Virtual Impactors: A Theoretical Study Virgil A. Marple" and Chung M. Chien Particle Technology Laboratory, Mechanical Engineering Department, University of Minnesota, Minneapolis, Minn. 55455
The characteristics of virtual impactors have been determined by the numerical solution of the Navier-Stokes equations and of the equations of motion of the particles. The effect of the nozzle Reynolds number, the fraction of flow passing through the collection probe, collection probe diameter, nozzle throat length, nozzle-to-collection probe distance, and collection probe inlet design on the small and large particle collection efficiencies has been studied. In addition, it was found that at the cutoff particle size there were significant losses on the inner surface of the collection probe. The results show that most parameters, with the exception of the nozzle Reynolds number, have little effect on the large particle collection efficiency. However, the effect on the small particle collection efficiency and collection probe losses was significant for many of these Darameters.
The virtual impactor is a device used for the inertial separation in airborne particles ( I , 2 ) . In this impactor, shown schematically in Figure la, a jet of particle-laden air is directed at a collection probe, which is slightly larger in diameter than the acceleration nozzle. The large particles cross the air streamlines and enter the collection probe, while the small particles follow the air streamlines into the side passage. T o remove the large particles from the collection probe, a fraction of the total flow passing through the nozzle of the virtual impactor is allowed to pass through the collection probe. This flow will be referred to as the minor flow, while the flow through the side passage will be referred to as the major flow. As is the case with real impactors, the virtual impactor's performance is characterized by a collection efficiency curve. For the ideal virtual impactor, the separation between the large and small particles should be perfectly sharp, as shown for the ideal case in Figure l b . Note, however, that in the virtual impactor there will always be some of the small particles with the large particles due to the air flow through the collection probe. In an actual virtual impactor, the efficiency curve is not quite so simple, since there are not only large particles passing through the collection probe and small particles passing through the side passage, but also a fraction of the particles impacting upon the inner surfaces of the collection probe. Thus, as shown in Figure lb, there are actually two efficiency curves separating the particles into the following three regions: (1) small particles passing out the side passage, (2) particles which are impacted upon the collection probe (losses), and (3) large particles passing through the collection probe. Since losses in the virtual impactor are highly undesirable, an impactor should be designed such that the displacement of the two efficiency curves shown in Figure 1b is as small as possible, with the two efficiency curves coinciding for the desirable case of a virtual impactor with no losses. It is the purpose of this paper to use numerical methods to determine the flow fields, particle trajectories, and, finally, the efficiency and loss curves for virtual impactors operating at different conditions of Reynolds numbers, fraction of flow passing through the collection probe, and virtual impactor design. 976
Environmental Science & Technology
Theoretical Techniques Although theoretical studies of virtual impaction devices have been made previously ( 3 , 4 ) ,the studies have assumed potential flow, which does not take viscous effects into account. The method used here to theoretically analyze the performance of the virtual impactor, which does include the viscous effects, is to first determine the flow field within the impactor by solving the full Navier-Stokes equations using numerical analysis techniques, and then to solve for the particle trajectory within this flow field by numerically integrating the particle's equation of motion. This method has been used successfully in a fundamental study of real impactors to determine the influence of various parameters on the characteristic collection efficiency curves ( 5 ) .Comparisons of the theoretical efficiency curves with those of experimental investigations (6-10) have shown that the agreement is good if the impactor inlet conditions, shape, and Reynolds number are similar. Since the flow field and particle trajectories are similar in real and virtual impactors, this theoretical technique should be equally successful in determining the efficiency curves of virtual impactors. The general method of solution of the flow field is to first express the Navier-Stokes equations in terms of the vorticity and the stream function. The resulting differential equations are then made dimensionless, with the radial, r , and axial, z , dimensions being in units of the nozzle throat diameter, Do. The Reynolds number: Re = PDoVo -
P
where p is the fluid density, Vo is the mean fluid velocity a t the nozzle throat, and 1.1 is the absolute viscosity of the fluid, will be a parameter in these equations. The dimensionless differential equations are next expressed in a finite difference form and solved by the method of relaxation over a grid of node points covering the field of interest, as shown in Figure 2a. (Note that the flow is symmetrical about the center line.) Although the solution is the determination of the vorticity and stream function values a t each node point, the values of the velocity components a t the node points can be calculated from the stream function values. For details on the derivation of the finite difference equations, boundary conditions, and relaxation technique, the reader is referred to a previous paper by Marple et al. ( 1I 1. After the flow field has been determined, it is then necessary to follow particle trajectories through the virtual impactor. T o accomplish this, the particle's equations of motion in the r and z directions are made dimensionless by again expressing the r and z dimensions in units of Do.For these equations, the Stokes number, St, defined by Fuchs ( 1 2 ) as the ratio of the particle stopping distance to Do/2, will be a parameter. The Stokes number is thus expressed as:
p,VoCD,2 (2) %Do where p p is the particle density, C is the Cunningham slip correction, and D, is the particle diameter. Since St is dimensionless, Equation 2 indicates that is a measure of the dimensionless particle diameter. The dimensionless equations of motion are next put in finite difference form and St =
0013-936X/80/0914-0976$01.OO/O
Jz
@ 1980 American Chemical Society
integrated numerically through the area of interest. This technique, which is described in detail by Marple and Liu ( 6 ) , is capable of describing the particle's trajectory once the particle's initial position and velocity have been given. The integration process is started by first assigning a specific value of to a particle, and giving the particle an initial velocity equal to the local fluid velocity a t a position near the entrance. By use of the Runge-Kutta integration method, the movement of the particle, AZ and A r , during a small increment of time, At, is determined. This gives the position of the particle a t the end of the time increment. This process is then repeated, and the movement of the particle through the impactor is followed until the particle either exits through the collection probe, exits to the side of the virtual impactor, or impacts upon the wall of the collection probe. The particle is considered impacted when the center of the particle comes within one particle radius, R,, of a surface. Since all dimensions of the virtual impactor are in units of the nozzle diameter, Do, the particle radius must also be expressed in units of DO.Thus, from Equations 1 and 2:
A detailed description of the numerical procedure and the computer program used has been given by Marple ( 5 ) . TOTAL FLOW
SIDE PASSAGE
Do
STREAMLINES-
S
L
'2
'
( a 1 GRID
MAJOR -FLOW
Y
DI PARTICLE TOO SMALL T O B E COLLECTED
c- COLLECTION PROBE
TRAJECTORY OF COLLECTED P A R T I C L E -MINOR FLOW
NOZZLE -EXIT PLANE
( 0 ) VIRTUAL IMPACTOR
100
OT
3
s? I s
REGION I
LL
za COLLECTION EFFICIENCY IN MAJOR FLOW
l-
8s
PARTICLE COLLECTION EFFICIENCY IN MINOR FLOW
RECIRCULATION REGION
O
s
I
'
PARTICE ?SEPARATION FOR IDEAL CASE
i
I
0'
PARTICLE DIAMETER, Dp REGION I - P A R T I C L E S IN THE MAJOR FLOW REGION 2-PARTICLES IMPACTED ON COLLECTION PROBE REGION 3-PARTICLES IN THE MINOR FLOW ( b ) COLLECTION EFFICIENCY CURVES
Figure 1. Nomenclature, streamlines, particle trajectories, and efficiency curves for a virtual impactor
Figure 2. Grid and flow field for virtual impactor at base case conditions
Volume 14, Number 8, August 1980
977
Table 1.
Values of Design Parameters Used in Study 41/41
Re
Dl100
LODO
SlDo
R0,deg
collectlp probe
\r\
1
10
B C
100
500 1000 0.05 5000b O . l O b 15000 0.15 0.25
1.16 1.33b 1.49
0.013
0.25
2.5b
1
30 456
PARTICLES STARTING AT 50% STREAMLINE
D Ab
2
. . . 1
a Collection probe A, thin wall; 6,infinite wall thickness; C, finite wall thickness; D, finite wall thickness with taper. (Collection probe designs are shown in Figures l l c , l l d , and I l e . ) Base values from which parameters are varied.
P
Resul t s As described above, the flow fields, particle trajectories, and corresponding efficiency curves of a virtual impactor will depend on the flow parameters, Re and Q1/Q0, the physical design parameters, DJDO, LO/&,, S/Do, and 8 0 , and the shape of the entrance to the collection probe. To initiate the parametric study, a set of base values for the parameters was chosen, and these are listed in italics in Table I. The other values of the parameters in Table I were then varied one a t a time to the values listed, while the remaining parameters remained a t the base values. The following is a discussion of the results for the base case, and then discussions of the effects of the various parameters. Base Case. The grid used for the base case, corresponding to the parameters in Table I, is shown in Figure 2a. A thinwalled tube collection probe, which simulates a collection probe with its wall tapered to a sharp edge at the entrance, was chosen, since this design introduces no new variables such as wall thickness or radius of curvature of the probe entrance. The flow field streamlines for the base case are shown in Figure 2b for streamlines corresponding to 5,10,20,40,80,and 100% of the flow inside that streamline. Note that the 0% streamline is at the center line, the 10%streamline intersects the collection probe (Q1/Qo = lo%),and the 100% streamline corresponds to the nozzle wall. Also note that the free streamline emitting from the nozzle reattaches to the surface defining the nozzle exit plane, forming a recirculation region between that surface and the free streamline. A second recirculation region is formed between the air flowing out the side passage and the lower surface, causing a 5% streamline to be indicated in this region. In Figure 3a, the particle trajectories for five particles with different values of Js,all starting a t the 50% streamline, are shown. These five particle trajectories include the three cases where particles (1)pass through the collection probe, (2) are impacted on the collector probe inner surface, and (3) pass through the side passage. Also included are the two critical trajectories between these three cases. For example, if the value of %/%of a particle is greater than 0.881, the particle will pass through the collection probe; if it is less than 0.677, the particle will pass through the side exit; and if it is between two critical values, the particle will impact on the collection probe inner surface. Another method by which the critical values of can be determined is to keep ficonstant while varying the inlet starting position of the particle, as shown in Figure 3b. This shows that particles with JE = 0.5 starting between the 24% streamline and the center line will pass through the collection probe, those starting between the 24 and 75% streamlines will impact on the collection probe, and those starting a t streamlines greater than 75% will pass out the side exit.
Jsf 0.575 0.677 0.779 0.881 0.983
PARTICLE TERM1NATION MAJOR FLOW CRlTl CAL IMPACT ON PROBE CRITICAL MINOR FLOW
~
( a 1 FIVE PARTICLES OF DIFFERENT SIZES STARTING AT THE SAME POINT
i
b
D
Environmental Science & Technology
.
U I
8
b
P
1.
I P
4s
978
r
1 ( b ) CRITICAL TRAJECTORIES FOR PARTICLES OF SIZE = 0.5
Figure 3. Particle trajectories at base conditions
By using data such as represented in Figure 3a for particles starting a t several positions, or as in Figure 3b for particles with different values of JE , the “large particle collection efficiency” and the “small particle collection efficiency” curves shown in Figure 4 can be determined. In either case, the collection efficiencies are the percent of particles emitting from the nozzle which pass through the collection probe or side passage, respectively. (The values of J E a s a function of these efficiencies are presented for all cases and are available as a supplementary material; see paragraph a t end of paper.) For the purpose of determining the percent of the particles being impacted on the inner surface of the collection probe, referred to as “collection probe loss” in this paper, it is best to construct the curve “100% - small particle collection efficiency”. This curve, along with the large particle collection efficiency curve, represents the two efficiency curves in Figure 1b. As stated before, the difference in efficiencies at any value of represents the collection probe loss, and thus the loss curve can be constructed. It should be noted in Figure 4 that collection characteristics of a virtual impactor can be specified by the large particle collection efficiency curve and any one of the other three curves. However, since collection probe losses ore of considerable importance, this curve will be used in this paper. Also note that the quantity of collection probe losses is quite large, being greater than has been reported in experimental investigations (13, 14). However, investigations of the computer outputs indicated that nearly all losses occurred a t the tip of the collection probe by particles that were traveling vertically upward directly adjacent to the probe inner surface. As will be shown later, it is possible to reduce these losses by proper contouring of the probe entrance. Therefore, the absolute values of the losses in this paper may be high, but the relative effects of the various parameters on these losses are of interest. Although the contour of the collection probe entrance affects the probe losses, it has little effect on the large particle collection efficiency curve. Thus, this curve should be considered as the most significant characteristic curve for virtual impactors, and it is the curve by which virtual impactors will be characterized in this paper. It is of interest to compare the large particle collection efficiency curve determined theoretically to the experimental results of Loo and Cork (13)in Figure 5 . The value of the parameters listed in Figure 5 for each curve indicates that the virtual impactors are similar. The comparison shows that the theory agrees well with the experimental results, with the
theoretical curve and experimental curve being nearly identical. Having confidence that the theoretical solution of the virtual impactor is indicating collection curves that agree well with experimental results for the base case, the other cases listed in Table I were analyzed by varying the value of each parameter one a t a time, while the other parameters were held a t the base values. The influences of these parameters are discussed in the following sections, with the flow fields presented in Figures 6,10, and 11,and the efficiency curves and loss curves presented in Figures 8 and 9, respectively. Influence of Reynolds Number. To determine the influence of the Reynolds number, cases were run for the different values of Re listed in Table I, while the other parameters were held constant a t base values. The flow fields are shown in Figure 6 for Re = 1,10,100,500,1000,and 15 000, and in Figure 2b for Re = 5000. In Figure 6, only the portion of the flow fields in the vicinity of the collection probe is shown, since the flow fields in other portions of the impactors are very similar to those in Figure 2b. I t should be noted that the flow may not be laminar for Re = 15 000 but is presented as a limiting case. Since the large particle collection efficiency curves are of primary interest, the flow fields within the collection probe are of most importance. I t is in this region where we observe the unexpected result that the penetration of the 10 and 20% streamlines into the collection probe is greater for the cases of Re = 100 and 500 instead of for the higher values of Re, where the inertial effects of the flow should be larger. The reason for this can be seen by inspecting the velocity profiles a t the nozzle exit plane shown in Figure 7 . At high values of Re (Re = 5000 and 15 000), the velocity profile is relatively uniform across much of the nozzle, making it difficult for any portion of the flow to penetrate into the air in the collection probe. For the cases of Re = 500 and 100, the velocity profile is more parabolic, and the relatively high velocity near the center line makes it possible for the jet to penetrate farther into the collection probe. However, if the value of Re is small (Re = lo), there is insufficient inertia in the jet to penetrate into the viscous air in the collection probe. Thus, Re in the range of 100 to 500 is the correct combination of nozzle velocity profile and inertia to obtain maximum penetration of the 10 and 20% streamlines into the collection probe. The large particle collection efficiency curves are shown in Figure 8a with Re as a parameter. The influence of the flow
PRESENT WORK
-ae
‘00-
I
1
80
I
-A
EXPERIMENTAL
100%-SMALL PARTICLE COLLECTION EFFICIENCY
E 40t
= t ’V
E
PROBE (LOSS)
P,/OO = 0.10
20
DllW = 1.33 TIW-2.5
\
\
SIW 8,:= 45. I
W 0 -I
6 1 0 0
0.1
b.
I
I
I
0.5
I0 I
1
1
1.0
i
01
Figure 4. Collection efficiency and loss curves for base conditions
05
20
10
Figure 5. Comparison of theoretical and experimental large particle collection efficiency curves at the following conditions:
2;o
JST
1
present theory Loo and Cork (73)
R~
ol/oo
si00
DlIDo
5000 6000
0.1 0.1
1.0
1.33 1.38
0.8
L o ~ D o 6, deg
2.5 0.8
Volume 14, Number 8, August 1980
45 45
979
I . I Re = I
I:
... 1 ;;;
..I
: :::, 1
: :: P
Re =500
i;‘
I
~
R e = 1000
S
Re
P
I
ti
-- 15,000
.earnlines
penetration into the collection probe can be seen in the relative position of the large particle collection efficiency curves. For the cases where the flow penetration into the collection probe is largest (Re = 100 and 500), the particle Stokes numbers must be larger in order for the particles to penetrate into the minor flow, since the Stokes number is defined as the particle stopping distance to the radius of the nozzle. Where the flow penetration into the collection probe is small, the required distance that the particle must travel, and thus the value of the Stokes number, will be smaller. It is of interest to note in Figure 8a that the efficiency curves are nearly identical for the cases of Re = 1and 10 and for Re = 500 and 15 000, indicating that lower or higher Reynolds numbers than those listed should have little effect on these curves. It is also interesting to note that the slopes of the penetration curves are greater for the larger values of Re, indicating better cutoff characteristics. However, this effect is small, which is different from real impactors, where the cutoff characteristics are much poorer for low values of Re than for large values (6). Concerning the influence of Re on the losses, Figure 9a shows that the influence is large. In general, the losses increase from a minimum of about 10%at low values of Re to about 60% a t high values, with the maximum losses occurring at the value of corresponding to 50% efficiency. Influence of Ql/QO. The flow fields corresponding to Ql/Qo values of 0.05,0.15, and 0.25 are shown in Figures loa, lob, and lOc, and should be compared to the base case of Q l / Q o = 0.10 in Figure 2b. As can be seen from these figures, more streamlines pass through the collection probe for large values of Ql/Qo, and the small amount of flow in the side passage leaves more room for recirculation in this area. The flow field for Ql/Qo = 0.05 shows reattachment to the lower surface of the side passage, much like the flow a t Re = 100 in Figure 6. Whether or not the flow field in this region is correct is uncertain. However, since the flow in the region has no effect on the flow field in the important area within the collection 980
Environmental Science & Technology
probe, a more detailed investigation of the flow in this region was not made. The corresponding large particle collection efficiency curves are shown in Figure 8b. The large particle collection efficiency curves indicate that a “sharper” cut between large particles collected and those which are not is obtained for smaller values
,r
DISTANCE FROM A X I S i
0.4
0.3
0.2
0.1
I
I
I
I
-> O
k
V
s W
10 100
>
---.-
W
0
a a
500 1000 - * -
5000 15000
-----
3.5
W
p
E
4 I
2”
I
>“ I. I X W
W J N N
e l-
.5 Q
> k V
s
W
>
d
X
LO 4
Figure 7. Influence of the Reynolds number on the axial velocity profile at the nozzle exit
*Ot
0 0.2
05
1.0
2 .o
a2
0.5
la1 EFFECT OF
0
a2
Re
0.5
I.O
2 .o
1.0
2.0
fi
fi lb) EFFECT
Ia
2.0
Or
a2
O,/O,
0.5
fi
AT
( c l EFFECT OF D l I D o
( d ) EFFECT
S/Do
OF
100
20
I
0.2
0.5
AT (e1 EFFECT OF
e,
1.0
2.0
02
(f)
1
I
05
,
,
,
I
, I0
20
EFFECT OF COLLECTION PROBE INLET
Figure 8. Large particle collection efficiency curves Volume 14, Number 8, August 1980
981
0.2
I.o
0.5
2.0
fi ( a ) EFFECT
fi
Re
OF
( b ) EFFECT
100
-ap
-
OF
O,/O,
l BO
BO
v) v)
s 60
0
a a
0
40
I-
V
w
J J
0
20
V
0 a2
0.5
1.0
2.0
fi
fi IC)
EFFECT
OF
D,/D,
I d ) EFFECT OF
m le) EFFECT
OF
fi Bo
(11 EFFECT PROBE
Figure 9. Collection probe loss curves
982
S/D,
Environmental Science & Technology
OF COLLECT INLET
.
;::
.
I
.-
I i:
. e
( e 1 DI /DO = 1.49
Figure 10. Influence of Q 7 / Q o ,& / D o ,
of Q1/Qo. Also, the cutoff size increases as Q1/Qo decreases. This is expected, since particles must pass through more air to enter the flow stream when Q1/Qo is small. The loss curves (Figure 9b) show more losses associated with the lower Q1/Qo values. This would be due to the larger percentage of the flow being exposed to the inlet of the collection probe where the losses generally occur, and should be decreased with proper inlet design. Effect of Dl/Do. Besides D J D o being equal to the base value of 1.33 (Figure 2b), D1/Do was also set a t 1.16 and 1.49 (Figures 10d and 10e). Although, as shown in Figures 8c and 9c, this parameter had only a small effect on the large particle collection efficiency and loss curves, there are substantial differences in the flow fields for these three cases. For example, the flow field attaches close to the nozzle exit when Dl/Do = 1.49 but does not attach when D1/Do = 1.16. Experiments (15)have shown that for values of DJDo on the order of 1.49, the reattaching flow does cause particles to impact on the nozzle exit plane surface, increasing the losses in the virtual impactor. Therefore, it is recommended that the value of D1/Do be kept less than 1.49, and preferably near 1.33. Effect of &/DO.T o investigate the effect of LolDo, the results of a case where Lo/Do is small (LolDo = 0.013) are compared to the base case. Although the streamlines emitting from the nozzle are not parallel to the nozzle axis when Lo/Do = 0.013 (Figure 10f) as they were when LolDo = 2.5 (Figure Zb), the effects on the large particle collection efficiency and the loss curves were negligible, and thus are not shown. Similar insensitivity to this parameter was found for real impactors (6).
Influence of S/Do. The influence of S/Do was determined by using values of S/Uo = 0.25,l (base), and 2. The flow fields shown in Figures l l a , I l b , and 2b appear to be quite different. However, in the region in the collection probe where the 10%
I:'
(f
1 Lo/ Do
0.013
and Lo/Doon the flow streamlines
streamline attaches to the probe wall, the flow fields are similar. Thus, as expected, the resulting large particle collection efficiency curves shown in Figure 8d are nearly identical. Again, this is different from the effect found for real impactors (6) where small values of SlDo have a large effect on collection efficiency. However, the probe loss curves shown in Figure 9d are influenced by S/Do, with larger losses being found with small S/Do values. It may be expected that additional losses will be experienced on the nozzle exit plane surface and on the backside of the collection probe for small S/Do values due to turbulence in the restricted area between the collection probe and nozzle exit plane. Influence of 00. The effects of the entrance angle on the large particle collection efficiency curve and the probe loss curve are shown in Figures 8e and 9e, respectively. Since the streamlines are very similar to those of the base case shown in Figure 2b, the streamlines for BO = 30" are not shown. In Figure 8e, it can be seen that the large particle collection efficiency curves are similar in shape for both angles, but the curve for 80 = 45" is shifted to smaller particle sizes. This is due to the particles being thrown closer to the center line for the larger 00 values, making the collection of particles in the probe slightly easier. The losses shown in Figure 9e indicate fewer losses for 00 = 45" than for 80 = 30". Thus, it appears that 00 should be at least 45". Influence of the Collection Probe Inlet Design. Four receiving tube configurations were tested, as shown in Figures 2b, l l c , l l d , and l l e . The base design was a thin wall shown as configuration A. In configuration B, the wall was infinite in width and in configuration C the wall had a finite thickness. In configuration D, the wall thickness was also finite, but the inner surface was tapered in an attempt to reduce losses. The flow fields for these configurations are shown in Figures Volume 14, Number 8, August 1980 983
I c
IC)
COLLECTION PROBE B
..
s
( d ) COLLECTION PROBE C
: Ib
O
2b, Ilc, l l d , and l l e , and the resulting large particle collection efficiency and probe loss curves are shown in Figures 8f and 9f, respectively. Since configuration B is essentially a real impactor impaction plate with a hole in its center, particles are collected on the plate, and thus losses are meaningless for this case and are not shown. I t is interesting to note that the configuration of the entrance of the collection probe had essentially no effect on the large particle collection efficiency curve. This would be expected, since this curve defines the separation of particles which impact upon the probe wall and those which pass through with the minor flow, and since the region of importance for this curve is inside the collection probe rather than a t its entrance. The losses, however, are influenced by the collection probe configuration. For configurations where there is a sharp edge a t the upper entrance, such as configurations A and C, the probe losses are quite large and the loss curves are similar. The losses that were found in these cases are from particles which are traveling vertically upward along the collection probe wall toward the major flow exit. The boundary layer in this region is very thin, and particles passing within one particle radius of this wall are collected as a loss. If there were some mechanism, such as aerodynamic forces, which would keep the particle from touching the wall, it would not be collected as a lost particle. In addition, losses should be reduced by replacing the sharp corner in configuration C with a tapered entrance (configuration D) so the particles could not be collected as easily a t the upper corner of the probe. As shown in Figure 9f, the losses were reduced. By replacing the taper with a radius, the losses should be reduced even further.
Conclusions An examination of the large particle collection efficiency curves reveals that thev all have essentiallv the same shalse and are not greatly influenced by any of the Parameters. The only parameters that appears to influence the shape is Ql/Qo, 984
Environmental Science & Technology
I
’ a
.
I
I
..
t.1.1,:
~
c
i
( e ) COLLECTION PROBE
D
.
IB
l
because the collection efficiency curves are asymptotic to the different values of QJQo a t low values of The reason none of the parameters has a large effect on the large particle collection efficiency curve is because this efficiency is governed by the flow field within the collection probe. The losses, however, are governed by the flow conditions a t the tip of the collection probe inlet, and, thus, are influenced by many of the parameters. This is especially true for the case of the collection probe inlet design, where losses were reduced by adding a taper to the probe inlet.
4s.
Literature Cited (1) Loo, B. W.; Jaklevic, J . M.; Goulding, F. S. In “Fine Particles: Aerosol Generation, Measurement, Sampling, and Analysis”; Liu, B. Y. H., Ed.; Academic Press: New York, 1976. (2) Dzubay, T. G.; Stevens, R. K. Enuiron. Sci. Technol. 1975, 9, 633-638. (3) Fourney, L. J.; Ravenhall, D. G.; Winn, D. S., J . Appl. Phys. 1978, 49(4). 2339-2345. (4) Ravenhall, D. G.; Fourney, L. J.; Jazayer, M. J . Colloid Interface Sci. 1978,65(1). (5) Marple, V. A. Ph.D. Thesis, University of Minnesota, Minneapolis, Minn., 1970. 16) M a d e , V. A,: Liu. B. Y. H. Enuiron. Sci. Technol. 1974. 8. 648-654. (7) Jaenicke, R.; Blifford, I. H. J. Aerosol Sci. 1974,5, 457. (8) Willeke, K.; McFeters, J. J. J . Colloid Interface Sci. 1975, 53, 121-127. (9) Schott, J . H. M.S. Thesis, University of Minnesota, Minneapolis, Minn. 1973. (10) Willeke, K. A m . Ind. Hqg. Assoc. J . 1975,36, 683-691. (11) Marple, V. A,; Liu, B. Y. H.; Whitby, K. T. J . Fluids Eng. 1974, 96, 394-400. (12) Fuchs, N. A. “The Mechanics of Aerosols”; Pergamon Press: New York, 1964; p 154. (13) Loo, B. W.; Cork. C. C. Lawrence Berkeley Labs, University of California, Berkeley, Calif., 1978, Paper LBL-8204. 114) McFarland. A. R.: Ortiz. C. A.: Bertch. R. W.. Jr. Enuiron. Sci. Technol. 1978,12, 679-682. (15) Loo, B. W., Lawrence Berkeley Labs, Berkeley, Calif., private communications, 1978.
Received for review December 26,1978. Accepted April 23,1980. This paper is Particle Technology Laboratory Publication No. 378. T h i s
uork was supported under Bureau of Mines Contract H0177026 through t h e T w i n Cities Mining Research Center. T h e financial support of t h e Bureau is gratefully acknowledged.
pages i n the microfilm edition of this uolume of t h e journal. Photocopies of the supplementary material from this paper or microfiche (105 X 148 mm, 24 X reduction, negatives) m a y be obtained from Business Omrations. Books and Journals Diuision. Amprirnn - - --- Chemical Society, 1155 16th St., N . W., Washington, D.C.20036. Full bibliographic citation (journal, title of article, author) and prep a y m e n t , check or money order for $5.50 for photocopy ($7.00 foreign) or $3.00 f o r microfiche ($4.00foreign), are required. ~
Supplementary Material Available: Tables S - I and S-II, listing values of fifor t h e large-particle and small-particle collection efficiency curues, respectively ( 3pages),will appear following these
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Water Vapor Effect on Photochemical Ozone Formation in a Propylene-NO,-Air System F. Sakamaki, H. Akimoto,. and M. Okuda The National Institute for Environmental Studies, P.O. Tsukuba-gakuen, lbaraki 305, Japan
The effect of water vapor on photochemical ozone formation in a propylene-NO,-air system has been studied by using an evacuable and bakable smog chamber. Both the propylene destruction rate due to the OH radical and the maximum NO decay rate increased 80%when the relative humidity increased from 0 to 60% for the initial concentrations of [C3H6]0 = 0.20, [NO10 = 0.06, and [NO210 = 0.02 ppm. The effects of impurity and nitrous acid formation were evaluated, but they could not explain the enhanced photooxidation rate upon the addition of water vapor. The third possibility of the enhancement of the reaction rate of a certain water-complexed free radical was suggested, but the effect was not quantified. The proportionality between [ 0 3 ] m a x and a square root of both [NOx]oand light intensity was established as in the case of the dry system. The [03Imax uncorrected for the wall decay decreased by only about 10%as the relative humidity was increased from 20 to 60% a t 30 "C. Although the presence of water vapor in the atmosphere might affect photooxidation kinetics in the polluted or unpolluted troposphere, its effect on atmospheric reactions has not been studied extensively, except in regard to aerosol formation. It has been reported (1-3) that the presence of water vapor in the photooxidation of hydrocarbon-NO,-air systems accelerates the oxidation rate of NO, and the effect has been ascribed (3-5) to the additional formation of OH radicals in the photolysis of nitrous acid formed in the reaction of NO, NO*, and HzO. However, quantitative validation of the explanation has not been made due to the lack of reliable and systematic experimental data. Water vapor effect on the maximum ozone concentration formed in the hydrocarbonNO,-air system is less certain, and even a qualitative trend has not yet been established. Thus, Dimitriades ( I ) noted that maximum ozone concentration in the CzHd-NO, system increased slightly as humidity increased from 1.5 to 11.7% (34 "C), while Wilson and Levy (2) reported that it decreased appreciably with the increase of humidity in the l-CdHB-NO, system. In a photochemical study using a dynamic flow cascade reactor, Nieboer and Duyzer (6) reported that the integrated dose of O3 in the C&-NO, system decreased slightly with an increase in relative humidity. 0013-936X/80/0914-0985$01 .OO/O
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In our previous paper ( 7 ) , the maximum concentration of ultimately reached ( [031max)in the photooxidation of the C3H6-NOx-dry air system has been analyzed as a function of reaction parameters such as light intensity and initial concentrations of C3H6 and NO,. Particularly, in the CsHc-excess region proportionality between [ 0 3 1 m a x and a generalized parameter, [03],,, was presented, and the proportionality coefficient was proposed to be an ultimate ozone formation potential of a specific hydrocarbon. Here, [O,],, is the photostationary concentration of 0 3 expected under the conditions of the same light intensity and same initial NO, concentration but in the absence of C&. In order to extend the above discussion of the ozone formation potential to the analysis of the ambient atmosphere, humidity effects on the photochemical ozone formation have to be studied in detail, since the presence of water vapor may invalidate the generalized relationship presented previously ( 7 ) . This paper reports photochemical ozone formation in the C3H6-NOX-humid air system studied by the evacuable and bakable photochemical smog chamber. The generalized relationship between [O3Imaxand [03],, was confirmed in the humidified system, too. The water vapor effects on the NO oxidation rates, C3H6 destruction rates, and [031max will be discussed. 0 3
Experimental
Reactions were carried out in the evacuable and bakable photochemical smog chamber described previously (7,8).A humidifier added heated water vapor to the purified dry air (H20 < 1 ppm, NO, < 2 ppb, THC < 30 ppbC) through a capillary. Prior to each run, the humidified pure air was introduced into the chamber a t about 770 torr. The premeasured amounts of C3Hs and NO, were then injected into the chamber through a l/B-in. 0.d. glass-lined stainless steel tube using the purified air as carrier gas. Before irradiation was started, the sample mixture was stirred by a fan for about 45 min in order to attain rather uniform initial conditions of NO, NO*, and HONO. All experiments were performed a t 30 f 1 "C. The light intensity, k,, expressed as the primary photolysis rate of NO2 (customarily defined as k l ) , was 0.22 f 0.02 min-l for the runs of humidity variation and 0.24 f 0.02 min-l for the runs of [C3H& or [NO,]o variation.
1980 American Chemical Society
Volume 14, Number 8, August 1980
985
