Velocity measurements in bubbly two-phase flows using laser doppler anemometry

,

,

VELOCITY MEASUREMENTS IN BUBBLY TWO-PHASE FLOWS USING LASER DOPPLER ANEMOMETRY

TEr.H~:SCHE BOnr2WDOl GElFT

VLiEGTUIGSCUWKUNDE rm~UC7~ :~EK Kluyverwzg 1 - DELFT T~r.J1!2jSC~!E HûEESC:-:COl C!:LIT VLiEGTUIG~DUWKUNDE ES3UOTHEEK K~uyv8rweD 1 - CELFT January,

1973.

(PART I) by

t,

APR.

1973

W. E.

R.Davies

VELOCITY MEASUREMENTS IN IDBBLY TWO-PHASE

FLOWS USING LASER DOPPLER ANEMOMETRY

(PART I)

by

W.E.R.

Davies

Submitted October, 1971.

Acknowledgement

The author wishes to express his thanks to Dr. I. I. Glass for suggesting the project and for his enthusiastic support throughout the work.

This research was made possible using the UTIAS facilities under the direction of Dr. G. N. Patterson and a financial contract from Atomie Energy of Canada Ltd

(AECL).

The assistance from Mr.

D.

D'Arcy ef AECL is very much appreciated.

Summary

Optical instrumentation utilizing the laser Doppier method has been employed to measure the velocity of steam and water in a two-phase flow. The apparatus is relatively simpie, not unduly sensitive to vibration and may be constructed to cover a large range of flow velocities.

1. 2.

3.

4.

5.

6.

" TABl@ OF CONTENTS , Sumrnary INTRODUCTION

'1'HE LASER DOPPLER METHOD

2.1 Local Oscillator Heterodyne Detector 2.2 Differential Doppler Detector

TWO-PHASE FLOW HETERODYNE MEASUREMENT TECHNIQ,UE EXPERIMENTAL APP ARATUS

RESULTS

&

DISCUSSlONS '

5.1 Recommenàati0n f0r Improving the Experimental Arrangement

SIGNAL HANDLING METHODS C ONCLUS I ONS References Figures iv PAGE 1 1 1 2 2 3 3

4

5

6

7

- - - ----

-1. INTRODUCTION

The laser Doppler method (Fig. 1) for the measurement of flow velocities in liquids and seeded gas flows has gained rapid acceptance in the laboratory

and promises to evolve as a standard technique in the industrial measurement field.

The method is capable of high spatial res~lution, is independent of the temperature

_ of the flow medium and does not perturb its motion. It mayalso be constructed to

cover a wide range of velocities. The instrumentation constant can be easily cal;

culated from the optical parameters making it an absolute technique with inherent linearity.

The laser Doppler method has been extensively covered in the open

lit-erature (see, for example, Refs. 1 to

5).

A great number of reports from various

research organizations are also available. For completeness a brief phenomenolo-gical description of the method is given.

2. THE LASER DOPPLER METHOD

When two light-beams, derived from the same* laser are focussed by a lens, (Fig. lb) to a common focal point, interference fringes are produced over a volume of coherence defined by a diffraction ellipsoid (qualitatively the common

volume at the intersection of the two beams). A particle traversing this region

will scatter radiation in apolar intensity pattern, which will be a function of the size of the scatterer. Figure lc is a plan view of the interference pattern viewed

perpendicular to the plane of the diagram.

For any fixed observation angle the scattered light intensity will appear modulated, as the particle traverses the interference fringe pattern. This modu-lation frequency, termed the Doppler heterodyne frequency (f

D

) ,

is a function of

the velocity component v of the particle and the fringe spaclng À/2sin(~/2)and is

gi ven by Eq. 1.

fTI=

2v sin9

X

/2 ( 1)

In this equation v is the velocity component in the direction k~ k~,

sc 1

Fig. la where k and k. are wave vectors specifying the direction of the two

sc 1

beams emanating from the focus. The laser wavelengthÀ and the intersection angle

e

should be measured in air.

There is a 1800 ambiguity in the velocity as measured by this tecpnique,

i.e~, a particle traversing the focal volume, will exhibit the same f

D regardless

whether it travels with a velocity

~

or

~

through the

foc~l

region.

2.1 Local Oscillator Heterodyne Detection

When the detector is placed as shown in Fig. la the system is operating

in what is termed 'the local oscillator heterodyne mode'. In ~his c~nfiguration

the intensity of the local oscillator is usually of the order of 10- of the .

strong beam. The precise attenuation is selected to give optimum modulation of

* In principle two separate lasers could be employed, however, available systems

do not have adequate frequency stability for this purpose and beat signals would result from the laserts frequencies drift.

the beat signal.

The light reaching the detector is a composite of radiation received

directly from the iaser and that arriving due to a scattering particle. The

inten-sity level of the local oscillator is such that it effectively swamps background

and shot-noise. This coherent detection system enables extremely small signals

to be measured af ter the De component has been rejected.

Apertures Sl and 8

2 are not vital to the operation of the system.

How-ever, it is good optical po11cy to use them to reject background radiation. The

optumum solid angle of view is automatically defined when all the incident laser

light in the weaker beam is collected.

2.2 Differentia1 Doppler Detection

This method, proposed by Mazumder and Wankum (Ref. 2) is sketched in

Fig. lb. In this case two beams of equal intensity are focussed to a common point

by the lens L10 Scattered light may be collected from any location which views

the focal reglltm, such as shown by the lens L

2• The stop S~. merely defines a

virtual image at the focus of Ll' serving tb reduce background radiation which

would appear as noise.

In this particular configuration heterodyning takes place between the

Doppler shifted scattered light from each ~eam. The Doppler sensitive direction,

given by k;c- k~ (F~g. la) is independent of the location of the col1ection lens

L

2 which may be placed anywhere in space with an unrestricted solid angle. The Doppler frequency f

D is again given by Eq. 1.

3.

TWO-PHASE FLOW HETERODYNE MEASUREMENT TECHNIQUE

For an understanding of the apparatus, to be described later, it is

convenient to consider the flowing water as a fluid carrying both bubbles of gas

and natural contaminants. The contaminants act as scatterers and are always either

present, or acquired in operation, in sufficient concentration to enable velocity

measurements to be made, even in distilled water. These particles are usually

smaller than the focal volume and except in extremely turbid media do not cause

undue interference by their presence outside the focal volume~

The passage of a scattering particle through the focus region witl

re-sult in a signal burst modulated both by f

D and the Gaussian intensity distribution

across the laser focus. Figure 4a shows a trace of such a signal. In practice

there may be more than one particle present in the focal volume at any ane time.

This will result in the superposition of the single burst signal from each particle

which may combine either constructively or destructively.

The duration of a burst wave train signal is dependent on the size of

the foca1 volume, as weU as the size and transit time of the particle. The

uncertainty ~fD of f

D in the frequency domain is similarly dependent on these

para-meters. It is pertinent to reca~l that the transform of even a perfect sine wave of finite length produces a Gaussian distribution of frequencies, the width of

the Gaussian being inversely re1ated to the wave train length.

The technique employed in this report cannot discriminate between bubbles

in the flow as produced by a fairly large void that adequate discrimination can be made. The identification of a bubble is made by amplitude analysis of the scattered signal.

The mechanism by which bubble scattering takes place in the present

method has not been determined, it may be due to Fresnel reflection àt the

non-uni.form gas-water interface or the coating of liquid contaminants surrounding the bubble. This is not of immediate consequence, since the signal from a bubble manifests itself either as a signal increase or decrease in the optical detector. This in turn depends.J(()ll whether the àetector' s view of the focal region is illumi-nated or occluded by the passage of the bubble, respectively.

4.

EXPERIMENTAL APPARATUS

The appàratus employed in this investigation is depicted in Fig. 2a and a photograph of the system is shown in Fig. 2b. The output (~ lmW) from a He-Ne

(À = 6328~) laser (Spectra-Physics Mod. 132) is split i~to two equal intensity beams which are then rotated into a vertical plane by the Dove prism and subse-quently focussed by lens Ll into the observation region in the fluid flow.

Beam No. 2 continues on through the fluid and its intensity is monitored by detector No. 1. The output of this unit is used to determine the liquid

velo-city taking advantage of the natural contaminants in the liquid. Lens L2' F~g. 2, is ana~ogous to L

2 in Fig. lb - it receives light that is backscattered from the focal region via lens Ll and mirror M2' Detector No. 2 has its sensitivity adjusted so that only scatterers above some predetermined size produce a useful signal.

In the tests carried out the fluid frow was provided by a copstant head

recirculation system. A pyrex tube (id 0.420

1n,

od 0.520 in, 24 in long) was used to confine the flow; viewing through the walls as shown, and traversing along the Y-axis, contributed minimal distortion. Gas bubbles were generated by means of electrolysis using two electrodes immersed in the liquide Steam-hubbles were obtained using a 1500 W immersion heater and a 1500 W heater tape, the latter

wrapped around the pyrex tube to just below the observation region. This was founa. to produce a uniform flow of steam hubbles.

5. RESULTS & DISCUSSIONS

Lens Ll was fitted to a traversing mechanism enabling velocity determi-nations to be carried out aD,rany point in a plane section of the flow perpendicular to the tube axis. The Hagen-Poiseuille velocity profile was calculated using the appropriate parameters for this experiment and experimental points are shown for the same conditions (Fig. 5). These points were obtained by reducing single sweep oscilloscope traces from detector No.~ (see Fig. 4a). For these measurements an ,

attenuator (10-2) was p~aced in the local oscillator beam before the flow.

The signa1 from detector No.2 was used to determine the bubble veloci~y.

Figure 4b is a sweep from a dual beam oscilloscope showillng the output signa1s from both detectors simultaneously. The upper trace from detector N02 shows a signa1 burst heterodyned at the Doppler frequency associated with the bubb1e velocity. The lower trace shows the simultaneous output from detector No.2; the passage of ~he bubble manifesting itself as a drop-out of the signal (downgoing signals

corresp~nd to an increase in light on the detectors).

The measurements in Figs. 4a and 4b were taken using an intersection angle 8 = 6.70• The Doppler frequency f

D =

49

kHz, derived from Fig. 4b (upper trace) corresponds to a velocity of 26.6 cm/sec. A weak modulation is still pre~ent on the lower trace, also at 49 KHz indicating that in this particular instance the bubble and water velocity were identical.

5.1 Recommendations

The optical arrangement used in the present experiment can be simplified and made more flexible. Figure 3 is a sketch of a suggested system,

The detectors should ideally be photomultipliers with their quantum efficiency maximized at the laser wavelength. (The S20 is the most suitable cathode for·the 6328~ line). Most photomultipliers have an upper frequency limit of about 300 MHz.

The beam splitter generates 3 beams; two of equal intensity for the differential Doppler measprement and a weaker beam for use as a local oscillator for detector No. 2.

The angle 8 (Fig.2) may be varied by choosing the appropriate beam splitter/lens Ll combmnation. The choice of the optimum value of 8 for each set of experiment al. conditions is important. Tqe frequency range whiii!h the detectors and electronic processing must handle is governed by the size of 8. It is also worthwhi le noting that the angle 8 is the dominant factor in defining the "y" dimension of the coherent volume. The other dimensions (for small values of 8) are governed predominantly by the focal length of the lens Ll and the laser beam size and divergence.

Table 1 lists two sets of test conditions employed in the study carried out here, the axes are defined in Fig. 2.

Table 1

.

Test Conditions

LTeome~r1.cal. AX1.al lJ1.menS1.ons 01·

-1 Coherent Volume

e dia of Focal d d Laser

Laser Beam Length L

e

6x=d"f. ó.y = sin 8/2

4.z

= s<bs8/2 Power

.J..

0.56 mm 115mm 6.70 140 I-L 2400 I-L 140 I-L ~l mW 0.56 mm 50 mm 150 60 I-L 450 I-L 60 I-L ~30 mW

*

(d is the e -1 amplitude diameter of the laser beam at the focus of Ll' inside the liquid filled tube, c.f. Ref. 6).

It is relevant to point out that increased spatial resolution is bought at the price of a higher f

D and more laser power. Detector No.l receives light _ that is scattered from the more intense beam and mixes this with thé reference light from the we aker beam. However, the radiation scattered from the observation region has apolar intensity pattern containing a sharp maximum in the forward direction. This type of scattering is covered JçYGuhe Mie theory, and it is known that the polar distribution is a function of the particle size for certain fixed parameters.

It is possible therefore, by employing a seeded flow with the appropriate seed particle diameter to scatter more light into the off-axis angles, thereby permitting the use of larger values of 9. Alternatively, an increase in 9 can be tolerated in an unseeded flow by raising the input laser power to the point where the scattering at the chosen angle is detectable.

6.

SIGNAL HANDLING METHODS

Electronic processing of the signals from the detectors may take several forms, a few of which are discussed below. The final choice for any system will depend on the nature of the flow being studied and the economics associated with the particular problem. A rough estimate of the cost of each method is given, although only items (a), (b) and (c) are c0mmercially available ~t the present time. --The predicted costs of items (d) and (e) include development.

(a) Oscilloscope:

A dual beam (not switched beam) , storage scope is ideal for measuring simultaneous outputs from detectors No.l and No.2. Data reduction fr0m single sweep traces is tedious and the use of only individual scans results in a great

waste of the information available. Synchronization of the signal on a repetitive basis is only possible for very high signal to noise ratios because of the hetero-dyned pulse train length.

The oscilloscope is highly recommended and is almost indispensable in ensuring that real heterodyning signal are being observed when setting up any system. (Cost: $1,

oqO.

min.).

(b

y

Spectrum Analyzer:

This instrument is essentially a swept frequency receiver. Thus only a finite time is spent observing each frequency interval. Because of the finite temporal length of the scattered wave train, any display of the photocurrent in the frequency domain, will result in a frequency spread. Thus, evan a laminar flow with a zero velocity ~radient will not be sharply defined in the frequency spectrum.

However, in some special situations (for example, where a distribution of velocities is present) it might be advantageous to obtain photographic records of the spectrum analyzer display and thus produce time averaged velocity distri~

butions • (Cost: $2,500. min). (c) Tracking Bandpass Filter:

This device will fo~low a signalof varying frequency pnce the signal falls within the bandpass of the filter. Because 0f noise problems and the finite wave train length this instrument has been used with very limited success for processing laser Doppler Signals. (Cost: $4,000. min).

(d) Frequency Conversion:

With appropriate bandpass filtering it is possible to design an inex-pensive, efficient, real-time analyzer that will handle quasi-continuous signals. Such a system requires good signal-to-noise conditions and is not well suited to t4e measurement of heterodyned transient signals.

It should be borne in mind that some of the transient scattering signals

reaching detector No.2 are not modulated. These can be created by an interface

which passes just outside the coherent volume; such unmodulated signals will cause

_ errors in a simple processing system. (Cost: $500. min).

(e) Discriminator:

The time-varying heterodyne signal is superimposed on waveferms associated with each scatterer and it is necessary to filter out these lower frequency com-ponents before signal handling can be carried out using discriminator techniques. Using pulse-height discrimination and appropriate circuitry the heterodyne signal may be converted to uniform pulses for direct or ratio counting.

An addi tional benefit accrues::from the discriminator method in that the

spatial resolution can be improved by selecting portiàms 0f pulse trains only above a pre-set amplitude. This ensures that processed signals originate from the central portion of the coherent volume.

This approach to signal handling has probably the greatest potential for

accuracy and flexibility in that almost any type of signal, continuous

or

transient

may be handled. However, because of the wide range of experimental conditions

that can be envisaged it is difficult to assess the cost of a system of this type.

(Cost: $5,o.Qo.. min).

7.

Co.NCL US Io.N8

The instrumentation described in this report can be carried to various levels of sophistication. A basic system using simple optics, 93lA type

photo-multipliers, a 1 mw laser and no signal handling would cost less than $1,0.00.. It

is stressed however that some form of automatic readout is highly desirable and for many types of flow would require the expenditure of about $9,000. This

com-bined with an industrial type optical system using 820. type photomultip~iers

would raise the total cost to around $12,0.0.0..

Any of the systems mentioned can be utilized to give velocity information not readily attainable by other methods and with the more elaborate instrumentation would prove the laser Doppler technique to be a powerful diagnostic tool in two-phase flows.

Undoubtedly new applications of the method would evolve with its use, for example, v0id size, count and fraction may be one possibility.

1. DeLange, O. E.

2. Mazumder, M. K.

Wankum, D. IL.

3. Huf'f'aker, R. M.

4. Greated, C.

Durrani,

'r. S.

5. Wilmshurst, T .. H.

6. Aàrian, R. J.

GOldstein, R. J.

REFERENCES

"Optical Heteredyne Detectie n", IEEE Spectrum 1968,

pp. 77-85.

IEEE J, of Quantum Electronics, June 1969, pp.316-317.

Applied Opties, Vol. 9, No.5 (1026), May t970, pp.

1026-1039.

J. of Phys. E. Sci. Insts., 1971, Vol.

4,

pp.24-26.

J.

0:f.

Phys. E. Sci. Insts., 1971, Vol.

4,

pp.77-80.

LASER

BEAM

SPUTTER

,

. WAVE

FRONTS

FIG. la LOCAL OSCILLATOR DOPPLER VELOCITY SYSTEM

DETECTOR

'

ELLIPTICAL

BOUNDARY

OF

INTERFERENCE

PATTERN

FORMED THROUGHOUT

COHERENT

FOCAL

VOLUME

LASER

M

_z

M._ I < ,

<

DOVE PRISM ---./

'1

---I--~--I

I

I

I

ld

Lzt-~-~

\

/

_ V _

DIFFERENTlAL DOPPLER DETECTOR 2

(meosures

bubble velocity )

L.

FIG. 2a SKETCH OF ÉXPERIMENTAL EQUIPMENT

x

LOCAL OSCILLATOR HETERODYNE DETECTOR I

(meosures liquid flow

velocity )

,

y

ï

e

O---HEATER - - - BUBBLE GENERATOR

tv

FIG. 2b VIEW OF EXPERIMENTAL EQUIPMENT

(a) He-Ne Laser; (b) Pyrex Pipe in Recirculating Flow System; (c) Lens L1; (d) Lens L2; (e) Detector No. 1;

(f)

Detector No. 2.

DETECTOR 2

----+- _____

"\..1 _..._- ...-...-I ~~

I

I

TRAVERSE

I I

~

_{\ I} \ I \ I \ / \ I

.

v.

t

= 0

o o o o 0 o 0 o • o o

DETECTOR

BUBBLE

_.

FIG.

3

SIMPLIFIED TWO-PHASE FLOW INSTRUMENTATION

FIG. 4a

FIG. 4b

HETERODYNE SIGNAL FROM WATER. VELOCITY 29 CM/SEC

SWEEP 100~SEC/CM, f

D - 54 KHz.

UPPER TRACE: REFLECTED SIGNAL FROM BUBBLE SURFACE . DETECTOR #2 IN FIG. 2

LOWER TRACE: SIGNAL DROP-OUT DETECTOR #1 IN FIG. 2 SWEEP 50~SEC/CM, f

o lIJ N 1.0 0.8

;i

0.6 :E a::

o

z

>-~

u

g

0.4 lLJ

>

0.2 • EXPERIMENTAL

HAGEN - POISEUILLE FLOW

MAXIMUM VELOCITY : 23.4

cm / sec

O~~----~----~---~----~---~----~---~----~~

- 0.2 - 0.1 0 0.1 0.2

DISTANCE FROM TUBE AXIS ( INCHES)

FIG. 5 LAMINAR PIPE FLOW PROFILE

I1rIAS TECJlllICAL NarE NO. 184

Institute for Aerospace Studies, University of T oronto

VELOCITY MEABtnIDIElITS IN BUBBLY TWO-PHABE rLCIWS tEIKl LASER DOPPLER ANDIa.lETRY (PART I)

Daviea, W. E. R. 7 pages 5 figures 1 table

1. Laser 2. Ane~try 3. Laser Doppler 4. Flow Measurements I. Davi.s, W. E. R. II. I1rIAS Technical Note No. 184

Optical instrumentation utilizing the laser Dappler method has been emplo)'ed to measure

the veloei t)' of steam and water in a two-pboae flow. The apparatua is relati vel)'

simple, not unduly aensltlve to vibratlon and may ba cODstructed to cover a large ranse

of flow veloeitiea.

~

I1rIAS TECJlllICAL NarE NO. 184

Institute for Aerospace Studies, University of T oronto

VELOClTY MEABtnIDIElITS IN BUBBLY TWO-PHABE rLCIWS tEIl«) LASER DOPPLER ANDIa.lETRY (PART I)

Davies, w. E. R. 7 pagea 5 figurea 1 table

1. Laser 2. Anemometry 3. Laser Doppier 4. Flow Messurements I. Daviea, W. E. R. II. I1rIAS Technical Note No. 184

Opt1cal 1nstrumentat1on util1zing the laser Dappler method has been emplo)'ed to measure

the veloe1t)' of steam and water in a two-phase flow. The apparatus is relat1vel)'

simple, not unduly sensltlve to vibration and 1IJI!J.y be cODstructed to cover a large range

of flow veloc1t1ea.

~

Available copies of ~his report: are limi~ed. Re~urn ~his card ~o UTIAS, if you require a copy. Available copies of ~his repor~ are limi~ed. Re~urn ~his card \0 UTIAS, if you require a copy.

I1rIAS TECJlllICAL NarE NO. 184

Jnst:i~ute for Aerospace Studies, Universi~y of T oron~o

VELOCITY MEABtnIDIElITS IN BUBBLY TWO-PlIASE rLCIWS tEIl«) LASER DOPPLER ANDIa.lETRY (PART Il

Davies, W. E. R. 7 pages 5 figurea 1 table

1. Laser 2. Anemometr)' 3. Laser Dappler 4. Flow Measurements

I. Davies, W. E. R. II. I1rIAS Technical Note No. 184

Optical instrumentation utilizing th. laser Dappler method has been emplo)'ed to meaaure

the veloclty ot steam and water in a. two-phaae flow. The apparatuB is relatively

simple, not un.duly aenaltive to vibratlon and may be conatructed to cover a 1arge range

of fiOlf velocit1ea.

~

... _,,,,, , ':1

Available copies of this report: are limited. Re~urn ~his card ~o UTIAS, if you require a copy.

I1rIAS TECJlllICAL NarE NO. 184

Ins~i~u~e for Aerospace Studies, University of T oronto

VELOClTY MEABtnIDIElITS IN BUBBLY TWO-PlIASE rLCIWS WIl«) LASER DOPPLER ANDIa.lETRY (PART Il

Davies, W. E. R. 7 pages 5 figures 1 table

1. Laser 2. Anemometr)' 3. Laser Doppler 4. Flow Meaaurementa

I. Dav1es, W. E. R. II. I1rIAS Technical Note No. 184

Opt1cal instrumentation utilizing the laser Dappler method has been emplo)'ed to meaaure

the velocity of steam and water in a two-phase flow. The apparatus is relatlvely

simple, not unduly sensitlve to vibration and llJAy be constructed to cover a large range

ot nov veloci ties.

~