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1、Particle Kinetics Impaction1Particle KineticsImpaction7.1 Curvilinear MotionWhen particles are transported by air currents, changes in the direction of these currents give rise to accelerating forces on the aerosol particles.-spinning of air tends to move the aerosol away from the axis of rotation;-

2、rapid changes of airflow around an obstacle can result in aerosol particles being deposited on that body.This may be one of the principal mechanisms by which particles are removed by nature from the atmosphere. Sampling and collection devices such as impactors or impingers are based on the use of ce

3、ntrifugal forces, as are such other devices as “cyclones” and aerosol centrifuges.Particle Kinetics Impaction2Particle KineticsImpactionConsider a particle being carried by a volume of air (or other gaseous medium) which is moving in a circular orbit with an angular velocity of , it experiences a ra

4、dial acceleration of (7.1)and a tangential velocity of (7.2)The particle will have a constant tangential velocity and will also experience an outward, accelerating force. At low radial velocities, it can be approximated by the Stokes resistance. Equating the radial accelerating force with this resis

5、ting force gives (7.3)Particle Kinetics Impaction3Particle KineticsImpactionSolving for assuming a spherical particle yields (7.4)an expression for the radial velocity of a particle.Particle Kinetics Impaction4Particle KineticsImpactionExample 7.1 A 10- -diameter unit-density sphere is held in a cir

6、cular orbit by an electric field. The orbit is 25cm in radius, and the particle moves around the center of the circle at a rate of 100r/min.a. What is the radial velocity of the particle at the instant the electric field is removed? From Eq. 7.4Particle Kinetics Impaction5Particle KineticsImpactionb

7、. How far will the particle move until its radial velocity is dissipated?The distance the particle will move is just the stop distance s:7.2 Impaction of ParticlesWhen air carrying particles suddenly changes direction, the particles, because of their inertia, tend to continue along their original pa

8、ths. If the change in air direction is caused by an object placed in the airstream, particles with sufficient inertia will strike the object. This process is known as impaction. It is the mechanism by which many large particles are removed from the atmosphere, it is one of the important mechanisms f

9、or removal of particles by the lungs, and it is important in air cleaning as well as aerosol sampling.Particle Kinetics Impaction6Particle KineticsImpactionConsider a simple model of impaction. Air issues from a long slot of width W at a velocity u. A surface is placed normal to the discharging flow

10、 a distance S away. With this configuration, air leaving the slot must make a 90turn before it escapes. Particles that fail to make this turn strike or “impact” on the surface and are assumed to be retained by that surface. As a crude first approach (Fuchs, 1964), it can be assumed that the streamli

11、nes of the air issuing from the slot are quarter-circles with their centers at C (see Fig.7.1a) and that S=W/2. Particle Kinetics Impaction7Particle KineticsImpactionFigure 7.1a Sketch of simple “ideal” impactor.At point B a particle has a tangential velocity given by and a radial velocity given by

12、(7.6)In a time dt the particle will be displaced toward the surface a distance (7.7)Where is the angle formed by the line connecting points B and C and the plane normal to the airflow which passes through point C (Fig.7.1b). As the streamlines turn from the slot to be parallel with the surface goes

13、from 0to 90. This change in angle can be expressed asParticle Kinetics Impaction8Particle KineticsImpaction (7.8)in traversing the full 90, the particle will be displaced a distance (7.9)That is, the particle will move one stop distance out of its initial streamline while losing all its original vel

14、ocity parallel to the slot. Since all particles that lie within a distance of the slot centerline are considered to be removed, the overall removal efficiency of the impactor will be (7.10)Particle Kinetics Impaction9Particle KineticsImpactionThis is, of course, only a very crude approximation for i

15、mpactor efficiency, since the actual flow field is much more complex, varying in configuration depending on the slot Reynolds number. In general, .Figure 7.1b Diagram of velocities at point B.This sample model does give some idea of how effective a particular impactor configuration can be, the estim

16、ate being reasonably good when but rapidly losing accuracy for .Particle Kinetics Impaction10Particle KineticsImpaction Example 7.2 A rectangular slot impactor has slot dimensions of 2.08-cm length and 0.358-cm width. Estimate the flow in liters per minute required for this impactor so that 15- -dia

17、meter unit-density spheres can be collected with near 100 percent efficiency.Particle Kinetics Impaction11Particle KineticsImpaction The quantity is an important dimensionless parameter in impactor studies, known as the Stokes number (7.11)This dimensionless parameter is used to describe impactor be

18、havior. For impactors with rectangular openings, W is the slit width; for circular openings W represents the diameter of the impactor opening. Thus the Stokes number is the ratio of the stop distance to the impactor opening half-width. Some authors prefer to use the impaction parameter , rather than

19、 the Stokes number, to describe properties (e.g., Green and Lane, 1964; Ranz and Wong, 1952). The impaction parameter is defined asParticle Kinetics Impaction12Particle KineticsImpaction (7.12)a factor which is one-half the Stokes number.Example 7.3 Compute Stk, for a circular jet impactor when cm a

20、nd the jet diameter is 0.1 mm.It is common practice to plot impactor efficiency as a function of either (e.g., Rao and Whitby,1978). This is done because the particle diameter is present in either term as d2, making the square root of the term proportional to the particle diameter.Particle Kinetics

21、Impaction13Particle KineticsImpactionImpactor operationThe characteristic behavior of impactors depends on factors such as nozzle-to-plate distance, nozzle shape, flow direction, and Reynolds numbers for both the jet and the particle. Other factors of importance include the probability that the part

22、icles will stick to the impaction surface and particle loss to the walls of the impactor. It is not surprising that with such a variety of possible variables it is quite difficult, if not impossible, to accurately predict impactor characteristics on purely theoretical grounds.Particle Kinetics Impac

23、tion14Particle KineticsImpactionCalculation of the jet or nozzle Reynolds number is straightforward for a round jet; thus Re=Wu/v, where W is the jet diameter, u the velocity in the jet, and v the kinematic viscosity viscosity. For a flow of Q cm3/s, (7.13)For a rectangular jet the “wetted perimeter

24、” concept must be used (Marple, 1970). That is, the opening width to be used in computing the Reynolds number is defined as = 4 (area/perimeter). For a rectangle of length L and width W, =2WL/(W+L),When (7.14)Particle Kinetics Impaction15Particle KineticsImpaction For a well-designed impactor, a typ

25、ical plot of impactor efficiency versus is shown in Fig.7.2. It can be seen that the efficiency curve may deviate from the ideal case. In the ideal case, for all efficiencies there would be a single value of .Figure 7.2 Typical impactor stage efficiency curve.Particle Kinetics Impaction16Particle Ki

26、neticsImpactionAnd hence a sharp size cut of the impactor. All particles larger than this size would be collected, and all smaller sizes would be passed. In actuality, this is not the case, and a range of particle sizes are collected with varying efficiencies. To represent the impactor stage collect

27、ion characteristic, it is often the practice to choose the 50 percent efficiency point as the representative cut point. The maximum slop at this point most nearly represents the ideal case. In Fig.7.2 both the actual and ideal cases would be considered to have the same characteristic cut size. What

28、constitutes a well-designed impactor? According to Marple and Rubow the minimum value of S/W should be no greater than 1 for a round impactor and 1.5 for a rectangular impactor. Particle Kinetics Impaction17Particle KineticsImpactionAs an upper limit, S/W ratios several times greater than these mini

29、mums are possible, but design values as close to the minimum as possible are preferred. However, some commercially available impactor designs use S/W ratios of about 0.5 . Figure 7.3 shows theoretical impactor performance when a number of parameters are varied, including jet-to-plate spacing, jet Re

30、ynolds number, and throat length-to-width ratio. These curves indicate that impactor efficiency is fairly insensitive to Reynolds number in the range 500Re3000 and that impactor efficiency is also relatively independent of S/W and T/W ratios, except for small values of S/W.Particle Kinetics Impactio

31、nParticle KineticsImpaction The calculations that produced the curves in Fig.7.3 were repeated in more detail by Rader and Marple (1985), who also included the effect of the physical size of the particle (interception distance) as it approaches the collection plate. Figure 7.4 shows similar curves f

32、rom this more recent work. Although differences in the results of the two sets of calculations are small, the newer curves are steeper and show a efficiency curves for rectangular and round impactors showing effects of jet-to-plate distances in Reynolds number Re and throat length T. 18Particle Kine

33、tics ImpactionParticle KineticsImpactionW is impactor width or impactor jet diameter. (From Marple and Willeke, 1979.)(a) Effect of jet-to-plate distance (Re=3000).(b)Effect of jet Reynolds number (T/W=1).(e)Effect of throat length(Re=3000). characteristic S shape that is found in experiment and is

34、most likely due to the inclusion of the interception distance in the calculations.19Figure 7.4 Revised impactor efficiency curves.(From Rader and Marple,1985.)Particle Kinetics Impaction20Particle KineticsImpactionExample 7.4 A round jet impactor is operated such that the jet Reynolds number is 3000

35、. Using Fig.7.3b, find the particle diameter (unit-density sphere ) that will be collected with 50 percent efficiency if the jet diameter is 0.3 cm. From Fig. 7.3b, at efficiency=50 percent and Particle Kinetics Impaction21Particle KineticsImpactionThe theoretical impactor performance data can also

36、be expressed in terms of the 50 percent cut size. Figure 7.5a shows plotted as a function of the S/W ratio, and Fig.7.5b shows the same ordinate plotted as a function of Re. These curves again illustrate that Stk50 is quite insensitive to changes in either S/W or Re, except in the extremes.理論沖擊器性能也能

37、用50%的切斷尺寸表示。圖7.5a 顯示 為S/W比值的函數(shù)。圖7.5b則顯示隨Re的變化。從這些曲線看到，除了極端情況之外， 對S/W 或 Re的變化是相當(dāng)不敏感的。 (7.15)Particle Kinetics Impaction22Particle KineticsImpactionFigure 7.5a The 50 percent cutoff Stokes number as a function of jet-to-plate distance. (From Marpe,1970.)Figure 7.5b The 50 percent cutoff Stokes number

38、as a function of Reynolds number. (From Marpe,1970.)Particle Kinetics Impaction21Particle KineticsImpactionImpactor 50 percent cut points can be estimated from the equation for rectangular jets of length L and width W or對于方形射流，50%切斷點(diǎn)的估計(jì)式為： (7.15)對圓形射流， 50%切斷點(diǎn)的估計(jì)式為： （7.16）從圖7.5a，對于 方形射流沖擊器， 的理論估計(jì)值為0.

39、71，對于圓形射流沖擊器，為0.46. Particle Kinetics Impaction23Particle KineticsImpactionParticle bounceThe surface on which particles impact is also an important factor in determining impactor efficiency. Particles which bounce off the impaction surface can be carried through the impactor and can distort measure

40、ment data. Particle bounce will lower the collection efficiency of a given impactor stage and will lower the apparent mean diameter of the aerosol measured.粒子所沖擊的表面性質(zhì)在確定沖擊器效率中也是一個很重要的參數(shù)。從沖擊表面彈出的粒子將通過沖擊器，不會被沖擊器捕獲，因此也就扭曲測量數(shù)據(jù)。粒子的彈跳將降低給定的沖擊器級的捕獲效率及降低所測量的氣溶膠表觀平均直徑。Particle Kinetics Impaction23Particle Ki

41、neticsImpactionAnother way of considering the effect of particle bounce is shown in Fig.7.6. A plot of collection efficiency versus substrate loading indicates efficiencies which never reach 100 percent. 圖7.6顯示了考慮粒子彈跳的另一種方式，顯示了收集效率與表面所加載體的量的關(guān)系?？煽吹?，收集效率不可能達(dá)到100%。Particle Kinetics ImpactionParticle Ki

42、neticsImpactionParticle bounce can be minimized by using collection media coated with such materials as Vaseline, L and H high-vacuum greases, stopcock grease, oil, or Apiezon (Moss and Kenoyer, 1986).Figure 7.6 Dependence of solid particle sticking efficiency for various surface treatments and subs

43、trate loading. (From Turner and Hering,1987.)24Particle Kinetics Impaction26Particle KineticsImpactionImpactors for very small particle sizesBy their very nature, impactors are high-pressure drop sampling devices. Air is drawn at high velocity through a fairly small nozzle to remove small aerosol pa

44、rticles. The particle diameter which can be collected with a 50 percent cut efficiency for a specific set of operating conditions can be determined by recalling that (7.17)Rearranging in terms of d gives (7.18)Particle Kinetics Impaction27Particle KineticsImpactionThen using the 50 percent Stokes nu

45、mber for the 50 percent cut size produces (7.19)Since the value of Stk50 is nearly constant for similar impactor designs and since the viscosity and particle density are constant, the only way to change the 50 percent cut size for an impactor is to vary the jet velocity, the jet width, or Cc.對類似的沖擊器

46、，Stk50的值幾乎不變，介質(zhì)粘度和粒子密度也是常數(shù)，因此改變沖擊器50%截斷直徑的唯一方法是改變噴射速度，噴嘴寬度，或修正系數(shù)Cc 。Particle Kinetics Impaction27Particle KineticsImpactionThe traditional cascade impactor is constructed with a series of jets of decreasing diameters, so that both W and v are varied. 傳統(tǒng)的瀑布型沖擊器由一系列直徑不斷減小的噴嘴構(gòu)造而成，因此寬度W和速度v都會變化。Air flow

47、s through one jet (or group of jets of the same diameter), removing particles with a certain 50 percent cut size, and then proceeds to a smaller size jet (or series of jets of the same diameter) where particles with a smaller cut size are removed. This process can be repeated until the velocity in t

48、he jet approaches sonic velocity, at which point a backup filter catches the remaining small particles which have penetrated the impactor. The partical lower limit for an impactor of an impactor of this type is about 0.4 .Particle Kinetics Impaction29Particle KineticsImpactionAs discussed by Hering

49、and Marple (1986), although traditional impactors are not adequate for this task, either low-pressure or microorifice impactors can collect particles with substantially smaller particle diameters than 0.4 .With microorifice impactors W is made very small, on the order of 50 to 150 . Velocities are s

50、till kept somewhere below 100m/s (Re 500 to 3000), but the number of orifices is increased to provide a reasonable total flow so that an adequate amount of sample is collected. Low-pressure impactors utilize the fact that Cc is a function of not only particle diameter but also, through the gas mean

51、free path, pressure. Therefore at low pressures Cc can be substantially larger than for the same size particle at atmospheric pressure.Particle Kinetics Impaction30Particle KineticsImpactionExample 7.5 A Hering impactor operates at a flow rate of 1 L/min. Both stage 3 and stage 6 of this single-jet

52、impactor have jet diameters of 0.99 mm and an S/W ratio of 0.5. Air enters stage 6 at 0.185 times atmospheric pressure and leaves with a p/p0 ratio of 0.146 (Hering and Marple,1986). Assuming Stk50=0.22 and a unit-density particle, calculate the appropriate d50.一個Hering沖擊器出口流量為1 L/min。沖擊器的第3級和第6級的噴嘴

53、直徑為0.99mm， 比值S/W為0.5. 空氣以0.185倍的大氣壓進(jìn)入第6級，然后以比值p/p0為0.146壓力離開。假定Stk50=0.22，粒子密度為單位密度，計(jì)算d50.Particle Kinetics Impaction30Particle KineticsImpactionThe average velocity in the jet is found by using Q/A adjusted for expansion and the lower or downstream pressure ratio. Then噴嘴出口氣流平均速度： Recalling Eq.7.19P

54、article Kinetics Impaction31Particle KineticsImpactionAll factors on the right-hand side of this equation are known except Cc, which depends on d50. Thus Cc is moved to the left-hand side of the equation, and the right-hand side is computed.Since Cc also depends on d50, to find d50 it is necessary t

55、o use an iterative procedure. A value for d50 is assumed and the associated Cc calculated by using the equationParticle Kinetics Impaction32Particle KineticsImpactionFor use a value of 0.0687 . The correct pressure ratio to use for computing Cc is the upstream pressure ratio since that is where the

56、effect of slippage on impaction will be most pronounced. However, as noted above, the jet velocity is computed by using the downstream pressure ratio since this represents the increased volume of air going through the impactor nozzle. After Cc is computed, is calculated and compared to that found by

57、 using Eq.7.19( )With several iterations the following values are determined: Particle Kinetics Impaction33Particle KineticsImpactionPressure drop in impactorsIn evaluating impactor operation it is important to know (or estimate) the pressure loss across each of the impactor stage. A simple approach

58、 is to assume that all the velocity pressure in the impactor jet is lose due to turbulence. Then the pressure drop P across an orifice can be written asHence , p, and v refer to the density, pressure, and velocity at atmospheric or some reference condition. The subscripts up and down refer to the pr

59、essure just upstream and downstream, respectively, of the impactor nozzle. For the first stage of an impactor, pup=p. For subsequent stages, the calculated pdown of stage n is set equal to the upstream pressure of stage n+1. If v is in units of centimeters per second and in units of grams per cubic

60、centimeter, then the units of p will be dynes per square centimeter.Particle Kinetics Impaction34Particle KineticsImpactionAnalysis of impactor dataThe most common configuration for impactors used for aerosol sampling is to have a series of jets of decreasing size, arranged so that the air passes in