Ijvd 08 elegant et al

Int. J. Vehicle Design, Vol. 50, Nos. 1/2/3/4, 2009 35

Copyright © 2009 Inderscience Enterprises Ltd.

Experimental investigation of electrostatic effects on ethanol and ethanol–diesel blend sprays in atmospheric ambiance

Daniel S. Elegant, Taekyu Kang and Dimitrios C. Kyritsis* Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, 1206 W. Green Street, Urbana, IL 61801, USA Fax: +1 217 244 6534 E-mail: [email protected] E-mail: [email protected] E-mail: [email protected] *Corresponding author

Abstract: An experimental investigation of electrosprays of ethanol–diesel blends and electrostatically assisted injection of ethanol and ethanol–isooctane blends was performed in order to investigate the potential of novel injection techniques for ethanol-containing fuels. An exponential dependence of liquid fuel conductivity on ethanol content was established. Simple, fundamental electrosprays of ethanol–diesel blends containing 5–15% ethanol exhibited micro-dripping atomisation and produced sprays that approached the monodispersity that has been observed for pure ethanol sprays operating in the cone-jet mode. Experiments were also performed with ethanol and ethanol–isooctane blends on a commercially available swirl-type fuel injector that was modified with a conductive cap in order to electrostatically charge the fuel emerging from it. Electrostatically charged ethanol sprays penetrated in the ambient gas significantly less at the early stage of injection, especially for lower injection pressures. In addition, the angle formed by the hollow cone of spray was larger for the charged sprays. Such effects are expectedly diminishing with decreasing alcohol content.

Keywords: diesel; e-diesel; electrostatic atomisation; ethanol.

Reference to this paper should be made as follows: Elegant, D.S., Kang, T. and Kyritsis, D.C. (2009) ‘Experimental investigation of electrostatic effects on ethanol and ethanol–diesel blend sprays in atmospheric ambiance’, Int. J. Vehicle Design, Vol. 50, Nos. 1/2/3/4, pp.35–49.

Biographical notes: Daniel S. Elegant received an BS in 2006 and a MS in 2007 both of them in Mechanical Engineering from the University of Illinois at Urbana-Champaign. Currently, he is employed as a Mechanical Engineer at Sargent and Lundy LLC, Chicago. His research in Illinois focused on electrostatically charged sprays of ethanol and ethanol-hydrocarbon blends.

Taekyu Kang graduated in Mechanical Engineering from the Hanyang University in Korea, received an MS (2005) and a PhD (2008) in Mechanical Engineering at the University of Illinois at Urbana-Champaign. Currently, he is employed as a Post-Doctoral affiliate in the same Department. During his

36 D.S. Elegant, T. Kang and D.C. Kyritsis

graduate study, he has experimentally and theoretically investigated stratified combustion, which led to six journal publications and six conference presentations. Prior to joining the University of Illinois in 2002, he worked as a Mechanical Design Engineer at LG Electronics, Seoul, Korea, where he obtained invaluable hands-on experience throughout the entire development processes including product design, prototype production, dye manufacturing and mass production. His areas of expertise are combustion, gaseous chemical reacting flow and laser diagnostics.

Dimitrios C. Kyritsis is an Associate Professor at the department of Mechanical Science and Engineering at the University of Illinois at Urbana-Champaign. He received his Diploma in Mechanical Engineering from the National Technical University of Athens, Greece in 1992 and his MA and PhD in Mechanical and Aerospace Engineering from Princeton University in 1995 and 1998, respectively. His research interests lie in the area of combustion physics and laser-based diagnostics with a particular emphasis in electrostatic atomisation of bio-fuels, autonomous, grid-independent power generation and flame dynamics.

1 Introduction

In internal combustion engines, liquid fuels typically atomise and mix with the oxidiser through pressure-driven injectors. In such injection schemes, the mass of injected fuel relates to engine load, whereas the way the fuel disperses in the combustion chamber is determined mainly by the fuel spray momentum. Since mass and momentum are fundamentally coupled, capability to effectively control fuel dispersion is therefore effectively restrained. If electrostatic charge was to be introduced to the fuel, an additional parameter that can be used for spray control and is independent of mass would be made available.

The potential of electrostatic effects for combustion applications was pointed out by Thong and Weinberg (1971) and, shortly thereafter, Kim and Turnbull (1976) and Kelly (1984) showed that electric charge can be injected in liquids of relatively low conductivity like hydrocarbons ( < 10 6 ( m) 1) with the use of sharp electrodes. This made available for application in the field of power generation, the ‘electrospray’ technology, which is widely used in analytic chemistry and yielded to its pioneer, J. Fenn, the 2002 Nobel Prize in Chemistry (Fernández De La Mora, 1992; Law, 2002; Fenn, 2003). The work that followed focused mainly on low flow rates in the context of using electrosprays for ‘liquid-fuel batteries’ that could act as portable power sources and deliver electric power on the order of 100 W. In this context, the fundamentals of hydrocarbon fuel electrosprays were studied by Tang and Gomez, (1996), catalytic micro-burners have been established (Kyritsis et al., 2004a,b) and an electrospray-based burner combined with a miniaturized stirling motor with an overall efficiency that exceeded 20% has recently been demonstrated (Gomez et al., 2007).

The combination of electrostatic phenomena with the relatively high flow rates and high injection pressures that are typical of engine operation has received relatively little research interest, probably because of the implicit assumption that, when present, inertial effects will dominate electrostatic ones. However, certain recent findings indicate that this assertion may not be universally valid. Rickard and Dunn-Rankin (2002) proposed a

Experimental investigation of electrostatic effects on ethanol 37

hybrid electrospray/air assisted-injection scheme that delivered good droplet size-control and Hetrick and Parsons (1997) discussed the possibility of ‘electrospray’ port injection in an internal combustion engine. Thomas, DiSalvo and Makar (2002) demonstrated computationally notable electrostatic effects during diesel injection and Bellan and Harstad (1998) showed that electrostatic charging of the diesel spray can affect soot formation. Regarding gasoline direct injection (GDI), the computational studies by Shrimpton and co-workers (2003), Shrimpton and Laoonual (2006) and Shrimpton and Rigit (2006) indicated that significant improvement of fuel dispersion is possible in a direct injection spark ignition engine when charge injection is achieved through narrow orifices. Experimental verification of this assertion has already been achieved in our previous work with electrostatically assisted gasoline injectors (Anderson et al., 2007a,b,c). Using relatively simple modifications to a commercially available GDI, it was shown that electrostatic effects can be observed in the resulting electrostatically charged sprays. In parallel, Al-Ahmed, Shrimpton and Mashayek (2007) have recently reported the establishment of an electrostatically assisted injector for use with bio-fuels at flow rates that were very high for the standards of electrically charged sprays.

Our objective in this work was to expand our recent findings with gasoline blends in the direction of diesel fuels. We are placing a particular focus on the case of ethanol–diesel blends that were recently shown to have severable favourable characteristics in terms of emissions (Degang et al., 2003; Rakopoulos et al., 2007). It is recognised, of course, that operation with such blends can be problematic because of the immiscibility of the two fuels and the significant decrease of the flash point even by minute ethanol contents (Hansen et al., 2005). However, a study of the behaviour of enhanced conductivity diesel sprays can be instrumental for the transition to blends of diesel with higher alcohols that do not present the problems of ethanol like butanol (Agathou et al., 2007). In order to achieve our objective, we followed two different routes of experimentation. Specifically, we characterised the fundamental phenomenology of ethanol–diesel electrosprays and we then proceeded to the characterisation of sprays from practical injectors using isooctane as a fuel surrogate.

2 Experimental apparatus and techniques

Ethanol can only be blended in low quantities and in a narrow temperature ranges in diesel without the presence of a co-solvent. In order to produce stable mixtures, GE Betz, DMX 10011 were used as a co-solvent. In order to test the feasibility of electrosprays of practical diesel fuel mixed with ethanol, we used European, ultra-low sulphur diesel. Table 1 shows the quantities of each mixture component in the blends that were used. It is clear that a practical logistic fuel is a multi-component mixture; the composition and physical properties of which can potentially vary within specifications from batch to batch. In order to perform droplet size and spray angle measurements from mixtures with constant properties, we used 2,2,4-trimethylpentane (isooctane) as the hydrocarbon-based component of the mixture for experiments with a practical injector. This could be mixed with ethanol without co-solvent.


Table 1 Mixture quantities of ethanol–diesel blends

Nominal ethanol content(% ethanol) Ethanol (g) Diesel (g) Co-solvent-DMX

10,011(g)

0 0 100 2 5 5 95 2 10 10 90 2 15 15 85 2

Conductivity of the fuel blends was measured with the apparatus shown in Figure 1. A 29.4 cm long, 4.5 mm ID insulated plastic cylinder was filled with the fuel being tested. Each end was capped with a conducting brass top that was in contact with the fuel inside. Applying voltages in the 1–7 kV range using a Fluke 410B high-voltage power supply and measuring the current with a Keithley instruments pico-amperometer, we could determine the cylinder resistance and then fuel conductivity using the relation:

1 fuel fuel1

/ : / ( )( / )

LV I R R R L A

A V I R (1)

where Rfuel is the resistance determined for the fuel cylinder, R1 is the resistance of a resistor that was inserted to the circuit in order to avoid shorts that would damage the amperometer, is the conductivity of the fuel, L is the length of the cylinder and A is the cross-sectional area of the cylinder.

Figure 1 Conductivity measurement apparatus


Figure 2 Schematic of fundamental electrospray set-up

The e-diesel electrospray set-up used in order to investigate the fundamental features of ethanol–diesel electrosprays is shown in Figure 2. The fuel was metered with a syringe pump at a constant volumetric flow rate of 15 mlitre hour 1. Stainless steel capillaries of 0.125 (3.2 mm) outer diameter, 400 m inner diameter, and 5 (127 mm) length were used. Their tips were electric-discharge-machined to a 45 angle in order to produce a point source for the electric field. The fluke high voltage power supply was electrically connected to the capillary which was suspended at a distance of 1.2 cm above a metal mesh on top of a collection dish. The mesh screen was grounded electrically. This produced an electric field between the capillary tip and the grounded screen.

In order to visualise the electrosprays, Mie scattering of a laser sheet was imaged. The light source used was a Quanta Ray Nd-YAG laser operating at its second harmonic at a wavelength of 532 nm and a firing frequency of 10 Hz. A laser sheet was produced by using a plano-convex lens with a focal length of 20 cm and a cylindrical negative lens with a focal length of 10 cm. The plano-convex lens was positioned so that its focal point was on the spray axis. The negative cylindrical lens was mounted such that the light diverged in the vertical direction and was positioned 15 cm from the spray axis. This resulted in a light sheet focused at the spray axis. Images were captured with a Canon EOS Digital Rebel camera with a Cannon EFS 18–55, f/5.6 lens set at a nominal focal length of 79 mm and two 12 mm extension tubes. To ensure that the camera captured one and only one laser pulse, the exposure time was set to 100 msec.

For practical injector experiments, a Mitsubishi MR560552 swirl-type injector was modified for electrostatic assistance of the sprays with an insert that introduced electrostatic charge to the fuel through a process described in detail by Anderson et al. (2007a,b). For the purposes of completeness, in Figure 3 we have shown the main idea of this insert which relies on the generation of a very strong electric field with the use of a sharp conical feature on a cap that is electrically insulated from the body of the injector. The injections that were studied here occurred at atmospheric ambiance.

A Dantec phase doppler anemometry particle analyser a 90° collection angle was used for droplet size measurements. The index of refraction used for ethanol was 1.36 and for diesel was 1.46 and the index of refraction of blends, which is a necessary input


for the size measurement was determined by the correlation proposed by Mutalik et al. (2006).

Figure 3 Injector cap insert for electrostatic assistance of the sprays

3 Results and discussion

3.1 Fundamental features of ethanol–diesel electrosprays

A fundamental feature of ethanol–diesel and ethanol–isooctane electrosprays is the very intense sensitivity of mixture conductivity on ethanol content as evidenced by the results of Figure 4. It is noted that the ordinate of the figure is logarithmic and that mixture containing 20% per mass ethanol has a conductivity more than two order of magnitudes higher than the one of pure diesel. This indicates that electrostatic phenomena on sprays can be enhanced drastically even with minor ethanol addition.

It is interesting to notice, however, that this result does not translate directly to the morphology of simple diesel–ethanol blend electrosprays shown in Figure 5 for a constant flow rate of 15 mlitre hour 1. Specifically, at an applied voltage of 5.5 kV, electrosprays of pure ethanol (Figure 5a) appear in the cone-jet mode as defined by Cloupeau and Prunet-Foch (1994). A cone shaped meniscus appears emitting a jet of droplets that fan out because of the applied electric field. As the voltage increases, the spray destabilises into multiple jets. Each jet produces small droplets that spread out due to Coulombic forces. This effect matches what was reported by Cloupeau and Prunet-Foch (1994) in their review of electrospray regimes. On the other hand, images of electrosprays formed using pure diesel are shown in Figure 5b. These sprays also


break-up into droplets, however, the droplets are much larger than those in the ethanol sprays. The structure of the spray is one of the field-enhanced dripping mode (Cloupeau and Prunet-Foch, 1994). In this mode, the electrostatic forces reduce the effective surface tension of the liquid leading to enhanced dripping. Notably, Figure 5c–e which shows electrosprays using 5, 10 and 15% ethanol per mass in diesel, respectively, resemble more the structure of Figure 5b than the one of Figure 5a, despite the drastic increase in conductivity shown in Figure 4. The sprays operate in the micro-dripping mode (Cloupeau and Prunet-Foch, 1994) and the cone-jet mode of Figure 5a are not established. Clearly, the drastic increase in conductivity for small ethanol contents is not enough to overcome the action of the surface tension of the fluid (which is roughly the same for ethanol and diesel).

Figure 4 Electrical conductivity (a) ethanol–diesel (b) ethanol–isooctane blends with varying ethanol content


Figure 5 (a) Ethanol electrospray (b) Diesel electrospray (c) 5% ethanol in diesel electrospray (d) 10% ethanol in diesel electrospray (e) 15% ethanol in diesel electrospray

Droplet size measurements for the sprays of Figure 5 were taken 7 mm directly beneath the tip of the capillary. This is clearly an arbitrary choice. Development of a detailed, spatially resolved map of droplet size measurements is beyond our scope here. Instead, we would like to highlight variations of droplet size with the applied voltage, which are not expected to change drastically with location in the spray. This expectation is supported by the lack of vapourisation in these low temperature sprays, as well as the resistance of charged droplets to coalescence (Of course, droplet break-up still remains a possibility). For a voltage range from 5.5 to 7 kV, 10,000 samples of droplets were registered in order to have sufficient sample statistics.

Figure 6 shows the results of droplet size measurements for ethanol–diesel blends of varying ethanol content (% mass) for applied voltages of 5.5–6 kV. Specifically, the probability distribution functions of the arithmetic mean diameter (d10) is reported. A key feature of electrosprays operating in the cone-jet mode highlighted by Tang and Gomez (1996) was the ability to produce monodisperse droplets (i.e. droplets that all have the same diameter, with a very narrow range of sizes around the mean diameter). At 5.5 kV, both the spray of pure ethanol and the 15% ethanol blend exhibit narrow size distributions that could be considered monodisperse to a very good approximation. Similarly, blends with 5 and 10% ethanol present reasonably narrow size distributions. It should be noted that the pure diesel droplet size distribution corresponds to a


polydisperse distribution (i.e. a distribution with a fairly wide range of sizes) centred around a much larger mean value of diameter. Clearly, despite their visual similarity with the sprays of pure diesel (Figure 5b), the sprays of low-ethanol content blends (Figure 5c–e) are more similar to the sprays of pure ethanol (Figure 5a). This result is in agreement with the results of Figure 4, which shows a strong sensitivity of the blend conductivity on ethanol content for low-ethanol content mixtures. It is also noted that, for a given flow rate, the droplet size is diminishing with increasing ethanol content.

Figure 6 Droplet diameter distributions for applied voltages (a) 5.5 kV (b) 6 kV and a flow rate of 15 mlitre hour 1 for varying ethanol content (% mass)


3.2 Spray structure from practical swirl injectors

Typical results of the spray evolution of a spray from a practical swirl injector are shown in Figure 7. This was a mixture of 95% per mass ethanol and 5% isooctane and the results are typical of the structure of the spray for all compositions. Data were taken for pure ethanol and for 5, 50 and 95% ethanol in isooctane. For each fuel type, sprays were imaged with 20, 35 and 50 psi (1.4, 2.4 and 3.4 bar) injection pressures. As shown in Figure 7, an early stage of poorly atomised spray is followed by the formation of a hollow cone typical of swirl injectors. It is, of course, realised that both the configuration of the particular injector and the employed injection pressures are not typical of diesel injection, but we would like to point to a few interesting features of spray structure that we observed in the easy-to-realise conditions of our experiment that deserve further exploration in more demanding environments. It is also noted that the structure of these sprays is similar to the structure of gasoline sprays reported by Anderson et al. (2007a,b,c).

Figure 7 shows certain differences between the electrostatically neutral spray and the spray that was charged with the application of a 3 kV voltage. In order to assess these differences quantitatively, we needed to measure spray penetration and spray width from the visualisation data. The pursuit of such measurements immediately raises the issue of a useful definition for these two quantities (i.e. penetration and width). Do, for example, the poorly atomised ligaments that appear in the data of Figure 7 count as ‘spray penetration’? Similar questions can be raised about ‘stray’ droplets that occasionally look disconnected from the main body of the spray. In order for these questions to be answered quantitatively, consistent algorithms have to be used for the detection of the edge of the spray.

Figure 7 Spray evolution of 95% ethanol–isooctane at 50 psi injection pressure

Times listed are msec after spray emergence

The visualisation data were processed as follows: two spray modes were defined, each with a different algorithm for its analysis. An ‘early’ mode was defined before the formation of the hollow cone. The time period after the establishment of the spray cone was defined as the ‘late’ mode. The algorithms for the quantitative analysis of both modes began by converting the images from colour to grayscale. This assigns each pixel of the image an intensity value between 0 and 255. These intensity values were then scaled from 0 to 1, where 0 was fully black and 1 corresponded to maximum intensity.


Next, all pixels with intensity greater than 0.2 were reassigned to an intensity value of 1, and the rest were set to 0. In order to eliminate points that were not part of the main body of the spray, all bright pixels that were not contiguous with a pixel selected within the main spray directly below the tip were discarded.

After this step, the two algorithms differed. The process for measuring ‘early’ penetration computed the distance from the injector tip to the bright pixel which was furthest from it in the vertical direction, shown as ‘A’ in Figure 8. The width was determined by the distance between the bright pixels furthest on the right and furthest on the left, shown as ‘B’ in Figure 8. In the ‘late’ spray, the penetration measurement was determined by the distance between the injector tip and the vertical coordinate where the spray had the greatest width, shown as ‘C’ in Figure 8. The width was determined for every vertical row of the spray. The maximum value, shown as ‘D’ in Figure 8, was reported as the width measurement.

The results of these calculations are shown in Figures 9 and 10 for pure ethanol and 95% ethanol blends. In the early injection phase, where the hollow cone has not yet developed, the penetration of the charged sprays of pure ethanol is smaller when compared to the uncharged sprays. This can be seen in Figure 9, which shows larger penetration for uncharged sprays at the early stage of injection. This is most pronounced in the 20 psi injection pressure, where electrical forces are relatively stronger compared to inertial forces than at higher injection pressures, and similar trends apply to 35 and 50 psi but less manifest. This smaller penetration can be potentially beneficial for engine operation, because diminished amounts of fuel injected in this early, undesirable, mode that precedes the fuel cone can lead to better quality of combustion.

The transition of charged and uncharged sprays into the hollow cone mode takes place at approximately the same time after injector actuation despite the differences in early penetration. The spray cone angle for each injection pressure is shown in Figure 10. The cone angle is defined as the angle formed by the spray based on the measurements of the ‘late’ phase of spray evolution:

1 / 22 tan DC

(2)

Figure 8 Early (left) and late (right) phases for the definition of spray penetration and width


Figure 9 Spray penetration measurements (a) pure ethanol (b) 95% ethanol–isooctane injections

20 psi injection pressure; , uncharged; , charged – 3 kV.


Figure 10 Cone angle results (a) pure ethanol (b) 95% ethanol–isooctane injections

, uncharged and , charged.

where D and C are defined in Figure 8. For the pure ethanol spray at 20 psi, the cone angle in the late spray is larger for the charged spray than for the uncharged spray as shown in Figure 10a. For the blends of 95% ethanol–isooctane (Figure 10b), there are no significant cone angle effects observed, and the 5 and 10% ethanol blends did not show cone angle effects either. Given that even the discernible effect of Figure 10a is minor, it


can be stated that the electrostatic effects on spray morphology are limited in the ‘early’ part of the injection, with only minor effects on the spray cone.

4 Conclusions

Ethanol–diesel blends have a conductivity that depends approximately exponentially on the ethanol content for blends with less than 20% per mass in ethanol. This manifested itself in an interesting manner in simple electrosprays that emanated from a capillary: although sprays with 5–15% per mass ethanol were visually similar to the ones of pure diesel, they produced approximately monodisperse sprays of finer droplets that were similar to the sprays of pure ethanol. Injection of electrostatically assisted ethanol–isooctane blends with a swirl-type injector showed that the electrostatically charged sprays penetrated an atmospheric ambiance less than uncharged sprays prior to cone formation. This result was observed in ethanol and 95% ethanol–isooctane and was most significant for 20 psi injection pressure. For sprays of pure ethanol, the cone that formed late in the spray had a wider angle for charged sprays compared to uncharged ones, but this difference was minor compared to the difference of the two sprays in the early part of injection, i.e. before the hollow spray cone was formed.

Acknowledgements

Authors would like to acknowledge the support of the US Department of Energy through the Graduate Automotive Technology Education (GATE) Center of Excellence at the University of Illinois, as well as of the US National Science Foundation (grant CTS 04-48968CAR – Dr. Phillip Westmoreland, contract monitor).

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Ijvd 08 elegant et al