AIRCRAFT PROPULSION UNIT COMPRISING A FUEL CELL

Abstract

Propulsion unit (1) of an aircraft (2) comprising a propeller (3) driven in rotation by an electric machine (4), the electric machine (4) being powered by a fuel cell (5) which comprises a core (6), the core (6) being cooled by a cooling system (7) which comprises a first heat exchanger (8) placed in a first ducted vein (9) in which air flows, characterized in that the cooling system (7) comprises a second heat exchanger (8) placed in a second ducted vein (9) in which air flows, the heat exchangers (8) and the veins (9) being annular, the veins (9) surrounding a central compartment (12) supplied with air in which the electric machine (4) and the core (6) of the fuel cell (5) are placed, the inlets (10) of the veins (9) preferably each having a so-called inlet section which is variable. Figure for abstract: 1

Description

Title of the invention: AIRCRAFT PROPULSION UNIT COMPRISING A FUEL CELL Technical field of the invention

[0001] The present invention relates to an aircraft propulsion unit comprising a fuel cell whose core is cooled by a cooling system, as well as to a method for adjusting such a cooling system. Technical background

[0002] An aircraft may comprise a propulsion unit equipped with an electric thruster comprising a propeller driven by an electric motor.

[0003] The electrical energy required to power the electric thruster is provided in part or in full by one or more electrical generators.

[0004] In the remainder of the application, we will focus on the particular case where the electric generator is a fuel cell better known by the acronym “PAC” or the English term “fuel cell”.

[0005] A fuel cell has the advantage of producing few polluting and noise emissions.

[0006] Such a fuel cell converts the chemical energy contained in a fuel / oxidant pair (for example the hydrogen / oxygen pair) into electrical energy. The fuel cell comprises in particular an electrical energy generating core which is formed of electrochemical cells.

[0007] The electrochemical reaction of the fuel / oxidant couple produces not only electricity but also heat which is important to evacuate, to allow its proper functioning and maximize its lifespan.

[0008] For this purpose, it is known to cool the core of the fuel cell with a cooling system which notably comprises a cooling circuit in which a heat transfer fluid circulates. To evacuate the thermal energy of the heat transfer fluid into the external environment, the cooling circuit comprises a heat exchanger placed in a ducted air stream. The air stream is generally of circular section and placed under the electric thruster.

[0009] Engine manufacturers note that such a propulsion unit architecture is not optimal.

[0010] Indeed, following various studies, engine manufacturers have noted that to meet the cooling requirements of the core in the most unfavorable case (namely takeoff in a hot environment), it is necessary to install a large heat exchanger, and consequently to have a large vein.

[0011] The installation of a large circular section vein under the electric thruster would not only cause significant aerodynamic drag but also a non-uniform supply of the vein (portions of vein located in the wake of the propeller and portions of vein located outside the wake of the propeller), to the detriment of the thermal efficiency of the heat exchanger and, more generally, the overall efficiency of the propulsion unit.

[0012] Furthermore, it is known to install a flap in the vein to limit the aerodynamic drag generated by the passage of air in the vein. The flap is generally in two states, namely open when the fuel cell is active and closed when the fuel cell is inactive.

[0013] Engine manufacturers note that such a shutter can be improved.

[0014] Indeed, for certain operating regimes such as cruising regime, the flow of air entering the vein is too high and only a portion of this air is necessary to cool the core of the fuel cell, the excess portion of air generating aerodynamic drag which is undesirable.

[0015] The objective of the present invention is therefore to provide a simple, effective and economical solution making it possible to respond to the aforementioned problems. Summary of the invention

[0016] The invention thus proposes an aircraft propulsion unit which extends around an axis X, the propulsion unit comprising a propeller which is rotatable around the axis X and driven in rotation by an electric machine, the electric machine being powered by a fuel cell which comprises a core generating electrical energy, the core of the fuel cell being cooled by a cooling system which comprises a first heat exchanger placed in a first ducted vein in which air flows from an inlet of the first vein and to an outlet of the first vein, characterized in that the cooling system comprises a second heat exchanger placed in a second ducted vein in which air flows from an inlet of the second vein and to an outlet of the second vein, the heat exchangers and the veins being annular around the axis X,the veins surrounding a central compartment supplied with air in which the electric machine and the heart of the fuel cell are placed, the inlets of the veins each preferably having a so-called inlet section which is variable.

[0017] Having several heat exchangers in separate veins makes it possible to supply each of the heat exchangers with fresh air (and in other words air not previously heated by its passage through a previous heat exchanger), which makes it possible to have a cooling system having a capacity high cooling capacity compared in particular to a cooling system comprising a single thick heat exchanger or to a cooling system comprising several heat exchangers placed in the same vein one after the other.

[0018] Having heat exchangers and annular veins makes it possible to install large heat exchangers while limiting the negative impacts (drag, mass, etc.) which are linked to their installation.

[0019] Such annular veins also make it possible to supply each of the heat exchangers homogeneously, all of the veins being in the wake of the propeller, to the benefit of the thermal efficiency of each of the heat exchangers and more generally of the overall efficiency of the propulsion unit.

[0020] The variable section inlets make it possible to precisely adapt the air flow rates entering each of the veins according to the cooling requirements of the core, and thus minimize the aerodynamic drag produced by the passage of air in each of the veins, during the different operating regimes of the propulsion group, to the benefit of its overall efficiency.

[0021] The propulsion unit according to the invention may comprise one or more of the following characteristics and / or steps, taken in isolation from one another or in combination with one another: - the propeller is mobile in rotation in a plane called rotation plane P which is perpendicular to the axis X, the inlets of the veins being arranged downstream of the propeller, the axial dimension Da between the rotation plane P of the propeller and the inlet of each vein being between 0.1 x R and 0.6 x R, where R is the radius of the propeller; - the inlet of each vein has a reference point Pr whose radial dimension Dr with respect to the X axis is between 0.4 x R and 0.95 x R, where R is the radius of the propeller, the reference point Pr being defined in an axial half-section of the propulsion group which passes through the X axis; - each vein comprises a diverging portion located directly upstream of the corresponding heat exchanger, so as to slow down the speed of the air entering the heat exchanger, and a converging portion located directly upstream of the outlet of the vein, so as to accelerate the air escaping from the vein and generate thrust; - at least one of the inlets of the veins is formed by a nozzle with variable section, the nozzle comprising a framework comprising a base from which extends an annular row of independent bands, the free ends of the bands being connected in common to a ring whose diameter is adjustable, the framework being covered at least partially with an envelope, so as to seal the nozzle; - at least one of the vein inlets comprises a sealing device comprising at least one adjustable flap; - the cooling system comprises a surface heat exchanger which is flush with an internal fairing of a nacelle of the propulsion group; - the cooling system comprises at least one water injector which is placed in one of the veins upstream of the corresponding heat exchanger; - the propulsion unit comprises an axial compressor placed in the compartment upstream of the electric machine and the core of the fuel cell, the axial compressor being supplied with air and comprising a rotor driven in rotation by the electric machine; - the propulsion unit comprises a cooler placed in the compartment between the axial compressor on the one hand and the electric machine and the core of the fuel cell on the other hand, the cooler cooling the air leaving the axial compressor; - the heat exchangers are axially offset from each other; - the cooling system comprises a third heat exchanger placed in a third ducted vein in which air flows from an inlet of the third vein and to an outlet of the third vein, the third heat exchanger and the third vein being annular around the X axis, the third vein surrounding the central compartment, the inlet of the third vein preferably having an inlet section which is variable.

[0022] The present invention also relates to a method for adjusting the cooling system of a propulsion unit as described above, the method comprising the step of: a) adjust the inlet section of the veins based on a measurement of the temperature of the fuel cell core, so as to adjust the air flow rates entering the veins according to the cooling requirements. Brief description of the figures

[0023] The invention will be better understood and other details, characteristics and advantages of the invention will appear more clearly on reading the following description given by way of non-limiting example and with reference to the appended drawings in which:

[0024] [Fig-1] [Fig.l] is an axial sectional view of a propulsion unit according to the invention;

[0025] [Fig.2] [Fig.2] is a detail view of [Fig.l] showing an axial compressor of the propulsion unit and its drive via a gear mechanism;

[0026] [Fig.3] [Fig.3] is a side view of the gear mechanism illustrated in the [Fig.2] ;

[0027] [Fig.4] [Fig.4] is a plan view of a cooling system for the core of a fuel cell of the propulsion unit illustrated in [Fig.l];

[0028] [Fig.5] [Fig.5] is a side view of a heat exchanger of the cooling system;

[0029] [Fig.6] [Fig.6] is a detail view of the heat exchanger illustrated in [Fig.5]

[0030] [Fig.7] [Fig.7] is a partial perspective view of a variable section nozzle;

[0031] [Fig.8] [Fig.8] is a perspective view of the variable section nozzle;

[0032] [Fig.9] [Fig.9] is a detail view illustrating the actuation of an adjustable ring of the variable section nozzle;

[0033] [Fig. 10] [Fig. 10] is a front view of the nozzle with a maximum opening section;

[0034] [Fig. 11] [Fig. 11] is a front view of the nozzle with a minimum opening section;

[0035] [Fig. 12] [Fig. 12] is an axial sectional view of a device for closing an entrance to a vein;

[0036] [Fig. 13] [Fig. 13] is an axial sectional view of the propulsion unit according to a first variant embodiment;

[0037] [Fig. 14] [Fig. 14] is an axial sectional view of the propulsion unit according to a second variant embodiment. Detailed description of the invention

[0038] In [Fig.l] a propulsion unit 1 of an aircraft 2 is schematically represented. The aircraft 2 may be, for example, an airplane or a drone.

[0039] The propulsion unit 1 extends around an axis X. The propulsion unit 1 comprises a propeller 3 which is rotatable around the axis X and driven in rotation by an electric machine 4. The electric machine 4 is powered by a fuel cell 5 which comprises a core 6 generating electrical energy. The core 6 of the fuel cell 5 is cooled by a cooling system 7 which comprises a first heat exchanger 8 placed in a first streamlined vein 9 in which air flows from an inlet 10 of the first vein 9 and to an outlet 11 of the first vein 9.

[0040] According to the invention, the cooling system 7 comprises a second heat exchanger 8 placed in a second streamlined vein 9 in which air flows from an inlet 10 of the second vein 9 and to an outlet 11 of the second vein 9. The heat exchangers 8 and the veins 9 are annular around the axis X. The veins 9 surround a central compartment 12 supplied with air in which are placed the electric machine 4 and the core 6 of the fuel cell 5. The inlets 10 of the veins 9 preferably each have a so-called inlet section which is variable.

[0041] By convention in the present application, the terms “upstream” and “downstream” are defined in relation to the direction of circulation of the air around the fairings 38, 39 of the nacelle 40, in the veins 9, in the passage 59 and in the compartment 12, when the propulsion unit 1 operates in “propulsor” mode.

[0042] Furthermore, by convention in the present application, “axial” or “axially” means any direction parallel to the X axis of the propulsion group 1, and “radial” or “radially” means any direction perpendicular to the X axis of the propulsion group 1.

[0043] Finally, in the present application, the elements common to the different embodiments bear identical numerical references.

[0044] As illustrated in the figures and in particular [Fig. 1], the propeller 3 is here unducted and rotatable about the axis X in a plane of rotation P which is perpendicular to the axis X. The propeller 3 is here driven in rotation directly by the electric machine 4. Alternatively, the propeller 3 could be driven indirectly by the electric machine 4, for example via a gear mechanism. The propeller 3 supplies air to the veins 9 but also to the central compartment 12 of the propulsion unit 1.

[0045] As illustrated in the figures, the central compartment 12 of the propulsion unit 1 is centered on the axis X and houses, from upstream to downstream, an axial compressor 13, a cooler 14, the electric machine 4, a power electronics box 15, the core 6 of the fuel cell 5 and an accessories box 16.

[0046] Advantageously, as illustrated in the figures, the central compartment 12 is defined externally by a casing 17.

[0047] Such a casing 17 can be made of composite material, so as to minimize the overall mass of the propulsion unit 1.

[0048] The central compartment 12 may comprise a converging portion upstream of its outlet, to generate additional thrust.

[0049] Advantageously, as illustrated in the figures, the propulsion unit 1 comprises an axial compressor 13 placed in the compartment 12 upstream of the electric machine 4 and the core 6 of the fuel cell 5. The axial compressor 13 is supplied with air and comprises a rotor 18 driven in rotation by the electric machine 4.

[0050] Such an axial compressor 13 compresses the air entering the compartment 12, in order not only to supply the core 6 of the fuel cell 5 according to the desired conditions (for example air supply between 1.5 and 2.5 bars) but also to cool on the surface the elements which are located in the compartment 12 downstream of the axial compressor 13.

[0051] As illustrated in the figures, the axial compressor 13 is located at the upstream end of the central compartment 12. The compressor 13 is supplied with air via an inlet axial which is formed at the upstream end of the compartment 12. The axial compressor 13 comprises a rotor 18 driven in rotation around the axis X by the electric machine 4 and a stator 19 which is integral with the casing 17.

[0052] Advantageously, the axial compressor 13 comprises at least one compression stage comprising a bladed moving wheel 21 which forms part of the rotor 18 and a bladed fixed rectifier 22 which forms part of the stator 19.

[0053] As illustrated in the figures and in particular [Fig.2], the axial compressor 13 here comprises two compression stages arranged axially one after the other.

[0054] Advantageously, the rectifier 22 of each stage comprises an annular row of variable-pitch vanes. Such variable-pitch vanes are better known by the English acronym “VSV” for “Variable Stator Vanes”. Such variable-pitch vanes make it possible to adapt the flow rate and the compression ratio of the axial compressor 13, depending on the power supply requirements of the core 6 of the fuel cell 5.

[0055] Advantageously, the timing of the blades of a rectifier 22 is adjusted in a synchronized manner via an adjustment device.

[0056] The adjustment device comprises, for example, a rotating movable ring which is common to all of the blades and a connecting rod which is specific to each of the blades. The ring is driven in rotation by one or more actuators. Each connecting rod comprises one end which is linked in rotation with the corresponding blade and one end which is articulated with the ring. The rotational driving of the ring by the actuator(s) makes it possible to adjust the setting of all of the blades of the rectifier 22 in a synchronized manner.

[0057] Advantageously, as illustrated in Figures 2 and 3, the rotor 18 of the axial compressor 13 is driven in rotation by the electric machine 4 via a gear mechanism 23. The gear mechanism 23 has a transmission ratio greater than 1, so that the rotational speed of the rotor 18 of the compressor 13 is greater than the rotational speed of the rotor 20 of the electric machine 4. Such a gear mechanism 23 is also called a “multiplier”.

[0058] The gear mechanism 23 can be arranged axially between the axial compressor 13 and the electrical machine 4, or even upstream of the axial compressor 13.

[0059] As illustrated in Figures 2 and 3, the gear mechanism 23 comprises an epicyclic train comprising a crown 24 secured to the rotor 20 of the electrical machine 4, an annular row of external satellites 25, an annular row of internal satellites 26 and a sun gear 27 secured to the rotor 18 of the axial compressor 13. The external satellites 25 are meshed (or engaged) with both the crown 24 and the internal satellites 26. The internal satellites 26 are meshed with both the external satellites 25 and the sun gear 27.

[0060] Advantageously, as illustrated in the figures, the propulsion unit 1 comprises a cooler 14 placed in the compartment 12 between the compressor 13 on the one hand and the electric machine 4 and the core 6 of the fuel cell 5 on the other hand, the cooler 14 cooling the air which leaves the compressor 13.

[0061] Such a cooler 14 is better known by the English designation “intercooler”. Such a cooler 14 cools the air leaving the compressor 13, in order not only to supply the core 6 of the fuel cell 5 according to the desired conditions, but also to effectively cool the surface of the elements which are located in the compartment 12 downstream of the cooler 14.

[0062] Advantageously, such a cooler 14 is an air / air heat exchanger.

[0063] As illustrated in the figures, the cooler 14 is placed in the compartment 12 downstream of the axial compressor 13 and upstream of the electrical machine 4.

[0064] The electric machine 4 is electrically powered directly or indirectly (for example via batteries) by the fuel cell 5.

[0065] As illustrated in the figures, the electrical machine 4 is placed in the compartment 12 downstream of the cooler 14 and upstream of the core 6 of the fuel cell 5. The electrical machine 4 comprises a rotor 20 which directly drives the propeller 3 and indirectly drives the rotor 18 of the axial compressor 13 via the gear mechanism 23. The outer skin of the stator of the electrical machine 4 is cooled on the surface by the air circulating in the compartment 12. The outer skin of the stator of the electrical machine 4 may comprise fins, in order to increase the exchange surface with the air.

[0066] The electrical machine 4 can be reversible, and thus be capable of operating in “motor” mode to produce mechanical energy (in particular to drive the propeller 3 and the rotor 18 of the axial compressor 13) and in “generator” mode to produce electrical energy.

[0067] The power electronics housing 15 is arranged axially between the electrical machine 4 and the core 6 of the fuel cell 5, so as to reduce the length of the cables connecting the electrical machine 4 and the core 6 of the fuel cell 5. The power electronics housing 15 may comprise various converters, batteries, etc.

[0068] The fuel cell 5 is better known by the acronym “PAC” or the English term “fuel cell”. The fuel cell 5 produces the electrical energy necessary in particular for the electrical power supply of the electric machine 4.

[0069] Advantageously, the fuel cell 5 is a hydrogen / oxygen cell, and in other words a cell whose fuel is hydrogen and the oxidant is oxygen.

[0070] The fuel cell 5 is for example a fuel cell known by the English acronym “PEMFC” for “Proton Exchange Membrane Fuel Cell”.

[0071] The core 6 of the fuel cell 5 is commonly called a “stack” in English.

[0072] Advantageously, the core 6 of the fuel cell 5 is formed of electrochemical cells.

[0073] The core 6 of the fuel cell 5 may comprise one or more stacks 28 of electrochemical cells.

[0074] The core 6 of the fuel cell 5 has a maximum admissible temperature which is predefined (for example 70°C). This maximum admissible temperature must be respected to allow its proper operation and maximize its lifespan.

[0075] Batteries may be used to store the electrical energy produced by the fuel cell 5.

[0076] As illustrated in the figures, the fuel cell 5 is here a hydrogen / oxygen cell. The core 6 of the fuel cell 5 is placed in the compartment 12 downstream of the electrical machine 4. The core 6 of the fuel cell 5 comprises several stacks 28 of electrochemical cells which are arranged axially one after the other. The core 6 is supplied with hydrogen by a hydrogen supply device which is connected to one or more hydrogen tanks. The core 6 is supplied with oxygen by an oxygen supply device which takes air from the compartment 12. The outer skin of the core 6 of the fuel cell 5 is surface cooled by the air circulating in the compartment 12. The outer skin of the core 6 of the fuel cell 5 may comprise fins, in order to increase the exchange surface with the air.The core 6 of the fuel cell 5 may comprise internal galleries (or passages) which allow thermal energy to be evacuated into the air of the compartment 12.

[0077] As illustrated in the figures, the accessory box 16 is arranged axially downstream of the core 6 of the fuel cell 5.

[0078] The accessory box 16 may include various pumps (heat transfer fluid pump 30, oil pump, hydrogen pump, water pump, etc.), a humidifier, an oil separator, etc.

[0079] Alternatively, the accessory box 16 could be arranged axially between the electric machine 4 and the core 6 of the fuel cell 5, so as to reduce the axial dimension of the transmission shaft which connects in rotation the electric machine 4 and the various accessories of the accessory box 16.

[0080] As indicated above, according to the invention, the heat exchangers 8 are annular around the axis X.

[0081] Each heat exchanger 8 makes it possible to evacuate the thermal energy from the core 6 of the fuel cell 5 into the air which passes through the corresponding vein 9.

[0082] Generally, the cooling system 7 can comprise between 2 and 6 heat exchangers 8, depending in particular on the cooling requirements of the core 6 of the fuel cell 5 and the space available for installing them.

[0083] As illustrated in [Fig.l], the cooling system 7 comprises two heat exchangers 8 each placed in a streamlined vein 9.

[0084] Alternatively, as illustrated in Figures 13 and 14, the cooling system 7 comprises three heat exchangers 8 each placed in a streamlined vein 9.

[0085] Advantageously, as illustrated in [Fig.4], the heat exchangers 8 are part of a cooling circuit 29 in which a heat transfer fluid circulates by means of a pump 30 (for example a volumetric or centrifugal pump).

[0086] The heat transfer fluid may be, for example, glycol or a glycol / water mixture.

[0087] Advantageously, as illustrated in [Fig. 4], the different stacks 28 of the core 6 of the fuel cell 5 are connected to the cooling circuit 29 in parallel with each other, to limit the flow rate of heat transfer fluid flowing in each of the stacks 28, and thus reduce the pressure losses and consequently the consumption of the heat transfer fluid pump 30.

[0088] Advantageously, each heat exchanger 8 comprises two independent paths, namely a first path in which the air from the corresponding vein 9 circulates and a second path in which the heat transfer fluid circulates.

[0089] The paths of a heat exchanger 8 may each comprise fins, in order to increase the exchange surface, for the benefit of the thermal efficiency of the heat exchanger 8 and the reduction of pressure losses.

[0090] Advantageously, the axial dimension of a heat exchanger 8 is less than or equal to 100 mm. It is noted that beyond the aforementioned axial dimension, the thermal efficiency of the heat exchanger 8 no longer increases.

[0091] Advantageously, a heat exchanger 8 comprises at least one tube 31 wound in the form of a spiral. Such a heat exchanger 8 is for example obtained by bending.

[0092] Advantageously, a heat exchanger 8 comprises fins 32 between the different turns of each tube 31, in order to increase the exchange surface.

[0093] As illustrated in the figures and in particular figures 5 and 6, the heat exchanger 8 comprises six tubes 31 wound in a spiral, the tubes 31 being arranged axially one after the other and fixed to each other (for example by welding or brazing). Each tube 31 here comprises six turns spaced radially from each other. The turns of a tube 31 are held in position relative to each other via four supports 33 (or reinforcements) distributed regularly around the axis X. Each tube 31 comprises fins 32 arranged in the interturn spaces, in order to increase the exchange surface.

[0094] Hollow and profiled (or tapered) arms can be introduced into the veins 9 for the passage of the heat transfer fluid pipes.

[0095] As indicated above, according to the invention, the veins 9 in which the heat exchangers 8 are placed are also annular around the axis X. The veins 9 surround the central compartment 12, and preferably each have an inlet section which is variable.

[0096] The inlets 10 of the veins 9 can also each have an inlet section which is fixed (or not variable), depending on the needs.

[0097] As illustrated in [Fig.l], the cooling system 7 comprises two separate and coaxial veins 9 around the axis X, each vein 9 housing a heat exchanger 8.

[0098] Alternatively, as illustrated in Figures 13 and 14, the cooling system 7 comprises three distinct and coaxial veins 9 around the axis X, each vein 9 housing a heat exchanger 8.

[0099] Advantageously, as illustrated in [Fig.l], the inlets 10 of the veins 9 are arranged downstream of the helix 3. The axial dimension Da between the plane of rotation P of the helix 3 and the inlet 10 of each vein 9 is between 0.1 x R and 0.6 x R, where R is the radius of the helix 3.

[0100] Such axial positioning of the inlet 10 of each vein 9 makes it possible to capture air having an optimal speed (namely between 50 and 1 10 m / s), so as to efficiently evacuate the thermal energy of the heat transfer fluid into the air of the vein 9.

[0101] Advantageously, as illustrated in [Fig.l], the inlet 10 of each vein 9 has a reference point Pr whose radial dimension Dr with respect to the axis X is between 0.4 x R and 0.95 x R, where R is the radius of the propeller 3. The reference point Pr is defined in an axial half-section of the propulsion group 1 which passes through the axis X.

[0102] The reference point Pr is here the radially median point of the inlet 10, when the inlet 10 has an inlet section which is minimal.

[0103] Such a radial positioning of the inlet 10 of each vein 9 makes it possible to capture air having an optimal speed (namely between 50 and 1 10 m / s), so as to efficiently evacuate the thermal energy of the heat transfer fluid into the air of the vein 9.

[0104] Alternatively, the reference point Pr could be the radially median point of the inlet 10, when the inlet 10 has an inlet section that is maximum. Or, the reference point Pr could be the radially inner point (or the radially outer point) of the inlet 10, when the inlet 10 has an inlet section that is minimum or maximum.

[0105] Advantageously, as illustrated in the figures, each vein 9 comprises a divergent portion 34 located directly upstream of the corresponding heat exchanger 8, so as to slow down the speed of the air entering the heat exchanger. heat 8, and a converging portion 35 located directly upstream of the outlet 11 of the vein 9, so as to accelerate the air escaping from the vein 9 and generate thrust.

[0106] The diverging portion 34 of the vein 9 forms a diffuser. The diverging portion 34 has a flow section which increases from upstream to downstream. The reduction in the air speed makes it possible to reduce the pressure losses linked to the passage of air in the heat exchanger 8.

[0107] The converging portion 35 of the vein 9 forms a converging nozzle. The converging portion 35 has a flow section which decreases from upstream to downstream. Such a converging nozzle makes it possible to accelerate the air escaping from the vein 9 and to generate thrust, the thrust at least partly compensating for the drag generated by the passage of the air in the heat exchanger 8.

[0108] Advantageously, as illustrated in the figures, the heat exchangers 8 are axially offset relative to each other.

[0109] Such an axial offset makes it possible to locate the diverging and converging portions 34, 35 of the veins 9, while minimizing the radial size of the propulsion group 1.

[0110] Advantageously, as illustrated in the figures, each vein 9 comprises an upstream part located upstream of the corresponding heat exchanger 8 and a downstream part located downstream of the corresponding heat exchanger 8.

[0111] Advantageously, as illustrated in the figures, each vein 9 is delimited radially by an internal wall 36 (or an internal conduit) and an external wall 37 (or an external conduit).

[0112] When the inlet 10 of each vein 9 has a variable inlet section, the inlet section varies between a minimum section and a maximum section.

[0113] To adjust the inlet section of a vein 9, the inlet 10 of the vein 9 can be formed by one or two nozzles 41 with variable section, each nozzle 41 forming at least in part the internal wall 36 or the external wall 37 of the vein 9.

[0114] Advantageously, as illustrated in Figures 7 to 11, a nozzle 41 with variable section comprises a frame 43 (or armature) comprising a base 44 from which extends an annular row of independent strips 45. The free ends of the strips 45 are connected in common to a ring 46 whose diameter is adjustable. The frame 43 is covered at least partially with an envelope 47 (or membrane), so as to seal the nozzle 4L

[0115] The frame 43 of the nozzle 41 can be made of composite material, so as to minimize the mass of the nozzle.

[0116] In the same way, the casing 47 can be made of polymer material, so as to minimize the mass of the nozzle.

[0117] Advantageously, as illustrated in Figures 7 to 11, the diameter of the ring 46 is adjustable via a mechanism 48 driven by an actuator 49 (for example a stepper type electric motor).

[0118] As illustrated in [Fig.9], the ring 46 is split and thus comprises two free ends 50 facing each other. The mechanism 48 comprises a driving screw 51 which is driven in rotation by the actuator 49, and two receiving nuts 52 engaged with the screw 51, each of the nuts 52 being fixed to a free end 50 of the ring 46. The rotational driving of the screw 51 by the actuator 49 causes the translation of each of the nuts 52 relative to each other, so as to increase or decrease the diameter of the ring 46 depending on the direction of rotation.

[0119] A nozzle 41 with variable section may comprise a defrosting device. The defrosting device comprises, for example, heating resistors distributed regularly around the periphery of the frame 43.

[0120] The free ends of the strips 45 can be connected to the ring 46 via elastic elements (return springs, polymer blocks) or tie rods.

[0121] The free ends of the bands 45 may each be in the form of a loop, the ring 46 passing through each of the loops.

[0122] [Fig. 10] is a front view of the nozzle 41 with a maximum opening section, the ring 46 having a maximum diameter. [Fig.l 1] is a front view of the nozzle 41 with a minimum opening section, the ring 46 having a minimum diameter.

[0123] Adjusting the diameter of the ring 46 thus makes it possible to adjust the opening section of the nozzle 4L.

[0124] The ring 46 of a nozzle 41 can form the upstream end of the internal wall 36 or of the external wall 37 of a vein 9, thus the adjustment of the diameter of the ring 46 makes it possible to adjust the inlet section of the vein 9.

[0125] Alternatively, to adjust the inlet section of a vein 9, the inlet 10 of the vein 9 may comprise a closing device 53 comprising at least one adjustable flap 55.

[0126] A shutter 55 adjustment can be movable in translation (sliding shutter) or movable in rotation (pivoting shutter).

[0127] As illustrated in [Fig.12], the inlet 10 of the vein 9 comprises a device shutter 53 comprising two annular rows of shutters 55 movable in rotation.

[0128] The inlet section of the vein 9 is minimal when the shutters 55 of the closure device 53 are closed. And conversely, the inlet section of the vein 9 is maximal ([Fig. 12]) when the shutters 55 of the closure device 53 are open.

[0129] The outlets 11 of the veins 9 may also each have an outlet section which is variable. Such an outlet with variable section not only makes it possible to generate thrust by accelerating the air escaping from vein 9, but also to control the intensity of this thrust.

[0130] To adjust the outlet section of a vein 9, the outlet 11 of the vein 9 could be formed by one or two nozzles 41 with variable section. Alternatively, the outlet 11 of the vein 9 could comprise a closing device 53 comprising at least one adjustable flap 55.

[0131] As illustrated in the figures, the propulsion group 1 comprises an annular air passage 59 around the axis X, this air passage 59 being arranged radially between the veins 9 (and more precisely the external vein 9) and a nacelle 40 of the propulsion group 1.

[0132] The air passage 59 is delimited radially by the external wall 37 of the external vein 9 and an internal fairing 38 of the nacelle 40.

[0133] Advantageously, as illustrated in the figures, the cooling system 7 comprises a surface heat exchanger 56 which is flush with the internal fairing 38 of the nacelle 40 of the propulsion unit 1.

[0134] Such a surface heat exchanger 56 provides additional cooling capacity for the core 6 of the fuel cell 5 while having a low impact on drag.

[0135] The cooling system 7 can obviously comprise one or more surface heat exchangers 56.

[0136] Advantageously, the surface heat exchanger(s) 56 are integrated or form part of the nacelle 40 of the propulsion unit 1, and in other words the surface heat exchanger(s) 56 are carried by the nacelle 40.

[0137] Advantageously, the surface heat exchanger(s) 56 form part of the cooling circuit 29 which comprises the main heat exchangers 8.

[0138] As illustrated in [Fig.l], the cooling system 7 comprises a single surface heat exchanger 56 which is annular about the X axis. The surface heat exchanger 56 is integrated into the nacelle 40 and is flush with the internal fairing 38 of the nacelle 40.

[0139] As illustrated in [Fig. 13], the cooling system 7 comprises three surface heat exchangers 56 arranged axially one after the other. The surface heat exchangers 56 are annular around the axis X. The surface heat exchangers 56 are integrated into the nacelle 40 and are flush with the internal fairing 38 of the nacelle 40. Unlike the cooling system 7 illustrated in [Fig. 13], the cooling system 7 illustrated in [Fig. 14] comprises only two surface heat exchangers 56, one of which has been replaced by a water tank 60, as described in the remainder of the description.

[0140] As illustrated in the figures, the nacelle 40 of the propulsion group 1 surrounds the veins 9, and is delimited radially by the internal fairing 38 and an external fairing 39 facing each other.

[0141] Advantageously, the cooling system 7 comprises at least one water injector 57 which is placed in one of the veins 9 upstream of the corresponding heat exchanger 8.

[0142] The air / water mixture provides a cooling capacity that is superior compared to air alone. The water will vaporize upon contact with the heat exchanger 8, and thus absorb a significant amount of heat from the heat transfer fluid, so as to significantly reduce the temperature of the heat transfer fluid at the outlet of the heat exchanger 8. The water vapor created is evacuated through the outlet 11 of the vein 9.

[0143] The cooling system 7 can obviously comprise one or more water injectors 57 in one or all of the veins 9.

[0144] Advantageously, the injector(s) 57 have a variable flow rate, so as to be able to adapt the flow rate of injected water according to the cooling requirements.

[0145] Advantageously, the injector(s) 57 are part of a water circuit which comprises a pump (for example a volumetric or centrifugal pump) and one or more water tanks 60.

[0146] Advantageously, the temperature of the injected water is low and lower than a predetermined value (for example 30°C). Low temperature water makes it possible to benefit from the high latent heat of vaporization of the water but also from the high heat capacity of the water.

[0147] Advantageously, the cooling system 7 comprises at least one radial or annular row 58 of water injectors 57 which is placed in one of the veins 9 upstream of the corresponding heat exchanger 8.

[0148] The water injected by the injectors 57 can be produced in part or in whole by the fuel cell 5, and resulting from the reaction between hydrogen and oxygen.

[0149] The water may be stored in one or more tanks 60. The tank(s) 60 may be filled periodically, for example before each takeoff. The tank(s) 60 are for example integrated into the nacelle 40. Each tank 60 may be associated with a water temperature sensor and a water level sensor. Each tank 60 may each comprise an isothermal wall, and in other words a thermally insulated wall. The tank(s) 60 may be cooled by a cooling device, the cooling device taking air for example from the air passage 59. Each tank 60 may comprise a filling orifice positioned at 12 o'clock by analogy with the dial of a clock, so as to facilitate its filling.

[0150] The water injector(s) 57 may be inclined relative to the X axis along the direction of air flow in the vein 9, so as to limit pressure losses (in particular aerodynamic disturbances) and optimize the homogeneity of the air / water mixture.

[0151] As illustrated in [Fig. 14], for each vein 9, the cooling system 7 here comprises several radial rows 58 of water injectors 57, the water injectors 57 being placed upstream of the corresponding heat exchanger 8. In each of the veins 9, the radial rows 58 of water injectors 57 are distributed regularly around the axis X. The cooling system 7 comprises a water tank 60 which is annular around the axis X. The water tank 60 is integrated into the nacelle 40 and is flush with the internal fairing 38 of the nacelle 40. The water tank 60 is arranged axially between the two surface heat exchangers 56.

[0152] Hollow and profiled (or tapered) arms can be introduced into the veins 9 for the passage of the water supply pipes of the injector(s) 57.

[0153] By way of example, the cooling system 7 of [Fig. 1] is intended for a propulsion unit 1 whose thrust-to-weight ratio varies between 0.2 and 0.25. And the cooling systems 7 of FIGS. 13 and 14 are intended for a propulsion unit 1 whose thrust-to-weight ratio is greater than 0.25.

[0154] The general characteristics described above (concerning for example the heat exchangers 8, the veins 9, etc.) can be applied to the embodiment illustrated in [Fig.l] as well as to the variant embodiments illustrated in Figures 13 and 14.

[0155] According to the invention, the cooling system 7 is configured to be adjusted by an adjustment method.

[0156] The adjustment method comprises the step of: a) adjusting the inlet section of the veins 9 from a measurement of the temperature of the core 6 of the fuel cell 5, so as to adjust the air flow rates entering the veins 9 according to the cooling requirements.

[0157] Such an adjustment method takes into account the temperature of the core 6 of the fuel cell 5 so as to quickly and precisely adapt the air flow rates entering the veins 9 according to the cooling requirements of the core 6, and thus minimize the aerodynamic drag produced by the passage of air in the different veins 9, during the different operating regimes of the propulsion unit 1, for the benefit of the overall performance of the propulsion unit 1 and of the aircraft 2 (consumption, flight autonomy, etc.).

[0158] The inlet sections of the veins 9 during step a) can be adjusted from additional parameters such as the temperature of the air upstream of the heat exchangers 8, the pressure of the air upstream of the heat exchangers 8, the speed flight speed of aircraft 2, flight altitude of aircraft 2, operating regime of propulsion unit 1 (takeoff regime, climb regime, cruise regime, descent regime and landing regime).

[0159] Advantageously, the measurement of the temperature of the core 6 of the fuel cell 5 which is used during step a) can be obtained by means of a temperature sensor which is specific to the core 6 of the fuel cell 5.

[0160] As an example, the minimum section of the inlet 10 of a vein 9 is equal to 0.05 x Sech, where Sech is the frontal section of the corresponding heat exchanger 8. And the maximum section of the inlet 10 of a vein 9 is equal to 0.8 x Sech-

[0161] For a fuel cell 5 whose core 6 has a maximum admissible temperature of 70°C, the inlet section of each vein 9 is approximately equal to 0.5 x Sech when the temperature of the core 6 is equal to 60°C, and the inlet section of each vein 9 is approximately equal to 0.8 x Sech when the temperature of the core 6 is equal to 68°C.

[0162] When the cooling system 7 comprises at least one water injector 57, the adjustment method comprises a step consisting of: b) adjusting the flow rate of water injected by the injector 57 into the vein 9 based on a measurement of the temperature of the core 6 of the fuel cell 5.

[0163] The flow rate of water injected during step b) can be adjusted from additional parameters such as the temperature of the air upstream of the heat exchangers 8, the pressure of the air upstream of the heat exchangers 8, the flight speed of the aircraft 2, the flight altitude of the aircraft 2, the operating regime of the propulsion unit 1 (takeoff regime, climb regime, cruise regime, descent regime and landing regime).

[0164] By way of example, for a fuel cell 5 whose core 6 has a maximum admissible temperature of 70°C, the injector(s) 57 are adjusted as follows: - zero flow rate when the temperature of the core 6 of the fuel cell 5 is below a first predetermined threshold value (for example 60°C); - flow controlled when the temperature of the core 6 of the fuel cell 5 is between the first threshold value and a second predetermined threshold value (for example 68°C); - maximum flow rate when the temperature of core 6 is beyond the second threshold value.

Claims

Claims

1. Propulsion unit (1) of an aircraft (2) which extends around an axis (X), the propulsion unit (1) comprising a propeller (3) which is rotatable around the axis (X) and driven in rotation by an electric machine (4), the electric machine (4) being powered by a fuel cell (5) which comprises a core (6) generating electrical energy, the core (6) of the fuel cell (5) being cooled by a cooling system (7) which comprises a first heat exchanger (8) placed in a first streamlined vein (9) in which air flows from an inlet (10) of the first vein (9) and to an outlet (11) of the first vein (9), characterized in that the cooling system (7) comprises a second heat exchanger (8) placed in a second streamlined vein (9) in which air flows from an inlet (10) of the second vein (9) and up to an exit (11) of the second vein (9),the heat exchangers (8) and the veins (9) being annular around the axis (X), the veins (9) surrounding a central compartment (12) supplied with air in which the electrical machine (4) and the core (6) of the fuel cell (5) are placed, the inlets (10) of the veins (9) each preferably having a so-called inlet section which is variable.,

2. Propulsion unit (1) according to claim 1, characterized in that the propeller (3) is rotatable in a so-called rotation plane (P) which is perpendicular to the axis (X), the inlets (10) of the veins (9) being arranged downstream of the propeller (3), the axial dimension (Da) between the rotation plane (P) of the propeller (3) and the inlet (10) of each vein (9) being between 0.1 x R and 0.6 x R, where R is the radius of the propeller (3).

3. Propulsion unit (1) according to one of the preceding claims, characterized in that the inlet (10) of each vein (9) has a reference point (Pr) whose radial dimension (Dr) with respect to the axis (X) is between 0.4 x R and 0.95 x R, where R is the radius of the propeller (3), the reference point (Pr) being defined in an axial half-section of the propulsion unit (1) which passes through the axis (X).

4. Propulsion unit (1) according to one of the preceding claims, characterized in that each vein (9) comprises a divergent portion (34) located directly upstream of the heat exchanger heat (8) corresponding, so as to slow down the speed of the air entering the heat exchanger (8), and a converging portion (35) located directly upstream of the outlet (11) of the vein (9), so as to accelerate the air escaping from the vein (9) and generate thrust.

5. Propulsion unit (1) according to one of the preceding claims, characterized in that at least one of the inlets (10) of the veins (9) is formed by a nozzle (41) with variable section, the nozzle (41) comprising a frame (43) having a base (44) from which extends an annular row of independent bands (45), the free ends of the bands (45) being connected in common to a ring (46) whose diameter is adjustable, the frame (43) being covered at least partially with an envelope (47), so as to seal the nozzle (41).

6. Propulsion unit (1) according to one of the preceding claims, characterized in that at least one of the inlets (10) of the veins (9) comprises a closing device (53) comprising at least one adjustable flap (55).

7. Propulsion unit (1) according to one of the preceding claims, characterized in that the cooling system (7) comprises a surface heat exchanger (56) which is flush with an internal fairing (38) of a nacelle (40) of the propulsion unit (1).

8. Propulsion unit (1) according to one of the preceding claims, characterized in that the cooling system (7) comprises at least one water injector (57) which is placed in one of the veins (9) upstream of the corresponding heat exchanger (8).

9. Propulsion unit (1) according to one of the preceding claims, characterized in that the propulsion unit (1) comprises an axial compressor (13) placed in the compartment (12) upstream of the electric machine (4) and the core (6) of the fuel cell (5), the axial compressor (13) being supplied with air and comprising a rotor (18) driven in rotation by the electric machine (4).

10. Propulsion unit (1) according to the preceding claim, characterized in that the propulsion unit (1) comprises a cooler (14) placed in the compartment (12) between the axial compressor (13) on the one hand and the electric machine (4) and the core (6) of the fuel cell (5) on the other hand, the cooler (14) cooling the air which leaves the axial compressor (13).

11. Propulsion unit (1) according to one of the preceding claims, characterized in that the heat exchangers (8) are axially offset relative to each other.

12. Propulsion unit (1) according to one of the preceding claims, characterized in that the cooling system (7) comprises a third heat exchanger (8) placed in a third streamlined vein (9) in which air flows from an inlet (10) of the third vein (9) and to an outlet (11) of the third vein (9), the third heat exchanger (8) and the third vein (9) being annular around the axis (X), the third vein (9) surrounding the central compartment (12), the inlet (10) of the third vein (9) preferably having an inlet section which is variable.

13. Method for adjusting the cooling system (7) of a propulsion unit (1) according to one of the preceding claims, the method comprising the step of: a) adjusting the inlet section of the veins (9) from a measurement of the temperature of the core (6) of the fuel cell (5), so as to adjust the air flow rates entering the veins (9) according to the cooling requirements.

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