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Impactor performance at small nozzle to collector distances
1. AcmsolSci. Vol. 29, Suppl. I, pp. S421-S422, 1998 8 1998 Published by Elsevier Science Ltd. All rights reserved Prhad in Great Britain 0021-850398 $19.00 + 0.00
IMPACTOR PERFORMANCE AT SMALL NOZZLE TO COLLECTOR DISTANCES F. J . GOMEZ-MORENO CIEMAT, Dept. Combustibles Fdsiles, Av. Complutense 22, 28040 Madrid, Spain
KEYWORDS
Inertial impactors; Reynolds number; radial acceleration; impactor geometry; short jets INTRODUCTION Marple’s (1970) numerical investigation on the performance of inertial impactors as a function of the ratio L/d,, between the nozzle to collector distance and the nozzle diameter demonstrated a deterioration of the capture efficiency curve for L/d,, < 1. Probably for that reason, not many experimental studies are available in this range. However, a variety of reasons, including turbulence avoidance at high Reynolds numbers Re, or controlling the cut via L (Gomez-Moreno et al., 1997) make such a study potentially useful. For L/d,, > 1, the onset of turbulence leads to loss of resolution for Re higher than a certain critical value Re*(Lld,J. Longer jets corresponding to larger L/d, become transitional and turbulent at lower Re than shorter jets. For L/d, < 1, the jet remains laminar even at Reynolds numbers of several thousands. But, at decreasing values of L/d,, below unity, an increasing fraction of the gas is accelerated radially rather than axially, leading to a poorer impactor resolution (May, 1975). At a sufficiently small L/d,,, the critical Stokes number should begin to increase. The objective of this work is to examine these effects in the domain l/2 < L/d, < 1 and 650 < Re < 1900. EXPERIMENTAL A variable-pressure impactor without focusing lenses was used, similarly as in Fernandez de la Mora et al. (1990). In order to vary Re in the impactor nozzle, a fixed aerosol flow rate was introduced through a critical orifice from atmospheric conditions into the low pressure region above the impactor nozzle. Clean air was added axisymmetrically around this aerosol by means of a porous metal tube upstream from the nozzle. With this system it was possible to vary the Re in the range from 650 to 1900. The nozzle was a thin-plate orifice with a diameter of 2 mm. The nozzle to collector distance L could be varied arbitrarily before each series of measurements. In this work L/d, takes the values 1.0, 0.75 and 0.5. S42 1
Abstracts
s422
of the 5th International
Aerosol Conference
1998
RESULTS The main results obtained are shown in Figure 1. R is the resolution, defined as R = S(1/2)/[S(O.8)-S(O.2)], where S(x) is the value of the Stokes number at which the collection efficiency is x. One sees that R decreases with decreasing L/d,,, a trend which persists for all the Re range studied. For more details see Gomez-Moreno (1998).
* * / I * L!dn=0.5
1AUdn=0.75 m Udn=l.O
600
600
1000
1200
1400
1600
1800
2coO
603
600
lOoI
1200
1400
1600
1600
2000
Fle
Re
Figure 1. Evolution of the resolution with Re for the L/d, values studied, suggesting that L/d,, should be kept as close to 1 as possible.
Figure 2. Dependence of S( l/2) on L/d,, There is a slight decrease at increasing Re, similarly as previously observed for L/d,, > 1.
In conclusion, the loss of resolution is already too large before S begins to be sensitive to L, so that the scheme of varying L to control the cut is not practical. However, the situation could be more favourable in the case of focusing impactors, where the resolution would be far less prone to deteriorate at reduced L/d,. ACKNOWLEDGEMENTS I am grateful to J. Rosell, I.G. Loscertales, J. Fernandez de la Mora for their valuable contributions to this work. NSF grant CTS-9319051 at Yale University (USA) and DGICYT project AMB96-1211-C02-02 are acknowledged for partial support. REFERENCES Femtidez de la Mora, J., Rao, N. and McMurry, P.H. (1990) Inertial impaction of fine particles at moderate Reynolds numbers and in the transonic regime with a thin-plate orifice. J. Aerosol Sci., 21, 889-909. Gomez-Moreno, F.J., Rosell, J. and Femandez de la Mora, J. (1997) Reynolds number effects on impactor performance for laminar and turbulent flows. AAAR Sixteenth Annual Conference Abstracts, 65. Gomez-Moreno, F.J. (1998) Ph.D. Thesis to be read, UNED. Marple, V.A. (1970) A fundamental study of inertial impactors. Ph.D. Thesis, U. Minnesota. May, K.R. (1975) Aerosol impaction jets. J. Aerosol Sci., 6,403-411.
IMPACTOR PERFORMANCE AT SMALL NOZZLE TO COLLECTOR DISTANCES F. J . GOMEZ-MORENO CIEMAT, Dept. Combustibles Fdsiles, Av. Complutense 22, 28040 Madrid, Spain
KEYWORDS
Inertial impactors; Reynolds number; radial acceleration; impactor geometry; short jets INTRODUCTION Marple’s (1970) numerical investigation on the performance of inertial impactors as a function of the ratio L/d,, between the nozzle to collector distance and the nozzle diameter demonstrated a deterioration of the capture efficiency curve for L/d,, < 1. Probably for that reason, not many experimental studies are available in this range. However, a variety of reasons, including turbulence avoidance at high Reynolds numbers Re, or controlling the cut via L (Gomez-Moreno et al., 1997) make such a study potentially useful. For L/d,, > 1, the onset of turbulence leads to loss of resolution for Re higher than a certain critical value Re*(Lld,J. Longer jets corresponding to larger L/d, become transitional and turbulent at lower Re than shorter jets. For L/d, < 1, the jet remains laminar even at Reynolds numbers of several thousands. But, at decreasing values of L/d,, below unity, an increasing fraction of the gas is accelerated radially rather than axially, leading to a poorer impactor resolution (May, 1975). At a sufficiently small L/d,,, the critical Stokes number should begin to increase. The objective of this work is to examine these effects in the domain l/2 < L/d, < 1 and 650 < Re < 1900. EXPERIMENTAL A variable-pressure impactor without focusing lenses was used, similarly as in Fernandez de la Mora et al. (1990). In order to vary Re in the impactor nozzle, a fixed aerosol flow rate was introduced through a critical orifice from atmospheric conditions into the low pressure region above the impactor nozzle. Clean air was added axisymmetrically around this aerosol by means of a porous metal tube upstream from the nozzle. With this system it was possible to vary the Re in the range from 650 to 1900. The nozzle was a thin-plate orifice with a diameter of 2 mm. The nozzle to collector distance L could be varied arbitrarily before each series of measurements. In this work L/d, takes the values 1.0, 0.75 and 0.5. S42 1
Abstracts
s422
of the 5th International
Aerosol Conference
1998
RESULTS The main results obtained are shown in Figure 1. R is the resolution, defined as R = S(1/2)/[S(O.8)-S(O.2)], where S(x) is the value of the Stokes number at which the collection efficiency is x. One sees that R decreases with decreasing L/d,,, a trend which persists for all the Re range studied. For more details see Gomez-Moreno (1998).
* * / I * L!dn=0.5
1AUdn=0.75 m Udn=l.O
600
600
1000
1200
1400
1600
1800
2coO
603
600
lOoI
1200
1400
1600
1600
2000
Fle
Re
Figure 1. Evolution of the resolution with Re for the L/d, values studied, suggesting that L/d,, should be kept as close to 1 as possible.
Figure 2. Dependence of S( l/2) on L/d,, There is a slight decrease at increasing Re, similarly as previously observed for L/d,, > 1.
In conclusion, the loss of resolution is already too large before S begins to be sensitive to L, so that the scheme of varying L to control the cut is not practical. However, the situation could be more favourable in the case of focusing impactors, where the resolution would be far less prone to deteriorate at reduced L/d,. ACKNOWLEDGEMENTS I am grateful to J. Rosell, I.G. Loscertales, J. Fernandez de la Mora for their valuable contributions to this work. NSF grant CTS-9319051 at Yale University (USA) and DGICYT project AMB96-1211-C02-02 are acknowledged for partial support. REFERENCES Femtidez de la Mora, J., Rao, N. and McMurry, P.H. (1990) Inertial impaction of fine particles at moderate Reynolds numbers and in the transonic regime with a thin-plate orifice. J. Aerosol Sci., 21, 889-909. Gomez-Moreno, F.J., Rosell, J. and Femandez de la Mora, J. (1997) Reynolds number effects on impactor performance for laminar and turbulent flows. AAAR Sixteenth Annual Conference Abstracts, 65. Gomez-Moreno, F.J. (1998) Ph.D. Thesis to be read, UNED. Marple, V.A. (1970) A fundamental study of inertial impactors. Ph.D. Thesis, U. Minnesota. May, K.R. (1975) Aerosol impaction jets. J. Aerosol Sci., 6,403-411.