Experimental Investigation of Combustion of ...

Paper # G11

Topic: Spray Combustion

5th US Combustion Meeting Organized by the Western States Section of the Combustion Institute and Hosted by the University of California at San Diego March 25-28, 2007.

Experimental Investigation of Combustion of Electrostatically Charged Ethanol Blended Gasoline Droplets E. K. Anderson1, A. P. Carlucci2, A. De Risi2, and D. C. Kyritsis1 1

Department of Mechanical Science and Engineering, University of Illinois at UrbanaChampaign, Urbana, Illinois 61801, USA 2

Department of Engineering for Innovation, University of Lecce, Lecce, Italy

By applying electrostatic charge to the fuel from a fuel injector, electrostatic assistance of fuel injector sprays is one possible means of improving the atomization and fuel distribution in an engine. A gasoline injector was modified to induce electrostatic charge on the fuel spray and particle imaging velocimetry (PIV) measurements of the spray velocity field were performed. These results were complemented by high speed video of combustion of electrostatically charged ethanol blended gasoline droplets suspended from a capillary. Ethanol blended gasoline was selected as a fuel because of its widespread use in automotive gasoline engines. The experiment took place at room temperature and pressure and under normal gravity and a spark ignition system was utilized. Non-charged droplets were burned under otherwise identical conditions to compare to the combustion of the charged droplets, and images of the droplets prior to combustion were taken to provide a measure of droplet diameter. High speed video of the combustion of the droplets was recoded and the features of the flames were compared for the charged and noncharged droplets.

1. Introduction Reliable operation of spark ignition engines requires an approximately stoichiometric air-fuel mixture in the vicinity of the spark [1]. However, once ignition has occurred, the flame can propagate through leaner mixtures, potentially decreasing the fuel consumption of the engine [2]. The use of gasoline direct injection (GDI) can allow stratified fuel mixtures to be achieved, resulting in a Direct Injection Stratified Charge (DISC) engine. However, difficulty in achieving precise control of fuel stratification has hindered adoption of DISC engines for automotive applications. The turbulent in-cylinder flow field and the wide range of engine speeds and loads encountered by automotive engines makes accurate control of fuel stratification difficult to achieve. Poor control of fuel stratification results in increased emissions as well as reliability problems [3]. The heart of the problem arises from the fact that with a pressure driven fuel injector, the fuel momentum, which determines the spray pattern and is therefore crucial to achieving accurate stratification, is coupled to the mass of fuel injected, which is determined by the load on the engine. By using electrostatic charge as an additional means of controlling the spray, fuel mass

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5th US Combustion Meeting – Paper # G11


and momentum can be decoupled. Electrical control of combustion processes was investigated as early as 1971 by Thong and Weinberg [4] and subsequent work by Kim and Turnbull [5] and Kelly [6] showed that electrostatic charge could be injected into low conductivity liquids, such as the liquid hydrocarbons present in gasoline, through the use of sharp electrodes. This technique can be used to apply “electrospray” technology for fueling in power generation applications. The already mature electrospray technology is widely used in the field of analytic chemistry [79]. Gomez and collaborators have extensively studied fuel electrosprays [10, 11] and employed this idea to design a “liquid fuel battery” which burns kerosene atomized by an electrospray technique [12, 13]. Gemci and Chigier also discussed the potential use of electrospray technology in micro combustors [14]. This “electrospray combustion” research employs electrostatic effects as the sole means of atomizing the fuel, which simplifies fueling but also limits the achievable fuel flow rate. The combination of electrostatic fuel atomization with pressure driven sprays for combustion applications has received little research interest, likely because of the assumption that inertial forces would dominate any electrostatic forces. However, a recent hybrid electrospray/air assisted injection design was shown to achieve good control of droplet size [15] and electrospray port injection in an internal combustion engine was discussed by Hedrick and Parsons [16]. Significant electrostatic effects during diesel injection were found computationally by Thomas et al. [17]. It was also found that electrostatic charging of the fuel spray can affect soot formation [18]. A computational study by Shrimpton [19] found that fuel dispersion in a Direct Injection Spark Ignition (DISI) engine could be improved through electrostatic charging of the fuel spray. Most of the studies, however, lack experimental verification. Our objective was to determine experimentally the effect of electrostatic charge on gasoline sprays of relevance to automotive gasoline engines, and the effect of the charge on the subsequent combustion of these droplets. A GDI injector was modified to apply electrostatic charge to the fuel spray and installed in an optically accessible spray chamber. Optical techniques described in [20, 21] were employed to measure the effect of charge on fuel dispersion and droplet size. With engine applications in mind, the combustion of electrostatically charged gasoline droplets was compared to combustion of non-charged droplets through the use of high speed video. 2. Experimental Apparatus and Techniques The hardware and procedures used to obtain the results is described in two separate sections below which focus separately on sprays themselves and combustion of charged droplets. For both the spray experiments and the combustion experiments, the same fuel, standard pump 87 octane (R+M)/2 gasoline, was used. Details regarding the fuel are available in [22]. 2.1 ELECTROSTATICALLY CHARGED SPRAYS The configuration of the apparatus for obtaining PIV velocity fields is shown in Fig. 1. A gasoline direct injector, Mitsubishi p/n MR560552, was modified to inject electrostatic charge into the fuel. To utilize the fact that electric field strength is locally enhanced by sharp edges on surfaces with high electric potential, the electrode shown in Fig. 2, featuring a sharp conical tip at the orifice where fuel emerges, was constructed. This electrode, made from copper, has 1 mm

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Figure 1: Configuration for PIV measurements. Figure 2: Injector cap.

side walls and a 0.5 mm base thickness. The sharp cone at the tip of the electrode protrudes 0.5 mm at an angle of 135 degrees with the base. The locally intense electric field that is generated from the sharp conical feature partially polarizes the fuel molecules and injects ions into it, thus transferring a net charge to the liquid. The electrode fits snugly around a 1 mm polyester insert, which electrically isolates the electrode from the injector. The polyester insert itself fits snugly around the end of the injector. To produce the charged sprays, 3 kV was applied to the copper electrode from a Fluke model 410B voltage generator. Details regarding the choice of angle for the sharp conical feature on the copper electrode are available in [20, 21]. PIV measurements are reported for injection pressures of 2.8 bar and 4.1 bar, and the sprays emerged into stagnant air at room temperature and pressure. These injection pressures are low for gasoline direct injection, but within the operating range of port injectors. Timing of the injection and the PIV system was accomplished with a Stanford Research Systems DG535 pulse generator. A National Semiconductor LM1949 chip was used to actuate the injector.

Figure 3: Measurement planes for PIV: r=0 and r=12 mm from the central axis of the injector.

The PIV (particle imaging velocimetry) equipment used was a LaVision FlowMaster HS high speed PIV system. Velocity fields were obtained along two planes in the spray, as shown in Fig. 3. Data in these planes were collected by aligning the PIV laser to illuminate one of three cross-sections of the spray located at r = 0 and 12 mm from the injector axis. These velocity fields were obtained at 0.4 ms time intervals after the start of injection.

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2.2 CHARGED DROPLET COMBUSTION The configuration for the apparatus for obtaining high speed video of charged and non-charged gasoline droplet combustion is shown in Fig. 4. A KD Scientific Model 100 syringe pump was used to pump known quantities of ethanol blended gasoline from a Hamilton Gastight #1750 0.5 ml syringe. The syringe pump was mounted vertically so that the syringe would point downwards. Downstream of the syringe, the fuel flowed through a steel 22 gauge syringe needle, a 3.2 cm section of electrically insulating teflon, and finally, it was suspended from a 2 cm length, 0.18 mm outer diameter capillary made of silicon carbide. For each droplet combustion experiment, a droplet was fed from the syringe and suspended from the silicon carbide capillary. Ignition was provided by a spark generated by an automotive coil. A Pyramid Model PS-36KX power supply generating 12 volts was used to power a Bosch 0 221 122 006 coil. The primary coil was triggered by a Gigavac model GR5LTA145 relay which received a timing signal from a Stanford Research Systems Model DG535 pulse generator. The droplet charging system consisted of a Fluke model 410B voltage generator which was set to 3 kV for charged droplet experiments and 0 kV for non-charged experiments for comparison. The high voltage lead from the voltage generator was connected to a Gigavac model GR5LTA145 high voltage relay to allow rapid switching of the high voltage. The relay was controlled with the pulse generator described above. The purpose of the high voltage switching system was to apply voltage to the droplet immediately after the spark event to prevent discharge of the high voltage circuit through the arc.

Figure 4: Droplet combustion experimental configuration.

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A Phantom v7 high speed camera was used to acquire video of the droplet combustion. Three Nikon extension tubes totaling 82.5 mm were used to increase magnification along with a Tiffen 52 mm zoom lens. Camera exposure time was set to 70 µs and the zoom and aperture settings were adjusted to provide a clear view of the entire flame. The camera was triggered by the same pulse generator controlling spark and the high voltage circuit. 3. Results and Discussion 3.1 PARTICLE IMAGING VELOCIMETRY (PIV) MEASUREMENTS Figure 5 provides results for the spray cross-sections shown in Fig. 3. As indicated in the figure, data were collected at 2.8 and 4.1 bar, and for each measurement plane, PIV images were acquired for both charged and non-charged sprays. To calculate the two-dimensional velocity fields shown in Fig. 5, the PIV technique involves calculating the cross-correlation functions of Mie scattering data [23]. This cross-correlation calculation allows quantitative velocity data to be extracted from two images obtained with a double-pulsed laser. When there are not enough droplets in a particular location to calculate the cross-correlation function accurately, no velocity

Figure 5: PIV velocity fields recorded at a plane including the spray axis (r=0 mm) for an injection pressure of 2.8 bar for the left two columns, r=0 mm and 4.1 bar for the middle two columns, and 2.8 bar and r=12 mm for the right two columns. The label 3 kV indicates a charged spray and 0 kV indicates a non-charged spray.

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vector is calculated and a blank pocket appears in the images of Fig. 5. The two left-most columns in Fig.5 show the temporal evolution of the spray for an injection pressure of 2.8 bar and the measurement plane through the center of the spray. The electrostatically charged sprays labeled 3 kV clearly show larger penetration and a slightly greater cone angle compared to the non-charged sprays, likely due to Coulombic repulsion. In addition to the differences in penetration and cone angle, the two columns on the left of Fig. 5 show that the non-charged sprays have the form of a well aligned stream of droplets traveling straight down and outward from the injector tip, whereas the charged sprays display a much less coherent velocity field. In contrast with the non-charged sprays, the charged sprays develop vortical motion of the droplets. Furthermore, the charged sprays exhibit pockets devoid of droplets within the spray. These features are likely due to mutual repulsion of the charged droplets and are of interest to engine applications because vortical motion and empty pockets within the sprays can enhance vaporization at the elevated temperatures within a combustion chamber. The two columns at the center of Fig.5 show the temporal evolution of sprays at an increased injection pressure of 4.1 bar. As was the case for the 2.8 bar sprays in the two left-most columns, the PIV laser sheet was positioned at a plane through the spray axis. These PIV velocity fields indicate that, as one would intuitively expect, the differences between electrostatically charged and non-charged sprays decrease with increasing injection pressure. At the increased injection pressure, both the charged and non-charged sprays develop vortical droplet motion and empty pockets with the sprays, especially after 1.6 ms after the start of injection. Comparing the 2.8 bar electrostatically charged spray in the second column to the 4.1 bar noncharged spray in the third column, it is remarkable that the charged spray at the lower pressure is very similar to the non-charged spray at the higher pressure. The lower pressure charged sprays occupy nearly the same area in the measurement plane and appear to feature even greater vortical motion than the higher pressure non-charged sprays. This feature of the charged sprays could potentially be very beneficial to fuel system design, since the same spray quality may be achievable at significantly reduced injection pressures, reducing the parasitic load on an engine caused by the fuel pump. In the last two columns of Fig. 5, PIV velocity fields are presented for the lower injection pressure of 2.8 bar but at a plane 12 mm away and parallel to the injector axis. At 1.6 and 3.6 ms after the start of injection, we see that there are two blank images in the fifth column, indicating that for the non charged spray, no velocity measurements are made until 4.8 ms after the start of injection. The charged droplets, however, have already reached this viewing plane at 1.6 ms. This is likely due to Coulombic repulsion causing the droplets to disperse outward from the spray axis. Also, as in the first two columns, it is apparent that the non-charged spray exhibits less vortical motion than the charged spray.

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Quantitative velocity data was extracted from the velocity fields of Fig. 5 and plotted in Fig. 6. The PIV technique provides two dimensional velocity fields, so only velocity information in the plane of the laser sheet can be extracted. Figure 6 displays the axial velocity along horizontal lines 7 and 11 mm below the injector tip 3.6 ms after the start of injection for 2.8 bar injection pressure. The figure shows several locations where the velocity drops to zero. The measurements of zero velocity indicate that not enough droplets were observed in the corresponding location and therefore an accurate velocity vector could not be calculated using PIV. Hence, these zero velocity points correspond to empty “pockets” within the spray rather than stationary droplets. The axial velocities of Fig. 6 appear to fluctuate much more for the electrostatically charged sprays because of the empty “pockets” within the charged sprays.

Figure 6: Axial velocity component as a function of radial location along lines intersecting the spray axis perpendicularly at 7 mm and 11 mm below the injector tip. Data was taken 3.6 ms after injection with an injection pressure of 2.8 bar. Non-charged sprays shown as (o) and charged sprays shown as (□). Zero-velocity points indicate locations were droplets are too spares for an accurate velocity measurement – not stagnant droplets.

Figure 6 also shows that outside the empty “pockets,” the droplets of the charged spray have consistently higher velocity. The increase in velocity of the charged sprays is on the order of 10%, although in some cases the difference is larger. It appears that Coulombic repulsion and the presence of an electrostatic field slightly increases droplet speed, thereby enhancing dispersion. 3.2 HIGH SPEED VIDEO OF DROPLET COMBUSTION

The investigation of electrostatically assisted sprays described in the preceding sections generates droplets that carry electrostatic charge. If an electrostatically assisted injection system of similar design were to be installed on an engine, the droplets injected into the cylinder would carry some charge and in addition there would be some electric field generated by the potential difference between the charge electrode and the metallic walls of the cylinder and combustion 7



chamber. Moreover, it is reminded that flames in general do generate electric charge, because of the high temperatures of the process that lead to the generation of a very dilute plasma [24]. For these reasons, experiments investigating the ignition and subsequent combustion of ethanol blended gasoline droplets have been initiated. Introduction of ethanol to the fuel provides a crucial link between this work and the emerging technology of biofuels. These fuels are particularly well suited to electrospray because of the polar –OH bond, which makes such liquids easier to polarize. The experimental configuration described in section 2.2 was used to generate droplets, ignite them, and record their combustion using a high speed video camera.

Figure 7: Images of ethanol blended gasoline combustion for various levels of droplet charge. All images are 100 ms after ignition.

Figure 7 provides images from the high speed videos of the combustion of charged and noncharged fuel droplets. For all videos, a droplet of approximately 10% ethanol and 90% petroleum gasoline fuel of 1 ± 0.2 mm diameter was generated by the syringe pump. Once a stable droplet was established, it was ignited using the spark electrodes which are visible as horizontal lines coming from the lower left and right sides of the images in Fig. 7. The high speed camera was triggered to begin recording 40 ms before the ignition event. The images of Fig. 7 show one frame from the high speed video of seven different droplets, each burned with a different level of electrostatic charge applied. All of these frames were captured 100 ms after the ignition event, so that the only experimental parameter that changes between

the images shown is voltage. Figure 7 shows that for positive voltages, the flame is attracted toward the spark electrodes, which are both electrically grounded immediately after the spark event. In addition, the flame is also attracted toward the ground plate which is just visible at the bottom of the images for the positive voltages, and slightly below the field of view for the rest of the images. This is because the camera was moved downward for the positively charged droplet combustion so that the flame would be entirely within the field of view. Combustion of negatively charged droplets, on the other hand, results in flames which are taller and narrower, with no apparent attraction to the spark electrodes or the ground plate. This result seems to suggest that the luminous soot we are

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seeing in the figures carries some electrostatic charge, causing it to be attracted or repelled from the surrounding electrically grounded objects. The fact that the soot surrounds the negatively charged capillary for the negatively charged droplet combustion and moves away from the positively charged capillary in the positively charged droplet combustion suggests that the soot may carry a positive charge. The unusually small flame of the -2 kV video is due to the rather large error in producing droplets of consistent initial diameter. This is due to difficulties pumping consistent amounts of fuel through the narrow diameter capillary. Small fuel leaks were found on occasion, and sometimes fuel continues to be ejected from the capillary even after the syringe pump has stopped moving. Clearly, these are preliminary results that simply establish the existence of electrostatic phenomena on the flame and render further investigation necessary. In terms of hardware, the experiment could be improved by developing a more accurate method of metering the liquid fuel. One idea is to install a small valve near the end of the capillary that would close after the desired droplet diameter is reached. By improving the accuracy of droplet generation, it should be possible to make better comparisons of burning rate and flame diameter between the various levels of electrostatic charge. 4. Concluding Remarks By simply adding an electrode to the tip of a commercially available fuel injector, it was shown that electrostatic effects can be observed in the resulting electrostatically charged sprays. The electrode was insulated electrically from the injector and featured a sharp conical orifice to inject charge into the low conductivity gasoline fuel. PIV measurements of spray velocity fields showed that the resulting charge and electrostatic field caused some phenomena that were only observed at higher injection pressures for non-charged sprays. In particular, vortical motion of the fuel droplets and pockets devoid of droplets were observed at lower injection pressures with the presence of electrostatic charge. In addition, the axial velocity components of the charged spray droplets were approximately 10% of those for the non-charged sprays, enhancing spray penetration and fuel dispersion. To complement the finding that electrostatic charge can enhance fuel dispersion in pressuredriven injector sprays, charged and non charged fuel droplets were burned. It was found by comparing the video of positively and negatively charged burning droplets that the soot appeared to carry positive charge. High speed video images also showed that there were morphological differences in the flames. For positively charged droplets, the flame was relatively small in the verctical direction and attracted to nearby electrical grounds. For negatively charged droplets, the flame was long in the vertical direction and thin in the horizontal direction and appeared to be repelled by the electrical grounds and attracted to the negatively charged capillary. Measurements of current transferred to a ground plate as fuel was fed through the charged droplet combustion apparatus and fuel injector sprays showed that the droplet combustion apparatus generated charge densities around 130 nC/ml while sprays generated charge densities of approximately 2 nC/mL.

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Acknowledgments The authors would like to thank the Donors of the American Chemical Society Petroleum Research Fund for support of this research (Grant # PRF 42287-G9-Dr. David L. Shutt, Acting Program Director) and the support of the U.S. Department of Energy Graduate Automotive Technology (GATE) Center of Excellence at the University of Illinois at Urbana-Champaign (Grant DOE DE-FG26-05NT42622). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]

Y. Takagi, Proceedings of the Combustion Institute 27 (1998) 2055-2068. H. R. Ricardo, U.S. Patent 1,271,942 (1918). S. Shiga, H. Ozone, H. T. C. Machacon, T. Karasawa, H. Nakamura, T. Udea, N. Jingu, Z. Huang, M. Tsue, M. Kono, Combustion and Flame 129 (2002) 1-10. K. C. Thong, F. J. Weinberg, Proceedings of the Royal Society of London 324 (1971) 201-215. K. Kim, R.J. Turnbull, Journal of Applied Physics 47 (1976) 1964-1969. A.J. Kelly, Journal of the Energy Institute 57 (1984) 312-320. J.B. Fenn, Angewandte Chemie – International Edition 42 (2003) 3871-3894. J. Fernandez De La Mora, The Journal of Fluid Mechanics 243 (1992) 561-574. S.E. Law (Ed.), Journal of Electrostatics 54 (2002) 217. K.Q. Tang, A. Gomez, Journal of Colloid and Interface Science 184 (1996) 500-511. A. Gomez, K.Q. Tang, Physics of Fluids 6 (1994) 405-414. D.C. Kyritsis, S. Roychoudhury, C. McEnally, C. Pfefferle, A. Gomez, Experimental Thermal and Fluid Science, 28 (2004) 97-104. D.C. Kyritsis, B. Coroton, F. Faure, S. Roychoudhury, A. Gomez, Combustion and Flame 139 (2004) 77-89. T. Gemci, N. Chigier, AIAA Paper Nr. 2003-675 (2003). M. Rickard, D. Dunn-Rankin, SAE Paper 2002-01-2754 (2002). R.E. Hedrick, M.H. Parsons, SAE Paper 97287 (1997). M.E. Thomas, R. DiSalvo, P. Makar, SAE Paper 2002-01-3053 (2002). J. Bellan, K. Harstad, Atomization and Sprays 8 (1998) 601-624. J.S. Shrimpton, International Journal for Numerical Methods in Engineering 58 (2003) 513-536. E.K. Anderson, A.P. Carlucci, A. de Risi, D.C. Kyritsis, Atomization and Sprays 17 (2007) 1-25. E.K. Anderson, A.P. Carlucci, A. de Risi, D.C. Kyritsis, International Journal of Vehicle Design in press (2007). http://www.bpdirect.com/pdfs/Amoco_Regular_Gasoline.pdf. R.J. Adrian, Experiments in Fluids 39 (2005) 159-169. B. Lewis, G. von Elbe, Combustion, Flames, and Explosions of Gases, 2nd ed., Academic Press, New York, 1961.

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