image processing Study of bubbly two-phase flows in a veritical pipe using ultrasound and

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Nuclear Science and Technology, Vol. 3, No. 3 (2013), pp. 13-24

Study of bubbly two-phase flows in a veritical pipe using ultrasound and image processing

Nguyen Tat Thang'-*, Hiroshige Kikura'", Duong Ngoc Hai''^'"', Hideki Murakawa*"""

'institute of Mechanics, Vietnam Academy of Science and Technology (VAST), 264 Doi Can, Ba Dinh, Hanoi, Vietnam

'Email: [email protected]; [email protected]

^ Tokyo Institute of Technology, 2-12-10-okayama, Meguro-ku, Tokyo, Japan

^ University of Engineering and Technology, VNU, l44XuanThuy, Cau Giay, Hanoi, Vietnam '"Email: [email protected]; [email protected]

''Kobe University, I-l, Rokkodai, Nada-ku, Kobe, Hyogo, Japan

""Email: [email protected] (Received 9 October 2013, accepted 2 December 2013)

Abstract: Two advanced methods, i.e. multiwave UVP (Ultrasonic Velocity Profile) and PIV (Particle Image Velocimetry) for measurement of bubbly two-phase flow in a vertical pipe have been developed. These methods are non-intrusive and able to measure instantaneous velocity profiles and 2D velocity fields of pipe flows. Thanks to measured instantaneous velocity profiles, characteristics and behaviors of two-phase flow in a pipe can be clarified. Test flow loops have been recently built at the Laboratory for Industrial and Environmental Fluid Dynamics, Institute of Mechanics, VAST. Air and tap water are used for generating bubbly counter-current two-phase flow. Measurements have been successfiilly conducted at varied liquid and gas superficial velocities in bubbly flow regime (void fi-action less than 10%). Measured results are highly useful for both practical and theoretical study of two-phase flows in engineering systems, especially in nuclear power plants. Moreover, the multiwave UVP measurement system, which is highly compact and able to work with opaque fluid and containers, can be applied to practical and online measurements in systems under operation.

Keywords: two-phase flow, bubbly flow, ultrasonic velocity profiler, image processing, nuclear power plant

I INTRODUCTION simulation and experimental measurement Numerical simulation based on approximate Two-phase flows are usually solution of mathematical models using PC encountered in various industrial and has been continuously improved. It has engineering systems. One typical example is advantages such as being possible for a wide two-phase flows in piping systems in thermal range of flow parameters etc. However, hydraulic system of nuclear reactors and numerical models have to adopt many kinds nuclear power plants. The characteristics and of assumptions including approximation, structures of two-phase flow in such systems averaging and model closure. Especially, have direct correlations with many scientific multi-phase flow models have to rely on and technical problems such as safe and experimental data for various kinds of optimization operation, and control of the correlations among phases. As a result, whole system. Hence, thorough experiments on two-phase flows play a understanding of two-phase flow has highly tremendously important part. Major important practical implication. Methods of advantage of experimental study is the studying pipe flows include numerical possibility to confimi measured data directly.

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STUDY OF BUBBLY TWO-PHASE FLOWS IN A VERITICAL PIPE USING..

It is possible to obtain flow parameters precisely. The data can be used not only for detailed study of liquid flow, for calibration and validation of numerical models but also for boundary conditions of the models.

Therefore, properly measured data is highly important to build up knowledge and data bank for two-phase flow [1].

Experimental methods for velocity measurement can be classified into intmsive methods and non-intrusive ones. In intmsive methods, insertion of measuring probes into the flow field must be implemented. Hence, intrusive effects cannot be neglected and experiment setup tends to be complicated.

Therefore, it is preferable to develop non- intmsive measurement methods such as laser Doppler anemometry (LDA), ultrasonic velocity profile (UVP), particle image velocimetry (PlV), nuclear magnetic resonance (NMR) etc. LDA is a point-wise measurement which requires an optical window. UVP is for velocity profile measurement. PIV and NMR methods can obtain results in 2D and/or 3D. However NMR method uses bulky hardware for safety protection and usually needs many operators for measurements. Therefore, it is not suitable for measurements in small experimental sites, and industrially on-site conditions. Hence, UVP and PIV methods are more usefiil measurement methods in both research and engineering [2]. The principle of UVP method which was originally applied to single-phase flow, is extracting velocity information carried in ultrasound when it is reflected from moving scatterers. Usually, micro scatterers are seeded into the flow fleld to facilitate the reflection of ultrasound. This method uses an ultrasonic transducer (TDX) which is excited by a pulser/receiver (P/R), to emit ultrasonic pulses. Echo sound is received and converted

into voltage signal by the same TDX. The signal is filtered and amplified by the P/R. It is then digitized using an analog to digital converter (ADC) and transferred into a PC.

Signal processing techniques are applied to the received signal to extract flow velocity along the sound path. Since ultrasound can penetrate various materials, UVP method can be used with opaque fluid and non-transparent containers [3, 4]. On the other hand, PIV method is basically based on optical principle to obtain flow images of seeding particles. The flow is usually exposed to a laser light sheet.

Statistical techniques such as auto-correlation and cross-correlation etc. are applied to the captured images to extract velocity of the flow. Therefore, PIV method must use an optical window for imaging the flow field. As a result, it cannot be applied to opaque liquid and containers 15].

In two-phase flow, instantaneous velocity profiles of bubbles and liquid are essential for analysis of turbulence characteristics, instantaneous void fraction distribution, bubbles' behavior and so on. These parameters have direct relationship with heat- mass transfer, neutron kinetics of nuclear reactors, safe and optimized design and operation of various thermal engineering systems. Therefore, in this study, for measurement of instantaneous velocity profiles of two-phase flows in a vertical pipe, we have developed a multiwave UVP measurement method which is based on a multiwave TDX [6]. The multiwave TDX is able to emit and receive two ultrasounds of 2 MHz and 8 MHz center frequencies simultaneously at the same position. 2 MHz frequency is used for measurement of gas. 8 MHz frequency is used for that of liquid. The signal processing method is based on the

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NGUYEN TAT THANG, HIROSHIGE KIKURA, DUONG NGOC HAI. HIDEKI MURAKAWA ultrasonic pulsed Doppler (UDM) method.

This measurement method is referred to as the multiwave UVP method. Test flow loops have also been developed for generation of single- phase and two-phase flows in vertical pipes of 26 mm l.D. (for single-phase flow only) and 50 mm I.D. (for both single-phase flow and two-phase flow). For two-phase flows, various counter-current flow regimes can be generated including bubbly flow, slug flow, annular flow etc. At this stage, we focus on measurement of bubbly counter-current flow currently generated in our test flow loop. In counter- current flow, velocity of bubbles and liquid has opposite directions. Therefore, it is easier to confirm measured results. However, all description of related techniques is applicable for both co-current and counter-current flows.

Co-current flow will be considered in next study. The multiwave UVP method has been used for measurement of bubbly flow regime with low void fraction to avoid excessive multiple reflection of ultrasound. Measured results were verified using PIV method. A statistical method which calculates the probability density function (PDF) of velocity of both phases at everj' measurement position has been applied for data analysis. It is confirmed that velocity of each phase has been successfully measured. Based on measured data of instantaneous velocity profiles of two- phase flows, analysis of flow characteristics is possible.

This paper has six chapters. Chapter I is Introduction with statement of background, problem, method and study objectives.

Chapter II mentions the development of measurement methods including multiwave UVP and PIV. Chapter III is about the specifications of measurement systems, test

flow loops and experimental conditions.

Chapter IV describes measured results of bubbly counter-current flow including average velocity profiles, velocity PDF of bubbles and liquid. Chapter V is for discussion. Chapter VI shows conclusions of this study.

n. DEVELOPMENT OF MULTIWAVE UVP METHOD

When UVP method is applied to bubbly two-phase flow, ultrasound will be back scattered by not only micro particles but also by liquid-gas interfaces. Normally, sound is very strongly reflected by gas-liquid interfaces. Therefore, bubble and liquid data both exist in measured result. It is difficult to clearly classify between liquid and gas velocities, especially when they are not much different. On the other hand, it has been observed that, in UVP method, when the measurement volume size changes, the measured data also changes. The measurement volume of UVP method is a cylindrical shape bounded by ulU-asonic beam, and has thickness equal to A^il/2 where A'^is the number of wave cycles of emitted pulses; A is the wavelength of the ultrasound. Using two different ultrasonic frequencies, it is possible to create measurement volumes of the same thickness (i.e. same spatial resolution) for measurement of two-phase flow. Difference in the volume sizes is only due to different beam widths. Experiments showed that, if an ultrasonic beam of diameter 10 mm and center frequency 2 MHz is used, it will mostly measure velocity of bubbles whose diameter is several millimeters [6]. At the same time, if an ultrasonic beam of diameter 3 mm and center frequency 8 MHz is used, measured result will mainly show liquid velocity. Based on this 15

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STUDY OF BUBBLY TWaPHASE FLOWS IN A VERITICAL PIPE USING..

idea, the multiwave UVP metiiod for measurement of bubbly two-phase flows has been developed.

A. Multiwave TDX

The multiwave TDX has a cylmdrical shape with a piezoelectric element of 8 MHz basic ultrasonic frequency, 3 mm diameter installed in the center. A piezoelectric element of 2 MHz basic frequency with hollow shape is set along the central axis. The inner diameter is 3 mm and the outer diameter is 10 mm. Each element is connected with BNC via lead wu-es (Fig. 1). The piezoelectric elements are made of composite oscillator with PZT and plastic. An epoxy resin with half thickness of the wavelength A is used as acoustic matching layer.

Since the composite oscillator has low acoustic impedance, the energy loss is low at acrylic wall surface comparing with the other materials. Furthermore, the element has an advantage that is difficult to decrease the sensitivity. An epoxy damper is set behind the elements. The multiwave TDX emits and receives ultrasounds independently for basic frequencies of 2 and 8 MHz. This unique design TDX enables signal processing to calculate velocity of liquid and gas simultaneously at the same positions

Fig.L Multiwave TDX.

B. Pulser/receiver and data acquisiticHi unit Usually, the ultrasonic pulsed Doppler method is used with a tone burst P/R for the excitation of ultrasonic TDX. However, we have proved that, by using low-cost, popular spike P/Rs in ultrasonic testing industry, measurement of instantaneous velocity profiles using pulsed Doppler method is also possible [7]. Therefore, two spike P/Rs, which were synchronized, have been used to excite two piezoelectric elements of the multiwave TDX simultaneously. After emission, reception of echo sound of both frequencies occurs at the same time. The captured signals of 2 and 8 MHz frequencies are converted into voltage signals by the corresponding piezoelectric elements respectively. Voltage signals then are amplified and filtered by the P/Rs. For data digitization and acquisition, a two channel, high speed ADC is used to connect with the two P/Rs. It is also synchronized with the P/Rs for data acquisition. Digitized signals of both channels are transferred into a PC for online signal processing to obtain instantaneous velocity profiles.

C. UDM signal processing

UDM method for single-phase flow has originally been developed by Takeda [8].

Recently, Kikura et al. [9] has succeeded in developing UDM method for bubbly two- phase flow. The principle is briefly described here. Based on the known sound speed in the working liquid, the range-gated technique calculates the distance, from the TDX surface, corresponding to the coming echo sound wave. As a result, corresponding to consecutive time delays of the reception of echo sound, measurement positions (measurement volumes or channels) along the

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NGUYEN TAT THANG. HIROSHIGE KIKURA, DUONG NGOC HAI, HIDEKI MURAKAWA sound path are completely determined. For the

calculation of velocity at a channel, a number of echo waves from the channel are analyzed to obtain Doppler shift fi^quency {fj) which is used to calculated flow velocity. Therefore, several emitted pulses must be used. Though each echo wave from a measurement channel does contain Doppler effect as a result of the movement of fluid, it is currently impossible to extract Doppler shift fi-equency by using just only one echo wave from a channel. The reason is mainly due to the limitation of die speed of the digitizer. UDM method uses a number, typically 16, 32, 128, 256 and so on, of echo waves to calculate velocity at a measurement channel. These number are a power of 2 due to the fact that fast Fourier analysis (FFT) is usually adopted in the signal processing, e.g. the quadrature demodulation process. In pulsed Doppler method, maximum measurable velocity v^^v 'S limited by the pulse repetition frequency Fptf due to Nyquist sampling theorem which states that fd<Fpr/2.

In addition, maximum measurable depth of UDM method also depends on Fprr which defines the pulse repetition duration

Tprf=}/Fprr. Echo sound can only be received up to some distance from the TDX surface (reception time restricted by 7^^.

D. Phase sqiaraticm mediod

In two-phase flow, ultrasound is back scattered by both seeding particles and gas- liquid interfaces. Since the echo signal from gas-liquid interfaces is much stronger than that from seeding particles, with an appropriate small gain setting for 2 MHz TDX, 2 MHz echo signal will contam only signal reflected from bubble surfaces. Therefore, it is assured that 2 MHz signal includes only bubble data.

However, 8 MHz frequenc\' signal always

contains bubble data if bubbles cross tiie 8 MHz sound beam. Therefore, bubble data must be eliminated from liquid data to obtain velocity profiles of only liquid.

For bubble data elimination, relation between bubble's presence and measured velocity of 2 and 8 MHz are considered including a) No bubble detected by both frequencies; b) One bubble crossing 2 MHz beam; and c) One bubble crossing 8 MHz beam.

i Multiwave 8MHz ultrasounc

UulUwave t , ^ ^ . Bubble vetDdly (ZMHz) 8MHz ultrasounc '

f W d velocity ( 8 V - l z )

j _ _ MWliwav BMHz ullrasounC

FlowwW>

BubCle " "

8MH2 uitiasoiind ofastiaciAti

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F ^ . Relative positions of a bubble and ultt^ound beams (left) and corresponding selection algorithms of valid liquid velocity (right).

In pattem a), only liquid velocity profile is measured by 8 MHz frequency. There is no bubble velocity (no bubble data) along the measurement line of 2 MHz frequency. If one bubble crosses the 2 MHz ultrasonic beam, case b), the bubble velocity is measured by 2 17

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MHz frequency; liquid velocity is measured by 8 MHz frequency at the same time (without bubble data since bubble does not cross the 8 MHz beam). When the bubble crosses the 8 MHz beam, case c), both frequencies will measure bubble velocity. In this case, 8 MHz frequency measures both liquid and bubble velocities. However, 8 MHz ultrasound is obstmcted by the bubble and echo signal coming from the area behind the bubble must be considered as noise. As a result, the measured velocity behind the bubble need to be deleted (Fig. 2). With the proposed algorithm for phase velocity selection, instantaneous velocity profiles of both liquid and bubble can be measured simultaneously along the measurement line. Using this method, relative velocity between bubble and liquid is no longer needed for the phase separation.

E. Verification of measured results using image processing

PIV method was used to verify measured results by multiwave UVP method. This method is based on digital image processing.

Its principle is now popular. A plane of the flow field is exposed to a laser sheet. Images of instantaneous distribution of seeding particles are captured using a digital camera.

The displacement ofthe particles between two adjacent exposures is estimated using statistical methods. Based on the particle displacement, velocity field of the flow is determined. For the estimation of velocity, particle images are divided into small rectangular areas known as interrogation windows. Statistical methods such as auto- correlation, cross-correlation etc. are applied to the interrogation windows to estimate particle displacement between two exposures.

Based on the displacement calculated for each interrogation window, spatially averaged flow velocit}' of interrogation windows is calculated [5]. In this study, an advanced Dantec 3D-PIV (stereo PIV) system in the Laboratory for Environmental and Industrial Fluid Dynamics, Institute of Mechanics, VAST has been exploited. The system includes a high power, pulse laser system (Dantec Dynamics, Nd-YAG laser, synchronized with PC and camera, maximum pulse power 1200 mJ, pulse emission time 4 ns, laser wavelengths 1064 and 532 nm), cameras (Dantec Flowsense EO 4M, high resolution 2048 x 2048, 8 bit grey scale, minimum 200 ns inter-frame time, maximum frame rate 20.4 Hz), and full function 2D/3D- PIV and digital image processing software (Dantec Dynamic Studio).

m . MULTIWAVE UVP SYSTEM, TEST FLOW LOOPS, EXPERIMENTAL SETUP

AND MEASUREMENT CONDITIONS A Multiwave ultrasonic UVP system

The multi-wave UVP system exploited in this study consists of:

Multi-wave TDX (Japan Probe Co. Ltd.):

2 MHz and 8 MHz center frequencies for emission and reception of 2 MHz and 8 MHz ultrasonic signal;

P/Rs (DPR300 and DPR35+, JSR Co., Ltd): each is connected to one TDX frequency element. Both P/Rs work in synchronization mode. The P/Rs generate negative electric spikes to excite the multi-wave TDX (both frequency components at the same time) with maximum pulse repetition frequency ( / ^ of 5 kHz, and minimum Fp,fOi 100 Hz;

Two-channel ADC (NI PCI-5112, 18

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NGUYEN TAT THANG, HIROSHIGE KIKURA, DUONG NGOC HAI, HIDEKI MURAKAWA National Instmment Co. Ltd.): 8 bits

resolution, maximum sampling rate 100 MHz, synchronized with both P/Rs.

PC (Intel Core i7 CPU, 4 GB RAM, Windows 7 OS): for hosting ADC and UDM signal processings and data analysis.

Fig3. Test flow loops for singe-phase and two-phase flow measurement.

B. Test flow loops

Test flow loops have been developed at the Laboratory for Industrial and Environmental Fluid Dynamics, Institute of Mechanics, VAST. A sketch ofthe flow loops is shown in Fig. 3 where: 1: floor tank; 2:

water flow-rate controlling valve; 3: bubble generator nozzle; 4: test pipe 50 mm I.D.; 5:

overflow weir of floor tank; 6: water circulation pump; 7: bypass valves; 8: pipe for water supply to the upper tank; 9: overflow weir of the upper tank; 10: upper overflow tank; 11: water box for housing ultrasonic TDX; 12: multi-wave TDX (Japan Probe Co.

Ltd.); 13: drainage pipe for overflow water down to floor tank; 14: water drainage valves;

15: main water drainage pipe; 16: air compressor; 17: main air flow-rate controlling valve; 18: air flow-rate regulator system (Cole

Parmer Co. Ltd.); 19: float valve for afr flow- rate measurement (Tokyo Keiso Co. Ltd.); 20;

P/Rs ofthe UVP system (JSR Ultrasonics Co.

Ltd.); 21: PC, ADC (National Instrument Co.

Ltd.) and UVP software etc.; 22: water flow- meter (Aichi Tokei Co. Ltd.); 23: test pipe (small diameter); 24: PIV camera; 25: high power pulse laser; 26: PIV software.

As shown in Fig. 3, there are two test pipes: large test pipe 4 with l.D. 50 mm, 3 m long and small test pipe 23 with l.D. 26 mm, 2.3 m long. Both are made of transparent acrylic that facilitates UVP measurement and optical flow visualization for PIV method.

Because the ratio of pipe length to pipe l.D. of large test pipe 4 is not enough for fiilly developed laminar flow, it is used for measurement of fully developed mrbulent single-phase flow and two-phase flow.

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/->^.--<

a)^^samm b)^^=^H c)i

Fig.4. Typical two-phase flow regimes observed m the flow loop with 50 mm I.D. pipe using a high speed camera (JVC Hybrid Camera GC-PXl, JVC Co. Ltd.) with fi^me rate 300 Hz (a: bubbly flow; b: slug

flow; c: annular flow).

Small test pipe 23 has higher ratio of pipe length to pipe I.D., it is used for measurement of fully developed laminar and turbulent single-phase flow. Water flow-rate in the test pipe 4 is controlled by a needle-type valve located at the outlet of the pipe, and is measured using a high precision propeller- type flow-meter. A funnel is attached to the inlet of test pipe 4 to facilitate the fully developed state of the flow in the pipe (Fig.

3). Overflow weirs in water tanks ensure a constant pressure difference between the tanks. For bubbly counter-current flow, bubbles are generated at the bottom part ofthe test pipe 4, and rise up. Water flows down from the upper tank to the lower tank solely under the effect of gravity.

from the pipe inlet to the position ofthe TDX;

D is the pipe iimer diameter. The water box is used also for optical visualization of the flow fleld and for PfV method.

C. &q)aimental setiq) and measurement omditions

Measurements were carried out in atmospheric pressure, and at room temperature. The multi-wave TDX was fixed at an inclined angle 45 degree [6] to the main flow direction using a TDX holder. The thickness of pipe wall at the measurement locations was I mm. Nylon powder (WS- 200P, Daicel-Evonic, Ltd.) with average diameter 80 pm, density 1.02 g/cm^ was used as ultrasonic scatterers for measurement of liquid velocity and for PIV particle image.

Measurement conditions are shown in Table I.

For test pipe 4, the multi-wave TDX is housed in a water box surrounding the test pipe. The position of the water box can be varied with L/D where L is the pipe length

Table I. Measuremrait conditions for bubbfy counter-current two-phase flow.

Liquid superficial velocity Gas superficial velocity

Liquid inlet length/Z). Gas inlet length/Z?

Average bubble size Results

Spatial/temporal resolution Measurement fi^quenc)' (both phases)

129 mm/s 0.68 mm/s 46,15 4-5 mm

- Measured result compared with that of PIV - Statistical assessment usin^ velocity PDF 0.75 mm / 4 ms

18 Hz

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nmUMAKAXA

NGUYEN TAT THANG. HIROSHIGE KIKURA, DUONG NGOC HAI, HIDEKI MURAKAWA

.1

IV. MEASURED RESULTS A typical bubble image ofthe flow which has been measured is presented in Fig. 5.

In this case, measurements have been carried out using both UVP method (for both phases at the same time) and PIV method (for gas phase only, i.e. without seeding particles).

Using the multi-wave UVP method, simultaneous measurements of liquid velocity and gas velocity have been conducted.

Instantaneous velocity profiles of liquid phase and gas phase have been obtained. Using instantaneous values, average velocity profiles of separated liquid phase and gas phase are shown in Fig. 6 (a). At present, it is not possible to conduct phase separation technique with PIV method based on our facility, measurements of only gas phase (without seeding particles) using our PIV method have been carried out.

In Fig. 6, r i s the radial distance from pipe center; ro is the pipe radius. In addition, Fig. 7 and Fig. 8 show velocity probability density fiinctions (PDFs) ofthe results measured by 8 MHz and 2 MHz frequencies ofthe muhiwave TDX. PDFs are shown for all measurement channels across the pipe diameter. As shown

in Fig. 7 (a) and (b), most of time, 8 MHz frequency measured liquid velocity (which has minus sign in this flow configuration). As predicted, some bubble velocity (positive velocity in this case) has been captured by 8 MHz frequency. Going away from the TDX surface (corresponding to higher channel numbers), the bubble frequency shows an accumulation. This might be due to the fact that, when a bubble crosses 8 MHz frequency beam, velocity measured by 8 MHz frequency in the far area behind the bubble shows fluctuation. Positive velocity will be captured as bubble velocity. In addition, in the distance area of the TDX, the noise level increases.

Such data must be eliminated using a phase separation method.

Fig^. A typical bubble image by high speed camera.

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Fig.6. Measurement of two-phase flow: (a) liquid and gas velocily profile measured by UVP method;

(b) gas velocity profile measured by UVP and PIV methods

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Fig.7. Velocity PDFs of 8 MHz frequency before phase separation: (a) 2D PDF disuibution; (b) PDF at the cemer ofthe pipe.

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Fig. 8. Velocity PDFs of 2 MHz frequency for gas phase: (a) 2D PDF distribution; (b) PDF at the center ofthe pipe compared with PDF of 8 MHz frequency.

At a single measurement channel at pipe center, the velocity PDF measured by 8 MHz frequency is shown in Fig. 7 (b). It shows clearly that mostly, liquid velocity is measured (highest peak is at around -180 mm/s), and a small part of PDF distribution is measured gas velocity (another small peak is at around 180- 200 mm/s). Similarly, the distribution of the PDF of measured velocity by 2 MHz frequency is shown in Fig. 8 (a) and (b). It is confirmed that mostly, 2 MHz frequency measured gas velocity. However, liquid velocity cannot be completely neglected by 2 MHz frequency, especially in the area close to the TDX surface. The PDF distribution measured by 2 MHz frequency at a selected channel at pipe center is depicted and compared with that measured by 8 MHz

frequency in Fig. 8 (b).

In order to obtain profiles of only liquid velocity and profiles of only gas (i.e. complete phase separation), the phase separation method proposed (section n.4) has been applied. Results after phase separation of 8 MHz frequency are shown in Fig. 9 (a) and (b). In Fig. 9 (a), it can be clearly seen that almost all bubble data has been eliminated from 8 MHz frequency. Fig. 9 (b) fiirther shows the liquid velocity distribution after phase separation. Only one peak of the velocity PDF exists ai around mean liquid velocity. The peak value increases as compared with the same peak before phase separation.

Therefore, it is confirmed that instantaneous velocity profiles of both phases have been measured.

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NGUYEN TAT THANG, HIROSHIGE KIKURA, DUONG NGOC HAI, HIDEKI MURAKAWA

Fig.9. VelociQ' PDFs of 8 MHz frequency after phase separation: (a) 2D PDF distribution; (b) PDF at the center ofthe pipe.

V. DISCUSSIONS

Using our muhiwave UVP method, instantaneous velocity profiles of both liquid and gas of bubbly counter-current fiow were obtained and separated. Results were verified using PIV method. By analysing the velocity PDFs of both phases, the measured results of instantaneous velocity of two-phase flow using multiwave UVP method have been confumed.

However, it can be seen that 2 MHz frequency still includes some liquid data. In addition, 8 MHz data also includes some velocity which is assumed as gas velocity but it does not correspond to any velocity measured in 2 MHz frequency. Note that 2 MHz frequency is assumed that it will capture all bubbles appear in the sound beam. Such velochies in 8 MHz data cannot be discriminated after applying phase separation algorithm. In order to increase the acciuacy of the measurement of two-phase flow, it is important to:

Completely avoid measiu^ment of liquid data in 2 MHz frequency or using some algorithm to eliminate liquid data from gas data measured by 2 MHz frequency;

Eiminate some velocity which seems to be gas velocity measured by 8 MHz fiequency but does not relate to any data in 2 MHz data.

In addition, it should be noted here that UVP method measures one-dimensional velocity of moving objects (i.e. velocity component in the soimd path direction). Three dimensional motion of bubbles may add some error to the measured results (Fig. 6 (b)). Three dimensional effect is stronger in the case of bubbly counter-current flow than the case of co- current flow. Elimination of 3D effect is one improvement of measurement accuracy in our future study.

VI. CONCLUSIONS Multiwave UVP method and PIV method for measurement of two-phase flow in a vertical pipe have been developed. Measured results using multiwave UVP method have been confirmed by PIV method. It has been shown that multiwave UVP method has many advantages and robustness for two-phase flow measurement.

Instantaneous velocity profiles of bubbly counter current two-phase fiows have been measured using multiwave UVP method.

Measured data have been analysed using velocity PDF of both phases. The PDFs confirms the measured data of each frequency before phase separation. By applying phase separation algorithm proposed, the PDFs show 23

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STUDY OF BUBBLY TWO-PHASE FLOWS IN A VERITICAL PIPE USING...

that separate instantaneous velocity profiles of each phase have been obtained. Further analyses will be carried out using measured data of both multiwave UVP method and image processing method in order to derive other two-phase flow characteristics such as local void fraction distribution etc. Based on measured results of instantaneous velocity profiles by multiwave UVP method, flow structures of two-phase fiow can be obtained.

ACKNOWLEDGEMENT The first author is grateful to Japan Society for the Promotion of Science - JSPS (within the framework ofthe JSPS Ronpaku - dissertation PhD program) and the scientific research project VASTOl.01/13-14 of Vietnam Academy of Science and Technology for financial support to carry out this research.

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[3]. Malcolm J. W. Povey, Ultrasonic techniques for fluids charactOTzation, Academic Press, 1997.

[4]. D. H. Evans and M. W. Norman, "Doppler ultrasound: physics, instrumentation, and signal processing", Chichester, Wiley, 2000.

[5]. Raffel, Markus, Christian E. Wiliert and Jijrgen Kompenhans, "Particle image velocimetry: a practical guide". Springer Verlag, 1998.

[6]. H. Murakawa, H. Kikura, and M. Aritomi,

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[7]. T. T. Nguyen, H. Murakawa, N. Tsuzuki and H. Kikura, "Development of muhiwave method using ultrasonic pulse Doppler method for measuring two-phase flow", Joumal of Japan Society of Experimental Mechanics, accepted for publication, 2013.

[8]. Y. Takeda, "Velocity profile measurement by ultrasound Doppler shift method". Int. J. Heat Fluid Flow, 7,pp.313-318, 1986.

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"Velocity profile measurement in bubbly flow using multi-wave ultrasound technique".

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