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Combustion and Flame 176 (2017) 318–325
Contents lists available at ScienceDirect
Combustion and Flame
journal homepage: www.elsevier.com/locate/combustflame
Hypergolicity and ignition delay study of pure and energized ethanol
gel fuel with hydrogen peroxide
B.V.S. Jyoti, Muhammad Shoaib Naseem, Seung Wook Baek
∗
Aerospace Engineering Department, Mechanical Engineering Building, KAIST, 291 Daehak-ro, Yuseong-gu, Daejeon 34141, South Korea
a r t i c l e i n f o
Article history:
Received 9 September 2016
Revised 17 November 2016
Accepted 17 November 2016
Keywords:
Ethanol
Gel
Propellant
Hypergolic
Ignition delay
Viscosity
a b s t r a c t
An experimental study of hypergolicity and ignition delay of pure and energized gelled ethanol with hy-
drogen peroxide was carried out. Experimental drop test results were obtained and discussed by using
Photron high speed camera imaging. This study represented a sufficient repeatability of ignition delay for
hypergolic gel bipropellant development. Gelled ethanol fuel (pure and energized with nano-Al/B/C parti-
cle substitution) mixture with metal catalysts were formulated to examine its hypergolicity with ignition
delays on the order of 1–30 ms in most of cases, which are comparable with the existing liquid hyper-
golic bipropellant systems. The minimum ignition delay time was recorded for boron case at 1.33 ms. And
the calculated activation energy for the gelled ethanol fuel with pure and energetic particle substitution
system resided within the range of 7–13 kJ/mole along with shear thinning behavior. Temperature profile
also indicated an exothermic nature of the propellant system with 10 0 0 to 160 0 K recorded. Parameters
such as apparent viscosity of the fuel, drop height and drop volume also played an important role for
the hypergolicity of the system in a drop experimentation. It was also observed that the formation of a
cage encapsulating the high temperature gases in a network formed by the gelling agent could result in
a longer ignition delay.
© 2016 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
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1. Introduction
The term hypergolicity is the spontaneous ignition of a fuel and
an oxidizer in contact. Hypergolic propellants are preferred for sev-
eral rocket propulsion missions, mostly when multiple and reliable
ignitions are required for mission success [1] .
Ethanol and hydrogen peroxide based hypergolic bipropellant
system can impart several properties that can contribute sys-
tem advantages when compared to the traditional hypergolic sys-
tem such as unsymmetrical dimethylhydrazine-Red fuming ni-
tric acid (UDMH-RFNA), monomethylhydrazine-nitrogen tetroxide
(MMH
–NTO) etc. [2] . It has high density, low vapor pressure, less
toxic, corrosive and ecofriendly nature. Additionally, the hydrogen
peroxide decomposition and combustion products with ethanol are
non-toxic and environmentally more friendly than traditional hy-
pergolic systems which are in current use. However, the hydrogen
peroxide provides some challenges too along with a number of ad-
vantages such as thermal stability, storage, handling and material
compatibility.
The interest in using an alternative and more ecofriendly fuel
and oxidizer system (such as ethanol and hydrogen peroxide) in
∗ Corresponding author. Fax: + 82423503710.
E-mail address: [email protected] (S.W. Baek).
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http://dx.doi.org/10.1016/j.combustflame.2016.11.018
0010-2180/© 2016 The Combustion Institute. Published by Elsevier Inc. All rights reserved
ocket propulsion application has been renewed over the past
ecades [2–7] . Ethanol was selected based on its application in
ast as a rocket fuel such as German V-2 missile, Jupiter C, Amer-
can Redstone Rocket, etc. In addition, the ethanol is a biofuel,
hich is least negative to the environment, easy to handle and
ransport, cheap and economical. Moreover, the hypergolic bipro-
ellant system based on catalytically promoted gelled ethanol fuel
nd hydrogen peroxide requires very less quantity of catalyst for
ts hypergolicity and produces non-toxic the combustion products
rom the reaction which are mostly water vapor and CO 2 gas with
ess than 1 mole% of metal catalyst oxide in solid form. On the
ontrary, a higher concentration of transition metal catalyst can re-
uce the exhaust velocity and specific impulse, since the exhaust
elocity is inversely proportional to the square root of the average
olecular weight of the exhaust product. In order to maximize the
pecific impulse, a reduction of the transition metal concentration
f the fuel is required without affecting the ignition delay time of
he hypergolic system. The use of catalysts for promoting decom-
osition and combustion of hydrogen peroxide (80% concentrated)
ith methanol fuel dates back to German use of calcium perman-
anate as a hypergolic combination in the Me163 [8] . Also, recently
study has been done in which fuel additives were introduced
or the purpose of ignition delay control specially for ethanol and
eroxide systems [9] . For keeping a prospective for rocket system,
critical concentration of catalyst was considered for the study.
.
B.V.S. Jyoti et al. / Combustion and Flame 176 (2017) 318–325 319
Fig. 1. Experimental setup with oxidizer injector at position 173 mm above the sample holder.
Fig. 2. Apparent viscosity study of gelled and energized ethanol fuel system.
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urrent study also employed this approach, using a metal catalyst
or hypergolic ignition.
The ignition delay time (ID) is a critical parameter for hy-
ergolic propulsion system design such as an injector design
nd starting sequence which leads to application limitations,
e.g. spacecraft system with a reaction control system, that uses
hrusters to provide attitude control, must have ignition delays
o allow multiple starts and stops within 100 ms to meet mis-
ion requirement). The time between fluid contact and ignition
s a function of mixing, chemical kinetics, heat transfer, initial
emperature, ambient temperature, and other local environmental
onditions. Especially, for a gel propellant system where viscosity
nd viscoelastic properties play a vital role [3–7,10–16] , the gel
ystem has to overcome flow resistance before mixing to initiate
gnition and chemical reaction [17–20] . Several studies have been
erformed to understand the effect of various parameters on ID
f hypergolic liquid systems. However, very less or few effort s
ave been made on gelled and metalized gelled propellants as
ar as its hypergolicity, ignition delay and combustion studies are
oncerned [21] . Such gelled systems (metalized and pure case)
ave a marked difference in their physio-chemical properties in
omparison to the traditional hypergolic liquid propellants which
ay lead to significant changes in their ignition and combustion
ehaviors. In past, the initial testing combinations for hypergolicity
tudy were done through a drop test by dropping one or more of
he propellant into a small volume of the other propellant while
bserving the subsequent reaction behavior [22–29] . Logically,
he drop testing should provide a best environmental condition
or repeatability of experimental results, although it should be
ecognized that the drop testing may differ considerably from the
nd application which includes aggressive mixing process [30] .
Many diagnostic tools have been used in the past in the drop
esting, such as photodiode, spectroscopy, shadowgraphy, Schlerien
nd high speed imaging. This article presents a discussion of hy-
ergolicity and ignition delay experimental studies with the use of
igh speed camera photography to determine decomposition, igni-
ion location and ignition delay times of gelled ethanol fuel (both
ure and energized) and metal catalyst mixture with hydrogen per-
xide. This would provide a significant insight into the ability to
rovide a reliable hypergolic ignition condition in practical condi-
ions in future.
In the present study the formulated ethanol gel fuel (both pure
nd energized) requires less wt% of the thickening agent for gela-
ion than the ones mentioned in the literature [3,4] . Regarding the
ypergolicity, the main focus is on studying the least wt% of the
ransitional metal catalyst required for achieving hypergolicity with
ase of suspension and reasonable ignition delay for a gel bipropel-
ant system. Finally, the current study examines the hypergolicity
nd reasonable ignition delay using a visco-elastic material with
arent fuel property of ethanol, whereas the previous papers pri-
arily used the liquid ethanol fuel with hydrogen peroxide for hy-
ergolicity.
. Materials and methods
.1. Materials
For the experimentation, the ethanol (99.8% pure, CAS No.
4-17-5, Sigma Aldrich Corp., South Korea) was used as a base fuel
ith cellulose derivative (propyl cellulose) as a gelling agent. The
rst fuel gel sample comprises 6 percent by weight of the gelling
gent (Molecular weight 370,0 0 0, Powder, with 20 mesh particle
ize), which is considered as a pure case, while for energized case,
percent by weight of gelling agent and 4 percent by weight
f nano-sized energetic particles were suspended [Al (100 nm), B
200–250 nm), and C (50 nm)], and all the samples were referred
n the nomenclature and hereafter will be represented as S P ,
Al , S B and S C , respectively. The percentages for gelling agent
nd energetic nano-particles are based on critical values for pure
nd energized cases. These critical values are ones at which the
el properties are maintained, i.e., if less gelling agent is added,
he formulated fuel will behave like liquid, which is not desired.
ence, an introduction of the thickening agent above the critical
alue for gelation can help to overcome the liquid like behavior.
he formulated gel fuel under normal conditions acts as semi-solid
hich behaves like a jelly, but with applied force or heat they
end towards liquid.
The oxidizer used was a propellant grade liquid hydrogen
eroxide (90%) and was stored in a refrigerator at a tempera-
ure of about 0–2 °C. Also the oxidizer was kept in a clean and
ight free environment in order to make sure no contamination
320 B.V.S. Jyoti et al. / Combustion and Flame 176 (2017) 318–325
Fig. 3. Arrhenius plot showing the viscosity-temperature relationship of ethanol gelled fuels.
Fig. 4. Gelled ethanol fuel (pure case) with CCAT.
Fig. 5. Gelled ethanol fuel (pure case) with MCAT.
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takes place. To achieve hypergolicity with the fuel (both in pure
and energized case), 1% catalyst was introduced such as copper
chloride hydrous (CuCl 2 .2H 2 O, 99.999% pure, CAS No. 7447-39-4,
Sigma Aldrich Corp., South Korea), or Manganese(II) acetylaceto-
nate (C 10 H 14 MnO 4 , CAS No. 14,024-58-9, Sigma Aldrich). These cat-
alysts will be represented as CCAT and MCAT respectively, here-
after.
2.2. Methods and experimental setup
Main focus of the study was to understand the gel viscosity
behavior, hypergolicity and ignition delay of the formulated gel
based bipropellant system. Temperature behavior due to chemical
reaction, which plays a vital role in the ignition delay time, was
also examined. Using the rotational rheometer (HAAKE RheoStress
60 0 0, by Thermo-Scientific, Germany), the viscosity behavior was
studied. The hypergolicity, ignition delay and the temperature pro-
file of the bipropellant system was observed and recorded using
a DSLR camera and a high speed camera, Photron SA-X2. A high
speed camera used was calibrated for 50 0 0 frames per second with
a total run time of 4.3678 s, capturing 21,839 frames. The ignition
delay time calculation is based on the frame number of the drop
mpact to the frame number of first visible flame multiplied by
he time taken for each frame. As for the temperature profile, the
type thermocouples by OMEGA Corporation were used with a
ead diameter of 0.127 mm with Teflon insulation, while for the
hermocouple wire, the diameter measured is 0.11 mm. The data
cquisition rate for measuring the temperature using thermocou-
les was set to 13 ms (sampling rate) from PC DAQ card. This is the
astest possible sampling rate achieved with the PC DAQ card sys-
em. These thermocouples are long enough so no additional copper
ires were added and directly connected to the data acquisition
ard PDAQ56. A setting was made in such a way that the acquisi-
ion delay is 13 ms, and the main focus of the DAQ card is to mea-
ure temperature profile of the flame, not to study ID. This DAQ
ard has 10 analog input channels for thermocouples with a mini-
um scan rate of 0.007 Hz up to 80 Hz. A very small bead size of
hermocouple might not introduce significantly a large error while
he radiation has a minimum effect. The temperature profile irreg-
larities were due to too slow data acquisition rate. The experi-
ental setup used for the hypergolicity and ignition delay studies
s shown in Fig. 1 . The ignition delay was measured based on the
ime from the oxidizer drop impact on the fuel bed to the point
hen a visible flame appears. In the ignition study the ambient
as is air. However, it does not participate in the initial ignition
ransient, while it plays a role in continuous combustion on the
uel bed.
Using an injector of 17 gauge (17 gauge represents the internal
iameter of the injector of 1.5 mm in diameter) with circular edge,
he oxidizer drop was introduced from the height of 173 mm, di-
ectly on top of the gelled fuel bed on a thin quartz glass slide.
he mass of fuel-catalyst mixture (99 wt% fuel and 1 wt% catalyst)
laced on the glass slide was about 0.06–0.07 g. To record the tem-
erature, thermocouples were placed at three different positions.
irstly, at the fuel bed, secondly at a vertical distance of 3 mm from
he bed and finally at about 9 mm from the bed. Directly perpen-
icular to the plane, the high speed camera was placed, focused at
he gelled fuel bed and the injector.
The injector was selected in order to produce a consistent drop
iameter of about 3 mm which is approximately 0.014 ml in vol-
me. However, the study was performed based on two parame-
ers, firstly a drop impact study and secondly by introducing two
ifferent volumes of oxidizer, one drop of 0.014 ml or multi-drops
f 0.05 ml, on the fuel bed bed (multi-drops represent a fixed
B.V.S. Jyoti et al. / Combustion and Flame 176 (2017) 318–325 321
Table 1
Ea of the gelled and metalized ethanol fuels.
Gel propellant Activation energy (Ea) (kJ/mol)
S P 7.1 ± 0.03
S Al 8.0 ± 0.01
S B 9.9 ± 0.01
S C 12.3 ± 0.04
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Fig. 6. S B gelled ethanol fuel with CCAT.
Fig. 7. S B gelled ethanol fuel with MCAT.
Fig. 8. Hypergolicity and ID study of ethanol/Al/CCAT with single drop.
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olume of 50 μl which is equal to 0.05 ml). This experimental setup
as designed to study the hypergolicity, ignition delay and tem-
erature profile of the formulated bipropellant system with pure
r energetic gelled fuel using two different catalysts and its overall
erformance.
. Results and discussion
.1. Apparent viscosity and activation energy study
The effect of applied shear rate (0–20 1/s) on the apparent vis-
osity of the gelled ethanol fuel is plotted in Fig. 2 . The applied
hear rate is shown to break down the gel network system so that
he apparent viscosity decreases in the formulated gelled and en-
rgized ethanol fuels with some yield point (shear thinning behav-
or).
In this experiment, a constant shear rate of 100 1/s was applied,
hen the temperature of the sample was varied from 0 to 70 °C so
hat the effect of the temperature on the gelled ethanol fuel for
ure and energized cases was investigated. Figure 3 shows the ap-
arent viscosity of all the gels measured as a function of the tem-
erature. It is to be noted that 70 °C is the boiling point of the
arent fuel (ethanol) over which it may cause the fuel to evapo-
ate and finally to be ignited. Figure 3 shows the Arrhenius plots
f ( ln ηap ) versus the inverse temperature for all of the ethanol gel
uel systems following Eq. (1 ).
ap = A. e Ea/RT (1)
here “A” is a constant related to molecular motion, while “Ea”
he activation energy for viscous flow at a constant shear rate, “R”
s the gas constant, and “T” is the absolute temperature in “K”.
The apparent viscosity as a function of temperature study indi-
ates that an exposure to higher temperatures provides the gel sys-
em with thermal energy, resulting in a decrease in the gel struc-
ural strength and a decrease in the apparent viscosity. Ea helps
o determine the minimum amount of energy required for struc-
ural network breakdown while incurring a flow initiation in the
el and also for deducing the sensitivity of a gel to the applied
emperature. For the current experimentation, Ea is comparatively
igher for S C than S B , S Al and Sp as seen in Table 1.
Table 1 shows that the pure ethanol gel has the lower activa-
ion energy than boron, aluminum and carbon based gels. A slight
mount of heat applied can cause the pure gel to gain energy
uickly as compared to carbon, boron and aluminum based gel
ases. With an increase in activation energy which affects the pre-
xponent values, a more energy is required for fuel gel network
o break down before getting exposed to the oxidizer. Since the
el propellant with a lower activation energy might ignite faster,
he gellant with a lower Ea may indicate a good suitability as the
hickening agent.
.2. Hypergolicity
Effort s were made to design a hypergolic gelled bipropellant
ystem using gelled ethanol fuel with pure and energized cases, by
ntroducing liquid hydrogen peroxide as an oxidizer in the pres-
nce of a suitable catalyst. It was observed that all the formulated
elled ethanol fuel (pure and energized), i.e., S P , S Al , S B and S C uel systems become hypergolic with the addition of 1 wt% CCAT
r MCAT catalyst.
In order to observe the hypergolicity of the formulated gelled
ipropellant system, a simple test was conducted using a quartz
up with a height of 20 mm and diameter of about 10 mm by in-
roducing a fixed volume of gelled fuel for each case, which is
.1 ml, whereas the oxidizer was introduced from a height of about
73 mm. It resulted in a positive display of hypergolic behavior for
oth catalysts as shown in Figs. 4 –7.
Hypergolicity, observed with a rapid decomposition followed by
gnition with varying delay times for respective fuel systems, was
ecorded by the DSLR camera ( Figs. 4 –7 ). The delay could be due
o the varying visco-elastic nature, suspended metal particles and
he gel network system of the pure and energized gel fuel.
Figures 4 –7 show the hypergolicity study for ethanol gel pure
nd boron case with CCAT/MCAT catalyst mixture. Similar study
as performed for the other cases too. It was observed that all the
ormulated gel compositions ignite spontaneously. Since the delay
s varying with the composition for both pure and energized case,
t is obvious that the rate of liberation of minimum ignition en-
rgy is the controlling factor in determining the ignition delay. It
ay, however, be mentioned that the minimum ignition energy,
hich is the threshold energy content of the system required to
nitiate ignition, is dependent on a number of factors such as na-
ure of fuel, its composition, exothermic reactivity with oxidizer,
ature of energetic gaseous products, nano-energetic particle addi-
ives, thickening agent, and type of catalyst too.
Detailed understanding of the ignition delay time of the respec-
ive gelled system and the temperature profile behavior is to be
iscussed in the following section. Due to the formation of cavi-
ation between fuel and oxidizer as well as bubbling of oxidizer
n the cup before ignition and stable combustion occur, this setup
322 B.V.S. Jyoti et al. / Combustion and Flame 176 (2017) 318–325
Fig. 9. Cage network breakdown and ignition delay.
Fig. 10. Hypergolicity and ID study of Ethanol/B/MCAT with multi drop (0.05 ml).
Table 2
Ignition delay of pure and metalized gelled ethanol fuel with hydrogen
peroxide.
Gelled fuel Oxidizer quantity Ignition delay (ms)
S P CCAT Single drop Rapid decomposition, No ignition
S P CCAT Multi drop 50
S P MCAT Single drop 13 .46
S P MCAT Multi drop 19 .8
S Al CCAT Single drop 4 .15
S Al CCAT Multi drop 28 .7
S Al MCAT Single drop 47 .7
S Al MCAT Multi drop 7 .0
S B CCAT Single drop 115 .0
S B CCAT Multi drop 90 .0
S B MCAT Single drop 37 .0
S B MCAT Multi drop 1 .33
S C CCAT Single drop 24
S C CCAT Multi drop 14 .35
S C MCAT Single drop 19 .2
S C MCAT Multi drop 18 .6
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may not be appropriate for the ignition delay studies, however it
gave a promising feedback of hypergolicity of the formulated gelled
bipropellant system, with a flame height averaging from 80 to
150 mm with bright bluish-green flame in case of fuel-CCAT mix-
ture and purple-yellowish flame in the case of fuel-MCAT mixture.
Also, bright energetic particle spikes could be observed in the en-
ergized cases as in Figs. 6 and 7.
3.3. Ignition delay
The results on the variation of ID for pure and energized
ethanol gels (Al, B, C) with CCAT and MCAT mixture are tabulated
in Table 2 . As previously mentioned, the drop testing can help in
the repeatability of replicating experiments, preliminary tests were
one to evaluate test parameters such as drop height, drop vol-
me, fuel volume and dropping oxidizer into fuel bed to ascer-
ain a measure of the sensitivity of parameters in the experiments.
hen dropping the peroxide into the fuel bed, the ignition loca-
ion was always just at the point of impact and above the fuel sur-
ace. In this drop test, experiment was performed with sufficient
mount of fuel for the oxidizer drop in order to avoid sensitivity
o the local mixing in our tests.
The process of ignition of a hypergolic bipropellant system is
irectly dependent on rate of chemical reaction and consequently
eat liberation. Lower ID could be due to a more liberation of heat
nergy whereas only a certain minimum amount of energy may
e required to initiate ignition. As for the energetic nano particle
ased gel system, the rate of reaction and heat liberation would
ncrease due to higher exothermic reactivity and higher value of
eat of combustion, which ultimately may become very faster due
o a rise in temperature, finally resulting in reducing the ignition
elay. However, the catalyst and metal type as well as its parti-
le size also play a vital role in determining the ignition delay in
nergized case.
The gelation of ethanol would result in an increase in magni-
ude of physical properties such as viscosity and surface tension,
hich would hinder the mixing process of fuel and oxidant. The
xidizer after coming into contact with the gelled fuel would react
ocally at the surface of the gel. This would cause the decomposi-
ion and oxidative degradation of the thickening agent molecules,
esulting in the exposure of fuel to oxidizer. The diffusion of the
xidizer into the gelled fuel may be initially slower but would in-
rease as the apparent viscosity decreases with a rise in tempera-
ure of the system. Since the net amount of oxidizer available for
eaction with gelled ethanol fuel would be less due to a utilization
f a certain proportion in degrading thickening agent molecules,
he ignition delay data in the case of gel would be higher than the
ure liquid fuel.
B.V.S. Jyoti et al. / Combustion and Flame 176 (2017) 318–325 323
Fig. 11. Temperature variation for various propellant systems at three vertical locations.
324 B.V.S. Jyoti et al. / Combustion and Flame 176 (2017) 318–325
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It was observed that when the gelled fuel was brought in con-
tact with liquid oxidizer, a copious amount of gases was liberated
first, showing the occurrence of exothermic reaction at the surface
of contact due to inter-diffusional mixing behavior of the propel-
lants through gel network before ignition, and then the hypergolic-
ity was noticed after the liquefaction of gelled propellant.
Table 2 presents a comparative summary showing the depen-
dence of ID on gelation, metallization, type of energetic nano par-
ticles, catalyst type and oxidizer volume. It is clearly seen that an
incorporation of thickening agent in ethanol gel fuel affected its
hypergolicity and ID. The addition of nano energetic particles in
the gel system further affected the hypergolic nature of the fuel
system and also ID data based on the type of energetic particles,
their sizes, catalyst type and respective exothermic reactivity of
metal particles blended in the ethanol gel system. In the case of
boron, the particle size is approximately 250 nm. Its bigger particle
size in comparison to Al and C, could be the reasons for a longer
ignition delay in comparison to the other pure and energetic gel
systems.
In the case of hypergolic bipropellant ignition in which the
hydrogen peroxide is utilized as an oxidizer, it could be character-
ized by two different reactions, i.e., the exothermic decomposition
of the peroxide and the local auto-ignition of heated oxygen and
the fuel vapor. The formulated hypergolic system is a catalytically
promoted hypergolic propellant system which is composed of
energetic gel fuel and transition metal salt mixture (CCAT/MCAT).
The suspended metal salt in the fuel catalytically decomposes
hydrogen peroxide, producing superheated oxygen and water
vapor. The exothermic decomposition of hydrogen peroxide pro-
vides the energy for heating and vaporizing the gel fuel and
superheated oxygen then reacts with fuel vapor to initiate ignition
and stable chemical reaction between the oxidizer and fuel, if the
auto-ignition temperature of the fuel vapor is exceeded [25–28] .
Among the catalytically driven fuels, the salts (transition met-
als) of CU(II) and Mn(III) suspended in a fuel are particularly re-
active with 90% H 2 O 2 . The exothermic decomposition of the hy-
drogen peroxide is a chemical process which uses a catalyst to
speed up desired oxidation reaction of propellants (fuel and per-
oxide). Hydrogen peroxide serves as an oxidizing agent in the
combustion of organic fuel. The driving force behind these reac-
tions is the conversion of oxygen in the −1 oxidation state to
−2 oxidation state. Furthermore, the hydrogen peroxide is weakly
acidic in nature (pKa = 11.65). Under basic conditions, the perox-
ide loses a proton and becomes much less stable. Hence, fuels,
that are strongly reducing agents and also basic in nature, would
facilitate this conversion and thus be very reactive with hydro-
gen peroxide [29] . Schumb et al . [31] describes several classes of
organic compounds that are reportedly hypergolic with hydrogen
peroxide. Among those mentioned were compounds containing hy-
droxyl groups, with electron rich areas on the molecule that can
act as reductants with hydrogen peroxide (such as ethanol with
pK a = 16, pK b = −1.9). In addition, the suspension of catalyst (tran-
sition metal salts; Cu
+ 2 , Mn
+ 3 ) can markedly increase the rapid
vaporization and temperature rise while promoting the exother-
mic reaction between fuel and peroxide. Another important feature
of catalytically promoted hypergolic propellant system is the mis-
cibility of fuel with the oxidizer. Miscibility is also an important
factor as the oxidizer must penetrate into the gel fuel to maxi-
mize contact area with the fuel and catalytic material. Hydrogen
peroxide is highly miscible in ethanol due to its polar nature. It
was observed experimentally that, when hydrogen peroxide was
introduced to pure liquid ethanol and gel ethanol, a rapid decom-
position occurred without ignition. But, when catalyst (transitional
metal salt such as CCAT/MCAT) was suspended into these systems,
the ignition was noticed after a rapid decomposition in gel fuel
system.
However, sometimes a rapid decomposition does not lead to hy-
ergolicity as seen in the pure ethanol gel with CCAT for the single
rop case in Table 2 , since the rate of liberation of minimum igni-
ion energy (known as threshold energy) was not sufficient enough
o initiate ignition. If ID is increasing with energetic nano parti-
le substitution, then it is evident that the increase in the ID with
ano particle addition is solely due to the presence of type of en-
rgetic particle and exothermic reaction by catalyst type and fuel
ixture.
When switching to multi drop from single drop test, the addi-
ional oxidizer concentration may increase the rate of reaction and
eat liberation, but simultaneously delay the attainment of local
inimum ignition energy level, which might be due to a dissipa-
ion of heat liberated to the excess of oxidizer for its vaporization
nd decomposition. Since the process of ignition of a bipropellant
ystem is directly dependent on the rate of chemical reaction and
onsequent heat liberation, a fact that some thermal energy out
f the heat liberated taken away by the excessive oxidizer would
ower the local enthalpy of the mixture (fuel-oxidizer vapor and
ecomposed energetic gaseous products), results in an increases
he ignition delay time. Since the delay is varying with composi-
ion as in Table 2 , it is obvious that the rate of liberation of min-
mum ignition energy is the controlling factor in determining the
D. However, due to the presence of catalyst promoting a rapid de-
omposition, it was noticed that the ignition delay for the ener-
ized ethanol gel fuel mixture with both the catalyst (CCAT and
CAT) is lower for multi drop than single drop test except for Al-
CAT case.
The observed ignition delay and temperature profile for all the
ystems in the existing setup are within the range of 1–30 ms and
0 0 0–160 0 K, with the maximum ignition temperature recorded
or S B CCAT (multi drop, 1620 K) and the minimum ignition de-
ay for S B MCAT (multi drop, 1.33 ms) ( Figs. 8 –11 and, Table 2 ).
egarding the temperature profile in Fig. 11 , for all the energized
ases with CCAT the higher temperature was recorded than that
ith MCAT. However, it was opposite for the temperature profile
ecorded for pure gel case. In Fig. 11 , initially a continuous rise
n temperature for all the formulated system was observed until
he maximum temperature was recorded at both positions, 2 and
. The measured ignition delay for S P , S Al , S B and S C hypergolic
ipropellant system resides well within the range of existing liquid
ypergolic bipropellant system as in Table 2 , except for the case
or S B CCAT (1 drop and multi drop) for which the observed igni-
ion delay was 115 and 90 ms. The longer ignition delay might be
ue to a prolonged breakdown time in the gel network as shown
n Fig. 9 . Moreover, the energetic nano particle type and size may
e also another reason for the delay time.
. Conclusions
An experimental test was performed to determine whether a
hear thinning ethanol gelled fuels (pure and energized) explored
n this study were hypergolic or not with hydrogen peroxide (90%).
ne percent of two different catalysts such as copper chloride
ydrous (CCAT) and Manganese(II) acetylacetonate (MCAT) was
ntroduced into fuels. Fuel-catalyst mixtures were tested and their
elative reactivity in terms of hypergolicity and ignition delays was
etermined by using the high speed imaging technique. All the
uel-catalyst mixtures were found to react with peroxide quickly
nd violently after various ignition delays.
The apparent viscosity study showed the shear thinning behav-
or of the gel fuels with experimentally estimated activation energy
ithin the range of 7–13 kJ/mol. The observed ignition delay was
ithin the range of 1–30 ms which is comparable with those ex-
sting in liquid hypergolic bipropellant systems except for S B CCAT
ixture case (1 drop and multi drop). Temperature profiles also
B.V.S. Jyoti et al. / Combustion and Flame 176 (2017) 318–325 325
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ndicated an exothermic nature of the candidate gel propellant
ith a high temperature level within 10 0 0–160 0 K range at atmo-
pheric conditions. Gelled ethanol fuel (pure and energized sys-
em) with CCAT or MCAT were found to be a promising hypergolic
ombination for bipropellant, having the low toxicity, high density,
nd reasonably short ignition delay.
cknowledgment
This work was supported by a National Research Foundation of
orea (NRF) grant funded by the Korea government (MEST) (No.
014R1A2A2A01007347 ). Dr. Jyoti was also supported by the Ko-
ean Research Fellowship Program funded by the Ministry of Sci-
nce, ICT and Future Planning through the National Research Foun-
ation of Korea (No. 2015H1D3A1061637 ).
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