Laminar Burning Velocity and Development of a Chemical Kinetic Model for Small Oxygenated Fuels

The thesis work was performed with the aim of increasing knowledge and understanding of the combustion of oxygenated fuels and intermediates. This was accomplished in two steps: experimental measurements of the laminar burning velocity to expand current databases and development of a reaction mechanism. In the first part of the project, the laminar burning velocity of oxygenated fuels and intermediates was measured using the heat flux method. Emphasis was placed on extending the experimental database for fuels and intermediates with limited or scattered experimental data. The laminar burning velocities of acetaldehyde and methyl formate were investigated experimentally and were compared with kinetic mechanisms from the literature. In addition, temperature dependence of the laminar burning velocity, expressed as SL=SL0(T/T0)α, was investigated both numerically and experimentally. It was found that a kinetic mechanism can overpredict the experimental laminar burning velocity yet still display good agreement with the experimentally determined temperature dependence. To investigate the temperature dependence further a sensitivity analysis of the α coefficient was performed. The sensitivity analysis provided a different view of the chemistry involved compared to the sensitivity of the laminar burning velocity. In the second part of the project, a contemporary detailed kinetic mechanism for the combustion of small oxygenated fuels and intermediates was developed. The mechanism was developed with the version 0.6 of the Konnov mechanism as a starting point. Reactions involved in the combustion of formaldehyde, methanol and acetic acid were reviewed and the most reliable rate constants were selected. The new kinetic mechanism was validated against experimental data from the literature covering a wide range of conditions including shock tube and flow reactors as well as burner stabilized and freely propagating flames. The sub mechanism for methanol and formaldehyde successfully reproduced experimental data from shock tube pyrolysis and flow reactor oxidation. The mechanism was in closer agreement with experimental data concerning the laminar burning velocity of methanol than version 0.6 of the Konnov mechanism was. Validation of the mechanism for acetic acid combustion included laminar burning velocities, measured here for the very first time by use of the heat flux method. The calculated velocities were about 3 cm/s higher than the experimental results. Further validation of the kinetic mechanism was achieved by simulating species profiles of burner stabilized acetic acid flames. While major species were reproduced successfully, minor species were either under-or-over predicted. Sensitivity analysis showed ketene to play an important role in the acetic acid combustion. The results of this project provide the scientific community with experimental data potentially useful for model validation as well as a new kinetic mechanism for small oxygenated fuels and intermediates

Laminar premixed flat non-stretched lean flames of hydrogen in air

Laminar burning velocity of lean hydrogen + air flames at standard conditions is still a debated topic in combustion. The existing burning velocity measurements possess a large spread due to the use of different measurement techniques and data processing approaches. The biggest uncertainty factor in these measurements comes from the necessity to perform extrapolation to the flat flame conditions, since all of the previously obtained data were recorded in stretched flames. In the present study, laminar burning velocity of lean hydrogen + air flames and its temperature dependence were for the first time studied in stretch-free flat flames on a heat flux burner. The equivalence ratio was varied from 0.375 to 0.5 and the range of the unburned gas temperatures was 278-358 K. The flat flames tended to form cells at adiabatic conditions, therefore special attention was paid to the issue of their appearance. The shape of the flames was monitored by taking OH* images with an EM-CCD camera. In most cases, the burning velocity had to be extrapolated from flat subadiabatic conditions, and the impact of this procedure was quantified by performing measurements in H-2 + air mixtures diluted by N-2. The effect of extrapolation was estimated to be of negligible importance for the flames at standard conditions. The measured burning velocities at 298 K showed an important difference to the previously obtained literature values. The temperature dependence of the burning velocity was extracted from the measured results. It was found to be in agreement with the trends predicted by the detailed kinetic modeling, as opposed to a vast majority of the available literature data. (C) 2015 The Combustion Institute. Published by Elsevier Inc. All rights reserved

Laminar burning velocity of diacetyl + air flames. Further assessment of combustion chemistry of ketene

Ketene is important intermediate in high-temperature chemistry of several oxygenates, such as acetone, acetic acid, and diacetyl. Ketene reactions appear in the sensitivity spectra of calculated burning velocities of the first two species. To provide independent experimental data for validation of the ketene sub-mechanism, the laminar burning velocities of diacetyl + air flames at 1 atm and initial gas temperatures of 298 K, 318 K, and 338 K were determined for the first time. Measurements were performed using the heat flux method in non-stretched flames, stabilised on a perforated plate burner at adiabatic conditions. Recently developed detailed kinetic mechanism for acetic acid flames with updated ketene sub-mechanism has been extended by reactions of diacetyl and includes revised rate constants for reactions of acetaldehyde and acetyl radical. The model was first compared with available experimental data on ketene pyrolysis and oxidation. Its performance in prediction of C2 species formation was improved by significant reduction of the previously estimated rate constants of ketene reactions with CH3 and CH2 radicals. The updated mechanism was then compared with the new measurements for diacetyl and earlier data for acetaldehyde, acetone and acetic acid flames. The model closely reproduces burning velocity of diacetyl + air in lean and rich mixtures while underpredicts in stoichiometric and slightly rich flames. Performance of the model for acetaldehyde + air flames was much improved as compared to the Konnov mechanism version 0.6. Good agreement of the modelling with experimental data for acetone + air flames was also demonstrated. The disparity between predicted burning velocities of acetic acid and recent measurements did not change. The model was further examined using sensitivity analysis for these flames to elucidate common reactions affecting its performance. It was concluded that the mechanism performance in prediction of the burning velocities of acetic acid flames could be improved by revision of reactions between CH2CO and OH radicals, while keeping its agreement with other flames studied. Remaining uncertainties in the ketene sub-mechanism are outlined

Laminar burning velocity of acetic acid + air flames

Laminar burning velocities of acetic acid + air flames at 1 atm and initial gas temperatures of 338 K, 348 K, and 358 K were determined using the heat flux method. Measurements were performed in non-stretched flames, stabilized on a perforated plate burner at adiabatic conditions. Due to experimental problems related to the corrosiveness of acetic acid towards the burner material, the uncertainty of the burning velocities was relatively high up to ± 2 cm/s. Seventy reactions pertinent to acetic acid and ketene have been reviewed and detailed reaction mechanism for acetic acid combustion was developed. The model over-predicts measured burning velocities by about 3 cm/s. The mechanism was also tested comparing with flame structure of the low-pressure flame of acetic acid (Leplat and Vandooren, 2012). Good agreement with the concentration profiles of major products was found, however several minor intermediates were over- or under-predicted by the model. To elucidate reactions responsible for the differences observed, the sensitivity analysis was performed. It was found that the calculated burning velocities are insensitive to the reactions of acetic acid and mostly governed by C1 chemistry typical for all hydrocarbons and by reactions of ketene. Possible modifications of the rate constants within the evaluated uncertainty factors were discussed

The effect of temperature on the adiabatic burning velocities of diluted hydrogen flames: A kinetic study using an updated mechanism

The effect of temperature on the adiabatic burning velocities of diluted hydrogen flames has been analyzed using an updated version of the Konnov detailed reaction mechanism for hydrogen. The contemporary choice of the reaction rate constants is provided with the emphasis on their uncertainties, and the analysis of the performance of the updated mechanism is presented and compared to the previous version for a wide range of validation cases: jet stirred and flow reactors; oxidation, decomposition and ignition in shock waves; ignition in rapid compression machines; laminar burning velocity and flame structure. An overall improvement of the mechanism performance was observed, particularly for the shock tube and flow reactor studies. Temperature dependence of the burning velocity, S-L, is commonly interpreted using the correlation S-L = S-L0 (T/T-0)(alpha). The updated mechanism was applied to study the behavior of the power exponent alpha for H-2 + O-2 + N-2 flames in a wide range of stoichiometry and dilution ratios. The simulations were compared to the available experimental results, either taken from the literature or evaluated in the present study from the existing burning velocity data. The equivalence ratio and N-2 content in the mixture were found to have significant influence on the temperature power exponent. The dependence of the temperature exponent on the fitting temperature range was observed and discussed. This effect was found to cause significant discrepancies in the burning velocities at high temperatures, if obtained with empirical correlation. (C) 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved

The temperature dependence of the laminar burning velocities of methyl formate plus air flames

Laminar burning velocities, S-L, of methyl formate and air flames were determined at atmospheric pressure and initial gas temperatures, T, of 298, 318, 338 and 348 K. Measurements were performed in non-stretched flames, stabilized on a perforated plate burner at adiabatic conditions, generated using the heat flux method. These new experimental data shed light on discrepancies seen in previously published results, and the temperature dependence of the laminar burning velocity of methyl formate was analysed using expression S-L = S-L0(T/T-0)(alpha). It was found that the power exponent, alpha, has a minimum close to equivalence ratio, phi, of 1.0. Both the laminar burning velocities and alpha coefficient were compared with predictions of the mechanisms of Glaude et al. (2005), Dooley et al. (2010) and Dievart et al. (2013). While the two latter mechanisms are in generally good agreement in lean mixtures, the Glaude mechanism over predicts the experimental burning velocities over the entire range of equivalence ratios. The temperature dependences predicted by the Glaude and Dievart mechanisms, however, are rather close and agree well with the measurements. To elucidate these differences and similarities in the performance of two mechanisms, the sensitivity analysis of the power exponent alpha was performed for the first time. It was demonstrated that examination of the temperature dependence of the burning velocity provides an independent approach for analysis of experimental data consistency. (c) 2015 Elsevier Ltd. All rights reserved

Oxy-fuel Combustion of Ethanol in Premixed Flames

First measurements of the adiabatic laminar burning velocities of lean ethanol + oxygen + carbon dioxide flames, at 1 atm and initial gas mixture temperatures of 298, 318, and 338 K, are presented. The oxygen content O-2/(O-2 + CO2) in the artificial air was 35%. The laminar burning velocities were determined using the heat flux method, where a non-stretched flame is stabilized under adiabatic conditions. The measurements were found in qualitative agreement with modeling performed using the Marinov model, the San Diego model, and the model by Leplat et al. In comparison to experimental data, the Marinov model gave the best quantitative agreement Notable quantitative differences between the models were analyzed using sensitivity analysis and reaction path diagrams. Reaction path analysis showed that the Marinov and the San Diego models have the same 12 most important species. Among the 12 most important species in the model of Leplat et al., 10 species are in common with the other two models. According to predictions of the Marinov model, combustion of ethanol in air and at oxy-fuel conditions proceeds via the same reaction path; however, the sensitivities of the key reactions are different

Combustion chemistry of methoxymethanol. Part II : Laminar flames of methanol+formaldehyde fuel mixtures

In the present study, the laminar burning velocities of mixtures of up to 16.4% (mol) formaldehyde in methanol, burning with air, were determined at atmospheric pressure using the heat flux method covering lean, stoichiometric and rich flames at initial gas mixture temperatures of 298, 318 and 338 K. Results published in the literature indicate that evaporation of CH2O+CH3OH fuel blends should lead to a gaseous mixture of formaldehyde, methanol and methoxymethanol, although the composition of these components in the gas phase was not well defined. To interpret the measurements performed in the present study, the detailed kinetic model developed by the group of Konnov was used. The recently updated mechanism was further extended by the reactions of methoxymethanol with the rate constants calculated in Part I of the present study. A comparison of the predictions of this mechanism with the new experimental data indicated that between 40% and 60% of CH2O present in the investigated CH2O+CH3OH mixtures were at 473 K evaporated as gaseous formaldehyde monomer, while the rest was released within CH3OCH2OH. Laminar burning velocity results suggest partial condensation of methoxymethanol in the CH3OH+CH2O fuel mixture with 5.84% formaldehyde at rich conditions and 298 K. These observations allowed evaluation of the partial pressure of CH3OCH2OH which was found to be between 0.35 and 0.52 kPa. The sensitivity and rate-of-production analyses revealed that the reduced reactivity with the increased amount of methoxymethanol in the fuel mixtures is explained by the conversion of CH3OCH2OH to CH3OCHOH radicals due to favored H-abstraction from the secondary hydrogen atoms predicted by ab initio calculations compared to other sites of methoxymethanol. Hydroxyl-methoxyl-methyl radicals further decompose forming slowly reacting formic acid and methyl radicals

Simultaneous one-dimensional fluorescence lifetime measurements of OH and CO in premixed flames

A method for simultaneous measurements of fluorescence lifetimes of two species along a line is described. The experimental setup is based on picosecond laser pulses from two tunable optical parametric generator/optical parametric amplifier systems together with a streak camera. With an appropriate optical time delay between the two laser pulses, whose wavelengths are tuned to excite two different species, laser-induced fluorescence can be both detected temporally and spatially resolved by the streak camera. Hence, our method enables one-dimensional imaging of fluorescence lifetimes of two species in the same streak camera recording. The concept is demonstrated for fluorescence lifetime measurements of CO and OH in a laminar methane/air flame on a Bunsen-type burner. Measurements were taken in flames with four different equivalence ratios, namely I center dot = 0.9, 1.0, 1.15, and 1.25. The measured one-dimensional lifetime profiles generally agree well with lifetimes calculated from quenching cross sections found in the literature and quencher concentrations predicted by the GRI 3.0 mechanism. For OH, there is a systematic deviation of approximately 30 % between calculated and measured lifetimes. It is found that this is mainly due to the adiabatic assumption regarding the flame and uncertainty in H2O quenching cross section. This emphasizes the strength of measuring the quenching rates rather than relying on models. The measurement concept might be useful for single-shot measurements of fluorescence lifetimes of several species pairs of vital importance in combustion processes, hence allowing fluorescence signals to be corrected for quenching and ultimately yield quantitative concentration profiles

Kinetics of premixed acetaldehyde plus air flames

Non-stretched laminar burning velocities, SL, of acetaldehyde + air mixtures at initial gas mixture temperatures, T, of 298, 318, 338, 348 and 358 K are reported for the first time. The flames were stabilized on a perforated plate burner at 1 atm using the heat flux method at conditions where the net heat loss from the flame to the burner is zero. Uncertainties of the measurements were analyzed and assessed experimentally. The overall accuracy of the burning velocities was estimated to be typically better than + 1 cm/s. Experimental results were compared with predictions of several kinetic models from the literature. Recent model of Leplat et al. (2011) [30] developed for acetaldehyde and ethanol oxidation showed the closest agreement with the measurements as compared to the Konnov and San Diego models. The effects of initial temperature on the adiabatic laminar burning velocities of acetaldehyde were interpreted using the correlation S-L = S-L0 (T/T-0)(alpha). Particular attention was paid to the variation of the power exponent alpha with equivalence ratio. The existence of a minimum in alpha in the slightly rich mixtures is demonstrated experimentally and confirmed computationally. The model of Leplat et al. was further analyzed using sensitivity analysis and it was concluded that the deviation of the modelled results when comparing with experiments is not a result of the fuel specific reactions but rather the sub-mechanisms of C1 and H-2/O-2. (C) 2014 The Combustion Institute. Published by Elsevier Inc. All rights reserved