The acoustic particle detector: mechanism of operation

Powder

Technology,

The Acoustic V_ P_ SINGHDepartment (Canada)

(Received

32 (1982)

107

107 - 117

Particle Detector:

Mechanism

of Operation

and M E. CHARLES

of

Chemrcal

December

Engmeering

1,198O;

and Applied

Chemistry.

Unwersity

of

Toronto.

Toronto

OnC. MS.9 1AQ

in revised Form June 25, 1961)

SUMMARY

When a small gas-borne particle is carrred through a constriction, such as a capillary tube, it may generate a click-ltke noise. It LS shown conclusively that this noise is directly associated with the generatioof a turbulent patch in the otherwise laminar fluid The patch may originate either in the jet downstream of the capillary or in the capillary itself. Either way, a pressure disturbance is created which is audible Whether or not a turbulent patch is generated (and therefore a click is heard) depends upon the capillary Reynolds number and the particle sire_ For example, while essentially all 40-urn particles produced a signal at a capillary Reynolds number of 14,000, only 60% produced signals at a Reynolds number of ll,OOO_ Knowledge of this behaviour may well provide the basis for the use of this deuice as a particle counter. Since the amplitude of the sound signal varied greatly forparticles of the same nominal xze under the same conditions. it is doubtful that the amplitude can be used as a measurement of particle size; however, knowledge of the efficiency of counting of particles of a giuen sEe as a funtion of Reynolds nvmber could in principle be used to determine particle size distrrbutions

particle, entrained in an airstream II-Ia tube, passes through a constriction_ This phenomenon was investigated in some det& by Langer [ 2 - 53, who has since developed and patented a device [ 61 as an acoustic particle detector. As shown in Fig. 1, the Langertype design consists of a cylindrical glass tube reducing to a length of capillary tubing via a conical section_ At the capillary exit, the geometry changes abruptly back to the dimensions of the larger upstream tube. Because of its unsophisticated nature, the device shows promise as a cheap, continuous particle detector and counter; if it could be used to indicate particle size the device would be even more useful. Langer has suggested it may be used in several applications, e g_ the counting of ice nuclei in the context of weather modification, the measurement of air

INTRODUCTION

A concept which may be of use in the detectlon, counting and sizing of particles has attracted the attention of several researchers It was reported by Gerhard Langer [l] in 1963

that a click-like

sound is heard when a

*Resent ad&es: Ontario Hydro, W. P. Dobson Research Laboratory. 800 Kipling Avenue, Toronto, Ont_ M8Z 554. Canada.

Fig. 1. The Langer-type

capillary constriction

@ Elsevier Sequoia/Printed

in The Netherlands

10s

quality in work areas where precision mstruments are assembled, and the monitoring of effluents from smoke stacks and fly ash near power stations_ However, Langer found that the device could not detect particles smaller than about 10 pm in diameter_ Also, a large scatter in the sound pressure pulse amplitudes was produced by particles of similar size_ This scatter hzu made it difficult to extend the usefulness of the device to particle size analysis. This objective has also been hindered by a lack of knowledge of the mechanism by which the sound is produced_ Both Langer, and Hofmann and Mohnep [7] investigated the mechanism of operation by making the jet at the exit of the capillary visible with a smoke tracer_ Hofrnann and Rlohnen reported that, with no particle passing through the device, the flow in the jet was laminar for a small distance downstream of the exit_ If a particle was leaving the capillary, the flow became turbulent over the entire jet. Hofmann and Mohnen postulated that a ‘wall separating larninar and turbulent flow moves from downstream to upstream within the jet_ This, in their opinion, created a compression wave. Langer disagreed with this explanation_ His smoke visualization studies showed that when a particle was leaving the capillary, the flow broke up momentarily only along a local streamhne and not over the whole cross-section and length of the jet. Langer concluded that “one may be dealing with a short-term, local pulsation in the laminar flow_ How this manifests itself as a sound rem&s a mystery” [5] _ Differences esist between the results of previous workers not only as to the possible mechanism of operation, but also as to the effect of particle shape_ Contrary to the results of Langer, Osborne et al [S, lo] showed that particle shape was important. Irregular sand particles gave larger signals than did smooth glass beads of similar mesh size. However, both investigators showed particle density to be en unimportant variable - a result inconsistent with those of the present study_ A knowledge of the mechanism of operation is important so that the large scatter in the pressure pulse amplitudes exhibited by particles of similar size may be reconciled_

Also, this knowledge may be useful in extending the range of particle sizes detected to smeller diameters, end the detector itself into a particle size analyser- The investigation of the mechanism by which sound is generated is the primary objective of this work; subsidiary objectives relate to the resolution of inconsistencres in the literature such as the effect of particle size_

FUNDAMENTAL

BACKGROUND

Although the accepted transition Reynolds number for pipe flow with high disturbance levels at the inlet is approximately 2100, Langer’s observation that the flow is laminar in the capillary at capillary Reynolds numbers up to 13,000 is not surprising because the conical inlet to the capillary causes fluid acceleration and hence stabilization of the flow. Furthermore, the smooth surfaces of the cone and capillary and the low upstream disturbance level favour laminar flow at elevated Reynolds numbers However, if the Reynolds number for the flow in the device proposed by Langer (Fig. 1) is increased sufficiently, the natural production of turbulent patches with the subsequent transition to fully turbulent flow in the capillw; will take place. Results will be presented to substantiate this. The natural production of turbulent patches in such a flow system was not reported by the previous workers because they pulled air through the device by means of vacuum pumps, achieved sonic velocities at the end of the capillary, and thereby prevented sufficiently high Reynolds numbers being attained for the natural transition to take place_ Sufficiently high Reynolds numbers may be achieved by feeding the air to the capillary at elevated pressures. It is proposed here that the main mechanism whereby the click-like sound is produced by the acoustic detector is through the initiation of a turbulent patch. In the absence of disturbances produced by particles, turbulent patches may onginate naturally at a sufficiently high Reynolds number. However, at lower Reynolds numbers, such patches may be promoted by the deliberate introduction of artificial disturbances of the flow, e-g_ by a particle, a probe, or an air current across the mouth of the detector.

109 Turbulent patches could be initiated in the capillary itself or in the jet at the exit of the capillary_ However, previous experimental and theoretical work [ll, 121 shows that the jet is more unstable than the capillary flow where the presence of walls provides a stabilizing effect_ Therefore, for a given capillary Reynolds number, a disturbance sufficiently small that it remains unchanged as it proceeds along the capillary (a neutral disturbance) is amplified m the exit section causing breakdown to a turbulent exit jet_ If the disturbance IS larger, a turbulent patch Hrlll be produced in the capillary and subsequently in the jet; if the disturbance is sufficiently small, it may not even affect the behaviour of ,he jet. Since a decrease in capillary Reynolds number stabilizes both the capillary and jet flows, an increased disturbance size is necessary for turbulent patch production as the Reynolds number Re, is decreased_ The Iengtll of the caplllaIy is an important practical consideration_ Langer reported that for capillary length-to-diameter (LJD,) ratios less than about 15 the signal was ‘lost’ in the background noise_ He recommended a value for LJD, of approximately 40 and such a value was used in the present investigation. The disturbances produced by spherical particles suspended in flowing fluids have been studied by previous workers in great detail. A particle entrained in an air stream sheds vortices which depend on the size, shape and density of the particle and on the relative velocity and properties of the fluid. Clift and Gauvin [ 13 J report that vortices are first shed from a sphere in a free stream at a partrcle Reynolds number of about 300, i-e_ Re,

= Dp/D,

- Re,

= 300

(1)

However, it is not possible to conclude defmitely at what particle Reynolds number vortex shedding will commence in the capillary, because of the unknown effect of the capillary wall and the effect of the upstream disturbances. Nevertheless, if these effects are neglected and a negligibly small particle velocity is assumed at the capillary inlet, eqn. (1) can, for example, be used to predict eddies preceding particles greater than 21 pm in diameter when the Reynolds number in the 1.5 mm diam. capill;rrv is 21,500. This predicted result is of the right order since it was found in the present investigation that

50% of glam spheres with a diameter of 22 pm produced sound signals at a Reynolds number of 21,500. The mechanism by which the generation of a turbulent patch results in an audible click is now considered. It is proposed that, for both jet and capillary flows, the situation is basically that of a developing parabolic laminar velocity profile being suddenly changed to a flatter turbulent proftie In the central part of the flow, i-e_ that cIose to the axis of the capillary or jet, the following (as yet undisturbed) laminar fluid is moving faster than the fluid in the turbulent patchThis ‘impact’ creates a compression, while near the capillary wall a rarefaction is produced by the inverse mechanism. The net effect is probably a compression propagating both upstream and downstream. Similarly, a net rarefaction IS produced at the leading edge of the parch_ The overall effect, since compression and rarefaction waves probably cancel out within the patch, 1s a compression wave propagated upstream and a rarefaction propagated downstream. Singh [ld] used water hammer theory to show that this mechanism predicts a compression propagating upstream of amplitude sufficient to account for the observed signals. For the case of the initiation of a turbulent patch in the capihary, the net signal is a combination of the above pressure pulse and other pressure pulses resulting from processes which are described below: (i) When a patch is formed in the capillary_ the frictional resistance for flow of the turbulent patch is greater than that of the same volume of fluid in larninar flow and this causes the flow to slow down over the whole cross-section_ This must lead to a pressure . disturbance_ (ii) For the situation when a patch is formed in the capillary and for a microphone connected via a probe to the capillary bore, an additional pressure pulse may be recorded when the patch passes the probe position This is because the pressure in the turbulent patch ~5 greater than the laminar regions adjacent to the patch This can be shown by means of a momentum balance: it has been experimentally observed that at a Reynolds number of approximately 2000 and at distances somewhat removed from the inlet of a pipe, a tzlrbulent patch proceeds along a

110

a pipe at a velocity equal to about 80% of the average fluid velocity [15] _ If the patch is stopped by superimposing a velocity of 0.8 times the average fluid velocity in the direction opposite to the fluid motion, and if friction is neglected, the momentum associated with the remaining parabolic laminar velocity pro5le is greater than that of the turbulent flow with a flatter profile. A momentum balance leads to the conclusion that the pressure in the patch is higher than in the laminar flow Ieading into the patch. (iii) When a turbulent patch which is formed in the capillary reaches the exit, the flow rate increases locally at the exit as a consequence of the decreased resistance. This produces a rarefaction propagating upstream. The net signal recorded by a microphone as a consequence of the several processes discussed above will be the superposition of the several individual pulses. The low disturbance level entrance of the Langer acoustic device enables stable, laminar flow to be obtained at high velocities which make flow visualization esperirnents difficult. However, if the Iaminar-turbulent transition mechanism of operation is valid, a simple capillary device made up of a 60-mm length of straight capillary tubing with no special entrance design and operated at Reynolds numbers less than 2100 should also serve to produce sound signals. This suggests that insight as to the mechanism of operation can be obtained by studying the phenomenon not only with Langer’s design, but also with simple capillaries which enable lower velocities to be used In addition to the main and associated mechanisms of turbulent patch initiation discussed above, at least one additional different mechanism exists whereby sound may be produced. A particle on entering the capil-

Fig_ 2_ Flow visualization

equipment.

lary is moving relatively _;lowly compared with the fluid and it causes an effective reduction in the cross-sectional area avarlable for fluid flow. This physical partial blockage of the flow occurs even if the flow is initially turbulent but is appreciable only for large particles. An upstream compression is expected. A comprehensive series of experiments was planned and carried out to confirm or disprove the above hypotheses.

E_XPEFUMENTAL

EQUIPMENT

Two distinct experimental arrangements were used. These were designed to provide important information through flow visualization on the one hand and measurement of pressure pulses, le. signal waveforms, on the other_ A. Flow visualization studies The equipment used in the determination of the appearance of the jet and the position of a particle when a noise signal was recorded is shown in Fig. 2. Air from a low pressure regulator was metered with a rotameter and a portion of the flow was routed to the smoke generator_ This generator consisted of a filter-tipped cigarette assembly through which the air was pushed, resulting in smoke with a very small particle size (=O 1 pm)_ The heated air was cooled in a U-tube to condense the volatlles and then passed through the acoustic device, which in all cases was a simple glass capillary. Provision was also made for the introduction of the particles one at a time. The noise signals generated by the particles passing through the capillary were received by a type 4133 Bruel and Kjaer half-inch condenser microphone. The normal incidence-

111

free field response of this microphone with the protecting g-rid 1s constant within + 2 dB in the frequency range 20 Hz to 40 kHz_ However, the response begins to fall off at 1 kHz for sound waves with 90” incidence_ The signal passed to a B & K type 2606 sound level meter and was monitored with a high impedance earphone. The signal was displayed on Lhe screen of a Tektronix type 503 oscilloscope which was positioned in such a manner that both the screen and the section of the capillary to be photographed by use of a movie camera were simultaneously in focus. The camera was a 16-mm high-speed model (HYCAM) and high-speed Eastman Kodak 4-X negative film was used. A typical frame speed was 5000 frames per second. Photographs of the signal waveforms produced by particles were also obtained with a Polaroid Oscilloscope camera (Tektronix C-12 model)_ B_ Signal waveform and amplitude studies A totally enclosed system in which the pressure at the entrance to the capillary could be increased up to 200 kPa is shown in Fig. 3_ High pressure air was filtered and humidified to alleviate electrostatic effects and then metered by one of two meters in parallel. A portion of the air was bypassed to the particle introduction system. This system consisted of an enclosed micromesh sieve on which presieved particles of the appropriate size were placed. Particles of diameters between 10 and 40 pm were mtroduced into the system in this manner. Larger particles were generated by a fluidizing arrangement [ 143. Inlet disturbances were reduced upstream of the device by the use of a 100 mm diam_,

Fig. 3. Signal amplitude and waveform equipment_

0.45 m long calming section. As recommended by Barnes and Peterson [16], three screens of moderate solidity ratio (0.22) were used in the dissipation of turbulence and in the elimination of variations in the velocity distribution. This led to stabilization of the flow so that laminar conditions were maintamed at Re, = 22,000. The acoustic device used in this apparatus was a Langer-type design similar to that shown in Fig. 1. The diameter at the inlet of the conical section was 25 mm and the cone was 67 mm long_ It was fitted with a 2 mm diam. microphone probe tube, 15 mm upstream from the cone/capillary junction. The 1.5 mm diam., 54 mm long capillary was also fitted with a 1 mm diam. microphone probe, 41 mm from the exit. However, the end of the capillary protruded some 19 mm into the 25 mm diam exit section as recommended by Hofmann and Mohnen [7] _ Particles passing through the acoustic device described in the preceding paragraph were collected by an unpactor which was situated downstream of the exit section. It was capable of collecting all particles above 5 pm in diameter for the particle densities and capillary Reynolds numbers employed in this investigation_ A surge chamber, downstream of the impactor, served to damp out fluctuatlons produced by the vacuum pumps

RESULTS AND DISCUSSION Experimental results were obtained using Langer-type acoustic devices, i-e capillaries with conical entrance sections. However, much useful information was also obtained using simple capillanes with abrupt entrances; data from these tests are presented first. Experiments with simple capillaries The jet issuing horn a simple capillary device, i.e_ one without a conical inlet, was visualized with smoke using the equipment shown in Fig_ 2. It was found that the flow was laminar for some distance downstream of the capillary exit at low flow rates. Further downstream from the caplllary, the jet became turbulent. A laminar jet at the caplllary exit showed that the flow in the capillary was also lammar, as expected at low Reynolds numbers.

112

With a microphone positioned near the capillary exit, click-like sounds were recorded on increasing the flow rate through the capillary_ These were apparently not due to particles since they persisted when the smoke generator was turned off and a O-025 ;rm pore size filter was inserted in the airstream just upstream of the capillary_ Capillaries with diameters between 1 and 5 mm with sharpedged inlets were used in these tests. For each capillary diameter, click-like sounds fnsr; appeared at a caplllarr Reynolds number of approsimately 2100_ This number is similar in magnitude to the experimentally accepted transition number for pipe flow when no effort is made to control inlet disturbances. The hypothesis that the sounds are associated with the laminar-turbulent transition is supported by the fact that increasing the air flow rate gradually caused more clicks per unit time. This is consistent with the results of other workers in that the patch birth rate increases with increasing Reynolds nulnberAlso, the amplitudes of the clicks were not constant but exhibited a spread, a result consistent with the statistical nature of turbulence_ It was concluded from these tests that the naturally produced clicks. t‘_e_ pressure pulses not produced by particles or probes, were due to the generation of turbulent patches during flow through the capillary and the jet. The appearance of the jet when a naturally produced sound was generated was photographed simultaneously with an oscilloscope display of the sound signal_ Rolls of film were run through the camera until a satisfactory record of the naturally produced sound signals was obtained_ Drawings based on a series of photographs are presented in Fig. 4 for which the Reynolds number of the flow through the 1.47 mm diam_ capillary of length-todiameter ratio 95 was 2200_ The microphone was positioned close to the jet downstream of the capillary. Frame 1 corn+ sponds to the situation just before a signal is initiated and frame 2 corresponds to the initiation of a signal_ Readily apparent is the fact that a changed appearance in the jet travels upstream_ The breakdown in the jet flow appears to be total, i-e_ over the whole length and diameter of the jet, before relaminarization occurs. It is concluded from the movement of the oscilloscope trace that the

Fig. 4. Drawings based on photographs of jet and oscilloscope trace positloa at time intervals during natural generatIon of a turbulent patch_ Frame speed SQQO/second_ D, = 147 mm, LJD, = 95; Re, = 2100.

Fig_ 5. Drawings based on photographs showing particle position, jet appearance and oscilloscope trace at time intervals. Frame speed SOOO/second. D, = 1.47 mm, L,ID,= 95; Re,= 950;particle. 48 - 65 mesh glass bead.

113 click-like sound is recorded at the same time the jet breaks down. The appearance of the jet when a 48 - 65 mesh glass bead was introduced into the same equipment, including the simple capillary, but number of 1000, is at a lower Reynolds shown in Fig. 5. The position of the particle, as well as the visualized jet and oscilloscope trace, is shown in this series of drawings, again based directly on photographs_ It is evident that bre&down from laminar flow in the jet was much more local than for the situation depicted in Fig 4. At a Reynolds number of 1000, the introduction of a particle was necessary to produce a sound and breakdown of laminar flow. The movement of the oscilloscope trace in Fig 5 when a turbulent patch was generated naturally at a Reynolds number of 2200 is shown in Fig_ 6 with a time base line_ For purpcses of comparison, Fig. 7 shows two traces obtained when two 48 - 65 mesh glass beads were introduced at the lower capillary Reynolds number of 1000. It is seen that the signal waveforms produced in the two sltuations are very similar_ In each case the microphone was positioned close to the jet leaving the capillary. From the observations of the appearance of the jet, it seems that the less stable the capillary flow, i-e_ the larger the capillary Reynolds number, the greater is the breakdown in the jet regardless of whether the turbulent patch is produced naturally or is triggered by the presence of a particle_ It is

Fig. 6. Waveform of naturally generated signal obtained with a simple capillary_ Sweep rate. O-5 msl cm;verticalscale:0.5 Vlun;D,= l-47 mm,L,/D,= 469 ,Re, = 2150.

Fig. 7. Waveform of signals obtained for two 4.3 65 mesh glass beads passing through a simple capillary Re, = 1000.

also to be expected that the larger the particle size, the greater the breakdown in the let_ This would explain the apparently mconsistent observations reported by Langer [5] and by Hofmann and hlohnen [7] _ Hofmann and Mohnen reported substantial breakdown, probably because larger particles and/or capillary Reynolds numbers were employed_ Langer reported only local breakdown, probably due to the use of capillary Reynolds number much lower than that corresponding to the natural transition or because relatively small particles were introducedIt was predicted in the discussion of the fundamental background that when a turbulent patch is formed in the capillary, a compression will be registered as the patch passes a microphone connected to the capillary via a probe tube. Furthermore, because of the increased pressure gradient for turbulent flow as compared to that for laminar flow, a rarefaction is to be expected when the patch leaves the capillary_ Evidence in support of the above hypothesis was obtained_ Figure S shows a photograph of the oscilloscope trace obtained with a microphone connected to the capillary bore for a turbulent patch produced naturally within a simple capillary. The first large compression signal, i-e_ the downwards displacement of the trace, is due to the turbulent patch passing the opening to the microphone. The second major signal - a rarefaction, or upwards displacement of the trace - occurs about 14 milliseconds after the first when

Fig. S. Signal sensed by microphone bore of simple capillary with p-pe

connected to of a partde.

the turbulent patch reaches the capllla~- es& Thus 1%ould indicate an average patch velocity which was 0.9 times the of 2OiO cm/set, averze fluid velocity of 2230 cm/set in the esperimentThE agrees \xiith kno\tn pipe transition phenomena [15] _ The second signal in Fig. S is complicated by the superposition of a response from the changed nature of the jet on the raefaction wave as the turhuient patch leaves the caprlla~-_

As expected, natural turbulent patch generation occurred in capillaries of the Langer type incorporated into the equipment shown in Fig. 3 with Reynolds numbers in the range l-I,000 - 22,000. Such high Reynolds numbers could only be achieved when the equrpment was operated in the ‘pressurized mode’. In the vacuum mode, with the Inlet of the devrce open to the atmosphere as depicted m Frg- 2. the flow was sonic at the exit of the capillary before Reynolds numbers large enough for natural transition could he obtained- The flow in the Langer-type capillary was larninar at Reynolds numbers much larger than for the simple caprkuy design because of rhe low disturbance entry condrtions and because the acceleration in the conical Inlet stabilized the flow_ The fact that turbulent patch generation occurred over a range of Reynolds numbers is consistent with the current understanding of the transitional phenomenon_

In the absence of particles, wire probes, etc., the flow was laminar and quret at a Reynolds number of 14,000 and less. This is indicated by the very low -amplitude smooth fluctuations in the oscilloscope trace shown in Fig_ 9(a)_ Figure 9(b) shows that at a slightly higher Reynolds number of 14,150, pressure pulses were obtained as turbulent patches were initiated. Resonance occurred as sho\vn m Fig. 9(c) as the number of turbulent patches formed in umt time coincided with the natural frequency of the system. _4t a Reynolds number of 14,950, the resonance suddenly disappeared, the flow was fully turbulent and produced flow noise as indicated u-r Fig. 9(d)_ The sharp fluctuations m the signd of Fig. 9(d) associated with turhulent flow contrast with the fluctuations associated with laminar flow shown in Fig.

9(a). Signal waveforms produced when two partrcles in the size range 3’i - 44 pm diam. were introduced at a Reynolds number of 14,000, i e just below the value of 14,150 required for natural turbulent patch generation, are shown in Fig. lo_ (It should be noted that although the amplitudes of the signals for the two

particles

are similar,

this

was

not

always so.) Furthermore, it x\as found that rf the flow in the capillary was initially turbulent, no signal above the flow noise depicted in Fig. 9(d) was obtained when similar partrcles were introduced_ This was not because the turbulent noise masked the signal, since If a signal had been present, its amplitude -as sho\\n in Fig_ 10 would have been much larger than the background noise_ These results confirm that the flow must be initially laminar for a signal to be obtained. Comparison of the signal amplitudes u-r Figs_ 9(b) (natural turbulent patch generation) and 10 (artificral patch generation) shows that, allowing for signal amplitude variations because of the statistical nature of turbulence, the amplitudes of the signals are very similar. This suggests that a 37 - 44 pm particle causes the formation of a turbulent patch in the capillary at a Reynolds number just below that required for natural patch generation_ It is again concluded that, because natural turbulent patch generation produces a pressure pulse and because a pressure pulse of similar amplitude is produced at a Reynolds number just below the natural transition value

(4

(b)

Cd)

Fig_ 9. Signals associated with natural turbulent patch generation usmg a capillary with a conical entrance. (c) Re, = 14,‘750; (d) RE, = 14,950. (a) Kc, = 14.000; (b) Kc, = 14.150; Sweep rate- 1 msfcm.

by a partrcle, the sound produced by the passage of a particle IS a consequence of the triggermg of a turbulent patch by the particle Experiments

wrtlz relatively

large particles m the discussion of background fundamentals that partral blockage of the capillary by a particle could be responsible for noise generatron in certain circumstances. Evrdence in support of this alternative mechanism for relatrvely large particles was obtained with flow visuahzation equipment incorporating a Langer-type cnplllary similar to that shown in Frg_ 2. X 24 - 28 mesh sand particle, occupying approximately 25% of the caprllary cross-sectional area, produced a signal as It entered the capillary. However, when partial flow blockage was

It was also hypothesized

Fig. 10. WaveTorm of signals obtained for two 37 - 44 pm particles passing through a capillary with a conical entrance. Sweep rate: 1 ms/cm; vertical scale 5 V/cm, Re, = 14,000_

116

indicated in this way, the total signal was a combination of several effects, all of which apparently generated pressure pulses as the pa.rticIe travelled through the device_ The transient pressure phenomena associated with the translation of a large particle through a capillary are much more complex than those documented for small particles_ Application

of the Lmzger acoustic device

The present study was motivated by the potential use of the Langer acoustic device in the counting and sizing of gas-borne particles. Once tht mechanisms by which the click-like sounds are generated by particles were well documented, attention shifted to the counting and sizing of particles_ Emphasis was placed on those smaller particles for which the sound signals were clearly directly related to the turbulent patch mechanism. As a counter, the detection efficiency, ie. the percentage of particles passing through the device which produced a signal, is of prime importance. The efficiency was determined by comparing the number of signals with the number of particles caught on an impactor plate smeared with petroleum jelly and incorporated into the equipment shown in Fig_ 3_ The efficiency is shown as a function of particle size range in Fig. 11 for glass beads and three values of capillary Reynolds number. Similar data were obtained with other panicles_ The efficiency is a strong function of particle size and Reynolds number- Extrapolation to an efficiency of

30

50

PartlcleSlze Fig_ 12. Signal amphtude entrance. Re, = 8300.

as a function

100

200

Fig_ 11. Detection efficiency as a function of particle size and Reynolds number for glass beads and a capillary with a conical entrance_

100% suggests that most, if not all, particles with a size greater than 30 pm would be detected at Reynolds number of 21,500 However, at a Reynolds number of 11,000 the size threshold for 100% efficiency is raised to 55 pm. The device could be made more sensitive, i_e detect smaller particles with essentially 100% efficiency, by operation at very high Reynolds numbers. The manner in which the efficiency varies with particle size and Reynolds number is entirely consistent with the basic mechanism of sound generation_ Larger particles and higher Reynolds numbers are more likely to provide the size of disturbance and lack of stabrhty required to produce turbulent patches. The sound signal amplitudes were measured from the photographic records of the oscil-

300

rn?

1000

(umj of particle size range for glas

beads and a capdIary

with a conical

117

loscope traces and, for a given set of conditions, plotted agamst particle size range. An example is shown in Fig. 12 for glass beads and a capillary Reynolds number of 8300. It is apmu-ent that the signal amplitude varies considerably for particles of the same nominal size. Again, this is consistent with the basic mechanism invohred, but unfortunately this does not provide much promise for the use of the device as a simple particle size analyser. However, Singh 1141 has shown how detection efficiency measurements might be used to indicate particle size indirectly. This approach is similar to that outlined more recently by Coover and Reist [ 171.

LIST

OF SYMBOLS

diameter particle diameter capillary length capillary Reynolds number = pfDeV/pf particle Reynolds number = p fDP V/pr average fluid velocity 111capillary fluid density fluid viscosity capillary

D,

*, L, Re, Rep V Pr Pcli

REFERENCES CONCLUSION

It is believed that the present investigation has provided a reasonably complete understanding of the fundamental mechanisms inherent in the operation of the acoustic particle detector. The turbulent patch mechanism of noise signal generation applies to particles which do not create a partial blockage 01 t.‘;: 1 ow The device may weil prove useful as a particie counter after calibration to determme efficiency a a function of particle size and Reynolds number. The random signal amplitude for particles of the same nominal size renders this parametei of doubtful utility in the sizing of particles; however, counts collected at several ReynoIds numbers, combined with knowledge of counting efficiencies. could provide particle size distributions_

1 G_ Langer, Res /Dew, 14 (1963) 40. 2 G. Langer, J Colioid Sci, 20 (1965) 602 3 G_ Langer, A further development of an acoustic particle counter, 5th Annu Am. Contamination Control Tech Meet., March 1966. Technol . 2 (1968169) 30i. 4 G. Langer, Powder G. Langer, Powder Technol. 6 j1972) 5. (1969). ; G_ Langer, US. Pat. 3.434.355 7 P. Hofmann and V. Mohnen, Staub. 38 (1968) 360_

8 B. F_ Osborne, Res_ Rep. ROOB. Mllltronics Limited, Peterborough, Ontario_ 9 B. F. Osborne, Res Rep. ROIO. Milltronics Limited, Peterborough, Ontario_ 10 R_ B. Hall, Res Rep. R012, Milltronics Limited, Peterborough, Ontario. 11 J. C_ Mollendorf and B Gebhart. L Fluid Mech.. 12 13

14 ACKNOWLEDGEMENTS

Financial assistance through an Air Management Branch Grant, Government of OntT%io, and the National Research Council of Canada is gratefully acknowledged.

15 16 17

61 (1973)

367_

R_ J. Leite, J Fluid Lfech., 5 (1959) 81. R. Clift and W. H. Gauvin, Proc. Powfech ‘71 (1st Int. Powder Technol and Bulk Granular Solids Conf.), sponsored by the Powder Advisory Centre. London, U 6.. 1971, p_ 47. V. P_ Singh, Ph. D. Thesis, Univ. of Toronto, 1975. I. J. Wygnanski and F_ H. Champagne, J. FIurd Mech., 59 (1973) 281. W. D. Baines and E_ G. Peterson. Trots Sot. Itlech. En&?_, 73 (1951) 464. S. R. &over and P. C. Reist. Environ. SIX Technol, 14 (1950) 951