Electrohydrodynamic Secondary Flow Research Articles

Experimental investigation on convective heat transfer enhancement by EHD in wire to plate configuration

The purpose of this study was to experimentally investigate the enhancement of forced convection heat transfer by corona wind in a channel scaled to potential future drying systems. An experimental rig was designed to investigate the thermal and dynamic effects of electro-hydrodynamic (EHD) induced air flow on the grounded bottom wall. The primary air flow, blown into the channel at controlled temperature and velocity, was disturbed by EHD secondary flow generated by a wire electrode. Using IR imaging, the heat transfer exchange at the wall was calculated in several configurations (i.e. electrodes location, wire-electrode to plate distance, applied voltage, primary air flow velocity), allowing the selection of optimal configurations according to the objective of minimum energy consumption or homogeneous transfer. Particle Image Velocimetry (PIV) characterization conducted on some configurations highlighted the vortices generated by the combination of the secondary and the primary air flows. Analysis of the flow patterns allowed to explain the enhancement of the local and average heat transfer coefficients. Higher enhancement could be obtained in the cross-wire configuration but led to heterogeneous treatment. For a more homogeneous one, the longitudinal wire configuration should be preferred thanks to the development of contra-rotating vortices moving in the channel.

The Influence of Electrohydrodynamic Secondary Flow on the Collection Efficiency and Deposition Pattern in ESP

Electrostatic precipitator (ESP) is widely used for dust removal from flue gas in industry. In the electrostatic precipitation, the electrohydrodynamic secondary flow (EHD) produced by corona ionization has an important influence on the characteristics of particle transport and the collection efficiency of ESP. In this work, a comprehensive ESP model with interaction of multiple physical fields is established to study the EHD effect in ESP. The numerical results show that the EHD generally can increase the streamwise velocity of airflow near the collection plate, which makes the removal performance of ESP worsen. Meanwhile, the EHD has a significant effect on the particle deposition pattern, especially at lower flue gas velocity. When the needle tip of discharge electrode points to the collection plate, the EHD can promote the circulation of airflow near the corona wire, increase the probability of particle charging, and then improve the collection efficiency of ESP.

Numerical study on electrohydrodynamic flow and fine-particle collection efficiency in a spike electrode-plate electrostatic precipitator

In this work, a numerical study on the electrohydrodynamic secondary flow and transport of fine particles has been carried out based on spike electrode-plate electrostatic precipitator (ESP). The effect of the inlet flow velocity, negatively applied voltage, and position of spike tip on airflow streamlines distribution are analysed in detail. The electrohydrodynamic (EHD) secondary flow patterns and particle collection efficiencies are examined for three different corona discharge electrode configurations, the spike tip locates at i) upstream, ii) downstream and iii) at both sides of the channel. The results show a complex and turbulent flow structure in the ESP with the spiked electrode. A pair of vortices is formed in the vicinity of the spiked electrode with the tip of the upstream-directed spikes, driving the main airflow toward the collecting plates and improving the collection efficiency of particles. Compared to the EHD secondary flow streamlines in the spike-plate type ESP one can deduce that the stronger vortices occur at higher Ehd/Re2 ratios. Furthermore, the distribution of deposited particles with the aforementioned spike tip locations reflected the real condition of particle deposition. The results help to improve the structural optimization of the spike-type ESP.

Experimental Study on EHD Flow Transition in a Small Scale Wire-plate ESP

Abstract The electrohydrodynamic (EHD) flow induced by the corona discharge was experimentally investigated in an electrostatic precipitator (ESP). The ESP was a narrow horizontal Plexiglas box (1300 mm×60 mm×60 mm). The electrode set consisted of a single wire discharge electrode and two collecting aluminum plate electrodes. Particle Image Velocimetry (PIV) method was used to visualize the EHD flow characteristics inside the ESP seeded with fine oil droplets. The influence of applied voltage (from 8 kV to 10 kV) and primary gas flow (0.15 m/s, 0.2 m/s, 0.4 m/s) on the EHD flow transition was elucidated through experimental analysis. The formation and transition of typical EHD flows from onset to the fully developed were described and explained. Experimental results showed that the EHD flow patterns change depends on the gas velocity and applied voltage. EHD flow starts with flow streamlines near collecting plates bending towards the wire electrode, forming two void regions. An oscillating jet forming the downstream appeared and moved towards the wire electrode as voltage increased. For higher velocities (≥0.2 m/s), the EHD transition became near wire phenomenon with a jet-like flow structure near the wire, forming a void region behind the wire and expanding as voltage increased. Fully developed EHD secondary flow in the form of counter-rotating vortices appeared upstream with high applied voltage.

Electrohydrodynamic secondary flow and particle collection efficiency in spike-plate multi-electrode electrostatic precipitator

In this work the results of electrohydrodynamic (EHD) secondary flow measurements in a spike-plate type electrostatic precipitator (ESP) are presented. In the investigated ESP two-sided and one-sided spike electrodes were used as a discharge electrodes. The results of 2D PIV measurements showed that flow pattern obtained for a different spike tips positions in respect to the primary flow direction significantly changing the flow structures in the ESP duct. The submicron dust particles collection measurements in this ESP were also performed. The obtained results showed that the collection efficiency of submicron dust particles depends on the generated EHD secondary flow.

Electrohydrodynamic flow patterns and collection efficiency in narrow wire-cylinder type electrostatic precipitator

Recently, narrow electrostatic precipitators (ESPs) have become a subject of interest because of their possible application for the cleaning of the exhaust gases emitted by diesel engines. Diesel engines emit fine particles, which are harmful to human and animal health. There are several methods for decrease particulate emission from a diesel engines, but up to now, these methods are not enough effective or very expensive. Therefore, an electrostatic precipitation was proposed as an alternative method for control of a diesel particulate emission. In this work, results of electrohydrodynamic (EHD) secondary flow and particle collection efficiency measurements in a narrow wire-cylinder type ESP are presented. The ESP was a glass cylinder (300 mm × 29 mm) equipped with a wire discharge electrode and two collecting cylinder-electrodes. A 0.23 mm in diameter and 100 mm long stainless-steel discharge wire electrode was mounted in the center of the cylinder, parallel to the main flow direction. The collecting electrodes were made of stainless steel cylinders, each with a length of 100 mm and inner diameter of 25.5 mm. An air flow seeded with a cigarette smoke was blown along the ESP duct with an average velocity of 0.9 m/s. The EHD secondary flow was measured using 2-dimensional particle image velocimetry (PIV) method. The PIV measurements were carried out in the wire electrode mid-plane, perpendicularly to the wire and the collecting electrodes. The results show similarities and differences of the particle flow in the wire-cylinder type ESP for a negative and a positive DC voltage polarity. The collection efficiency was calculated from the fractional particle concentration. The fractional particle concentration was measured using the optical aerosol spectrometer. The results of the fractional collection efficiency confirmed the common view that the collection efficiency of fine particles in the ESP increases with increasing voltage and it is higher for negative voltage polarity and decreases when decreasing particle diameter.

Electrohydrodynamic secondary flow and particle collection efficiency in a one-sided spike-plate type electrostatic precipitator

In this work, results of electrohydrodynamic secondary flow and particle collection efficiency measurements in an electrostatic precipitator under positive and negative voltage polarity are presented. The electrostatic precipitator had two plate collecting electrodes and a single spike discharge electrode. The spike tips were only on one side of the discharge electrode, therefore called a one-sided spike electrode. The electrohydrodynamic secondary flow pattern and collection efficiency measurements were carried out for two one-sided spike electrode positions: with the spike tips directed either upstream or downstream the primary flow. The results of the flow pattern measurement showed different flow patterns for different spike tips positions in respect to the primary flow direction. The electrostatic precipitator collection efficiency measurements clearly showed difference in the particle collection for both spike tips positions. Since the other electrostatic precipitator working parameters were the same, it can be concluded that the observed change of the electrohydrodynamic secondary flow was the main reason of the difference in the electrostatic precipitator collection efficiency.

Numerical calculations of submicron particle removal in a spike-plate electrostatic precipitator

A 3D hybrid Finite Element (FE)-Flux Corrected Transport (FCT) numerical algorithm is proposed to evaluate the electrical characteristics of a laboratory scale single stage spikeplate electrostatic precipitator and to predict the collection of submicron particles with diameters in the range of 0.25-1.5 μm. The precipitator consists of two parallel collecting plates with a spiked electrode mounted at the center, parallel to the planes and excited with a high negative dc voltage. The complex interaction between the electric field, fluid dynamics and the particulate flow in this precipitator are taken into account in the simulation. Particles are assumed to be charged by combined field and diffusion charging mechanisms. Motion of submicron particles under electrostatic and aerodynamic forces in turbulent flow is calculated using a Lagrangian-type Discrete Random Walk (DRW) model and User Defined Functions (UDFs) feature of the commercial FLUENT 6.2 software. The electrohydrodynamic secondary flow patterns, particle migration velocity patterns and particle collection efficiencies are examined for three different corona discharge electrode configurations: spikes located only on one side of the electrode with the tips directed either upstream or downstream of the channel, and spikes located on both sides. For a given particle concentration at inlet, the particle deposition rate along the channel and the average charge-to-mass ratios are evaluated for different particle sizes and applied voltages. Finally, the influence of particle concentration on particle collection efficiency for the ESP with spikes on two sides is investigated for -30 kV applied voltage. In selected cases, the numerical results are compared with the existing experimental data.

Electrohydrodynamic flow and particle collection efficiency of a spike-plate type electrostatic precipitator

In this work, the results of electrohydrodynamic (EHD) secondary flow and particle collection efficiency measurements in a spike-plate type electrostatic precipitator (ESP) are presented. The EHD secondary flow was measured using 2- and 3-dimensional particle image velocimetry (PIV) method under the negative DC voltage. The PIV measurements were carried out in several cross-sectional planes along and across the ESP duct. The results show a complex and turbulent flow structure in the ESP. The EHD secondary flow significantly depends on applied voltage and measuring cross-sectional plane position in respect to the spike tip. The partial collection efficiency of the ESP was measured for negative and positive DC voltage. The particle concentration with and without discharge was measured at the ESP exit using an optical aerosol spectrometer.

CFD modelling of electrohydrodynamic gas flow in an electrostatic precipitator

Modelling of the flow velocity fields for electrohydrodynamic flow in a wire-to-plate type electrostatic precipitator was achieved. From calculations for the flow employing different flow models, the Chen-Kim k − ε turbulent model appeared to be the most appropriate choice to obtain a quantitative image of the resulting mean flow field and downstream wake flow of the rear wire, although this was obtained from a qualitative analysis owing to the lack of experimental verification. The flow velocity field pattern showed a strong electrohydrodynamic secondary flow, which was clearly visible in the downstream regions of the corona wire despite the low Reynolds number for the electrode (ReCW = 12.4). Secondary flow vortices were also caused by the electrohydrodynamic with increases in the discharge current. Pressure drop increased gradually according to the increase of the electrohydrodynamic number.

Numerical modeling of near corona wire electrohydrodynamic flow in a wire-plate electrostatic precipitator

Numerical modelling of the flow velocity fields for the near corona wire electrohydrodynamic (EHD) flow was conducted. Solutions of the steady, two-dimensional momentum equations have been computed for a wire-plate type electrostatic precipitator (ESP). The equations were solved in the conservative finite-difference form on a fine uniform rectilinear grid of sufficient resolution to accurately capture the momentum boundary layers. The numerical procedure for the differential equations was used by SIMPLEST algorithm. The CFD code coupled with Poisson's electric field, ion transport equations and the momentum equation with electric body force, was used for the numerical simulation with the Chen-Kim k-epsiv turbulent model. The numerical results show that EHD secondary flow was clearly visible in the downstream regions of the corona wire despite the low Reynolds number for the electrode (Re cw=12.4). Secondary flow vortices caused by the EHD increases with increasing discharge current or EHD number, hence pressure drop of ESP increases

On the modeling of electro-hydrodynamic flow in a wire-plate electrostatic precipitator

Modeling of the flow velocity fields for the electrohydrodynamic (EHD) flow in a wire-to-plate type electrostatic precipitator (ESP) was achieved. Solutions of the steady, two-dimensional Navier-Stokes equations have been computed. The equations were solved in the conservative finite-difference form on a fine uniform rectilinear grid of sufficient resolution to accurately capture the momentum boundary layers. The numerical procedure for differential equations was used by SIMPLEST [Michel, 2002], a derivative of Patankar’s SIMPLE algorithm, to bring rapid convergence. The Phoenics (Version 3.5.1) CFD code, coupled with Poisson’s and ion transport equations and electric body force in the momentum equation, developed in this study, was used for the numerical simulation. From calculations for the flow employing different flow models, the Chen-Kimk-e turbulent model appeared to be the most appropriate choice to obtain a quantitative image of the resulting mean flow field and downstream wake flow of the rear wire, although this was obtained from a qualitative analysis due to the lack of experimental verification. The flow velocity field pattern showed a strong EHD secondary flow, which was clearly visible in the downstream regions of the corona wire despite the low Reynolds number for the electrode (ReCW=12.4). Secondary flow vortices were also caused by the EHD with increases in the discharge current

NUMERICAL MODELING OF WIRE ELECTROHYDRODYNAMIC FLOW IN A WIRE-PLATE ESP

Numerical modeling of the flow velocity fields for the near corona wire electrohydrodynamic (EHD) flow was conducted. The steady, two-dimensional momentum equations have been computed for a wire-plate type electrostatic precipitator (ESP). The equations were solved in the conservative finite-difference form on a fine uniform rectilinear grid of sufficient resolution to accurately capture the momentum boundary layers. The numerical procedure for the differential equations was used by SIMPLEST algorithm. The Phoenics (Version 3.5.1) CFD code, coupled with Poisson's electric field, ion transport equations and the momentum equation with electric body force were used for the numerical simulation and the Chen-Kim turbulent model numerical results that an EHD secondary flow was clearly visible in the downstream regions of the corona wire despite the low Reynolds number for the electrode (). Secondary flow vortices caused by the EHD increases with increasing discharge current or EHD number, hence pressure drop of ESP increases.

EHD flow in a wide electrode spacing spike–plate electrostatic precipitator under positive polarity

In this work, results of two- and three-dimensional particle image velocimetry (PIV) measurements of the flow velocity fields in a wide spacing spike–plate electrostatic precipitator (ESP) under positive polarity are presented. A DC voltage of positive polarity (up to 28 kV) was applied to the spike electrode. The average gas flow velocity was 0.6 m/s. The PIV measurements were carried out in four planes perpendicular to the plate electrodes. Three parallel planes passed along the ESP while one plane passed across the ESP duct. The results show that electrohydrodynamic (EHD) secondary flow with relatively strong vortices exist in the ESP. The EHD secondary flow pattern depends on applied voltage and measuring plane position in respect to the spike tip. The strongest vortices occur in the plane passing through the tip of the upstream-directed spike. These relatively strong EHD vortices may hinder collection of the particles in the diameter range of 0.1–1 μm in the wide electrode spacing spike–plate ESPs.

Electrohydrodynamic Secondary Flow

Secondary flow in an electrostatic precipitator in the form of electric wind takes the form of jets emitted from corona wires. Depending on the magnitude of electric Reynolds number of the systems, jet speed may reach several times that of mean gas flow.