Abstracts 1353

MOTION OF MICRON SPHEROIDAL PARTICLES IN VERTICAL SHEAR FLOWS

D. Broday, M. Shapiro, M. Fichman and C. Gutfinger

Faculty of Mechanical Engineering, Technion, Haifa

Transverse drift of spherical particles has been found when fluid inertia cannot be uniformly neglected in the model of particle motion. The lift force acts on neutrally buoyant non-spherical particles in viscous fluids, although it does not have any net influence on particle motion when inertia is absent from the model. The usual way to investigate the effect of inertia is to look at neutrally buoyant particles moving in a flow characterized by a non-zero Reynolds number. However. in this way one cannot differentiate between the contribution of the particle and fluid inertia, both of which should be accounted for. Normally, aerosol particles are much heavier than the air. Therefore the air inertia can be neglected. Furthermore, the coupled translationallrotational motions of non-spherical particles result in several features which are not observed for spheres. Namely, a net transverse drift for non-spherical particles may occur even when fluid inertia is vanishingly small, i.e. in slow gravitational settling of micron fibers. The present work deals with investigation of the motion of non-neutrally buoyant prolate spheroidal particles in shear air flows under these conditions.

Using generalized Faxen law, we calculate the hydrodynamic forces and moments acting on such particles and their trajectories. These calculations were carried out for cases where particle orientations are free or fixed (by means of an external torque exerted by a strong orienting field), for inertial and inertialess particles. Particle inertia is found to cause their lateral migration across the streamlines. This motion differs considerably from the trajectories calculated for inertialess particles. Neutrally buoyant spheroids, freely rotating or with fixed orienta- tions, in shear flows translate along the streamlines, Non-neutrally buoyant inertialess and free spheroids in a simple shear flow translate along periodic trajectories with no net lateral drift.

In contrast, inertial particles in similar flow conditions drift laterally towards locations characterized by higher local velocities in the upward direction. Non-neutrally buoyant inertial particles with fixed orientations may perform an unstable motion for which the drift velocity grows exponentially. Conditions for the occurrence of this unstable motion are formulated analytically in terms of particle and flow parameters. In general, the rate of the drift depends on the particle shape (via its aspect ratio), and its inertia. Since the trajectories of spheroidal particles are sensitive to their shape, modeling of particle motion using an equivalent sphere approach yields trajectories which differ considerably from those of spheroids.

SIZE DISTRIBUTION OF DUST IN THE VICINITY OF WORKING EQUIPMENT IN COTTONSEED OIL MILL

D. Wolf and S. Tabak

Department of Agricultural Engineering, Technion ~ Israel Institute of Technology, Haifa

Handling of cottonseed and cottonseed cake in oil production industry is accompanied by air pollution both inside the mill and in the outside environment. Emission of cottonseed particles leads also to loss of raw material, with high concentration of oil, protein, etc. The sources of dust dispersion include: cleaning of seeds from contamination, delinting, automatic weighing, hulling, separation of kernels from hulls, and rolling of meals before press. Intensive dispersion of cottonseed cake dust into the environment occurs during processes of its handling, displacement, charging and discharging.

Mass concentration of particles and their size distribution were studied in points evenly placed near the equipment. Measurements were taken in three height levels: the lower zone, the human respiration zone (1.52 m from a floor) and the upper zone.

In order to determine mass concentration, air was drawn through membrane filters, and the filter weight was compared to the weight before sampling. When the particulate matter is collected in sufficient amounts, its concentration was determined. Dust particles were sized using a standard optical microscope.

Field studies were made at six cottonseed oil mills. The average mass concentration at different points of workshop environment was found to range from 40.0 to 213 mg m 3, which far exceeds the limits of the cotton dust standard. Particle size distribution was approximated by exponential function. Particles average size was found to be less than 8 mm.

A COMBINED SYSTEM FOR DUST COLLECTION

V. Galperin

Laboratory of Transport Processes in Porous Materials, Faculty of Mechanical Engineering, Technion ~ Israel Institute of Technology, Haifa 32000

A new combined dust collection system, which consists of three stages, is presented. Dry cyclones are used in the first and the second stages and a wet scrubber in the third one. Major part of the dust is removed at the first two

1354 Abstracts

stages in a form of technological product (powder). At the same time they serve for preliminary purification of the dust-laden gas. The final gas cleaning is made in the third stage.

The system peculiarity is a possibility of wide adjustment of inlet and outlet pipe sizes in the cyclones and of the gas passage size in the scrubber. Due to these features a high collection efficiency is achieved without large energy consumption, relative to known similar systems. In comparison with other dust control methods, such as fabric filters and electrostatic precipitators. the system is much less expensive and simple in maintenance. It may be used practically for all kinds of dust in a wide range of gas flow rate.

The combined dust collection system has been successfully used in industry of Russia, Kazakhstan, Byelorussia. Technical data obtained for the system during repeated tests are given. The system has been designed in several sizes to clean gases at the rate from 1000 up to 85 000 m3 h-r. The total system efficiency is no less than 99.9% and depends on dust properties and gas flow parameters.

INFLUENCE OF AEROSOL DISTRIBUTION ON SOLAR UV FLUX ON THE SURFACE

E. H. Steinberger and A. Dvorkin

Department of Atmospheric Sciences, The Hebrew University, Jerusalem 91904

A one-dimensional numerical model has been developed for computing the solar flux in the wavelength region 290-385 nm at ground level. The UV flux depends on I”, namely the flux at the top of the atmosphere, the total ozone content, aerosol amounts and size distribution and the solar zenith angle. The aerosol size-distribution function chosen was the Junge model, however, the parameters of the distribution could be varied, thereby changing the size range of the particles and the total amount of aerosol.

It has been found that smaller particles reduce the UV flux reaching the ground more strongly than larger particles, and this for the same amount of total aerosol. In addition, the direct UV flux is more strongly influenced by aerosols than the scattered radiation.

APPORTIONMENT OF AEROSOLS, INCLUDING SULFATES, IN THE HAIFA URBAN AREA - AN UPDATE

Y. Mamane and E. Loshin

Faculty of Civil Engineering. Technion-Israel Institute of Technology, Haifa 32000

This study deals with receptor modeling of fine (< 2.5 pm) and coarse 2.5510 pm) particles in Haifa in order to identify the main sources and their impact on ambient particle concentration. The main results of the second year of this program are presented.

The following analysis was performed: (1) Meteorological and statistical analysis of existing ambient air quality data collected by the Haifa District Environmental Towns Association; (2) Measurements of respirable suspended particles (RSP) concentrations and chemical analysis of filter samples, including Pb, SO,, and other ionic species; (3) Collection of fine and coarse particle fraction with a dichotomous sampler and X-ray fluorescence (XRF) analysis of the filters; (4) Scanning electron microscopy (SEM) of samples collected by the dichotomous sampler.

In the winter of 1995/1996 the dichotomous sampler was placed in Neve-Shaanan, Haifa, and collected 30 pairs (fine and coarse) samples on Teflon and Nucleopore filters for 24 h each. The filters were analyzed for elemental content (30 elements) by XRF at the Atmospheric Research Exposure Assessment Laboratory (AREAL), U.S. Environmental Protection Agency. Also 500 selected particles were individually analyzed with a scanning electron microscope (SEM).

The following results have been obtained from XRF and SEM analyses: Ca was the most abundant element in the coarse fraction (60% abundance versus 22% in the fine fraction). Al

and Si were found in lower concentrations in the coarse fraction (22% both). In the fine concentration they contribute 14% (both elements). All values are the ratio of a given element to the sum of the XRF major elements (not including elements with Z 1 11).

Sulfur dominates the fine fraction: 53% versus 8% in the coarse fraction. Electron microscopy analyses of the filters were consistent with these results.

Chlorides were found at both fractions in small quantities. Contents of V and Ni are highly correlated, indicating the contribution of heavy oil combustion (the Haifa oil

fired power plant and the Refineries), Apportionment of ambient particles in Haifa using the Chemical Mass Balance Receptor Model (CMB-7.0) yield

inadequate source resolution, Apparently the use of source profiles collected in the U.S.A. may not be suitable for the Haifa Airshed.