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American Institute of Aeronautics and Astronautics
1
Simulations of Plasma-Assisted Combustion Flames in
Coaxial Microwave Reactors
Joseph W. Zimmerman
1
, Andrew D. Palla
2
, Darren M. King
3
, David L. Carroll
4
CU Aerospace, Champaign, IL, 61820
and
Constandinos M. Mitsingas
5
, Rajavasanth Rajasegar
6
, and Tonghun Lee
7
University of Illinois, Urbana, IL, 61801
The atmospheric coaxial direct-coupled microwave torch configuration offers a
convenient experimental format for validating multiphysics simulations of plasma-assisted
combustion (PAC). The optical accessibility of this configuration allows for the application
of various diagnostics to the PAC flame such as planar laser-induced fluorescence (PLIF) to
determine quantitative two-dimensional density distributions of radicals (e.g. OH, CH, NO),
Rayleigh scattering thermometry (RST) for temperature profiles, and particle image
velocimetry (PIV) to characterize the flame flow-field, as well as assessment of plasma
nonequilibrium effects via probe measurements and spectroscopic emission measurements.
Furthermore, the flexible system enables premixed and non-premixed configurations to be
addressed, and the most recent modifications have added tangential swirling flows. Here,
recent changes to the experimental apparatus to combine PAC with swirl-stabilization are
overviewed, and initial PLIF measurements of OH radicals are presented, as well as
measurements of flame stability limits and acoustics. Cold flow simulations of the swirl-
stabilized chamber using BLAZE Multiphysics simulation suite are presented.
Nomenclature
EEDF = electron energy distribution function
E/N = electric field to gas density ratio (reduced electric field)
fn = acoustic mode frequency
Le = effective acoustic length
n = mode number
ne = electron density
SLPM = standard liters per minute
Te = electron temperature
Tg = gas temperature
U = flow velocity
Vs = sound speed
t = turbulent kinematic viscosity
p = primary fuel-to-oxidizer equivalence ratio
1
Senior Scientist, 301 N. Neil St., Suite 502, AIAA Member.
2
Senior Physicist, 301 N. Neil St., Suite 502, AIAA Member.
3
Senior Engineer, 301 N. Neil St., Suite 502
4
President, 301 N. Neil St., Suite 400, AIAA Fellow.
5
Research Assistant, Mechanical Science and Engineering, 3001 Mech. Eng. Lab
6
Research Assistant, Mechanical Science and Engineering, 3001 Mech. Eng. Lab
7
Assoc. Prof., Mechanical Science and Engineering, 3001 Mech. Eng. Lab
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54th AIAA Aerospace Sciences Meeting
4-8 January 2016, San Diego, California, USA
AIAA 2016-0190
Copyright © 2015 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.
AIAA SciTech
American Institute of Aeronautics and Astronautics
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I. Introduction
tudies focused on the application of nonequilibrium plasmas to problems in ignition and combustion are
prevalent [1, 2]. Application areas range from automotive engine technologies for improving fuel efficiency to
aerospace propulsion where researchers are developing technologies for stabilizing combustion in hypersonic
engines, improving performance envelopes and emissions characteristics of subsonic jet engines, and stabilizing
flames in lean-fuel conditions [2]. Configurations under study include longitudinal DC field coupling [3], RF
plasma-ignition [4], nanosecond-pulsed dielectric barrier discharges (ns-DBDs) [5], laser-induced breakdown with
microwave power deposition [6], and waveguide coupled microwave systems [7-9]. There is also interest in
applying similar nonequilibrium plasma devices to fuel synthesis and chemical production technologies that use
carbon-dioxide or methane captured from industrial and other anthropogenic sources [10, 11].
A series of recent experiments was devised to investigate coupling of microwave power into methane-air
mixtures jetting out from coaxial microwave cavities [7-9]. The plasma-assisted combustion flames generated in
these microwave torch-type configurations can be characterized with Rayleigh scattering for temperature
determination, 2D planar laser-induced fluorescence (PLIF) for characterization of radical distributions, particle
image velocimetry (PIV) to characterize flow-fields and flame-fronts, as well as probe measurements and
spectroscopic techniques to establish electronic characteristics, excited species contents, and non-equilibrium
behaviors. Rao et al. [7] investigated plasma re-ignition of premixed CH4/air in a coaxial reentrant type cavity, and
demonstrated the capability of increasing microwave power deposition to extend flow rate blowoff limits (for lean,
stoichiometric, and rich conditions). Hammack et al. [8] characterized the influence of power deposition on OH
content and temperature of PAC flames from a direct-coupled microwave applicator and characterized the plasma-
associated heating of stoichiometric premixed CH4/air discharge compared to that in an air discharge. More recent
work by Rajasegar et al. has shown the influence of continuous microwave power on swirl-stabilized flames,
characterizing the impact of swirl flow on flame stand-off distance and lean blow-off (LBO) limit, and also
demonstrating the impact of microwave power coupling to lower the LBO limit (by >40%) and increase the OH
radical content (~150%) with addition of microwave power [9]; this work also illustrated significant vibrational
excitation of N2 by the plasma (Tvib ~ 6100 K). Similar characterizations of PAC devices have been made with
pulsed DBDs [12] and with microwave coupling to flat flames within waveguides [13]. Such nonequilibrium plasma
experiments, with well-defined geometries and comprehensive diagnostic suites, offer excellent opportunities for
validation of the multiphysics modeling capability for PAC applications.
This paper summarizes the recent results from a combined study where robust multiphysics modeling is being
applied directly to a microwave-enhanced swirl-flow configuration currently under investigation with detailed
experiments. The current work extends from recently reported results which simulated a free coaxial jet with power
deposition (DC approximation) [14]. The swirl-stabilized PAC chamber apparatus and the associated diagnostics are
discussed. A region of swirl-flow flame stability that will serve as the simulation validation case is overviewed,
showing the LBO limit, and the measurements of OH radicals in the swirl-stabilized flame. Near these limits, the
influence of plasma power deposition is shown, demonstrating the lowering of the LBO limit, the change in the
flame character from a lifted flame to an anchored flame at the coax, and the impact of power deposition on OH
radical content. High-temperature microphone measurements in the new chamber are also reported, showing the
impact of the continuous microwave plasma on the intensity of the acoustic modes of the chamber. The initial cold-
flow simulation results of the swirl-stabilized chamber for baseline flow conditions are also reported.
II. Experimental Configuration and Diagnostics
A. Swirl-Stabilized PAC Reactor
A microwave waveguide plasma system (developed in collaboration with Amarante Technologies) was used to
directly couple microwave energy into flames with various geometries; the configuration is illustrated in Figure 1.
The system allows for complete access of the plasma-enhanced flame for laser and optical diagnostics. Microwave
radiation can be generated in excess of 1 kW but also as low as 30 W. Microwave energy is generated by a
research-grade 2.45-GHz magnetron directly mounted to a WR284 waveguide and powered by an SM840 power
supply. The microwave transmission inside the WR284 waveguide is in TE 10 mode. A directional coupler mounted
next to the magnetron head is used to measure the incident power. A circulator directs reflected energy into a
dummy load with a coupler, so that reflected energy can also be measured. Both the magnetron and dummy load are
water cooled. The three-stub tuner and sliding short are used to adjust the electric field and impedance to match the
load of the plasma applicator and the nozzle.
S
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The nozzle has a solid electrode that protrudes inside the WR284 waveguide. Microwave energy travels through
the nozzle, acting as a coaxial guide, in transverse electromagnetic mode. The internal nozzle geometry is optimized
for plasma breakdown by delivering a high electric field at the nozzle tip. A plasma discharge is initiated by
adjusting the maximum E-field position of the standing wave inside the waveguide using the sliding short. As the
standing wave is repositioned, the E-field at the nozzle tip also varies. When the E-field reaches the breakdown
threshold, gas around the electrode tip will be ionized, generating a plasma discharge that sits entirely above the
surface of the nozzle, accessible for direct optical measurements. This system has been developed to produce a
direct-coupled plasma-enhanced flame accessible to optical and laser diagnostics. The torch would likely be more
efficient with a truncated inner electrode, where the outer electrode (nozzle body) extended further than the inner
solid electrode; however, a heavy interest in complete access to the plasma ignition and coupling point directed the
decision to use a “flush” configuration, where the electrodes end at the same length.
A 1st generation swirl-stabilized PAC chamber has been assembled for initial testing, Figure 2. The swirl
chamber is constructed around the existing 2.45 GHz waveguide-coupled coaxial microwave applicator, and consists
of a 1.5”-diameter circular cylindrical swirl injector section which is 0.5” high, with four 2.5 mm diameter tangential
injectors spaced at 90 intervals about the circumference of the outer wall. Above the circular section, the combined
annular jet and swirling flow enters a 1.5” x 1.5” rectangular cross-section fused-silica tube with rounded corners
measuring 12” long. This configuration mimic realistic combustion geometries and to prevent entrainment of outside
air into the jet. In the present work, only air is injected through the swirl inlet ports. This dilutes the main jet
boundary and thereby lowers the overall operating equivalence ratio of the burner. The swirl number of the flow
entering the combustor is varied by changing the proportion of the tangential to the axial flow rates. The axial (fuel
and air) and tangential (air) flow rates are metered individually by three laminar-flow, differential pressure mass
flow controllers (MKS mass controller). This configuration allowed for generation of stable lifted flames at a height
of 5 to 25 mm from the nozzle exit.
Figure 1. Schematic of microwave applicator system for
swirl-stabilized PAC.
Figure 2. Swirl-stabilized PAC chamber. Photos at upper
right are for 10:1 SLPM Air:CH4 through the main
annular jet, with a 40 SLPM air flow through the swirler.
B. PLIF Diagnostic Setup
The excitation of OH is made using the Q1(8) transition from the A2 Σ+ - X2 Π (1, 0) band, which requires
narrowband UV light near 283 nm. The measurements are conducted using a Sirah Precision Scan dye laser pumped
using the frequency-doubled output of a Spectra Physics Quanta-Ray Pro pulsed Nd:YAG laser at 532 nm
wavelength at 10 Hz. The output of the dye laser is 10 ns pulses at 566 nm, which is subsequently frequency
doubled through a BBO crystal followed by a compensator to a final frequency of 283 nm. The laser is pulsed at 10
Hz and has a spectral line width of about 0.1 cm−1 at 283 nm. The pulse energy is recorded digitally using a fast
photodiode and an oscilloscope and attenuated to ensure operation within the linear fluorescence regime. The laser is
expanded into a sheet, and the fluorescence signal was collected at 90° using a La Vision Imager Intense 10 Hz
CCD camera fitted with a La Vision IRO intensifier unit with a Cerco 45 mm focal length f/2.8 UV lens and a high-
transmission (> 80% at 310 nm) Asahi narrow band pass filter centered at 310 nm with a full width half maximum
of 10 nm. A flat flame burner was used to calibrate the OH PLIF signal from intensity units to number density. A 0-
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D simulation of the flat flame was performed with CHEMKIN to approximate the OH concentration in the flame
and produce a calibrated profile. A schematic of the laboratory diagnostic setup is shown in Figure 3.
Figure 3. Laser diagnostics setup for PAC with microwave torch system.
C. High-temperature Probe Microphone
As part of the analysis of the impact of plasma on methane combustion flames, a high temperature microphone
was used to monitor sound emissions from the combustion chamber. This approach is prompted by recent
experiments which have analyzed acoustic emissions and demonstrated the influence of pulsed plasma power
deposition on the flame stability [15]. A PCB Piezotronics Model #377B26 high-temperature ICP® Probe
Microphone along with a compatible Model #480B21 Signal Conditioner was used for these acoustic measurements.
The PCB #377B26 has a frequency response range of 2.0 Hz to 20 kHz with sensitivity of 2.15 mV/Pa (measured at
250 Hz), and can handle temperature up to 800 C at the probe tip. An EZM Electronics FG-100 digital
programmable function generator has been purchased to serve as a frequency reference for the microphone system.
The frequency response and amplification of the signal conditioner were confirmed using the FG-100 source, a
Sperry DM-6450, a Harmon-Kardon HK-206 speaker, and a Tektronix TDS-3034B oscilloscope to measure the
microphone signal. During plasma experiment, the output from the signal conditioner was monitored with National
Instruments data acquisition module and LabView software. For measurements discussed here, the probe tip was
placed at the exit (top) of the quartz chamber (see Figure 2).
III. Experimental Measurements
A. Swirl-flow Flame Stability Limits
Figure 4 shows the primary equivalence ratio at LBO determined as a function of swirler flow rate for the swirl-
stabilized PAC reactor (w/o plasma). During this test the air flow through the coax was held constant at 10 SLPM,
while methane flow was varied to change the equivalence ratio {}. Flow conditions to
the upper right of this curve resulted in a stable flame in the reactor, while conditions at the lower left resulted in
extinction, where a flame could not be held in the reactor.
Once these LBO limits were defined, OH measurements were made for the stable flame conditions near the
threshold level varying the swirl flow rate and equivalence. Figure 5 shows the calibrated 2-D PLIF results as the
equivalence ratio varied from 0.85 to 1.3 for swirler flow rates of 20 to 50 SLPM. The images are of dimensions 500
pixels by 1024 pixels corresponding to 27.4 mm wide by 56.01 mm high. For the data set shown here (w/o plasma),
OH concentration reached as high as 3.2x1016 cm-3. The scale is shown below the table of images. Increases in either
swirl flow or equivalence ratio (fuel-to-air through coax) from the LBO limit resulted in an increase in the OH
concentration at the core. However, at high values of swirl or equivalence ratio, the OH level decreased as the flame
volume increased. The annular shape of the jet is evident throughout this data set, resulting in a symmetrical lobed
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shape in the 2-D plane, with a triangular void at the centerline above the solid center electrode (center body wake).
The lobed symmetry became more pronounced as the swirler flow rate increased.
Figure 4. Lean blow off limit as a function of the
(outer) swirler flow rate. Premixed coaxial Air flow
rate held constant at 10 SLPM.
Figure 5. 2-D OH PLIF mapping of low-flow swirl flame lean
blow off limits.
B. Plasma-Assisted Combustion Flame Measurements
With the low-flow LBO limits of the swirl-stabilized flame well defined, the influence of plasma power
deposition near these limits was examined, measuring OH concentrations with PLIF as the microwave power in the
coaxial reactor varied from 35 to 120 W. Figure 6 shows the PLIF results for conditions of 10 SLPM air in coaxial,
p = 1.0 (1.05 SLPM CH4), and an outer tangential swirl flow of 30 SLPM air. For these particular flow conditions,
a flame can be sustained without plasma power (0 W) at
p = 1.0, but is unstable for the lower fuel-to-air ratios. The
flame stands well off the electrodes (0W), and the OH distribution has a dual-lobed shape.
When 35 W is applied, the OH distribution attaches downward, with increased OH concentration just above the
center electrode which further increases along the centerline, then decreases at the top of the flame. As power
deposition increases, the OH level increases within the core and the height of the OH distribution increases. With
plasma deposition, the transverse distribution of OH has a single peak, while in the 0 W case the transverse profile
has a double peak with symmetry about the centerline, as in a number of cases shown in Figure 5. For the
p = 0.85
and 0.95 cases, the 0 W case is unstable, but plasma-assisted flames are held in the 35 to 120 W range (Note that for
a few cases, some white spots appear at the center of the flame. This is due to the measured concentration being
higher at these points than the scale that was used in data reduction; the scale was chosen to allow good comparisons
of the OH flame shape throughout the table).
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Figure 6. 2-D OH PLIF showing the influence of plasma power on swirl lean blow off limits. Swirl flow rate maintained
at 30 SLPM.
C. Acoustic Measurements
High-temperature microphone measurements indicate that the combustor chamber behaves similarly to a
closed-end cylinder resonator, and resonates with evenly spaced frequencies on odd harmonics only, driven by the
broadband sound caused by the flow in the combustion chamber. The resonant frequencies of the cylinder are
approximately
. (Eqn. 1)
Here, n is the mode number, Vs is the sound speed, and Le is the effective acoustic length of the tube, assumed equal
to the actual length plus 40% of the effective diameter. This results in the measurement of up to 5 resonant
longitudinal modes in the 1 foot (0.3048 m) long quartz chamber below the FFT sampling cutoff of 4000 Hz. For
operation without plasma, the bulk of the acoustic power is in the fundamental mode. The addition of microwave
plasma power deposition results in a significant decrease in the coupling of the fundamental mode.
The peak magnitude of each resonant mode is shown as a function of mode number (for n = 1, 3, 5, 7, 9) in
Figure 7. For plasma power deposition of 0 W, 50 W, and 100 W. The flame could not be stabilized for 10 and 20
SLPM swirl flow without plasma, but a flame could be sustained when microwave power was introduced. In the no
plasma (0 W) case, most of the measured acoustic power was in the fundamental mode, and this fell off with
increasing mode number and frequency. The introduction of microwave power deposition in the reactor resulted in a
decrease in the magnitude of the fundamental mode from approximately 1400 mPa to below 800 mPa. Some effect
of plasma in the n = 3 mode was also observed, showing a decreasing magnitude in the 50 SLPM swirl flow case,
but an increase for the 30 and 40 SLPM cases. The impact on higher modes is negligible.
As the swirl flow in the tube increases, the peak of each mode shifts to a lower value. The frequencies of n = 1
and n = 3 modes are plotted as a function of swirl flow for varied microwave power input in Figure 8. The drop in
frequency of the fundamental (n = 1) and n = 3 modes is due to a decrease in the effective speed of sound due to
increased heating of the gas and change in composition. Plasma power deposition increased the resonant frequencies
(and sound speed). As shown in Figure 9 there was also a significant shift in the fundamental mode with increased
fuel-air equivalence ratio, which indicates an increase in average sound speed (and temperature). A >10% shift in
the fundamental mode frequency over the range of methane flows tested (0.89 to 1.37 SLPM) was observed. An
average sound speed can be deduced from the mode frequencies, assuming the shift in modes can be modeled with
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Eqn. 1 (by using either the peak frequency of the fundamental mode, or slope of a linear fit to the first few mode
frequencies). Sound speeds of 500-600 m/s are indicated by this method.
(a) no plasma
(b) 50 W
(c) 100 W
Figure 7. Peak pressure fluctuation magnitude for each resonant mode as a function of mode number for varied outer
swirler air flow: (a) no plasma, (b) 50 W, (c) 100 W. The coaxial flow is a stoichiometric mix of 10:1.05 SLPM air:CH4.
Figure 8. Peak frequency for n = 1 and n = 3 as a function
of swirl flow for varied microwave power deposition. The
coaxial flow is 10:1.05 SLPM air:CH4.
Figure 9. Fundamental frequency peak as a function of
methane flow. Conditions are 10 SLPM air through the
coax, and 30 SLPM air through the outer swirler.
When the equivalence ratio is varied in the PAC flame, an interesting effect is observed showing an increase in
the magnitude of higher order acoustic modes. Figure 10 shows the peak magnitude as a function of mode number
for increasing methane flow. In the no plasma case, increasing the methane flow reduced the magnitude of the
fluctuation in the fundamental mode (n = 1). The introduction of 50 W of plasma power decreased the fundamental
mode further. However, with plasma, there was a strong increase in the n = 3 mode (near 1100 Hz) with increased
methane flow.
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(a) no plasma
(b) 50 W
Figure 10. Peak pressure fluctuation magnitude for each resonant mode as a function of mode number for varied
methane flow: (a) no plasma and (b) 50 W. The coaxial flow is 10 SLPM Air with varied SLPM methane as indicated, and
the outer swirl flow is 30 SLPM.
IV. Multiphysics Simulation Development for PAC
The advanced BLAZE Multiphysics™ Simulation Suite (BLAZE-7, http://blazemultiphysics.com), which
was developed by CU Aerospace, has been utilized for high-fidelity, multi-dimensional modeling of a plasma-
assisted combustion test chamber. The initial application of BLAZE-7 to simulate plasma assisted combustion in a
free-jet microwave torch configuration was reported by Palla et al. [14]. Additional applications to plasmas and laser
systems have been reported [16-19].
BLAZE-7 is comprised of a number of modular inter-operable and highly scalable MPI-based parallel finite-
volume models for the analysis of complex physical systems dependent upon laminar and turbulent fluid-dynamic
(incompressible and compressible subsonic through hypersonic regimes), molecular transport with gas kinetics, non-
equilibrium gas- and plasma-dynamic, electrodynamic, thermal, optical (radiation transport and wave optics), and
fluid/solid interaction physics using any modern computational platform (Windows, Mac, Unix/Linux). BLAZE is
compatible with a number of free, open-source, yet commercial quality grid generation and post-processing software
packages, which greatly reduces training and operating costs. The cases shown here applied Gmsh [20] for grid
generation and ParaView [21] for post-processing of solutions. BLAZE is also compatible with state of the art
commercial grid generation and post-processing solutions. Additionally, BLAZE users can create, compile, and
include their own complete models into BLAZE without any knowledge of parallel programming, input/output,
grid/mesh formats, sparse linear system solution schemes, etc. as this functionality is provided to the user by the
simulation engine via an easy to use application programming interface (API). Strict, object-oriented coding
practices using the C++ programming language allow rapid design iteration. The generic coding practices followed
in the BLAZE-7 development generally conform to DO178B/C standards.
A number of industry standard, yet challenging example cases have been run with which the BLAZE
multiphysics software package was validated, including the driven cavity, the backwards facing step, the rotating
cylinder, unsteady von Karman vortex shedding, NASA turbulent airfoils, gas- and plasma-kinetic simulations,
coupled gas-kinetic / optical field simulations, supersonic shock and hypersonic flow simulations, heat exchanges,
neutral chemistry and plasma-driven hydrocarbon combustion simulations, and wave-optics simulations among
others (see [16] for results).
V. Cold Flow Simulation of Swirl-Flow PAC Chamber
D. Problem Setup and Approach
In order to develop the mesh required for resolution of the physics associated with the test chamber, initial
calculations focused on simulation of a cold flow configuration in which combustion chemistry and plasma effects
were not modeled. The simulated geometry consisted of an axial exhaust jet with a diameter of 0.16 in., tangential
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swirler tubes with interior diameters of 0.16 in., a cylindrical swirler region below the exhaust jet plane with a
diameter of 1.5 in. and a depth below the exhaust plane of 0.5 in., and a shroud downstream of the injection plane
with a square cross-section with a side length of 1.7 in., Figure 11; a close-up image of the multi-block grid
geometry is shown in Figure 12.
Figure 11. Plasma-assisted combustion test chamber
showing axial nozzle and tangential swirler flow tubes.
Figure 12. Close-up of plasma-assisted combustion test
chamber multi-block geometry.
The domain downstream of the injection plane (in the box shroud region) has a length in the axial direction of 6
in., Figure 13. In order to simplify the analysis of grid studies a curvilinear multi-block structured mesh was
developed which conforms to the modeled geometry. The baseline mesh was comprised of 153 blocks and 249,856
hexahedra, Figure 13 and Figure 14.
Figure 13. Plasma-assisted combustion test chamber multi-
block geometry.
Figure 14. Plasma-assisted combustion test chamber multi-
block curvilinear structured mesh.
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E. Simulation Results and Discussion
Initial cold flow simulations utilized the BLAZE Navier-Stokes, Spalart-Allmaras Turbulence, Poisson Wall
Distance, and Molecular Transport models. The wall distance model was utilized to close the low Reynolds number
turbulence model equations and the molecular transport model was utilized to simulate transport of N2, CO2, O2,
CH4, O, and H. Fluid molecular dynamic viscosity, thermal conductivity, heat capacity and multi-component
Fickian molecular diffusion coefficients were calculated as a function of finite volume cell molecular mixture and
temperature. All modeled scalar transport variables used second order fluxes. Calculations further assumed an
exhaust jet flow rate of 10 SLPM of a stoichiometric air/methane mixture at 293.15 K and 1 atm and the swirler
tubes assumed a total flow rate across all tubes of 10 SLPM of air at 293.15 K and 1 atm. The resulting baseline
simulation was able to resolve the turbulent free jet, swirl effects, and recirculation regions, Figure 15a.
Calculations were also able to resolve the mixing of the jet air / methane mixture with the swirler air flow, Figure
15b. As expected, the map of methane concentration shows increased density in the wake of the center electrode,
and voids near the lower corners of the chamber. Subsequent calculations will further refine the cold flow simulation
and also focus on neutral- and plasma-assisted-combustion cases.
Figure 15. Plasma-assisted combustion test chamber baseline cold flow case BLAZE predicted (a) streamlines colored by
turbulent kinematic viscosity and (b) methane concentration on two orthogonal cross-sectional flow planes.
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VI. Conclusion and Future Work
The experimental work here with a swirl-stabilized PAC chamber has identified validation cases for simulating
the influence of nonequilibrium plasma on methane combustion in BLAZE Multiphysics. The results identify the
swirl-stabilized lean-flame blow off (LBO) limits, and demonstrate the extension of these limits by coupling of
plasma power into the coax. The OH PLIF measurements show a fundamental change between the lifted flame in
the swirl-stabilized case, and an attached flame in the PAC case which anchors in the wake of the center electrode
body. Replicating this fundamental behavior in simulations is one of the goals of future work.
High-temperature microphone measurements showed that the experimental PAC chamber resonates as a closed-
end cylinder, and that the introduction of plasma power (at levels small compared to the thermal power) results in a
significant decrease in the acoustic power of the fundamental mode (yielding a similar result to the pulsed discharge
case in a similar geometry [15]). Also, in the PAC flame, the adjustment of equivalence ratio changes the
distribution of power in the resonant modes, increasing the magnitude of harmonics compared to that of the
fundamental as methane is increased. The shifts in frequencies the fundamental mode and harmonics are consistent
with gas heating as plasma power is increased (or cooling as swirl flow is increased).
Initial modeling of cold flow mixing of air and methane in the swirl-stabilized reactor showed the complex
interaction of the coaxial center jet and the tangential flows, and the increased methane concentration in the wake of
the jet is first step toward illustrating the character of the PAC flame. Future work with the inclusion of neutral
combustion and air-methane plasma kinetics will be made to replicate the experimental results shown here. Some
near-term goals of the current combined experimental and computational effort are: (i) further explore the
relationship between stability limits and acoustics, (ii) make direct comparisons between cold-flow simulations and
PIV measurements in the swirl-flow apparatus, (iii) introduce vibrational kinetics for primary species to the existing
BLAZE PAC kinetics database [14], and (iv) model the swirl-stabilized PAC flame and compare the results to
recent experimental observations (e.g. ref. 9) of OH radical distributions, vibrational temperatures, and the impacts
of swirl and plasma power deposition on LBO.
Acknowledgments
This work was supported by Air Force STTR contract #FA8650-15-C-2547 “Nonequilibrium Plasma-Assisted
Combustion Efficiency Control in Vitiated Air.” The authors wish to thank the program manager Lt. Anthony
Trombley of Air Force Research Laboratory for helpful interactions and guidance, and co-investigators Gregory
Elliott and Marco Panesi of University of Illinois Aerospace department for their collaboration. CU Aerospace
would like to thank Igor Adamovich for lending his recommendations on kinetics packages.
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