Showing papers on "Shock tube published in 2023"

Interactions in Ammonia and Hydrogen Oxidation Examined in a Flow Reactor and a Shock Tube.

Abstract: Ammonia (NH3) is a promising fuel, because it is carbon-free and easier to store and transport than hydrogen (H2). However, an ignition enhancer such as H2 might be needed for technical applications, because of the rather poor ignition properties of NH3. The combustion of pure NH3 and H2 has been explored widely. However, for mixtures of both gases, mostly only global parameters such as ignition delay times or flame speeds were reported. Studies with extensive experimental species profiles are scarce. Therefore, we experimentally investigated the interactions in the oxidation of different NH3/H2 mixtures in the temperature range of 750-1173 K at 0.97 bar in a plug-flow reactor (PFR), as well as in the temperature range of 1615-2358 K with an average pressure of 3.16 bar in a shock tube. In the PFR, temperature-dependent mole fraction profiles of the main species were obtained via electron ionization molecular-beam mass spectrometry (EI-MBMS). Additionally, for the first time, tunable diode laser absorption spectroscopy (TDLAS) with a scanned-wavelength method was adapted to the PFR for the quantification of nitric oxide (NO). In the shock tube, time-resolved NO profiles were also measured by TDLAS using a fixed-wavelength approach. The experimental results both in PFR and shock tube reveal the reactivity enhancement by H2 on ammonia oxidation. The extensive sets of results were compared with predictions by four NH3-related reaction mechanisms. None of the mechanisms can well predict all experimental results, but the Stagni et al. [React. Chem. Eng. 2020, 5, 696-711] and Zhu et al. [Combust. Flame 2022, 246, 115389] mechanisms perform best for the PFR and shock tube conditions, respectively. Exploratory kinetic analysis was conducted to identify the effect of H2 addition on ammonia oxidation and NO formation, as well as sensitive reactions in different temperature regimes. The results presented in this study can provide valuable information for further model development and highlight relevant properties of H2-assisted NH3 combustion.

Numerical Simulation of a Shock Tube in Thermochemical Non-Equilibrium

Abstract: An efficient method is developed for calculating the non-equilibrium properties of the test gas in a shock tube in the shock frame of reference. The one dimensional method is based on the parabolised Navier-Stokes equations, resulting in a form similar to a stagnation line problem but with appropriate consideration of the mass loss to the boundary layer present in a shock tube. Gas properties are determined using Park’s two temperature model. Transport properties are evaluated using second order Chapman-Enskog theory. The centreline solution is coupled to an artificial radial pressure profile which mimics the effect of a boundary layer. The method was tested on a variety of air cases ranging from 5.5 km/s to 9.6km/s, and demonstrated improved modelling of the non-equilibrium regions compared to a Rankine-Hugoniot solver. A 6.1 km/s, 13.3 Pa test case relevant for Titan entry demonstrates the necessity of appropriately modelling mass loss to the boundary layer in a shock tube. The effect of shock structure and mass loss to the boundary layer in a non-equilibrium flow within a shock tunnel is shown to substantially impact the test gas properties and non-equilibrium radiance profiles in the UV/Vis and Vis/IR regions.

Laser Absorption Spectroscopy Measurements of Post-Shock Non-Equilibrium Species in the NASA Ames Electric Arc Shock Tube

Abstract: The NASA Ames Electric Arc Shock Tube (EAST) is a unique facility capable of generating high enthalpy, shock-heated impulse gas flows representative of the kinetic and radiative conditions encountered by atmospheric entry vehicles. The facility maintains a successful history with emission spectroscopy, providing measurements of absolute radiance upon which various validation studies are anchored. The central objective of this work is to develop a comprehensive tunable diode laser absorption spectroscopy (TDLAS) sensing capability to provide complementary experimental insights to existing emission techniques. More specifically, a fast-scanning TDLAS-based diagnostic targeting the cyano radical (CN) in the near-infrared (near-IR) near 926.6 nm is employed in mixtures of 2.2% CH4 in N2 by mole for a sweep of incident shock velocities ranging 3.0-5.5 km/s and fill pressures 0.3-1.15 Torr. Laser scan rates up to 500 kHz probe multiple absorption features to provide quantitative measurements of temperature and species number density profiles behind the incident shock. In all instances, the temperature trend is consistent between emission-inferred and TDLAS-inferred measurements. However, at the lower-velocity conditions, there is an appreciable offset between the Data Parallel Line Relaxation Code (DPLR) simulation and experimental measurements. A similar behavior is observed in number density profiles, with emission measurements providing inferences of the CN(B) and CN(A) state, while TDLAS measurements provide an additional inference of the CN(X) state. In the fast-scanned experiments, measurement inferred number densities suggest a much faster production of CN at early times than captured by the model. This discrepancy motivates ongoing, additional spectroscopic and kinetics CN experiments to resolve the accuracy of modeling assumptions.

Pyrolysis of Methyl Formate and the Reaction of Methyl Formate with H Atoms: Shock Tube Experiments and Statistical Rate Theory.

Abstract: Methyl formate (MF) is the smallest carboxylic ester and currently considered a promising alternative fuel. It can also serve as a model compound to study the combustion chemistry of the ester group, which is a typical structural feature in many biodiesel components. In the present work, the pyrolysis of MF was investigated behind reflected shock waves at temperatures between 1430 and 2070 K at a nominal pressure of 1.1 bar. Both time-resolved hydrogen atom resonance absorption spectroscopy (H-ARAS) and time-resolved time-of-flight mass spectrometry (TOF-MS) were used for species detection. Additionally, the reaction of MF and perdeuterated MF-d4 with H atoms was investigated at temperatures between 1000 and 1300 K at nominal pressures of 0.4 and 1.1 bar with H-ARAS. In the latter experiments, ethyl iodide served as precursor for H atoms. Rate coefficients of seven parallel unimolecular decomposition channels of MF and five parallel reaction channels of the MF + H reaction were calculated from statistical rate theory on the basis of molecular and transition state data from quantum chemical calculations. These calculated rate coefficients were implemented into an MF pyrolysis/oxidation mechanism from the literature, and the experimental concentration-time profiles of H (from ARAS) as well as MF, CH3OH, HCHO, and CO (from TOF-MS) were modeled. It turned out that the literature mechanism, which was originally validated against flow-reactor experiments, ignition delay times, and laminar burning velocities, was generally able to fit also the concentration-time profiles from the shock tube experiments reasonably well. The agreement could still be improved by substituting the original rate coefficients, which were estimated from structure-reactivity relationships, by the values calculated from statistical rate theory in the present work. Details of the channel branching are discussed, and the updated mechanism is given, also in machine-readable form.

Laminar Flame Speed, Ignition Delay Time, and CO Laser Absorption Measurements of a Gasoline-like Blend of Pentene Isomers.

Abstract: The combustion properties of a gasoline-like blend of pentene isomers were determined using multiple types of experimental measurements. The representative mixture (Mix A) is composed of 5.7% 1-pentene (1-C5H10), 39.4% 2-pentene (2-C5H10), 12.5% 2-methyl-1-butene (2M1B), and 42.4% 2-methyl-2-butene (2M2B) (% mol). Laminar flame speeds were measured at equivalence ratios of 0.7-1.5 in a constant-volume combustion chamber, and ignition delay times (including both OH* and CH* diagnostics) as well as CO time-history profiles were performed in shock tubes, in highly diluted mixtures (0.995 He/Ar), at a stoichiometric condition for temperatures ranging from 1350 to 1750 K, and at near-atmospheric pressure. Two additional unbalanced mixtures removing either 2M2B (Mix B) or 2-C5H10 (Mix C) were studied in a shock tube to collect CO time histories, representing the most stringent validation constraints, as these two pentenes constitute the biggest proportions in Mix A and exhibit opposite behaviors in terms of reactivity due to their chemical structure differences. Numerical predictions using a recent validated chemical kinetics mechanism encompassing all pentene isomers from Grégoire et al. ( Fuel2022, 323, 124223) are presented. The use of a complex blend of four pentene isomers in the present paper provided a capstone test of the current mechanism's ability to model pentene-isomer combustion chemistry, with very good results that reflect positively on the current state of the art in pentene isomer kinetics modeling.

Two-Temperature Modeling of Nonequilibrium Relaxation and Dissociation in Shock-Heated Oxygen

Abstract: Two-temperature models for coupled vibrational relaxation and dissociation in shock-heated oxygen are assessed using low-uncertainty measured data from reflected shock tube experiments. A computationally efficient multistep technique is developed to model the unsteady dynamics of shock reflection in a relaxing and dissociating gas. The developed technique is then benchmarked through comparison with unsteady computational fluid dynamic simulations. Results from the benchmarking effort demonstrate that the adopted multistep modeling procedure accurately captures the dominant gas dynamic effects influencing the state of the test gas at the measurement location. A parametric study is then performed to assess several combinations of possible two-temperature modeling approaches for nonequilibrium oxygen dissociation. The current assessment demonstrates that the widely adopted Park model is inconsistent with the measured data, while the recently developed modified Marrone and Treanor (MMT) model demonstrates promising agreement with the data. The results of the present study clearly indicate that the MMT model is more appropriate for two-temperature modeling of nonequilibrium oxygen dissociation than the legacy Park model. Patterns in the parametric comparison also suggest that the approximate treatment of non-Boltzmann vibrational state distributions within the MMT model may require improvement.

Shock-Particle Curtain Interactions at High Mach Number

Abstract: Shock-particle interactions at elevated Mach numbers are studied using the Sandia free-piston High-Temperature Shock Tube (HST). Like previous studies in a lower-strength facility, the particle curtain was comprised of 100-micron glass at an initial volume fraction of approximately 20%. Shock-particle interactions were investigated using high-speed imaging, where the incident shock Mach number ranged from 3 – 4.2. The corresponding post-shock velocities were 760 – 1170 m/s, nearly doubling the range available in previous experiments. The spread of the particle was curtain compared to that in the literature. The scaling law of DeMauro et al. (2019) effectively collapsed the curtain spread over experiments ranging approximately one order of magnitude of post-shock velocities.

Multiwavelength Speciation in Pyrolysis of n-Pentane and Experimental Determination of the Rate Coefficient of nC5H12 = nC3H7 + C2H5 in a Shock Tube.

Abstract: We report the application of a multiwavelength speciation strategy to the study of n-pentane (nC5H12) pyrolysis behind reflected shock waves in a shock tube. Experiments were conducted with 2% nC5H12/0.8%CO2/Ar (by mole) between 1150 and 1520 K in the pressure range of 1-2 atm. Utilization of laser absorption spectroscopy at eight wavelengths allowed time-resolved measurements of n-pentane, ethylene, methane, heavy alkenes, and temperature. The measured time histories were compared against the predictions of four recently developed chemical kinetic models for heavy hydrocarbons. It was found that none of the models reconciled the measured species time histories simultaneously. Sensitivity analysis was conducted to identify key reactions influencing the evolution of ethylene and other major pyrolysis products. The analysis revealed that the unimolecular decomposition of n-pentane into n-propyl and ethyl radicals has a dominating influence over the evolution of ethylene in the temperature range of 1150-1450 K. The rate coefficient of this reaction was then adjusted to match the measured ethylene time histories for each experiment. The rate coefficients thus determined, were fit against temperature using an Arrhenius expression given by k1(T) = 3.5 × 1014 exp(-67.2 kcal/RT) s-1. The average overall 2σ uncertainty of the measured rate coefficient was found to be ±35%, resulting primarily from uncertainties in the rate coefficients of secondary reactions. The measured rate coefficient, when used with the models, leads to a significant improvement in the prediction of species time histories. Further improvements in the model are possible if the rate coefficients of relevant reactions pertaining to small hydrocarbon chemistry are determined with an improved accuracy, and less uncertainty. To the best knowledge of the authors, this is the first experimental determination of the rate coefficient of C5H12 → nC3H7 + C2H5.

Examination of Mars2020 shock-layer conditions via infrared laser absorption spectroscopy of CO2 and CO

Abstract: A mid-infrared laser absorption diagnostic was deployed to study a simulated Mars2020 shock layer in the Electric Arc Shock Tube (EAST) facility at NASA’s Ames Research Center. Rapid RF-diplexing techniques enabled quantitative temperature and number density measurements of CO2 and CO with μs-resolution over an incident shock velocity range of 1.39 – 3.75 km/s. Two interband cascade lasers were utilized at 4.17 and 4.19 μm to resolve eight CO2 asymmetric stretch fundamental band (��3) transitions from two different vibrational levels: 000 (ground state) and 010 (first excited bending mode). The probed rotational levels span across J” = 58 to J” = 140. Results are compared to DPLR simulations of the shock layer using kinetic mechanisms of Johnston et al. and Cruden et al. At shock velocities below 3.1 km/s, the agreement between the measurements and the Johnston mechanism is typically within 5% for temperature and within 10% for number density. At shock velocities above 3.1 km/s, the CO2 measurement becomes sensitive to a thin boundary layer and corrections of this effect are presented. On test cases with enough energy to dissociate CO2, a quantum cascade laser scanned the P(2, 20), P(0, 31), and P(3, 14) transitions of the CO fundamental band at 4.98 μm. CO formation rate is measured to be close to the Johnston kinetic mechanism at low velocities, and then trending towards the Cruden kinetic mechanism at high velocities. On a few low velocity test cases, rovibrational relaxation of the Martian atmosphere is probed with μs resolution.

Influence of Thermochemical Nonequilibrium on Expansion Tube Air Test Conditions - A Numerical Study

Abstract: Using a Lagrangian solver, thermochemical nonequilibrium simulations are performed for the entire range of practical operating conditions of expansion tubes to isolate the influence of nonequilibrium and identify key features in large-scale facilities. Particular attention is given not only to the influence of the nonequilibrium unsteady expansion but also to the influences of the nonequilibrium region behind the primary shock and non-ideal secondary diaphragm rupture. The nonequilibrium unsteady expansion is found to be the most influential process on the test flow - it can significantly influence the flow properties and cause significant temporal variations in the properties during the test time. The nonequilibrium unsteady expansion is also found to accelerate the secondary shock and contact surface. The non-ideal secondary diaphragm rupture is found to increase the amount of nonequilibrium in the test flow due to the generation of a reflected shock. The nonequilibrium region behind the primary shock may be considered negligible in most conditions. Regarding the creation of thermochemical equilibrium test conditions, important factors for achieving this include having a high acceleration tube fill pressure, large-scale facility, and high total enthalpy. The combined effects of viscosity and nonequilibrium are postulated, and the results are supported by experimental works which report consistent findings. To provide an idea of the sensitivity of the numerical configuration, simulations of fixed-volume reactors at various de-excitation conditions are performed using different nonequilibrium models.

Studies on the Kinetics of the CH + H2 Reaction and Implications for the Reverse Reaction, 3CH2 + H

Abstract: The reaction of CH radicals with H2 has been studied by the use of laser flash photolysis, probing CH decays under pseudo-first-order conditions using laser-induced fluorescence (LIF) over the temperature range 298–748 K at pressures of ∼5–100 Torr. Careful data analysis was required to separate the CH LIF signal at ∼428 nm from broad background fluorescence, and this interference increased with temperature. We believe that this interference may have been the source of anomalous pressure behavior reported previously in the literature (BrownswordR. A.; J. Chem. Phys.1997, 106, 7662−7677). The rate coefficient k1 shows complex behavior: at low pressures, the main route for the CH3* formed from the insertion of CH into H2 is the formation of 3CH2 + H, and as the pressure is increased, CH3* is increasingly stabilized to CH3. The kinetic data on CH + H2 have been combined with experimental shock tube data on methyl decomposition and literature thermochemistry within a master equation program to precisely determine the rate coefficient of the reverse reaction, 3CH2 + H → CH + H2. The resulting parametrization is kCH2+H(T) = (1.69 ± 0.11) × 10–10 × (T/298 K)(0.05±0.010) cm3 molecule–1 s–1, where the errors are 1σ.

Experimental and Kinetic Study of the Effect of Nitrogen Dioxide on Ethanol Autoignition

Abstract: The contribution of NO2 to the ethanol ignition delay time was investigated behind reflected shock waves. The experiments were performed at a pressure of 0.20 MPa, temperature range of 1050–1650 K, equivalence ratio of 0.5/1.0/1.5, and ethanol/NO2 mixing ratios of 100/0, 90/10, and 50/50. The experimental results showed that the addition of NO2 decreased the ignition delay time and promoted the reactivity of ethanol under all equivalence ratios. With an increase in NO2 blending, the effect of equivalence ratio on the ethanol ignition delay time decreased, and with an increase in temperature, the effect of NO2 in promoting ethanol ignition weakened. An updated mechanism was proposed to quantify NO2-promoted ethanol ignition. The mechanism was validated based on available experimental data, and the results were in line with the experimental trends under all conditions. Chemical kinetic analyses were performed to interpret the interactions between NO2 and ethanol for fuel ignition. The numerical analysis indicated that the promotion effect of NO2 is primarily due to an increase of the rate of production and concentration of the radical pool, especially the OH radical pool. The reaction NO + HO2 ⇔ NO2 + OH is key to generating chain-initiating OH radicals.

Rotational and Vibrational Temperature Measurements of CN Χ2Σ+ Behind Shock Waves using Ultrafast-Laser-Absorption Spectroscopy in the UV

Abstract: This article describes the characterization of the temporal evolution of non-Boltzmann CN X formed behind strong shock waves in N2-CH4 mixtures at conditions relevant to entry into Titan's atmosphere. This was done using a novel broadband ultrafast-laser-absorption-spectroscopy (ULAS) diagnostic. An ultrafast (femtosecond) light source was utilized to produce 55 fs pulses near 385 nm at a repetition rate of 5 kHz and a spectrometer with a 2400 lines/mm grating was utilized to spectrally resolve the pulses after passing through the Purdue High-Pressure Shock Tube. This enabled broadband single-shot absorption measurements of CN to be acquired with a spectral resolution and bandwidth of ≈0.02 nm and ≈6 nm (≈402 cm-1 at these wavelengths), respectively. A line-by-line absorption spectroscopy model for the B←X system of CN was developed and utilized to determine six internal temperatures (two vibrational and four rotational) of CN from the (0,0), (1,1), (2,2) and (3,3) absorption bands. Measurements were acquired behind reflected shock waves in 5.65% CH4 and 94.35% N2 with an initial pressure of 1.56 mbar and incident shock speed of ≈2.1 km/s. For this test condition, the chemically and vibrationally frozen temperature of the mixture behind the reflected shock was 5000 K and the pressure was 0.6 atm. The high repeatability of the shock-tube experiments (0.3% variation in shock speed across tests) enabled multi-shock time histories of CN mole fraction and six internal temperatures to be acquired with a single-shot time resolution of less than 1 ns. The measurements revealed that CN X is non-Boltzmann rotationally and vibrationally for greater than 200 μs, thereby strongly suggesting that chemical reactions are responsible for the non-Boltzmann population distributions.