experimental observation of hypergolic ignition of...
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Experimental Observation of Hypergolic Ignitionof Superbase-Derived Ionic Liquids
Jianling Li∗ and Wei Fan†
Northwestern Polytechnical University, 710072 Xi’an, People’s Republic of China
Xinyan Weng,‡ Chenglong Tang,§ Xuhui Zhang,¶ and Zuohua Huang**
Xi’an Jiaotong University, 710049 Xi’an, People’s Republic of China
and
Qinghua Zhang††
Chinese Academy of Engineering Physics, 621900 Mianyang, People’s Republic of China
DOI: 10.2514/1.B36441
The hypergolic ignition behaviors of four newly synthesized ionic liquids with dicyanamide anion (ionic liquids 1 and
3) and cyanoborohydride anion (ionic liquids 2 and 4) were experimentally investigated. The results showed that
successful hypergolic ignition of both group ionic liquids were achieved with white fuming nitric acid and red fuming
nitric acid. Two distinct flame initiation processes illustrated by several stages were observed for each group of ionic
liquids. Specifically, the ionic liquid 1 drop showed immerging, exploding, and ignition: the drop immerged and reacted
with the oxidizers underneath the liquid surface, producing gas vapor andwhite foam like intermediates on the surface;
then, the surface exploded by fast accumulation of heat and vapor underneath, followed by flame kernels formation in
the adjacent gas phase. The ionic liquid 2 drop showed bouncing and igniting: it just created a crater on the oxidizer
surface and then sat on theLeidenfrost vapor layer generated by impaction and reaction. Ionic liquids 3 and 4 showed a
similar flame initiation process, respectively, to that of ionic liquids 1 and 2. Furthermore, the explosion delay time and
ignition delay time of the four ionic liquids were recorded, both the previous and present experimental repeatabilities
were analyzed, and it was shown that heat loss control was important to the experimental repeatability.
I. Introduction
NASA and commercial launch vehicles use four types ofpropellants, including petroleum, cryogenics, solids, and liquid
hypergolic [1]. Liquid hypergolic propellants are those fuel andoxidizer pairs that react and release enough heat to ignitesimultaneously upon contact with each other. These types ofpropellants offer significant advantages over their counterparts due totheir unique ignition and combustion behaviors. For instance, liquidhypergolic propellant rockets are typically simple and reliablebecause they need no ignition system, which also contributes to ahigher thrust-to-weight ratio. In addition, unlike cryogenics, storageconditions for these propellants have no low-temperature require-ment; thus, they are called storable liquid propellants. Furthermore,compared to solids, it can be restarted, throttled, and shut down bysimply controlling the valves of the reactant supply lines.Hypergolicity was first found in Germany around the middle of the1930s, and testing and research on hypergolic propellants in terms ofreactivity, ignition mechanism comprehension, and oxidizerselection spread to other countries after World War II. The mostwidely used liquid hypergolic propellants in rocket engines aremonomethylhydrazine (MMH), unsymmetrical dimethylhydrazine(UDMH), and Aerozine (mixture of UDMH and straight hydrazine),
all of which use nitric tetroxide (N2O4) as the oxidizer due to theirhigh specific impulses. However, these hydrazine-based hypergolicpropellants are extremely toxic and carcinogenic, with high vaporpressure and a low flash point. Thus, the storage and handling areexpensive and of high risk. Searching for less toxic and easily storablegreen hypergolic propellants to reduce cost and increase handlingsafety has raised significant attention in the past few decades [2–10],and finding the replacement of hydrazine-based propellants is also amajor objective of several current research projects in the UnitedStates [11] and the European Union [12].To evaluate the performance of a liquid hypergolic propellant,
several issues need to be addressed from a fundamental combustionpoint of view. First, selection of an appropriate oxidizer pair isimportant because different fuel/oxidizer combinations may result incompletely different hypergolicity, and some combinationsmay evenfail to hypergolically ignite. This is due to the reactive nature of thefuel and oxidizer. Second, the specific impulse of the developed greenhypergolic propellant needs to be evaluated such that its propulsionperformance is not compromised by the benign nature. Finally, theignition delay time (IDT) needs to be evaluatedwithwell-defined testconditions and accuracy. The ignition delay time is defined as thetime interval between the instant of fuel–oxidizer contact and the firstobservation of a flame kernel. The IDT is one of the most importantengineering parameters for hypergolic propellant development. Along IDT may result in the so-called hard-start problem [13–15]:accumulation of explosive intermediate species may lead todetonation upon ignition, which is catastrophic. Thus, accuratedetermination of the ignition delay times is fundamentally important.However, previous ignition delay times data had difficulties inrepeatability and laboratory-to-laboratory comparisons [8,9].Recently, energetic ionic liquids (ILs) have emerged and been
considered to be ideally suited as a potential hydrazine replacement[16–26] because energetic ILs (which are kinds of salts in the liquidstate at room temperature) have extremely low vapor pressure, highthermal stability, and high energy density. In addition, ILs areinsensitive toward destructive stimuli such as impact, friction, orelectrostatic discharge. Furthermore, they can be produced with lowsolubility in water and high hydrolytic stability for environmentalreasons. All these properties and characteristics make energetic ILstorage and handling easy, and they have been constantly called green
Received 24 August 2016; revision received 19 March 2017; accepted forpublication 8 May 2017; published online 9 June 2017. Copyright © 2017by the American Institute of Aeronautics and Astronautics, Inc. All rightsreserved. All requests for copying and permission to reprint should besubmitted to CCC at www.copyright.com; employ the ISSN 0748-4658(print) or 1533-3876 (online) to initiate your request. See also AIAA Rightsand Permissions www.aiaa.org/randp.
*School of Power & Energy; [email protected].†School of Power & Energy; [email protected].‡State Key Laboratory of Multiphase Flow in Power Engineering;
[email protected].§Associate Professor, State Key Laboratory of Multiphase Flow in Power
Engineering; [email protected] (Corresponding Author).¶State Key Laboratory of Multiphase Flow in Power Engineering;
[email protected].**State Key Laboratory of Multiphase Flow in Power Engineering;
[email protected].††[email protected].
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JOURNAL OF PROPULSION AND POWER
Vol. 34, No. 1, January–February 2018
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hypergolic propellants. The first IL reported was ethylammoniumnitrate, which was reported in 1914 by Walden; see the review paper[17]. In the past few decades, the synthesis of new ILs with higher-energy density has been themost exciting progress in chemical science[19–25].As attractive as energetic ILs are, only a few of them have been
reported to be hypergolically ignitable. Schneider et al. [16] reportedthat several nitrocyanamide–anion-based ionic liquids would reactwith white fuming nitric acid (WFNA) and lead to hypergolicignition. However, the quality of the reported photographs was nothigh enough to show the initial contact of the dropletwith the oxidizerin order to reveal the initiation mechanism of flame kernel. McCraryet al. [26] recently investigated the hypergolic ignition of 38 ionicliquids comprising 13 cations, and they found that the reactivity ofthese ionic liquids correlated strongly with the electron density in thecation and subtle changes in the chemical structure of cation wouldgreatly influence the hypergolic ignition.One contributor of this work recently synthesized 14 new
superbase-derived ionic liquids [24] with remarkably improvedthermal stability. Fundamental understanding of their ignitabilitybehaviors should favor the possible application of these hypergolicfuels. As such, the first objective of the present work is toexperimentally test the hypergolicity of someof these ionic liquids andoxidizer combinations to see if they can achieve hypergolic ignitionwith selected oxidizers. The hypergolicity of a fuel is perhaps the mostsensitive to the oxidizer with which it reacts, and an unmatchedcombination will definitely lead to failure of ignition. In addition, wenote that hypergolic ionic liquids are liquid salt that contain anions andcations. How the chemical structure of the ionic liquids affects thehypergolic initiation process is a question that has not been answered.Thus, our second objective is to experimentally explore if there are anyspecific rules in controlling the flame initiation process of the ionicliquids. To access this, we will focus on the initial behavior uponcontact of the selected ionic liquid fuel and liquid oxidizer. The reasonis that the hypergolic ignition is controlled by complex coupling of thehydrodynamics of liquidmixing, a liquid phase reaction that generatesintermediate products and heat, and a phase change that is induced bylow-temperature heat release; thus, a phenomenological investigationmay provide direct support for understanding this complicatedprocess. Furthermore, meaningful parameters that characterize theliquid phase reaction such as the ignition delay time are importantbecause they represent the liquid phase reactivity in terms of rate ofheat release and intermediate species accumulation. Thus, our thirdobjective is to quantify the liquid phase reactivity by using somewell-defined parameters such as the ignition delay time. In this work, wehave also shown that the previously proposed explosion delay time(EDT) in [6,7,10] is a physicallymeaningful parameter to represent thelow-temperature reaction timescale. In the following, we will specifyour experimental apparatus and procedures first. Then, experimentalobservations and a detailed analysis on the flame initiation will bepresented, followed by quantification of the experimental data byintroducing some physically sounding parameters.
II. Experimental Specifications
A. Experimental Apparatus
In this work, we will investigate the hypergolic ignition of fourselected superbase-derived ionic liquids. Wewill use the drop testingmethod for the study because this method is simple, quick, and lowcost; more important, it offers the experimental condition that can bemost accurately controlled. Figure 1 shows the schematic drawing ofthe experimental system. Ionic liquid droplets are generated at the tipof a hypodermic needle (mounted on a three-dimensional positioner)and fall down vertically into the target oxidizer pool in a deep Petridish. The initial stage of fuel droplet interaction and the oxidizerliquid pool was recorded by a Phantom V611 high-speed cameraoperating at a rate of 10,000 frames per second (fps). A long-focusmicroscope was attached to this camera to ensure high spatialresolution (∼0.01 mm∕pixel).Another high-speed camera (PhantomV1) with an operating rate of 10,000 fps was used to simultaneouslyand directly capture the subsequent ignition and flame propagation
with a larger targeting view area. Unlike the shadow light source forV611, the scattering light sourcewas used for a V1 camera so that theillumination of the ignition process could be identified. The dropletrelease height was adjustable, and the velocity before impaction wasobtained from the V611 camera and confirmed by the V1 camera.The V611 and V1 cameras were perpendicular.
B. Test Liquids
Ionic liquid has an extremely low vapor pressure and no corrosion.As a consequence, its experimental handling is easy and safe.However, great caution is required when handling the liquid oxidizer.All the preparationswere conducted in a fuming hood, and an effectiveventilation above the experimental area was installed. Three oxidizers(purchased from the Xi’an Aerospace Propulsion Institute) weretested: white fuming nitric acid (WFNA, 97.5 wt % HNO3, less than2 wt % water, and 0.5 wt % maximum NO2), red fuming nitric acid(RFNA, 84 wt % HNO3, 14� 1 wt% NO2 and 1 ∼ 2 wt% water),and N2O4. Before the experiment, the Petri dish filled with oxidizerwas covered by a glass plate to minimize the effect of oxidizervaporization. The plate was then removed exactly before the ionicliquid drop fels down. The Petri dish was large and deep enough tocontain sufficient liquid oxidizer. Because the amount of the oxidizerpool (around ∼150 ml) was much more than the fuel droplets(∼0.01 ml), we assumed that several fuel drops did not contaminatethe oxidizer pool, andwe did not have to change the oxidizer for everydropping. However, after about five droppings, we changed thereplaced the pool liquids with new and fresh oxidizer. The four ionicliquids tested in this work were synthesized by Zhang et al. [24], andthe chemical formulas and structures are listed in Table 1. Ionic liquids1 and 3 have the samedicyanamide anions �N�CN�−2 �, and ionic liquids2 and 4 have the same cyanoborohydride anions (BH3CN
−). A total of102 dropping tests have been performed (70 forWFNA, 22 for RFNA,and 10 for N2O4). The properties of these liquids and the testconditions including the oxidizer, drop impact velocity, and number ofeach repeated test are also given in Table 1.
C. Uncertainty Analysis
In this work, the size of an impacting droplet was measured usinghigh-speed video cameraV611 captured images. The impact velocityof the droplet was obtained from the droplet photographs in twoframes before impact by their spatial distance (around 200 pixels) andtime interval.We estimated that a maximum error in determination ofthe droplet contour was about�2 pixels (equivalent to 0.02mmwitha 0.01 mm∕pixel spatial resolution). The uncertainty of the dropletdiameter D0 was then estimated to be�0.04 mm, and the deviationof the droplet velocity U was less than 2% when the distance thedroplet travelled was 2 mm (200 pixels). The Weber number wasdefined as We � ρU2D0∕σ, where ρ and σ are, respectively, thedensity and surface tension of the ionic liquids. As a consequence, the
Fig. 1 Sketch of the experimental system.
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Weber number uncertainty was estimated to be ΔWe∕We �ΔD0∕D0 � 2 ΔU∕U, which was less than 6%.The uncertainty of the ignition delay time and explosion delay time
(EDT) mainly comes from the determination of the impact timing,ignition timing, and explosion timing. Because the ignition andexplosion phenomenon can be well recognized within three framesand the high-speed camera sampling rate is 10,000 fps, the errors ofignition and explosion timing are then approximately �0.3 ms,which is negligibly small compared to the IDT and EDT.
III. Results and Discussion
A. Experiments of Nonreactive Case
It is noted that, in the hypergolic propellant rocket engines, ignitionis affected by impinging andmerging the hypergolic fuel and oxidizerdroplets or jets. The ignition and the resulting heat release rate wouldthen depend on the initial contact dynamics and the physicalmixing rate of the merged mass [27]. Because of the importance ofthese physical processes, before we proceed with the hypergolicexperiment, we have dropped typical ionic liquid into a water pool asa nonreactive base case for comparison with the upcoming reactivecase. Previously, there have been extensive studies on interfacephenomenon in terms of a droplet impact with the same or differentfluid, including the impacting of a droplet with another droplet[28–30], with liquid film [31,32], or with a liquid pool [33–36]because, in the absence of a reaction, the impacting process isgoverned by the physical properties of the fluids and the impactinertia. These studies show that, for given fluids, the impactingdynamics and outcome depend on the impaction inertia, which istypically quantified by theWeber numberWe � ρD0U
20∕σ, where ρ,
D0, U0, and σ are, respectively, the density, diameter, velocity, andsurface tension of the impacting droplet. In addition,with the increaseof the Weber number, the impacting will result in four differentoutcomes: 1) soft coalescence, 2) bouncing, 3) hard coalescence, and4) splashing.Figure 2 shows a typical ionic liquid droplet impacting into a
distilled water pool with the droplet velocities from 1.25 to1.80 ms−1. For the droplet velocity studied here in this work, it justimpacts and coalesces with the liquid pool. No bouncing orsecondary splashing droplets were observed; thus, the impactionoutcomewas coalescence. In addition, because soft coalescence onlyoccurs for a very weak impaction case when the impacting Webernumber is on the order of O�0.1� [27,28], the present coalescenceimpaction lies in the hard coalescence region, which was alsoevidenced by the strong outwardly propagating capillary wave uponcoalescence: As the droplet approaches the liquid pool, it quicklymerges with the pool surface and penetrates into the pool while a
swelling capillary wave is raised and propagates outwardly, and then
its height goes up to a maximum and decreases while it keeps on
expanding; ultimately, the surface stays at rest (for a complete
impaction process, see Supplemental Video 1).We note that, for all the tested ionic liquids, the nonreactive
impaction processes were quite similar, and we have defined the
spreading diameter of the swelling wave. In addition, we can see that
the physical time of the impaction process is on the order of 100 ms.
We attempted to quantify the physical impacting dynamics through
some meaningful parameters. The primary diameter of the spreading
swelling wave was then nondimensionalized by the initial droplet
diameter (β in Fig. 3). The reason we selected this parameter was that
it actually represented the extent of the vertical penetration length of
the impacting droplet, which we were not able to measure in the
present test rig. A larger β represented a smaller penetration length to
some extent because the initial droplet-impacting inertia should be
balanced by the energy of the spreading wave and the vertical
penetration kinetics.Figure 3 shows the nondimensionalized diameter of the spreading,
swellingwave evolution for the tested velocity range. It was seen that,
as time proceeds, β increaseswith a decreased rate. This is because, astime proceeds, the initial inertia of the impacting droplet gradually
dissipates through the fluid motion in both the vertical and horizontal
directions. In addition, it seems that, with the increase of the initial
droplet velocity, β slightly decreases. This is probably because a
higher inertia of the droplet results in a deeper penetration, and there
was relatively less inertia dissipated in the horizontal direction
through the spreading, swelling wave.
B. Immerging, Exploding, and Ignition
Figure 4 shows selected images of IL1 drop impacts into the
WFNA pool simultaneously captured by two cameras. Figures 4a–4c
are from the long-focus microscope V611 camera, and Figs. 4d–4f
are from the V1 camera. The detailed process of this impact can be
seen in Supplemental Videos 2 and 3. After the droplet impact on the
liquid pool surface, the following processes were observed:1) The droplet immerged into the deep pool and a swelling wave
was generated and propagated outwardly, which was similar to thenonreactive case. However, later, at around 27.2ms, a bulge appearedat the exact location of the impact (Fig. 4b) and its surface becamehighly disturbed (Fig. 2c). This was because the liquid phase reactionunderneath the surface generated enough heat to raise the temperatureto the boiling point of the WFNA. The vaporized oxidizer and gasproducts were accumulated to blow up the bulge.2) At 52.2ms, somewhite foamlike products appeared at the bulge
surface and some gray smoke began to raise and grow in area (please
Table 1 Chemical structures, physical properties, and drop test conditions of the ILs
Physical properties [24] Test condition
IL no. Chemical formula Chemical structure ρ, g ⋅ cm−3 μ, mPa ⋅ s σ,a N ⋅m−1 Oxidizer U0, ms−1 No. test
1 C12H17N5 1.12 46.4 0.065 WFNA 1.25∕1.55∕1.8 11∕7∕6RFNA 1.25∕-∕- 3∕-∕-N2O4 1.25∕-∕- 2∕-∕-
2 C11H20BN3 1.00 87.4 0.060 WFNA 1.25∕1.55∕1.8 5∕6∕8RFNA 1.25∕1.55∕1.8 3∕2∕2N2O4 1.25∕-∕- 2∕-∕-
3 C12H15N5 1.14 121.7 0.068 WFNA 1.25∕1.55∕1.8 2∕8∕2RFNA 1.25∕1.55∕1.8 3∕3∕3N2O4 1.25∕-∕- 1∕-∕-
4 C11H18BN3 1.03 332.8 0.071 WFNA 1.25∕1.55∕1.8 5∕5∕5RFNA 1.25∕-∕- 3∕-∕-N2O4 1.25∕-∕- 5∕-∕-
aObtained by balancing the surface tension holding the droplet onto the needle tip and gravity at the instant when the droplet is detached from the
needle, as σ � ρgD30∕�6dnozzle�.
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refer to Supplemental Video 3). The gray smoke was very similar tothe aerosol cloud reported by Wang and Thynell [9] for the MMHdrop test into a nitric acid liquid pool, and we believe it was certain
kinds of fine intermediate products/fuel particles in the gas vapor ofoxidizer. In addition, the foamlike products were hypothesized to besome spongy solid that could attract oxidizer vapor.3) At 63.6 ms, the foam surface exploded and many smaller white
“foam balls”were ejected (Fig. 4d) because the gas under the surfaceaccumulated to a sufficiently high pressure. In addition, more graysmoke was generated at a faster rate. These foam balls can attractoxidizer vapor to further react with the remnant fuel inside.4) One or more ejected white foam balls quickly became black
(Fig. 4e), which indicated new intermediate products due to thecontinuous increase in local temperature. Immediately after that,flame kernels were formed at a certain distance away from the foamball and quickly propagated through the gray smoke along the surfaceand upwardly to engulf other foam balls (Fig. 4f).For IL1 drop impacts in the RFNA pool, the process was very
similar to that of the WFNA: the drop immerged and reacted withthe liquid oxidizer underneath the surface, which led to white a
foam bulge appearing and growing in size at the disturbed surface,which led to bursting of the bulge foam and ejection of smallerfoam balls, which led to white foam balls becoming black, which
led to the flame kernel being generated. However, the timedurations of the aforementioned stages were different (see
Supplemental Video 4). In addition, when the IL 1 drop impactedthe N2O4 pool, no hypergolic ignition was observed, although wedid see some sudden ejection of reddish brown gas caused by a
reaction underneath the surface.Fig. 3 Normalized capillary wave diameter as a function of normalizedtime.
Fig. 2 Images and time sequences of typical Ionic liquid drop impacting onto a water pool.
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C. Bouncing, Leidenfrost Layer, and Ignition
Because ILs 2 and 4 have different anions from ILs 1 and 3, we
have also analyzed the initial droplet contact and flame initiation
process. Surprisingly different phenomena in terms of the initialcontact dynamics and the subsequent flame initiation were observed.
Figure 5 shows selected images upon the IL2 drop impact on theWFNA pool. Figures 5a–5c are from a long-focus microscope V611
camera, and Figs. 5d–5f are from aV1 camera (Supplemental Videos
5 and 6). It is seen that, after impact, the following process wasobserved:1)A crownlikewavewas formed immediately after the impact, and
the impact surface was crater shaped. Gas vapor and a secondarydroplet were ejected along the crater surface (Fig. 5b) at around2.5 ms after impact. The rim of the crown was then descending andexpanding outwardly, the center crater contracted back, and thisprocess was accompanied by continuous ejection of a secondarydroplet and vapor from the crater surface. The mechanism was that,upon contacting the surface, the interface mixtures reacted andproduced heat and a gaseous product. Then, the local temperatureincrease vaporized the WFNA and a vapor layer composed of thegaseous product and WFNA vapor was then quickly formed toprevent the droplet frommixingwith the liquidWFNA; this layerwasthe so-called Leidenfrost vapor layer, which was similar to theobservations ofDambach et al. [6] andForness et al. [10] for anMMHdroplet.2) When the center crater gradually contracted upwardly, a foam-
shelled droplet gradually jumped up (Fig. 5d) and bounced off thesurface. After reaching the maximum height, the droplet fell downfreely onto the surface again and deformed to a disk shape; it thenresumed the spherical shape sitting on the surface (please refer toSupplemental Video 5). Mechanistically, the Leidenfrost vapor layerlevitated droplet or foam ball was actually the remnant of the IL2shelledwith some foam intermediate (spongy solid) product, and thisfoam ball was much similar to the foam ball ejected in stage 3 for theIL1, as illustrated previously.As the IL droplet keeps on reactingwith
the oxidizer surface, gas products or oxidizer vapor form a film andpropagate radially along the oxidizer surface (please refer toSupplemental Video 6), which acts as the reactive Leidenfrostvapor layer.3) Because the spongy structured foam shell can attract oxidizer
vapor to continuously react with the inner remnant fuel, it then growsand ejects gray gas vapors. After sitting on and reacting with thevapor layer for a some induction time, thewhite foam ball turns blackand the ejection of the gray vapor is suddenly accelerated (Fig. 5e,and complete Video 6), indicating new intermediate products due tothe continuous increase in the local temperature. Immediately afterthat, a flame is initiated that quickly propagates and engulfs the grayvapor and the foam ball (Fig. 5f).We have also conducted experiments for IL3 and 4, and their flame
initiation processes are, respectively, similar to that of IL1 and IL2.The typical ignition processes of IL3 and IL4 are illustrated inSupplemental Videos 7 and 8. Thus, we conclude here that anions inthe ionic liquid dominate the initial flame initiation process. We haveto note that recent work by Forness et al. [10] and Dambach et al. [6]have raised the issue of experimental repeatability for MMH andsignificant data scatters reported before. They showed that distinctimpact outcomeswere observed even if the same initial condition andprocedures were carefully followed. In addition, the probability ofsuccessful hypergolic ignition reported in [6,10] was from 0 to 100%for different combinations of hydrazines and oxidizers. Here, in ourwork, we are confident that the experimental observations are veryrepeatable. For all 103 dropping tests, the hypergolic ignitions of allthe ILs with the WFNA and RFNA were achieved (100% ignitionprobability). However, none of the ILs showed ignition with N2O4
(0% ignition probability). In addition, except for the different timedurations of each stage described, the phenomena that were observedin each stage were almost the same: IL1 and IL3 [with the sameN�CN�−2 anion] showed explosion and ignition after immerged withthe WFNA and RFNA pool, whereas with N2O4, there was noignition or it was too long to be recorded by the camera. The IL2 and
bulgedisturbed surface
foam ball
t/ms=0 27.2 34.6
63.6 85.5 131.1 Fig. 4 Selected images of drop test for IL1 drop impacts with WFNA pool.
t/ms=0 2.5 4.7
225.9 712.4 766.8
vapor droplets
foam ball
Fig. 5 Selected images of drop test for IL2 drop impacts with WFNA pool.
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IL4 (with the same BH3CN−1 anion) showed reaction with the
Leidenfrost layer gas and then ignition after a long induction timeafter impacted and bounced off of the WFNA and RFNA poolsurface; whereas with N2O4, there was no ignition or ignition takestoo long to be recorded by the high speed camera. This differencebetween the repeatability of the outcomes (ignition or nonignition) ofthe fuel–oxidizer impact between [6,10] and the present experimentmay be attributed to two reasons. First, the impact angle (shape of theoxidizer surface) in [6,10] shows an important effect; whereas in ourwork, the impact is vertical. Second, the miscibility between the fueland oxidizer in this work and that in [6,10] may be different, whichpossibly leads to different mixing and ignition behaviors.
D. Explosion Delay Time and Ignition Delay Time
We have carefully examined the successful hypergolic ignitionvideos. The majority of the tested oxidizer is the WFNA, and wefound that there are significant inconsistencies in the reactiontimescale.To quantitatively illustrate the overall liquid phase reactivity, we
adopted two induction time definitions: the explosion delay time andthe ignition delay time. The explosion delay time is defined as theinitial propellant contact and the first sign of violent liquid expulsionfrom the contact region. The ignition delay time is the time intervalbetween the contact of a drop on the liquid oxidizer surface andthe instance of the first observation of a flame. Figure 6 shows theexplosion delay time and ignition delay time as a function of theWeber number: We � ρD0U
20∕σ. The oxidizer pool is WFNA,
the scatters are experimental measurements, and the lines are linearfitting. We note that the data in Fig. 6a were from a long-focusmicroscope V611 camera so that fine spatial resolution could bereached, and data in Fig. 6b were obtained from a V1 camera with alarger captured view. It seems that the EDT was very consistent forsimilar Weber number We impactions, and a slight decrease in theEDTwas found with an increasing Weber numberWe. For instance,the explosion delay times of IL1withWFNAare, respectively, 64, 52,and 24 ms, for We ≈ 60, 100, and 140. In addition, dicyanamideanion ILs (1 and 3) show a considerably higher EDT thancyanoborohydride anion ILs (2 and 4). This is because IL2 and IL4drop very quickly, and they generate a significant amount of gasvapor and secondary droplets upon contacting the pool surface;whereas IL1 and IL3 immerge into the pool; see the comparisonbetween Figs. 4b and 5b. However, faster explosion does notnecessarily lead to faster ignition, as can be elegantly seen in Fig. 6b.It is seen that the IDTof IL1 and IL3were very similar, on the order ofseveral tens of milliseconds, and they show very good consistency.However, for IL2 and IL4, although with good personnel handling, asignificant discrepancy in the IDT was observed. The IDT For IL2and IL4 for We ≈ 140, respectively, spanned a range between 100and 800 ms and 100 and 500 ms. We have done a great manyrepeating experiments, and both the EDTand IDT presented here arethe averaged values. Table 2 shows the averaged values of the EDTand IDT, together with the standard deviation.The only factor that lead to a local temperature increase that
favored the hypergolic ignitionwas the heat release due to preignitionreactions. These reactions included the liquid phase reaction,
reactions of liquid fuel with vaporized oxidizer, and reactionsbetween vaporized fuel or intermediate species with the gas phaseoxidizer. It was expected that the liquid phase reaction came first torelease enoughheat so that the liquid oxidizerswere evaporated; then,liquid fuel or intermediate species might react with gaseous oxidizerto producemore a gaseous intermediate and further increase the localtemperature. If the temperature was high enough, the vaporized ionicliquids and intermediate species might be accumulated to sufficientlyhigh concentration such that a gas phase reaction and ignitionmight occur.However, during the sequences of exothermic reactions, there may
be some other factors that bring down the local temperature such asdecomposition reactions; the evaporation heat absorption; and mostimportant, the heat loss to the surrounding environment. If the heatrelease exceeds those heat loss factors, hypergolic ignition occurs. Aswe observed in the experiments, for IL1 and IL3, the liquid phasereaction happened underneath the surface, for which the heat releasejust resulted in the evaporation of the oxidizer and acceleration ofreaction sequences, and there was no heat loss to the surroundingenvironment. However, for IL2 and IL4, although the initialexplosion was faster than IL1 and IL3, the fuel was oxidized on theLeidenfrost vapor layer; thus, significant heat loss to the environmentwas expected, and this heat loss, neither convective nor radiative,could not bemonitored experimentally in thiswork. This explains thelower IDT for IL1 and IL3 and the poor repeatability in the IDTof IL2and IL4 in Fig. 6b.
IV. Conclusions
The hypergolicity of four ionic liquids (two dicyanamide anions andtwo cyanoborohydride anions) was experimentally examined in thiswork. Different oxidizers have been tested to see if the specific ionicliquid (IL) and oxidizer combinations show successful hypergolicignition. It was seen that the four ILs all hypergolically ignited withwhite fuming nitric acid and red fuming nitric acid, but neither of themignited with N2O4. In addition, two groups of ILs showed asignificantly distinct flame initiation process. The dicyanamide anions
Table 2 Explosion delay time and ignition delay time of ILs withWFNA
IL no. U0, ms−1 Explosion delay time, ms Ignition delay time, ms
1 1.25 64(6) 79(16)1.55 52(8) 66(8)1.8 24(2) 59(7)
2 1.25 1(0) 395(293)1.55 1(0) 555(202)1.8 1(0) 504(329)
3 1.25 55(0) 72(6)1.55 22(10) 61(21)1.8 38(23) 88(34)
4 1.25 17(0) 280(177)1.55 4(1) 172(73)1.8 4(1) 215(66)
Fig. 6 EDT and IDT as a function of Weber numberWe for different ionic liquid drops.
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ILs’ drops showed immerging, exploding, and ignition uponcontacting the pool surface: the drop immerged and reacted with theoxidizers underneath the liquid surface, producing gas vapor andwhitefoamlike intermediates on the surface; then, the surface exploded byfast accumulation of heat and vapor underneath, followed by theoxidation and ignition of those foamlike intermediates by the flamekernels formed in the adjacent gas phase. The cyanoborohydride aniondrops showed bouncing and igniting: they just created a crater on theoxidizer surface and sat on the Leidenfrost vapor layer generated byimpaction and reaction; the remnant drop was wrapped by whitefoamlike intermediates,whichwere further oxidized and ignited by theflame kernel formed in the adjacent gas phase. These behaviorsindicated that the flame initiation process of ILs was dominated bytheir anions. Furthermore, the drop test videos showed a veryrepeatable flame initiation process; this good repeatability, that was notreached by previous studies on hydrazine, was believed to be aconsequence of the thermal stability nature of ionic liquids. However,the ignition delay time for cyanoborohydride anion ionic liquidsshowed poor quantitative inconsistency because of the heat loss to theenvironment that could not be monitored.
Acknowledgments
This work is supported by the Science Challenge Project (No.TZ2016001), the National Natural Science Foundation of China(91541107 and 11572258), the Fundamental Research Funds for theCentral Universities, the opening project of State Key Laboratory ofExplosion Science and Technology (Beijing Institute of Technology)(No. KFJJ17-04M). Liquid preparation by Wenquan Zhang and ShiHuang is very much appreciated.
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