Abstrict A fluid flow meter having a paddle wheel rotatably mounted in a
cylindrical cavity and an inlet conduit and an outlet conduit tangentially
aligned with the periphery of the cavity for directing a fluid to
flow through it. The paddle wheel's axis of rotation is displaced
from the cavity's centerline, in the direction of the inlet conduit
and substantially perpendicular to the flow of fluid entering the
cavity, such that the flow meter operates effectively over an extended
range of Reynolds numbers, spanning both turbulent and laminar flows.
Claims We claim:
1. A fluid flow meter comprising:
a housing having means defining a substantially cylindrical cavity;
inlet and outlet conduit means closely spaced to each other and
aligned substantially tangentially with the periphery of the cavity,
for directing a fluid to flow through a major portion of the cavity,
from the inlet conduit means to the outlet conduit means;
paddle wheel means mounted in the cavity for rotation in accordance
with the fluid flow, the paddle wheel means including a plurality
of substantially flat, evenly-spaced paddles; and
means for sensing the angular velocity of the paddle wheel means
and producing a corresponding signal;
wherein the paddle wheel means is mounted for rotation about an
axis displaced from but parallel to the longitudinal axis of the
cavity such that the fluid flow rotates the paddle wheel means in
a prescribed direction over an extended range of Reynolds numbers,
spanning both turbulent and laminar flow regimes.
2. A fluid flow meter as defined in claim 1 wherein:
the axis of rotation of the paddle wheel means is displaced from
the longitudinal axis of the cavity in the direction of the inlet
conduit means, substantially perpendicular to the direction of flow
of the fluid entering the cavity; and
the paddle wheel means projects at least part way into an imaginary
longitudinal extension of the inlet conduit means into the cavity.
3. A fluid flow meter as defined in claim 2 wherein the axis of
rotation of the paddle wheel means is located about one-third of
the way from the longitudinal axis of the cavity to the edge of
the imaginary longitudinal extension of the inlet conduit means
into the cavity, and the paddle wheel means projects at least about
halfway into the imaginary extension of the inlet conduit means.
4. A fluid flow meter as defined in claim 1 wherein the inlet
conduit means and the outlet conduit means are substantially parallel
to each other, but oppositely directed, and the fluid travels substantially
180 degrees in one plane through the cavity, from the inlet conduit
means to the outlet conduit means.
5. A fluid flow meter as defined in claim 4 wherein the outlet
conduit means has a cross sectional area greater than that of the
inlet conduit means.
6. A fluid flow meter as defined in claim 1 wherein the inlet
conduit means and the outlet conduit means both have substantially
circular cross sections.
7. A fluid flow meter as defined in claim 1 wherein the substantially
cylindrical cavity included in the housing is free of any fluid-diverting
baffles.
8. A fluid flow meter as defined in claim 1 wherein each paddle
of the paddle wheel means includes a hole through its base end.
9. A fluid flow meter as defined in claim 1 wherein:
the paddle wheel means includes a rotatable shaft having a needle
point at each end; and
the housing includes first and second bearing means located on
opposite sides of the cavity, each bearing means including
a seat for receiving the shaft needle point at one end of the rotatable
shaft, the seat being adapted to permit low-friction rotation of
the needle point, and
an annular seal for encircling a portion of the shaft adjacent
its needle point to prevent minute debris from entangling in the
bearing means.
10. A fluid flow meter comprising:
a housing having means defining a cylindrical cavity;
means defining an inlet conduit aligned tangentially with the cylindrical
cavity, for delivering a fluid thereto;
means defining an outlet conduit aligned tangentially with the
cylindrical cavity, for removing fluid therefrom, the inlet and
outlet conduits being substantially parallel to each other and displaced
from each other such that the direction of the fluid flow is changed
by about 180 degrees;
a paddle wheel having a plurality of substantially flat, evenly-spaced
paddles and mounted in the cylindrical cavity for rotation in accordance
with the flow of fluid from the inlet conduit to the outlet conduit;
and
means for sensing the angular velocity of the paddle wheel and
producing a corresponding signal;
wherein the paddle wheel is mounted for rotation about an axis
displaced from the centerline of the cylindrical cavity, in the
direction of the inlet conduit and substantially perpendicular to
the longitudinal axis of the inlet conduit, the paddle wheel projecting
at least part way into the flow pattern of the fluid entering the
cavity from the inlet conduit, whereby the paddle wheel is rotated
by the fluid in a prescribed direction over an extended range of
Reynolds numbers, spanning both turbulent and laminar flow regimes.
11. A fluid flow meter as defined in claim 10 wherein the axis
of rotation of the paddle wheel is located about one-third of the
way from the centerline of the cavity to the edge of an imaginary
longitudinal extension of the inlet conduit into the cavity, and
the paddle wheel projects at least about halfway into the imaginary
extension of the inlet conduit.
12. A fluid flow meter as defined in claim 10 wherein:
the outlet conduit has a cross sectional area greater than that
of the inlet conduit; and
the inlet conduit and the outlet conduit both have substantially
circular cross sections.
13. A fluid flow meter as defined in claim 10 wherein:
the paddle wheel includes a rotatable shaft having a needle point
at each end; and
the housing includes first and second bearing means located on
opposite sides of the cylindrical cavity, each bearing means including
a seat for receiving the shaft needle point at one end of the rotatable
shaft, the seat being adapted to permit low-friction rotation of
the needle point, and
an annular seal for encircling a portion of the shaft adjacent
its needle point, to prevent minute debris from entangling in the
bearing means.
14. A fluid flow meter as defined in claim 10 wherein the diameter
of the paddle wheel is about sixty percent that of the cylindrical
cavity.
15. A fluid flow meter comprising:
a housing having means defining a cylindrical cavity;
means defining an inlet conduit aligned tangentially with the cylindrical
cavity, for delivering a fluid thereto;
means defining an outlet conduit aligned tangentially with the
cylindrical cavity, for removing fluid therefrom, wherein the inlet
and outlet conduits are substantially parallel to each other and
displaced from each other such that the direction of fluid flow
is changed by about 180 degrees, and wherein the outlet conduit
is larger than the inlet conduit and both have substantially circular
cross sections;
a paddle wheel having a rotatable shaft with a needle point at
each end and a plurality of substantially flat, evenly-spaced paddles,
each paddle having a hole through its base end, the paddle wheel
being mounted in the cylindrical cavity for rotation in accordance
with the flow of fluid from the inlet conduit to the outlet conduit;
wherein the housing includes first and second bearing means located
on opposite sides of the cylindrical cavity, each bearing means
including
a seat for receiving the shaft needle point at one end of the rotatable
shaft, the seat being adapted to permit low-friction rotation of
the needle point, and
an annular seal for encircling a portion of the shaft adjacent
its needle point, to prevent minute debris from entangling in the
bearing means; and
means for sensing the angular velocity of the paddle wheel and
producing a corresponding signal;
wherein the paddle wheel is mounted for rotation about an axis
located about one-third of the way from the centerline of the cylindrical
cavity to the edge of an imaginary longitudinal extension of the
inlet conduit into the cavity and the paddle wheel projects about
halfway into the imaginary extension of the inlet conduit, such
that the paddle wheel is rotated by the fluid in a prescribed direction
over an extended range of Reynolds numbers, spanning both turbulent
and laminar flow regimens.
Description BACKGROUND OF THE INVENTION
This invention relates generally to fluid flow meters, and more
particularly to flow meters of a type having a paddle wheel rotatably
mounted within a substantially cylindrical cavity.
An example of a fluid flow meter of this particular type is described
in U.S. Pat. No. 4173144 to Edwin Pounder. It includes a paddle
wheel mounted for rotation in a cylindrical cavity, and inlet and
outlet conduits for directing a fluid to flow through the cavity
and rotate the paddle wheel at an angular velocity corresponding
generally to the flow rate. The paddle wheel rotates about an axis
aligned with the cavity's longitudinal axis, and a special optical
sensor senses its angular velocity to provide a signal indicative
of the fluid flow rate.
Although flow meters like the one described above have generally
performed satisfactorily, especially at relatively high flow rates,
they are believed to be limited in the range of Reynolds numbers
over which they can operate effectively. The devices are generally
effective when measuring flows having relatively high Reynolds numbers,
i.e., turbulent flow, but are generally ineffective for flows having
relatively low Reynolds numbers, i.e., laminar flow. This can be
a drawback in flow meter applications such as soft drink dispensers,
where the viscosity of the fluids being dispensed can vary significantly
and the flow can be either laminar or turbulent.
It should therefore be appreciated that there is a need for an
improved paddle wheel type flow meter that can operate effectively
for fluid flows covering an extended range of Reynolds numbers.
There is particularly a need for a flow meter that can operate effectively
for a range of Reynolds numbers spanning both turbulent and laminar
flow regimes. The present invention fulfills this need.
SUMMARY OF THE INVENTION
The present invention is embodied in an improved paddle wheeltype
fluid flow meter that operates effectively over an extended range
of Reynolds numbers. The flow meter includes a housing having means
defining a substantially cylindrical cavity, and inlet conduit means
and outlet conduit means aligned substantially tangentially with
the periphery of the cavity for directing a fluid to flow through
it. The flow meter further includes paddle wheel means mounted in
the cavity for rotation in accordance with the fluid flow, and means
for sensing the angular velocity of the paddle wheel means and producing
a corresponding signal. In accordance with the invention, the paddle
wheel means is mounted for rotation about an axis displaced from
the longitudinal axis of the cavity such that the fluid flow rotates
the paddle wheel means in a prescribed direction over an extended
range of Reynolds numbers.
More particularly, the inlet conduit means and the outlet conduit
means are substantially parallel to each other, and the fluid travels
in one plane substantially 180 degrees through the cavity, from
the inlet to the outlet. Also, the outlet conduit means has a larger
cross sectional area than the inlet conduit means, to reduce pressure
losses, and both conduit means have cross sections that are preferably
substantially circular. The paddle wheel includes a plurality of
substantially evenly-spaced paddles, each having a hole at its base
end.
To most effectively extend the operating range of the flow meter,
the paddle wheel is displaced from the longitudinal axis of the
cavity in the direction of the inlet conduit means, substantially
perpendicular to the direction of fluid flow entering the cavity.
At least a portion of the paddle wheel projects into an imaginary
extension of the inlet conduit means into the cavity. In the preferred
embodiment, the paddle wheel has a diameter approximately 60 percent
that of the cavity, and it rotates about an axis located above one-third
of the way from the longitudinal axis of the cavity to the edge
of the imaginary extension of the inlet conduit means. The flow
meter therefore operates effectively over a dynamic range of Reynolds
numbers of at least about 400 to 1 spanning both turbulent and
laminar flow regimes.
In another aspect of the invention, the paddle wheel rotates in
a special needle bearing that includes a seal to prevent minute
debris from becoming lodged in the bearing.
Other aspects and advantages of the present invention will become
apparent from the following description of the preferred embodiment,
taken in conjunction with the accompanying drawings, which illustrate,
by way of example, the principles of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of a fluid flow meter embodying the
present invention;
FIG. 2 is a sectional view of the flow meter taken in direction
of arrows 2--2 in FIG. 1;
FIG. 3 is a sectional view of the flow meter taken in the direction
of arrows 3--3 in FIG. 2;
FIG. 4 is a perspective view of the paddle wheel depicted in FIGS.
2 and 3;
FIG. 5 is a sectional view of the paddle wheel taken in the direction
of arrows 5--5 in FIG. 4;
FIG. 6 is a schematic view of the flow meter's cylindrical cavity,
showing the flow pattern for relatively high Reynolds numbers;
FIG. 7 is a schematic view of the flow meter's cylindrical cavity,
showing the flow pattern for relatively low Reynolds numbers; and
FIG. 8 is a graph showing the ratio of paddle wheel tip speed to
inlet flow speed as a function of 1n(Reynolds number), for three
different paddle wheel locations, one being its location in the
preferred embodiment of FIGS. 1-7.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to the drawings, and particularly to FIGS. 1-3 there
is shown an improved paddle wheeltype fluid flow meter that operates
effectively over an extended range of Reynolds numbers, spanning
both turbulent and laminar flows. The flow meter includes a housing
comprising a base block 11 and a cap 13 that are fastened together
and appropriately configured to form a cylindrical cavity 15 between
them. An inlet conduit 17 supplies fluid to the cavity and an outlet
conduit 19 carries the fluid away from it. The two conduits are
substantially parallel to each other and oriented tangentially to
the periphery of the cylindrical cavity such that the fluid travels
substantially 180 degrees in one plane through the cavity.
The flow meter further includes a paddle wheel 21 that is mounted
in the cylindrical cavity 15 for rotation in accordance with the
flow rate of the fluid flowing through it. An optical sensor 23
located in the base 11 adjacent the cavity detects angular movement
of the paddle wheel and provides a corresponding signal indicative
of its angular velocity.
In accordance with the invention, the axis of rotation of the paddle
wheel 21 is displaced from the longitudinal axis of the cylindrical
cavity 15 in the direction of the inlet conduit 17 and substantially
perpendicular to the inlet conduit's longitudinal axis. A portion
of the paddle wheel projects into an imaginary extension of the
inlet conduit into the cavity. This off-center displacement of the
paddle wheel increases the range of Reynolds numbers over which
the flow meter is effective, and in particular enables the flow
meter to operate effectively in both turbulent and laminar flow
regimens.
More particularly, and with particular reference to FIGS. 2 and
3 the base block 11 includes an annular recess having a planar
bottom wall 25 that defines one side of the cylindrical cavity 15
and a circular side wall 27 that defines the periphery of the cavity.
The cap 13 includes an annular projection that fits part way into
the recess of the base block, to form the cavity. A planar bottom
wall 29 of the cap forms the cavity's second side wall. An O-ring
31 is disposed in an annular seat 33 formed in the cap projection,
to prevent fluid from flowing outwardly from the cavity into the
space between the cap and the base block. Three screws 35 fasten
the cap and base block securely together.
The inlet conduit 17 and the outlet conduit 19 both have circular
cross sections, with the inlet conduit having a diameter about half
the thickness of the cylindrical cavity 15 and the outlet conduit
having a diameter substantially equal to the cavity thickness. This
size difference minimizes choking and provides reduced pressure
loss through the flow meter.
As shown in FIGS. 4 and 5 the paddle wheel 21 includes four evenly-spaced
paddles 37 and a stainless steel shaft 39 at the center of the paddles
with a needle point 41 on each of its ends. A hole is formed through
the base of each paddle. Bearings 43 and 45 (FIG. 3) are disposed
in recesses formed in the cavity's two side walls 25 and 29 respectively.
The needle points of the paddle wheel shaft are received in seats
in the two bearings, to keep the paddle wheel properly centered
in the cavity 15 and to take any side load imposed on the paddle
wheel by the moving fluid.
The bearings 43 and 45 further include integral seals for enveloping
portions of the paddle wheel shaft 39 spaced from the respective
need points 41. This prevents minute debris from becoming entangled
in the needle bearings. There is sufficient clearance, however,
to permit the fluid to lubricate the bearings. These seals also
function as journal bearings for taking the flow load.
As shown in FIG. 2 the paddle wheel 21 has a diameter about 60
percent that of the cylindrical cavity 15 and rotates about an axis
displaced from the center of the cavity, in the direction of the
inlet conduit 17. In particular, this rotation axis is displaced
about one-third of the way from the cavity's center to the edge
of an imaginary extension of the inlet conduit, as shown by the
dotted line 47. The paddle wheel therefore projects about halfway
into the flow pattern of the inlet conduit. This paddle wheel placement
has been found to extend significantly the range of Reynolds numbers
over which the flow meter operates effectively, spanning both turbulent
and laminar flows. The flow meter operates over a dynamic range
of Reynolds numbers of at least about 400 to 1 and has substantially
linear performance curve for a dynamic range of at least about 150
to 1.
The optical sensor 23 includes an LED light source 49 and a photodiode
51 located adjacent to each other in a recess 53 formed in the base
block 11. The base block is preferably formed of a transparent polycarbonate
material, to permit light from the light source to pass through
it. One suitable optical sensor is a Model OPB 708 reflective object
sensor, manufactured by TRW.
When the paddle wheel 21 rotates, the tips of its paddles 37 successively
reflect light from the LED light source 49 to the photodiode 51.
The frequency of the signal output by the photodiode therefore indicates
the angular velocity of the rotating paddle wheel. Forming the paddle
wheel of a highly reflective material, or painting the paddle tips
with a highly reflective paint, increases the intensity of the light
reflected to the photodiode and enchances the sensor's performance,
especially when the fluid passing through the cavity 15 has at least
some opacity.
FIGS. 6 and 7 are schematic views of the cylindrical cavity 15
showing the flow pattern for a very high Reynolds number and turbulent
flow (FIG. 6) and for a very low Reynolds number well below the
laminar-turbulent transition (FIG. 7). It will be observed that
for turbulent flow, a vortex is created within the cavity, whereas
for the low Reynolds number laminar flow, there is no vortex and
the fluid flows directly from the inlet conduit 17 to the outlet
conduit 19. Locating the paddle wheel 21 in the position indicated
insures that the moving fluid causes it to rotate in the same counter
clockwise direction for flows spanning the entire range of conditions
between those represented in FIG. 6 and 7.
FIG. 8 is a graph depicting the ratio of paddle wheel speed to
inlet flow speed as a function of Reynolds number. One curve is
for a flow meter having its paddle wheel coaxial with the cylindrical
cavity's centerline, a second curve is for the flow meter of the
preferred embodiment (FIGS. 1-7), and a third curve is for a flow
meter having its paddle wheel rotating about an axis aligned with
the edge 47 of the imaginary extension of the inlet conduit 17.
It is generally the case that moving the paddle wheel's rotation
axis toward the inlet conduit 17 extends the flow meter's operating
range to flows having lower Reynolds numbers. If it's moved as far
as the edge of the inlet conduit's flow stream, however, the flow
meter becomes somewhat non-linear, especially in the transition-region
between turbulent and laminar flows. It should be appreciated, however,
that if the fluid's viscosity is known, any non-linearity in the
flow meter's performance curve can be compensated for by simply
adjusting the optical sensor's output signal by a prescribed factor.
The location of the paddle wheel in the preferred embodiment represents
a good compromise location that only only extends the effective
Reynolds number range but also provides improved linearity.
It should be appreciated from the foregoing description that the
present invention provides an improved flow meter of a type having
a paddle wheel rotatably mounted in a cylindrical cavity, and an
inlet conduit and an outlet conduit for directing a fluid to flow
through the cavity. The paddle wheel's axis of rotation is displaced
from the cavity's centerline, in the direction of the inlet conduit,
such that the flow meter operates effectively over a wider range
of Reynolds numbers, spanning both turbulent and laminar flows.
Although the invention has been described in detail with reference
to the presently preferred embodiment, it should be understood by
those of ordinary skill in the art that various modifications can
be made without departing from the scope of the invention. Accordingly,
the invention is not to be limited except by the appended claims.
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