Patent Publication Number: US-2021194326-A1

Title: Fin and insert cooling device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to foreign French patent application No. FR 1915037, filed on Dec. 20, 2019, the disclosure of which is incorporated by reference in its entirety. 
     FIELD OF THE INVENTION 
     The invention relates to the cooling of an element producing heat. The invention is applicable in the field of electrical machines and power electronics. Indeed, it is known that electrical elements and power electronics generate losses which are reflected by a production of heat which has to be dissipated. The dissipation of this heat, guaranteeing good operation of the abovementioned elements, therefore becomes a priority. The invention is particularly applicable in the field of aeronautics in which the trend is to increase the quantity of electrical equipment and therefore the electrical power onboard. 
     BACKGROUND 
     As stated previously, electrical machines can generate a lot of heat. Furthermore, the cooling required to dissipate this heat is limited by certain constraints. The bulk and the weight added by a possible cooling device are notably limited by the specifics of a vehicle with an electrical machine embedded. For example, for vertical take-off and landing aircraft, electrical or hybrid, it is desirable to obtain a device capable of cooling the electrical elements that has a low weight and is compact.
         There are many devices that allow electrical machines to be cooled. Two approaches are these days distinguished:   The first approach making it possible to resolve this problem of cooling of the electrical elements relies on the use of a flow of air capable of dissipating the heat by convection. The cooling device, according to this first approach, takes the form of a heat sink provided with fins capable of exchanging heat with the electrical machine. The flow of air sweeping the electrical machine then exchanges thermally with these fins, which receive the heat originating from the electrical machine. However, this solution is limited by the maximum quantity of heat that this device can dissipate. Thus, to increase the cooling capabilities of this type of device, it is necessary to increase the bulk of the device, which does not seem optimal.   Another approach focuses more on the use of a coolant circulating in channels of the electrical machine. The fluid absorbs the heat generated by the electrical machine. Outside the electrical machine, the fluid circulates in an exchanger that allows the fluid to be cooled. The heat-transfer fluid is chosen to have a significant heat capacity. The cooling of the electrical machine is thereby enhanced. However, the bulk added around the electrical machine via the exchangers, the circulation pump and the fluid makes this approach more complicated and less desirable for a person skilled in the art seeking to lighten his or her vehicle.       

     Consequently, the cooling of an electrical machine is these days limited by constraints of weight and bulk that the prior art is not capable of overcoming. 
     It is therefore necessary to obtain an efficient cooling means of reduced weight and bulk. 
     For this, the invention proposes taking up the first approach and improving the efficiency thereof. To this end, the invention proposes combining cooling fins and heat pipes. 
     SUMMARY OF THE INVENTION 
     More specifically, the subject of the invention is a cooling device having a surface configured to allow the circulation of a heat-transfer fluid along the surface in a first direction, an exchange of heat being able to take place by convection between the cooling device and the fluid, the device comprising: 
     n cooling fins, n being an integer greater than or equal to one, each cooling fin forming a protuberance of the device, extending primarily in a plane containing the first direction, 
     at least one insert extending primarily in a second direction of the plane distinct from the first direction, the insert having, over its greater length, a thermal resistance lower than the thermal resistance of the cooling fin along the same length. 
     According to one aspect of the invention, the insert is a heat pipe. 
     According to one aspect of the invention, the insert has a tube form and in which a characteristic dimension of a section of the tube is greater than a thickness of the corresponding cooling fin defined outside of a section of the cooling fin at right angles to the first direction and passing through the insert. 
     According to one aspect of the invention, the n cooling fins, n being an integer greater than or equal to two, extend along the same abscissae in the first direction and along different ordinates on an axis of the section, the inserts having the same abscissa in the first direction. 
     According to one aspect of the invention, then cooling fins each comprising at least one insert extend along the same abscissae in the first direction and a different ordinate along an axis of the section, the inserts having an abscissa offset by a length greater than or equal to the characteristic dimension of the section of the inserts in the first direction. 
     According to one aspect of the invention, the insert is embedded in the cooling fin accommodating it. 
     According to one aspect of the invention, the greater length of the insert is equal to or greater than a greater length of the cooling fin in a section of the cooling fin at right angles to the first direction. 
     According to one aspect of the invention, the cooling fin has a smaller section at the end. 
     According to one aspect of the invention, several of the n cooling fins are aligned in the first direction. 
     According to one aspect of the invention, the cooling device comprises: 
     a first cooling fin extending along an abscissa in the first direction and an ordinate along an axis of the section, 
     a second cooling fin extending along the abscissa in the first direction and an ordinate different from the first cooling fin along the axis of the section, 
     one of the n, n being an integer greater than or equal to three, cooling fins comprising at least one insert disposed in the cooling fin and extending in the second direction, the cooling fin having an ordinate lying between the ordinates of the first and the second cooling fins along the axis of the section. 
     According to one aspect of the invention, a thermal grease is applied between the insert and the corresponding cooling fin. 
     According to one aspect of the invention, the electrical machine comprises a field frame on which windings bear and equipped with the cooling device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood and other advantages will become apparent on reading the detailed description of an embodiment given by way of example, the description being illustrated by the attached drawing in which: 
         FIG. 1  represents an electrical machine provided with a cooling device according to the invention. 
         FIGS. 2 a  and 2 b    represent a view focused around two cooling fins of the cooling device. 
         FIG. 3 a    represents a second embodiment of the cooling device. 
         FIG. 3 b    represents a focused view of an accommodating enclosure according to the second embodiment of the cooling device. 
         FIG. 3 c    represents a schematic view of a variant of the second embodiment of the cooling device. 
         FIGS. 4 a  and 4 b    represent a third embodiment of the cooling device. 
     
    
    
     In the interests of clarity, the same elements will bear the same references in the different figures. 
     DETAILED DESCRIPTION 
     In the present description, a direction D 1  is defined by the direction of a flow of air  12  directed by a dedicated propeller or fan wheel. 
       FIG. 1  represents an electrical machine  2  provided with a cooling device  4 . It is possible to implement the invention with any other type of heat source, with or without rotating parts, whether this heat source is electrical or mechanical. Generally, an electrical machine  2  capable of generating power is composed of a moving part or rotor  6  rotating about an axis  62  and a fixed part or stator  8 . During these phases of production of electrical power in generator mode, or mechanical power in motor mode, losses  10  are generated in the form of heat that has to be dissipated. 
     The cooling device  4 , intended to dissipate this heat, is formed by a tubular heat sink  41 , produced for example in an aluminium alloy. The heat sink  41  comprises cooling fins  44  that are substantially flat. Each cooling fin  44  extends primarily in a plane P containing the direction D 1 . Each cooling fin  44  extends also in a radial direction D 2  with respect to the axis of rotation  62  of the rotor  6 . In the example represented, the direction D 2  is at right angles to the direction D 1 . In other words, the plane P is defined by D 1  and D 2 . There is therefore a plane P for each cooling fin  44 , since each direction D 2  is specific to its cooling fin  44 . The cooling fins  44  take the form of thin rectangular or trapezoidal blades that are run over in their lengthwise direction by a flow of air  12  in the direction D 1 . 
     The heat sink  41  comprises a tubular base  42  from which emerge the cooling fins  44 . The outer surface of the base  42  and of the cooling fins  44  forms a heat exchange surface  424  between the heat sink  41  and the flow of air  12 . An inner surface  422  of the base  42  of the heat sink  41  is in contact with an outer wall  82  of the stator  8 . The cooling device  4  encloses the electrical machine  2  by being secured to the outer wall  82  of the stator  8 . 
     In this way, the dissipation of the heat  10  generated by the electrical machine  2  takes place in the outward direction, that is to say radially with respect to the axis of rotation  62  of the rotor  6 . The heat  10  is dissipated in the direction D 2 , first of all by conduction at the point of contact between the outer wall  82  of the stator  8  and the inner surface  422  of the base  42 , then, still by conduction within the heat sink  41 , from its base  42  to its cooling fins  44  and finally by convection between the heat sink  41  and the flow of air  12  at the heat exchange surface  424 . 
     In the present invention, each cooling fin  44  comprises at least one heat pipe  9  extending primarily in a third direction D 3  of the plane P distinct from the direction D 1 . In the example represented, the third direction D 3  coincides with the direction D 2 . It is also possible to arrange the heat pipes  9  extending in a third direction D 3  that is inclined with respect to the direction D 2 , the third direction D 3  of each heat pipe  9  remaining distinct from the direction D 1 . To arrange each heat pipe  9 , the cooling fin  44  concerned comprises, in its length L defined in the direction D 2 , one or more accommodating enclosures  442  in each of which there is inserted a heat pipe  9  that are represented more specifically in  FIGS. 2 a  and 2 b   . In the example represented, each cooling fin  44  comprises several accommodating enclosures  442  aligned in the direction D 1  of the flow of air  12 . The heat pipes  9  can take any bar form. 
     A heat pipe is a hermetic enclosure enclosing a fluid in the state of equilibrium. The heat pipe is a heat conductor which allows the heat to be guided between its two ends via changes of state of the fluid inside the heat pipe. More specifically, in fact, the vaporization of the fluid in the liquid state, inside the heat pipe, allows the absorption of thermal energy emitted by a heat source which is transferred, following the circulation of this same vapour within the heat pipe, to a dissipation zone where the fluid reverts to its initial state by condensation. The heat pipes  9  can be heat pipes available on the market. 
     More generally, the heat pipe  9  forms an insert whose thermal resistance is lower than that of the heat sink  41 . In the context of the invention, it is possible to replace the heat pipe with a bar produced in a material whose thermal conductivity is greater than that of the material in which the heat sink  41  is produced. As an example, the insert can be formed by a copper bar inserted into a heat sink made of aluminium alloy. Advantageously, it is possible to envisage placing inserts with a density lower than that of the cooling fins  44  in order to lighten the cooling device  4 . For example, the use of inserts made of aluminium alloy in a heat sink made of steel would favour the heat exchanges and would significantly reduce the weight of the cooling device  4 . 
       FIGS. 2 a  and 2 b    represent a view focused around two cooling fins  44  of the cooling device  4 . In  FIG. 2 a   , the two cooling fins  44  each comprise two heat pipes  9 . In  FIG. 2 b   , the two cooling fins  44  are represented in cross section through a plane P 1  defined as a plane passing through the heat pipes  9  at right angles to the direction D 1 . 
     The cooling fins  44  and the base  42  form the heat sink  41 , advantageously in a single piece. The heat sink  41  can be obtained in different ways, such as moulding or material removal for example. 
     The heat pipes  9  essentially comprise an evaporation zone  92  at a first end, intended to be disposed in proximity to the heat zone, that is to say as close as possible to the outer wall  82  of the stator  8 , a condensation zone  94  at the second end, intended to facilitate the heat exchange with the flow of air  12  and a transition zone  96  between the evaporation zone  92  and the condensation zone  94 . In the example represented, the heat pipes  9  have a cylindrical form around their third direction D 3 . The heat pipes  9  have a length L 2  defined in the third direction D 3 . The length L 2  represents the greatest length of the heat pipe  9  considered. 
     As stated previously, the heat pipes  9  can take any bar or tube form. The accommodating enclosure  442 , which comprises lateral edges  444 , represented in  FIG. 2 b   , is in direct contact with the heat pipe  9  accommodating it. Thus, in order to maximize the contact between the heat pipe  9  and the accommodating enclosure  442  via the lateral edges  444 , the lateral edges  444  take a form similar to the form of the heat pipes  9 . Thus, for heat pipes  9  that have a cylindrical form, the lateral edges  444  take a circular form giving the accommodating enclosure  442  the form of a hollow cylinder capable of accommodating the heat pipe of cylindrical form. For the case of a heat pipe  9  of parallelepipedal form, the accommodating enclosure  442  that relates to it takes the appropriate form allowing the accommodation of this parallelepipedal heat pipe  9  while maximizing the direct contact. Thus, each heat pipe  9  is inserted into the accommodating enclosure  442  that relates to it. 
     On the left-hand part of  FIG. 2 b   , the heat pipe  9  has a length L 2  less than the length L of the corresponding cooling fin  44 . In other words, the heat pipe  9  is embedded in the cooling fin  44 . That makes it possible to prevent potential contact between the heat pipe  9  and outside elements carried by the flow of air  12  which could damage, or even destroy, the condensation zone  94 . The cooling fin  44  then becomes a protective carapace of the heat pipe  9  inserted into the corresponding cooling fin  44 . 
     Alternatively, as represented on the right-hand part of  FIG. 2 b   , the heat pipe  9  can have a length L 2  equal to or greater than the length L of the corresponding cooling fin  44 . In other words, the condensation zone  94  of the heat pipes  9  extends beyond the top end  446  of the corresponding cooling fin  44 . The condensation zone  94  is then in immediate contact with the flow of air  12 , which enhances the heat exchange with the flow of air  12 . 
     Nevertheless, the heat pipes  9  of tubular form, that have a length L 2  greater than the length L of the corresponding cooling fin  44  and that extend in the third direction D 3 , are and remain embedded in the corresponding cooling fin  44  over a greater proportion of their length L 2  in the third direction D 3  of each heat pipe  9 . More specifically, the length L of the cooling fin  44  is at least equal to half the length L 2  of the accommodated heat pipe  9 . That way, the rigid structure of the cooling fins  44  ensures protection of the heat pipes  9  which are fragile exchangers. 
     Furthermore, in order to avoid having air imprisoned between the heat pipes  9 , the accommodating enclosures  442  of the cooling fins  44  and the outer wall  82  of the stator  8 , which can be a brake to the heat exchanges, a thermal grease  14  is applied around the contact zones, that is to say around the point of contact  98  between the heat pipes  9  and the outer wall  82  of the stator  8  and on lateral edges  444  of the accommodating enclosures  442 , an edge in direct contact with the lateral walls  99  of the heat pipes  9 . The thermal grease  14  thus allows the effect of galvanic corrosion between the heat pipes  9 , the accommodating enclosures  442  of the cooling fins  44  and the outer wall  82  of the stator  8  to be limited. 
     Indeed, since the heat pipes  9 , the heat sink  41  and the stator  8  are not necessarily obtained from the same material, it is possible for a corrosion phenomenon to occur, which would be damaging to the correct operation of the cooling device  4 . The presence of the thermal grease  14  thus allows a free choice of the materials in contact, notably for the heat pipe  9  and the cooling fin  44  which accommodates it.
           FIG. 3 a    represents a second embodiment of the cooling device  4  in which several cooling fins  44  are aligned on the base  42  of the heat sink  41  in the direction D 1  of the flow of air  12 . The second embodiment allows the heat exchange zone between the heat sink  41  and the flow of air  12  to be increased. This second embodiment is described more particularly in  FIG. 3   b.        FIG. 3 b    is a view focused on the accommodating enclosure  442 , according to the second embodiment, intended to accommodate a heat pipe  9 . In  FIG. 3 b   , a characteristic dimension of the section of the heat pipe  9 , that is to say a diameter D of the heat pipe  9 , in the case of heat pipes of tubular form, is greater than a thickness E of the corresponding cooling fin  44  defined outside of the section P 1  of the cooling fin. Thus, the accommodating enclosures  442  form a protuberance  445  of the cooling fins  44  in a direction substantially at right angles to the direction D 1  of the flow of air  12 . This protuberance  445  allows the flow of air  12  to be locally modified and thus local turbulences  122  to be generated around the protuberances  445  which significantly increase the heat exchanges between the cooling fins  44  and the flow of air  12  running along them. Alternatively, as represented in  FIG. 1 , the diameter D of the heat pipes is less than any thickness of the cooling fins  44 . That makes it possible to prevent any pressure drop in the circulation of the flow along the cooling fin  44 .       

     Furthermore, each accommodating enclosure  442  of a cooling fin  44  is spaced apart from another adjacent accommodating enclosure  442  on this same cooling fin  44  by a length at least equal to the characteristic dimension of the section of the tube of the heat pipe  9 , namely the diameter. Preferentially, each accommodating enclosure  442  of a cooling fin  44  is spaced apart from another adjacent accommodating enclosure  442  on this same cooling fin  44  by a length greater than the characteristic dimension of the section of the tube of the heat pipe  9 . Thus, each heat pipe  9  is distant in the first direction D 1  from another heat pipe  9  by a length equal to or greater than the characteristic dimension of the section of the tube of the heat pipe  9 . This spaced-apart disposition of the heat pipes  9  in the cooling fins  44  allows the heat exchanges to be augmented between the cooling fins  44  and the flow of air  12  (represented in  FIG. 1 ) sweeping it while guaranteeing a good rigidity of the supporting structure, namely the cooling fin  44 . Indeed, since the heat pipes are fragile exchangers, the insertion of a large number of heat pipes  9  in the cooling fins  44  can be detrimental to the good rigidity of the cooling fins  44  and make them fragile, or even subject to possible breakages. Furthermore, this spaced-apart disposition also allows the number of heat pipes  9  inserted into the cooling fins  44  to be limited, thus limiting the cost associated with the use of these heat exchangers. 
     Another advantage to this spaced-apart disposition of the heat pipes  9  in the cooling fins  44  is the adaptability of the cooling device to accommodate a plurality of different heat pipes already available on the market and to not be obliged to adapt a new type of specific heat pipe to the cooling fins  44 . 
     Furthermore, it is also possible to envisage directly modifying tubular heat sinks having fins used in the market and directly introducing heat pipes  9  therein by drilling the fins, for example, in order to obtain the accommodating enclosures  442 . In that way, it is possible to limit the cost of production of the cooling device  4  by “recycling” other tubular heat sinks intended to cool the same components. 
       FIG. 3 c    represents a variant of this second embodiment of the cooling device  4  in which the cooling fins  44  are placed staggered facing the cooling fins  44  which precede them with respect to the flow of air  12 . That means that, for two cooling fins  44   a  and  44   c  of a row of fins  54  extending along abscissae  541   a  that are identical in the direction D 1  and that each have a distinct ordinate along an axis A 1  at right angles to the direction D 2  of the plane P 1 , respectively  641   a  and  641   c , there is a cooling fin  44   b  of a row of fins  55  characterized by an abscissa  541   b  and by an ordinate  641   b  along the axis A 1  of the plane P 1  which lies between the ordinates  641   a  and  641   c  of the cooling fins  44   a  and  44   c  of the row of fins  54 . 
     This staggered disposition allows the flow of air  12  to be deflected for each row of fins  54 ,  55  that the flow of air  12  encounters. In this way, end turbulences  124  are formed, increasing the heat exchanges between the cooling fins  44  and the flow of air  12 . 
     Furthermore, in  FIG. 3 c   , the cooling fins  44   a  and  44   c  comprise accommodating enclosures  442   a ,  442   b  for the cooling fin  44   a  and accommodating enclosures  442   g ,  442   h  for the cooling fin  44   c . Each pair of accommodating enclosures ( 442   a ,  442   g ) and ( 442   b ,  442   h ) is defined by an abscissa  551   a  for the pair of accommodating enclosures ( 442   a ,  442   g ) and by an abscissa  551   b  for the pair of accommodating enclosures ( 442   b ,  442   h ). Thus, for cooling fins  44   a  and  44   c  extending along one and the same abscissa  541   a  in the direction D 1  and along ordinates  641   a  and  641   c  that are different along the axis A 1 , the heat pipes  9  inserted into the accommodating enclosures  442   a  and  442   g  have the same abscissa  551   a  in the direction D 1 . Likewise, the heat pipes  9  inserted into the accommodating enclosures  442   b  and  442   h  have the same abscissa  551   b  in the direction D 1 . 
       FIGS. 4 a  and 4 b    represent a third embodiment of the cooling device  4 . 
     According to this embodiment, the accommodating enclosures  442  of a cooling fin  44  are placed staggered with the accommodating enclosures  442  of the adjacent cooling fins  44  of one and the same row of fins  54 . 
     “Staggered” can be understood to mean that, for the cooling fin  44   a  of the row of fins  54 , the accommodating enclosures  442   a ,  442   b  and  442   c , which are characterized by an abscissa  542   a ,  542   b ,  542   c  in the direction D 1  and by an ordinate  642   a  on the axis A 1  of the plane P 1  passing through the cooling fin  44   a  at right angles to the direction D 1 , have coordinates in terms of abscissa in the direction D 1  and of ordinate on the axis A 1  that differ from accommodating enclosures  442   g ,  442   h ,  442   i  of a cooling fin  44   c  adjacent to the cooling fin  44   a  in the row of fins  54 . 
     Likewise, the accommodating enclosures  442   g ,  442   h ,  442   i  of the cooling fin  44   c , which are characterized by abscissae  542   g ,  542   h ,  542   i  in the direction D 1  and by ordinates  642   c  on the axis A 1 , have coordinates in terms of abscissa in the direction D 1  and of ordinate on the axis A 1  that differ from accommodating enclosures  442   j ,  442   k ,  442   m  of a cooling fin  44   d  adjacent to the cooling fin  44   c  in the row of fins  54 . 
     This staggered disposition of the accommodating enclosures  442   a ,  442   b ,  442   c ,  442   g ,  442   h ,  442   i  of the cooling fins  44   a ,  44   c  allows the flow of air  12  running along the cooling fins  44   a ,  44   c  to be regulated by making the speed of the flow of air  12  uniform during the phase of heat exchange between the cooling fins  44  and the flow of air  12 . 
     Furthermore, in all the embodiments, the cooling fins  44  can be provided with reduced sections  46 . These reduced sections  46  can take the form of a cone, of a parabola of any eccentricity or any other form promoting the blowing of the flow of air  12  along the cooling fins  44 .