FLOW PATTERN IDENTIFICATION

The simplest way to determine the gas-liquid flow pattern is to merely observe them flowing along transparent pipes. This allows for personal judgement and interpretation which prevents the flow patterns from being objectively and precisely ascertained. Where this is not feasible because of high gas and liquid flow rates, high speed photography is often employed. These methods are of no use within actual industrial pipeline that are generally not transparent Hernandez- Perez (2008). Other techniques are briefly described below.

The method of Hubbard and Dukler (1966) for flow regime determination involved the use of spectral analysis to examine the observed pressure fluctuations. The approach depends on the concept that the gas/liquid flow patterns were characterised by fluctuations in wall pressure. The power spectral density (PSD) of digitised time response obtained from a pressure transducer

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located flush to the wall of the flow pipe was computed using the autocorrelation method. Three types of power spectral distributions were obtained and used to categorise the various flow regimes measured for horizontal air/water pipe flows. These are illustrated in Figure 2.8 and are as follows: (1) separated flows; containing a peak at zero frequency; this type of response is obtained from stratified and wavy flows, (2) dispersed flows; possessing a flat and relatively uniform spectrum and (3) intermittent flows; with a characteristic peak; this is obtained for plug and slug flows.

Figure 2.8: Flow identification by Power Spectrum Density of pressure gradient Hubbard and Dukler (1966). Adapted from Hewitt (1978)

The conclusions of this research work represented the first attempt to objectively classify flow patterns and was followed by the studies performed by Nishikawa et al. (1969) and Kutataledze (1972). Subsequent investigations, including that of Tutu (1982) and Matsui (1984), analysed the time variation of pressure gradient and pressure fluctuations respectively. Tutu (1982) used the probability density distribution to identify the flow patterns observed in vertical flow systems. However, Keska and Williams (1999) concluded that the pressure system they investigated did not offer an improved flow pattern recognition method when

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compared to capacitive and resistive systems. Jones and Zuber (1975) pioneered the use of the photon attenuation technique, to measure the time-varying, cross- sectional averaged void fraction. This system utilised a dual x-ray beam device for a two-phase mixture of air and water, flowing vertically. It was found that the probability density function (PDF) of the void fraction fluctuations shown in Figure 2.9 could be used as an objective and quantitative flow pattern discriminator. Vince and Lahey (1982) obtained a series of chordal-averaged void fraction measurements using a dual beam x-ray system for low pressure air/water flow in a vertical tube. Their results were used to generate corresponding PDF and PSD functions of the recorded signals. It was concluded that the computed moments were sensitive to the velocity of the liquid phase. Costigan and Whalley (1997) further developed the PDF methodology of Jones and Zuber using segmented impedance electrodes and successfully classified flow patterns into six: discrete bubble, spherical cap bubble, stable slug, unstable slug, churn and annular flow as shown in Figure 2.10. From Figure 2.10, (i) a single peak at low void fraction represents bubble flow, (ii) a double peak feature with one at low void fraction whilst the other at a higher void fraction represents slug flow (iii) a single peak at low void fraction accompanied by a broadening tail represents spherical cap bubble whilst (iv) a single peak at a high void fraction with a broadening tail represents churn flow and (v) a single peak at high void fraction is defined as annular flow.

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Figure 2.9: Flow pattern identification by Probability Distribution Function of void fraction Jones and Zuber (1975)

Figure 2.10: Void fraction traces and corresponding PDFs from Costigan and Whalley (1997)

2.3.1 Electrical tomography:

The field of electrical tomography may be subcategorised into two distinct areas based upon the method by which the electric field is produced, using either electrical conductance or capacitance. The choice of the potential application of each method is based primarily on the electrical properties of the fluids, and in particular their conductance.

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2.3.1.1 Conductance tomography:

Conductance tomography consists of multiple conductance probes flush-mounted and evenly distributed around an interior section of the flow pipe. There are essentially two methods of measurements, either by the use of a constant current to measure the resulting potential at other electrodes, or by the application of a constant potential between two electrodes and to measure the resultant induced current. Since there is a need for the electrodes to be in direct electrical contact with the conducting fluid, tomographic imaging of certain flow patterns, for example slug and churn flows, cannot be achieved with this flush-mounted method.

To overcome this shortfall Reinecke et al. (1998) proposed an extension of the conductance approach which employs wire-mesh electrodes. Their arrangement, shown in Figure 2.11, consisted of three planes of 29 thin wires each with a diameter of 0.1 mm. The measurement planes are set 3 mm apart and the wires of two successive planes are offset by an angle of 60o.

By measuring the impedance between all pairs of adjacent wires in the same plane a projection of the conductivity distribution along the direction of the wires is obtained. For each plane, the impedance measurement is carried out with a high frequency (1000 Hz) alternating current, with the sampling of the individual electrode pairs performed by a multiplex unit. This process results in three independent projections, which are then transformed into a conductivity distribution and then further interpreted as the inherent void fraction distribution Hernandez-Perez (2008).

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Figure 2.11: Schematic representation of the measuring chain for wire mesh tomographic measurement technique by Reinecke et al. (1998).

Prasser et al. (1998) concluded that the main disadvantage of the approach developed by Reinecke et al. (1998) was the image reconstruction step, both in terms of the relatively large computational time and the undetermined nature of the equations needed to be solved. In view of this, Prasser et al. (1998) presented a new wire mesh sensor for fast tomographic imaging without the need for time consuming and potentially inaccurate image reconstruction procedures. The sensor, shown schematically in Figure 2.12, uses two electrodes planes 1.5 mm apart, one for transmitting and the other for receiving signals. Each plane consisted of sixteen 0.12 mm diameter electrode wires, producing a grid of 1616 measurements points evenly distributed across the pipe cross-section. The grid had a free area of approximately 96 %, with a negligible pressure drop. In one measurement cycle, the transmitter electrodes are activated by a multiplex circuit in successive order, Hernandez-Perez (2008).

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Figure 2.12: simplified scheme of the two-plane electrode-mesh devise used by Prasser et al. (1998)

2.3.1.2 Capacitance tomography:

Electrical capacitance tomography (ECT) technology is a process tomography technology that provides a new way to resolve void fraction measurement. The results of many research studies using this method have been reported in the literature. However, ECT technology is still at the stage of a developing laboratory research methodology. The ECT is a non-invasive technique since the sensing electrodes are not in contact with the fluid under observation but are located around the pipe exterior. The imaging parameter, the electrical permittivity, is the dielectric property of each of the phases in the two-phase system. An ECT image may be subsequently reconstructed based on the permittivity distribution obtained from the measurements made by the electrical capacitance taken between the possible pairs of electrodes, Hernandez-Perez

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(2008). ECT has recently been used by Baker (2003) to study two phase horizontal pipe flow.