Laser Absorption Technique Research Articles

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.

Direct Measurement of Dissolved Gas Using a Tapered Single-Mode Silica Fiber.

Dissolved gases in the aquatic environment are critical to understanding the population of aquatic organisms and the ocean. Currently, laser absorption techniques based on membrane separation technology have made great strides in dissolved gas detection. However, the prolonged water-gas separation time of permeable membranes remains a key obstacle to the efficiency of dissolved gas analysis. To mitigate these limitations, we demonstrated direct measurement of dissolved gas using the evanescent-wave absorption spectroscopy of a tapered silica micro-fiber. It enhanced the analysis efficiency of dissolved gases without water-gas separation or sample preparation. The feasibility of this sensor for direct measurement of dissolved gases was verified by taking the detection of dissolved ammonia as an example. With a sensing length of 5 mm and a consumption of ~50 µL, this sensor achieves a system response time of ~11 min and a minimum detection limit (MDL) of 0.015%. Possible strategies are discussed for further performance improvement in in-situ applications requiring fast and highly sensitive dissolved gas sensing.

One-dimensional direct absorption sensor for flat flame characterization

Laser absorption techniques are widely used in combustion research to provide a detailed characterization of reactive media. Recently, the improvements of high-speed infrared camera have enabled the development of laser absorption imaging (LAI) techniques with application to combustion environment. In this paper, we present the development and implementation of the first one-dimensional direct absorption (1D-DA) speciation sensor in the near-infrared, absorption transition near 1391.7 nm, for measuring water in a flat flame. Our 1D-DA sensor relies on a simple structure based on standard optical components, and a novel post-processing method was developed to correct diffraction-like signal deformation. With our 1D-DA sensor, we characterized two C2H4-O2-Ar flames with different equivalence ratios, and validated the results using both the traditional DA method based on a single pixel detector, and Cantera flame simulations using USC-Mech II. Our sensor achieves a spatial resolution better than 70μm along along the vertical direction, with an uncertainty estimated to be around 13%.Novelty and Significance Statement1. First one-dimensional direct absorption (1D-DA) speciation sensor applied to a near-IR line-scan camera.2. Novel approach to remove diffraction pattern related to laser light coherence.3. High spatial resolution along along the vertical direction.4. Strong potential for future tomographic applications

Development of interference-free rotational and vibrational thermometry for studies on shock-heated thermochemical non-equilibrium CO

A mid-infrared interference-free laser absorption technique for simultaneously measuring rotational temperature, vibrational temperature, and CO concentration was developed for application to shock-tube studies on thermochemical non-equilibrium CO over 1000–3000 K. Three transition lines in the fundamental vibrational band of CO (P(0, 21), near 4.87 μm, P(1, 21), near 4.93 μm, and P(0, 37), near 5.05 μm) were selected. The P(0, 21)/P(1, 21) line pair was used for vibrational temperature measurements whereas the P(0, 21)/P(0, 37) line pair was used for rotational temperature measurements. Spectroscopic parameters for developing the technique were measured: line strengths and collisional broadening data in Ar were obtained at 1040–2940 K. Validation experiments for the thermometry system were performed in shock-heated thermal-equilibrium CO/Ar mixtures at 1050–3010 K and 1.1–2.8 bar. The time-dependent rotational and vibrational temperatures were measured during the vibrational relaxation processes of CO. The technique showed high sensitivity in detecting the rotational and vibrational temperatures. The measured rotational temperature agreed well with the temperature calculated using the measured pressure and isentropic relationship. The measured vibrational temperature showed good agreement with the predictions using the Landau and Teller theory and Millikan and White relationship. The time-dependent CO concentration during the oxidation processes of n-heptane over a wide temperature range (1350–2750 K) was measured considering n-heptane as one of the alternative fuels for the scramjet. The interference-free laser absorption strategy showed good flexibility in detecting the CO concentration at ultra-high temperatures. The measured results showed overall good agreement with the predictions from two detailed mechanisms and one skeletal mechanism. The reactivity of n-heptane was found to be insensitive to the temperature increase at ultra-high temperatures (>2100 K).

Exploring cesium properties internal to the LANSCE H- ion source using resonant absorption spectroscopy

The Los Alamos Neutron Science Center (LANSCE) H- ion source has provided stable output for decades of LANL mission needs, but its maximum beam output has remained the same at ∼15 mA. A roadblock to improving beam output has been a lack of thorough understanding of the internal mechanisms of LANSCE H- ion source. The LANSCE H- Ion Source Laser Diagnostic Stand (HLDS) was recently built and commissioned to explore these internal mechanisms using laser absorption techniques, to measure and diagnose dynamic H- and cesium densities. The cesium density probe is based on resonant absorption of a continuous wave diode laser tuned though the D2 line of cesium (∼852 nm). The diagnostic capabilities of HLDS will be reviewed, and measurements using the cesium laser diagnostic will be presented.

High-speed mid-infrared laser absorption spectroscopy of CO_2 for shock-induced thermal non-equilibrium studies of planetary entry

A high-speed laser absorption technique is employed to resolve spectral transitions of CO_2 in the mid-infrared at MHz rates to infer non-equilibrium populations/temperatures of translation, rotation and vibration in shock-heated CO_2 - Ar mixtures. An interband cascade laser (DFB-ICL) resolves 4 transitions within the CO_2 asymmetric stretch fundamental bands (Delta v_3 = 1) near 4.19 upmu hbox {m}. The sensor probes a wide range of rotational energies as well as two vibrational states (00^00 and 01^10). The sensor is demonstrated on the UCLA high enthalpy shock tube, targeting temperatures between 1250 and 3100 K and sub-atmospheric pressures (up to 0.2 atm). The sensor is sensitive to multiple temperatures over a wide range of conditions relevant to Mars entry radiation. Vibrational relaxation times are resolved and compared to existing models of thermal non-equilibrium. Select conditions highlight the shortcomings of modeling CO_2 non-equilibrium with a single vibrational temperature.

Development and validation of a hybrid constraint spectral thermometry for laminar sooting flames.

A hybrid constraint spectral thermometry (HCST) method based on combined light extinction and spectral soot emission was developed for temperature measurements in sooting flames. Light extinction measurements were performed in the 635-1064 nm spectral range to constrain the fitting of the soot optical property E(m) as a function of the wavelength for separately measured spectral emission data. This hybrid constraint methodology helps to avoid complete reliance on either measurement, which significantly increases the immunity of the temperature determination to measurement noises. After numerically validating the performance of HCST, measurements were performed for a series of representative sooting premixed flames. Additional measurements were conducted with the proven tunable diode laser absorption technique to provide a reference temperature, and satisfactory agreements demonstrate the accuracy and robustness of the proposed HCST method.

Laser-absorption-spectroscopy-based temperature and NH3-concentration time-history measurements during the oxidation processes of the shock-heated reacting NH3/H2 mixtures

As a carbon-free fuel, NH3 has been considered as a potential and promising alternative energy carrier in the future transportation system. This study simultaneously measured the temperature and NH3-concentration time histories during the oxidation processes of the shock-heated NH3/H2 mixtures by combining laser absorption techniques. The v" = 0, P8 and v" = 1, R21 lines in the fundamental vibrational band of CO were selected for temperature time-history measurements. Nine transition lines in the v2 fundamental vibrational band of NH3 near 8.91 µm were selected for NH3-concentration time-history measurements. The temperature- and pressure-dependent NH3 absorption cross-section highly diluted in argon was measured at 1000–2200 K and 0.8–3.9 bar, respectively. The time-dependent temperature and NH3 concentration were then measured and compared with the predictions based on four mechanisms (Mathieu & Petersen, 2015; Glarborg et al., 2018; Wang et al., 2020; Shrestha et al., 2021). The four mechanisms were substantially slow in predicting the NH3-concentration time histories for the stoichiometric NH3/H2 mixtures. The Glarborg et al. mechanism and the Shrestha et al. mechanism showed very good agreements with the experiments for the fuel-rich NH3/H2 mixtures, especially in predicting the NH3-concentration time-history profiles in the burnout state. Comprehensive kinetics analyses illustrated that the NH3 addition evidently changed the mixture reactivity and formation and consumption processes of H and OH radicals. This study updated the rate coefficients of NH3 + OH = NH2 + H2O in the Glarborg et al. mechanism and the modified mechanism showed improved performances in predicting the NH3-concentration profiles.

Kinetic insights into the reaction of hydroxyl radicals with 1,4-pentadiene: A combined experimental and theoretical study

Olefinic hydrocarbons are one of the main intermediates of the oxidation of large hydrocarbons, and they are also found in distillate transportation fuels. Combustion characteristics of olefinic species, particularly non-conjugated ones, are not well understood. This work reports chemical insights into the reaction of OH radicals with 1,4-pentadiene (14PTDN), an important non-conjugated diolefin. High-temperature rate coefficients of OH + 14PTDN were measured in a shock tube over T = 881 – 1314 K and p ∼ 1150 Torr. The progress of the reaction was monitored by detecting OH radicals near 307 nm using a UV laser-absorption technique. OH radicals were generated by the fast thermal decomposition of tert‑butyl hydroperoxide (TBHP). Stochastic RRKM-based Master Equation (RRKM-ME) simulations were carried out on the potential energy surface computed at M06–2X/aug-cc-pVTZ level of theory to gain mechanistic insights. The theoretical model captured high-temperature experimental data from this work and previously reported data at low temperatures (294 - 468 K). This combined experimental and theoretical study reveals: (i) no discernible pressure dependence of the total rate constants, (ii) mechanism shift occurs with temperature, (iii) similar to conjugated dienes + OH reactions, the addition of OH radicals to 14PTDN dominates compared to abstraction pathways for T < 500 K, (iv) among the bimolecular product channels originating from the OH-adducts, only vinyl alcohol + allyl radical is important, contributing ∼28% of the reactant consumption at 0.1 Torr and 400 K, (iv) beyond 1000 K, the addition channel contributes negligibly small for all pressures. To our knowledge, this is the first detailed experimental and theoretical kinetic study of the reaction of 1,4-pentadiene with OH radicals. The expression for the theoretical total rate coefficients (in units of cm3 molecule−1s−1) is provided as k(T)=0.172×T−3.82exp(−125.6KT)+1.49×10−20T3.09exp(−53.7KT) over T = 200 – 2000 K and P = 760 Torr.

Isopropanol dehydration reaction rate kinetics measurement using H2O time histories

AbstractH2O formation during thermal decomposition of isopropanol was measured behind reflected shock waves at temperatures ranging from 1127 to 1621 K at an average pressure of 1.42 atm using a laser absorption technique. Of the five modern chemical kinetics models used to compare the H2O time histories, the model from Li et al (Combust. Flame 2019;207:171‐185) showed the best overall agreement. Sensitivity and rate of production analyses using the Li et al model (as well as those from AramcoMech 3.0, CRECK, and Togbe et al (Energy Fuel. 2011;25:676‐683)) showed unimolecular dehydration of isopropanol, iC3H7OH ⇌ H2O + C3H6 (R1), is nearly the sole reaction controlling H2O production at early times, allowing for an a priori measurement of the forward rate constant k1. The Arrhenius expression k1 (s–1) = 2.60 × 1013 exp(−31 120 K/T) was determined to represent best the data from this study. According to the models and previous experimental investigations, the pressure investigated is well within the high‐pressure limit (HPL) for this reaction. Additional, higher‐pressure experiments also confirmed the HPL assumption. An uncertainty analysis was performed by varying secondary reactions within their uncertainties and examining their effect on the overall prediction, establishing an uncertainty within ±20% for all but the highest temperature cases, which have a maximum uncertainty of ±40%. Experiments conducted with a radical trapper, toluene, showed little influence from radical chemistry, suggesting this estimated uncertainty is fairly conservative. Experimental data from Heyne et al (Z Phys Chem. 2015;229:881‐907) were found to be in good agreement with the rate measurements from this study and, therefore, a second Arrhenius expression, k1 (s–1) = 2.11 × 1013 exp(−30 820 K/T), was found to represent both datasets well. This second expression has a larger temperature range of 976‐1621 K. The present study provides the first high‐temperature data collected for this reaction, adding to the limited data available for isopropanol in the literature.

On the redox reactions between allyl radicals and NOx

NOx mitigation is a central focus of combustion technologies with increasingly stringent emission regulations. NOx can also enhance the autoignition of hydrocarbon fuels and can promote soot oxidation. The reaction between allyl radical (C3H5) and NOx plays an important role in the oxidation kinetics of propene. In this work, we measured the absolute rate coefficients for the redox reaction between C3H5 and NOx over the temperature range of 1000–1252 K and pressure range of 1.5–5.0 bar using a shock tube and UV laser absorption technique. We produced C3H5 by shock heating of C3H5I behind reflected shock waves. Using a Ti:Sapphire laser system with frequency quadrupling, we monitored the kinetics of C3H5 at 220 nm. Unlike low-temperature chemistry, the two target reactions, C3H5 + NO → products (R1) and C3H5 + NO2 → products (R2), exhibited a strong positive temperature dependence for this radical-radical type reaction. However, these reactions did not show any pressure dependence over the pressure range of 1.5–5.0 bar, indicating that the measured rate coefficients are close to the high-pressure limit. The measured values of the rate coefficients resulted in the following Arrhenius expressions (in unit of cm3/molecule/s):k1(C3H5+NO)=1.49×10−10exp(−6083.6KT)(1017−1252K)k2(C3H5+NO2)=1.71×10−10exp(−3675.7KT)(1062−1250K)To our knowledge, these are the first high-temperature measurements of allyl + NOx reactions. The reported data will be highly useful in understanding the interaction of NOx with resonantly stabilized radicals as well as the mutual sensitization effect of NOx on hydrocarbon fuels.

A Shock-Tube Study of the Rate Constant of PH3 + M ⇄ PH2 + H + M (M = Ar) Using PH3 Laser Absorption.

Phosphine (PH3) is a highly reactive and toxic gas. Prior experimental investigations of PH3 pyrolysis reactions have included only low-temperature measurements. This study reports the first shock-tube measurements of PH3 pyrolysis using a new PH3 laser absorption technique near 4.56 μm. Experiments were conducted in mixtures of 0.5% PH3/Ar behind reflected shock waves at temperatures of 1460-2013 K and pressures of ∼1.3 and ∼0.5 atm. The PH3 time histories displayed two-stage behavior similar to that previously observed for NH3 decomposition, suggesting by analogy that the rate constant for PH3 + M ⇄ PH2 + H + M (R1) could be determined. A simple three-step mechanism was assembled for data analysis. In a detailed kinetic analysis of the first-stage PH3 decomposition, values of k1,0 were obtained and best described by (in cm3·mol-1·s-1) k1,0 = 7.78 × 1017 exp(-80,400/RT), with units of cal, mol, K, s, and cm3. Agreement between the 1.3 and 0.5 atm data confirmed that the measured k1,0 was in the low-pressure limit. Agreement of the experimental k1,0 with ab initio estimates resolved the question of the main pathway of PH3 decomposition: it proceeds as PH3 ⇄ PH2 + H instead of PH3 ⇄ PH + H2.

Laser-based CO concentration and temperature measurements in high-pressure shock-tube studies of n-heptane partial oxidation

This paper presents a laser-based absorption technique for measuring temperature and CO concentration in high-pressure shock tubes. Two fundamental vibrations of CO (v" = 0, P8, 4.73 µm and v" = 1, R21, 4.56 µm) were selected for high-temperature sensitivity with a reduced influence from pressure broadening compared to previous work. Single-pass absorption (80 mm path length) was measured with two quantum-cascade lasers. The technique was demonstrated by measuring time-resolved temperature for non-reactive mixtures at 1100–1960 K and 1.2–9.7 bar. During partial oxidation of n-heptane, temperature and CO concentrations were measured with 4 µs time resolution at 1360–1670 K and 5.8–8.2 bar. Interference from broadband CO2 absorption was quantified and subtracted. Measured data in the burnout state are in excellent agreement with predictions from kinetics mechanisms (Mehl et al. Proc Combust Inst 33:193, 2011; Zhang et al. Combust Flame 172:116, 2016) over the entire range of operating conditions, which validates the performance of the current laser-absorption technique in reactive-mixture measurements. Additionally, time-resolved temperature and CO-concentration measurements agree well with predictions based on the Mehl et al. mechanism.

Laminar Flame Speed Experiments of Alternative Liquid Fuels

Abstract New laminar flame speed experiments have been collected for two alternative liquid fuels. Understanding the combustion characteristics of these synthetic fuels is an important step in developing new chemical kinetics mechanisms that can be applied to real fuels. Included in this study are two synthetic Jet fuels: Syntroleum S-8 and Shell GTL. The precise composition of these fuels is known to change from sample to sample. Since these are low-vapor pressure fuels, there are additional uncertainties in their introduction into gas-phase mixtures, leading to uncertainty in the mixture equivalence ratio. An in-situ laser absorption technique was implemented to verify the procedure for filling the vessel and to minimize and quantify the uncertainty in the experimental equivalence ratio. The diagnostic utilized a 3.39-μm HeNe laser in conjunction with Beer's law. The resulting spherically expanding, laminar flame experiments were conducted over a range of equivalence ratios from φ = 0.7 to φ = 1.5 at initial conditions of 1 atm and 403 K in the high-temperature, high-pressure (HTHP) constant-volume vessel at Texas A&amp;M University. The experimental results show that both fuels have similar flame speeds with a peak value just under 60 cm/s. However, it is shown that when comparing the results from different datasets for these real fuels, equivalence ratio may not be the best parameter to use. Fuel mole fraction may be a better parameter to use as it is independent of the average fuel molecule or fuel surrogate used to calculate equivalence ratio in these real fuel/air mixtures.

Experimental study of ethanol oxidation behind reflected shock waves: Ignition delay time and H2O laser-absorption measurements

The oxidation of ethanol was investigated in shock tubes by measuring ignition delay times and water time history profiles. A laser absorption technique was used in the latter case. Ignition delay times were investigated for three equivalence ratios, 0.5, 1.0, and 2.0, and for pressures ranging from 1.3 to 53 atm. The fuel concentration and the type of diluent used were also varied. The ignition delay times were in very good agreement with results from the literature for the very few overlapping conditions, and most of the experimental conditions have never been studied before. A high level of accuracy was observed for the predictions of the ignition delay times from detailed kinetics mechanisms. Inhomogeneous ignition events were observed in some specific conditions, validating the notions of thermal diffusivity and flame thickness developed in the literature for the reasons behind such inhomogeneous ignition. The water time-history profiles showed the formation of water in two stages. These profiles were also modeled using well-known detailed kinetic mechanisms from the literature. Severe discrepancies were often observed for laser-absorption measurements, and this was the case with most of the models. A numerical analysis (sensitivity and rate-of-production) was conducted using the two best-performing literature mechanisms (namely, AramcoMech3.0 and CRECK).

Ignition delay time and H2O measurements during methanol oxidation behind reflected shock waves

To improve detailed chemical kinetics models, the oxidation of methanol was investigated behind reflected shock waves in shock tubes. Ignition delay times of methanol–air mixtures, with Ar as diluent, were studied between 940 and 1540 K in a heated shock tube, for pressures up to 14.9 atm and for equivalence ratios of 0.5, 1.0, and 2.0. Water profiles were measured by utilizing a laser absorption technique in the 1350-to-1600-K temperature range, at an average pressure of 1.3 atm and for similar equivalence ratios. The present study shows the ignition delay times of methanol to be in very good agreement with results from the literature (Fieweger et al., 1997), whereas the other conditions have never been investigated before. The ignition delay time data are also in good agreement with modern detailed kinetics mechanisms such as the AramcoMech 3.0 model. The water time-history profiles were modeled using well-known literature mechanisms. Discrepancies were observed between these kinetics mechanisms, and poor predictions were observed for the lower temperatures investigated. Sensitivity and rate-of-production analyses were performed using 3 literature mechanisms (namely, AramcoMech 3.0, Princeton, and JetSurfII). Discrepancies were found among the models when predicting important reactions dominating the oxidation of methanol as well as the rate-of-production of H2O.

The unimportance of the reaction H2 + N2O ⇆ H2O + N2: A shock-tube study using H2O time histories and ignition delay times

The hydrogen-nitrous oxide (H2–N2O) system is of paramount importance in safety considerations for nuclear waste management and semiconductor manufacturing. For less-dilute H2–N2O mixtures, a reaction of key importance in determining ignition delay times is the direct reaction between H2 and N2O to give H2O and N2, although this reaction has received only one direct investigation in the work of Kosarev et al. (2007). To examine the importance of the title reaction, new H2O time histories were obtained using a laser absorption technique at 1.39 µm in a mixture of 0.222% N2O/1.778% H2/Ar between 1414 and 1811 K near 1.2 atm. Additionally, the ignition delay time measurements of Kosarev et al. (2007) were repeated using endwall emission and pressure diagnostics. The new datasets show excellent agreement with the predictions of a recent mechanism when the title reaction is reduced by a factor of 30 relative to the work of Kosarev et al. Furthermore, the accurate mechanism predictions of H2–N2O ignition delay time data available in the literature (other than those of Kosarev et al.) were not degraded when the proposed value of k1 = 7.0×1013exp(−16,356/T) was used (k1 in cm3 mole−1 s−1, T in K). This expression for k1 should be viewed as an upper limit on the rate for the title reaction and should make the title reaction negligibly important in future NOx modeling efforts; the true value of k1 is likely even lower than this expression. A factor of 2 uncertainty is assigned to this expression for k1. Finally, it is recommended that the data of Kosarev et al. (2007) be neglected in future mechanism validations as these data appear to be inaccurate, in part due to non-ideal pressure rise effects and choice of emission diagnostic.

Ethanol pyrolysis kinetics using H2O time history measurements behind reflected shock waves

Abstract The thermal decomposition of ethanol has been studied under pyrolytic conditions behind reflected shock waves in the 1250 to 1677 K temperature range, at an average pressure of 1.31 atm for a mixture highly diluted in Ar. A laser absorption technique was utilized to measure H2O time-histories, and the detailed kinetics mechanism (AramcoMech2.0) was selected among various models from the literature based on its a priori agreement with the experimental data in the present study. Sensitivity and rate-of-production analyses were performed and showed that the C2H5OH→C2H4+ H2O (R1) decomposition pathway is almost the sole reaction contributing to H2O formation at the early times under the present conditions, allowing an a priori direct measurement of its rate coefficient k1. The rate coefficient was determined to be defined as the Arrhenius equation k1 (s−1) = 3.37 × 1011 exp (–27174 K/T), which is in very good agreement with Kiecherer et al. (2015), where k1 was also directly determined under similar conditions. Secondary chemical reactions taking place in the thermal decomposition have very low influence in the H2O formation during the time-frame selected, leading to an uncertainty for k1 of approximately 20%. The full H2O time histories are useful for validating the full ethanol kinetics mechanism for future validation.

A shock tube kinetic study on the branching ratio of methanol + OH reaction

Methanol (CH3OH) is the simplest alcohol and is considered to be a future fuel, produced by solar-driven reduction of carbon dioxide. The reaction of methanol and hydroxyl radicals is important in both combustion and atmospheric systems because this reaction is the dominant consumption pathway for methanol oxidation. Hydrogen abstraction at the CH3 or OH site of CH3OH leads to different radical intermediates. The relative importance of these two channels is critical for combustion modeling as the subsequent chemistries of the product radicals (CH3O and CH2OH) are markedly different. In this work, we measured overall rate coefficients for the reaction of methanol (CH3OH), methanol-d3 (CD3OH) and methanol-d1 (CH2DOH) with OH radicals over the temperature range of 900 − 1300 K and pressures near 1.3 atm by employing shock tube/UV laser absorption technique. Combining our results with literature data, we recommend following three-parameter Arrhenius expressions (cm3 molecule−1 s−1):k1(CH3OH+OH)=3.25×10−13(T300K)2.55exp(297.8KT)210−1344Kk2(CD3OH+OH)=4.69×10−13(T300K)2.24exp(−59.8KT)293−1287KUsing our measured total rate coefficients, we determined site-specific H-abstraction rate coefficients and hence, branching ratios of the two abstraction channels. Our results show that abstraction at the CH3 site is the dominant channel, contributing more than 80% throughout our temperature range. Our calculated site-specific rate coefficients (per H atom) over 900–1300 K are given by (cm3 molecule−1 s−1):k1aH(CH2OHchannel)=2.55×10−11exp(−2287.1K/T)k1b(CH3Ochannel)=4.30×10−11exp(−3463.2K/T)

A shock tube kinetic study of allyl + allyl and allyl + OH recombination reactions at high temperatures

In this work, we investigated the self-reaction of allyl radicals and its cross-reaction with hydroxyl radical by employing shock tube and laser absorption techniques. We carried out the experiments behind reflected shock waves over the temperature range of 800–1200 K and pressures of 1.1–2.5 bar. We generated allyl (C3H5) and OH radicals by fast thermal decomposition of allyl iodide (C3H5I) and tert-butyl hydroperoxide (TBHP), respectively, and monitored reaction progress by detecting OH near 306.69 nm and C3H5 near 220 nm using UV laser absorption. At the detection wavelength, we measured the temperature dependence of the absorption cross-sections of C3H5 and C3H5I. Rate coefficient for the self-recombination reaction of allyl radicals showed a small negative temperature dependence and no noticeable fall-off behavior over 1.15–1.96 bar giving a mean value of kC3H5+C3H5=(1.0±0.2)×10−11cm3molecule−1s−1. Likewise, the cross-reaction of allyl and OH radicals did not exhibit discernible pressure and temperature dependence under our experimental conditions indicating a barrierless reaction, and an average value of kC3H5+OH=(9.3±0.7)×10−11cm3molecule−1s−1 best illustrates our measured rate coefficients. These measurements represent the first direct experimental determinations of the rate coefficients for these important radical-radical reactions at high temperatures and pressures.