DEVICE AND METHOD FOR COOLING A HEAT EXCHANGER

Description

[0001] The invention relates to an aircraft propulsion system and a method for cooling a heat exchanger of a fuel cell of an aircraft propulsion system outside a flight phase of an aircraft, wherein the aircraft propulsion system has an air line with at least one compressor for supplying compressed air for operating the fuel cell.

[0002] In aircraft engines with fuel cells, large amounts of heat are generated which must be dissipated via heat exchangers to prevent the fuel cells from overheating. Accordingly, heat exchangers in such aircraft engines are usually arranged so that propeller downwash air and / or ram air can be used to cool the at least one heat exchanger. On the ground, outside of an aircraft's flight phase, no cooling air or insufficient cooling air is available. During operation of the fuel cells outside of an aircraft's flight phase, alternative cooling of the heat exchangers is therefore required in order to dissipate sufficient heat from the fuel cells. Additional fans arranged on the aircraft to generate a cooling air flow on the ground are superfluous during the flight phase and tend to have a detrimental aerodynamic effect.Examples of fuel cell propulsion systems are known from documents GB2545246A and US20170211474A1. A flow amplifier is known from document US20090032130A1. US 2017 / 166320 A1 discloses an auxiliary power unit with a gas turbine and a fuel cell.

[0003] Based on this, it is an object of the present invention to propose an improved aircraft propulsion system for an aircraft with a device and a method for cooling a heat exchanger of a fuel cell of the aircraft propulsion system outside of a flight phase of the aircraft. This is achieved according to the invention by the teaching of the independent claims. Advantageous embodiments of the invention are the subject of the dependent claims.

[0004] To achieve this objective, a first aspect proposes an aircraft propulsion system for an aircraft with a device for cooling a heat exchanger of a fuel cell of the aircraft propulsion system outside of a flight phase of the aircraft. The aircraft propulsion system has an air line with at least one compressor for supplying compressed air for operating the fuel cell. The device has a flow amplifier arranged in the region of the heat exchanger, which is configured to direct air onto a cooling surface of the heat exchanger, and a branch in the air line arranged downstream of the at least one compressor, via which branch compressed air can be directed to the flow amplifier.

[0005] Similar to conventional kerosene-powered turbomachines, an aircraft propulsion fuel cell requires compressed air to achieve high power density. Electrochemical fuel cells convert fuel and oxidizer into electrical energy and a reaction product. Ambient air, typically used as an oxidizer in aircraft propulsion, is supplied to the fuel cell via an air line that, in addition to the necessary lines, connections, and valves, also features at least one compressor for compressing air drawn in from the environment. One or more compressors or compressor stages arranged in series and / or parallel can be provided to compress the air.Since the reaction of fuel and oxidizer generates both heat and electrical power, a fuel cell stack must be cooled even after reaching an operating temperature to avoid damage to the fuel cells.

[0006] Such cooling is typically achieved with the aid of at least one heat exchanger connected to a fuel cell coolant circuit, over which a fan and / or ram air flow flows during flight phases. In particular, an air flow flowing over a cooling surface of the heat exchanger absorbs heat and dissipates it, particularly convectively, from the heat exchanger. Although "the heat exchanger" is specified in the claims and the description, the heat exchanger can, of course, also comprise several heat exchanger devices arranged spatially adjacent to one another and / or distributed in a distributed manner, which can, in particular, also (each) have several cooling surfaces. In this context, a cooling surface is defined as any (surface) surface arranged on the heat exchanger which is heated by the heat energy to be dissipated and from which heat can be dissipated by an air flow passing over it.Since the fuel cells of the aircraft propulsion system are operated at least partly outside of the flight phase, for example to provide propulsion energy for taxiing on the runway or to supply power to the aircraft on the ground, there is no sufficient fan or ram air flow available during these phases to dissipate heat energy from the heat exchanger of the fuel cells.

[0007] To direct the air to a cooling surface of the heat exchanger, the device of the proposed aircraft propulsion system has a flow amplifier arranged in the region of the heat exchanger. Outside of flight phases, the fuel cells are typically operated at reduced power, which is why only a reduced air supply to the fuel cells is required. Accordingly, free compressor power is then available, which can be used to cool the heat exchanger of the fuel cells. For this purpose, the device of the proposed aircraft propulsion system has a branch arranged in the air line of the fuel cell downstream of the at least one compressor, via which branch compressed air can be branched from the air line and directed to the flow amplifier.Depending on the compressor's design and performance, the branching can be arranged downstream of a first or subsequent compressor, and in particular also depending on the control integration of the branching circuit during the transition into or out of a flight phase. For example, only a portion of the compressed air available at the branching can be extracted via the branching, or the air available in a sub-branch of the compressor can be directed to the flow amplifier.

[0008] The compressed air diverted via the branch is guided from there to a flow amplifier arranged in the region of the heat exchanger, which is designed to direct the air onto a cooling surface of the heat exchanger. The flow amplifier is arranged in the region of the heat exchanger and is therefore in a suitable position for directing the amplified air flow onto at least one cooling surface of the heat exchanger. In particular, the flow amplifier has an air outlet opening with suitable outlet geometries and / or air guiding devices in order to form an air flow that is advantageous, in particular with regard to flow velocity and direction, for the efficient removal of thermal energy from the heat exchanger. In particular, the compressed air guided through the flow amplifier has a higher flow velocity after exiting the flow amplifier than before the flow amplifier.Such a flow amplifier can have different geometries and flow cross-sections, depending on the specific application. Accordingly, the flow amplifier can be adapted to the available space, which is why such flow amplifiers can be installed, for example, in confined spaces while still achieving good airflow guidance. In particular, a flow amplifier is designed and arranged in such a way that it does not have aerodynamically detrimental effects during the flight phase.

[0009] As a result, the proposed aircraft propulsion system with the device enables the removal of thermal energy from a heat exchanger of a fuel cell of the aircraft propulsion system outside of the aircraft's flight phase without the need for additional, complex devices for generating an airflow, which are superfluous during the flight phase or have aerodynamically detrimental effects. To remove the thermal energy from the heat exchanger outside of the flight phase, an existing compressor capacity, not required on the ground, is used to compress the air and can be used to remove the excess thermal energy. This way, only a few additional devices are required for cooling the fuel cell heat exchanger outside of the flight phase.

[0010] In one embodiment of the aircraft propulsion system, the flow amplifier operates according to the Venturi and / or Coanda principle to direct the air to the cooling surface of the heat exchanger. In addition, the device can also direct ambient air to the cooling surface of the heat exchanger in addition to the compressed air. For example, the flow amplifier can be designed to utilize the effect of a (Venturi) jet pump. The air flow supplied by the compressor forms the propulsion medium, allowing additional ambient air to be drawn in and accelerated in the area of ​​the flow amplifier before the resulting amplified air flow flows to the cooling surface of the heat exchanger.Alternatively, or in combination with this design, the flow amplifier can, for example, have an outlet opening utilizing the Coanda principle, in which the air flow is guided over the surface of a suitably widening wall, against which the air flow adheres. This creates a pressure drop in the center of the outlet opening, through which additional ambient air can be drawn in and accelerated before the resulting amplified air flow flows to the cooling surface of the heat exchanger. By using the Venturi and / or Coanda principle, the amount of heat that can be removed by the heat exchanger can be increased.

[0011] In one embodiment of the aircraft propulsion system, a flow control valve is arranged upstream of the flow amplifier, with which the air flow supplied to the flow amplifier can be regulated. Using a flow control valve, the flow of a flow amplifier can be easily regulated, in particular to adapt it to the cooling air requirements of the heat exchanger. For example, a flow control valve can be arranged in the branching of the air line.

[0012] In one embodiment of the aircraft propulsion system, the flow amplifier is at least partially annular. In such a design, the air flow can be guided, for example, through an annular channel having an outlet opening on its radial inner side directed substantially parallel to the direction of the rotation axis. In this design, a negative pressure is created at the center of the axially extending air flow thus formed, through which ambient air is drawn in, thereby amplifying the air flow directed toward the heat exchanger. With such a design, a directed air flow with an at least partially circular cross-section can be formed.

[0013] In one embodiment of the aircraft propulsion system, the heat exchanger is arranged in the propulsion nacelle. This position places it in close proximity to the fuel cells arranged in the nacelle and their coolant supply. In this embodiment of the device, the flow amplifier can be arranged, for example, in the inlet of the propulsion nacelle, so that an air flow flowing out of the flow amplifier flows axially through the propulsion nacelle. When flowing through the propulsion nacelle, the air flow can use the flow paths provided for cooling air during the flight phase to reach the at least one cooling surface of the heat exchanger and to transport the heat absorbed there out of the propulsion nacelle.

[0014] In one embodiment of the aircraft propulsion system, the compressor has a multi-stage design, and the branching is arranged after at least a first stage. The multi-stage compressor for supplying air to the fuel cells can have several compressor stages arranged in series and / or parallel, which, outside of flight phases, are only partially used, for example, to supply other aircraft equipment with cooling air. For example, outside of flight phases, individual compressor stages can be switched off because they are not required to compress the smaller requested air volume. Accordingly, the branching for branching off compressed air for the flow amplifier is arranged such that the supply to the flow amplifier is guaranteed and can be suitably integrated into the compressor control system.

[0015] To achieve the object, a second aspect proposes a method for cooling a heat exchanger of a fuel cell of an aircraft engine outside of a flight phase of an aircraft, wherein the aircraft engine has an air line with at least one compressor for supplying compressed air for operating the fuel cell. The method comprises the following steps: Compressing air with at least one compressor of the aircraft engine; directing the compressed air through a flow amplifier onto a cooling surface of the heat exchanger; absorbing the thermal energy from the cooling surface by the air flow; and dissipating the thermal energy with the air flow.

[0016] In the proposed method, air is first compressed using a compressor in the aircraft engine. This can, for example, be a compressor used to compress the air required to operate the fuel cell, which is supplied to the fuel cell via an air line. Since fuel cells are usually only operated at reduced power outside of flight phases, only a reduced air supply to the fuel cells is required. This free compressor power is therefore available, which can be used to compress air to cool the heat exchanger of the fuel cells. In the proposed method, the compressed air is fed to a flow amplifier, which is arranged in particular in the region of the heat exchanger and directs the air to at least one cooling surface of the heat exchanger.

[0017] The flow amplifier models the flow profile of the airflow, particularly with regard to flow velocity and direction, so that as much heat energy as possible can be absorbed and dissipated by the cooling surface of the heat exchanger. This allows for efficient removal of heat energy from the heat exchanger. In particular, the compressed air passing through the flow amplifier exhibits a higher flow velocity after exiting the flow amplifier than before the flow amplifier.

[0018] The proposed method thus enables low-effort removal of thermal energy from a heat exchanger of a fuel cell of an aircraft propulsion system outside of an aircraft's flight phase, at least partially utilizing existing aircraft propulsion systems. The method can be carried out, in particular, with the previously described aircraft propulsion system, which may have the features of one or more of the embodiments described herein. The method may accordingly also have the features and advantages described herein.

[0019] In one embodiment of the method for cooling a heat exchanger, the flow amplifier directs ambient air to a cooling surface of the heat exchanger in addition to the compressed air. The flow amplifier draws in ambient air, which is accelerated by the air flow directed to it. The thus additionally amplified air flow then flows to the cooling surface of the heat exchanger to absorb and dissipate heat energy from the cooling surface(s). In this way, the amount of heat that can be removed by the heat exchanger can be increased.

[0020] Further features, advantages and possible applications of the invention will become apparent from the following description taken in conjunction with the figures. Fig. 1 shows a schematic representation of an exemplary device of an aircraft propulsion system according to the invention for cooling a heat exchanger of a fuel cell of an aircraft propulsion system outside of a flight phase of an aircraft; Fig. 2 shows a schematic sectional representation of an exemplary flow amplifier; and Fig. 3 shows a schematic representation of a flow diagram of the method according to the invention.

[0021] Fig. 1shows a schematic representation of an exemplary device 10 of an aircraft propulsion system according to the invention for cooling a heat exchanger 12 of a fuel cell 16 of an aircraft propulsion system outside of a flight phase of an aircraft. The aircraft propulsion system has an air line 20 for supplying the fuel cell 16 with air. In the illustrated embodiment, the air line 20 has three compressors 21, 22, 23 or compressor stages 21, 22, 23 for supplying compressed air for operation of the fuel cell 16. Two of the compressors 21, 22 or compressor stages 21, 22 are connected in parallel in the exemplary embodiment, and a further compressor 23 or compressor stage 23 is connected downstream of them in series. This compressor 23 orCompressor stage 23 is additionally driven by a turbine 24, which is imparted with rotational energy by the air flow passing through the cathode(s) 17 of the fuel cell(s) 16, before the air escapes from the air line 20 via an air outlet 25. The ambient air is supplied to the air line 20 via an air filter 26.

[0022] The fuel cell 16 is cooled via a cooling circuit 30, which includes a coolant reservoir 31, a filter 32, a coolant pump 33, and an air-cooled heat exchanger 12 for cooling the fuel cell 16. The coolant is pumped by the coolant pump 33 through the fuel cell 16, where it absorbs thermal energy. During a flight phase of the aircraft, an airflow flows over the cooling surfaces of the heat exchanger, dissipating the flow energy generated in the fuel cell 16.

[0023] To dissipate excess thermal energy from the fuel cell 16 outside of flight phases, the device 10 for cooling the heat exchanger 12 has a flow amplifier 40 arranged in the region of the heat exchanger 12, which is configured to direct air onto a cooling surface 13 of the heat exchanger 12. Compressed air is supplied to the flow amplifier 40 from the air line 20, which primarily serves to supply the fuel cell 16 during the flight phase and is usually not fully utilized outside of the flight phase. For this purpose, a branch 28 is arranged in the air line 20 after at least one compressor 21, 22, via which branch, in the exemplary embodiment, compressed air from the compressors 21 and 22 can be directed to the flow amplifier 40. In the exemplary embodiment, a flow control valve 29 is arranged upstream of the flow amplifier 40, with which the air flow supplied to the flow amplifier 40 can be regulated.

[0024] Fig. 2shows a schematic sectional view of an exemplary flow amplifier 40, which is ring-shaped. The air flow branched off from the air line 20 is guided through an annular air duct 41, which has an outlet opening 42 on its radial inner side, directed essentially parallel to the direction of the axis of rotation. The air flow is guided over the surface of the widening wall, against which the flow 44 conforms according to the Coanda principle. In the process, a negative pressure forms in the center of the flow amplifier 40, through which additional ambient air 45 is sucked in axially and accelerated. The amplified air flow thus formed then flows to the cooling surface 13 of the heat exchanger 12 to absorb thermal energy and thus dissipate it from the heat exchanger 12.

[0025] Fig. 3shows a schematic representation of a flow diagram of the method according to the invention for cooling a heat exchanger 12 of a fuel cell 16 of an aircraft propulsion system outside a flight phase of an aircraft, wherein the aircraft propulsion system has an air line 20 with at least one compressor 21, 22, 23 for supplying compressed air for operation of the fuel cell 16.

[0026] The method according to the invention comprises the following steps: In a first step a), air is compressed by at least one compressor 21, 22, 23 of the aircraft engine and, in a second step b), is directed through a flow amplifier 40 onto a cooling surface 13 of the heat exchanger 12. In one embodiment of the method, in a step b1), the flow amplifier 40 can direct ambient air, in addition to the compressed air, onto a cooling surface 13 of the heat exchanger 40. In a step c), the air flow absorbs the thermal energy from the cooling surface 13 of the heat exchanger 12 and, in a further step d), dissipates it from the cooling surface of the heat exchanger. LIST OF REFERENCE SYMBOLS

[0027] 10Device 12Heat exchanger 13Cooling surface 16Fuel cell 17Cathode 20Air line 21Compressor (stage) 22Compressor (stage) 23Compressor (stage) 24Turbine (stage) 25Air outlet 26Air filter 28Branching 29Flow control valve 30Cooling circuit 31Coolant reservoir 32Filter 33Coolant pump 40Flow amplifier 41Air duct 42Outlet opening 44Flow 45Ambient air

Claims

1. Flight propulsion system for an aircraft, comprising a fuel cell (16), an air line (20) having at least one compressor (21, 22, 23) for supplying compressed air for operating the fuel cell (16), and a device (10) for cooling a heat exchanger (12) of the fuel cell (16) outside a flight phase of the aircraft, characterized by a flow amplifier (40), which is arranged in the region of the heat exchanger (12) and is designed to direct air onto a cooling surface (13) of the heat exchanger (12) and through a branch (28) arranged in the air line (20) so as to be downstream of the at least one compressor (21, 22, 23), via which branch compressed air can be directed to the flow amplifier (40).

2. Flight propulsion system according to claim 1, characterized in that the flow amplifier (40) operates according to the Venturi and / or Coanda principle in order to direct the air onto the cooling surface (13) of the heat exchanger (12).

3. Flight propulsion system according to claim 2, characterized in that the flow amplifier (40) is designed to direct ambient air (45) in addition to the compressed air onto the cooling surface (13) of the heat exchanger (12).

4. Flight propulsion system according to any of the preceding claims, characterized in that a flow control valve (29) is arranged upstream of the flow amplifier (40), by means of which the air flow supplied to the flow amplifier (40) can be regulated.

5. Flight propulsion system according to any of the preceding claims, characterized in that the flow amplifier (40) is at least partially annular.

6. Flight propulsion system according to any of the preceding claims, characterized in that the heat exchanger (12) is arranged in the propulsion nacelle.

7. Flight propulsion system according to claim 6, characterized in that the flow amplifier (40) is located in the inlet of the propulsion nacelle.

8. Flight propulsion system according to any of the preceding claims, characterized in that the compressor (21, 22, 23) is constructed so as to have several stages and the branch (28) is arranged downstream of at least a first stage (21, 22, 23).

9. Method for cooling a heat exchanger (12) of a fuel cell (16) of a flight propulsion system outside a flight phase of an aircraft, wherein the flight propulsion system has an air line (20) having at least one compressor (21, 22, 23) for supplying compressed air for operation of the fuel cell (16), characterized by the steps of: - compressing air by means of at least one compressor (21, 22, 23) of the flight propulsion system; - directing the compressed air through a flow amplifier (40) onto a cooling surface (13) of the heat exchanger (12); - absorbing the thermal energy from the cooling surface (13) by means of the air flow; and - dissipating the thermal energy by means of the air flow.

10. Method for cooling a heat exchanger according to claim 9, characterized in that the flow amplifier (40) directs both the compressed air as well as ambient air (45) onto a cooling surface (13) of the heat exchanger (12).