Optical phase shift fluid flow velocity measurement mechanism

An optical shift fluid flow velocity measurement mechanism is shown and described. A fluid flows over a bluff body to produce a vortex street. Downstream of the bluff body a light beam emission source directs a beam of light towards a lenslet including a light transmission gradient. Light transmitted through the lenslet is perceived by a photovoltaic cell which generates an electrical signal. Vortices in the vortex stream disrupt the light beam, affecting the amount of light transmitted through the lenslet and perceived by the photovoltaic cell, thereby changing the electrical signal. The frequency of the changes in the electrical signal can then be correlated to the velocity of the fluid flow.

BACKGROUND

The present disclosure relates to fluid flow measurement systems. Measuring the velocity of fluid flows is essential for countless applications, many of which including those in the gas, oil, and nuclear fields, require systems which can be used in highly corrosive environments and which have minimal maintenance requirements.

Previously described methods of measuring fluid flow include pressure-based systems that restrict fluid flow in order to measure its rate from the resulting pressure difference, or mechanical methods that rely on a moving mechanical apparatus (e.g. a pinwheel, rotor, or the like) placed in the fluid stream. Other previously described methods include vortex flow meters, wherein a bluff body is placed in the path of the fluid. Vortices are created as the fluid passes the bluff body. The vortices trail behind the bluff body and the frequency of the vortex shedding off the bluff body is detected, often via a piezoelectric crystal or other sensor, which produces a small, but measurable, voltage pulse each time a vortex is created. The frequency of the pulse is measured and the fluid flow velocity is calculated by V=f L/S where f is the frequency, L is the characteristic length of the bluff body. And S is the Strouhal number, which is essentially a constant for a given body shape within its operational limits. Still further methods of measuring flow rates include optical flow meters which typically measure the actual speed of particles in the fluid flow. In this instance small particles in the fluid pass through two laser beams spaced a known distance apart from each other. A signal is generated when a particle passes through the first laser beam and a second signal is generated when the same particle passes through the second laser beam. Fluid flow velocity is calculated by V=D/T where D is the distance between the laser beams and T is the interval between the generation of the first and second signals. All of these existing methods have limitations. Restriction of the flow rate can be detrimental to a given system or process, while mechanical flow measurement devices are sometimes less restrictive to the flow itself, but their moving parts can fail. Previously described vortex flow meters may rely on expensive sensors and electronics that may be destroyed is corrosive environments and previously described optical flow meters that rely on the ability to detect the same particle passing through different laser beams may be expensive or impractical in various environments. Accordingly, there remains a need for inexpensive fluid flow measurement systems which can be used in chemically harsh environments and which require minimal to no maintenance.

SUMMARY

The present disclosure provides a novel flowmeter that employs a bluff body and a light beam directed across, and generally perpendicular to, the fluid flow, wherein the focal point of the light beam shifts in response to periodic disturbances in the flow created by the bluff body and wherein an electronic signal is generated in response to the shifting movement of the focal point of the light beam.

DETAILED DESCRIPTION

According to an embodiment the present disclosure provides an inexpensive, solid state flowmeter that can be completely sealed so that it is suitable for use in a chemically harsh environment. Turning toFIG. 1, a side-view schematic of an exemplary flowmeter is shown. InFIG. 1, fluid flows in the x-direction through conduit10. It should be understood that conduit10may take the form of a channel, pipe, or any other suitable body through which fluid can flow. Flowmeter12includes a bluff body14. It will be understood that bluff body14may take the form of any shape for which velocity data is known or calculable and which produces a vortex street within the fluid stream (e.g., an elliptic cylinder instead of a circular one). According to a specific embodiment, bluff body14takes the form a cylinder, though it can likewise take almost any other shape with a defined width. Velocity data for flow around cylinders is well-established and freely available. Specifically, it is known that a pattern of counter-rotating vortices (the Benard-von Karman wake) emerges behind a bluff body and persists for a wide range of freestream flow velocities, with the Strouhal number (the vortex shedding frequency nondimensionalized by the bluff body diameter and freestream velocity) remaining nearly constant through a large part of that range. According to some embodiments, it may be desirable for bluff body or cylinder14to be heated. Heating the bluff body can increase refractability without adding significant costs or moving parts to the system. In general, the less compressible the medium being measured, the more desirable it may be to heat the bluff body. Methods for heating the bluff body include electrical heating or heating via circulation of warm fluid through channels inside the body.

Downstream of bluff body14, flowmeter12further includes a light beam emission source16, which is configured to direct a beam of light18across (that is perpendicular to) the direction of fluid flow. (In the schematic the direction of the light beam is shown as direction Y.) According to a specific embodiment, the light beam emission source16is an inexpensive laser diode. Recent innovations in laser diode technology have dramatically decreased the costs and space requirements associated with light beam emission, making them both cost-effective and practical for commercial field applications.

Regardless of its source, light beam18is directed at lenslet20, which is situated across the fluid flow path from light beam emission source16such that fluid traveling through conduit10passes through light beam18.FIG. 2is an isolated view of lenslet20. It will be seen that lenslet20includes distinctive radial tint, or gradient of emissivity. Specifically, lenset20is designed to include an outwardly radiating circular light transmission gradient24. According to an embodiment, the gradient is such that the center26of the gradient is absolutely opaque (that is absolutely no light in the wavelength of light beam18is transmitted through the center most portion of the lenslet) and the outer portion28of the gradient is absolutely clear (that is maximum light in the wavelength of light beam18is transmitted through the lenslet). It should be appreciated that according to some embodiments, the gradient may be reversed, that is the center of the gradient may be absolutely clear and the outer portion of the gradient may be absolutely opaque, as shown inFIG. 3. According to some embodiments a smoothly linear gradient may be used. However, in an alternate embodiment, a stepped gradient may be used, with the size of the steps being inversely related to signal resolution.

For the purposes of the present disclosure it will be noted that the center of the gradient may or may not be placed at the actual physical center of the lenslet. Typically, however, any area outside of the gradient circle should be coherent with the outer edge of the gradient. For example, if the gradient extends from a clear center to an opaque outer portion, then the portion of the lenslet outside of the gradient circle should also be opaque.

Lenslet20may be formed of any suitable material capable of producing a light transmission gradient including, but not limited to, plastic, glass, or any material that is translucent to the given wavelength of light. Suitable methods for applying a light transmission gradient to the lenslet include, but are not limited to molded or machined semi-opaque materials, plating, sputtering, films, or dispersions applied as coatings.

Returning toFIG. 1, it can be seen that behind lenslet20is a signal generator30configured to respond to fluctuations in light transmission through lenslet20and produce a detectable signal. According to a specific embodiment, signal generator30is a photovoltaic cell. The photovoltaic cell may be any suitable photovoltaic cell including those commonly commercially available from silicon, gallium arsenide, amorphous, gels, films, or any other materials that convert light into electrical energy.

In some embodiments it may be desirable to include a diffusion medium32between the signal generator30and the lenslet20in order to spread out the point of light beam18, so that its light can be more easily absorbed and converted into electricity without, for example, burning the photovoltaic cell. Suitable diffusion mediums include, but are not limited to agar, polyacrylamide gel, a stack of plastic or glass sand-blasted lenses, or any other translucent material manufactured, cast, formed, machined, or shaped with reflective or refractive inclusions.

Signal generator30is in electronic communication with an electronic mechanism31, which may be, for example, an oscilloscope or other apparatus configured to detect the signal produced by signal generator30and display it in an observable format, a digital signal processor (DSP), or any other analog wave receiver such as a computer. The system may further include additional displays and/or other instrumentation suitable for the intended use of the flow meter including, for example, a computer or other control system.

While the embodiment shown inFIG. 1provides a flowmeter that is used within a confined channel or the like, it should be appreciated that the flowmeter of the present disclosure can be used on a vehicle or other body traveling in a free stream flow or across which a free stream flows. For example, the presently described flowmeter could be used instead of or in addition to the pitot tube that is traditionally used on aircrafts. In this embodiment, one end of the flowmeter (e.g. the light beam emission source or “sender”) might be located on the surface of the vehicle and oriented so as to direct a beam of light to the “receiver” (e.g. the lenslet and diode, with or without a diffusion medium) which is located some distance away from the surface of the vehicle (or vice versa). According to one embodiment, the bluff body could form some, none, or all of the physical structure used to position the sender or the receiver away from the vehicle. For example, the bluff body could extend perpendicularly away from the surface of the vehicle. The distal end of the bluff body could then include or otherwise form some type of attachment and/or positioning device for the sender or receiver. Alternatively, a positioning device other than the bluff body could otherwise be supported downstream of the measurement instrument. An advantage to using the flowmeter described herein instead of, or in addition to, a pitot tube is that the currently described flowmeter would be more resistant to icing than the pitot tube. Furthermore, characteristic signatures in the signal produced by the presently described flowmeter could alert crews to the presence of icing conditions that might affect the readings produced by the pitot tube.

As shown inFIGS. 4-6, the periodic disturbances created by the bluff body change the aim of the focal point of the light beam with respect to the light transmission gradient on the lenslet. The light transmission gradient, in turn, effects the amount of light that is transmitted through the lenslet and captured by the signal generator, which as explained above, may be, for example, a photovoltaic cell. Referring toFIGS. 4 and 5, it can be seen that in this exemplary embodiment, laser source39aims laser beam34such that it will hit the clear center36on lenslet35when the laser beam is unperturbed by any disturbances in the air flow (e.g., when the fluid flow velocity is zero or if the bluff body is removed) (FIG. 4). According to some embodiments, the diode may be adjusted, using known means, so that it can be tuned to different materials/fluids etc. When the laser beam is unperturbed by any disturbances in the air flow, all the light from laser beam34is transmitted through the clear portion of the lenslet so that it hits photovoltaic cell40. When fluid starts flowing in the x direction through conduit33(if present), the cyclical shedding of vortices42behind the bluff body37causes refraction phase shift of laser beam34away from the center of the lenslet (FIG. 5), moving the beam intermittently into the tinted region of the gradient, and thereby reducing the amount of light that passes through the lenslet, causing the electrical current generated by photovoltaic cell40to fluctuate in a wave pattern (FIG. 6) consistent with the vortex shedding frequency, which, in turn, correlates to fluid flow velocity. InFIG. 6, point “a” shows with highest current generation by the photovoltaic cell, which corresponds to a period of high light beam transmission through the lenslet, which, in turn, correlates with the time period between vortices. Conversely, point “b” shows the lowest current generation by the photovoltaic cell, corresponding to a period when the light beam was most refracted and least transmitted through the lenslet, thereby indicating the presence of a vortex between the light beam source and the lenslet.

The refraction phase shift is caused by density fluctuations in the wake, which are due to pressure variations (low pressure being associated with vortex cores) or pressure variations combined with buoyancy effects (due to heated cylinder). It will be noted that, if desired, the properties (wavelength, intensity, etc.) of light beam18may be selected to match the optical properties of the medium (such as preferential pass-through wavelength).

Furthermore, once a baseline signal is determined, changes in the signal (e.g. alterations in the wave shapes such as peaking, shifting, or loss of amplitude) may indicate changing conditions within the system such as contamination, plaque buildup, temperature changes or the like.

It will be appreciated that the various embodiments of the flowmeter described herein contains no moving parts. Furthermore, there is no need for the detector or other electronic parts to make contact with the fluid flow. Accordingly, the system described herein is particularly useful and relevant for highly caustic or harsh environments. Furthermore, the parts used in the present detector tend to be long lived, substantially reducing the need for maintenance. In general, the two life-span limiting factors for the system is the life span of the laser beam generator and contamination of the lenslet. However, if contamination occurs at a known rate, even this can be accounted for in the velocity calculation.

The low cost of the presently described flowmeters allow for multiple flowmeters to be used in series, while the long vortex street following a single bluff body allows that multiple sensor arrays may be used in series to form a single complex meter, where one or more of these arrays may be operated at any given time, to allow for error checking or to even more substantially reduce the need for maintenance, as might be desired, for example, in nuclear reactors or oil or gas pipelines in remote and hard to reach areas.

All patents and publications referenced or mentioned herein are indicative of the levels of skill of those skilled in the art to which the invention pertains, and each such referenced patent or publication is hereby incorporated by reference to the same extent as if it had been incorporated by reference in its entirety individually or set forth herein in its entirety. Applicants reserve the right to physically incorporate into this specification any and all materials and information from any such cited patents or publications. The specific methods and compositions described herein are representative of preferred embodiments and are exemplary and not intended as limitations on the scope of the invention. Other objects, aspects, and embodiments will occur to those skilled in the art upon consideration of this specification, and are encompassed within the spirit of the invention as defined by the scope of the claims. It will be readily apparent to one skilled in the art that varying substitutions and modifications may be made to the invention disclosed herein without departing from the scope and spirit of the invention. The invention illustratively described herein suitably may be practiced in the absence of any element or elements, or limitation or limitations, which is not specifically disclosed herein as essential. The methods and processes illustratively described herein suitably may be practiced in differing orders of steps, and that they are not necessarily restricted to the orders of steps indicated herein or in the claims. As used herein and in the appended claims, the singular forms “a,” “an,” and “the” include plural reference unless the context clearly dictates otherwise.

Under no circumstances may the patent be interpreted to be limited to the specific examples or embodiments or methods specifically disclosed herein. Under no circumstances may the patent be interpreted to be limited by any statement made by any Examiner or any other official or employee of the Patent and Trademark Office unless such statement is specifically and without qualification or reservation expressly adopted in a responsive writing by Applicants.