Applications in Energy and Combustion Science 15 (2023) 100164
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Applications in Energy and Combustion Science
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Nanosecond repetitively pulsed discharges and conventional sparks of
ammonia-air mixtures in a fan-stirred cruciform burner: Flammability
limits and ignition transition
S.S. Shy *, V.T. Mai, Y.R. Chen, H.Y. Hsieh
Department of Mechanical Engineering, National Central University, Tao-yuan 320317, Taiwan
A R T I C L E I N F O
A B S T R A C T
Keywords:
Premixed ammonia/air combustion
Nanosecond repetitively pulsed discharges
Conventional sparks
Laminar and turbulent minimum ignition
energies
Flammability limits
Ignition transition
Ammonia, an efficient hydrogen carrier, is crucial for achieving net-zero emissions. However, its low reactivity,
as manifested through small unstretched laminar flame speed (SL,0 ~ 7 cm/s) and large laminar flame thickness
(δL ~ 2.85 mm) at atmospheric pressure and stoichiometric conditions together with narrow flammability limits,
makes it difficult for initiation. As such, how to measure accurately ammonia minimum ignition energies under
laminar and turbulent conditions (MIEL and MIET) and identify ammonia flammability limits are important to
understand fundamental challenges and restricted usage for practical applications. We apply both nanosecondrepetitively-pulsed-discharges (NRPD) and conventional sparks (CS) via the same stainless-steel electrodes of 1mm diameter with sharp ends at a fixed gap of 2 mm in the same fan-stirred cruciform burner to identify
flammability limits of ammonia/air mixtures for spherical flame initiation. The burner is capable of generating
near-isotropic turbulence having roughly equal magnitudes of r.m.s. turbulent fluctuating velocities in all three
directions (u′) with negligible mean velocities. We find that NRPD operated at a pulse repetition frequency of 40
kHz can promote ignition or decrease the MIE on fuel lean and fuel rich sides as compared to that of CS.
However, even using 2,000 pulses with a total ignition energy of 4.4 J for NRPD, no self-sustained flame
propagation can be observed at ϕ = 0.65 and/or ϕ = 1.44 that marks lean and/or rich flammability limits for
spherical flame initiation. Moreover, we also find a turbulent ignition transition for the stoichiometric ammonia/
air mixture, of which the increasing slopes of MIET/MIEL versus u′/SL change drastically from gradually to
exponentially at a critical value of (u′/SL)c ≈ 13 for both NRPD and CS. Finally, these results should be useful for
future practical applications of premixed ammonia/air combustion.
1. Introduction
Ammonia is an efficient hydrogen carrier that is essential for
achieving net-zero carbon emissions [1]. This is because the properties
of ammonia are very similar to those of propane (e.g., low liquefied
pressure of around 10 atm at room temperature and high condensation
temperature of − 33.3 ◦ C at 1 atm [1,2]). As such, liquid ammonia can be
directly shipped via current liquefied petroleum gas carriers [3], making
it stand out for much easier storage and transportation than the lique­
fication of hydrogen that requires an extremely low temperature of
− 252.9 ◦ C with very high-cost materials to store it in liquid form [3].
However, using ammonia as a fuel has some drawbacks. Its low reac­
tivity, as manifested through small unstretched laminar flame speed (SL,
0 ~ 7 cm/s) and large laminar flame thickness (δL ~ 2.85 mm) at
atmospheric pressure and stoichiometric conditions having narrow
flammability limits, makes it difficult for initiation. Therefore, how to
measure accurately ammonia minimum ignition energies under laminar
and turbulent conditions (MIEL and MIET) and expand ammonia flam­
mability limits by using plasma-assisted nanosecond repetition pulse
discharges (NRPD) are the important steps to understand fundamental
challenges and restricted usage for practical ammonia combustion ap­
plications. This motivates the present work.
This study applies both NRPD and conventional sparks (CS) via the
same stainless-steel pin-to-pin electrodes in the same fan-stirred cruci­
form burner under well-controlled experimental conditions to measure
values of MIEL and MIET of ammonia/air mixtures. The fan-stirred
cruciform burner is capable of generating stationary near-isotropic
turbulence having roughly equal magnitudes of r.m.s. turbulent
* Corresponding author.
E-mail address: sshy@ncu.edu.tw (S.S. Shy).
https://doi.org/10.1016/j.jaecs.2023.100164
Received 26 March 2023; Received in revised form 1 June 2023; Accepted 19 June 2023
Available online 28 June 2023
2666-352X/© 2023 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/bync-nd/4.0/).
S.S. Shy et al.
Applications in Energy and Combustion Science 15 (2023) 100164
fluctuating velocities in all three directions (u′) with negligible mean
velocities (see [4–7] and references therein). Further, the
plasma-assisted NRPD is a promising energy deposition technology that
can be used to enhance ignition and combustion (see a recent review
article by Lacoste [8] at the 39th International Symposium on Com­
bustion). The present study applies a train of NRPD with in-situ mea­
surements of ignition energies (Eig) for each pulse discharge at a pulse
repetition frequency (PRF) of 40 kHz. The selection of PRF = 40 kHz is
based on previous NRPD studies that have shown that the cumulative
ignition enhancement effect of successive NRPD could occur at some
certain PRFs roughly around 10–40 kHz for spherical flame initiation
depending on various variables such as the number of pulse (Np), the
total ignition energy (Etot), the distance between electrodes (dgap), and
the flow conditions (e.g., [9–12]). For comparison, CS ignition experi­
ments with near-square voltage and current waveforms at fixed pulse
duration times (τcs = 500 μs and/or 1 ms) having controllable Eig of high
accuracy are also conducted. Moreover, MIE is an important parameter
for ignition in various combustion devices and for the risk assessment of
accidental explosion in industrial and aviation safety [13]. Note that
MIE is a probabilistic variable, not a threshold value, which is commonly
defined as the Eig at 50% ignitability, Eig(50%), that can be only deter­
mined statistically by repeating many ignition experiments under the
same conditions, detailed in [14].
Hence, we perform well-controlled laminar and turbulent ignition
experiments using ammonia as a fuel to address the following two
fundamental questions, as the two objectives of the present study. (1)
Could NRPD extend ammonia flammability limits in comparison with
CS? (2) Is ignition transition a universal phenomenon that also exists in
turbulent premixed ammonia/air combustion regardless of different fuel
types? The first objective is to obtain measured MIEL data over a range of
the equivalence ratio (ϕ), as wide as possible, for lean and rich flam­
mability limits using both NRPD and CS with specified conditions, as
described in the next section. Thus, MIE measurements can be repro­
duced by other groups if needed in the future. The second objective is to
scrutinize the effect of u′ on MIET using the stoichiometric ammonia/air
mixture and to see whether the increasing slopes of MIET with u′ could
display a drastic change from gradually to exponentially when u′ is
greater than a critical value of u′c. Such a phenomenon was called as
ignition transition or monotonic MIE transition found in some CS pre­
mixed turbulent ignition studies using a stationary homogeneous tur­
bulence for a variety of fuels (e.g., methane, propane, n-butane,
isooctane, primary reference fuel, hydrogen; please see [15,16] and
references therein). The monotonic MIE transition has also been found
independently in the laser-induced premixed turbulent ignition study
using a wind-tunnel decaying homogeneous turbulence for lean
methane/air mixtures [17]. In short, whether the monotonic MIE tran­
sition also exists in randomly-stirred ammonia/air mixtures either using
NRPD or using CS is still an open issue, deserving a thorough
investigation.
2. Experimental methods
The same NRPD methodology as that reported in [11,12,18] for a
lean premixed n-butane/air spherical flame initiation is applied in the
present ammonia laminar and turbulent ignition studies, which are
rather rare in the literature. For completeness, a brief description of the
experimental setup is included upon here. Ignition experiments using
ammonia as a fuel are conducted in a large double-chamber, con­
stant-temperature/pressure, fan-stirred 3D cruciform burner with an
averaged inside diameter of 300 mm. As shown on the left top of Fig. 1,
the inner cruciform burner is equipped with a pair of counter-rotating
fans and perforated plates to generate a near-isotropic turbulence flow
field with negligible mean velocities within in an experimentation
domain of 150 × 150 × 150 mm3. The inner cruciform burner is resided
in a very large outer safety pressure vessel (not shown). Detailed in­
formation on the dual-chamber, fan-stirred 3D cruciform burner (see
Fig. 1 in [19]) and its associated turbulence characteristics (e.g.,
energy-weighted r.m.s. turbulence fluctuation velocities in all three di­
rections and the integral length scale of turbulence) can be found in our
previous studies (e.g., [19,20]).
Concerning the NRPD ignition system, a custom-made FID GmbH
equipment (FPG 20- 100NK) having a maximum open-circuit-voltage
(OCV) of 30 kV is used to discharge a train (number) of nanosecond
pulses of 3–5 ns FWHM. In this study, OCV = 28 kV is applied at a
selected PRF = 40 kHz controlled by a delay/pulse function generator
(GWINSTEK AFG-2225) via the external trigger signals. Note that NRPD
voltage and current signal waveforms of each pulse are measured by the
high voltage probe (Tektronix P6015A) and the Pearson current monitor
(Model 6585) and recorded in the oscilloscope (Tektronix MDO3054).
As can be found from the Tektronix instruction manual, the minimum
frequency of the high voltage probe can vary from 25 MHz to 75 MHz
when shortening the cable length from 25-ft to 10-ft. This is because of
the cable inductance between the high voltage probe and the oscillo­
scope. The longer the length of cable is, the higher the inductance is. To
reach a sufficiently higher frequency detection, the transfer cable is
shortened as possible as we can in our measurement system and the data
recorded by the oscilloscope have a resolution of 0.4 ns with a time
window of 150 ns for each discharge pulse measurement. For NRPD
ignition studies, the same high voltage probe (Tektronix P6015A) and
the Pearson current monitor (Model 6585) have also been used by other
Fig. 1. Left-top: A simplified schematic dia­
gram of the inner cruciform burner. Leftbottom: Two sets of sequential schlieren im­
ages for laminar spherical flame initiation of
the stoichiometric NH3/air mixture using NRPD
at PRF = 40 kHz, where “Go” and “No Go” coexist at the same Etot ≈ 80 mJ. Right: Two
typical NRPD’s current, voltage, and energy
waveforms from a train of pulses at PRF = 40
kHz using a pair of 1 mm-diameter stainless
steel electrodes with sharp ends at a spark gap
of dgap = 2.0 mm.
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Applications in Energy and Combustion Science 15 (2023) 100164
research groups (e.g., Ju & co-workers [21] and Chen & co-workers
[22]).
As already described in Ref. [12] and the references therein, NRPD
may have three parts of impedance: (i) the internal impedance inside the
NRPD power source, (ii) the characteristic impedance of the trans­
mission line (cable) connected to the ignition circuit depending on its
line length and properties, and (iii) the load impedance at the gap of
electrodes of the electric ignition circuit. These impedances can influ­
ence the energy transfer from the pulses of a NRPD source to the plasma
inside the pin-to-pin electrodes. Therefore, to minimize the pulse
reflection, it is important to match the internal impedance with the sum
of the load impedance and the characteristic impedance from the
transmission line. The present custom-made NRPD equipment (FID
GmbH FPG 20–100NK) has an internal impedance of 75 Ω. Efforts have
been taken to match a 75 Ω for the load impedance of the electric
ignition circuit. In addition, it is equally important to measure the
breakdown voltage in situ directly on the positive electrode of the
ignition circuit (the negative electrode is connected to the ground) for all
cases using the precision high voltage probe without the extra exposure
connecting cable (line). As such, the phase difference of voltage and
current waveforms induced by the characteristic impedance of the
transmission line (cable) connected to the ignition circuit may be
minimized, as can be seen from the first major voltage and current peaks
of 1st and 2nd pulses in the right part of Fig. 1 which have almost no
phase difference. However, the impendence of the spark gap could vary
slightly after the breakdown and thus the subsequent voltage and cur­
rent waveforms are somewhat irregular and have a little phase
difference.
As to the CS ignition system, a precision high-voltage pulse generator
with a maximum breakdown voltage of 25 kV (HV-M25K) together with
adjustable loading resistances and a small damping resistor of 100 Ω in a
homemade ignition circuit was used to generate near-square voltage and
current waveforms at fixed pulse duration times of 500 μs and/or 1 ms,
similar to that used in our previous MIE transition studies [15,23]. When
dgap is sufficiently small, typically less than 1 mm, heat losses to elec­
trodes and the surrounding are important especially for a long ignition
duration of 1 ms [15,23,24]. However, the effects of heat losses can be
drastically reduced when dgap ≥ 2 mm [15,23,24]. Therefore, a large
electrode gap of dgap = 2.0 mm is used in the present study.
The right part of Fig. 1 presents two typical NRPD’s current, voltage,
and energy waveforms from a train of pulses at PRF = 40 kHz using a
pair of 1 mm-diameter stainless steel electrodes with sharp ends at a
spark gap of dgap = 2.0 mm; except for the first pulse having Eig = 0.8 ±
0.1 mJ, all subsequent pulses have nearly the same Eig = 2.2 ± 0.15 mJ
with uncertainties of about 7%. In the left bottom of Fig. 1, two sets of
time-evolution schlieren images for laminar spherical flame initiation of
the stoichiometric NH3/air mixture using NRPD at PRF = 40 kHz are
presented to show that “Go” and “No Go” events can co-exist even when
the same Etot ≈ 80 mJ (Np = 37 pulses) is used, showing the statistic
nature of spark ignition. These schlieren images are recorded by a highspeed, high-resolution CMOS camcorder (Phantom V711) operated at a
frame rate of 10,000 frames/s with the exposure time of 1 μs, 800 × 800
pixels, and a field of view of 60 × 60 mm2 except for the last image at 40
ms in the “Go” event having 100 × 100 mm2. For the “No Go” event, the
initial kernel at 1 ms is similar to that in the “Go” event, but the kernel
dissipates almost completely at 15 ms (no self-sustained flame
propagation).
Fig. 2 shows a typical example for the determination of MIE at 50%
ignitability using the logistic regression method, where a pair of 1-mm
diameter electrodes with sharp ends are used in the ammonia/air
mixture at ϕ = 1 in quiescence using NRPD at PRF = 40 kHz, yielding
MIEL = Eig(50%) = 79.1 mJ. Each MIE datum is obtained by 30–40
repeated runs using a range of total ignition energy, as shown in Fig. 2,
where “Go” and “No Go” indicated by “o” and “x” symbols can co-exist
within an overlapping region. Such a high value of MIEL indicates that
the premixed ammonia/air mixture is rather difficult to ignite
Fig. 2. A typical example of the laminar MIE (MIEL) determination at 50%
ignitability using the logistic regression method for the stoichiometric
ammonia/air mixture.
successfully because of its low reactivity. Also, the same electrodes are
used in CS measurements of ammonia/air mixtures to investigate lean
and rich flammability limits in comparison with those obtained by
NRPD. Besides, all values of MIEL and MIET presented in this study are
obtained by using the same logistic regression method (Fig. 2), which
are used to investigate ammonia flammability limits and ignition tran­
sition (monotonic MIE transition).
The flammability limits of ammonia/air mixtures under atmospheric
conditions have been obtained experimentally in the past (e.g., [25,26]).
Unfortunately, these previous results of ammonia flammability limits
were very different, depending on a number of factors (e.g., the methods
used to measure, the electrodes’ geometry and configuration, the mea­
surement vessel and its material, Eig, and most importantly the flam­
mability criterion used to determine flammability limits). For instance,
the US Bureau of Mines method applied vertical glass tubes of 20–80 mm
diameters with 1–1.5 m heights to measure the flammability limits of
chemicals based on the flammability criterion that a visual flame can
propagate not only away from the electrodes but also through the full
length of the tube [25,26]. Lean and rich flammability limits (LFL and
RFL) of ammonia were reported, i.e., at LFL = 16.6 vol.% (ϕ = 0.72) and
RFL = 27.2 vol.% (ϕ = 1.35) [25] or LFL = 15 vol.% (ϕ = 0.62) and RFL
= 28 vol.% (ϕ = 1.36) [26]. But the EN 1839 tube method (European
standard) using a vertical glass tube of 80 mm in diameter and 300 mm
in length reported quite different results, i.e., LFL = 15.4 vol.% (ϕ =
0.64) and RFL = 33.6 vol.% (ϕ = 1.77), in which the flammability cri­
terion was defined when the visual detachment flame from the elec­
trodes can propagate 100 mm of the length of the tube [26]. These
aforesaid results in [25,26] suggested that flammability limits strongly
depend on the flammability criterion. Therefore, it is important to define
the present flammability criterion for spherical flame initiation that is
different with previous studies (e.g., [25,26]), as discussed in the next
section.
3. Results and discussion
3.1. The flammability criterion by NRPD and CS for spherical flame
initiation
The flammability criterion for the present spherical flame initiation
using both NRPD and CS is defined when two combustion stages are
completed. For simplicity, we only present the case of NRPD. First, the
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Applications in Energy and Combustion Science 15 (2023) 100164
centrally-ignited, outwardly-propagating laminar flame kernels (see the
first three images for the “Go” event in Fig. 1 at 1 ms, 10 ms and 20 ms
with a view field of 60 × 60 mm2) can successfully develop into a selfsustained flame propagation (the fourth image at 40 ms with a view
field of 100 × 100 mm2). To better demonstrate the time evolution of the
development of self-sustained laminar spherical flame propagation,
Fig. 3 presents five flame images at different times varying from 25 ms to
100 ms, all images with the same large view field of 100 × 100 mm2
under exactly the same conditions as that for the “Go” event in Fig. 1.
Second, such a self-sustained laminar expanding flame can eventually
consume all reactants (the premixed ammonia/air mixture) inside the
entire volume of the cruciform burner (not shown). On the other hand,
when the flame kernel is faded out and/or it cannot develop into a selfsustained flame propagation (the “No Go” event in Fig. 1), this belongs
to a failure ignition event that is used to identify corresponding values of
ϕ for lean and rich flammability limits. Furthermore, the averaged
critical flame radius <R>c can be estimated from flame speed data
d<R>/dt versus <R>(t) obtained from the time evolution of flame
kernel formation and its subsequent self-sustained spherical flame
development (see Fig. 3), where <R>(t) = [A(t)/π]0.5, A(t) is the area
enclosed by the expanding flame kernel and/or front tracked from highspeed schlieren images, and t is time. For the stoichiometric NH3/air
mixture as in the case of Fig. 3, the value of <R>c is around 12 mm, as
indicated in the first image of Fig. 3, at which the time to reach <R>c is
around tc ≈ 25 ms that is much longer than typical hydrocarbon fuels
with tc ~ 10 ms [27]. Again, Fig. 3 reveals the low reactivity of the
ammonia/air mixture even at ϕ = 1 having a rather small flame speed, as
substantiated by such a long elapse time of 75 ms at which the laminar
spherical flame takes to grow from 〈R〉 = 12.2 mm to 〈R〉 = 40.3 mm.
Fig. 4. Laminar minimum ignition energy as a function of the equivalence ratio
for ammonia/air mixtures using both NRPD at PRF = 40 kHz (●) and CS at τcs
= 500 μs (△, △ superimposed by “x”). To identify lean and rich flammability
limits for spherical flame initiation, the maximum Eig ≈ 500 mJ at τcs = 1 ms is
used for CS and 2000 pulses with Etot ≈ 4.4 J are applied for NRPD.
and 575 mJ, respectively, at which all data of “Go” (open symbol) and
“No Go” (cross symbol) are also plotted in Fig. 4. As seen, the lowest
values of Etot for successful ignition (open symbol) are respectively
438.6 mJ at ϕ = 0.66 and 482.6 mJ at ϕ = 1.42, all less than 500 mJ
(please also see Fig. S1 in the Supplementary Materials). Thus, NRPD
operated at PRF = 40 kHz can promote the ignition or decrease the MIE
of ammonia/air mixtures for fuel lean and rich sides as compared to CS.
It is anticipated that the non-equilibrium plasma-assisted NRPDs can
generate and accumulate more highly reactive species and radicals
around the neighborhood of the spark gap than that of CS especially for
rich ammonia/air mixtures through complex chemical kinetics [2]. Note
that for the present NRPD study, we find that ignition is always failed at
lower PRFs ≤ 10 kHz even when 2000 pulses (≈ 4.4 J) are applied.
Although the true effect of PRF is still waiting to be explored, it is
anticipated that the ammonia flammability limits would not change
much with PRF when PRFs are around 40 ~ 100 kHz.
Second, we now discuss different measured values of MIEL between
NRPD and CS as presented in Fig. 4. Values of MIEL measured by NRPD
at ϕ = 0.75 and ϕ = 1.2 are found to be roughly the same as that
measured by CS, where the NRPD datum at ϕ = 1.2 is purposely shifted a
little to the right for clarity. However, it is unexpected to find that values
of MIEL measured by NRPD are much higher than those obtained by CS
within the range of ϕ = 0.8–1.1. For instance, the lowest value of MIEL
occurs at ϕ = 0.9, same for both NRPD and CS, where MIEL(NRPD) = 70.7
mJ >> MIEL(CS) = 23.6 mJ (Fig. 4). What causing such high values of
MIEL within ϕ = 0.8–1.1 when using NRPD as compared to those of CS?
Probably, the observation of the early development of the initial NRPD
kernels can be used to explain the above question, as displayed in Fig. 5
where all images have the same small view field of 30 × 30 mm2. Note
3.2. Ammonia flammability limits by NRPD for spherical flame initiation
Fig. 4 presents values of MIEL as a function of ϕ for both CS and NRPD
under the same experimental conditions including such as the same
electrodes, the same cruciform burner, the same ammonia/air mixtures.
First, let us examine possible flammability limits of ammonia/air mix­
tures for spherical flame initiation. For CS having near-square voltage
and current waveforms, the largest value of Eig at a fixed pulse duration
time of τcs = 500 μs is 250 mJ. But when using τcs = 1 ms, Eig can be
increased to a maximum value of 500 mJ in the present work due to the
equipment limitation. Note again that this study applies a large dgap =
2.0 mm, and heat losses to the electrodes and the surrounding can be
drastically reduced. This has been substantiated by measurements of
values of MIEL as a function of dgap, in which values of MIEL at dgap = 2.0
mm are much smaller than those at dgap = 0.3 mm and 0.58 mm (please
see Fig. 2a in [15]).
By applying discharged from CS at dgap = 2 mm, failure ignition of
ammonia/air mixtures (“No Go” for at least 10 repeated runs) is iden­
tified at ϕ = 0.7 and/or 0.68 for the lean flammability limit and at ϕ =
1.25 and/or 1.35 for the rich flammability limit with 0% ignitability (see
the “x” symbol in Fig. 4). As to NRPD, we discover that even using 2000
pulses with a total ignition energy as high as Etot ≈ 4.4 J, ammonia/air
mixtures at ϕ = 0.65 (LFLNRPD) and at ϕ = 1.44 (RFLNRPD) cannot be
ignited, as shown by the gray areas in Fig. 4, that mark lean and rich
flammability limits. At ϕ = 0.66 and 1.42, we find that MIEL = 608 mJ
Fig. 3. Same conditions as the “Go” event in Fig. 1, but plotted for the demonstration of self-sustaned flame propagation from 25 ms to 100 ms.
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Fig. 5. (a) Same conditions as the “Go” event in Fig. 1, but plotted for the early development of the embryonic NRPD kernels from 0.5 ms to 4.0 ms in a smaller field
of view (30 × 30 mm2) to show some local extinctions of weak kernel parts at the upper side as indicated by white dashed ellipses at the last three images. (b) Same as
(a), but for the other repeated run showing the statistical nature of local extinctions of weak kernel parts at the lower side.
that Figs. 5(a) and 5(b) have the same conditions as in Fig. 1 for the “Go”
event, having Etot ≈ 80 mJ (Np = 37 pulses). At t = 0.5 ms (the first
image in Fig. 5a and b), 20 pulses have just been discharged to form
added-up wrinkling flame kernels with multi-layer and non-symmetric
appearance due to the synergistic flow recirculation effect (e.g., [9,11,
12]) and/or the jetting phenomenon effect (e.g., [10]). In comparison,
the CS flame kernel at t = 0.5 ms was a smooth and symmetric ellipse
(please see Fig. 3 in [11]). For high reactivity fuels, such as methane,
propane, n-butane, hydrogen used in previous studies (e.g., [9–12]
among many others), the aforementioned synergistic flow recirculation
and/or jetting phenomenon effects were reported to enable a noticeable
enhancement on ignition. But this is not the case for the ammonia/air
mixture due to its very low reactivity. The accumulated radicals inside
multi-layer non-symmetric kernels at t = 0.5 ms due to the jetting
phenomenon effect can split into two portions at a later time, i.e. a weak
expanding multi-layer corrugation flame kernel and a strong expanding
Fig. 6. Ammonia MIE transition at ϕ = 1. (a) Values of MIE plotted against u′ using CS at τcs ≈ 500 μs. (b) Similar to (a), but using NRPD at PRF = 40 kHz. (c)
Normalized MIET/MIEL as a function of u′/SL for both NRPD (●) and CS (△).
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Applications in Energy and Combustion Science 15 (2023) 100164
flame front, as shown in the second image in Figs. 5(a) and 5(b) at t = 1
ms. On the one hand, the former for the weak one is faded out at t = 2 ms
and t = 3 ms and quenched at t = 4 ms, as marked by the dashed ellipses
in the third, fourth, and fifth images of Figs. 5(a) and 5(b). On the other
hand, the latter for the strong one continues to expand and becomes a
self-sustained propagating flame. Indeed, the local quench of the split­
ting weak multi-layer corrugation flame kernel takes away a significant
portion of the input ignition energy that makes successful ignition very
difficult. This is why the value of MIEL(NRPD) has to be as large as 79.1 mJ
at ϕ = 1 in order to achieve 50% successful ignition for the
low-reactivity ammonia/air mixture. In comparison, MIEL(NRPD) = 79.1
mJ >> MIEL(CS) = 26.9 mJ for the same stoichiometric ammonia/air
mixture because of no splitting of initial flame kernel for CS. It should be
noted that the splitting of weak and strong portions of added-up wrin­
kling flame kernels for NRPD has no preferential direction; it seems to be
random as weak and strong portions can switch directions, as can be
seen from Figs. 5(a) and 5(b).
The splitting phenomenon, as shown in Figs. 5(a) and 5(b), can also
be observed for fuel lean and fuel rich ammonia/air mixtures outside the
range of ϕ = 0.8–1.1. Actually, such splitting phenomenon can be
observed for all values of ϕ varying from 0.66 to 1.42; within this range
of ϕ, it is anticipated that the facilitating effect of non-equilibrium
plasma can prevail the splitting effect.
circle symbol) as a function of u′ are shown in Fig. 6(a) and 6(b),
repetitively, alongside the data of “Go” (empty circle symbol) and “No
Go” (cross symbol) at two different values of u′ to show the overlapping
region and the statistic nature of spark ignition. When u′ < u′c ≈ 0.9 m/s
in the pre-transition, values of MIET for CS (NRPD) increase gradually
with u′ from 26.9 mJ (79.1 mJ) at u′ = 0 to 51 mJ (91 mJ) at u′ ≈ 0.9 m/s;
both CS and NRPD showing the same modest increase (Figs. 6a and 6b).
But when u′ > u′c ≈ 0.9 m/s in the post-transition, values of MIET for CS
(NRPD) increase exponentially to 150.2 mJ (150 mJ) at u′ ≈ 1.16 m/s (u′
≈ 1.25 m/s); both CS and NRPD showing the same exponential increase
(Figs. 6a and 6b). In Fig. 6(a) for CS at u′ ≈ 1.4 m/s, the stoichiometric
ammonia/air mixture cannot be ignited successfully with 0% ignit­
ability even when Eig = 250 mJ is applied. Fig. 6(c) presents the
normalized MIET/MIEL data as a function of u′/SL for both NRPD (solid
circle symbol) and CS (empty triangle symbol). In the pre-transition,
MIET/MIEL increases gradually with u′/SL where MIET/MIEL ~ (u′/
SL)0.09 (NRPD) and/or MIET/MIEL ~ (u′/SL)0.34 (CS). As to the posttransition when u′/SL > 13, MIET/MIEL increase drastically with u′/SL
where MIET/MIEL ~ (u′/SL)1.53 (NRPD) and/or MIET/MIEL ~ (u′/SL)3.88
(CS).
Fig. 7 displays schlieren images of centrally-ignited, outwardlypropagating turbulent stoichiometric ammonia/air flames at two
different values of u′ = 0.69 m/s (a) and u′ = 1.06 m/s (b), where both
cases show “Go” and “No Go” using the same turbulent MIET. In the posttransition regime where u′ = 1.06 m/s > u′c ≈ 0.9 m/s, local quench of
broken flame kernels can be clearly observed (see the dashed circles on
the last two images for the “Go” case in Fig. 7(b) at 30 ms and 40 ms); the
broken flames can develop into randomly-propagating distributed-like
flames for successful ignition events, as shown in Fig. S2 of the Sup­
plemental Materials.
3.3. Ignition transition of stoichiometric ammonia/air mixture using both
NRPD and CS
Fig. 6 demonstrates that ignition transition or the monotonic MIE
transition also exists for the ammonia/air mixture for the first time,
where the transition occurs at a critical value of u′c ≈ 0.9 m/s. Both CS’s
MIET data at τcs ≈ 500 μs (triangle symbol) and NRPD’s MIET data (solid
Fig. 7. Schlieren images of centrally-ignited, outwardly-propagating turbulent stoichiometric ammonia/air flames: (a) u′ = 0.69 m/s and (b) u′ = 1.06 m/s. Both
cases show “Go” and “No Go” using the same turbulent Eig(50%).
6
S.S. Shy et al.
Applications in Energy and Combustion Science 15 (2023) 100164
4. Conclusions
the online version, at doi:10.1016/j.jaecs.2023.100164.
In this study, well-controlled laminar and turbulent ignition experi­
ments using ammonia as a fuel alongside nanosecond repetitively pulsed
discharges and conventional sparks as ignition sources are conducted in
the dual-chamber fan-stirred cruciform burner capable of generating
near-isotropic turbulence. We find that NRPD operated at a pulse
repetition frequency of 40 kHz can promote the ignition of ammonia/air
mixtures on fuel lean and rich sides in comparison with those of CS. Note
that even using 2000 pulses with a total ignition energy of 4.4 J for
NRPD, no self-sustained flame propagation can be observed at ϕ = 0.65
and/or ϕ = 1.44 that marks lean and/or rich flammability limits for
spherical flame initiation (Fig. 4).
Unexpectedly, values of MIEL(NRPD) are found to be greater than
those obtained by CS within the range of ϕ = 0.8–1.1. This is explained
by the observation of the early development of the initial NRPD kernels
(Fig. 5). Such added-up wrinkling NRPD kernels with multi-layer and
non-symmetric appearance having accumulated radicals split into two
portions at a later time for the ammonia/air mixture due to its very low
reactivity, i.e. a weak expanding multi-layer corrugation flame kernel
and a strong expanding flame front for successful ignition. The former is
faded out and quenched quickly, while the latter can become a selfsustained propagating flame provided that the input total energy is
sufficiently large. The local quench of the splitting weak multi-layer
corrugation flame kernel takes away a significant portion of Etot that
makes successful ignition more difficult for NRPD than for CS. Specif­
ically, MIEL(NRPD) = 79.1 mJ >> MIEL(CS) = 26.9 mJ for the same stoi­
chiometric ammonia/air mixture because of the splitting of initial flame
kernels for NRPD due to the low reactivity of ammonia. Further, the
splitting of weak and strong portions of added-up wrinkling flame ker­
nels for NRPD has no preferential direction; it seems to be random as
weak and strong portions can switch directions.
Is ignition transition a universal phenomenon [28] that also exists in
turbulent premixed ammonia/air combustion regardless of different fuel
types? The answer seems to be positive. We find similar turbulent
ignition transition for the stoichiometric ammonia/air mixture, where
the increasing slopes of MIET/MIEL versus u′/SL change drastically from
gradually to exponentially at a critical value of (u′/SL)c for both NRPD
and CS.
Finally, the present MIE transition measurements of low-reactivity
ammonia/air mixtures should be important for our fundamental un­
derstanding of spark ignition behavior in premixed turbulent combus­
tion, which may offer ignition characteristics for two-stage ammonia
combustion and liquid ammonia spray combustion in future ammonia
gas turbine power generation applications.
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Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Data availability
Data will be made available on request.
Acknowledgments
The financial support from the Ministry of Science and Technology
(now National Science and Technology Council), Taiwan, under grants
(MOST 109-2221-E-008-088-MY3) is greatly appreciated.
Supplementary materials
Supplementary material associated with this article can be found, in
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