2 FIELD OF THE INVENTION
The invention relates to an optical prooe for multiphase flow.
The invent'on further relates to an optical probe, which can identify the flow patterns in two phase systems on the basis of the difference in optical properties of the phases. The probe according to the invention is suitable for liquid - liquid and gas-liquid systems and can also distinguish between separated and dispersed flows. Also, it is suitable for any flow geometry and gives an accurate estimation of the chordal average phase distribution due to its narrow probing area.
BACKGROUND OF THE INVENTION
Owing to the importance of flow patterns in understanding the hydrodynamics and heat and mass transfer characteristics in multiphase flow, several methods have been proposed in literature for identification of patterns. However, most of them have been used in gas-liquid flows and cannot be extended to liquid-liquid systems.
The commonly used techniques for gas-liquid flow are such as visual method, which is the simplest method for determining flow pattern is by visual observation of the flow through the walls of a transparent test section. The eye simply detects the variation of voids; the photographic methods are useful at high flow-rates in gas liquid flows.

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Photographic methods are useful as has been demonstrated by Raissan (1965). Hsu & Graham (1963), Bergles and Suo (1966) and Hewitt and Roberts (1969) at high flow-rates, in gas-liquid flows. This method enables the observations of the instantaneous local behavior. Video photography and its frame wise analysis can represent the lengthwise view. However, photographic methods are limited by the size of the field of view.
Method based on pressure measurement employ the measurement of pressure drop/gradient. Early attempts were used to characterize the change in flow pattern by observing the change in slope of the time averaged pressure gradient. A number of preconditions are necessary like frictional pressure drop should be small, there should be no drastic change in void fraction along channel length, manometer line should be filled up be a single phase fluid.
Methods based on photon attenuation give an idea of the chordal average void fraction. It includes the measurement of the attenuation of a single beam from a continuous X-ray source. The probability density analysis of the time varying signals, has been employed. The random signals from an IR source have been subjected to power spectral analysis. However, the risk of high-energy radiation is there. Moreover it is costly and gamma ray absorption has some limitations for liquid-liquid systems.
According to methods based on impedance, an impedance probe can operate either is resistance (conductance) or in capacitance mode. If the liquid phase is continuous and electrically conducting, then the probe is used in the resistive mode. If the gas phase is continuous or the liquid phase is non-conducting, then the probe is used in the capacitance mode. A large variety of probe designs are

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possible. Arc electrode probe is used to detect gas-liquid stratified flow. Ring electrode probe estimates the average void fraction across a cross-section while the grid electrodes are well suited for volume average voidage in bubbly flow. Parallel wire probe is suitable to detect annular, stratified and slug flow. Needle probe is popular for local void estimation. Multiple needles can be used to understand bubble geometry. In this technique prominent difference in conductance/capacitance between the phases is necessary. Further it does not have a very good applicability in liquid-liquid cases since the oil phase wets the probe and drastically affects its response even or wall mounted probes.
In probes using the hot film anemometer, the core of the anemometer is an exposed hot wire either heated up by a constant current or maintained at a constant temperature. The heat lost to fluid convection is a function of the presence of bubbles. By measuring the change in wire temperature the constant current or the current required to maintain a constant wire temperature, the heat lost can be obtained. The heat lost can then be converted into in proportion of void fraction in accordance with connective theory. This method has been used to detect cap bubbles in kerosene-water vertical flow. However, in order to obtain reliable results it is important to have an accurate calibration of the probe.
In optical fibre probes, the incident light, passing down the optical fibre, is totally reflected and returned back when the probe is in gas phase. So it is capable of detecting minute void and it is suitable for local measurement. Fibre-optic sensors are used for immiscible-fluid discrimination in multiphase flows. The method is not very useful for liquid-liquid flows.

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To overcome the limitations of local measurement, tomographic imaging techniques have been adopted to scan the entire flow passage. Tomographic imaging of a flow passage may be obtained by a variety of basic void measurement techniques like impedance, optical, radiation attenuation etc. Identification of flow regimes and estimates of some parameters of two-phase flow are possible through image reconstruction. X-ray and gamma ray tomography is used to detect gas fraction in gas liquid two phase flow. Electrical impedance tomography is used to detect bubble distribution in gas/liquid or liquid-liquid flow.
Ultrasonic flow meters measure the traveling times (transmit time models) or the frequency shifts (Doppler models) of ultrasonic waves in a pre-configured acoustic field that the flow is passing through to determine the flow velocity. In the same way, the presence of void fraction can also be determined. Ultrasonics is also being used to measure the flow film thickness. The technique adopted a rotating reflector, capable of measuring time-dependent spatial distribution of liquid film thickness around a simulated nuclear fuel rod. Ultrasonic transmission technique is based on a time of flight, to the measurement of spatial and time-dependent film thickness of a liquid film flowing over a horizontal plate in a rectangular duct. This technique is not very sensitive to the change in the thickness of one fluid layer or the presence of droplets.
Nuclear magnetic Resonance is also used to detect flow pattern by doping the liquid with a paramagnetic salt.

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It is known to use Electrical probes (conductivity/resistivity) or different geometries such as point electrode (Serizawa et al., 1975), arc electrode (Cheng et al., 2002), ring electrode (Andreussi & Bendiksen, 1989), wire electrode (Miya et al., 1971), strip electrode (Das & Das, 2002) have been widely used in two phase flows. Double sensor conductivity probes were used to measure local interfacial area (Wu & Ishii, 1999; Hibiki & Ishii, 1999; Hibiki et al., 2001). Lee et al., 2002 constructed a double tip conductivity probe to measure radial profiles of local void fraction and vapor velocity in a subcooled boiling flow of water through a vertical concentric annulus having a heated inner tube.
Mouza et al. (2000) described one photometric technique to determine liquid film thickness. The optical system employed in the work comprised of the following parts - a small sized diode laser source of high directivity and relatively high and stable light intensity and a silicon photodiode, which is used as light detector. The thickness of the liquid layer is determined by measuring the intensity of the laser beam passing from the pipe bottom to the liquid film. The light absorption is enhanced by adding methylene blue dye to water.
In horizontal liquid-liquid flow, Valle and Kvandall (1995) measured the insitu flow configuration and local phase holdup using conductivity probe. They studied flow patterns in detail with the use of wall mounted conductivity probes and a sampling tube and observed entrainment on one phase into the other and the onset of the stratified wavy-entrained pattern. Nadler & Mewes (1995) studied flow pattern along with phase continuity by conductivity probe. Local phase fraction was measured by Vedapuri et al., (1997) by isokinetic probes. They also

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recorded responses for different flow regimes. After crossing the initial hurdles of instrumentation in horizontal liquid-liquid flow, conductivity probes as well impedance probes were in vogue for the estimation of flow regime or for the determination of local phase holdup (Kurban et al, 1997, Valle & Utvik, 1997, Angeli & Hewitt, 1998, Angeli & Hewitt, 2000, Lovick & Angeli, 2004, Angeli & Hewitt 2002). Soleimani (1999) used high frequency impedance probe and gamma densiometer to estimate phase distribution and flow pattern. Multi point sampling probe was used by Fairuzov et al. (2000) to measure the average volume fraction. The identification of the dual continuous flow pattern boundaries was achieved with the use of impedance and a conductivity probe by Lovick & Angeli (2004). They used two probes namely a conductivity probe and a high-frequency impedance probe. The impedance probe gave the distribution of the two phases in a pipe cross section while the conductivity probe provided phase continuity information and showed whether one (dispersed flow) or both (dual continuous flow) phases are continuous. From the above studies, it was evident that stratified flow with a complete separation of the liquids may prevail for a limited range of relatively low flow rates where the stabilizing gravity force due to a finite density difference is dominant.
The flow patterns and phase fraction in liquid-liquid inclined flow have been identified by conductivity or impedance probe (Vigneaux et al., 1988, Flores, 1997, Angeli et al., 2002).
Due to prevalence of dispersions, studies were focused to drop size distribution in vertical liquid-liquid flow. A wide variety of measurement techniques were employed, such as a laser diffraction technique, a laser back scatter technique.

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Photographic techniques, an image processing technique to determine the sizes of water droplets injected into a continuous oil phase.
The main drawback with most of the popular processes is the intrusive nature of detection. This disturbs the flow phenomena and is particularly disadvantageous for liquid-liquid cases where the oil/organic phase often wets the probe and alters its response. Further the conductivity probe has been reported to be suitable only when the water phase is continuous and fails for oil continuous flow patterns. Hence, the need remains to develop an instantaneous and non intrusive method for determination flow patterns in two phase systems.
OBJECTS OF THE INVENTION
It is therefore, an object of this invention to propose a probe for multiphase flow, which is non-intrusive.
It is further object of this invention to propose a probe for multiphase flow, which does not depend on the physical properties of the fluids.
Another of this invention to propose a probe for multiphase flow, which can differentiate between separated and dispensed flows,
Yet another of this invention to propose a probe for multiphase flow, which gives instantaneous response and is accurate.
These and other objections of the invention will be apparent from the essentially description, when read in conjunction with the accompanying drawings.

9 SUMMARY OF THE INVENTION
Accordingly, there is provided a non-intrusive optical probe releasably along the cross-section of a tubular test-piece (Ts) constructed of perspex enabling visual observation of multiphase flow phenomena occurring inside the tube (Ts), the flow-rate, mixing ratio, flow-pattern including associated parameters of the multiphase flow system being individually generatable, measurable, separable and controllable by the devices of said multiphase flow system, the optical probe identifying the flow patterns including estimating the depth of the multiphase flows in stratified flow and the bubble characteristics in bubbly flow, the probe comprising a laser source adaptable as a source of monochromatic laser light, a photodiode sensor disposed on the tubular test piece (Ts) at a location being diametrically opposite to the laser source for receiving the light incident; an operational amplifier converting the light incident to voltage signals and arranging amplification of the output-voltage signals; a data acquisition means operably connected to the operational amplifier for continuous recordal of the data in respect of the voltage signals; and a post processing computer device having a memory means with pre-stored data in respect of signals corresponding to individual and combined flow at different velocities, the computer device receiving the real-time data from the data acquisition means for comparing with the pre-stored data, thereby outputting the flow patterns of the multiphase flow.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
Figure 1 shows the optical probe according to the invention;

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Figure 2 shows the operational amplifier circuit of the optical probe of Figure 1;
Figure 3 shows signals for smooth stratified flow and dispersed droplets;
Figure 4(a) show the schematic diagram of the devices in a horizontal (a) and
& (b) vertical (b) rig test for liquid-liquid and gas-liquid systems;
Figure 5 a pictorial view of the devices in the horizontal and vertical set-up;
Figure 6(a) show the probe signals and their PDF curves for different patterns; &(b)
Figure 6(c) show the probe signals and their PDF curves at different mixing
& (d) ratio;
Figure "/(a; show the three layer pattern from stratified wavy at a constant
& (b) kerosene velocity;
Figure 7 (c) shows the pattern of figure 7(a) but with different kerosene/water flow;
Figure 8 shows the probe signals and PDF values for 'oil dispersed in water' and 'annular flow';
Figure 9 shows patterns for slug flow (a) and churn flow (b).

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DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION
As shown in Figure 1, the optical probe comprises of:
a) A point semiconductor laser source (1), having 2 mW ~660 nm
wavelength and 2 mm beam diameter, which is used as a source of
monochromatic laser light.
b) A photo diode sensor (2) located at the diametrically opposite point to
detect light after its passage through the test section (Ts). Its test
conditions are Vce=5V; H=2mW/cm2 and the operating temperature
range in -55C to 125C. The detector is placed inside a dark box to omit
the effect of external light source.
c) An amplifier arrangement (3) after the detector (2). The detailed circuitry
of the amplifier is shown in figure lb. The operational amplifier (3) is a
key building block in analog integrated circuit design. The OPAMP (3) is
configured with several transistors and passive elements (resistors and
capacitors) and arranged such that its low frequency voltage is very high.
With this OPAMP and several resistors and capacitors the output has been
amplified. The details of the processing circuit (3) is shown in figure 2.
Figure l(a) shows a horizontal scanning by the optical probe which may
also be adapted in lieu of the vertical scanning shown in Figure 1.
d) A data acquisition means (4) transfer the signals to a computer (5).

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The light incident on the photodiode (2) is converted to a voltage signal and recorded continuously in a PC (5) via the data acquisition means (4) through the processing circuit (3). The signal depicts the variation in the intensity of light falling on the diode (2). Signals have been pre-recorded for single phase water and kerosene flow through the pipe, and stored in a memory in the PC (5). A higher voltage is obtained for only water as compared to that of only kerosene flow. It has been noted that the same value of voltage has been obtained for all velocities of either phase thus indicating that the amount of light attenuated by the individual liquids is independent of its velocity. All the signals obtained for two phase flow are normalized with respect to Vmax, the voltage obtained for pure water flow, to facilitate a comparative study.
The amount of light incident on the photodiode (2) depends on the fraction absorbed and scattered by the two phase mixture. The amount attenuated depends on the absorption coefficient of the fluids. It is higher for blue kerosene as compared to water. Therefore, for a water-kerosene mixture, the amount attenuated increases with the increase in the kerosene fraction in the pipe. Apart from attenuation, scattering becomes predominant with the onset of the droplets and wavy interfaces. Typical signals obtained for smooth stratified flow and dispersed droplets are shown in Figure 3. In smooth stratified flow lack of disturbances at the interface gives a smooth response whereas in dispersion scattering of light causes a fluctuating lower voltage response. Thus, the optical probe has been observed to be a very effective tool for flow indicator to identify the flow patterns and transitions. It is a relatively simple, non-intrusive, reliable and inexpensive method.

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Further, the probe can be rotated along the periphery of the test-piece (Ts) to obtain the radial variation of voidage at any cross-section. It can also be.slided in a "tuning fork type of arrangement" through the axial height of heightwise scanning of the distribution. Determination of liquid height in stratified flow and measurement of bubble velocity and dimensions can be possible by this gadget. The presence of Taylor bubble or spherical bubble and its swarm velocity in a vertical tube can be determined by the use of two such probes at a known distance apart.
The probe can be used in both a horizontal and vertical peripheral scanning for liquid-liquid including gas-liquid systems. Water and blue kerosene is selected as the test fluids for liquid-liquid systems and air-water as the fluids for gas-liquid systems. The schematic diagram of the devices adapting the probe is shown in figures 4(a) and (b). Both the devices has an entry section (ENS), a test section (TS) and an exit section (EXS) in order, in the direction of flow. The test liquids are pumped through separate rotameter tanks (WR, KR) from their respective storage tanks (WT, KT) to a mixing section (M). The flow rates of the two fluids are separately controlled by a set of control valves (QVi, QV2)
The test section (TS in figure 4) is 0.0254 m) in diameter and 2.13 m in length. The device has been constructed of perspex to enable visual observation of the flow phenomena occurring inside the tube (Ts). The two fluids flow to a separator tanks (WT, KT) using the effect of gravity. Two separate pumps (PI and P2) draw the liquids from their respective tanks (WT, KT).

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The device is installed in the test section (Ts) at the region (M). It is disposed at a distance of about 2.0 m from the entry section. A photograph of the device is given in figure 5. The diode laser (1) is being used as a source of monochromatic laser light. The light passes through the flow passage and a photo diode sensor (2) located at the diametrically opposite points detects it. The incident is converted to a voltage signal and recorded continuously in the data acquisition means (4) through the processing circuit (3) with a sampling frequency of 23 Hz for duration of 3 minutes.
Initially the signals have been recorded for flow of either of the phases alone in the conduit. The response has been noted for different velocities of both the liquids in the vertical as well as the horizontal devices. It has been noted that a higher voltage has been recorded for water flow as compared to that of only kerosene flow in the conduit. The response has been observed to be insensitive to velocity of the liquids and orientation of the pipe (Ts). The voltage signals recorded ror the simultaneous flow of the two phases under different flow conditions have been normalized with respect to that obtained for only water in order to facilitate a comparative study.
The probe signals and their PDF curves for the different patterns are depicted in figure 6. The PDFs have been quantified by means of the statistical moments (mean, variance, skewness and kurtosis). While the mean gives us the average value of the distribution, the variance is a measure of the distribution about the mean. The skewness (third moment) characterizes the asymmetry of the PDF and the kurtosis (fourth moment) indicates the flatness of the distribution compared with the Gaussian one.

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The flow is smooth stratified with complete separation of the two liquids at low velocities of the two liquids as shown in figure 6. This is denoted by a smooth response of the probe signal and a single PDF peak (figure 6) with minimum spread. As the flow rate of either phase is increased, waviness sets in at the interface. This is evident from the increase in fluctuations of the signal and the increased spread in the PDF curve. The mean value of the signal is higher (figure 6) than stratified smooth due to more water entry (Usw>Usk) and less cross sectional area of kerosene to absorb light. On further increase of water velocity the waves touch the upper wall and cause plugs of kerosene to be intercepted by water bridges and the plug flow water pattern occurs.
This pattern occurs at a higher velocity but the mean value of the response is than in the previous case. This arises because scattering of light by the large plugs of oil with high residence time become important along with its attenuation. This is further accompanied by an increase in the spread of the PDF curve (figure 6). The same phenomena continues with further increase in water velocity. The flow becomes dispersed with a decrease in the size of the kerosene droplets which are uniformly distributed in the water phase. This is depicted by a PDF curve with increased spread and a peak at lower voltage values.
The same phenomena is also encountered when the experiment is repeated at higher kerosene velocities. As the water velocity is increased at a constant kerosene velocity, the three layer pattern from stratified wavy at higher kerosene velocities is entered. The response shifts to lower voltage but exhibits higher oscillation. This is due to predominant effect of scattering and increased turmoil at the interface. The value of standard deviation and negative kurtosis of three layer flow in figure 7 reflects its increased spread and low peakedness. For

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dispersed flow the peak is sharp (figure 7) and the spread of the histogram is low (low oscillation) due to intense scattering by the array of droplets, which propagate through the conduit at high velocities. They have a small residence time and high population, which results in insufficient light passage.
In liquid-liquid vertical flow, mainly two types of flow patterns are observed namely 'oil dispersed in water' at low oil flow rates and 'annular flow' with kerosene in the core and water as a film in the annulus at higher kerosene superficial velocities. The probe signals and PDFs are distinctly different for the two cases as shown in figure 8. The signal occurs at higher voltage and has more fluctuations in dispersed flow. The PDF in both the cases is unimodal but it occurs at a higher voltage for dispersed flow and shifts to lower voltage value with a drastic reduction in the spread for "core annular flow".
This optical probe has been tested for different conditions of air-water two phase flows. Two representative cases namely the identification of the slug and slug-churn flow patterns have been presented in figure 9. In this case the voltage signals have been normalized with respect to the voltage obtained for air. The signal and the bimodal peak of figures 9(a) highlight the intermittent character of slug flow, which is destroyed, with the onset of the churn flow pattern.

17 We Claim
1. A non-intrusive optical probe releasably along the cross-section of a tubular test-piece (Ts) constructed of perspex enabling visual observation of multiphase flow phenomena occurring inside the tube (Ts), the flow-rate, mixing ratio, flow-pattern including associated parameters of the multiphase flow system being individually generatable, measurable, separable and controllable by the devices of said multiphase flow system, the optical probe identifying the flow patterns including estimating the depth of the multiphase flows in stratified flow and the bubble characteristics in bubbly flow, the probe comprising:
- a laser source (1) adaptable as a source of monochromatic laser
light,
- a photodiode sensor (2) disposed on the tubular test piece (Ts) at a
location being diametrically opposite to the laser source (1) for
receiving the light incident;
- an operational amplifier (3) converting the light incident to voltage
signals and arranging amplification of the output-voltage signals;
- a data acquisition means (4) operably connected to the operational
amplifier (3) for continuous recordal of the data in respect of the
voltage signals; and

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a post processing computer device (5) having a memory means with pre-stored data in respect of signals corresponding to individual and combined flow at different velocities, the computer device receiving the real-time data from the data acquisition means (4) for comparing with the pre-stored data, thereby outputting the flow patterns of the multiphase flow.
2. The optical probe as claimed in claim 1, wherein the probe is installed at a
region (M) in the tubular test piece (Ts).
3. The optical probe as claimed in claim 1, wherein the probe is rotatable
along the periphery of the tubular test piece (Ts), and wherein the probe
is slidable through the axial height for heightwise scanning of the
distribution.
4. The optical probe as claimed in any one of preceding claims, wherein the
data recordal at the acquisition means (4) is carried-out with a sampling
frequency of 20 to 25 Hz for a duration of 2 to 5 minutes.
5. A non-intrusive optical probe for identifying the flow pattern including
estimating the depth of a multiphase flow in stratified flow and the bubble
chaiacteristic in bubbly flow as herein described and illustrated with
reference to the accompanying drawings.
Dated this 4th day of OCTOBER 2006