Laminar flame speed measurements of ethanol, iso-octane, and their binary blends at temperatures up to 1020 K behind reflected shock waves

Abstract

A shock tube augmented with a laser-spark ignition system was used to measure the atmospheric-pressure (1 atm ±2%) laminar flame speeds of neat ethanol and iso-octane as well as ethanol/iso-octane blends in a 21% O2-79% Ar (so-called “airgon”) oxidizer. Low-temperature (453 K–524 K) validation experiments of atmospheric-pressure, stoichiometric ethanol/air were first performed for comparison with literature. Two fuel blends with 50% and 85% ethanol (by volume) in iso-octane (E50 and E85, respectively) were then studied in airgon at high unburned-gas temperatures (637 K–1020 K) and at various equivalence ratios (0.75, 1, and 1.2). The low-temperature ethanol/air measurements showed good agreement with literature data, confirming the accuracy of flame speeds measured using a shock tube. High-temperature fuel/airgon flame speeds were simulated with various kinetic models, and significant model disagreement was found, highlighting the value of the new flame speed data in the high-temperature regime. To make the present fuel/airgon measurements compatible with practical engineering applications using air as the oxidizer, the ratio of fuel/airgon and fuel/air flame speeds was calculated using 1D simulations for each experiment, then subsequently used to perform mixture-scaling for the fuel/airgon data to obtain equivalent fuel/air flame speeds. Fuel/air laminar flame speed correlations as a function of the unburned-gas temperature were determined for the studied fuel blends and equivalence ratios. Finally, flame speed sensitivity analyses conducted for high-temperature ethanol/airgon and iso-octane/airgon flames revealed three key governing reactions. Assigning common rate expressions to these reactions across different kinetic models improved the agreement between model-predicted and experimentally measured ethanol flame speeds, but did not have a similar effect for iso-octane, suggesting the need for further exploration of iso-octane-specific reaction rates to facilitate better model agreement.