Patent Publication Number: US-2023132708-A1

Title: Airflow guiding device for the sound- and pressure-optimized supply of an airflow to an inlet nozzle of a fan, fan device, generator, and wind power installation

Description:
BACKGROUND 
     Technical Field 
     The invention relates to an airflow guiding device for the sound- and pressure-optimized supply of an airflow to an inlet nozzle of a fan. The invention furthermore relates to a fan, a generator, and a wind power installation. 
     Description of the Related Art 
     Wind power installations emit sound during operation. A reason for this, for example, is that there are vortices of the wind at the rotor blades of the wind power installations. In particular, units within wind power installations also constitute substantial sources of noise during operation. A substantial source of noise includes, for example, fans which generate an airflow for example for cooling an (electric) generator of wind power installations. The conveying and deflecting of the airflow within the generator and in particular at the fans regularly gives rise to vortices which emit sound. 
     People and animals perceive the sound as a noise with a certain volume in a certain frequency range. The volume is characterized by the sound pressure level. In order to protect people and animals, wind power installations may not exceed a certain volume (limit value) during operation because of legal stipulations. The permissible limit value which a wind power installation may not exceed customarily depends on the location of the wind power installation, at which the latter is operated. For example, the limit value is generally lower in the vicinity of localities than away from localities. 
     In order to prevent a wind power installation from exceeding the permissible limit value during operation, fans can be operated at a reduced rotational speed. However, this also reduces the cooling power of the fans, and therefore the wind power installation frequently also has to be operated at a reduced rotational speed in order to avoid overheating the generator, in particular the stator and/or the rotor. 
     In order to improve the efficiency of wind power installations, the hub height and the diameter of the aerodynamic rotor of wind power installations is being increased ever further. However, with the increasing height of the wind power installations, the sound emission thereof generally also increases. A reason for this is not only the higher sound emission at the rotor blades, but also the greater need for cooling power for cooling the generator. Relatively large or a greater number of fans which can provide an airflow with the required cooling power are therefore required. The sound emission of wind power installations can thereby also increase. The starting point is therefore that the previously outlined problem will become more intense in the future, in particular also in view of the average temperature increasing on average. 
     BRIEF SUMMARY 
     One embodiment relates to an airflow guiding device for the sound- and pressure-optimized supply of an airflow to an inlet nozzle of a fan, in particular a radial fan, wherein the inlet nozzle has a nozzle input opening with an incident flow angle, the airflow guiding device having: a frame device which extends along a longitudinal axis in an axial direction with a height between an inlet opening, through which the airflow flows into the frame device during operation of the airflow guiding device, and an outlet opening, through which the airflow flows out of the frame device during operation of the airflow guiding device, wherein the inlet opening has an inlet cross section with a minimum inlet cross-sectional width, and the outlet opening has an outlet cross section with a minimum outlet cross-sectional width which is smaller than the minimum inlet cross-sectional width, and the frame device comprises a flow duct which connects the inlet opening and the outlet opening in terms of flow, wherein the outlet opening is designed to be connected in terms of flow to the inlet nozzle of the fan in the operating state. 
     Provided are an airflow guiding device, a fan, in particular a fan of a wind power installation, a generator, in particular a generator for a wind power installation, and a wind power installation, which reduce or eliminate one or more of the disadvantages mentioned of existing solutions. 
     The airflow guiding device is configured for the sound-optimized and pressure-optimized supply of an airflow to an inlet nozzle of a fan, in particular a radial fan, wherein the inlet nozzle has a nozzle input opening with an incident flow angle. 
     Fans, in particular radial fans, are flow machines which convey air or other gases, for example. Such fans conventionally have a fan housing in which a drivable impeller is mounted rotatably. For example, the impeller can be driven with an electric motor. The driven, i.e., rotating, impeller generates an airflow which it sucks up through an input opening of the fan housing (suction side) and conveys same out through an output opening of the fan housing (delivery side). The input opening of the fan housing is conventionally configured as an inlet nozzle with the nozzle input opening. 
     The inlet nozzle is preferably circular. It can also be preferred for the inlet nozzle to be polygonal, for example rectangular or square. The inlet nozzle is characterized by the shape and size of the opening cross section and the incident flow angle. In particular, the incident flow angle is the incident flow angle at which the air is sucked into the fan at the outer edge of the inlet nozzle. The manufacturers of the fans conventionally predetermine the incident flow angle by the design of the inlet nozzle. The incident flow angle is therefore in particular dependent on the type of fan. The incident flow angle describes, in a preferred manner, the angle at which the air flows into the inlet nozzle. In particular, the incident flow angle is that angle at which the air at the edge of the inlet nozzle flows into the inlet nozzle through the nozzle input opening. The incident flow angle is in particular that angle by which the air flowing into the inlet nozzle at the edge of the inlet nozzle, in particular the nozzle input opening, is inclined with respect to a surface normal which extends orthogonally to the nozzle input opening. In particular, the incident flow angle is that angle by which the air flowing into the inlet nozzle at the edge of the inlet nozzle, in particular the nozzle input opening, is inclined with respect to an axis of rotation of the impeller of the fan, the axis of rotation extending orthogonally to the nozzle input opening. 
     The airflow guiding device has a frame device which extends along a longitudinal axis with a height in an axial direction between an inlet opening and an outlet opening. The frame device is preferably formed from profiled frame elements. The profiled frame elements comprise, for example, a U-shaped cross section and/or an I-shaped cross section and/or a T-shaped cross section. The frame device is preferably formed from metal, in particular steel or aluminum, and/or plastic. During operation of the airflow guiding device, the airflow flows through the inlet opening into the frame device and flows out of the frame device through the outlet opening. The outlet opening is connected in terms of flow to the inlet nozzle of the fan during operation of the airflow guiding device. 
     The inlet opening has an inlet cross section with a minimum inlet cross-sectional width, and the outlet opening has an outlet cross section with a minimum outlet cross-sectional width. The minimum outlet cross-sectional width is smaller than the minimum inlet cross-sectional width. The minimum outlet cross-sectional width is in particular a minimum outlet cross-sectional diameter. The minimum inlet cross-sectional width is in particular a minimum inlet cross-sectional diameter. 
     Furthermore, the frame device comprises a flow duct which connects the inlet opening and the outlet opening in terms of flow. The flow duct is preferably tubular. In particular, the flow duct is cylindrical. It can alternatively be preferred that the flow duct has a polygonal, in particular square or rectangular, cross section. 
     The height h of the frame device according to the following formula: 
     
       
         
           
             h 
             ≥ 
             
               
                 D 
                 - 
                 d 
               
               
                 2 
                 · 
                 
                   tan 
                   ⁡ 
                   ( 
                   β 
                   ) 
                 
               
             
           
         
       
     
     corresponds to a ratio of a difference of the minimum inlet cross-sectional width D and a diameter of the inlet nozzle, in particular a nozzle input opening of the inlet nozzle, d and double the value of the tangent of the incident flow angle β or is greater than the ratio. 
     Furthermore, the airflow guiding device has a grid element which is arranged on the inlet opening. The grid element is in particular a mesh grid. In particular, the grid element is composed of steel. The grid element preferably has a mesh width of at least 0.5 mm, 1 mm, 2 mm, 5 mm, 10 mm or more. Additionally or alternatively, the grid element preferably has a mesh width of at maximum 0.5 mm, 1 mm, 2 mm, 5 mm or 10 mm. In particular, it is preferred that the grid element is composed of a wire mesh or comprises the latter. The wire of the wire mesh preferably has a wire diameter of at least 0.2 mm, 0.5 mm, 1 mm, 1.5 mm, 2 mm or more. Additionally or alternatively, the wire of the wire mesh has a wire diameter of at maximum 0.2 mm, 0.5 mm, 1 mm, 1.5 mm or 2 mm. 
     The grid element is preferably fastened to the frame device on the outside. The grid element is preferably fastened to the frame device by means of a screw connection and/or clamping connection. Additionally or alternatively, it may be preferred for the grid element to be fastened in an integrally bonded manner, for example with a welded connection, to the frame device. 
     The arrangement of the grid element on the inlet opening prevents relatively coarse dirt from being sucked into the fan and conveyed into the generator. Furthermore, the grid element prevents relatively large objects, for example a tool, or individuals, from entering the fan. The grid element is also designed to this extent as an engagement guard. The grid element therefore firstly brings about a longer service life and secondly prevents injuries to individuals. 
     It goes without saying that, depending on the height of the frame device, the incident flow area through which air flows into the airflow guiding device through the grid element at the inlet opening varies. The height of the frame device is preferably selected in such a manner that the incident flow area corresponds to at least 50% of the overall area of the inlet opening. In particular, the height of the frame device is selected in such a manner that the incident flow area corresponds to at least 75% of the overall area of the inlet opening. Particularly preferably, the height of the frame device is selected in such a manner that the incident flow area corresponds to at least 100% of the overall area of the inlet opening. 
     The invention is based on the finding by the inventors that a small distance between a grid element and the inlet nozzle leads to a comparatively high loss of pressure since the area of the grid element through which the flow passes is comparatively small. The arrangement of a frame device with the height according to the invention leads to the area of the grid element through which the flow passes becoming substantially larger because of the incident flow angle and the loss of pressure becoming correspondingly smaller. The cooling power of a fan can therefore be considerably increased by the arrangement of the airflow guiding device with the fan power remaining the same. 
     Fans can therefore be operated at a higher rotational speed and can realize a higher cooling power without exceeding the legal limit values. 
     By means of the arrangement of such an airflow guiding device, the cooling power can therefore be considerably improved such that complicated and costly re-equipment and/or retrofitting of existing wind power installations can be dispensed with. 
     According to a preferred embodiment of the airflow guiding device, it is provided that the inlet cross section is circular or polygonal, in particular square. Additionally or alternatively, it is provided that the outlet cross section is circular, in particular tubular, or polygonal, in particular square. The outlet cross section preferably corresponds to the inlet cross section. Particularly preferably, the minimum outlet cross-sectional width of the outlet cross section is greater than the diameter of the cross section of a nozzle input opening of a fan of the inlet nozzle. 
     This has the advantage, in particular if the minimum outlet cross-sectional width of the outlet cross section is greater than the diameter of the cross section of the nozzle input opening of the inlet nozzle, that the air sucked up by the fan is not swirled. Furthermore, this has the effect that the air can be supplied with a sound reduction to the fan. 
     According to a further preferred development of the airflow guiding device, it is provided that the frame device has a tubular outer wall and/or a tubular inner wall and/or the frame device is double-walled. 
     This has the advantage that the frame device can be adapted to the cross section of the inlet nozzle of the fan. This minimizes vortices and reduces the emitted sound. 
     Furthermore, it is provided, according to a preferred development of the airflow guiding device, that the tubular outer wall is annular, wherein preferably the tubular outer wall has a circular or a polygonal, in particular square, cross section. Additionally or alternatively, it is provided that the tubular inner wall is annular, wherein preferably the tubular inner wall has a circular or a polygonal, in particular square, cross section. 
     Furthermore, it is provided, according to a preferred development of the airflow guiding device, that the inner wall is arranged spaced apart from the outer wall and forms a cavity, in particular an annular cavity, with a cavity volume. 
     According to a further preferred development of the airflow guiding device, it is provided that the inner wall has at least one through duct which connects the flow duct in terms of flow to the cavity, wherein the inner wall is preferably configured as a perforated sheet wall, wherein particularly preferably the inner wall has a plurality of through ducts which are arranged spaced apart equidistantly from one another. 
     According to a further preferred development of the airflow guiding device, it is provided that the at least one through duct defines a free inner wall opening cross-sectional area of the inner wall, wherein at least one third, at least a half or at least two thirds of the inner wall is/are formed by the free inner wall opening cross-sectional area. The free inner wall opening cross-sectional area is dependent in particular on the number and the size of the opening cross-sectional area of the at least one through duct. 
     The inner wall opening cross-sectional area can be realized, for example, by an inner wall which is in the form of a perforated sheet or perforated plate in order to obtain resonator openings distributed as homogeneously as possible over the area. The inner wall preferably has a wall thickness of at least 0.5 cm, 1 cm, 1.5 cm, 2 cm or more. 
     Furthermore, in a preferred embodiment of the airflow guiding device, it is provided that a sound absorption element is arranged between the inner wall and the outer wall, in particular in the cavity. The sound absorption element is in particular a closed-cell polyethylene foam. Furthermore, the airflow guiding device has the advantage that the emitted sound is considerably minimized. 
     Furthermore, according to a preferred embodiment, it is provided that the airflow guiding device has a flow rectifier which is arranged on the outlet opening, in particular is arranged upstream of the outlet opening in the flow direction of the airflow. The flow rectifier minimizes vortices in a preferred manner and thereby reduces the emitted sound. Additionally or alternatively, it is provided in this embodiment that a grid element is arranged upstream of the inlet opening in the flow direction of the airflow. It is preferably provided that the grid element covers the inlet cross section of the inlet opening. Additionally or alternatively, it is preferably provided that the grid element has a plurality of inlet ducts which connect the surroundings of the airflow guiding device in terms of flow to the flow duct. Furthermore, additionally or alternatively, it is preferably provided that the at least one inlet duct defines a free inlet opening cross-sectional area of the grid element, wherein at least one third or at least a half or at least two thirds of the grid element is/are formed by the free inlet opening cross-sectional area. 
     According to a further preferred development of the airflow guiding device, it is provided that the airflow guiding device is configured as a Helmholtz absorber, wherein the cavity is configured as a resonance body. It is preferably provided in this embodiment that the cavity is closed air-tightly in the axial direction in relation to the surroundings and, by means of the outer wall, the cavity is closed air-tightly in the radial direction in relation to the surroundings. Additionally or alternatively, it is preferably provided that the airflow guiding device has a cavity locking unit which is configured to close the cavity air-tightly in the axial direction in relation to the surroundings. 
     According to a further preferred development of the airflow guiding device, it is provided that the free inner wall opening cross-sectional area of the inner wall, a wall thickness of the inner wall, and the cavity volume of the cavity are designed depending on a dominant frequency f of the fan according to the following formula: 
     
       
         
           
             f 
             = 
             
               
                 c 
                 
                   2 
                   ⁢ 
                   π 
                 
               
               ⁢ 
               
                 
                   
                     S 
                     0 
                   
                   
                     
                       V 
                       0 
                     
                     · 
                     L 
                   
                 
               
             
           
         
       
     
     The dominant frequency f is in particular the blade passing frequency BPF which depends on the rotational speed n of the impeller of the fan and on the number of vanes k of the impeller according to the following formula: 
     
       
         
           
             BPF 
             = 
             
               
                 n 
                 60 
               
               * 
               k 
             
           
         
       
     
     Provided is a fan device having a fan, in particular a radial fan, with an inlet nozzle which has a nozzle input opening with an incident flow angle, and an airflow guiding device according to the previously described aspect or according to one of the previously described embodiments, which is arranged on the nozzle input opening of the inlet nozzle. 
     It is preferably provided that the airflow guiding device is arranged upstream of the fan in the flow direction of the airflow. Additionally or alternatively, it is furthermore preferred that the outlet cross section corresponds to a cross section of the nozzle input opening. 
     According to a further aspect, the object mentioned at the beginning is achieved by a generator according to claim  13 . The generator is in particular a generator of a wind power installation. The generator has at least one previously described fan device. 
     According to a further aspect, the object mentioned at the beginning is achieved by a wind power installation according to claim  14 . The wind power installation has a previously described generator and/or a previously described fan device. 
     For further advantages, embodiment variants and embodiment details of the further aspects and of the possible developments thereof, reference is also made to the description above regarding the corresponding features and developments of the airflow guiding device. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       Preferred exemplary embodiments are described by way of example with reference to the attached figures, in which: 
         FIG.  1    shows a schematic three-dimensional view of an exemplary embodiment of a wind power installation; 
         FIG.  2    shows a schematic sectional view in a side view of an exemplary embodiment of a fan device; 
         FIG.  3    shows a schematic top view of the fan device illustrated in  FIG.  2   ; 
         FIG.  4    shows a schematic sectional view in a side view of a further exemplary embodiment of a fan device; and 
         FIG.  5    shows a schematic top view of the fan device illustrated in  FIG.  4   . 
     
    
    
     In the figures, identical or substantially functionally identical elements are provided with the same reference signs. General descriptions relate in general to all of the embodiments, unless differences are explicitly indicated. 
     DETAILED DESCRIPTION 
     The explanation of the invention on the basis of examples with reference to the figures is substantially schematic, and, for the sake of better illustration, the elements which are explained in the respective figure may be exaggerated in it and other elements may be simplified. Thus, for example,  FIG.  1    schematically illustrates a wind power installation  100  as such, and therefore a fan device  5  or the airflow guiding device  1  and a generator cannot be seen in detail. 
       FIG.  1    shows a schematic, three-dimensional view of a wind power installation  100 . The wind power installation  100  has a tower  102  and a nacelle  104  on the tower  102 . The tower  102  can consist here of tower segments which are arranged on one another. An aerodynamic rotor  106  having three rotor blades  108  and a spinner  110  is provided on the nacelle  104 . During the operation of the wind power installation  100 , the aerodynamic rotor  106  is caused to rotate by the wind and thus also rotates an electrodynamic rotor of a generator that is coupled directly or indirectly to the aerodynamic rotor  106 . The generator is arranged in the nacelle  104  and generates electrical energy. The fan device  5 , comprising a fan  3 , and the airflow guiding device  1  enable cooling of the generator with a sound-optimized and pressure-optimized airflow supply. 
       FIGS.  2  and  3    show a preferred exemplary embodiment of a fan device  5  for a generator of a wind power installation  100 .  FIG.  2    shows a schematic sectional view in a side view of the exemplary embodiment of the fan device  5 .  FIG.  3    shows a schematic top view of the fan device illustrated in  FIG.  2   . 
     The fan device  5  has a radial fan  3  and an airflow guiding device  1 . The radial fan  3  has a nozzle input opening  4  with an incident flow angle β. During operation, the radial fan  3  generates an airflow S which flows into the radial fan  3  through the nozzle input opening  4  of the inlet nozzle  2 . The incident flow angle β describes in particular the angle at which the air flows at the edge of the nozzle input opening  4  into the inlet nozzle  2 . For the sound- and pressure-optimized supply of the airflow S to the inlet nozzle  2  of the radial fan  3 , the airflow guiding device  1  is arranged on the nozzle input opening  4  of the inlet nozzle  2  such that the frame device  10  is connected in terms of flow to the radial fan  3 . 
     For the sound- and pressure-optimized supply of the airflow S, the airflow guiding device  1  has a frame device  10  which extends with a height h in the axial direction A along a longitudinal axis L between an inlet opening  11  and an outlet opening  12 . 
     The inlet opening  11  comprises an inlet cross section with a minimum inlet cross-sectional width D and the outlet opening  12  comprises an outlet cross section with a minimum outlet cross-sectional width d. The inlet opening and outlet opening  11 ,  12  are configured in such a manner that the minimum outlet cross-sectional width b corresponds to the minimum inlet cross-sectional width D of the inlet opening  11 . In the preferred embodiment of the airflow guiding device  1 , the inlet cross section and the outlet cross section have a square cross-sectional area. It may be preferred for the inlet cross section and the outlet cross section to have a round, in particular a circular, cross-sectional area. The inlet opening  12  is configured in particular corresponding to the nozzle input opening  4  of the inlet nozzle  2 . A flow rectifier  18  is preferably arranged upstream of the outlet opening  12  in the flow direction S of the airflow S. 
     A flow duct K extends between the inlet opening  11  and the outlet opening  12 , the flow duct connecting the inlet opening  11  and the outlet opening  12  in terms of flow. The flow duct K preferably has a duct cross section which corresponds to the inlet cross section and/or to the outlet cross section. In particular, the duct cross section has a square or round, in particular a circular, cross-sectional area. The flow duct K has a substantially constant duct cross section along the longitudinal axis L in the axial direction A. The flow duct K preferably has a duct width which corresponds to the minimum inlet cross-sectional width D of the inlet opening  11 . It may be preferred for the duct cross section of the flow duct K at the inlet opening  11  to correspond to the inlet cross section with the minimum inlet cross-sectional width D and at the outlet opening  12  to correspond to the outlet cross section with the minimum outlet cross-sectional width b. In particular, it may be preferred that the duct cross section of the flow duct decreases from the inlet opening  11  in the direction of the outlet opening  12 . However, it is provided here that the minimum outlet cross-sectional width b is greater than the diameter of the nozzle input opening  4  of the inlet nozzle  2 . The flow duct K extends substantially with the height h in the axial direction along the longitudinal axis L. 
     The frame device  10  is double-walled and has a tubular outer wall  13  and a tubular inner wall  14 . In the present embodiment, both the outer wall  13  and the inner wall  14  have a square cross section. The inner wall  14  is arranged within the outer wall  13  in a manner spaced apart therefrom. By this means, an annular cavity  15  with a cavity volume V 0  is formed between the outer wall  13  and the inner wall  14 . In the present embodiment, a sound absorption element  17  made from a closed-cell polyethylene foam is arranged in the cavity  15  formed between the inner wall  14  and the outer wall  13 . 
     The inner wall  14  is configured as a perforated sheet wall which has a plurality of through ducts  16  which connect the flow duct K to the cavity  15  in terms of flow. The flow ducts  16  are arranged equidistantly from one another, i.e., the distances between adjacent through ducts  16  are identical. The through ducts  16  define a free inner wall opening cross-sectional area S 0  of the inner wall  14 . In the present case, at least half of the inner wall  14  is formed by the free inner wall opening cross-sectional area S 0 . 
     The airflow guiding device  1  furthermore has a grid element  19 . The grid element  19  is arranged on the inlet opening  11  upstream of the inlet opening  11  in the flow direction S of the airflow S. The grid element  19  has a plurality of inlet ducts  20  which connect the surroundings U of the airflow guiding device  1  in terms of flow to the flow duct K. The inlet ducts  20  preferably form a free inlet opening cross-sectional area S 1  of the grid element  19  such that more than two thirds of the grid element  18  are formed by the free inlet opening cross-sectional area S 1 . 
       FIGS.  4  and  5    show a further preferred exemplary embodiment of a fan device  5  for a generator of a wind power installation  100 .  FIG.  4    shows a schematic sectional view in a side view of the further exemplary embodiment of the fan device  5 .  FIG.  4    shows a schematic top view of the fan device  5  which is illustrated in  FIG.  5   . 
     The exemplary fan device  5  illustrated in  FIGS.  4  and  5    is substantially based on the fan device  5  which is illustrated by way of example in  FIGS.  2  and  3   . The exemplary fan device  5  illustrated in  FIGS.  4  and  5    differs essentially from the fan device  5  illustrated by way of example in  FIGS.  2  and  3    in that no sound absorption element  17  is arranged in the cavity  15 . 
     In this preferred embodiment, the airflow guiding device  1  is configured as a Helmholtz absorber, wherein the cavity  15  is configured as a resonance body. For this purpose, the cavity  15  is closed air-tightly in a radial direction R and in the axial direction A in relation to the surroundings U. Furthermore, a cavity locking unit  21  is provided which closes the cavity  15  air-tightly in the axial direction A on the side of the inlet opening  11  in relation to the surroundings U. On the side of the outlet opening  12 , a base plate which has the outlet opening  12  closes the cavity  15  air-tightly on the side of the outlet opening  12  in the axial direction A in relation to the surroundings U. 
     In the case of the airflow guiding device  1  configured here as a Helmholtz absorber, the free inner wall opening cross-sectional area S 0  and a wall thickness of the inner wall  14  and the cavity volume V 0  of the cavity are designed depending on a dominant frequency f of the fan  3  according to the following formula 
     
       
         
           
             f 
             = 
             
               
                 c 
                 
                   2 
                   ⁢ 
                   π 
                 
               
               ⁢ 
               
                 
                   
                     S 
                     0 
                   
                   
                     
                       V 
                       0 
                     
                     · 
                     L 
                   
                 
               
             
           
         
       
     
     LIST OF DESIGNATIONS 
     
         
         
           
               1  Airflow guiding device 
               2  Inlet nozzle 
               3  Fan 
               4  Nozzle input opening 
               5  Fan device 
               10  Frame device 
               11  Inlet opening 
               12  Outlet opening 
               13  Outer wall 
               14  Inner wall 
               15  Cavity 
               16  Through duct 
               17  Sound absorption element 
               18  Flow rectifier 
               19  Grid element 
               20  Inlet duct 
               21  Cavity locking unit 
               100  Wind power installation 
               102  Tower 
               104  Nacelle 
               106  Rotor 
               108  Rotor blades 
               109  Stator 
               110  Spinner 
             β Incident flow angle 
             A Axial direction 
             b Minimum outlet cross-sectional width 
             D Minimum inlet cross-sectional width 
             d Diameter of the inlet nozzle or of the nozzle input opening 
             h Height of the frame device 
             V 0  Cavity volume 
             K Flow duct 
             L Longitudinal axis 
             R Radial direction 
             S Airflow/flow direction of the airflow 
             S 0  Free inner wall opening cross-sectional area 
             S 1  Free inlet opening cross-sectional area 
             U Surroundings 
           
         
       
    
     The various embodiments described above can be combined to provide further embodiments. These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.