis number are adopted to explain the change in Markstein length for spherical flame in lean and rich mixtures reformed by the low temperature chemistry. This work fills an important gap in laminar premixed flame research relevant to practical combustion systems and can provide useful insight into local turbulent flame behaviors and phi-sensitivity in engine combustion.

Shock Tube Study of Ignition Delay Times for Hydrogen–Oxygen Mixtures

This paper presents the results of measurements of ignition delay times in hydrogen–oxygen mixtures highly diluted with argon. The experiments were carried out behind an incident shock wave at temperatures from 870 to 2500 K, pressures from 0.5 to 1.5 atm, and equivalence ratios from 0.1 to 2.0. The results obtained were processed in terms of the partial pressure of the combustible mixture stoichiometric part that is consumed in the combustion process. An almost linear dependence of the ignition delay time on the reciprocal value of the partial pressure was found for both rich and lean mixtures. The measured data are compared with calculations based on the previously developed kinetic model and experimental data from other authors.

Multiple impact effects of helium-driven shocks on thin fiber-metal laminates

Fiber Metal Laminates (FMLs) have garnered considerable attention and are increasingly being utilized in the development of protective armors for explosion and ballistic scenarios. While most research has focused on assessing the response of FMLs to single impacts, real battlefield situations often require shielding structures to endure multiple impacts. Thus, this study revolves around the creation of hybrid FMLs designed for shock shielding purposes. The primary focus is on how these laminates withstand repetitive impacts from high-intensity shock waves, aiming to pinpoint the optimal sequence that offers the highest resistance against multiple shock impacts. To establish effective shielding, a multi-layered FML configuration is employed. This configuration incorporates AA6061-T6 facing plates, ballistic-grade synthetic materials like aramid/epoxy ply, and ultra-high molecular weight polyethylene (UHMWPE)/epoxy ply. Additionally, a paperboard/epoxy lamina is introduced to induce functional grading based on layerwise shock impedance mismatches. Shock impact experiments are conducted using a shock tube equipped with helium as the driver gas. Critical shock parameters, including Mach Number, positive impulse, and peak overpressure, are meticulously evaluated. For validation purposes, a numerical model is employed to project the damage profile as a function of radial distance across different laminate sequences. The study unveils that ply deformations are strongly influenced by the arrangement of core layers, particularly the positions of the paperboard and UHMWPE layers within the core structure. To contextualize the findings, the shock impact results obtained from this study are compared with those from prior experiments that employed nitrogen-driven shocks.

Mars2020 Entry Shock Layer Thermochemical Kinetics Examined by Megahertz-Rate Laser Absorption Spectroscopy

A mid-infrared laser absorption diagnostic was deployed to examine the evolution of thermophysical properties across a simulated Mars2020 shock layer in the Electric Arc Shock Tube (EAST) facility at NASA Ames. Rapid laser tuning techniques using bias-tee circuitry enabled quantitative temperature and number density measurements of CO2 and CO with microsecond resolution over a shock velocity range of 1.30–3.75 km/s. Two interband cascade lasers were utilized at 4.17 and 4.19 μm to resolve rovibrational CO2 lines spanning across J′′=58 to J′′=140 in the asymmetric stretch fundamental bands. In test cases with enough energy to dissociate CO2, a quantum cascade laser scanned multiple transitions of the CO fundamental bands near 4.98 μm. The results are compared to the Data Parallel Line Relaxation (DPLR) code and Lagrange shock tube analysis (LASTA) simulations of the shock layer. A numerical simulation of the compressible boundary layer is used to account for measurement sensitivities to this flow feature in the EAST facility. Temperature and species transients are compared to multiple chemical kinetic models. The laser absorption data presented in this work can be used to refine the models used to simulate the aerothermal environment encountered during Mars entry.

Impact of exhaust gas recirculation and nitric oxide on the autoignition of an oxygenated gasoline: Experiments and kinetic modelling

Exhaust gas recirculation (EGR) and NOx affect the autoignition of gasoline in internal combustion engines. In this work, ignition delay times of an oxygenated gasoline (Euro 6 E10) are investigated with EGR and NOx addition. The experimental data are of interest for gasoline surrogate modeling and for predicting fuel ignition behavior in various engine combustion modes. We studied Euro 6 E10 reactivity with EGR additions of 20 % and 40 % (by mass) doped, in select cases, with high levels of NOx (874, 1501, 3174, and 5568 ppm, by mol%). Experiments were performed in two high-pressure shock tubes (HPSTs) and two rapid compression machines (RCMs) over a wide range of temperature (658–1610 K), equivalence ratio (0.5, 1.0) and at pressures of 20 and 30 bar. Results show that EGR addition inhibits Euro 6 E10 gasoline reactivity in the intermediate- and low-temperature regimes, while it is minimally affected at high-temperatures. In contrast, a strong reactivity-promoting effect of NOx is observed at temperatures greater than 825 K for all equivalence ratios investigated, while an inhibiting effect is seen at lower temperatures with a high doping of NOx (1501 ppm). For fuel-lean cases, where Euro 6/EGR mixtures are sensitized with 3174 – 5568 ppm NOx, a significant reactivity-promoting effect is observed across the entire range of conditions investigated, except at temperatures below 830 K, where the NOx-inhibiting effect dominates. A gasoline surrogate model is developed by combining a detailed gasoline surrogate mechanism with appropriate sub-models and making minor updates. The proposed model captures well the influence of EGR and EGR/NOx on Euro 6 E10 autoignition over the wide range of conditions studied here. Finally, sensitivity analyses were conducted to identify key reactions contributing to the perturbative effects of EGR and EGR/NOx on oxygenated gasoline ignition.

Ignition of Various Lubricating Oil Compositions Using a Shock Tube

Abstract With new restrictions imposed on gas turbine efficiencies and power outputs, lubricating oils are used at higher temperatures and harsher conditions leading to potential, unintended combustion. To establish an understanding of lubricating oil's resistance to combustion, a new spray injector system was utilized in the High-Pressure Shock Tube (HPST) Facility at the TEES Turbomachinery Laboratory at Texas A&M University. Two gas turbine oils (Mobil DTE 732 and Castrol Perfecto X32), a base mineral oil, and a surrogate (n-hexadecane) were tested at postreflected shock conditions at equivalence ratios near 2.5. Castrol Perfecto X32 was also characterized at an equivalence ratio near 1.2. All of the lubricating oils displayed ignition between temperatures of 1152 and 1383 K and near atmospheric pressures. To characterize combustion, two different definitions of ignition delay time (IDT) were considered: sidewall OH* chemiluminescence and sidewall pressure. Both definitions were used to create temperature-dependent correlations for each of the lubricating oils. In general, both definitions provided similar results within the accuracy of the measurements. One trend from the data herein is that the brand-name oils (Mobil DTE 732 and Castrol Perfecto X32) provided ignition delay times that were similar to each other but slightly larger than the corresponding mineral oil and n-hexadecane results. This difference could be attributed to the additives that are present in the brand-name oils.

Evaluation of the relative importance of preheat from hohlraum x rays and a radiative shock on a low-density foam

Indirectly driven shock-tube experiments were performed on the Omega Laser Facility to evaluate the relative importance of hohlraum x ray and radiative shock preheat sources on a low-density foam. X rays emitted from the hohlraum and a subsequent shock wave are channeled into a low-density foam sample, which contains a plastic preheat-witness disk placed downstream of the foam. Simultaneous radiographic measurements of the shock speed in the foam and the expansion rate of the witness disk due to preheat allow for the observation of effects from the hohlraum's low-energy and high-energy x-ray spectrum. We showed, from simulations, that low-energy x rays from the hohlraum are preferentially absorbed near the ablator surface (where the hohlraum and the shock tube meet), while higher-energy x rays largely pass through the ablator and foam and are volumetrically absorbed by the witness disk. Reproducing the experimentally measured shock speed and expansion of the witness disk simultaneously, we extracted the temperature evolution of preheated foam from the simulation and evaluated the relative importance of preheat sources on a low-density foam from hohlraum x-ray radiation and radiative shock. We found that radiation from the shock front was more effective at preheating the low-density foam than the high-energy x rays from the hohlraum. This shock-tube preheat experiment is important for understanding the results of the MARBLE experiments at the National Ignition Facility because initial conditions of foam-filled MARBLE capsules are sensitive to preheat.

A Comparative Study on the Optimal Modeling of Laminated Glass

This study addresses the challenging task of modeling laminated glass responses to extreme loading scenarios for the design and analysis of protective structures. The primary objective is to seek an optimal modeling approach that balances accuracy and computational efficiency. To achieve this, the failure modeling of laminated glass layups comprising thin and thick panels with three and eleven layers is investigated under blast loading conditions. Various simulation techniques are employed, including the finite element method (FEM) with element erosion/deletion, smoothed particle hydrodynamics (SPH), and a hybrid approach involving the conversion of elements into particles. The feasibility and limitations of each technique are examined, considering both accuracy and computational cost. Experimental results from arena and shock tube testing scenarios assess the deployed modeling techniques and the presented comparisons. Emphasis is placed on mesh sensitivity and the significance of adaptive meshing in capturing fracture patterns. The present paper suggests that utilizing hybrid techniques results in optimal modeling outcomes. Furthermore, the stability of the modeling results under diverse blast conditions is confirmed. This article contributes to the field by offering insights into modeling laminated glass responses to extreme loading, emphasizing the use of hybrid techniques to strike a balance between accuracy and computational efficiency. This research enhances the understanding of protective structure design and analysis, highlighting the critical importance of computational methods in this context. Doi: 10.28991/CEJ-2023-09-11-018 Full Text: PDF

On the decomposition mechanism of propanal: rate constants evaluation and kinetic simulations

The reactivity of aldehydes has been the subject of considerable interest in chemical kinetics, with propanal often chosen as the representative species. Despite its relevance, the reactivity of propanal is currently estimated from analogy and fitting of experimental data measured in limited temperature and pressure ranges, while the few literature theoretical studies have focused more on the exploration the potential energy surface (PES) than on the estimation of rate constants. The purpose of this work is to reinvestigate the propanal decomposition kinetics using the ab initio transition state theory based master equation approach with the intent of: (1) Determining accurate rate constants of key reaction channels; (2) Updating and validating an existing kinetic model by simulating available experimental data on propanal pyrolysis. It is found that propanal decomposition at the initial stages of pyrolysis occurs through four unimolecular barrierless reactions to form CHO + C2H5, CH2CHO + CH3, CH3CHCHO + H, and CH3CH2CO + H, and a termolecular pathway leading to the formation of C2H4 + CO + H2. High pressure rate constants were determined for each barrierless reaction channel using Variable Reaction Coordinate Transition State Theory and used to estimate phenomenological temperature and pressure dependent channel specific rate constants integrating the 1 dimensional master equation over the whole PES. The decomposition rate constants so determined are in agreement with the few available experimental data and significantly faster than previous literature estimates. The estimated kinetic parameters were finally implemented into the CRECK kinetic mechanism, leading to an improved agreement with shock tube pyrolysis data from the literature.

An Experimental and Detailed Kinetics Modeling Study of Norbornadiene in Hydrogen and Methane Mixtures: Ignition Delay Time and Spectroscopic CO Measurements

High-energy-density compounds such as norbornadiene (NBD) are being considered as potential cost-effective fuel additives, or partial replacements, for high-speed propulsion applications. To assess the ability of NBD to influence basic fuel reactivity enhancement and to build a database for developing future NBD kinetics models, ignition delay times were measured in two shock-tube facilities at Texas A&M University for H2/O2, CH4/O2, H2/NBD/O2, and CH4/NBD/O2 mixtures (ϕ = 1) that were highly diluted in argon. The reflected-shock temperatures ranged from 1014 to 2227 K, and the reflected-shock pressures remained near 1 atm for all of the experiments, apart from the hydrogen mixtures, which were also tested near 7 atm, targeting the second-explosion limit. The molar concentrations of NBD were supplemented to the baseline mixtures representing 1–2% of the fuel by volume. A chemiluminescence diagnostic was used to track the time history of excited hydroxyl radical (OH*) emission, which was used to define the ignition delay time at the sidewall location. Spectroscopic CO data were also obtained using a tunable quantum cascade laser to complement both the ignition and the chemiluminescence data. The CH4/O2 mixtures containing NBD demonstrated reduced ignition delay times, with a pronounced effect at lower temperatures. Conversely, this additive increased the ignition delay time dramatically in the H2/O2 mixture, which was attributed to changes in the fundamental chemistry with the introduction of molecules containing carbon bonds, which require stronger activation energies for ignition. Correlations were developed to predict the ignition delay time, which depends on species concentration, temperature, and pressure. Additionally, one tentative mechanism was tested, combining base chemistry from NUIGMech 1.1 with pyrolysis and oxidation reactions for NBD using the recent efforts from experimental and theoretical literature studies. The numerical predictions show that the rapid decomposition of NBD provides a pool of active H-radicals, significantly increasing the reactivity of methane. This study represents the first set of gas-phase ignition and CO time-history data measured in a shock tube for hydrogen and methane mixtures containing the additive NBD.

Experimental and modeling study of ignition delay times of natural gas with CO2 dilution

Pressurized oxy-fuel combustion has attracted considerable attention as an efficient technology with lower emissions. Natural gas, which primarily consists of methane, ethane, and propane, is a widely used fossil fuel owing to its safety and low cost. The auto-ignition of 90 %methane/9 %ethane/1 %propane mixture diluted in CO2 or N2 were investigated using a shock tube at temperatures of 1280–1645 K, pressures of 2 and 10 bar, and equivalence ratios of 0.5–2.0 in this study. An updated model named OXY-NG can accurately predict the ignition delay times, species yields, and laminar flame speeds of natural gas mixtures in oxy-fuel and air combustion modes. The ignition delay times increased as the equivalence ratios increased related to the recombination of CH3 radicals at 2 bar but decreased as the equivalence ratios increased owing to reactions involving HO2 at 250 bar. The diluent CO2 has inhibitory effects on ignition compared with N2, and these effects were minor under high pressure and fuel-rich conditions when diluted in CO2. The effects on natural gas mixture ignition owing to chemical and physical properties of CO2 are discussed in detail.

Strongly conservative discretization of governing equations in cylindrical coordinates

Cylindrical coordinates are often used in computational fluid dynamics (CFD), particularly when one considers axisymmetric flow. Although cylindrical coordinates have several advantages in accurately describing the rotational flow as well as the complex boundary, there are limits to how far they can be taken. These are (1) the governing equations are mostly in a non-conservative form in current standard models, which cannot preserve the conservation of numerical simulations, (2) the singularity along the axis at the coordinate origin (z-axis), leading to difficulty reproducing the flow across the z-axis, and (3) the time step is extremely shortened to satisfy the CFL condition near the z-axis because the numerical cell thereof is narrow in the azimuthal direction for a given angular resolution. A strongly conservative model is proposed in this paper to overcome these difficulties. First, the strongly conservative form of the governing equations is derived by considering changes in the direction of the base vector within a cell, and a concept of direction error is defined to reveal the inherent error of the standard model. Second, inspired by the derivation of the first, a strongly conservative method for evaluating the flux across each cell surface is proposed, preserving the conservation of discretization and enlarging the time step. Finally, the proposed model is tested by three classical problems, i.e., uniform flow, Sod shock tube problem, and expanding outflows. The results imply that the proposed model is strongly conservative for the cylindrical coordinate system, and it can overcome the limits of the standard model and eliminate the inherent error in the standard model. Furthermore, the strongly conservative model and method are extended to arbitrary coordinates for the numerical conservation in general curvilinear coordinates of the problem with complex boundaries.

Radiation properties of air behind strong shock wave

The problem of descent spacecrafts safety is closely related to the correct assessment of heat fluxes to their surface. This work is devoted to an experimental study of radiative heat fluxes that play an important role during the entry of a spacecraft into the Earth's atmosphere at superorbital velocity.The radiation properties of high-temperature air were measured in the detonation driven shock tube DDST-M at the gas pressure 0.25 Torr ahead of the shock wave in the shock wave velocity range from 7.7 to 11.4 km/s. The applied registration method fixates the time-integrated distribution of the radiation intensity of the shock heated air passing by the measuring cross section. The measurements were carried out in the spectral range from 190 to 1100 nm. Simultaneously, the evolution of the radiation power at certain wavelengths that are selected using monochromators was recorded. Time spectrograms at certain wave lengths make it possible to determine the effective process time that can be used to recalculate the measured integral radiation intensity to the radiation power that traditionally measures by 3-D spectroscopy method. A new spectral model is presented that performs a line-by-line calculation of the emission and absorption spectra of atoms and molecules. The experimental results are compared with the calculated data obtained using the spectral model, as well as data from other measurements.

ANCHORAGE DESIGN FOR BLAST-RESISTANT WINDOWS

This paper presents a summary of experimental research on the performance of blast-resistant window anchors and the development of a design procedure for the anchors. The experimental results, obtained from forty-six window tests conducted under simulated blast loading using a shock tube are reported. Two analysis procedures that were developed for anchor design are described. The first procedure involves a two-degree-of-freedom dynamic analysis. The second procedure involves a single degree of freedom analysis that can be implemented manually. The development of both methods is presented for the anchorage design of windows subjected to blast loads.

Probing the kinetic sensitization effect of NO2 on ethylene oxidation

Exhaust gas recirculation (EGR) has been widely equipped in modern engines for pollutant emission abatement. NOx present in EGR can greatly influence fuel combustion properties in engine operation. Ethylene (C2H4) is well-known a critical intermediate formed via β-scission reactions of alkyl radicals in the pyrolysis and oxidation of hydrocarbons, thus investigating the mutual effects of C2H4 and NOx is beneficial to engine design as well as hierarchically developed strategy of the chemical kinetic model. This study investigated the auto-ignition behavior of C2H4/air mixtures containing different levels of NO2 (0 ppm, 200 ppm, 500 ppm, 3000 ppm) in a high-pressure shock tube at pressures of 1 atm and 24 atm, temperatures of 820–1300 K and different equivalence ratios of 0.5, 1.0 and 2.0. The results showed that the addition of NO2 significantly promoted auto-ignition at high pressure, while only limited effects at low pressure. A detailed kinetic mechanism was proposed based on a hierarchically developed strategy, and it successfully reproduced the experimental data over the entire test conditions. Flux and sensitivity analyses were implemented to explore the sensitization effect of NO2 addition on the ignition kinetics of C2H4. The analysis revealed that NO2 mainly affects the reaction pathway of ĊH2CHO radical, leading to the formation of HONO. The additional generation of ȮH radicals through HONO = NO + ȮH and NO + HȮ2 = NO2 + ȮH accelerate the auto-ignition of C2H4. At low pressure, the decrease in the rate constant of HONO decomposition weakens the role of NO2.

CO Laser Absorption Measurements During Syngas Combustion at High Pressure

Abstract Syngas is a desirable fuel for combustion in the Allam-Fetvedt cycle, which involves combustion under supercritical-CO2 conditions. While some work has been conducted in collecting ignition delay times (IDT) at the extreme pressures required by these systems, significant model deficiencies remain. Additionally, considerable barriers in terms of nonideal gas dynamic effects have been shown for these experiments in shock tubes. Further investigation into the fundamental combustion kinetics of H2/CO/CO2 mixtures is required. Time-resolved speciation measurements for target species have been shown to better aid in improving the understanding of underlying chemical kinetics than global ignition delay time measurements. Therefore, laser absorption measurements of CO were measured behind reflected shock waves during combustion of syngas at 5 and 10 bar and temperatures between 1080 and 2100 K. The mixtures investigated utilized H2-to-CO ratios of 1:1 and 1:4, respectively, each at stoichiometric conditions, allowing for discussions of the effect of initial fuel composition. A ratio of fuel to CO2 of 1:2 was also utilized to represent commercially available syngas. The mixtures were diluted in helium and argon (20% He, 76.5% Ar) to minimize thermal effects and to expedite CO thermal relaxation during the experiment. The resulting CO time histories were then compared to modern chemical kinetics mechanisms, and disagreement is seen for this system, which is assumed to be fairly well known. This study elucidates particular chemistry that needs improvement in moving toward a better understanding of syngas combustion at elevated pressures.

An Adaptive Mesh Refinement–Rotated Lattice Boltzmann Flux Solver for Numerical Simulation of Two and Three-Dimensional Compressible Flows with Complex Shock Structures

An adaptive mesh refinement–rotated lattice Boltzmann flux solver (AMR-RLBFS) is presented to simulate two and three-dimensional compressible flows with complex shock structures. In the method, the RLBFS, which has a strong shock-capturing capability and can effectively eliminate the shock instability phenomenon, is applied to solve the flow filed by reconstructing the fluxes at each cell interface adaptively with the mesoscopic lattice Boltzmann model. To locally and dynamically improve the resolution of intricate shock structures and optimize the required computational resources, a block-structured adaptive mesh refinement (AMR) technique is introduced. The validity and effectiveness of the proposed method are confirmed through a range of two and three-dimensional numerical cases, including the shock tube problem, the four-wave Riemann problem, explosion within a rectangular box, and the vorticity induced by a shock. The results obtained using the AMR-RLBFS exhibit excellent agreement with published data and demonstrate high accuracy in capturing complex shock structures. The computational efficiency of the AMR-RLBFS can be also improved significantly compared to the RLBFS on uniform grids. Furthermore, the numerical outcomes underscore the capability of the AMR-RLBFS to eliminate shock instability effects while efficiently capturing a broader spectrum of small-scale vertical structures. These findings highlight the ability of AMR-RLBFS to improve the computational efficiency and capture intricate shock structures effectively, making it a valuable tool for studying a wide range of compressible flows from aerodynamics to astrophysics.

Experimental and Modeling Study on the Ignition Kinetics of Nitromethane behind Reflected Shock Waves.

Nitromethane (NM) is the simplest nitroalkane fuel and has demonstrated potential usage as propellant and fuel additive. Thus, understanding the combustion characteristics and chemistry of NM is critical to the development of hierarchical detailed kinetic models of nitro-containing energetic materials. Herein, to further investigate the ignition kinetics of NM and supplement the experimental database for kinetic mechanism development, an experimental and kinetic modeling analysis of the ignition delay times (IDTs) of NM behind reflected shock waves at high fuel concentrations is reported against previous studies. Specifically, the IDTs of NM are measured via a high-pressure shock tube within the temperature from 900 to 1150 K at pressures of 5 and 10 bar and equivalence ratios of 0.5, 1.0, and 2.0. Brute-force sensitivity analysis and chemical explosive mode analysis in combination with reaction path analysis are employed to reveal the fundamental ignition kinetics of NM. Finally, a skeletal mechanism for NM is derived via the combination of directed relation graph-based methods, which demonstrates good prediction accuracy of NM ignition and flame speeds. The present work should be valuable for understanding the combustion chemistry of NM and the development of the fundamental reaction mechanism of nitroalkane fuels.

Time-varying uncertainty evaluation of the shock wave pressure measurements with small samples in an aerospace shock tube

Shock tube is a crucial device for the dynamic calibration of pressure sensors in aerospace and defense fields. Accurate measurement of the shock wave pressure (SWP) in the shock tube and reliable evaluation of the measurement uncertainty are the basis for improving the calibration accuracy of pressure sensors. However, the repeated measurement results of the SWP in shock tube have typical features of time-varying and small samples, leading to a poor reliability of the measurement uncertainty evaluation. To address this issue, an improved method is proposed to accurately evaluate the time-varying uncertainty of the SWP measurement results with small samples in an aerospace shock tube. The time-varying SWP measurement signals are firstly decomposed into four components by the multicomponent extraction based on signal decomposition algorithms. For the repeated SWP measurement data with small samples, a sample size expansion based on the bootstrap method is presented to reduce the mean estimation error of the four components. A time-varying uncertainty evaluation model of the four components is established based on the conjugate prior Bayesian theory. The reliable evaluation of the time-varying uncertainty of the SWP measurements is finally realized according to the uncertainty propagation principle. Simulations and the SWP measurement experiments in an aerospace shock tube are carried out to validate the performance of the proposed method. The results show that the proposed method works well in the time-varying uncertainty evaluation of the SWP measurements with small samples. Furthermore, the comparative experiments demonstrate the superiority of the proposed method over the Bessel method and the Bayesian method for the SWP measurement uncertainty evaluation in both accuracy and reliability.