CHEMICAL ENGINEERING TRANSACTIONS  
 

VOL. 52, 2016 

A publication of 

 

The Italian Association 
of Chemical Engineering 
Online at www.aidic.it/cet 

Guest Editors: Petar Sabev Varbanov, Peng-Yen Liew, Jun-Yow Yong, Jiří Jaromír Klemeš, Hon Loong Lam 
Copyright © 2016, AIDIC Servizi S.r.l., 

ISBN 978-88-95608-42-6; ISSN 2283-9216 
 

Electrified Water Sprays Generation for Gas Pollutants 

Emission Control 

Lucia Mannaa, Francesco Di Natale*a, Claudia Carotenutob, Amedeo Lanciaa 

aUniversity of Naples, Department of Chemical, Material and Production Engineering, P.le Tecchio 80, 80125 Napoli, Italy 

bThe Second University of Naples, Department of Information and Industrial Engineering, Via Roma 29, 81031, Aversa, 

Caserta, Italy 

francesco.dinatale@unina.it 

In this study, we investigated the induction charging of water sprays and the effect of electric field on the 

breakup mechanism of liquid sprays. Two different kind of hollow cone hydraulic spray nozzles were used. 

Our investigation aimed to estimate the Droplet Charge to Mass Ratio (D-CMR) and the dependence of 

breakup length on electric potential. The experiments revealed that the D-CMR increased with the potential 

until to a maximum, then started to decrease. At the same time, the breakup length decreased more than 

0.5 cm compared to the uncharged value, suggesting that the electric field actually influenced the jet break-up 

dynamics. For V<Vmax, an induction charging model and a fluid-dynamic one were adopted to compare the 

theoretical droplet specific charge to experimental one. The experimental data are well described with a 

corrective factor of 0.243 by the induction charging model. 

1. Introduction 

The electrostatic charging of sprays is a technique used to improve the dispersion of liquids in gas thanks to 

the coupling of electric forces with the hydrodynamic or aerodynamic forces adopted in the conventional 

hydraulic or air-assisted sprays. Besides, charged droplets have peculiar behavior that allows their deposition 

on target surface also against the conventional gravitational forces. For example, electrified sprays of 

pesticides can be used to wet both the upper and the bottom sides of leaves, thanks to the electrostatic 

attraction between the charged drops and the leaves surfaces, which are at ground potential (Law 2001). 

Application of electrified sprays spreads over different fields, from medicine to agriculture (Law 2001), to 

industrial process and air pollution control (Jaworek et al. 2006). Recently, electrified spray scrubbers were 

proposed as an alternative to conventional technologies (Di Natale and Carotenuto, 2015) for marine diesel 

engines emission control (Di Natale et al., 2013). 

There are three main technological approaches to spray charging: exposure to corona ions; contact charging 

(i.e. electrospray) and induction charging. Induction charging is the best compromise between water 

throughput and charging level. In fact, electrospray is suitable for low flow rate applications, while corona 

tends to produce low charging levels. 

Law (1978) pointed out that for induction charging to be effective, the fluid must be sufficiently conductive to 

allow charge movement in the time frame of droplet formation. Quantitatively, the charge relaxation time of the 

fluid, τ, must be smaller than the formation time of the droplet, τD, evaluable from the ratio of breakup length 

and velocity of liquid jet. 

In this sense, thanks to its high conductivity, tap water may be a very good candidate for electrification. 

However, water is an ionic liquid and its physical properties are subject to substantial change in presence of 

electric field (Chaplin, 2009): on the overall, electrified spray of water are more instable than those of organic 

fluids. A side effect of this result is that electrified water sprays are scarcely studied in the pertinent literature. 

Cross et al. (2003) investigated the removal of particles electrically charged by means of an inductively 

charged water spray. It was estimated that the charging level of water decreased with the flow rate because of 

the lower residence time of liquid in electric field. On the contrary, the droplets charge reached a maximum, 

                                

 
 

 

 
   

                                                  
DOI: 10.3303/CET1652071 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Please cite this article as: Manna L., Di Natale F., Carotenuto C., Lancia A., 2016, Electrified water sprays generation for gas pollutants 
emission control, Chemical Engineering Transactions, 52, 421-426  DOI:10.3303/CET1652071   

421



which was related to the occurrence of gas breakdown. Di Natale et al. (2015) performed experimental tests 

on a pilot scale wet electrostatic scrubber for gas cleaning and showed that water electrification could be 

reliably achieved and that interactions of oppositely charged droplets and particles could give substantial 

improvement of particle removal inside a water scrubber. Krupa et al. (2013) compared the spray current 

measured with a hollow cone spray and a full cone one at different voltages. The current increased with 

potential up to a maximum. The maximum value was higher for the hollow cone thanks to the different charge 

distribution on liquid jet. It was due to the diverse mechanism of jet disruption that led to finest droplets 

formation that could acquire higher charge. 

One of the main assumption of these studies is the observation that water sprays electrified by induction 

mostly preserve their droplet size distribution regardless of the charging levels (Krupa et al., 2013). In our 

opinion, this evidence mainly indicates that secondary spray atomization remains largely unchanged by 

electrification, giving rise to similar droplets. Apparently, this imply that the primary jet breakup is unaffected by 

the application the electric field. Differently, Laryea and No (2004) studied the breakup spraying parameters as 

spray angle and breakup length by atomizing kerosene in a pressure-swirl hollow cone. The results underlined 

that break-up length and the spray angle changed with increasing the applied voltage and the operating 

pressure. These findings were related to the charging of droplets, because they repulsed each other modifying 

the spray trajectory and the equilibrium between electric and aerodynamic forces. The modifications of jet 

break up length and of primary atomization mechanism influence the charging level that can be actually 

achieved by induction processes. At the best of our knowledge, there is no evidence of such phenomena for 

induction electrification of water. 

In this work, we report experimental results on the induction charging of water, with specific reference to the 

charge levels and the primary jet atomization characteristics associated to the exposure to electric fields of 

different intensities. The charging level of liquid drops was evaluated by the ratio of droplets current I drop and 

the exerted flow rate QL: 

D − CMR =  
𝐼𝑑𝑟𝑜𝑝

𝑄𝐿
  (1) 

Experimental results are interpreted in light of two mechanistic models available in the pertinent literature, 

namely the LISA model (Ashgriz, 2011) for the jet breakup and the induction charging model of Artana et al. 

(1993) used to describe experimental droplet charging. 

2. Materials and Methods 

The lab-scale experimental rig was made by a prototype of Droplet Charging Unit (DCU). The experiments 

were performed with tap water whose conductivity was 0.750 S/m. The DCU was composed by a Teflon box 

20 × 6.2 × 9 cm, placed on a supporting structure. On its left there was the high tension connection. Two 

cylindrical wires of 1 mm diameter and distant 130 mm extended out from the Teflon box to support the 

electrode. It was a smooth circular torus with internal diameter of 110 mm and external diameter of 130 mm. In 

the middle of the charging unit, there was a stainless steel cylindrical tube of 2 cm large at which the nozzle 

was connected. The tube height could be changed to modify the distance Z between the nozzle exit and the 

centre of the electrode. Except for the ring and the supporting sticks, all the structure was grounded. The high 

voltage was provided by a DC power supply. The liquid was sprayed using the volumetric pump and it was 

regulated through an inverter. The sprayed droplets were collected in a Faraday Cage in order to measure 

their current. The cage was a cylindrical enclosure formed by a mesh of iron filaments and both dimensions 

and position had been preliminarily optimized to reduce water losses during measurements and electrical 

interferences. A representation of the experimental assembly is showed in Figure 1.  

Two Lechler hydraulic nozzles (model 216.364 and 216.404) were used. Hereafter they are indicated as 

Nozzle 1 and 2, respectively. Their geometric and hydrodynamic features as spray angle θ and orifice 

diameter D0 are resumed in Table 1. 

Two experimental investigations were performed. Firstly, the break-up length Lb and spray angles θ of nozzles 

were evaluated by optical tests as function of the applied voltage. Photos were taken with a NIKON P300 at 3 

and 6 bar. The images were elaborated with the software Image Pro Plus®. An example of optical tests results 

is shown in Figure 2 for two applied voltages. The second investigation aimed to measure the charge acquired 

from sprayed droplets. The charging tests were executed at 3 and 6 bar and the distance Z was fixed as 

breakup length at 0 kV. The current of droplets Idrop was measured at the Faraday cage, following the method 

of D'Addio et al. (2013), that was tested for a lab scale unit (D'Addio et al., 2014) and applied to a pilot scale 

WES unit in Di Natale et al. (2015). The droplet charge to mass ratio was calculated according to Eq(1). 

422



 

Figure 1: Flow sheet of experimental rig: 1 water tank, 2 Pump, 3 Inverter, 4 Flow meter, 5, Pressure gauge, 6 

Spray, 7 Electrode, 8 Faraday Cage, 9 Water collector, 10 Electrometer, 11 HV power supply 

Table 1: Performance data of tests nozzle) 

QL, L/min 

Nozzle Model Spray angle θ, ° D0, m 1 bar 2 bar 3 bar 4 bar 

216.364 60 14 53 71 81 9 

216.404 60 2 85 116 14 158 

 

The break-up length and spray angle were used to estimate liquid sheet geometric parameters using 

Linearized Instability Sheet Atomization (LISA) model (Ashgriz, 2011). The model assumed that the liquid bulk 

was transformed into a liquid sheet subjected to a spatial and temporal instability in a form of a wavy 

disturbance due to the aerodynamic interaction between the liquid and its surrounding gas. The wave grew 

until to reach a critical wavelength at which the sheet broke into ligaments and, then, in large droplets. This 

first phase was called primary atomization. The formed droplets broke into smallest fragments due to 

disruptive aerodynamic forces (secondary atomization). 

 

Figure 2: Breakup length and spray angle. In the pictures the Nozzle 1 at 3 bar is reported at 0 kV (left) and at 

Vmax (right). The dotted line divides the primary and secondary atomization zones 

In particular, the LISA model provides an estimation of the liquid sheet thickness that has a role in spray 

charging. Indeed, the model of Artana et al. (1993) considers the system liquid jet-electrode as an electrical 

circuit and assumes that the D-CMR is a function of Electric field intensity and sheet thickness a according to 

the formula: 

D − CMR =  
𝜀0E(V)

𝑎
{

𝜀0𝜀𝑟

𝜎𝑇𝑑
(1 − exp (

−𝜎𝑇𝑑

𝜀0𝜀𝑟
)) − 1}  (2) 

where E is the electric field obtained from computational studies with COMSOL Multiphysics, ε0 and εr are the 

vacuum and relative liquid permittivity respectively, σ is the liquid conductivity and Td is droplet formation time. 

Regarding that for water Td<<1, Eq(2) can be simplified in the formula used in our investigation: 

D − CMR =  
𝜀0𝜀𝑟𝐸(𝑉)

𝑎
  (3) 

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3. Experimental Results 

In Figure 3, the droplets current Idrop, the D-CMR and the breakup length Lb for Nozzle 1 and 2 respectively are 

reported for both pressures. 

 

 

Figure 3: Comparison of Droplet charge to mass ratio, droplets current and breakup length at different 

voltages for Nozzle 1 (left) and 2 (right). Grey symbols: D-CMR/Idrop, white symbols: Lb. ▼: 6 bar; ■: 3 bar. 

Regarding the trend of current and D-CMR, it can be observed that they increased almost linearly with a slope 

β as function of applied potential until to a maximum voltage called Vmax, then they started to decrease. The 

highest charging levels were obtained at 6 bar for both nozzles. In this case, the highest current appeared 

when the liquid flow rate was higher. Indeed, the same trend was observed by comparing the behavior of two 

nozzles: an increase of 50 % of current for the nozzle 2 at Vmax was measured. The breakup length showed a 

not negligible dependence on electric field. It decreased significantly and then from 8 kV it assumed a 

constant value. This behavior could be explained by assuming that the liquid sheet is perturbed by both 

aerodynamic and electric forces. The spray angle slightly increased by about 5 ° by increasing V because of 

repulsion of charge droplets. 

Table 2 summarizes the main experimental results. 

Table 2: Summary of experimental results 

Nozzle 
P 

[bar] 

QL 

[L/min] 

β 

10- 3 [1/Ω] 

Vmax 

10- 3 [V] 

Imax 

10- 6 [A] 

Lb (V0) 

10- 2 [m] 

Lb (Vmax) 

10- 2 [m] 

θ (V0) 

[°] 

θ (Vmax) 

[°] 

1 
3 81 3 105 286 25 176 60 60 

6 116 39 12 418 1 75 60 65 

2 
3 14 40 125 445 25 22 60 60 

6 196 56 12 609 2 174 60 62 

4. Discussion 

The optical tests results were used to evaluate the atomization parameters following LISA model 

(Ashgriz, 2011). Starting from the optical evaluation of Lb, the half sheet thicknesses at exit (a0) and breakup 

(ab) points, the breakup time (τb) and the primary droplets diameter (dD) were evaluated. 

Table 3: Breakup parameters for both nozzles in uncharged conditions  

Nozzle 
P 

[bar] 

a0 

10-4 [m] 

ab (V0) 

10- 5 [m] 

ab (Vmax) 

10- 5 [m] 

dD (V0) 

10- 4 [m] 

dD (Vmax) 

10- 4 [m] 

τb (V0) 

10- 3 [s] 

τb (Vmax) 

10-4 [s] 

1 
3 23 249 311 661 723 113 897 

6 23 510 709 610 692 417 292 

2 
3 27 367 450 860 909 150 121 

6 27 437 564 580 630 890 681 

 

 

424



By comparing the results, it was observed that the half sheet thickness a0 increased with diameter d0 and as 

consequence the larger a0, the bigger half sheet thickness at breakup point. It is worth underlining that the 

thickness ab and the breakup time followed the same trend of breakup length with pressure: as P increased, 

the liquid sheet broke earlier at a lower time and had a larger thickness. To take into account for the effect of 

charging potential on liquid sheet breakup, we introduced a dependence of surface tension on the droplet 

charge, while liquid density and viscosity were assumed as constant. The surface tension was calculated as a 

function of droplets experimental charge QD and its Rayleigh limit QR (Taflin et al., 1989) as follows (Tang, 

1994): 

𝜎𝑒 = 𝜎𝐿 (1 −
𝑄𝐷
𝑄𝑟

)                                                                                                                                                                                  (4) 

It was varied with potential until Vmax. Starting from the droplet diameter evaluated from LISA, the volume 

VDwas calculated. From VD and D-CMR, the droplet charge and Rayleigh limit were known. The final values 

were estimated with an iterative calculation until to achieve an error less than 2 %. 

The values of thickness ab were used in the induction charging model to evaluate the theoretical D-CMR 

(Eq.3). For each potential from 0 kV to Vmax, the sheet thickness was set as LISA value ab(V), while the 

electric field was evaluated along the direction of liquid sheet from nozzle exit to the actual breakup point. In 

Figure 4, the ratio the D-CMRexp and the D-CMRth is graphed. 

D-CMR Model predictions [ C/kg]

0 200 400 600 800 1000

D
-C

M
R

 E
x
p

e
ri

m
e

n
ts

 [
C

/k
g

]

0

50

100

150

200

250

Nozzle 1 at 3 bar

Nozzle 1 at 6 bar

Nozzle 2 at 3 bar

Nozzle 2 at 6 bar

 

Figure 4: Comparison between theoretical and experimental droplet charge to mass ratio at different potential 

It is worth to underline that experimental data of Nozzle 2 at 3 bar from 0 kV to 4 kV are not plotted because 

this evaluation is restricted to the linear range of D-CMR (V). 

All data were fitted with a linear regression to estimate the slope that is the proportional factor between 

experimental and theoretical evaluations. The results of regression was a coefficient of 0.243 ± 8.80∙10-3 and 

a coefficient of determination R2 of 0.9364. It can be observed that all data are enclosed in a defined range. It 

implies that the model describes experimental values satisfactory. 

5. Conclusions 

In this work, a lab-scale electrified spray was studied in order to evaluate the charging level on liquid droplets 

charged by induction. We used 60 ° hollow cone hydraulic spray nozzles and a toroidal ring as induction 

charging electrode. The experiments were aimed to estimated break up length of the spray nozzles in 

uncharged and charged conditions and droplet charging efficiency as a function of the charging potential. At 

low potential the droplet charge to mass ratio (D CMR) increased linearly with the charging potential, V. Then 

a maximum D-CMR level was achieved at Vmax and for V>Vmax D-CMR started to decrease. 

425



For V<Vmax, the induction charging model was used to describe data obtained from charging tests. It was 

found that the experimental data were well described by induction charging model and a corrective factor of 

0.243 was necessary to have a perfect coincidence between them. To this end, future work may benefit for the 

implementation of further investigations on the electro-hydrodynamic of liquid jets to identify the parameters 

this proportionality is related to. 

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