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Deflagration-to-detonation transition via the distributed photo ignition

of carbon nanotubes suspended in fuel/oxidizer mixtures

Daniel J. Finigan, Brian D. Dohm, Jeffrey A. Mockelman, Matthew A. Oehlschlaeger

Department of Mechanical, Aerospace, and Nuclear Engineering, Rensselaer Polytechnic Institute, Troy, NY, USA

a r t i c l e i n f o

Article history:Received 9 June 2011

Received in revised form 18 August 2011

Accepted 27 September 2011

Available online 20 October 2011

Keywords:

Deflagration-to-detonation transition

Detonation tube

Ignition

Photo ignition

Nanoparticles

a b s t r a c t

Here the promotion of flame acceleration and deflagration-to-detonation transition (DDT) using the dis-tributed photo ignition of photo-sensitive nanomaterials suspended in fuel/oxidizer mixtures is demon-

strated for the first time. Distributed photo ignition was carried out by suspending single-walled carbon

nanotubes (SWCNTs) with Fe impurity in quiescent C2H4/O2/N2 mixtures and flashing them with an

ordinary Xe camera flash. Following the flash, the distributed SWCNTs photo ignite and subsequently

provide a quasi-distributed ignition of the C2H4/O2/N2 mixture. In a closed detonation tube the quasi-dis-

tributed photo ignition at one end of the tube leads to the promotion of flame acceleration and DDT and,

for sensitive C2H4/O2 mixtures, appears to lead to direct detonation initiation or multiple combustion

fronts. The DDT run-up distance, the distance required for the transition to detonation, was measured

using ionization sensors and was found to be approximately a factor of 1.5 to 2 shorter for the distrib-

uted photo ignition process than for traditional single-point spark ignition. It is hypothesized that the

increased volumetric energy release rate resulting from distributed photo-ignition enhances DDT due

to the decreased ignition delay and greater early-time flame area and turbulence levels, which in turn

result in accelerated formation and amplification of the leading shock and accelerated DDT.

2011 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

1. Introduction

Flame acceleration and deflagration-to-detonation transition

(DDT) have been the subject of numerous studies, see pertinent

reviews [14], motivated by the extreme danger undesired detona-

tion can pose to industrial processes involving combustible gases

andthe potentialfor utilizingthe rapidenergyreleaseand highover-

pressures resulting from detonation for high-efficiency high-speed

propulsion cycles [57] (e.g., pulse detonation engines PDEs).

Due to the unsteady nature of propulsion cycles reliant on detona-

tion, the time and length scales associated with the ignition and

formation of a detonation wave in confined geometries is critical

to the performance of detonation engines [8].A detonation wave can be directly initiated using a high-energy

source or by flame acceleration resulting in deflagration-to-detona-

tion transition (DDT),wherethe flame is initiatedusinga traditional

low-energy ignition source(e.g., spark). Because of thevery high en-

ergy requirementsfor thedirect initiation of a detonation in gaseous

fuel/air mixtures (order of kilojoules) [9], DDT is the most practical

means by which to generate a detonation in a propulsion engine.

Following a localized deposition of energy (e.g., spark), DDT occurs

through several flame acceleration steps. First a laminar flameforms

from the ignition kernel and quickly becomes wrinkled due to the

LandauDarrieus instability, intrinsic to freely expanding flames

[3]. The wrinkled flame develops into a fully turbulent flame brush

which accelerates with increasing levels of turbulence and corre-

sponding growth in flame surface area [3]. As the turbulent flame

brush accelerates, compression waves are generated ahead of the

flame, which coalesce into a leading shock wave [3]. Finally the

accelerating flame transitions into a detonationwave. The finaltran-

sitionfrom a high-speed turbulentflame/shockfront to a detonation

is thought to involve a localizedexplosion somewhere in or ahead of

theturbulent flamebrushor inthe boundarylayer, dueto theattain-

ment of autoignition conditions, and the establishment of an induc-

tion-time gradient enabling the SWACER (shock wave amplificationby coherent energy release) mechanism originally proposed by Zel-

dovich [3,10,11]. Many studies suggest that the localized explosion

occurs within a quenched volume of reactants within the turbulent

flame brush or in the boundary layer [2].

The performanceof pulse detonation engines (PDEs), where DDT

is the means of detonation initiation, is dependent on the requisite

time and length scales for DDT run-up, due to the requirement of

sufficientengine lengthfor DDT run-upand thelimitations thetime

required for DDT run-up place on PDEcycle frequency. Many efforts

have been made to promote flame accelerationand DDTthroughthe

use of obstacles to induce turbulent fluctuations in the unburned

gas ahead of the accelerating flame and thereby increase the flame

0010-2180/$ - see front matter 2011 The Combustion Institute. Published by Elsevier Inc. All rights reserved.doi:10.1016/j.combustflame.2011.09.017

Corresponding author. Address: 110 8th St., JEC 2049, Troy, NY 12065, USA.

E-mail address: [email protected] (M.A. Oehlschlaeger).

Combustion and Flame 159 (2012) 13141320

Contents lists available at SciVerse ScienceDirect

Combustion and Flame

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c o m b u s t fl a m e
http://dx.doi.org/10.1016/j.combustflame.2011.09.017mailto:[email protected]://dx.doi.org/10.1016/j.combustflame.2011.09.017http://www.sciencedirect.com/science/journal/00102180http://www.elsevier.com/locate/combustflamehttp://www.elsevier.com/locate/combustflamehttp://www.sciencedirect.com/science/journal/00102180http://dx.doi.org/10.1016/j.combustflame.2011.09.017mailto:[email protected]://dx.doi.org/10.1016/j.combustflame.2011.09.017

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area andincreasethe rate of flame acceleration. Studies using a wide

range of obstacle geometries have been reported, including helical

spirals, orifice plates, dimples, baffles, and swept ramps [1215].

Of course the use of obstacles to promote DDT results in a drag pen-

alty to engine thrust.

Other non-fluid mechanic DDT promotion efforts include the

addition of sensitive fuel/O2 mixtures at the location of ignition to

start a detonation that then propagates into a fuel/air mixture[16]. Multipoint or distributed ignition sources that promote great-

er levelsof initial volumetric heat release and/or shock wave ampli-

fication have also been investigated for DDTpromotion. Frolovet al.

[17] proposed a concept for promoting DDT based on triggering 11

electric sparks, spaced down the length of a detonation tube,

sequentially to amplify the leading shock wave developed from

the coalescence of compression waves emanating from the acceler-

ating flame. Their system, although effective, required a high volt-

age source (2500 kV) and a total spark energy deposition of

1.68 MJ/m2. Ciccarelli et al. [18] used four circumferentially-spaced

spark igniters to initiate multiple ignition kernels at the head end of

a detonation tube and demonstrated 30% reductions in DDT run-up

distance using the four-point ignition method in conjunction with

orifice plate obstacles. Wang et al.[19] and Zhukov and Starikovskii

[20] have demonstrated ignition and DDT promotion using a high-

voltage nanosecond transient plasma ignition source (corona

discharge) that creates several high-energy ignition kernels around

the plasma zone. Wang et al. [19] demonstrated reductions in igni-

tion delay and rise times of a factor of two to three for the distrib-

uted transient plasma ignition when compared to single sparks in

a flowing PDE. The literature contains various mechanistic explana-

tions for the influence of ignition distribution on flame acceleration

and DDT [17,18,20], it can be generalized that the increased rate of

volumetric energy release due to the distribution of ignition sites

results in greater early-time flame surface area, which increases

the rate of turbulence generation and flame acceleration leading

to amplification of the compression from the accelerating turbulent

flame brush, and, hence, results in accelerated leading shock forma-

tion and DDT.

Recently, Berkowitz and Oehlschlaeger [21] investigated a dis-

tributedignitionmethod for the quasi-homogenous ignition of com-

bustible gaseous mixtures using the photo ignition of single-walled

carbon nanotubes (SWCNTs) containing Fe impurity suspended in

fuel/air mixtures. The photo-ignition of SWCNTs containing metal

impurities was first discovered by Ajayan et al. [22]. In subsequentstudies it was determined that the photo ignition phenomenon is

dependent on the presence of metal nanoparticle impurities in the

SWCNTs and the SWCNTs simply act to stabilize the naturally pyro-

phoric metal nanoparticles [2325]. It was also shown that the

photo ignition of these Fe-containing nanomaterials results in peak

temperatures in excess of 1500 C based on nanoscale characteriza-

tion of the products [24,25]. The exposure of the photo-sensitive

nanomaterials to a low-energy Xe camera flash, when suspended

in combustible fuel/oxidizer mixtures, results in the rapid heating

and oxidation of the nanomaterials followed by the ignition of the

fuel/oxidizer mixture. Because the nanomaterials can be distributed

throughout any given volume and exposed to a spatially-diffusive

light source, the ignition of the fuel/oxidizer mixture can be highly

distributed. Berkowitz and Oehlschlaeger demonstrated the quasi-

homogenous ignition of ethylene/air mixtures through high-speed

camera images (see Fig. 1), which show the luminosity from the

photo-ignitingnanomaterials, inlm sizedclumps, andfromthe vol-umetric combustion of the ethylene/air. Experiments in a closed

combustion chamber also demonstrated reductions in ignition

delay and rise times by up to a factor of two when compared to sin-

gle-point spark ignition [21]. In other studies, the photo ignition of

SWCNTs has been demonstrated for combustion applications by

Chehroudi and Danczyk for the ignition of single fuel droplets in

air [26] andManaa et al.[27] for the ignition of solidexplosives. Che-

hroudi and Danczyk also patented the concept of using the photo

ignition of carbon nanotubes in distributed ignition applications

[28,29]. Distributed photo ignition has potential in combustion

Fig. 1. Images of the distributed photo ignition of a stoichiometric C 2H4/air mixture at 1 bar containing suspended single-walled carbon nanotubes with Fe impurity (70% byweight), from Berkowitz and Oehlschlaeger [21].

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applications where a degree of distribution and/or control over the

location and timing of ignition are desired. Here we demonstrate

that thedistributed photo ignition of SWCNTs with Fe impurity sus-

pended in gaseous fuel/oxidizer mixtures can be used to promote

flame acceleration and DDT in a confined geometry.

2. Experimental method

Experiments were performedin a closeddetonation tube to com-

paratively study DDT resulting from the distributed photo ignition

of carbon nanotubes with Fe impurity and a traditional single-point

spark ignition. A schematic of the detonation tube is shown inFig. 2.

The carbon steel tube is 1 m long with a 7.62 cm (3 in.) inner diam-

eter and closed on both ends. The tube inner diameter is constant

with average tube bore roughness ofRa = 0.41.6 lm. For the mea-surement of combustion wavetrajectories to determineDDT run-up

distances, 12 ion sensors were axially spaced every tube diameter

(7.62 cm, 3 in.) along the inside of the detonation tube side wall.

The ion sensors were miniature spark plugs (Rimfire Mini Viper

Z2, 0.5 mm spark gap, 5 mm outer thread diameter) supplied with

9 V from an alkaline battery and connected in series to a computer-

ized data acquisition system (1 MHz National Instruments system,automated LabView software interface). Upon passage of the com-

bustion wave at each ion sensor location, the voltage monitored

by the data acquisition system sharply drops from the open circuit

value ($9 V) due to the completion of the circuit at the ion sensor

spark gap by the partially-ionized gases present in the combustion

products; typical experimental ion sensor signals are shown in

Fig. 3. Although the detonation tube was outfitted with 12 sensors,

only 7 were connected during a given experiment, due to the chan-

nel limitation of the data acquisition system. Sensors were strategi-

cally selected for each experiment to capture the DDT run-up

distance.

Measurements of DDT run-up distance were carried out for

both the spark and photo ignition of quiescent stoichiometric

C2H4/O2/N2 mixtures with three levels of N2 dilution (0%, 20%,and 40%), at initial pressures ranging from 25 to 170 kPa, and an

initial temperature of 297 2 K. Mixture compositions and initial

pressures were chosen such that DDT would occur within the

1 m tube length. The C2H4/O2/N2 mixtures were made in a holding

tank via partial pressures and allowed to diffusively mix for 24 h

before use. Prior to experiments the detonation tube was evacu-

ated to 5 103 Torr and filled with the C2H4/O2/N2 mixture to

the desired pressure specified with a 1000 Torr Baratron MKS

manometer.

Combustion was initiated at the head end of the detonation tube

by either an automotive spark plug located in the tube end wall

(modified MSD 6A capacitive discharge ignition controller, Cham-

pion model QL82YC spark plug, 1 mm gap, single 105 mJ sparks)

or by the photo ignition of suspended nanomaterials. Photo ignition

was achieved by injecting 2 mg of single-wall carbon nanotubes

(SWCNTs) containing 70% Fe impurity by weightthrough a diffusive

air-blast style injector located in the tube side wall 3 cm from the

end wall and exposing the nanomaterials to a Xe flash. The as-pro-duced SWCNTs with Fe impurity (no purification) were synthesized

by Nano-C in a pre-mixed combustion process where the introduc-

tion of a Fe catalyst precursor allows for the SWCNT growth. The

resulting Fe impurity is specified by Nano-C as 70% by weight

which is approximately 10% Fe by volume. The air-blast injection

resulted in a distributed suspension of SWCNT-Fe clumps through-

out approximately the first 6 cm of the detonation tube. The unifor-

mity of the suspension could not be quantified due to insufficient

optical access to the ignition zone, where high pressures are

achieved, but we have previously shown that quasi-homogenous

photo ignition of gaseous fuel/oxidizer can be achieved within a

spherical volume with a diameter of approximately 34 cm using

this relatively crude particle injection method [21]. The injected

nanomaterials are suspended in aggregate clumps ofl

m dimen-

sions due to adhesion and entanglement of the flame synthesized

SWCNTs. The C2H4/O2/N2 mixtures were used for air-blast injec-

tion to ensure homogeneity of the gas mixture. Following injection,

the suspended SWCNTs were exposed to a Xe camera flash

($300 mJ of visible light, $1 ms flash duration), housed within

the tube and located at the head end, resulting in a quasi-distrib-

uted ignition phenomenon, as illustrated in Fig. 1. Both the spark

and photo ignition hardware and experimental procedures used

in this study were identical to those previously detailed by Berko-

witz and Oehlschlaeger [21].

Following the initiation of combustion at the head end of the

tube by either spark or photo ignition, the acceleration of the com-

bustion wave from high-speed flame to detonation was monitored

using the ion sensors (Fig. 3). The distance versus time trajectories

provided by the ion sensors can be converted into wave velocity

versus time, of course the resulting wave velocity is averaged over

the sensor axial spacing.Example wave trajectoryand wavevelocity

Fig. 2. Detonation tube experimental setup.

Fig. 3. Example raw ion sensor signals for the photo ignition of a 30.3 kPa

stoichiometric C2H4/O2 mixture. S1S7 labels indicate the ion sensor signals for

sensors located sequentially from the ignition end of the detonation tube. In this

case DDT occurred prior to sensor 3.

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measurements are shown in Fig. 4 for both spark and photo ignition

experiments. In all experiments a sharp rise in wave velocity corre-

sponding to DDT was observed at some axial location in the tube. At

the axial location of DDT the transition in measured wave velocity

was typically from 5001000 m/s to greater than 2000 m/s within

one sensor spacing or tube diameter (7.62 cm). The ChapmanJoug-

uet (CJ) detonation velocities for the studied mixtures and initial

conditions are 21402360 m/s, per calculations carried out using

the STANJAN thermochemical equilibrium routine [30]. The DDT

run-up distance was defined as the axial location, from the ignition

end of the tube, where the measured combustion wave velocity

equaled or surpassed the CJ detonation velocity. Because velocity

measurements (e.g., Fig. 4) are reported at the mid-point between

sensor locations, the DDT run-up distance is defined as the axial

location of the upstream sensor for the first sensor pair where the

measured wave velocity equaled or surpassed the CJ velocity. Using

the measured wave velocity profiles, DDT run-up distances were

measured for C2H4/O2/N2 mixtures, selected because they provided

DDT within the 1 m tube length.

3. Results and discussion

Wave velocity measurements and corresponding DDT run-up

distances for spark and photo ignition experiments are shown in

Figs. 5 and 6, respectively; the measured DDT run-up distances

are also given in Tables 1 and 2 and all measured velocity profilesand calculated CJ detonation velocities are given in Table 3.

Measurements of both wave velocity and DDT run-up distances

were highly reproducible, as illustrated by the overlapping data

for repeated conditions in Fig. 6 and Tables 13. The high level of

apparent reproducibility is partly due to the 7.62 cm limit in reso-

lution of DDT run-up distance imposed by the ion sensor spacing.

The measurements show a reduction of DDT run-up distance with

increasing pressure and an increase in DDT run-up distance with

increasing N2 dilution, consistent with trends found in the litera-

ture [31,32].

The results shown in Figs. 5 and 6 also illustrate shorter DDT

run-up distances for distributed photo ignition compared to

single-point spark ignition. The measured reductions in DDT

run-up distance are approximately a factor of 1.5 to 2 for photoignition, with respect to spark ignition. Greater relative reductions

for photo ignition were generally observed for longer DDT

distances, occurring at lower initial pressures and greater N2 dilu-

tion. The magnitude of reduction in DDT run-up distance is consis-

tent with the factor of 2

reduction in ignition delay and ignitionrise times reported by Berkowitz and Oehlschlaeger for the photo

Fig. 5. Velocitydistance measurements for the spark and photo ignition of three

stoichiometric C2H4/O2/N2 mixtures (0%, 20%, and 40% N2) at varying initial

pressure.

Fig. 4. Example wave trajectories (top) and wave velocity profiles (bottom) for the

spark and photo ignition of stoichiometric C2H4/O2 mixtures at 30.5 and 30.3 kPa,

respectively.


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ignition of stoichiometric C2H4/air mixtures in comparison to sparkignition [21].

We hypothesize that the promotion of DDT observed for photo

ignition occurs because the early-time heat release resulting from

photo ignition is distributed volumetrically and therefore is greater

in magnitude than that from a single-point spark ignition [21]. This

presumably results in larger early-time flame area, increased insta-

bility and wrinkling of the early flame, faster transition to turbu-

lence, and higher rates of turbulent flame acceleration, all of

which will lead to faster leading shock formation, increased shock

amplification, and accelerated DDT. This hypothesis is in concert

with the experimental observations of Sinibaldi et al. [33] who

showed that the location of ignition in a tube influences the

early-time flame area which correlates with flame acceleration

and DDT run-up distance.

In addition to the observed reduction in DDT run-up distance,

the measured wave velocity profiles for the highest pressure photo

ignition cases for 0% and 20% N2 dilution show what appear to be

extremely overdriven detonation waves at the ignition end of the

detonation tube. In these cases the magnitude of the measured

wave velocities are far greater than that measured for any of the

DDT cases; in some experiments the measured detonation veloci-

ties at DDT were in excess of 2500 m/s but not greater than

3000 m/s. In the case of experiments performed for stoichiometric

C2H4/O2 at 50 kPa and 70 kPa, the wave velocities measured at the

first sensor pair (sensors located 7.62 and 15.24 cm from the

ignition end wall) were in the range of 60007000 m/s, followed

by decay within one ion senor location (one tube diameter) to a

velocity near that of a CJ detonation. These extremely high wavevelocities might indicate that multiple combustion fronts have

been formed by the distributed photo ignition and/or that a deto-

nation wave may have been directly initiated by the distributed

photo ignition. Berkowitz and Oehlschlaeger [21] showed that

the photo ignition event can be quasi-homogenous, which under

highly-sensitive fuel/O2 conditions could lead to quasi-volumetric

explosion at the ignition end of the tube, directly following the

flash, resulting in direct initiation of a detonation due to the high

energy release. Matsui and Lee [9] reported a critical energy of

around 100 mJ for the direct single-point initiation of a detonation

Fig. 6. DDT run-up distance as a function of initial pressure for both the spark and

photo ignition of three stoichiometric C2H4/O2/N2 mixtures (0%, 20%, and 40% N2).

Calculated ChapmanJouguet (CJ) detonation velocities also illustrated.

Table 1

Measured DDT run-up distances for the spark ignition of the three stoichiometric C 2H4/O2/N2 mixtures studied.

Spark, 0% N2 Spark, 20% N2 Spark, 40% N2

P (kPa) DDT distance (m) P (kPa) DDT distance (m) P (kPa) DDT distance (m)

25.0 0.762 50.1 0.8382 130.2 0.8382

25.0 0.762 50.1 0.8382 132.4 0.8382

29.6 0.381 50.6 0.8382 148.9 0.6858

30.1 0.381 69.9 0.4572 149.6 0.6858

30.5 0.381 70.1 0.4572 169.6 0.4572

39.9 0.2286 84.9 0.3048

39.9 0.2286 85.0 0.3048

40.0 0.2286 101.4 0.0762

49.9 0.1524 101.7 0.0762

50.1 0.1524

50.1 0.1524

70.0 0.1524

70.0 0.1524

70.1 0.1524

Table 2

Measured DDT run-up distances for the photo ignition of the three stoichiometric

C2H4/O2/N2 mixtures studied.

Photo, 0% N2 Photo, 20% N2 Photo, 40% N2

P(kPa) DDT distance (m) P(kPa) DDT distance (m) P(kPa) DDT distance (m)

24.9 0.4572 50.0 0.4572 129.6 0.6858

25.1 0.4572 50.1 0.4572 148.9 0.381

30.1 0.2286 50.2 0.4572 150.3 0.381

30.3 0.2286 70.0 0.3048 169.6 0.3048

40.1 0.1524 70.1 0.3048

40.1 0.1524 84.9 0.1524

40.3 0.1524 85.0 0.1524

50.2 0.0762 101.5 0.0762

50.2 0.0762 101.6 0.0762

50.3 0.0762

70.0 0.0762

70.1 0.0762

70.1 0.0762


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in stoichiometric C2H4/O2 at an initial pressure of 1 atm. While the

105 mJ spark does not provide direct initiation for stoichiometric

C2H4/O2 mixtures at initial pressures of 50 kPa or 70 kPa, the

exposure of the suspended nanomaterials to the Xe flash (approx-

imately 300 mJ of visible optical energy with a 1 ms flash duration)

results in unique initial velocity profiles, perhaps indicative of

direct detonation initiation resulting from a volumetric explosion.Further investigation of the dynamics of the photo ignition and

detonation formation phenomena, through high-speed optical

imaging and multiple dynamic pressure measurements, is needed

to determine if direct detonation initiation is possible and further

understand flame acceleration and DDT from distributed photo

ignition for cases where it is not.

The two-fold reduction in DDT run-up distances demonstrated

here using the distributed photo ignition of suspended nanomate-rials in gaseous fuel/oxidizer mixtures are similar in magnitude to

Table 3

Combustion wave velocity profile measurements and calculated ChapmanJouguet (CJ) detonation velocities. All experiments performed at an initial temperature of 297 2 K.

Spark ignition (left) Flash ignition (right)

P (kPa) CJ vel. (m/s) Axial location (cm) (sensor mid-points) P (kPa) CJ vel. (m/s) Axial location (cm) (sensor mid-points)

Wave velocity (m/s) Wave velocity (m/s)

/ = 1.0 C2H4/O2, 0% N2 / = 1.0 C2H4/O2, 0% N225.0 2307 49.53 57.15 64.77 72.39 80.01 87.63 24.9 2307 26.67 34.29 41.91 49.53 57.15 64.77

560 680 635 896 2822 2822 1058 828 918 2628 2458 2309

25.0 2307 49.53 57.15 64.77 72.39 80.01 87.63 25.1 2308 26.67 34.29 41.91 49.53 57.15 64.77

491 635 615 866 2309 2822 680 712 662 2309 2930 2627

29.6 2315 11.43 19.05 26.67 34.29 41.91 49.53 30.1 2316 11.43 19.05 26.67 34.29 41.91 49.53

377 432 501 552 2721 2930 680 686 2721 2822 2309 2309

30.1 2316 11.43 19.05 26.67 34.29 41.91 49.53 30.3 2316 11.43 19.05 26.67 34.29 41.91 49.53

334 403 582 692 2241 2822 630 837 2721 2721 2309 2309

30.5 2317 11.43 19.05 26.67 34.29 41.91 49.53 40.1 2330 11.43 19.05 26.67 34.29 41.91 49.53

416 448 591 770 2309 2540 965 2721 2381 2381 2309 2309

39.9 2329 11.43 19.05 26.67 34.29 41.91 49.53 40.1 2330 11.43 19.05 26.67 34.29 41.91 49.53

401 686 2241 2381 2309 2241 865 2309 2381 2309 2309 2309

39.9 2329 11.43 19.05 26.67 34.29 41.91 49.53 40.3 2330 11.43 19.05 26.67 34.29 41.91 49.53

336 526 2241 2621 2309 2241 940 2721 2309 2309 2309 2309

40.0 2330 11.43 19.05 26.67 34.29 41.91 49.53 50.2 2340 11.43 19.05 26.67 34.29 41.91 49.53

433 646 2241 2381 2241 2241 5861 2721 2458 2381 2381 2381

49.9 2340 11.43 19.05 26.67 34.29 41.91 49.53 50.2 2340 11.43 19.05 26.67 34.29 41.91 49.53

540 2931 2540 2931 2005 2309 5080 2540 2381 2309 2381 2381

50.1 2340 11.43 19.05 26.67 34.29 41.91 49.53 50.3 2340 11.43 19.05 26.67 34.29 41.91 49.53

595 2822 2458 2381 2241 2309 4233 2721 2540 2241 2458 238150.1 2340 11.43 19.05 26.67 34.29 41.91 49.53 70.0 2356 11.43 19.05 26.67 34.29 41.91 49.53

629 2822 2458 2241 2458 2241 4010 2540 2381 2381 2381 2309

70.0 2356 11.43 19.05 26.67 34.29 41.91 49.53 70.1 2356 11.43 19.05 26.67 34.29 41.91 49.53

940 2721 2458 2458 2309 2381 6927 2930 2381 2005 3048 2309

70.0 2356 11.43 19.05 26.67 34.29 41.91 49.53 70.1 2356 11.43 19.05 26.67 34.29 41.91 49.53

802 2540 2309 2381 2458 2381 4010 2628 2381 2381 2381 2381

70.1 2356 11.43 19.05 26.67 34.29 41.91 49.53

953 2540 2381 2627 2117 2381

/ = 1.0 C2H4/O2/N2, 20% N2 / = 1.0 C2H4/O2/N2, 20% N250.1 2230 49.53 57.15 64.77 72.39 80.01 87.63 50.0 2330 41.91 49.53 57.15 64.77 72.39 80.01

569 582 866 726 819 2117 856 2059 2822 2381 2241 2309

50.1 2230 49.53 57.15 64.77 72.39 80.01 87.63 50.1 2230 41.91 49.53 57.15 64.77 72.39 80.01

485 488 615 668 693 2721 856 2721 2381 2241 2309 2241

50.6 2231 49.53 57.15 64.77 72.39 80.01 87.63 50.2 2230 34.29 41.91 49.53 57.15 64.77 72.39

501 610 712 762 712 2822 786 847 2177 2721 2309 2241

69.9 2244 34.29 41.91 49.53 57.15 64.77 72.39 70.0 2244 11.43 19.05 26.67 34.29 41.91 49.53

640 907 2721 2540 2381 2177 907 674 819 2241 2721 2381

70.1 2244 34.29 41.91 49.53 57.15 64.77 72.39 70.1 2244 11.43 19.05 26.67 34.29 41.91 49.53

651 620 2540 2721 2458 2177 540 573 876 2241 2822 2381

84.9 2252 19.05 26.67 34.29 41.91 49.53 57.15 84.9 2252 11.43 19.05 26.67 34.29 41.91 49.53

610 605 2381 2721 2309 2309 828 2458 2540 2241 2241 2241

85.0 2252 11.43 19.05 26.67 34.29 41.91 49.53 85.0 2252 11.43 19.05 26.67 34.29 41.91 49.53

615 540 699 2628 2628 2458 886 2309 2540 2540 2309 2241

101.4 2259 11.43 19.05 26.67 34.29 41.91 49.53 101.5 2259 11.43 19.05 26.67 34.29 41.91 49.53

3436 2822 2458 2241 2309 2241 3464 2540 2628 2381 2241 2309

101.7 2260 11.43 19.05 26.67 34.29 41.91 49.53 101.6 2260 11.43 19.05 26.67 34.29 41.91 49.53

3313 2458 2627 2309 2309 2309 4763 3175 2309 2540 2309 2241

u = 1.0 C2H4/O2/N2, 40% N2 u = 1.0 C2H4/O2/N2, 40% N2130.2 2142 49.53 57.15 64.77 72.39 80.01 87.63 129.6 2141 49.53 57.15 64.77 72.39 80.01 87.63

540 560 646 699 747 2458 635 625 591 2177 2721 2627

132.4 2143 49.53 57.15 64.77 72.39 80.01 87.63 148.9 2147 34.29 41.91 49.53 57.15 64.77 72.39

778 467 581 718 645 2381 929 2241 2721 2381 2177 2116

148.9 2147 49.53 57.15 64.77 72.39 80.01 87.63 150.3 2148 26.67 34.29 41.91 49.53 57.15 64.77

482 657 867 2540 2628 2540 640 740 2381 2721 2309 2241

149.6 2147 49.53 57.15 64.77 72.39 80.01 87.63 169.6 2152 11.43 19.05 26.67 34.29 41.91 49.53

548 552 657 2117 2381 2458 540 600 556 2259 2721 2628

169.6 2152 19.05 26.67 34.29 41.91 49.53 57.15

582 548 504 595 2822 2381


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those realized using distributed transient plasma ignition [19]. The

photo ignition method has advantages for detonation engines, in

that it provides DDT promotion with a very low optical power

([22] reported photo ignition with as little as 100 mW/cm2 flash

power, here we used a Xe flash with 300 mJ of visible energy and

a duration of 1 ms) and therefore could be implemented with sim-

ple light sources (e.g., flash lamp, light-emitting diode, diode laser)

requiring low-mass low-voltage power supplies. However, compli-cating its application to detonation engines, the photo ignition

method utilizes photo-sensitive nanomaterials that would have

to be distributed into either the oxidizer or fuel stream.

Further research is needed to evaluate the photo ignition meth-

od demonstrated here for DDT promotion and ignition in general.

For this study, exposing SWCNTs with Fe impurity with to the flash

from a Xe camera flash was implemented simply because it was

known to produce the photo ignition phenomenon. However, other

metal and carbon-based nanomaterials are also known to exhibit

photo ignition (e.g., graphene oxide [34]) and initial experimental

studies performed in our laboratory suggest that nanomaterial

selection is important for optimizing ignition, flame acceleration,

and DDT promotion. Similarly, the optimization of the light source

has yet to be considered and could provide further gains. Impor-

tantly, the photo ignition demonstrations presented here were car-

ried out for sensitive C2H4/O2/N2 mixtures. These studies need to

be extended to fuel/air conditions where the photo ignition tech-

nique may need to be combined with other fluid mechanic means

of promoting DDT (e.g., orifice plates, helical spirals, ramps) for suf-

ficiently short DDT run-up distances for engine applications.

4. Summary

The promotion of deflagration-to-detonation transition (DDT)

using the distributed photo ignition of photo-sensitive nanomate-

rials suspended in fuel/oxidizer mixtures has been demonstrated

for the first time. Single-wall carbon nanotubes (SWCNTs) with

70% Fe impurity by weight were suspended at one end of a closed

detonation tube filled with C2H4/O2/N2 mixtures. The SWCNTs

were exposed to a Xe camera flash causing them to photo ignite

and subsequently produce a volumetrically distributed ignition of

the C2H4/O2/N2 mixture. The distributed photo ignition leads to

enhanced flame acceleration and deflagration-to-detonation tran-

sition (DDT). Combustion wave velocity measurements made with

ion sensors show that photo ignition provides DDT run-up

distances that are around a factor of 1.5 to 2 shorter than for

traditional single-point spark ignition. We hypothesize that the in-

creased volumetric energy release rate resulting from distributed

photo-ignition enhances DDT due to greater early-time flame area

and turbulence levels, resulting in accelerated formation and

amplification of the leading shock and accelerated DDT. For the

most sensitive C2H4/O2 mixtures studied, photo ignition yieldsextremely high combustion wave velocity measurements immedi-

ately following ignition, with velocities approximately 3 greater

than the ChapmanJouguet detonation velocity, suggesting that

the detonation was either directly initiated or that multiple com-

bustion fronts are formed by the distributed ignition. Further study

is needed to understand the mechanism for detonation formation

for these cases.

Acknowledgments

We are grateful for the support of the US Office of Naval Re-

search with Dr. Gabriel Roy as technical monitor and to Heesik

Yoo, Aaron Ide, Stephen Kim, and Garrett Ellsworth for initial setupof the experiment.

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