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) generating electrical energy, the core (6) of the fuel cell (5) being cooled by a cooling system (7) which comprises a heat exchanger (8) placed in a faired vein (9) in which air flows from an inlet (10) to an outlet (11) of the vein (9), the heat exchanger (8) and the vein (9) being annular, the vein (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 inlet (10) of the vein (9) preferably having an inlet section which is variable and the outlet (11) of the vein (9) preferably having an outlet 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 heat exchanger placed in a streamlined vein in which air flows from an inlet of the vein and to an outlet of the vein, characterized in that the heat exchanger and the vein are annular around the axis X, the vein surrounding a central compartment supplied with air in which the electric machine and the core of the fuel cell are placed,the inlet of the vein preferably having a so-called inlet section which is variable and the outlet of the vein preferably having a so-called outlet section which is variable.

[0017] Having an annular heat exchanger and vein makes it possible to install a large heat exchanger (and therefore to have a large exchange surface) while limiting the negative impacts (drag, mass, etc.) which are linked to its installation.

[0018] Such an annular vein also makes it possible to supply the heat exchanger in a homogeneous manner, the entire vein being in the wake of the propeller, for the benefit the thermal efficiency of the heat exchanger and more generally the overall efficiency of the propulsion unit.

[0019] The variable section inlet and outlet make it possible to precisely adapt the air flow rates entering and leaving the vein according to the cooling requirements of the core, and thus minimize the aerodynamic drag produced by the passage of air in the vein, during the different operating regimes of the propulsion group, to the benefit of its overall efficiency.

[0020] The variable section outlet is used to form a converging nozzle intended to accelerate the air escaping from the vein and generate thrust, and thus partially compensate for the drag generated by the passage of air in the vein.

[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 inlet of the vein being arranged downstream of the propeller, the axial dimension Da between the rotation plane P of the propeller and the inlet of the vein being between 0.1 x R and 0.6 x R, where R is the radius of the propeller; - the inlet of the vein has a reference point Pr whose radial dimension Dr with respect to the X axis is between 0.6 x R and 0.9 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; - the vein comprises a diverging portion located directly upstream of the 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; - the inlet of the vein and / or the outlet of the vein are each 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; - the vein inlet and / or the vein outlet each comprise a closure 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 the vein upstream of the 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 includes a cooler placed in the central compartment between the axial compressor on the one hand and the electric machine and the heart of the fuel cell on the other hand, the cooler cooling the air leaving the axial compressor.

[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 and the outlet section of the vein from a measurement of the temperature of the fuel cell core, so as to adjust the air flows entering and leaving the vein 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 view similar to [Fig.l] in which the inlet and outlet sections of the vein are minimal;

[0026] [Fig.3] [Fig.3] is a view similar to figures 1 and 2 in which the inlet and outlet sections of the vein are maximum;

[0027] [Fig.4] [Fig.4] is a detail view showing an axial compressor of the propulsion unit and its drive via a gear mechanism;

[0028] [Fig.5] [Fig.5] is a side view of the gear mechanism illustrated in [Fig.4];

[0029] [Fig.6] [Fig.6] is a plan view of a cooling system for the core of a fuel cell of the propulsion unit;

[0030] [Fig.7] [Fig.7] is a side view of the heat exchanger of the cooling system;

[0031] [Fig.8] [Fig.8] is a detail view of the heat exchanger illustrated in [Fig.7]

[0032] [Fig.9] [Fig.9] is a partial perspective view of a variable section nozzle;

[0033] [Fig. 10] [Fig. 10] is a perspective view of the variable section nozzle;

[0034] [Fig. 11] [Fig. 11] is a detail view illustrating the actuation of an adjustable ring of the variable section nozzle;

[0035] [Fig. 12] [Fig. 12] is a front view of the nozzle with a maximum opening section;

[0036] [Fig. 13] [Fig. 13] is a front view of the nozzle with a minimum opening section;

[0037] [Fig. 14] [Fig. 14] is an axial sectional view of the propulsion unit according to a first variant embodiment;

[0038] [Fig. 15] [Fig. 15] is an axial sectional view of the propulsion unit according to a second variant embodiment, the inlet and outlet sections of the vein being minimal;

[0039] [Fig. 16] [Fig. 16] is a view similar to [Fig. 15] in which the inlet and outlet sections of the vein are maximum. Detailed description of the invention

[0040] Figures 1 to 3 schematically show a propulsion unit 1 of an aircraft 2. The aircraft 2 may be, for example, an airplane or a drone.

[0041] 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 heat exchanger 8 placed in a ducted vein 9 in which air flows from an inlet 10 of the vein 9 and to an outlet 11 of the vein 9.

[0042] According to the invention, the heat exchanger 8 and the vein 9 are annular around the axis X. The vein 9 surrounds 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 inlet 10 of the vein 9 preferably has a so-called inlet section which is variable and the outlet 11 of the vein 9 preferably has a so-called outlet section which is variable.

[0043] 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 vein 9 and in the compartment 12, when the propulsion unit 1 operates in “propulsor” mode.

[0044] Furthermore, by convention in the present application, “axial” or “axially” means any direction parallel to the axis X of the propulsion group 1, and “radial” or “radially” means any direction perpendicular to the axis X of the propulsion group 1.

[0045] Finally, in the present application, the elements common to the different embodiments bear identical numerical references.

[0046] As illustrated in the figures, 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 vein 9 but also to the central compartment 12 of the propulsion unit 1.

[0047] 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.

[0048] Advantageously, as illustrated in the figures, the central compartment 12 is defined externally by a casing 17.

[0049] Such a casing 17 can be made of composite material, so as to minimize the overall mass of the propulsion unit.

[0050] The central compartment 12 may comprise a converging portion upstream of its outlet, to generate additional thrust.

[0051] 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.

[0052] Such an axial compressor compresses the air entering the compartment, in order not only to supply the core of the fuel cell 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 downstream of the axial compressor.

[0053] 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 axial inlet 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 electrical machine 4 and a stator 19 which is integral with the casing 17.

[0054] 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.

[0055] As illustrated in the figures and in particular [Fig.4], the axial compressor 13 here comprises two compression stages arranged axially one after the other.

[0056] 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 variable allow the flow rate and compression ratio of the axial compressor to be adapted, depending on the power requirements of the fuel cell core.

[0057] Advantageously, the timing of the blades of a rectifier 22 is adjusted in a synchronized manner via an adjustment device.

[0058] 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 in a synchronized manner the setting of all of the blades of the rectifier 22.

[0059] Advantageously, as illustrated in Figures 4 and 5, 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”.

[0060] Advantageously, the gear mechanism 23 is arranged axially between the axial compressor 13 and the electrical machine 4.

[0061] As illustrated in Figures 4 and 5, 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.

[0062] Advantageously, as illustrated in the figures, the propulsion unit 1 comprises a cooler 14 placed in the central 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.

[0063] Such a cooler 14 is better known by the English designation “intercooler”. Such a cooler cools the air leaving the compressor, in order not only to supply the core of the fuel cell according to the desired conditions, but also to effectively cool the surface of the elements which are located in the compartment downstream of the cooler.

[0064] Advantageously, such a cooler 14 is an air / air heat exchanger.

[0065] As illustrated in the figures, the cooler 14 is placed in the compartment 12 downstream of the axial compressor 13 and upstream of the electric machine 4 and the core 6 of the fuel cell 5.

[0066] The electric machine 4 is electrically powered directly or indirectly (for example via batteries) by the fuel cell 5.

[0067] 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.

[0068] 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.

[0069] 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.

[0070] 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.

[0071] 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.

[0072] The fuel cell 5 is for example a fuel cell known by the English acronym “PEMFC” for “Proton Exchange Membrane Fuel Cell”.

[0073] The core 6 of the fuel cell 5 is commonly called “stack” in English.

[0074] Advantageously, the core 6 of the fuel cell 5 is formed of electrochemical cells.

[0075] The core 6 of the fuel cell 5 may comprise one or more stacks 28 of electrochemical cells.

[0076] The core 6 of the fuel cell 5 has a maximum admissible temperature which is predefined. This maximum admissible temperature must be respected to allow its proper functioning and maximize its lifespan.

[0077] Batteries may be used to store the electrical energy produced by the fuel cell 5.

[0078] 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.

[0079] The accessory box 16 may include various pumps (heat transfer fluid pump, oil pump, hydrogen pump, water pump, etc.), a humidifier, an oil separator, etc.

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

[0081] The 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 vein 9.

[0082] Advantageously, as illustrated in the figures and in particular [Fig.6], the heat exchanger 8 is 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).

[0083] The heat transfer fluid may be, for example, glycol or a glycol / water mixture.

[0084] Advantageously, as illustrated in [Fig. 6], 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, and thus reduce the pressure losses and consequently the consumption of the heat transfer fluid pump.

[0085] Advantageously, the heat exchanger 8 comprises two independent paths, namely a first path in which the air from the vein 9 circulates and a second path in which the heat transfer fluid circulates.

[0086] The paths of the 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 and the reduction of pressure losses.

[0087] Advantageously, the axial dimension of the 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 no longer increases.

[0088] Advantageously, the 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.

[0089] Advantageously, the heat exchanger 8 comprises fins 32 between the different turns of each tube 31, in order to increase the exchange surface.

[0090] As illustrated in the figures and in particular figures 7 and 8, 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.

[0091] Hollow and profiled (or tapered) arms can be introduced into the vein 9 for the passage of the heat transfer fluid pipes.

[0092] As indicated above, according to the invention, the vein 9 in which the heat exchanger 8 is placed is also annular around the axis X. The vein 9 surrounds the central compartment 12, and preferably has inlet and outlet sections which are variable.

[0093] The inlet 10 and outlet 11 of the vein 9 may also have inlet and outlet sections which are fixed (or not variable), depending on requirements.

[0094] When the inlet 10 of the vein 9 has a variable inlet section, the inlet section of the vein 9 varies between a minimum section ([Fig.l] (in solid line), 2, 14 (in solid line) and 15) and a maximum section ([Fig.l] (in broken line), 3, 14 (in broken line) and 16).

[0095] In the same way, when the outlet 11 of the vein 9 has a variable outlet section, the outlet section of the vein 9 varies between a minimum section ([Fig.l] (in solid line), 2, 14 (in solid line) and 15) and a maximum section ([Fig.l] (in broken line), 3, 14 (in broken line) and 16).

[0096] Advantageously, as illustrated in Figures 1 and 14, the inlet 10 of the vein 9 is 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 the vein 9 being between 0.1 x R and 0.6 x R, where R is the radius of the helix 3.

[0097] Such axial positioning of the inlet of the vein makes it possible to capture air having an optimal speed (namely between 50 and 1 lOm / s), so as to efficiently evacuate the thermal energy of the heat transfer fluid into the air of the vein.

[0098] Advantageously, as illustrated in Figures 1 and 14, the inlet 10 of the vein 9 has a reference point Pr whose radial dimension Dr with respect to the axis X is between 0.6 x R and 0.9 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 group 1 which passes through the axis X.

[0099] 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.

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

[0101] 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.

[0102] Advantageously, as illustrated in the figures, the vein 9 comprises a divergent portion 34 located directly upstream of the heat exchanger 8, so as to slow down the speed of the air entering the heat exchanger 8, and a convergent 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.

[0103] The divergent portion 34 of the vein 9 forms a diffuser. The divergent 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.

[0104] 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. The thrust generated by the acceleration of the air makes it possible to partially compensate for the drag generated by the passage of the air in the heat exchanger.

[0105] Advantageously, as illustrated in the figures, the vein 9 comprises an upstream part located upstream of the heat exchanger 8 and a downstream part located downstream of the heat exchanger 8.

[0106] Advantageously, as illustrated in the figures, the vein 9 is delimited radially by an internal wall 36 (or an internal conduit) and an external wall 37 (or an external conduit).

[0107] As illustrated in the figures, the external wall 37 of the vein 9 is here formed by an internal fairing 38 of a nacelle 40 of the propulsion group 1.

[0108] The inlet 10 of the vein 9 can be formed by one or two nozzles 41 with variable section. In the same way, the outlet 11 of the vein 9 can be formed by one or two nozzles 42 with variable section.

[0109] As illustrated in Figures 1 to 3 and 14, the inlet 10 of the vein 9 is formed by the external wall 37 of the vein 9 and an internal inlet nozzle 41 with variable section, the internal inlet nozzle 41 forming at least in part the internal wall 36 of the vein 9.

[0110] As illustrated in Figures 1 to 3 and 14, the outlet 11 of the vein 9 is formed by the external wall 37 of the vein 9 and an internal outlet nozzle 42 with variable section, the internal outlet nozzle 42 forming at least in part the internal wall 36 of the vein 9.

[0111] Advantageously, as illustrated in Figures 9 to 13, a nozzle 41, 42 with variable section comprises a frame 43 (or reinforcement) comprising a base 44 from which extends an annular row of independent strips 45, the free ends of the strips 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 (or membrane), so as to seal the nozzle 41, 42.

[0112] The frame 43 of the nozzle 41, 42 can be made of composite material, so as to minimize the mass of the nozzle.

[0113] In the same way, the casing 47 can be made of polymer material, so as to minimize the mass of the nozzle.

[0114] Advantageously, as illustrated in Figures 9 to 13, the diameter of the ring 46 is adjustable via a mechanism 48 driven by an actuator 49 (for example a stepper type electric motor).

[0115] As illustrated in [Fig.l 1], 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.

[0116] A nozzle 41, 42 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.

[0117] The free ends of the strips 45 can be connected to the ring 46 via elastic elements (return springs, polymer blocks) or tie rods.

[0118] 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.

[0119] [Fig. 12] is a front view of the nozzle 41, 42 with a maximum opening section, the ring 46 having a maximum diameter. [Fig. 13] is a front view of the nozzle 41, 42 with a minimum opening section, the ring 46 having a minimum diameter.

[0120] Adjusting the diameter of the ring 46 thus makes it possible to adjust the opening section of the nozzle 41, 42.

[0121] As illustrated in Figures 1 to 3 and 14, the ring 46 of the inlet nozzle 41 forms the upstream end of the internal wall 36 of the vein 9, thus adjusting the diameter of the ring 46 (inlet nozzle 41) makes it possible to adjust the inlet section of the vein 9.

[0122] In the same way, the ring 46 of the outlet nozzle 42 forms the downstream end of the internal wall 36 of the vein 9, thus the adjustment of the diameter of the ring 46 (outlet nozzle 42) makes it possible to adjust the outlet section of the vein 9.

[0123] The inlet section of the vein 9 is minimal ([Fig.2]) when the diameter of the ring 46 of the inlet nozzle 41 is maximal. And conversely, the inlet section of the vein 9 is maximal ([Fig.3]) when the diameter of the ring 46 of the inlet nozzle 41 is minimal.

[0124] In the same way, the outlet section of the vein 9 is minimal ([Fig.2]) when the diameter of the ring 46 of the outlet nozzle 42 is maximal. And conversely, the outlet section of the vein 9 is maximal ([Fig.3]) when the diameter of the ring 46 of the outlet nozzle 42 is minimal.

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

[0126] In the same way, to adjust the outlet section of the vein 9, the outlet 11 of the vein 9 may comprise a closing device 54 comprising at least one adjustable flap 55.

[0127] A shutter 55 adjustment can be movable in translation (sliding shutter) or movable in rotation (pivoting shutter).

[0128] As illustrated in Figures 15 and 16, the inlet 10 of the vein 9 comprises a closure device 53 comprising two annular rows of flaps 55 movable in rotation. In the same way, the outlet 11 of the vein 9 comprises a closure device 54 comprising two annular rows of flaps 55 movable in rotation.

[0129] The inlet section of the vein 9 is minimal ([Fig. 15]) when the shutters 55 of the closure device 53 are closed. And conversely, the inlet section of the vein 9 is maximal ([Fig. 16]) when the shutters 55 of the closure device 53 are open.

[0130] In the same way, the outlet section of the vein 9 is minimal ([Fig. 15]) when the shutters 55 of the closure device 54 are closed. And conversely, the outlet section of the vein 9 is maximal ([Fig. 16]) when the shutters 55 of the closure device 54 are open.

[0131] Advantageously, as illustrated in the figures, the cooling system 7 comprises a surface heat exchanger which is flush with the internal fairing 38 of the nacelle 40 of the propulsion unit 1.

[0132] Such a surface heat exchanger 56 provides additional cooling capacity for the fuel cell core while having a low impact on drag.

[0133] 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.

[0134] Advantageously, the surface heat exchanger(s) 56 form part of the cooling circuit 29 which comprises the main heat exchanger 8.

[0135] As illustrated in the figures, the cooling system 7 comprises five 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.

[0136] As illustrated in the figures, the nacelle 40 of the propulsion group 1 surrounds the vein 9, and is delimited radially by the internal fairing 38 and an external fairing 39 facing each other.

[0137] Advantageously, the cooling system 7 comprises at least one water injector 57 which is placed in the vein 9 upstream of the heat exchanger 8.

[0138] The air / water mixture offers a superior cooling capacity compared to air alone. The water will vaporize upon contact with the heat exchanger, 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. The water vapor created is evacuated through the outlet of the vein.

[0139] 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.

[0140] 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.

[0141] 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.

[0142] Advantageously, the cooling system 7 comprises at least one radial or annular row 58 of water injectors 57 which is placed upstream of the heat exchanger 8.

[0143] 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.

[0144] The water may be stored in one or more tanks. The tank(s) may be filled periodically, for example before each takeoff. The tank(s) are for example integrated into the nacelle 40. The tank(s) may be associated with a water temperature sensor and a water level sensor. The tank may comprise an isothermal wall, and in other words a thermally insulated wall. The tank(s) may be cooled by a cooling device, the cooling device taking air from the duct 9 for example. Each tank may comprise a filling orifice positioned at 12 o'clock by analogy with the dial of a clock, so as to facilitate its filling.

[0145] 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.

[0146] As illustrated in [Fig. 14], the cooling system 7 here comprises several radial rows 58 of water injectors 57 which are placed in the vein 9 upstream of the heat exchanger 8, these radial rows 58 of water injectors 57 being distributed regularly around the axis X.

[0147] Hollow and profiled (or tapered) arms can be introduced into the vein 9 for the passage of the water supply pipes of the injector(s) 57.

[0148] The various cooling systems 7 explained in this application are in particular intended for a propulsion group 1 whose thrust-to-weight ratio varies between 0.1 and 0.25.

[0149] According to the invention, the cooling system 7 is configured to be adjusted by an adjustment method.

[0150] The adjustment method comprises the step of: a) adjusting the inlet section and the outlet section of the vein 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 and leaving the vein 9 according to the cooling requirements.

[0151] Such an adjustment method takes into account the temperature of the core of the fuel cell so as to quickly and precisely adapt the air flow rates entering and leaving the vein according to the cooling requirements of the core, and thus minimize the aerodynamic drag produced by the passage of air in the vein, during the different operating regimes of the propulsion unit, for the benefit of the overall performance of the propulsion unit and the aircraft (consumption, flight autonomy, etc.).

[0152] The inlet and outlet sections of the vein 9 during step a) can be adjusted from additional parameters such as the temperature of the air upstream of heat exchanger 8, air pressure upstream of heat exchanger 8, 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).

[0153] 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.

[0154] As an example, the minimum section of the inlet 10 of the vein 9 is equal to 0.05 x Sech, where Sech is the frontal section of the heat exchanger 8. And the maximum section of the inlet 10 of the vein 9 is equal to 0.8 x Sech-

[0155] For a fuel cell 5 whose core 6 has a maximum admissible temperature of 70°C, the inlet section of the 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 the vein 9 is approximately equal to 0.8 x Sech when the temperature of the core 6 is equal to 68°C.

[0156] 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.

[0157] 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 exchanger 8, the pressure of the air upstream of the heat exchanger 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).

[0158] By way of example, for a fuel cell 5 whose core 6 has a maximum admissible temperature of 70°C, the injector 57 is 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 heat exchanger (8) placed in a streamlined vein (9) in which air flows from an inlet (10) of the vein (9) and to an outlet (11) of the vein (9), characterized in that the heat exchanger (8) and the vein (9) are annular around the axis (X), the vein (9) surrounding a central compartment (12) supplied with air in which the electric machine (4) and the heart (6) of the fuel cell (5),the inlet (10) of the vein (9) preferably having a so-called inlet section which is variable and the outlet (11) of the vein (9) preferably having a so-called outlet 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 inlet (10) of the vein (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 the 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 the vein (9) has a reference point (Pr) whose radial dimension (Dr) with respect to the axis (X) is between 0.6 x R and 0.9 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 the vein (9) comprises a divergent portion (34) located directly upstream of the heat exchanger (8), so as to slow down the speed of the air entering the heat exchanger (8), and a convergent 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 the inlet (10) of the vein (9) and / or the outlet (11) of the vein (9) are each formed by a nozzle (41, 42) with variable section, the nozzle (41, 42) 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 at least partially covered with an envelope (47), so as to seal the nozzle (41, 42).

6. Propulsion unit (1) according to one of claims 1 to 4, characterized in that the inlet (10) of the vein (9) and / or the outlet (11) of the vein (9) each comprise a closing device (53, 54) 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 the vein (9) upstream of the 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 central 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. 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 and the outlet section of the vein (9) from a measurement of the temperature of the core (6) of the fuel cell fuel (5), so as to adjust the air flows entering and leaving the vein (9) according to the cooling requirements.

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