Dynamic Weakening (Extinction) of Simple Hydrocarbon-Air Counterflow Diffusion Flames by Oscillatory Inflows

Abstract

This study of laminar non-premixed HC-air flames used an Oscillatory-input Opposed Jet Burner (OOJB) system developed from a previously well-characterized 7.2-mm Pyrex-nozzle OJB system. Over 600 dynamic Flame Strength (FS) measurements were obtained on unanchored (free-floating) laminar Counterflow Diffusion Flames (CFDF's). Flames were stabilized using plug inflows having steady-plus-sinusoidal axial velocities of varied magnitude, frequency, f, up to 1600 Hz, and phase angle from 0 (most data) to 360 degrees. Dynamic FS is defined as the maximum average air input velocity (U air , at nozzle exit) a CFDF can sustain before strain-induced extinction occurs due to prescribed oscillatory "peak-to-peak" velocity inputs superimposed on steady inputs.Initially, dynamic flame extinction data were obtained at low f, and were supported by 25-120 Hz HotWire cold-flow velocity data at nozzle exits. Later, expanded extinction data were supported by 4-1600 Hz Probe Microphone (PM) pk/pk P data at nozzle exits. The PM data were first obtained without flows, and later with cold stagnating flows, which better represent speaker-diaphragm dynamics during runs. The PM approach enabled characterizations of Dynamic Flame Weakening (DFW) of CFDF's from 8 to 1600 Hz. DFW was defined as % decrease in FS per Pascal of pk/pk P oscillation, namely, DFW = -100 d(U air / U air,0Hz ) / d(pkpk P). The linear normalization with respect to acoustic pressure magnitude (and steady state (SS) FS) led to a DFW unaffected by strong internal resonances.For the C 2 H 4 /N 2 -air system, from 8 to 20 Hz, DFW is constant at 8.52 + 0.20 (% weakening)/Pa. This reflects a "quasi-steady flame response" to an acoustically induced dU air /dP. Also, it is surprisingly independent of C 2 H 4 /N 2 mole fraction due to normalization by SS FS. From 20 to ~ 150 Hz, the C 2 H 4 /N 2 -air flames weakened progressively less, with an inflection at ~ 70 Hz, and became asymptotically insensitive (DFW ~ 0) at ~ 300 Hz, which continued to 1600 Hz. The DFW of CH 4 -air flames followed a similar pattern, but showed much greater weakening than C 2 H 4 /N 2 -air flames; i.e., the quasi-steady DFW (8 to ~ 15 Hz) was 44.3 %/Pa, or ~ 5x larger, even though the 0 Hz (SS) FS was only 3.0 x smaller. The quasi-steady DFW's of C 3 H 8 -air and C 2 H 6 -air were intermediate at 34.8 and 20.9 %Pa, respectively. The DFW profiles of all four fuels, at various frequencies, correlated well but non-linearly with respective SS FS's. Notably, the DFW profile for C 3 H 8 -air fell more rapidly in the range > 15 to 60 Hz, compared with the 1-and 2-carbon fuels. This may indicate a shift in chemical kinetics, and/or O 2 transport to a flame that moved closer to the fuel-side.

Opposed Jet Burner Approach for Characterizing Flameholding Potentials of Hydrocarbon Scramjet Fuels

“…(5) A series of HC-air extinction results will be shown, including pure and N 2 -diluted methane and propane vs air from an early horizontal brass (5.3 mm) nozzle-OJB; and then N 2 -diluted ethylene (20 -80%) vs air from a "standard" vertical contoured-Pyrex nozzle-OJB (7.2 mm). The latter system has also been used extensively to characterize the dynamic weakening of the five gaseous HC-air systems [26][27][28][29], using steady plus oscillatory velocity inputs at frequencies from 8 to 200 Hz, where virtually all the weakening occurs, and then up to 1600 Hz. Note for all the above N 2 -diluted H 2 and HC results, extinction limits will show asymptotic approaches to nearly constant values as pure fuel is approached.…”

“…The FS of ethylene was high enough that the fuel mass flowmeter just exceeded its operational limit. The same OJB system has also been used extensively to characterize dynamic flame weakening caused by sinusoidal velocity inputs of various (driven) amplitudes superimposed on steady inflows, with (mostly) in-phase frequencies ranging from 8 to 1600 Hz [26][27][28][29].…”

“…The expanded sets of extinction data also help provide a more accurate and comprehensive means of globally validating reduced chemical kinetic models for HCs, at temperatures appropriate to strain-induced OJB extinctions and to the loss of non-premixed idealized, incipient flameholding in more complex scramjet combustors. The study concludes with a comprehensive normalization of the nozzle and tube OJB data that characterize steady (and dynamic [26][27][28][29]) strain-induced extinction of a very wide range of non-premixed fuel-air systems, near atmospheric pressure.…”

Unsteady Extinction of Opposed Jet Ethylene / Methane HiFIRE Surrogate Fuel Mixtures vs. Air

“…31, 33 The present results include four major sets of dynamic extinction data from the gaseous HC-air systems of ethylene, methane, and 64/36 and 15/85 molar mixtures. The FS extinction data, obtained as functions of applied pk/pk voltage magnitudes to twin speakerdrivers, were normalized initially by Hot Wire velocity magnitude data for cold dynamic flows.…”

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

“…A4), from a "flowfield-standardized" contoured-Pyrex nozzle-OJB (7.2 mm), characterized a three-fold increase in FS as N 2 dilution diminished from 80% to 20%, and was followed by an asymptotic approach to steady state FS for pure C 2 H 4 . Also, the same system was used to characterize the dynamic response of five gaseous HC-air systems [37][38][39][40], in which steady plus oscillatory velocity inputs caused unique and systematically decreasing extents of Dynamic Flame Weakening (for each fuel), at frequencies from 8 to 200 Hz. Notably, from 300 to 1600 Hz (limit of the study), weakening due to lagging diffusion response of the flame was negligible, as flames became totally insensitive to oscillatory inputs.…”