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Novel low voltage EHD spray nozzle for atomization of water in the cone jet mode
ARTICLE IN PRESS
Journal of Electrostatics 65 (2007) 490–499 www.elsevier.com/locate/elstat
Novel low voltage EHD spray nozzle for atomization of water in the cone jet mode Orest Lastowa,b,, Wamadeva Balachandranb a
AstraZeneca R&D Lund, S-221 87 Lund, Sweden School of Engineering and Design, Brunel University, Uxbridge, Middlesex UB8 3PH, UK
b
Received 2 February 2006; received in revised form 27 August 2006; accepted 10 November 2006 Available online 11 December 2006
Abstract Due to the high surface tension and high conductivity, water is unsuitable for electrohydrodynamic (EHD) atomization using a DC electric field in air. The high local electric field, that is required to atomize water, is likely to generate corona discharge and consequently destabilize the atomization process. This study describes a novel low voltage EHD spray nozzle that can be used to atomize water and weak saline solutions in the stable cone jet mode. The properties of the atomization have been investigated together with the generated droplet size distribution. The nozzle operates at very low flow rates (0.5–4.0 ml/min). Due to the high dielectric constant of water and the low flow rate, the atomization takes place outside the applicability range of the scaling laws. The experimental results show that the droplet size is approximately constant when the flow rate is increased from 0.5 to 4.0 ml/min. The atomization of water was numerically simulated using computational fluid dynamics (CFD). The simulation results agree reasonably well with the experimental results with respect to the liquid cone shape and droplet size. r 2006 Elsevier B.V. All rights reserved. Keywords: EHD atomization; Water; CFD; Cone jet mode
1. Background Atomization of water using electrohydrodynamic (EHDs) is not widely reported in the literature. In a review of 49 published papers conducted by Grace and Marijnissen [1], only four references to atomization of pure water were found. The inherent physical properties of water, i.e. surface tension and conductivity, make water unsuitable for EHD atomization in the cone jet mode using DC field [2–4]. In order to overcome the effect of the high surface tension, the typical applied DC electric field should be of the order of 106 V/m (10–30 kV). This gives electrical breakdown in the surrounding air, leading to corona discharge around the edges of the nozzle and the liquid cone apex [2,5]. The corona discharging at the nozzle edge destabilizes the liquid cone and the droplet size distribution becomes very wide and not reproducible. In addition, due Corresponding author. AstraZeneca R&D Lund, S-221 87 Lund, Sweden. E-mail address: [email protected] (O. Lastow).
0304-3886/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.elstat.2006.11.004
to high conductivity of water, the charges relax to the surface very fast (a few ms) initiating uncontrolled disruption of the liquid jet. Several ways to reduce or avoid discharging have been reported in the literature [6]. Some of the different approaches are discussed below. To avoid corona discharges from the sharp edges of a nozzle the electric field strength near the edges can be reduced by blunting the edges. Marijnissen [7] developed a specially designed nozzle with no sharp edges. However, the liquid cone apex and jet have inherently very sharp geometries and corona discharge occurs from the apex. The corona discharge from the cone apex is sufficient to destabilize the atomization. Water can also be atomized using a co-axial nozzle when water and methanol are mixed at the point of atomization. This approach was first suggested by Smith et al. [8,9] for mass spectroscopy applications. Du¨lcks and Juraschek [10] used a similar coaxial nozzle to atomize aqueous solutions. To avoid corona discharge, the atomization can be performed in an insulating gas. This can be done either by placing the whole set-up in an environment filled with a
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suitable inert gas or having a co-axial sheath flow around the nozzle and spray [4,5,11,12]. The gas used is often CO2. Other suitable gases are SF6 and S2F10 [3,12]. Using an inert gas makes it possible to apply voltages up to at least 20 kV. Tang and Gomez [4] used CO2 as sheath gas to atomize water at flow rates of 5.8–42.4 ml/min. They obtained droplets in the size range of 5–12 mm. They used a capillary with inner diameter of 0.12 mm with a potential of 10–20 kV. The spray was generated in the stable cone jet mode giving a monodisperse aerosol. To decrease the high surface tension of water, a surfactant can be added to decrease the surface energy. At equilibrium, the liquid surface is covered with surfactant molecules oriented perpendicular to the surface. When a droplet is formed, a new surface is created by the remaining liquid. Directly after droplet formation, this surface is not covered by surfactants and non-equilibrium conditions occur. In the surface generation process, the surfactant molecules are continuously transported by gradient driven diffusion from the bulk to the surface. The molecules are then adsorbed onto the surface and reoriented perpendicular to the surface with the hydrophilic group facing inwards [13]. This process continues until an equilibrium surface tension is reached, i.e. a state of lowest free energy. As the surfactant concentration increases at the surface, the surface tension decreases. This leads to a time-dependent surface tension. The surface tension normally reported from EHD experiments and used in modelling (e.g. in scaling laws) is the static equilibrium value. The time constant for the entire process depends on the surfactant– liquid system and can range from seconds to minutes. When atomizing water with a flow rate of 1 ml/min and a droplet size of 2 mm, the hydrodynamic time constant is 0.1 ms (10 MHz) at the point of droplet break-up. The surfactant transport time constant is several orders of magnitude greater than the hydrodynamic time constant, which suggests that an equilibrium surface tension is not reached during the process. Smith [3] reports that by adding the non-ionic surfactant Hodag 1035-L, giving a surface tension of about 0.05 N/m, the voltage required to obtain atomization was reduced from 31 to 24 kV. This reduction is not sufficient to avoid corona discharge. The atomization can be stabilized by using AC voltage superimposed on the DC voltage [14,15]. The AC frequency must be synchronized with the natural frequency of the droplet break-up. Balachandran et al. [16] used superimposed sinusoid AC voltage to atomize water. They used low frequencies of 500–1800 Hz and generated droplets in the size range of 390–500 mm. The flow rate used was 1.9 ml/min and the inner diameter of the capillary was 0.23 mm. This gives a droplet size of the same size as the capillary diameter. The superimposed AC voltage amplitude was 3 kV peak-to-peak. The DC voltage was 8 kV. Although the total voltage exceeded 11 kV, no corona discharge was reported. The droplet frequency and size suggest that the atomization was performed in the microdripping mode. To use superimposed AC to generate
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droplets of 2 mm, a frequency of 10 MHz must be used. Sample and Bollini [17] produced water droplets in the size range 120 mm using a harmonic AC of 750 Hz. The atomization process was very similar to micro dripping. None of the approaches discussed above can be used for cone jet mode atomization of water into micron-sized droplets. A novel design of a low voltage nozzle is presented and discussed below.
2. Low voltage nozzle For low flow rates (ml/min) and a nozzle consisting of a thin stainless steel capillary with an inner diameter in the range of 0.11–0.20 mm, a stable spray of water with very narrow droplet size distribution can be generated with a moderate voltage. A proprietary nozzle [18] was designed and built to investigate EHD atomization of water. The principal design of the low voltage nozzle is shown in Fig. 1. By arranging the counter electrode as a concentric ring around the capillary, the distance between the capillary nozzle and the counter electrode can be decreased. The electric field strength is then increased and the applied voltage can be lowered. The voltage of the ring shaped outer electrode is elevated to a constant potential of 2 kV relative to ground to give a more stable atomization. The potential difference between the capillary and the outer electrode is only 2 kV, which is a comparatively low value for EHD atomization. Two different stable wetting modes could be observed, see Fig. 2. One of the two wetting modes observed is inner wetting of the capillary as shown in Fig. 2 (left). In this case the base of the liquid cone is equal to the inner diameter of the capillary. The second type is outer wetting, Fig. 2 (right), which means that the liquid cone extends from the outer edge of the capillary, which also defines the base of the liquid cone. The cone angle is less for the outer wetting mode. The inner wetting angle is similar to the Taylor angle of 49.31 [19]. The two different wetting modes appear for the same voltage and the same flow rate. The spray can operate for tens of minutes in one mode and then transform to the other mode and after several minutes return to the original 1 μl/min +4 kV +2 kV
∅ 0.11 mm
∅ 4 mm
Fig. 1. Geometry of the low voltage nozzle with typical values of flow rate, diameters and voltages.
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Fig. 2. Inner wetting of capillary (left) and outer wetting (right). The flow rate is 1 ml/min and potential difference 2 kV.In both cases, the inner diameter of the capillary is 0.11 mm and the jet diameter is about 5 mm.
state. The outer wetting mode appears to be less stable than the inner wetting mode. Possibly, it is small variations in pressure or flow rate that make the cone to transform from one mode to the other. The operating window and droplet size distribution of the low voltage nozzle are discussed below. A numerical simulation of the dynamic atomization process is presented in Section 4.
PC 1
Syringe pump
PC 2
HV DC/DC 1 HV DC/DC 2
3. Experimental characterization of EHD atomization of water The performance of low voltage nozzle atomization of water and weak saline solutions was investigated. The spray mode, window of operation, droplet size distribution and spray current were investigated. The experimental setup is described in Fig. 3. The liquid was fed to the capillary by a syringe pump, PHD 2000 Infusion from Harvard Apparatus. A 3 ml plastic syringe was used. A PC data acquisition card supplied voltages 0–10 V to two high voltage DC/DC converters (Spellman MP series high voltage power supply modules). The voltage was thereby converted by a factor of 1000. The acquisition card was controlled by a LabVIEW 5.0 application. The droplet size distribution and velocity were measured using a 2-D Particle Dynamics Analyzer (PDA), Dantec 2D Fibre PDA. The results are presented as size distributions of a collection of a large number of individual particles, typically 50k. The properties of the used liquids can be found in Table 1. For visual characterization, a long distance microscope system, Infinity K2, was placed in the same way as the PDA system. The current was measured using a Faraday pail connected to an electrometer (Keithley 6517).
PDA reciever
LASER
Electrometer
Fig. 3. Experimental set-up.
3.1. Visual characteristics The shape of the cone can be seen in Fig. 2. The stability of the atomization was studied using a digital high speed imaging system, Red Lake MotionScope. The high speed imaging set-up was similar to the one shown in Fig. 3. The frame rate used was 125 fps. A sample image from the video sequence is shown in Fig. 4. The cone and jet were completely stable for the duration of the video sequence. A typical duration of one sequence was about 500 ms. During this period more than 106 droplets were generated. A number of high speed recordings were also made of sprays that were not stable, to serve as a reference to the stable sprays. Visual observations revealed that unstable sprays were also pulsating. The window of operation is defined as the range of voltage and flow rate for a specific liquid, which gives a
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Table 1 Liquid properties Liquids
Conductivity s (S/m)
Dielectric constant, k
Surface tension, g (mN/m)
Viscosity m (Pa s)
Density r (kg/m3)
Water Water+0.0050% NaCl
1.3E4 1.0E2
80.2 87.2
72 72
9.9E4 9.9E4
998 998
Noymer and Garel [20]. The low voltage nozzle could only operate with positive potential applied to the liquid. If the potential was reversed and negative potential was applied, no stable spray could be achieved. This phenomenon has also been observed by Jaworek and Krupa [21,22]. They report two different sets of spraying modes depending on the polarity, attributing the different behaviours to corona discharging disturbing the liquid cone. The polarity sensitivity is assumed to be due to the mobility difference between negative and positive ions in air, negative potentials are more likely to generate corona discharging than positive potentials. The window of operation was also determined for saline solutions. A stable spray could, however, only be achieved at the lowest flow rates (see Fig. 5). The voltage window was also very narrow and only a low concentration saline solution could be sprayed. The maximum reproducibly sprayable conductivity and dielectric constant, using the low voltage nozzle design, was 1.2E2 S/m and 92.7, respectively. The flow rates 0.5–1.0 ml/min could only be atomized in the voltage range of 2.21–2.25 kV. 3.2. Droplet size distribution Fig. 4. One frame in a series of high speed imaging frames. The potential difference is 2 kV and the flow rate is 1 ml/min. The inner diameter of the capillary is 0.11 mm and the jet diameter is about 5 mm.
stable spray with a narrow droplet size distribution measured by PDA, see Fig. 5. At voltages above the upper line and below the lower line, no stable atomization could be obtained. The grey areas show voltage and flow rate ranges where atomization in the cone jet mode can be obtained. The vertical axis displays the voltage difference between the capillary and the outer electrode. The outer electrode was kept constant at 2 kV. Water could be sprayed reproducibly with a flow rate up to 4 ml/min. Spraying with flow rates of up to 8 ml/min was occasionally achieved but could not be routinely reproduced. Water could also be sprayed using a capillary of 0.2 mm inner diameter. The window of operation was in this case considerably smaller in terms of flow rates and voltages and not as reproducible. All further experiments reported here were performed using a 0.11 mm capillary. The spray also displayed some degree of hysteresis behaviour. When increasing the voltage or flow rate from inside the window of operation to the outer limit, a stable jet could be obtained for values outside the initial limit. Similar hysteresis behaviour was observed by
The droplet size distribution was measured for both water and saline. The distribution of saline droplets was very narrow (see Fig. 6). The droplet size distribution was found to be independent of flow rate. For all flow rates the mean size was 1.0 mm with a geometrical standard deviation (gsd) of 1.4. When pure water was atomized, the size distribution showed some similarities with the saline solution results in terms of shape and independence of flow rate, see Fig. 7. There was a clear difference between the droplet frequencies for the different flow rates. The measurements were continued until about 50k droplets were registered. For a flow rate of 0.5 ml/min the measurements took about 30 min. When spraying at 4.0 ml/min the measurement took only about 5 min. A minor change in the size distribution could be observed as the flow rate was changed. The distributions became wider for 1.0 and 4.0 ml/min. This can be attributed to the dynamic properties of the spray. Since the spray is very sensitive to pressure variations, it proved difficult to keep the pressure stable for the whole duration of the measurement. It can also be the case that the spray temporarily operated in the outer wetting mode. The gsd for all the measurements was about 1.6. The high gsd is due to the fact that the number fraction does not completely
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Fig. 5. Window of operation of a low voltage nozzle with water (light grey) and saline (dark grey).
of 30 mm. These droplets contribute to the high gsd values. This behaviour is typical and was found for all flow rates. The relationship between flow rate and droplet size was studied and described by Gan˜a´n-Calvo [23,24]. He used a hybrid experimental–numerical method to derive a law describing this relationship 0:5 Q . d ¼ d0 Q0
Saline, 2.2 kV 70 0.50 μl/min 0.75 μl/min 1.00 μl/min
Number fraction (%)
60 50 40 30 20 10 0 0
2
4
6
8
d (μm)
Fig. 6. Droplet size distribution of atomized saline solution 0.0050% NaCl. Note that the 0.75 ml/min line is covered by the 1.0 ml/min line.
Water, 2.0 kV 35 Number fraction (%)
30 25
0.5 μl/min 1.0 μl/min 2.0 μl/min 4.0 μl/min
20 15 10 5 0 0
2
4
6
8
d (μm)
Fig. 7. Droplet size distribution of atomized water.
decrease to zero for droplet sizes above 5 mm. In Fig. 8 it can be clearly seen that on few occasions spitting instabilities ejected a number of large droplets in the order
This relationship is widely used in EHD atomization research and is commonly called scaling laws. Hartman refined these scaling laws using a purely numerical model [25,26] 0:48 Q d ¼ d0 . Q0 The applicable range of the scaling laws is limited by a maximum dielectric constant kmax and a minimum flow rate Qmin. The scaling laws presented by Gan˜a´n-Calvo are limited to liquids with ko31.2 (water k ¼ 80.2) [27]. Chen and Pui [28] and Ku and Kim [29] investigated the Qmin of liquids with high dielectric constant. For water, a lower limit of Qmin ¼ 3.8 ml/min was suggested. As the only difference between the Hartman scaling laws and the Gan˜a´n-Calvo laws is the exponent it is reasonable to assume the same applicable range. This means that the flow rates used in this work to atomize water are below the range governed by the scaling laws. In Fig. 9 the experimental droplet sizes are compared to the scaling laws. It is obvious that the water and saline atomization do not obey the scaling laws. In Fig. 9 the measured droplet sizes are compared to droplet sizes predicted by the scaling law if the flow rates had been above Qmin and kokmax. For the highest flow rate, the scaling law predicts a droplet size three times higher than the experimental result.
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Fig. 8. Measured size of each droplet at 1 ml/min. The total number of droplets was about 45 000 and the measurement time was about 20 min.
Fig. 9. Experimental droplet size compared with scaling laws.
The experiments were reproduced on several occasions, with different experimental set-ups and different nozzles, yielding similar results. Tang and Gomez have reported droplet size of atomized water as a function of flow rate for a CO2 sheath gas nozzle [12]. The voltage used was at least five times higher (10–20 kV) than in this work. The flow rate was 2–48 ml/min and the capillary diameter was almost the same, 0.12 mm. In general, the droplet size increases with the flow rate. However, in [12] it was reported that for the flow rates about 2 and 4 ml/min the droplet size is almost the same, 1.6–1.7 mm. The droplet sizes obtained in the present work are consistent with these findings. 3.3. Spray current The current was measured as shown in Fig. 3. The method would measure the total current including ions generated by the electric field. When the flow rate was zero, while the voltage was on, the current was also zero. This indicates that the nozzle itself does not generate ions. The small geometries of the liquid cone and jet give a high electric field that can generate ions. The computational
fluid dynamics (CFD) calculations below showed that the electric field around the apex and jet is in the order of 10 MV/m, which is above the ionization threshold of dielectric breakdown in air (3 MV/m). An attempt was made to quantify the amount of ions. A thin earthed copper wire was placed perpendicularly to the spray, just before the rim of the Faraday pail. The objective was to collect the majority of the ions if present. This would result in a decrease of the measured total current. No such current drop could, however, be observed. The inability to collect the ions suggests that the ion concentration was low. The measured total current was more or less independent of the flow rate. The measured spray current value was about 44 nA. For a 2.5 mm droplet, this equals a droplet charge in the order of 1014 C, which is the same order of magnitude as the Rayleigh limit. The current is close to the maximum current, in an EHD water spray, suggested by Tang and Gomez [12]. Tang and Gomez observed currents in the same order as the present work. They concluded that the spray operated in a corona assisted cone jet mode. There is, however, no evidence the water atomization in this work operates in the corona assisted cone jet mode. The matter needs, however, to be investigated further. 4. CFD simulation of water atomization In [30] Lastow and Balachandran presented a novel CFD simulation method of the EHD atomization process. The model can be used to predict the shape, stability regions and droplet sizes. This approach was used to model the atomization of water using the low voltage nozzle. The flow rate range in the simulations was the same as in the experiments. The shape of the cone is in general terms qualitatively consistent with experimental observations. The result from the CFD simulation can be seen in Fig. 10. The cone angle is approximately 501. This is consistent with the theoretical Taylor angle of 49.31 [19]. The tangent of the liquid surface is, however, not constant. The cone
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angle decreases close to the jet. The shape is visually consistent with experimental observations, see Fig. 11. As can be seen in Fig. 10, the cone is drawn up into the capillary. The receding behaviour of the cone is discussed
in [30] and is attributed to the numerical artefact of unrealistically sharp edges of the capillary walls.
4.1. Velocity field
Fig. 10. CFD simulation of water at 1 ml/min.
An overview of the velocity field inside the liquid cone is shown in Fig. 12. In the velocity field, two toroid shaped vortices can be seen. See detailed picture of the vortices in Fig. 13. The liquid moving down in the main flow direction is divided twice by the two toroid shaped vortices. In the velocity fields of ethanol and heptane described in [30], only one vortex was found. In the first bifurcation, the majority of the liquid is drawn into the inner vortex and the remaining liquid is recirculated by the outer vortex and mixed with the main flow. The second bifurcation is on the boundary of the torus of the inner vortex, close to the base of the jet. A majority of the liquid is drawn back up and is mixed with the main flow. The remaining liquid is drawn into the jet (see Fig. 13). These results are consistent with the findings of Shtern and Barrero [31]. In the jet, the velocity field profile is initially bi-directional. In the centre of the jet, the liquid moves upstream, whereas the peripheral liquid moves downstream. As the liquid in the jet is accelerated by the Coulombic force, the velocity profile changes and becomes uni-directional. Hayati et al. [32] studied the flow pattern inside the liquid cone using lycopodium particles and photography. They observed axisymmetric circulating patterns inside the cone. Particles were observed moving down along the surface towards the cone apex and then turning and moving up along the symmetry line. These results are consistent with the back flow seen in the CFD results.
Fig. 11. Cone shape at 1 ml/min predicted by CFD model (left) compared to experimentally obtained shape (right). The dashed line indicates the CFD shape. The figure to the right is identical to Fig. 2 (left).
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4.2. Droplet size Since no droplet break-up model was included in this CFD model, no droplet size distribution can be readily obtained. It can be assumed that a varicose break-up takes place and generates monodisperse droplets. The jet diameter can then be converted into a droplet diameter using the Rayleigh scaling factor of 1.89 [33]. This approach was also used in [30] to compare the droplet size of ethanol to experimental results. In the experiments, the droplet size distribution was not monodisperse but had a width. The width of the distribution cannot be obtained
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from the CFD results. When comparing the CFD results to experiments, only one droplet size value can be used. In this work the number mean value is used as a comparative number. The droplet size was determined at the same four flow rates as in the experiments. The results can be found in Fig. 14. The CFD results are generally consistent with the experimental data. The scaling law values are, however, neither consistent with the CFD results nor with the experimental data. The CFD model underestimates the droplet size for the lowest flow rate by about 1 mm. The accuracy of the lower end droplet size predictions could be increased by refining the computational mesh. The spatial resolution of the mesh is not sufficient to capture the smallest droplets.
5. Summary and conclusions
Fig. 12. Velocity vector field, u, at 1 ml/min.
A novel low voltage nozzle has been designed to atomize water [18]. The nozzle operates at low flow rates (ml/min) and at a low voltage of 2 kV. The water spray generated has a narrow size distribution. The nozzle can also atomize low concentration saline solutions. The window of operation of the nozzle using water and weak saline was investigated. The conductivity of saline is two orders of magnitude higher than of water. The conductivity difference reflects on the spray properties. The saline showed a very narrow window of operation. The window for water was wider, having a voltage range of a few hundreds of volts for the lowest flow rate and about 50 V for the highest flow rate. The stability of the atomization was verified using high-speed imaging. The droplet size distribution was measured using PDA instrument. The water and saline sprays showed a droplet size distribution that was
Fig. 13. Detailed views of the velocity vector field in Fig. 12. The bold line to the right is the capillary wall.
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Acknowledgements The authors wish to thank Dr. Jan Marijnissen, TUDelft and Dr. Tomek Ciach, Warsaw University for many valuable comments and support. We also wish to express our gratitude to Bjo¨rn Ullbrand for the help provided with the CFD calculations. References
Fig. 14. Water droplet size calculated using CFD compared to experimental values and scaling laws.
independent of flow rate (0.5–4 ml/min). The saline aerosol had a mean droplet diameter of 1 mm. The water aerosol had a mean droplet size of 2.1–2.9 mm. The droplet frequency increased with flow rate but the size remained constant. This is not consistent with the droplet size scaling law, which predicts the droplet size to triple when moving from the lowest flow rate to the highest. Similar results were reported by Tang and Gomez [12], who obtained the same droplet size when the flow rate was doubled from 2 to 4 ml/min. The flow rate used in these experiments is below that minimum flow rate and above the maximum dielectric constant for which the scaling laws are applicable. This leads to the conclusion that water does not obey the scaling laws when atomized in the cone jet mode at low flow rates. Measurements of the total current were consistent with the observations of Tang and Gomez [12]. The atomization described by Tang and Gomez operated in the corona assisted cone jet mode. Since no ion current could be detected, there are no indications that the atomization in the present work operates in the corona assisted cone jet mode. The conclusion is that the atomization is in the classical cone jet mode. This needs, however, to be investigated further e.g. by using a coupled mass spectrometer as described by Tang and Gomez. The EHD atomization of water was successfully simulated using CFD. The simulations were performed for the four different flow rates used in the experiments. The cone angle obtained compared well to the Taylor angle. The cone shape also showed good agreement with the obtained images of the cone. The velocity field inside the cone showed two co-axial toroidal vortices. The inner vortex drives the majority of liquid upstream in the centre of the cone. A minority of the liquid passes the vortex and reaches the jet. In the jet, the liquid undergoes a strong acceleration to reach an equilibrium velocity after a short distance downstream. The droplet size, predicted by the CFD simulation, was compared to experimental values and agreed well. Overall, the CFD approach produced simulations consistent with experimental results.
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ARTICLE IN PRESS O. Lastow, W. Balachandran / Journal of Electrostatics 65 (2007) 490–499 [24] A.M. Gan˜a´n-Calvo, Cone-jet analytical extension of Taylor’s electrostatic solution and the asymptotic universal scaling laws in electrospraying, Phys. Rev. Lett. 79 (1997) 217–220. [25] R. Hartman, Electrohydrodynamic atomization in the cone jet mode. From physical modeling to powder production, Ph.D. Thesis TUDelft, 1998. [26] R. Hartman, D. Brunner, D. Camelot, J. Marijnissen, B. Scarlett, Electrohydrodynamic atomisation in the cone-jet mode physical modeling of the liquid cone and jet, J. Aerosol Sci. 30 (7) (1999) 823–849. [27] A.M. Gan˜a´n-Calvo, The surface charge in electrospraying: its nature and its universal scaling laws, J. Aerosol Sci. 30 (7) (1999) 863–872. [28] D.-R. Chen, D.Y.H. Pui, Experimental investigation of scaling laws for electrospraying: dielectric constant effect, Aerosol Sci. Technol. 27 (1997) 367–380.
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[29] B.K. Ku, S.S. Kim, Electrospray characteristics of highly viscous liquids, J. Aerosol Sci. 33 (2002) 1361–1378. [30] O. Lastow, W. Balachandran, Numerical simulation of electrohydrodynamic (EHD) atomization, J. Electrostatics 64/12 (2006) 850–859. [31] V. Shtern, A. Barrero, Striking features of fluid flows in Taylor cones related to electrosprays, J. Aerosol Sci. 25 (6) (1994) 1049–1063. [32] I. Hayati, A. Bailey, T.H.F. Tadros, Investigation into mechanism of electrohydrodynamic spraying of liquids #2, J. Colloid Interface Sci. 117 (1987) 222–230. [33] C. Weber, Zum Zerfall eines Flussigkeitsstrahles, Z. Angew. Math. Mech. 11 (1931) 136–154.
Journal of Electrostatics 65 (2007) 490–499 www.elsevier.com/locate/elstat
Novel low voltage EHD spray nozzle for atomization of water in the cone jet mode Orest Lastowa,b,, Wamadeva Balachandranb a
AstraZeneca R&D Lund, S-221 87 Lund, Sweden School of Engineering and Design, Brunel University, Uxbridge, Middlesex UB8 3PH, UK
b
Received 2 February 2006; received in revised form 27 August 2006; accepted 10 November 2006 Available online 11 December 2006
Abstract Due to the high surface tension and high conductivity, water is unsuitable for electrohydrodynamic (EHD) atomization using a DC electric field in air. The high local electric field, that is required to atomize water, is likely to generate corona discharge and consequently destabilize the atomization process. This study describes a novel low voltage EHD spray nozzle that can be used to atomize water and weak saline solutions in the stable cone jet mode. The properties of the atomization have been investigated together with the generated droplet size distribution. The nozzle operates at very low flow rates (0.5–4.0 ml/min). Due to the high dielectric constant of water and the low flow rate, the atomization takes place outside the applicability range of the scaling laws. The experimental results show that the droplet size is approximately constant when the flow rate is increased from 0.5 to 4.0 ml/min. The atomization of water was numerically simulated using computational fluid dynamics (CFD). The simulation results agree reasonably well with the experimental results with respect to the liquid cone shape and droplet size. r 2006 Elsevier B.V. All rights reserved. Keywords: EHD atomization; Water; CFD; Cone jet mode
1. Background Atomization of water using electrohydrodynamic (EHDs) is not widely reported in the literature. In a review of 49 published papers conducted by Grace and Marijnissen [1], only four references to atomization of pure water were found. The inherent physical properties of water, i.e. surface tension and conductivity, make water unsuitable for EHD atomization in the cone jet mode using DC field [2–4]. In order to overcome the effect of the high surface tension, the typical applied DC electric field should be of the order of 106 V/m (10–30 kV). This gives electrical breakdown in the surrounding air, leading to corona discharge around the edges of the nozzle and the liquid cone apex [2,5]. The corona discharging at the nozzle edge destabilizes the liquid cone and the droplet size distribution becomes very wide and not reproducible. In addition, due Corresponding author. AstraZeneca R&D Lund, S-221 87 Lund, Sweden. E-mail address: [email protected] (O. Lastow).
0304-3886/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.elstat.2006.11.004
to high conductivity of water, the charges relax to the surface very fast (a few ms) initiating uncontrolled disruption of the liquid jet. Several ways to reduce or avoid discharging have been reported in the literature [6]. Some of the different approaches are discussed below. To avoid corona discharges from the sharp edges of a nozzle the electric field strength near the edges can be reduced by blunting the edges. Marijnissen [7] developed a specially designed nozzle with no sharp edges. However, the liquid cone apex and jet have inherently very sharp geometries and corona discharge occurs from the apex. The corona discharge from the cone apex is sufficient to destabilize the atomization. Water can also be atomized using a co-axial nozzle when water and methanol are mixed at the point of atomization. This approach was first suggested by Smith et al. [8,9] for mass spectroscopy applications. Du¨lcks and Juraschek [10] used a similar coaxial nozzle to atomize aqueous solutions. To avoid corona discharge, the atomization can be performed in an insulating gas. This can be done either by placing the whole set-up in an environment filled with a
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suitable inert gas or having a co-axial sheath flow around the nozzle and spray [4,5,11,12]. The gas used is often CO2. Other suitable gases are SF6 and S2F10 [3,12]. Using an inert gas makes it possible to apply voltages up to at least 20 kV. Tang and Gomez [4] used CO2 as sheath gas to atomize water at flow rates of 5.8–42.4 ml/min. They obtained droplets in the size range of 5–12 mm. They used a capillary with inner diameter of 0.12 mm with a potential of 10–20 kV. The spray was generated in the stable cone jet mode giving a monodisperse aerosol. To decrease the high surface tension of water, a surfactant can be added to decrease the surface energy. At equilibrium, the liquid surface is covered with surfactant molecules oriented perpendicular to the surface. When a droplet is formed, a new surface is created by the remaining liquid. Directly after droplet formation, this surface is not covered by surfactants and non-equilibrium conditions occur. In the surface generation process, the surfactant molecules are continuously transported by gradient driven diffusion from the bulk to the surface. The molecules are then adsorbed onto the surface and reoriented perpendicular to the surface with the hydrophilic group facing inwards [13]. This process continues until an equilibrium surface tension is reached, i.e. a state of lowest free energy. As the surfactant concentration increases at the surface, the surface tension decreases. This leads to a time-dependent surface tension. The surface tension normally reported from EHD experiments and used in modelling (e.g. in scaling laws) is the static equilibrium value. The time constant for the entire process depends on the surfactant– liquid system and can range from seconds to minutes. When atomizing water with a flow rate of 1 ml/min and a droplet size of 2 mm, the hydrodynamic time constant is 0.1 ms (10 MHz) at the point of droplet break-up. The surfactant transport time constant is several orders of magnitude greater than the hydrodynamic time constant, which suggests that an equilibrium surface tension is not reached during the process. Smith [3] reports that by adding the non-ionic surfactant Hodag 1035-L, giving a surface tension of about 0.05 N/m, the voltage required to obtain atomization was reduced from 31 to 24 kV. This reduction is not sufficient to avoid corona discharge. The atomization can be stabilized by using AC voltage superimposed on the DC voltage [14,15]. The AC frequency must be synchronized with the natural frequency of the droplet break-up. Balachandran et al. [16] used superimposed sinusoid AC voltage to atomize water. They used low frequencies of 500–1800 Hz and generated droplets in the size range of 390–500 mm. The flow rate used was 1.9 ml/min and the inner diameter of the capillary was 0.23 mm. This gives a droplet size of the same size as the capillary diameter. The superimposed AC voltage amplitude was 3 kV peak-to-peak. The DC voltage was 8 kV. Although the total voltage exceeded 11 kV, no corona discharge was reported. The droplet frequency and size suggest that the atomization was performed in the microdripping mode. To use superimposed AC to generate
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droplets of 2 mm, a frequency of 10 MHz must be used. Sample and Bollini [17] produced water droplets in the size range 120 mm using a harmonic AC of 750 Hz. The atomization process was very similar to micro dripping. None of the approaches discussed above can be used for cone jet mode atomization of water into micron-sized droplets. A novel design of a low voltage nozzle is presented and discussed below.
2. Low voltage nozzle For low flow rates (ml/min) and a nozzle consisting of a thin stainless steel capillary with an inner diameter in the range of 0.11–0.20 mm, a stable spray of water with very narrow droplet size distribution can be generated with a moderate voltage. A proprietary nozzle [18] was designed and built to investigate EHD atomization of water. The principal design of the low voltage nozzle is shown in Fig. 1. By arranging the counter electrode as a concentric ring around the capillary, the distance between the capillary nozzle and the counter electrode can be decreased. The electric field strength is then increased and the applied voltage can be lowered. The voltage of the ring shaped outer electrode is elevated to a constant potential of 2 kV relative to ground to give a more stable atomization. The potential difference between the capillary and the outer electrode is only 2 kV, which is a comparatively low value for EHD atomization. Two different stable wetting modes could be observed, see Fig. 2. One of the two wetting modes observed is inner wetting of the capillary as shown in Fig. 2 (left). In this case the base of the liquid cone is equal to the inner diameter of the capillary. The second type is outer wetting, Fig. 2 (right), which means that the liquid cone extends from the outer edge of the capillary, which also defines the base of the liquid cone. The cone angle is less for the outer wetting mode. The inner wetting angle is similar to the Taylor angle of 49.31 [19]. The two different wetting modes appear for the same voltage and the same flow rate. The spray can operate for tens of minutes in one mode and then transform to the other mode and after several minutes return to the original 1 μl/min +4 kV +2 kV
∅ 0.11 mm
∅ 4 mm
Fig. 1. Geometry of the low voltage nozzle with typical values of flow rate, diameters and voltages.
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Fig. 2. Inner wetting of capillary (left) and outer wetting (right). The flow rate is 1 ml/min and potential difference 2 kV.In both cases, the inner diameter of the capillary is 0.11 mm and the jet diameter is about 5 mm.
state. The outer wetting mode appears to be less stable than the inner wetting mode. Possibly, it is small variations in pressure or flow rate that make the cone to transform from one mode to the other. The operating window and droplet size distribution of the low voltage nozzle are discussed below. A numerical simulation of the dynamic atomization process is presented in Section 4.
PC 1
Syringe pump
PC 2
HV DC/DC 1 HV DC/DC 2
3. Experimental characterization of EHD atomization of water The performance of low voltage nozzle atomization of water and weak saline solutions was investigated. The spray mode, window of operation, droplet size distribution and spray current were investigated. The experimental setup is described in Fig. 3. The liquid was fed to the capillary by a syringe pump, PHD 2000 Infusion from Harvard Apparatus. A 3 ml plastic syringe was used. A PC data acquisition card supplied voltages 0–10 V to two high voltage DC/DC converters (Spellman MP series high voltage power supply modules). The voltage was thereby converted by a factor of 1000. The acquisition card was controlled by a LabVIEW 5.0 application. The droplet size distribution and velocity were measured using a 2-D Particle Dynamics Analyzer (PDA), Dantec 2D Fibre PDA. The results are presented as size distributions of a collection of a large number of individual particles, typically 50k. The properties of the used liquids can be found in Table 1. For visual characterization, a long distance microscope system, Infinity K2, was placed in the same way as the PDA system. The current was measured using a Faraday pail connected to an electrometer (Keithley 6517).
PDA reciever
LASER
Electrometer
Fig. 3. Experimental set-up.
3.1. Visual characteristics The shape of the cone can be seen in Fig. 2. The stability of the atomization was studied using a digital high speed imaging system, Red Lake MotionScope. The high speed imaging set-up was similar to the one shown in Fig. 3. The frame rate used was 125 fps. A sample image from the video sequence is shown in Fig. 4. The cone and jet were completely stable for the duration of the video sequence. A typical duration of one sequence was about 500 ms. During this period more than 106 droplets were generated. A number of high speed recordings were also made of sprays that were not stable, to serve as a reference to the stable sprays. Visual observations revealed that unstable sprays were also pulsating. The window of operation is defined as the range of voltage and flow rate for a specific liquid, which gives a
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Table 1 Liquid properties Liquids
Conductivity s (S/m)
Dielectric constant, k
Surface tension, g (mN/m)
Viscosity m (Pa s)
Density r (kg/m3)
Water Water+0.0050% NaCl
1.3E4 1.0E2
80.2 87.2
72 72
9.9E4 9.9E4
998 998
Noymer and Garel [20]. The low voltage nozzle could only operate with positive potential applied to the liquid. If the potential was reversed and negative potential was applied, no stable spray could be achieved. This phenomenon has also been observed by Jaworek and Krupa [21,22]. They report two different sets of spraying modes depending on the polarity, attributing the different behaviours to corona discharging disturbing the liquid cone. The polarity sensitivity is assumed to be due to the mobility difference between negative and positive ions in air, negative potentials are more likely to generate corona discharging than positive potentials. The window of operation was also determined for saline solutions. A stable spray could, however, only be achieved at the lowest flow rates (see Fig. 5). The voltage window was also very narrow and only a low concentration saline solution could be sprayed. The maximum reproducibly sprayable conductivity and dielectric constant, using the low voltage nozzle design, was 1.2E2 S/m and 92.7, respectively. The flow rates 0.5–1.0 ml/min could only be atomized in the voltage range of 2.21–2.25 kV. 3.2. Droplet size distribution Fig. 4. One frame in a series of high speed imaging frames. The potential difference is 2 kV and the flow rate is 1 ml/min. The inner diameter of the capillary is 0.11 mm and the jet diameter is about 5 mm.
stable spray with a narrow droplet size distribution measured by PDA, see Fig. 5. At voltages above the upper line and below the lower line, no stable atomization could be obtained. The grey areas show voltage and flow rate ranges where atomization in the cone jet mode can be obtained. The vertical axis displays the voltage difference between the capillary and the outer electrode. The outer electrode was kept constant at 2 kV. Water could be sprayed reproducibly with a flow rate up to 4 ml/min. Spraying with flow rates of up to 8 ml/min was occasionally achieved but could not be routinely reproduced. Water could also be sprayed using a capillary of 0.2 mm inner diameter. The window of operation was in this case considerably smaller in terms of flow rates and voltages and not as reproducible. All further experiments reported here were performed using a 0.11 mm capillary. The spray also displayed some degree of hysteresis behaviour. When increasing the voltage or flow rate from inside the window of operation to the outer limit, a stable jet could be obtained for values outside the initial limit. Similar hysteresis behaviour was observed by
The droplet size distribution was measured for both water and saline. The distribution of saline droplets was very narrow (see Fig. 6). The droplet size distribution was found to be independent of flow rate. For all flow rates the mean size was 1.0 mm with a geometrical standard deviation (gsd) of 1.4. When pure water was atomized, the size distribution showed some similarities with the saline solution results in terms of shape and independence of flow rate, see Fig. 7. There was a clear difference between the droplet frequencies for the different flow rates. The measurements were continued until about 50k droplets were registered. For a flow rate of 0.5 ml/min the measurements took about 30 min. When spraying at 4.0 ml/min the measurement took only about 5 min. A minor change in the size distribution could be observed as the flow rate was changed. The distributions became wider for 1.0 and 4.0 ml/min. This can be attributed to the dynamic properties of the spray. Since the spray is very sensitive to pressure variations, it proved difficult to keep the pressure stable for the whole duration of the measurement. It can also be the case that the spray temporarily operated in the outer wetting mode. The gsd for all the measurements was about 1.6. The high gsd is due to the fact that the number fraction does not completely
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Fig. 5. Window of operation of a low voltage nozzle with water (light grey) and saline (dark grey).
of 30 mm. These droplets contribute to the high gsd values. This behaviour is typical and was found for all flow rates. The relationship between flow rate and droplet size was studied and described by Gan˜a´n-Calvo [23,24]. He used a hybrid experimental–numerical method to derive a law describing this relationship 0:5 Q . d ¼ d0 Q0
Saline, 2.2 kV 70 0.50 μl/min 0.75 μl/min 1.00 μl/min
Number fraction (%)
60 50 40 30 20 10 0 0
2
4
6
8
d (μm)
Fig. 6. Droplet size distribution of atomized saline solution 0.0050% NaCl. Note that the 0.75 ml/min line is covered by the 1.0 ml/min line.
Water, 2.0 kV 35 Number fraction (%)
30 25
0.5 μl/min 1.0 μl/min 2.0 μl/min 4.0 μl/min
20 15 10 5 0 0
2
4
6
8
d (μm)
Fig. 7. Droplet size distribution of atomized water.
decrease to zero for droplet sizes above 5 mm. In Fig. 8 it can be clearly seen that on few occasions spitting instabilities ejected a number of large droplets in the order
This relationship is widely used in EHD atomization research and is commonly called scaling laws. Hartman refined these scaling laws using a purely numerical model [25,26] 0:48 Q d ¼ d0 . Q0 The applicable range of the scaling laws is limited by a maximum dielectric constant kmax and a minimum flow rate Qmin. The scaling laws presented by Gan˜a´n-Calvo are limited to liquids with ko31.2 (water k ¼ 80.2) [27]. Chen and Pui [28] and Ku and Kim [29] investigated the Qmin of liquids with high dielectric constant. For water, a lower limit of Qmin ¼ 3.8 ml/min was suggested. As the only difference between the Hartman scaling laws and the Gan˜a´n-Calvo laws is the exponent it is reasonable to assume the same applicable range. This means that the flow rates used in this work to atomize water are below the range governed by the scaling laws. In Fig. 9 the experimental droplet sizes are compared to the scaling laws. It is obvious that the water and saline atomization do not obey the scaling laws. In Fig. 9 the measured droplet sizes are compared to droplet sizes predicted by the scaling law if the flow rates had been above Qmin and kokmax. For the highest flow rate, the scaling law predicts a droplet size three times higher than the experimental result.
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Fig. 8. Measured size of each droplet at 1 ml/min. The total number of droplets was about 45 000 and the measurement time was about 20 min.
Fig. 9. Experimental droplet size compared with scaling laws.
The experiments were reproduced on several occasions, with different experimental set-ups and different nozzles, yielding similar results. Tang and Gomez have reported droplet size of atomized water as a function of flow rate for a CO2 sheath gas nozzle [12]. The voltage used was at least five times higher (10–20 kV) than in this work. The flow rate was 2–48 ml/min and the capillary diameter was almost the same, 0.12 mm. In general, the droplet size increases with the flow rate. However, in [12] it was reported that for the flow rates about 2 and 4 ml/min the droplet size is almost the same, 1.6–1.7 mm. The droplet sizes obtained in the present work are consistent with these findings. 3.3. Spray current The current was measured as shown in Fig. 3. The method would measure the total current including ions generated by the electric field. When the flow rate was zero, while the voltage was on, the current was also zero. This indicates that the nozzle itself does not generate ions. The small geometries of the liquid cone and jet give a high electric field that can generate ions. The computational
fluid dynamics (CFD) calculations below showed that the electric field around the apex and jet is in the order of 10 MV/m, which is above the ionization threshold of dielectric breakdown in air (3 MV/m). An attempt was made to quantify the amount of ions. A thin earthed copper wire was placed perpendicularly to the spray, just before the rim of the Faraday pail. The objective was to collect the majority of the ions if present. This would result in a decrease of the measured total current. No such current drop could, however, be observed. The inability to collect the ions suggests that the ion concentration was low. The measured total current was more or less independent of the flow rate. The measured spray current value was about 44 nA. For a 2.5 mm droplet, this equals a droplet charge in the order of 1014 C, which is the same order of magnitude as the Rayleigh limit. The current is close to the maximum current, in an EHD water spray, suggested by Tang and Gomez [12]. Tang and Gomez observed currents in the same order as the present work. They concluded that the spray operated in a corona assisted cone jet mode. There is, however, no evidence the water atomization in this work operates in the corona assisted cone jet mode. The matter needs, however, to be investigated further. 4. CFD simulation of water atomization In [30] Lastow and Balachandran presented a novel CFD simulation method of the EHD atomization process. The model can be used to predict the shape, stability regions and droplet sizes. This approach was used to model the atomization of water using the low voltage nozzle. The flow rate range in the simulations was the same as in the experiments. The shape of the cone is in general terms qualitatively consistent with experimental observations. The result from the CFD simulation can be seen in Fig. 10. The cone angle is approximately 501. This is consistent with the theoretical Taylor angle of 49.31 [19]. The tangent of the liquid surface is, however, not constant. The cone
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angle decreases close to the jet. The shape is visually consistent with experimental observations, see Fig. 11. As can be seen in Fig. 10, the cone is drawn up into the capillary. The receding behaviour of the cone is discussed
in [30] and is attributed to the numerical artefact of unrealistically sharp edges of the capillary walls.
4.1. Velocity field
Fig. 10. CFD simulation of water at 1 ml/min.
An overview of the velocity field inside the liquid cone is shown in Fig. 12. In the velocity field, two toroid shaped vortices can be seen. See detailed picture of the vortices in Fig. 13. The liquid moving down in the main flow direction is divided twice by the two toroid shaped vortices. In the velocity fields of ethanol and heptane described in [30], only one vortex was found. In the first bifurcation, the majority of the liquid is drawn into the inner vortex and the remaining liquid is recirculated by the outer vortex and mixed with the main flow. The second bifurcation is on the boundary of the torus of the inner vortex, close to the base of the jet. A majority of the liquid is drawn back up and is mixed with the main flow. The remaining liquid is drawn into the jet (see Fig. 13). These results are consistent with the findings of Shtern and Barrero [31]. In the jet, the velocity field profile is initially bi-directional. In the centre of the jet, the liquid moves upstream, whereas the peripheral liquid moves downstream. As the liquid in the jet is accelerated by the Coulombic force, the velocity profile changes and becomes uni-directional. Hayati et al. [32] studied the flow pattern inside the liquid cone using lycopodium particles and photography. They observed axisymmetric circulating patterns inside the cone. Particles were observed moving down along the surface towards the cone apex and then turning and moving up along the symmetry line. These results are consistent with the back flow seen in the CFD results.
Fig. 11. Cone shape at 1 ml/min predicted by CFD model (left) compared to experimentally obtained shape (right). The dashed line indicates the CFD shape. The figure to the right is identical to Fig. 2 (left).
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4.2. Droplet size Since no droplet break-up model was included in this CFD model, no droplet size distribution can be readily obtained. It can be assumed that a varicose break-up takes place and generates monodisperse droplets. The jet diameter can then be converted into a droplet diameter using the Rayleigh scaling factor of 1.89 [33]. This approach was also used in [30] to compare the droplet size of ethanol to experimental results. In the experiments, the droplet size distribution was not monodisperse but had a width. The width of the distribution cannot be obtained
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from the CFD results. When comparing the CFD results to experiments, only one droplet size value can be used. In this work the number mean value is used as a comparative number. The droplet size was determined at the same four flow rates as in the experiments. The results can be found in Fig. 14. The CFD results are generally consistent with the experimental data. The scaling law values are, however, neither consistent with the CFD results nor with the experimental data. The CFD model underestimates the droplet size for the lowest flow rate by about 1 mm. The accuracy of the lower end droplet size predictions could be increased by refining the computational mesh. The spatial resolution of the mesh is not sufficient to capture the smallest droplets.
5. Summary and conclusions
Fig. 12. Velocity vector field, u, at 1 ml/min.
A novel low voltage nozzle has been designed to atomize water [18]. The nozzle operates at low flow rates (ml/min) and at a low voltage of 2 kV. The water spray generated has a narrow size distribution. The nozzle can also atomize low concentration saline solutions. The window of operation of the nozzle using water and weak saline was investigated. The conductivity of saline is two orders of magnitude higher than of water. The conductivity difference reflects on the spray properties. The saline showed a very narrow window of operation. The window for water was wider, having a voltage range of a few hundreds of volts for the lowest flow rate and about 50 V for the highest flow rate. The stability of the atomization was verified using high-speed imaging. The droplet size distribution was measured using PDA instrument. The water and saline sprays showed a droplet size distribution that was
Fig. 13. Detailed views of the velocity vector field in Fig. 12. The bold line to the right is the capillary wall.
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Acknowledgements The authors wish to thank Dr. Jan Marijnissen, TUDelft and Dr. Tomek Ciach, Warsaw University for many valuable comments and support. We also wish to express our gratitude to Bjo¨rn Ullbrand for the help provided with the CFD calculations. References
Fig. 14. Water droplet size calculated using CFD compared to experimental values and scaling laws.
independent of flow rate (0.5–4 ml/min). The saline aerosol had a mean droplet diameter of 1 mm. The water aerosol had a mean droplet size of 2.1–2.9 mm. The droplet frequency increased with flow rate but the size remained constant. This is not consistent with the droplet size scaling law, which predicts the droplet size to triple when moving from the lowest flow rate to the highest. Similar results were reported by Tang and Gomez [12], who obtained the same droplet size when the flow rate was doubled from 2 to 4 ml/min. The flow rate used in these experiments is below that minimum flow rate and above the maximum dielectric constant for which the scaling laws are applicable. This leads to the conclusion that water does not obey the scaling laws when atomized in the cone jet mode at low flow rates. Measurements of the total current were consistent with the observations of Tang and Gomez [12]. The atomization described by Tang and Gomez operated in the corona assisted cone jet mode. Since no ion current could be detected, there are no indications that the atomization in the present work operates in the corona assisted cone jet mode. The conclusion is that the atomization is in the classical cone jet mode. This needs, however, to be investigated further e.g. by using a coupled mass spectrometer as described by Tang and Gomez. The EHD atomization of water was successfully simulated using CFD. The simulations were performed for the four different flow rates used in the experiments. The cone angle obtained compared well to the Taylor angle. The cone shape also showed good agreement with the obtained images of the cone. The velocity field inside the cone showed two co-axial toroidal vortices. The inner vortex drives the majority of liquid upstream in the centre of the cone. A minority of the liquid passes the vortex and reaches the jet. In the jet, the liquid undergoes a strong acceleration to reach an equilibrium velocity after a short distance downstream. The droplet size, predicted by the CFD simulation, was compared to experimental values and agreed well. Overall, the CFD approach produced simulations consistent with experimental results.
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