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Collection efficiency of aerosol particles by raindrops
J. Aerosol Sci., Vol. 19, No. 7, pp. 855 - 858, 1988 Printed in Great Britain
0021-8502/88 $3.00. + 0.00 Pergamon Press plc
COLLECTION EFFICIENCY OF AEROSOL PARTICLES BY RAINDROPS H.-G. Horn, H. Bonka, E. Gerhards, B. Hieronimus, M. Kalinowski, L. Kranz, M. Maqua Lehrgebiet Strahlenschutz in der Kerntechnik, RWTH Aachen Templergraben 55 D-5100 Aachen, F. R. Germany
Introduction For particlebound radionuclides emitted by nuclear facilities, the wet deposition due to washout is one of the significant pathways into the foodchain or to the 9round. An important input for the calculation of the deposition due to washout is the collection efficiency of aerosol particles by raindrops. A review of published models and 10~ data concernin 9 the wet deposition showed that, especially in the range of ,I 10o particle diameters from approx. 0.5 --0.2 mm i to 5 #m, the reported collection effi10-I ciencies spread over more than four X=lmm ~f~ orders of magnitude, see Fi9ure 1. In Fi9ure 1, there are also plotted 10"2 three curves for the collection efficiency as a function of the aerody104 namical particle diameter (dae) and the diameter of the raindrop (X). The1Gr4 se curves are derived from publications of Ranz and Marshall (1952) for • ~, 9" ~ 2 ,~Z,5 collection due to diffusion, of Fuchs i(7 s (1964) for collection due to interception, and of Langmuir and Blodgett 10 -s (1945) for collection due to inertial 10-3 10 .2 10 -I 10 0 101 l~rn 10z impaction. The equations for intercepdQo tion and impaction are formulated for potential flow (NRe-> oo). We use these equations, if a conservative calculation Fi9. I: Published experimental results and model calculations for the coflection efficiency of aerosol particles by raindrops, of the washout is needed for radiation from Bonka et. al. (1984) and Horn, Maqua and Bonka (1988) protection purposes, see e. 9. Horn, Maqua and Bonka (1988). Up to now, many detailed models to describe the collection of aerosol particles by raindrops were developed. However, the wide spreadin9 of the experimental results makes it difficult to decide which of these models are useful in the field of radiation protection. In order to achieve clearer results, own experiments and model calculations were carried out.
~
Experiments and model calculations Two basically different approaches were made to measure and calculate the collection efficiency: In the first case, the raindrops were simulated by solid models. The flow around the model drops was analysed, followed by the calculation of the collection efficiency. The calculations then were compared with experiments which also used solid drop models. In the second case, water drops of known size were accelerated to their terminal velocity, before they fell through an aerosol chamber. The collection efficiency was determined by countin 9 the particles on the collected drops. For the experiments and calculations concerning the solid drop models, spherical models as well as oblate spheroids were used. The oblate spheroids represent the shape of drops with diameters larger than 1 mm,
855
856
H.-G. HORN et al.
which get deformed due to the fluiddynamical and gravitational forces. The shapes of the oblate spheroids were taken from Pruppacher and Pitter (1971). Figure 2 shows the flow around an oblate spheroid with a diameter of 6 mm. The photograph shown in Fi9ure 2 was taken at NRe=1130. Flow fields like the one shown in Figure 2 were transformed into two-dimensional grids of flow-velocity vectors. Particle t r a jectories were then calculated with the aid of a computer program. For larger particles, where the collection by drops is mainly due to inertial impaction, the fluiddynamic forces were used to calculate the trajectories. The dashed line in Figure 6 shows the result of this calculation for the raindrop model shown in Figure 2. The same calculations were carried out for spheriFiq.2: Flow around an oblate spheroid, photographed in cal raindrops. Using the potential flow theory a water channel. Largest diameter X=6mm, to calculate the velocity vectors lead to collection efficiencies due to inertial impaction that NRe = 1130. lay above those calculated for the oblate spheraids (dotted and dashed line in Figure 6). The calculations did not take account of any collection on the backside of the raindrop. The collection of small particles is dominated by the Brownian diffusion. In order to use the flow field measured in the w a t e r channel (or calculated from the potential flow theory) for the calculation of the collection efficiency, the Monte-Carlo method was chosen. Durin 9 the transport of the particles towards the drop, the direction of their 10 0 mean displacement from the streamlines was randomly simulated. X=Imm This seemed to be adequate for the statistical impacts with air molecules causin 9 the diffusion process. A result of these calculations is shown in Fi9ure 3 for spherical drops with 1 mm diameter. range of vorious pubtiThe ran9e of the result of published collection efficiencies by shed models diffusion, also shown in Fi9ure 3, refers to different models which lO-Z are based on the analogy between mass and heat transfer, see e. 9. Horn, Maqua and Bonka (1988). Considering the totally different approach, the a9reement is surprisin91y 9oocl. However, the 10-3 Monte-Carlo calculations are undergoin 9 further examinations in Monte- Carlo order to find out whether numerical improvements (e. 9. a distribucatcutation tion function for momentum exchange between particles and 10-~, molecules to model a random length of displacement) will change the results.
lO-1
lO-S collection by diffusion
10-6 10-3
I 10-2 /Jm d
Fiq.3: Monte-Carlo calculation o f the collection efficiency by diffusion, compared with other models.
Solid models of spherical and oblate-spheroid haped raindrops were also exposed to an airflow loaded with particles. This experiments were carried out in a small vertical wind tunnel (diameter 100 mm, length 2 0 0 0 mm). The scematical setup of the experiment is shown in Figure 4. The particles used were either lycopodium spores (dae=32 I~m) produced by a fluidized bed aerosol generator or monodisperse PSL particles (dae--3.4 f/m) produced by a spray generator. The experiments w e r e carried out with a geometrical scalin 9 factor of 5, which made it necessary to transform the results using the Stake's Number. The experimental conditions therefore were chosen in such a way that the inertial
Collection efflclency of aerosol particles by raindrops
857
impaction was expected to be the dominant collection mechanism. In order to eliminate blow off and bounce off of the particles, the models' surfaces were prepared with adhesive tapes. The results [m~ -fitter probe of these experiments are shown as circ( removeddunngexperiments) ~ . les (spherical models) and squares (oblate "0l spheroid shaped models) in Fi9ure 6. It mode[ can be seen that the results of the measurements are in agreement with the aerosol in calculations based on the flow field. How~, (~ btower ever, it must be noted that, in contrary to the calculations, the experiments 9ave honeycomb grit[ hi9her collection efficiencies for the I l oblate spheroids and not for the spheres. oeroso[neutroThe experiments also showed that, under generofor [izer -- fitter the chosen conditions, the backside collection of the lar9er particles by the Fiq.4: Experimental setup for the measurement of the collecmodels was less than 5% of that on the tion of aerosol particles by solid raindrop models front side, while for the smaller PSL particles the distribution on the model's surface was nearly uniform. For the lycopodium spores, the maximum deposition was found at the rim of the upstream-side dishin 9 of the oblate spheroid.
~--flowmefer
~
A disadvanta9e of experiments with solid models of raindrops is the statical behaviour of the models in the airflow. In reality, raindrops show inner circulations of w a t e r which influence the flow-boundary layer at the surface of the drop. The measurement of the collection efficiency of aerosol particles by fallin 9 water drops therefore should yield in results that are closest to reality. In Figure 5, the setup used for experiments with fallin 9 drops is shown. Before enterin 9 the aerosol chamber, the drops are accelerated to their terminal velocity. The setup allows the 9eneration and acceleration of monodisperse drops with diameters from 0.5 to approximately 3 mm. The particle concentration in the aerosol chamber is continuously monitored durin 9 the experiments. A lot of work was necessary to eliminate all sources of error in this experiments (inhomocjenous particle distribution and inacceptably hi9h airflow in the aerosol chamber, separation of drops that had washed particles from the walls of the drop sampler etc.). Surprisingly, all measures taken to avoid errors resulted in step by step lower collection efficiencies. Each triangle in Fi9ure 6 represents the mean value of a series of experiments. This f i r s t results are believed to be free of errors. The measurements now show a 9ood reproducibility, too. All collection efficiencies measured in the aerosol chamber up to now are in the lower ran9e of the published results of other authors, as can be seen by a comparison between Fi9ures 6 and 1.
1 aerosol chamber 2 drop-accelerator pipe 3 blower 4
drop-waterfeed
5 tear-off alrstream 6 clean air supply (TSI model 3074) 7 aerosol diluter (Palas VKL 10) 8 9 10 11 12
r i - "~"T " -#" ;-
optical particle counter (Royco 5000)
computer display neutralizer (TSI model 3012)
aerosol 9enerator (TSI model 3460) 13 sheath air 14 drop sampler 8
"
Fiq.5: Experimental setup for the measurement of the collection efficiency of aerosol particles by water drops accelerated to their terminal velocity
AS 19:7-G
858
H.-G. HORN et al.
10o
Legend: ....
E 10"
J ~ /
10.z
7 / ~ t:'~'
equations derived from Langmulr and Blodgett (1945) and Fuchs (1964) for NR=-) oo, and Ranz and Marshall (1952) . . . . calculated for spheres, X:Smm, flow accordln 9 to the potential flow theory (NR=-~ m) - - - calculated from water channel experiments with dished oblate spheroids, X=6mm (compare FI9.2) Wind [] o •
10"2
10"1
10~ pm 10z
100 d (:m
tunnel measurements (solid drop models): oblate spheroids, X=6mm spheres, X=6rnm spheres, X=2.Smm
Aerosol chamber experiments (falling drops): • a) X=0.5 mm • b) )(=1.0 mm • c) X=2.0 rnm • d) X=2,8mm
Fig.G: Calculated and experimental results compared with calculations using potential flow theory (NRe-> co) Conclusions The calculations and measurements of the collection efficiency of aerosol particles by raindrops
showed that, in any case, the results are in the lower range of the published experimental data. The calculation of the collection efficiencies is conservative with respect to radiation protection purposes if the equations of Langmuir and Blodgett (1945) for impaction and Fuchs (1964) for interception (both for potential flow theory), and those derived from the publications of Ranz and Marshall (1952) for diffusion are used. References: Bonka, H., et.al. (1984) "Zum Transport von an Aerosolteilchen 9ebundenen Radionukliden in die Vegetation nach Emission aus kerntechnischen Anlagen", Research Report to the Fed. Min. of the Interior Fuchs, N.A. (1964) "The Mechanics of Aerosols", Pergamon Press, Oxford Horn, H.-G., Maqua, M, Bonka, H. (1988) "Masse und trockene Ablagerun9 radioaktiver Stoffe auf die Vegetation und den Erdboden", Research Report to the Fed. Min. of Environment, Conservation a. Reactor Safety Langmuir, I., Bledgett, K.B. (1945) "Mathematical Investigation of Water Droplet Trajectories", General Electric Research Laboratory, Report No. RI-325 Pruppacher, H.R., Pitter, R.L. (1971) "A Semi-empirical Determination of the Shape of Cloud and Rain Drops", J. Atmospheric Sci. 2:86 Ranz, W.E., Marshall, W.R.jr. (1952) "Evaporation from Drops I,II", Chem. En9. Progr. 48:141 and 173 T h i s w o r k w a s s p o n s o r e d by t h e F e d e r a l M i n i s t e r o f E n v i r o n m e n t , C o n s e r v a t i o n and R e a c t o r S a f e t y u n d e r No. S t . S c h . 9 2 0 . W e t h a n k t h e I n s t i t u t e o f A e r o d y n a m i c s , R W T N A a c h e n , For t h e k i n d a s s i s t a n c e a t t h e water channel.
0021-8502/88 $3.00. + 0.00 Pergamon Press plc
COLLECTION EFFICIENCY OF AEROSOL PARTICLES BY RAINDROPS H.-G. Horn, H. Bonka, E. Gerhards, B. Hieronimus, M. Kalinowski, L. Kranz, M. Maqua Lehrgebiet Strahlenschutz in der Kerntechnik, RWTH Aachen Templergraben 55 D-5100 Aachen, F. R. Germany
Introduction For particlebound radionuclides emitted by nuclear facilities, the wet deposition due to washout is one of the significant pathways into the foodchain or to the 9round. An important input for the calculation of the deposition due to washout is the collection efficiency of aerosol particles by raindrops. A review of published models and 10~ data concernin 9 the wet deposition showed that, especially in the range of ,I 10o particle diameters from approx. 0.5 --0.2 mm i to 5 #m, the reported collection effi10-I ciencies spread over more than four X=lmm ~f~ orders of magnitude, see Fi9ure 1. In Fi9ure 1, there are also plotted 10"2 three curves for the collection efficiency as a function of the aerody104 namical particle diameter (dae) and the diameter of the raindrop (X). The1Gr4 se curves are derived from publications of Ranz and Marshall (1952) for • ~, 9" ~ 2 ,~Z,5 collection due to diffusion, of Fuchs i(7 s (1964) for collection due to interception, and of Langmuir and Blodgett 10 -s (1945) for collection due to inertial 10-3 10 .2 10 -I 10 0 101 l~rn 10z impaction. The equations for intercepdQo tion and impaction are formulated for potential flow (NRe-> oo). We use these equations, if a conservative calculation Fi9. I: Published experimental results and model calculations for the coflection efficiency of aerosol particles by raindrops, of the washout is needed for radiation from Bonka et. al. (1984) and Horn, Maqua and Bonka (1988) protection purposes, see e. 9. Horn, Maqua and Bonka (1988). Up to now, many detailed models to describe the collection of aerosol particles by raindrops were developed. However, the wide spreadin9 of the experimental results makes it difficult to decide which of these models are useful in the field of radiation protection. In order to achieve clearer results, own experiments and model calculations were carried out.
~
Experiments and model calculations Two basically different approaches were made to measure and calculate the collection efficiency: In the first case, the raindrops were simulated by solid models. The flow around the model drops was analysed, followed by the calculation of the collection efficiency. The calculations then were compared with experiments which also used solid drop models. In the second case, water drops of known size were accelerated to their terminal velocity, before they fell through an aerosol chamber. The collection efficiency was determined by countin 9 the particles on the collected drops. For the experiments and calculations concerning the solid drop models, spherical models as well as oblate spheroids were used. The oblate spheroids represent the shape of drops with diameters larger than 1 mm,
855
856
H.-G. HORN et al.
which get deformed due to the fluiddynamical and gravitational forces. The shapes of the oblate spheroids were taken from Pruppacher and Pitter (1971). Figure 2 shows the flow around an oblate spheroid with a diameter of 6 mm. The photograph shown in Fi9ure 2 was taken at NRe=1130. Flow fields like the one shown in Figure 2 were transformed into two-dimensional grids of flow-velocity vectors. Particle t r a jectories were then calculated with the aid of a computer program. For larger particles, where the collection by drops is mainly due to inertial impaction, the fluiddynamic forces were used to calculate the trajectories. The dashed line in Figure 6 shows the result of this calculation for the raindrop model shown in Figure 2. The same calculations were carried out for spheriFiq.2: Flow around an oblate spheroid, photographed in cal raindrops. Using the potential flow theory a water channel. Largest diameter X=6mm, to calculate the velocity vectors lead to collection efficiencies due to inertial impaction that NRe = 1130. lay above those calculated for the oblate spheraids (dotted and dashed line in Figure 6). The calculations did not take account of any collection on the backside of the raindrop. The collection of small particles is dominated by the Brownian diffusion. In order to use the flow field measured in the w a t e r channel (or calculated from the potential flow theory) for the calculation of the collection efficiency, the Monte-Carlo method was chosen. Durin 9 the transport of the particles towards the drop, the direction of their 10 0 mean displacement from the streamlines was randomly simulated. X=Imm This seemed to be adequate for the statistical impacts with air molecules causin 9 the diffusion process. A result of these calculations is shown in Fi9ure 3 for spherical drops with 1 mm diameter. range of vorious pubtiThe ran9e of the result of published collection efficiencies by shed models diffusion, also shown in Fi9ure 3, refers to different models which lO-Z are based on the analogy between mass and heat transfer, see e. 9. Horn, Maqua and Bonka (1988). Considering the totally different approach, the a9reement is surprisin91y 9oocl. However, the 10-3 Monte-Carlo calculations are undergoin 9 further examinations in Monte- Carlo order to find out whether numerical improvements (e. 9. a distribucatcutation tion function for momentum exchange between particles and 10-~, molecules to model a random length of displacement) will change the results.
lO-1
lO-S collection by diffusion
10-6 10-3
I 10-2 /Jm d
Fiq.3: Monte-Carlo calculation o f the collection efficiency by diffusion, compared with other models.
Solid models of spherical and oblate-spheroid haped raindrops were also exposed to an airflow loaded with particles. This experiments were carried out in a small vertical wind tunnel (diameter 100 mm, length 2 0 0 0 mm). The scematical setup of the experiment is shown in Figure 4. The particles used were either lycopodium spores (dae=32 I~m) produced by a fluidized bed aerosol generator or monodisperse PSL particles (dae--3.4 f/m) produced by a spray generator. The experiments w e r e carried out with a geometrical scalin 9 factor of 5, which made it necessary to transform the results using the Stake's Number. The experimental conditions therefore were chosen in such a way that the inertial
Collection efflclency of aerosol particles by raindrops
857
impaction was expected to be the dominant collection mechanism. In order to eliminate blow off and bounce off of the particles, the models' surfaces were prepared with adhesive tapes. The results [m~ -fitter probe of these experiments are shown as circ( removeddunngexperiments) ~ . les (spherical models) and squares (oblate "0l spheroid shaped models) in Fi9ure 6. It mode[ can be seen that the results of the measurements are in agreement with the aerosol in calculations based on the flow field. How~, (~ btower ever, it must be noted that, in contrary to the calculations, the experiments 9ave honeycomb grit[ hi9her collection efficiencies for the I l oblate spheroids and not for the spheres. oeroso[neutroThe experiments also showed that, under generofor [izer -- fitter the chosen conditions, the backside collection of the lar9er particles by the Fiq.4: Experimental setup for the measurement of the collecmodels was less than 5% of that on the tion of aerosol particles by solid raindrop models front side, while for the smaller PSL particles the distribution on the model's surface was nearly uniform. For the lycopodium spores, the maximum deposition was found at the rim of the upstream-side dishin 9 of the oblate spheroid.
~--flowmefer
~
A disadvanta9e of experiments with solid models of raindrops is the statical behaviour of the models in the airflow. In reality, raindrops show inner circulations of w a t e r which influence the flow-boundary layer at the surface of the drop. The measurement of the collection efficiency of aerosol particles by fallin 9 water drops therefore should yield in results that are closest to reality. In Figure 5, the setup used for experiments with fallin 9 drops is shown. Before enterin 9 the aerosol chamber, the drops are accelerated to their terminal velocity. The setup allows the 9eneration and acceleration of monodisperse drops with diameters from 0.5 to approximately 3 mm. The particle concentration in the aerosol chamber is continuously monitored durin 9 the experiments. A lot of work was necessary to eliminate all sources of error in this experiments (inhomocjenous particle distribution and inacceptably hi9h airflow in the aerosol chamber, separation of drops that had washed particles from the walls of the drop sampler etc.). Surprisingly, all measures taken to avoid errors resulted in step by step lower collection efficiencies. Each triangle in Fi9ure 6 represents the mean value of a series of experiments. This f i r s t results are believed to be free of errors. The measurements now show a 9ood reproducibility, too. All collection efficiencies measured in the aerosol chamber up to now are in the lower ran9e of the published results of other authors, as can be seen by a comparison between Fi9ures 6 and 1.
1 aerosol chamber 2 drop-accelerator pipe 3 blower 4
drop-waterfeed
5 tear-off alrstream 6 clean air supply (TSI model 3074) 7 aerosol diluter (Palas VKL 10) 8 9 10 11 12
r i - "~"T " -#" ;-
optical particle counter (Royco 5000)
computer display neutralizer (TSI model 3012)
aerosol 9enerator (TSI model 3460) 13 sheath air 14 drop sampler 8
"
Fiq.5: Experimental setup for the measurement of the collection efficiency of aerosol particles by water drops accelerated to their terminal velocity
AS 19:7-G
858
H.-G. HORN et al.
10o
Legend: ....
E 10"
J ~ /
10.z
7 / ~ t:'~'
equations derived from Langmulr and Blodgett (1945) and Fuchs (1964) for NR=-) oo, and Ranz and Marshall (1952) . . . . calculated for spheres, X:Smm, flow accordln 9 to the potential flow theory (NR=-~ m) - - - calculated from water channel experiments with dished oblate spheroids, X=6mm (compare FI9.2) Wind [] o •
10"2
10"1
10~ pm 10z
100 d (:m
tunnel measurements (solid drop models): oblate spheroids, X=6mm spheres, X=6rnm spheres, X=2.Smm
Aerosol chamber experiments (falling drops): • a) X=0.5 mm • b) )(=1.0 mm • c) X=2.0 rnm • d) X=2,8mm
Fig.G: Calculated and experimental results compared with calculations using potential flow theory (NRe-> co) Conclusions The calculations and measurements of the collection efficiency of aerosol particles by raindrops
showed that, in any case, the results are in the lower range of the published experimental data. The calculation of the collection efficiencies is conservative with respect to radiation protection purposes if the equations of Langmuir and Blodgett (1945) for impaction and Fuchs (1964) for interception (both for potential flow theory), and those derived from the publications of Ranz and Marshall (1952) for diffusion are used. References: Bonka, H., et.al. (1984) "Zum Transport von an Aerosolteilchen 9ebundenen Radionukliden in die Vegetation nach Emission aus kerntechnischen Anlagen", Research Report to the Fed. Min. of the Interior Fuchs, N.A. (1964) "The Mechanics of Aerosols", Pergamon Press, Oxford Horn, H.-G., Maqua, M, Bonka, H. (1988) "Masse und trockene Ablagerun9 radioaktiver Stoffe auf die Vegetation und den Erdboden", Research Report to the Fed. Min. of Environment, Conservation a. Reactor Safety Langmuir, I., Bledgett, K.B. (1945) "Mathematical Investigation of Water Droplet Trajectories", General Electric Research Laboratory, Report No. RI-325 Pruppacher, H.R., Pitter, R.L. (1971) "A Semi-empirical Determination of the Shape of Cloud and Rain Drops", J. Atmospheric Sci. 2:86 Ranz, W.E., Marshall, W.R.jr. (1952) "Evaporation from Drops I,II", Chem. En9. Progr. 48:141 and 173 T h i s w o r k w a s s p o n s o r e d by t h e F e d e r a l M i n i s t e r o f E n v i r o n m e n t , C o n s e r v a t i o n and R e a c t o r S a f e t y u n d e r No. S t . S c h . 9 2 0 . W e t h a n k t h e I n s t i t u t e o f A e r o d y n a m i c s , R W T N A a c h e n , For t h e k i n d a s s i s t a n c e a t t h e water channel.