Acoustic emission and gas-phase measurements in

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Acoustic Emission and Gas-Phase Measurements in Two-Phase Flow A Addali, S Al-lababidi, H Yeung, D Mba and F Khan Proceedings of the Institution of Mechanical Engineers, Part E: Journal of Process Mechanical Engineering 2010 224: 281 DOI: 10.1243/09544089JPME359 The online version of this article can be found at: http://pie.sagepub.com/content/224/4/281

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TECHNICAL NOTE

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Acoustic emission and gas-phase measurements in two-phase flow A Addali1 , S Al-lababidi1 , H Yeung1 , D Mba1∗ , and F Khan2 1 School of Mechanical Engineering, Cranfield University, Cranfield, UK 2 The Fluid Engineering Centre, BHR Group Limited, Cranfield, UK The manuscript was received on 15 February 2010 and was accepted after revision for publication on 12 April 2010. DOI: 10.1243/09544089JPME359

Abstract: The two-phase liquid/gas slug flow regime phenomenon can be encountered over a range of gas and liquid flowrates. Monitoring of slugs and measurement of their characteristics, such as the gas void fraction, are necessary to minimize the disruption of downstream process facilities. This article presents experimental results correlating acoustic emission measurements with gas void fraction in a two-phase water/air flow regime. It is concluded that the gas void fraction can be determined by the measurement of acoustic emission, which hitherto has not been investigated. Keywords: acoustic emission, slug flow, two-phase flow, gas void fraction

1

SLUG FLOW MECHANISM AND ACOUSTIC EMISSION

In the oil and gas production process, a multi-phase slug flow regime is normally encountered for a range of pipe inclinations and over a wide range of gas and liquid flowrates. Slug flow is characterized by a complex dynamic structure, which consists of aerated slugs of liquid that travel down the pipeline at the local gas velocity. The mechanism of slug initiation has been experimentally investigated by many authors [1–8]. The idealized picture of a ‘stable slug’ flow in a horizontal pipe is presented in Fig. 1. Section ‘F’ represents the front region of the liquid slug body (LSB) and section ‘T’ represents the tail region of the LSB. In a stable slug flow, the LSB length, LLSB , and the elongated bubble (EB) length, LEB , remain essentially constant in the downstream direction. The gas in the EB moves at a velocity VGEB , which is faster than the average mixture velocity Vmix, in the LSB. As a result, the liquid is shed from the back of the LSB to form the liquid film layer along the EB. The liquid in the film at the EB nose may be aerated. Also, the bubbles in the LSB coalesce with the EB interface and are gradually absorbed, which in the fullness of

time results in the liquid film becoming un-aerated. The mixture velocity is the sum of the liquid velocity and the gas velocity (VSL + VSG ). At the same time, the gas bubbles are fragmented from the tail of the EB and re-entrained into the front section of the LSB ‘F’ at a defined rate, GE . The fragmentation of the EB tail and the entrainment of the bubbles into the front section of the LSB are due to the dispersing forces induced by the flow of the liquid film as it plunges into the liquid slug front. From the description of the slug flow formation, dispersed gas bubbles can be generated in the LSB region through the formation, coalescence, breakage, and collapse of bubbles processes. The entrained gas bubbles in the liquid experience a transient pressure as they move through the hydrodynamic pressure field of the liquid. The transient pressure causes the gas bubbles to oscillate at their natural frequencies, one consequence of which is the generation of sound [9]. 1.1

Acoustic emission technique

email: [email protected]

Acoustic emission (AE) is used to describe the spontaneous elastic energy released by a process in the form of transient elastic waves. AEs are generated within a material manifest as elastic waves on the surface of the material and cover a broad frequency range typically from 20 kHz to 1 MHz [10]. As a non-destructive testing tool AE has been successfully applied to a range of industries [11–13]. In addition, the AE technology

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Proc. IMechE Vol. 224 Part E: J. Process Mechanical Engineering

∗ Corresponding

author: School of Mechanical Engineering, Cran-

field University, Building 52, Cranfield, Bedfordshire MK43 0AL, UK.

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A Addali, S Al-lababidi, H Yeung, D Mba, and F Khan

T

F Liquid Slug Body (LSB)

EB tail

Elongated Bubble (EB)

EB nose

GB GT

VT

VGEB

VLSB(L) VLSB(G)

GE

LLSB

Fig. 1

HLF

VLF

LEB

Schematic description of an idealized developed slug flow

has also gained considerable recognition as a complimentary tool for machine condition monitoring [14]. The earliest known references relating emitted sound from two-phase gas/liquid flow to the presence of air bubbles were published by Bragg in the early 1920s [15]. Thereafter, numerous investigations on the sound emitted from two-phase flow have focused on understanding the bubble characteristics, including bubble size and shape [16–19]. Studies on the dynamics of bubble behaviour [17–20] have shown bubble formation, bubble coalescence, and/or division result in bubble oscillations at the bubble resonant frequencies that are dependent on the radius of the bubble and mode of excitation. In two-phase gas/liquid flow regimes, gas bubbles entrained in the liquid will generate sound pressures when excited by external pressure fluctuations such as experienced within the slug. Such excitation, in addition to the formation, coalescence, breakage, and collapse of bubbles in the slug (as described earlier), will result in volumetric bubble oscillations at various modes of the bubble. All such pressure pulsations, dependent on the magnitude, will excite a broad frequency range extending to the AE spectrum. It is also known that the event of a bubble collapse results in the release of AE energy [21]. The objective of this study was to develop a correlation to predict the gas void fraction (GVF) in two-phase air/water slug flow as a function of the absolute AE energy and slug velocity.

Although several techniques are commercially available for measuring GVFs in two- and multi-phase flows, they are sometimes fraught with difficulty. Depending on the technique used, the accuracy of void fraction measurement may also vary. Gammaray attenuation and electrical impedance are the most generally adopted techniques for void fraction

measurements The gamma-ray densometer and conductivity rings suffer from saturated output signal at higher GVFs and require the construction of complicated, dedicated facilities. The signal of impedance methods (conductance probe) shows high sensitivity and dependency on liquid temperature, for instance a 1 ◦ C increase in temperature can increase the conductivity measurement by 2.5 per cent [22]. Other techniques such as dilation and photographic methods are suitable for laboratory and research use only. The separation method is the most traditional method used for void fraction measurements, where an expensive separator facility is employed to physically separate and then measure the two-phase flow components. The basic principle for electrical impedance methods for component fraction (void fraction) is measuring the electrical impedance or the dielectric constant of the mixed flow. Hence, the mixed flow is characterized as an electrical conductor, and by applying a well-calibrated relationship between the void fraction and the conductivity and permittivity of the oil, gas, and water components, the information about the void fraction of these components can be determined. Generally, measurements of the electrical impedance are carried out across the pipe diameter (using, e.g. contact or non-contact electrodes). The fast response of this method makes it possible to employ it for measurements during both steady-state and transient situations. However, it suffers from two significant limitations – it cannot be used at high gas fraction ranges and is flow regime dependent. Also, due to the nature of its online installation, it is considered as an invasive measurement technique that requires special arrangements in order to install it, unlike the proposed passive AE technique that is flow regime independent and non-invasive. Several other methods are described in the literature such as arc electrodes [23], ring electrodes [24], helical electrodes [25], and rotating field electrodes [26]. The application of AE in the measurement of GVF offers another measurement technique, but in this


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1.2

Conductivity measurement technique

Acoustic emission and gas-phase measurements in two-phase flow

regard the technology can be applied non-intrusively and rapidly, offering an option that is not available at present.

2

EXPERIMENTAL SET-UP AND PROCEDURE

A purposely built experimental facility that can simulate a two-phase flow was employed (see Fig. 2). The majority of the piping system was made from ABS (class E) pipes; however, two Perspex sections were installed to allow visual observations of the flow. Measurements of GVF were undertaken with conductivity probe section followed by stainless-steel pipe of 750 mm length and 8 mm thickness, thereby allowing a direct correlation between the AE measured from the

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stainless-steel pipe and the measured GVF. The flow loop pipeline was of sufficient length to allow the formation of fully developed slugs. Water was supplied to the flow loop using a centrifugal pump with a maximum capacity of 40 m3 /h and a maximum discharge pressure of 5 bar. The water flow was measured using an electromagnetic flow meter with 0–20 m3 /h range. Air was injected into the liquid flow through a 0.5 in (13 mm) pipe fitted with an airflow meter. The experiments covered a range of superficial water velocities (VSL ) of 0.3–1.2 m/s at increments of 0.1 m/s and superficial air velocities (VSG ) of 0.2– 1.4 m/s at increments of 0.2 m/s at each constant VSL (see Fig. 3). The VSL and VSG values were achieved by throttling the valves downstream of the flow meters, and every test condition was maintained for 120 s Air tank Air Flowmeter

compressor

½”

PP1 T T1

Air Air injection Mag Flowmeter P P3

T T3

Wat Water tan er tank k

Water

Pump

Conductivity sensor

Steel pipe

Fig. 2 Two-inch air/water horizontal flow test facility

Fig. 3 JPME359

Flow regime highlighting the test region Proc. IMechE Vol. 224 Part E: J. Process Mechanical Engineering

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Fig. 4 Two-inch test section (AE sensor and preamplifier) and conductivity sensor

during which period data were acquired for each test condition. All tests were undertaken at ambient temperature and pressure conditions. Comparisons between the conductivity sensor measurement and AE were undertaken for every test condition. The conductivity probe measuring section is a 0.5 m long Perspex pipe of 50 mm inner diameter, equipped with two pairs of flush-mounted ring electrode

Fig. 5

conductivity probes, as seen in Fig. 4. However, only one ring will be utilized in this investigation. The probes discussed here are of twin-ring electrodes type. They consist of two stainless-steel ring electrodes with a width of (Sp ) 3.7 mm and spaced at 17 mm (= De ) apart, as shown in Fig. 5. An electronic circuit is used to measure the electrical impedance between the electrodes. Probes based on this technique have been used by Andreussi and Bendiksen [27] and Fossa et al. [28]. Such probes can be operated either in the conductance (lower AC frequency) or in the capacitance (very high AC frequency) mode. Labview software and AT-M10 data acquisition card of National Instruments were used to acquire and store the data continuously in the computer hard disk. Calibration of the probe was performed by connecting the electrode pairs to the conductivity electronic box device that supplied a 7 kHz AC carrier signal. The aspect ratios of the probe De /d = 0.34 and Sp /d = 0.074 were chosen based on the design recommendations by Fossa [29], and d was equal to 0.05 m. The gas– liquid phase distribution was achieved by introducing

Scheme of flush-mounted stainless-steel conductivity ring electrodes

1 0.9 0.8

Liquid Holdup

0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

G* (Dimensionless Conductance)

Fig. 6

Calibration of conductivity ring probe


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known liquid volumes into the horizontally positioned test pipe. Tap water was used and great care was taken to check the inclination of the pipe at each measurement. A total of 48 measurements were performed in order to cover the liquid fraction range 0–1. At each measurement, both the weight of the water excluding the weight of the conductivity ring and the corresponding value in volts was recorded. As a result, a calibration curve for the probe was obtained, as shown in Fig. 6.

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The correlated liquid hold-up (E) as a function of the normalized output voltage was fixed at ‘1’ when the pipe was full and ‘0’ when the pipe is empty, resulting in the following relationship E = −1.489 × (G ∗ )4 + 1.475 × (G ∗ )3 + 0.368 × (G ∗ )2 + 0.623 × (G ∗ )

(1)

where G ∗ is the dimensionless conductance.

1.2

1 Film Region

Liquid Holdup

0.8 Slug Body

0.6

0.4

0.2

0 0

2

4

Fig. 7

6

8 Time (second)

10

12

14

Slug trace by conductivity probes

3.00E+03

2.50E+03

AE Abs. Energy (Atto joules)

Slug nose

2.00E+03

1.50E+03 Slug Body

Film Region

1.00E+03

5.00E+02

0.00E+00 0

1

2

3

4

5

6

7

Time (second)

Fig. 8 Typical AE signal from slug flow JPME359


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A sample trace collected from the conductivity probe under slug flow conditions is presented in Fig. 7. A commercially available AE acquisition system was used for acquiring the data from a Pico-type AE sensor with a broadband operating frequency range of 150– 750 kHz. The AE sensor was non-invasively mounted by means of industrial superglue onto the stainlesssteel pipe, as shown in Fig. 4. This orientation of the AE sensor was previously investigated and justified by Addali et al. [30]. Sensor sensitivity was evaluated using the pencil lead fracture technique [31]. The output AE signals from the sensor were pre-amplified at 60 dB and the AE absolute energy parameter (attoJoules, 10−18 ) was recorded at 10 ms sample rate. The absolute energy is defined as a measure of the true energy and is derived from the integral of the squared voltage signal (raw signal) divided by the reference resistance (10 k) over the sampling interval of the AE signal. A typical AE energy signal collected from the AE sensor under slug flow conditions is presented in Fig. 8. Clearly, the levels of AE energy increased with the passing of the slug over the sensor. Interestingly, the largest amplitude was associated with the slug nose and the lowest AE energy was associated with the fluid film region (see Fig. 8). AE levels associated with the slug body were less than that at the slug nose. These observations were as expected, given that the rate of bubble collapse and formation at the slug nose is higher than that in the slug body, which in turn is significantly higher than that in the fluid film.

3

RESULTS AND DISCUSSION

Having taken the measurement of AEs for various gas and liquid velocities, a correlation was then established between AE and GVF.

3.1

Gas bubbles in the slug body

Adamson [32] stated that the surface free energy per unit interfacial area is equal to the interfacial surface tension between a liquid phase and a gas phase. Assuming that gas bubbles are all spherical with a diameter, dbubble , the total surface free energy of the discrete gas bubbles (Esurface ) in the LSB was proposed by Brauner and Ullmann [33] as Esurface =

6σ dbubble

A(1 − ELSB )LLSB

(2)

where σ is the interfacial surface tension, A is the internal cross-sectional area of the pipe, LLSB is the length of the LSB, and ELSB is the liquid hold-up in the slug body. From reference [34], a critical bubble diameter, dbubble , is given as

dbubble

0.4σ =2 (ρL − ρG )g

1/2 (3)

where, ρL and ρG are the liquid and gas densities, respectively, and g is the gravitational force. The slug length LLSB was calculated as a function of the pipe diameter D [3] as LLSB = 32D

(4)

Values employed in estimating Esurface included: Slug length = 1.6 m and bubble diameter = 0.003 451 m Zhang et al. [35] assumed that the surface free energy of the discrete gas bubbles, based on the maximum amount of gas the liquid slug can hold, is proportional to the turbulent kinetic energy in the LSB. This assumption is used in this article to relate

4

Surface Free Energy (Joule)

3.5 3 2.5 2 1.5 1 0.5 0 0.1

0.2

Fig. 9

0.3 0.4 0.5 0.6 Gas Void Fraction Measurement by Conductivity Sensor

0.7

0.8

Surface free energy and measured GVF in the slug body


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AE to surface free energy. The GVF measured using the conductivity sensor is used to calculate the surface free energy in air/water slug flow conditions as per equation (2) and is plotted in Fig. 9. As expected, the relationship between the surface free energy and GVF is linear for the given slug length, bubble diameter, and interfacial surface tension. This is because the surface free energy is proportional to the amount of gas bubbles held in the slug body. From the experiments it was noted that at a fixed superficial water velocity, for example, VSL = 0.8 m/s,

287

increasing the superficial gas velocity resulted in an increase of the measured absolute AE energy (see Fig. 10). This was observed for all VSL levels investigated. This was not surprising and showed that an increase in the bubble content, and its associated bubble dynamics, resulted in an increase of AE generated. It was interesting to note that an increase in VSL for a fixed VSG resulted in a relative decrease in surface free energy, while a simultaneous increase in AE energy was observed. Figure 11 describes the relationship between AE energy measured from the

200

150

100 1.2

50

1.1 1.0 0.9 0.8

0

0.3

Fig. 10

0

0.4

0.6

0.5

0.8

V SG

1.4

0.6

1 1.2

0.7

/s )

0.4

L (m

0.2

VS

Abs Acoustic Energy (10^-18 Joule)

250

(m /

s)

Contribution of the liquid and air velocities on the increase of the measured absolute acoustic energy

250 Vsl=1.2 m/s

Vsg=1.4 m/s Abs Acoustic Energy (10^-18 Joule)

200

Vsl=1.1 m/s

Vsl=1.0 m/s

150 Vsl=0.9 m/s

Vsg=0.8 m/s 100 Vsl=0.8 m/s

Vsl=0.7 m/s

50

Vsl=0.6 m/s

0 0.5

0.7

0.9

1.1

1.3

1.5

1.7

1.9

2.1

2.3

2.5

Surface Free Energy (Joule)

Fig. 11

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Contribution of the turbulence kinetic energy on the increase of the absolute acoustic energy Proc. IMechE Vol. 224 Part E: J. Process Mechanical Engineering

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AE sensor and the surface free energy calculated from equation (2). This suggests that there are two mechanisms responsible for the generation of AE. When the water’s superficial velocity increases, the intensity of the turbulence diffusion increases and, as a result, the associated absolute AE energy increases even though the calculated surface energy decreases, as illustrated in Fig. 11. Similarly, an increase in the water’s superficial velocity reduces the GVF for a defined superficial air velocity; this GVF is directly correlated to the free surface energy. Therefore, the authors believe that there are two processes influencing the generation of AE: the free surface energy that is a measure of the bubble content in the liquid (air velocity) and the influence of turbulent diffusion

(turbulent kinetic energy) due to high superficial liquid velocities. In the latter instance, the reduction in AE associated with the surface free energy is less than the increase in AE associated with the turbulence diffusion process. Figure 10 illustrates the above-mentioned processes between liquid and air velocities, and the associated absolute AE energy. 3.2

AE and GVF correlation in the slug body

Figure 12 presents the measured GVF taken by the conductivity sensor and the associated absolute AE energy suggesting a non-linear relationship. Figure 12 provided the basis for establishing a relationship between the GVF as a function of the AE energy.

0.8 y = 0.1875x 0.3776

GVF (Measured by Conductivity)

0.7

Vsl=1.2 m/s

= 0.148x y = 0.1941xy0.3395

0.3375

y = 0.1241x 0.355 y = 0.0817x 0.4129 0.4591

y = 0.0571x

0.514

y = 0.037x

Vsl=1.1 m/s

0.6 0.5

Vsl=1.0 m/s

0.4

Vsl=0.9 m/s

0.3

Vsl=0.8 m/s

0.2

Vsl=0.4 m/s

0.1 Vsl=0.3 m/s

0.0 0

50

100

150

200

250

Abs Acoustic Energy (10^-18 Joule)

Fig. 12

Absolute AE energy and measured GVF

0.8 VSL=0.3 m/s

GVF predicted by AE Correlation

0.7

VSL=0.4 m/s VSL=0.5 m/s

0.6

VSL=0.6 m/s

0.5

VSL=0.7 m/s

0.4 VSL=0.8 m/s

0.3

VSL=0.9 m/s

0.2

VSL=1.0 m/s VSL=1.1 m/s

0.1

VSL=1.2 m/s

0 0.0

0.5

Fig. 13

1.0

1.5 2.0 Mixture Velocity (m/s)

2.5

3.0

GVF predicted from the proposed AE model, equation (5)


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1 This study

Beggs (1973)

0.9 Predicted Gas Void Fraction

0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Measured Gas Void Fraction by Conductivity

Fig. 14

Comparison of measured results with that of Beggs and Brill

A multiple exponential regression resulted in the following relationship c d ε = aAE b Vmix VSG

(5)

where ε is the GVF in slug body, a = 0.768, b = 0.003, c = −0.690, and d = 0.744. Figure 13 shows the obtained GVF in the LSB from the developed correlation (5) as a function of the absolute AE energy and slug velocities. The results of the proposed model, equation (5), were compared with a proposed model by Beggs and Brill [36], see Fig. 14 where a standard deviation of the proposed model and the data obtained from reference [36] of less than 9 per cent was obtained. Beggs’ model was developed for the whole spectrum of flow situations using 1 and 1.5 pipe sizes at various angles from the horizontal. The following correlation was developed by Beggs and Brill for the intermittent flow ε=

0.5351 0.845 ELSB 0.0173 Frm

(6)

where Frm is the Froude number, calculated as Frm = 2 (Vmix /gD). It must be noted that the developed model is based on the measured set of data for the VSL range between 0.3 and 1.2 m/s and VSG range between 0.2 and 1.2 m/s and at a fixed diameter. However, this is the first reported correlation, and it is envisaged that other investigators will address the influence of pipe diameters. 4

CONCLUSIONS

The applicability of the AE technology to measure the GVF has been demonstrated. A correlation is JPME359

developed as a function of the absolute AE energy and slug velocities for the range of liquid and gas velocities. In order to validate the applicability of the model for other flow and physical conditions, further experimental investigations are required. The work currently being undertaken investigates the GVF prediction for varying viscosity, pipeline orientation, and surface roughness of the pipe. © Authors 2010 REFERENCES 1 Kordyban, E. S. and Ranov, T. Mechanism of slug formation in horizontal two-phase flow. J. Basic Engng, 1970, 92, 857–864. 2 Graham, B., Wallis, G. B., and Dobson, J. E. The onset of slugging in horizontal stratified air–water flow. Int. J. Multiph. Flow, 1973, 1(1), 173–193. 3 Taitel, Y. and Dukler, A. E. A model for predicting flow regime transitions in horizontal and near horizontal gasflow. J. Am. Inst. Chem. Engng, 1976, 22(1), 47–55. 4 Mishima, K. and Ishii, M. Theoretical prediction of onset of horizontal slug flow. Trans. ASME, J. Fluids Engng, 1980, 102, 441–445. 5 Nydal, O. J., Pintus, S., and Andreussi, P. Statistical characterisation of slug flow in horizontal pipes. Int. J. Multiph. Flow, 1992, 18(3), 439–453. 6 Barnea, D. and Taitel, Y. A model for slug length distribution in gas-liquid slug flow. Int. J. Multiph. Flow, 1993, 19(5), 829–838. 7 Fan, Z., Lusseyran, F., and Hanratty, T. J. Initiation of slugs in horizontal gas–liquid flows. J. Am. Inst. Chem. Engng, 1993, 39, 1741–1753. 8 Hale, C. P. Slug formation, growth and decay in gas– liquid flow. PhD Thesis, Imperial College, London, UK, 2000. 9 Strasberg, M. Gas bubbles as sources of sound in liquids. J. Acoust. Soc. Am., 1956, 28(1), 20–26. Proc. IMechE Vol. 224 Part E: J. Process Mechanical Engineering

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