Electrohydrodynamic Flow Patterns in a Narrow Electrostatic … · 2017. 1. 20. · Electrohydrodynamic Flow Patterns in a Narrow Electrostatic Precipitator with Longitudinal Wire

I. INTRODUCTION

Since decades electrostatic precipitators (ESPs) have been used for dust particles collection. Recently narrow ESPs have become a subject of interest because of their possible application in diesel engines. Diesel engines emit fine particles, in size range of 7.5 10-3 – 1.0 μm [2], that are harmful for human and animal health. Such measures as diesel engine modification, special fuel additives, alternative fuels, aftertreatment systems or diesel particulate filters (DPFs) decrease particulate emission from diesel engines [2-6]. However, the first three measures are not effective for reduction of diesel soot. On the other hand, the DPFs removal efficiency is very high (90% to 95% [4]), but DPFs are very expensive because of their high-energy consumption and high maintenance costs.

Several authors proposed electrostatic precipitation as an alternative control of diesel particulate emission [7-12]. The precipitation of particles in the duct of an ESP depends on the dust-particle properties, electric field, space charge, particle physical parameters and electrode geometry. The interaction between the electric field and

charge and the flow results in considerable turbulences of the flow [13-20], which seems to lower the fine particle collection efficiency. According to some authors [17-20], the turbulence should be reduced to improve the fine particle collection efficiency. Improving other factors, such as ESP electrode geometry or ESP operating conditions, which influence the flow patterns in ESPs, may also increase the fine particle collection efficiency. Data on the flow patterns in ESPs are essential for studying the performance of ESPs.

The collection efficiency increases with increasing applied voltage, but the increasing of applied voltage causes increase of the vorticity, what is not profitable phenomenon in electrostatic precipitation. The negative voltage polarity is more efficient in the particle collection than the positive one [21, 22]. For the both applied voltage polarities, the collection efficiency decreases when decreasing particles diameter. This tendency is most distinct for sub-micron particles in the range from 0.25 µm to 0.5 µm, as results from the mismatching between particle diameter dependences of particle charging and charged particle migration velocity [21, 22]. Here, it must be noted that the partial particle collection efficiency usually exhibits a minimum for particle size between 0.2 µm to 0.5 µm in the majority of ESPs [21, 22].

Electrohydrodynamic Flow Patterns in a Narrow Electrostatic Precipitator with Longitudinal Wire Electrode for Various Electrode Geometries

A. Niewulis1, J. Podliński1, and J. Mizeraczyk1, 2

1Centre for Plasma and Laser Engineering, The Szewalski Institute of Fluid Flow Machinery, Polish Academy of Sciences, Poland

2Department of Marine Electronics, Gdynia Maritime University, Poland

Abstract—Recently narrow electrostatic precipitators (ESPs) have become a subject of interest because of their

possible application in diesel engines. In this paper results of 2-dimensional (2D) Particle Image Velocimetry (PIV) measurements of the flow patterns in a narrow ESP for various electrode geometries are presented. The PIV measurements were carried out in the observation plane that is perpendicular to the ESP duct. The ESP was a glass duct, 120 mm long, 30 mm wide and 30 mm high. The electrical electrode set consisted of a single wire discharge electrode and either single or two or four plane collecting electrodes. The discharge electrode, made of a stainless steel wire of a diameter of 0.23 mm and length of 100 mm, was placed longitudinally to the flow. The collecting plane electrodes (30 mm × 120 mm) were made of aluminum tape of a thickness of 50 μm glued on dielectric supporting plates. Depending on the electrode geometry, either a single collecting plane electrode was placed on the bottom wall (geometry 1), either two or four collecting plane electrodes were placed on the bottom and top walls (geometry 2), or on the bottom and side walls (geometry 3), or on all four walls (geometry 4) of the ESP. The DC voltage of 10 kV was applied to the wire electrode through a 10 M resistor, while the collecting electrodes were grounded. Air flow seeded with a cigarette smoke was blown along the ESP duct with an average velocity of 0.9 m/s. The PIV measurements were carried out in a so-called electrode mid-plane, perpendicularly to the wire and the collecting electrodes. The obtained results showed that the particle flow in the ESP had strongly three dimensional (3D) character due to both the applied voltage and narrow cross section of the ESP duct. For the electrode geometries 1, 2 and 3 large and very regular vortex structures were observed in the mid plane. Although our measurements were two dimensional the analysis of the obtained results showed that the observed vortices exhibit three dimensional character, having the form of a spiral vortex moving along the ESP. These spiral vortices may decrease the particles collection efficiency, as it was calculated by Dumitran [1]. However, for the electrode geometry 4 the vortex structures were smaller and irregular, which may results in adifferent particle collection efficiency. The particle collection efficiency for all presented electrode geometries is under investigation.

Keywords—Narrow electrostatic precipitator, ESP, EHD flow, 2D PIV, flow measurement

Corresponding author: Jerzy Mizeraczyk e-mail address: [email protected] Presented at the Sixth International Conference on Applied Electrostatics, ICAES’2008, in September 2008

139 International Journal of Plasma Environmental Science & Technology, Vol.4, No.2, SEPTEMBER 2010

In this paper results of 2-dimensional (2D) Particle Image Velocimetry (PIV) measurements of the flow patterns in a narrow ESP for various electrode geometries are presented. The PIV measurements were carried out in the observation plane that is perpendicular to the ESP duct.

II. EXPERIMENTAL SET-UP

The experimental apparatus used in this work

consisted of an ESP, high voltage power supply and standard 2D PIV equipment [23-24] for the measurement of velocity field (Fig. 1).

The ESP was a narrow glass duct, 120 mm long, 30 mm wide and 30 mm high (Fig. 2). The electric electrode set consisted of a single wire discharge electrode and either single or two or four plane collecting electrodes. The discharge electrode, made of a stainless steel wire of a diameter of 0.23 mm and length of 100 mm, was placed along the ESP, longitudinally to the flow. The plate collecting electrodes (120 mm long and 30 mm wide) were made of aluminum tape of a thickness of 50 μm glued on dielectric supporting plates. The dielectric supporting plates of thickness of 1 mm were placed inside the channel.

Depending on the electrode geometry, either a single collecting plane electrode was placed on the bottom wall (geometry 1), either two or four collecting plane electrodes were placed on the bottom and top walls (geometry 2), or on the bottom and side walls (geometry 3), or on all four walls (geometry 4) of the ESP (Fig. 3).

The DC voltage (negative or positive) of 10 kV was applied to the wire electrode through a 10 M resistor, while the collecting electrodes were grounded.

The experiments were carried out with cigarette smoke as tracer particles. Most particles of the cigarette smoke were the sub-micron particles of a diameter from 0.25 µm to 0.3 µm. 99 % of particles had a diameter below 1 µm, as measured by optical aerosol spectrometer (GRIMM 1.109). Air flow seeded with a cigarette smoke was blown along the ESP duct with an average velocity of 0.9 m/s.

The PIV measurements were carried out in the wire electrode mid-plane, perpendicularly to the wire and the

Fig. 1. Experimental set-up.

Fig. 2. Electrode geometry 1 of the ESP with observation plane

marked at x = 0 mm.

Fig. 3. Electrode geometries of the ESP; a) geometry 1; b) geometry 2; c) geometry 3, d) geometry 4.

Niewulis et al. 140

collecting electrode/electrodes. All the velocity fields presented in this paper resulted

from the averaging of 100 measurements, which means that each velocity map was time-averaged. The time associated with the 100 measurements was 20 s.

III. RESULTS AND DISCUSSIONS Results of the 2D PIV measurements for all electrode

geometries are shown in Figs. 4-12. They show the flow patterns: the velocity fields (and corresponding flow streamlines) and vorticity contour in the mid plane (the y- z plane) of the ESP. At the primary flow average velocity of 0.9 m/s, the Reynolds number was Re = 1720. The Reynolds number was calculated using a formula [25]:

Re = V L / were the primary flow velocity V = 0.9 m/s, characteristic length L = 0.03 m (4 times the cross-sectional area, divided by the wetted perimeter; for all presented geometries the value of characteristic length was rounded up to the 2 decimal place) and air dynamic viscosity = 1.57 105 m2/s. The flow patterns in the ESP for the electrode geometry 1 when no voltage was applied are show in Fig. 4. It is seen from Fig. 4, that a slight movement of the seed particles exist in the y-z plane, the velocity y- and z-components are very low (below 0.1 m/s). The vorticity reaches value lower than ± 20 s-1 and any vortex structures are not form in the y z plane. Very similar results not shown in this paper were obtained for the other electrode geometries (2, 3 and 4) of the ESP. A. Negative operating voltage

Figs. 5-8 show the flow patterns in the y-z plane for all ESP geometries for a negative voltage of 10 kV. It is worthy of mention that at constant operating voltage the discharge current differs when charging the ESP geometry (no difference for geometries 2 and 3 due to the some number of the collecting electrodes). Due to the voltage applied, the flow patterns changed significantly when compared to those shown in Fig. 4 (when no voltage was applied). A secondary flow, caused by the electrohydrodynamic (EHD) forces, is formed in the y-z plane, i.e. transversely to the flow duct.

The particle flow velocity fields measured by PIV method reflect of the dust particles in the ESP. When no voltage is applied, the submicron dust particles followed the gas main flow and, in this case, the measured velocity field of the particles corresponds to the gas flow velocity field. The particle flow velocity fields (and the corresponding flow streamlines) in Figs. 5-8 differ slightly from the gas flow velocity fields (and the corresponding flow streamlines). To obtain the gas flow patterns one has to subtract the particle drift velocity from the measured particle velocity. However, this requires a complex computing of the drift of the charged

particles in the electric field with a space charge in the flowing gas. However, the drift velocity of fine particles is considerably smaller than the gas velocity in the whole ESP duct, except in the area around the wire where the electric field is very high. This means that the gas flow patterns in the ESP can be reasonably well represented by the particle flow patterns presented in Figs. 5-8. The flow patterns for the electrode geometry 1 show in Fig. 5 are for the total discharge current of 25 μA. At this current, the EHD number was 2.3 106. The EHD number was calculated using a formula [25]:

Ehd = I L3 / (2 i A) (2) where: I = 25 μA (total discharge current), L = 0.03 m (characteristic length), = 1.57 10-5 m2/s (air dynamic viscosity), = 1.205 kg/m3 (air density), i = 2.70 10-4 m2/Vs (ion mobility), and A = 0.0036 m2 (discharge area for geometry 1). The ratio of the EHD number to the Reynolds number squared (Ehd/Re2) was 0.8. Ehd/Re2 describes the ratio of the electric forces to the inertial force. If Ehd/Re2 > 1, the electric force dominates over the inertial force, however if Ehd/Re2 < 1 the inertial force dominates over the electric forces.

Fig. 4. Results of 2D PIV measurement in the ESP for electrode geometry 1 when no voltage was applied: a) the flow velocity field and b) the vorticity contour. - show the direction of the primary

flow.


It is seen from Figs. 5 a) and b) that due to the EHD forces the particles flow from the ESP center (i.e. from the discharge wire electrode) towards the plate electrode. Then, near the plate electrode the particles turn towards the side walls, and move along them. Finally, the particles turn again towards the wire electrode. As a result, two vortices, rotating in opposite directions, are formed in the y–z plane. The highest vorticity is in the center of the flow circulation (Fig. 5). The flow vorticity

ranged from -450 s-1 to +420 s-1. The positive vorticity determines the counter-clockwise circulation of the flow, whereas the negative vorticity corresponds to the clockwise circulation. The flow patterns in the ESP for the electrode geometry 2 (2 plate electrodes facing each other) are presented in Fig. 6. The average total discharge current was 95 μA i.e. it was higher than for the geometry 1 at the same operating voltage (-10 kV). At this current, the

Fig. 5. Results of 2D PIV measurement in the ESP for electrode geometry 1 for a negative voltage of 10 kV: a) the flow velocity

field, b) flow streamlines, and c) the vorticity contour. The average total discharge current 25 μA.



Niewulis et al. 142

EHD number was 4.4 106, and the ratio Ehd/Re2 was 1.5 (when using formula (2) A = 0.007 m2). Figs. 6 a) and b) show similar effects, caused by the applied voltage, as in the ESP having the geometry 1 (Fig. 5). The particle trajectories are strongly bent towards the plate electrodes, then they turn towards the side walls, and moved along them, and finally they turn again towards the wire electrode. However, in the ESP with two plate electrodes facing each other (mounted on

the bottom and top walls) four vortices are formed. Each vortex forms in a quarter of the transverse cross section of the ESP duct. The vortex structures are regular and symmetric. The highest values of the flow vorticity (Fig. 6 c)) were -900 s-1 for the clockwise rotation and +600 s-1 for counter-clockwise rotation of the flow. Figs. 7 a)-c) show the flow patterns at the average total discharge current of 95 μA. The EHD number was 4.4 106, and the ratio Ehd/Re2 was 1.5. (A = 0.007 m2).


field, b) flow streamlines, and c) the vorticity contour. The average total discharge current 95 μA




It is seen from Figs. 7 a) and b) that in the ESP having two plate electrodes mounted next to each other (on the bottom and side walls) two vortex structures form in the opposite corners of the ESP transverse cross section. As always when voltage is applied the particles move towards plate electrodes. Then the particles move along the plate electrodes, next turn towards the side walls. Around the middle of the side walls the particles turn towards the wire electrode. The flow vorticity in the center of the vortex is high and ranges from -600 s-1 to +750 s-1. The transverse flow patterns for the electrode geometry 4 (4 plate electrodes) are presented in Figs. 8 a) and b). At the total discharge current of 235 μA the EHD number was 5.5 106, and the ratio Ehd/Re2 was 1.9 (A = 0.014 m2). It is seen from Figs. 8 a) and b) that the particles are forced to move towards the plate electrodes. However, when comparing the particle velocity towards the plate electrodes for all four electrode geometries (geometries 1, 2, 3 and 4), one sees that the particle velocity is the lowest in the cause of geometry 4. In the cause of 4 plate electrodes, the 4 strong streams of the particles towards the plate electrodes cannot forces regular self consistent vortices as in the cause of the geometry 2, when 2 plate electrodes facing each other were used (Fig. 6). This is because after reaching the plate electrode the stream meets oncoming two neighboring streams which results in scattering of the streams. Small and irregular vortex structures are formed in all corners of the ESP cross section. The flow vorticity (Fig. 8 c)) is -350 s-1 for the clockwise rotation and +300 s-1 for the counter-clockwise rotation of the flow. In general, the particle flow in the geometry 4 seems to be more chaotic then in other geometries. This may be caused by the different collecting electrode geometry and higher discharge current than in the other geometries. B. Positive operating voltage Figs. 9-12 show the transverse flow patterns in the ESP for positive voltage of 10 kV. The changing of voltage polarity from negative to positive did not essentially change the flow patterns in all presented geometries comparing to those for the negative polarity. The transverse flow patterns in the ESP for the electrode geometry 1 for positive voltage polarity are presented in Figs. 9 a)-c). At the total discharge current of 10 μA (i.e. lower than for negative polarity) the EHD number was 1.3 106. The EHD number was calculated using formula (3), however for positive polarity the ion mobility was i = 2.00 10-4 m2/Vs. The ratio of the EHD number to the Reynolds number squared (Ehd/Re2) was 0.4. Figs. 9 a) and b) show similar effects, caused by the applied voltage, as in the ESP with the geometry 1 for negative polarity (Figs. 5 a) and b)). The particle move towards the plate electrode and two vortices are formed near the plate electrode. However, the particle velocity towards the plate electrode direction as well as the flow vorticity are

lower (from -300 s-1 to +350 s-1) than those for the negative voltage. This may be caused by lower discharge current (10 μA against 25 μA for negative voltage). Results of the measurement in the ESP for electrode geometry 2 for positive voltage are presented in Figs. 10 a)-c). The total discharge current was 65 μA. At this current, the EHD number was 4.1 106. The ratio of the EHD number to the Reynolds number squared (Ehd/Re2) was 1.4.

Fig. 9. Results of 2D PIV measurement in the ESP for electrode geometry 1 for a positive voltage of 10 kV: a) the flow velocity field,

b) flow streamlines, and c) the vorticity contour. The average total discharge current 10 μA.

Niewulis et al. 144

Similarly as for negative voltage polarity (Figs. 6 a) and b)) four very symmetric and regular vortex structures are form for positive polarity. However, the highest vorticity (-800 s-1 and +800 s-1) is for positive polarity. The transverse flow patterns in the ESP for the electrode geometry 3 for positive polarity are presented in Fig. 11. At the total discharge current of 65 μA the EHD number was 4.1 106. The ratio of the EHD

number to the Reynolds number squared (Ehd/Re2) was 1.4. It is seen from Figs. 11 a) and b) that the flow pattern confirms the features of the particle flow for negative polarity (Figs. 7 a) and b)). Two very symmetric and regular vortices are formed in the corners of the ESP cross section. In the center of the flow circulation, the vorticity (Fig. 11 c)) were -750 s-1 for the clockwise

Fig. 10. Results of 2D PIV measurement in the ESP for electrode geometry 2 for a positive voltage of 10 kV: a) the flow velocity





rotation and +720 s-1 for counter-clockwise rotation of the flow. Results presented in Fig. 12 show the transverse flow patterns in the ESP for the electrode geometry 4 for positive polarity. At the total discharge current of 200 μA the EHD number was 6.3 106. The ratio of the EHD number to the Reynolds number squared (Ehd/Re2) was 2.1. Similarly as for negative voltage polarity (Figs. 8 a)

and b)), several small and irregular vortex structures were form in the ESP cross section for positive voltage (Figs. 12 a) and b)). However, these structures circulated slower than at negative voltage (the vorticity was -280 s-1 and +250 s-1).

IV. CONCLUSION

The presented results of 2D PIV measurements of

the particle velocity fields in the narrow longitudinal ESP showed that the applied voltage (either negative or positive) and the electrode geometry significantly change the particle flow patterns. For the electrode geometries 1, 2 and 3 large and very regular vortex structures were formed by the EHD forces in the plane perpendicular to the main flow. Although our measurements were two dimensional the analysis of the obtained results suggests that those vortices exhibit three dimensional character, having the form of a spiral vortices moving along the ESP. These spiral vortices may decrease the particles collection efficiency, as it was calculated by Dumitran [1]. For the electrode geometry 4 the vortex structures were smaller and irregular, which may result in different particle collection efficiency. The particle collection efficiency for all the presented electrode geometries is under investigation. All our results are presented for 0.03 mm wide square duct. However, we may not always obtain the same flow patterns for each electrode geometry in the narrow ESP having different cross section area. An experiment could be carried to elucidate this point.

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Electrohydrodynamic Flow Patterns in a Narrow Electrostatic … · 2017. 1. 20. · Electrohydrodynamic Flow Patterns in a Narrow Electrostatic Precipitator with Longitudinal Wire