Patent ID: 12194475

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Referring to the figures, an ESP particle collector1according to embodiments of the invention comprises an inlet section4, a collector section6, and an electrode arrangement8. The particle collector may further comprise a particle charger2arranged upstream of the inlet section4configured to electrically charge the particles of the gas stream entering the inlet section4.

The particle charger is configured to impart a small charge on the particles contained in the gas stream to be sampled preferably in a range of about 1 elementary charge per 10 nm (1 nm=10−9m) to about 1 elementary charge per 50 nm diameter of a particle. Preferably the charge is in a range of 1 elementary charge per 10 nm diameter to about one elementary charge per 40 nm diameter for instance around 1 elementary charge per 20 nm diameter. The relatively small charge allows the particles to be charged with a low generation of reactive species such as ions and radicals such as ozone, in order to ensure low chemical interference on the particles contained in the gas stream. Various per se known particle chargers may be used, such known chargers using field charging, diffusion charging, or ultraviolet charging, provided that they have a low reactive species generation on the particles in the gas stream.

An example of a charger that may be used for the invention is for instance described in Han [5] which describes a wire-wire charger with a low ozone production.

The charging of the particle stream, although optional in embodiments of the invention, advantageously assists in improving uniforms spatial distribution of particles on the collector plate.

The inlet section4comprises a flow tube10defining a gas flow channel12therein bounded by a guide wall24that is preferably of a generally axisymmetric shape. The flow tube that may be generally cylindrical as illustrated in embodiment ofFIG.1or may have other axisymmetric shapes for instance as illustrated inFIGS.2and3. The flow tube may however also have non-axisymmetric cross-sectional profiles such as polygonal (square, pentagon, hexagon or other polygons).

The flow tube10extends between an entry end14and a collector end16that serves as an inlet to the collector section6.

The entry end14comprises an inlet28for the particle gas stream and a sheath flow inlet portion26for generating a sheath flow around the particle gas stream. By the term “particle gas stream” it is meant the gas stream containing the particles to be collected in the collector section6.

The sheath flow inlet portion26comprises a sheath flow gas inlet27, a gas chamber29and a sheath flow gas outlet31surrounding the centre of the flow channel12and configured to generate and annular sheath flow along the wall24of the flow channel12surrounding the particle gas flow. The chamber29serves to contain a volume of gas with a low or essentially no pressure gradient within the chamber with respect to the sheath gas inlet, such that the radial nozzle defining the sheath flow outlet31generates an even circumferential sheath flow.

The flow rates of the sheath flow and particle gas flow may be calibrated such that the two gas streams have laminar flow properties and the boundary layer between the sheath flow stream and particle gas stream remains laminar substantially without mixing. The gas flow streams are configured such that the Reynolds number is below 2200, preferably below 500, for instance around 200.

The flow tube10has an overall length D that is configured to ensure that the velocity of the sheath gas stream and particle gas stream at the interface therebetween accelerates such that the velocity profile of the gas stream within the flow channel collector end is a substantially continuous single rounded profile with a substantially flatter profile compared to the gas stream as the entry end. In effect, at the sheath flow outlet, the laminar flow profile is substantially parabolic and joins the particle gas stream at the boundary interface with a velocity close to zero that accelerates as the gas stream flows away from the sheath flow outlet.

The sheath flow separating the particle gas flow from the guide wall24reduces or avoids deposition of particles on the guide wall24and has further advantages in improving spatial uniformity of the particle deposition in the collector section6, reducing also chemical interference, reducing size dependence in the collection and improving collection efficiency. This is not only because it reduces the gradient in axial velocity of the particle gas stream that flows on to the collector plate, but also due to the separation of the gas stream from the flow channel walls, it reduces interference of the charge particles with the flow channels walls.

The collector section6comprises a housing18coupled to the flow tube10, and a collector plate20mounted therein on a collector plate holder22.

In an embodiment, the inlet section4may be coupled removably to the collector section6for instance by means of an assembly ring33. In an embodiment, the collection section6comprises a removable cap35allowing access to a chamber inside the housing18for insertion and removal of the collector plate20.

The collector plate may for instance comprise a transparent disc, for instance made of a crystal such as a Silicon, Zinc Selenide, or Germanium crystal, that may be used for optical analysis, for instance infrared spectroscopy. In such applications, the collector disc may be removably mounted within the housing for placement in observation of a spectroscopic instrument for analysing the particles deposited on the collector plate20. In other applications it would however be possible to integrate this spectroscopic optical instrument or other measuring instruments within the housing18of the particle collector for automated measurement of the particles collected on the collector plate. The collector plate20may comprise a filler material21arranged around the collector plate20. The gas stream flow over the collector plate is thus defined not only by the collector end16of the flow tube10but also the radius of the collector plate20and the filler material21therearound.

The electrode arrangement8comprises at least a base electrode8apositioned adjacent or on an underside25of the collector plate20, below the collection surface23where particles are deposited. The electrode arrangement8further comprises a counter-base electrode8bpositioned at a certain separation distance L2above the collector plate20and which may be arranged substantially parallel to the base electrode8asuch that an electrical field is generated between the electrodes8a,8b.

In embodiments, the electrode arrangement may optionally further comprise a tube electrode8caround the collector end16forming the inlet to the collector section6. The tube electrode8cmay be at the same voltage as the counter-base electrode8bor at a different voltage therefrom separated by an insulating element from the counter-base electrode8b.

It may be noted that the various electrodes may be at a certain voltage with respect to ground or one of the electrodes may be connected to ground and the other at a potential different from ground.

The inlet channel at the collector end has a radius defined as L1. The collector plate has a radius defined as L3. The base electrode has a radius defined as L4. The distance between the counter-base electrode8aand the collector plate20has a separation distance defined as L2.

At least two ratios, namelythe ratio L1/L2between the inlet channel collector end radius L1and counter-base electrode to collector plate separation distance L2named hereinafter for convention as ratio_1, andthe ratio L1/L3between the inlet channel and radius L1and the collection plate radius L3named hereinafter by convention ratio_3
are within certain ranges that according to an aspect of the invention allow to provide a high spatial uniformity and low size dependence, as well as a high collection efficiency of particles to be sampled on the collector plate20.

An optimal ratio_1(L1/L2) affects the variation in the electric field under the inlet tube which may be optimized to improve spatial uniformity and collection efficiency.

A lower bound value for an optimal ratio_3may be constrained by any value where impaction affects the final deposition pattern, however collection mass flux is generally higher if this ratio is more than 1. An upper bound value may be constrained by a fixed limit on operating voltage (and maximum electric field strength) and on ratio1above, for example by,

ratio3≤VmaxEmax×ratio1collection⁢disc⁢radius
The upper bound value may also be constrained by a desired efficiency, for example by,

ratio3≤1efficiency×radial⁢sheath⁢position⁢⁢lim_s.

Advantageously, another ratio L1/L4of interest for high spatial uniformity and low chemical interference is a ratio between the radius L1of the inlet channel collector end and the base electrode radius L4, named hereinafter by convention as ratio_2. The ratio_2controls the electric field concentration effects on the collector plate's edges. An optimal ratio_2may thus serve to improve spatial uniformity and lowers the electric field strengths in some regions, in particular to lower the variation in electric field strength under the inlet tube.

According to an aspect of the invention, the ratio_1(L1divided by L2) is in a range of 0.3 to 1.8, preferably in a range of 0.8 to 1.2.

According to an aspect of the invention, the ratio_2(L1/L4) is less than 1, preferably less than 0.7, for instance 0.5 or lower.

According to an aspect of the invention, the ratio_3(L1divided by L3) is preferably in a range of 0.05 to 20, preferably in a range of 0.1 to 5.

The electric field generated between the base electrode8aand counter-base electrode8bis preferably in a range of 0.1 kV per mm to 3 kV per mm, preferably from 0.5 kV per mm to 1.5 kV per mm for instance around 1 kV per mm, with an absolute voltage on any electrode that is less than 10 kV, to reduce chemical interference while ensuring high collection efficiency.

The inner radius Ls of the sheath flow relative to outer radius L1of the sheath flow at the collector end16forming the inlet to the collector section6, is defined herein as ratio hill s (Ls/L1). Ratio hill s is in a range of 0.1 to 0.9, preferably in a range of 0.1 to 0.6, for instance around 0.4, to ensure a sheath flow layer sufficient to provide a good separation between the gas particle stream and the flow channel wall24as well as ensuring that the particle gas stream impinging upon the collector plate20allows optimal uniform spatial distribution of the particles on the collector plate.

The above mentioned ratios are important in achieving the following advantages of embodiments of the invention:

AdvantageFeaturesSpatially uniformSheath flow: the method of introducing sheath flow described herein resultsdeposition patternin high spatial uniformity in the deposition pattern.Defining the geometric length ratios greatly reduces effect of impaction withlarger inlet tube radius size.The absence of electrodes/sheets in the inlet flow tube avoids disturbance inthe gas flow stream.Low size-Defining the geometric length ratios: mainly, increasing the inlet tube radiusdependenceL1 relative to collection plate radius L3 will lower particle size dependence.However, this could generally mean a loss of collection efficiency. Hence,the value is limited in the range where the collected mass flux is higher onthe collection plate.Sheath flow: This is an artificial method of tuning this ratio described above,as even for a larger tube, a sheath flow limits the incoming particles to acertain radial distance.Low ChemicalDefining the geometric length ratios: Define separation distance L2 requiredinterferenceto maintain a low electric field strength, and keep deposited particles furtheraway from high-voltage counter-base electrode 8b. Moreover, increasing theratio of inlet radius L1 to the base electrode 8a radius L4 is useful forreducing local electric field strengths.Sheath flow: This keeps particle laden air streams farther away from thehigh-voltage counter-base electrode 8b in the collection region.High collectionFocusing particles to the center using a) tube electrode 8c and b) sheath flowefficiencyHigh collectionDefining the geometric length ratios: mainly, increasing the inlet tube radiusmass fluxL1 to collection plate radius L3 will increase the flow rate that one canachieve for keeping the same lower size-dependence, but the efficiency willgo down.Note:Constraints exist on this ratio because of the constraints onthe ratio of the inlet tube radius L1 to the separation distance L2.Sheath flow: depending on the axial velocity profile, this increases theoperable flow rate limit for a similar size-dependence performance.Moreover, the increase in collection efficiency also directly effects thecollection mass flux.

Embodiments of the invention may advantageously be used in various applications, including:Aerosol sample collection for spectroscopy and spectrometry—or other types of chemical analyses—for studies in air quality, atmospheric science, or industries that involve particle generation (e.g., fabrication and manufacturing, construction, e-cigarettes) where worker or customer safety is a consideration.Seeding applications for subsequent epitaxial film growth of bulk/film crystals can prove useful in membrane technology and nanocrystal technology.Health studies where collection of biological sample is needed for subsequent optical analysis or other in-vitro/in-vivo studies.Coating applications.

An unexpected finding by the inventors of the present invention is that the particle velocity distribution at the plane of deposition (just before collection on the collector plate20) is not a direct representation of the final distribution of deposited particles. This finding contradicts conventional thinking such as found in the work of Dixkens and Fissans [1] and Preger et al. [2], but is an important consideration predicting and therefore optimizing particle deposition patterns.

Examples of Implementation

Use Case: “Aerosol Sampling Device for Quantitative Spectroscopic Analysis”

It is important to characterize the composition of aerosol particles in air, which causes adverse health effects and millions of deaths each year. Aerosol, or particulate matter (PM), is difficult to characterize because of its wide range of particle sizes (few nanometers to several micrometers); constituents (various organic and inorganic compounds); concentration (one to hundreds of μg/m3, for PM<2.5 μm); morphology; state (liquid or solid); and time-dependent modification.

An ideal collector would enable collecting an aerosol sample that is an identical copy of the aerosol in air at an instant of time. Such a collector, when used with an ideal characterization method, will allow an ideal quantitative measurement of the composition of the aerosol. However, most conventional particle collectors modify or preferentially sample certain size ranges, chemical composition, morphology or state. Furthermore, collected sample is characterized for the constituents and/or their composition using numerous spectrometric techniques, which can induce further modifications. For example, most spectroscopic techniques require collecting aerosol on a surface for a prolonged period to make a confident claim about its constituents' composition.

Infrared (IR) spectroscopy is a non-destructive method, which provides useful chemical information about the constituents. Current methods for collecting samples use filters that are made of material which interferes with the IR spectra and thus lowers detection capabilities. Hence, collection on an IR-transparent substrate (for example, chalcogenide crystals) is desirable. A particle collector according to embodiments of the invention that achieves the advantages mentioned above allows to make a good quantitative measurement using IR-spectroscopy. Specifically, “Low size-dependence”, “Low chemical interference” and “High collection efficiency” is required to collect an aerosol sample that is identical to the aerosol in air, “High spatial uniformity in deposition pattern” is required to reduce optical artefacts or spectrometer dependence, and “High collection mass flux” is required to reduce the collection time needed for making a confident claim.

Electrostatic precipitation (ESP) is a versatile method of collection and does not suffer from high pressure drop (which can modify the aerosol chemical composition, for example in filtration), or from bounce-off effects (which preferentially samples the size range and liquids, for example in impaction). ESP is a common device for dust removal but is also used for particle deposition.

Example 1: Referring to the exemplary embodiment illustrated schematically inFIG.5a, the plot in5a(ii), which is aligned along the vertical axis (z) with the particle collector schematically shown inFIG.5a(i), shows results from particle deposition simulations done using a simulation program (COMSOL Multiphysics). The difference between the outer radius of deposition of various sizes is low and the spatial deposition is close to the ideal profile. In this example the collector plate radius is L3=12.7 mm.

Inlet and operating conditions:1. Inlet flow condition—Sheath flow: Sheath flow is used starting from a radial position that is lims=⅓, i.e the radial position of commencement of sheath, Ls is ⅓rdthat of the inlet tube radius (L1) at the collector end. However, if needed, the sheath limit can be varied while it is lower than 0.5. This is important for the spatial uniformity of the final deposition.2. Inlet charge condition—Particles are pre-charged before entering the collector section. The charging is selected to be at a level such that different sized particles are charged to a level corresponding to about 1 elementary charge for every 20 nm particle diameter.3. Operating condition—electric field strength and voltage: The voltage was fixed at 10 kV on the counter-base electrode, while the base electrode is grounded. This leads to an electric field strength of 1 kV/mm. However, if needed, the voltage on the counter-base electrode can be varied while the electric field is lower than the breakdown strength in air (around 3 kV/mm), and while the absolute voltage on any electrode is lower than 10 kV, preferably.4. Operating condition—flow rate: For the given collector disc size (L3=12.7 mm), given electric field strength of 1 kV/mm and the given charge condition of 1 elementary charge every 20 nm particle diameter, the flow rate is tuned such that the maximum deposition flux is obtained. This happens around 2 LPM (liters per minute) of aerosol (particle) flow. With the given sheath flow limit, lims=⅓, this aerosol flow rate corresponds with a sheath flow rate of 7.5 LPM. If the sheath flow limit is changed to lims=0.5 as shown inFIG.5c, then the only change for operating condition would be to change the sheath flow to 4.6 LPM. If there is any change in the electric field strength, the degree of charge and the area of collection, the aerosol flow rate can be changed in direct proportion to either of those changes, in order to keep operating at the maximum deposition flux limit.

Example 2: This example shown inFIG.5b, has the same collection plate radius and differs from Example 1 above mainly in the ratio3value L1/L3.

Inlet and operating conditions:1. Inlet flow condition—Sheath flow: Sheath flow limit is lims=½, instead of the ⅓ in the previous example 1.2. Inlet charge condition—Same as Example 1, at 1 elementary charge for every 20 nm particle diameter.3. Operating condition—electric field strength and voltage: The electric field strength was the same as that in Example 1, at 1 kV/mm. As ratio3is halved, the operating voltage in order to keep the same electric field strength was also halved, thus, 5 kV was applied on the counter-base electrode, while the base electrode is grounded.4. Operating condition—flow rate: Same aerosol flow as Example 1, as the electric field strength, charge condition and the collector plate radius L3is the same. Thus, 2 LPM of aerosol flow. However, With the given sheath flow limit, lims=½, this aerosol flow rate corresponds with a sheath flow rate of 4.6 LPM. If the device in Example 1 is operated with a sheath limit of lims=½ as well, then the sheath flow would have been kept the same at 4.6 LPM.
Equations of Radial ESP Systems Affecting Operating Conditions

Referring toFIG.6, the basic equation for the radial ESP system is:

rfRc=r0R⁢(RRc)⁢1+vinvelec⁢(r0R)-1⁢f_v⁢(r0)⇒rfRc=(r0R⁢RRc)2+3⁢(Q⁢μe⁢E0⁢Rc)⁢(Dpn⁢Cc⁢Rc)⁢(r0R)⁢fν¯⁢(r0)where, Q=Total flow rate (aerosol flow rate+sheath flow rate),E0=Electrostatic field strength,Dp=Particle diameter,Cc=Drag correction factor,Rc=Collection plate radius (L3),R=Inlet tube radius (L2),e=charge on an electron,n=no. of elementary charges on the particle of size Dp, andμ=dynamic viscosity of air.

The equation is different for the two representative theoretical inlet flow profiles illustrated inFIG.6: a) plug-flow and b) parabolic flow, based on the different expression offv(r0) and Q for both the cases. For the limiting case of

r0R=r0,maxR=lims
the final position for the outermost particle for both the equations become:

rf,maxRc=(r0,maxR⁢RRc)2+3⁢(Qa⁢μeE0⁢Rc)⁢(Dpn⁢Cc⁢Rc)⁢(r0,max/R)2[2-(r0,max/R)2(lims)2[2-(lims)2])Parabolic-flow inletrf,maxRc=(r0,maxR⁢RRc)2+3⁢(Qa⁢μeE0⁢Rc)⁢(Dpn⁢Cc⁢Rc)⁢(r0,max/R)2lims2)Uniform-flow inletwhere, Qa= Aersol flow rate (particle containing air stream)where, Qa=Aerosol flow rate (particle containing air stream)

Furthermore, this equation is in terms of the aerosol flow rate, which is the flow rate of interest as it contains the particle and if possible maximizing this flow rate while keeping the collection efficiency high would be ideal. Some key implications of the analytical model:1. Results and analysis are scalable—as the model is dimensionless: The analytical model generalizes device performance in one geometry to a wide range of others due to its dimensionless form. For a given inlet flow condition (parabolic or uniform) and a fixed sheath position (lims), there are mainly 4 dimensionless parameters:

(r0,maxR⁢RRc)
—relating to geometry,

(Qa⁢μe⁢E0⁢Rc)
—relating to operating parameters,

(Dpn⁢Cc⁢Rc)
—relating to particle properties and

(rf,maxRc)
—relating to particle collection performance. All these four parameters scale with the collection plate radius (L3=Rc).2. For operation conditions, Qa/E is present in a term, meaning that doubling the electric field strength and the aerosol flow rate would result in the exact same aerosol collection performance.3. For particle based dependence, D/n is present in a term, meaning that if the amount of charge on a particle scales proportional to its diameter (which is many times the case), then there is negligible effect on particle size performance because of charge alone. However, the size dependence emerges because the drift correction, Ccranges over orders of magnitude for particle sizes between 100 nm and 1 μm. This is the mathematical representation of the size-dependence in the ESP device.4. Collection performance is related to the outermost final potion of the particle on the collector plate,

(rf,maxRc),
as larger this value, the more spread out the collection and thus lower the collection efficiency. If the final spatial deposition is uniform, then the collection efficiency can be represented as

η=π⁢Rc2π⁢tf,max2=(rf,maxRc)-2.5. Most importantly, this analytical model is original in that it includes this vast number of geometric, operating and performance parameters, allowing using it to propose geometries for a desired performance and operating condition, or to find operating conditions for a given geometry and desired performance or to simply evaluate the performance of a given device operating at certain conditions.
Factors Important for “High Spatial Uniformity in Deposition Pattern”.1. Inlet condition—Sheath flow and axial velocity profile: The axial velocity profile under laminar flow conditions (mostly the case in this invention), can either be parabolic-like (when near fully developed for example), or plug-flow-like (when entering a sudden contraction, or exiting a nozzle for example). Both conditions are possible and result in different spatial deposition pattern—and ultimately the spatial uniformity. For the case of plug-flow-like inlet some sheath might be required depending on how the plug-flow is developed (orifice, nozzle, flow straightener, others).For parabolic-flow-like inlet makes the deposition uniform—the closer to the center the sheath flow starts, the greater is the effect of making the final deposition uniform. A radial starting position of 0.5 (as a ratio to the inlet tube radius, L1) is desirable for parabolic flow. This helps achieve spatial uniformity, even for a parabolic like axis velocity at the inlet. An example of using no sheath vs using a sheath flow for parabolic-flow-like inlet, is shown inFIGS.7,7b.2. Device geometry—Ratio of the inlet tube radius L1to the separation distance L2(ratio1): As this ratio changes, the radial distribution of the electric field strength over the collector plate changes. This change in electric field strength just under the inlet tube is evident in all the simulations (COMSOL Multiphysics simulations). An example of the effect of ratio1(for values equal to 1.00 and 4.00) is shown inFIGS.8a,8b. A very high value results in non-uniformity in deposition because of the non uniformity in the electric field strength. The average and the variation of the electric field strength (normalized to maximum) over the collector plate is shown inFIG.9a, as a function of both ratio1and ratio3. ratio1<1.5 results in low variations. Moreover, there is a clear advantage of using lower values of ratio1as the average electric field strength over the collector plate is high over a wide range of ratio3.
Factors Important for “Low Size-Dependence”.1. Device geometry—Ratio of the inlet tube radius to the collector plate radius (ratio3): Some points that affect ratio3are discussed here.Firstly,FIG.12, shows that the values of (lims)×ratio3>1.1 reduces the efficiency to below 50%, which is not desirable. Hence, desirable values are

ratio3⁢<1.1lims.
For example, if radial sheath position (position where sheath begins as a ratio of the inlet radius, L1) is 0.5, then ratio3<2.2. This consideration of efficiency is high in priority, though it can be overruled if low efficiency is justified for the process.Secondly, very low values of the inlet radius L1, can result in impaction of particles, which is not desired. With a smaller radius inlet tube there are higher chances of irregularities affecting the deposition. Furthermore, as ratio1would be fixed, making the inlet radius small, would also bring the electrodes closer the collector plate, which increases the likelihood of electrical discharge. For these reasons, operating at low inlet radius sizes is not desirable. As these considerations are quite subjective, the actual scale of the collector plate is needed to make better estimate on this value. Tentatively, if we are on the scale of 10 s of mm for the collector plate, then a ratio of ratio3>0.1 is desirable, with a higher value being better. The consideration of the effect of impaction and electrical discharge possibility is of high priority.Lastly,FIG.9b, apart from showing the effect of ratio1also shows range of ratio3values that can adversely affect the electric field strength above the collector plate (and hence, the uniformity). Very low ratios ratio3<0.1 have a low variation and a high average value of the electric field strength. Similarly, higher values, ratio3>2 also reduces the variations because of ratio1(though the average electric field strength is not as high at these values). This consideration, though important, can also be solved by choosing the correct, ratio1values, and is hence of lower priority.2. Inlet condition—Sheath flow: Particles are focused towards the center because of the tube electrode. Details of the extent of focusing is shown and discussed inFIG.14. The effect is different for different particle sizes and smaller particles are focused more and hence, induces size-dependence. The closer to the center the sheath flow starts, i.e. lower the value of lims, the extent of size dependence is lower (for both uniform flow and parabolic flow inlet). Thus, lower values of limsis desirable.
Factors Important for “Low Chemical Interference”.1. Operating condition—Electric field strength (E0): Very high electric field strengths are undesired as chemical interference can increase through generation of reactive free radicals that react with the particles. The electrical breakdown of air is around 3 kV/mm. An average electric field strength is the ratio of the applied voltage (between counter-base electrode and the base electrode), and the separation distance L2. However, the presence of edges and of charged particle inside this electric field can enhance the local electric field strength values. Hence, a factor of safety (of 1.5 or 2 for example) should be used to limit the design electric field strength. Furthermore, some studies on streamer discharge also mention onset conditions from electric field strength of 2.28 kV/mm [4].In the two embodiment examples 1 and 2, a safety factor of 3 is used on the breakdown voltage in which manner it is also below the 2.28 kV/mm limit.2. Operating condition—Counter-base electrode Voltage: Apart from an electrical discharge stemming from the local electric field strength, there are a few processes which also limit the voltage directly, to a degree. For example, Trichel discharge from electrodes (generally sharp) with high negative potential or streamer discharge from electrodes (generally sharp) with high positive potential have similar onset conditions [3]. Trichel discharge has been shown to have lesser dependence on the separation distance and onset from above 10 kV in magnitude. For these reasons, the examples 1 and 2 are to be operated at 10 kV and 5 kV respectively.3. Inlet condition—Sheath flow: The closer to the center the sheath flow starts (i.e. lower the value of lims), the further away particles are kept from the high voltage on the tube electrode and the counter-base electrode. Some studies [6] have shown that any ozone produced between electrodes has a hyperbolic concentration profile which decreases further away from the discharge electrode. Thus, lower values of limsis desirable.4. Device geometry—Ratio of the inlet tube radius to the base electrode radius (ratio2): The sudden increase in electric field strength values because of ratio2>0.5, is undesirable also because it might result in possible chemical modification. Thus, values of ratio2<0.5 is desirable.
Factors Important for “High Collection Efficiency” and “High Collection Mass Flux”.1. Inlet condition—Charge: It is assumed that the particles are charged prior to introducing into the device. Any charger that charges the particles using field charging/diffusion charging/UV charging can be used. The number of elementary charges on a particle charged using a combination of field charging and space charging is approximately directly proportional to the particle size. In the examples 1 and 2, it has been assumed that 1 elementary charge per 20 nm diameter is present. The charger used in Examples 1 and 2 is a wire-wire charger per se known as a part of a bioaerosol sampling device that has low ozone generation (hence, low chemical interference).2. Operating condition—Flow Rate and collection flux: The relationship between the total operating flow rate and the particle-laden aerosol flow rate (Qa) is shown inFIG.6. An example of determining the operating aerosol flow rate for a design such that size-dependence is low and collection flux is high is shownFIG.11. The variables affecting this flow rate is illustrated inFIG.8.FIG.11shows the minimum aerosol flow rate limit for different designs on the same collector plate are and the same charging and electric field conditions. Note that the focusing effect because of tube electrode is present in these calculations. Surprisingly, the volume flux of deposition is nearly a constant i.e. by changing the geometry (sheath position and ratio3) the proportion of change in the flow rate limit for a said size-dependence is the same as the proportion change in the collection spot area on the collector plate.Hence, we can see that for a given charge condition, particle size range and electric field strength, the collection volume flux is more or less a constant at φmax=Qa/(πRc2=0.3936 LPM/cm2. Moreover, on this value, there is absolutely no variation because of the collector plate radius (over orders of magnitude) and very small variation if the sheath position changed as shown inFIG.13.For the examples 1 and 2, as the collector plate radius is 12.7 mm, we operate the device at 0.3936×3.14×1.272≅2 LPM. The charging condition used was 1 elementary charge for every 20 nm diameter, and the particle size range was from 100 nm to 2.5 μm (though the difference for a range of 100 nm to 1 μm was very little).The collection mass flux can be calculated by multiplying the collection volume flux with the particle concentration and the mass density of each particle.3. Device geometry—Tube electrode: The presence of tube electrode results in particles being focused towards the center. This effect is very prominent and results in increase in collection efficiency (as the particles are closer to the center). The extent of focusing is different for different flow rate, electric field strength and particle size. If the device is operated at the flow rate limit (as discussed above), then for a given collector plate radius, the drift effect is shown for different geometric parameters. For a parabolic flow inlet profile,FIG.14(i) and (ii) shows the extent of drift (as percentage drift away from the initial position in the tube, sheath position lims), and the size-based variation in the extent of drift (shown as the ratio if median absolute deviation (MAD) and the median). The size-based dependence is not desirable.5. Inlet condition—Sheath flow: For a given ratio3value, the sheath position can be lowered in order to operate at a higher efficiency. As shown inFIG.12, values of ratio3×lims>0.8 (approximately), the maximum collection efficiency decreases as the minimum operating flow rate for “acceptable” size-dependent variation is high. Thus, for a given ratio3value, limscan be lowered till ratio3×lims<0.8, if possible. Furthermore, lower limswould result in lower size-dependent variation because of the tube electrode focusing. Thus, lower values of limsis desirable.4. Device geometry—Ratio of the inlet tube radius to the separation distance (ratio1): As shown inFIGS.8a,8b, a higher value of ratio1results in a more non-uniform electric field strength, which not only changes the spatial uniformity but also the collection efficiency (asFIG.11bhas the final particle deposition more spread out than inFIG.11a). This is because the high non-uniformity of the electric field strength is coupled with lower values (especially closer to the center), which results in less particle deposition in the region under the tube.
Other Factors that are Important in the Particle Collector Device Design.
Upper Limit on the Collector Plate Radius (L3) to Keep the Flow Laminar

The analytical model is valid for the case where flow is laminar. Hence, for any given combination of L3value and ratio3value, the operating flow rates can be adjusted such that the Reynold's number (Re) is within laminar limit. However, if we operate at the collection volume flux limit (which is related to velocity), and with Re<1800 (such that the flow is laminar), we have an upper limit of collector plate radius (L3) for various sheath flow positions (lims) and ratio3. Some examples of the limit is shown inFIG.15if the system is made to operate at the volume flux limit (φmax) where the upper limit is because of Reynolds number and the lower limit is to avoid particle impaction. As the volume flux limit is higher for higher electric field strength, three different operating electric field strengths are used to find the limits on the collector plate radius.

Re=4⁢ρ⁢Qπ⁢μ⁢D,where,Q=πφmax⁢Rc2lims2(2-lims2),andD=2⁢Rc(ratio3)
Lower Limit on the Collector Plate Radius (L3) to have Negligible Impaction

The analytical model is valid for the case where particles are not impacting onto the surface. Hence, for any given combination of L3value and ratio3value, the operating flow rates can be adjusted such that the Stokes number (St) is low (lower than 0.1 as then the impaction efficiency is lower than 1%). However, if we operate at the collection volume flux limit (which is related to velocity), and with St<0.1 (such that impaction is negligible), we have a lower limit of collector plate radius (L3) for various sheath flow positions (lims) and ratio3. The examples inFIG.15have the lower limit to have negligible impaction (i.e. impaction efficiency around 1%) for particles with density of 1 g/cc and diameter 2.5 μm.

St=4⁢ρ⁢Dp2⁢Q9⁢π⁢μ⁢D3
Materials

Various considerations in choosing exemplary materials for various parts of embodiments of the invention are presented below:

ExamplePartRequired propertiesOptional propertiesmaterialsInlet tubeLow static electricity affinity: ToConducting: ImportantSteel,avoid local electric fields.when inner wall is inAluminum,Smooth inner surface: Flowproximity of chargedCopper, ABS,profile should not be affected.particles.Polycarbonate,Nitrile Rubber,etc.TubeHigh conductivity.Low thermal expansion:SS, Tungsten,electrodeLow corrosion potential: TheIf the electrodes getsPlatinum, Gold,andmaterial should not ablateheated this can be usefulSilver, Copper,counter-considerably under high voltage.to consider.etc.baseelectrodeBaseHigh conductivity.Low thermal expansion:Gold, Nickle,ElectrodeLow corrosion potential: TheIf the electrodes getsTin, Silver, etc.material should not ablateheated this can be usefulconsiderably under high voltageto consider.nor degrade through galvanicVery high thermalcorrosion.conductivity: As chargeLow oxidation potential.will flow through a solid-solid contact.CollectorConductivity: A level ofHighlyplateconductivity that can help carrydependent on theaway the charge from theuser.deposited particles is required.MostLow corrosion potential: Theconductors,material should not ablatesemiconductorsconsiderably through the(eg., Silicon,particles depositing on itsZinc Selenide,surface.Germanium),and someinsulators mightalso be used.FillerRelative permittivity comparableConductivity: A level ofWide range ofto that of the collector plateconductivity that canmaterialsmaterial.help carry away thepossible ABS.charge from thedeposited particles isrequiredDielectricLow conductivity: This wouldHigh relativeHigh-karoundact as an insulation around thepermittivity: This woulddielectrics arecounter-electrodes.not dampen the electricpreferable. Verybase andfield strength.thin layer oftubelow-k dielectricelectrodeswould also findapplication.

LITERATURE REFERENCES

1. Dixkens, J., & Fissan, H. (1999). Development of an electrostatic precipitator for off-line particle analysis. Aerosol Science and Technology, 30(5), 438-453. https://doi.org/10.1080/0278682993044802. Preger, C., Overgaard, N. C., Messing, M. E., Magnusson, M. H., Preger, C., Overgaard, N. C., . . . Magnusson, M. H. (2020). Predicting the deposition spot radius and the nanoparticle concentration distribution in an electrostatic precipitator. Aerosol Science and Technology, 0(0), 1-11. https://doi.org/10.1080/02786826.2020.17169393. Rees, J. a. (1973). Chapter 5 Electrical breakdown in gases. High Voltage Engineering: Fundamentals, V, 294. https://doi.org/10.1016/B978-0-7506-3634-6.50006-X4. Heiszler, M. (1971). Dissertation. Iowa State University. Analysis of streamer propagation in atmospheric air. https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=5458&context=rtd5. Han, T. T., Thomas, N. M., & Mainelis, G. (2017). Design and development of a self-contained personal electrostatic bioaerosol sampler (PEBS) with a wire-to-wire charger. Aerosol Science and Technology, 51(8), 903-915. https://doi.org/10.1080/02786826.2017.13295166. Jodzis, S., & Patkowski, W. (2016). Kinetic and Energetic Analysis of the Ozone Synthesis Process in Oxygen-fed DBD Reactor. Effect of Power Density, Gap Volume and Residence Time. Ozone: Science and Engineering, 38(2), 86-99. https://doi.org/10.1080/01919512.2015.1128320

LIST OF REFERENCES IN THE DRAWINGS

Particle gas stream3Sheath gas stream5Particle collector1Particle charger2Inlet section4Flow tube10Flow channel12Guide wall24Inlet end14Sheath flow inlet portion26sheath flow gas inlet27gas chamber29sheath flow gas outlet31Particle inlet28Collector end16Collector section6Housing18Collector plate20Particle collection side23Underside25Transparent (e.g. crystal) discCollector plate holder22Filler material21Outlet30Assembly ring33Cap35Electrode arrangement8a,8b,8cBase electrode (attracting electrode)8aCollector inlet electrode(s) (repulsing electrodes)Counter-base electrode8bTube electrode8cInlet channel collector end radius L1Counter-base electrode to collector plate separation distance L2Collector plate radius L3Collector plate filler radius L4Particle stream radius (inner radius of sheath stream) r