Abstrict A microwave fluid flow meter is described utilizing two spaced
microwave sensors positioned along a fluid flow path. Each sensor
includes a microwave cavity having a frequency of resonance dependent
upon the static pressure of the fluid at the sensor locations. The
resonant response of each cavity with respect to a variation in
pressure of the monitored fluid is represented by a corresponding
electrical output which can be calibrated into a direct pressure
reading. The pressure drop between sensor locations is then correlated
as a measure of fluid velocity. In the preferred embodiment the
individual sensor cavities are strategically positioned outside
the path of fluid flow and are designed to resonate in two distinct
frequency modes yielding a measure of temperature as well as pressure.
The temperature response can then be used in correcting for pressure
responses of the microwave cavity encountered due to temperature
fluctuations.
Claims We claim as our invention:
1. Apparatus for measuring the rate of liquid flow within a fluid
conduit comprising:
a first microwave sensor having a resonant microwave cavity directly
responsive to the fluid pressure internal of the conduit to provide
a representative electrical output indicative thereof, said microwave
cavity being exposed to the interior of the fluid conduit at a first
given fixed location therealong outside the path of fluid flow;
a second microwave sensor having a second resonant microwave cavity
directly responsive to the fluid pressure internal of the conduit
to provide a representative electrical output indicative thereof,
said second microwave cavity being exposed to the interior of the
fluid conduit at a second given fixed location therealong outside
the path of fluid flow and spaced from said first location; and
means for comparing the respective outputs from said first and
second sensors and responsive thereto to provide a measurement of
fluid flow within the conduit.
2. The apparatus of claim 1 wherein said first and second microwave
cavities are respectively formed, in part, integral with the wall
of the fluid conduit.
3. The apparatus of claim 2 wherein at least a portion of the fluid
conduit wall at said first and second locations respectively form
at least a section of one wall of said corresponding first and second
microwave cavities, the remaining walls of said first and second
cavities being maintained exterior of the fluid conduit.
4. The apparatus of claim 2 wherein corresponding portions of the
conduit wall common to said first and second microwave cavities
are constructed in a manner to change one geometric dimension of
said first and second cavities in response to a pressure gradient
across said common wall portions.
5. The apparatus of claim 1 wherein said first and second cavities
respectively comprise a single hollow member having a wall section
responsive to pressure to move in a manner to change the effective
dimension of the hollow member along a prescribed axis, said first
and second cavities being designed to resonate at two resonant frequency
modes respectively dependent upon a unique combination or permutation
of and collectively dependent on both the temperature and pressure
of the fluid measured, and wherein said first and second sensors
are responsive to the two resonant modes of said corresponding first
and second cavities to provide electrical outputs indicative of
both pressure and temperature so that the respective pressure measurements
can be compensated for variations in the corresponding cavity temperatures.
Description The advent of the fast breeder reactor employing liquid metal technology
has generated a need for apparatus capable of determining the sodium
flow rate encountered through the individual fuel subassemblies,
process channels and closed loop test assemblies. Design limitations
require such sensors to operate in a very limited space under severe
environmental conditions of temperature, pressure and radiation
levels. When necessary, installation, removal or replacement of
the sensor must be achieved with a minimum of effort and time.
State-of-the-art flow sensors presently available are unable to
meet the aforedescribed specifications. In fact, presently proposed
sensors partially obstruct flow of the fluid during the measurement
process. This obstruction causes turbulent flow, a pressure drop
due to the flow meters, and even a possible coolant blockage if
failures occur.
A critical need exists for better methods of coolant flow detection
in all nuclear reactors. A flow restriction through one or more
fuel channels in the core can result in a failure or melting of
the fuel. To monitor for such occurrences, flow or coolant temperature
detection equipment have been used in the past. Temperature detection
methods are limited because of the corresponding time lag between
blockage and increased coolant temperature. Flow monitors are ideal
since they detect immediately any change in flow. Immediate detection
is vital if one ever hopes to avert fuel melting or at least minimize
fuel damage. Unfortunately, reliable and accurate flow meters are
not a common feature in reactors today. Many of the problems occur
because of the requirement for several, usually mechanical, devices
prone to frequent failure as well as numerous instrumentation leads
and connectors, also prone to failure.
Accordingly, apparatus is desired having the capability of providing
such measurements without disturbing the flow monitored. Additionally,
such sensors must exhibit the ability to maintain a high degree
of sensitivity and stability under severe environmental conditions
as well as provide a fast response time as required in liquid metal
applications.
SUMMARY OF THE INVENTION
Briefly, this invention employs a plurality of spaced microwave
sensors positioned along a fluid flow path to provide a measurement
of fluid velocity. The individual sensors include a microwave cavity
having a resonant frequency response dependent upon the static pressure
of the fluid monitored. The respective cavity resonant frequencies
are translated into representative electrical outputs calibrated
into a pressure response. The pressure drop between sensors is then
interpreted as a measure of the fluid's velocity between monitoring
locations.
The sensor microwave cavity can be designed to resonate in two
distinct frequency modes having a specific dependence on the fluid
parameters that will yield a measure of temperature as well as pressure.
The temperature response can then be employed in correcting for
temperature dependent pressure responses of the microwave cavity.
In the preferred embodiment the sensors are positioned outside
the fluid path to avoid flow obstructions.
BRIEF DESCRIPTION OF THE DRAWINGS
For a better understanding of the invention, reference may be had
to the preferred embodiment, exemplary of the invention, shown in
the accompanying drawings, in which:
FIG. 1 is a schematic diagram of an exemplary flow rate metering
system contemplated by this invention shown in a fast breeder reactor
environment monitoring one of the external coolant loops; and
FIG. 2 is a sectional view of the dual property sensor and waveguide
assembly employed in FIG. 1.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The apparatus of this invention utilizes a pair of spaced microwave
sensors positioned along a fluid flow path to measure the fluid's
velocity. The preferred embodiment to follow is illustrated an extremely
adverse fast breeder reactor environment which particularly points
out the versatility and specific benefits provided by the inventive
concepts presented.
A liquid metal fast breeder reactor vessel and head enclosure 10
are illustrated in FIG. 1 having a heat generative core 12 and coolant
flow inlet and outlet means 14 and 16 formed integral with and through
the vessel walls. The coolant flow outlet pipe 16 commonly called
the hot-leg of the primary loop of the reactor, conducts the heated
coolant to intermediate heat exchange means, not shown, commonly
employed to create steam which is used to drive apparatus designed
for the production of electricity. The cooled coolant exiting the
heat exchanger is returned through cool-leg conduit 14 to the core
region of the reactor for recirculation. The sensors of this invention,
18 and 20 are illustrated positioned at two-spaced locations along
the hot-leg of the coolant piping 16 to monitor the static pressure
of the fluid sodium at the two sensor locations. The corresponding
pressure measurements obtained are then compared to give an indication
of the fluid coolant flow rate within the conduit 16.
In its preferred form the sensors 18 and 20 respectively include
individual metal cavities 22 of particular internal dimensions designed
to resonate at microwave frequencies. The specific design of the
cavities, as well as the sensor waveguide assembly 26 is illustrated
is more detail in FIG. 2. Of importance to this embodiment, there
are specific values of (2a//L).sup.2 for a cylindrical cavity (where
a = the inside radius and L = the side length of the cylinder) for
which the cavity resonates in two distinct (degenerate) modes collectively
dependent upon both pressure and temperature for the same exciting
frequency. Simultaneous and independent excitation and detection
of these modes can be achieved, as fully described in copending
application Ser. No. 328220 filed Jan. 31 1973 to allow simultaneous
detection of both pressure and temperature.
The embodiment illustrated in FIG. 2 shows the resonant cavity
22 as having a flexible end wall 24 formed as an integral part of
the piping wall 16. A change in pressure within the piping will
result in a displacement of the end wall 24 effectively changing
the longitudinal dimension L of the cavity and thus altering the
resonant frequency of the pressure dependent mode. The resonant
energy is communicated to the processing electronics 28 shown in
block form coupled to the waveguide 26 in FIG. 1. The electronics
function to convert the microwave energy to a corresponding electrical
output representative of the monitored coolant's pressure as indicated
by block 30. The pressure responses obtained in monitoring the sensor
locations are then compared and analyzed, as schematically shown
by block 32 to obtain a measurement of the flow rate of the fluid
within the conduit 16. A separate temperature measurement 34 can
be obtained, as explained in the aforecited reference, from the
corresponding response of the temperature dependent mode to provide
a correction for temperature responsive pressure readings of the
cavity. A more detailed understanding of the specific apparatus
employed can be had from the aforecited reference and a comparison
of pressure responses can be obtained by a simple implementation
of differential amplifier circuitry presently within the state of
the art.
After the pressure responses are obtained they are correlated using
a mathematical relationship relating the pressure drop between sensors
to the fluid flow rate as illustrated by the following example.
Assuming a 4 inch diameter pipe or tube containing flowing liquid
sodium at 700.degree.F, the Reynolds number for different velocities
of flow can be calculated from the relationship: ##EQU1##
For velocities of 10 feet/second, 15 feet/second, and 20 feet/second,
the corresponding Reynolds numbers calculated are R.sub.10 = 9.6
.times. 10.sup.5 R.sub.15 = 14.4 .times. 10.sup.5 and R.sub.20
= 19.2 .times. 10.sup.5. Given a relative roughness for a standard
commercial pipe of .epsilon./D of 0.0004 a Moody diagram determines
the friction factor to be f.sub.10 = 0.0164 f.sub.15 = 0.0162
and f.sub.20 = 0.0161 for the above velocities, respectively.
The pressure drop along the pipe can be related to velocity using
the relationship: ##EQU2## where: .rho. = density
f = friction factor
L = the spacing between sensors
g = the force of gravity.
Again, assuming a fluid of liquid sodium at 700.degree.F, and with
the pressure sensors along the pipe spaced 10 feet apart, the calculated
pressure drop for different velocities of flow are illustrated by
the following table.
______________________________________ For 10% Velocity Pressure
Drop Change in Flow Rates ______________________________________
6 ft/sec 0.11 psi 10 ft/sec 0.29 psi 0.06 psi 11 ft/sec 0.35 psi
15 ft/sec 0.64 psi 0.13 psi 16.5 ft/sec 0.77 psi 20 ft/sec 1.12
psi 0.24 psi 22 ft/sec 1.36 psi ______________________________________
Thus, detection sensitivity, depending upon the allowed spacing
between sensors, will be greater at higher velocities. For sensors
spaced 10 feet apart, pressure resolution will be approached at
the lower limit of 6 feet/second, which is within the design specifications
for liquid metal fast breeder reactors. The above calculations prove
that the apparatus of this invention will readily be applicable
to monitoring the flow at the core sodium inlet and outlet locations.
Alternate applications to the individual subassemblies comprising
the core appear similarly feasible. Tables compiled in the same
manner as that presented above can form a basis for calibrating
the readout electronics according to the pressure drop between sensors
to provide a direct readout of the flow rate as indicated by reference
character 32.
Thus, while the embodiment illustrated as being exemplary of this
invention has been shown positioned external to the coolant loop
piping of the reactor with the sensor cavity having one wall formed
integral with the pipe walls to avoid obstructions to the fluid
flow, it should be understood that the sensors can be positioned
at any desired location along the fluid flow path, either internal
or external to the reactor vessel. For example, in monitoring for
coolant flow blockage within the core it is desirable to position
the sensors directly along the path of flow within the fuel assemblies,
so that an immediate response can be obtained and acted upon to
limit damage to the fuel.
Accordingly, a liquid flow rate monitor has been described which
in its preferred form will yield output signals proportional to
both temperature and static pressure, and therefore can be employed
to sense those fluid physical properties as well as flow. Inasmuch
as two sensors are employed to measure three parameters, flow rate,
pressure and temperature, additional reliability is achieved in
obtaining the added parameter responses. Calibration and operational
conditions of the pressure sensors when immersed in the monitored
fluid can be ascertained simply by providing appropriate valving
and specific reference gas pressures as taught in the aforecited
reference.
Furthermore, experimental results have verified the indicated pressure
resolution and time responses for given pressure variations, and
the sensors have been shown to be applicable to the severe environments
encountered in a fast breeder application. Of particular significance,
the upper operational temperature limit for the sensor exceeds the
1200.degree.F requirement for this specific application, while the
sensor remains insensitive to thermal instability and thermal drift
when the simultaneous temperature response is employed for compensation.
The few components necessary for sensor fabrication (cavity, diaphragm
and waveguide) will simplify instrumentation, while component material
choices will insure provision of a sensor relatively insensitive
to decalibrations due to the severe environment. Finally, the pressure
sensors can be placed outside the path of flow, reducing turbulence
which might otherwise contribute to inaccurate static pressure readings
as well as affect fluid performance within the monitored system. |