Wind Ions Research Articles

Ionic-Wind-Enhanced Raman Spectroscopy without Enhancement Substrates.

Raman spectra are often masked by strong fluorescence, which severely hinders the applications of Raman spectroscopy. Herein, for the first time, we report ionic-wind-enhanced Raman spectroscopy (IWERS) incorporated with photobleaching (PB) as a noninvasive approach to detect fluorescent and vulnerable samples without a substrate. In this study, ionic wind (IW) generated by needle-net electrodes transfers charges to the sample surface in air on the scale of millimeters rather than nanometers in surface-enhanced Raman spectroscopy. Density functional theory calculations reveal that the ionic particles in IW increase the susceptibility of the sample molecules, thus enhancing the Raman signals. Meanwhile, the incorporation of IW with PB yields a synergistic effect to quench fluorescence. Therefore, this approach can improve the signal-to-noise ratio of Raman peaks up to three times higher than that with only PB. At the same time, IWERS can avoid sample pollution and destruction without substrates as well as high laser power. For archeological samples and a red rock as an analogue to Mars geological samples, IWERS successfully identified weak but key Raman peaks, which were masked by strong florescence. It suggests that IWERS is a promising tool for characterizations in the fields of archeology, planetary science, biomedicine, and soft matter.

Ionic Wind Driven Fog Collection in Windless Environment

Fog collection is a promising technique to harvest water from the atmosphere. However, the fog collection in the windless environment is still a challenge. An ionic wind-driven fog collector was numerically designed. Numerical results demonstrated that the discharge electrode location significantly influenced the ionic wind flow pattern and fog collection rate. The fog collection rate corresponding to a nearly unidirectional flow pattern was several times of that corresponding to the vortices dominant flow pattern. An enhancement of 6.7 times in the fog collection rate was realized by increasing the input power. The fog collection rate increased linearly with the liquid water content under a fixed power consumption. A fog collector prototype was fabricated. The measured ionic wind velocity and fog collection rate supported the numerical design. In a windless foggy environment with a liquid water content of 0.62 g m–3, a collection rate of 0.46 L h–1 was achieved.

Model of Multistaged Ducted Thrusters for High-Thrust-Density Electroaerodynamic Propulsion

Electroaerodynamic (EAD) thrusters are a means of producing a propulsive force in air that does not require any moving parts and is nearly silent. In these devices, ions generated from atmospheric air are accelerated by an electric field across two electrodes at different potentials, resulting in an ionic wind and a thrust force. It has been demonstrated that EAD is a feasible form of aircraft propulsion; however, substantial performance improvements are needed for practical applications. Here, multistaged ducted (MSD) EAD thrusters, which have the potential to provide higher thrust density than previously demonstrated, are proposed and modeled. An MSD thruster contains multiple sets of electrode pairs in series, enclosed in a duct and fitted with an inlet and a nozzle. One-dimensional momentum theory is combined with models for two limiting cases for the pressure generated by each stage: ideal one-dimensional EAD stages and wire-to-airfoil corona-discharge stages. The model evaluates how geometric and electrical parameters affect the performance of MSD thrusters under both sets of assumptions. If pressure losses per stage are kept small, the results show that MSD thrusters can provide order-of-magnitude improvements in thrust density and efficiency as compared to single-stage thrusters, potentially broadening the type of missions that can be performed by EAD thrusters.

The effect of electrically pre-stressing of high DC electric field on performance of SDBD thruster in near space

The space charge on the surface of solid dielectric has great influence on the surface dielectric barrier discharge (SDBD). In this paper, the surface space charge of SDBD is effectively regulated by the effect of electrically pre-stressing of high DC electric field to improve the dynamic performance of ion wind thruster. The conduction current, surface charges, thrust and thrust-to-power ratio are measured respectively for the ion wind thruster with and without the effect of electrically pre-stressing. The experimental results show that compared with the ion wind thruster without the effect of electrically pre-stressing, the instantaneous polarization current of the SDBD ion wind thruster with the effect of electrically pre-stressing is reduced by 42.7%, and the attenuation constant of the relaxation polarization current is reduced by about 28.8%, which effectively reduces the relaxation polarization loss of the thruster under the alternating electric field. In addition, the discharge time of anode and cathode increases respectively by about 20.0% and 43.3%, and the variation of surface space charge increases by about 50.6% in one voltage cycle. The saturation thrust of the thruster is increased from 1070 μN/m to 1660 μN/m by about 55.1%. The maximum thrust-to-power ratio of the thruster is increased from 993 μN/kW to 2125 μN/kW by about 114.0%.

Experimental and numerical analyses of electrohydrodynamic force according to air pressure

Electrohydrodynamic (EHD) force produced by corona discharge is considered as a new thrust for solar-powered aircraft and stratosphere balloons in near space. However, its performance at low air pressures remains to be clarified. An experiment of measuring the EHD force at 0.02 atm–1.0 atm (1 atm = 1.01325×105 Pa) is carried out with the wire-to-cylinder geometric structure. The ion distribution is analyzed by using the drift-diffusion model with two-dimensional numerical simulation. The experimental result shows that the EHD force is not linearly related to the corona discharge current at low air pressures. Numerical simulation finds that the proportion of ions in the counter-direction electric field increases from approximately 0.36% to 30% when the pressure drops from 1.0 atm to 0.2 atm. As a result, the EHD force with a constant power supply drops faster than the previous theoretical prediction in the ground experiment, suggesting that the consideration of counter-direction EHD force is necessary to improve the ionic wind propulsion efficiency in near-space applications.

Simulation of Current Pulses and Sound Waves Resulting from Partial Discharges in a Needle-Plane Geometry in Air

Partial discharges can occur in different types of electric equipment and cause progressive insulation deterioration, so there is interest in monitoring partial discharges for assessing the state of the isolation of an electric system and to predict failures. Techniques to detect partial discharges, such as detecting ultrasonic emission, have been proposed in the literature, but as various effects can co-occur during a discharge, identifying characteristics in sound and pressure waveforms and correlating then with discharge characteristics is difficult. Simulations can assist in these correlations by allowing the isolation of the different phenomena. In this work the drift diffusion model, including photoionization, is coupled with the linearized compressible Navier-Stokes equations to simulate ultrasonic waves produced by partial discharges. Previous works have used the incompressible Navier-Stokes equations, so they can simulate ionic wind produced by the Trichel pulses, but no sound. In the literature, simulations have focused either on streamers or Trichel pulses. In this work both discharges and produced sound waves are successfully simulated for the needle-plane geometry in air. The electric current and charge per pulse are compared with experimental results reported in the literature for the same discharge conditions. The simulations have demonstrated that the sound waves depart from the electrode tip for the Trichel pulses, and for the streamers two sound waves are produced, one from the electrode tip and the other from the whole discharge length. Differences in the wave front with respect to the relative position to the electrode tip were analyzed, showing that near the discharge spot the sound wave is not a spherical wavefront. The sound wave for one of the discharges in the Trichel pulse regime was compared with experimental results in the literature. Results are in good agreement with the experimental data found in the literature. Both current and sound waves were successfully predicted and correlated with the discharge, results that can be used to help in the detection of partial discharges.

Experimental mapping of the ozone distribution in a pulsed positive corona discharge to estimate the efficiency of ozone production

The spatial and temporal evolution of the ozone density has been measured in a 1 cm electrode gap tip to plane positive pulsed corona discharge in air at atmospheric pressure with no gas flow. It was expected to observe a higher ozone density in the vicinity of the discharge gap but ozone has been found at a quasi uniform density level up to 10 cm from the electrode gap after only 30 s of ignition. Such a wide distribution of ozone in the reactor cell is attributed to strong turbulence mostly triggered by the ionic wind. The mapping of the ozone density associated to the power measurement led to the estimation of the ozone production yield in term of mass per electrical energy.

Experimental research of capture enhancement mechanism of submicron particles by designing two-stage electrostatic precipitators with various ratios of charger and collector units

In this paper, six kinds of two-stage ESPs with various ratios of charger and collector units are designed to research the capture enhancement mechanism of submicron particles. The working conditions are optimized by studying the influence of gas velocity, charger current, and electric field strength of collector. Results show that the electrostatic field characteristics of the two-stage ESP with Ra = 1/6 is beneficial to improve the collection efficiency while slightly less stable. The indicator factor of particle charging enhancement effect of two-stage ESP with Ra = 6/1 fluctuates between 0.06 and 0.1, which suggests that the two-stage ESP with a higher proportion of charger units cannot achieve the satisfying performance for the limitation of ionic wind effect. Under comprehensive consideration of ionic wind effect and ozone emission, the two-stage ESP with Ra = 2/5 is selected at the first priority by exerting the advantages of fully particle charging and enough collection space, which has a strong potential in both particle charging enhancement and electric field enhancement. However, for improving the collection efficiency of submicron particle with a size ranging from 0.2 to 0.5 µm which suffers from both of weak particle charging and agglomeration effect, the two-stage ESP with Ra = 4/3 is not recommended.

The Effect of SiC-MOSFET Characteristics on the Performance of Dielectric Barrier Discharge Plasma Actuators with Two-Stroke Charge Cycle Operation

The low-voltage operation of a dielectric-barrier-discharge (DBD) plasma actuator with a simple electric circuit has the potential to put it into industrial applications. However, there is an issue that the efficiency of the low-voltage operated DBD plasma actuator is lower than that of the high-voltage operated one. In this study, the characteristics of silicon carbide (SiC) metal-oxide-semiconductor field-effect transistors (MOSFETs), which are used in the electric circuit, are investigated with a focus on the on-state resistance. The on-state resistance of the SiC-MOSFET affects the rise time of the applied voltage in our experimental condition. The energy consumption by applying a pulse voltage to the DBD plasma actuator increases with increasing the on-state resistance. Flow visualization with particle image velocimetry measurement reveals that a DBD plasma actuator with the SiC-MOSFET whose on-state resistance is the lowest induces the highest velocity of the ionic wind. Also, low on-state resistance is preferable in terms of the thrust-to-power ratio. These findings contribute to the development of an optimal power supply for DBD plasma actuators for industrial applications.

Numerical Simulation of Ethanol Air Diffusion Flame Quenching under Transverse AC Electric Field

The electric field fire extinguishing technology is an efficient, clean, and new fire extinguishing technology that can be operated at a long distance. In order to study the synergistic mechanism of “electric-flow-heat” in the process of transverse AC electric field fire extinguishing, the ionic wind formed by the influence of electric field on each charged particle during the burning process of ethanol–air diffusion flame is simulated by the non-premixed combustion model, and the experimental phenomenon of flame quenching in the transverse AC electric field is reproduced by means of numerical simulation. The accuracy of the numerical model was verified by comparing the temperature and flow velocity in the region obtained from the simulation with the data measured in the experiment. According to both simulated and experimental phenomena, we present a hypothesis of how the flame is quenched under the influence of an electric field. The next research directions are: (1) improving the accuracy of numerical simulation by building fine models; (2) studying the dynamic mechanism of real flames by particle image velocimetry technology.

Toroidal counter electrode for ionic propulsion

Significant attention has recently been given to applications of ionic wind to atmospheric propulsion. Rotational ionic engines (RIE) have also demonstrated to have potential for in-atmosphere propulsion in negative polarity. However, such devices have not yet produced enough thrust for a rotary ionic drone to be developed. We demonstrate here that a toroidal counter electrode can increase the RIE's performance by up to 7.8 times greater than in previous configurations (upper limit not determined). The RIE is designed with pin emitters extended on the trailing edge of a 12.6 cm two-blade plastic propeller placed above a toroidal counter-electrode which provided axial thrust up to 288.55 m Nat 23.15 N/m2, 4.2 m/s bulk airflow speed within the propeller plane, and 251 m3/h flow rate. The new design generates axial thrust due to the linear acceleration of ions between electrodes, and also due to the induced rotary motion of the propeller which captures the energy and momentum of ions accelerated in the propeller rotational plane. Thrust to power ratio can be measured by the ratio of voltage to current or propeller kinetic energy to power. A 4-RIE array matched the thrust (1 N) of a four-blade drone with similar blade size.

Effect of the high voltage waveform on the ionic wind produced by a needle-to-plate dielectric barrier discharge

Although corona discharges are used in many industrial applications because of their ability to produce chemical species, the ionic wind they induce is less known and often ignored. Therefore, the present study aims at investigating the ionic wind produced by a corona discharge ignited between a high voltage needle and a grounded plate electrode covered by a dielectric material. More specifically, the work focuses on the influence of the high voltage waveform on the temporal behavior of the ionic wind. The results highlight that the high voltage waveform plays a key role on the dynamics of the flow produced inside the discharge. On the one hand, for the sine, triangle and sawtooth waveforms, there is a flow acceleration during both the positive and the negative half-cycles, the positive discharge being more effective in velocity production. On the other hand, for the square waveform, the increase in velocity occurs during the rises and falls of the voltage, because of the strengthening of the electric field due to the ions remaining from the previous half-cycle at the wall of the dielectric material.

Electrohydrodynamic drying of probiotics

Drying and moisture content is known to influence long-term storage stability of probiotics. In this study, electrohydrodynamic drying (EHD) of probiotics using ionic wind was investigated. The EHD drying process was found to be dependent on several parameters such as polarity and voltage of the electric field, collector type, surrounding temperature and gas, and use of excipient. By using a moving mesh flat electrode collector to facilitate the airflow during EHD processing, the water evaporation and cell viability were 76% and 61%, respectively. Moreover, comparison of the EHD drying process with the freeze-drying, reveals that the survival of probiotics and the water evaporation rates were similar, as well as the surface properties of the cells (zeta potential and hydrophobicity).

Ionic wind of a needle-to-plate dielectric barrier discharge

The present study aims at investigating the ionic wind produced by a dielectric barrier discharge ignited between a needle and a grounded plate electrode covered by a dielectric material. The needle is powered by a sine high voltage vAC(t) and the electrode gap is equal to 15 mm. Preliminary measurements of the discharge current highlighted that two different discharges take place during one voltage cycle. During the positive going-cycle, there is a positive corona discharge composed of streamers, their frequency (close to 5 kHz) and magnitude (a few mA) increasing with the magnitude of the applied voltage. During the negative-going cycle, a negative corona discharge occurs, its current highlighting the presence of Trichel pulses whose the frequency varies between 100 kHz and 500 kHz.Then we investigated the ionic wind produced by the DBD with the help of a 20 kHz PIV system (particle image velocimetry) in order to access to time-averaged and time-resolved data. First, we charaterized the effect of the magnitude of vAC(t) and its frequency value fAC on the time-averaged ionic wind. Secondly, we investigated the temporal behavior of the ionic wind as a funtion of the high voltage frequency. Two main behaviors have been highlighted. At low frequency (up to 200 Hz), the sine high voltage results in a pulsed ionic wind, with a velocity peak at each half cycle: there is a first flow acceleration due to the positive corona discharge and a second one during the negative corona, the velocity during the positive discharge being higher than the one measured during the negative discharge. At higher frequency (from 500 Hz up to 1000 Hz), the positive ionic wind takes over the negative one and the velocity peak due to the negative discharge is very weak. At 2000 Hz, there is only one single flow acceleration per voltage cycle, because the ionic wind produced by the positive corona discharge takes totally over the ionic wind produced by the negative corona because it is more intense and faster.

Mitigating reverse emission in electroaerodynamic thrusters

Electroaerodynamic (EAD) thrusters have the potential to become a quieter and solid-state alternative to propellers in unmanned aircraft. An EAD thruster produces ions from atmospheric air and accelerates them across an electric field between two electrodes: an emitter, where ions are produced; and a collector, where they are neutralized. As ions drift from the emitter to the collector, they collide with neutral air molecules and transfer momentum to them, resulting in an ionic wind and a thrust force. Reverse emission from the collector is a major non-ideal effect in EAD systems that can reduce the efficiency of EAD thrusters: it increases the power draw of the device and eventually leads to electrical breakdown and sparking. Here, we perform a parametric study with a wire-to-cylinder electrode geometry using positive and negative corona discharges as well as wire-to-wire dielectric barrier discharges as ion sources. We show that reverse ion emission is primarily caused by a gas discharge at the tips of the cylindrical collectors, which increases current without contributing to thrust and produces audible noise. We identify three ways to mitigate reverse emission: increasing the collector diameter, increasing the collector span with respect to the emitter span, and attaching hemispherical end caps to the collector. By mitigating reverse emission, we show that the power draw of an EAD thruster can be significantly reduced (by up to 64% in our particular configuration) while still delivering the same thrust.

Negative corona discharge and flow characteristics of a two-stage needle-to-ring configuration ionic wind pump for temperature and relative humidity

Ionic wind pumps have the advantages of a compact structure, no moving parts, and low power consumption. The effects of temperature and humidity on the current–voltage and flow characteristics of ionic wind pumps are still unclear. This study experimentally investigates the effects of temperature and humidity on the breakdown voltage, discharge current, and velocity of a two-stage needle-to-ring type ionic wind pump with a negative corona discharge. It is found that the coupling effects of temperature, humidity, and water vapor condensation determine the current–voltage and flow characteristics of ionic wind pumps. The temperature and vapor condensation effects can enhance the discharge current and outlet velocity and weaken the breakdown voltage. However, the humidity effect can weaken the discharge current and outlet velocity, and improve the breakdown voltage. Moreover, there exists a competition between humidity and vapor condensation effects on breakdown voltage, current, and velocity. The increase in temperature intensifies the humidity effect and weakens the condensation effect; thus, the temperature changes the contribution of humidity and condensation effects in the competition. A working spectrum of ionic wind is provided in the temperature and humidity coupling environment, including a vapor condensation-controlled zone, a sensitive humidity zone for breakdown voltage and velocity, and an insensitive humidity zone for breakdown voltage and velocity. The spectrum can provide theoretical guidance for designing ionic wind pumps at different temperatures and humidity.

Fluid flow and heat transfer characteristics of corona wind in cross-cut finned channel

An efficient heat exchanger should exhibit superior heat transfer performance with low pressure drop. However, in the case of forced convection such as fan cooling, the heat transfer performance is proportional to the area of the extended surface and the airflow rate, which accompanies a pressure drop and imposes a limit on performance improvement. Herein, flow acceleration using corona wind is proposed as a method to simultaneously solve the problem of pressure drop in heat exchangers and improve heat transfer performance. Further, the concept of accelerating the flow of local regions around the surface by installing a wire electrode on the fins of a heat exchanger for corona wind generation is implemented. In addition, an original form of heat exchange fins that combine wire electrodes and cross-cut fins is presented. By controlling the flow velocity direction of the corona wind in the downstream direction of the mean stream, the pressure drop is suppressed and maximize the heat-transfer performance. Detailed experimental data on the velocity and temperature of the corona wind from the wire to the cross-cut fins with various wire materials, positions, and dimensionless design parameters in the channel are presented. In addition, the flow patterns of the accelerated corona wind are analyzed using particle image velocimetry on the cross-cut fins. The results confirm that the corona wind can be applied to a cross-cut fin array and can significantly enhance the heat-transfer performance of the fins.

Influence of needle electrode arrangement on gas transportation performance of the multi-needle ionic wind pump

The ionic wind pump with multi-electrode arrangement is helpful in enlarging its application. However, the arrangement has an inherent mutual interference among the electrodes during discharging process, which have a vital influence on the transportation performance of the ionic wind pump. Meanwhile, the relevance between the mutual interference and the intensity of the ionic wind has not been clearly revealed. In this study, the gas transportation experiments of the multi-needle electrode ionic wind pump are conducted. The intensity variation of the ionic wind with mutual interference is observed in the experiments. A simulation model based on the plasma transportation is proposed to describe the multi-needle discharging process with mutual interference effect. The EHD force is used as the indicator parameter for the intensity of the ionic wind. The experimental results show that the measured mass flow rate of the ionic wind pump is enhanced under dual-row needles and weakened under triple-row needles. Based on the simulation results, the above phenomenon is attributed to variations of the space charge and electric field induced by multi-needle discharging. The added needle electrodes reduce the electric field intensity per needle while increasing the net negative charge in the space. Due to the discharging interference among the needles, the increment of the net negative charge is not proportional to the number of needles. As a result, comparing the three arrangements from the simulations, the electrohydrodynamic (EHD) force of each needle, which is the electric field intensity multiplying by space charge density, increases approximately 45.8% under dual needles and decreases approximately 38.1% under triple needles. The conclusions obtained in this paper can help to guide the design and optimization of the multi-needle ionic wind pump.

Phase Mixing and the 1/f Spectrum in the Solar Wind

The origin and evolution of the 1/f power law observed in the energy spectrum of solar coronal and solar wind fluctuations at scales of around an hour is not entirely understood. Several existing theories aim at explaining it, involving both linear and nonlinear mechanisms. An often overlooked property of the solar corona and solar wind is their highly inhomogeneous nature. In this paper we investigate the linear evolution of pure Alfvén and surface Alfvén waves propagating through a plasma that is inhomogeneous across the magnetic field. The inhomogeneity is given by density, which we model to be 2D colored noise, with power spectral slopes ranging from −2 to −1. Alfvén waves propagate independently on individual magnetic field lines, and eventually get completely out of phase through the process of phase mixing, leading to unrealistic spectra. When the coupling between the inhomogeneous background and the propagating waves is fully accounted for, transverse waves such as surface Alfvén waves (also referred to as kink or Alfvénic) appear, showing collective wave behavior of neighboring magnetic field lines with different Alfvén speeds. We show that the linear cascade of surface Alfvén wave energy, induced by phase mixing and resonant absorption, leads to a perpendicular wave energy spectrum, which tends to the perpendicular power spectrum of the background density. Based on our model, we propose that a perpendicular density power spectrum of 1/f in the solar corona can induce, through linear processes, the 1/f spectrum of the fluctuations that is observed at the largest scales.

Local hotspot thermal management improved by ionic wind generator coupled with porous materials

Ionic wind is induced by corona discharge caused by extremely inhomogeneous electric field between two electrodes. Due to its compact size, energy saving, flexible design, and easy integration, ionic wind technique exhibits great potential to improve hotspot cooling, especially for compact electronic devices. However, the heat removal capability towards high power density is relatively low due to the limited heat transfer area and low air thermal conductivity. The principle and designs are still lacking for local hotspot cooling. Here, an ionic wind generator with a novel architecture coupled with porous medium is conceived to enhance cooling performance. It is demonstrated that when a porous medium block is coupled, heat transfer can be significantly boosted due to the modified heat and mass transfer in the porous block. It is also found that electrode configuration significantly influences the device performance. There is a trade-off between heat transfer area and airflow impediment in the device. The geometry and property parameters of the porous block are critical for the cooling performance, which is attributed to the modified flow field by changing porous structure. Based on this principle, another ionic wind generator improved by a coupled heat sink with porous fins is proposed, whose performance can be further enhanced by optimizing its design parameters. The optimized heat sink structure can maintain the max temperature of the cooling surface below 83.2 °C at a heat flux of 90 kW/m2. This study provides a promising way for air-side thermal management of hotspot. The findings are helpful for understanding EHD-induced airflow heat transfer in porous medium and shed new light on designing ionic wind generators with better performance.