MASS RATE OF FLOW METER

Abstract: An angular momentum type flowmeter comprises a housing receiving a swirl generator (33), a rotor (37) and a restrained reaction turbine (40). The rotor has a plurality of fluid flow passages (61) therethrough, each said passage having a convergent inlet section (62) and a divergent outlet section (63). The fluid discharge from the rotor thereby has a flow profile that is more nearly constant over a range of operating temperatures.

Description: MASS RATE OF FLOW METER Background of the Invention The present invention relates to mass rate of flow meters of the angular momentum type having a swirl generator for imparting angular momentum to a fluid stream to be measured, a rotor that is rotated by the fluid stream, and a restrained reaction turbine for removing the imparted angular momentum. Such a flowmeter is disclosed in United States Patent No. 4,056,976 (Hildebrand et al). That patent shows a mass flowmeter in which a swirl generator imparts angular momentum to the fluid to be measured. Immediately downstream from -the swirl generator is an unrestrained rotor. Swirling fluid from the swirl generator is directed through passageways in the rotor provided by multiple cylindrical tubes, causing the unrestrained rotor to rotate at the average angu¬ lar velocity of the fluid stream. The restrained reaction turbine is similar to the rotor, having passageways formed by multiple tubes through which fluid from the rotor flows. The angular momentum of the fluid stream angularly displaces the turbine until the bias of a restraining spring balances the torque on the turbine, allowing measurement of the mass flow rate by means not material to the present O PI invention. When such flowmeters are used to measure the flow of fuel in an aircraft, both the fuel and the flowmeter are subjected to a wide range of operating temperatures. The effect on the fuel is manifested, in part, by a change in fuel viscosity which, in turn, can lead to inaccurate indications of flow measurements. More specifically, it appears that the momentum imparted to the rotor and leakage through the flowmeter past the rotor and turbine become depen¬ dent upon viscosity, and, therefore temperature. Referring first to the changes in momentum, prior flowmeters utilize plural, cylindrical tubes to form the rotor passageways. Such construction is shown in United States Patent No. 2,148,150 (Cornell). Given a constant viscosity, the pattern of fluid flow, or flow profile, becomes nearly con¬ stant over a wide range of flow rates and establishes a radius of gyration. Thus, it is possible to cali- brate the flowmeter because the radius of gyration remains nearly constant. However, when the viscosity changes, the flow profile and the radius of gyration also change, and these changes manifest themselves as errors in the readings. Now referring to leakage through the flowmeter, it is desirable that the rotor and turbine move with as little friction as possible. This allows the angular velocity of the rotor to be solely a function of the angular velocity of the fuel leaving the rotor. It also allows the displacement of the turbine to be determined by the restraining spring. Thus, the outer peripheries of rotor and turbine must not con¬ tact the inner surfaces of the housing. Moreover, 0 , A s y 1 the need to minimize friction restricts the possible sealing methods that might be used to prevent leakage, especially at low flow rates. To date flowmeters are constructed with a small, annular clearance bet- ween the housing and each of the rotor and turbine. Some leakage does occur through this clearance., but in the past this has been compensated during calibra¬ tion. In current applications of these flowmeters, this leakage can introduce inaccuracies into flow indications. As the viscosity of the fuel changes, the percentage of fuel leaking through the clearance can also change in an unpredictable manner. Moreover, at low fuel flow rates, the fuel that passes through the clearance, and therefore is not measured, can become a significant percentage of the total fuel passing through the flowmeter. _# These changes in leakage are not easily compensated and so it is pos¬ sible that inaccurate readings can be obtained. Summary of the Invention According to the present invention there is provided a mass ' rate of flow meter comprising a swirl generator (33) to impart angular momentum to a fluid stream to be measured, a rotor (37) downstream from the swirl generator, said rotor having at least one fluid flow passage (61) therethrough and being jour- nalled (41,42) for rotation about an axis, and a restrained reaction turbine (40) downstream from the rotor to remove angular momentum from fluid dis- charged from the rotor, characterised in that the or each said passage (61) has a convergent inlet section (62) to receive fluid flow from said generator and a divergent outlet section (63) for discharge of the fluid stream. This construction of the passages can produce in the fluid discharged from the rotor a flow profile that is more nearly constant over a wide range of operating temperatures. In accordance with another aspect of this invention the clearance between the flowmeter housing and at least one of the rotor and the turbine includes a double labyrinth seal. These seals minimize leakage between the housing and each of the rotor and turbine without altering the characteristic friction of the rotor and turbine. Embodiments of the invention will now be des¬ cribed, by way of example, with reference to the accompanying drawings. Brief Description of the Drawings FIG. 1 is a longitudinal section of a mass rate of flow meter embodying this invention; FIG. 2 is a sectional view of part of the rotor of the flowmeter of FIG. 1; FIG. 3 is a perspective view of a portion of the rotor shown in FIG. 2; and FIG. 4 is a sectional view of another portion of the flowmeter of FIG. 1. Description of an Illustrative Embodiment FIG. 1 shows a flowmeter according to this invention. It comprises a housing 10 having an inlet port 11 and an outlet port 12 at the ends of the housing 10 which, with other elements of the flowmeter, defines a generally annular passage for a fluid, such as aircraft fuel. The passage is generally disposed along a longitudinal axis 13. A first sens¬ ing coil assembly 14 generates first timing, or start, pulses and is affixed to the housing 10. The assembly 14 has a longitudinal axis that is perpendicular to the axis 13 and is secured in a shield 15. A second sensing coil " assembly 16 generates second, or stop, timing pulses and is also affixed to the housing 10. The assembly 16 has a longitudinal axis that is coincident with the axis 13 and includes a sensing c.oil 17 that is located against a flange 20 at the outlet port 12. Conductors from both the first sensing coil assembly 14 and the second sensing coil assembly 16 terminate at a connector assembly (not shown). Both the coil assemblies 14 and 16 are isolated from the flow of fluid through the hous¬ ing 10. A first, or turbine, inner assembly 22 is radially positioned in the housing 10 by a coherent fit in the end flange 20 and is axially positioned by the end flange 20 and a retaining ring 21. The assembly 22 also supports a spring mechanism 23. At the inlet port 11, a second, or rotor, inner assem¬ bly includes a flow straightener, that comprises a plurality of ' longitudinally extending, circumferen- tially spaced vanes 25. The flow straightener is positioned in a tapered bore 26 and is mounted to one end of a stationary shaft 30A. An aligned, rotat- able shaft 30B is supported by the assembly 22 and lies on the longitudinal axis 13. A forward strut element in the rotor assembly comprises a stationary annulus 31 and a plurality of struts 32 that extend inwardly from the annulus 31 and that support a swirl generator 33. The annulus 31 radially positions the rotor assembly and coacts with a retaining ring to axially position the rotor 0 PI r -y. VV1P00 assembly in the housing. The swirl generator 33 supports the shaft 30A. A flanged ring 34 is carried on the outer surface of the vanes 25 and supports one end of a variable diameter conduit 35 that inc- ludes a plurality of spring fingers encircling the swirl generator 33 and that acts as a flow responsive valve. A second ring 36 clamps the conduit 35 and the ring 34- . to the vanes 25. This ring 36 also coacts with the housing 10 to radially position the shaft 30A. A rotor 37 is journalled on shaft 30A and is supported and positioned thereon by thrust bearings 41 and 42. A turbine 40 is axially spaced from rotor 37 and is secured to rotatable shaft 30B. Shaft 30B is journalled for rotation in assembly 22 by thrust bearings 43 and 44. A flat band, helical spring (not shown) in the spring mechanism 23 is clamped between the turbine shaft 30B and assembly 22 to restrain rotation of the turbine 40 about the axis 13. An outer annulus 45 on the rotor 37 supports a group of permanent bar magnets 46 in the periphery of the rotor 37. Each time the magnets 46 rotate past the sensing coil assembly 14, a start pulse is induced in the coil assembly 14 that indicates the passage of a predetermined point on the rotor 37 (i.e., the location of the magnets 46) past a predetermined point on the housing 10 (i.e., the location of the coil assembly 14). Another group of permanent bar magnets 47 also mounts to the outer annulus 45 of the rotor 37. More specifically, the annulus 45 has an annular extension 50 that extends toward and overlaps a por- tion of the turbine, specifically the ends of turbine blades 51 on the turbine. Grooves 52 are cut in the outer surface of the extension 50 to carry the bar magnets 47 which are closely spaced to each other. In addition to the turbine blades 51, the turbine 40 carries an exciter blade 53 of a permeable material and a diametrically opposed, non-permeable, balancing blade (not shown). An outer ring band, or shroud, 54 fits over the turbine blades 51, the exciter blade 53, and the balancing blade. The band 54 engages a flux collecting ring 55 of a permeable material between the band 54 and a radial extension 56 on the turbine 40. The ring 55 bears against a tab 57 from the exciter blade 53 and a similar tab from the balancing blade. Each time the magnets 47 pass the exciter blade 53, flux linkages are coupled to the coil 17 through the exciter blade 53 and the flux collection ring 55 and induce an electrical stop pulse in the sensing coil 17 that indicates the passage of another predetermined point on the rotor 37 (i.e., the loca¬ tion of the magnet 47) past a predetermined point on the turbine (i.e., the position of the exciter blade 53). The time between the start and stop pulses is representative of mass flow rate. FIG. 2 is a sectional view taken through the upper portion of the unrestrained rotor 37 and the _ O PI . W1P0 housing 10. The rotor passages are constructed to improve the performance of the flowmeter over wide temperature variations by reducing changes in flow profile that would otherwise occur in fluid discharg- ing from the rotor relative to that entering the rotor. The unrestrained rotor 37 includes the outer annulus 45 and a hub 60 between which the fluid stream is conveyed through tapered passages, such as the passage 61. These passages are arranged in a circle thereby to form an annulus of passages around the rotor 37. Each tapered passage has an inlet section 62 that receives the fluid stream from the swirl gener¬ ator and an outlet section 63 through which fluid flows toward the restrained reaction turbine 40. The inlet section 62 of the tapered passages 61 is formed by a section 64 of the outer -annulus 45 that converges with a section 65 of the hub 60. The outlet section 63 of the tapered passages 61 is formed by a section 66 of the outer annulus 45 that diverges from a section 67 of the hub 60. A splitter vane. " 70 may be interposed in the diverging outlet section of the tapered passage. This vane 70 is centred in the outer section 63 of the tapered passage 61 for maintaining flow stability. Referring to FIG. 3, radially aligned, circum- ferentially spaced grooves 71 and 72 are machined into the outer annulus 45 and the inner hub 60, respec¬ tively. At the inlet end of the rotor, each groove 72 is notched through to form a radial slot 73. Each pair of radially aligned grooves 71 and 72 sup¬ ports a divider plate 74 that defines the circumferen¬ tial limits of each passage. _ Q_ Each divider plate 74 has a tab 75 that fits into a corresponding one of the radial slots 73. There also is formed in each divider plate 74 a longi¬ tudinally extending slot 76. When all the plates 74 are installed, the locus of the slots 76 defines a circumferential support for a cylinder 77. The portion of the cylinder 77 between adjacent divider plates 74 constitutes the vane 70 in the passage. The reduction of the inlet section 62 to a small diameter at 80 in FIG. 2 causes all the fluid to pass through an area that more accurately defines a constant radius of gyration. Moreover, the fluid flow through the narrow passage at 80 tends to mini¬ mize any eddies or other Influences that detract from a uniform and constant flow profile even as viscosity changes. The diverging outlet section 63 permits the flow to expand, thereby lowering the uniform velocity of fluid presented to the turbine 40. The splitter vane 70 assists in maintaining flow stability by preventing further change in flow profile. As a result, the-angular velocity of the rotor 37 becomes less dependent upon viscosity so the effect of this source of possible error is greatly reduced. Leakage is reduced by the use of double laby¬ rinth seals that are shown in FIG. 4. A double laby¬ rinth comprises two sets of seals each of which includes two parallel, offset channels and an inter¬ connecting perpendicular channel between them. Fluid must therefore pass through two ninety-degree bends to pass through the seal. Thus, in a double labyrinth seal the fluid must pass through four ninety-degree bends. BURE4 0. PJ WJPO ■ Referring first to ■ the rotor 37, a double labyrinth seal 81 is disposed in the clearance between the outer annulus 45 of the rotor 37 and the housing 10. More specifically, the outer annulus 45 includes outer surfaces 82 and 83 of a maximum radius. These surfaces are separated from corresponding inner sur¬ faces 84 and 85 of the housing 10 by clearances 86 and 87. A circumferential channel 90 is machined in the outer annulus 45 and a radially extending sealing ring 91 is formed in the housing 10. The radial depths of the channel 90 and sealing ring 91 exceed the radial clearance 87 and approximate the radial clearance 86. Thus, a longitudinal clear¬ ance passage 92 is formed between the outer surface of the channel 90 and the sealing ring 91. The pas¬ sage 92 is parallel to and radially offset from the passage 87 and is interconnected to it by radially extending passage 94. The passage 92 is parallel to the passage 86 and interconnected to it by passage 93. In FIG. 4, the passages 86, 93, and 92 form one labyrinth seal; the passages 92, 94, and 87, the second labyrinth seal. Thus, the double labyrinth seal 81 of the rotor is formed by two axially dis¬ placed labyrinth seals. Additionally, an annular notch 110 is machined in the outer annulus 45 of the rotor 37 to a . radial depth greater than that of the circumferential channel 90. There is a similar construction at the turbine 40, except for the notch 110. A radially extending sealing ring 95 in the housing 10 and a channel 96 in the outer surface of the ring 54 provide double labyrinth seal 97. Specifically, the sealing ring 95 interrupts longitudinal passages 100 and 101 (pas- -gU R 0.M W1P sages 100 and 87 interconnect). Longitudinal passage 103 is formed between the sealing ring 95 and the channel 96, and is parallel to passages 100 and 101 and radially offset from passage 101. A passage 102 interconnects the passages 100 and 103; a radial passage 104 interconnects passages 103 and 101. Fluid must flow from the passage 100 through the passage 102. to the passage 103. From the passage 103, fluid flows through radial passage 104 to passage 101. Each of the double labyrinth seals 81 and 97 interrupts axial or longitudinal leakage between the rotating rotor 37 and turbine 40 and the housing 10. This, by itself, tends to reduce leakage even when the turbine 40 is stationary. When the rotor 37 turns, an even greater impediment to longitudinal flow is provided because the fluid in the clearances tends to rotate with the rotor and produces a barrier- like area to such flow. The reductions in leakage through these clearances are significant. The leakage becomes low as a percentage of flow even at low flow rates. Moreover, leakage tends to become essentially Independent of fuel viscosity. As a result, changes in leakage do not adversely affect the fuel flow rate measurement. The notch 110 in the outer annulus 45 of the rotor 37 further decreases leakage by providing an additional interruption to axial leakage. That is, some fluid in the passage 92 diverts into the notch 110 instead of flowing axially through the passage 92. Moreover, the notch causes turbulence in the fluid flow that also decreases leakage. As the rotor 37 turns, the notch 110 produces an additional impedi- ment to axial flow by affording another area where accumulating fluid has a barrier-like effect. This double labyrinth seal construction also facilitates assembly of the flowmeter. Referring to FIG. 4, the radius of the rotor 37 at the passage 86 is selected to allow the rotor to clear the sealing ring 91. Consequently the rotor 37 can readily be inserted axially into position within the housing 10. When two separately sealed rotating bodies are inserted within a housing, the same ease of assembly can be realized. In ' FIG. 4 the rotor 37 and the turbine 40 constitute separate rotating bodies. The rotor 37 can clear the sealing ring 95 associated with the turbine 40 because the radius of the rotor 37 at the passage 87 is less than the radius at the sealing ring 95. Similarly, the radius of the shroud 54 on the turbine 40 at the passage 100 is less than the radius of the sealing ring 95. Consequently both the rotor 37 and turbine 40 clear the sealing ring to facilitate the assembly of both elements in the housing 10. The described flowmeter thus operates with reduced sensitivity to changes of operating tempera¬ ture and to changes in flow rate.

Claims: CLAIMS : 1. A mass rate of flow meter comprising a swirl generator (33) to impart angular momentum to a fluid stream to be measured, a rotor (37) downstream from the swirl generator, said rotor having at least one fluid flow passage (61) therethrough and being journalled (41,42) for rotation about an axis, and a restrained reaction turbine (40) downstream from the rotor to remove angular momentum from fluid dis- charged from the rotor, characterised in that the or each said passage (61) has a convergent Inlet section (62) to receive fluid flow from said generator and a divergent outlet section (63) for discharge of the fluid stream. 2. A flowmeter according to claim 1 character- ised by a splitter vane (70) disposed substantially centrally in and extending along said divergent outlet section (63) of the or each said passage (61). 3. A flowmeter according to claim 1 wherein said rotor comprises a hub (60) and an outer annular member (45), and characterised in that said convergent inlet section (62) comprises mutually convergent surfaces of said hub and said annular member, and in that said divergent outlet section (63) comprises mutually divergent surfaces of said hub and said annular member. 4. A flowmeter according to claim 3 wherein there are a plurality of circumferentially spaced longitudinally extending said passages (61), and characterised by a plurality of circumferentially spaced radially extending divider plates (74) bounding said plurality of passages. 5. A flowmeter according to claim 4 character- ised in that each said divider plate (74) has a longi¬ tudinal slot (76) at said outlet section, and a hollow cylinder (77) is received in said slots to form a splitter vane (70) extending between said divider plates in each said passage (61). 6. A flowmeter according to any one of claims 1 to 5 characterised by double labyrinth sealing means (81,97) between a housing (10) of the flowmeter and at least one of said rotor and said turbines. 7. A mass rate of flow meter comprising a housing (10), a swirl generator (33) in the housing to impart angular momentum to a fluid stream to be measured, a rotor (37) downstream from the swirl generator for rotation by fluid from the swirl genera¬ tor, said rotor being journalled for rotation within and spaced from said housing, a restrained reaction turbine (40) downstream from the rotor to remove angular momentum from fluid discharged from the rotor, said turbine being journalled for rotation within and spaced from said housing, characterised by double labyrinth sealing means (81,97) formed by said housing (10) and at least one of said rotor (37) and said turbine (40) to impede flow of fluid between the housing and at least one of said rotor and said tur¬ bine.

Inventors: WYLER J
Applicants: GEN ELECTRIC, GENERAL ELECTRIC CO
Application year: 1980
Main classification: G01F1
IPC codes: G01F1