Discharge current of water electrospray with electrical conductivity under high-voltage and high-flow-rate conditions

Highlights

• Water electrospray is studied under high-flow-rate and high-voltage conditions.

• Spray shape determined by visualizing spray trajectory and corona discharge.

• Discharge characteristics according to appearance of Trichel pulse in waveforms.

• Analysis of optimal operating conditions with power consumption in a single needle.

Abstract

Recently, several attempts have been made to implement electrospray for electrostatic precipitators (EPs) to reduce water consumption and prevent dust from being rescattered from dry EPs. In particular, the electrospray droplet size in EPs should be sufficiently large to collect dust in the exhaust gas flow, which requires a considerably larger electrospray flow rate than that required for typical applications. In addition, in the case of water electrospray, a high supply voltage is required because of its high surface tension. In this study, the characteristics of water-based electrospray were investigated under a high voltage and high flow rate. Three types of fluids—distilled water, tap water, and saline solution—were used to test for different conductivities under an electric charge with a negative current. The corona discharge and spray trajectories were visualized to determine the spray stability. To verify the influence of the spray current, the waveform of the electrospray was also analyzed under both dry- and wet-spray conditions. From this comparison, a portion of the spray current was estimated out of the total discharge current, which is the sum of the corona discharge and the spray current. The flow rate and voltage of the test were limited at 20 mL/min and −50 kV, respectively. In contrast with previous low flow rate studies, our results showed that the discharge current increased rapidly under flow rates above 6 mL/min, leading to arc discharge. Thus, it is important to adjust the flow and conductivity to guarantee stable and efficient water-based electrosprays.

Introduction

Electrospray or electrohydrodynamic atomization (EHDA) is the process of atomizing a liquid by applying a high voltage to the liquid. It is a convenient method for dispersing the liquid into small charged droplets with a relatively small electric force. Therefore, this method been utilized in various applications, such as for the fabrication of ion sources for mass spectrometry and in surface coating. A variety of previous studies including up-to-date ones have been attempted to identify a range of stable operating cone-jet modes for EHDA [1], [2], [3], [4]. For the detailed application of electrospray at the cone-jet mode, Wang et al. [4] summarized various field like were introduced for bio-molecules analysis, paint and print spraying, fuel injection, film coating, space propulsion, electro-spinning, and so on. These researchers observed the spray shapes and established formulas for calculating the droplet size, current, and voltage in that mode. However, they are limited in that only very low flow rates have been analyzed. In addition, they focused on submicron-sized droplets and flow-rate ranges of microliters per hour to at most a few microliters per minute. In this study, electrospray was applied in an electrostatic precipitator (EP) to remove dust. For this application, the water droplets should be sufficiently large to capture dust particles. Thus, the size of the droplets should be in the order of millimeters, and the flow rate should be in milliliters per minute. Various operating conditions exist under which the meniscus appears in a long jet form and not as a cone, thus the cone-jet mode is not observed. Under these high-flow-rate conditions, spray forms that do not appear under low flow rate conditions are observed. When the spray forms change, other properties such as discharge currents or droplet sizes are altered. However, very little research has been conducted under high-flow-rate conditions, and the conditions that have been studied involve flow rates that are less than 5 mL/min. Therefore, it is necessary to study the spray shapes or characteristics of electrospray under high-flow-rate conditions.

Thus far, electrosprays have been investigated only at voltages below the corona onset voltage because the corona discharge impinges on the cone-jet mode, thereby making the spray unstable. However, the original principle of EPs is based on corona discharge to enhance electric adhesion of dust. And electrosprays that nebulize the charged droplets are used in this study to improve the dust collection efficiency. Therefore, the characteristics of electrospray with corona discharge need to be studied. Additionally, in order to use water as an operating fluid of the electrospray, a high voltage is required to atomize water because of its high surface tension. For the same reason, water electrospray has not been a primarily interest due to corona discharge. Another important reason why the electrospray of water has not been studied was a lack of its application. However, water has a great potential to be applied in various fields because of its usefulness of environmental- and bio-friendly properties as well as its cost effectiveness. To expand the scope that has limitations in previous studies, the effects of flow rate and electrical conductivity on spray forms and discharge characteristics under high-flow-rate conditions of water electrospray were investigated extensively.

In 1917, Zeleny [5] observed the characteristics of oscillations of the meniscus at the end of a capillary or needle. The steady-state mode, after being rediscovered by Vonnegut and Neubauer [6] in 1952, has attracted the attention of various authors. Taylor [7] was the first to theoretically demonstrate the formation of a conical meniscus. The conical form of a meniscus represents the force balance at a liquid’s surface under the electric field, surface tension, and hydrostatic pressure. He found that the condition of an equipotential surface was a half angle of a cone, which is 49.3°, and designated this as a “Taylor cone.”

Cloupeau and Prunet-Foch [8] defined the “cone-jet mode,” which simply denotes the aspect of the meniscus and jet, in 1989; this expression has often been used since it was coined. In 1994, Cloupeau and Prunet-Foch [9] classified the modes of various applied voltages at atmospheric pressure, such as dripping, spindle, cone-jet, and multi-jet. Ragucci et al. [10] presented electrospray modes and stability regimes with electric voltage, flow rate, initial jet velocity, and Weber number changes. In recent years, various authors have attempted to visualize electrosprays for mode classification. These studies have used techniques such as short-time snapshot methods using stroboscopic techniques [11], [12] and time-resolved methods using high-speed cameras [13].

By visualizing an electrospray, Faraji et al. [14] observed the cone shape as the conductivity of solution (distilled water and KCl) was increased. The authors identified that increasing the solution conductivity elongated the cone further and reduced the velocity of droplets. Another study based on electrical conductivity was conducted by Yazdekhasti et al. [15], who studied the diameter of droplets and generated EHD modes of electrospray based on a dimensionless number of diameter and electric charge. They observed no significant effects on modes of the fluids but obtained an equation for dimensionless droplet diameter.

Smith [16] studied the effects of liquid properties, particularly conductivity, surface tension, and viscosity, and showed that a conductivity >0.1 S/m produces droplets that cannot be visually observed because their light-scattering is very low. The cone-jet mode cannot be obtained if the surface tension of a liquid is >0.05 N/m. Furthermore, liquids with surface tensions of >0.073 N/m, such as distilled water, cannot display the cone-jet mode in air because the atomizing onset voltage is higher than the corona onset voltage. Corona discharge is known to deform the cone-jet mode; thus, most studies have been conducted at voltages under the corona onset voltage.

However, recently, electrospray has been used in various applications for other operation modes, such as for fuel atomization, in dust removal, for agricultural treatment, and in medical fields. In this study, we aim to remove dust, particularly the Particulate Matter (PM) of exhaust gas, from an EP. This is based on the principle that charged water droplets capture fine dust with electrical attraction, become neutralized, and then, move to the discharge plate. As described earlier in the text, a very large flow rate is required for dust collection compared to previous studies (up to 200 times). Atomizing the water and using high flow rates of fluid require a relatively high voltage. Under these conditions, the electrospray has an atypical shape, implying that the cone-jet mode cannot be observed and corona discharge is always observed.

Water electrospray has been investigated in several studies [11], [12], [17], [18], [19]. Jaworek and Krupa [11] used the geometrical forms of the meniscus or jet, droplet size, droplet charge, and volumetric flow rates as distinguishing criteria to classify spraying modes. Park et al. [12] attempted to classify the EHD spraying modes of water in air at atmospheric pressure using dimensionless numbers representing various criteria such as electrical conductivity, surface tension, flow rate, and voltage. Cho et al. [17] described the modes of water electrospray using an experimental approach featuring a horizontal nozzle under operating conditions of high flow rate and high voltage.

Jaworek and Krupa [18] imaged the corona discharge generated near the surface of the water jet, identified the current–voltage characteristics of electrospray accompanied by corona discharge, and compared these characteristics with those of a dry capillary. Lopez-Herrera et al. [19] conducted experiments with water in air at atmospheric pressure to measure the average current emitted from a liquid meniscus by spraying modes (including the cone-jet mode) with glow discharge (positive voltage). They also explored how the characteristics of current varied over time. Mora and Loscertales [20] defined “electrospray current” as the charge streaming from the Taylor cone tip per unit time and measured the electrospray current based on five variables—liquid, surrounding gas, geometric properties, liquid flow rate, and the difference in voltage between the electrodes. They attempted to derive an experimental equation for electrospray current applicable to cone-jet spray.

For application in EP, dust collection efficiency, discharge current, and power consumption are more important than the operation mode. These previous studies were limited to the cone-jet mode and featured either nanoscale or microscale conditions. The results and equations of these previous studies therefore cannot be used for this study. No study has focused on a water electrospray with corona discharge at high-flow-rate, high-voltage, and high-liquid-conductivity conditions. In this study, the shape of the spray and corona discharge were observed using a camera, and the discharge current characteristics of water electrospray were experimentally identified under high-flow-rate and high-voltage conditions and various conductivities of the liquid.