Content of this thesis

In chapter 2, literature related to the previous designs and numerical studies of the corona unipolar chargers is reviewed. In chapter 3, the experimental methods for evaluating the performance of the charger with axial sheath air designed in this study are presented first, followed by the numerical methods for predicting the flow, electric potential, ion concentration, and charged particles concentration fields.

Chapter 4 first covers the experimental results for the optimum operating condition of

the charger with axial sheath air and the comparison of experimental data and numerical results. Then the numerical results for the performance evaluation of two designs of the charger with radial sheath air are presented. Finally, the predicted extrinsic charging efficiency of the charger with radial sheath air is compared with the highest extrinsic charging efficiency of the previous unipoalr chargers (Chen and Pui 1999; Kimoto et al.

2010). In chapter 5, the conclusions of this thesis are drawn and future work is recommended.

CHAPTER 2

LITERATURE REVIEW

A variety of unipolar chargers have been designed to generate ions by radioactive sources (Adachi et al. 1985, 1992; Romay et al. 1991; Romay and Pui 1992a;

Wiedensohlor et al. 1994). In the charger of Adachi et al. (1992), as shown in Figure 2.1, the charger consisted of a Po²¹⁰ radioactive source placed between two screen electrode enclosed by a Plexglass tube. When a positive voltage was applied to the inlet screen electrode and the out electrode was grounded, a uniform electric field was established between the electrodes. Negative ions were attracted to the inlet electrode, while positive ions flowed to the outlet electrode.

Figure 2.1 Schemataic diagram of the unipolar charger using a radioactive source developed by Adachi et al. (1992). (from Intra and Tippayawong, 2011)

In a later work, Chen and Pui (1999) designed a unipolar charger with the parallel configuration of aerosol and ion flow, as shown in Figure 2.2. A relatively weak electrical field was applied to keep the same polarity ions and collect the opposite

polarity ions from a Po²¹⁰ radioactive source. The design further used a sheath air surrounding the aerosol flow to keep charged particles in the core region to minimize electrostatic loss. The extrinsic charging efficiency was found as high as 22% for 3 nm, 48% for 5 nm, and 65% for 10 nm particles. So far, this design has the highest extrinsic charging efficiency for particles smaller than 5 nm in diameter among all unipolar chargers. However, the issue of tight safety regulations on using radioactive sources remains. Moreover, the unipolar charger using radioactive sources required an electric field to separate the positive and negative ions is also referred to as a relatively complicated design. These technical features complicate the structure of the particle charging apparatus, resulting in the increase of cost and causing the apparatus unable to be miniaturized for the use in a portable particle measuring instrument. Charging nanoparticles with unipolar ions produced by corona discharge instead of by radioactive sources is therefore a preferred option.

Figure 2.2 Schemataic diagram of the unipolar charger using a radioactive source developed by Chen and Pui (1999). (from Intra and Tippayawong 2011)

2.1 Previous corona unipolar chargers

Numerous corona-based unipolar chargers were designed to achieve high charging efficiency using either a wire or a needle as the discharge electrode. Compared to the typical unipolar chargers using radioactive sources, the advantages of the corona unipolar chargers are the higher ion concentration and the elimination of radioactive sources. Intra and Tippayawong (2009, 2011) presented a detailed review of the corona-based unipolar chargers including the operating principles and physical characteristics of these chargers. The following paragraphs give a brief overview of the different designs of corona unipolar chargers and a comparison of the charging efficiency based on the work of Intra and Tippayawong (2009, 2011). The corona-wire chargers without sheath air are introduced first, followed by the corona-wire chargers with sheath air and corona-needle chargers.

In the corona-wire chargers without sheath air, Hewitt (1957) was one of the first to develop a corona-wire diffusion charger, as shown in Figure 2.3. The Hewitt charger consisted of a cylinder with a concentric wire along the axis. The corona discharge and aerosol flow region were separated by a metallic mesh and an alternating voltage (AC) was applied between the mesh and the outer electrode of the charger to reduce particles loss. Hewitt reported that the electric field strength resulted in high electrostatic loss for particles with a diameter as small as 70 nm. An improvement on the corona-wire charger based on the Hewitt’s design was carried out by Kruis and Fissan (2001). It was called the twin Hetwitt charger, as shown in Figure 2.4. The charging zone of this charger was separated from two ion production zones by the metal wire screens connected to two square-wave generators to prevent the expansion of aerosol flow into the corona discharge zone. Ions were produced in two cylindrical sections with an Au wire placed in the center of the metal cylinder. It was reported that the extrinsic

charging efficiency of this charger as high as about 5% for 5 nm particles.

Figure 2.3 Schemataic diagram of the corona-wire charger developed by Hewitt (1957).

(from Intra and Tippayawong 2009)

Figure 2.4 Schemataic diagram of the twin Hetwitt charger developed by Kruis and Fissan (2001). (from Intra and Tippayawong 2011)

Biskos et al. (2005a) later developed a Hewitt-type corona charger, shown in Figure 2.5, consisted of two concentric electrodes with a tungsten wire placed along the axis.

The generated ions migrated toward the inner electrode due to the high electric field.

The inner electrode was made of a metallic mesh to allow the ions to flow in the charging zone. An AC voltage was applied on the outer electrode forced ions to enter the charging zone without causing charged particles to deposit on the wall of the charger while the perforated inner electrode was grounded. No extrinsic charging efficiency was reported for this charger.

Figure 2.5 Schemataic diagram of the corona-wire unipolar charger developed by Biskos (2005a). (from Intra and Tippayawong 2011)

In the corona-wire chargers with sheath air, the unipolar charger used in the commercial electrical aerosol analyzer (EAA, model 3030, TSI Inc.) was designed by Liu and Pui (1975). As shown in Figure 2.6, the charger consisted of two concentric metal cylinders with a tungsten wire placed along the axis of the cylinders. A high voltage was applied to the wire to produce a corona discharge from the wire. The ions were forced through the coaxial screen opening on the inner cylinder into the annular

gap region between the cylinders. The aerosol stream flowing in the annular gap was exposed to the unipolar ions. Axial sheath air was used to minimize particles from entering the corona cylinder. The performance of this charger in the nanoparticle range was reported by Pui et al. (1988). Significant charged particle loss was found below 10 nm. An improvement on this charger was designed by Büscher et al. (1994). As shown in Figure 2.7, an AC electric field was applied in the charging zone to reverse the direction of the charged particles before they deposit on the wall. Axial sheath air was also used to prevent particles from entering the corona discharge zone. With this improvement, the charger achieves a 4% extrinsic charging efficiency for 5 nm particles.

Figure 2.6 Schemataic diagram of the corona-wire unipolar charger developed by Liu and Pui (1975). (from Intra and Tippayawong 2011)

Figure 2.7 Schemataic diagram of the corona-wire charger developed by Büscher et al.

(1994). (from Intra and Tippayawong 2009)

Cheng et al. (1997) developed a high-volume corona charger, shown in Figure 2.8, having flow rate of 0.6–6.0 m³/s with radial sheath air through the grounded porous wall of the metal tube to minimize the electrostatic loss, while the annulus aerosol flow was parallel to the six discharge wires mounted around a Teflon rod holder in the axial direction of the charger. The loss of charged particles in the submicron size range in the charger can be reduced by using radial sheath air at the flow rate of 2.4 m³/s or greater, but the extrinsic charging efficiency was not reported.

Figure 2.8 Schemataic diagram of the corona-wire unipolar charger developed by Cheng et al. (1997). (from Intra and Tippayawong 2011)

As shown in Figure 2.9, Biskos et al. (2005b) improved the design of Biskos et al.

(2005a) with axial sheath air to achieve a high aerosol penetration and laminar flow inside the charger. The extrinsic charging efficiency of this charger was reported as high as about 24.9% for 10 nm particles. Recently, Tsai et al. (2008, 2010) developed a unipolar with multiple discharge wires to enhance the extrinsic charging efficiency by using axial sheath air near the wall of the charger to reduce the electrostatic loss of nanoparticles. This charger is shown schematically in Figure 2.10. The charger had a Teflon core to fix four gold wires and the outer stainless steel casing was grounded. To reduce charged particle loss in the charging zone, a clean sheath air was introduced from the 0.1 mm annular slit formed by the aluminum shroud and the outer casing. The performance of the charger was evaluated under different operating conditions including sheath air flow rates, corona voltages, and particles sizes. The highest extrinsic charging efficiency was 2.8%–86.3% for particles of 2.5–50 nm in diameter at the applied

Figure 2.9 Schemataic diagram of the corona-wire unipolar charger developed by Biskos (2005b).

Figure 2.10 Schemataic diagram of the corona-wire unipolar charger developed by Tsai et al. (2008, 2010).

In the corona-needle chargers, Whitby (1961) developed the first needle-type corona charger, shown in Figure 2.11, which consisted of an arrangement of a sharp needle at high voltage upstream of a sonic orifice to generate the ions within a non-conductive housing. Clean air entered at the inlet then passed through the orifice. It was reported that the ion concentration can be up to 10¹⁷ ions/m³ in the charging zone. A simple corona-needle charger for charging nanoparticles was proposed by Hernandez-Sierra et al. (2003), as shown in Figure 2.12. The design was a cylinder tube with tapered ends and divided into three sections. The first and second sections were made of methacrylate and the third (outlet) section of aluminum. A circular piece made of Teflon, placed between the two methacrylate sections, contained a series of orifices through the aerosol flows. The central piece was used to hold a stainless steel needle electrode, ending in a sharp tip coaxial with the cylinder. The outlet metallic section was grounded. It was reported that the extrinsic charging efficiency of this charger as high as about 2.1% for 2.7 nm particles.

Figure 2.11 Schemataic diagram of the corona-needle unipolar charger developed by Whitby (1961). (from Intra and Tippayawong 2011)

Figure 2.12 Schemataic diagram of the corona-needle unipolar charger developed by Hernandez-Sierra et al. (2003). (from Intra and Tippayawong 2009)

Later on, an improvement on the charger of Hernandez-Sierra et al. (2003) was made by Alonso et al. (2006) by modifying aerosol inlet geometry as well as the manner holding the discharge electrode shown schematically in Figure 2.13. It consisted of an inner stainless steel electrode ending in a sharp tip. The electrode is coaxial with a grounded metal cylinder which inner wall has a conical shape. The extrinsic charging efficiency of the charger of Alonso et al. (2006) was 1.8% for 3 nm particles.

Figure 2.13 Schemataic diagram of the corona-needle unipolar charger developed by Alonso et al. (2006). (from Intra and Tippayawong 2009)

A corona-needle miniature unipolar charger for a personal particle sizer was proposed by Qi et al. (2008). As shown in Figure 2.14, the charger consisted of two major components. The outer included a radial inlet tube and axial outlet tube. The second was the corona discharge module, consisting of a pointed tungsten needle electrode placed coaxially in the outer tube capped with a perforate dome. The corona discharge module was installed in the case at the end opposite the axial exit tube. It was reported that the extrinsic charging efficiency of this charger was 4% for 5 nm particles.

Figure 2.14 Schemataic diagram of the corona-needle unipolar charger developed by Qi et al. (2008). (from Intra and Tippayawong 2009)

Similar to the configuration of the charger of Qi et al. (2008), Li and Chen (2011) developed a corona-needle unipolar charger consisted of an outer metal case and a corona discharger tube module capped with a metal screen at one end, as shown in Figure 2.15. A pointed tungsten needle was used in the tube module to produce ions.

The corona discharge tube module case was on the ion-driven voltage which was much lower than that applied to the needle. Ions produced in the tube module were driven through the metal screen by a weak electric field into the charging zone. The

geometrical arrangement of the tube module and the aerosol exit section allowed establishing the ion-driving field approximately in the longitudinal direction, which achieved the implementation of parallel electric and aerosol flow fields. The ion concentration in the charging zone can be controlled by varying the strength of the ion-driving field. It was reported that the extrinsic charging efficiency of this charger as high as about 8.8% for 5 nm particles.

Figure 2.15 Schemataic diagram of the corona-needle unipolar charger developed by Li and Chen (2011).

Whitby (1961) first introduced the concept of applying a sonic jet flow to direct unipolar ions out from the corona discharge zone in the development of an ion generator.

Medved et al. (2000) used a similar principle in the design of a unipolar charger, shown in Figure 2.16, which was later modified and used in the electrical aerosol detector (EAD, model 3070A, TSI Inc.) and nanoparticle surface area monitor (NSAM, model 3550, TSI Inc.). Ions were generated at a corona needle tip in a small ion-genetration chamber connected to a mixing chamber via an orifice. An air flow transferred the ions

into the mixing chamber and an opposing aerosol flow promoted mixing the aerosol and the ions. However, the issue of particle loss in ion-particle flow mixing was often encountered in the above chargers.

Figure 2.16 Schemataic diagram of the corona-needle unipolar charger developed by Medved et al. (2000). (from Intra and Tippayawong 2009)

With a careful flow mixing arrangement, Qi et al. (2007) developed a mixing-type unipolar charger consisted of completely separated corona ionization and charging chambers, as shown in Figure 2.17. Sonic-jet flow was also used to inject unipolar ions into the charging chamber. Instead of injecting ions perpendicular to or against the aerosol flow directions in the previous chargers of the same type (Medved et al., 2000;

Marquard et al., 2006a), the charger injected sonic-jet flowed from each of the two ionizers into the charging zone at a 45° angle to the charger axis. The use of angular impingement of dual jet flows in the charger not only canceled the jet energy in the radial direction but also facilitated the exit of charged particles. The extrinsic charging efficiency of the charger of Qi et al. (2007) was reported as 5.7% for 4 nm particles.

Recently, Kimoto et al. (2010) also developed a mixing-type unipolar charger, shown in Figure 2.18, consisted of a high-pressure corona ionizer to generate unipolar ions and a

small charging chamber (0.5 cm³ volume) where the ions were mixed with nanoparticles without an external electric field at negative pressure. Up to now, the charger of Kimoto et al. (2010) showed the highest extrinsic charging efficiency among all existing corona-based unipolar chargers for particles smaller than 10 nm in diameter.

The measured extrinsic charging efficiency was up to 59.7 % for 5 nm particles.

Figure 2.17 Schemataic diagram of the corona-needle unipolar charger developed by Qi et al. (2007).

Figure 2.18 Schemataic diagram of the corona-needle unipolar charger developed by Kimoto et al. (2010).

2.2 Numerical studies of the corona unipolar chargers

Diffusion charging has been studied theoretically and various models are available.

Detailed overview on the diffusion charging models in all aerosol regimes can be seen from the literature (Romay and Pui 1992a; Biskos et al. 2004; Marquard 2007). In the transition regime (Knudsen number Kn1), the birth-and-death charging model (Boisdron and Brock 1970) with the ion-particle combination coefficient estimated by Fuchs diffusion charging theory (Fuchs 1963) was used to predict the charge distribution for the unipolar charger, assuming Ni t condition is given where Ni is the ion concentration (ions/m³) and t is the charging time (sec) (Biskos et al. 2005b; Marquard et al. 2005; Qi et al. 2007, 2009; Vivas et al. 2008; Li and Chen 2011). The model assumes that ion concentration in the charging region is spatially uniform, considering neither the transport of ions and particles nor particle loss in the charger. These assumptions which are difficult to validate, especially for charging devices with complicated geometrical, electrical, and hydrodynamic conditions (Marquard et al.

2006b), could lead to inaccurate predictions. By considering the transport effects of ions and particles, some numerical models (Aliat et al. 2008, 2009; Alonso et al. 2009) were able to simulate unipolar diffusion charging based on Fuchs theory in a tube flow with the simple plug flow assumption for the flow field. However, the models are not applicable to other unipolar chargers with more complicated geometry. Huang and Alonso (2011) obtained particle trajectories through the combined mechanisms of diffusion and field charging to calculate nanoparticle electrostatic loss in the corona-needle unipolar charger for particles ranging from 3–30 nm in diameter. But the charging efficiency was not calculated. Kimoto et al. (2010) developed a theoretical model based on Fuchs theory to predict the extrinsic charging efficiency of an efficient small mixing-type unipolar charger. The measured charging efficiency for particles

smaller than 10 nm was much higher than the theoretical results due to the well-mixed flow assumption.

Table 2.1 and 2.2 summarize the charging performances and numerical studies of the above mentioned corona-based unipolar aerosol chargers, respectively. In summary, the charger performance depends on the extrinsic charging efficiency. An efficient unipolar charger should be further designed and developed for charging nanoparticles. In addition, an accurate numerical model which can be used to facilitate the design of an efficient unipolar nanoparticle charger and predict charging efficiency is needed to develop.

Table 2.1 Charging performances of corona-based unipolar aerosol chargers.

Table 2.2 Numerical studies of the corona-based unipolar aerosol chargers.

Researcher Numerical method Particle

charging model Assumption / Limit Biskos et al. (2005b);

Alonso et al. (2009) Eulerian method Fuchs theory (1963)

Simple plug flow for the flow field in the charger.

Kimoto et al. (2010) Lagrangian method Fuchs theory (1963)

Complete mixing of particles with ions in the

charging chamber.

Huang and Alonso (2011) Lagrangian method Combined charging model

The charging efficiency is not available.

CHAPTER 3 METHODS

3.1 Experimental method

3.1.1 Design of unipolar charger with axial sheath air

Figure 3.1 shows the schematic diagram of the unipolar charger with axial sheath air which is a modification of the nanoparticle charger with multiple discharging wires developed by Tsai et al. (2010). The charger consists of a gold wire of 50 μm in diameter and 2 mm in length as the discharge electrode, on which a high D.C. voltage is applied from the top of the charger. The outer stainless steel cylindrical casing of 30 mm in diameter is grounded. The space between the gold wire and the stainless steel casing is the charging zone where aerosol charging takes place. The aerosol flow was introduced into the charger from the bottom and a filtered high-speed sheath air flow with the velocity of 0.9–7.1 m/s was introduced from an annular slit of 0.1 mm gap formed by the Teflon shroud and the outer casing to minimize charged particle loss. The charged particles were accelerated to exit the charger quickly through another annular slit of 0.1 mm gap after the charging zone.