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Combustion and Flame xxx (2013) xxx–xxx

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Combustion and Flame

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Experimental investigation of decomposition and evaporationcharacteristics of HAN-based monopropellants

0010-2180/$ - see front matter � 2013 The Combustion Institute. Published by Elsevier Inc. All rights reserved.http://dx.doi.org/10.1016/j.combustflame.2013.09.026

⇑ Corresponding author. Fax: +82 42 350 3710.E-mail address: [email protected] (S.W. Baek).

Please cite this article in press as: C.H. Hwang et al., Combust. Flame (2013), http://dx.doi.org/10.1016/j.combustflame.2013.09.026

Chang Hwan Hwang a, Seung Wook Baek a,⇑, Sung June Cho b

a Propulsion and Combustion Laboratory, School of Mechanical, Aerospace and Systems Engineering, Korea Advanced Institute of Science and Technology (KAIST), 373-1 Guseong-dong,Yuseong-gu, Daejeon 305-701, Republic of Koreab Clean Energy Technology Laboratory, Department of Applied Chemical Engineering, Chonnam National University, 77 Yongbong-ro, Buk-gu, Gwangju 500-757, Republic of Korea

a r t i c l e i n f o a b s t r a c t

Article history:Received 28 March 2013Received in revised form 13 June 2013Accepted 28 September 2013Available online xxxx

Keywords:Hydroxylammonium nitrate (HAN)MonopropellantDropletEvaporationDecomposition

Hydroxylammonium nitrate (HAN)-based monopropellants are among the most promising candidates foreco-friendly rocket engine propellants. They are not carcinogenic or mutagenic, and their thermal decom-position reactions are sufficiently exothermic for military and aerospace applications. Here, an experi-mental analysis was performed to investigate the characteristics of HAN-based mixtures formonopropellant applications. Three kinds of propellants were prepared for this study: a HAN–water solu-tion, a HAN–water solution with methanol added at a stoichiometric ratio, and a third solution wherewater was added to further dilute the solution. Two different experimental techniques were used underatmospheric pressure of nitrogen gas environment: thermal analysis and droplet evaporation analysis.Thermal and catalytic decomposition were analyzed using thermo gravimetric analysis. Droplet evapora-tion was analyzed using a cylindrical vessel equipped with a heating system and a droplet feed, and ahigh-speed charge-coupled device (CCD) camera. These data were post-processed to calculate the tempo-ral variation of the droplet diameters.

� 2013 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

1. Introduction

Hydroxylammonium nitrate (HAN)-based monopropellants areionic liquids, and are one of the most promising candidate materi-als for environmentally friendly propellants for space and militaryapplications. Hydrazine has been popular over the last few dec-ades. However, it is difficult to handle because of its toxicity; fororal ingestion of hydrazine by mice, LD50 = 60 mg/kg [1]. HAN-based monopropellants have an LD50 of 325 mg/kg for oral inges-tion by mice [2], and are not carcinogenic or mutagenic. In addi-tion, their combustion products present a relatively smallinhalation hazard. The specific impulse, Isp, is comparable to hydra-zine (220–240 s); furthermore, the freezing point is lower and thedensity is higher [3]. These desirable properties of HAN-based pro-pellants have attracted much recent attention for thruster and gunpropellant applications.

To use HAN-based liquids as monopropellants, the evaporationand decomposition (or combustion) characteristics must beknown. In the initial stages of development of HAN-based propel-lants, the focus was on use as a propellant for ballistics applica-tions. Zhu and Law [4] and Call et al. [5] tested the explosion anddroplet combustion characteristics of LP-1845, a mixture of HAN,

triethanol ammonium nitrate (TEAN), and water. They observedfreely falling propellant droplets exposed to the post-combustionregion, where the temperature was 900 �C and the pressures inthe range 0.101–0.405 MPa. The droplets exhibited expansionand micro-explosion behaviors. Zhu and Law concluded that themicro-explosions were caused by chemical reactions, not waternucleation. Call et al. observed that the droplets reacted with a yel-low flame after gradual expansion at 0.405 MPa. Kounalakis andFaeth [6] determined the high-pressure combustion characteristicsof LP-1845 and LP-1846 theoretically. Vosen [7,8] reported thecombustion characteristics of HAN-water mixtures LP-1845 andLP-1846 using a strand burner, and measured their mass decompo-sition rates at pressures up to 30 MPa. The decomposition rate wasfound to be proportional to the HAN concentration, and dependedonly weakly on pressure [7]. In addition, instabilities at the liquid/gas interface were suppressed during combustion at pressuresgreater than 25 MPa for LP-1846 [8].

HAN-based propellants were first considered for rocket engineapplications in the 1990s. Jankovsky [3] examined the theoreticalperformance of HAN-based propellants using the NASA ChemicalEquilibrium with Applications program, and compared the physi-cal characteristics of liquid propellants with hydrazine. Meinhardtet al. [9] developed a 1.0 lbf (4.45 N) HAN thruster and performed aground test. They tested several mixtures of HAN–water solutionsand hydrocarbon fuels with catalysts. Preheating of the catalyst

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was required for complete catalytic combustion at 600 �F (315 �C).They achieved an Isp in the range 190–195 s for a HAN/glycine pro-pellant. Wucherer et al. [10] performed firing tests of a 1 lbf HAN-based monopropellant thruster employing a Shell 405 catalyst.They concluded that a new catalyst material was required due tothe high combustion temperature of HAN-based propellants – upto 4000 �F (2204 �C). Chang et al. [11] measured the burn rate ofHAN/methanol-based monopropellants using a strand burner atpressures in the range 0.74–7.3 MPa. The burn rate of HAN284-MEO17 (a mixture of 77.2% HAN, 17.2% methanol, and 4.9% water)increased with pressure, while the burn rate of HAN269MEO15 (amixture of 69.7% HAN, 14.8% methanol, and 14.9% water) did notincrease at pressures over 1.93 MPa. Farshchi et al. [12] character-ized the droplet combustion of HAN–water solutions and HAN-based propellants using a wire-loop igniter at pressures in therange 0.101–1.013 MPa in nitrogen and air. Small bubbles formedinside the droplets due to a liquid-phase reaction of the propellant.Residues detected after decomposition were found to be smaller athigher pressures. Furthermore, no visible flames were observed inthe nitrogen environment, although most of propellants exhibitedvisible flames in air. Wei and Shaw [13] reported the effects ofpressure on the reduced-gravity combustion of HAN/methanolpropellants in air. The burn rate decreased with the amount ofmethanol in the propellant composition. They characterized theaerosol formation after the flame was extinguished, and concludedthat a condensed-phase HAN reaction occurred at higher concen-trations of HAN. Courthéoux et al. [14] examined the decomposi-tion characteristics of HAN and hydrazinium nitroformate (HNF)using thermo gravimetric analysis (TGA) instrument and a batchreactor. The decomposition temperatures of the solutions in-creased with the water content. They concluded that the decompo-sition of these ionic solutions occurred faster when the water wasremoved because the ionic reagents could make contact. They alsoinvestigated the catalytic decomposition of HAN and HNF with Si-modified alumina-supported platinum catalysts. The catalyticdecomposition temperature (107 �C) was much lower than thethermal decomposition temperature (174 �C) for a 40% HAN–watersolution in TGA result.

The experimental environments for the droplet research de-scribed above [4,5,12,13] deviate from the spray conditions in arocket engine. When a propellant is sprayed from an injector, thedroplet experiences a sudden change of environment, in particulartemperature. In this work, we employed a droplet analysis appara-tus that could effect such an abrupt temperature change. Details ofthe droplet experiment apparatus are described in Section 2. Thepurpose of this study was to investigate the evaporation anddecomposition characteristics of HAN-based propellants. Toachieve this objective, the evaporation and thermal decompositionbehaviors of HAN-based propellant droplets were observed at var-ious temperatures. In addition, thermal analysis using TGA wascarried out to interpret the droplet behaviors.

Table 1Concentration and mass fraction of each HAN-based monopropellant.

Concentration (mol/m3) Mass fraction

HAN H2O MeOH HAN H2O MeOH

83.9% HAN 21 21.49 0 0.839 0.161 0(83% HAN) + MeOH 21 22.94 14 0.701 0.144 0.156(55% HAN) + MeOH 21 91.64 14 0.490 0.401 0.109

2. Experiments

2.1. HAN-based monopropellant preparation

To characterize the droplet evaporation and thermal behaviors,three different samples of HAN-based monopropellants were syn-thesized from hydroxylamine, nitric acid, and methanol. The threedifferent concentrations were a 83.9 wt.% HAN–water solution(83.9% HAN), a 83 wt.% HAN–water solution blended with metha-nol in stoichiometric ratio ((83% HAN) + MeOH), and a 55 wt.%HAN–water solution blended with methanol ((55% HAN) + MeOH).These HAN-based monopropellants were provided by the CleanEnergy Technology Laboratory at Chonnam National University,

Please cite this article in press as: C.H. Hwang et al., Combust. Flame (2013), h

Gwangju, Republic of Korea. The concentration and mass ratio ofeach solution are given in Table 1. The mixture ratio was deter-mined by assuming stoichiometric reaction that there will be nonitrogen oxides remained on product side but nitrogen, water va-por and carbon dioxide. Eqs. (1) and (2) describe the blends of(83% HAN) + MeOH (blend type 1) and (55% HAN) + MeOH (blendtype 2), respectively.

21ðNH3OHÞðNO3Þ þ 22:94H2Oþ 14CH3OH

! 21N2 þ 92:94H2Oþ 14CO2 ð1Þ

21ðNH3OHÞðNO3Þ þ 91:64H2Oþ 14CH3OH

! 21N2 þ 161:64H2Oþ 14CO2 ð2Þ

The stoichiometric ratio between HAN and methanol does notchange between Eqs. (1) and (2); therefore, the blend type 2 canbe viewed as the blend type 1 diluted with water.

2.2. Experimental method

The thermal analysis was implemented using a Setsys 16/18differential scanning calorimeter (Setaram). The terminal heatingtemperature was set to 300 �C and atmospheric pressure of nitro-gen gas environment was maintained. The flow rate of nitrogengas was 30 mL/min in order not to disturb on the temperature pro-file of sample. For thermal analysis, an alumina pan was used toprevent catalytic decomposition with the container [14]. In the cat-alytic decomposition analysis, 5 mg of the catalyst was placed in-side the aluminum pan, and then 5 mg of monopropellant wasinjected. The iridium catalyst was provided by Clean Energy Tech-nology Laboratory; detailed information on the catalyst can befound in Ref. [15]. To prevent catalyst pellet loss by sudden expan-sion inside the container, the aluminum pan was covered with acap.

In previous reports of HAN-based propellant droplet analysis[4,5,12,13], the environmental temperature was not precisely con-trolled. We performed our analysis using droplet evaporationinstrumentation in which the temperature and pressure can becontrolled [16]. A schematic diagram of our experimental setupis shown in Fig. 1. The droplet experiment was performed insidea vessel, and data were recorded using a high-speed charge-cou-pled device (CCD) camera. An electric furnace, which was guidedby a vertical rail, was installed inside the vessel, and a plunger mi-cro-pump and micro-needle were placed at the bottom of the ves-sel. The electric furnace was rectangular, and the bottom had anopening for the droplets. Electric coil heaters were installed atthe two sides of the furnace wall, which was insulated, and theassembly was covered by a ceramic shield to prevent direct radia-tion by the heating element. The other two walls had quartz win-dows to observe the behavior of the droplets. A quartz fibersupport was located at the bottom of the vessel interior, whichcould be seen through the quartz windows in the electric furnaceand vessel. Further details of the droplet evaporation apparatuscan be found in Ref. [16].

The droplet experiment was conducted under atmosphericpressure of nitrogen gas environment. The ambient temperature

ttp://dx.doi.org/10.1016/j.combustflame.2013.09.026


Fig. 1. Schematic diagram showing the experimental apparatus. 1: Cylindricalvessel, 2: guide bar, 3: furnace bottom hole, 4: electric furnace, 5: quartz glasswindow on furnace, 6: temperature controller, 7: furnace lever, 8: air vessel, 9:quartz glass window on cylindrical vessel, 10: backlight source, 11: quartz fiber, 12:droplet, 13: shock absorber, 14: droplet generator, 15: droplet lever, 16: plungermicro-pump, 17: high-speed CCD camera.

C.H. Hwang et al. / Combustion and Flame xxx (2013) xxx–xxx 3

inside the vessel was varied from 100 �C to 600 �C in steps of 50 �C.Before starting the experiment, the vessel was purged with nitro-gen, and the nitrogen gas inside the vessel was refreshed after eachdroplet evaporation experiment. After purging the vessel, theambient temperature inside the furnace was stabilized, and thena droplet was suspended at the end of the quartz fiber. Dropletswere generated using the micro-pump and micro-needle. Smalldroplets were formed at the tip of the micro-needle; the droplet le-ver was activated to move the droplet to the quartz fiber. Thediameters of the suspended droplets were in the range 0.7–1.0 mm. The evaporating experiment proceeded by adjusting thetemperature of the electric furnace to expose a droplet to the de-sired temperature. At the same time, images of the droplet evapo-rating were recorded using the high-speed CCD camera.

The recorded droplet behavior images were post-processed.Sequential droplet images were recorded with a 0.1 mm diameterSiC fiber as a reference. The number of pixels corresponding to thedroplet was calculated, and the droplet diameter inferred by com-paring this with number of pixels corresponding to the SiC fiber. Toextract droplet diameter from the captured images, a flexible im-age-processing code was developed using Visual Basic. In this code,a threshold value for pixel gray level was carefully set to count thepixels in the droplet zone. The details of calculating procedurewere discussed in Refs. [16–18]. The developed image processingcode was used to calculate the effective diameter of droplet andit gives diameter according to threshold value of pixel gray level.The evaporation history of the droplets was quantified in termsof the square of the diameter of each droplet as a function of time.To compare droplet histories, the squared diameter was normal-ized by the initial droplet diameter, i.e., D2=D2

i ; the evaporationtime was also normalized similarly, i.e., t=D2

i . The square of thediameter decreases linearly with time, which is described by thewell-known D2 law [19,20], as follows:


D2 ¼ D2i � Cv t ð3Þ

where

Cv ¼ �dðD2Þ

dtð4Þ

is the evaporation coefficient, and was extracted from the temporalvariation of the squared droplet diameter using linear regression.The evaporation coefficient is dependent on the thermo-physicalproperties of the liquid itself, as well as the surroundings [19,20].

3. Results and discussion

The two different experiments performed in this study can bedistinguished by their heating methods. During our thermal analy-sis, the propellant experienced a gradual increase in temperature.However, in the droplet experiment, the propellant was exposedto a sudden temperature change. The reaction and evaporationcharacteristics of the propellants were analyzed by examiningthe results from these two different experiments. The interpreteddata from the results will be given in the order of 83.9% HAN,and the blend types 1 and 2.

3.1. Thermal analysis

The mass of the propellant and catalyst pellet used in the ther-mal analysis is indicated in Figs. 3 and 5. The sample mass of pro-pellant was determined to prevent catalyst pellet fragmentation.The mass ratio of propellant to catalyst was set to be 1:1. At initialthermal analysis with catalyst, the residual mass was only 20% ofinitial mass. The intense catalytic decomposition was found tocause sudden expansion of product gas inside the catalyst pelletsand this resulted in fragmentation and escape of catalyst pelletsfrom the container. A comparison of the initial catalyst pellets withthe fragments is shown in Fig. 2. Furthermore, the appropriatemass of each sample was determined from several initial tests toensure that the temperature profile was not affected by the heatreleased from the thermal decomposition of the propellant.Whereas the sample mass at first experimental thermal analysiswas 12.31 mg, it was later reduced to about 5 mg. Also, the samplepan was covered with a drilled cap to prevent escape of catalystpellets. Consequently, the thermal analysis after these modifica-tions was successfully implemented without catalyst fragmenta-tion and large perturbation of temperature profile. Therefore, thesample mass for further thermal analysis is regulated to be about5 mg. However, the sample pan was not covered for thermaldecomposition analysis.

3.1.1. Thermal decompositionThe thermal decomposition results are shown in Figs. 3 and 4.

The masses shown in Fig. 3 were normalized by the initial massto allow a quantitative comparison of different samples. The tem-perature profiles were programmed so that the propellant washeated at a rate of 6 �C/min, following an initial stabilization atroom temperature. During heating, the more volatile components,water and methanol, started to evaporate at temperatures belowtheir respective boiling points. The evaporation for the blend type2 was markedly different to that of the other propellants because itwas not affected by the diffusion limit [21,22] owing to the largerfraction of water and methanol. The pan opened only on one sideto allow the evaporation into surroundings; this facilitated obser-vation of the diffusion-limited phenomena.

As shown in Fig. 3, there was a negative gradient in the massversus time curve following the onset of heating, which corre-sponded to evaporation. Before the temperature reached at theboiling point of water, methanol evaporated mainly together with



Fig. 2. Comparison of (a) the normal catalyst pellets with (b) the pellet fragments following sudden expansion inside the catalyst pore during catalytic decomposition.

0 500 1000 1500 2000 2500 3000 3500 4000

0

20

40

60

80

100

0

50

100

150

200

250

300

350

83.9 % HAN(83 % HAN)+MeOH(55 % HAN)+MeOH

Wei

ght (

%)

Time (sec)

Tem

pera

ture

(o C)

(a)

0 500 1000 1500 2000 2500 3000 3500 4000

0

5

10

40

60

80

100 83.9 % HAN(83 % HAN)+MeOH(55 % HAN)+MeOH

Mas

s de

crea

sing

rate

(%/m

in)

Time (sec)

(b) Fig. 3. Temporal variation of thermally decomposing propellants (a) reduced massin percent with temperature profile, (b) decreasing rate of mass. Weight ofpropellant; (1) 83.9% HAN: 4.87 mg, (2) (83% HAN) + MeOH: 4.95 mg, (3) (83%HAN) + MeOH: 4.83 mg.

-500 0 500 1000 1500 2000 2500 3000 3500 4000 4500

-200

-100

0

100

0

30

60

90

120

150

180

160.5 oC170.3 oC

150.0 oC


Hea

t rel

ease

rate

(mW

/min

)

Time (sec)

Rel

ease

d he

at (m

W)

(a)

0 300 600 900 1200 1500 1800-10

-8

-6

-4

-2

0

2

4

-10

-5

0

5

10

15

0

20

40

60

80

100

120


Hea

t rel

ease

rate

(mW

/min

)

Time (sec)

Rel

ease

d he

at (m

W)

Tem

pera

ture

(o C)

(b)Fig. 4. (a) Temporal variation of the heat release rate, as well as the total releasedheat from the thermally decomposing propellants and temperature profile. (b)Magnified view of graphs during evaporation.


some portion of water. There was a distinctive gradient change onmass curve near the boiling temperature of water. After removal ofwater, there was a sudden change in mass, which corresponds tothermal decomposition of HAN. The thermal decomposition afterthe removal of water was also reported by other research group[14]. The thermal decomposition temperature was defined by thepoint of the positive peak heat release rate, which was 150.0 �C,160.5 �C, and 170.3 �C for the 83.9% HAN and the blend types 1and 2.

In Fig. 4, the evaporation period of water and methanol canbe identified from the negative value of released heat, i.e.


endothermic phenomena. The thermal decomposition of HAN,then, occurs by producing heat release peaks.

3.1.2. Catalytic decompositionFigures 5 and 6 show the results for catalytic decomposition of

propellants. In contrast to the thermal decomposition analysis, thetemperature profiles were programmed so that there was no initialstabilization time because of the lower decomposition tempera-ture. This minimized the evaporation of water before catalyticdecomposition occurred. Therefore, the discontinuity in the massas a function of time for the catalytic decomposition shown inFig. 5 was suppressed as much as possible. In Fig. 5, the final mass



0 500 1000 1500 2000 2500 3000 3500

50

60

70

80

90

100

0

50

100

150

200

250

300

350


Wei

ght (

%)

Time (sec)

Tem

pera

ture

(o C)

(a)

0 500 1000 1500 2000 2500 3000 3500

0

2

4

20

40

60

80


Mas

s de

crea

sing

rate

(%/m

in)

Time (sec)

(b) Fig. 5. Temporal variation of catalytically decomposing propellants (a) reducedmass in percent with temperature profile, (b) decreasing rate of mass. Weight ratioof propellant to catalyst; (1) 83.9% HAN: 5.15 mg to catalyst: 4.86 mg, (2) (83%HAN) + MeOH: 4.91 mg to catalyst: 4.95 mg, (3) (83% HAN) + MeOH: 5.17 mg tocatalyst: 4.82 mg.

-500 0 500 1000 1500 2000 2500 3000 3500-1200

-900

-600

-300

0

300

600

900

0

100

200

300

400

50088.7 oC

96.2 oC

78.7 oC

Hea

t rel

ease

rate

(mW

/min

)

Time (sec)

Rel

ease

d he

at (m

W)


Fig. 6. Temporal variation of the heat release rate as well as the total released heatfrom the catalytically decomposing propellants.

Fig. 7. Normalized squared diameter of the evaporating HAN-based monopropel-lant droplets as a function of normalized time at 100 �C.


was 50% of the starting mass because the catalyst pellets remainedinside the sample pan.

The catalytic decomposition temperature of the three samplesfollowed a similar trend as in the thermal decomposition data. Inthe catalytic decomposition of the HAN-based propellant, itoccurs even before the elimination of volatile component. Thecatalytic decomposition temperature becomes higher for lower


concentration of HAN. This implies that higher activation energyfor lower concentration of HAN is needed for the interaction be-tween HAN molecule and iridium catalyst which leading to selfstanding decomposition. Therefore, the catalytic decompositionhappened at the temperatures of 78.7 �C, 88.7 �C, and 96.2 �C forthe 83.9% HAN and the blend types 1 and 2.

3.2. Droplet evaporation

The droplet evaporation experiments were performed at tem-peratures in the range 100–600 �C at 50 �C intervals. The dropletbehaviors are distinguished whether strong puffing phenomenonoccurs or not. Therefore, the experimental data are categorized intotemperature ranges of r 100 �C and 150 �C, s 200–350 �C, and t

400–600 �C. The temporal history of droplets were collected anddrawn with the results which have the effective diameter in therange 800 ± 50 lm.

3.2.1. 100 �C and 150 �CFigures 7 and 8 show the droplet behavior of each propellant at

temperatures of 100 �C and 150 �C. These results revealed thatthere were two distinct stages in the droplet histories: a steady-state evaporating period of the volatile components, and a dropletexpansion period. At 100 �C, the temporal variation of the normal-ized droplet diameter decreased, with some fluctuations, after aninitial steady state evaporating period. The degree of fluctuationwas quite different between the three propellant mixtures. Themethanol has a higher vapor pressure (methanol: 350.98 kPa,water: 100.72 kPa, both at 100 �C) and a lower boiling point thanwater (64.7 �C at 101.3 kPa) [23]. Thus, the droplet expansion forthe blend type 1 starts earlier, at around 23 s/mm2, compared to40 s/mm2 for the 83.9% HAN, as shown in Fig. 7. Furthermore,the expansion of the droplet was faster and the fluctuations morepronounced for the blend type 1. However, the expansion periodfor the droplets of blend type 2 was more stable than that of theother mixtures, which was attributed to the increased water con-centration inside the droplet.

At 150 �C, which is well above the boiling point of water, theexpansion and fluctuation of the droplets are more pronouncedso that puffing phenomenon occurs for the blend type 1. The puff-ing and post-puffing behavior of droplets can be explained througha diffusion-limit model [21,22]. After a very short initial heatingperiod, the droplets undergo a steady-state evaporating period,during which they are in a diffusion-limited regime because of



Fig. 8. Normalized squared diameter of the evaporating HAN-based monopropel-lant droplets as a function of normalized time at 150 �C.

Fig. 9. Normalized squared diameter of the evaporating HAN-based monopropel-lant droplets as a function of normalized time at temperatures in the range 200–300 �C.


the relatively large concentration of the less volatile componentsnear the surface. Following the stable period, more volatile compo-nents trapped inside the droplet undergo nucleation. This nucle-ation results in gasification of volatile components and causesintense internal pressure to build up. This results in fast escapeof gas generated inside a droplet (i.e., puffing). The droplet of blendtype 1 does not evaporate or thermally decompose after strongpuffing near 40 s/mm2. This reveals that the water and methanolare almost eliminated by strong puffing. The droplets of 83.9%HAN and blend type 2 did not experience strong puffing as in thedroplet of blend type 1. The post-puffing behavior for 83.9% HANdroplet shows that the diameter of droplet increases gradually.However, the case for blend type 2 shows evaporating behaviorwith fluctuation after puffing. This means that the remaining watercontents inside droplet is not enough to build up pressure suffi-ciently so that it does not escape from droplet as for 83.9% HANdroplet. The water content in the droplet of blend type 2 is notcompletely eliminated by puffing so that some exists inside thedroplet. Thus, the droplet of blend type 2 can show evaporatingbehavior after puffing. These phenomena can be explained byexamining the vapor pressure of HAN based propellants.

The vapor pressure of HAN is not available in the literature;however, we can estimate it from the physical properties of theHAN-based propellants LP-1845 (63.23% HAN, 19.96% TEAN,16.81% water) and LP-1846 (60.79% HAN, 19.19% TEAN, 20.02%water) [24]. The reported vapor pressures of LP-1845 and LP-1846 are 10.83 kPa and 11.88 kPa, respectively, at 65 �C, whereasthe vapor pressures of methanol and water are 102.52 kPa and24.86 kPa at this temperature. Furthermore, the vapor pressure ofHAN-based propellants can be calculated by Raoult’s law [25].The vapor pressure values for 83.9% HAN and blend types 1 and2 are calculated as 12.57 kPa, 34.61 kPa and 29.32 kPa respectively,at 65 �C. Consequently, the evaporation of HAN-based propellantdroplets can be described in the framework of a diffusion-limitedmodel.

3.2.2. 200–350 �CIn this temperature range, all the HAN-based propellant drop-

lets exhibited strong puffing phenomena. Furthermore, three dis-tinct stages of the evaporation could be discerned: steady-stateevaporation, puffing, and steady-state decomposition of theremaining HAN. Figure 9 shows the droplet behavior of each pro-pellant mixture at temperatures of 200 �C, 250 �C, and 300 �C whileFig. 10 shows sequential images of an evaporating 83.9% HAN


droplet with effective diameter of 820 lm at 300 �C. Althoughthe experiment was also performed at an ambient temperatureof 350 �C, these data are not shown here. Puffing occurred firstfor the droplet of blend type 1, slightly later in the 83.9% HANdroplets, and later still for the droplet of blend type 2. This wascaused by the variation in the concentration of the more volatilespecies in the droplets. In particular, the droplet of blend type 2 re-sulted in the longest steady-state evaporation period because thewater and methanol in the droplet could easily evaporate becauseof a large portion of the water and methanol and the time to reachthe diffusion limit was longer than the other propellant mixtures.

After evaporation of the water and methanol near the dropletsurface, the droplet experienced expansion due to gasification in-side the droplet. Following the sudden expansion of droplet, astrong puffing phenomenon took place. This strong puffing wascaused not only by droplet heating from surrounding, but alsothe heat release from the thermal decomposition of HAN. It is obvi-ous that the HAN–water solution should be thermally decomposedunder 200 �C from previous research [14] and the thermal analysisresults in this study.

Following the puffing phenomenon, a linear trend in the nor-malized squared diameter as a function of time was observed;the rate of change was larger as the temperature increased. Theremaining droplet was expected to consist of pure HAN togetherwith its decomposition products, because water and methanol can-not exist in a liquid phase at 300 �C. During the later stages of thedroplet evaporation, the square of the diameter decreased linearlywith time, in a manner similar to what occurs when a single-mate-rial liquid droplet burns or evaporates [19]. Therefore, it isconcluded that the final steady-state period represents the decom-position process of pure HAN.

3.2.3. 400–600 �CFigure 11 shows the normalized squared diameter as a function

of time in this temperature range for the three different mixtures,while Fig. 12 shows sequential images of an evaporating the drop-let of blend type 2 with effective diameter of 790 lm at 600 �C. Thethree distinct periods of droplet behavior were no longer observedfor this temperature range. A reduction in the size of the dropletsbefore the puffing was not observed, except for the case of thedroplet of blend type 2 at 400 �C. The droplet diameters remained



0.0s 2.9s 3.4s 3.9s 4.4s

4.6s 4.8s 5.0s 6.5s 38.6s

Fig. 10. Sequential images of the evaporating 83.9% HAN droplet with initial effective diameter of 820 lm at 300 �C.

Fig. 11. Normalized squared diameter of the evaporating HAN-based monopropel-lant droplets as a function of normalized time at temperatures in the range 400–600 �C.


almost constant for a short period of time following exposure tothe high-temperature environment. Then they expanded slowly,rather than shrinking. This can be explained by considering that

0.0s 0.15s 0.49s

1.19s 1.23s 1.39s

Fig. 12. Sequential images of the evaporating (55% HAN) + MeOH


the nucleation and gasification of the more volatile contents insidethe droplets happened much faster than the evaporation of waterand methanol near the droplet surfaces [16]. The ensuing puffingbehavior was similar to the previous temperature regimes, buthappened earlier and faster at the higher temperatures.

The role of methanol is not so dramatic in thermal analysis be-cause the HAN-based propellants are heated very slowly. However,the presence of methanol plays an important role in the dropletexperiment, when the propellant droplet is exposed abruptly tohigh temperature surroundings. The propellant droplet of blendtype 1 showed faster puffing or micro explosion phenomena com-pared to 83.9% HAN propellant droplet. This can augment theatomization or fragmentation of the droplet in a real thrusterand even the decomposition efficiency. The results for (55%HAN) + MeOH propellant droplet shows that large content of waterdelays the puffing phenomenon.

3.2.4. Decomposition rate of the HAN dropletsFigure 13 shows the decomposition rates at different tempera-

tures from HAN droplet evaporation data extracted from the linearpart of the droplet shrinkage curves. As described previously, thislinear part corresponds to the decomposition of pure HAN. The lin-ear regression rates were calculated and the average values wereplotted with error bar. The difference in the gradient at a giventemperature was considered to be the effect of the droplet sizeafter the puffing phenomenon has occurred. After the puffing, thedroplet of blend type 2 tended to have smaller droplet sizes thanthe other mixtures, and this resulted in higher evaporation rates.

0.82s 1.10s

3.05s 4.05s

droplet with initial effective diameter of 790 lm at 600 �C.



200 300 400 500 600 7000.00

0.02

0.04

0.06

0.08 83.9 % HAN(83 % HAN)+MeOH(55 % HAN)+MeOH

Cv (

mm

2 /s)

Temperature (oC)

Fig. 13. Decomposition rate of the remaining HAN droplets following puffing attemperatures in the range 200–600 �C.


4. Conclusions

The evaporation and decomposition characteristics of threeHAN-based monopropellants were investigated using thermalanalysis and droplet evaporation. The base monopropellant was83.9% HAN–water solution. A HAN-based monopropellant blendedwith methanol in a stoichiometric ratio was also examined, alongwith a third propellant further diluted with water. The dropletexperiments were performed at atmospheric pressure and at tem-peratures in the range 100–600 �C. Images of droplets were post-processed to calculate the diameter of the evaporating droplets,and the decomposition rate was obtained using linear regressionof the final section of the droplet evaporation history. Our majorresults can be summarized as follows.

(1) The thermal decomposition temperatures of 83.9% HAN,(83% HAN) + MeOH, and (55% HAN) + MeOH were 150.0 �C,160.5 �C, and 170.3 �C, respectively. Thermal decompositionof HAN occurred after the water and methanol had beenevaporated from the droplets.

(2) In the presence of an iridium catalyst, the catalytic decom-position temperatures were 78.7 �C, 88.7 �C, and 96.2 �C for83.9% HAN, (83% HAN) + MeOH, and (55% HAN) + MeOH,respectively. The presence of water and methanol disturbedthe interaction between HAN molecules and catalysts.

(3) The droplet evaporation data at temperatures of 100 �C and150 �C showed a two-stage evaporation history. After an ini-tial period where the more volatile species near the dropletsurface evaporated, the droplet exhibited diffusion-limitedbehavior. This resulted in nucleation inside the droplets,and fluctuations of the droplet surfaces were observed.

(4) Strong puffing phenomena were observed for each of thethree propellant mixtures at temperatures over 200 �C. Lin-ear regression of the temporal variation of the droplet


following the puffing led to conclusion that the remainingmaterial was pure HAN together with its decompositionproducts.

(5) The thermal decomposition rate of the HAN droplets wascalculated from the gradient of linear droplet shrinkagecurves at atmospheric pressure and at temperatures in therange 250–600 �C.

Acknowledgments

The authors gratefully acknowledge the financial support pro-vided by Defense Acquisition Program Administration and Agencyfor Defense Development under the contract ADD13010413.

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