Particle Image Velocimetry Measurements of Airflow in Electrohydrodynamic Device for Dust Particle Collection

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1 82 International Journal of Plasma Environmental Science & Technology, Vol.7, No.1, MARCH 2013 Particle Image Velocimetry Measurements of Airflow in Electrohydrodynamic Device for Dust Particle Collection J. Podliński 1, A. Berendt 1, and J. Mizeraczyk 1, 2 1 Centre for Plasma and Laser Engineering, The Szewalski Institute of Fluid Flow Machinery, Polish Academy of Sciences, Poland 2 Department of Marine Electronics, Gdynia Maritime University, Poland Abstract In this paper the pumping effect of air in the electrohydrodynamic (EHD) device for dust particle collection is presented. To induce an airflow in this EHD device corona discharge was used. The discharge was generated between the one-sided spike electrodes and the plate electrodes. The asymmetric electric field and generated discharge resulted in a unidirectional airflow through the EHD device. The current-voltage characteristics were measured and the corona discharge power was calculated. The EHD flow and the average airflow induced in the device duct were measured using 2D PIV method. The results showed that the investigated EHD device was capable of producing significant unidirectional airflow with velocities up to 0.64 m/s. Keywords Electrohydrodynamics, EHD, fluid flow measurement, PIV, flow patterns I. INTRODUCTION Inhalation of aerosols, bacteria or viruses as well as dust particles can be harmful for human health. Especially dangerous are submicron dust particles, which can contain traces of toxic elements like sulphur and nitrogen oxides, ammonia and metals like mercury, arsenic and zinc. Due to the low weight and small dimension of submicron dust particles they can float relatively long in the air. Moreover, they can easily penetrate into human respiratory system when inhaled. Thus, submicron dust particles removal from the air is very important and new devices for air cleaning are needed. We propose to use an electrohydrodynamic (EHD) device for air pumping [1-7] and simultaneous dust particles removal. Principle of operation of this device is similar to standard electrostatic precipitators (ESPs), which are used as dust particle collectors. However, in this case external fan for primary flow generation is not needed. When a strong electric field exists between a sharp object at a high voltage and a grounded electrode, a corona discharge occurs resulting in ionization of the gas molecules. An organized ion flux in an electric field initiates an ion-driven wind of neutral molecules (electrohydrodynamically induced gas flow is generated). When the electrodes configuration results in asymmetrical electric field, a unidirectional gas flow can be generated. This phenomenon is called EHD gas pumping. EHD flow and its influence on performance of ESPs was a subject of investigations performed in many laboratories [8-16]. Our group at the Centre of Plasma and Laser Engineering Polish Academy of Science in Gdansk has also contributed to this field [17, 18]. The Corresponding author: Janusz Podlinski address: janusz@imp.gda.pl experience gained during our studies allowed us to design an EHD device for simultaneous air pumping and suspended dust cleaning. In the presented investigations we focused on the measurements of a unidirectional flow velocity, which was induced in the EHD device duct. We also studied complex airflow patterns produced around the discharge electrodes of the EHD device. II. EXPERIMENTAL SET-UP The experimental set-up for measurements of the flow in the EHD device is presented in Fig. 1. It consisted of the EHD device, the high voltage supplier and the apparatus for flow measurements by 2D PIV method. The EHD device (Fig. 2) used in this experiment consisted of a transparent acrylic box (1600 mm long, 200 mm wide and 100 mm high), two smooth stainlesssteel plate electrodes (200 mm 1100 mm) placed at the top and bottom of the device and four stainless-steel spike electrodes mounted in the plane in the middle between the plate electrodes. Each spike electrode had 6 spikes on the one side (One-Sided Spike Electrode with 6 spikes OSSE6). These electrodes were 200 mm long, 1 mm thick and 25 mm wide (Fig. 3). The distance between the plate electrodes and the spike electrodes was 50 mm. The plate electrodes were grounded. DC high voltage (HV) was applied to the spike electrodes through ballast resistors. When the high voltage was applied the corona discharge started from the spike tips of these electrodes. During our experiments high voltage was applied to 1, 2 or 4 spike electrodes. To prevent interactions between discharges [19] generated by the successive OSSE6 electrodes, the distance of 150 mm between them was fixed. For checking if the air pumping effects induced by the consecutive discharges are independent (similarly as in [20]), two electrode Presented at the 8th Conference of the French Society of Electrostatics, SFE2012, in July 2012

2 Podliński et al. 83 Fig. 1. Scheme of experimental set-up for flow velocity measurements in the EHD device for air pumping and dust particle collection. voltage U applied to the spike electrodes was calculated basing on the voltage u and the current I measured on the HV generator. Assuming equal division of the current between all active spike electrodes the voltage applied to each spike electrode can be calculated using formula: U u- Fig. 2. Scheme of the EHD device for air pumping and dust particle collection. Fig. 3. Scheme of the spike discharge electrode used in the investigated EHD device. arrangements with 2 OSSE6 were investigated. The first arrangement - when the high voltage was applied to the electrodes number 1 and 2 (called the active electrodes), and the second - when the high voltage was applied to the electrodes number 1 and 4 (the electrode numbering is shown in Figs. 1 and 2). To the EHD device inlet 3 m long PCV pipe of 160 mm inner diameter was connected. Before each PIV measurement this pipe and the EHD device duct were filled up with a cigarette smoke (99% of particles had diameters smaller than 1 µm [17]) which was used as a seeding for PIV investigations. The EHD device was not equipped with any external fan, thus, the flow generated in this device was induced only by the corona discharge. The negative or positive DC high voltage applied to the spike electrodes was supplied by a Spellman high voltage generator model SL300. Each HV electrode was supplied through a resistor R = 10 MΩ. The high voltage generator was equipped with an average output current and an output voltage meters. The value of the high I R number _ of _ HV _ electrodes (1) Then, basing on the calculated voltage U and the average discharge current I the discharge power P was calculated as P = U I. The flow velocity fields in the EHD device were measured using 2D PIV system [21]. The 2D PIV apparatus consisted of a double Nd:YAG laser system (λ = 532 nm), a cylindrical telescope, CCD cameras and a PC computer. The PIV investigations were carried out in the plane fixed by a laser sheet shaped by the cylindrical telescope. The laser sheet was introduced in the centre of the EHD device duct, perpendicularly to the plate (grounded) electrodes. The pairs of images of the seeding particles dissipating the laser light were recorded by the two FlowSense M2 cameras. The CCD camera sensor size was 1600 pixels 1186 pixels. The time between the consecutive images used to determine a single velocity field depended on the flow conditions and usually ranged from 0.6 ms to 4 ms. For each tested case a set of 100 pairs of PIV instantaneous images were captured and transmitted to the PC computer with a Dantec FlowManager software. The flow velocity fields were calculated using an adaptive cross-correlation algorithm. The interrogation window for the cross-correlation procedure was 32 pixels 32 pixels (horizontal vertical). For successful tracing the particles moving from one interrogation window to the neighbouring one an overlap of 25% of the neighbouring interrogation windows was set. This ensured a satisfactory spatial resolution of the measured velocity field maps. Finally, basing on the 100 instantaneous flow velocity field images a time-averaged flow velocity field map was calculated. In our investigations the PIV technique was used for two purposes: firstly, to observe the EHD flow structures generated near the discharge electrodes, which generated

3 84 International Journal of Plasma Environmental Science & Technology, Vol.7, No.1, MARCH 2013 a unidirectional airflow; and secondly, to measure the total air flow rate in the EHD device (measured at the EHD device inlet). To investigate the EHD flow structures the first camera observed a relatively wide region between the discharge electrodes number 1 and 3 (camera observation area was 284 mm wide and 210 mm high). This resulted in the spatial resolution of velocity vector maps of 4.25 mm 4.25 mm. The second camera used to measure the unidirectional flow induced in the EHD device observed much smaller region at the EHD device inlet (camera observation area was 150 mm 112 mm). In this case the spatial resolution of velocity vector maps was 2.25 mm 2.25 mm. The standard deviation of the time-averaged flow velocities (calculated from 100 instantaneous images) was less than 5%. A comment on the PIV technique is required in this place. The PIV technique is based on tracing the seeding particles movement suspended in the airflow. In general, it is assumed that the particles follow the airflow. Therefore, the velocity maps measured by the PIV are regarded as airflow velocity maps. However, strictly speaking the PIV maps show the particle flow velocity fields. In our experiments we used submicron particles as seeding. The submicron particles follow the airflow except the areas where the electric forces are very high (i.e. around the spike tips) [10]. Therefore, in these areas the airflow velocity field may slightly differ from that measured by the PIV, i.e. from the particle velocity field. This discrepancy does not influence the accuracy of determining the average velocity induced in the EHD device (and the airflow rate) because it was measured at the EHD device inlet where the electric field is relatively low and both velocity fields, of airflow and of particle flow are similar. III. RESULTS The current - voltage characteristics of the EHD device with the single active spike electrode are presented in Fig. 4. In the case of negative voltage polarity the voltage applied to the electrode was 20 kv, 25 kv, 27.5 kv and 30 kv. When positive voltage of Fig. 4. Current-voltage characteristics of the EHD device with 1 spike electrode kv was applied the discharge current increased rapidly and an intensive lighting was observed between several spikes and plate electrodes. It indicated that the breakdown streamers occurred [22]. When the positive voltage reached 30 kv accidental sparks occurred. Therefore, in the case of positive voltage polarity investigations of the flow in the EHD device were carried out for the applied voltage not exceeding 29.5 kv. The examples of the measured flow velocity vector maps and the streamlines in the area between the discharge electrodes number 1 and 3 are presented in Figs In Fig. 5 the flow pattern generated when the high voltage was applied to the single spike electrode (number 2) is presented. In Figs. 6 and 7 the flow pattern measured when the high voltage was applied to all 4 spike electrodes is shown. In both cases the applied voltage was 27.5 kv (negative polarity). As it can be seen in Figs. 5-7, behind the spike tips the strong jet-like flow was generated. The jet was directed to the outlet of the EHD device duct (in z-direction) and spread towards the plate electrodes. The velocity of the jet was relatively high (the velocity z-component in the jet centre was up to 1.65 m/s). The jet induces the unidirectional flow through the EHD device duct. However, the air is sucked by the jet not only from the inlet of the EHD device, but also from its interior, i.e. from the neighbourhood of the plate electrodes. This results in large recirculation areas between the spike and the plate electrodes. As a Fig. 5. Flow patterns measured by 2D PIV method in the area between the discharge electrodes number 1 and 3. The high voltage of 27.5 kv (negative polarity) was applied to the spike electrode number 2.

4 Podliński et al. 85 Fig. 6. Flow patterns measured by 2D PIV method in the area between the discharge electrodes number 1 and 3. The high voltage of 27.5 kv (negative polarity) was applied to all 4 spike electrodes. Fig. 7. Streamlines in the area between the discharge electrodes number 1 and 3. The high voltage of 27.5 kv (negative polarity) was applied to all 4 spike electrodes. consequence, only a part of the air is directed to the EHD device outlet. The remaining air revolves in the recirculation areas. When 4 discharge electrodes are used, all individual flows generated by the consecutive discharges accumulate and a significant net unidirectional airflow in the EHD device duct is induced. In Figs. 5-7 a small asymmetry of the flow patterns is seen. This is typical of the experimental PIV investigations of EHD flows. The flow pattern asymmetry observed in Figs. 5-7 is usually caused by a geometrical asymmetry of the electrode arrangement resulted from the imperfections introduced when EHD device is assembled. We found that even a small asymmetry in the electrodes arrangement (of the order of a fraction of mm) can influence the expected symmetry of EHD flow pattern (see also [23]). In Fig. 8 an example of obtained time-averaged velocity vector map of the flow at the EHD device inlet is presented (airflow measured upstream the spike electrodes). As it is seen the flow in the observed area was smooth. Basing on the time-averaged velocity vector maps the flow velocity profiles in the investigated EHD device were determined. The position of velocity profiles is marked with a broken line in Fig. 8 (at z = 134 mm). An example of the velocity profile obtained when the high voltage of 20 kv was applied to 4 spike electrodes is presented in Fig. 9. An average velocity of the unidirectional airflow induced in the EHD device duct was calculated as an arithmetical average of the velocities which determined the velocity profile. Fig. 8. Time-average velocity vector map of the flow in the EHD device duct. The negative high voltage of 20 kv was applied to 4 spike electrodes. Fig. 9. Velocity profile of the airflow in the EHD device duct. High voltage of 20 kv (negative polarity) was applied to 4 spike electrodes. The average airflow velocity was 0.37 m/s.

5 86 International Journal of Plasma Environmental Science & Technology, Vol.7, No.1, MARCH 2013 The experimental data obtained during measurements of the current-voltage characteristics, calculated corona discharge power and average values of the unidirectional airflow velocity in the EHD device duct are listed in the Table I. As it can be seen, the highest average airflow velocity of 0.64 m/s was obtained when the negative voltage of 30 kv was applied. Taking into account the EHD device duct cross-section area (0.02 m 2 ) and the average velocity of the induced airflow the volumetric flow rate can be calculated. The maximum average volumetric airflow rate attained in our EHD device was m 3 /s. The average airflow velocity induced in the EHD device duct for a different discharge power is shown in Fig. 10. As it is seen, average velocity of induced airflow increases with increasing discharge power. However, it can be noticed that the velocity increase is lower for a higher discharge power. It is particularly noticeable in the case of positive polarity of the applied high voltage. Probably, it was caused by the change in the discharge character (breakdown streamers from the spike electrodes were observed). TABLE I EXPERIMENTAL DATA OBTAINED DURING CURRENT VOLTAGE AND FLOW VELOCITY MEASUREMENTS IN THE EHD DEVICE. THE VALUES OF APPLIED VOLTAGE IN THE CASE OF POSITIVE POLARITY IN BRACKETS. Electrodes 1xOSSE6 2xOSSE6 1&2 2xOSSE6 1&4 4xOSSE6 Current ( A) Discharge power (W) Average velocity (m/s) Voltage (kv) Negative Positive Negative Positive Negative Positive (29.5) (29.5) (29.5) (29.5) Fig. 10. Average flow velocity induced in the EHD device for the different discharge power.

6 Podliński et al. 87 Analysing the experimental data in Table I and graphs in Fig. 10 it was noticed that doubling a number of active spike electrodes (at the same voltage value and polarisation) the average airflow velocity in the EHD device duct increased by about 50%. In Fig. 10 it is also clearly seen that both tested electrode configurations with 2 active spike electrodes result in nearly the same values of the average airflow velocities. It means that the 150 mm distances between consecutive spike electrodes were appropriate and the corona discharges from neighbouring electrodes did not disturb each other. IV. SUMMARY The EHD device for air pumping and dust cleaning was presented in this paper. The 2D PIV method was used for investigations of the EHD secondary flow patterns near the spike electrodes and the unidirectional airflow induced in this device. The flow patterns measured near the discharge electrodes revealed strong jet-like flows behind the tips of the spike electrodes and large recirculation areas between the spike and the plate electrodes. We expect that reducing these recirculation areas could be beneficial for inducing a strong unidirectional airflow in the presented EHD device. The airflow measured upstream the spike electrodes showed that our EHD device was capable of inducing unidirectional airflow with average velocity up to 0.64 m/s, what means that maximum average pumping rate was about 13 l/s. Likely the airflow induced in our EHD device could be increased in two ways: by increasing the discharge power and by increasing the number of the spike electrodes. However, obtained current-voltage characteristics showed that for a high discharge power the induced airflow velocity increases slower (with a tendency to saturation, i.e. the achievement of maximum value which cannot be increased by increasing discharge power). It was especially visible when the voltage of positive polarity was applied. Therefore, increasing the number of spike electrodes for induced airflow increase seems to be favourable. Moreover, in such case the discharge volume and the collecting area increase what should be beneficial from particle collection efficiency point of view. It was shown that presented EHD device could be used for efficient air pumping. Further experiments are foreseen to determine all the factors that influence the airflow induced in the EHD device. Also dust collection efficiency measurements will be performed. ACKNOWLEDGMENT The research was supported by the Ministry of Science and Higher Education (grant PB 7475/B/T02/2011/40). REFERENCES [1] O. Stuetzer, "Ion drug pressure generation," Journal of Applied Physics, vol. 30, pp , [2] H. Yanada, S. Hakama, T. Miyashita, and N. 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7 88 International Journal of Plasma Environmental Science & Technology, Vol.7, No.1, MARCH 2013 [20] R. Hanaoka, I. Takahashi, S. Takata, T. Fukami, and Y. Kanamaru, "Properties of EHD Pump with Combination of Rodto-Rod and Meshy Parallel Plates Electrode Assemblies," IEEE Transactions on Dielectrics and Electrical Insulation, vol. 16, pp , [21] M. Raffel, C. E. Willert, and J. Kompenhans, Particle image velocimetry - A practical guide, Springer-Verlag Berlin Heidelberg, [22] M. Goldman and R. S. Sigmond, "Corona and Insulation," IEEE Transactions on Electrical Insulation, vol. EI-17, pp , [23] R. B. Lakeh and M. Molki, "Patterns of Airflow in Circular Tubes Caused by a Corona Jet With Concentric and Eccentric Wire Electrodes," Journal of Fluids Engineering-Transactions of the Asme, vol. 132, , 2010.

Instantaneous and time-averaged velocity fields in a wide electrostatic precipitator

Instantaneous and time-averaged velocity fields in a wide electrostatic precipitator J. Podlinski, Pierre Atten, J. Mizeraczyk To cite this version: J. Podlinski, Pierre Atten, J. Mizeraczyk. Instantaneous