Combustion and Flame 176 (2017) 318–325

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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: swbaek@kaist.ac.kr (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

i

w

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t

c

a

A

K

2

r

e

d

R

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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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