MXPA06005448A - Optical device and method for sensing multiphase flow - Google Patents

Abstract

The invention provides a method for measuring the velocity of a multiphase fluid flowing in a pipe. The method comprises directing at least two collimated beams of light from an illuminator through the multiphase fluid by means of transparent portions of the pipe, the at least two collimated beams spaced apart in a direction of flow of the multiphase fluid by a predetermined distance;detecting scattered, deflected and attenuated light with at least two photodetectors to produce at least two signals, the at least two photodetectors associated with the at least two collimated beams;calculating a cross-correlation function between the at least two signals to determine a time delay between the signals;and, calculating the average velocity of the multiphase fluid by taking the ratio of the predetermined distance to the time delay.

Description

OPTICAL DEVICE AND METHOD FOR DETECTING THE FLOW OF MULTIPLE PHASES Technical Field The present invention relates to optical flow meters for detecting the velocity of fluids, including mixtures of gaseous and liquid fractions such as vapor, which move in a tube. BACKGROUND OF THE INVENTION The need for measurements of velocity and velocity of steam flow, for example, is a known problem in industrial control since steam is widely -used as an energy carrier in many processes and because measurement of the Steam flow is a complicated task. The main reason for this complication is the presence of two fractions in the flow, a gaseous phase or vapor, which is mixed with a liquid phase (water). The liquid phase moves in the tube in the form of water droplets of various sizes, fluctuating water aggregates and water condensate which is collected at the bottom of the tube if the steam quality is low. Each of these components moves with different speed. The proportion between these components varies with time, the water aggregates can be combined and the water condensate can be collected suddenly and is accelerated by the flow creating a "vibration effect". Furthermore, the quality of the steam changes along the pipe depending on the outside temperature of the pipe, the insulation of the pipe, the curvature of the pipe, etc. All these factors make it difficult to measure the flow of steam. Several solutions have been proposed to measure the vapor flow. Some are based on tracking the electrical properties of steam and water by measuring capacitance of the fluid at various points along the tube or tracking variations. in the density of the fluid with ultrasound. The main disadvantage of these methods is the high inconsistency with the operating temperature. High-power industrial boilers operate at temperatures above 350 ° C which are beyond the limit of capacitive and ultrasonic methods. Other solutions based on gamma irradiation methods could be applicable for vapor measurement; however, gamma radiation is expensive and this creates a risk for operational personnel. Cross-correlation methods for non-invasive measurements of fluid flow using optical means are known in the art. Optical methods are usually not adversely affected by high temperatures since light sources and photodetectors can be located remotely from the hot measuring zones. US Patent No. 6,611,319 (ang) describes an optical flow meter which is based on the recording of intermittent (flashing) light due to small changes in the refractive index with changes in temperature. The mobile fluid is transilluminated by a single light source and the directed light is measured by two separate photodetectors along the direction of flow. A cross-correlation function between the signals of these photodetectors is calculated and a position of their maximum is determined. This position provides the average time that is necessary for the flow to move from one photodetector to the other. Consequently, the ratio of the distance between the photodetectors to the delay time gives an estimate of the average flow velocity. A similar correlation technique has been described in WO 02 / 077578A1 (Hyde) for measuring the gas flow in long tubes using the attenuation of light by the gas stream. The different constituents in the mobile gas may have different absorption in the infrared region, which will cause the modulation of the light passing through the tube. However, both Wang's scintillation method and Hyde's infrared absorption method require long optical trajectories to accumulate sufficient flow abnormalities. Such methods require a minimum pipe diameter of approximately one meter to carry out reliable flow measurements. Diameters such as these are too large for steam pipes where the maximum diameter is 30 cm (12 inches) and the sizes of most pipes are 5 cm (2 inches) to 15 cm (6 inches). further, the highly divergent beam of light from a single light source used in the Wang scintillation method extends the time delay because different portions of the fluctuating flow cross the beam at different locations. This reduces the accuracy of the measurement. The collimated beams used in Hyde's infrared absorption method are not affected by this effect, but the vapor does not absorb much light. In particular, high quality steam is highly transparent over a wide range of wavelengths. The non-scattered light, therefore, has a very low modulation depth due to the high intensity of the direct light from the light source. In addition, none of the optical methods described above has been applied to detect the quality of the steam, which is as important as the velocity and flow measurements. Therefore, there is a need for an apparatus and method for detecting the flow velocity of gas and liquid mixtures, as they appear, for example, for steam moving in small tubes.

Brief Description of the Invention An object of the present invention is to provide an optical device and a method for detecting the vapor flow in industrial tubes. Another object of the invention is to provide such an optical device and a method that will be suitable for vapors of varying quality. Still another object of the invention is to provide such an optical device and a method that can simultaneously detect the quality of the vapor. According to a preferred embodiment of the present invention, two or more narrow collimated light beams are directed towards a fluid flowing inside a tube, through transparent windows in the walls of the tube. The beams are separated along the direction of flow. Droplets of liquid and other components of the flow, which move through the tube, cross the beams and scatter and deflect the light. The scattered and deflected light passes through transparent windows on the opposite side of the tube and focuses on the photodetectors by means of an optical system. The scattered light can be collected by means of another optical system for monitoring purposes and for absorption measurements if the steam quality is low.

In one embodiment, the signals from the photodetectors are digitized and separated into different groups by filtering them with bandpass filters. The groups are associated with different vapor components such as, for example, miniature, medium, and large droplets, and water aggregates. The differentiation is based on the bandwidths of the frequency; the lower frequencies correspond to the larger droplets and the higher frequencies correspond to the smaller droplets. The cross-correlation functions are calculated for each group and the time delays between the signals of the separated beams are determined. The average velocity of each vapor component is calculated as the ratio of the separation distance between the beams over the time delay. The local velocity of each component of the vapor is detected by illuminating the flow with two beams from a variety of directions and collecting the deviated and scattered light from a variety of directions of the measurement zones through the tube. According to another embodiment of the invention, additional vertical beams are sent through the horizontally arranged tube, thus the level of liquid condensate is measured by detecting the absorption of the light. The flow intensity of each flow component is determined by calculating the dispersion of the filtered signals. The total flow rate is calculated as the sum of all the components of the flow measured in all measurement zones through the tube. Gaseous or vapor fractions are defined as the fastest flow fraction that moves in the tube. According to another embodiment of the invention, a method for measuring the velocity of a multiphase fluid flowing in a tube is provided. The method comprises the steps of: directing a pair of collimated light beams from an illuminator through the multiphase fluid, by means of transparent portions of the tube, the pair of collimated beams separated in a direction of the flow of the multiphase fluid at a predetermined distance; detecting scattered light, deflected and attenuated with a pair of photodetectors to produce a pair of signals, each of the pair of photodetectors is associated with one of the pair of collimated beams; calculate a cross-correlation function between the pair of signals to determine a time delay between the signals; and, calculate the average velocity of the multiphase fluid taking the ratio of the predetermined distance to the time delay. According to another embodiment of the invention, collimated beams are focused in one direction along the direction of flow to create two sheets of light which are oriented perpendicular to the flow. The light plates can be focused by means of a cylindrical lens. Photodecerators are associated with light plates and these record light, dispersed by liquid droplets. The fluid velocity is determined by the cross-correlation technique while the amount of the liquid fraction is determined from the scattering of the signals from the photodetectors. According to another embodiment of the invention, a collimated beam of light illuminates the fluid in addition to two sheets of light, which are used for measuring the velocity of the fluid. The collimated beam is used for the calculation of the liquid fraction of the fluid by measuring the scattering of the signal from a reference photodetector associated with. the beam collimated. Alternatively, the liquid fraction can be calculated based on a ratio of the signals recorded by the reference photodetector as long as the fluid is illuminated at two different wavelengths. The optical device and method described are suitable for measuring the multiphase flow such as steam in a small tube and provide qualitative analysis of the moving medium, such as the quality of the vapor. The method is highly sensitive since it detects microscopic water droplets in high quality steam as well as large droplets and water aggregates in low quality steam. In this specification, reference is made to the measurement of water and steam, but the invention applies equally to all transparent liquids and their corresponding gases. The invention can be applied to mixtures of water and hydrocarbons such as, for example, natural gas, which move in a tube. The detection is provided by the calculation of the cross-correlation between the signals of several photodetectors arranged along the tube, which register the light scattered by the gas fraction and diverted and absorbed by the liquid fraction. The present invention as well as its numerous advantages will be better understood by means of the following non-restrictive description of the possible modalities made with reference to the attached drawings. BRIEF DESCRIPTION OF THE DRAWINGS In the drawings, which illustrate the non-limiting embodiments of the invention; Figure 1 is a schematic representation of an apparatus for detecting the multiphase flow according to an embodiment of the invention; Figure 2 is a schematic representation of a portion of an optical system suitable for use with the apparatus of Figure 1; Figure 3A is an example of the signal detected by a photodetector, indicating the presence of miniature and large water droplets; Figure 3B is an example of the signal detected by a photodetector, indicating the presence of water droplets of medium size and water aggregates; Figure 4A is a schematic representation of the signals of two photodetectors, indicating a time shift, caused by the movement of the light beams along the flow; Figure 4B is an illustration of the cross-correlation function between the two signals of Figure 4A; Figure 5 is an example of four cross-correlation functions corresponding to four different components in the steam flow; Figure 6 is a schematic representation of an apparatus for detecting the multiphase flow using a channel of two vertical beams for the measurement of the water condensate according to another embodiment of the invention; Figure 7 is a schematic representation of an apparatus for detecting the multiphase flow with an additional measurement zone through the tube, according to another embodiment of the invention; Figure 8 is a schematic representation of several measurement zones through the tube using various optical collection systems, according to another embodiment of the invention; Figure 9 is a schematic of an optical, coaxial, multifocal picking system, which collects light from the various measuring zones of Figure 8; Figure 10 is a schematic of an optical, coaxial, multifocal collection system, which collects light from the various measurement zones of Figure 8, working in the backscatter mode; Figure 11 is a block diagram illustrating an example of signal processing in a single channel with two photodetectors and several bandpass filters; Figure 12 is a block diagram illustrating an example of a calculation of liquid and gas flow rates based on m measurement channels and n flow components; Figure 13 is a schematic representation of an apparatus for detecting multiphase flow using two light sheets created in the tube, according to another embodiment of the invention; Figure 14A is an example of a signal measured by one of the photodetectors of Figure 13 while measuring high quality steam; Figure 14B is an example of a signal measured by one of the photodetectors of Figure 13 while measuring low quality steam; Figure 15 is a schematic representation of an apparatus for detecting the multiphase flow using two light sheets created for the measurement of the fluid velocity and an additional collimated beam for the measurement of the liquid content, according to another embodiment of the invention; and Figure 16 is an example of the signal relationships recorded by the reference photodetector of Figure 15 at two different wavelengths. Detailed Description of the Invention Throughout the following description, specific details are explained to provide a more complete understanding of the invention. However, the invention can be practiced without these particulars. In other cases, well-known elements have not been shown or described in detail to avoid unnecessarily obscuring the invention. Therefore, the specification and drawings should be considered in an illustrative sense, rather than restrictive.

Figure 1 shows a first embodiment of an optical device for detecting multiphase or multi-phase flow. An illuminator 10 generates two narrow collimated light beams 12 and 14, which enter a tube 16 that transports fluid through a lighting window 18. Beams 12 and 14 may comprise infrared, ultraviolet, or visible light. Beams .12 and 14 are arranged spaced apart at a distance d along the direction of fluid flow. The bundles 12 and 14 are dispersed, deflected and absorbed by the fluid moving through the tube 16. The scattered and deflected light passes through a pickup window 19 in the opposite wall of the tube. The light that passes through the acquisition window 19 is collected by an optical system (not shown in Figure 1), and focuses on an array 20 of photodetectors. The arrangement 20 of photodetectors. The photodetector arrangement 20 preferably comprises the photodetectors 20a and 20b positioned to receive the scattered and deflected light collected by the optical system at different angles, thus, the photodetectors 20a and 20b can measure the light scattered and biased to different degrees from the optical axis of beams 12 and 14, respectively. The array of photodetectors preferably also comprises the photodetectors 20c and 20d positioned to receive the direct light (without scattering) collected by the optical system, such that the photodetectors 20c and 20d can measure the light absorption of the beams 12 and 14, respectively, by the flow. Figure 2 schematically illustrates a portion of an optical system 22 suitable for use with the embodiment of Figure 1. The illustrated portion of the optical system 22 comprises the optical collectors 24 and 26, the droplets 21 of liquid moving in the tube cross the beam 12. The droplets 21 scatter and deflect a portion of the light in the beam 12 in a manner dependent on the ratio of its size and the wavelength of the light and its refractive index. Smaller droplets with sizes comparable to the wavelength of light predominantly scatter light at large solid angles, while larger droplets deviate light for the most part at small angles relative to the horizontal. The scattered and deflected light 23 is dispersed within the tube and a portion of it passes through the window 19 and is focused by the optical collector 24 on the photodetector 20a. the direct light 27 (without dispersion, but attenuated) passes through the window 19 and is removed by absorption or focused by the optical collector 26 in the photodetector 20c. recording direct light provides monitoring of the power of illuminator 10 and indicates the attenuation of light due to absorption by very large water droplets. The signals generated by the photodetectors according to the invention typically consist of a plurality of components of different frequencies. Figures 3A and 3B illustrate examples of typical signals from a photodetector, such as the photodetector 20a or 20b, which record the scattering light within a small angle relative to the horizontal to the optical axis. The graphs of Figures 3A and 3B trace the signals of a photodetector (in millivolts) against time (in seconds x 10"4) The exemplifying signals can be separated into a high frequency component 32, and a frequency component 34 medium and a low frequency component 36, as well as the ultra-slow 38 and 39 components (solid line), all created by the different components of the fluid flow.Components 32, 34, and 36 are created respectively by the droplets of miniature water (less than 1 micron in size), medium (approximately 1 micron in size), large (larger than 2 microns in size), and components 38 and 39 are created by water aggregates that fluctuate in the tube The arrangement of two beams shown in Figure 1 leads to a temporary displacement of the signals.When the elements of the fluid flow through the tube 16 they pass through the beam 12 before the beam 14, and the resulting fluctuations in thesignal of the photodetector 20a before they occur in the signal of the photodetector 20b. The displacement is shown schematically in Figure 2A for the two photodetectors, PDl and PD2. the photodetectors PDl and PD2 can comprise any pair of photodetectors which register the light of two light beams separated by the distance d along the direction of the flow of the fluid. A cross-correlation function between the two signals of PDl and PD2 will have a maximum in an elapsed time t (time delay), as illustrated in Figure 4B, defined as where d is the separation between the beams, and V is the average velocity of the flow. The signals of the photodetectors 20a and 20b can be filtered by means of the bandpass filters, as described below, to isolate the components of the signals at different frequencies, which are generated by the different components of the flow. The time delay t is different for the different components of the flow. The miniature water droplets move with the speed of water vapor while the aggregates move at a much slower rate, causing a stagnant effect. The exemplary cross-correlation functions corresponding to the flow components 32, 34, 36, and 38/39 described above, are illustrated in Figure 5. In Figure 5, the time elapsed for miniature, medium, and large droplets, and the water aggregates are 20. 30, 46 and 70 μs, respectively, these time delays correspond to the average speeds V of 50, 33, 22 and 14 m / s for the flow components 32, 34, 36, and 38/39, respectively, for the separation d = 0.1 mm. Figure 6 illustrates another embodiment of the invention wherein an additional pair of beams 62 and 64 generated by another illuminator 66 are sent through a window 68 within the tube 16 in the vertical direction. Doing 62 and 64 are coupled with a second optical pick-up system (not shown in Figure 6) located under the tube in the same manner as beams 12 and 14 in Figure 1. In addition to the scattered and deflected light, the second optical acquisition system focuses the beams 62 and 64 directly on a pair of photodetectors, thus providing detection of the attenuation of the beam in the condensation 69 of water at the bottom of the tube 16. The attenuation is related to the depth h of the condensate 69 of water according to Beer's law: I = I0 exp (-_ t?) where I0 is the intensity of the beam without any water condensate, I is the measured intensity of the beam, it is already the extinction coefficient. The cross-sectional area A of the water condensate in the tube 16 is related to the depth h as A * R2 arccos (l-? R) - (R-h) (2Rh-tf where R is the radius of the tube. The speed of the water condensate F_ond can be calculated by multiplying this area by the speed of the water condensate *? -? ßaá _ =? AVrew »? I The acquisition of the deviated and scattered light from a small angle relative to the horizontal along the beam spanning the tube 16 causes the integration of the signal since the flow velocity varies through the section of tube 16. therefore, in accordance with yet another embodiment of the present invention shown in Figure 7, in addition to the integral evaluation of the average velocity profile, the flow is analyzed in the local measurement zone 70. Beams 72 and 74 of light scattered by water droplets at an almost perpendicular angle are captured with an optical system 76 in a photodetector unit 78 comprising a pair of photodetectors. The measuring zone 79 can be located in several locations through the tube 16. An example of four measuring zones along the diameter of the tube 16 are shown in Figure 8. The zones could be located at several distances from the center of the tube. One of the preferred distances is a = 3/4 R. The local flow velocity mediated in this location is the closest to the average velocity in the tube 16 with virtually no effect of the flow profile.

The measurement of the multipoint flow can be simplified if a transparent section 89 is mounted in the tube 16. This section can represent a section of a glass tube having the same internal diameter as the tube 16. measuring multiple points it could be achieved using a multifocal optical system such as the one shown in Figure 9. The optical system 110 consists of several optical components of various apertures, therefore, the optical power of the system varies with the number of components. In the example of Figure 9Four measurement zones 112, 114, 116, and 118 are optically associated with four photodetectors 122, 124, 126, and 128, respectively, which measure the scattered light and deviate from the measurement zones. The scattered and deflected light can be captured in a backscatter mode as shown in Figure 10. Photodetectors 132, 134, 136, and 138 measure light scattered and deflected from measuring areas 118, 116, 114, and 112 respectively , by means of a multifocal optical system 130. The advantage of the backscatter mode is the low amount of background noise of light scattered on the tube walls since the light that is recorded has been scattered at high angles. Figure 11 illustrates schematically a signal processing means according to an embodiment of the invention. The electrical signals of the photodetectors 140 and 142, which may be a pair of photodetectors spaced along the direction of the flow, are amplified (the amplifiers are not shown) and digitized in analog and digital converters 144 and 146. Preferably, the digital signals are further processed by the digital signal processor (DSP). The signals are filtered by the bandpass filters 150, 152, 254, 160, 162, and 164. The number of digital filters may vary depending on the number of components of the fluid to be discriminated, which in turn is determined by the desired accuracy. The frequency bandwidth of the filter is determined by the number of bands to be selected and the maximum speed of the fluid. For example, to discriminate four components of the steam flow with a maximum velocity of 50 m / s, the preferred bandwidths are; ? f? = 0 to 10 Hz; ? f2 = 10 to 100 Hz; ? f3 = 100 Hz at 1 kHz; and? f4 = l at 100 kHz. The cross-correlation functions are calculated for each pair of signals filtered with the same bandwidth using the 170, 172, 174, correlation procedures crossed (CC). The time delay t_ for each CC function is determined as a position of the maximum CC. Local velocities are calculated for each component of the flow using the relation: V, = < f / r, As described above, the fastest measured of the flow components are the miniature water droplets which move at the speed of the steam fraction in the tube 16. In addition to the velocities, the amount of each of the Flow components can be measured from the intensities of the filtered frequency components of the signals. For example, the low-intensity fluctuations recorded at the bandwidth? F3 = 100 to 1,000 Hz, indicate that the number of medium-sized droplets (the size of about 1 micron for saturated vapor) is low. The intensity of the signal at each bandwidth I is representative of the amount of liquid fraction of each component of the flow. The ratio between I. and the quantity of the corresponding flow components is preferably established by the calibration in a regulated flow environment. The intensities are preferably found as dispersions of the signals where U (is the spectral density of the signal, which represents the distribution of the signal spread in the frequency domain.) The spectral density can be measured using Fourier transforms or by Any other known means After calculating the velocity of the flow components and the intensities of the corresponding signals (which are indicative of the quantity of the corresponding flow component) for each measurement zone or "channel", the final velocities of the Liquid (water) and gas (vapor) are found as shown in Figure 12. The number of channels depends on the number of beam ares that illuminate the fluid flow and the number of measurement zones for each pair of beams. The time delay t and intensity I are calculated for each of the n components for each of the m channels, and are provided to a 200 d calculator. e flow speed. The flow rate calculator 200 also receives as inputs the pressure and temperature data and the condensate level 69 at the bottom of the tube, as calculated from the absorption data as described above.

The vapor fraction of the fastest of the n flow components is calculated, which are the nth components when? Fn is the bandwidth of the highest frequency, over all m channels as follows: steam =? linVin M The total amount of the liquid and vapor fractions is calculated as a sum over all the components of the flow: The quality of the steam can be determined by the relationship: Steam mass) _ F vapor Q = Steam. { more?) + Water (mass) Total Ft The above equations are corrected by the pressure and temperature factors. The liquid condensate can be taken into account by calculating the cross section of the tube filled with liquid at a level (depth) h based on the absorption measurement described above with reference to Figure 6. According to another embodiment of the invention, the Beams 12 and 14 collimated (Figure 1) are focused in the direction of flow using a cylindrical lens, thus, two thin sheets 292 and 204 are created as shown in Figure 13. The sheets are oriented perpendicularly to the direction of the flow and separate along the direction of flow in the same way as the collimated beams of Figure 1. The direct light shown as arrows 206 and 208 is obscured by a non-transparent mask 210. The optical system 212 captures only the light scattered by the water droplets in the tube 16. the light is captured on the photodetectors 214, 216, Each photodetector is associated with a sheet of light. A viewing area 218 of the photodetector 214 is determined by the size of the sensitive area of the photodetector, and the pickup aperture and focal length of the optical system 212. A display area 220 of the photodetector 216 is determined in the same way. Since the vapor is associated with the high temperature, the photodetectors 214 and 216 can be replaced with optical fibers coupled to the photodetectors outside the device. The advantage of this modality is twofold: first, the light is more concentrated than in the collimated beams; and, second, the system operates in dark field without strong direct light that could saturate the photodetectors. This improves the detection capability of the device and allows the use of sensitive avalanche photodiodes (APD) such as photodetectors 214, 216, if the steam quality approaches 100% or the steam is overheated.

The light sheets 202, 204 may be provided in various locations within the tube 16. The sheets may be located in the center of the tube, with the velocity of the center line being measured using the cross-correlation technique. The centerline velocity must be converted to the average velocity to calculate the total flow of the fluid. This conversion can be done by calculating the Reynolds number through the known temperature and pressure of the fluid. Alternatively, the sheets can be located at 1/4 radius from the tube wall. This location eliminates the need for speed conversions since the velocity measured at this point represents the integral velocity along the tube 16. In addition to determining the flow velocity through the cross correlation calculation, the signal processing means for this embodiment can be used to calculate the scattering of the signals of the photodetectors 214, 216. Figure 14A and Figure 14B show the signal of one of the photodetectors while monitoring the steam moving at a speed of 20 m / s and it has a quality of 94% and 56%, respectively. Higher water contents or lower vapor quality result in an increase in signal dispersion from 0.52 to 0.97 in this example.

According to another embodiment, a collimated beam 230 is added to two sheets of light as shown in Figure 15. The two sheets of light are used to determine the flow velocity in a manner as described above. The light of the collimated beam 230 is attenuated by the fluid flowing in the tube 16, and this is captured by an optical system 232 in a reference photodetector 234. In addition to the speed measured using the cross-correlation technique, the signal processing means for this mode can be used to calculate the signal dispersion of the reference photodetector 234. Alternatively, the collimated beam 230 of the embodiment of Figure 15 can be produced by an illuminator having a plurality of light sources, of which at least one light source generates light of a first wavelength which is very absorbent in the fluid in the tube 16, and at least one of the other light sources generates light of a second wavelength which is less absorbent in the fluid in the tube 16. the light sources can be switched using multiplexing in time or a number of reference photodetectors, each associated with the narrowband optical filter, can be used to select an appropriate light source. Light sources may include light-emitting diodes (LEDs) or lasers that emit in the visible range, which are transparent to water. For example, red LEDs or lasers with a wavelength close to 660 nm. The attenuation of the water increases in the region near the IR, therefore, another spectral band to determine the water content can have a wavelength surrounded at 1360 nm or longer. Figure 16 shows the exemplary signal ratios (660 nm over 1300 nm) recorded for variable quality steam moving in a 2-inch tube at V = 34 m / s. The ratio of the signal increases with the quality of the steam in this example. The modalities described above can be achieved by a variety of modes. Preferably, said light emitters are used in the illuminators as light sources. In particular, the green, red and near infrared LEDs are suitable for this application, since they are well coupled with sensitive and easily available sensitive Si photodiodes, which can be used as photodetectors. UV and blue LEDs provide better dispersion efficiency due to their shorter wavelength, however, this can lead to high background light scattered within the tube 16 and on an optical window 18 and 19, in particular. It may be desirable to use optical fiber components in the optics of illumination and capture for steam pipes, since the operating temperatures for steam pipes may be too high for direct contact with light sources and photodetectors. Semiconductor lasers are more suitable for optical fibers since they provide high coupling efficiency with thin fibers. Intensive laser beams (with powers ranging from 1 to 10 mW) can be sent through the window over long distances using cheap lasers visible and near the infrared. The liquid fractions in the flow effectively disperse and deflect the propagation light, which can be captured in the photodetectors. The amount of light picked up by the optical system such as the dark field system shown in Figure 9 can reach 1 W in a 5.08 centimeter (2 inch) steam line. Such light intensity provides a signal-to-noise ratio in excess of 104 in the PIN photodiodes or avalanche photodiodes. Therefore, time delays can be measured with high accuracy. The devices and methods described above provide good premediation of the flow rate through the tube 16. This reduces the error caused by the uncertainty of the flow profile and the deviations of the flow profile, which are a major source of inaccuracies for the flow profile. the ultrasonic flow meters. The described methods are not affected by impacts or vibrations since they are not based on interference. The present invention provides robust and reliable devices, and methods, which are not affected by turbulence. Unlike other non-invasive flow measurement techniques, such as, for example, ultrasonic methods, flow turbulence is used in the present invention for the modulation of scattered and deflected light. The depth of the modulation increases with the level of turbulence. As will be apparent to those skilled in the art in the light of the foregoing description, many alterations and modifications to the practice of this invention are possible without departing from the spirit and scope thereof. For example, although the embodiment of Figure 1 described above employs two beams and four photodetectors, it should be understood that more than two beams could be used and the cross-correlation functions could be applied to something more than the two beams. Also, the signals could be measured by a number of photodetectors, as long as there is at least one photodetector for each beam. Accordingly, the scope of the invention should be considered in accordance with the substance defined by the following claims.

Claims (19)

1. CLAIMS 1. A method for measuring the velocity of a multiphase or multi-phase fluid flowing in a tube, characterized in that it comprises: (a) directing a pair of collimated light beams from an illuminator through the multiphase fluid by means of transparent portions of the tube, said pair of collimated beams separated in a direction of the flow of the multiphase fluid by a predetermined distance; (b) detecting the scattered, deflected and attenuated light with a pair of photodetectors to produce a pair of signals, each of said pair of photodetectors associated with one of said pair of collimated beams; (c) calculating a cross correlation function between said pair of signals to determine a time delay between the signals; and (d) calculating the average velocity of the multiphase fluid by taking the ratio of the predetermined distance to the time delay between the signals; and (e) passing the signal pair through a plurality of bandpass filter to isolate a plurality of pairs of corresponding frequency components, each of the plurality of pairs of corresponding frequency components corresponding to one of a plurality of flow components. A method according to claim 1, characterized in that it further comprises, for each of said plurality of pairs of corresponding frequency components: (a) calculating a cross-correlation function between the pair of corresponding frequency components to determine a time delay between the corresponding frequency components; and (b) calculating the velocity of the corresponding flow component by taking the ratio of the predetermined distance to the time delay between the corresponding frequency components. 3. A method according to claim 2, characterized in that it further comprises determining an intensity of each of said pairs of frequency components and calculating a corresponding one amount of said plurality of flow components from said intensity. 4. A method according to claim 3, characterized in that it further comprises determining a flow velocity of each of said plurality of flow components by multiplying the velocity of each flow component by the intensity of the corresponding pair of frequency components. A method according to claim 4, characterized in that, a vapor fraction of said multiphase flow is calculated as a flow velocity of the fastest of said plurality of flow components. 6. A method according to claim 5, characterized in that it further comprises determining a total flow rate of said multiphase flow by summing the flow rates of the said plurality of flow components. 7. A method according to claim 6, characterized in that it further comprises calculating a multiphase flow quality by taking a ratio of the fraction of the vapor to the total flow rate. 8. An apparatus for measuring the velocity of a multiphase fluid flowing in a tube, the multiphase fluid comprising a liquid phase and a gaseous or solid phase, the apparatus characterized in that it comprises: (a) an illuminator to generate a pair of beams collimated light and directing said beams through the multi-phase fluid by means of transparent portions of the tuno, said pair of collimated beams separated in a direction of the flow of the multiphase fluid by a predetermined distance; (b) a pair of photodetectors positioned through the tube from said illuminator, each of said pair of photodetectors optically associated with one of said pair of collimated beams to detect scattered, deflected, and attenuated light from the associated beam and general a signal; and, (c) a signal processing means for processing the signals of said pair of photodetectors and calculating the cross-correlation functions between the signals to determine a time delay, and to calculate the velocity of the multiphase fluid taking a ratio of the predetermined distance with respect to the time delay, said signal processing means comprising a plurality of bandpass filter for isolating a plurality of frequency components from each one of the pair of signals. 9. An apparatus according to claim 8, wherein said illuminator comprises a first illuminator for generating a first pair of collimated beams, and wherein said pair of photodetectors comprises a first pair of photodetectors, the apparatus characterized in that it further comprises: a) a second illuminator for generating a second pair of collimated light beams and directing said second pair of beams through the multiphase fluid at an angle to said pair of beams generated by said first illuminator; and (b) a second pair of photodetectors positioned through the tube from said second illuminator, each of said second pair of photodetectors optically associated with one of said second pair of collimated beams to detect scattered, deflected and attenuated light from the beam associated and generate a signal, wherein said signals of said second pair of photodetectors are processed by said signal processing means. 10. An apparatus according to claim 9, characterized in that, the angle is perpendicular. 11. An apparatus according to claim 9, characterized in that it further comprises at least one optical system for focusing the scattered light at an angle perpendicular from said pair of collimated beams of at least one measurement area on the at least one photodetector. An apparatus according to claim 8, characterized in that it further comprises a multifocal optical system for focusing the scattered light at a small angle relative to the horizontal from said pair of collimated beams from a plurality of measurement zones on a plurality of photodetectors. An apparatus according to claim 8, characterized in that it comprises a multifocal optical system for focusing the scattered light at an angle close to 180 degrees from said pair of collimated beams from a plurality of measurement zones on a plurality of photodetectors. 14. A method for measuring the velocity of a multiphase fluid flowing in a tube, the method, characterized in that it comprises: (a) directing a pair of light sheets from an illuminator through the multiphase fluid by means of transparent portions of the tube, said pair of light sheets oriented perpendicular to a direction of the flow of the multiphase fluid and separated in the direction of flow by a predetermined distance; (b) detecting scattered and deflected light with a pair of photodetectors to produce a pair of signals, each of said pair of photodetectors associated with one of said pair of light sheets; (c) calculating a cross correlation function between said pair of signals to determine a time delay between the signals; (d) calculating the average velocity of the multiphase fluid by taking the ratio of the predetermined distance to the time delay; and (e) calculating a quantity of liquid fraction in the multiphase fluid based on the dispersion of the signals of said photodetectors. 15. A method according to claim 14, characterized in that it further comprises: (a) detecting at least one collimated beam in a direction generally parallel to said pair of light sheets; (b) detecting the deflected and attenuated light from said collimated beam with a reference photodetector to produce a signal associated with said collimated beam; and (c) calculating the amount of liquid fraction in the multiphase fluid based on the signal dispersion of said reference photodetector. 16. A method according to claim 14, characterized in that it further comprises: (a) directing at least one collimated beam in a direction generally parallel to said pair of light sheets, said collimated beam comprising light from a first wavelength with high absorbance in a liquid fraction and light of a second wavelength with low absorbance in the liquid fraction; (b) detecting the attenuated light with the reference photodetectors to produce a first signal corresponding to the light of said first wavelength and a second signal corresponding to the light of said second wavelength; Y, (c) calculating the amount of liquid fraction in the multiphase fluid based on a ratio of said first and second signals. 17. An apparatus for measuring the velocity of a multiphase fluid flowing in a tube, the multiphase fluid comprising a liquid phase and a gas phase or solid phase, the apparatus characterized in that it comprises: (a) an illuminator to generate a pair of light sheets and directing said sheets of light through the multiphase fluid by means of transparent portions of the tube, said pair of light sheets oriented perpendicularly to a flow direction of the multiphase fluid and separated in the direction of flow by a predetermined distance; (b) a pair of photodetectors positioned through the tube from said illuminated, each of said pair of photodetectors optically associated with one of said light sheets to detect light scattered from the associated light sheet and which generates a signal; and, (c) a signal processing means for processing the signals of said pair of photodetectors, calculating the cross-correlation functions between the signals to determine a time delay, calculating the velocity of the multiphase fluid taking a ratio of the predetermined distance to the time delay, and to calculate an amount of liquid fraction in the multiphase fluid based on the scattering of the signals of said photodetectors. 18. An apparatus according to claim 17, characterized in that it further comprises: (a) a reference illuminator for generating a collimating and directing said collimator through the multiphase fluid by means of transparent portions of the tube; (b) a reference photodetector positioned through the tube from said reference illuminator and optically associated with said collimated beam to detect the attenuated light of said collimated beam and generate a signal, and; (c) a means for processing reference signals to process said signal from said reference photodetector and calculating the amount of liquid fraction in the multiphase fluid based on the dispersion of said signal. 19. An apparatus according to claim 17, characterized in that it further comprises: (a) a reference illuminator for generating a collimated and directed said collimated through the multiphase fluid by means of transparent portions of the tube, said collimated beam comprising light of a first wavelength with high absorbance in a liquid fraction and light of a second wavelength with low absorbance in the liquid fraction; (b) reference photodetectors positioned through the tube from said illuminator and optically associated with said collimated beam to detect the attenuated light of said collimated beam and generate a first signal corresponding to the light of said first wavelength and a corresponding second signal in light of said second wavelength, and (c) a means of processing reference signals to process said first and second signals and calculating the amount of liquid fraction in the multiphase fluid based on a ratio of said first and second signals. signs