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Measurement of density and mass flow of particles in a dilute suspension in general flow
Powder Technology, 25 (1980) 177 - 184 @ Elsevier Sequoia S-A., Lausanne - Printed in the Netherlands
177
Measurement of Densit- and Mass Flow of Particles in a Dilute Suspension in General Flow* ELT. CHAO,
H. PEREZ-BLANCO,
Department of Mechanical III. 61801 (U.S.A.) (Received
May 16,1979;
J_ H. SAUNDERS
and S. L. SO0
and Industrial Engineering.
in revised form September
For the measurement of density and mass flow of the particle phase in an air suspension in pipe flow with tangential motion, new instrumentation needs arose. For these measurements, the earlier fiber optics probe and electrostatic ball probe have to be modified. SimuRaneous measurements of particle density and mass flow permits determination of mean velocity of the particle phase. For the measurement of particle density, a ‘twin diode’ optical probe has been introduced. In this design, the source and detector are two miniature diodes, one as a source and the other as a detector mounted on two arms of a probe holder. The probe is insensitive to yaw angles of +-90” and pitch angles of ?30”. The probe sensitivity approaches 0.01 kg/ms. An important innovation of the electrostatic counter probe is by filtering and co~unting the electric pulses of particles impacting on a probe due to charge transfer. The probe diameter can be as small as 1.6 mm. Sensitivity of the order of 0.002 kg/m2 s has been indicated. This probe is insensitive to direction of motion of particles and has potential for use in fluctuating or nonsteady flows. 1. INTRODUCTION
For our study of modeling and scaling of dilute suspensions, two types of systems have been tested to determine the motions of phases and distributions or particles: the case of flow-gravity interaction in the free board
76-01665
and 78-12796.
by NSF
grants ENG
of Illinois at Urbana-Champaign,
Urbana,
13, 1979)
SUMMARY
*This study was supported
University
of a fluidized bed and the case of flowcentrifugal force field interaction in pipe flow with a rotating section [l]. The challenging nature of measurements in the above systems in comparison with those in our earlier studies of fully developed pipe flow [ 2] or entrance into flat plates [33 is readily noted. One significant aspect is the departure from predominantly axial flow in the previous experiments on pipe flow and channel flow_ We need to account for (1) large flow fluctuation at low velocities in the free board of a fluidized bed and (2) the prominent tangential component in pipe flow with a rotating section. For measurements of local particle density and local mass flow of particles, both the fiber optics probe and the electrostatic ball probe [2, 31 have been modified. By combining signal 5ltering and counting, the electrostatic probe has been developed to become a reliable sensor suitable for unsteady flow and a range of flow directions. When used in monodispersed suspensions of low mass loading such that simultaneous collision is of rare occurrence and when the charge transfer upon impact gives rise to a signal significantly higher than the background noise, the electrostatic counter probe may be adopted- as a primary standard in that it requires no experimentally determined calibration constant.
2. TWIN-DIODE MEASUREMENT
OPTICAL PROBE FOR OF LOCAL DENSITY
OF
PARTICLES
Measurement of local particle density has remained a challenge in multiphase flow instrumentation. Available methods consist of
those based on electric capacitance 14 - 63 or light scattering from a light beam or fiber optics based on attenuation or scattering [ 7, 2, S] _ Several generations of fiber optics configurations have been developed [3, 9]_ Three major Iirnitations remain:
Therefore, by placing the particies between a light source and detector and by a suitable calibration, the density of a suspension may be measured_
probe
2-2 Probe
(1) It is a secondary standard azrd cahbration is limited to isokinetic sampling at nearly identical velocity of phases as the primary standard. (2) Calibration is affected by collection of particles on the optical surface(3) Because of the need for the light shield, flow direction is limited primarily to the Iongitudinai direction. The newly developed ‘twin diode’ opticaI density probe is not subject to limitations (1) and (3). Because of the highly directional character of the diode, the effect of particie collection on probe calibration is lessened but not eliminated. The variability of this soiling effect is kept within bounds by coating the opticai surfaces with a commercial antistatic transparent film. 2.2 Fundamental relations The scatteting of radiation as a meas.nement of the density of a suspension has been witieIy used previously_ These determinations are based on the simple exponentiaI attenuation reIation [lo] I i = i0 e-OL
(1)
where L is the optical path length between the light source and the detector, i, is the intensity of the emitter and i is the intensity at the detector. The scattering coefficient is reIated to the number density of the particles N, by
CT
(T = SAN,
(2)
configumtion
In previous designs 121, the emitter and detector were pIaced outside the tubular probe containing the particulate suspension. The light was conducted in and out of the probe by means of opticaI fibers. In the present design, the source of radiation and the detector are actually placed inside the device containing the suspension. This avoids some of the limitations of the fiber optics concept, namely the need for a tubular shield, the need for careful alignment and polishing of the optical surfaces and a prior knowiedge of the direction of particle flow. However, one crucial problem, that of keeping the optical surfaces clean, remains. The source and detector are two miniature Ga-As-P diodes, manufactured by Hewlett Packard (HETM 6000). They belong to the general group of LED-s and emit visibie radiation when forward biased. These diodes offer several desirable features: (a) sma.U size, with overah dimensions of 2.9 X 2.2 X 2-5 mm as shown in Fig. 1, (b) high frequency response, up to 5 MHz and a maximum rise time of 70 ns, and (c) low forward voltage, 1.4 - 1.5 V. The emi&on peaks at about 700 nm at 25 “C wiLh a bandwidth of 20 nm at half peak Ill] _ The diode emits photons as a result of electron-hoIe recombination for a forward biased p-n junction_ Conversely,
scattering crosssection of the particles over the waveband of the light source. From eqns. (1) and (2), one can write where
Sk is the average
i = i0 emKPp
(3)
where pp is &e average particle phase density of the suspension under consideration, and H is a constant which depends on L, the wavelength of the radiation, the size, and the material density of the individual particles and their average scattering cross-section.
Fig. 1. Miniature Ga-As-P diode. Al1 dimensions are in mm; ranges show tolerance cf manufacturer_
# 2 set screws Fig. 2. Diode holder(dimensionsin mm). when photons of proper energy strike at a p-n junction, they are absorbed and create electron-hole pairs, producing a reverse bias current in a ciosed circuit or an open circuit voltage. The Ga-As-P diodes were employed in the present study as an emitter as well as a detector_ The diode holder was made of aluminum and is shown in Fig. 2. Each diode was placed in a t&on socket which, in turn, was pressed into the 4 mm cylindrical cavity of the holder. The resistance between the diode leads and the holder was checked and found to exceed 20 MSL. Figure 3 is a photograph of the probe assembly. The aluminum foil covering the back side of each holder was grounded and served as an electrical as well as a mechanical shield for the leads. The 3.1 mm steel rod enabled traversing the probe assembly across the flow field. Since the direction of the flow incident upon the density probe is not known in the case of a swirling flow, its sensitivity to yaw and pitch angle was measured. It was found that the probe assembly shown in Fig. 3 is insensitive to yaw angles of 190” and pitch angles of 230”. 2_3_ Electronic ckcuitry The output of the Ga-As-P diode detector requires appropriate treatment in order to obtain meaningful interpretation. One has the choice of me asuring the open circuit voltage or the reverse bias current in a closed circuit. The oper- circuit output voltage, vat, &f the diode under an incident light of %ensity iis vat = KIT
ln
(1 + I@)
where LXI and Kz are constants and T is the
(4)
Fig. 3. Twin diode particle density probe assembly_ [Coin is U.S. lo-cent piece, 18 mm in diameter.)
absolute temperature. Hence, if iK2 3- 1, as is usually the case [ll] , eqn. (4) becomes Combining eqns. (3) and (5) gives V out = WZ-&P~)T
(6)
where K3 and K4 are constads. Clearly, uat is dependent on suspension temperature. The changes in uWt due to the changes in temperature that took place in a normal operation were measured and found to 3e greater than the actual signal for typical particle mass loadings_ This mode of operation was thus abandoned. The other distinct output is the photocurrent which is the variable being measured now. The photocurrent I from the diode when subjected to an incident light of intensity i is [ll] I=K5i
(7)
where the constant K5 depends, in general, on the quantum efficiency. Thus, from eqns. (3) and (7), one obtains I = Ks emKf%
(8)
180
Fig 4. Photocurrent measuring circuit; [l + (Rz/R1)] = 25; f = [1/2x&C] = 5 Hz, where C denotes capacitance and f denotes cut-off frequency_
The electronic circuitry currently being used is shown in Fig. 4. It has two stages: a photocurrent amplifier with current to voitage conversion and a continuous averaging stage with further amplification. The voltage output from the first stage, VI, is related to I and the diode dark current 1, according to
v, = lo’(1tln)
(2)
An alternative means of processing and measuring the photocurrent from the diode is to employ a ‘chopper’ together with a high quality, narrow bandpass filter and a true r.m.s. voltmeter for the readout. The noise can be more effectively eliminated but the components are more expensive. The deposition on the optical surface tends to reach saturation in a dilute suspension_ The variability of the degree of saturation can be controlled to some extent by coating the optical surfaces with an antistatic film as has been noted earlier. Thus, by calibrating under the said condition, this source of error is minimized_ A variable power supply for the emitter diode was designed and built for frequent calibration checks during operation. This checking was carried out by comparing the probe output with the output for a normalized condition or with isokinetic sampling. Figure 5 shows typical measurements made in a horizontal pipe flow system.
The CR0 trace of VI showed extremely rapid fluctuations, as expected. An averaging circuit with a further gain of 25 was employed to make possible the measurement of final output voltage V, with an inexpensive voltmeter. The averaging circuit served also the useful purpose of filtering out the unwanted high frequency noise_ Referring to Fig. 4, one has V,
=
25 X 10’(1+1,)
(10)
Since the dark current is a function of temperature, the temperature sensitivity of the probe was studied. It was determined that for gIass beads flowing in the present system, the change of voltage with temperature AV/AT amounted to -O.OlSV/C at a voltage signal of 5 v as the suspension temperature changed from 22 “C to 28.6 “C. This range of temperature change produced by internal dissipation was typical for the present system. Since the output voltage for Iatex particles is one order of magnitude higher under otherwise identical conditions, the effect of temperature change is of even less consequence. Calibration was done by locating the density probe at the center-line of the straight section of a closed loop pipe flow system using a range of air velocities and particle mass ioading. The density was evaluated from the measured mass flux and centerline air velocity.
I
-OS*
,
I
04
1
06
$
08
KCJ/I2
Fig. 5. Typical measurements of particle density by the twin diode probe (51 mm horizontal pipe, glass beads 88 - 66 pm, mean velocity 12 m/s).
3.
ELECTROSTATIC
MEASUREZW3NTS
COUNTER OF MASS
PROBE
FOR
FLOW
The electrostatic ball probe [S, 121 has been used as a secondary standard for measuring mass flow of particles in a suspension. The primary standard has been the isokinetic sampling probe. By counting
181
electric pulses produced by particle impact on a prcbe due to charge transfer, together with appropriate filtering of signals to minimize the effect of noise, it is possible to determine the local time-averaged mass flow of particles. The technique has been demonstrated to be successful for essentially monodispersed dilute suspensions. Extension to a suspension with groups of particles of sufficiently large difference in sizes seems possible. 3.1 Basic concept It has been demonstrated by a number of investigators that virtually all particles in our size range of interest (20 - 200 m) transfer charge to a metal surface upon impact [ 123, provided the surface is of different material from the particles. Previously, this phenomenon has been used to measure local particle mass flow [ 133 _ Particles transferred charge to a conducting sphere upon impact and the sphere is discharged to ground through an electrometer. The resulting current is calibrated against particie mass flux. However, the reproducibility of data was often hampered by variations in charge transfer from particle to particle and by the system’s sensitivity to noise. In the present study, a modification on the probe read-out has been made. As each particle hits the ball probe (Fig. 6), an individual pulse of current can be identified_ Through suitable electronic circuitry, the number of pulses in a given time can be counted. This is a direct measure of
yLhre.
Long
Cooxml Cobb Two Shelds
wth
Fig. 7_ Calibration probe.
system,
electrostatic
counter
the number of particle-probe collisions which can be related to local particle mass flow. Thus, the present mass flow measurement technique combines the ball probe and the impact counter system [ 14]_ This electrostatic counter probe offers several advantages: (1) relative insensitivity to flow direction, (2) isokinetic condition is not required; within limits, it is applicable to unsteady and fluctuating flows, (3) relative independence of charge transfer variations from particle to particle, (4) highly repeatable, (5) minimal flow disturbance, (6) measurements can be taken quickly. 3.2 Probe circuitry
Fig_ 6. Electrostatic counter pK>be for mass flow measurement in rotating pipe sydem.
0 61mm da . I7m-n
calibration
and signal processing
For convenience, calibration was done with a small wire probe, 0.61 mm diam. by l-7 mm long, installed in the midsection of a thin walled tube, 4 mm i-d. by 12 mm long, as illustrated in Fig. 7. This electrically grounded tube formed the entrance of a sanpling tube and filter which, in turn, was connected to a vacuum pump via a throttling valve, capable of producing a range of sampling velocities. All particles entering the tube were collected in a porous thimble and the mass of particles collected was determined by weighing. A typical sampling time was one minute. Calibration was done with glass beads, 66 88 m, and latex particles, 88 - 105 /.nn_ The particle mass flow rates, as determined from weighing, average particle size, particle material density and sampling time compare
182
favorably with those obtained from probe count, ratio of the probe frontal area to the cross-sectional area of the entrance tube and the impaction fraction based on Langmuir and Blodgett [ 15]_ The electronic circuit is shown schematically in Fig. S(a), which includes a photograph of the CR0 trace for an almost noise-free single pulse produced by a latex particle of approximately 100 ~.rm diam. Amplifier No. 1 is a low drift amplifier, wired in a current-to-voltage mode. Connecting two identical FET operational amplifiers in a tracking configuration as shown in Fig. S(b) provides this desirable characteristic. Upon the collision of a particle with the probe, the transferred charge wiII discharge to ground via the 1 Ma resistor. The voltage drop across the resistor is amplified 200 times. Amplifier No. 2 is a simple voltage amplifier with a gain of 10 in an inverting configuration. The use of two amplifiers in tandem is to achieve flat frequency response from d-c. to approximately 5 kHz. The output then goes to a bandpass fiIter tuned to pass signa& with frequencies in the range 80 - 1300 Hz. The lower cutoff frequency was dictated by the 60 Hz noise prevailing in the laboratory. Amplifier No. 3 has a variable gain ranging
Fig. S(a). Schematic for processing trace: 2 V per division, time base:
signal from electrostatic 0.5 m/s per division.
from zero to 20 and is also in inverting configuration. Hence, the signal would resume its normal mode after being inverted twice and the combined amplification may have a maximum of 40,000 times. For latex particles 75 - 80 camdiam. and traveling at 10 - 15 m/s, typical voltage signals at this stage are approximately 15 V at a gain of 15,000. The Schmitt trigger serves the important function of providing an unambiguous pulse each time the output signal of amplifier No. 3 rises above a preselected threshold level. This pulse is then sent to the counter. In spite of the use of the bandpass filter, noise oftentimes persists in the signals as has been revealed by the CRO. To eliminate any false counting due to noise, a Schmitt trigger with variable threshoid is used as i&r&rated in Fig. S(c). When the signal reaches the first threshold voltage VI, it is immediately reduced to V,, and the difference, VI - V,, is made to be much greater than the halfamplitude of the noise. In this way, f&e triggering due to noise is eliminated. The desired voltage difference, VI - V,, can be readily ascertained by observing the CR0 trace of the output signal and then set by adjusting the 0 - 25 kX2 potentiometer of the Schmitt trigger circuitry shown in Fig. S(b).
counter
probe_
Upper
trace:
5 V pez division,
lower
183
Schntt
Fig. S(b).
Fig.
8(c).
Tnqgt=s
Components
Operating
The operation of the variable threshold design is schematically illustrated in Fig. 8(c). The pulse signature produced by a particleprobe collision is quite interesting. Each signature consists of a positive and a negative pulse of approximately the same height, typical of the response of an RC circuit to a finite voltage pulse. Hence, even if the sign of the charge transferred varies during a run, the pulse count remains the same. As long as the particle pulse height is above the noise level, a collision can be registered. The results of calibration for glass beads using air velocities ranging from 7.3 to 14 m/s are shown in Fig_ 9. The particle mass fluxes determined by filtering and weighing compare very favorably with those by the counter probe. Equally satisfactory results have also been obtained for the latex particles. The calibrated wire probe and the ball probe were then placed along the centerline of a 2 in. (51 mm) pipe flow loop, with the ball probe located approximately eleven pipe diameters downstream. The particle mass flux as determined from the electrical outputs of both probes agreed well. The conducting element of the electrostatic ball probe used in the fluidized bed is a l/8 in. (3 mm) steel ball insulated over onehalf its surface so that only upward mass flow is registered. For the rotating pipe, a l/16 in.
_
circuitry.
conditions
of
the
Schmitt
trigger
showing
the
importance
of variable
threshold
voltage.
184 I
4. DISCUSSION
I
I
/ /'G
5 -6 x "4"E Y (r g 3-
-
z / 'o"/ +%
z B cz-
15
o"'& / / ,
370 Q/
s u
,4 $g*; ’ -gp I I
J OO
Somphng
I 4
A
h
Collechon.
5
g/cm2sx10-*
Fig_ 9. Mass flow probe calibration (numerals adjacent to data points designate nominal air velocity in m/s).
The direction for maximum count for mass flow gives the direction of mean flow of the particle phase with its speed given by the ratio of mass flow to the particle phase density. Further work will include determination of mass flow of each range of particle size by selective electronic filtration based on pulse heights. Applications of these probes to dense suspensions remain to be developed, and the upper limits of particle concentration for meaningful measurement by these probes remain to be determined. Measurements of reversing flow adjacent to the bubbling surface in the free board of a fiuidized bed will be the subject of a subsequent paper. REFERENCES
14 _ 12
:
:
l
:
-
.
IO
.
.
$08 Cl
. .
06-O
I
04-
-
I
Alumrum Oxide Particles 54-45pm W,,=23 7 m/s - s=o - s=o4 &=8 2 X 10e3 Kg/m’s
_
I 02 ----Pg 00
-10 ica of Pye
-05
r/r0
00
05
10 %s
Fig. 10. Typical measured results of particle mass flow in 127 mm diam. horizontal pipe flow system. Wu and Go are, respectively, the axial air velocity and particle mass flux at centerline. S = RDl2Wo. D is the pipe diameter and S2 is the angular velocity of rotation_ Measurement station is one pipe diameter downstream of the rotating section_
(1.6mm) steel ball is used to ensure adequate insensitivity to flow direction. Figure 10 gives a typical set of measurements in pipe flow with and without a rotating section using the ball probe.
1 B. T. Chao, S. T. Leung, H. Perez-Blanco, J. H. Saunders and S. L. Soo, Proc. Int. Conf. on Pneumatic Conveying (Invited Lecture), Int. Powder Inst. (London), 18 Jan., 1979. 2 S. L. Soo, G. J. Trezek, R. C. Dimick and G. F. Hohnstreiter, Ind. Eng. Chem. Fundam.. 3 (1964) 98 - 106. 3 S. L. Soo, J. J. Stukel and J. M. Hughes, Environ. Sci. Technot.. 3 (1969) 386 - 393. 4 J. H. Daniel and F. S. Bracket& J. Appl. Phys.. 22 (1951) 542 - 554. 5 K. Min. B. T. Chao and M. E. Wyman, Rev- SciInstrum.. 34 (1963) 529 - 531. 6 D. Van Zoonen, Proc_ Symp. on Interaction between Fluids and Particles. Inst. Chem. Eng. (London), 1962, p_ 64. 7 R. E. Rosensweig, H. C. Hottel and G. C. Williams, Chem. Eng. Sci., 15 (1961) 111 - 125. 8 S. L. Soo, Fluid Dynamics of Multiphase Systems, Blaisdell Publ. Co., Waltham, Mass., 1967. 9 J_ C. Haas, M.S. Thesis, Univ. of Illinois at Urbana-Champaign, 1968. 10 ‘I’. K. Siegel and B. Woertz, Ind. Eng. Chem.. 31 (1939) 1034 - x041. Designer’s 11 Hewlett Packard Co., Optoelectronics CataIog, 1978. 12 L. Cheng and S. L. Soo, J. Appl. Phys.. 42 (1970) 585 - 591. 13 L. Cheng, S. L_ Soo and S. K_ Tung, J. EngPower, 92 (1970) 135 - 149. 14 S. L. Soo and J. A. Regalbuto, Can. L Chem. Eng.. 38 (5) (1960) 160 - 166. 15 I. Langmuir and K. Blodgett, Tech. Rep. 5418, Air Materiel Command, U.S.A.A.F. (1946).
177
Measurement of Densit- and Mass Flow of Particles in a Dilute Suspension in General Flow* ELT. CHAO,
H. PEREZ-BLANCO,
Department of Mechanical III. 61801 (U.S.A.) (Received
May 16,1979;
J_ H. SAUNDERS
and S. L. SO0
and Industrial Engineering.
in revised form September
For the measurement of density and mass flow of the particle phase in an air suspension in pipe flow with tangential motion, new instrumentation needs arose. For these measurements, the earlier fiber optics probe and electrostatic ball probe have to be modified. SimuRaneous measurements of particle density and mass flow permits determination of mean velocity of the particle phase. For the measurement of particle density, a ‘twin diode’ optical probe has been introduced. In this design, the source and detector are two miniature diodes, one as a source and the other as a detector mounted on two arms of a probe holder. The probe is insensitive to yaw angles of +-90” and pitch angles of ?30”. The probe sensitivity approaches 0.01 kg/ms. An important innovation of the electrostatic counter probe is by filtering and co~unting the electric pulses of particles impacting on a probe due to charge transfer. The probe diameter can be as small as 1.6 mm. Sensitivity of the order of 0.002 kg/m2 s has been indicated. This probe is insensitive to direction of motion of particles and has potential for use in fluctuating or nonsteady flows. 1. INTRODUCTION
For our study of modeling and scaling of dilute suspensions, two types of systems have been tested to determine the motions of phases and distributions or particles: the case of flow-gravity interaction in the free board
76-01665
and 78-12796.
by NSF
grants ENG
of Illinois at Urbana-Champaign,
Urbana,
13, 1979)
SUMMARY
*This study was supported
University
of a fluidized bed and the case of flowcentrifugal force field interaction in pipe flow with a rotating section [l]. The challenging nature of measurements in the above systems in comparison with those in our earlier studies of fully developed pipe flow [ 2] or entrance into flat plates [33 is readily noted. One significant aspect is the departure from predominantly axial flow in the previous experiments on pipe flow and channel flow_ We need to account for (1) large flow fluctuation at low velocities in the free board of a fluidized bed and (2) the prominent tangential component in pipe flow with a rotating section. For measurements of local particle density and local mass flow of particles, both the fiber optics probe and the electrostatic ball probe [2, 31 have been modified. By combining signal 5ltering and counting, the electrostatic probe has been developed to become a reliable sensor suitable for unsteady flow and a range of flow directions. When used in monodispersed suspensions of low mass loading such that simultaneous collision is of rare occurrence and when the charge transfer upon impact gives rise to a signal significantly higher than the background noise, the electrostatic counter probe may be adopted- as a primary standard in that it requires no experimentally determined calibration constant.
2. TWIN-DIODE MEASUREMENT
OPTICAL PROBE FOR OF LOCAL DENSITY
OF
PARTICLES
Measurement of local particle density has remained a challenge in multiphase flow instrumentation. Available methods consist of
those based on electric capacitance 14 - 63 or light scattering from a light beam or fiber optics based on attenuation or scattering [ 7, 2, S] _ Several generations of fiber optics configurations have been developed [3, 9]_ Three major Iirnitations remain:
Therefore, by placing the particies between a light source and detector and by a suitable calibration, the density of a suspension may be measured_
probe
2-2 Probe
(1) It is a secondary standard azrd cahbration is limited to isokinetic sampling at nearly identical velocity of phases as the primary standard. (2) Calibration is affected by collection of particles on the optical surface(3) Because of the need for the light shield, flow direction is limited primarily to the Iongitudinai direction. The newly developed ‘twin diode’ opticaI density probe is not subject to limitations (1) and (3). Because of the highly directional character of the diode, the effect of particie collection on probe calibration is lessened but not eliminated. The variability of this soiling effect is kept within bounds by coating the opticai surfaces with a commercial antistatic transparent film. 2.2 Fundamental relations The scatteting of radiation as a meas.nement of the density of a suspension has been witieIy used previously_ These determinations are based on the simple exponentiaI attenuation reIation [lo] I i = i0 e-OL
(1)
where L is the optical path length between the light source and the detector, i, is the intensity of the emitter and i is the intensity at the detector. The scattering coefficient is reIated to the number density of the particles N, by
CT
(T = SAN,
(2)
configumtion
In previous designs 121, the emitter and detector were pIaced outside the tubular probe containing the particulate suspension. The light was conducted in and out of the probe by means of opticaI fibers. In the present design, the source of radiation and the detector are actually placed inside the device containing the suspension. This avoids some of the limitations of the fiber optics concept, namely the need for a tubular shield, the need for careful alignment and polishing of the optical surfaces and a prior knowiedge of the direction of particle flow. However, one crucial problem, that of keeping the optical surfaces clean, remains. The source and detector are two miniature Ga-As-P diodes, manufactured by Hewlett Packard (HETM 6000). They belong to the general group of LED-s and emit visibie radiation when forward biased. These diodes offer several desirable features: (a) sma.U size, with overah dimensions of 2.9 X 2.2 X 2-5 mm as shown in Fig. 1, (b) high frequency response, up to 5 MHz and a maximum rise time of 70 ns, and (c) low forward voltage, 1.4 - 1.5 V. The emi&on peaks at about 700 nm at 25 “C wiLh a bandwidth of 20 nm at half peak Ill] _ The diode emits photons as a result of electron-hoIe recombination for a forward biased p-n junction_ Conversely,
scattering crosssection of the particles over the waveband of the light source. From eqns. (1) and (2), one can write where
Sk is the average
i = i0 emKPp
(3)
where pp is &e average particle phase density of the suspension under consideration, and H is a constant which depends on L, the wavelength of the radiation, the size, and the material density of the individual particles and their average scattering cross-section.
Fig. 1. Miniature Ga-As-P diode. Al1 dimensions are in mm; ranges show tolerance cf manufacturer_
# 2 set screws Fig. 2. Diode holder(dimensionsin mm). when photons of proper energy strike at a p-n junction, they are absorbed and create electron-hole pairs, producing a reverse bias current in a ciosed circuit or an open circuit voltage. The Ga-As-P diodes were employed in the present study as an emitter as well as a detector_ The diode holder was made of aluminum and is shown in Fig. 2. Each diode was placed in a t&on socket which, in turn, was pressed into the 4 mm cylindrical cavity of the holder. The resistance between the diode leads and the holder was checked and found to exceed 20 MSL. Figure 3 is a photograph of the probe assembly. The aluminum foil covering the back side of each holder was grounded and served as an electrical as well as a mechanical shield for the leads. The 3.1 mm steel rod enabled traversing the probe assembly across the flow field. Since the direction of the flow incident upon the density probe is not known in the case of a swirling flow, its sensitivity to yaw and pitch angle was measured. It was found that the probe assembly shown in Fig. 3 is insensitive to yaw angles of 190” and pitch angles of 230”. 2_3_ Electronic ckcuitry The output of the Ga-As-P diode detector requires appropriate treatment in order to obtain meaningful interpretation. One has the choice of me asuring the open circuit voltage or the reverse bias current in a closed circuit. The oper- circuit output voltage, vat, &f the diode under an incident light of %ensity iis vat = KIT
ln
(1 + I@)
where LXI and Kz are constants and T is the
(4)
Fig. 3. Twin diode particle density probe assembly_ [Coin is U.S. lo-cent piece, 18 mm in diameter.)
absolute temperature. Hence, if iK2 3- 1, as is usually the case [ll] , eqn. (4) becomes Combining eqns. (3) and (5) gives V out = WZ-&P~)T
(6)
where K3 and K4 are constads. Clearly, uat is dependent on suspension temperature. The changes in uWt due to the changes in temperature that took place in a normal operation were measured and found to 3e greater than the actual signal for typical particle mass loadings_ This mode of operation was thus abandoned. The other distinct output is the photocurrent which is the variable being measured now. The photocurrent I from the diode when subjected to an incident light of intensity i is [ll] I=K5i
(7)
where the constant K5 depends, in general, on the quantum efficiency. Thus, from eqns. (3) and (7), one obtains I = Ks emKf%
(8)
180
Fig 4. Photocurrent measuring circuit; [l + (Rz/R1)] = 25; f = [1/2x&C] = 5 Hz, where C denotes capacitance and f denotes cut-off frequency_
The electronic circuitry currently being used is shown in Fig. 4. It has two stages: a photocurrent amplifier with current to voitage conversion and a continuous averaging stage with further amplification. The voltage output from the first stage, VI, is related to I and the diode dark current 1, according to
v, = lo’(1tln)
(2)
An alternative means of processing and measuring the photocurrent from the diode is to employ a ‘chopper’ together with a high quality, narrow bandpass filter and a true r.m.s. voltmeter for the readout. The noise can be more effectively eliminated but the components are more expensive. The deposition on the optical surface tends to reach saturation in a dilute suspension_ The variability of the degree of saturation can be controlled to some extent by coating the optical surfaces with an antistatic film as has been noted earlier. Thus, by calibrating under the said condition, this source of error is minimized_ A variable power supply for the emitter diode was designed and built for frequent calibration checks during operation. This checking was carried out by comparing the probe output with the output for a normalized condition or with isokinetic sampling. Figure 5 shows typical measurements made in a horizontal pipe flow system.
The CR0 trace of VI showed extremely rapid fluctuations, as expected. An averaging circuit with a further gain of 25 was employed to make possible the measurement of final output voltage V, with an inexpensive voltmeter. The averaging circuit served also the useful purpose of filtering out the unwanted high frequency noise_ Referring to Fig. 4, one has V,
=
25 X 10’(1+1,)
(10)
Since the dark current is a function of temperature, the temperature sensitivity of the probe was studied. It was determined that for gIass beads flowing in the present system, the change of voltage with temperature AV/AT amounted to -O.OlSV/C at a voltage signal of 5 v as the suspension temperature changed from 22 “C to 28.6 “C. This range of temperature change produced by internal dissipation was typical for the present system. Since the output voltage for Iatex particles is one order of magnitude higher under otherwise identical conditions, the effect of temperature change is of even less consequence. Calibration was done by locating the density probe at the center-line of the straight section of a closed loop pipe flow system using a range of air velocities and particle mass ioading. The density was evaluated from the measured mass flux and centerline air velocity.
I
-OS*
,
I
04
1
06
$
08
KCJ/I2
Fig. 5. Typical measurements of particle density by the twin diode probe (51 mm horizontal pipe, glass beads 88 - 66 pm, mean velocity 12 m/s).
3.
ELECTROSTATIC
MEASUREZW3NTS
COUNTER OF MASS
PROBE
FOR
FLOW
The electrostatic ball probe [S, 121 has been used as a secondary standard for measuring mass flow of particles in a suspension. The primary standard has been the isokinetic sampling probe. By counting
181
electric pulses produced by particle impact on a prcbe due to charge transfer, together with appropriate filtering of signals to minimize the effect of noise, it is possible to determine the local time-averaged mass flow of particles. The technique has been demonstrated to be successful for essentially monodispersed dilute suspensions. Extension to a suspension with groups of particles of sufficiently large difference in sizes seems possible. 3.1 Basic concept It has been demonstrated by a number of investigators that virtually all particles in our size range of interest (20 - 200 m) transfer charge to a metal surface upon impact [ 123, provided the surface is of different material from the particles. Previously, this phenomenon has been used to measure local particle mass flow [ 133 _ Particles transferred charge to a conducting sphere upon impact and the sphere is discharged to ground through an electrometer. The resulting current is calibrated against particie mass flux. However, the reproducibility of data was often hampered by variations in charge transfer from particle to particle and by the system’s sensitivity to noise. In the present study, a modification on the probe read-out has been made. As each particle hits the ball probe (Fig. 6), an individual pulse of current can be identified_ Through suitable electronic circuitry, the number of pulses in a given time can be counted. This is a direct measure of
yLhre.
Long
Cooxml Cobb Two Shelds
wth
Fig. 7_ Calibration probe.
system,
electrostatic
counter
the number of particle-probe collisions which can be related to local particle mass flow. Thus, the present mass flow measurement technique combines the ball probe and the impact counter system [ 14]_ This electrostatic counter probe offers several advantages: (1) relative insensitivity to flow direction, (2) isokinetic condition is not required; within limits, it is applicable to unsteady and fluctuating flows, (3) relative independence of charge transfer variations from particle to particle, (4) highly repeatable, (5) minimal flow disturbance, (6) measurements can be taken quickly. 3.2 Probe circuitry
Fig_ 6. Electrostatic counter pK>be for mass flow measurement in rotating pipe sydem.
0 61mm da . I7m-n
calibration
and signal processing
For convenience, calibration was done with a small wire probe, 0.61 mm diam. by l-7 mm long, installed in the midsection of a thin walled tube, 4 mm i-d. by 12 mm long, as illustrated in Fig. 7. This electrically grounded tube formed the entrance of a sanpling tube and filter which, in turn, was connected to a vacuum pump via a throttling valve, capable of producing a range of sampling velocities. All particles entering the tube were collected in a porous thimble and the mass of particles collected was determined by weighing. A typical sampling time was one minute. Calibration was done with glass beads, 66 88 m, and latex particles, 88 - 105 /.nn_ The particle mass flow rates, as determined from weighing, average particle size, particle material density and sampling time compare
182
favorably with those obtained from probe count, ratio of the probe frontal area to the cross-sectional area of the entrance tube and the impaction fraction based on Langmuir and Blodgett [ 15]_ The electronic circuit is shown schematically in Fig. S(a), which includes a photograph of the CR0 trace for an almost noise-free single pulse produced by a latex particle of approximately 100 ~.rm diam. Amplifier No. 1 is a low drift amplifier, wired in a current-to-voltage mode. Connecting two identical FET operational amplifiers in a tracking configuration as shown in Fig. S(b) provides this desirable characteristic. Upon the collision of a particle with the probe, the transferred charge wiII discharge to ground via the 1 Ma resistor. The voltage drop across the resistor is amplified 200 times. Amplifier No. 2 is a simple voltage amplifier with a gain of 10 in an inverting configuration. The use of two amplifiers in tandem is to achieve flat frequency response from d-c. to approximately 5 kHz. The output then goes to a bandpass fiIter tuned to pass signa& with frequencies in the range 80 - 1300 Hz. The lower cutoff frequency was dictated by the 60 Hz noise prevailing in the laboratory. Amplifier No. 3 has a variable gain ranging
Fig. S(a). Schematic for processing trace: 2 V per division, time base:
signal from electrostatic 0.5 m/s per division.
from zero to 20 and is also in inverting configuration. Hence, the signal would resume its normal mode after being inverted twice and the combined amplification may have a maximum of 40,000 times. For latex particles 75 - 80 camdiam. and traveling at 10 - 15 m/s, typical voltage signals at this stage are approximately 15 V at a gain of 15,000. The Schmitt trigger serves the important function of providing an unambiguous pulse each time the output signal of amplifier No. 3 rises above a preselected threshold level. This pulse is then sent to the counter. In spite of the use of the bandpass filter, noise oftentimes persists in the signals as has been revealed by the CRO. To eliminate any false counting due to noise, a Schmitt trigger with variable threshoid is used as i&r&rated in Fig. S(c). When the signal reaches the first threshold voltage VI, it is immediately reduced to V,, and the difference, VI - V,, is made to be much greater than the halfamplitude of the noise. In this way, f&e triggering due to noise is eliminated. The desired voltage difference, VI - V,, can be readily ascertained by observing the CR0 trace of the output signal and then set by adjusting the 0 - 25 kX2 potentiometer of the Schmitt trigger circuitry shown in Fig. S(b).
counter
probe_
Upper
trace:
5 V pez division,
lower
183
Schntt
Fig. S(b).
Fig.
8(c).
Tnqgt=s
Components
Operating
The operation of the variable threshold design is schematically illustrated in Fig. 8(c). The pulse signature produced by a particleprobe collision is quite interesting. Each signature consists of a positive and a negative pulse of approximately the same height, typical of the response of an RC circuit to a finite voltage pulse. Hence, even if the sign of the charge transferred varies during a run, the pulse count remains the same. As long as the particle pulse height is above the noise level, a collision can be registered. The results of calibration for glass beads using air velocities ranging from 7.3 to 14 m/s are shown in Fig_ 9. The particle mass fluxes determined by filtering and weighing compare very favorably with those by the counter probe. Equally satisfactory results have also been obtained for the latex particles. The calibrated wire probe and the ball probe were then placed along the centerline of a 2 in. (51 mm) pipe flow loop, with the ball probe located approximately eleven pipe diameters downstream. The particle mass flux as determined from the electrical outputs of both probes agreed well. The conducting element of the electrostatic ball probe used in the fluidized bed is a l/8 in. (3 mm) steel ball insulated over onehalf its surface so that only upward mass flow is registered. For the rotating pipe, a l/16 in.
_
circuitry.
conditions
of
the
Schmitt
trigger
showing
the
importance
of variable
threshold
voltage.
184 I
4. DISCUSSION
I
I
/ /'G
5 -6 x "4"E Y (r g 3-
-
z / 'o"/ +%
z B cz-
15
o"'& / / ,
370 Q/
s u
,4 $g*; ’ -gp I I
J OO
Somphng
I 4
A
h
Collechon.
5
g/cm2sx10-*
Fig_ 9. Mass flow probe calibration (numerals adjacent to data points designate nominal air velocity in m/s).
The direction for maximum count for mass flow gives the direction of mean flow of the particle phase with its speed given by the ratio of mass flow to the particle phase density. Further work will include determination of mass flow of each range of particle size by selective electronic filtration based on pulse heights. Applications of these probes to dense suspensions remain to be developed, and the upper limits of particle concentration for meaningful measurement by these probes remain to be determined. Measurements of reversing flow adjacent to the bubbling surface in the free board of a fiuidized bed will be the subject of a subsequent paper. REFERENCES
14 _ 12
:
:
l
:
-
.
IO
.
.
$08 Cl
. .
06-O
I
04-
-
I
Alumrum Oxide Particles 54-45pm W,,=23 7 m/s - s=o - s=o4 &=8 2 X 10e3 Kg/m’s
_
I 02 ----Pg 00
-10 ica of Pye
-05
r/r0
00
05
10 %s
Fig. 10. Typical measured results of particle mass flow in 127 mm diam. horizontal pipe flow system. Wu and Go are, respectively, the axial air velocity and particle mass flux at centerline. S = RDl2Wo. D is the pipe diameter and S2 is the angular velocity of rotation_ Measurement station is one pipe diameter downstream of the rotating section_
(1.6mm) steel ball is used to ensure adequate insensitivity to flow direction. Figure 10 gives a typical set of measurements in pipe flow with and without a rotating section using the ball probe.
1 B. T. Chao, S. T. Leung, H. Perez-Blanco, J. H. Saunders and S. L. Soo, Proc. Int. Conf. on Pneumatic Conveying (Invited Lecture), Int. Powder Inst. (London), 18 Jan., 1979. 2 S. L. Soo, G. J. Trezek, R. C. Dimick and G. F. Hohnstreiter, Ind. Eng. Chem. Fundam.. 3 (1964) 98 - 106. 3 S. L. Soo, J. J. Stukel and J. M. Hughes, Environ. Sci. Technot.. 3 (1969) 386 - 393. 4 J. H. Daniel and F. S. Bracket& J. Appl. Phys.. 22 (1951) 542 - 554. 5 K. Min. B. T. Chao and M. E. Wyman, Rev- SciInstrum.. 34 (1963) 529 - 531. 6 D. Van Zoonen, Proc_ Symp. on Interaction between Fluids and Particles. Inst. Chem. Eng. (London), 1962, p_ 64. 7 R. E. Rosensweig, H. C. Hottel and G. C. Williams, Chem. Eng. Sci., 15 (1961) 111 - 125. 8 S. L. Soo, Fluid Dynamics of Multiphase Systems, Blaisdell Publ. Co., Waltham, Mass., 1967. 9 J_ C. Haas, M.S. Thesis, Univ. of Illinois at Urbana-Champaign, 1968. 10 ‘I’. K. Siegel and B. Woertz, Ind. Eng. Chem.. 31 (1939) 1034 - x041. Designer’s 11 Hewlett Packard Co., Optoelectronics CataIog, 1978. 12 L. Cheng and S. L. Soo, J. Appl. Phys.. 42 (1970) 585 - 591. 13 L. Cheng, S. L_ Soo and S. K_ Tung, J. EngPower, 92 (1970) 135 - 149. 14 S. L. Soo and J. A. Regalbuto, Can. L Chem. Eng.. 38 (5) (1960) 160 - 166. 15 I. Langmuir and K. Blodgett, Tech. Rep. 5418, Air Materiel Command, U.S.A.A.F. (1946).