Abstrict An improved cylindrical conduit for directing fluid flow to the
swirl generator of an angular momentum type rate of flow meter commonly
employed in measuring fuel flow rate in aircraft engines. The conduit
includes a first set of longitudinally extending, resilient fingers
and a second set of concentric sealing fingers that seal the resilient
fingers in the first set. At low flow rates, the sealing fingers
prevent leakage between the resilient fingers and the conduit directs
substantially all fluid to the swirl generator.
Claims What is claimed as new and desired to be secured by Letters Patent
of the United States is:
1. In a mass rate of flow meter of the angular momentum type, having
a swirl generator for imparting angular momentum to the measured
fluid stream, a restrained reaction turbine located downstream from
the swirl generator for removing the imparted angular momentum,
the improvement of a substantially cylindrical conduit means for
controlling fluid flow to and coaxial with the swirl generator comprising:
A. multiple, longitudinally extending, independently deflectible,
resilient fingers which deflect in response to fluid pressure for
conducting fluid to the swirl generator; and
B. sealing means contiguous to said resilient fingers for substantially
reducing fluid leakage between said resilient fingers.
2. A mass rate of flow meter as recited in claim 1 wherein said
sealing means comprises compliant means for maintaining a sealing
relationship as said resilient fingers deflect.
3. A mass rate of flow meter as recited in claim 1 wherein said
resilient fingers are outwardly deflectible and said sealing means
comprises independently deflectible means contiguous to an inner
cylindrical surface of the conduit and overlapping the resilient
fingers.
4. A mass rate of flow meter as recited in claim 3 wherein the
deflectible means comprises multiple, longitudinally extending,
deflectible sealing fingers, each said sealing finger having substantially
the same circumferential dimension as a resilient finger of the
conduit and overlapping a pair of contiguous resilient fingers.
Description BACKGROUND OF THE INVENTION
This invention relates to mass rate of flow meters of the angular
momentum type having a swirl generator for imparting angular momentum
to a measured fluid stream and a restrained reaction turbine for
removing the imparted angular momentum. More particularly, the invention
relates to such flowmeters having an improved construction for preventing
leakage and optimizing fluid flow at low flow rates.
A mass rate of flow meter of the angular momentum type is disclosed
by Hildebrand et al in U.S. Pat. No. 4056976 issued Nov. 8 1977
titled "Mass Rate of Flow Meter" and assigned to the same
assignee as the present invention. In this prior art flowmeter,
a swirl generator includes radially extending skewed vanes that
impart angular momentum to the fluid passing between the vanes.
Swirling fluid from the swirl generator passes through multiple
tubes in an unrestrained rotor located downstream. Fluid leaving
the rotor has the same angular velocity as the rotor. The restrained
reaction turbine also includes multiple tubes that form longitudinal
flow channels. As swirling fluid leaves the rotor, the restrained
reaction turbine removes the angular momentum from fluid flowing
through the longitudinal channels. The torque it experiences in
doing so is balanced by a restraining means including a biased spring
that permits measurement of flow rate by methods not material to
the present invention.
U.S. Pat. No. 3538767 issued Nov. 10 1970 titled "Flowmeter
Fluid Drive", and assigned to the same assignee as the present
invention, discloses another type of flowmeter that incorporates
a torque motor for restraining the turbine. This flowmeter includes
a cylindrical conduit within the flowmeter casing for conducting
fluid to the swirl generator. The conduit comprises longitudinally
extending resilient fingers that maintain a substantially cylindrical
shape of the conduit at low flow rates in order that the conduit
directs as much incoming fluid as possible to the swirl generator.
At high flow rates, the same resilient fingers deflect outwardly
in response to fluid pressure to control the angular momentum imparted
to the fluid.
Thus the total fluid flow downstream from the swirl generator is
a mixture of flows that have gone through the vanes on the swirl
generator or bypassed them under the control of the conduit. This
control prevents the swirl velocity of the fluid entering the reaction
turbine from becoming excessive at high flow rates.
In flowmeters of the type described in the foregoing U.S. Pat.
No. 4056976 timing circuits sense start and stop pulses induced
in a pair of coils and use these pulses to determine rotor speed
and to determine deflection of the restrained reaction turbine.
The amplitude and width of these pulses vary with rotor speed. In
the unrestrained rotor flowmeters, however, the rotor can have a
wider range of angular velocities (e.g., from 1 to 6 revolutions
per second). Thus, the timing circuits must either include circuitry
for compensating these variations or operate with inaccuracies.
At low flow rates, it has been found that even compensated flowmeters
of this type tend to have non-linear errors that are not readily
compensated. Several sources of these inaccuracies have been found.
One source is fluid leakage between the individual resilient fingers
of the cylindrical conduit. This leakage limits the percentage of
fluid flow affected by the swirl generator at low flow rates. As
a result, the angular velocity of the rotor decreases as the flow
rate decreases. Moreover, the angular velocity can reach a level
at which accurate measurements of the start and stop pulse timing
become difficult to achieve.
SUMMARY
Therefore, it is an object of the present invention to provide
an improved mass rate of flow meter of the angular momentum type
that enables more accurate measurements at low flow rates.
A further object of this invention is to provide an improved mass
flow meter of the angular momentum type that reduces fluid leakage
between the resilient fingers of a cylindrical conduit that directs
fluid through and around the swirl generator.
Yet a further object of this invention is to minimize the changes
in angular velocity of an unrestrained rotor in mass rate of flow
meters over a wide range of flow rates.
In accordance with this invention, the above objects are achieved
by improving the substantially cylindrical conduit that directs
fluid to and around the swirl generator. More specifically, the
improvement includes conduit means comprising longitudinally extending,
independently resilient fingers and sealing means contiguous to
said resilient fingers for substantially reducing leakage between
said resilient fingers.
This invention is pointed out with particularity in the appended
claims. The above and further objects and advantages of this invention
may be better understood by referring to the following detailed
description taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a longitudinal view in cross-section of a mass rate of
flow meter embodying this invention;
FIG. 2. is an exploded perspective view of certain construction
details of the first and second substantially cylindrical conduits
and the swirl generator;
FIG. 3. is a side view, partly in section, of the first and second
substantially cylindrical conduits mounted in supporting flanges;
and
FIG. 4 is a cross-sectional view of the first and second substantially
cylindrical conduits.
DESCRIPTION OF AN ILLUSTRATIVE EMBODIMENT
FIG. 1 illustrates an exemplary flowmeter that incorporates 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 sensing 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 coil 17 that is disposed at 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 a fluid through the housing 10.
A first inner, or turbine, assembly is radially positioned on the
housing 10 by a housing end flange 20 and an end assembly 22 and
is axially positioned by a retaining ring 21. The end assembly 22
also supports a spring mechanism 23. At the inlet port 11 a second
inner, or rotor, assembly includes a flow straightener 24 that comprises
a plurality of longitudinally extending, circumferentially spaced
vanes 25. The flow straightener 24 is positioned in a tapered bore
26 and is mounted to one end of a shaft 30A. An aligned shaft 30B
is supported by the end 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 31A to axially position the rotor assembly on the housing 10.
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 includes a plurality
of spring fingers that encircle the swirl generator 33. The conduit
35 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 and a turbine 40 are journaled on shafts 30A and 30B
respectively in an axially spaced relationship. Thrust bearings
41 and 42 support and position the rotor 37 on the shaft 30A; thrust
bearings 43 and 44 the turbine 40 on the shaft 30B. A flat band,
helical spring (not shown) in the spring mechanism 23 is clamped
between the turbine 40 and the shaft 30B to restrain rotation of
the turbine 40 about the shaft 30B.
An outer annulus 45 on the rotor 37 supports a group of permanent
bar magnets 46 in the periphery of the rotor 37. These magnets are
disposed to produce a north-south magnetic axis along a chord near
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 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 portion
of the turbine, specifically the ends of turbine blades 51 on the
turbine. Longitudinal grooves 52 are cut in the outer surface of
the extension 50 to carry longitudinaly extending, closely spaced,
radially poled magnets 47. These magnets 47 also produce a field
with a north-south magnetic axis lying along a chord near the periphery
of the rotor 37.
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 band, or shroud,
54 fits over the turbine blade 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 location 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 flow rate.
Referring to FIGS. 2 and 3 the swirl generator 33 has a base 60
and a nose portion 61. On the periphery of the nose portion 61 are
multiple skewed vanes 62 adjacent to slots in the base 60. The fluid
stream impinges first on the nose portion 61 where it is directed
radially outwardly toward the slots between the skewed vanes 62
in the base 60 of the swirl generator 35.
Further referring to FIG. 2 the cylindrical conduit 35 surrounds
and bears against the swirl generator 33 at rest. At low flow rates,
the conduit 35 maintains its cylindrical position, continues to
bear against the base 60 of the swirl generator 33 and directs
incoming fluid through the vanes 62 of the swirl generator 33.
The discharge end 63 of the conduit 35 has multiple longitudinally
extending, independently deflectible resilient fingers 64 that deflect
outwardly in response to fluid pressure developed at high flow rates.
As a result of this outward deflection, some fluid is directed through
the swirl generator 33 while other fluid is directed around the
swirl generator where it will not be influenced.
As previously discussed, the conduit 35 is affixed to the outside
of the flanged ring 34. A second ring 36 fits over the conduit 35
and the first ring 34. The second ring 36 clamps the ring 34 to
the vanes 25 of the flow straightener 24.
Further referring to FIGS. 2 3 and 4 the preferred embodiment
of the present invention comprises the resilient fingers 64 in combination
with compliant sealing means 65 contiguous to said resilient fingers
64. Outwardly deflectible sealing fingers 66 are concentric with
the conduit 35 and contiguous to the inner cylindrical surface of
the conduit 35. In this embodiment, each deflectible sealing finger
66 has substantially the same circumferential dimension as a resilient
finger 64 of the conduit 35 and overlaps a pair of contiguous resilient
fingers 64. The sealing means 65 fits over the flanged ring 34 inside
the conduit 35.
During operation at low flow rates, the sealing fingers 66 significantly
reduce leakage between the resilient fingers 64. As flow rate increases,
the resilient fingers 64 deflect outwardly. The modulus of elasticity
for the sealing fingers 66 is selected so that the sealing fingers
66 remain in contact with the fingers 64 as they deflect without
impeding either the deflection or the flow of fluid. Thus, the sealing
fingers 66 constitute one embodiment of an elastic sealing means
that is contiguous to a cylindrical surface defined by the resilient
fingers at low flow rates and that is sufficiently compliant to
maintain a sealing relationship even as the resilient fingers 64
deflect.
A flowmeter that incorporates the sealing means of the present
invention has a rotor velocity at low flow rates that is significantly
greater than the velocity achieved in prior flowmeters. Specifically,
rotor velocities in the range of three revolutions per second (RPS)
have been achieved as compared to much lower rotor velocities (i.e.,
in the order of one RPS) measured in a prior flowmeter. Moreover,
this structure of the conduit 35 has provided a related advantage.
There is a certain minimum flow rate that is required to start turning
the rotor. A conduit 35 constructed in accordance with this invention
reduces this minimum. Consequently the minimum measurable flow rate
also is reduced, so the flowmeter is more sensitive.
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