CA2057960A1 - Reduced hydrodynamic bearing seal air cycle machine - Google Patents

Abstract

BEARING COOLING ARRANGEMENT FOR AIR CYCLE MACHINE
ABSTRACT

In air cycle machines with hydrodynamic bearings, the mass flow rate of coolant (363 passing through a series of bearings (32, 28, 30) is metered before the coolant passes through the final bearing (30). The final bearing operates at an internal pressure equivalent to that present in the region (72) into which coolant exhausts after exiting the final bearing. Since coolant in the final bearing is at the same pressure as air immediately downstream from the bearing, no rotating seal is required at the outlet of the bearing cooling flowpath.

Description

BEAkING COOLING ARRANGEMENT FOR AIR CYCLE MACHINE

Technical Field This invsntion relates to air cycle machines having hydrodynamic bearings.

Background Art Environmental control systems for aircraft typically employ air cycle machines and heat exchangers to cool and condition high pressure air supplied by either the engines or the auxiliary power unit. A compressor and fan in these machines are powered by a shaft connected to a turbine. The pressurized supply air passes first into the compressor.
Outlet flow from the compressor, heated and further pressurized by the compression step, is chilled as it passes through the warm path of a heat exchanger. To sufficiently reduce the temperature of the air passing through the warm path, the fan draws cooler ambient air through the cooling path of the heat exchanger. Chilled air exiting the warm path of the heat exchanger is then expanded in the turbine to further cool it before it enters the aircraft cabin.
Since the cabin air is maintained at a lower pressure than the supply air, properly designed systems pro~ide conditioned air at temperatures low enough to cool both the cabin and the aircraft avionics.
To support the shaft connecting the turbine to the compressor and the fan, air cycle machines typically use three bearings. Two of these three bearings are journal bearings, and are configured to prevent the shaft from shifting radially~ The third, a thrust bearing, fixes the axial orientation of the shaft. For optimum machine performance, very small clearances ~etween the stators fixed to the machine housing and the tips of the fan and compressor blades must be maintained. Since the compressor ....

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and turbine rotors, to which the blades attach, are connected to the shaft, should the baarings allow more than slight amounts of free play, the shaft would shift when loaded and the blade tips would contact the stator surfaces encircling them.
As they offer minimal free play and reliable operation at high speed, hydrodynamic fluid film journal and thrust bearings are used to locate the shaft radially and axially, respectively. The inner race of each of these bearings connects to, or is a part of, the shaft, and the outer race of each attaches to the housing. When the shaft rotates, hydrodynamic forces are generated in fluid contained in the space between the inner and outer races of each bearing.
These forces combine to yield a high pressure region in each bearing sufficient to oppose loads applied to the shaft.
To ensure that the magnitude of these hydrodynamic bearing forces remains constant during operation, the clearance between the inner and outer races must be maintained within a fairly narrow range. However, the hydrodynamic effect responsible for producing the high pressure region between the races of a rotating hydrodynamic bearing also generates heat. To minimize nonuniform thermal expansion and regulate inner race-outer race clearance, coolant is used to carry this heat away from the bearings.
US Patent 4,500,143 describes a roller bearing and journal assembly that employs oil and air both to regulate the clearance between inner and outer races and to lubricate the system. Cool pressurized oil circulates through passages adjacent to both the inner race of the roller bearing and the journal encircling the bearing. The flow rate of the cooling oil is selected to limit, during hot operation, the thermal expansion of the inner and outer surfaces to within a specified range. Holes drilled radially into the cooling passages at periodic intervals bleed a portion of this oil flow into the bearing chamber, 2~!~7~
directly lubricating and cooling the rollers comprising the bearing. To prevent the inner race from being overcooled to the point where the roller bearing and journal clearance increases beyond the specified range, warm air is introduced in~o a second passageway adjacent to the inner race. By applying air in this fashion, only the inner race expands, and the bearing-journal clearance r~mains su~ficiently small.
In US Patents 4,503,683 and 4,507,939 a shaft lG supporting a turbine, compressor, and fan in an air cycle machine is axially and radially constrained by one air thrust, and and two air journal, bearings. A portion of the turbine inlet air is extracted, serving as a coolant that lubricates, cools, and supports these three bearings. A
first portion o~ this coolant flows first into the thrust bearing cooling flowpath. A labyrinth seal at one end of the thrust bearing forces the coolant to exhaust from the other end. The slightly warmed coolant then flows directly into the inlet of the first journal bearing cooling flowpath. A labyrinth seal at the outlet of this cooling flowpath meters the mass flow rate of air passing through both the thrust and the journal bearing cooling flowpaths.
This seal is critical, as flow exiting the first journal bearing cooling flowpath exhausts directly into the fan circuit. Without this facility for metering the cooling circuit flow, an excessive mass of air is extracted from the turbine inlet and wasted. Additionally, with no seal, the pressure of the coolant in both bearing flowpaths drops to the air pressure in the fan circuit, which is approximately equivalent to ambient pressure. As the density of the coolant at ambient pressure is too low to adequately support the bearings, the inner race contacts the outer rac~, causing excessive friction and potentially damaging wear.
A second portion of the coolant extracted from the inlet of the turbine is delivered to the second journal .

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bearing cooling flowpath. The second journal bearing has labyrinth seals at both ends. The first of these seals allows no flow, and the second seal meters the amount of coolant allowed to flow throuyh this second journal bearing, similar to the way the seal on the first journal bearing meters flow through the thrust and first journal bearings.
The inlet to the second journal bearing flowpath is located adjacent to the first seal. Coolant therefore flows along the length of the bearing, exhausting through the second seal into the fan circuit.
Other, less relevant patents that generally relate to hydrodynamic bearing applications are US Patents 4,306,755 and 4,580,406.

Disclosure of Invention Objects of the invention include improvements in hydrodynamic bearing cooling circuits.
Further objects of the invention include extracting entrained particulates from bearing coolant air prior to injection into the bearing cooling ~lowpath.
According to the present invention, the mass flow rate and pressure of coolant flowing through a hydrodynamic bearing cooling circuit is metered upstream of the cooling circuit.
According further to the invention, coolant circulates through an annular plenum upstream of the bearing cooling circuit, wherein the coolant velocity is reduced to cause any entrained particulate matter to drop out of the coolant flow.
In prior art systems, ~he flow of coolant in hydrodynamic bearing cooling circuits is throttled by a labyrinth seal, or metering holes encircling a labyrinth seal, located at the outlet of the cooling circuit.
Regardless of the throttliny means chosen in these systems, a labyrinth or other rotating seal must always be included '7 ~ ~ ~
at the circuit outlet to seal the space between the inner and outer bearing races and prevent excessive flow through the circuit. The pressure of the coolant flowing through the circui~ is t~erefore maintained by properly adjusting the flow through these labyrinth seals or metering holPs.
In the present invention, the mass flow rate of the coolant is metered before it enters the final bearing cooling flowpath in the circuit. The metering or throttling orifi~es are sized to ensure adequate, but not excessive, coolant flow through the entire circuit. Since metering is done before the final bearing cooling flowpath in the circuit, the labyrinth seal and/or metering hole arrangement at the circuit outlet, as taught in the prior art, is no longer necessary. In the present invention, therefore, pressure in this final bearing flowpath is nearly equal to the air pressure in the region receiving the exhausted coolant. The operating pressure of these bearings is critical, since a minimum coolant density is required to generate hydrodynamic forces sufficient to oppose the anticipated shaft loads. To ensure proper bearing operation, the cooling circuit outlet therefore exits into a region upstream of the compressor having pressure no lower than the minimum desired coolant pressure.
The benefits of reducing the total number of seals required to ensure proper flow through a hydrodynamic bearing cooling circuit are numerous. Since fewer parts are required to produce air cycle machines comprising these bearings, both the complexity and cost of manufacturing these machines decrease. Unlike static seals, rotating seals attempt to seal a region defined by two bodies in motion with respect to each other. Reducing the number of these seals therefore results in fewer contact and wear points, improving the overall reliability of the machine.
Reducing the number of fragile labyrinth seals also reduces the risk of damaging them when the shaft is installed or 2 1~ ~ r~
remove~, resulting in easier machine assembly and disassembly. Elimination o~ seals also allows the overall shaft length to be shortened. This both lowers the weight of the machine and increases the fundamental natural frequency of the shaft, raising the maximum speed at which the machine can be run without exciting shaft oscillations.
The foregoing and other objects, features, and advantages of the present invention will bacome more apparent in the light of the following detailed description of exemplary embodiments thereof, as illustrated in the accompanying drawings.

Brief Description of Drawings FIG. 1 is schematic diagram of an air cycle machine incorporating the present invention;
FIG. 2 is a broken away side view showing a cooling circuit for hydrodynamic bearings in the air cycle machine;
FIG. 3 is an enlarged view taken along the line 3-3 in FIG. 2, showing a second gas foil journal bearing in greater detail;
FIG. 4 is a sectional view, taken on the line 4-4 in FIG. 3, showing, not to scale, a schematic end view of the second gas foil journal bearing; and FIG. 5 is a sectional view, taken on the line 5-5 in FIG. 2, showing the configuration of a flow tube that extracts coolant to supply the cooling circuit.

Best Mode for Carrying Out the Invention Referring now to FIG. 1, in an aircraft air cycle machine, a hollow shaft 10 connects a turbine 12, a compressor 14, and a fan 16. The compressor 14 further compresses supply air 18 delivered to the compressor inlet from either the aircraft engine compressor bleed system (not shown~ or an auxiliary power unit (not shown). Outlet air 20 exiting the compressor 14, heated in the compression 20 ~7 ~ 6 D
step, passes subsequently to the warm path of a heat exchanger 22. To lower ~he temperature of the outlet air 20, cooler ambient air 24 is drawn through the cooling path of the heat exchanger 22 by the fan 16. Cooled air 26 exiting the warm path of the heat exchanger 22 then passes to the turbine 12. Expanding ~his air 26 in the turbine 12 not only produces the power necessary to drive the compressor 14 and ~an 16, but chills it as well, allowing it to be used to cool and condition the aircraft cabin (not shown~.
Referring now to FIG. 2, a first gas foil journal bearing 28, located between the compressor 14 and the turbine 12, and a second gas foil journal bearing 30, located at the forward end of the machine between the compressor 14 and the ~an 16, radially locate and support the shaft 10. A gas foil thrust bearing 32, located at the aft end of the machine near the inlet of the turbine 34, ensures that proper orientation is maintained when the shaft 10 is axially loaded. Each bearing is composed of an inner and an outer race. FIGS. 3 and 4 show expanded views of the second journal bearing 30. The inner race 31 is formed integrally with the shaft 10. The outer race 33 has groves cut into its outside surface. Compression of O-rings 35 installed in these grooves 33 against an inner surface of the housing 39 fixes the second journal bearing 30 with respect to the housing. A foil pack 41 separates the inner 31 from the outer 33 raceO
The first journal bearing 28 has an inner race/outer race and foil pack configuration identical to the second journal bearing 30. Thrust beaxing 32 construction is based upon similar principleq, but the inner race, outer race, and foil pack are all planar disks, instead of cylindrical sleeves. When the shaft 10 rotates, hydrodynamic forces inside these gas foil bearings 28, 30, 32 combine to produce a pressuri2ed region at the inner race-outer race interface, $ ~ a both driving the foil pack away from contact with the inner race and opposing any axial or radial loads tending to force the shaft ~0 from its desired orientation.
Both to produce the necessary hydrodynamic pressures and to minimize axial and radial free play, during shaft rotation the inner race-outer race clearance in each bearing is small. Should this clearance increase, the hydrodynamic pressures developed may be insufficient to support the shaft 10. Should the clearance decrsase, the inner race could contact the outer race, causing wear and friction.
Clearance fluctuation is a concern, as developing and maintaining the pressurized region within each of these bearings generates heat which could cause the inner and outer races to expand. Coolant 36 therefore passes over the races in each bearing, controlling the thermal expansion of each component to maintain inner race-outer race clearance within some predetermined critical range.
Referring to FIG. 2, a portion of the air in the turbine inlet 34 serves as this coolant 36. Turbine inlet air is both the coolest, having passed through the heat exchanger 22l as shown in FIG. 1, and the highest pressure air available in the system. Typically, when the machine is operated at sea level conditions, the pressure of the air in the turbine inlet 34 ranges between 40 and 50 psig (280 to 350 kPa). The pressure and density of the coolant 36 is important, since it not only cools the bearing, but lubricates and supports it as well. The greater the coolant pressure, the greater the hydrodynamic forces generated within each bearing, and the larger the loads that can be handled by the bearing. Based upon anticipated peak bearing loading, therefore, each bearing 28, 30, 32 has a critical coolant pressure which must be met or exceeded to ensure proper operation of the machine.
Referring to FIGS. 2 and 5, coolant 36 flows from the turbine inlet through a flow tube 38. The design and 2~7~
orientation o~ this flow tuhe 38 prevent dirt, water, and other particulates entrained in the turbine inlet air from entering and clogging the cooling circuit 40 that delivers coolant 36 to each ~earing 28, 30, 32 in series. The inlet end 42 of the flow tube 38 is scarf cut and extends perpendicularly into the ~low of air 44 circulating through the turbine inlet 34. The flow tube 38 is rotated such that the opening 42 formed by the scarf cut faces away from the airflow 44. With this configuration, only entrained particles able to rapidly change direction can remain in the coolant flow 36 passing through the tube 38. Since only lighter and smaller particles are able to change direction quickly enough, orienting the inlet 42 of the flow tube 38 in this fashion removes a significant percentage of the particles large enough to clog the bearing cooling circuit ~0 .
To further minimize the concentration of air~orne contaminants, the outlet 46 of the flow tube is directed into an annular plenum 48 that separates the turbine 12 from the compressor section 14. Before entering the inlet holes 50 in the gas foil thrust bearing 32 located adjacent to the flow tube outlet 46, the coolant 36 circulates circumferentially through nearly the entire plenum 48. As the flow area of the plenum 48 is considerably greater than the flow area of the bearing cooling circuit 40, the velocity of the coolant 36 passing through the plenum 48 is very low. The coolant 36 no longer moves quickly enough to support any but the lightest and smallest particles, so most of the remaining airborne contaminants fall into a collection reservoir 52 at the lowest point in the plenum 48.
Coolant 36 enters the thrust bearing inlet hole 50 and splits at the outside edge of the inner race to flow over both the forward 56 and the aft 54 inner race surfaces. Air flowing over the aft 54 surface exhausts into a cavity at 2~7~

the backface of the turbine rotor 58 through a labyrinth seal 60 on the a~ side of the bearing 32. To ensure adequate cooling flow through the other bearings 28, 30, the gap between the labyrinth seal 60 and the shaft 10 7 S sized to allow only roughly one-third of the coolant to pass over the aft surface 54 of thP thrust bearing 32. The remaining two-thirds of the coolant passes over the the forward inner race surface 56 before exiting through the thrust bearing outlet holes 62.
Coolant 36 leaving the thrust bearing outlet holes 62 flows directly in~o inlet holes 64 on the aft end of the compressor journal bearing 2~, traversing forward over the length of the bearing. A labyrinth seal 66 at the forward end of the bearing 2~ prevents coolant 36 from passing into a cavity at the backface of the compressor rotor 67. All coolant 36 therefore exhausts through outlet holes 68 in the inner race, passing directly into the hollow interior of the shaft 10.
Referring to FIGS. 2, 3, and 4, through the hollow interior of the shaft 10, coolant 36 flows forward from the compressor journal bearing 28 to the fan journal bearing 30.
Throttle orifices 70 in the shaft wall serve as fan journal bearing inlet holes, allowing the coolant 36 to enter the fore end of that bearing. A labyrinth seal 74 at the forward end of the fan journal bearing 30 prevents coolant 36 from entering the fan circuit 76. As there is no seal at the aft end of the fan journal bearing 30, coolant 36 flows from the forward to the aft end of the bearing, exhausting directly into the compressor inlet 72.
Sizing the throttle orifices 70 so that a desired mass flow rate through then is maintained when operating at baseline conditions requires that the pressure at both the inlet and outlet of the orifices be known at these operating conditions. The inlet pressure is the pressure of coolant contained within the hollow shaft lO~ As the coolant has ~ ~P~3 passed through the thrust 32 and first journal bearing 28 cooling flowpaths be~ore reaching the hollow shaft cavity, the coolant 36 pressure at the orifice inlets is typically two to three psi (15 to 20 kPa) below the turbine inlet pressure. To determine the pressure at the outlets of the orifices 70, the pressure drop across the second journal bearing cooling flowpath needed to maintain the desired mass flow rate of coolant 36 through that flowpath is calculated.
The pressure at the outlet of the orifi~es 70 is therefore the compressor inlet pressure typically around 35 psig (240 kPa~, plus the calculated pressure drop. Based on these inlet and outlet pressures, throttling orifices 70 are selected that allow the desixed mass flow rate of coolant 36 to circulate through the bearing cooling circuit ~0.
Although the invention has been shown and described with respect to exemplary embodiments thereof, it should be understood by those skilled in the art that various changes, omissions, and additions may be made therein and thereto, without departing from the spirit and scope of the invention.

Claims (15)

1. An air cycle machine comprising:
a compressor receiving a flow of cool air from a compressed air source;
means for delivering coolant extracted from a first region downstream of said compressor to a second region upstream of said compressor;
a hydrodynamic bearing having a cooling flowpath, said cooling flowpath being disposed in said delivering means;
and means, disposed in said delivering means upstream of said cooling flowpath, for reducing the pressure of said coolant to the pressure of air contained in said second region.

2. The machine according to claim 1, wherein said reducing means includes a throttling orifice.

3. The machine according to claim 2, wherein said throttling orifice is disposed at the inlet of the cooling flowpath of said hydrodynamic bearing.

4. The machine according to claim 1, further comprising a turbine connected to said compressor by a shaft.

5. The machine according to claim 4, wherein said first region is the inlet of said turbine.

6. The machine according to claim 1, wherein said second region is the inlet of said compressor.

7. The machine according to claim 1, wherein said hydrodynamic bearing is a gas foil journal bearing.

8. An air cycle machine comprising:

a compressor receiving a flow of cool air from a compressed air source:
a turbine connected to said compressor by a shaft;
a hydrodynamic journal bearing having a rotating seal at exactly one end;
said journal bearing having a cooling flowpath, such that flow introduced at the sealed end passes through the space between the bearing inner race and outer race and exhausts from the unsealed end;
means for delivering a portion of the flow from the inlet of said turbine to the inlet of a throttling orifice;
means for introducing flow from the outlet of said throttling orifice to the inlet of said cooling flow path at said sealed end of said journal bearing; and means for supplying flow exhausting from said unsealed end of said journal bearing to the inlet of said compressor.

9. The machine according to claim 8, wherein said rotating seal is a labyrinth seal.

10. The machine according to claim 8, wherein said hydrodynamic journal bearing is a gas foil journal bearing.

11. A method for cooling hydrodynamic journal bearings in an air cycle machine, comprising:
extracting a flow of coolant from the airflow downstream of a compressor;
delivering said coolant to a throttling orifice;
cooling said hydrodynamic journal bearing with the coolant passed through said throttling orifice; and exhausting the coolant warmed in said cooling step upstream of said compressor.

12. A method for cooling hydrodynamic journal bearings in an air cycle machine having a compressor and a turbine, comprising:
extracting coolant from the inlet of said turbine;
delivering said coolant to a throttling orifice;
cooling said hydrodynamic journal bearing with coolant passed through said throttling orifice; and exhausting coolant warmed in said cooling step into the inlet of said compressor.

13. A system for extracting and removing entrained particles from air in the inlet of a turbine comprising:
an annular plenum having an inlet and an outlet, said plenum inlet configured such that air introduced at said plenum inlet circulates circumferentially through said plenum before exiting via said plenum outlet; and means for delivering a portion of the air in said turbine inlet to said plenum inlet.

14. The system according the claim 13, wherein said plenum inlet is located adjacent to said plenum outlet.

15. The system according to claim 13, wherein said delivering means is a flow tube having an outlet, connected to said plenum inlet, and a scarf cut inlet, said scarf cut inlet oriented 50 as to face away from the airflow circulating through said turbine inlet.