Gerhard Kasper

Dynamics and Measurement of Smokes. I Size Characterization of Nonspherical Particles

Aerodynamic, volume equivalent, and mobility equivalent diameters are the cornerstones of a practical approach to the dynamics of nonspherical aerosol particles. Together, they also compose the relevant set of equivalent diameters for particle deposition and characterization by aerodynamic, electric mobility, or diffusion methods. The relationships between D AE, D VE, and D ME are discussed on the basis of a review of the dynamic shape factor concept, including the effect of slip. Emphasis is given to aggregates, especially in chainlike configurations.

Bipolar diffusion charging of fibrous aerosol particles—I. charging theory

Possible models for the bipolar diffusion charging mechanism of long fibrous or chainlike aerosol particles are discussed and compared numerically. While few specific approaches for elongated particles are available, reviews of equilibrium charging and steady-state ion flux theories suggest an approximation of particle shape by conducting spheroids. A “charging equivalent diameter” DQE is introduced, which leads to a particularly simple expression for the Boltzmann charge distribution. In the transition region of Knudsen numbers and for the image charge effect, no rigorous solution is available for elongated particles. The model of Laframboise, J. G. and Chang, J. S., J. Aerosol Sci. 8, 331 (1977) appears to be limited to particle dimensions much larger than the ion mean free path. Calculations with a modified Fuchs-Natanson model indicate that image force and non-continuum effects contribute marginally above d1 ⋍ 0.04 μm.

Bipolar diffusion charging of fibrous aerosol particles—II. charge and electrical mobility measurements on linear chain aggregates

Charge distributions on chain aggregates with thicknesses of less than 0.1 μm and length-to-diameter ratios of 10 to several hundred were derived from measurements of electrical mobility distributions using a differential mobility analyzer (DMA), in combination with a condensation nuclei counter and an electrometer as particle detectors. The experiments are backed by measurements of chain size distributions with aerosol centrifuge and electron microscope, by DMA transfer function measurements, and by an investigation into the symmetry of the charging mechanism. Good agreement is found with the Boltzmann charge distribution when using a charging equivalent diameter DQE derived for conducting prolate spheroids.

Dynamics and Measurement of Smokes. II The Aerodynamic Diameter of Chain Aggregates in the Transition Regime

Aerodynamic diameters of linear Fe2 O3 chain aggregates with up to 160 primary particles ranging in mean diameter from 0.035 to 0.12 μm were measured with a cylindrical aerosol centrifuge. Aerodynamic diameters closely follow the semiempirical expression were ρ/ρo is the ratio of particle density to unit density; DI and og are the mean diameter and geometric standard deviation of the primary particles, respectively; and n is the average number of primary particles in a chain found at the location corresponding to D AE. The coefficient k is a function of D I only. In the continuum limit (DI

An experimental study of Brownian coagulation in the transition regime

0.02 μm) κ appears to reach a constant value that is consistent with earlier measurements by Stober et al. (1970). The results suggest a predominant settling orientation normal to the chain axis for centrifuge measurements. Values for the slip coefficient as a function of chain thickness and length are derived, and imperfections in the chain configuration are explained in terms of a coefficient X.

Dynamics and Measurement of Smokes III—Drag and Orientation of Chain Aggregates in an Electrical Mobility Spectrometer

Size distribution measurements of polydisperse iron oxide chain aggregates (600–1300 A thick with aspect ratios up to 700) were made and compared, using an aerosol centrifuge, a cascade impactor, and an electrical aerosol analyzer, with supporting data from electron microscopy. Number and volume frequency distributions are compared and analyzed on the basis of means and widths only. Appropriate conversions between aerodynamic and mobility equivalent diameters are made. Preferred particle orientation due to flow field and electrostatic field is taken into account. An estimate for the increase of average charge per fiber over that of spheres is given for the EAAs diffusion charger.

Dynamics and Measurement of Smokes IV—Comparative Measurements with an Aerosol Centrifuge and an Aerodynamic Particle Sizer APS33 Using Submicron Chain Aggregates

Abstract Brownian coagulation of aerosols was studied in the transition regime at Knudsen numbers ranging from 1.2 to 7.4. Coagulation constants were obtained from the change in total concentration in a flowing system by using nebulized NaCl aerosols of mean mobility equivalent diameters between 0.018 and 0.105 μm in air at atmospheric pressure. Size distributions and concentrations were measured with an Electrical Aerosol Analyzer (TSI). The coagulation constants account for the polydispersity of the aerosol, diffusional losses in the coagulation tube, and particle losses in the ducts leading to the EAA. The EAAs response to particle sizes was compared to data from a transmission electron microscope and an electrical mobility classifier. Deviations of 10–15% were found in the size range between 0.03 and 0.05 μm. The experimental rate of coagulation agrees very closely with that predicted by Smoluchowskis continuum theory with the slip correction and the Fuchs correction factor included. The results do not support a correction factor recently proposed by Davies.

Hot‐wire thermal precipitator with low inlet losses and low size selectivity

Simultaneous measurements of electric mobility and geometric size have permitted determination of the drag force of chain aggregates as a function of Knudsen number for the orientation prevalent in a differential electric mobility classifier (DMA). The measurements indicate that chains are oriented along the field lines (i.e., in the ∥ orientation) in a DMA. In combination with earlier measurements of drag in the ⊥ orientation with an aerosol centrifuge (Part II; Kasper, 1982b), these data give a complete description of the translational drag on fibrous particles in the Knudsen number regime below about 10. Indirectly, these measurements also present a validation of the shape factor concept and the equivalent diameters D AE, D VE, and D ME associated with it (Part I; Kasper, 1982a).

On the coagulation rate of aerosols with spatially inhomogeneous particle concentrations

The internal hydrodynamic flow patterns of the Cylindrical Aerosol Centrifuge and the APS33 impart different orientations to fibrous aerosol particles. The difference in orientation results in different aerodynamic diameters and a shift in size distributions. Theoretically, this difference is on the order of 20 to 30%; measured differences agree very closely with this value. *APS33 is a product name of TSI, Inc.