Opposed Jet Burner Extinction Limits: Simple Mixed Hydrocarbon Scramjet Fuels vs Air

Opposed Jet Burner tools have been used extensively by the authors to measure Flame Strength (FS) of laminar non-premixed H 2 -air and simple hydrocarbon (HC)-air counterflow diffusion flames at 1-atm.FS represents a strain-induced extinction limit based on air jet velocity.This paper follows AIAA-2006-5223, and provides new HC-air FSs for global testing of chemical kinetics, and for characterizing "idealized flameholding potentials" during early scramjet-like combustion.Previous FS data included six HCs, pure and N 2 -diluted; and three HC-diluted H 2 fuels, where FS decayed very nonlinearly as HC was added to H 2 , due to H-atom scavenging.This study presents FSs on mixtures of (candidate surrogate) HCs, some with very high FS ethylene.Included are four binary gaseous systems at 300 K, and a "hot" ternary system at ~ 600 K.The binaries are methane + ethylene, ethane + ethylene, methane + ethane, and methane + propylene.The first three also form two ternary systems.The hot ternary includes both 10.8 and 21.3 mole % vaporized nheptane and full ranges of methane + ethylene.Normalized FS data provide accurate means of (1) validating, globally, chemical kinetics for extinction of non-premixed flames, and (2) estimating (scaling by HC) the loss of "incipient" flameholding in scramjet combustors.The n-heptane is part of a proposed "baseline simulant" (10 mole % with 30% methane + 60% ethylene) that "mimics" the ignition of endothermically cracked JP-7 "like" kerosene fuel, as suggested by Colket and Spadaccini in 2001 in their shock tube "Scramjet Fuels Autoignition Study."Presently, we use FS to gauge idealized flameholding, and define HC surrogates.First, FS was characterized for hot nheptane + methane + ethylene; then a hot 36 mole % methane + 64% ethylene surrogate was defined that mimics FS of the "baseline simulant" system.A similar hot ethane + ethylene surrogate can also be defined, but it has lower vapor pressure at 300 K, and thus exhibits reduced gaseous capacity.The new FS results refine our earlier "idealized reactivity scale" that shows wide ranging (50 x) diameter-normalized FSs for various HCs.These range from JP-10 and methane to H 2 -air, which produces an exceptionally strong flame that agrees within ~1% of recent 2-D numerically simulations.Finally, we continue advocating the FS approach as more direct and fundamental, for assessing idealized scramjet flameholding potentials, than measurements of "unstrained" laminar burning velocity or blowout in a Perfectly Stirred Reactor.