To increase power output, improve fuel economy, and meet strin-gent emission regulations, scientists of the automotive, or moregenerally, the energy industry, focus their interest on the domainof techniques related to the design or operating conditions of en-gines (Pulkrabek 2004; Rakopoulos et al. 2004a, b, 2009; Abu-Jraiet al. 2009; Levendis et al. 1994); i.e., to diesel or spark ignitionengines (Giakoumis et al. 2012; Alkidas 2007; Rakopoulos et al.2008; Rakopoulos 2012) that are well established as dominatingpowertrain solutions in the world market. Particularly for the reduc-tion of pollutant emissions, the interest is concentrated on thedomain of fuel-related techniques, such as the use of alternativegaseous fuels that are friendly to the environment, such as methane,syngas, or hydrogen (Barlow et al. 2001, 2009; Rakopoulos andMichos 2008; Vancoillie et al. 2012), or oxygenated fuels thatpresent reduced particulate emissions (Rakopoulos et al. 2011,2012). In various countries, considerable attention has been paidto the development of alternative fuel sources, with emphasis onbiofuels that are oxygenated by nature, which possess the advan-tages of being renewable, biodegradable, and nontoxic, such asdiesel engines fueled with blends of diesel fuel with vegetable oilsand biodiesels, ethanol, n-butanol, or diethyl ether (Giakoumis et al.2013; Rakopoulos 2013; Rakopoulos et al. 2013).
Recent research has shown the possibility of efficientlyproducing butanol with the use of fermentation microorganisms(clostridia); namely, Ezeji et al. (2007) and Tashiro et al. (2007),who had earlier presented results on the coproduction of butanolwith ethanol and acetone (Tashiro et al. 2005), demonstrated bio-butanol production with the use of the microorganism chlostridiumbeijerinckii and highlighted the potential use of biobutanol as analternative fuel derived from renewable sources. Butanol has asignificantly increased energy density over to the currently widelyused bioethanol. Specifically, the energy densities of butanol andethanol are 36.4 and 24.8 MJ=kg, respectively, whereas the energycontent of gasoline is 44.9 MJ=kg. Notably, butanol mixes readilywith diesel fuel, unlike ethanol.
Butanol is particularly appropriate for the technology of electro-static atomization because of the −OH bond in its molecule, whichbecomes polarized under an electrostatic field of elevated intensity,converting butanol into a relatively conductive molecule. In thefield of electrostatic atomization, promising results have recentlybeen reported in terms of utilizing this technology for butanol(Agathou et al. 2007). Comparative studies of butanol with otherfuels of automotive interest have been reported (Agathou andKyritsis 2010, 2012a), and further results on butanol blends havebeen given (Agathou and Kyritsis 2012b). Early work was per-formed in the context of direct injection (Anderson et al. 2007a)and focused on the effect of the electrostatic field on the structureof electrostatically assisted sprays (Anderson et al. 2007b) and theperformance of sprays of gasoline-ethanol blends (Anderson andKyritsis 2007). The fundamental combustion properties of butanolhave been compared with those of well-established fuels such asmethane and heptane (Agathou and Kyritsis 2011, 2012c), andthe actual performance of biobutanol diesel blends was studiedin diesel engines (Rakopoulos et al. 2010b). The advantages ofbiobutanol blending in diesel are evident when it is compared tovegetable oil, biodiesel, or ethanol (Rakopoulos et al. 2007, 2010a).The behavior of butanol/gasoline blends in spark-ignition engineshas also been reported (Gu et al. 2012; Irimescu 2012).
The possibility of automotive application of the electrospray(e-spray) technology has been discussed for early injection in
1Senior Engineer, Hellenic Petrol, 5 Chimaras St., 15125 Athens,Greece; formerly, Dept. of Mechanical Science and Engineering, Univ.of Illinois at Urbana-Champaign, 1206 W. Green St., Urbana, IL 61801.
2Associate Professor, Dept. of Mechanical Science and Engineering,Univ. of Illinois at Urbana-Champaign, 1206 W. Green St., Urbana, IL61801; and Professor, Dept. of Mechanical Engineering, Khalifa Univ.of Science, Technology and Research, P.O. Box 127788, AbuDhabi, UnitedArab Emirates (corresponding author). E-mail: [email protected];[email protected]
Spark-ignited (SI) engines (Shrimpton 2003) and for transientatomization (Kourmatzis and Shrimpton 2011). More studies inthe automotive field include charge injection atomizers (Shrimptonand Kourmatzis 2010) and electrostatic atomization insertion intoCompression-ignition (CI) engines (Thomas et al. 2002). Togetherwith the work of Anderson and Kyritsis (2007), these works seemto indicate a possibility for the practical application of automotive,electrostatically manipulated sprays if appropriate microfabricatedinjectors are made available.
For low conductivity liquids, namely hydrocarbons, there is aregion of stable e-spray operation termed the cone-jet mode, inwhich the e-spray consists of three major sections: a conical me-niscus, which is formed at the outer surface of the nozzle and is also
known as a Taylor cone; a liquid ligament, which breaks up furtherdownstream because of Rayleigh instability; and a spray of nearlyuniform sized droplets (Tang and Gomez 1994, 1996; Gomez andTang 1994). Gomez proposed a mesoscale burner based on kero-sene e-sprays (Kyritsis et al. 2004a, b), in which two features of thee-spray were highlighted: First, when operated in cone-jet mode,the e-spray can produce practically monodisperse sprays; second,the droplet size of the monodisperse e-sprays can be controlled bythe mass flow rate per spray. The morphology of the e-sprays wasextensively studied by Cloupeau and Prunet-Foch, both in the cone-jet mode regime (1989) and in other functioning modes (1994).
The objective of this paper is to experimentally investigatewhether these particularly attractive features of the e-spray can
(a) (b)
Fig. 1. Electrospray configuration: (a) typical e-spray; (b) system creating the e-spray
Fig. 2. Electrospray imaging results for small flow rates of butanol
be achieved for butanol. Previous studies on butanol e-sprays(Agathou et al. 2007; Agathou and Kyritsis 2010, 2012a, b) sug-gested an e-spray operation away from the stable cone-jet regimeand practically produced polydisperse sprays. In these distribu-tions, the 10% ratio of the SD over the average diameter, whichis widely used as a criterion for monodispersity, was not achieved;rather, this ratio had a value on the order of 35%. The purpose ofthis study was to investigate whether this could be cured by utiliz-ing smaller flow rates to achieve monodisperse sprays.
Experimental Methodology
E-Spray Experimental Setup
Fig. 1(a) shows a typical e-spray and Fig. 1(b) shows a schematicof the system realizing the e-spray. The apparatus consisted of asyringe pump that fed the injector with butanol and controlledthe flow rate. Commercially available glass capillaries were usedas injectors, after applying a layer of silver on their outer surfacesthat acted as a conductor. The capillaries had outer and inner
diameters of 1 mm and 30 μm, respectively, and were charged ata potential on the order of several kV provided by a high voltagepower supply. The charged spray was collected at an electricground in the form of a steel mesh placed 11 mm from the capillaryexit. The liquid was drained in a liquid beaker placed beneath theelectric ground. The liquid under consideration was pure 1-butanol(n-butanol). Butanol has a relatively high electrical conductivitycompared to commercially available hydrocarbons. For this reason,e-spraying was possible without the addition of conductivityenhancers.
Measurements of Droplet Size and Velocity: PhaseDoppler Anemometry
For spray droplet size and velocity measurements, a Dantec fiberphase Doppler anemometer (PDA) was used (Dantec Dynamics,Skovlunde, Denmark). It was powered by a 5 WAr-ion laser andwas run in one-dimensional (1D) PDA mode, utilizing only514.5 nm of the Ar-Ion laser and collecting at a forward scatteringangle of 71.4°. For each measurement, 10,000 data points were se-lected, which was sufficient for adequate statistics. Droplet size andvelocity measurements were performed at a location on the central
spray axis located 11 mm from the glass capillary exit. The collec-tion point location is shown in Fig. 1(b). Measurements wereobtained at only one location, because the goal was to establishthe possibility of monodisperse sprays, not the spatial distributionof droplet size.
High-Speed Visualization
Visualization of the butanol e-sprays was achieved with a VisionResearch Phantom v7.0 high-speed video camera (VisionResearch, Wayne, NJ). Three Nikon extension tubes totaling82.5 mm in length (Nikon, Tokyo, Japan) were used to increasemagnification, along with a Tiffen 52 mm zoom lens (Tiffen,Happauge, NY). Camera exposure time was set to 96 μs andthe zoom and f-number (defined as the ratio of focal length ofthe diameter of the aperture) settings were adjusted to provide aclear view of the entire spray. Laser light from an Oxford LaserSystems LS 20-50 copper-vapor laser (Oxford Lasers, Shirley,MA) operating at 510.6 nm with a frequency of 10 kHz was di-rected toward the doublet of the concave spherical-cylindrical lensthat was used to generate the laser sheet. Both the high speed cam-era and the laser were synchronized by a Berkeley Nucleonics500C042 pulse generator (Berkeley Nucleonics, San Rafael, CA).To acquire images of a two-dimensional (2D) cross section of thespray, the laser sheet was aligned so that its top would be just belowthe capillary tip and the bottom would be approximately 200 mmbelow the orifice; thus, the whole spray was captured. The thick-ness of the laser sheet was approximately 1 mm.
Results and Discussion
E-Spray Phenomenology
The establishment of cone-jet sprays requires a finer increment offlow rate and applied voltages than those used previously (Agathouet al. 2007; Agathou and Kyritsis 2010, 2012a, b) so that they canbe tracked, especially for a fuel that has not been investigated fromthis aspect before and has potentially novel modes of e-spray op-eration. During earlier experiments (Agathou et al. 2007; Agathouand Kyritsis 2010, 2012a, b), the applied flow rate was in the rangeof 5–30 mL=h, the voltage varied from 4–7 kV, and the e-sprayswere unsteady and polydisperse. The distance between the capillarytip and the ground electrode (liquid beaker) was 11 mm; it was kept
constant during the current experiments and equal to the one usedbefore (Agathou et al. 2007; Agathou and Kyritsis 2010, 2012a, b).In this work, smaller butanol flow rates were investigated:1–4.5 mL=h. The typical structure of the ensuing e-sprays is shownin Fig. 2. These data were collected in the range of the minimumvoltage that could support an e-spray, i.e., 3.3–3.5 kV. For a lowervoltage, atomization was not achieved and only the fuel liquidligament was observed. For those low flow rates, the e-sprays weresteady with a conical stable meniscus at the outside surface of thecapillary tip, exactly as expected from the cone-jet mode. In somecases, satellite droplets were present that deviated from the mainbody of the spray; in the rest of the cases, the fan of droplets wasnarrower than the sprays reported by Tang and Gomez (1996), andGomez and Tang (1994). Atomization led to an e-spray structuresimilar to the results of Tang and Gomez (1996) and Gomez andTang (1994), in which the established sprays operated in the cone-jet mode with a stable meniscus and a time independent spray.
E-Spray Structure: Droplet Size Measurements
Droplet size distribution results were obtained for flow rates from 1to 4.5 mL=h with an increment of 0.5 mL=h. The applied voltagesvaried from 3.4 to 4.5 kV, with an increment of 0.1 kV. The goalwas to investigate the possibility of monodisperse sprays, the exist-ence of which is conditioned on the stability of the conical menisciat the injector tip. Probability density function results of the average
droplet diameter are presented in Fig. 3 for 12 values of appliedvoltage. In data processing, the average droplet diameter d10 ispresented.
Contrary to the insensitivity of droplet size on applied voltageobserved in previous works (Agathou et al. 2007; Agathou andKyritsis 2010, 2012a, b; Tang and Gomez 1996; Gomez and Tang1994), a dependence on voltage was clearly observed for low flowrates of butanol. To capture this dependence, small increments ofvoltage were selected. The investigation took place for the voltagesthat did generate a spray. For example, for the lowest flow rates(1–2 mL=h), an e-spray could not be established for a voltage lowerthan 4.5 kV. Clearly, the electrostatic repulsion in the liquid was notsufficient to break the liquid meniscus.
First, at the lowest voltages (3.4 and 3.5 kV), the droplet sizedistribution for flow rates ranging from 1 to 3 mL=h is narrow.Hence, near the voltage limit where the e-sprays start forming,the sprays demonstrate a behavior that resembles a cone-jet andis expected to produce monodisperse droplets. However, after adeparture from the narrow distributions (Cases 3, 4, and 5), thee-sprays return to an almost cone-jet behavior for voltages ofapproximately 4.3 kV; this occurs for all flow rates, not only forthe lower ones, as in the case of low voltages (Case 1). In thesecases, a typical ratio of the SD of the distributions over the meandroplet diameter is equal to 18%, which is fairly close to the 10%threshold typically used as a criterion for monodispersity. As acomparison, in the results of Tang and Gomez (1996) and Gomezand Tang (1994), this ratio is typically equal to 6–8%.
Fig. 4 shows the dependence of droplet size on voltage for aconstant flow rate. Again, contrary to previous literature findings(Agathou et al. 2007; Agathou and Kyritsis 2010, 2012a, b; Tangand Gomez 1996; Gomez and Tang 1994), this relation is not mon-otonic. An increase of droplet size is clear that peaks in the vicinityof 3.7 kV, abruptly decreases to a minimum value in the region of4.3 kV and then another increasing trend. The voltages for whichthe minima are shown in Fig. 4 correspond to the narrowest dropletsize distributions of Fig. 3. Results for heptane (Tang and Gomez1996; Gomez and Tang 1994) indicate an insensitivity of dropletsize to voltage for flow rates smaller than approximately 10 mL=h.
The effect of liquid flow rate on average droplet size under con-stant voltage is shown in Figs. 5(a and b). The results indicate anincrease in droplet size with increasing flow rate. These findingsagree with the already established finding that droplet size can bedecreased with diminishing flow rates (Tang and Gomez 1996;Gomez and Tang 1994). A power law dependence, as reportedby Tang and Gomez (1996) and Gomez and Tang (1994), wasobtained with a satisfactory regression coefficient for many of thecases investigated here. The equation used for the regression was ofthe form
d10 ¼ α ×mβ ð1Þwherem = flow rate; α = constant; and β = exponent. The values ofβ and the corresponding regression coefficients (R2) are presentedin Table 1. In most cases, β is very small (<0.3), which actuallyindicates a very mild dependence of droplet size on flow rate.
E-Spray Structure: Velocity Measurements
Velocity distributions are provided in Fig. 6 for six of the casespresented in Fig. 3. The panels presented in the figure correspondto the regions where a behavior closer to cone-jet was observed inthe previous section; that is, the cases that have the lowest ratios ofSD over the mean droplet diameter. The sprays showing this quasi-monodisperse nature developed in the region of approximately 3.4and 4.3 kV. Narrow velocity distributions are shown in the cases of
the lowest flow rates for voltages of 3.4 and 3.5 kV, whereas withincreasing flow rate the distributions become equally broad, show-ing insensitivity to voltage. This insensitivity is also observed forall flow rates of voltages 3.6, 4.2, 4.3, and 4.4 kV.
The effects of voltage and flow rate on the average velocityare shown in Fig. 7. An increase in droplet velocity is shownwith increasing voltage, irrespective of flow rate. Contrary to thenegligible flow rate effect, the voltage effect is substantial andtranslates to higher velocities for higher applied voltages. Reynolds(R) and Weber (W) numbers for the droplet motion were calculatedand are presented in Figs. 8 and 9 for constant flow rate andvoltage, respectively. Fig. 8 shows that R and W increase withincreasing flow rate, which is observed for all voltages. However,in Fig. 9, the relation between R and W with voltage is not mon-otonic. Instead, two monotonic regions and a velocity decrease atapproximately 4 kVare shown for all flow rates, which may imply atransition to a different flow regime for all cases.
The values ofW allow prediction of the droplet breakup mecha-nism according to the analysis of Pilch and Erman (1987). In par-ticular, the flow regime corresponding toW between 100 and 300 isthat of sheet stripping, in which the droplet breakup mechanism isbased on the creation of an ellipse from the initial droplet, whichbreaks down into a series of satellite droplets around its two edges.
Regions withW ≤ 350 correspond to voltages below 3.6 kV, wherethe narrowest droplet size distributions are visible. This mechanismof the generation of satellite droplets through an unstable sheet cancontribute to the formation of almost uniform size droplets. In starkcontrast to this, for W ≥ 350, droplet breakup occurs in the cata-strophic regime (Pilch and Erman 1987). In this scenario, secon-dary droplets develop in a cloud around the surface of the initialdroplet as they reduce its size in an arbitrary manner. For the flowrates under investigation, although the droplet size distributionsrevealed an ∼18% uniformity in size distribution, which wassignificantly improved compared to higher butanol flow rates(Agathou et al. 2007; Agathou and Kyritsis 2010, 2012a, b), theywere still polydisperse. The source of polydispersion can be attrib-uted to the breakup mechanism, which is the catastrophic regimethat does not favor the creation of uniformly sized droplets.
Nondimensional Controlling Parameters
In this section, the possibility is investigated of producing empiricalcorrelations among the three variables under consideration: thedroplet diameter, the voltage, and the flow rate. According toFernandez de la Mora et al. (1990), the nondimensional dropletdiameter is a function of the nondimensional voltage, the flow rate,and the dielectric constant, ε, of the liquid:
d�fðQ�;V�; εÞ ð2ÞThus, for a specific liquid, d� is a function of V� and Q�. The
formulation of the nondimensional parameters is dependent on themode of operation of the e-spray; for a stable cone-jet regime, it isas follows (Fernandez de la Mora et al. 1990):
Q� ¼ ρ · λ · Qε · ε0 · σ
ð3Þ
V� ¼ V − V0
V0
ð4Þ
d� ¼�ρ · λ2 · d3
ðε · ε0Þ2 · σ�1=3
ð5Þ
where ρ = density of the liquid; ε0 = electric permittivity of air; andV0 = onset voltage, which was assumed to be 3 kV for this analysis.Eqs. (3) and (5) come from dimensional analysis. As indicated inthe previous sections, e-sprays of low butanol flow rates present astable behavior, although the 10% criterion used in previousstudies to establish monodispersity was not satisfied. Conductinga surface fit over the nondimensional data, an empirical relationwas generated that correlated d�, V�, and Q� by utilizing athird-order polynomial:
The regression coefficient of the correlation is R2 ¼ 93.14% andall coefficients of the polynomial are reported with 95% confidencebounds. This correlation basically presents all of the current resultsin one equation, without revealing any fundamental characteristicsof the underlying physics.
Conclusions
Biobutanol e-sprays can be achieved in the cone-jet mode in a narrowregime of voltages near the onset of electrostatic atomization forflow rates on the order of 1 to 4.5 mL=h. The ratio of SD to averagedroplet diameter ranges from 18 to 23%. This is substantially largerthan the 10% criterion typically used to establish monodispersion,but it is also significantly smaller than the corresponding values mea-sured for larger flow rates (Agathou et al. 2007; Agathou and Kyritsis2010, 2012a, b), which reached 35%. Thus, substantially narrowerdroplet distributions can be achieved by operating at low flow ratesand voltages near the onset of the e-spray. High speed visualizationshowed a stable e-spray behavior under these conditions withoutoscillations of the liquid meniscus from which the spray ensued.The dispersion in droplet size is attributed to the high Weber numberof the droplet motion, which can cause secondary atomization.
Acknowledgments
The authors would like to acknowledge the support of the U.S.DepartmentofEnergy through theGraduateAutomotiveTechnologyEducation (GATE) Center of Excellence at the University of Illinois.
Notation
The following symbols are used in this paper:d = average droplet diameter (m);d� = nondimensional droplet diameter;Q = volumetric flow rate (m3=s or mL=h);Q� = nondimensional flow rate;q = total electric charge on the droplet surface (C);R = Reynolds number (Ud=v);U = velocity of the droplet (m=s);
UAVE = average velocity of the droplet (m=s);V = applied voltage (V);V� = nondimensional voltage;Vo = onset voltage (V);W = Weber number (ρU2d=σ);ε = liquid dielectric constant;ε0 = electric permittivity of the surrounding medium (s=Ωm);λ = electrical conductivity (1=Ωm);ν = kinematic viscosity (m2=s);ρ = density (kg=m3); andσ = surface tension (N=m).
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