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

Abstract

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.

Uncertainty propagation of chemical kinetics parameters and binary diffusion coefficients in predicting extinction limits of hydrogen/oxygen/nitrogen non-premixed flames

“…Notwithstanding the large amount of data available on ignition delays and flame propagation, only limited experimental data exist on non-premixed extinction limits of hydrogen counterflow flames [4][5][6]. Specifically, the uncertainty bounds of extinction limits of hydrogen flames that are not well established, which is one of the key focuses of this work.…”

Planar laser-induced fluorescence imaging of OH in a supersonic combustor fueled with ethylene and methane

“…For this experiment, the combustor was operated with both ethylene and methane as fuels. Ethylene and methane were selected because they essentially bound the entire range of hydrocarbon-fuel reactivity (based on ignition delay time or extinction strain rate) [9,10]. Injection and flameholding/combustor geometry were chosen for performance reasons.…”

HIFiRE Direct-Connect Rig (HDCR) Phase I Scramjet Test Results from the NASA Langley Arc-Heated Scramjet Test Facility

“…The facility fuel system delivered the flight fuel mixture, which is a molar mixture of 64% ethylene-36% methane (a surrogate for endothermically-cracked JP-7 7 ). A fuel system schematic is shown in Figure 4.…”

“…The test fuel is a mixture of ethylene and methane, which serves as a surrogate for an endothermically cracked JP-7 fuel. 7 The HF2 science objectives are listed in Table 1. The HF2 payload is shown in Figure 1.…”