Stephen Walton - Academia.edu (original) (raw)

Papers by Stephen Walton

Proceedings of the Combustion Institute, 2009

Ignition studies of two C 5 esters were performed using a rapid compression facility. Methyl buta... more Ignition studies of two C 5 esters were performed using a rapid compression facility. Methyl butanoate and ethyl propanoate were chosen to have matching molecular weights and C:H:O ratios while varying the length of the constituent alkyl chains. The effect of functional group size on ignition delay time was investigated using pressure time-histories and high-speed digital imaging. Low-temperature, moderate-pressure conditions were selected for study due to the relevance to low temperature combustion strategies and internal combustion engine conditions. The experiments covered a range of conditions: T=935-1117 K, P=4.7-19.6 atm, and φ=0.3-0.4. The experimental data are compared to previous high temperature studies and chemical modeling. A new mechanism for methyl butanoate and ethyl propanoate ignition is presented. The modeling and experimental data are in excellent agreement for methyl butanaote and yield good agreement for ethyl propanoate.

An Imaging Study of Compression Ignition Phenomena of Iso-Octane, Indolene, and Gasoline Fuels in a Single-Cylinder Research Engine

Journal of Engineering for Gas Turbines and Power, 2008

High-speed imaging combined with the optical access provided by a research engine offer the abili... more High-speed imaging combined with the optical access provided by a research engine offer the ability to directly image and compare ignition and combustion phenomena of various fuels. Such data provide valuable insight into the physical and chemical mechanisms important in ...

Atomic Spectrometry Update?Atomisation and Excitation

Journal of Analytical Atomic Spectrometry, 1987

... Spatial drifts attributed to the E X B interaction were found to influence the analyte - subs... more ... Spatial drifts attributed to the E X B interaction were found to influence the analyte - substrate interaction as well as the SBR. Profound effects on the radiative properties of the transient discharge were obtained with field strengths of only a few kilogauss. ...

Atomic Spectrometry Update?Atomic Emission Spectrometry

Journal of Analytical Atomic Spectrometry, 1996

... Revista bibliográfica. ; Análisis multielemento. ; Elemento inorgánico. ; Espectrometría emis... more ... Revista bibliográfica. ; Análisis multielemento. ; Elemento inorgánico. ; Espectrometría emisión. ; Arco eléctrico. ; Descarga luminiscente. ; Descarga chispa. ; Cátodo hueco. ; Atomización termoeléctrica. ; Vaporización. ; Nebulización. ; Espectrometría MIP. ; Espectrometría ICP. ; ...

Experimental investigation of the intermediates of isooctane during ignition

International Journal of Chemical Kinetics, 2007

Fuel, 2011

The current work presents the results of an experimental study of the intermediates formed during... more The current work presents the results of an experimental study of the intermediates formed during ignition of methyl butanoate (C 5 H 10 O 2 ) and air mixtures. A rapid-sampling system and the University of Michigan rapid compression facility were used to acquire gas samples at conditions of P = 10.2 atm and T = 985 K using mixtures of v mb = 0.96%, v O 2 = 20.79%, v N 2 = 52.89%, and v Ar = 25.25% (mole fraction, percent basis); corresponding to / = 0.30 and an inert gas to O 2 molar ratio of 3.76. The samples were analyzed using gas chromatography. Quantitative measurements of mole fraction time-histories of methane, ethane, propane, ethene, propene, and 1-butene are compared with model predictions based on a reaction mechanism developed in previous work. The methane and ethene time-histories are in excellent agreement (within $20%), while propene and ethane are underpredicted by the model. Sensitivity analysis shows ignition is controlled primarily by competition between H 2 O 2 and HO 2 kinetics at these conditions. Reaction path analysis shows the methyl butanoate fuel consumption is dominated by H-atom abstraction by OH.

Combustion and Flame, 2005

Autoignition of iso-octane was examined using a rapid compression facility (RCF) with iso-octane,... more Autoignition of iso-octane was examined using a rapid compression facility (RCF) with iso-octane, oxygen, nitrogen, and argon mixtures. The effects of typical homogeneous charge compression ignition (HCCI) conditions on the iso-octane ignition characteristics were studied. Experimental results for ignition delay times, τ ign , were obtained from pressure time-histories. The experiments were conducted over a range of equivalence ratios (φ = 0.25-1.0), pressures (P = 5.12-23 atm), temperatures (T = 943-1027 K), and oxygen mole fractions (χ O 2 = 9-21%), and with the addition of trace amounts of combustion product gases (CO 2 and H 2 O). It was found that the ignition delay times were well represented by the expression τ ign = 1.3 × 10 −4 P −1.05 φ −0.77 χ −1.41 O 2 exp(33,700/R (cal/mol/K) T ),

Combustion and Flame, 2006

Iso-octane ignition delay times (τ ign ) and hydroxyl (OH) radical mole fraction (χ OH ) time his... more Iso-octane ignition delay times (τ ign ) and hydroxyl (OH) radical mole fraction (χ OH ) time histories were measured under conditions relevant to homogeneous charge compression ignition engine operating regimes using the University of Michigan rapid compression facility. Absolute quantitative OH mole fraction time histories were obtained using differential narrow-line laser absorption of the R 1 (5) line of the A 2 Σ + ← X 2 Π i (0, 0) band of the OH spectrum (ν 0 = 32606.56 cm −1 ). Ignition delay times were determined using pressure and OH data. Diluted iso-octane/argon/nitrogen/oxygen mixtures were used with fuel/oxygen equivalence ratios from φ = 0.25 to 0.6 for τ ign measurements and from φ = 0.25 to 0.35 for χ OH measurements. The pressures and temperatures after compression ranged from 8.5 to 15 atm and from 945 to 1020 K, respectively, for the combined τ ign and χ OH data. The maximum mole fraction of OH during ignition and the plateau value of OH after ignition are compared with model predictions using different iso-octane oxidation mechanisms. Sensitivity and rate of production analyses for OH identify reactions important in iso-octane ignition under these lean, intermediate-temperature conditions. The OH time histories show significant sensitivity to the OH + OH + M = H 2 O 2 + M, CH 3 + HO 2 = CH 3 O + OH, and CH 3 + HO 2 = CH 4 + O 2 reactions, which have rate coefficients with relatively high uncertainties. Improved predictions of the OH time histories can be achieved by modifying the rate coefficient for these reactions. The enthalpy of formation used for OH also has a significant effect on the predicted ignition delay times.

Combustion and Flame, 2007

High-speed digital imaging has been used in rapid compression facility (RCF) studies to investiga... more High-speed digital imaging has been used in rapid compression facility (RCF) studies to investigate ignition phenomena of iso-octane/air mixtures. Sequential images were captured for each experiment. The results indicate the existence of two ignition regimes. In one domain, ignition is rapid, typically less than 76 μsec, and ignition occurs simultaneously throughout the test volume. In the other domain, reaction fronts form and propagate within the test volume prior to volumetric ignition. The data span equivalence ratios from φ = 0.20 to 1.98, with inert/O 2 gas ratios from 1.38 to 5.89, pressures from 8.7-16.6 atm and temperatures from 903-1020 K. The transition between the two regimes is discussed in the context of the mixture composition and experimental conditions. The analysis shows that the fuel mole fraction is a key parameter dictating the boundary between the modes of ignition. Below a critical mole fraction limit, volumetric ignition is observed; above the critical limit, reaction fronts are consistently present prior to volumetric ignition. The ignition delay times for both ignition regimes are well reproduced using a homogenous simulation with detailed reaction chemistry, when the state conditions are modified to account for the presence of the reaction fronts. The results are also discussed in terms of theory proposed for modes of ignition.

Proceedings of the Combustion Institute, 2007

Ignition studies of simulated syngas mixtures of hydrogen (H 2 ), carbon monoxide (CO), oxygen (O... more Ignition studies of simulated syngas mixtures of hydrogen (H 2 ), carbon monoxide (CO), oxygen (O 2 ), nitrogen (N 2 ), and carbon dioxide (CO 2 ) were performed using a rapid compression facility. Experiments were conducted using pressure time-histories and high-speed imaging to measure ignition delay times (s ign ), over a broad range of conditions relevant to current and proposed gas-turbine technologies, and which included fuel compositions consistent with typical gasification facilities. Specifically, the s ign data spanned pressures from P = 7.1 to 26.4 atm, temperatures from T = 855 to 1051 K, equivalence ratios from / = 0.1 to 1.0, oxygen mole fractions from v O 2 ¼ 15% to 20% and H 2 :CO ratios from H 2 :CO = 0.25 to 4.0 (mole basis). Regression analysis yielded the following best-fit to the composite data set: s ign ¼ 3:7 Â 10 À6 P À0:5 / À0:4 v À5:4 O 2 expð12; 500=R ½cal=mol=K T Þ In this expression, s ign is the ignition delay time [ms], P is pressure [atm], T is temperature [K], / is the equivalence ratio (based on the H 2 and CO to O 2 molar ratio), and v O 2 is the oxygen mole fraction. The uncertainty in the measured values for s ign is estimated as less than 30%. The experimental data are in good agreement with model predictions based on a recently proposed detailed reaction mechanism for H 2 and CO.