Air bearing arrangement for fuel cell compressors having an expander

Abstract

The invention relates to a turbomachine (1), in particular for a fuel cell system (100) of a vehicle (200), for example of a utility vehicle, the turbomachine comprising a rotor shaft (19), an expander wheel (15) that is fastened on the rotor shaft (19), and an air bearing arrangement (21) that is designed to support the rotor shaft (19) such that said shaft is rotatable about a rotor axis (X), wherein a flow path (27) is formed between the expander wheel (15) and the air bearing arrangement (21). It is proposed that a flow generator (29) is arranged in the flow path (27) between the air bearing arrangement (21) and the expander wheel (15) and is designed to generate, on the basis of a rotation of the rotor shaft (19), an air flow directed toward the expander wheel (15), in order to build up a sealing pressure.

Description

[0001] Hanover, September 21, 2022 IP, Adria, Schulz / Ek SR 2022P00182DE EM 2021E00351 DE Air bearing arrangement for fuel cell compressors with an expander The present application relates to a turbomachine, in particular for a fuel cell system of a vehicle, such as a commercial vehicle, having a rotor shaft, an expander wheel fastened to the rotor shaft, and an air bearing arrangement configured to support the rotor shaft rotatably about a rotor axis, wherein a flow path is formed between the expander wheel and the air bearing arrangement. Turbomachines of the type described above are generally known. Air bearing arrangements are preferably used in these turbomachines because they enable contactless mounting of the rotor shaft.The contactless bearing, in turn, requires that there is no hermetic seal between a chamber in which the expander wheel is located and the cavity in which the rotor shaft and at least the rotating parts of the air bearing arrangement are arranged, but rather the above-mentioned flow path is formed. During operation of the turbomachine within a fuel cell system, air previously compressed by the turbomachine enters a fuel cell as a reactant supply for the fuel cell process. After passing through the fuel cell, a reactant mixture with a lower oxygen content, so-called cathode exhaust gas, leaves the fuel cell and re-enters the turbomachine to be expanded in an expander chamber by means of the expander wheel. The air reaching the expander wheel has a hydrostatic inlet pressure that is lower than the pressure previously provided by the turbomachine.However, the inlet pressure in the expander chamber is still above ambient pressure. Therefore, there is an overpressure in the expander chamber surrounding the expander wheel. The cathode exhaust gas contains water in droplet form and water-saturated air. While droplet-shaped water can be removed using conventional means such as water separators, the water bound in the air cannot be easily separated. Expansion by the expander wheel causes water to condense out of the air. Due to the overpressure in the expander chamber and / or capillary action, this condensate can enter the flow path and move towards the air bearing arrangement. This creates unwanted friction in the bearing and, with it, potentially higher rotational resistance, which must be overcome by the turbomachine's electric motor. In addition, wear due to the water ingress increases significantly.These phenomena are undesirable in practice. The invention was therefore based on the object of achieving an improvement in a turbomachine of the type described above so that the disadvantages described above are overcome as far as possible. In particular, the invention was based on the object of further developing the turbomachine of the type described above in such a way that improved protection against the penetration of water or other contaminants into the air bearing arrangement is achieved without impairing the efficiency of the turbomachine. The invention achieves the underlying object by proposing a turbomachine according to claim 1.In particular, the invention proposes that a flow generator be arranged in the flow path between the air bearing arrangement and the expander wheel and be configured to generate an air flow directed towards the expander wheel depending on the rotation of the rotor shaft. The invention is based on the finding that the risk of water penetrating the air bearing arrangement increases the higher the pressure gradient between the expander wheel and the area of ​​the air bearing arrangement. However, the overpressure prevailing in the chamber around the expander wheel increases the faster the rotor shaft rotates. The power of the turbomachine also increases as a function of the rotor speed. If the turbomachine has one or more expander stages, this would be the drive power or recuperation power, which can be used to support an upstream compressor stage. In a compressor arrangement, this would also be the compressor power itself.The invention therefore proposes coupling the flow generator to the rotor shaft in such a way that the higher the rotor shaft speed, the more air flow the flow generator generates in the direction of the expander wheel. Within the scope of the invention, turbomachines are understood to mean both integrated compressor-expander arrangements in which one or more compressor stages are operated on a common shaft with the expander wheel, as well as arrangements in which the expander wheel is arranged in the system mechanically decoupled from any compressor stages. Both variants represent preferred embodiments of the invention. As a result, at low speeds, the air flow generated by the flow generator in the direction of the expander wheel is also very low.In this regard, the system takes advantage of the fact that the pressure at the expander wheel always lags behind the compressor-side pressure of the turbomachine by a certain amount of time, particularly when the turbomachine has one or more compressor stages upstream of the expander stage. This is because the air compressed by the turbomachine must first pass through other system components, in particular the fuel cell, before entering the expander wheel. The fact that the rotor shaft must first start up at the start of operation of the turbomachine in order to reach its predetermined operating speed is therefore not detrimental. As long as the speed is low, there is no critical overpressure at the expander wheel. However, when a higher overpressure is applied to the expander wheel after a few seconds, the rotor shaft is already at the required operating speed, and the flow generator can provide the necessary flow towards the expander wheel.In a preferred embodiment, the expander wheel is arranged in an expander chamber into which the cathode exhaust gas is conveyed at a predetermined hydrostatic inlet pressure, wherein the flow generator is designed to build up a barrier pressure on a pressure side facing the expander wheel, which barrier pressure is equal to or greater than the inlet pressure of the expander chamber. This pressure ratio can be easily determined in preliminary tests by calibration because both the hydrostatic inlet pressure into the expander chamber and the barrier pressure provided by the flow generator are directly dependent on the speed of the rotor shaft. In a further preferred embodiment, the barrier pressure exceeds the inlet pressure by 0.5 bar absolute or more, preferably by 1.0 bar absolute or more, particularly preferably in a range of 1.0 bar absolute - 2.0 bar absolute, when the rotor shaft reaches or exceeds a predetermined speed.In preferred embodiments, the aforementioned predetermined speed lies above the lift-off speed of the rotor shaft. The lift-off speed is preferably in a range of 10,000 rpm and 30,000 rpm, preferably between 12,000 rpm and 18,000 rpm. In a further preferred embodiment, the flow generator has a number, preferably a plurality, of recesses and / or projections formed on the rotor shaft. By means of the recesses and / or projections, turbulence is achieved by rotation of the rotor shaft, which builds up a dynamic pressure that can spread through the flow path towards the expander chamber. In a first preferred variant, the flow generator has a plurality of recesses in the form of grooves, which are preferably distributed evenly over the circumference of the rotor shaft.The grooves can, for example, be introduced directly into the surface of the rotor blades or applied to a correspondingly designed sleeve on the rotor shaft. Likewise, in a variant according to the invention, it is preferred that the flow generator has a plurality of projections in the form of ribs, preferably evenly distributed over the circumference, which protrude from the rotor shaft. In a further preferred variant, the rotor shaft has both recesses and projections, which are preferably evenly distributed over the circumference of the rotor shaft. In a further preferred embodiment, the recesses and / or projections are aligned parallel to the rotor axis or at an angle relative to the rotor axis.The turbomachine is preferably designed to rotate the rotor shaft in a preferred direction of rotation, wherein the angle of the recesses and / or projections has a pitch opposite to the direction of rotation. In other words, the recesses and / or projections are oriented to rise to the left on a clockwise rotating rotor shaft, and to rise to the right on a counterclockwise rotating rotor shaft. In this way, due to the orientation of the flanks of the recesses / protrusions, a turbulence acting in the direction of the expander wheel is generated, with which the blocking pressure can be built up. The angle is preferably in a range of 10° to 80° relative to the rotor axis. In this way, unlike spiral grooves, for example, which apply a supporting force acting orthogonally to the rotor axis by way of air cushion generation, it is achieved that the recesses or projections exert their effect in the axial direction relative to the rotor axis.In a further preferred embodiment, the recesses and / or projections are formed relative to a surface of the rotor shaft and have a radial extent relative to the surface in a range of up to 20 µm. In other words, the radial extent defines the depth of the recesses or the height of the projections relative to the surface of the rotor shaft. The recesses and / or projections can run straight along the angle described above, but they can also have a curved course, wherein in the case of a curved course, the angle is preferably defined as the angle of a secant that runs between the axial end points of a respective recess or projection.In a further preferred embodiment, the flow generator is assigned an air supply line, which is provided separately from the air bearing arrangement. The air supply line is preferably designed as an intake line that is fluidically connected to the environment. The air supply line preferably comprises one or more bores, and more preferably a partially or completely circumferential groove. The air supply line reliably prevents the flow generator from sucking air out of the bearing arrangement when the rotor shaft rotates, since the suction side of the flow generator, opposite the barrier pressure, can be supplied with air via the air supply line.In a further preferred embodiment, the flow generator and the air bearing arrangement are arranged adjacent to one another in the direction of the rotor axis or, preferably, spaced apart from one another, wherein the flow generator is arranged on a side of the air bearing arrangement facing the expander wheel. The embodiments described above have described the turbomachine with reference to a single flow generator. In a further preferred embodiment, however, it is provided that the flow generator is only a first flow generator, and the turbomachine further comprises a second flow generator which is arranged opposite the air bearing arrangement relative to the first flow generator, wherein the second flow generator is configured to generate an air flow directed away from the air bearing arrangement depending on a rotation of the rotor shaft.The second flow generator is particularly preferably provided to compensate for or counteract the axial force exerted by the first flow generator, which acts between the housing of the turbomachine and the rotor shaft. Both flow generators generate a barrier pressure directed away from the air bearing arrangement. If the turbomachine has a compressor wheel on the rotor shaft opposite the expander wheel, which it does in a preferred embodiment, this is to be understood that the second flow generator is designed to build up a barrier pressure in the direction of the compressor wheel. The functioning of the second flow generator is otherwise the same as the functioning of the first flow generator.The preferred embodiments relating to the flow generator of the turbomachine and described above are therefore also preferred embodiments of the second flow generator in the turbomachine. In an exemplary preferred embodiment, the second flow generator has a number, preferably a plurality, of recesses and / or projections formed on the rotor shaft. The recesses and / or projections of the second flow generator are preferably aligned parallel to the rotor axis or relative to the rotor axis at an angle that is opposite to the angle of the first flow generator and is preferably equal in magnitude to the angle of the first flow generator.In a further preferred embodiment, the recesses and / or projections of the second flow generator have a smaller radial extent than the recesses and / or projections of the first flow generator. The invention is described in more detail below with reference to the attached figures using a preferred exemplary embodiment. Fig. 1 shows a schematic representation of a turbomachine according to a preferred exemplary embodiment, and Fig. 2 shows a detailed view of the turbomachine according to Fig. 1. Fig. 1 shows a turbomachine 1 which is part of a fuel cell system 100 of a vehicle 200, preferably a commercial vehicle. The turbomachine 1 has a compressor wheel 3 which is arranged in a compressor chamber 5.The compressor chamber 5 has an inlet 7, which is designed to supply air to the compressor chamber 5, for example via an intake tract (not shown), at an inlet pressure p1. This air is then compressed to an outlet pressure p2 by rotation of the compressor wheel 3 in the compressor chamber 5 and discharged via an outlet 9 of the compressor chamber 5. The turbomachine 1 is fluidly connected to a fuel cell 101 of the fuel cell system 100 and is designed to convert the air compressed by the compressor wheel in a fuel cell reaction in a generally known manner. The air represents the cathode-side reactant. The compressed air is supplied to the fuel cell 101 at the pressure p2. After passing through the fuel cell 101, an air / water mixture is discharged from the fuel cell 101 as the so-called cathode exhaust gas with a hydrostatic pressure p3, which is lower than p2.The fuel cell 101 is fluidly connected to an expander chamber 11 of the turbomachine 1, more precisely to an inlet 13 of the expander chamber 11. The expander chamber 11 is assigned to the turbomachine 1 and has an expander wheel 15 in its interior. As a result of the cathode exhaust gas entering at the pressure p3 as inlet pressure, the expander wheel 15 is flowed against and, in a generally known manner, an expansion of the cathode exhaust gas occurs here, so that the cathode exhaust gas leaves the expander chamber 11 through an outlet 17 at a pressure p4 which is approximately or equal to the ambient pressure p. U The pressure of the cathode exhaust gas p3 is lower than the pressure p2 after passing through the compressor chamber 5. However, it is still above the intake pressure p1, which is equal to or higher than the ambient pressure p Uwould be. The compressor wheel 3 and the expander wheel 15 are connected by means of a rotor shaft 19 and are each fastened to the rotor shaft 19 in a rotationally rigid manner. The rotor shaft 19 is driven in a generally known manner by an electric machine 21 so as to rotate about a rotor axis X, in the present exemplary embodiment according to Fig. 1 clockwise. The rotor shaft 19 is rotatably mounted in a compressor housing 23 by means of an air bearing arrangement 21, to which the expander chamber 11 and the compressor chamber 5 are preferably also assigned. The air bearing arrangement 21 has at least a first air bearing 21a, which can be a radial air bearing, and a second air bearing 21b, which can also be a radial air bearing. The air bearing arrangement 21 preferably also has one or more axial air bearings (not shown), which also support the rotor shaft 19 and the parts rotating with it in the axial direction relative to the axis X.For clarity, only the radial air bearings 21a, 21b are shown here. For aerostatic support, the air bearing assembly 21 is preferably fluidly connected via an air bearing flow path 25 to a compressed air source, which is configured to supply compressed air at a pressure p for support. Lto be blown into the air bearing arrangement 21 in order to support the load-bearing capacity of the air bearing 21 as long as the rotor shaft 19 has not yet reached its required lift-off speed to form a sufficiently load-bearing air cushion. Between the air bearing 21 and the expander wheel 15, or the expander chamber 11, a first flow path 27 is formed, which is designed as an annular gap between the compressor housing 23 and the rotor shaft 19. This first flow path 27 is ultimately justified by the fact that a contact-free movement of the rotor shaft 19 relative to the compressor housing 23 is desired. The flow path 27 is thus a potential gateway for water and possibly solid particles, which could penetrate towards the air bearing arrangement 21 due to the excess pressure resulting from the inlet pressure p3 within the expander chamber 11 and / or due to capillary action. To prevent this, a space is provided between the air bearing arrangement 21 and the expander chamber 11, orA first flow generator 29 is arranged on the expander wheel 15 and is designed in the manner described below to generate a blocking pressure p. S on its side facing the expander wheel 15 within the flow path 27, depending on the speed of the rotor shaft 19. The flow generator 29 is designed to generate the barrier pressure p S (see Fig.2) at a height of p S> p3 to prevent water droplets and / or particles from moving through the flow path 27 to the air bearing arrangement 21. Thus, the first flow generator 29 creates a contact-free fluid and particle seal based on the generation of a local overpressure in the flow path 27 on the side facing the expander wheel 15 from the flow generator 29. The first flow generator 29 is further fluidically connected by means of an air supply line 31. Air can be supplied via the air supply line 31 to the intake side of the first flow generator 29, i.e., on the side facing the air bearing arrangement 21, as seen from the first flow generator 29. This air supply line can be used to supply air, for example, from the environment at ambient pressure p UThis effectively prevents bearing air from being cannibalized by the air bearing arrangement 21 due to rotation of the rotor shaft 19 by means of the first flow generator 29, so that the first flow generator 29 does not impair the load-bearing capacity of the air bearing arrangement 21. The first flow generator 29 has a number, preferably a plurality, of recesses and / or projections 33 which are provided on the rotor shaft 19 and rotate with the rotor shaft 19 at its speed about the axis X. In the illustrated embodiment, the recesses and / or projections 33 are designed as grooves which have a predetermined radial extent t1, namely a depth defining the grooves, and are arranged at an angle α1 to the rotor axis X. The angle α1 is counterclockwise with respect to the rotor axis X, i.e. oriented opposite to the direction of rotation of the rotor shaft 19, so that the blocking pressure p S(cf. Fig. 2) is generated on the "correct" side of the flow generator 29. The turbo machine 1 further comprises, in addition to the first flow generator 29, a second flow generator 35 which is arranged so as to act between the air bearing arrangement 21 and the compressor wheel 3, or the compressor chamber 5, in a second flow path 37. The second flow path 37 is also designed as an annular gap between the compressor housing 23 and the rotor shaft 19, for the same structural reasons as the first flow path 27. In essentially the same mode of operation as the first flow generator 29, the second flow generator 35 also has a number of projections and / or recesses 39 which are arranged on the rotor shaft 19. The projections and / or recesses 39 are preferably designed as grooves.The projections / recesses 39 have a radial extent t2, in the case of grooves also a measure of the groove depth, which is preferably less than the radial extent t1 of the recesses / protrusions of the first flow generator 29. By differentiating the radial extents t1, t2, a partial compensation of the axial forces generated by the flow generators is achieved, which can also be matched to the axial forces emanating from the compressor wheel 3 or expander wheel 15 in the direction of the axis X. The alignment of the projections or recesses 39 takes place in Fig.1 at an angle α2, which is oriented opposite to the angle α1 of the first flow generator 29, with the rotor shaft 19 rotating to the right, i.e. also rising to the right. Consequently, the second flow generator 35 generates its blocking pressure on the other side relative to the first flow generator 39, i.e. on the side opposite the compressor wheel 3 orthe side of the flow path 37 facing the compressor chamber 5, in order to at least minimize the entry of unwanted particles from this side into the flow path 37 and, above all, to ensure the above-mentioned, at least partial axial force compensation. In Fig. 2, the right-hand side of the turbo machine 1 according to Fig. 1 is shown on a larger scale. The invention is explained in more detail with regard to the first flow generator 29, wherein the technical functions can also be transferred analogously to the second flow generator 35, the separate illustration of which is omitted here for the sake of clarity. When the shaft 19 rotates about the axis X in the clockwise direction of rotation, a blocking pressure p is generated by the opposite orientation of the projections or recesses 33 by the angle α1 on the flank side lagging in the direction of rotation, i.e. on the right-hand side in Fig. 2. Sdue to the air turbulence in the flow path 27, which under unobstructed flow conditions would cause an air flow towards the expander chamber 11. At least the dynamic or blocking pressure pS is generated, which at a sufficiently high speed of the rotor shaft 19 is higher than the hydrostatic inlet pressure p3 in the expander chamber 11. The blocking pressure p Sis preferably 1.0 bar or more higher than the hydrostatic pressure p3 in the expander chamber 11. In order to track and / or at least relieve the suction side of the flow generator 29, the side of the flow generator 29 facing away from the expander wheel 15 is preferably assigned to one or more partially circumferential, one or more grooves 41, which are fluidly connected to the air supply line 31 in order to compensate for the locally occurring intake negative pressure pA by air tracking. This is preferably done with air intake from the environment. The first flow generator 29 is spaced apart from the air bearing 21 by a distance δ1 in the direction of the axis X.This distance δ1 creates, in a sense, a neutral zone in which no structural elements are present on the rotor shaft 19, and preferably also not on the compressor housing 23, in order to minimize mutual interference between the flow generator 29 and the air bearing arrangement 21. This also minimizes any adverse effect of the flow generator 29 or the air turbulence it generates on the air bearing arrangement 21, which could be constructed, for example, as a spiral groove bearing or a foil bearing. Likewise, the second flow generator is preferably spaced axially by a distance δ2 from the air bearing arrangement 21 in the direction of the X axis, see Fig. 1.

[0002] Reference symbols (part of the description): 1 Turbomachine 5 Compressor chamber 7 Inlet 9 Outlet 11 Expander chamber 15 Expander wheel 17 Outlet 19 Rotor shaft 21 Air bearing 21a, b Air bearing arrangement 23 Compressor housing 25 Air bearing flow path 27 Flow path, expander side 29 First flow generator 31 Air supply line 33 Projections / recesses, first flow generator 35 Second flow generator 37 Flow path, compressor side 39 Projections / recesses, second flow generator 100 Fuel cell system 101 Fuel cell 200 Vehicle X axis p1 Pressure, compressor inlet p2 Pressure, compressor outlet p3 Inlet pressure, expander, cathode exhaust gas p4 Outlet pressure, expander, cathode exhaust gas pS Sealing pressure, flow generator p A Intake pressure, flow generator pU Ambient pressure α1Angle, first flow generator α2Angle, second flow generator δ1 Distance, first flow generator δ2Distance, second flow generator

Claims

Patent claims:

1. Turbomachine (1), in particular for a fuel cell system (100) of a vehicle (200), such as a commercial vehicle, with a rotor shaft (19), an expander wheel (15) fastened to the rotor shaft (19), and an air bearing arrangement (21) which is designed to support the rotor shaft (19) rotatably about a rotor axis (X), wherein a flow path (27) is formed between the expander wheel (15) and the air bearing arrangement (21), characterized in that a flow generator (29) is arranged in the flow path (27) between the air bearing arrangement (21) and the expander wheel (15) and is designed to generate an air flow directed towards the expander wheel (15) depending on a rotation of the rotor shaft (19). 2.Turbomachine (1) according to claim 1, wherein the expander wheel (15) is arranged in an expander chamber (11) into which a cathode exhaust gas of a fuel cell (101) is conveyed with a predetermined hydrostatic inlet pressure (p3), characterized in that the flow generator (29) is designed to generate a barrier pressure (p3) on a pressure side facing the expander wheel (15). S ) which is equal to or greater than the inlet pressure (p3) of the expander chamber (11).

3. Turbomachine (1) according to claim 2, characterized in that the barrier pressure (pS) exceeds the inlet pressure by 0.5 bara or more, preferably by 1.0 bara or more, particularly preferably by 1.0 bara to 2.0 bara, when the rotor shaft (19) reaches or exceeds a predetermined speed.

4. Turbomachine (1) according to one of the preceding claims, characterized in that the flow generator (29) has a number, preferably a plurality, of recesses and / or projections (33) formed on the rotor shaft (19).

5. Turbomachine (1) according to claim 4, characterized in that the recesses and / or projections (33) are aligned parallel to the rotor axis (X) or at an angle (α1) relative to the rotor axis (X).

6. Turbomachine (1) according to claim 5, characterized in that the turbomachine (1) is configured to rotate the rotor shaft (19) in a preferred direction of rotation, wherein the angle (α1) of the recesses and / or projections (33) has a pitch opposite to the direction of rotation.

7. Turbomachine (1) according to claim 5 or 6, characterized in that the angle (α1) to the rotor axis (X) is in a range from 10° to 80°.Turbomachine (1) according to one of claims 4 to 7, characterized in that the recesses and / or projections (33) are formed relative to a surface of the rotor shaft (19) and have a radial extension (t1) relative to the surface in a range from ... to ...

9. Turbomachine (1) according to one of the preceding claims, characterized in that an air supply line (31) is assigned to the flow generator (29), which is provided separately from the air bearing arrangement (21).

10. Turbomachine (1) according to one of the preceding claims, characterized in that the flow generator (29) and the air bearing arrangement (21) are adjacent to one another in the direction of the rotor axis (X). are arranged adjacent to one another or spaced apart from one another by a distance (δ1), wherein the flow generator (29) is arranged on a side of the air bearing arrangement (21) facing the expander wheel (15).

11. Turbomachine (1) according to one of the preceding claims, characterized in that the flow generator (29) is a first flow generator (29), and the turbomachine (1) has a second flow generator (35) which is arranged opposite the air bearing arrangement (21) relative to the first flow generator (29), wherein the second flow generator (35) is designed to generate an air flow directed away from the air bearing arrangement (21) depending on a rotation of the rotor shaft (19).

12. Turbomachine (1) according to claim 11, characterized in that the second flow generator (35) has a number, preferably a plurality, of recesses and / or projections (39) formed on the rotor shaft (19).Turbomachine (1) according to claim 11 or 12, wherein the recesses and / or projections (39) are aligned parallel to the rotor axis (X) or relative to the rotor axis (X) at an angle (α2) that is opposite to the angle (α1) of the first flow generator (29), and is preferably equal in magnitude to the angle (α1) of the first flow generator (29).

14. Turbomachine (1) according to one of claims 11 to 13, characterized in that the recesses and / or projections (39) of the second flow generator (35) have a smaller radial extent (t2) than the recesses and / or projections (33) of the first flow generator (29).

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