Abstract:
An electrical machine, especially permanent magnet machine, is comprised of a stator and a rotor rotatable relative to the stator. The rotor and stator are separated from each other by an air gap. A boundary layer control maintains a desired boundary layer thickness in the air gap. The boundary layer control maintains optimal cooling, which minimizes the electrical machine&#39;s overall dimensions while maximizing its power density.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This disclosure relates to an air-cooled electrical machine, especially a permanent magnet machine that utilizes boundary layer control to improve cooling and increase power density. 
         [0002]    Advanced power applications, like for example, miniature turbine generators, dental hand-pieces, precision tools, ultra high speed motors, etc., require high speed electrical machines with rotors that must be capable of operating at very high speeds, i.e. speeds in excess of 250,000 rpm, while also maintaining structural integrity. Concerns with this type of application include the extreme centrifugal forces, which cause large mechanical stresses in the rotors, as well as potentially insufficient cooling for both the rotor and stator. 
         [0003]    A typical permanent magnet rotor uses a metal/composite laminated retaining sleeve which allows the high-speed rotor to be positioned at a very small distance, i.e. a small air gap, from an inner wall of the associated stator. It is desirable that this air gap be minimal to avoid eddy current losses in the conductive sleeve; however, from a thermodynamic perspective it is desirable that this air gap be larger to provide a better heat transfer coefficient between the rotor and stator. Thus, historically these two concepts have been at odds. 
       SUMMARY OF THE INVENTION 
       [0004]    In one exemplary embodiment, an electrical machine comprises a stator and a rotor rotatable relative to the stator about an axis. The rotor and stator are separated by an air gap. A boundary layer control maintains a desired boundary layer thickness in the air gap. 
         [0005]    In a further embodiment of the above, the boundary layer control comprises a suction feature. 
         [0006]    In a further embodiment of any of the above, the suction feature comprises a plurality of suction holes formed within an inner surface of the stator. 
         [0007]    In a further embodiment of any of the above, the stator comprises a cylinder having an outer surface spaced radially outwardly from the inner surface, and the plurality of suction holes extend through a thickness of the stator from the inner surface to the outer surface. 
         [0008]    In a further embodiment of any of the above, the plurality of suction holes are spaced circumferentially about the inner surface of the stator. 
         [0009]    In a further embodiment of any of the above, the plurality of suction holes are spaced axially apart from each other along a length of the stator extending along the axis. 
         [0010]    In a further embodiment of any of the above, the plurality of suction holes are spaced circumferentially about the inner surface of the stator, and wherein the plurality of suction holes are spaced axially apart from each other along a length of the stator extending along the axis. 
         [0011]    In a further embodiment of any of the above, the air gap has a radial thickness that is greater than zero and less than 1.50 mm (0.06 inches). 
         [0012]    These and other features of this application will be best understood from the following specification and drawings, the following of which is a brief description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  schematically illustrates an electrical machine with boundary layer control. 
           [0014]      FIG. 2A  is a cross-sectional side view of a boundary layer thickness in an electrical machine without boundary layer control. 
           [0015]      FIG. 2B  is a cross-sectional side view of a boundary layer thickness in an electrical machine with boundary layer control. 
           [0016]      FIG. 3  is a schematic illustration of one example application for the electrical machine of  FIG. 2B . 
       
    
    
     DETAILED DESCRIPTION 
       [0017]      FIG. 1  schematically illustrates an electrical machine  10 , such as a permanent magnet machine for example, that includes a rotor  12  and a stator  14 . Rotor shaft ends  16  are supported by bearings  18  such that the rotor  12  rotates about an axis A relative to the stator  14 . One or more magnets  20  and a retaining sleeve  22  are mounted for rotation with the rotor  12 . How the permanent magnet machine  10  operates to generate driving power is known and will not be discussed in detail. 
         [0018]    In one example, the stator  14  comprises a cylindrical body having an inner peripheral surface  24  and an outer peripheral surface  26  spaced radially outwardly of the inner peripheral surface. An outer surface  28  of the sleeve  22  is radially spaced from the inner peripheral surface  24  of the stator  14  by an air gap  30 . 
         [0019]    A boundary layer control  32  is used to maintain a desired boundary layer thickness in the air gap  30 . Boundary layer generally refers to a layer of reduced velocity in fluids, such as air for example, that is immediately adjacent to a surface of a solid past which the fluid is flowing. In one example, the boundary layer control  32  comprises a suction feature. In one example, the suction feature comprises a plurality of holes  34  that are formed in the stator  14 . The holes  34  extend through a thickness of the stator  14  from the inner peripheral surface  24  to the outer peripheral surface  26 . The holes  34  are circumferentially spaced about the inner peripheral surface  24  and extend along a length of the stator  14 . 
         [0020]      FIG. 2A  shows an example that does not utilize boundary layer control. The rotor  12  and stator  14  are separated by a first gap t 1  Specifically between the outer surface  28  of the sleeve  22 , and the peripheral surface  24  of the stator  14 , respectively. The boundary layer B is defined by a corresponding thickness d 1  that sufficiently fills the gap, t 1 , such that d 1 =t 1 .  FIG. 2B  shows an example that uses boundary layer control via suction. In this example, the boundary layer, B, is defined by a thickness d 2  that is significantly less than d 1 . This, in turn, allows the air gap, t 2 , between the stator  14  and rotor  12  to be significantly reduced to a gap thickness t 2  which is significantly less than t 1 . 
         [0021]    The subject electrical machine is capable of operating at very high speeds, i.e. in excess of 250,000 rpm, and at very high temperatures, i.e. in excess of 290 degrees Celsius (554 degrees Fahrenheit). Using boundary layer control vastly improves cooling and allows the air gap to be minimized to increase power density. 
         [0022]    In one example, the working gas is air or nitrogen (N 2 ); however, other gases could also be used. High pressure gas is pumped into the stator  14  in a direction along the axis A (as indicated by arrows  40 ) of the rotor  12  and is then discharged through a check valve  44  to the ambient as shown in  FIG. 1 . The inlet gas pressure is higher than the outlet gas pressure and the resulting pressure differential provides the coolant flow to the stator  14 . The holes  34  provide for suction through which exits the stator  14  is indicated by arrows  42 . 
         [0023]    The type of fluid flow&#39;s regime is identified by a Reynolds number. The Reynolds number, Re, is a dimensionless number, ρV1/μ, where V is the fluid velocity, ρ is the density, μ is the viscosity, and 1 is a characteristic dimension of the system. The value of Re indicates the regime of the fluid flow. When a certain Re is exceeded, instable flow can be generated. For example, a configuration such as shown in  FIG. 1 , i.e. the viscous flow between two concentric cylinders, of which the inner cylinder is in motion and the outer cylinder is at rest (i.e., Couette flow), demonstrates an example of a typical unstable flow stratification caused by centrifugal forces. When such flow instabilities occur (due to reaching a certain critical Re), certain toroidal flow vortices, known as Taylor vortices, can appear whose axes are located along the circumference of the inner cylinder and which rotate in alternately opposite directions. Instability in the flow is not desirable as it adversely affects the operating efficiency of the machine. 
         [0024]    The condition for the onset of instability is given by the Taylor number, Ta, which is: 
         [0000]        Ta =( U   i   /d )/ν*( d/R   i )&gt;41.3
 
         [0025]    Where: d=the radial width of the gap; R i  is the inner radius of the inner cylinder, i.e. the rotor; U i  is the peripheral velocity of the inner cylinder; and ν is the kinematic viscosity (ν=μ/ρ, which is the ratio of the viscosity μ to the density ρ). There are three defined Taylor number, Ta, ranges of flow between cylinders: 
         [0026]    Ta&lt;41.3 (laminar Couette flow) 
         [0027]    41.3&lt;Ta&lt;400 (laminar flow with Taylor vortices) 
         [0028]    Ta&gt;400 (turbulent flow) 
         [0029]    For high speed applications, such as those contemplated here, it is expected that the predominant flow will include high levels of turbulence, i.e. Ta&gt;400. Under such extreme conditions, the velocity gradient in the narrow air gap is very high resulting in a wide variety of shear stresses. However, it is desirable to minimize the boundary layer thickness in the annulus (i.e., the “air gap”  30 ) between the rotor and the stator, thus suppressing the formation of the undesirable Taylor vortices. As shown, the condition for the onset of this instability is given by keeping Ta&lt;41.3 (i.e. laminar Couette flow regime). This type of flow minimizes the torque coefficient between the two cylinders resulting in lower “pumping” losses. 
         [0030]    Suction is used in order to achieve this boundary layer control. The effect of suction is the removal of the slowest (decelerated) fluid particles from the boundary layer before they can cause a separation leading to turbulence and inefficient heat transfer. By applying suction (see  FIG. 2B ) at discrete locations along the inner peripheral surface  24  of the stator  14 , a new, i.e. thinner boundary layer is formed within the air gap  30 , which is capable of overcoming the adverse pressure gradient that forms behind the suction openings. This leads to a decrease in the pressure drag, which is reduced due to the absence of flow separation. 
         [0031]    Controlling the boundary layer thickness in this manner provides a sufficient amount of cooling between the rotor and stator. Further, the thinner boundary layer minimizes the parasitic loss of “windage effects,” while also allowing for a smaller air gap thickness, which is critical in increasing the power density of the machine  10 . In one example, the subject boundary layer control allows the gap  30  to be reduced within a range that is greater than 0 and less than 1.50 mm (0.06 inches). 
         [0032]    It should be understood that using suction to control boundary layer thickness is just one method of control and that other methods and apparatus can be used to control the boundary layer thickness. For example, injecting different types of gases into the air gap or generating acceleration through the air gap can also be used to control boundary layer thickness as needed. 
         [0033]    Using boundary layer control within the permanent magnet machine or any other cylindrical-rotor electric machine results in a more compact machine size, a high mechanical reliability, an effective heat transfer, and intensive cooling capability. The configuration uses very few moving parts and is durable in adverse ambient conditions. Further, the electrical machine is capable of operating at high speeds and there are no thermal limitations due to active fuel and/or air cooling (see  FIG. 3 ). 
         [0034]      FIG. 3  is a schematic illustration of one example application for the electrical machine  10 . Active monitoring and control are needed to avoid excessively high air pressures to minimize pneumatic instabilities. Such a control can be utilized for an aircraft environmental control system  60 . The system  60  includes an electronic engine control (EEC)  62 , which is part of the on-board Full Authority Digital Engine Control (FADEC) system that monitors engine pressure ratio and shaft (spool) speeds. Aircraft gas turbine engines can include two or more shafts (spools), which connect fan, compressor, and turbine components as known. 
         [0035]    In one example of a twin-spool gas turbine engine, a low speed shaft  64  interconnects a fan  66 , a low pressure compressor  68 , and a low pressure turbine  70 , while a high speed shaft  72  interconnects a high pressure compressor  74  and a high pressure turbine  76 . As known, airflow is compressed by the low pressure compressor  68  then the high pressure compressor  74 , mixed and burned with fuel in a combustor, then expanded over the high pressure turbine  76  and low pressure turbine  70 . The turbines  70 ,  76  rotationally drive the respective low speed shaft  64  and high speed shaft  72  in response to the expansion of the hot products of the combustion process. 
         [0036]    Since the electronic engine control  62  normally monitors the speeds of the shafts  64 ,  72 , it is an ideal application for an active control implementation for the engine based controls. High pressure air can be supplied as a byproduct gas from an on-board air separation module that supplies a nitrogen enriched air stream to an on-board nitrogen generating system (NGS). The air separation module (ASM) is part of an aircraft fuel inerting system where the nitrogen enriched air steam is an airflow product that results after nitrogen has been separated from the ambient air and pumped into an aircraft fuel tank  78 . This provides a safe inerting environment with displaced volatile fuel vapors. 
         [0037]    In order to prevent compressed air from reaching elevated working temperatures, a fuel-cooling loop is used to circulate outside of the rotor. Cold fuel from the tank  78  provides an effective heat sink medium for dissipating compressed air heat through convective/conductive heat transfer. Heated fuel can then be utilized for burning directly in the combustor to provide better fuel atomization, mixing, and burning as the fuel is pre-heated. Alternatively, if not needed, the pre-heated fuel can be returned to the tank  78  and mixed with the resident colder fuel. If this increases fuel temperature above a desired level (typically limited by the coking-resistance properties of the fuel), an efficient air-to-fuel heat exchanger  80  can be used to cool down the fuel using inlet ambient cold ram air as the heat sink. Shown in  FIG. 3  is a counter-flow heat exchanger ( 80 ), but any other highly-efficient, compact, and light-weight heat exchanger can be used. The resulting heated air can be discharged overboard as shown in  FIG. 3 . 
         [0038]    As shown in  FIG. 3 , the electrical machine  10  is controlled by the electronic engine control  62  and is coupled to a power source  82  and is grounded at  84 . The machine  10  drives shaft  86 , which is supported by a pair of bearings  88 . Cooling flow is circulated to the bearings  88  from the tank  78  along path  90  and is returned to the tank  78  along path  92 . Compressed nitrogen  94  (either pumped directly from the ASM and/or from an on-board N 2  storage tank) is also circulated through the bearings  88  for cooling purposes and is vented via check valves  96 . 
         [0039]    Although an embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.