0021-8502/93 $6.00 + 0.00 Pergamon Press Lid
J. Aeros~l S~i., Vol. 24, Suppl. 1, pp. $511-$512, 1993 Printed in Great Britain.
39 P 05
Modeling of an Aerosol Transport System E. Karg gsf-Forschungszentrum, D-85758 Oberschleil3heim
Introduction Aerosols transported through tubes loose part of their particle mass by different physical mechanisms. Aim of this paper is to estimate the number of aerosol particles transmitted through an aerosol transport system by including the most important mechanisms of particle deposition in a computer model. The model is available as DOS software and is used to optimize the design oftransport systems for aerosol monitoring or to check whether an existing system delivers an aerosol sample to a monitor which is representative for its size range. Method The particle transmission xi,dof a flow element i for particles of size d is defined as the ratio of the particle concentration at the outlet to the one at the inlet. It is related to particle deposition 6 by eq. (1). Aerosol particles can be deposited by various mechanisms m. The most important mechanisms are sedimentation (Thomas, 1958), impacXi, d = 1 - 6 i , d 50
(1)
em ~>
t 5
0 O
tion (Cheng and Wang, 1981), diffusion (Gormley and Kennedy, 1949) and nonisokinetic sampling (Belayev and Levin, 1974). The transmission xt of a system of arbitrary length and complexity is approximated by a combination of flow elements: horizontal and vertical tubes, 90 ° bends and thinwalled isoaxial sampling nozzles. It is calculated by eq. (2) (Brockman, 1993) using i l:t,d = H l - I z d i m
Aerosol
(2)
Sizer
flow elements with m mechanisms each.
Fig. 1 Schematic aerosol transport system
A polydisperse particle distribution can be modeled by calculating eq. (2) for a sufficient number of particle sizes. The channel distribution of any particle sizing equipment can be used. $511
The influences of turbulence, electrostatic and phoretic forces and different sampling inlets will be included in a future model.
Aerosol Transmission
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O.O
Results 0.5
-
0,4
-
O.3
-
O
in ~ Fig. 2 Aerosol transmission for a tube of 2.5 cm inner diameter and a sampling volume flow rate of 5 lpm. Aerosol Transmission
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0.7
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0.3 0,1 -o-~-
0.062~
,
O. t;15
l
.
0.25
0.5
.
.
This method has been extensively tested to eliminate errors in the computer program. As an example the aerosol transmission of the system in fig. 1 is shown in fig. 2. A spectrometer covering a particle size range between 0.06 and 32 p~m samples at a flow rate of 5 liters per minute through a sampling inlet of 2.5 cm inner diameter. The transmission is rapidly decreasing for particle sizes above eight micrometers (fig. 2). Increasing the tube diameter to 5 cm and using an eight times higher transport flow rate leading to slightly subisokinetic sampling conditions (Vtransport'Vsampling = 1.6"1) at the sensor, the aerosol transmission of the system is highly improved (fig. 3).
.
X
2
4
S
Particle Diameter in ~m Fig 3 Aerosol transmission for a tube of 5 c m inner diameter, a transport volume flow rate of 40 Ipm and isoaxial subisokinetic sampling at the sensor of 5 Iprn.
References Brockman J.E.(1993) Sampling and transport of aerosols. In: Aerosol measurement: Principles, techniques and application. K. Wilteke and P.A. Baron (Eds.). Van Nostrand Reinhold, New York.
Belayev S.P. and M.L. Levin (1974) Techniques for collection of representative aerosol samples. J. Aerosol Sci, 5(4), 325-338. Cheng J.S. and C.S. Wang (1981) Motion in bends of circular pipes. Atmos. Environ. 15(3), 301-306. Gormley P. and M. Kennedy (1949) Diffusion from a stream flowing through a cylindrical tube. Proceedings of the Royal Irish Academy 52A, 163-169 Thomas J,W. (1958) Gravity settling of particles in a horizontal tube. J. Air Pollution Control Assoc. 8, 32-34.