Particle Impaction Patterns from a Circular Jet

Transcription

1 Aerosol Science and Technology ISSN: (Print) (Online) Journal homepage: Particle Impaction Patterns from a Circular Jet Virendra Sethi & Walter John To cite this article: Virendra Sethi & Walter John (1993) Particle Impaction Patterns from a Circular Jet, Aerosol Science and Technology, 18:1, 1-10, DOI: / To link to this article: Published online: 11 Jun Submit your article to this journal Article views: 221 View related articles Citing articles: 24 View citing articles Full Terms & Conditions of access and use can be found at Download by: [ ] Date: 03 January 2018, At: 14:59

2 Particle Impaction Patterns from a Circular Jet Virendra Sethi* and Walter ~ ohd Air and Idustrial Hygiene Laboratoly, California Depaitment of Health Seruices, 2151 Berkeley Way, Berkeley, CA The surface distributions of particles that have impacted from a circular jet have been measured by optical microscopy and image analysis. Monodisperse, 3-pm ammonium fluorescein particles were impacted onto greased surfaces at various velocities from the nozzle. The spot radius is approximately equal to the nozzle radius at JSf = 0.8. At JSt = 1.6, the spot radius is about half of the nozzle radius; convergence of the flow into the nozzle results in a focusing effect. The INTRODUCTION The design of a particle impactor requires consideration of the configuration of the accelerating nozzle. Initially, the particles tend to follow the converging flow into the nozzle. Then the particle trajectories depart increasingly from the streamlines as the flow accelerates, the departure depending on the curvature of the streamlines and the particle inertia. At the impaction plate where the flow bends through a right angle, particles with sufficient inertia impact. The point of impaction and the angle of impaction depend on the initial starting point and the Stokes number. Marple (1970) has calculated particle trajectories in impactors by solving nu- * Present address: Department of Civil and Environmental Engineering, University of Cincinnati, Cincinnati, OH ' To whom correspondence should be addressed. Aerosol Science and Technology 18:l-10 (1993) Elsevicr Science Publishing Co., Inc. surface density has a peak which moves out to larger radius with decreasing Stokes number, reaching the nozzle radius at about JSt = 0.5. The radii enclosing 10.5% and 70.3% of the particles as a function of JSt are in good agreement with the theoretical predictions of Marple (1970); however, the upturn of the experimental data for JSt < 0.5 is somewhat sharper. Empirical formulas were fitted to the data. merically the Navier-Stokes equations and the equation of motion of the particles. He verified the theoretical calculations with a limited number of measurements of the point of impact for an impactor with a rectangular slit and a rounded nozzle entrance. We are not aware of any data on the particle impaction points for impactors with circular nozzles. Such impaction patterns are of interest for the optimization of impactor design and for the use of impactors, where the deposit pattern may be important to the application. Recently, our group has used a circular jet impactor to study particle resuspension induced by impacting particles (John et al., 1991). For these studies it was necessary to know the impaction patterns in detail. These have been measured by impacting the particles onto a greased surface and analyzing the particle positions with an automated image analysis system. The automation makes it feasible to mea-

3 V. Sethi and W. John FIGURE 1. Schematic drawing of the impactor used for the measurements. The nozzle diameter, D, was mm.

4 Particle Impaction Patterns DILUTION I AIR ---+ TEST CHAMBER 4-b IMPACTION SURFACE 1 EXHAUST CP~X'~~ MULTI- PERSONAL 1 PARTICLE AMPLIFIER CHANNEL COMPUTER ANALYZER FIGURE 2. Experimental arrangement. sure the impaction patterns for a wide range of Stokes numbers. These patterns may be compared to Marple's calculations and used for practical purposes. EXPERIMENTAL --- u T t MANOMETER The impactor configuration is shown in Figure 1. The jet-to-plate distance was set at half the nozzle diameter to facilitate comparison to Marple's calculations. The nozzle configuration differs slightly from that of Marple (see Figure 6): the convergence angle is 52" vs. Marple's 60, the throat length is 2.52 times the nozzle diameter vs. 1.0 for Marple, and our nozzle is tapered up at 47". We do not believe these differences to be very significant for the performance. The experimental layout is illustrated in Figure 2. Monodisperse, solid, smooth spheres of ammonium fluorescein were produced by a vibrating orifice aerosol generator, using the techniques of Vanderpool and Rubow (1988). The aerosol - PLENUM NEUTRALIZER BERGLUND-LIU VIBRATING ORIFICE Kr-85 was charge neutralized, accumulated in a plenum, diluted with clean air, and then introduced to the impactor. The particle size and concentration were monitored by a Climet Model 208 optical counter (Climet Instrument Co., Redlands, CA.).' The data were recorded by a personal computer. The test chamber was pumped by the optical particle counter at a constant flow rate of 5 L/min. To vary the aerosol flow rate through the nozzle, clean bypass air was introduced at the top of the chamber through a porous metal plate to smooth out the flow. The flow through the nozzle was monitored by measuring the pressure drop as shown in Figure 2, using a precision water manometer (Debro Miniscope MIN 5, Debro-Werk, Diisseldorf-Oberkassel, FRG). The pressure drop was calibrated in terms of flow rate by tem- ' The mention of commercial products is not to be construed as an endorsement of such products.

5 V. Sethi and W. John porarily connecting a bubble meter in the line leading to the chamber. The impaction surfaces were polished aluminum SEM (scanning electron microscope) stubs, 9.43 mm in diameter. The top surface was coated with Vaseline (petroleum jelly) by dipping in a solution 1:1.5 vol/vol Vaseline/toluene. The excess was wicked off into tissue paper from the side. The resulting grease coating was effective in preventing any observable particle bounce and presented a smooth background for image analysis. All of the data were taken with 3.0-pm ammonium fluorescein particles. The Stokes number was varied by changing the flow rate through the nozzle. A measurement sequence began with the impaction surface retracted from the nozzle. After the flows were adjusted to the desired settings, the particle count rate was taken. Then the impaction surface was inserted rapidly into the particle beam for a timed exposure. The insertion time was few tenths of a second, negligible compared to the exposure time of several minutes. The distribution of particles on each surface was measured by an image'analysis technique which has been described in detail by John et al. (1991). The specimen was viewed by a video camera through an optical microscope to provide an image for analysis by a Kevex Delta Class Analyzer (Kevex Instruments, San Carlos, CA). The image was digitized and processed to remove background, and the coordinates of the center of each particle were determined. These data were then transferred to a personal computer for final data reduction. RESULTS AND DISCUSSION Particle Deposition Patterns Figure 3 shows computer plots of the particle deposit patterns for three values of,/st. The Stokes number, St, was evaluated 2 Nozzle I I I Jst = I I I - (0) FIGURE 3. Computer plots of the deposit patterns for three different values of the Stokes number. from the equation: where p, is the particle density (1.35 g/cm3), C is the slip factor, v is the average jet velocity, Dp is the particle

6 Particle Impaction Patterns 5 diameter (3.0 pm), p is the viscosity of air, and dj is the nozzle diameter (1.484 mm). At JSt = 1.6 (Figure 3a), the spot diameter is about half the nozzle diameter. At JSt = 0.8 (Figure 3b), the spot diameter is approximately equal to the nozzle diameter. At,/St = 0.48 (Figure 3c), the spot has developed into a ring. This value of,/st is near the 50% cutpoint, which we found to be at,/st = The collection of 50% of.the particles is dependent on the size of the impaction plate, a point not ordinarily mentioned in discussions of impactors. The radial profiles of the surface density of the deposits computed from the data are plotted in Figure 4. The total number of particles for each profile was normalized to Qualitatively, there is a minimum on the nozzle axis, the density increasing with radial distance to maximum and then falling off rapidly. The radial position of the peak increases with JSt. At JSt = 1.6, the peak is at of the nozzle radius; at JSt = 0.5, the peak is near the nozzle edge. As JSt decreases < 0.5, the peak moves rapidly to larger radius (Figure 5). Comparison to Marple Theory Figure 6 shows the impactor configuration and particle trajectories calculated by Marple (1970). The particle impaction points are in qualitative agreement with the data presented above. At high Stokes number, there is a focusing effect produc Radial Position (mm) FIGURE 4. Plots of the measured particle surface density vs. radial position for various Stokes numbers.

7 6 V. Sethi and W. John Radial Position (mm) FIGURE 5. Plots of the measured particle surface density vs. radial position for,/st < 0.5. ing a beam spot smaller than the nozzle diameter. At lower Stokes number, the impaction points move to larger radii. It is possible to make a more quantitative comparison to Marple's theoretical results by utilizing his table of radial impaction points vs. Stokes numbers for the two trajectories shown in Figure 6. Assuuming uniform flow at the initial particle positions at the entry, 10.5% of the flow is enclosed between trajectory A and the axis, and 70.3% of the flow is between trajcetory E and the axis. For comparison to theory, we have calculated from our data at each Stokes number the radii enclosing 10.5% and 70.3% of the deposited particles. The resulting radii, normalized to the nozzle radius, are plotted as experimental points in Figure 7, and Marple's theoretical results are plotted as solid lines. For trajectory A, enclosing 10.5% of the flow, the agreement between experiment and theory is close. For trajectory E, enclosing 70.3% of the flow, the agreement is good for JSt > 1; for smaller JSt, the theory tends to overestimate the radial distance, especially in the vicinity of the sharp upturn below JSt = 0.5. The more gradual upturn of the theoretical line might be due to the limited resolution afforded by the grid over which the Navier-Stokes equations were numerically solved. The later impactor calculations by Rader and Marple (1985) employing a finer grid showed sharper cutoff curves; however, trajectory data which could be used for the present comparison were not

8 Particle Impaction Patterns FIGURE 6. Impactor configuration used by Marple (1970) for theoretical calculations of particle trajectories. Lines marked A and E are numerically computed particle trajectories.

9 V. Sethi and W. John --- o Experimental 70.3% I I A Experimental 10.5% - Theoretical (Marple, 1970) Jst FIGURE 7. Comparison between the present experimental results and the theoretical results of Marple (1970) for the radii enclosing 10.5% of the particles vs. Stokes number (end of trajectory marked A in Figure 6). Results are also shown for the radii enclosing 70.3% of the particles (end of trajectory marked E in Figure 6). presented. For trajectory E, the agreement for JSt would be improved if the assumption of uniform flow at the initial point were replaced by a more realistic flow which has lower velocities near the wall. Another source of differences between theory and experiment is in the effect of changing Reynolds numbers, Re. The theoretical calculations were made with Re = For the experiments, as JSt varied from 0.46 to 1.60, Re ranged from 386 to Examination of the various theoretical results vs. Reynolds number indicates the effect of Reynolds number on impaction parameters to be small in this range, but it is difficult to estimate the effect quantitatively. Finally, we have mentioned the small differences between our impactor configuration and that of Marple. Considering all the factors, we believe the agreement between experiment and theory to be quite good. Empirical Formulas The theory does not provide analytical expressions for the particle surface density. Such expressions can be useful for practical purposes. A purely empirical ex-

10 Particle Impaction Patterns pression was fitted to the data by eye for three JSt spanning the range: where S is the particle surface density in number/mm2 and r is the radial position in mm. The coefficients are tabulated in Table 1. In Figure 8, the empirical expressions are plotted as lines for comparison to the data points. SUMMARY AND CONCLUSIONS The surface distributions of particles that have impacted from a circular jet have been measured by microscopy and image analysis. At JSt = 1.6, the spot radius is about one-half the nozzle radius, i.e., focusing is produced by the flow convergence into the nozzle. For lower Stokes number, the peak in the surface density moves out to larger radius, reaching the nozzle radius at about JSt = 0.5. TABLE 1. Numerical Values of Coefficients in Eauation 2 Jst 1.6 r Fit Radial Position (mm) FIGURE 8. Fits of an empirical function (Eq. 2) for the particle surface density as a function of the radial position. Measured data points are shown.

11 10 V. Sethi and W. John The radii enclosing 10.5% and 70.3% of the particles as a function of JSt are in good agreement with the theoretical predictions of Marple; however, the upturn of the experimental data for JSt < 0.5 is somewhat sharper. Empirical formulas were fitted to the data for JSt = 0.48, 0.8, and 1.6. This project was supported by Grant No. CTS from the US. National Science Foundation. REFERENCES John, W., Fritter, D. N., and Winklmayr, W. (1991). J. Aerosol Sci. 22: Marple, V. A. (1970). A Fundamental Study of Inertial Impactors. Ph.D. dissertation, University of Minnesota. Particle Technology Laboratory Publication 144. Rader, D. J. and Marple, V. A. (1985). Aerosol Sci. Technol. 4: Vanderpool, R. W., and Rubow, K. L. (1988). Aerosol Sci. Technol. 9: Received December 27, 1991; accepted April 20, 1992.