Abstract
Adiabatic laminar burning velocities and post-flame NO mole fractions for neat and blended ethanol and n-heptane premixed flames were experimentally determined using a heat flux burner and laser-induced fluorescence. The flames were stabilized at atmospheric pressure and at an initial temperature of 338 K, over equivalence ratios ranging from 0.6 to 1.5. These experiments are essential for the development, validation and optimization of chemical kinetic models, e.g. for the combustion of gasoline-ethanol fuel mixtures. It was observed that the addition of ethanol to n-heptane leads to an increase in laminar burning velocity that is not proportional to the ethanol content and to a decrease of NO formation. Such a NO reduction is due to the slightly lower flame temperatures of ethanol, which decrease the production of thermal-NO at 0.6 < Φ < 1.2, while under fuel-rich conditions this behavior is due to the lower concentrations of CH radicals, which decrease the production of prompt-NO. At Φ > 1.3, the lower NO formation through the prompt mechanism in the ethanol flames is partially offset by a lower rate of NO consumption through the reburning mechanism. New experimental results were compared with predictions of the POLIMI detailed chemical kinetic mechanism. An excellent agreement between measurements and simulated results was observed for the laminar burning velocities over the equivalence ratio range investigated; however, discrepancies were found for the NO mole fractions, especially under rich conditions. Further numerical analyses were performed to identify the main causes of the observed differences. Differences found at close-to stoichiometric conditions were attributed to an uncertainty in the thermal-NO mechanism. In addition, disagreement under rich conditions could be explained by the relative importance of reactions in hydrogen cyanide consumption pathways.
Highlights
In recent years, the constantly fluctuating prices of crude oil, the depletion of its worldwide reserves and the more stringent govern mental regulations on pollutant emissions, have stimulated a growing interest in the search for alternative fuels, with particular attention on biofuels
Ethanol as engine fuel is not a novel concept as it has been used since the end of 19th century and, nowadays, ethanol-based fuels are increasingly being used in “flex-fuel” spark-ignition (SI) engines because of their higher octane number compared with gasoline [2], or in compression ignition (CI) engines that use dual-injection strategies [3]
Numerous experimental studies claimed that the use of ethanol-enriched fuels significantly re duces emissions of carbon monoxide, unburned hydrocarbons and soot compared to gasoline- and diesel-fueled engines, mainly due to the leaning effect caused by the oxygen content in ethanol; ethanol addition may adversely affect the production of harmful carbonyl species [12] and nitrogen oxides (NOx) [7]; these concerns could become a significant barrier to ethanol market expansion
Summary
The constantly fluctuating prices of crude oil, the depletion of its worldwide reserves and the more stringent govern mental regulations on pollutant emissions, have stimulated a growing interest in the search for alternative fuels, with particular attention on biofuels. When ethanol is added to gasoline or diesel fuel, it increases the H/C atom ratio of the fuel and the availability of oxygen for the combustion process, leading to a coupled shift in temperature, fuel–air ratio and combustion duration and this, in turn, influences both thermaland prompt-NO formation mechanisms in a rather complex way that depends on the oxygen-sensing feedback control and catalyst [8,9,14,17,50] Another reason for the observed inconsistencies is that the way in which this delicate balance impacts the increase/decrease of NOx emissions depends on vehicle type, engine speed and load, compression ratio, fueling method, conversion efficiency and internal exhaust gas recirculation [5,9,13,17,24,26]. To accurately assess each step of the com bustion process and provide a reliable prediction of the interplay between the fuel structure and NOx formation mechanisms, it is neces sary to experimentally and computationally study chemical details under controlled conditions In this context, adiabatic premixed laminar flames are very useful tools. Results are analyzed to provide some insights about the effect of physicochemical properties of the fuel on dominant NO formation pathways
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