Helmut Büttner

In situ optical particle counter with improved coincidence error correction for number concentrations up to 107 particles cm−3

Abstract An optical particle counter of the type first described by Umhauer (1983) was modified for the purpose of in situ measurements at high concentrations to achieve very low coincidence errors. Therefore, the optically defined measuring volume was reduced to 36 × 52 × 35 μm 3 and additionally a digital signal processing (DSP) as described by Sachweh (1991) was applied to measure the fraction of coincident signals for different aerosols (quartz, Monospheres and glycerin) and number concentrations up to 2 × 10 6 particles cm −3 . Thus, errors from coincidence events in particle size distribution and number concentration could be corrected successfully. The fraction of coincident signals could directly be measured by the DSP. The results were found to be in good agreement with the theoretical model derived by Raasch and Umhauer (1984). From these experiments the upper concentration limit could be assessed to be approximately 10 7 particles cm −3 . A lower 50% size detection limit of 0.2 μm was also determined for PSL spheres in comparison with a condensation particle counter. Performance comparisons are also reported for a commercial HC15 counter (Polytec) based on the same operating principle but “normal” sensing volume of 110 × 220 × 220 μm 3 .

PARTICLE SHAPE AND STRUCTURE ANALYSIS FROM THE SPATIAL INTENSITY PATTERN OF SCATTERED LIGHT USING DIFFERENT MEASURING DEVICES

Abstract The spatial intensity pattern of scattered light from nonspherical particles was investigated numerically and experimentally in order to obtain—beside size—sensitive shape information. The discrete dipole approximation (DDA) was used to calculate the light scattering pattern for some basic types of particle shapes. From these calculations the lower size limit where shape information can successfully be detected from light scattering was found at a size parameter of approximately 1. It can be even lower if elongated particles (cylinders) are present. For the exemplary studies three different laboratory instruments available to the authors were utilized. The comparison of experimental and numerical results yielded good correlations, which confirmed the selected theoretical approach. Thus, it is possible to develop experimental setups for specific applications only on basis of theoretical data. From the experiments we found that azimuthal scattering at a constant scattering angle is a promising setup for shape characterization, which can be adapted to specific applications with high flexibility. For supermicron particles the surface structure also contributes to the scattering pattern provided the characteristic size of surface elements substantially exceeds the wavelength of the light source.

Dimensionless representation of particle separation characteristic of cyclones

Abstract In particular the collection efficiencies were measured as a function of flow rate, cyclone dimensions and particle size. For this purpose a fast, accurate and problem adapted measuring technique has been used, which enables the determination of grade efficiency curves by measuring the size distributions in the cyclone up- and downstream with optical particle counters. The extended experimental data from this parameter study were analysed by the methods of dimensional analysis and theory of models. An evaluation of all measuring results for two cyclone designs has been resulted in an empirical, nondimensional correlation of the collection characteristic, a dimensionless grade efficiency curve. Deviating from geometric similarity this correlation includes a variation of cyclone outlet diameter. Grade efficiencies of the cyclones are a definite function of the dimensionless numbers Stokes and Reynolds number and of the dimensionless cyclone outlet diameter. Analysis of own and published data has shown that this experimental correlation includes the influence of the temperature and that cyclone body diameter do not influence efficiency. The influence of cyclone height on flow behaviour and collection characteristic could be quantified as well. The range, in which prediction of collection efficiencies is possible, is marked in a state diagram Reynolds number versus dimensionless cyclone height.

Aerosol Measurement in Low‐Pressure Systems with Standard Scanning Mobility Particle Sizers

The scanning mobility particle sizer (SMPS) is one of the best known instruments for measuring particle size distributions in the submicron range. The SMPS consists of two parts: an electrostatic aerosol classifier (differential mobility particle analyser, DMA), followed by a counting device, in general a condensation particle counter (CPC). Unfortunately, commercial measurement devices such as the TSI DMA Model 3071 and the TSI CPC Model 3022 (TSI Inc., St. Paul, MN, USA), can be used only at nearly atmospheric pressure in the sampling line or in slight overpressure mode, but not in low-pressure systems. A modification in the sampling line is shown which enhances the operating range of a standard SMPS system to low pressure. Samples taken under standard and low-pressure conditions show good agreement in the measured particle size distributions and concentration. The behaviour observed in experimental studies agrees well with theoretical predictions.

Calibration of optical particle counters: Comparison between theoretically and experimentally derived results

Abstract The quality of the correlation between numerically and theoretically derived results for calibration of OPCs strongly depends on the signal evaluation method. There exists an influence of the spectral functions of the optical components, which has to be included into calculation. The weighting function for the geometry of optical arrangement is negligible for particles smaller than 4μm. Only one transfer parameter enables the correct numerical calib- ration for different materials. For calibration of unknown aerosols only a small number of experimental data points has to be measured and a wide range of sizes can numerically be determined.

EXPERIMENTAL INVESTIGATION OF A WET DUST SCRUBBER: DUST COLLECTION BASED ON TURBULENT DIFFUSION

ABSTRACT The “nozzle scrubber” is a wet scrubber in which the scrubbing water is dispersed in the dust laden gas stream by means of one or more pneumatic nozzles. This scrubber is distinguished by an excellent collection efficiency for submicron dust at an unusually low energy and water consumption. No well-defined theory exists for this process. The collection efficiency in the “nozzle scrubber” depends primarily on turbulent diffusion respectively on the interaction of particles and droplets induced by turbulence, and not on inertial separation as in the case of the venturi scrubber. A light scattering device was used to measure the particle distributions. The experimental set-up was built up in a technical scale. The influence of operation parameters, especially water consumption, residence time, and pressurized air, on the grade efficiency has been demonstrated by their systematic variation. The contribution of turbulent diffusion to the collection efficiency has been confirmed.