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Induction charging of water spray produced by pressure atomizer
International Journal of Heat and Mass Transfer 135 (2019) 631–648
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International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt
Induction charging of water spray produced by pressure atomizer A. Marchewicz ⇑, A.T. Sobczyk, A. Krupa, A. Jaworek Institute of Fluid Flow Machinery, Polish Academy of Sciences, Gdansk, Poland
a r t i c l e
i n f o
Article history: Received 2 November 2018 Received in revised form 6 February 2019 Accepted 7 February 2019
Keywords: Charged spray Electrostatic scrubbing Induction charging Liquid atomization
a b s t r a c t Charged sprays are widely used in various branches of industry, for example, for surface coating or gas cleaning by wet electrostatic scrubbers. The selection of appropriate system for charged spray generation is of crucial importance for obtaining high cleaning efficiency of a scrubber. This paper presents comparative experimental studies of parameters of charged sprays produced by various types of pressure atomizers with charging of droplets by induction, using three types of induction electrodes. The spray systems were characterized in terms of droplet size distribution and specific electric charge carried by the generated droplets. The induction charging process was investigated experimentally for the following parameters: voltage of induction electrode, inter-electrode distance, supply water pressure. It was determined that nozzles of lower flow rate generate sprays of higher specific charge, because the Sauter Mean Diameter for these nozzles is smaller. It was also found that the specific charge of droplets increases with increasing voltage applied to the induction electrode, but only to a certain voltage magnitude. Above this voltage level, the specific charge decreases due mainly to the shielding effect of space charge of produced droplets and the corona discharge from induction electrode, which neutralizes the droplets’ current. It was shown that the specific charge of droplets decreases with increasing Sauter Mean Diameter of those droplets. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Liquid dispersed in gaseous phase is commonly used by industry, particularly in such processes as fuel combustion, spray coating, paint spraying, air conditioning, fire extinguishing, liquid metal atomization (for metal powder production), cooling of metals, crop spraying or flue gas cleaning. All of those processes require sprays of specific parameters, optimized with respect to the process progress. For instance, fuel dispersed for the combustion needs fine droplet of the size in the range 5 mm Dv0,5 80 mm, where Dv0,5 is the volume median diameter, but in the case of cooling processes, the dispersed liquid droplets can be coarser, in the range of 90 mm Dv0,5 2000 mm [32]. For scrubbing processes, droplets of the size of about 0.2–1 mm are used. Spray nozzles of various types (pressure nozzles, pneumatic nozzles, ultrasonic etc.) are the main devices used for spray generation [7]. Nozzles of the same type differ in their construction and dimensions, and, therefore, the physical aspects of the process of liquid dispersion are different, and hence, the droplet size distribution can also be different. Novel technologies have been developed in recent years, which require electrically charged droplets in order to improve the ⇑ Corresponding author. E-mail address: [email protected] (A. Marchewicz). https://doi.org/10.1016/j.ijheatmasstransfer.2019.02.013 0017-9310/Ó 2019 Elsevier Ltd. All rights reserved.
performances of those processes by only a small amount of additional energy needed to this goal. Electrospraying and mechanical atomization with induction charging are the most effective techniques used for the production of charged sprays [22]. Electrospraying allows generation of highly charged spray with fine droplets of narrow size distribution, but with low liquid flow rates (0.1–100 ml/h). On the other hand, induction charging provides less charged spray in comparison to electrospraying, but with significantly higher liquid flow rate. The high liquid flow rate is important for many industrial applications where spray have to be deposited on large areas (for example, crops spraying and spray painting) or where the volume flow rate of sprayed liquid plays an important role for the industrial process, for example, in the case of flue gas scrubbing [13,23,25]. Coarser droplets produced by the nozzles used in this research may be beneficial for the above mentioned purposes, due to slower evaporation of such droplets. For each of these applications, the induction charging process has to be optimized to achieve as high specific charge of droplets as possible. This paper focuses on the optimization of induction charging of water spray in terms of droplets’ charge maximization for the application to flue gas cleaning by electrostatic scrubbers. Because of practical reasons the commercial nozzles have been used in these investigations. Nonetheless, the results may be applied to many other industrial applications, in which these types of spray
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nozzles can be used, and where the magnitude of charge imparted to droplets play an important role in the process. The spraying process is crucial for induction charging, but detailed modelling of spray nozzles, taking into consideration all physical parameters involved in the process of spraying, is up to date very difficult. The phenomenon of disintegration of liquid jet or film is strongly dependent on the liquid properties, jet velocity and vibration, therefore, both internal and external forces have to be taken into consideration in such analysis. These forces have a disruptive or integrating character. In the case when the inertial and aerodynamic forces associated with a high kinetic energy of liquid flowing from a nozzle are sufficiently high, to overcome the surface tension force, a droplet is detached from the liquid
jet. Depending on the size of droplet and the Weber number for this droplet, further disintegration of the droplet in the gaseous phase is possible. It is usually assumed that the critical Weber number for droplet disintegration is 12 [18]. On the other hand, droplets in a spray may collide with each other and coagulate. For these reasons only statistical methods are used for spray characterization [7]. Statistical methods, such as droplet size distribution, can provide enough data for the assessment of microstructure of the spray needed for a specific application. More complex problems can occur when the sprayed droplets are electrically charged. In that case, also the electrostatic forces between droplets have to be considered as additionally influencing the liquid jet dispersion and the droplets motion.
Fig. 1. Schematic of experimental set-up for droplet size distribution measurement (a), for the measurement of spray current (b).
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Fig. 4. Pressure atomizer nozzle with ring induction electrode.
Fig. 2. Photograph of the spray column with Faraday cage inside.
Charged sprays can be produced by various methods. Charging the droplets by ion current is used when the spray already exists, while the induction charging, contact charging or electrospraying are the methods used for simultaneous charging during the spray generation. Charging by ion current occurs when the droplets flow through an ionized gas, for example, the corona discharge region. The droplets are charged by ions or electrons after their collision with the droplets [35,36]. The induction charging occurs during the liquid atomization, when the electric charge is induced at the surface of liquid jet flowing from the nozzle. The electric field necessary for the induction charging is produced by an electrode maintained at high electric potential, which is placed in the vicinity of grounded nozzle [4]. The contact charging is a method of depositing of an electric charge on liquid jet and droplets by the
Table 1 Technical specification of the nozzles used in the experiments (data from catalogue [27], except ‘‘water flow rate” columns). Nozzle 216.404p is modified version of 216.404 hollow cone nozzle. Nozzle model
Spray cone type
Spray cone angle [°]
Water inlet type
Water flow rate at 3 bar [l/min]
Water flow rate at 3 bar (measured) [l/min]
Water flow rate at 6 bar (measured) [l/min]
460.523 460.524 460.484 460.444 216.404 216.404p 460.403 302.364.30
Full cone Full cone Full cone Full cone Hollow cone Hollow cone Full cone Hollow cone
45 60 60 60 60 60 45 60
Axial Axial Axial Axial Axial Axial Axial Tangential
2.35 2.35 1.88 1.47 1.22 1.22 1.18 0.77
2.28 2.39 2.07 1.48 1.28 1.29 1.14 0.79
3.21 3.17 2.71 1.96 1.72 1.82 1.63 1.08
Fig. 3. Schematic of pressure swirl nozzles used in the study: axial full cone nozzle (a); axial hollow cone nozzle (b); tangential hollow cone nozzle (c).
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connection of atomizer nozzle to a high voltage source [38]. No other induction electrode is necessary in that case, and the electric field lines originating at the surface of liquid jet are closed to the nearby grounded objects or the target substrate. Electrospraying is a specific process of liquid atomization, by which only the electrical forces acting on the liquid meniscus at the outlet of a capillary nozzle are responsible for liquid atomization. No other mechanical forces are imposed on the jet in that case [15,33]. Because of the shear stress produced at the jet surface by the electric field, the jet is stretched from the tip of the meniscus, and disperses into fine droplets due to mechanical instabilities [15,20,21,24,33,34]. All of these methods have several industrial applications, however, this paper is focused at the induction charging only, because this method provides a reasonable level of droplet charge by relatively high flow rates necessary for industrial applications. One of such applications is the process of cleaning of contaminated gases produced by various sources: power plants, industry and transport. Recently, the emission of particulate matter (PM)
by power plants and transport became a severe concern [11,31]. New regulations regarding the emission of particles smaller than 2.5 lm (PM2.5) have come into force. One of the largest contributors in terms of PM emission by transport sector is the maritime transport [14]. Among various solutions used for the removal of PM from exhaust gases emitted by marine Diesel engines is the electrostatic scrubbing. In an electrostatic scrubber, the electrically charged particles are deposited onto oppositely charged water droplets, due to Coulomb attraction between them. This effect allows obtaining higher collection efficiency of such devices compared to conventional inertial scrubbers. In electrostatic scrubbers, the process of droplet charging is crucial for the efficient particle removal, which was shown in many studies [1,9,8,12,13,22,25]. The efficiency of gas cleaning by electrostatic scrubber depends mainly on the size and charge of droplets used for this process. Droplets should be charged to a level as high as possible, and the droplet concentration should be high enough to enable a high probability of their collision with particles. For these purposes, the induction charging is considered as the optimal one because of a high liquid
Fig. 5. Normalized number and volume size distributions of droplets for full cone (a–d, f) and hollow cone (e, g) nozzles; water pressure: 6 bar. Results ordered with decreasing water flow rate.
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Fig. 5 (continued)
flow rate available, and relatively high magnitude of electric charge imparted to the droplets, which can be as high as 1/5 of the Rayleigh limit [25]. In order to accomplish the induction charging process, a ring induction electrode, coaxial with the axis of spray nozzle and located in the vicinity of the nozzle outlet, is usually used, while the nozzle is grounded. This electrode is connected to a high-voltage power supply, in order to produce an electric field at the surface of the liquid jet flowing from the nozzle. The measurements of droplets size distribution and the electric charge of droplets produced by various types of water-spray atomizers are presented in this paper. Two types of pressure atomizers were tested depending on the spray plume: hollowcone (droplets are ejected from the conical liquid film produced by the nozzle, and the droplets move mainly in the vicinity of lateral surface of this cone) and full-cone (droplets fill the whole spray-cone volume). Ring or cylindrical induction electrodes were used for the charging of droplets by induction during their atomization. This paper is focused on experimental evaluation of the quality of water spray generated by several commercial pressure swirl atomizers and maximization of the spray current con-
veyed by the charged droplets. The research is aimed at the optimization of the spray system for its application to the wet electrostatic scrubbing of gases in various industrial processes. The spray nozzles were tested for the range of water flow rate between 1 and 3.2 l/min (at 6 bar pressure), which is adequate for the application to electrostatic scrubbers [13,25]. 2. Experimental Experimental setup used in this research is shown schematically in Fig. 1. It comprised of a water tank and water pump supplying the spray nozzle with water. The pipeline from the water pump is ended with a connector allowing an easy replacement of the tested nozzle. The water was supplied to the nozzle by diaphragm pump Hydra-cell G-03 (nominal flow rate 200 l/h at 1395 rpm, by 50 Hz power supply). The flow rate of the pump was controlled by frequency inverter Nordac SK 500e (Nord Drive Systems). The water flow rate was measured by electromagnetic flow meter Promag 50H08 (Endress + Hauser), and the pressure at the nozzle inlet by pressure meter Cerabar M PMC51 (Endress + Hauser). Water
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Fig. 5 (continued)
temperature was measured by thermometer RTD TR10 (Endress + Hauser). The measurements of droplet size distribution and spray current were carried out in two separate steps. The measuring system used for the measurement of droplet size distribution was AWK D made by KAMIKA INSTRUMENTS (Poland). This device consisted of an optical probe for the detection of water droplets, an electronic interface, and a computer for data storage and processing. A scheme of experimental setup for droplet size distribution is shown in Fig. 1a. The optical probe consisted of infrared laser, photodetector and inlet orifice. The optical parts were mounted in a stainless steel casing. During the measurement, the spray flows into the measuring volume through the inlet orifice. Each spray droplet shadows partially the laser sheet focused on the photodetector, decreasing the incident light intensity. The maximum decrease in the light intensity for a given droplet is proportional to the droplet diameter. Thus the photodetector generates an electric signal of amplitude proportional to the droplets diameter. Consequently, the electronic interface determined the droplets size from the signals generated by each droplet, and classifies them into
256 size classes. The distance between the nozzle outlet and the inlet aperture of the analyser was 60 cm. Fig. 1b shows a scheme of experimental setup for the measurement of specific charge of droplets. For the measurement of total spray current conveyed by the droplets, the nozzle was mounted in a spray column of inner diameter of 400 mm and height of 2000 mm made of PMMA. A grounded fine mesh was stretched at the inner wall of this column in order to remove the electric charge from the deposited droplets. A Faraday cage of inner diameter of 210 mm, made of a two-layer grid of dense copper mesh was used for the measurement of spray current. The cage was suspended on dielectric threads in order to avoid the leakage current from cage. The distance between the inlet of this cage and the nozzle outlet was set in such a way as the spray cone could entirely flow into this cage. Fig. 2 shows a photograph of the spray column with electrostatic spray nozzle and Faraday cage used for the measurement of electric charge of droplets. Seven models of commercially available pressure swirl nozzles produced by Lechler GmbH have been tested. The technical data for these nozzles are listed in Table 1. Fig. 3 presents the schematic
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Fig. 5 (continued)
Table 2 Sauter Mean Diameter (SMD) of droplets dispersed by various spray nozzles, for two water pressures and at two positions of the measuring probe. Nozzle model
Sauter mean diameter [lm]
Nozzle type Measuring probe position Water pressure [bar]
460.523 460.524 460.484 460.444 216.404 216.404p 460.403 302.364.30
Full cone Full cone Full cone Full cone Hollow cone Hollow cone Full cone Hollow cone
of three types of pressure swirl nozzles used in this study. Black arrows indicate of the streamlines of water flowing through nozzle. Nozzles of this type are described in the literature [7], and are widely used in various industrial applications. The flow rate of all of the nozzles was lower than 3.2 l/min for the water pressure at the nozzle inlet of 6 bar. One of the nozzles, model 216.404, has been modified in order to check the effect of the nozzle shape on charging efficiency, which is discussed in Section 3.5. The modified nozzle has been marked with letter ‘‘p” after the model number of the nozzle (i.e., 216.404p). The droplet size distribution has been measured in two different positions of the probe: in the axis of the nozzle and 20 cm from the axis, for all types of tested nozzles. The charging process was investigated for three types of induction electrodes: torus of inner diameter of 100 mm, and two cylindrical of inner diameters of 80 mm and 65 mm, and height of 30 mm each. The torus electrode was bent from a brass pipe of the diameter of 12 mm. The induction electrode was placed coaxially with the nozzle. The electrode was mounted on a support to allow changing its vertical position along the nozzle axis, in order to vary the position of the electrode relative to the nozzle outlet d (cf. Fig. 1b). High voltage was applied to the induction electrode from power supply Spellman HV unit (SL30PN300) of negative polarity, while the nozzle was grounded. The spray current was measured with a digital picoammeter (Keithley 486) connected to the Faraday cage. The spray current has been measured for the following distances between the upper plane of the induction electrode and the plane placed at the outlet of the nozzle (d): 0, 10, 20, 30 and 40 mm. A photograph of spray
Axial 3
Axial 6
20 cm off axis 3
20 cm off axis 6
307 301 265 292 257 269 241 285
295 282 248 273 254 246 247 229
329 356 304 325 313 313 293 317
304 319 321 320 305 308 291 321
nozzle with a ring induction electrode, taken during the measurements, is shown in Fig. 4. Tap water used in these measurements was provided by the same means as in the droplet size distribution tests. Water temperature during tests varied between 10 and 15 °C. Water pressure, flow rate and temperature were measured by the same measuring devices as in the droplet size distribution tests. The air from the spray column was smoothly removed through a pipe mounted at the bottom of the column with the flow rate of 130 m3/h (the mean gas velocity of about 0.3 m/s) in order to remove the water mist from the column. The conductivity of water was measured in order to verify whether a sufficient amount of electrical charge can be delivered to the surface of liquid film before droplet detachment. At 10 °C the electric conductivity (r) was 4.19 mS/m. The time constant associated with electric phenomena (electric charge relaxation time) is:
sel ¼
e r
ð1Þ
where e is the electric permittivity of water. For the measured conductivity, the value of sel was equal to 0.16 ls. The hydrodynamic time constant characterizing the motion of liquid film ejected from the nozzle is estimated from the following equation
sh ¼
dout pdout ¼ u 4Q
3
ð2Þ
where dout is the diameter of orifice assumed to be the characteristic length, u is the liquid velocity at the nozzle outlet, and Q is the
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water flow rate. For the 460.523 nozzle, liquid flow rate through the nozzle at a pressure of 6 bar, and orifice diameter of 1.6 mm was 3.21 l/min, and the value of hydrodynamic time constant can be estimated to be 61 ls. The electric charge relaxation time constant was therefore 380 times shorter than the hydrodynamic time constant, hence the tap water used in the following experiments can be considered as a perfect conductor.
3. Results and discussion 3.1. Droplet size distribution All the spray nozzles have been tested for two water pressures, 3 and 6 bar. The size distribution of droplets was measured for two positions of the optical probe. Fig. 5 presents the results of measurements of normalized number and volume size distributions of droplets for the investigated nozzles, obtained by water pressure of 6 bar, for two positions of the probe: 0 and 20 mm from the noz-
zle axis. The droplet size distributions presented in Fig. 5a–d and f are for nozzles with full spray cone, and in Fig. 5e and g for the nozzles with hollow spray cone. Sprays generated by all tested nozzles had much larger number of small (<200 lm) droplets in the spray cone axis than at the off-axis probe position. As a consequence, the Sauter Mean Diameter (SMD) is significantly smaller in the spray cone axis than at the off-axis probe position. The values of SMD of droplets dispersed by various spray nozzles, for two water pressures and at two positions of the measuring probe in the spray cone are given in Table 2. Fig. 6 compares the effect of water pressure (3 and 6 bar) on the cumulative volume size distributions of droplets for three selected nozzles: one of full-cone and two of hollow-cone type. The smallest SMD of droplets for all tested nozzles was obtained in the axis of the spray cone for the hollow cone nozzle 302.364.30, at a water pressure of 6 bar. However, the SMD increased by 24% when the pressure was decreased to 3 bar (cf. Fig. 6b). For comparison, for full cone nozzle 460.484, the SMD increased by 7% (cf. Fig. 6a), and for hollow cone nozzle 216.404 with axial water inlet the
Fig. 6. Example of cumulative volume size distributions of droplets for (a) full cone 460.484 and hollow cone nozzles: (b) 302.364.30, (c) 216.404 for various water pressures and probe positions.
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Fig. 6 (continued)
Fig. 7. Specific charge of spray generated by single fluid pressure swirl nozzles: (a), (b), (g) 302.364.30 for various distances between the nozzle outlet and the induction electrode (d). Water pressure: 6 bar, induction electrode distance: 30 mm. Results ordered with decreasing water flow rate.
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Fig. 7 (continued)
SMD increased by only 1%, when the supply water pressure was reduced to 3 bar (cf. Fig. 6c). 3.2. Specific charge of droplets The most common parameter used for characterization of the electric charge level of charged spray is the specific charge. The specific charge of a single droplet is the ratio of the charge on this droplet to the mass of this droplet. The mean value of the specific charge for a spray can be estimated from the ratio of spray current to the mass flow rate of sprayed liquid [2,10,25,26]:
I ¼ _ m
PN 2 4prq Nrs 2 Q ðrs Þ i¼1 4prq ðiÞr i ¼ ¼ 4 PN 4 3 3 m pqNrs d ðr s Þ i¼1 3 pqr i 3
ð3Þ
where rq ðiÞ is the surface charge density on the i-th droplet of radius r i , rs is the Sauter mean radius, q is the water density, N is the total number of droplets in the spray, Q(rs) is the charge of droplet of Sauter mean radius, and md(rs) is the mass of this droplet. The specific charge presented in the following figures is determined
from the spray current measured by the Faraday cage and the mass flow rate of sprayed water. The problem of the magnitude of charge on a single droplet was discussed by [22]. This charge was determined from the value of surface charge density induced on a liquid film at the nozzle outlet. The magnitude of surface charge density rlf at a liquid film subjected to electric field is:
rlf ¼ e0 En
ð4Þ
where En is the electric field magnitude perpendicular to the surface of liquid film. The electric field depends on the induction electrode potential, the induction electrode geometry and the distance between induction electrode and nozzle outlet. The charge on the i-th dropletQ ðiÞ is equal to the total charge accumulated on the fragment of liquid film from which the droplet is formed at the moment of droplet detachment:
Q ðiÞ ¼ Ai rlf ðAi Þ ¼ Ai e0 En where Ai is the area of the fragment of liquid film.
ð5Þ
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Fig. 7 (continued)
From the above assumptions results that the droplet charge is estimated to be proportional to the electric field on the surface of liquid film at the moment of droplet detachment, and, for a fixed electrode geometry and a given inter-electrode distance, is proportional to the voltage applied to induction electrode. For the mass of droplet independent of electric field, also the specific charge of droplets is proportional to the voltage applied to the induction electrode. This model neglects the space charge and the electric discharges from the nozzle and induction electrode, and can only be applied to voltages below the specific charge saturation. The results of measurement of the specific charge vs. voltage applied to the induction electrode are presented in Fig. 7. For low voltages, the specific charge increases nearly proportionally to the voltage, but the characteristic gradually departure from the straight line. Such observations were also reported by other papers on the induction charging [10,17,25,30]. The saturation of specific charge can be caused by several effects. First of all, the deposition of water mist on the induction electrode was observed during the atomization process (even at 0 kV voltage). This phenomenon was intensified with increasing supply voltage. Droplets detached from
the induction electrode are charged with opposite polarity to those charged by induction, which could decrease the total current flowing to the Faraday cage. Secondly, for voltages higher than a certain magnitude, small liquid jets were ejected from the water film formed at the induction electrode towards the nozzle. At the tip of these jets a faint light of the corona discharge was observed in a dark room. It is probable that the charge carried by these droplets and gaseous ions emitted from these jets are also responsible for the decrease in the current measured by the Faraday cage for higher voltages. The corona discharge was also observed by Higashiyama et al. [17] and Castle and Inculet [10]. Another effect on specific charge saturation that has to be taken into consideration is the effect of space charge of the generated charged droplets reducing the electric field on the surface of liquid film, and consequently, reducing the charge of newly detaching droplets. The maximal value of specific charge of droplets measured by the Faraday cage was higher for nozzles designed for lower flow rates. Nozzles designed for higher flow rates of water, generated sprays of larger values of number concentration of droplets. Because of higher space charge density of droplets, the shielding
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Fig. 7 (continued)
effect of this charge in the vicinity of the breakup region of the liquid film is more important for the charging process than for nozzles of lower flow rates. The maximum specific charge was higher for the hollow cone nozzles. This happened due to the shape of water film at the outlet of spray nozzle before its breakup. The film has a distinctive conic-helical shape (see photo in Fig. 4). Longer liquid film to breakup allows higher electric charge to be induced by the electric field produced by the induction electrode. Results presented in Fig. 7 also show the dependence between the specific charge of droplets and the position of induction electrode (d – see Fig. 1b). The measuring points obtained for the optimal position of the induction electrode, for which the specific charge was the maximal, are presented by filled markers in each of these figures. For almost all nozzles, the optimal distance between nozzle outlet and induction electrode was about 30 mm. Further increase in the distance caused only a decrease in the value of specific charge. The specific charge for hollow cone nozzle 302.364.30 presented in Fig. 7g was measured for one induction electrode position only, because of tangential inlet construction of this nozzle. 3.3. Effect of supply water pressure on the specific charge Fig. 8 presents the effect of voltage applied to the induction electrode on the value of specific charge for different supply water pressures, and for the optimal induction electrode position. The maximum specific charge did not change significantly with the pressure in the tested range (Fig. 8a, b), and for most of the nozzles, the specific charge slightly decreases with the pressure increasing. This small decrease in the specific charge could be caused by the influence of the space charge of droplets on the electric field at the liquid film. For higher water pressure, the water flow rate is also higher, the droplets are finer (SMD is lower, cf. Fig. 6), therefore, the space charge density is higher. Moreover, the smaller droplets are easier deflected by the electric field and deposited on the induction electrode. Then, the water film forms more jets, which are dispersed as small droplets of charge opposite to that generated by the nozzle, which reduces the spray current. Thus, the value of specific charge measured by the Faraday cage is a matter of balance between the effect of the generated space charge, the oppositely charged droplets detaching from the induction elec-
trode, and the ionic current generated by corona discharge from the induction electrode. The space charge density of charged droplets depends on the number concentration of droplets, their velocity, and the charge carried by the droplets. The effect of supply water pressure was more significant for full cone nozzles with 45° spray cone angle, because of higher droplets concentration and higher space charge density. 3.4. Effect of spray cone angle for nozzles of similar flow rate Manna et al. [29,30], using one of the nozzles tested in this paper, the hollow-cone nozzle 216.404, observed that the angle of liquid cone at the nozzle outlet increases, and the length of this liquid cone decreases with increasing voltage applied to the induction electrode. These findings suggest that the electric field induced on the liquid sheet generates an additional electric force, which deforms the film, and amplifies the wave oscillations of this film. By this way, the electric field can influence the spray mechanism. Similar effect was also observed by Higashiyama et al. [17] who measured the droplet size distribution during the spraying with and without the induction charging of droplets. In the macroscopic scale, those experiments showed that the inductively charged spray tends to disperse outwards the nozzle axis due to electrostatic repulsion of droplets, and that the droplet size distribution of electrically charged spray along radial distance from the axis is more uniform than the droplet size distribution of uncharged spray. The effect of charged spray dispersion due to Coulomb repulsion suggests that it is likely that droplets of highest electrical mobility tend to precipitate on the induction electrode, causing the emission of water jets, which reduce the charging efficiency. Moreover, Higashiyama et al. [17] also reported on small droplets deposition on the induction electrode and the corona discharge over certain induction electrode voltage. A comparison of generated specific charge for nozzles with similar water flow rate but of different spray cone angle is shown in Fig. 9. It has to be noted that the maximum specific charge of droplets for these nozzles is different, despite a similar droplet size distribution. The maximum specific charge for the 460.523 nozzle (cone angle-45°) was higher by 48.7% than for the 460.524 nozzle (cone angle-60°), while the SMD for 460.523 was higher by only 1.7%, for the same experimental conditions (supply water pressure:
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3 bar, induction electrode voltage: 17 kV, induction electrode distance: 30 mm). This indicates that in the case of nozzle with larger spray cone angle, where the detachment of droplets occurs closer to the induction electrode, the electric field is high enough to deflect the droplets’ trajectories, thereby leading to the droplets’ deposition on the induction electrode and consequently decreasing the measured spray current.
3.5. Effect of spray nozzle geometry The hollow cone nozzle model 216.404 has been modified in order to check the effect of the tip of the nozzle on the electric field distribution at the surface of liquid film, and on the specific charge of spray (the modified nozzle was labelled in the following as 216.404p). The geometry of the nozzle before and after its modification is drawn schematically in Fig. 10. It was assumed that the machined nozzle will allow producing a higher electric field at the nozzle tip and the liquid film, due to a sharper cone at the nozzle outlet. The higher electric field should cause an increase of the
643
electric charge on the liquid film before its disruption to droplets. The results of measurements of the droplet size distribution and specific charge characteristics for the modified nozzle are shown in Fig. 11. A slight shift in the specific charge characteristics towards a lower voltage applied to the induction electrode, in comparison to the non-modified version of this nozzle, can be noticed. However, no significant increase in the maximum value of specific charge has been obtained. The only significant difference observed was a faster decrease of the specific charge for voltages higher than that at the maximum (Fig. 11). This effect was probably caused by a higher corona discharge current from a sharper tip of the machined nozzle.
3.6. Effect of induction electrode geometry Besides the inter-electrode distance, spray cone angle and water pressure, the geometry of induction electrode is also important parameter in the process of spray charging. Most of the previous research has been focused on the ring induction electrode with cir-
Fig. 8. Specific charge of spray generated by the nozzles: (a) 460.484, (b) 216.404 and (c) 460.523 vs. induction electrode voltage for various water pressures. Distance between the nozzle and induction electrode (d) was set to optimal position for each electrode.
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Fig. 8 (continued)
Fig. 9. Comparison of specific charge characteristics for two full cone nozzles of different spray cone angles, for similar water flow rate and charging conditions.
cular cross section (torus electrode) or square/rectangle cross section [3,17,19,29,30]. Depending on practical application and a type of spray nozzle used, the inner diameter of induction electrode varied between 5 and 110 mm. Several papers considered also the induction charging of droplets for flat-spray nozzles [16,37], and centrifugal atomizers [5,6]. Machowski and Balachandran [28] have optimized numerically the shape of induction electrode in order to increase the effect of induction charging, but those simulation results have not been compared with experiments. Fig. 12 compares the results of measurements of specific charge of droplets obtained for various types of induction electrode: torus and two cylindrical, of inner diameter of 80 mm and 65 mm. Each measurement was carried out for the optimal inter-electrode distance (d), for which the maximal specific charge was. Fig. 13 presents the magnitude of specific charge at the maximum versus the inter-electrode distance (d), for spray generated by the nozzle 460.484. The cylindrical induction electrode of inner diameter of 80 mm induced the highest specific charge on the droplets. The optimal distance between electrodes (d) was different than for
Fig. 10. Outline of the shape of hollow cone nozzle model 216.404, before (left) and after (right) modification.
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645
Fig. 11. Comparison of: (a) droplet size distribution and (b) specific charge characteristics for modified (216.404p) and non-modified (216.404) nozzles for the optimal charging conditions.
the torus electrode. The negative value of the distance means that upper plane of the induction electrode is placed above the plane of the spray nozzle outlet. The maximum specific charge obtained in the case of charging with cylindrical induction electrode of 65 mm diameter was significantly lower than for other tested electrodes, therefore this electrode have been tested only for one interelectrode distance. During the measurements, this electrode was immediately covered with water film, hence the corona discharge was present at much lower voltage (9 kV) than for two other cases. 3.7. Optimization of induction charging process in terms of the specific charge The experimental results show that the selection of nozzle for a specific industrial application of an electrically charged spray (e.g., for wet electrostatic scrubbing) is a matter of balance between the maximum specific charge of droplets and the optimal diameter of droplets in the produced spray. Table 3 summarizes the optimal
droplet charging conditions for all tested nozzles with respect to the maximum specific charge of droplets. The maximum specific charge of droplets has been obtained for the nozzle 460.403. For this case, the optimal inter-electrode distance was 20 mm by a voltage of 17 kV and water pressure of 3 bar. However, the flow rate of water for this nozzle was not very high (1.14 l/min), and in the case of wet electrostatic scrubbing applications, the use of nozzles of lower flow rates requires a larger number of those nozzles in order to obtain the assumed scrubbing efficiency that could lead to the problems with nozzle arrangement inside a scrubber column. Fig. 14 presents the maximum specific charge and Sauter Mean Diameter vs. water flow rate, for all tested nozzles. The regression lines indicate that for the single fluid pressure swirl nozzles operating at 6 bar of water pressure, SMD was higher for the nozzles with higher flow rate, but the maximum specific charge decreased accordingly. The mass of a droplet is proportional to its diameter cubed, and the charge is proportional to the droplet surface, there-
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Fig. 12. Specific charge of spray vs. induction electrode voltage, generated by the nozzle 460.484 for various types of induction electrode at their optimal distance.
Fig. 13. Maximum specific charge of spray vs. inter-electrode distance for the nozzle 460.484 for various types of induction electrode and various water pressures.
Table 3 The optimal charging conditions for various single fluid pressure swirl nozzles in terms of the specific charge. Nozzle model
Nozzle type
Spray cone angle [°]
Water pressure (flow rate) [bar] ([l/min])
Sauter mean diameter [lm]
Maximal specific charge [lC/kg]
Optimal electrode voltage [kV]
Optimal electrode distance [mm]
460.523 460.524 460.484 460.444 216.404 216.404p 460.403 302.364.30
Full cone Full cone Full cone Full cone Hollow cone Hollow cone Full cone Hollow cone
45 60 60 60 60 60 45 60
3 4 3 5 6 3 3 4
306.5 300.8 265.4 273.4 254.3 268.8 241.4 284.6
171.1 128.7 158.7 169.8 230.2 239.0 260.5 196.7
17 15 14 15 14 12 16 10
30 30 20 40 20 30 20 30
(2.28) (2.68) (2.08) (1.82) (1.72) (1.23) (1.14) (0.90)
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Fig. 14. The maximum specific charge and Sauter Mean Diameter vs. water flow rate for eight tested single fluid pressure swirl nozzles. Water pressure: 6 bar.
Fig. 15. The maximum specific charge vs. Sauter Mean Diameter for eight tested single fluid pressure swirl nozzles. Water pressure: 6 bar.
fore the specific charge of droplets generated by the nozzles of higher flow rates is lower. The maximum specific charge of droplets decreases with increasing SMD that was shown in Fig. 15. 4. Conclusions In this paper, a set of commercial single-fluid pressure-swirl spray nozzles with charging by induction have been tested with respect to the droplet size distribution and specific charge of generated droplets. The nozzles have been selected for the water flow rate falling in the range required for electrostatic scrubbing applications, which is usually assumed to be between 1 and 3.2 l/min for a single spray nozzle [13]. The obtained results show that the specific charge of droplets increases almost proportionally with increasing voltage applied to the induction electrode but only to a certain voltage magnitude. Above that voltage, the specific charge characteristic declines from the straight line due to several phenomena, which may occur in
such conditions. One of such phenomenon is the presence of space charge of the charged spray in the vicinity of the liquid film at the nozzle outlet. This space charge has negative effect on the spray charging process, because it changes the electric field distribution and reduces its magnitude at the liquid film at the nozzle outlet. Secondly, the water mist deposited on the induction electrode has been observed. For higher induction electrode voltages, water deposited on the surface of this electrode has formed fine liquid jets, which could produce oppositely charged droplets via electrospraying phenomenon. Further increase in the induction electrode voltage lead to the occurrence of corona discharge from these liquid jets, and it is probable that ionic current of corona discharge can neutralize the charge of the droplets. Moreover, the oppositely charged water droplets dripping from the induction electrode had reduced the spray current measured by the Faraday cage placed beneath the nozzle. The geometry of induction electrode is an important parameter in the process of spray charging. It was shown that the specific
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charge of droplets charged by cylindrical induction electrode of inner diameter of 80 mm was higher than by using torus electrode or a cylindrical one of smaller diameter. The optimal distance of the induction electrode maximizing the specific charge was determined for each nozzle. The regression line of the maximal specific charge obtained for each nozzle vs. Sauter Mean Diameter measured for eight tested nozzles indicated that the specific charge of droplets decreases with increasing Sauter Mean Diameter of those droplets. In the case of the tested nozzles the specific charge of droplets varied between about 120 and 240 mC/kg. It was also shown that the specific charge of droplets was higher for nozzle of lower spray cone angle. Conflict of interest None. Acknowledgements The paper is a result of the statutory research at the Institute of Fluid Flow Machinery, Polish Academy of Sciences, within the project No. O1/T3/Z4. References [1] K. Adamiak, A. Jaworek, A. Krupa, Deposition efficiency of dust particles on a single, falling and charged water droplet, IEEE Trans. Indus. Appl. 37 (2001) 743–750. [2] T.C. Anestos, J.E. Sickles, R.M. Tepper, Charge to mass distributions in electrostatic sprays, IEEE Trans. Ind. Appl. (1977) 168–177. [3] A. Atten, S. Oliveri, Charging of drops formed by circular jet breakup, J. Electrostat. 29 (1992) 73–91. [4] A.G. Bailey, Electrostatic Spraying of Liquids, Research Studies Press Limited, 1988. [5] W. Balachandran, A. Bailey, The influence of electrostatic fields on the centrifugal atomisation of liquids, J. Electrostat. 10 (1981) 189–196. [6] W. Balachandran, A.G. Bailey, The dispersion of liquids using centrifugal and electrostatic forces, IEEE Trans. Ind. Appl. (1984) 682–686. [7] L. Bayvel, Z. Orzechowski, Liquid Atomization, Taylor and Francis, 1993. [8] C. Carotenuto, F. Di Natale, A. Lancia, 2009. Electrostatic Enhanced Water Scrubbing for Particulate Abatement in Combustion Systems – Modelling Analysis and Preliminary Design Criteria Combustion Colloquia. [9] C. Carotenuto, F. Di Natale, A. Lancia, Wet electrostatic scrubbers for the abatement of submicronic particulate, Chem. Eng. J. 165 (2010) 35–45. [10] G. Castle, I. Inculet, Induction charge limits of small water droplets, in: The 8 Th International Conference on Electrostatics 1991, Oxford, Engl, 04/10-12/91, 1991, pp. 141–146. [11] R. Chirico, P. DeCarlo, M. Heringa, T. Tritscher, R. Richter, A. Prévôt, J. Dommen, E. Weingartner, G. Wehrle, M. Gysel, et al., Impact of aftertreatment devices on primary emissions and secondary organic aerosol formation potential from inuse diesel vehicles: results from smog chamber experiments, Atmos. Chem. Phys. 10 (2010) 11545. [12] L. D’Addio, F. Di Natale, C. Carotenuto, W. Balachandran, A. Lancia, A lab-scale system to study submicron particles removal in wet electrostatic scrubbers, Chem. Eng. Sci. 97 (2013) 176–185.
[13] F. Di Natale, C. Carotenuto, L. D’Addio, A. Jaworek, A. Krupa, M. Szudyga, A. Lancia, Capture of fine and ultrafine particles in a wet electrostatic scrubber, J. Environ. Chem. Eng. 3 (2015) 349–356. [14] F. Di Natale, C. Carotenuto, L. Manna, M. Esposito, F. La Motta, L. D’addio, A. Lancia, Water electrified sprays for emission control in energy production processes, Int. J. Heat Technol. 34 (2016) S597–S602. [15] A.M. Gañán-Calvo, J.M. López-Herrera, M.A. Herrada, A. Ramos, J.M. Montanero, Review on the physics electrospray: from electrokinetics to the operating conditions of single and coaxial Taylor cone-jets, and AC electrospray, J. Aerosol Sci. (2018). [16] J.L. Hensley, X. Feng, J.E. Bryan, Induction charging nozzle for flat fan sprays, J. Electrostat. 66 (2008) 300–311. [17] Y. Higashiyama, S. Tanaka, T. Sugimoto, K. Asano, Size distribution of the charged droplets in an axisymmetric shower, J. Electrostat. 47 (1999) 183– 195. [18] W.C. Hinds, Aerosol Technology: Properties, Behavior, and Measurement of Airborne Particles, John Wiley & Sons, 2012. [19] I.I. Inculet, G.P. Castle, Induction charging of flat sprays for space charge generation, in: Industry Applications Conference, 2001. Thirty-Sixth IAS Annual Meeting. Conference Record of the 2001 IEEE, 2001, pp. 1946–1949. [20] A. Jaworek, A. Krupa, Classification of the modes of EHD spraying, J. Aerosol Sci. 30 (1999) 873–893. [21] A. Jaworek, A. Krupa, Jet and drops formation in electrohydrodynamic spraying of liquids. A systematic approach, Exp. Fluids 27 (1999) 43–52. [22] A. Jaworek, A. Krupa, Charged Sprays Generation and Application. Sprays: Types, Technology and Modeling, 2011, pp. 1–100. [23] A. Jaworek, A. Krupa, A.T. Sobczyk, A. Marchewicz, M. Szudyga, T. Antes, W. Balachandran, F. Di Natale, C. Carotenuto, Submicron particles removal by charged sprays, Fundam. J. Electrostat. 71 (2013) 345–350. [24] A. Jaworek, A.T. Sobczyk, Electrospraying route to nanotechnology: an overview, J. Electrostat. 66 (2008) 197–219. [25] A. Krupa, A. Jaworek, A.T. Sobczyk, A. Marchewicz, M. Szudyga, T. Antes, Charged spray generation for gas cleaning applications, J. Electrostat. 71 (2013) 260–264. [26] G.N. Laryea, S.-Y. No, Development of electrostatic pressure-swirl nozzle for agricultural applications, J. Electrostat. 57 (2003) 129–142. [27] Lechler, Lechler- Nozzles Catalogue, 2016. (accessed 01.12.2016). [28] W. Machowski, W. Balachandran, Design of electrostatic fog generator using a reverse field modelling technique, in: Industry Applications Conference, 1997. Thirty-Second IAS Annual Meeting, IAS’97., Conference Record of the 1997 IEEE, 1997, pp. 1784–1789. [29] L. Manna, C. Carotenuto, R. Nigro, A. Lancia, F. Di Natale, Primary atomization of electrified water sprays, Canad. J. Chem. Eng. 95 (2017) 1781–1788. [30] L. Manna, F. Di Natale, C. Carotenuto, A. Lancia, Electrified water sprays generation for gas pollutants emission control, Chem. Eng. 52 (2016). [31] S. Nakao, M. Shrivastava, A. Nguyen, H. Jung, D. Cocker III, Interpretation of secondary organic aerosol formation from diesel exhaust photooxidation in an environmental chamber, Aerosol Sci. Technol. 45 (2011) 964–972. [32] G.G. Nasr, A.J. Yule, L. Bendig, Industrial Sprays and Atomization: Design, Analysis and Applications, Springer Science & Business Media, 2002. [33] J. Rosell-Llompart, J. Grifoll, I.G. Loscertales, Electrosprays in the cone-jet mode: from Taylor cone formation to spray development, J. Aerosol Sci. (2018). [34] O.V. Salata, Tools of nanotechnology: electrospray, Curr. Nanosci. 1 (2005) 25– 33. [35] T. Seto, T. Orii, H. Sakurai, M. Hirasawa, S.-B. Kwon, Ion beam charging of aerosol nanoparticles, Aerosol Sci. Technol. 39 (2005) 750–759. [36] B. Walch, M. Horányi, S. Robertson, Charging of dust grains in plasma with energetic electrons, Phys. Rev. Lett. 75 (1995) 838. [37] S. Zhao, G. Castle, K. Adamiak, The effect of space charge on the performance of an electrostatic induction charging spray nozzle, J. Electrostat. 63 (2005) 261– 272. [38] S. Zhao, G.P. Castle, K. Adamiak, Comparison of conduction and induction charging in liquid spraying, J. Electrostat. 63 (2005) 871–876.
Contents lists available at ScienceDirect
International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt
Induction charging of water spray produced by pressure atomizer A. Marchewicz ⇑, A.T. Sobczyk, A. Krupa, A. Jaworek Institute of Fluid Flow Machinery, Polish Academy of Sciences, Gdansk, Poland
a r t i c l e
i n f o
Article history: Received 2 November 2018 Received in revised form 6 February 2019 Accepted 7 February 2019
Keywords: Charged spray Electrostatic scrubbing Induction charging Liquid atomization
a b s t r a c t Charged sprays are widely used in various branches of industry, for example, for surface coating or gas cleaning by wet electrostatic scrubbers. The selection of appropriate system for charged spray generation is of crucial importance for obtaining high cleaning efficiency of a scrubber. This paper presents comparative experimental studies of parameters of charged sprays produced by various types of pressure atomizers with charging of droplets by induction, using three types of induction electrodes. The spray systems were characterized in terms of droplet size distribution and specific electric charge carried by the generated droplets. The induction charging process was investigated experimentally for the following parameters: voltage of induction electrode, inter-electrode distance, supply water pressure. It was determined that nozzles of lower flow rate generate sprays of higher specific charge, because the Sauter Mean Diameter for these nozzles is smaller. It was also found that the specific charge of droplets increases with increasing voltage applied to the induction electrode, but only to a certain voltage magnitude. Above this voltage level, the specific charge decreases due mainly to the shielding effect of space charge of produced droplets and the corona discharge from induction electrode, which neutralizes the droplets’ current. It was shown that the specific charge of droplets decreases with increasing Sauter Mean Diameter of those droplets. Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction Liquid dispersed in gaseous phase is commonly used by industry, particularly in such processes as fuel combustion, spray coating, paint spraying, air conditioning, fire extinguishing, liquid metal atomization (for metal powder production), cooling of metals, crop spraying or flue gas cleaning. All of those processes require sprays of specific parameters, optimized with respect to the process progress. For instance, fuel dispersed for the combustion needs fine droplet of the size in the range 5 mm Dv0,5 80 mm, where Dv0,5 is the volume median diameter, but in the case of cooling processes, the dispersed liquid droplets can be coarser, in the range of 90 mm Dv0,5 2000 mm [32]. For scrubbing processes, droplets of the size of about 0.2–1 mm are used. Spray nozzles of various types (pressure nozzles, pneumatic nozzles, ultrasonic etc.) are the main devices used for spray generation [7]. Nozzles of the same type differ in their construction and dimensions, and, therefore, the physical aspects of the process of liquid dispersion are different, and hence, the droplet size distribution can also be different. Novel technologies have been developed in recent years, which require electrically charged droplets in order to improve the ⇑ Corresponding author. E-mail address: [email protected] (A. Marchewicz). https://doi.org/10.1016/j.ijheatmasstransfer.2019.02.013 0017-9310/Ó 2019 Elsevier Ltd. All rights reserved.
performances of those processes by only a small amount of additional energy needed to this goal. Electrospraying and mechanical atomization with induction charging are the most effective techniques used for the production of charged sprays [22]. Electrospraying allows generation of highly charged spray with fine droplets of narrow size distribution, but with low liquid flow rates (0.1–100 ml/h). On the other hand, induction charging provides less charged spray in comparison to electrospraying, but with significantly higher liquid flow rate. The high liquid flow rate is important for many industrial applications where spray have to be deposited on large areas (for example, crops spraying and spray painting) or where the volume flow rate of sprayed liquid plays an important role for the industrial process, for example, in the case of flue gas scrubbing [13,23,25]. Coarser droplets produced by the nozzles used in this research may be beneficial for the above mentioned purposes, due to slower evaporation of such droplets. For each of these applications, the induction charging process has to be optimized to achieve as high specific charge of droplets as possible. This paper focuses on the optimization of induction charging of water spray in terms of droplets’ charge maximization for the application to flue gas cleaning by electrostatic scrubbers. Because of practical reasons the commercial nozzles have been used in these investigations. Nonetheless, the results may be applied to many other industrial applications, in which these types of spray
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nozzles can be used, and where the magnitude of charge imparted to droplets play an important role in the process. The spraying process is crucial for induction charging, but detailed modelling of spray nozzles, taking into consideration all physical parameters involved in the process of spraying, is up to date very difficult. The phenomenon of disintegration of liquid jet or film is strongly dependent on the liquid properties, jet velocity and vibration, therefore, both internal and external forces have to be taken into consideration in such analysis. These forces have a disruptive or integrating character. In the case when the inertial and aerodynamic forces associated with a high kinetic energy of liquid flowing from a nozzle are sufficiently high, to overcome the surface tension force, a droplet is detached from the liquid
jet. Depending on the size of droplet and the Weber number for this droplet, further disintegration of the droplet in the gaseous phase is possible. It is usually assumed that the critical Weber number for droplet disintegration is 12 [18]. On the other hand, droplets in a spray may collide with each other and coagulate. For these reasons only statistical methods are used for spray characterization [7]. Statistical methods, such as droplet size distribution, can provide enough data for the assessment of microstructure of the spray needed for a specific application. More complex problems can occur when the sprayed droplets are electrically charged. In that case, also the electrostatic forces between droplets have to be considered as additionally influencing the liquid jet dispersion and the droplets motion.
Fig. 1. Schematic of experimental set-up for droplet size distribution measurement (a), for the measurement of spray current (b).
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Fig. 4. Pressure atomizer nozzle with ring induction electrode.
Fig. 2. Photograph of the spray column with Faraday cage inside.
Charged sprays can be produced by various methods. Charging the droplets by ion current is used when the spray already exists, while the induction charging, contact charging or electrospraying are the methods used for simultaneous charging during the spray generation. Charging by ion current occurs when the droplets flow through an ionized gas, for example, the corona discharge region. The droplets are charged by ions or electrons after their collision with the droplets [35,36]. The induction charging occurs during the liquid atomization, when the electric charge is induced at the surface of liquid jet flowing from the nozzle. The electric field necessary for the induction charging is produced by an electrode maintained at high electric potential, which is placed in the vicinity of grounded nozzle [4]. The contact charging is a method of depositing of an electric charge on liquid jet and droplets by the
Table 1 Technical specification of the nozzles used in the experiments (data from catalogue [27], except ‘‘water flow rate” columns). Nozzle 216.404p is modified version of 216.404 hollow cone nozzle. Nozzle model
Spray cone type
Spray cone angle [°]
Water inlet type
Water flow rate at 3 bar [l/min]
Water flow rate at 3 bar (measured) [l/min]
Water flow rate at 6 bar (measured) [l/min]
460.523 460.524 460.484 460.444 216.404 216.404p 460.403 302.364.30
Full cone Full cone Full cone Full cone Hollow cone Hollow cone Full cone Hollow cone
45 60 60 60 60 60 45 60
Axial Axial Axial Axial Axial Axial Axial Tangential
2.35 2.35 1.88 1.47 1.22 1.22 1.18 0.77
2.28 2.39 2.07 1.48 1.28 1.29 1.14 0.79
3.21 3.17 2.71 1.96 1.72 1.82 1.63 1.08
Fig. 3. Schematic of pressure swirl nozzles used in the study: axial full cone nozzle (a); axial hollow cone nozzle (b); tangential hollow cone nozzle (c).
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connection of atomizer nozzle to a high voltage source [38]. No other induction electrode is necessary in that case, and the electric field lines originating at the surface of liquid jet are closed to the nearby grounded objects or the target substrate. Electrospraying is a specific process of liquid atomization, by which only the electrical forces acting on the liquid meniscus at the outlet of a capillary nozzle are responsible for liquid atomization. No other mechanical forces are imposed on the jet in that case [15,33]. Because of the shear stress produced at the jet surface by the electric field, the jet is stretched from the tip of the meniscus, and disperses into fine droplets due to mechanical instabilities [15,20,21,24,33,34]. All of these methods have several industrial applications, however, this paper is focused at the induction charging only, because this method provides a reasonable level of droplet charge by relatively high flow rates necessary for industrial applications. One of such applications is the process of cleaning of contaminated gases produced by various sources: power plants, industry and transport. Recently, the emission of particulate matter (PM)
by power plants and transport became a severe concern [11,31]. New regulations regarding the emission of particles smaller than 2.5 lm (PM2.5) have come into force. One of the largest contributors in terms of PM emission by transport sector is the maritime transport [14]. Among various solutions used for the removal of PM from exhaust gases emitted by marine Diesel engines is the electrostatic scrubbing. In an electrostatic scrubber, the electrically charged particles are deposited onto oppositely charged water droplets, due to Coulomb attraction between them. This effect allows obtaining higher collection efficiency of such devices compared to conventional inertial scrubbers. In electrostatic scrubbers, the process of droplet charging is crucial for the efficient particle removal, which was shown in many studies [1,9,8,12,13,22,25]. The efficiency of gas cleaning by electrostatic scrubber depends mainly on the size and charge of droplets used for this process. Droplets should be charged to a level as high as possible, and the droplet concentration should be high enough to enable a high probability of their collision with particles. For these purposes, the induction charging is considered as the optimal one because of a high liquid
Fig. 5. Normalized number and volume size distributions of droplets for full cone (a–d, f) and hollow cone (e, g) nozzles; water pressure: 6 bar. Results ordered with decreasing water flow rate.
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635
Fig. 5 (continued)
flow rate available, and relatively high magnitude of electric charge imparted to the droplets, which can be as high as 1/5 of the Rayleigh limit [25]. In order to accomplish the induction charging process, a ring induction electrode, coaxial with the axis of spray nozzle and located in the vicinity of the nozzle outlet, is usually used, while the nozzle is grounded. This electrode is connected to a high-voltage power supply, in order to produce an electric field at the surface of the liquid jet flowing from the nozzle. The measurements of droplets size distribution and the electric charge of droplets produced by various types of water-spray atomizers are presented in this paper. Two types of pressure atomizers were tested depending on the spray plume: hollowcone (droplets are ejected from the conical liquid film produced by the nozzle, and the droplets move mainly in the vicinity of lateral surface of this cone) and full-cone (droplets fill the whole spray-cone volume). Ring or cylindrical induction electrodes were used for the charging of droplets by induction during their atomization. This paper is focused on experimental evaluation of the quality of water spray generated by several commercial pressure swirl atomizers and maximization of the spray current con-
veyed by the charged droplets. The research is aimed at the optimization of the spray system for its application to the wet electrostatic scrubbing of gases in various industrial processes. The spray nozzles were tested for the range of water flow rate between 1 and 3.2 l/min (at 6 bar pressure), which is adequate for the application to electrostatic scrubbers [13,25]. 2. Experimental Experimental setup used in this research is shown schematically in Fig. 1. It comprised of a water tank and water pump supplying the spray nozzle with water. The pipeline from the water pump is ended with a connector allowing an easy replacement of the tested nozzle. The water was supplied to the nozzle by diaphragm pump Hydra-cell G-03 (nominal flow rate 200 l/h at 1395 rpm, by 50 Hz power supply). The flow rate of the pump was controlled by frequency inverter Nordac SK 500e (Nord Drive Systems). The water flow rate was measured by electromagnetic flow meter Promag 50H08 (Endress + Hauser), and the pressure at the nozzle inlet by pressure meter Cerabar M PMC51 (Endress + Hauser). Water
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Fig. 5 (continued)
temperature was measured by thermometer RTD TR10 (Endress + Hauser). The measurements of droplet size distribution and spray current were carried out in two separate steps. The measuring system used for the measurement of droplet size distribution was AWK D made by KAMIKA INSTRUMENTS (Poland). This device consisted of an optical probe for the detection of water droplets, an electronic interface, and a computer for data storage and processing. A scheme of experimental setup for droplet size distribution is shown in Fig. 1a. The optical probe consisted of infrared laser, photodetector and inlet orifice. The optical parts were mounted in a stainless steel casing. During the measurement, the spray flows into the measuring volume through the inlet orifice. Each spray droplet shadows partially the laser sheet focused on the photodetector, decreasing the incident light intensity. The maximum decrease in the light intensity for a given droplet is proportional to the droplet diameter. Thus the photodetector generates an electric signal of amplitude proportional to the droplets diameter. Consequently, the electronic interface determined the droplets size from the signals generated by each droplet, and classifies them into
256 size classes. The distance between the nozzle outlet and the inlet aperture of the analyser was 60 cm. Fig. 1b shows a scheme of experimental setup for the measurement of specific charge of droplets. For the measurement of total spray current conveyed by the droplets, the nozzle was mounted in a spray column of inner diameter of 400 mm and height of 2000 mm made of PMMA. A grounded fine mesh was stretched at the inner wall of this column in order to remove the electric charge from the deposited droplets. A Faraday cage of inner diameter of 210 mm, made of a two-layer grid of dense copper mesh was used for the measurement of spray current. The cage was suspended on dielectric threads in order to avoid the leakage current from cage. The distance between the inlet of this cage and the nozzle outlet was set in such a way as the spray cone could entirely flow into this cage. Fig. 2 shows a photograph of the spray column with electrostatic spray nozzle and Faraday cage used for the measurement of electric charge of droplets. Seven models of commercially available pressure swirl nozzles produced by Lechler GmbH have been tested. The technical data for these nozzles are listed in Table 1. Fig. 3 presents the schematic
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Fig. 5 (continued)
Table 2 Sauter Mean Diameter (SMD) of droplets dispersed by various spray nozzles, for two water pressures and at two positions of the measuring probe. Nozzle model
Sauter mean diameter [lm]
Nozzle type Measuring probe position Water pressure [bar]
460.523 460.524 460.484 460.444 216.404 216.404p 460.403 302.364.30
Full cone Full cone Full cone Full cone Hollow cone Hollow cone Full cone Hollow cone
of three types of pressure swirl nozzles used in this study. Black arrows indicate of the streamlines of water flowing through nozzle. Nozzles of this type are described in the literature [7], and are widely used in various industrial applications. The flow rate of all of the nozzles was lower than 3.2 l/min for the water pressure at the nozzle inlet of 6 bar. One of the nozzles, model 216.404, has been modified in order to check the effect of the nozzle shape on charging efficiency, which is discussed in Section 3.5. The modified nozzle has been marked with letter ‘‘p” after the model number of the nozzle (i.e., 216.404p). The droplet size distribution has been measured in two different positions of the probe: in the axis of the nozzle and 20 cm from the axis, for all types of tested nozzles. The charging process was investigated for three types of induction electrodes: torus of inner diameter of 100 mm, and two cylindrical of inner diameters of 80 mm and 65 mm, and height of 30 mm each. The torus electrode was bent from a brass pipe of the diameter of 12 mm. The induction electrode was placed coaxially with the nozzle. The electrode was mounted on a support to allow changing its vertical position along the nozzle axis, in order to vary the position of the electrode relative to the nozzle outlet d (cf. Fig. 1b). High voltage was applied to the induction electrode from power supply Spellman HV unit (SL30PN300) of negative polarity, while the nozzle was grounded. The spray current was measured with a digital picoammeter (Keithley 486) connected to the Faraday cage. The spray current has been measured for the following distances between the upper plane of the induction electrode and the plane placed at the outlet of the nozzle (d): 0, 10, 20, 30 and 40 mm. A photograph of spray
Axial 3
Axial 6
20 cm off axis 3
20 cm off axis 6
307 301 265 292 257 269 241 285
295 282 248 273 254 246 247 229
329 356 304 325 313 313 293 317
304 319 321 320 305 308 291 321
nozzle with a ring induction electrode, taken during the measurements, is shown in Fig. 4. Tap water used in these measurements was provided by the same means as in the droplet size distribution tests. Water temperature during tests varied between 10 and 15 °C. Water pressure, flow rate and temperature were measured by the same measuring devices as in the droplet size distribution tests. The air from the spray column was smoothly removed through a pipe mounted at the bottom of the column with the flow rate of 130 m3/h (the mean gas velocity of about 0.3 m/s) in order to remove the water mist from the column. The conductivity of water was measured in order to verify whether a sufficient amount of electrical charge can be delivered to the surface of liquid film before droplet detachment. At 10 °C the electric conductivity (r) was 4.19 mS/m. The time constant associated with electric phenomena (electric charge relaxation time) is:
sel ¼
e r
ð1Þ
where e is the electric permittivity of water. For the measured conductivity, the value of sel was equal to 0.16 ls. The hydrodynamic time constant characterizing the motion of liquid film ejected from the nozzle is estimated from the following equation
sh ¼
dout pdout ¼ u 4Q
3
ð2Þ
where dout is the diameter of orifice assumed to be the characteristic length, u is the liquid velocity at the nozzle outlet, and Q is the
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water flow rate. For the 460.523 nozzle, liquid flow rate through the nozzle at a pressure of 6 bar, and orifice diameter of 1.6 mm was 3.21 l/min, and the value of hydrodynamic time constant can be estimated to be 61 ls. The electric charge relaxation time constant was therefore 380 times shorter than the hydrodynamic time constant, hence the tap water used in the following experiments can be considered as a perfect conductor.
3. Results and discussion 3.1. Droplet size distribution All the spray nozzles have been tested for two water pressures, 3 and 6 bar. The size distribution of droplets was measured for two positions of the optical probe. Fig. 5 presents the results of measurements of normalized number and volume size distributions of droplets for the investigated nozzles, obtained by water pressure of 6 bar, for two positions of the probe: 0 and 20 mm from the noz-
zle axis. The droplet size distributions presented in Fig. 5a–d and f are for nozzles with full spray cone, and in Fig. 5e and g for the nozzles with hollow spray cone. Sprays generated by all tested nozzles had much larger number of small (<200 lm) droplets in the spray cone axis than at the off-axis probe position. As a consequence, the Sauter Mean Diameter (SMD) is significantly smaller in the spray cone axis than at the off-axis probe position. The values of SMD of droplets dispersed by various spray nozzles, for two water pressures and at two positions of the measuring probe in the spray cone are given in Table 2. Fig. 6 compares the effect of water pressure (3 and 6 bar) on the cumulative volume size distributions of droplets for three selected nozzles: one of full-cone and two of hollow-cone type. The smallest SMD of droplets for all tested nozzles was obtained in the axis of the spray cone for the hollow cone nozzle 302.364.30, at a water pressure of 6 bar. However, the SMD increased by 24% when the pressure was decreased to 3 bar (cf. Fig. 6b). For comparison, for full cone nozzle 460.484, the SMD increased by 7% (cf. Fig. 6a), and for hollow cone nozzle 216.404 with axial water inlet the
Fig. 6. Example of cumulative volume size distributions of droplets for (a) full cone 460.484 and hollow cone nozzles: (b) 302.364.30, (c) 216.404 for various water pressures and probe positions.
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Fig. 6 (continued)
Fig. 7. Specific charge of spray generated by single fluid pressure swirl nozzles: (a), (b), (g) 302.364.30 for various distances between the nozzle outlet and the induction electrode (d). Water pressure: 6 bar, induction electrode distance: 30 mm. Results ordered with decreasing water flow rate.
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Fig. 7 (continued)
SMD increased by only 1%, when the supply water pressure was reduced to 3 bar (cf. Fig. 6c). 3.2. Specific charge of droplets The most common parameter used for characterization of the electric charge level of charged spray is the specific charge. The specific charge of a single droplet is the ratio of the charge on this droplet to the mass of this droplet. The mean value of the specific charge for a spray can be estimated from the ratio of spray current to the mass flow rate of sprayed liquid [2,10,25,26]:
I ¼ _ m
PN 2 4prq Nrs 2 Q ðrs Þ i¼1 4prq ðiÞr i ¼ ¼ 4 PN 4 3 3 m pqNrs d ðr s Þ i¼1 3 pqr i 3
ð3Þ
where rq ðiÞ is the surface charge density on the i-th droplet of radius r i , rs is the Sauter mean radius, q is the water density, N is the total number of droplets in the spray, Q(rs) is the charge of droplet of Sauter mean radius, and md(rs) is the mass of this droplet. The specific charge presented in the following figures is determined
from the spray current measured by the Faraday cage and the mass flow rate of sprayed water. The problem of the magnitude of charge on a single droplet was discussed by [22]. This charge was determined from the value of surface charge density induced on a liquid film at the nozzle outlet. The magnitude of surface charge density rlf at a liquid film subjected to electric field is:
rlf ¼ e0 En
ð4Þ
where En is the electric field magnitude perpendicular to the surface of liquid film. The electric field depends on the induction electrode potential, the induction electrode geometry and the distance between induction electrode and nozzle outlet. The charge on the i-th dropletQ ðiÞ is equal to the total charge accumulated on the fragment of liquid film from which the droplet is formed at the moment of droplet detachment:
Q ðiÞ ¼ Ai rlf ðAi Þ ¼ Ai e0 En where Ai is the area of the fragment of liquid film.
ð5Þ
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Fig. 7 (continued)
From the above assumptions results that the droplet charge is estimated to be proportional to the electric field on the surface of liquid film at the moment of droplet detachment, and, for a fixed electrode geometry and a given inter-electrode distance, is proportional to the voltage applied to induction electrode. For the mass of droplet independent of electric field, also the specific charge of droplets is proportional to the voltage applied to the induction electrode. This model neglects the space charge and the electric discharges from the nozzle and induction electrode, and can only be applied to voltages below the specific charge saturation. The results of measurement of the specific charge vs. voltage applied to the induction electrode are presented in Fig. 7. For low voltages, the specific charge increases nearly proportionally to the voltage, but the characteristic gradually departure from the straight line. Such observations were also reported by other papers on the induction charging [10,17,25,30]. The saturation of specific charge can be caused by several effects. First of all, the deposition of water mist on the induction electrode was observed during the atomization process (even at 0 kV voltage). This phenomenon was intensified with increasing supply voltage. Droplets detached from
the induction electrode are charged with opposite polarity to those charged by induction, which could decrease the total current flowing to the Faraday cage. Secondly, for voltages higher than a certain magnitude, small liquid jets were ejected from the water film formed at the induction electrode towards the nozzle. At the tip of these jets a faint light of the corona discharge was observed in a dark room. It is probable that the charge carried by these droplets and gaseous ions emitted from these jets are also responsible for the decrease in the current measured by the Faraday cage for higher voltages. The corona discharge was also observed by Higashiyama et al. [17] and Castle and Inculet [10]. Another effect on specific charge saturation that has to be taken into consideration is the effect of space charge of the generated charged droplets reducing the electric field on the surface of liquid film, and consequently, reducing the charge of newly detaching droplets. The maximal value of specific charge of droplets measured by the Faraday cage was higher for nozzles designed for lower flow rates. Nozzles designed for higher flow rates of water, generated sprays of larger values of number concentration of droplets. Because of higher space charge density of droplets, the shielding
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Fig. 7 (continued)
effect of this charge in the vicinity of the breakup region of the liquid film is more important for the charging process than for nozzles of lower flow rates. The maximum specific charge was higher for the hollow cone nozzles. This happened due to the shape of water film at the outlet of spray nozzle before its breakup. The film has a distinctive conic-helical shape (see photo in Fig. 4). Longer liquid film to breakup allows higher electric charge to be induced by the electric field produced by the induction electrode. Results presented in Fig. 7 also show the dependence between the specific charge of droplets and the position of induction electrode (d – see Fig. 1b). The measuring points obtained for the optimal position of the induction electrode, for which the specific charge was the maximal, are presented by filled markers in each of these figures. For almost all nozzles, the optimal distance between nozzle outlet and induction electrode was about 30 mm. Further increase in the distance caused only a decrease in the value of specific charge. The specific charge for hollow cone nozzle 302.364.30 presented in Fig. 7g was measured for one induction electrode position only, because of tangential inlet construction of this nozzle. 3.3. Effect of supply water pressure on the specific charge Fig. 8 presents the effect of voltage applied to the induction electrode on the value of specific charge for different supply water pressures, and for the optimal induction electrode position. The maximum specific charge did not change significantly with the pressure in the tested range (Fig. 8a, b), and for most of the nozzles, the specific charge slightly decreases with the pressure increasing. This small decrease in the specific charge could be caused by the influence of the space charge of droplets on the electric field at the liquid film. For higher water pressure, the water flow rate is also higher, the droplets are finer (SMD is lower, cf. Fig. 6), therefore, the space charge density is higher. Moreover, the smaller droplets are easier deflected by the electric field and deposited on the induction electrode. Then, the water film forms more jets, which are dispersed as small droplets of charge opposite to that generated by the nozzle, which reduces the spray current. Thus, the value of specific charge measured by the Faraday cage is a matter of balance between the effect of the generated space charge, the oppositely charged droplets detaching from the induction elec-
trode, and the ionic current generated by corona discharge from the induction electrode. The space charge density of charged droplets depends on the number concentration of droplets, their velocity, and the charge carried by the droplets. The effect of supply water pressure was more significant for full cone nozzles with 45° spray cone angle, because of higher droplets concentration and higher space charge density. 3.4. Effect of spray cone angle for nozzles of similar flow rate Manna et al. [29,30], using one of the nozzles tested in this paper, the hollow-cone nozzle 216.404, observed that the angle of liquid cone at the nozzle outlet increases, and the length of this liquid cone decreases with increasing voltage applied to the induction electrode. These findings suggest that the electric field induced on the liquid sheet generates an additional electric force, which deforms the film, and amplifies the wave oscillations of this film. By this way, the electric field can influence the spray mechanism. Similar effect was also observed by Higashiyama et al. [17] who measured the droplet size distribution during the spraying with and without the induction charging of droplets. In the macroscopic scale, those experiments showed that the inductively charged spray tends to disperse outwards the nozzle axis due to electrostatic repulsion of droplets, and that the droplet size distribution of electrically charged spray along radial distance from the axis is more uniform than the droplet size distribution of uncharged spray. The effect of charged spray dispersion due to Coulomb repulsion suggests that it is likely that droplets of highest electrical mobility tend to precipitate on the induction electrode, causing the emission of water jets, which reduce the charging efficiency. Moreover, Higashiyama et al. [17] also reported on small droplets deposition on the induction electrode and the corona discharge over certain induction electrode voltage. A comparison of generated specific charge for nozzles with similar water flow rate but of different spray cone angle is shown in Fig. 9. It has to be noted that the maximum specific charge of droplets for these nozzles is different, despite a similar droplet size distribution. The maximum specific charge for the 460.523 nozzle (cone angle-45°) was higher by 48.7% than for the 460.524 nozzle (cone angle-60°), while the SMD for 460.523 was higher by only 1.7%, for the same experimental conditions (supply water pressure:
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3 bar, induction electrode voltage: 17 kV, induction electrode distance: 30 mm). This indicates that in the case of nozzle with larger spray cone angle, where the detachment of droplets occurs closer to the induction electrode, the electric field is high enough to deflect the droplets’ trajectories, thereby leading to the droplets’ deposition on the induction electrode and consequently decreasing the measured spray current.
3.5. Effect of spray nozzle geometry The hollow cone nozzle model 216.404 has been modified in order to check the effect of the tip of the nozzle on the electric field distribution at the surface of liquid film, and on the specific charge of spray (the modified nozzle was labelled in the following as 216.404p). The geometry of the nozzle before and after its modification is drawn schematically in Fig. 10. It was assumed that the machined nozzle will allow producing a higher electric field at the nozzle tip and the liquid film, due to a sharper cone at the nozzle outlet. The higher electric field should cause an increase of the
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electric charge on the liquid film before its disruption to droplets. The results of measurements of the droplet size distribution and specific charge characteristics for the modified nozzle are shown in Fig. 11. A slight shift in the specific charge characteristics towards a lower voltage applied to the induction electrode, in comparison to the non-modified version of this nozzle, can be noticed. However, no significant increase in the maximum value of specific charge has been obtained. The only significant difference observed was a faster decrease of the specific charge for voltages higher than that at the maximum (Fig. 11). This effect was probably caused by a higher corona discharge current from a sharper tip of the machined nozzle.
3.6. Effect of induction electrode geometry Besides the inter-electrode distance, spray cone angle and water pressure, the geometry of induction electrode is also important parameter in the process of spray charging. Most of the previous research has been focused on the ring induction electrode with cir-
Fig. 8. Specific charge of spray generated by the nozzles: (a) 460.484, (b) 216.404 and (c) 460.523 vs. induction electrode voltage for various water pressures. Distance between the nozzle and induction electrode (d) was set to optimal position for each electrode.
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Fig. 8 (continued)
Fig. 9. Comparison of specific charge characteristics for two full cone nozzles of different spray cone angles, for similar water flow rate and charging conditions.
cular cross section (torus electrode) or square/rectangle cross section [3,17,19,29,30]. Depending on practical application and a type of spray nozzle used, the inner diameter of induction electrode varied between 5 and 110 mm. Several papers considered also the induction charging of droplets for flat-spray nozzles [16,37], and centrifugal atomizers [5,6]. Machowski and Balachandran [28] have optimized numerically the shape of induction electrode in order to increase the effect of induction charging, but those simulation results have not been compared with experiments. Fig. 12 compares the results of measurements of specific charge of droplets obtained for various types of induction electrode: torus and two cylindrical, of inner diameter of 80 mm and 65 mm. Each measurement was carried out for the optimal inter-electrode distance (d), for which the maximal specific charge was. Fig. 13 presents the magnitude of specific charge at the maximum versus the inter-electrode distance (d), for spray generated by the nozzle 460.484. The cylindrical induction electrode of inner diameter of 80 mm induced the highest specific charge on the droplets. The optimal distance between electrodes (d) was different than for
Fig. 10. Outline of the shape of hollow cone nozzle model 216.404, before (left) and after (right) modification.
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Fig. 11. Comparison of: (a) droplet size distribution and (b) specific charge characteristics for modified (216.404p) and non-modified (216.404) nozzles for the optimal charging conditions.
the torus electrode. The negative value of the distance means that upper plane of the induction electrode is placed above the plane of the spray nozzle outlet. The maximum specific charge obtained in the case of charging with cylindrical induction electrode of 65 mm diameter was significantly lower than for other tested electrodes, therefore this electrode have been tested only for one interelectrode distance. During the measurements, this electrode was immediately covered with water film, hence the corona discharge was present at much lower voltage (9 kV) than for two other cases. 3.7. Optimization of induction charging process in terms of the specific charge The experimental results show that the selection of nozzle for a specific industrial application of an electrically charged spray (e.g., for wet electrostatic scrubbing) is a matter of balance between the maximum specific charge of droplets and the optimal diameter of droplets in the produced spray. Table 3 summarizes the optimal
droplet charging conditions for all tested nozzles with respect to the maximum specific charge of droplets. The maximum specific charge of droplets has been obtained for the nozzle 460.403. For this case, the optimal inter-electrode distance was 20 mm by a voltage of 17 kV and water pressure of 3 bar. However, the flow rate of water for this nozzle was not very high (1.14 l/min), and in the case of wet electrostatic scrubbing applications, the use of nozzles of lower flow rates requires a larger number of those nozzles in order to obtain the assumed scrubbing efficiency that could lead to the problems with nozzle arrangement inside a scrubber column. Fig. 14 presents the maximum specific charge and Sauter Mean Diameter vs. water flow rate, for all tested nozzles. The regression lines indicate that for the single fluid pressure swirl nozzles operating at 6 bar of water pressure, SMD was higher for the nozzles with higher flow rate, but the maximum specific charge decreased accordingly. The mass of a droplet is proportional to its diameter cubed, and the charge is proportional to the droplet surface, there-
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Fig. 12. Specific charge of spray vs. induction electrode voltage, generated by the nozzle 460.484 for various types of induction electrode at their optimal distance.
Fig. 13. Maximum specific charge of spray vs. inter-electrode distance for the nozzle 460.484 for various types of induction electrode and various water pressures.
Table 3 The optimal charging conditions for various single fluid pressure swirl nozzles in terms of the specific charge. Nozzle model
Nozzle type
Spray cone angle [°]
Water pressure (flow rate) [bar] ([l/min])
Sauter mean diameter [lm]
Maximal specific charge [lC/kg]
Optimal electrode voltage [kV]
Optimal electrode distance [mm]
460.523 460.524 460.484 460.444 216.404 216.404p 460.403 302.364.30
Full cone Full cone Full cone Full cone Hollow cone Hollow cone Full cone Hollow cone
45 60 60 60 60 60 45 60
3 4 3 5 6 3 3 4
306.5 300.8 265.4 273.4 254.3 268.8 241.4 284.6
171.1 128.7 158.7 169.8 230.2 239.0 260.5 196.7
17 15 14 15 14 12 16 10
30 30 20 40 20 30 20 30
(2.28) (2.68) (2.08) (1.82) (1.72) (1.23) (1.14) (0.90)
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Fig. 14. The maximum specific charge and Sauter Mean Diameter vs. water flow rate for eight tested single fluid pressure swirl nozzles. Water pressure: 6 bar.
Fig. 15. The maximum specific charge vs. Sauter Mean Diameter for eight tested single fluid pressure swirl nozzles. Water pressure: 6 bar.
fore the specific charge of droplets generated by the nozzles of higher flow rates is lower. The maximum specific charge of droplets decreases with increasing SMD that was shown in Fig. 15. 4. Conclusions In this paper, a set of commercial single-fluid pressure-swirl spray nozzles with charging by induction have been tested with respect to the droplet size distribution and specific charge of generated droplets. The nozzles have been selected for the water flow rate falling in the range required for electrostatic scrubbing applications, which is usually assumed to be between 1 and 3.2 l/min for a single spray nozzle [13]. The obtained results show that the specific charge of droplets increases almost proportionally with increasing voltage applied to the induction electrode but only to a certain voltage magnitude. Above that voltage, the specific charge characteristic declines from the straight line due to several phenomena, which may occur in
such conditions. One of such phenomenon is the presence of space charge of the charged spray in the vicinity of the liquid film at the nozzle outlet. This space charge has negative effect on the spray charging process, because it changes the electric field distribution and reduces its magnitude at the liquid film at the nozzle outlet. Secondly, the water mist deposited on the induction electrode has been observed. For higher induction electrode voltages, water deposited on the surface of this electrode has formed fine liquid jets, which could produce oppositely charged droplets via electrospraying phenomenon. Further increase in the induction electrode voltage lead to the occurrence of corona discharge from these liquid jets, and it is probable that ionic current of corona discharge can neutralize the charge of the droplets. Moreover, the oppositely charged water droplets dripping from the induction electrode had reduced the spray current measured by the Faraday cage placed beneath the nozzle. The geometry of induction electrode is an important parameter in the process of spray charging. It was shown that the specific
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charge of droplets charged by cylindrical induction electrode of inner diameter of 80 mm was higher than by using torus electrode or a cylindrical one of smaller diameter. The optimal distance of the induction electrode maximizing the specific charge was determined for each nozzle. The regression line of the maximal specific charge obtained for each nozzle vs. Sauter Mean Diameter measured for eight tested nozzles indicated that the specific charge of droplets decreases with increasing Sauter Mean Diameter of those droplets. In the case of the tested nozzles the specific charge of droplets varied between about 120 and 240 mC/kg. It was also shown that the specific charge of droplets was higher for nozzle of lower spray cone angle. Conflict of interest None. Acknowledgements The paper is a result of the statutory research at the Institute of Fluid Flow Machinery, Polish Academy of Sciences, within the project No. O1/T3/Z4. References [1] K. Adamiak, A. Jaworek, A. Krupa, Deposition efficiency of dust particles on a single, falling and charged water droplet, IEEE Trans. Indus. Appl. 37 (2001) 743–750. [2] T.C. Anestos, J.E. Sickles, R.M. Tepper, Charge to mass distributions in electrostatic sprays, IEEE Trans. Ind. Appl. (1977) 168–177. [3] A. Atten, S. Oliveri, Charging of drops formed by circular jet breakup, J. Electrostat. 29 (1992) 73–91. [4] A.G. Bailey, Electrostatic Spraying of Liquids, Research Studies Press Limited, 1988. [5] W. Balachandran, A. Bailey, The influence of electrostatic fields on the centrifugal atomisation of liquids, J. Electrostat. 10 (1981) 189–196. [6] W. Balachandran, A.G. Bailey, The dispersion of liquids using centrifugal and electrostatic forces, IEEE Trans. Ind. Appl. (1984) 682–686. [7] L. Bayvel, Z. Orzechowski, Liquid Atomization, Taylor and Francis, 1993. [8] C. Carotenuto, F. Di Natale, A. Lancia, 2009. Electrostatic Enhanced Water Scrubbing for Particulate Abatement in Combustion Systems – Modelling Analysis and Preliminary Design Criteria Combustion Colloquia. [9] C. Carotenuto, F. Di Natale, A. Lancia, Wet electrostatic scrubbers for the abatement of submicronic particulate, Chem. Eng. J. 165 (2010) 35–45. [10] G. Castle, I. Inculet, Induction charge limits of small water droplets, in: The 8 Th International Conference on Electrostatics 1991, Oxford, Engl, 04/10-12/91, 1991, pp. 141–146. [11] R. Chirico, P. DeCarlo, M. Heringa, T. Tritscher, R. Richter, A. Prévôt, J. Dommen, E. Weingartner, G. Wehrle, M. Gysel, et al., Impact of aftertreatment devices on primary emissions and secondary organic aerosol formation potential from inuse diesel vehicles: results from smog chamber experiments, Atmos. Chem. Phys. 10 (2010) 11545. [12] L. D’Addio, F. Di Natale, C. Carotenuto, W. Balachandran, A. Lancia, A lab-scale system to study submicron particles removal in wet electrostatic scrubbers, Chem. Eng. Sci. 97 (2013) 176–185.
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