Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N15/00—Investigating characteristics of particles; Investigating permeability, pore-volume or surface-area of porous materials
  • G01N15/02—Investigating particle size or size distribution
  • G01N15/0255—Investigating particle size or size distribution with mechanical, e.g. inertial, classification, and investigation of sorted collections

Definitions

• Aerosol particle classifiers [0001] Aerosol particle classifiers
• Aerosol classifiers are used to produce a monodisperse aerosol, that is, they select a narrow range of particles from a larger distribution of particles
• This method is used for many applications including, nano-particle generation, measuring distributions of particles in air, measuring the deposition of particles in filters and other devices, sampling ambient aerosols, and many others
• These measurements are often done in research areas as diverse as nano-technology, pharmaceutical research, health-effects studies, inhalation toxicology, bio-aerosol detection, filter testing, indoor-air quality studies, industrial hygiene, energy and combustion research, automotive emissions measurements, and atmospheric and climate-change research
• the DMA classifies particles based on their electrical mobility, that is, the motion of a charged particle in an electrostatic field
• the particles are classified by their electrical mobility, which is related to the number of electric charges on the particle and the drag experienced by the particle, which is a function of the particle's size and shape
• an equivalent diameter called the electrical mobility equivalent diameter is defined for these particles, which have the same electrical mobility of a spherical particle of the same size
• an electric charge must be placed on these particles using charging methods such radioactive-source charge neutralizers or corona discharge
• charging methods such radioactive-source charge neutralizers or corona discharge
• particles may obtain one, two, three, or more positive charges, one, two, three, or more negative charges or no charge at all
• the electrical mobility of the particles is a function of the number of charges
• a method of classification of particles suspended in a fluid comprising the steps of providing a carrier flow of a fluid, supplying particles into suspension in the carrier flow, providing an acceleration to the flow at an angle to the velocity of the flow to cause the particles to follow trajectories determined by the acceleration and drag on the particles caused by the fluid, and classifying the particles according to the trajectories of the particles
• the particles may be classified for example by splitting a flow containing the particles or by detecting impacts of the particles on boundaries of a flow channel containing the flow
• the fluid may be a gas such as air
• the carrier fluid may be caused to rotate around an axis by the rotation of one or more conveying flow channels
• the acceleration may be centripetal acceleration
• the step of supplying particles into suspension in the carrier flow may comprise merging a flow of a fluid containing suspended particles into the carrier flow
• the step of classifying particles according to the trajectory of the particles may comprise splitting the carrier flow into two or more flows
• the step of classifying the particles according to the trajectory of the particles may comprise supplying a surface at which particles may impact depending on their trajectory
• an apparatus for classifying particles suspended in a fluid comprising elements defining one or more carrier flow channels, a source of a carrier fluid flow into the carrier flow channel, a source of particles connected to supply the particles into suspension in the carrier fluid in the carrier flow channel, a drive connected to operate on the elements defining the carrier flow channel to supply an acceleration to the elements defining the flow channel at an angle to the flow of fluid through the carrier flow channel, and a classification system for classifying the suspended particles according to their trajectories [0009]
• the carrier fluid may be a gas
• the carrier flow may be caused to flow through one or more flow channels caused to rotate around an axis
• the flow channels may be sectors or the whole of an annular space defined by inner and outer walls which are surfaces of revolution around an axis close to the axis of rotation
• the surfaces of revolution may be substantially cylindrical in shape
• the drive may comprise a motor connected to cause rotation of the elements defining the carrier
• Fig 1 is a schematic of an Aerosol Particle Classifier (APC) (not to scale) with a cylindrical flow path,
• API Aerosol Particle Classifier
• Fig 2 is a diagram showing details of the particle trajectory and flows between the cylinders in the embodiment of Fig 1,
• Fig 3 A is a graph of the normalized transfer function of the APC of Fig 1,
• Fig 3B is a graph of the transfer function of the APC of Fig 1 in terms of aerodynamic diameter for the operating conditions given in the description
• Fig 4 is a schematic of an APC (not to scale) with a partial cylinder flow path
• FIG. 5 is a schematic of an APC (not to scale) with a curved flow path with boundaries shaped as surfaces of revolution,
• FIG. 6 is a schematic of an APC (not to scale) with detectors on an outer cylinder defining the flow path
• Fig 7A is a schematic showing an aerodynamic classifier with a particle counter
• Fig 7B is a schematic showing a particle charger with an aerodynamic classifier of the embodiment of Fig 6
• Figs 1 and 2 show diagrams of an exemplary embodiment of the APC, generally denoted by 100
• the APC disclosed here comprises elements defining a carrier flow channel, here two concentric cylinders, an inner cylinder 102 and an outer cylinder 104 rotating in the same direction and at a similar rotational speed (normally the two cylinders would be rotating at the same rotational speed although different speeds can also be used, see below)
• Other surfaces of revolution axially symmetric shapes, synonymously surfaces of rotation
• a flow channel may be defined by partial cylinders, e g sectors of a cylinder that do not extend in a full circle around the central axis of the cylinders, or partial surfaces of revolution, and by substantially radial surfaces between the inner and outer surfaces If a flow channel is defined by partial cylinders or partial surfaces of revolution then the surfaces could form a single element defining the flow channel Referring to Fig 1, in the embodiment shown the cylinders are attached to a rotating shaft 120 mounted on bearings
• a sheath flow 108 with flow rate Q sb is also introduced from a source of carrier fluid flow into the carrier flow channel between the two cylinders.
• an initial flow channel 126 acts as a source of carrier fluid by introducing the sheath flow into the carrier flow channel. It is assumed that flow is laminar and incompressible, which is a reasonable assumption for the geometry, flow rates, and gas pressure used in normal operation.
• the flow is axial in the frame of reference of the rotating cylinders, and tangential to the cylinders or to an imaginary cylinder coaxial with the cylinders; in an embodiment with cylinders rotating at different speeds the flow may still be tangential.
• the particles would travel between the two cylinders between the inner cylinder wall and the aerosol streamline 110.
• the particles experience a centrifugal force in the direction of the outer cylinder and a drag force toward the centre of rotation.
• the centrifugal force both supplies the particles into the carrier flow and imparts a component of velocity across the carrier flow.
• the particles are not pre-mixed.
• the particles will also travel in the axial direction carried along by the aerosol flow and sheath flow.
• r is the radial position of the particle
• ⁇ is the rotational speed of the cylinders
• m is the mass of the particle
• d v is the diameter of the particle
• ⁇ is the viscosity of the carrier gas
• C c is the Cunningham slip correction factor for the particle
• u is the velocity of the carrier gas in the axial direction. It will be assumed that the velocity profile is uniform (i.e., u is constant).
• the particle relaxation time, ⁇ is defined as,
• p v is the Secret particle density
• p Q is unit density (1000 kg/m J )
• d ae is the so-called aerodynamic diameter of the particle.
• r m is the initial position of the particle when it enters the classifier.
• a classification system classifies the suspended particles according to their trajectories.
• particles are classified according to whether their trajectories bring them through sampling exit 114.
• the transfer function of the instalment (the distribution of particles that leave the classifier at any given operating condition) can be found by determining the trajectory of the particles.
• a sample flow 112 with flow rate Q s exits the classifier through sampling exit 114.
• the sample flow is part of the sheath flow.
• the remainder of the sheath flow and the aerosol flow exit the classifier as exhaust flow 116. Defining r, as the outer radius of the inner cylinder, r ⁇ as the inner radius of the outer cylinder, i ⁇ as the outer radius of the aerosol flow, and r .
• the largest particle i.e., the largest ⁇
• the largest particle i.e., the largest ⁇
• Particles with ⁇ > r max will intercept the outer cylinder wall before reaching the exit slit and will adhere to the cylinder surface, while particles with ⁇ ⁇ r mm will flow past the exit slit and be carried out of the instalment with the exhaust flow.
• the particles adhere to the wall of the outer cylinder due to van der Waals forces (Friedlander, 2000) and will remain there until the cylinder is cleaned. (Like the DMA, under normal operating conditions and aerosol concentrations, the cylinder will only need to be cleaned once every few months.) Between the maximum and minimum relaxation times, only a fraction of the particles will be classified.
• a particle must migrate into the sample flow, defined by the sample streamline (r 4 ⁇ r ⁇ A), by the time the particle has reached the end of the classifier.
• For particles with ⁇ > r mm only particles with an initial radial position /; ⁇ r ⁇ ⁇ will be classified, where /; is called the critical radius.
• the limiting trajectory for ⁇ > ⁇ mm will be the particle that starts at /; and reaches r ,. Substituting this condition into Eq. 3 and solving for the aerosol fraction, /J, that is classified reveals,
• the particles starting at the critical radius, /; must reach r ⁇ by the end of the classifier.
• the fraction of the aerosol, /-, that is classified is,
• the normalized transfer function is shown in Fig 3A, where the normalized particle relaxation time is defined as ⁇ / ⁇
• ⁇ r (r mix - r mm )/2
• this embodiment of the APC will produce a very narrow, or
• the APC would be able to classify particles over an extremely wide range for example 10 nm to 10 ⁇ m using rotational speeds ranging from 20,000 to 95 rpm and smaller particle sizes could be classified by using higher rotational speeds (by comparison the DMA is typically used over a range of approximately 2 5 to 1,000 nm)
• Fig 1 uses concentric cylinders to classify the particles
• partial cylindrical sections such as sectors of a cylinder may be used as elements defining one or more carrier flow channels or other long channels attached to a rotating shaft may be used as the carrier flow channels
• the analysis of the transfer function will change with these different geometries, where the cylindrical geometry is the simplest to analyze
• an APC 200 is shown with a rotating flow channel (multiple flow channels may be included around the central axis, but only one is shown in the figure)
• the flow channel may be defined by an inner partial cylindrical section 202 and an outer partial cylindrical section 204 although other shapes than partial cylindrical sections are possible
• the embodiment may operate in a similar way as the embodiment of Fig 1 except that the flow channel does not extend all the way around the central axis
• rotating shaft 220, bearings 222, pulley 224, sheath flow 208, aerosol flow 206, initial flow channel 226, sample flow 212 and exhaust flow 216 may be similar to their counterparts of Fig 1 and cooperate similarly, except that the portions
• the above description describes how the APC can be used to produce a monodisperse aerosol based on the particle's aerodynamic diameter, much like how a DMA is used to produce a quasi-monodisperse aerosol based on the particle's electrical mobility diameter DMA's are often combined with a condensation particle counter (CPC) to measure the number concentration of the quasi-monodisperse aerosol Typically, the voltage controlling the electrostatic field in the DMA is 'scanned' over the range of the instalment, and by completing a data inversion of the CPC data, the size distribution of the aerosol can be determined This combination of DMA and CPC is normally called, a Scanning Mobility Particle Sizer (SMPS) The same method can be employed by combining the APC with a CPC, or any other particle counting device, and by continuously 'scanning' the rotational speed or intermittently stepping the rotational speed That is, a particle counter could be placed in or connected to the particle classification system or the outlet of the particle classification
• SMPS Sc
• a detector may comprise a conductor connected to an electrometer circuit, for example in an annular embodiment, the detector may comprise a conducting ring connected to an electrometer The ring may be electrically isolated from both the remainder of the surface defining the flow channel and any other detection rings which may be present The detectors may be situated at different axial locations along the outermost surface of the flow channel Referring to Fig 6, an APC 400 is shown having detectors 428 located in the flow channel at which particles may impact depending on their trajectory In the embodiment shown the flow channel is defined by cylinders as in Fig 1, although other shapes would also work In this embodiment the detectors may be rings, electrically connected to electrometer circuits, extending around the inside of the outer cylinder No sampling exit is necessary to classify particles when detectors are used to detect impact
• Particle Classifier APC
• APC Particle Classifier
• APC monodisperse aerosol without classifying multiply-charged particles like the DMA, APM, or CPMA.
• An APC could be combined in series with a DMA or CPMA in order to measure other important particle properties including: mobility diameter, particle mass, effective density, fractal-like dimension, and dynamic shape factor.

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• Chemical & Material Sciences (AREA)
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• 2010
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