Abstract:
A fluid cooling device for the propulsion unit of a propfan type aircraft including a compressed air intake at the air compressor of the turbomachine, an air vein capable of conveying the collected compressed air to a cooler, and a system for conveying the heat of the lubricant to the cooler is provided. The cooler includes a matrix body provided with a plurality of ducts for a cooling agent, the ducts extending along a first inner surface up to a second outer surface of the matrix body such that the collected pressurized air serving as a cooling agent can pass through the matrix body, the matrix body of the cooler forming a portion of the outer skin of the propulsion unit; and a set of fins extending from the outer surface towards the outside of the propulsion unit, and oriented mainly parallel to the air flow direction when the aircraft is in flight.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the field of aeronautical equipment. It relates more specifically to heat dissipation systems, and even more particularly heat sinks for turbomachine-type aircraft propulsion units. 
     2. Description of the Related Art 
     A large number of aircraft propulsion units comprise a turbomachine; this is the case, for example, for propfan-type aircraft propulsion units, where this turbomachine drives two counter-rotating rotors. 
     In these propulsion units, a gearbox (gears between the turbomachine axle and the rotors) transmits the mechanical energy generated by the turbomachine to the rotors. 
     Although it has very high efficiency, this gearbox dissipates part of the energy created by the propulsion unit into heat by friction. This heat is transmitted in particular to the gearbox lubricant. 
     Moreover, the turbomachine itself generates significant heat dissipation mainly by mechanical friction, also through its lubricant. 
     It is clear that this heat must be dissipated to the outside environment to cool the propfan. 
     Various solutions have been developed to perform this cooling. 
     A first solution, close to the state of the art of turbofans (see for example patent application FR2006/0052654), consists of collecting air over a compressor stage (low pressure so that temperatures are not too high), positioning an air/oil exchanger in this air flow, positioning an air regulator valve, and having the air exit tangentially to the outer surface of the nacelle to recover a portion of the thrust. On thermal design points (ground on a hot day, for example), the valve is completely open and, despite the fact that the airplane is immobile, the air exchanger is supplied. In flight, to limit the impact on performance, the air valve is partially open in order to adjust the air flow. The major drawbacks of this solution are:
         Significant impact on the engine&#39;s performance due to the collection of air (the turbomachine must be sized taking this loss of flow into account)   Acoustic impact of ejecting air upstream of the propellers and on a discrete position (non-annular)   Distortion of the nacelle&#39;s external aerodynamic flow, which can lead to a reduction in the propellers&#39; efficiency.       

     BRIEF SUMMARY OF THE INVENTION 
     The objective of this invention is therefore to propose a turbomachine cooling device that overcomes the aforementioned drawbacks by providing a sufficient level of cooling on the ground and in flight while limiting the aerodynamic impact during flight phases. 
     A second objective of the invention is to use a reduced volume, the invention having to be used in particular in a propulsion unit nacelle. 
     To this end, the invention envisages a fluid cooling device for so-called propfan-type aircraft propulsion unit, said propulsion unit comprising a turbomachine that drives in rotation at least one rotor, said turbomachine being supplied with outside air by an air inlet, said turbomachine comprising a compressor with at least two stages, and generating heating of a lubricant during its operation, 
     said fluid cooling device comprising a pressurized air intake, an air vein able to convey the pressurized air to a volumetric/surface cooler, means of conveying the lubricant&#39;s heat to the volumetric/surface cooler, 
     said volumetric/surface cooler comprising: 
     firstly, a matrix body provided with a plurality of ducts for a cooling agent, said ducts extending along a first so-called inner surface up to a second so-called outer surface of the matrix body, such that the pressurized air serving as a cooling agent can pass through the matrix body, 
     and, secondly, a set of fins extending from the outer surface of the matrix body towards the outside of the propulsion unit, and oriented mainly parallel to the direction of airflow when the aircraft is in flight. 
     In other words, the volumetric/surface cooler operates in two possible modes: firstly, as a volumetric heat exchanger, by offering a very large area of contact with the cooling agent, within the ducts of the matrix body, secondly, as a surface heat exchanger, thanks to the fins. 
     In this case, preferably, the matrix body is sized so as to be sufficient to perform the desired cooling by itself when the aircraft is at low or zero speed, within preselected environmental conditions, and the fins are sized so as to be sufficient to perform the desired cooling by themselves when the aircraft is in flight, within preselected environmental and speed conditions, 
     These fins perform two functions. 
     In effect, with regard to these fins, the cooler works as a surface heat exchanger, unlike its operation by means of its portion comprising a matrix body crossed by a large number of ducts. 
     Secondly, these fins contribute to protecting the cooler&#39;s matrix body against the impacts of objects. 
     Preferably, the cooler&#39;s matrix body forms a portion of the outer skin of the propulsion unit. 
     In effect, this arrangement removes the need for an air vein on exit from the matrix body, and thereby reduces the device&#39;s size. In addition, it is known that the airflow in flight, perpendicular to the ducts of the matrix body, is little disturbed by the outer surface of this matrix body. The loss of load generated is low, in the light of the resulting simplification. 
     According to an advantageous implementation, the compressed air intake is positioned downstream of the air compressor&#39;s 1 st  or 2 nd  stage. 
     Alternatively, the cooling device comprises an additional air inlet positioned substantially forward of the propulsion unit. 
     These arrangements allow a source of pressurized air to be supplied at ambient temperature; this air has to absorb the excess heat in the turbomachine&#39;s lubricants. 
     According to various arrangements that may be implemented together:
         the matrix body comprises a plurality of strips delimiting the ducts,   the ducts of the matrix body are oriented substantially perpendicular to the outer surface of the matrix body.       

     In order to control the cooling mode, the cooling device comprises means of controlling the flow of compressed air passing through the ducts of the cooler&#39;s matrix body. 
     The goal here is to make it possible to control the cooler between its volumetric heat exchange mode and its surface heat exchange mode. 
     The invention also envisages an aircraft comprising a device as described. 
     In a second aspect, the invention envisages a method designed to use a device as described, said method comprising operations, when the aircraft is on the ground, of commanding the passage of a maximum flow of compressed air through the cooler, and, when the aircraft is in flight, of commanding the passage of a reduced or zero flow of air through the ducts of the cooler&#39;s matrix body. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The description that will follow, 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 an engine of a type called “propfan,” to which the invention can be applied, 
         FIG. 2  illustrates such a propulsion unit in a very schematic cross-section view, 
         FIG. 3  is a detail view of  FIG. 2 , centered on the front part of the propulsion unit, which highlights the main elements of the device according to the invention, 
         FIG. 4  illustrates a detail view of a volumetric/surface cooler utilized in the invention, 
         FIG. 5  illustrates schematically the data processed by the electronic control unit of the cooling device according to the invention, 
         FIG. 6  illustrates the operation of the cooling device according to the invention when the aircraft is on the ground, 
         FIG. 7  then illustrates in a detailed view the air circulation in the cooler in this mode of operation, 
         FIG. 8  likewise illustrates the operation of the cooling device according to the invention when the aircraft is in flight, 
         FIG. 9  then illustrates in a detailed view the air circulation in the cooler in this mode of operation, 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention is intended to be used in an airplane propulsion unit  1 , for example of the type called “propfan”, as shown in  FIG. 1 . Such engines 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  here comprises two counter-rotating rotors  3   a ,  3   b  each comprising a set of equidistant blades  4   a ,  4   b  and positioned in the rear part 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 axle drives the axles  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 a hot annular channel  10  having an outlet located at the rear of the two rotors  3   a ,  3   b . In a variant, these gases can also be discharged upstream of the two rotors. 
     The realization details of propfans and their components—rotors, turbomachine, transmission, and their dimensions, materials etc.—are outside the framework of this 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. 
     During the aircraft&#39;s flight, outside air, at a temperature of between +30° C. immediately next to the ground and −50° C. at altitude, circulates along the outer skin of the propulsion unit, substantially in the direction opposite to the longitudinal axis X of movement of the aircraft. 
     At the same time, the propulsion unit generates a significant heat discharge, part of which is evacuated by the hot annular channel  10  and another part, transferred to the oil circuits of the engine and the gearbox, must be evacuated by an ad-hoc cooling device. 
     The turbomachine  8  comprises, conventionally, a multistage compressor allowing incremental increases in the pressure of air entering the turbomachine. 
     The cooling device according to the invention takes advantage of the presence of this compressor, and thus comprises an air intake  11 , of a type known per se, positioned, in this non-limiting example, downstream of the second stage of the compressor of the turbomachine  8 , as seen in  FIG. 3 . This arrangement is intended to provide air that is already sufficiently pressurized (1.5 to 2.5 relative bars), but as yet little warmed by compression, unlike the air at the following stages of the compressor. 
     The position of the collection point naturally depends on the specific characteristics of the turbomachine  8  in question and its compressor, but this position is dictated by the need for air at a sufficient pressure, to bring a predefined airflow to a cooler, and at a sufficiently low temperature, while not disturbing the correct operation of the compressor and more generally of the turbomachine  8 . 
     This air intake  11  comprises a regulator valve  12 , here illustrated schematically, designed to control the flow of pressurized air collected at the air intake  11  between a value close to zero and a maximum value determined by the cooling requirement of the gearbox and/or engine oil. 
     An air vein  13  positioned downstream from the regulator valve  12  conveys the flow of collected pressurized air towards a volumetric/surface cooler  14 . 
     This volumetric/surface cooler  14  is designed to operate in two main heat exchange modes: 1/one on the ground, or during take-off, when the flow of outside air is low or zero and a heat exchange carried out over a very large surface installed in a small volume (by misuse of language, the exchange is said to be volumetric) is preferable, 2/the other in flight, when the flow of outside air is significant and allows heat exchange over a small surface (the exchange is then said to be surface). 
     This volumetric/surface cooler  14  is for example of the type described in the Airbus Deutschland German patent application no. 10 2009 013 159.0 filed on 16 Mar. 2009, not yet published, and incorporated here in this description by reference. 
     According to this application, summarized here for purposes of clarification, the volumetric/surface cooler  14  comprises ( FIG. 4 ), firstly, a matrix body  15 , in which a set of ducts  20  have been installed for the passage of the cooling agent, here the pressurized air brought by the air valve  13 . 
     This matrix body  15  is composed, for example, of assembled strips, which thus delimit the ducts  20  for the passage of pressurized air. 
     The matrix body  15  is made of a material with high heat conductivity, e.g. a metal alloy or composite material suitable for this purpose. 
     The matrix body  15  serves as a heat transfer between the fluid to be cooled (the gearbox lubrication oil in this case) and the pressurized air at a temperature close to ambient temperature. In this non-limiting example, the matrix body  15  comprises a set of tubes (not shown in the figures) embedded in its volume and in which the fluid to be cooled circulates. 
     In the case where cooling several fluids simultaneously is wished, various networks of tubes are installed in the volume of the matrix body  15 . 
     The matrix body  15  is positioned, in this example, so that its outer surface  16  extends the outer surface  6  of the propulsion unit nacelle, locally replacing this outer surface  6 . The matrix body  15  of the cooler thus forms a portion of the outer skin of the propulsion unit. The shape of the body matrix is therefore here generally cylindrical, and in all cases determined by the shape of the outer surface of the propulsion unit nacelle at the place where the matrix body must be installed. The matrix body  15  here has a substantially rectangular shape, arched to match the profile of the surface  6  of the propulsion unit nacelle. 
     The inner surface  17  of the matrix body is, in this example, substantially parallel to its outer surface  16 , the length of the ducts  20  being thus quasi-constant, and equal in this example to the distance between these inner  17  and outer  16  surfaces, if the ducts  20  are perpendicular to these surfaces. 
     The dimensions of the matrix body  15  are determined by the cooling requirement when the airplane is on the ground or at low speeds, the flow of pressurized cold air available and the exchange surface installed within the matrix body  15 . The calculation is known to experts and is therefore not detailed further here. For cooling during take-off an electrical generator equipping an aircraft having about 150 seats and two propulsion groups, the front surface of the heat exchanger is approximately 80 in 2  for an approximate thickness of 3 in. The maximum airflow passing through the heat exchanger is approximately 0.45 kg/s. The volumetric/surface cooler  14  comprises secondly a set of fins  18  starting from the outer surface  16  of the matrix body  15  and protruding at the outer surface  16  of the matrix body. 
     These fins  18  act as surfaces guiding the airflow, and are oriented substantially parallel to the flow lines of an air stream flowing over the outer surface  16  of the matrix body  15  when the aircraft is in flight, i.e. substantially along the longitudinal axis X. 
     The fins  18  are realized here in the same material as the matrix body  15 , to the surface of which they are fastened by the greater part of their lower edge. 
     The dimensions of these fins  18  are determined by the cooling requirement when the airplane is in flight, and by the flow of outside air and the temperature of the air flowing along the surface of these fins  18 . The details of such a calculation are known to experts. For cooling during cruising an electrical generator equipping an aircraft having about 150 seats and two propulsion groups, the exchange surface of the fins is approximately 0.5 m 2  for fins approximately 50 mm high. 
     In addition, these fins  18 , in this example, have a profile curved in the direction of the outer surface  16  of the matrix body  15 . The fins  18  protect the matrix body  15  and in particular the outer surface  16  of the matrix body  15  from external impacts, e.g. bird-strikes, hail, etc. 
     The cooling device is controlled by an electronic control unit  19 , (shown in  FIG. 5 ), of a type known per se, whose primary function is setting the regulator valve  12  according to various input information. 
     In this non-limiting example, as inputs this electronic control unit  19  receives temperature data for the oil circuits that the cooling device must regulate, as well as for the outside air and the air in the air vein  13 . It also receives status information concerning the regulator valve  12 . 
     It transmits control data, e.g. temperature of the oil circuits and position of the regulator valve, to the aircraft&#39;s cockpit, from which it also receives instructions. 
     This electronic control unit  19  may be installed at the propulsion unit, in the immediate vicinity of the volumetric/surface cooler  14 . Alternatively, the electronic control unit  19  may be part of the various pieces of electronic equipment located in the cockpit, or simply be one of the functions provided by one of the multi-purpose computers usually found in aircraft. 
     In operation, when the aircraft is on the ground ( FIGS. 6 and 7 ) or in taxiing, takeoff or approach phases, with the propulsion units operating, the thermal discharge from the propulsion group is very large and the airplane&#39;s speed is low or zero. 
     During these phases, the flow of outside air is low and not sufficient for cooling only by the fins  18 . The electronic control unit  19  therefore sets the regulator valve  12  substantially into the maximum open position, allowing the matrix body  15  to be traversed by the pressurized air collected at the compressor. This ensures a heat exchange between the hot matrix body  15  and the cold pressurized air, causing the desired cooling of the matrix body  15  and the fluids circulating inside or connected to it by thermal conduction. 
     As the climb progresses and evolves towards level flight, the speed of the aircraft increases and the outside air temperature decreases. Accordingly, the collection of air at the compressor is reduced by gradual closing of the regulator valve  12  controlled by the electronic control unit  19 , and the cooling is increasingly performed only by the fins  18 . 
     Subsequently, when the aircraft is in steady flight ( FIGS. 8 and 9 ), the cooling is performed normally by the external fins  18 , and the regulator valve thus remains closed, thereby eliminating the collection of air from the compressor, and therefore reducing the increased fuel consumption that otherwise arises from this power draw 
     Of course, in the event of special conditions (high outside temperatures, overload of the electrical generators, etc.), even in this flight phase it is possible, on the basis of oil temperature information received, to order via the electronic control unit  19  the opening of the control valve  12 , so as to circulate air in the matrix body and thereby increase the overall cooling capacity of the cooling device. 
     It is apparent from the description that the cooling device allows the engine components to be cooled in all flight phases while allowing the surface exchanger to be used during cruising phases. The engine&#39;s aerodynamic performance is thus improved. 
     The fact of managing the opening of the regulator valve  12  during the flight allows the power draw on the compressor to be controlled, and to be reduced whenever possible, which translates into reduced consumption. 
     In addition, the present invention takes advantage of the presence of the compressor, which allows pressurized air to be supplied, without having to add a specific fan dedicated to making air pass through the matrix body. This results in increased simplicity for the device, as well as smaller dimensions and also lower fuel consumption. 
     The scope of this invention is not limited to the details of the forms of embodiment considered above as an example, but on the contrary extends to modifications in the reach of the expert. 
     In a variant, it is possible to replace, or supplement, the air collection point  11  by a second air inlet, positioned forward of the propulsion unit nacelle, close to the main air inlet  7 . This second air inlet is also equipped with a regulator valve  12 ′. 
     If the air collection point  11  is replaced by the second air inlet, a fan is integrated within the new air vein  13 ′ to provide a sufficient flow of cooling air when the aircraft is on the ground. 
     In contrast, if the second air inlet co-exists with the air collection point  11  on the compressor, this second air inlet is particularly usable when the speed of the aircraft is sufficient to provide a significant flow of cooling air, for example above Mach 0.2. With the aim of further reducing the power draw on the compressor, it is possible to combine the two sources based on the flight modes, modifying the electronic control unit  19  accordingly. 
     In another embodiment, the matrix body  15  comprises ducts  20  that are no longer perpendicular to its outer surface  16 , but on the contrary are slanted toward the rear of the propulsion unit, so as to reduce the drag caused by the matrix body  15 . 
     A matrix body  15  with a rectangular, arched shape has been mentioned in the description. Of course, other geometries are possible, in particular for taking into account availability in terms of volume in the vicinity of the outer surface  6  of the propulsion unit nacelle. One arrangement that can be envisaged comprises several matrix bodies  15  distributed at regular angular intervals around the nacelle, so as to limit the disturbance on the blades created by the airflow created at the exit from said matrix body  15 . 
     In another variant, not shown, the matrix body  15  is not flush with the outer surface  6  of the propulsion unit nacelle, but is positioned a few centimeters under this surface, and only its fins  18  pass the outer surface  6 . In this case, the ducts  20  are slanted and an air outlet vein conveys the air leaving the matrix body  15  to an air outlet point at the outer surface  6  of the nacelle.