Paper # 070HE-0225 Topic: Spray Flames

8th US National Combustion MeetingOrganized by the Western States Section of the Combustion Institute

and hosted by the University of UtahMay 19-22, 2013.

Experimental Characterization of 1-Butanol ElectrosprayCombustion

Michael Pennisi1 Dimitrios C. Kyritsis123

1Department of Mechanical Science and Engineering,University of Illinois Urbana-Champaign, 1206 W. Green St., Urbana IL, 61801

2Department of Mechanical Engineering,Khalifa University of Science and Technology, and Research Abu Dhabi, UAE3International Institute for Carbon Neutral Energy Research (WPI-I2CNER),

Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan

An experimental investigation is presented on the combustion of bio-butanol electrosprays. High-resolution images were used to establish spray flame phenomenology and features such as shape,size, and stability of the spray flame. Phase Doppler Anemometry droplet size measurements wereperformed in both reacting and non-reacting electrosprays at several voltage and flow rate conditions.Analysis of the droplet sizing information revealed a bi-modal droplet size distribution. Evaporationtime estimates were used to rationalize the results. Large droplets could pass through the flame withoutcompletely evaporating. Electrosprays of n-butanol were demonstrated to sustain flames stabilized withthe assistance of electrostatics. Furthermore, these electrospray flames had characteristics substantiallydifferent from the spray flames of non-charged fuels.

1 Introduction

Electrospray is an injection system which causes liquid atomization using electrostatic forces,without the use of a large pressure differential. Typically, a capillary is charged at a high electricpotential and liquid is passed through it. Charge is introduced to the liquid during this passage. Agrounding electrode is placed a few centimeters from the charged capillary and an electrostatic fieldis thus established between the capillary and the ground. Electrostatic forces produce three distinctregions in the spray. The liquid emerges from the tip of the capillary and forms a meniscus calleda Taylor Cone [1]. From the tip of this meniscus, a liquid stream, or ligament, emerges. Becauseof Rayleigh Instability [2], this ligament breaks into smaller ligaments, and finally into droplets.Under certain conditions, a fan of droplets of nearly uniform size is produced, referred to as the“cone jet mode” [3]. Previous electrospray combustion research has been primarily conducted withtraditional hydrocarbon fuels, such as heptane. Gomez and his collaborators at Yale Universityperformed studies with heptane in a counter flow burner, injecting through an electrospray fromthe bottom and studying the effect on the flat flame produced in their configuration [4]. Specificallymentioned in this paper was the inability for direct electrospray combustion, citing the breakdownof the electric field because of the high concentration of chemi-ions in the flame. Additionally,

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8th US Combustion Meeting – Paper # 070HE-0225 Topic: Spray Flames

the larger droplets were observed to have a higher average velocity than the smaller ones, and,therefore, had lower resonate time in the flame [4]. In a subsequent study, the same group soughtto detect and capture evidence of droplet fission in the flames of a counter-flow burner. Theirtheory sought to explain the rapid loss of larger sized droplets, given that evaporation effects wereinsufficient to account for this finding. Convincing evidence was presented in several images ofabnormally close droplets in areas with otherwise sparse populations [5]. This would suggest thatone larger droplet broke apart into several smaller droplets as a result of Rayleigh Limit inducedfission. Shrimpton and colleagues have performed numerous studies on electrospray and dropletcharacteristics, most recently evaluating Reynolds and Weber number effects of both laminar andturbulent regimes on spray atomization. This group introduced a modified Weber number to takeinto account the effectice reduction of surface tension in the presence of the electric field [6]. Jidoet al. [7] performed a study in which kerosene mixed with varying quantities of methanol, diesel,and water were mixed and injected through an electrospray onto a horizontal flame holder andignited. Flames produced with the electrospray were reported to be brighter and longer comparedto flames produced with a standard pressure-driven injector [7].

Electrospraying is particularly appropriate for bio-alcohols that have an electric conductivity ofat least four orders of magnitude higher than the ones of hydrocarbons. With recent advances inlarge scale bio-butanol production [8, 9], butanol may become a mainstream transportation fuelin the future. Butanol has many advantages over ethanol, which is currently the most popularliquid biofuel. In addition to having a higher energy density than ethanol (36.4 vs 24.8 MJ/kg,respectively), butanol requires less molecular breakdown during production, which translates intoless energy needed in production. Furthermore, butanol can be easily transported in pipelines andit can be mixed with diesel fuel without the need for additives. Promising combustion benefits withthe use of electrosprays have been demonstrated in enhancing fuel injection in internal combustionengines [10, 11], and power generation [12] with a particular emphasis on micro-combustion [13,14]. Research into electrospray enhancement of fuel injectors for use with ethanol-gasoline andbutanol-gasoline blends has been conducted [11, 15], showing benefits in emission reduction. Morerecently butanol electrosprays have been compared to those of ethanol and conductivity enhancedheptane by Agathou and Kyritsis [16, 17].

Published literature lacks fundamental studies of butanol electrospray combustion. With this study,we would like to offer some initial experimental data about this combustion phenomenon. Flamephenomenology was described, and the effect of the presence of the flame on defining parametersof the spray, such as droplet size and velocity, was examined and compared to previous studies onelectrospray flames. Of particular interest was the dual effect of electrostatics on both atomizationand flame stabilization.

2 Experimental Apparatus

The experimental apparatus is shown schematically in Figure 1. The electrospray is formed byconnecting a steel capillary connected to a syringe pump and high voltage source. The capillaryhad an inner diameter of 127µm and a conical tip that was machined using an Electrical DischargeMachine to enhance the electric field at the nozzle.

The flow of liquid butanol ran opposite to gravity, and it was metered with a KD-Scientific syringe

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8th US Combustion Meeting – Paper # 070HE-0225 Topic: Spray Flames

514.5 nm Laser

532 nm Laser

PDA Detector Steel Capillary

Cylindrical Lens and

resulting laser sheet

Butanol Electrospray

(Blue)

SLR/High Speed

Camera

Figure 1: Laser sheet through the air-stabilized flame

pump. n-Butanol was used in all of the tests because this is the dominant isomer in bio-butanolmixtures. The distance between the capillary tip and grounded mesh was fixed at 125 cm for alltests. Voltages of 6.5-7.5kV and 10-20 ml/hr were found to produce self-sustaining air-stabilizedflames. At each flow rate, the potential field was varied at 6.5, 7.0 and 7.5 kV. With the fixed gapof 125 cm, this translates to 520, 560, and 600 V/m. At each voltage setting, the butanol flow ratewas varied at 10, 15, and 20 ml/hr. An air-stabilized flame is defined as a flame that burns at asubstantial distance above the capillary tip, anchored at a location that is essentially determined bythe fuel flow rate at a given voltage. A 2D Dantec PDA system was used to capture both dropletdiameter and velocity. The system was equipped with a BSA P60 processor connected to 58N70detector. The laser used in the PDA system was a Spectra Physics Stabilite 2017 Argon-ion.

3 Bio-butanol Electrospray Flame Phenomenology

Similar to the observations by Kim et al [18], flame anchoring was done through electrostatics andoccurred at a substantial distance from the capillary tip. In this study, the flames were observedto stabilize between 20 and 35 mm from the tip of the capillary and were primarily a functionof butanol flow rate. The flame had two distinct modes of operation. The first was a traditionaldiffusion flame with one large flame surrounding a group of vaporizing droplets. This is referredto as the “group combustion mode”. The flame fluctuated in size but was typically about 6.5 cmtall and 3.5 cm wide with a conical shape. In this mode, the droplets entered from the base of thecone, evaporated, and burned along the exterior of the cone. The second mode only consisted ofa collection of burning droplets, without a orange luminous flame sheet (Fig. 2). This mode wasunstable and often occurred with some flickering of the entire collection of burning droplets, rightbefore extinction of the spray flame.

Most of the spray flames produced at the lower voltage settings were stable, meaning that theywere self-sustained by timely fuel evaporation, without extinction. Increasing the voltage to 600

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8th US Combustion Meeting – Paper # 070HE-0225 Topic: Spray Flames

V/m created an unstable flame structure. The flame frequently alternated between the purely groupcombustion flame (left image in Fig. 3 on the following page) to the mixed group and single dropletcombustion, which closely resembles internal group combustion (right image in Fig. 3). This typeof behavior was not seen in the 520 and 560 V/m cases.

Figure 2: Single droplet combustion modewith laser sheet present

The primary difference between these combustionmodes is the ratio between droplet spacing anddroplet size. As the droplets become bigger andspread farther apart, the single droplet combustionmode is preferred (Fig. 2). If the droplets are smalland close to each other, the droplets burn as a group(left image in Fig. 3), with an intermediate statebeing internal group combustion (right image inFig. 3). Clearly, electrostatic control can drasticallyaffect flame morphology. Furthermore, in all of thetest conditions, some of the droplets either deviatedaway from the flame before reaching it or passed ata distance from the flame that prevented completeevaporation and subsequent ignition.

Figure 4 shows a non-burning spray at 600 V/m.Under this elevated voltage, an almost dual spraystructure appears, that is much more pronouncedthan in lower voltage cases. Increasing the exposuretime to 1/80 s, a narrow central core spray (redarrows) and a much broader outer spray (bluearrows) are clearly visible. When the flame isburned in the mode shown on the left panel in Fig.3, it could primarily burn on the fuel from the innerspray cone, whereas the much broader flame on theright panel received fuel from both the inner andouter spray cones. The existence of two cones isconsistent with the findings of Tang and Gomez [3],who observed what they termed “satellite droplets”in their flames.

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8th US Combustion Meeting – Paper # 070HE-0225 Topic: Spray Flames

Figure 3: Different combustion modes for 20 ml/hr butanol flow rates and a 7.5 kV applied potential

Inner Cone

Outer Cone

Figure 4: Electrospray Structure: 20 ml/hr butanol flow rate and 7.5 kV applied potential, no flamepresent 5

8th US Combustion Meeting – Paper # 070HE-0225 Topic: Spray Flames

Figure 5: Comparison between 10 (left), 15 (middle), 20 (right) ml/hr butanol flow rates under a 6.5kV potential

The effect of changing the flow rate under a constant voltage on phenomenology is shown in Fig. 5.As the flow rate decreases, the flame decreases in size and moves closer to the capillary tip. Thechange in combustion modes, as a function of flow rate, is most likely because of a change in theratio of droplet size to inter-droplet spacing. External group combustion requires a larger spacing-to-diameter ratio than single droplet combustion. The droplet diameter and inter-droplet spacingshould decrease with decreasing flow rate [3]. As the flow rate decreases, the amount of chargedliquid also decreases. This reduces the total electrical force, causing the spray to disperse anda narrower spray to form. This smaller spray would then, in turn, form a narrower flame withdroplets inside the flame envelope and that are closer together. Indeed, this is visible in the imagesof the spray flames above. The droplets in the 10 ml/hr case are hard to distinguish from oneanother and are closer together than the 15 and 20 ml/hr cases, where the droplets in the flame areeasily distinguished.

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8th US Combustion Meeting – Paper # 070HE-0225 Topic: Spray Flames

4 Droplet Size Analysis

Droplet size measurements were taken as close to the base of the flame as possible, which wereabout 30 mm from the tip of the capillary. Many combinations of voltage and flow rate were tested;however, only results acquired with a voltage of 560 V/m and mass flow rate of 15 ml/hr case willbe presented for brevity. Probability density functions of droplet size are presented both with andwithout a flame present; with diameter graphed versus volumetric density in Fig. 6. Significantdifferences can be seen between the two cases. The size distribution when the flame is not presentis notably different from the monodisperse sprays observed in previous studies in the cone-jet mode[4, 5].

Figure 6: Probability mass function without combustion (left) and with combustion (right)

In the reacting case, the distribution becomes much more peaked around a central value - in thiscase about 42µm. Both the number of droplets larger and smaller than this central size havedecreased, most likely a result of evaporation of the droplets as they approach the high temperatureregion of the flame. This would reduce the size of the largest droplets and nearly eliminate thesmallest droplets.

The velocity component along the spray axis versus droplet diameter is presented in Fig. 7. Awide range of velocities for each droplet size is observed; however, larger droplets on average havea lower velocity. Furthermore, during combustion, the droplets with the median size now have alarger range of velocities. This could be due to the large droplets evaporating but remaining at thesame velocity, meaning their diameter, but not their velocity, is decreasing. Furthermore, thesedroplets have the largest surface area exposed to the high temperatures near the flame and wouldvaporize at a faster rate.

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8th US Combustion Meeting – Paper # 070HE-0225 Topic: Spray Flames

Figure 7: Comparison between Droplet Diameter and Vertical Velocity (combusting left, non-combusting right)

The captured flame images and droplet velocity distributions reveal two important characteristicsof these burning electrosprays. First, droplets are not all completely vaporized by the time theyreach the flame. Second, droplets moving at a high rate of speed (>10 m/s) are observed inside theflame envelope that surrounds the spray. With these two pieces of information, a residence time inthe flame for total droplet evaporation can be established [5]. In particular, the following analysisof Turns [19] is used to provide a plausible explanation to droplets passing through the flame. Theequations used in this model are described below.

kg is an averaged specific heat for the medium [19], in this case butanol and air.

kg = 0.4 ∗ kC4H10O+ 0.6 ∗ kAir (1)

Bq is the ratio of the enthalpy in the vaporized butanol vs the enthalpy of vaporization, where T∞ isthe adiabatic flame temperature for a butanol-air flame, Tboil is the boiling temperature for butanolat 1 atm, Cpg is the specific heat of the butanol vapor, and Hfg is heat of vaporization of butanol.

Bq =Cpg ∗ (T∞ − Tboil)

Hfg(2)

K is known as the evaporation constant.

K =8kg

ρC4H10O∗ Cpg

∗ ln(Bq + 1) (3)

Finally, the evaporation time tD is given by, where D is the initial droplet diameter:

tD =D2

0

K(4)

The results of this calculation are found in Table 1, based on the observed flame size of 6.5 cm. Thecritical speed, as defined here, is the maximum speed a droplet can be traveling to ensure complete

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8th US Combustion Meeting – Paper # 070HE-0225 Topic: Spray Flames

vaporization before leaving the flame region. Therefore, droplets traveling faster than this velocitywould pass through the flame before vaporizing.

Droplet Diameter (µm) Evaporation Time (ms) Critical Speed (m/s)

50 4.76 13.755 5.76 11.360 6.86 9.4865 8.05 8.0870 9.33 6.9675 10.1 5.33

Table 1: Evaporation time and critical speed to completely vaporize

Indeed, 65-75 micron droplets moving faster than the calculated critical speed were observed atthe base of the spray flames. This model provides some justification of the observations of dropletspassing through the flame before completely vaporizing. This model does not take into account thepossibility of droplets slowing down as a result of the flame. Further study of the changes in thedroplet velocity within the spray flame structure will be needed to conclusively prove the dropletevaporation theory of why droplets are observed passing through the spray flame.

5 Conclusion

Reactive bio-butanol electrosprays can operate both in the group combustion and the individualdroplet combustion mode. In the individual droplet combustion mode, droplets cross the flamesheet, ignite there and continue combusting as single droplets in a non-stagnant medium. Changingthe applied electric field and fuel flow rate resulted in changing the stable position of the sprayflame as well as the droplet combustion mode of the spray. In the non-reacting electrospray, analmost bi-modal droplet size distributions were observed, but during combustion, the numbers ofboth the largest and smallest sized droplets were reduced. Furthermore, the comparison of dropletvelocity with and with out a flame present revealed a narrowing of the droplet velocities witha flame present. Combining the measured droplet diameter and velocity information, a simpledroplet evaporation model was used to suggest that some droplets could pass through the flameregion without completely vaporizing.

6 Acknowledgments

The authors would like to acknowledge the support of the U.S. Department of Energy throughthe Graduate Automotive Technology Education (GATE) Center of Excellence at the University ofIllinois. DCK gratefully acknowledges the support of the International Institute for Carbon NeutralEnergy Research (WPI-I2CNER), sponsored by the World Premier International Research CenterInitiative (WPI), MEXT, Japan.

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8th US Combustion Meeting – Paper # 070HE-0225 Topic: Spray Flames

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