Particle resuspension burst from dust layers induced by impacting objects

J Aerosol Sci. Vol. 30, Suppl. I, pp. $731-$732, 1999 O 1999 Published by Elsevier Science Ltd. All fights t~erved Printed in Great Bntain 0021-8502/99/$ - see front matter

Pergamon

PARTICLE RESUSPENSION BURST FROM DUST LAYERS INDUCED BY IMPACTING OBJECTS L. M~dler, W. Koch, Fraunhofer-lnstitute of Toxicology and Aerosol Research, NikolaiFuchs-StraBe 1, Hannover, Germany

Keywords: Resuspension, multilayer Introduction Surfaces are secondary sources of dust that may contribute substantially to the airborne aerosol concentration in the indoor, the workplace and the clean-room environment. Particles can be released from surfaces by various mechanisms: resuspension by a direct air-flow, vibrations, direct mechanical momentum transfer from impacting objects. In this contribution, we focus on highly instationary particle resuspension by means of resuspension bursts from dusty surfaces caused by the impact of flat objects. In our experimental system, the resuspension related to the instationary radial flow developing between the falling object and the surface is the dominating resuspension mechanism. In a previous study we established correlations between the area affected by particle resuspension i.e. the zone of influence of impacting disks and experimental parameters such as the disk radius, the mass and the drop height (M~dler and Koch, 1997). The experiments were restricted to disks of maximum diameter of 14.4 cm. Here we report on experiments aiming at quantifying and characterizing the amount of resuspended particles. The correlations obtained previously were extrapolated to design experiments also with disks of larger size up to 0.5 m in diameter.

Experimental method The amount, m(dp), of particles of size dp released by the drop of disks on the dust covered surface was determined from time and size resolved concentration measurements in a volume, V, encapsulating the release process. A situation of well stirred mixing inside the volume was established by the air momentum generated initially by the falling object and by internal recirculation of air due to the operation of a highvolume filter sampling unit inside the test volume. Under these conditions, m(dp) is calculated from m = Co(alp)V, where Co(d~) is extracted from the expected time behaviour of the concentration: C(dp,t) = Co(dp) Exp[-(y+~(d~)) t] by extrapolation to t=O. Here T = Q/V is the dilution rate due to aerosol sampling (total sampling flow-rate: Q) and p(d~) is the size dependent particle removal rate due to wall losses: ~(d~) = v~/h if sedimentation is the dominant deposition mechanism (h = chamber height). The following instruments were used for concentration analysis: a high volume filter sampler (flow rate can be varied from 40 to 115 m3/h), a 9-stage Berner impactor and a RESPICON-TM (Koch et al. 1997) for continuous monitoring of three size fractions: < 4.5 pro, 4.5-10 pm, and, >lOpm. Steel disks of different radii (R=O.1, O.15, and 0.25 mm) but the same thickness (20 ram) were dropped from a height of 1.8 m onto a layer of quartz dust (MMAD= 12 pm, Og= 2.3). The dust cover was varied between 200 and 4440 g/m 2, which means multilayers in all cases. Prior to the release experiments the dust was deposited on the surface by brushing it through a sieve (sieve sizes = 1 ram) resulting in a homogenous layer only on a macroscopic scale. The dust particles were not deagglomerated before deposition.

$731

$732

Abstracts of the 1999 European Aerosol Conference

Results A typical recording of a release experiment is shown in Fig. 1 where, as an example, the concentration curves of the size fraction smaller than 4.5 pm and the fraction larger than 10 IJm is shown. After a short mixing time, the curves follow the expected exponential behaviour and the numeric values of the slopes are in quantitative agreement with the loss rates determined by the dominating mechanisms causing the concentration decrease: dilution for the small particle fraction, dilution and sedimentation for the large particles. In all experiments carried out the size distribution of the released dust was nearly identical to the size distribution of the dust forming the layer. We further got the surprising result that the absolute amount of released dust is independent of the layer thickness. This indicates that only a thin surface layer of the overall dust layer contributes to resuspension. A strong reduction of the resuspension burst was measured when reducing the disk radius from 0.25 m to 0.15 and 0.1 m. This is caused by a reduction in the momentum flux of the air flow developing between the falling disk and the dust layer due to both the reduced weight and the reduced diameter of the disk. Based on the correlations derived in the previous study (M~dler and Koch, 1997) the amount, rn, of released dust should correlate with R according to: m=a(R+RT6)2. Fig. 2 shows the experimental results and the best fit to the theoretical curve. 100 Co

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Fig. 2: Correlation to the scaling -up of previous studies

References L. M~dler, W. Koch, J. Aerosol. Sci. 28, S85-$86, 1997 W. Koch, W. Dunkhorst, H. L6dding, Gefahrstoffe-Reinhaltung der Luft, 57, 177-185, 1997 Financial support from the GRS mbH under BMU grant # SR 2075/8-1 are gratefully acknowledged