Abstract:
A deicing device for propfan-type aircraft propulsion unit blades, wherein the propulsion unit includes a turbomachine that drives in rotation at least one rotor including a plurality of blades arranged around an annular crown moving with the blades, which forms with its outer wall part of the outer envelope of the propulsion unit, the outer envelope being subjected to atmospheric conditions outside the propulsion unit, the turbomachine generating a flow of hot gases that exit via an annular hot vein, which is concentric with the moving annular crown, and defined for part of its surface by an inner wall of the moving annular crown. The deicing device includes: a mechanism transforming thermal energy into electrical energy, within the moving annular part; a mechanism transferring the generated electrical energy towards the rotor blades; and a mechanism transforming the electrical energy into thermal energy onto at least a part of the surface of the blades.

Description:
BACKGROUND OF THE INVENTION 
     The present invention falls within the field of aeronautical equipment. It concerns more specifically deicing devices. In this case, it relates in particular to the problem of deicing propeller blades. 
     During the various phases of flight, particularly on the ground, at take-off, climbing or landing, aircraft are regularly subjected to icing atmospheric conditions (cold surface+ambient humidity), which cause ice deposits to be created on various parts of the fuselage. These ice deposits modify the aircraft&#39;s aerodynamic performance, increase its mass and reduce its maneuverability. 
     Various anti-icing devices (that prevent ice forming on a surface of the aircraft) and deicing devices (that detach pieces of ice once they have formed) have been developed over decades and are already known to experts. For example, for the leading edges of wings, they use heating resistors that cause the ice to melt and to break into pieces removed by the airflow. In the same way, inflatable membranes are used intermittently to break ice while it is forming. 
     It is obvious that similar problems of fighting icing by anti-icing or deicing occur for the propeller blades in the case of propeller-driven airplanes. In this case, heating resistors are generally used, with an electrical generator installed in the propeller shaft and a transfer of current towards cables passing through this shaft towards the various blades (see patent document WO 97/24261, for example). 
     The amount of power required to ensure permanent deicing of the blades then leads to the preferred choice of heating the blades one after the other, in cyclical fashion. This mode of deicing at regular intervals reduces the electrical power needed and the size of the generator. 
     In contrast, in the case of propulsion units known under the generic name “propfan”, comprising two counter-rotating propellers with an open rotor (not faired) driven by a differential gearbox which is itself driven by a turbomachine, the propellers are arranged in annular fashion around the core of this turbomachine and this arrangement prevents the use of the devices mentioned previously. 
     Rotating contact devices are known in addition that ensure the transmission of electrical power between a fixed shaft and a moving annular part by using electro-conductive brushes fixed on the shaft that slide on an annular track of the rotating part. 
     In this case, the power to be transferred to device the blades of a propfan is close to some twenty kilowatts, which implies devices of significant size. One of the main drawbacks of these rotating contact systems is linked to the speed of the brushes in relation to the moving track, this speed being in general close to one hundred meters per second and depending naturally on the diameter of the annular track and on the speed of rotation of this part. 
     The consequence of this for all these rotating contact systems is rapid wear of the brushes, leading to reduced performance and a requirement for frequent and costly maintenance. The absence of lubrication for these brushes (for reasons of complexity) also contributes to reducing this lifespan significantly. 
     In the case of the front propeller of a propfan, the diameter of the turbomachine&#39;s core leads to a relative speed of the moving part in relation to the stationary part of the order of four hundred meters per second, which makes systems using brushes and a moving track unusable in practice, as this exceeds the specifications of devices available on the market. 
     The situation is further exacerbated in the case of propfans by the counter-rotating characteristic of the two propellers. 
     Lastly, propfans are characterized by the high temperature of the exhaust flow they generate, about 800° C. at the outlet; this gas flow passes between the propulsion unit&#39;s shaft and the two propellers and makes it difficult to install materials that may suffer in high-temperature conditions. 
     BRIEF SUMMARY OF THE INVENTION 
     The objective of the present invention is therefore to propose a device for deicing/anti-icing of blades for propfan-type propulsion units, which avoids the drawbacks mentioned above. 
     To this end, the invention envisages a deicing device for propfan-type aircraft propulsion unit blades, where said propulsion unit comprises a turbomachine that drives in rotation at least one rotor comprising a plurality of blades arranged around an annular crown moving with these blades, which forms with its outer wall part of the outer envelope of the propulsion unit, said outer envelope being subjected to the atmospheric conditions outside the propulsion unit, said turbomachine generating a flow of hot gases that exit via an annular hot vein, which is concentric with the moving annular crown and defined for part of its surface by the inner wall of said moving annular crown, 
     the deicing device comprising:
         means of transforming the thermal energy into electrical energy, within the moving annular part,   means of transferring the electrical energy generated towards the rotor blades,   means of transforming the electrical energy into thermal energy onto at least a part of the surface of said blades.       

     Preferably, the means of transforming thermal energy into electrical energy comprise an electrical generator made of a set of Seebeck-effect thermal diodes, arranged between the inner wall and the outer wall of the annular crown, which act, respectively, as hot source and cold source for these diodes, said thermal diodes being laid out in series and parallel groups so as to achieve as the output of the electrical generator a voltage and amperage compatible with the deicing requirements of the rotor blades. 
     According to an advantageous embodiment, the thermal diodes are of Pb 0.5 Sn 0.5 Te type. 
     According to an advantageous embodiment, caloducts are provided between one of the annular crown&#39;s walls and one surface of the thermal diodes. 
     Alternatively, the device for deicing blades comprises means of channeling either hot air from the hot annular vein or outside air towards a wall on which the thermal diodes are installed. 
     It is understood that it is necessary to cater for the distance that exists in the propulsion unit between the hot and cold areas, in view of their use as thermal diode hot and cold sources. 
     According to various arrangements that may be used together:
         the electrical generator extends in annular fashion over substantially the whole of the inner perimeter of the annular crown,   in the case of a rotor comprising n blades, each 360°/n sector of the electrical generator supplies electrical energy to one blade with a suitable amount of power for its deicing,   the device comprises means of controlling the temperature difference between the hot and cold sources of the thermal diodes, controlled according to the instructions of an electronic control unit for the current generated by the diodes.       

     It is understood that this last arrangement allows a feedback process to be created that maximizes in real-time the energy yield of the thermal diodes. 
     To optimize the use of the electrical energy generated by the electrical generator, this last comprises favorably an electronic control unit to which all the diodes supply the generated current; this electronic control unit is designed to measure the available electrical power and distribute it amongst the blades and to select a cyclical supply mode for the blades in case the amount of power generated is below a predefined threshold. 
     The threshold will be selected such as to characterize the moment when the amount of power generated is insufficient for a permanent parallel supply to all the blades. 
     Preferably, the heating resistors of the blades are permanently supplied and therefore all the blades are supplied simultaneously. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The description that follows, given solely as an example of an embodiment of the invention, is made with reference to the figures included in an appendix, in which: 
         FIG. 1  shows a propfan-type propulsion unit, to which the invention can be applied, 
         FIG. 2  illustrates such a propulsion unit in a very schematic cross-section view, 
         FIG. 3  shows schematically the assembly principle of a Seebeck-effect electrical generator, 
         FIG. 4  shows the electrical power density that can be obtained with a commercially available thermal diode, depending on the available temperature difference, 
         FIG. 5  illustrates a deicing device for blades according to the invention, 
         FIG. 6  illustrates an implementation example of the thermal diodes on the two rotors of the propfan. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention is destined to be used in an airplane propulsion unit  1 , for example of the type called “propfan”, as shown in  FIG. 1 . Such propulsion units are envisaged for future aircraft. In the example of implementation illustrated here, two propfan propulsion units  1  are attached by engine pylons, on both sides of the rear part of an aircraft fuselage  2 . 
     Each propfan propulsion unit  1  comprises here two counter-rotating rotors  3   a ,  3   b  each comprising a set of blades  4   a ,  4   b , which are equidistant and arranged at the rear of the propulsion unit  1 . The blades  4   a ,  4   b  of each rotor  3   a ,  3   b  protrude from an annular crown  5   a ,  5   b , which is mobile with this rotor, the outer surface of which is located in the continuity of the outer envelope  6  of the propulsion unit. 
     As shown schematically in  FIG. 2  the propfan propulsion unit  1  comprises an air inlet  7  that supplies a turbomachine  8 . This turbomachine  8  comprises an axial portion driven in rotation when the turbomachine is running. In turn, this shaft drives the shafts  9   a ,  9   b  of the blades  4   a ,  4   b  of the two counter-rotating rotors  3   a ,  3   b  via mechanical transmissions not shown in  FIG. 2 . 
     The hot gases generated by the turbomachine  8  when in operation are discharged through an annular hot vein  10  having its outlet located at the rear of the two rotors  3   a ,  3   b.    
     The realization details of “propfans” and their components—rotors, turbomachine, transmission—as well as their dimensions, materials etc. are beyond the scope of the present invention. The elements described here are therefore provided only for information purposes, to facilitate understanding of the invention in one of its non-limiting examples of implementation. 
     As is apparent from the description above, during the aircraft&#39;s flight, outside air, at a temperature of between +30° C. near the ground and −50° C. at altitude, circulates along the annular crowns  5   a ,  5   b  of the propellers, substantially in the direction opposite to the longitudinal axis X of movement of the aircraft. 
     At the same time, the gases circulating within the hot air vein  10  are at a temperature usually ranging between 600 and 800° C. 
     The deicing device according to the invention takes advantage of this significant temperature difference at a few centimeters distance by using Seebeck-effect assemblies that allow thermal energy to be transformed into electrical energy. 
     As a preliminary, it is stated that the thermoelectric effect (Seebeck effect) is defined by the potential difference between the two terminals of a conductor when they are subjected to different temperatures. This effect is used in temperature measurements using thermocouples. This is the opposite of the Peltier effect, in which applying a potential difference between the terminals of a conductor causes the creation of a temperature difference between these terminals. 
     Seebeck-effect power generation generally implies creating closed circuits comprising a set of conductors  11 , linked two by two by junctions  12 ,  13 , one of them subjected to a first temperature T 1  and the other to a second temperature T 2  ( FIG. 3 ). 
     Various studies show that the materials that provide the best thermoelectric yield are those that fulfill requirements of high electrical conductivity, low thermal conductivity and high Seebeck coefficient. Several materials suitable for use in thermal diodes are currently known. Amongst these, are Mercury Cadmium Telluride (Hg 0.86 Cd 0.14 Te), Bismuth telluride (Bi 2 Te 3 ), Silicon nano threads, etc. 
     It should also be noted that thermoelectric materials each have favorable characteristics within a given temperature range (Lead telluride around 550-750K, Bismuth telluride around 250-350K, etc.) Therefore, in cases where the temperature difference between hot (T 2 ) and cold (T 1 ) areas, which is the case for example in this implementation example on a propfan propulsion unit (T 1  close to 250K and T 2  close to 1000K) it is advantageous to use several superposed materials to make up each conductor  11 . 
     The yield for conversion between thermal energy and electrical energy is currently of the order of 30% of the ideal Carnot yield for a 300K temperature difference between the hot and cold areas, which corresponds to a theoretical yield of 13% approximately (13% of the thermal energy converted to electrical energy). With a 700K temperature difference, an 18% yield is obtained. 
     Because of the mass characteristics of commercially-available diodes, a yield of 30% of the Carnot ideal corresponds to a power-to-weight ratio of 1,000 watts/kg of installed diodes. 
       FIG. 4  illustrates the electric power density that can be achieved per unit of area, depending on the available temperature difference, for a commercially available Pb 0.5 Sn 0.5  type thermal diode. It can be seen that for temperature gradient values of the order of 350K, electrical power of 10 watts/cm 2  can be obtained. 
     These values demonstrate the compatibility of using thermal diodes with the constraints on available size, volume and mass and of required deicing power for a propfan propulsion unit. 
     Indeed, this last generates at least 200 kW of thermal power, evacuated via the annular hot air vein  10 . 
     Deicing a rotor of the propfan requires approximately 10 kW of electrical power. Therefore, deicing the two rotors  3   a ,  3   b  of the propfan requires 20 kW. The mass of the thermal diodes required to supply this electrical power is therefore approximately 20 kg. 
     This value is compatible with existing mass constraints on aircraft and with volumetric constraints linked to the design of the rotors  3   a ,  3   b  of propfans. Effectively, each annular crown  5   a ,  5   b  has an inner wall  15   a ,  15   b  and an outer wall  14   a ,  14   b , separated by some twenty centimeters approximately, for an annular crown  5   a ,  5   b  inner diameter of several tens of centimeters and an annular crown  5   a ,  5   b  width (along the longitudinal axis X) of a few tens of centimeters. 
     The deicing device for rotor blades  4   a ,  4   b  according to the invention ( FIG. 5 ) is described here for the forward rotor  3   a  of the propulsion unit under consideration. An identical device is envisaged for the aft rotor  3   b . The device uses the space available within the annular crown  5   a  of each rotor  3   a  to place a thermal diode  22  electrical generator  16 . 
     It comprises in addition, in this non-limiting example, a set of cables  17  designed to transfer the generated electrical energy towards the blades  4   a  of the rotor  3   a.    
     Lastly, each blade  4   a  is fitted with a set of resistors  18  to heat the areas to device or anti-ice, e.g. the leading edge of blade  4   a , etc. 
     The electrical generator  16  is made of a set of thermal diodes  22  installed between the inner wall  15   a  and the outer wall  14   a  of the annular crown  5   a .  FIG. 6  illustrates an implementation example of these thermal diodes  22  on the two rotors  3   a ,  3   b  of the propfan. 
     The thermal diodes  22  under consideration are, for example, of Pb 0.5 Sn 0.5 Te, delivering a 13% yield approximately. 
     Depending on the normal operating thickness of the thermal diodes  22  under consideration between the cold source (the outer wall  14   a ,  14   b  of each annular crown  5   a ,  5   b ) and the hot source (the inner wall  15   a ,  15   b  of each annular crown  5   a ,  5   b  opposite the annular hot vein  10 ), caloducts  21  of know type are installed between one of the walls of the annular crown  5   a  and a surface of the thermal diodes  22 . On the cold source side, the device comprises advantageously a heat sink  23 , for example in the form of metal blades parallel to the airflow (i.e. to the aircraft&#39;s longitudinal axis) creating a large thermal exchange surface with the outside environment. 
     It is clear that, as a variant, it is possible to invert this layout. The thermal diodes  22  are then positioned near the outer wall  14   a ,  14   b  of each annular crown  5   a ,  5   b . In this case, the heat sinks  23  are installed within the annular hot vein  10 , on the inner wall  15   a ,  15   b  of each annular crown  5   a ,  5   b  and the caloducts  21  transport the heat from this hot source towards the thermal diodes  22 . 
     These thermal diodes  22  are laid out in series and parallel groups by means known per se to achieve, at the output of the electrical generator  16 , a voltage and amperage compatible with the deicing requirements of the rotor  3   a  blades  4   a.    
     Preferably, the two walls  14   a ,  15   a  of the annular crown  5   a  (the walls  14   b ,  15   b  of the annular crown  5   b , respectively) are made of a metallic material or in any event, a very good thermal conductor. The inner wall  15   a  is, for example, made of titanium and the outer wall  14   a  of aluminum. The lateral walls  19   a ,  20   a  of this annular crown  5   a  are made of a material with low thermal conductivity so that the thermal flow goes preferably past the thermal diodes  22 . 
     In the implementation considered here as an example, the electrical generator  16  extends over the whole of the inner perimeter of the annular crown  5   a  and over a width of approximately ten centimeters of said annular crown  5   a.    
     In the case of a rotor  3   a  comprising  12  blades  4   a , each 30° sector of the electrical generator  16  supplies electrical energy to one blade with approximately 1 kW of power for its deicing. The mass of the thermal diodes  22  represented is of the order of 1 kg per 30° sector of the annular crown  5   a . More generally, for n blades, each 360°/n sector supplies electrical energy to one blade  4   a.    
     This supply&#39;s transfer cable  17  goes through or very close to the shaft  9   a  of the blade  4   a  to follow its changes when its settings change during the flight. 
     The transfer cable  17  supplies a set of heating resistors  18  of a type know per se; these heating resistors  18  (as well as their layout on the surface of the blade) and this transfer cable  17  are similar to those used in the case of current transfer between the stator (forward part of the propulsion unit) and the rotor  3   a  by rotating contacts. 
     In operation, the yield of the electrical generator  16  becomes significant as soon as the turbomachine  8  is started since the temperature difference T 2 −T 1  is already, on the ground, several hundreds of degrees K. In the operating mode chosen here as an example, the heating resistors  18  for the blades  4   a  are permanently supplied and all the blades  4   a  are supplied simultaneously; this is made possible by the available power of 10 kW per rotor approximately. In devices of the previous state of the art, the blades  4   a  were generally supplied in cyclical fashion, one after the other because of the lower available power. Besides the fact that anti-icing performance was significantly decreased, this procedure brought about the need for an electronic control unit for this cyclical supply, which increased the mass of the whole. 
     It is apparent from the description that the electrical generator according to the invention removes the problems caused by fast rotating contacts such as used in the previous state of the art. It utilizes an energy resource that is partially lost by taking advantage of the heat generated by the propulsion unit that passes under the crown of the rotor. 
     This facilitates the maintenance of the blade deicing mechanism. 
     In addition, the absence of moving parts in this generator causes increased reliability. 
     Lastly, its installation requires no significant changes to the propulsion unit. 
     The scope of this invention is not limited to the details of the embodiments considered above as an example, but on the contrary extends to modifications in the reach of the expert. 
     A transfer of electrical energy from the generator  16  to the heating resistors  18  of the blade  4   a  by a cable  17  going through the shaft  9   a  of the blade  4   a  was mentioned in the description. Alternatively, the current transfer towards the blade  4   a  is realized, at the output of the electrical generator  16 , by a conductive brush and conductive track on the shaft of the blade  4   a , both of types known per se, the relative speeds of these two parts being here very low. 
     In a variant, it is of course possible to replace the heating resistors  18  used for deicing the blades by any other deicing means using an electrical energy source, without changing the utilization principle of this invention. In the same manner, it is possible to retain cyclical deicing of the blades  4   a , for example, for cases of particular icing conditions. 
     In another variant, to facilitate the installation of the thermal diodes  22 , ducts channeling either hot air from the hot annular vein  10  or outside air towards a wall on which the thermal diodes  22  are installed are used instead of the caloducts  21  designed to reduce the distance between the hot source (inner wall  15   a  of the annular crown  5   a ) and the cold source (outer wall  14   a  of the annular crown  5   a ). 
     In another variant a temperature control device is fitted between the hot and cold sources of the thermal diodes  22 . In effect, these thermal diodes  22  have an optimal yield point for a given temperature difference and any variance from this temperature difference causes a decrease in the electrical current generated. 
     Such a control device can comprise air ducts that mix hot and cold air towards the hot source of the diodes, in accordance with the instructions of an electronic control unit for the current generated by the diodes. This creates a feedback device that maximizes in real time the energy yield of the thermal diodes  22 , whatever their age and the change in their maximum yield point. 
     It was mentioned in the description that each blade  4   a  is supplied by a sector of the thermal diodes  22 . Alternatively, it can be decided, for the sake of redundancy, that all the thermal diodes  22  supply the generated current to a single electronic control unit (not shown in  FIG. 5 ) that measures the available electrical power and distributes it to the blades  4   a , or even selects a cyclical supply mode for the blades  4   a , in cases where the generated power is insufficient for all the blades to be supplied permanently and in parallel. 
     In the same way, the electronic control unit communicates, by means not detailed here because they are outside the scope of this invention, the power generated by the thermal diodes  22  towards the airplane&#39;s pilots.