-
Available online at www.sciencedirect.comProceedings
ScienceDirect
Proceedings of the Combustion Institute 35 (2015) 2101–2108
www.elsevier.com/locate/proci
of the
CombustionInstitute
Cavity ignition in supersonic flow by sparkdischarge and pulse
detonation
Timothy M. Ombrello a,⇑, Campbell D. Carter a, Chung-Jen Tam
b,Kuang-Yu Hsu c
a U.S. Air Force Research Laboratory, Wright-Patterson Air Force
Base, OH 45433, USAb Taitech, Beavercreek, OH 45430, USA
c Innovative Scientific Solutions, Inc., Dayton, OH 45459,
USA
Available online 18 August 2014
Abstract
Ignition of an ethylene fueled cavity in a supersonic flow was
achieved through the application of twoenergy deposition
techniques: a spark discharge and pulse detonator (PD).
High-frequency shadowgraphand chemiluminescence imaging showed that
the spark discharge ignition was passive with the ignition ker-nel
and ensuing flame propagation following the cavity flowfield. The
PD produced a high-pressure andtemperature exhaust that allowed for
ignition at lower tunnel temperatures and pressures than the
sparkdischarge, but also caused significant disruption to the
cavity flowfield dynamics. Under certain cavity fuel-ing conditions
a multiple regime ignition process occurred with the PD that led to
decreased cavity burningand at times cavity extinction. Simulations
were performed of the PD ignition process, capturing thedecreased
cavity burning observed in the experiments. The PD exhaust
initially ignited and burned the fuelwithin the cavity rapidly.
Simultaneously, the momentary elevated pressure from the detonation
caused ablockage of the cavity fuel, starving the cavity until the
PD completely exhausted and the flowfield couldrecover. With
sufficiently high cavity fueling, the decrease in burning during
the PD ignition process couldbe mitigated. Cavity fuel injection
and entrainment of fuel through the shear layer from upstream
injectionallowed for the spark discharge ignition process to
exhibit similar behavior with peaks and valleys of heatrelease (but
to a lesser extent). The results of using the two energy deposition
techniques emphasized theimportance of cavity fueling and flowfield
dynamics for successful ignition.Published by Elsevier Inc. on
behalf of The Combustion Institute.
Keywords: Scramjet; Cavity ignition; Supersonic combustion;
Pulse detonation
http://dx.doi.org/10.1016/j.proci.2014.07.0681540-7489/Published
by Elsevier Inc. on behalf of The Combu
⇑ Corresponding author. Address: 1950 Fifth Street,Building 18A,
Wright-Patterson AFB, OH, USA. Fax:+1 937 656 4659.
E-mail address: [email protected] (T.M.Ombrello).
1. Introduction
Successfully igniting high-speed air-breathingcombustors, such
as supersonic combustionramjets, is a significant challenge because
of therestrictive reactive environment. While the limitedresidence
times in these combustors can be miti-gated through the use of
cavities and struts [1,2],
stion Institute.
http://dx.doi.org/10.1016/j.proci.2014.07.068mailto:[email protected]://dx.doi.org/10.1016/j.proci.2014.07.068http://crossmark.crossref.org/dialog/?doi=10.1016/j.proci.2014.07.068&domain=pdf
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Fig. 1. Schematic of cavity based flow path.
2102 T.M. Ombrello et al. / Proceedings of the Combustion
Institute 35 (2015) 2101–2108
the low pressures and temperatures in the super-sonic flow at
takeover flight speeds (Mach num-bers
-
T.M. Ombrello et al. / Proceedings of the Combustion Institute
35 (2015) 2101–2108 2103
flow), a step that dramatically improves imagequality.
Chemiluminescence (primarily from CH*
and C2*, with soot emission inhibited by highfuel–air mixing
rates) was collected from the samefield of view as the
shadowgraph.
A simple automotive style spark ignition sys-tem and plug was
used to provide a single10 mJ/pulse of energy for the spark
discharge.The PD was comprised of a 61-cm-long and1.03-cm-ID
stainless steel tube with compressionfittings. The fuel (C3H8) and
oxidizer (N2O) werechosen to produce detonations in the small
diam-eter tube and at the static pressure in the tunnel.The PD tube
was overfilled with the reactive mix-ture to ensure that a
detonation reached the cav-ity. The excess reactants that were
injected intothe cavity from overfilling prior to the
detonationemerging were found to not change the cavityignition
process. The reactive mixture in the PDwas ignited with the same
automotive spark dis-charge and each firing produced a
detonationwave with near Chapman–Jouguet conditions. Incontrast to
the spark discharge, the energyreleased from each detonation was
�100 J. Whilethe PD could be operated at greater than 10
Hzrepetition rate, single firings were used for the cav-ity
ignition study. More details are provided inRef. [10].
3. Results
3.1. Shadowgraph and chemiluminescence
The spark discharge and PD produced verydifferent cavity
ignition processes because of howthey deposit energy and interact
with the flow.The ignition process with a spark discharge
reliesheavily upon the cavity flowfield to spread the ini-tial
flame kernel and therefore is classified as apassive device. On the
other hand, a detonationis disruptive to the cavity flow but still
relies uponthe cavity dynamics to sustain the flame. Becauseof the
interaction of the ignition devices with theflow, the cavity
burning depended strongly onthe fueling condition. To best capture
the ignitionprocess with these two devices, shadowgraphyallowed for
interrogation of the density fluctua-tions and chemiluminescence
imaging provided apseudo marker of the heat release. While the
mea-surements were not taken simultaneously, goodrepeatability
between each ignition sequenceallowed for meaningful comparison
between theshadowgraph and chemiluminescence imaging.
A representative set of images from the high-frame-rate videos
for both ignition devices areshown in Fig. 2 with a fueling rate
from the slotupstream of the cavity (Qslot) of 144 slpm (stan-dard
liters/min) and 104 slpm from the holes inthe cavity closeout ramp
(Qcavity). Chemilumines-cence images are displayed in false-color
to
provide good contrast. For the spark discharge,the ignition
kernel is present in the shadowgraphimage by 0.01 ms based on the
observed densitygradients but is difficult to see with
chemilumines-cence because, presumably, the heat release is low.By
0.09 ms the ignition kernel had moved towardsthe front of the
cavity, following the recirculationpattern, but had grown little.
In stark contrast,when the PD was fired into the cavity under
thesame conditions, the cavity disruption was dra-matic. Within
0.01 ms, the detonation plume hadreached the shear layer because of
the high veloc-ity (�2000 m/s) exiting the tube. While the
deto-nation had significant momentum flux that filledthe cavity
rapidly, it was isolated to the cavityand downstream (i.e., little
upstream propaga-tion). Furthermore, the initial detonation
plumereached halfway across the core flow above thecavity by 0.09
ms.
Later in time, the distinct difference in the cav-ity ignition
process becomes clearer (Fig. 3).Within 1 ms, the ignition kernel
from the sparkdischarge had grown and moved to the front ofthe
cavity and was being entrained into the shearlayer. This allowed
for rapid circulation of thecombustion products towards the end of
the cav-ity and then down the ramp to ignite the fuel by 2–2.5 ms.
At that point, the cavity gases had com-pleted one cycle, and there
was significant heatrelease. In shadowgraph images, this was
shownby the thickening and movement of the shear layerthat created
a shock at the front edge of the cavity(starting at �1–1.5 ms). By
3 ms, the cavityappeared to be in a stable/steady state
burningprocess with a shear layer flame, as well as burn-ing within
the cavity. On the other hand for thePD, the strong detonation
plume had significantlydecayed by 1 ms, and there was the
appearance ofcavity burning and a strong shock at the frontedge of
the cavity. By 3 ms, the cavity burninghad decreased and the PD was
nearing the endof its exhaust process. This resulted in less
heatrelease in the cavity, as shown by the weakershock at the front
edge of the cavity. At this point,the large disruption from the
detonation had sig-nificantly affected the cavity cycling and
thereforechanged the burning process.
The cavity burning for the spark dischargechanged little between
3 and 4 ms (Figs. 3 and 4)and also did not change significantly
thereafter.Therefore, 3 ms after the spark discharge, the cav-ity
had settled into a quasi-stable burning processthat appeared to be
independent of the ignitionevent. This makes sense since the cavity
had cycleda couple of times (� 1–1.5 ms/cycle). For the PD,on the
other hand, the cavity burning steadilydecreased between 3 and 5 ms
and indeed appearednearly extinguished at 5 ms before achieving
quasi-stable burning at approximately 8 ms. While thecavity was
successfully ignited using both devices,the processes were vastly
different.
-
Fig. 2. Shadowgraph and chemiluminescence images of spark
discharge and PD cavity ignition for Qslot/Qcavity = 144/104 slpm
from 0.01 to 0.09 ms.
Fig. 3. Shadowgraph and chemiluminescence images of spark
discharge and PD cavity ignition for Qslot/Qcavity = 144/104 slpm
from 1 to 3 ms.
2104 T.M. Ombrello et al. / Proceedings of the Combustion
Institute 35 (2015) 2101–2108
In an attempt to go beyond simple qualitativeimages, the total
chemiluminescence from the cav-ity versus time was plotted. A
control volume ofthe cavity only was used for the measurement,and
Fig. 5 shows the results for the spark dis-charge and the PD with
Qslot/Qcavity = 0/66 slpm.The images were normalized to the peak
chemilu-minescence coming from the PD. The total emis-sion, and
hence heat release in the cavity,followed different paths for the
two ignitiondevices, but it converged to the same level by
approximately 8 ms. The heat release from sparkdischarge
ignition was gradual with a local peakat 4 ms, while the PD had
five distinct regimes(shown by the numbering in Fig. 5). Initially
therewas significant emission (regime I), from PDchemiluminescence
and soot incandescence, fol-lowed by a rapid decay and then a
diminishedand relatively constant level of emission (regimeII). The
duration of regime II was approximatelyone cavity cycle time that
led to a “cavity ignitiondelay time.” In regime III, there was a
rapid
-
Fig. 4. Chemiluminescence images of spark discharge and PD
cavity ignition for Qslot/Qcavity = 144/104 slpm from 4 to12
ms.
Fig. 5. Average integrated chemiluminescence in cavityduring
ignition versus time with Qslot/Qcavity = 0/66 slpmfor spark
discharge and PD.
Fig. 6. Average integrated chemiluminescence in cavityduring
ignition versus time for PD at different Qslot andQcavity.
T.M. Ombrello et al. / Proceedings of the Combustion Institute
35 (2015) 2101–2108 2105
increase in heat release from burning the fuel inthe cavity.
Once the fuel was consumed, therewas a long lull in integrated
emission of 3–4 ms(regime IV) that was needed to replenish a
com-bustible mixture. At that point (regime V), theheat release in
the cavity increased to a quasi-steady value that was independent
of the ignitionevent.
When different fuel flow rates in the cavity wereused, the
distinct burning regimes changed dramat-ically. In Fig. 6, the
result with Qslot/Qcavity =0/66 slpm was compared to that with
Qslot/Qcavity= 0/59 slpm, 0/136 slpm, and 126/38 slpm.
WithQslot/Qcavity = 0/59 slpm, regime I and II werepresent, but
regime III was not, and the cavityextinguished. When Qcavity was
increased to136 slpm or when Qcavity decreased to 38 slpmand Qslot
was 126 slpm, there was not as muchdecay in regime II, regime III
was much longer intime, and regime IV was suppressed. The
resultsindicated that sufficiently high cavity fueling(either
directly into the cavity or through entrain-ment via the shear
layer) could mitigate thedecrease in heat release caused by the
disruptionfrom the detonation. Furthermore, it was alsofound that
while the PD was disruptive to the cav-ity flow, the high level of
energy deposition allowed
for ignition across a wider range of T0 and P0 in theflowpath
when compared to the spark discharge.To wit: the PD was capable of
igniting the cavityat a T0 of 500 K, while the spark discharge
couldnot achieve ignition until 570 K for the same pres-sure and
fueling conditions.
The spark discharge also demonstrated similardecreased heat
release behavior when using bothlower and higher fueling rates. In
Fig. 7 with Qslot/Qcavity = 0/52 slpm, the heat release process
wasmore gradual, and there were two peaks beforethe cavity achieved
steady burning. The first localpeak in heat release came from the
first cycling ofthe cavity after ignition. This preconditioned
thecavity for the second heat release peak which wassignificantly
larger. The time between these peaksaligned with the 1–1.5 ms
cavity cycling time.Interestingly, when the Qcavity was decreased
to45 slpm and upstream fuel injection (Qslot = 115 -slpm) was used,
the same multi-peak behaviorwas present, but there was a lull in
emission afterthe second larger heat release peak. There was thena
gradual rise in heat release until steady cavityburning was
achieved. This cavity ignition behav-ior was similar to that with
the PD where therewas a peak in heat release from burning a
signifi-cant amount of fuel in the cavity followed by a lull
-
Fig. 7. Average integrated chemiluminescence in cavityduring
ignition versus time for spark discharge atdifferent Qslot and
Qcavity.
Fig. 8. Initial conditions for PD exhaust model.
2106 T.M. Ombrello et al. / Proceedings of the Combustion
Institute 35 (2015) 2101–2108
in heat release. While the exact nature of the sparkdischarge
ignition process was dependent on Qcav-ity, cavity flame extinction
was not observed for theentire range of Qcavity (42–110 slpm) and
Qslot (0–126 slpm) employed here.
3.2. Numerical simulations
With the limited set of measurements that werepossible to
examine the cavity ignition process,numerical simulations were
utilized. Here, thefocus was on the PD. Specifically, an
explanationof the decay in heat release after the initial
cavityignition process was sought. Fortunately, a previ-ous
investigation of the PD exhausting into a M-2cross-flow provided
validation through high-frame-rate shadowgraph and planar
laser-inducedfluorescence (of NO) [10]. For the simulations, 3-D
unsteady calculations using the CFD++ codewere used [11]. The
turbulence was modeled usingthe two-equation cubic j-e model, which
has non-linear terms that account for normal-stress anisot-ropy,
swirl, and streamline-curvature effects.Three turbulent Schmidt
numbers were employed,Sct = 0.5, 0.7, and 1.0, to bound the problem
andobserve any ignition/mixing differences. The deto-nation and
reaction inside the PD were not com-puted [10], but rather the
exhaust process wassimulated with the boundary conditions shownin
Fig. 8. This was reasonable since the majorityof the chemical
reactions from the PD were occur-ring inside the tube. In order to
replicate the prod-ucts of the detonation from the experiments,
thegas composition was derived from the Chap-man–Jouguet
calculations with the species N2,H2O, CO, CO2, O2, H2, O, H, and
OH. For thechemical reactions inside the cavity with C2H4,the TP2
reduced kinetic model was used and noturbulence-chemistry
interaction was taken intoaccount [12]. The computational domain
con-sisted of the full width and height of the test
section with a total of 12.4 million cells. Thenumerical
approach was divided into two steps.First a steady-state solution
was performed forthe supersonic flow path without the interactionof
the PD tube. Second, the PD flow field was ini-tiated at the cavity
floor, similar to a shock tubeproblem. At this time, the simulation
was per-formed in a time-accurate manner.
Initially the simulations were validated in thecavity geometry
in terms of capturing the timescales and mixing processes. A
comparison ofshadowgraph images (integrated) and density gra-dients
(synthetic shadowgraph in the plane of thePD) showed some variation
across the range ofSct (0.5–1.0). Specifically, the computation
withSct = 0.5 showed too much mixing, while that atSct = 1.0 showed
too little mixing (relative toexperimental observation), therefore
boundingthe problem. Nevertheless, all three Sct were stillused for
the ignition simulations to see the effecton reactivity. A Sct of
0.7 appeared to producethe best agreement with experiments (and
alsomatched the previous validation studies [10]). Fig-ure 9 shows
the comparison between the experi-ments and numerical simulations
with no fuelinjection. While the early time scales (first0.12 ms)
agreed well, there appeared to be dis-agreement at later times.
Specifically, by 2.8 msthe simulations had relaxed to steady state
whilethe experiments showed the PD still exhausting.In fact, the
simulations at 1.4 ms agreed with theexperiments at 2.8 ms. This
result emphasized adeficiency in the PD model developed in the
previ-ous study [10]: agreement between the simulationsand
experiment were sought only for the first0.4 ms where it was most
difficult to match the den-sity gradients and plume structure.
Consequently,the simulations predicted PD blow-down
timesapproximately half that of the experiments. Whilethe initial
conditions of pressure, temperature, andvelocity of the PD model
could be adjusted to cap-ture the overall timescale of the exhaust
processmore accurately, the initial exhaust time scalesand
structure were in good agreement, and there-fore the PD evolution
was deemed reasonable forthe current ignition simulations.
-
Fig. 9. Comparison of shadowgraph and density gradients from
simulations of cavity flow without fueling.
Fig. 10. Psuedo-integrated T, P, and YC2H4 in cavityversus time
from simulations with inset simulated imagesof YC2H4.
T.M. Ombrello et al. / Proceedings of the Combustion Institute
35 (2015) 2101–2108 2107
Since there was no direct methodology to com-pare the
chemiluminescence images to the simula-tions, the time dependence
of the T, P (staticvalues) and C2H4 mass fraction (YC2H4) were
usedto monitor the ignition process. These values wereextracted
from the numerical simulations inplanes at every 6.4 mm spanning
the cavity, pro-ducing 23 planes. At each time step, the T, P,and
YC2H4 were averaged to produce a pseudo-integrated view of the
cavity, to compare to thechemiluminescence images of PD ignition
inFigs. 5 and 6. For the case with Qslot/Qcavity = 0/66 slpm, there
was an initial rapid decay ofYC2H4 in the cavity. This aligned with
regime Iin Figs. 5 and 6. Here, the detonation forced a
sig-nificant amount of C2H4 out of the cavity (asshown by inset
image (b) above the plot inFig. 10), and T and P increased because
of the ele-vated values within the detonation plume. Inregime II,
there were relatively constant valuesof T, P, and YC2H4 in the
cavity. This could beassociated with a “cavity ignition delay time”
tocycle hot detonation products and ignite the fuelthat remained in
the cavity. The fuel then startedto burn rapidly in regime III with
a decrease inYC2H4 and increase in T while maintaining a con-stant
P. The rapid burning in regime III is shownby the deficiency of
C2H4 in image (c) in Fig. 10and also by the spike in heat release
shown bythe increased chemiluminescence in Fig. 5. It isimportant
to note that up to this point the cavityflow had been significantly
disrupted by the deto-nation, specifically by elevated P from the
detona-tion. While this was beneficial to reaction rates toenhance
ignition, it was also detrimental becauseit decreased and, at
times, stopped the ramp fuel-ing, since the fuel injection from the
closeout rampwas not choked. The simulations were, of
course,designed to replicate the experimental fuel injec-tion
set-up by defining the fuel mass flow rateand temperature
approximately 5 cm upstreamof the exit of the fuel injection into
the cavity. Thislocation was approximately where the fuel plenumwas
located, and therefore allowed for pressurevariations in the cavity
to change Qcavity at the
ramp face. Indeed, the approximate 15% increasein the average
cavity P from the detonation signif-icantly decreased Qcavity. In
regimes I, II, and III,Qcavity was reduced, the cavity cycling and
flowwas significantly disrupted, and the detonationwas able to
ignite and burn most of the fuel withinthe cavity. This led to
decreased heat release inregime IV because most of the fuel and air
wereconsumed and there was not sufficient time toreplenish a
reactive mixture. Simultaneously, thePD was at the end of its
exhaust cycle, and thusP began to decrease in the cavity. This in
turn(i) allowed for more fuel to flow into the cavityas shown in
regime IV of Fig. 10 and after a per-iod of time (ii) increased
heat release and chemilu-minescence as shown in Fig. 5. After a
fewmilliseconds, the cavity flow was able to cycle afew times and
regain its normal steady state burn-ing. Image (d) in Fig. 10 shows
this behavior witha distribution of fuel throughout the cavity
front,along the floor, and near the leading edge of theshear layer.
As noted above, the time scales dif-fered by a factor of �2, due to
the PD behaviors(simulated vs. observed). Nevertheless, the
overall
-
2108 T.M. Ombrello et al. / Proceedings of the Combustion
Institute 35 (2015) 2101–2108
trends were correctly captured and gave insightinto mechanisms
for the observed ignitionregimes. The high-pressure of the
detonation withseveral milliseconds of duration disrupted the
cav-ity flow and the non-choked fuel injection led toless fuel.
4. Summary and conclusions
Cavity ignition in a supersonic flow wasachieved using two
vastly different forms of energydeposition devices that caused
different levels offlow disruption. Spark discharge ignition was
amore passive process, following the flow to achievesteady cavity
burning. Conversely, PD ignitionwas extremely disruptive to the
cavity flow dynam-ics with its high-pressure and -temperature
plume.The disruption was found to be advantageousunder certain
conditions (such as at lower T0 andP0), but was also seen to be
somewhat detrimentalbecause of decreased cavity burning and
nearextinction from the flow field interaction. High-frame-rate
shadowgraph and chemiluminescenceimaging allowed for an
unprecedented look atthe transient ignition process and provided
ameans for comparison to simulations. While thesimple model
developed of the PD exhaust processpredicted shorter time scales,
it correctly capturedall burning regimes of the ignition
processobserved in the experiments. Most importantly,it provided
insight to the mechanism of decreasedheat release because of the
high-pressure PDexhaust momentarily diminishing fuel flow to
thecavity. Therefore, a smaller PD with shorterblow-down time that
produces less pressure risein the cavity, as well choked fuel
injection to thecavity has the potential to mitigate any
decrease
in burning and therefore a greater chance for suc-cessful
ignition over a wide range of conditions.
Acknowledgements
This work was partially supported by the AirForce Office of
Scientific Research under Dr. Chi-ping Li.
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Cavity ignition in supersonic flow by spark discharge and pulse
detonation1 Introduction2 Experimental setup3 Results3.1
Shadowgraph and chemiluminescence3.2 Numerical simulations
4 Summary and conclusionsAcknowledgementsReferences