Temperature dependence of the Laminar burning velocity of methanol flames

To better understand and predict the combustion behavior of methanol in engines, sound knowledge of the effect of the pressure, unburned mixture temperature, and composition on the laminar burning velocity is required. Because many of the existing experimental data for this property are compromised by the effects of flame stretch and instabilities, this study was aimed at obtaining new, accurate data for the laminar burning velocity of methanol–air mixtures. Non-stretched flames were stabilized on a perforated plate burner at 1 atm. The heat flux method was used to determine burning velocities under conditions when the net heat loss from the flame to the burner is zero. Equivalence ratios and initial temperatures of the unburned mixture ranged from 0.7 to 1.5 and from 298 to 358 K, respectively. Uncertainties of the measurements were analyzed and assessed experimentally. The overall accuracy of the burning velocities was estimated to be better than ±1 cm/s. In lean conditions, the correspondence with recent literature data was very good, whereas for rich mixtures, the deviation was larger. The present study supports the higher burning velocities at rich conditions, as predicted by several chemical kinetic mechanisms. The effects of the unburned mixture temperature on the laminar burning velocity of methanol were analyzed using the correlation uL = uL0(Tu/Tu0)α. Several published expressions for the variation of the power exponent α with the equivalence ratio were compared against the present experimental results and calculations using a detailed oxidation kinetic model. Whereas most existing expressions assume a linear decrease of α with an increasing equivalence ratio, the modeling results produce a minimum in α for slightly rich mixtures. Experimental determination of α was only possible for lean to stoichiometric mixtures and a single data point at equivalence ratio= 1.5. For these conditions, the measurement data agree with the modeling results

The effects of dilution with nitrogen and steam on the laminar burning velocity of methanol at room and elevated temperatures

Dilution of flames using recirculated exhaust gas or water vapor is an important NOx reduction technique in various combustion applications. Dilution, however, can also lead to combustion instabilities due to reduced burning velocities. For methanol/air flames this subject is largely unexplored. Therefore, the current work examines the effects of diluting methanol/air flames with nitrogen and water vapor both experimentally and by simulation. Using the heat flux method, the laminar burning velocity u(l) of methanol/air mixtures was measured at p = 1 bar, T-u = 298-358 K, phi = 0.7-1.5, molar water vapor contents up to 20% and molar excess nitrogen contents of almost 10%. Simulations were performed using the methanol oxidation mechanism of Li et al. (IJCK 39: 109 (2007)). Excellent agreement between experimental and computed results was found for lean mixtures. For rich mixtures the mechanism of Li et al. overestimated the laminar burning velocity up to 5%. The effect of dilution on u(l) was well predicted for both diluents. The relative impact of thermal and chemical effects of dilution was estimated computationally. For both N-2 and H2O the chemical effect was shown to be negligible for diluents ratios considered here (<20%). Based on the modeling results, an explicit correlation was proposed that describes the effect of dilution on u(l) in terms of diluent molar content, diluent specific molar heat capacity, equivalence ratio and unburned mixture temperature. Very good agreement was obtained between the correlation and the modeling data. The effects of unburned mixture temperature on the laminar burning velocity of methanol were analyzed using the correlation u(l) = u(l0) . (T-u/T-u0)(alpha). The current experimental results showed that the power exponent alpha reached a minimum for phi = 1.2, which was well reproduced by the modeling. The modeling results indicated that alpha increases as the mixture gets more diluted

Laminar burning velocities of primary reference fuels and simple alcohols

Laminar burning velocities for methanol, ethanol, and binary and quaternary mixtures of these with isooctane and n-heptane, have been determined using the heat flux method on a flat flame adiabatic burner. Measurements were done for an equivalence ratio range between 0.7 and 1.5 and for a range of temperatures between 298 K and 358 K at atmospheric pressure. The present study expands the available data on laminar burning velocities of alcohol-hydrocarbon blends and validates simple mixing rules for predicting the laminar burning velocity for a wider range of fuel blends of hydrocarbons with methanol and/or ethanol. It is shown that simple mixing rules that consider the energy fraction of the blend's components are accurate enough to predict the experimentally determined laminar burning velocity of the mixtures. (c) 2013 Elsevier Ltd. All rights reserved