Variable pressure pitot pump with reduced heating of pumped fluid

A variable pressure and variable flow rate pump in accordance with the invention includes a stationary pump housing (1), a rotating pump housing (10), rotatably mounted with respect to the stationary pump housing including a fluid receiving chamber (12) for receiving fluid to be pumped which is driven by a drive (11); a pitot probe (5) rotatably mounted in the stationary pump housing having a tip (5a) disposed in the fluid receiving chamber radially offset from an axis of rotation of the probe for receiving fluid from the fluid receiving chamber; a brake (15) for selectively braking a rotational speed of the probe to control the rotational speed of the probe independently of a rotational speed of the rotating pump housing to provide a fluid output from the output with a variable pressure and flow rate; a thermal insulator (44) mounted between the brake and the fluid receiving chamber for thermally insulating the fluid receiving chamber from heat generated by the brake; and a control (C) coupled to the brake for controlling activation of the brake to control a rate of rotation of the probe to control the pressure and flow rate of the fluid produced at the output.

DESCRIPTION 
1. Technical Field 
The present invention relates to a pitot pump which provides variable 
pressure and variable flow rates. 
2. Background Art 
In certain applications, such as a fuel pump for an aircraft, it is 
desirable to vary an output pressure and flow rate of a centrifugal pump 
independently of speed. In aircraft fuel pumps it is conventional practice 
to throttle, recirculate or bypass the flow of the pump so as to vary the 
output pressure and flow thereof. However, this approach results in 
unacceptably high fuel temperature rise due to poor efficiency. 
U.S. Pat. Nos. 2,440,624, 3,791,757, 4,073,596, 4,281,962, 4,339,923 and 
Austrian Patent 230,159 disclose centrifugal pumps with rotating tubes. 
Pitot pumps have the advantage of reducing heating of fuel in aircraft 
applications which can be a severe problem with non-pitot pump fuel pumps 
where minimum flow conditions are encountered. Moreover, pitot pumps 
themselves are known to generate high internal heat levels during constant 
high speed operation of a pump. U.S. Pat. No. 4,073,596 discloses the 
cooling of the oil supply so that the lubricity of the oil applied to the 
bearings is not diminished by high temperatures which extends the life of 
the bearings. 
In aircraft fuel pump applications it is extremely important to prevent the 
temperature of the fuel from exceeding a temperature limit. If the 
temperature limit is exceeded, fuel coking can rapidly occur which can 
cause malfunction or failure of combuster injection nozzles. 
DISCLOSE OF INVENTION 
U.S. patent application Ser. No. 605,424 entitled "Variable Pressure Pitot 
Pump" filed on Oct. 28, 1990 discloses an improved pitot pump having a 
preferred application of a fuel pump in an aircraft. FIG. 1 illustrates 
the pitot pump disclosed in Ser. No. 605,424. A fluid pump P is provided, 
including a stationary housing 1 and a rotating housing comprised of 
connected members 10 and 10a. The rotating housing 10, 10a rotates in 
response to torque applied from a rotary drive 11 about a rotational axis 
centered on the rotating housing 10, 10a. The stationary housing 1 
includes a conventional bearing support 4 for a freely rotating pitot tube 
5. An inlet seal 6, of a conventional construction, is provided at an 
inlet end of the stationary housing 1, with an outlet seal 7, also of 
conventional construction provided at the outlet end of the stationary 
housing. The pitot tube 5, which is rotatably journalled by bearings 4 in 
the stationary housing 1 terminates in a probe tip 5a disposed in a 
cylindrical chamber 12 defined by the rotating housing parts 10, 10a. The 
fluid to be pumped such as aircraft propulsion fuel is introduced into the 
chamber 12 through a fluid inlet port 2. A maximum pressure of the pump P 
is developed at the probe tip 5a and at the pump outlet 3 when the probe 
is prevented from rotating. When the probe 5 is permitted to rotate 
freely, rotational speed of the probe will be substantially equal to the 
rotational speed of the housing 10, 10a driven by the rotary drive 11. 
Consequently, very little pressure develops at the probe tip 5a and at the 
pump outlet 3. Control of the rotational speed of the probe 5 permits the 
probe to rotate at selected intermediate speeds to control the pump outlet 
pressure and flow rate. A brake 15, including a plurality of magnets 20, 
controls the rotational velocity of the probe 5. The magnets 20 are 
disposed about a periphery of an end of the probe 5 opposite the tip 5a. 
The coils are mounted in the stationary housing 1 in opposition to the 
magnets so that a magnetic field created by current flow through the coils 
21 applies braking torque to the probe 5 through interaction of the 
magnetic field produced by the magnets 20 with the magnetic field produced 
by the coil 21. A controller C controls the current which is applied to 
the coil 21 in response to pressure or flow signals which are detected by 
a conventional sensor S which is coupled to the fluid pumped from the 
outlet 3 in a manner not illustrated. Furthermore, it should be understood 
that the aforementioned brake 15 alternatively may be a friction brake or 
a hydrodynamic brake which is controlled by the controller C in response 
to the sensed pressure or flow rate sensed by sensor S. 
The pump P with the pitot probe 5 independently braked by the brake 15 
provides a pumping mechanism for a low specific speed low NPSH (net 
positive suction head) application such as that required for an aircraft 
fuel pumping system. Varying the rotation of the probe 5 by activation of 
the brake 15 permits the pump to have a variable pressure and flow 
independent of the shaft speed produced by the rotary drive 11. 
The pitot pump disclosed in U.S. patent application Ser. No. 605,428 has a 
disadvantage of coupling heat generated by the brake 15 to the fluid 
entering the inlet port 2 through thermal conductivity through the 
boundary wall 24. As a result, the temperature of the fuel flowing into 
the chamber 12 is raised to a point where the temperature of the pumped 
fuel at the outlet 3 could be at a temperature where coking would occur. 
The present invention is an improvement of the pitot pump disclosed in Ser. 
No. 605,428 by providing a mechanism for minimizing the temperature rise 
of the pumped fluid caused by activation of the brake. With the invention, 
the stationary housing is split into two parts with thermal insulation 
being provided between the two parts. As a result, the flow of heat 
generated by the activation of the brake is minimized with a preferred 
application of the invention having only the pitot tube itself thermally 
connecting the two parts of the housing. As a result, a chamber is defined 
radially between an outside surface of the pitot tube which is filled with 
air which extends to an inner radius of the thermal insulation which is 
preferably annular with first and second ends of the annular thermal 
insulation being in contact with the first and second parts of the 
stationary housing. Furthermore, in accordance with the invention an air 
fan may be added to the brake to duct cooling air into contact with the 
brake to minimize the temperature rise in the brake to minimize the flow 
of heat between the first and second parts of the housing. Additionally, a 
heat exchanger may be coupled to the fluid outlet of the pump which is 
coupled to the cool air being drawn into contact with the brake to provide 
further cooling of the fluid pump from the outlet of the pump. As a 
result, the temperature of the pumped fluid provided by the pump may be 
minimized. However, in an application where the pump is used to pump 
propulsion fuel for an aircraft, it is desirable to minimize the 
temperature rise of the fuel, keeping it below the coking temperature. 
Therefore, the heat load represented by cooling of the fluid outputted by 
the pump should not produce a rise in the temperature of the air ducted 
into contact with the brake which results in the fuel temperature within 
the pump rising above the coking temperature. 
A variable pressure and variable flow rate pump in accordance with the 
invention includes a stationary pump housing; a rotating pump housing 
rotatably mounted with respect to the stationary pump housing including a 
fuel receiving chamber for receiving fluid to be pumped which is driven by 
a drive; a pitot probe rotatably mounted in the stationary pump housing 
having a tip disposed in the fluid receiving chamber radially offset from 
an axis of rotation of the probe for receiving fluid from the fluid 
receiving chamber and having an output which outputs a fluid output from 
the probe; a brake for selectively braking a rotational speed of the probe 
to control the rotational speed of the probe independently of a rotational 
speed of the rotating pump housing to provide the fluid output from the 
output with a variable pressure and flow rate; a thermal insulator mounted 
between the brake and the fluid receiving chamber for thermally insulating 
the fluid receiving chamber from heat generated by the brake to minimize 
temperature rise in the fluid; and a control coupled to the brake for 
controlling activation of the brake to control a rate of rotation of the 
probe to control the pressure and flow rate of fluid produced at the 
output. The thermal insulator comprises an annulus having one end 
thermally coupled to the brake and another end thermally coupled to an 
outside surface of the fluid receiving chamber. The stationary pump 
housing comprises a first section in which the brake is disposed; a second 
section in which the fluid receiving chamber is disposed; and wherein the 
pitot probe is rotatably mounted in the first and second sections with the 
thermal insulator and the pitot probe defining a chamber extending 
radially from the probe to the insulator. A fan is rotatably driven by the 
probe for blowing air at a temperature cooler than the brake into contact 
with a surface thermally coupled to the brake for cooling the brake. A 
heat exchanger is coupled to air flowing to the fan which is cooler than 
the fluid outputted by the fan and to the fluid outputted by the output 
for cooling the fluid outputted by the output.

BEST MODE FOR CARRYING OUT THE INVENTION 
FIG. 2 illustrates a first embodiment of the present invention. Like 
reference numerals identify like parts throughout the drawings. The first 
embodiment 100 differs from the pitot pump illustrated in FIG. 1 in that 
the stationary housing 1 is split into a first section 40 and a second 
section 42 which are connected together by the pitot probe 5 and an 
annular insulator 44. A first end 46 of the annular insulator is in 
surface contact with the first section 40 and is thermally coupled to the 
brake 15. A second end 48 is thermally coupled to an outside surface of 
the fluid receiving chamber 12. The annulus 44 is chosen to have 
sufficient strength to provide structural support for the first and second 
sections 40 and 42 and further to provide a high degree of thermal 
insulation between heat generated by the operation of the brake 15 and the 
fluid receiving chamber 12 as a consequence of the chamber being filled 
with air. The outside surface 50 of the pitot probe 5 and the inside 
surface 52 define a radially extending chamber 54 which provides further 
insulation between heat generated by the operation of the brake 15 and the 
fluid receiving chamber 12 as a consequence of the chamber being filled 
with air. As a result of the insulation provided by the annulus 44 and the 
chamber 54, the temperature of the fluid being pumped entering the inlet 2 
is prevented from being substantially raised by the operation of the brake 
in substantially retarding the rotation of the pitot probe 5. 
The pitot probe 5 is rotatably supported in the first section 40 by a pair 
of roller bearings 60 and is rotatably supported in the second section 42 
by a bushing 62. As a result, the probe 5 is freely rotatable within the 
stationary housing 1. The rotational speed of the pitot probe 5 is varied 
in accordance with the sensed pressure or flow rate sensed by sensor S 
which is applied to controller C which applies a control signal to the 
coil 21 to produce a desired pressure or flow rate at the output 3. 
Moreover, like the pitot pump illustrated in FIG. 1, the magnetic brake 15 
may be replaced with a friction brake or a hydraulic brake with suitable 
control of the braking provided by the combination of the sensor S and the 
controller C in a manner analogous to the magnetic braking provided by the 
magnetic brake 15. 
FIG. 3 illustrates a second embodiment 200 in accordance with the present 
invention. The second embodiment 200 differs from the first embodiment 100 
illustrated in FIG. 2 in that a fan 202 rotatably driven by the pitot 
probe 5 blows air at a temperature cooler than the brake into contact with 
a surface thermally coupled to the brake for cooling the brake. The arrows 
204 represent the flow of cooler air into the inlet 206 of the fan. A 
plurality of blades 208 are attached to a hub 210 which is joined to the 
pitot probe 5. The blades 208 work in a manner analogous to the fan 
mechanism on an exercise bicycle. The fan, together with a controlled 
discharge throttle or inlet guide vanes, can settle the braking function. 
The magnets 20 may be mounted on the tip of the blades 202 so as to 
interact with the magnetic field produced by the coil 21. A heat exchanger 
212 may be optionally provided to cool the fuel pump from the outlet by 
contact with the cold air 204 which is drawn into the inlet 206 of the fan 
202. However, it should be noted that the use of the heat exchanger 212 
should only occur in circumstances where the temperature of the fuel 
pumped by the pump P is maintained below a temperature at which coking 
occurs. Cooling of the pumped fuel after it has been raised to a 
temperature where the onset of coking occurs is not advantageous with the 
desired mode of operation being to always maintain the temperature of the 
fuel below the coking temperature within the pump. However, if the cool 
air source providing the air 202 is sufficient to maintain the temperature 
of the brake 15 below a temperature at which the fluid would be heated to 
a temperature at which coking occurs, operation of the heat exchanger may 
be utilized without having a detrimental effect on the fluid being pumped. 
It should be understood that the heat exchanger 212 has been illustrated 
schematically with various physical configurations of a heat exchanger 
being possible including the heat exchanger being symmetrically disposed 
with regard to the inlet 206 of the fan. 
While the invention has been described in terms of its preferred 
embodiments, it should be understood that numerous modifications may be 
made thereto without departing from the spirit and scope of the invention. 
It is intended that all such modifications fall within the scope of the 
appended claims. For example, while a preferred application of the present 
invention is the pumping of fuel to a propulsion engine in an aircraft, it 
should be understood that the invention is not limited thereto.