Reflected Shock Wave Experiments Research Articles

High-speed imaging of inhomogeneous ignition in a shock tube

Homogeneous and inhomogeneous ignition of real and surrogate fuels were imaged in two Stanford shock tubes, revealing the influence of small particle fragmentation. n-Heptane, iso-octane, and Jet A were studied, each mixed in an oxidizer containing 21% oxygen and ignited at low temperatures (900–1000 K), low pressures (1–2 atm), with an equivalence ratio of 0.5. Visible images (350–1050 nm) were captured through the shock tube endwall using a high-speed camera. Particles were found to arrive near the endwalls of the shock tubes approximately 5 ms after reflection of the incident shock wave. Reflected shock wave experiments using diaphragm materials of Lexan and steel were investigated. Particles collected from the shock tubes after each experiment were found to match the material of the diaphragm burst during the experiment. Following each experiment, the shock tubes were cleaned by scrubbing with cotton cloths soaked with acetone. Particles were observed to fragment after arrival near the endwall, often leading to inhomogeneous ignition of the fuel. Distinctly more particles were observed during experiments using steel diaphragms. In experiments exhibiting inhomogeneous ignition, flames were observed to grow radially until all the fuel within the cross section of the shock tube had been consumed. The influence of diluent gas (argon or helium) was also investigated. The use of He diluent gas was found to suppress the number of particles capable of causing inhomogeneous flames. The use of He thus allowed time history studies of ignition to extend past the test times that would have been limited by inhomogeneous ignition.

Strategies for obtaining long constant-pressure test times in shock tubes

Several techniques have been developed for obtaining long, constant-pressure test times in reflected shock wave experiments in a shock tube, including the use of driver inserts, driver gas tailoring, helium gas diaphragm interfaces, driver extensions, and staged driver gas filling. These techniques are detailed here, including discussion on the most recent strategy, staged driver gas filling. Experiments indicate that this staged filling strategy increases available test time by roughly 20 % relative to single-stage filling of tailored driver gas mixtures, while simultaneously reducing the helium required per shock by up to 85 %. This filling scheme involves firstly mixing a tailored helium–nitrogen mixture in the driver section as in conventional driver filling and, secondly, backfilling a low-speed-of-sound gas such as nitrogen or carbon dioxide from a port close to the end cap of the driver section. Using this staged driver gas filling, in addition to the other techniques listed above, post-reflected shock test times of up to 0.102 s (102 ms) at 524 K and 1.6 atm have been obtained. Spectroscopically based temperature measurements in non-reactive mixtures have confirmed that temperature and pressure conditions remain constant throughout the length of these long test duration trials. Finally, these strategies have been used to measure low-temperature n-heptane ignition delay times.

A shock tube study of CH3OH + OH → Products using OH laser absorption

The rate coefficient for the reaction of methanol (CH3OH) with the hydroxyl (OH) radical was determined in reflected shock wave experiments at temperatures of 961–1231K and pressures of 1.18–1.48atm. Pseudo-first order reaction conditions were achieved with mixtures of CH3OH and tert-butyl hydroperoxide (TBHP) diluted in argon. Rapid thermal decomposition of TBHP at high temperatures was used as a fast OH radical source, and OH time-histories were measured using laser absorption of OH radicals near 306.7nm. The rate coefficient of the title reaction was found from best-fit comparison of simulated OH profiles with measured data, using simulations from the Wang et al. USC-Mech II reaction mechanism (2007). A detailed uncertainty analysis was performed, with estimated uncertainties of −10%/+17% at 961K and −12%/+18% at 1231K. The current results agree well with the results of Vandooren and Van Tiggelen (1981), Hess and Tully (1989), and Xu and Lin (2007). The three-parameter Arrhenius expression k1(210–1231K)=5.71×104T2.62exp(343/T[K]) cm3mol−1s−1 is recommended from a fit to the combined data of Hess and Tully (1989), Jiménez et al. (2003), Dillon et al. (2005), and the current work.

Kinetic Analysis of Ethyl Iodide Pyrolysis Based on Shock Tube Measurements

ABSTRACTThe optimization of a kinetic mechanism of the pyrolysis of ethyl iodide was carried out based on data obtained from reflected shock wave experiments with H‐ARAS and I‐ARAS detection. The analysis took into account also the measurements of Michael et al. (Chem. Phys. Lett. 2000, 319, 99–106) and Vasileiadis and Benson (Int. J. Chem. Kinet. 1997, 29, 915–925) of the reaction H2 + I = H + HI. The following Arrhenius parameters were determined for the temperature range 950–1400 K and the pressure range 1–2 bar: C2H5I → C2H5 + I: log10(A) = 13.53, E/R = 24,472 K; C2H5I → C2H4 + HI: log10(A) = 13.67, E/R = 27,168 K; H + HI → H2 + I: log10(A) = 13.82, E/R = 491 K; C2H5I + H →C2H5 + HI: log10(A) = 15.00, E/R = 2593 K (the units of A are cm3, mol, s). The joint covariance matrix of the optimized Arrhenius parameters was also determined. This covariance matrix was converted to the temperature‐dependent uncertainty parameters f of the rate coefficients and also to the temperature‐dependent correlation coefficients between pairs of rate coefficients. Each fitted rate coefficient was determined with much lower uncertainty compared to the estimated uncertainty of the data available in the literature.

Rate Constant Measurements for the Overall Reaction of OH + 1-Butanol → Products from 900 to 1200 K

The rate constant for the overall reaction OH + 1-butanol → products was determined in the temperature range 900 to 1200 K from measurements of OH concentration time histories in reflected shock wave experiments of tert-butyl hydroperoxide (TBHP) as a fast source of OH radicals with 1-butanol in excess. Narrow-linewidth laser absorption was employed for the quantitative OH concentration measurement. A detailed kinetic mechanism was constructed that includes updated rate constants for 1-butanol and TBHP kinetics that influence the near-first-order OH concentration decay under the present experimental conditions, and this mechanism was used to facilitate the rate constant determination. The current work improves upon previous experimental studies of the title rate constant by utilizing a rigorously generated kinetic model to describe secondary reactions. Additionally, the current work extends the temperature range of experimental data in the literature for the title reaction under combustion-relevant conditions, presenting the first measurements from 900 to 1000 K. Over the entire temperature range studied, the overall rate constant can be expressed in Arrhenius form as 3.24 × 10(-10) exp(-2505/T [K]) cm(3) molecule(-1) s(-1). The influence of secondary reactions on the overall OH decay rate is discussed, and a detailed uncertainty analysis is performed yielding an overall uncertainty in the measured rate constant of ±20% at 1197 K and ±23% at 925 K. The results are compared with previous experimental and theoretical studies on the rate constant for the title reaction and reasonable agreement is found when the earlier experimental data were reinterpreted.

High‐temperature rate constants for H/D + C2H6 and C3H8

AbstractThe reactions of H/D with C2H6 and C3H8 have been studied with both shock‐tube experiments and ab initio transition‐state theory calculations. Rate constants for the reactions of D with C2H6 and C3H8 have been measured in reflected shock wave experiments over the temperature range, 1128–1299 K, at pressures 0.3–1 atm. D atoms are detected using atomic resonance absorption spectrometry. The measured D‐atom profiles are sensitive to only the thermal dissociation of the D‐atom source molecule, C2D5I, and to the title reactions. Since the dissociation has been previously studied in this laboratory, modeling the temporal evolution of the D‐atom profiles allows determinations of the rate constants for the title reactions. Over the T range, 1128–1228 K, rate constants from the present experiments for D + C2H6 can be represented by the Arrhenius expression equation image The experimental total rate constants for D + C3H8 over the T range, 1128–1299 K, can be represented by the Arrhenius expression equation image The title reactions have been studied at the CCSD(T)/aug‐cc‐pv∞z level of theory using saddle point geometries at B3LYP/6‐311++G(d,p) and MP2/6‐311++G(d,p) levels of theory. The reaction endothermicities are in good agreement with current Active Thermochemical Tables values. Theoretical rate constants were estimated using transition state theory. The theoretical rate constants are in good agreement with the present experiments and lower temperature literature data. Over the T range of the present experiments, the theoretically predicted isotope effects are close to unity. © 2012 Wiley Periodicals, Inc. Int J Chem Kinet 44: 194–205, 2012

The use of driver inserts to reduce non-ideal pressure variations behind reflected shock waves

Non-ideal shock tube facility effects, such as incident shock wave attenuation, can cause variations in the pressure histories seen in reflected shock wave experiments. These variations can be reduced, and in some cases eliminated, by the use of driver inserts. Driver inserts, when designed properly, act as sources of expansion waves which can counteract or compensate for gradual increases in reflected shock pressure profiles. An algorithm for the design of these inserts is provided, and example pressure measurements are presented that demonstrate the success of this approach. When these driver inserts are employed, near- ideal, constant-volume performance in reflected shock wave experiments can be achieved, even at long test times. This near-ideal behavior simplifies the interpretation of shock tube chemical kinetics experiments, particularly in experiments which are highly sensitive to temperature and pressure changes, such as measurements of ignition delay time of exothermic reactions.

High‐temperature shock tube study of the reactions CH3 + OH → products and CH3OH + Ar → products

AbstractThe reaction between methyl and hydroxyl radicals has been studied in reflected shock wave experiments using narrow‐linewidth OH laser absorption. OH radicals were generated by the rapid thermal decomposition of tert‐butyl hydroperoxide. Two different species were used as CH3 radical precursors, azomethane and methyl iodide. The overall rate coefficient of the CH3 + OH reaction was determined in the temperature range 1081–1426 K under conditions of chemical isolation. The experimental data are in good agreement with a recent theoretical study of the reaction. The decomposition of methanol to methyl and OH radicals was also investigated behind reflected shock waves. The current measurements are in good agreement with a recent experimental study and a master equation simulation. © 2008 Wiley Periodicals, Inc. 40: 488–495, 2008

High-Temperature Measurements of the Reactions of OH with Toluene and Acetone

The reaction of hydroxyl [OH] radicals with toluene [C6H5CH3] was studied at temperatures between 911 and 1389 K behind reflected shock waves at pressures of approximately 2.25 atm. OH radicals were generated by rapid thermal decomposition of shock-heated tert-butyl hydroperoxide [(CH3)3-CO-OH], and monitored by narrow-line width ring dye laser absorption of the well-characterized R1(5) line of the OH A-X (0,0) band near 306.7 nm. OH time histories were modeled by using a comprehensive toluene oxidation mechanism. Rate constants for the reaction of C6H5CH3 with OH were extracted by matching modeled and measured OH concentration time histories in the reflected shock region. Detailed error analyses yielded an uncertainty estimate of +/-30% at 1115 K for the rate coefficient of this reaction. The current high-temperature data were fit with the lower temperature measurements of Tully et al. [J. Phys. Chem. 1981, 85, 2262-2269] to the following two-parameter form, applicable over 570-1389 K: k3 = (1.62 x 10(13)) exp(-1394/T [K]) [cm3 mol(-1) s(-1)]. The reaction between OH radicals and acetone [CH3COCH3] was one of the secondary reactions encountered in the toluene + OH experiments. Direct high-temperature measurements of this reaction were carried out at temperatures ranging from 982 to 1300 K in reflected shock wave experiments at an average total pressure of 1.65 atm. Uncertainty limits were estimated to be +/-25% at 1159 K. A two-parameter fit of the current data yields the following rate expression: k6 = (2.95 x 10(13)) exp(-2297/T [K]) [cm3 mol(-1) s(-1)].

Rate Constants, 1100 ≤ T ≤ 2000 K, for H + NO2 → OH + NO Using Two Shock Tube Techniques: Comparison of Theory to Experiment

Rate constants for the reaction H + NO2 → OH + NO have been measured over the temperature range 1100−2000 K in reflected shock wave experiments using two different methods of analysis. In both methods, the source of H-atoms is from ethyl radical decomposition in which the radicals are formed essentially instantaneously from the thermal decomposition of C2H5I. The first method uses atomic resonance absorption spectrometry (ARAS) to follow the temporal behavior of H-atoms. Experiments were performed under such low [C2H5I]0 that the title reaction could be chemically isolated, and the decay of H-atoms was strictly first-order. The results from these experiments can be summarized as k = (1.4 ± 0.3) × 10-10 cm3 molecule-1 s-1 for 1100 ≤ T ≤ 1650 K. The second method utilizes a multipass optical system for observing the product radical, OH. A resonance lamp was used as the absorption source. Because this is the first OH-radical kinetics investigation from this laboratory, extensive calibration was required. This procedure resulted in a modified Beer's law description of the curve-of-growth, which could subsequently be used to convert absorption data to OH-radical profiles. Rate constants by this method required chemical simulation, and the final result can be summarized as k = (1.8 ± 0.2) × 10-10 cm3 molecule-1 s-1 for 1250 ≤ T ≤ 2000 K. Because the results from the two methods statistically overlap, they can be combined giving k = (1.64 ± 0.30) × 10-10 cm3 molecule-1 s-1 for 1100 ≤ T ≤ 2000 K. The present results are compared to earlier work at lower temperatures, and the combined database yields the temperature dependence over the large range, 195−2000 K. The combined results can be summarized as k = (1.47 ± 0.26) × 10-10 cm3 molecule-1 s-1 for 195 ≤ T ≤ 2000 K. The reaction is subsequently considered theoretically using ab initio electronic structure calculations combined with modern dynamical theory to rationalize the thermal rate behavior.

A shock tube study of reactions of CN with HCN, OH, and H2 using CN and OH laser absorption

Quantitative, narrow-line laser absorption measurements of CN time-histories at 388.444 nm were acquired in high-temperature pyrolysis and laser photolysis shock tube experiments. The data were analyzed using a detailed kinetics mechanism to determine the rate coefficients of the reactions for temperatures between 940 and 1860 K. Two independent experimental approaches were utilized: laser photolysis (at 193 nm) of dilute C2N2/HCN/argon and C2N2/H2/argon mixtures in reflected shock wave experiments, and shock heating of HNO3/HCN/argon mixtures in incident and reflected shock wave experiments. Laser absorption measurements of OH at 306.687 nm were also taken in the HNO3/HCN/argon experiments The results are in good agreement with rate coefficient determinations from previous studies at different temperatures. The expression derived by Yang et al (1992) from their k2 measurements in combination with those of Szekely et al (1983), is recommended for the broad temperature range 300–3000 K. The uncertainty factors f and F give the limiting values of the rate coefficient kmin = f kbest fit, kmax = Fkbest fit. The recommended expression for the rate coefficient of reaction (3) also valid for temperatures 300–3000 K, is taken from the transition state theory analysis of the CN + H2 reaction by Wagner and Bair (1986). The rate coefficient for reaction (1) was measured to be 4.0 × 1013cm3mol−1 s−1(f = 0.61, F = 1.40) for the temperature range 1250–1860 K. © 1996 John Wiley & Sons, Inc.

Quenching of reaction in gas-sampling probes to measure scramjet engine performance

The problem of chemical quenching in gas-sampling probes was studied as a reactive, Rayleigh, and Fanno flow. The typical lengths of friction, convective heat transfer, and reactions in the probes are evalnated under the testing conditions of ramjet and scramjet engines. A reduced chemical kinetic scheme was adopted, and the reaction times were evaluated from the ignition times derived by reflected shock wave experiments. Thereby, criteria for quenching in probes were obtained analytically. The critical Damköhler number ( Da ) for quenching in an H 2 -air reaction was found to be 23 (1500 K) to 10.5 (3000 K). A 2.6-fold greater Da was derived for CH 4 -air mixtures. Because reactions in CH 4 -air mixtures can be quenched even above 2200 K, combustion performance in a CH 4 -fueled ramjet combustor was measured from gas analysis. However, quenching of H 2 unburned gas in more difficult than quenching of CH 4 , Cooling with supersonic expansion is essential to freeze reactions in H 2 -air mixtures in scramjets tested under a total temperature higher than 2000 K. A constant Mach number expansion is recommended downstream of the sampling orifice. The contour of the expansion region and the decay of total pressure were determined taking into account friction and cooling effects. Reactions with unburned H 2 can be quenched if the sampling probe works as a supersonic diffuser operating in the supercritical mode.

Simultaneous laser absorption measurements of CN and OH in a shock tube study of HCN + OH → products

AbstractSimultaneous, quantitative, narrow‐line laser absorption measurements of CN time‐histories at 388.444 nm and OH time‐histories at 306.687 nm have been made in incident and reflected shock wave experiments using dilute mixtures of nitiric acid (HNO3) and HCN in argon. The thermal decomposition of HNO3 serves as a rapid source of OH upon shock‐heating, and the OH subsequently reacts predominantly with the HCN in the test gas mixture. The rate coefficient for the reaction equation image was determined in the temperature range 1120–1960 K via detailed kinetics modeling of the simultaneously acquired CN and OH measurements. These data are in good agreement with lower temperature measurements of the rate of the reverse reaction (−1a) when recent values of the heats of formation of CN and HCN are used. The expression valid for temperatures 500 to 2000 K, effectively represents the experimental measurements. The estimated uncertainty of the expression for k1a is ±30%, based on the experimental uncertainties of the individual rate coefficient studies. Analysis of the decay region of the experimental OH time‐histories yielded the total rate coefficient k1 (all product channels) for the reaction of HCN with OH for temperatures ranging from 1490 to 1950 K. These measurements are consistent with a previous theoretical analysis of the three primary addition‐isomerization‐dissociation processes for the HCN + OH reaction at combustion temperatures when the contribution to k1 from reaction (1a) is included. © 1995 John Wiley & Sons, Inc.

Thermal decomposition of CH2Cl2

The thermal decomposition of CH 2 Cl 2 has been investigated in reflected shock wave experiments at temperatures between 1400 and 2300 K and at three different loading pressures with various initial CH 2 Cl 2 concentrations. The resulting product Cl atoms are monitored by the atomic resonance absorption spectrometric (ARAS) technique. A reaction mechanism is used to numerically simulate the measured Cl-atom profiles in order to obtain rate constants for the two primary dissociation reactions: (1) CH 2 Cl 2 →CHCl+HCl, and (2) CH 2 Cl 2 →CH 2 Cl+Cl. The experimental second-order Arrhenius expressions for the two reactions are k 1 /[Kr]=2.26×10 −8 exp(−29,007 K/T ) cm 3 molecule −1 s −1 and k 2 /[Kr]=6.64×10 −9 exp(−28,404 K/T ) cm 3 molecule −1 s −1 , with standard deviations of ±43 and 40%, respectively. The results are compared to theoretical calculations using the semiempirical Troe formalism. The best fits to the experimental data are obtained with threshold energy and collisional energy transfer parameters of E 1 o =73.0 kcal mole −1 and Δ E 1down =630 cm −1 . Similar values for reaction (2) are E 2 o =Δ H 2 o (OK)=78.25 kcal mole −1 and Δ E 2down =394 cm −1 .

A study of ethane decomposition in a shock tube using laser absorption of CH3

AbstractThe rate coefficient for the unimolecular reaction, C2H6 → CH3 + CH3, was measured in reflected shock wave experiments using narrow‐linewidth laser absorption of methyl radicals at 216.6 nm. The experiments were conducted in the falloff regime at the conditions 1350 to 2110 K, 0.58 to 4.4 atm, in 50 to 500 ppm C2H6/Ar and 190 ppm C2H6/N2 mixtures. At temperatures below 1500 K, the measured rate coefficients are in good agreement with the expression of Wagner and Wardlaw (1989). Above 1500 K, the measurements fall increasingly below their predictions. © 1993 John Wiley & Sons, Inc.

A shock tube study of reactions of atomic oxygen with isocyanic acid

AbstractReactions of atomic oxygen with isocyanic acid (HNCO) have been studied in incident and reflected shock wave experiments using HNCO/N2O/Ar mixtures. Quantitative time‐histories of the NH(X3Σ−) and OH(X2Πi) radicals were measured behind the shock waves using cw, narrow‐linewidth laser absorption at 336 nm and 307 nm, respectively. The second‐order rate coefficients of the reactions: equation image and equation image were determined from early‐time NH and OH formation rates, with least‐squares two‐parameter fits of the results given by: and cm3 mol−1 s−1. The minimum and maximum rate constant factors (ƒ,F) define the lower and upper uncertainty limits, respectively.An upper limit on the rate coefficient of equation image was determined to be: .

A shock tube study of methane decomposition using laser absorption by CH 3

The pyrolysis of methane was studied in the temperature range 1790 to 2325K, pressure range 0.56 to 3.76 atm, and CH4 mol fraction range 200 ppm to 1.0% in reflected shock wave experiments. The rate coefficient, k1, for methane decomposition, CH4 + Ar → CH3 + H + Ar was determined by comparing measurements of CH3 concentrations made using a narrowlinewidth CW laser absorption technique with CH3 concentrations calculated using a reaction mechanism adapted from Miller and Bowman (1989). Reactant mixture compositions and the experimental conditions were selected so that the early-time CH3 concentration profiles were sensitive almost exclusively to the rate of reaction (1). The least-squares fit to 25 early-time CH3 profiles yielded a bimolecular rate coefficient expression k1 = 8.17 × 1016 exp (−44030/T[K])[cm3/mol/s]. The measured bimolecular rate coefficients were found to be independent of pressure in the range studied. The present determination is similar to the previously-published values summarized by Cobos and Troe (1990) and differs from the recent determination of Klemm et al. (1991).

A shock tube study of reactions of carbon atoms with hydrogen and oxygen using excimer photolysis of C3O2 and carbon atom atomic resonance absorption spectroscopy

The reactions of C({sup 3}P) with H{sub 2} and O{sub 2} were studied at high temperature in reflected shock wave experiments. C atoms were detected at 156.1 nm by atomic resonance absorption spectroscopy (ARAS). Controlled levels of C atoms for calibrating the ARAS diagnostic were made by high-temperature pyrolysis of highly dilute CH{sub 4}/Ar mixtures. For kinetics experiments, C atoms were formed by pyrolysis of C{sub 3}O{sub 2} above 2,300 K and by ArF excimer photolysis of C{sub 3}O{sub 2} below 2,050 K. Formation of C atoms in the presence of excess H{sub 2} or O{sub 2} permitted first-order rate coefficient determinations of the reactions C({sup 3}P) + H{sub 2} {yields} CH + H (1) and C({sup 3}P) + O{sub 2} {yields} CO + O(2), yielding k{sub 1} = 4.0 {times} 10{sup 14} exp({minus}11,700 K/T) ({plus minus} 50%) cm{sup 3} mol{sup {minus}1} s{sup {minus}1}, over the temperature range 1,525-2,540 K and pressure range 0.5-1.2 atm, and k{sub 2} = 1.2 {times} 10{sup 14} exp({minus}2,010 K/T) ({plus minus} 50%) cm{sup 3} mol{sup {minus}1} s{sup {minus}1}, over the temperature range 1,500-4,200 K.

Reaction kinetics of NH in the shock tube pyrolysis of HNCO

AbstractThe high temperature kinetics of NH in the pyrolysis of isocyanic acid (HNCO) have been studied in reflected shock wave experiments. Time histories of the NH(X3Σ−) radical were measured using a cw, narrow‐linewidth laser absorption diagnostic at 336 nm. The second‐order rate coefficients of the reactions: equation image were determined to be: cm3−mol−1−s−1, where f and F define the lower and upper uncertainty limits, respectively. The data for k1a are somewhat better fit by:

Effect of NO 2 on the ignition delay of CH 4-air mixtures

The ignition delay for methane-air mixtures is found to be reduced by the addition of nitrogen dioxide. Reflected shock wave experiments indicate that a reduction of 1 2 – 1 3 in ignition delay can be realized when 1–2% NO 2 is added to stoichiometric mixtures at a temperature range of 1300–1800°K and pressures around 14 atms.