Konnov Mechanism Research Articles

The temperature dependence of the laminar burning velocity of ethanol flames

The Heat Flux method was extended for the first time towards liquid fuels and used to determine burning velocities under conditions when the net heat loss from the flame to the burner is zero. Non-stretched flames were stabilized on a perforated plate burner at 1atm. Uncertainties of the measurements were analyzed and assessed experimentally. The overall accuracy of the burning velocities was estimated to be better than ±1cm/s. Excellent reproducibility of the experiments over an extended period of time was demonstrated. Measurements of the adiabatic burning velocity of ethanol+air flames in the range of initial mixture temperatures from 298 to 358K are presented. Experimental results are in a good agreement with the recent literature data obtained in constant volume bombs. Both the ethanol combustion mechanism of Saxena and Williams and the Konnov mechanism significantly over-predict ethanol laminar burning velocities in lean and near-stoichiometric mixtures. The effects of initial temperature on the adiabatic laminar burning velocities of ethanol were interpreted using the correlation SL=SL0 (T/T0)α. Particular attention was paid to the variation of the power exponent α with equivalence ratio at atmospheric pressure. Experimental data and proposed empirical expressions for α as a function of equivalence ratio were summarized. They were compared with the predictions of detailed kinetic models. The existence of a minimum in α in the slightly rich mixtures is demonstrated experimentally and confirmed computationally.

Laminar burning velocities of three C 3H 6O isomers at atmospheric pressure

Laminar flames of three C 3H 6O isomers (propylene oxide, propionaldehyde and acetone), representative of cyclic ether, aldehyde and ketone species important as intermediates in oxygenated fuel combustion, have been studied experimentally and computationally. Most of these flames exhibited a non-linear dependency of flame speed upon stretch rate and two complementary independent techniques were adopted to provide the most reliable burning velocity data. Significant differences in burning velocity were noted for the three isomers: propylene oxide + air mixtures burned fastest, then propionaldehyde + air, with acetone + air flames being the slowest; the latter also required stronger ignition sources. Numerical modelling of these flames was based on the Konnov mechanism, enhanced with reactions specific to these oxygenated fuels. The chemical kinetics mechanism predicted flame velocities in qualitative rather than quantitative agreement with the measurements. Sensitivity analysis suggested that the calculated flame speeds had only a weak dependency upon parent fuel-specific reactions rates; however, consideration of possible break-up routes of the primary fuels has allowed identification of intermediate compounds, the chemistry of which requires a better definition.

Adiabatic laminar burning velocities of CH4+H2+air flames at low pressures

Experimental measurements of the adiabatic burning velocity in methane + hydrogen + air flames using the Heat Flux method are presented. The hydrogen content in the fuel was varied from 0 to 20%. Non-stretched flames were stabilized on a perforated plate burner from 20 to 100 kPa. The equivalence ratio was varied from 0.8 to 1.4. Adiabatic burning velocities of CH4 + H2 + air mixtures were found in good agreement with the literature results at atmospheric pressure. Also low-pressure measurements in CH4 + air flames performed earlier were accurately reproduced. The effects of enrichment by hydrogen on the laminar burning velocity at low pressures have been studied for the first time. Calculated burning velocities using the Konnov mechanism are in satisfactory agreement with the experiments over the entire range of conditions. Pressure dependences of the burning velocities for the three fuels studied could be approximated by an empirical exponential correlation.

Implementation of the NCN pathway of prompt-NO formation in the detailed reaction mechanism

This work presents revised detailed reaction mechanism for small hydrocarbons combustion with possibly full implementation of available kinetic data related to the prompt NO route via NCN. It was demonstrated that model predictions with the rate constant of reaction CH + N 2 = NCN + H measured by Vasudevan and co-workers are much higher than experimental concentrations of NO in rich premixed flames at atmospheric pressure. Analysis of the correlations of NO formation with calculated concentrations of C 2O radicals strongly supports the inclusion of reaction between C 2O and N 2 and reduction of the rate constant of reaction between CH and N 2. Rate constants of the reactions of NCN consumption were mostly taken from the works of Lin and co-workers. Some of these reactions affect calculated profiles of NCN in flames. Proposed modifications allow accurate prediction of NO formation in lean and rich flames of methane, ethylene, ethane and propane. Agreement of the experiments and the modeling was much improved as compared to the previous Release 0.5 of the Konnov mechanism. Satisfactory agreement with available measurements of NCN radicals in low pressure flames was also demonstrated.

The effects of enrichment by H 2 on propagation speeds in adiabatic flat and cellular premixed flames of CH 4 + O 2 + CO 2

Experimental studies of adiabatic flat and cellular premixed flames of (CH 4 + H 2) + (O 2 + CO 2) are presented. The hydrogen content in the fuel was varied from 0% to 35% and the oxygen content in the oxidizer was 31.55%. These mixtures could be formed when oxy-fuel combustion technology is combined with hydrogen enrichment. Non-stretched flames were stabilized at atmospheric pressure on a perforated plate burner. A heat flux method was used to determine propagation speeds under conditions when the net heat loss of the flame is zero. Adiabatic burning velocities of methane + hydrogen + carbon dioxide + oxygen mixtures were found in satisfactory agreement with the detailed kinetic modeling employing the Konnov mechanism. Under specific experimental conditions the flames become cellular; this leads to significant modification of the flame propagation speed. The onset of cellularity was observed throughout the stoichiometric range of the mixtures studied. Visual and photographic observations of the flames were performed to quantify their cellular structure. The results obtained in the present work in (CH 4 + H 2) + O 2 + CO 2 mixtures are in good accordance with the previous observations for different fuels, CH 4, C 2H 6 and C 3H 8. The enrichment by hydrogen leads to: the increase of the laminar burning velocities; the increase of the number of cells observed; the decrease of the mean cell diameter. The flame acceleration due to cellularity was not affected by the hydrogen enrichment.

The effects of composition on the burning velocity and NO formation in premixed flames of C 2H 4 + O 2 + N 2

Experimental measurements of the adiabatic burning velocity and NO formation in C 2H 4 + O 2 + N 2 flames are presented. The oxygen content in synthetic air varied from 18% down to 14%. Non-stretched flames were stabilized on a perforated plate burner at 1 atm. A heat flux method was used to determine the burning velocities under the conditions when the net heat loss of the flame is zero. Adiabatic burning velocities of ethylene + nitrogen + oxygen mixtures were found in good agreement with the modeling. Sampling measurements were performed at 10–20 mm from the burner and concentrations of stable species, CO, CO 2, O 2 and NO x were recorded. The concentrations of CO, CO 2 and O 2 were compared with the modeling to reveal that the range of the flame conditions was less affected by the ambient air entrainment. The dependencies of [NO] as a function of equivalence ratio clearly possess two maxima: in stoichiometric mixtures due to Zeldovich thermal-NO mechanism and in rich mixtures at equivalence ratio close to 1.4 due to Fenimore prompt-NO mechanism. Dilution by nitrogen decreases [NO] at any equivalence ratio. Numerical predictions of the concentrations of NO in a post-flame zone of lean and stoichiometric flames are in good agreement with the experiments when downstream heat losses to the environment were taken into account. The predictions of the Konnov mechanism in rich ethylene flames reproduce trends of the experimental data with an under-prediction of about 10–15 ppm.