Network Reference - Particle Sizer
Particulates are defined as liquids or solids suspended in a gaseous medium. They come in a wide range of sizes and originate from many different man-made stationary and mobile sources as well as from natural sources. They may be emitted directly by a source or formed in the atmosphere by the transformation of gaseous precursor emissions. Several methods have been developed for off-line and on-line particle size determination. A typical approach for off-line measurement is to examine a single particle under a microscope. It provides specific data about aerosols such as accurate size and shape but is time-consuming and impractical for applications where real-time information is required (aircraft engine exhaust characterization, smoke stack emissions monitoring, etc.). Another cumbersome technique involves the transmission of visible light through a section of filter paper before and after the sampled gas is drawn through it. The optical density of particle deposit is determined from the ratio of intensities measured on the filter with and without the deposit. In its most advanced implementation, a clean portion of a filter is periodically moved into the sampling position, thereby allowing diurnal variations in particle concentrations to be recorded.
Real-time measurement of particle size became possible with the advent of lasers and microcomputers. In addition to near instantaneous response, these devices have the advantages of high sensitivity and the avoidance of physical contact with the particles. A standard on-line particle size measurement technique begins with the formation of a particle jet through a nozzle. At the exit of the nozzle, one or more continuous wave (CW) laser beams are used to measure: (1) particle light scattering properties, (2) light extinction by particles or (3) particle velocity. While other on-line measurement instruments have been developed such as the Electric Single-Particle Aerodynamic Relaxation Time Analyzer (E-SPART), cascade impactor with piezoelectric crystals, fibrous-aerosol monitor, diffusion battery particle size analyzer and differential-mobility particle sizer, these tend to be yet more complex and less portable than the representative instruments herein discussed. Excessive complexity and high-costs, both initial and recurring, will continue to limit many concepts to laboratory investigations.
Section below describes the technical approach of the NAL Research's particle sizer. The equation of motion for a particle in a flow field can be described by Newton's Second Law,
where Fp, mp and ap are particle force, mass and acceleration, respectively. Through rearranging and simple decomposition of variables, one can easily show the expression becomes
where is Up the particle velocity. Forces acting on the particle are drag and gravitational forces. Let α be the angle between the flow and the vector normal to the gravitational field,
with the particle cross-sectional area represented by A, drag coefficient of the particle defined as CD, ρg is density of gaseous medium, Ueff is the effective velocity experienced by the particle, U∞ is velocity of the flow field and g is the gravitational constant. Even though the last term in Equation [3] is small and can be neglected, it is kept throughout the derivation. Providing Rep ≤ 1.0 or within the Stokes regime, the relationship between drag coefficient and Reynolds number is given by,
where the Reynolds number is defined as,
and μg is viscosity of gaseous medium. Substituting Equations [5], [4] and [3] into Equation [2] and assuming spherical particles with particle diameter dp we obtain
By substituting the definition of particle aerodynamic diameter into Equation [6], the equation of motion for a spherical particle in a Stokesian flow thus becomes
We are left with a simple expression in Equation [7]. One can measure particle aerodynamic diameter by measuring both particle velocity at two different locations and gas velocity, provided the assumption of Rep ≤ 1.0 holds. This forms the basis of the NAL Research's particle sizer, which directly measures the particle velocity gradient with a pair of single-component miniature LDVs separated by a known distance and gas velocity with a pressure probe enabling Equation [7] to be solved for particle aerodynamic diameter. Gas velocity is scanned within the Stokes regime to measure particles of different sizes. The biggest advantage of the NAL Research's particle sizer is that when the atmospheric viscosity and flow speed are known, two direct measurements of particle velocity provide velocity gradient information which yields particle aerodynamic diameter. No other parameters are needed, including manufacturing tolerances. Operation is largely independent of ambient conditions. The system does not need calibration resulting in low cost and simple operation.