Charged Spray Research Articles

Factors affecting deposition in electrostatic pesticide spraying

In agricultural pesticide spraying, the major goals are to increase the pesticide deposition on target crops and to reduce off-target losses. There is often a conflict between the conditions required for achieving both. It has been confirmed that charged sprays employing smaller droplet sizes can enhance the deposition by space charge and image forces and thus reduce the amount of pesticide use by as much as 50%. For air-assisted induction-charging pesticide spraying, a high charge-to-mass ratio is usually desired for good electrostatic control of the droplet's trajectories which can increase the deposition and the “wrap-around” effect. However, spray drift may increase with droplet charge-to-mass ratio and nozzle-to-target distance if they are not properly controlled. In this paper, a numerical simulation technique is employed to model the charged droplet trajectories towards a spherical target for different droplet sizes, charge-to-mass ratios and nozzle-to-target distances. By determining the droplet trajectories and number of droplets deposited on the spherical target, the conditions required to achieve the optimum deposition may be demonstrated.

Experimental investigation of a maximum entropy assumption for acceleration terms within a poly‐disperse moment framework

AbstractMoment transport methods are being developed to model poly‐dispersed multiphase flows by transporting statistical moments of the particle size–velocity joint probability density function (JPDF). A common feature of these methods is the requirement to reproduce or approximate the form of the JPDF from the transported moments for calculation of body force terms and other source terms. This paper examines the application of a maximum entropy technique against phase Doppler anemometry data sets from an electrostatically charged kerosene spray and also an automotive pressure swirl atomizer. An assessment of which moments are required to reproduce the JPDFs using a maximum entropy assumption to a sufficient level of accuracy is made. It is found that it is possible to reproduce the JPDFs to a high level of accuracy using a large number of moments; however, this incurs large computational overheads. If the moments to be transported are chosen on the basis of physical reasoning (such as the relationship between size and velocity due to drag) it is possible to reduce the number of moments to those which would be conserved via balance equations. This permits an approximation to the JPDF commensurate with the closure level of the moment transport method and thus the closure model method is naturally scalable with the degree of information from available conservation equations. Copyright © 2008 John Wiley & Sons, Ltd.

Effectiveness of Electrostatically Charged Water Spray in Reducing Dust Concentration in Enclosed Spaces

This research investigated the potential of electrostatically charged water spray in reducing dust concentration in airspaces under controlled laboratory conditions. Test particles (i.e., corn starch and NaHCO3) were each aerosolized into in an enclosed experimental chamber (L = 3.6 m, W = 2.4 m, H = 2.4 m). Charged water droplets were then sprayed into the chamber. An Aerodynamic Particle SizerTM (APS) spectrometer was used to measure the particle concentration and size distribution. In addition, a tapered element oscillating microbalance (TEOM) was used to measure the mass concentration. From the APS and TEOM data, the particle removal efficiency of the charged water spray was determined. The particle removal efficiency of the charged water spray was significantly greater than that for the uncharged water spray. Results also showed that longer charged spray durations (i.e., 4 and 6 min) were more effective than the shorter spray duration (i.e., 2 min) in removing airborne particles. In addition, the polarity of the charged water droplets (negative vs. positive) did not significantly affect the removal efficiency of the charged water spray.

Synopsis of experimentally determined effects of electrostatic charge on gasoline sprays

The effects of applying electrostatic charge to gasoline fuel sprays from a gasoline direct injection (GDI) injector have been investigated. The injector was modified by the addition of an electrically isolated electrode installed on its tip. The morphology of electrostatically charged and non-charged sprays was characterized using a Mie scattering visualization technique, Fraunhofer diffraction measurements of droplet size, and particle imaging velocimetry (PIV) measurements of the velocity field. The PIV measurements showed that the electrostatic charging of the sprays enhanced the vortical motion of the droplets and produced empty pockets within the spray. Furthermore, the charged sprays penetrated further and were wider than the non-charged sprays. The results of the PIV measurements were complemented by Fraunhofer diffraction (Malvern) measurements, which showed significantly increased consistency of droplet size for the electrostatically charged case. The extent of charging of the fuel sprays was determined with combined measurements of the mass and the charge carried by the sprays. In parallel, the electric field generated by the presence of the charged electrode was computed by numerically solving the Laplace equation at the vicinity of the injector tip. With this data, the relative magnitudes of the electrostatic and inertial forces were compared.

Reactive Desorption Electrospray Ionization Linear Ion Trap Mass Spectrometry of Latest-Generation Counterfeit Antimalarials via Noncovalent Complex Formation

Desorption electrospray ionization mass spectrometry (DESI MS) is rapidly becoming accepted as a powerful surface characterization tool for a wide variety of samples in the open air. Besides its well-established high-throughput capabilities, a unique feature of DESI is that chemical reactions between the charged spray microdroplets and surface molecules can be exploited to enhance ionization. Here, we present a rapid screening assay for artesunate antimalarials based on reactive DESI. Artesunate is a vital therapy for Plasmodium falciparum malaria, but artesunate tablets have been counterfeited on a very large scale in SE Asia, and more recently in Africa. For this reason, faster and more sensitive screening tests are urgently needed. The proposed DESI assay is based on the formation of stable noncovalent complexes between linear alkylamines dissolved in the DESI spray solution and artesunate molecules exposed on the tablet surface. We found that, depending on amine type and concentration, a sensitivity gain of up to 170x can be obtained, in comparison to reagent-less DESI. Hexylamine (Hex), dodecylamine (DDA), and octadecylamine (ODA) produced proton-bound noncovalent complexes with gas-phase stabilities, increasing in the order [M + Hex + H]+ < [M + DDA + H]+ < [M + ODA + H]+. Tandem MS experiments revealed that complex formation occurred by hydrogen bonding between the amine nitrogen and the ether-like moieties within the artesunate lactone ring. After the reactive DESI assay was fully characterized, it was applied to a set of recently collected suspicious artesunate tablets purchased in shops and pharmacies in SE Asia. Not only did we find that these samples were counterfeits, but we also detected the presence of several wrong active ingredients. Of particular concern was the positive detection of artesunate traces in the surface of one of the samples, which we quantified with standard chromatographic techniques.

ELECTROSTATIC EFFECTS ON GASOLINE DIRECT INJECTION IN ATMOSPHERIC AMBIANCE

An experimental investigation of electrostatic effects on gasoline injection from a commercially available injector into atmospheric ambiance is reported. The injector was modified with the addition of an insulated insert that introduced a sharp edge, through which an electric charge was introduced into the fuel. Visualization experiments were performed in order to characterize spray morphology both for electrostatically charged and noncharged sprays. Supported by PDA measurements of droplet size and velocity, these results indicate a faster transition of charged sprays from an initial stage of a narrow, columnlike spray to the desirable, hollow-cone spray. Light Fraunhofer diffraction measurements indicate slow temporal variation of the droplet size during injection and also that charged sprays have a considerably better repeatability of droplet size. These measurements were combined with a measurement of the average induced charge density in order to yield estimates of the electrostatic and inertial forces exerted on the droplets.

Experimental investigation of the possibility of automotive gasoline spray manipulation through electrostatic fields

An experimental evaluation of the possibility of using 'electrospray' in automotive technology. To this extent, a commercially available Gasoline Direct Injection (GDI) injector was modified in order to induce electrostatic charge in the non-conducting fuel spray. Spray morphology was characterised with visualisation experiments and spray structure was probed with Particle Image Velocimetry (PIV) and Fraunhofer light diffraction droplet size measurements. PIV data showed that electrostatically charged sprays presented pockets that were sparsely populated by droplets and areas of increased vortical motion at an injection pressure where the non-charged sprays were much more coherent. In this sense, electrostatic charging at relatively low pressures aided in the generation of flow patterns that are exhibited at higher pressures for non-charged sprays. Measurements of droplet diameter using a Fraunhofer light diffraction technique provided indications of increased resistance to droplet coalescence and a much smaller cyclic variation of droplet size for the electrostatically charged sprays.

Field Evaluation of Application Variables and Plant Density for Bell Pepper Pest Management

Bell peppers are a valuable vegetable crop in the U.S. Management of pepper insects and diseases relies on chemical control options. Because there are relatively few chemical options available for pepper pest management, it is critical to make efficacious application of pesticides. Foliar spray coverage and spray retention in the middle and bottom of bell pepper canopies were compared using fluorescent dyes and food coloring, respectively. Several delivery systems were evaluated, including air-induction and twin-fan nozzles, air-assisted delivery with conventional hydraulic nozzles, and pneumatic atomization nozzles producing electrically charged sprays. Plant spacing between and within rows was evaluated in the second year of the application trial. Travel speeds of 6.4 and 12.9 km h-1 were also evaluated. Faster travel speeds did not significantly affect spray retention in twin-row canopies. The electrostatic sprayer produced the greatest differences in deposits between the middle and bottom of the canopy. Although there was no more than a 25 cm difference between leaves sampled from the middle and bottom canopy locations, spray retention on foliage in the bottom canopy locations had significantly lower retention than the middle canopy for almost all sprayer types. Despite differences in atomization characteristics, the performance of the twin-fan nozzle and the air-induction nozzle treatments were similar. Air-assisted delivery provided no advantage in the amount of spray retained on the foliage, but it produced more desirable spray quality on foliage and resulted in more spray retained on whole fruit.

Electrospray droplet sources for thin film deposition

Electrospraying utilises electrical forces for liquid atomisation. Droplets obtained by this method are highly charged to a fraction of the Rayleigh limit. The advantage of electrospraying is that the droplets can be extremely small, down to the order of 10’s nanometres, and the charge and size of the droplets can be controlled to some extent be electrical means. Motion of the charged droplets can be controlled by electric field. The deposition efficiency of the charged spray on an object is usually higher than that for uncharged droplets. Electrospray is, or potentially can be applied to many processes in industry and in scientific instruments manufacturing. The paper reviews electrospray methods and devices, including liquid metal ion sources, used for thin film deposition. This technique is applied in modern material technologies, microelectronics, micromachining, and nanotechnology.

Electrostatic interaction of free EHD jets

Three spray systems with multiple nozzles arranged linearly, of different geometrical configurations were tested experimentally: (1) spray nozzles without induction electrodes facing a distant plate, (2) spray nozzles with two parallel bar induction electrodes, and (3) spray nozzles with individual ring induction electrodes. The systems were set to operate at the precession mode of spraying. In all the systems, except that of individual induction electrodes, the precession mode of spraying was first set at peripheral nozzles, and next, for higher voltages, at central nozzles. In the system with ring induction electrodes, the precession mode of spraying was generated for all nozzles at the same voltage level. The size and charge distributions of the droplets were measured for each spray system. The charge distribution of the droplets was measured with a ring induction probe, and it was compared with the charge distribution determined numerically from the size distribution of the droplets, based on the Rayleigh limit formula. It was shown that both these distributions correlate quite well. Free droplets suspended and moving irregularly within the centre of the charged spray plume were observed in the system with individual rings.

Predictions of Charge Drift in a Concept Electrosprayed DISI Engine

Limited to nonvaporizing spray cases, this work discusses the transport of charged droplets within a cylinder of a motored axisymmetric model electrosprayed direct injection spark ignition (eDISI) engine with electrified walls. The concept engine investigated here is assumed to operate with an electrostatic atomizer previously studied for application in fuel burners [Yule et al., 1994, Fuel, 74(7), pp. 1094–1103]. A split/multiple injection strategy is employed in which three pulses of 5mg each are made at crank angles of 80, 150 and 300 deg ATDC of the intake, which fall within the intervals for stable combustion of either early or late injection modes of operation of DISI engines [Jackson et al., 1997, SAE Paper No. 970543]. The direct Simulation Monte Carlo (DSMC) approach embodied in an in-house CFD research code is used to simulate the discrete phase flow with the electrical charge distribution for different instants within the computational cells being computed by simple addition of the droplet charges residing in particular cells at particular instants of time. It is shown in the half engine cycle investigated that the use of charged sprays in eDISI engines may help to reduce the in-cylinder wall-wetting phenomenon. In addition, pockets of highest electrical charge are found to populate the region near the spark plug by 345 deg CA, which may be a path for improved combustion efficiency.

EFFICACY OF DIAMOND COMBINATIONS AGAINST TARNISHED PLANT BUG ON COTTON, 2005

Cottonseed were planted on a Commerce silt loam soil on 20 Jun. Plot size was four rows (centered on 40 inches) by 50 ft. Treatments were replicated four times in a RCB. Treatments were applied on 18 Aug with a high clearance sprayer and CO2 charged spray system calibrated to deliver 6 gpa through Teejet TX-8 hollow cone nozzles (2 per row). Treatment efficacy was determined by sampling the center two rows of each plot with a sweep net (25 sweeps/row) at 5 and 7 DAT. Data for each row were averaged and all data were subjected to ANOVA and means separated according to Fisher’s Protected LSD (P = 0.05). No rainfall occurred from the application date until the end of sampling. Plots were mechanically harvested using a spindle type harvester on 26 Oct. Yields were converted to lb seed cotton/acre.

SPRAY CHARACTERISTICS OF CHARGE INJECTION ELECTROSTATIC ATOMIZERS WITH SMALL-ORIFICE DIAMETERS

Here the characteristics of charged sprays of insulating liquids generated by an improved charge injection (electrostatic) atomizer design are described, and the experimental database previously available in the literature is extended to a smaller range of orifice diameters and to more viscous liquids. Previously identified “subcritical” and “supercritical” electrohydrodynamic (EHD) regimes for the atomizer are confirmed to be present for the viscosity and orifice diameter ranges studied here, showing that these EHD regimes appear to be generic to the atomization method. The jet breakup dynamics and length are qualitatively and quantitatively studied using imaging and phase Doppler anemometry (PDA), and the general spray plume characteristics are quantitatively described in terms of droplet velocity and diameter probability density functions (PDFs). Radial spray charge and mass flow rates are quantified as using a purpose-built collecting system. By appropriately normalizing the data, the degree of self-similarity between different spray data sets is clearly evident and proves that the near-axis droplets are poorly charged and that the mean specific charge increases with radial displacement, again in a self-similar manner.

Simulation of electrosprays in model direct-injection spark-ignition engine in-cylinder flows

The calculated effects on injected charged sprays of dielectric and conductive in-cylinder wall materials are presented for a half-cycle of an axisymmetric flow model direct-injection spark-ignition (DISI) engine. A plain orifice electrostatic atomizer, previously used in experiments for application in fuel burners, is embodied into the EPISO code and this is assumed to pump a charged spray while working at moderate pressures of 5 MPa and fuel deliveries of 5 cm3/s, the maximum rates currently reported in electrostatic atomization of hydrocarbons. The transition mode operation of DISI engines is selected for the study and this consists of multiple pulses of 5 mg each, occurring at 80, 150, and 300° crank angle with the engine running at 3000 r/min. In the case of the third pulse, which impinges on the piston surface, the wall impaction model of Park and Watkins is used particularly when an electric potential of 1 kV is applied on this surface for it helps to reduce excessive fuel film build-up. Particles impinging on the cylinder roof and liner are treated with the stick impaction model of Naber and Reitz. A simple axisymmetric engine geometry of flat piston and cylinder heads is considered and computations of an uncharged spray are taken as a reference case. It is found in the study, firstly, that charge improves mixture preparation when dielectric in-cylinder surfaces are used, secondly, that the need for charge drainage in metallic surfaces produces poor spray characteristics in comparison with an ideal charged spray with boundary electric fields (this is shown by the large impingement of drop parcels relative to an ordinary spray as well as to a charged spray with electric boundary fields) and, thirdly, that charge reduces the fuel film thickness on the piston surface during late injection.

The effect of ionisation of spray in cooling air on the wetting characteristics of finned tube heat exchanger

A major concern when introducing a water spray to pre-cool the air flowing into a finned tube heat exchanger is that wetting of the heat exchanger surface may cause corrosion. In an attempt to prevent droplets from impacting and wetting the heat exchanger, the use of an electrostatically charged spray was investigated. Experiments were performed whereby electrostatically charged spray was sprayed on a heat exchanger with an electric charge having the same polarity as the droplets. The results indicated that droplet deposition decreased significantly as the charge on the droplets was increased. However, total prevention of deposition could not be achieved, since the equipment used could not produce high enough voltages. This concept shows some promise, and it is recommended that further research be performed on it.

Emission limited electrostatic atomization and the fine structure constant

Quadrupole mass spectrometer measurements of charged Octoil sprays reveal that operation in what is interpreted to be the emission limited regime is accurately represented by Z=exp(ε0∕ε−rs∕f), where: Z is the fraction of the emission limited charge level, rs the coupling parameter, and f the inverse fine structure constant (137.04). The coupling parameter, defined as 1∕(a0√πn), where a0 is the first Bohr radius, and n the surface charge density (m−2), is a nondimensional measure of intercharge spacing, and a fundamental descriptor of two-dimensional (2D) electron structures. The appearance of the coupling parameter, and the fine structure constant as empirical correlating constants for the data implies that the surface 2D electrons are in a gaseous or liquid state. This is consistent with the view that the emission limit—Rayleigh limit transition reflects a phase change of the 2D surface electron cloud from a gas∕liquid to a crystalline state. For Octoil, the data indicate that this transition occurs when spray droplet radii are about 0.9μm. This corresponds to emission field strength of 0.13GV∕m. Bimodal droplet size distributions characterized by approximately fixed energy levels of one and two Rydbergs [1 Rydberg (Ry)=13.6eV], and noncontinuous charging behavior is observed. In all instances studied, surface charge energy levels are high, and are conveniently ordered in terms of the Rydberg.

Dielectric charged drop breakyup at sub-rayleigh limit conditions

The maximum charge a drop may hold, for an electrically isolated, electrically conducting drop, in vacuum, is defined by the Rayleigh Limit. For spray plumes of electrically charged drops this condition is clearly not met due to the space charge field. We would like to simulate such spray plumes and to simulate drop break up within them, using stochastic methods. Since many simulated particles are required a dynamic drop stability analysis is clearly not computationally feasible. Based upon a static analysis, and a thorough review of the previous experimental data on charged drop stability, it is shown that for dielectric drops in the presence of significant electric fields, and particularly those within spray plumes, the maximum charge a drop may hold is less than the Rayleigh Limit. Typical values of stable drop charge of 70-80% of the conducting drop Rayleigh Limit are predicted, and this is supported by a majority of recent experimental work. We present an explanation of the sub-Rayleigh Limit drop fission within charged spray plumes for dielectric drops, based upon a static, rather than a dynamic analysis. This permits sub-Rayleigh Limit drop fission to be incorporated into stochastic particle simulations.

ELECTROSTATIC EFFECTS ON AGRICULTURAL AIR-ATOMIZED SPRAYS AND DEPOSITION. PART II: A COMPUTATIONAL STUDY

This article reports on a computational investigation of inductively charged sprays used in crop spraying. The charged sprays are produced using an air-atomizing induction-charging nozzle. In complementary research, the spray structures are experimentally characterized based on drop size and velocity data obtained using laser-based techniques, both in the free spray and in the near-target vicinity. The experimental deposition study used spectrophotometry analysis to quantify the drop residue on a grounded target. The study reported on here comprises the computational simulation of free spray and deposition aspects of charged and uncharged sprays, and uses the data obtained in the parallel investigation to determine the ability of the computational model to accurately predict such sprays. Experimental test results demonstrate the dominance of hydrodynamic effects within the charged free sprays, whereas, in the vicinity of a grounded target, the sprays are influenced by the attractive electrostatic force fields created, leading to greater drop depositions on the target than for an uncharged spray. These trends are confirmed by the computational results. However, there are differences of detail. The model generally underpredicts the effects of the electric fields on the spray motion near the target. This is probably due to an incorrect mass flux profile at the injector, leading to too little mass and, hence, charge being predicted in the target vicinity.

Design and Development of a Computerised Instrumentation System for Spray Charge Measurement

Electrically charged sprays can provide greater control of droplet transport and can reduce spray wastage. In developing countries, the main lacuna in the development of electrostatic charging systems is the non-availability of relevant research facilities, particularly concerning instrumentation. A simple, easy to build instrumentation setup was developed using off-the shelf components and products available in Indian market. It consisted of a Faraday's cage to collect the spray charge, a charge-voltage converter circuit to convert the charge to a corresponding voltage and a computerized data logging system to record the spray charge level at continuous time intervals. The developed system was evaluated on an electrostatic charging attachment to a knapsack mist-blower. The system clearly showed that the chargemass ratio induced on the spray fluid increased linearly with charging voltage upto a critical value of 3.5 to 4.0 kyo It increased as the flow rate was decreased from 300ml min-i to 60 ml l min-i. The time-wise record of charging phenomenon helped to understand aspects such as electrode wetting, shorting and electrode loading patterns.

The effect of space charge on the performance of an electrostatic induction charging spray nozzle

In an electrostatic induction-charging spray nozzle, it is generally believed that the space charge formed by the charged spray cloud suppresses the nozzle's internal induction-charging field, causing the space charge cloud current to reduce as the nozzle-to-target distance increases. Experiments reported here show that the space charge does not affect the total current induced upon the liquid; rather, it affects the distribution between the space charge cloud current and the electrode leakage current. The main reason for this reduction of the space cloud current is that a small number of highly charged droplets escape the air entrainment by the repulsion of the space charge field and are repelled radially or are deposited on the nozzle's external surface, adding to the electrode leakage current. The stronger the space charge field, the more droplets are repelled to the nozzle's surface. The result of this “feedback” phenomenon is that under constant atomizing air pressure and within a certain voltage range, the space charge effect makes the target current to be constant at a certain distance away from the nozzle, irrespective of the electrode voltage. Raising the electrode voltage will not increase the target current.