Patent Publication Number: US-6911636-B2

Title: Cooling fan for microwave oven

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a cooling fan for cooling a magnetron and a high voltage transformer disposed in a electric device chamber of a microwave oven, and more particularly, the present invention relates to a cooling fan for a microwave oven, which comprises a mixed flow fan having a specified hub angle, or an axial flow fan having optimized design parameters such as a ratio of an outer diameter of the fan to a width of a electric device chamber defined in a cabinet, a hub ratio, a sweep angle, a pitch angle, a maximum camber rate, etc. 
   2. Description of the Related Art 
   Generally, a microwave oven serves as an electric home appliance which is operated to induce increased molecular motion of water contained in a food item using high-frequency electromagnetic waves, cause molecules of water to vibrate, and generate heat within the food item to thereby cook the food item in a short period of time. 
   A space defined in a cabinet of a microwave oven is divided into a cooking chamber in which a food item is cooked, and a electric device chamber in which various electric devices are disposed. 
     FIG. 1  is a side cross-sectional view illustrating a state wherein a conventional cooling fan is disposed in a electric device chamber of a microwave oven. 
   As shown in  FIG. 1 , in the electric device chamber of the microwave oven, there are disposed up and down a magnetron  12  for radiating high frequency waves into a cooking chamber (not shown) defined in a cabinet  2  and a high voltage transformer  14  for applying a high voltage to the magnetron  12 . A cooling fan  20  is arranged behind the magnetron  12  and high voltage transformer  14  to supply airflow and cool them. The cooling fan  20  is driven by a motor  16 . 
   Conventionally, the cooling fan  20  comprises an axial flow fan which sucks air through air suction holes  2   a  defined in a rear wall of the cabinet  2  and discharges the air in an axial direction. The axial flow fan  20  has a hub  22  which is coupled to an output shaft  16   a  of the motor  16  to be integrally rotated therewith and a plurality of blades  24  which are installed on a circumferential outer surface of the hub  22  to be spaced apart one from another by a predetermined angle. 
   The axial flow fan  20  is arranged behind the magnetron  12  and high voltage transformer  14  which are disposed up and down in the electric device chamber. Concretely speaking, in order to ensure that airflow is evenly distributed over the magnetron  12  and high voltage transformer  14 , the axial flow fan  20  is located between the magnetron  12  and high voltage transformer  14  along a vertical direction. 
   However, the conventional microwave oven constructed as mentioned above suffers from defects in that, since the magnetron  12  and high voltage transformer  14  are disposed up and down in the electric device chamber and the axial flow fan  20  is arranged behind them, a some portion of the airflow discharged at a high velocity from the axial flow fan  20  simply passes through a space which is defined between a lower end of the magnetron  12  and an upper end of the high voltage transformer  14 . As a consequence, a cooling efficiency of the cooling fan  20  cannot but be deteriorated. 
     FIG. 2  is a perspective view independently illustrating the conventional cooling fan  20  of  FIG. 1 , and  FIG. 3  is a partial side view illustrating a distal end of the blade  24  which forms a part of the cooling fan  20  shown in FIG.  1 . 
   In the conventional axial flow fan  20 , a ratio between an outer diameter of the fan  20  and a width of the electric device chamber defined in the cabinet  2 , as measured on a z-axis of  FIG. 1 , is 0.74, a hub ratio between outer diameters of the hub  22  and the axial flow fan  20  is 0.23, a sweep angle is 0˜32°, and a pitch angle is 31˜45°. 
   Here, the pitch angle denotes an angle which is defined between a straight line connecting a leading edge LE to a trailing edge TE of the blade  24  and a line diametrically extending through the hub  22 . Therefore, the pitch angle indicates a degree to which the blade  24  is inclined with respect to a plane perpendicular to the output shaft  16   a  of the motor  16 . 
   Nevertheless, the conventional axial flow fan  20  configured as described above has disadvantages in that, when the axial flow fan  20  is driven by the motor  16  to be rotated at 2856 RPM, a volume flow rate sucked into the cabinet  2  is 1.73 CMM, and under this condition, a noise level reaches 43.1 dB[A], whereby considerable air suction noise is generated during rotation of the blades  24 . 
   Also, when considering the fact that, as shown in  FIG. 3 , each blade  24  has a positive-pressure acting surface  24   b  on which a positive pressure is applied due to air suction and a negative-pressure acting surface  24   c  on which a negative pressure is applied due to air discharge, and a blade tip  24   a  formed between distal ends of the positive-pressure acting surface  24   b  and negative-pressure acting surface  24   c  has a slightly curved cross-section, since a static pressure regaining phenomenon quickly occurs in the air flowing from the positive-pressure acting surface  24   b  over the blade tip  24   a  toward the negative-pressure acting surface  24   c , a noise level is further increased. 
   SUMMARY OF THE INVENTION 
   Accordingly, the present invention has been made in an effort to solve the problems occurring in the related art, and an object of the present invention is to provide a cooling fan for a microwave oven, which is adapted for directing axially sucked airflow to be evenly distributed over a magnetron and a high voltage transformer, thereby improving a cooling efficiency. 
   Another object of the present invention is to provide a cooling fan for a microwave oven, which is configured to have optimized design parameters such as a ratio of an outer diameter of the fan to a width of a electric device chamber defined in a cabinet, a hub ratio, a sweep angle, a pitch angle, a maximum camber rate, etc., thereby suppressing a noise level and increasing a volume flow rate. 
   In order to achieve the first mentioned object, according to one aspect of the present invention, there is provided a cooling fan for a microwave oven comprising: a mixed flow fan having a specified hub angle. 
   Further, in order to achieve the second mentioned object, according to another aspect of the present invention, there is provided a cooling fan for a microwave oven comprising: an axial flow fan having optimized design parameters such as a ratio of an outer diameter of the fan to a width of a electric device chamber defined in a cabinet, in which a magnetron and a high voltage transformer are disposed, a hub ratio, a sweep angle, a pitch angle, a maximum camber rate, etc. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above objects, and other features and advantages of the present invention will become more apparent after a reading of the following detailed description when taken in conjunction with the drawings, in which: 
       FIG. 1  is a side cross-sectional view illustrating a state wherein a conventional cooling fan is disposed in a electric device chamber of a microwave oven; 
       FIG. 2  is a perspective view independently illustrating the conventional cooling fan of  FIG. 1 ; 
       FIG. 3  is a partial side view illustrating a distal end of a blade which forms a part of the cooling fan shown in  FIG. 1 ; 
       FIG. 4  is a side cross-sectional view illustrating a state wherein a cooling fan in accordance with a first embodiment of the present invention is disposed in a electric device chamber of a microwave oven; 
       FIG. 5  is a perspective view independently illustrating the cooling fan according to the first embodiment of the present invention; 
       FIG. 6  is a side cross-sectional view illustrating a state wherein a cooling fan in accordance with a second embodiment of the present invention is disposed in a electric device chamber of a microwave oven; 
       FIG. 7  is a perspective view independently illustrating the cooling fan according to the second embodiment of the present invention; 
       FIG. 8  is a front view of the cooling fan according to the second embodiment of the present invention; 
       FIG. 9  is a side view of the cooling fan according to the second embodiment of the present invention; 
       FIG. 10  is a partial side view illustrating a distal end of a blade which forms a part of the cooling fan according to the second embodiment of the present invention; 
       FIG. 11  is a cross-sectional view illustrating a state wherein the blade of  FIG. 10  is cut in a widthwise direction; 
       FIG. 12  is a graph comparing cooling fans of the present invention and conventional art with each other in terms of rotational speed and volume flow rate; and 
       FIG. 13  is a graph comparing an axial flow fan of the present invention with comparative axial flow fans in terms of volume flow rate and noise level. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   Reference will now be made in greater detail to a preferred embodiment of the invention, an example of which is illustrated in the accompanying drawings. Wherever possible, the same reference numerals will be used throughout the drawings and the description to refer to the same or like parts. 
     FIG. 4  is a side cross-sectional view illustrating a state wherein a cooling fan in accordance with a first embodiment of the present invention is disposed in a electric device chamber of a microwave oven, and  FIG. 5  is a perspective view independently illustrating the cooling fan according to the first embodiment of the present invention. 
   As shown in  FIG. 4 , in the electric device chamber of the microwave oven, there are disposed up and down a magnetron  62  for radiating high frequency waves into a cooking chamber (not shown) defined in a cabinet  52  and a high voltage transformer  64  for applying a high voltage to the magnetron  62 . A mixed flow fan  70  is arranged behind the magnetron  62  and high voltage transformer  64  to direct axially sucked airflow in a manner such that the airflow is evenly distributed over the magnetron  62  and high voltage transformer  64 . The mixed flow fan  70  is driven by a motor  66 . 
   As shown in  FIG. 5 , the mixed flow fan  70  has a hub  72  which is coupled to an output shaft  66   a  of the motor  66  to be integrally rotated therewith and a plurality of blades  74  which are installed on a circumferential outer surface of the hub  72  to be spaced apart one from another by a predetermined angle. 
   The hub  72  is formed in a manner such that a hub angle θ measured between a center of the output shaft  66   a  of the motor  66  and the circumferential outer surface of the hub  72  is within the range of 20˜40°. 
   The hub  72  of the mixed flow fan  70  is formed in a manner such that a first prolongation  11  of a line, which extends on the circumferential outer surface of the hub  72  and exists on a first plane orthogonal to an axis of the hub  72 , passes through a first center of gravity G 1  of the magnetron  62 , and a second prolongation of a line, which extends on the circumferential outer surface of the hub  72  and exists on a second plane orthogonal to the axis of the hub  72 , passes through a second center of gravity G 2  of the high voltage transformer  64 . 
   As a consequence, if the mixed flow fan  70  is operated, air sucked in an axial direction through air suction holes  52   a  defined in a rear wall of the cabinet  52  is guided by the hub  72  toward the magnetron  62  and high voltage transformer  64  which are disposed up and down in the electric device chamber. At this time, no portion of the airflow supplied by the mixed flow fan  70  simply passes through a space which is defined between a lower end of the magnetron  62  and an upper end of the high voltage transformer  64 . Therefore, since the airflow as a whole passes through the magnetron  62  and high voltage transformer  64  in a evenly distributed state to cool them, a cooling efficiency of the cooling fan is improved. 
     FIG. 6  is a side cross-sectional view illustrating a state wherein a cooling fan in accordance with a second embodiment of the present invention is disposed in a electric  110  device chamber of a microwave oven,  FIG. 7  is a perspective view independently illustrating the cooling fan according to the second embodiment of the present invention,  FIG. 8  is a front view of the cooling fan according to the second embodiment of the present invention, and  FIG. 9  is a side view of the cooling fan according to the second embodiment of the present invention. 
   As shown in  FIG. 6 , the cooling fan according to this second embodiment of the present invention comprises an axial flow fan  100  arranged behind a magnetron  92  and a high voltage transformer  94  which are disposed up and down in the electric device chamber defined in a cabinet  82 . 
   Of course, as shown in  FIGS. 6 and 7 , the axial flow fan  100  has a hub  102  which is coupled to an output shaft  96   a  of a motor  96  to be integrally rotated therewith and a plurality of blades  104  which are installed on a circumferential outer surface of the hub  102  to be spaced apart one from another by a predetermined angle. 
   As shown in  FIGS. 6 ,  8  and  9 , the axial flow fan  100  is formed in a manner such that a ratio of an outer diameter D 2  of the fan  100  to a width L 1  of the electric device chamber defined in the cabinet  82 , as measured on a z-axis of  FIG. 6 , is no less than 0.8, and a hub ratio of an outer diameter D 1  of the hub 102 to the outer diameter D 2  of the fan  100  is in the range of 0.28˜0.32. 
   Concretely speaking, in the axial flow fan  100  according to this preferred embodiment of the present invention, the width L 1  of the electric device chamber of the cabinet  82  corresponds to 145 mm, the ratio of the outer diameter D 2  of the fan  100  to the width L 1  of the electric device chamber of the cabinet  82  corresponds to 0.83, and the hub ratio corresponds to 0.3. As a consequence, the axial flow fan  100  has an outer diameter D 2  of 120.35 mm, and the hub  102  has an outer diameter D 1  of 36.105 mm. 
   Meanwhile, in view of various considerations such as a cooling efficiency, a volume flow rate, an air pressure, and so forth, it is preferred that the axial flow fan  100  has five blades  104 . In the case that the axial flow fan  100  is rotated in a clockwise direction, each blade  104  has a contour in which both side surfaces are curved in a counterclockwise direction. 
   Here, a front side surface of each blade  104 , which is positioned upstream in the clockwise rotating direction of the axial flow fan  100 , serves as a front edge  104   a , and a rear side surface of each blade  104 , which is positioned downstream in the clockwise rotating direction of the axial flow fan  100 , serves as a rear edge  104   c . A protruded part  104   b  is formed at a distal end of the front edge  104   a  in a manner such that it projects forward when viewed in the clockwise rotating direction. 
   An outer peripheral surface of each blade  104 , which connects distal ends of the front and rear edges  104   a  and  104   c  with each other, serves as an outer edge (hereinafter, referred to as a “blade tip”) ( 104   d ), and an inner peripheral surface of each blade  104 , which is opposed to the blade tip  104   d  and is secured to the circumferential outer surface of the hub  102 , serves as an inner edge (hereinafter, referred to as a “blade root”) ( 104   e ). 
   An apex of the front edge  104   a  serves as a leading edge LE, and an apex of the rear edge  104   c  serves as a trailing edge TE. 
   Each blade  104  is formed to have a predetermined curvature between the front and rear edges  104   a  and  104   c  in a manner such that rear end portions of the blade tip  104   d  and blade root  104   e  on the rear edge  104   c  are positioned closer to an exit end of the hub  102  than front end portions of the blade tip  104   d  and blade root  104   e . Each blade  104  has a positive-pressure acting surface  104   g  on which a positive pressure is applied due to air suction and a negative-pressure acting surface  104   f  on which a negative pressure is applied due to air discharge. 
   Each blade  104  of the present invention is formed to have a sweep angle α of 26˜30°. 
   The sweep angle α denotes a degree to which the blade  104  is inclined in the clockwise rotating direction. The sweep angle α is defined by a first line X 1  which connects centers of the blade tip  104   d  and blade root  104   e  with each other and a second line X 2  which connects centers of the blade root  104   e  and hub  102  with each other. 
   The blade  104  has a first pitch angle β of 43° when measured at the blade root  104   e  and a second pitch angle β of 29.7° when measured at the blade tip  104   d . Therefore, each blade  104  has a pitch angle β which varies within the range of 43˜29.7° between the blade tip  104   d  and blade root  104   e.    
   The pitch angle β indicates a degree to which the blade  104  is inclined with respect to a plane perpendicular to the output shaft  96   a  of the motor  96 . Thus, the pitch angle β denotes an angle which is defined between a straight line connecting the leading edge LE to the trailing edge TE of the blade  104  and a y-axis orthogonal to an x-axis serving as a rotational axis. 
   The pitch angle β functions to disperse a great load exerted to the blade tip  104   d  while the axial flow fan  104  is rotated, in such a way as to minimize flow separation. Hence, the pitch angle β serves as a design parameter which suppresses a noise level upon fluid flow and increases a volume flow rate. 
   The blade  104  has a width of 42.94 mm when measured between the distal ends of the front and rear edges  104   a  and  104   c . Two adjoining blades  104  are separated from each other by a first distance of 6˜8 mm at their blade roots  104   e , and by a second distance of 23˜25 mm at their blade tips  10   d . Also, the two adjoining blades  104  are separated from each other by a third distance of 11˜13 mm at points dl which correspond to 0.75 of a length between the blade tip  104   d  and the blade root  104   e  when measured from the blade roots  104   e , and by a fourth distance of 8˜10 mm at points d 2  which correspond to 0.95 of the length between the blade tip  104   d  and the blade root  104   e  when measured from the blade roots  104   e.    
     FIG. 10  is a partial side view illustrating a distal end of the blade which forms a part of the cooling fan according to the second embodiment of the present invention, and  FIG. 11  is a cross-sectional view illustrating a state wherein the blade of  FIG. 10  is cut in a widthwise direction. 
   As shown in  FIG. 10 , the blade  104  is formed in a manner such that it is curved from the positive-pressure acting surface  104   g  toward the negative-pressure acting surface  104   f  over a distance of 8 mm measured from the blade tip  104   d  toward the blade root  104   e , to have a preselected curvature. 
   Consequently, since a static pressure regaining phenomenon slowly occurs in the air flowing from the positive-pressure acting surface  104   g  over the blade tip  104   d  toward the negative-pressure acting surface  104   f , it is possible to minimize generation of vortex flow at the blade tip  104   d.    
   As shown in  FIG. 11 , the blade  104  has a maximum camber position CP of 0.53 which is constantly maintained from the blade tip  104   d  to the blade root  104   e . Also, the blade  104  has a first maximum camber rate of 4% when measured at the blade root  104   e  and a second maximum camber rate of 9.3% when measured at the blade tip  104   d.    
   The maximum camber position CP denotes a position on the blade  104 , which is farthest separated from a chord CL as a straight line connecting the leading edge LE and trailing edge TE with each other. A maximum camber C is represented by a distance between the chord CL and the blade  104 . The maximum camber rate is a percentage representation of a ratio between the maximum camber C and a length of the chord CL. 
   In TABLE 1, the blade  104  according to the present invention, configured as mentioned above, is compared with the conventional blade for an axial flow fan. That is to say, design parameters of the present blade B and conventional blade A are compared with each other in TABLE 1. 
     FIG. 12  is a graph comparing cooling fans of the present invention and conventional art with each other in terms of rotational speed and volume flow rate, and  FIG. 13  is a graph comparing the axial flow fan of the present invention with comparative axial flow fans in terms of volume flow rate and noise level. 
   
     
       
         
             
             
             
           
             
                 
               TABLE 1 
             
             
                 
                 
             
             
                 
               Conventional 
                 
             
             
                 
               axial flow fan 
               Present axial 
             
             
                 
               (A) 
               flow fan (B) 
             
             
                 
                 
             
           
          
             
                 
             
          
         
         
             
             
             
             
          
             
                 
               Number of blades 
               5 
               5 
             
             
                 
               Width of 
               145 
               145 
             
             
                 
               electric device 
             
             
                 
               chamber (mm) 
             
             
                 
               Outer diameter 
               108 
               120 
             
             
                 
               (mm) 
             
             
                 
               Inner diameter 
               25 
               36 
             
             
                 
               (mm) 
             
             
                 
               Outer 
               0.74 
               0.83 
             
             
                 
               diameter/width 
             
             
                 
               of electric 
             
             
                 
               device chamber 
             
             
                 
               Outer 
               0.23 
               0.3 
             
             
                 
               diameter/inner 
             
             
                 
               diameter 
             
             
                 
               Sweep angle (°) 
               32 
               28 
             
             
                 
               Pitch angle (°) 
               45˜31 
               43˜29.7 
             
             
                 
                 
             
          
         
       
     
   
   As can be readily seen from  FIG. 12 , when a noise level is 43.1 dB[A], the conventional axial flow fan A has a rotational speed of 2856 RPM and a volume flow rate of 1.73 CMM, and the present axial flow fan has a rotational speed of 2495 RPM and a volume flow rate of 2.29 CMM. 
   At the same noise level, in the case of the present axial flow fan B, the rotational speed is reduced by no less than 10% and a volume flow rate is increased by no less than 30%. Accordingly, it is to be readily understood that the present axial flow fan B is significantly improved in terms of cooling efficiency and noise level. 
   In  FIG. 13 , in the case of a first comparative axial flow fan F 2 , only a width ratio is changed to 0.74 while the other design parameters have the same values as the present axial flow fan F 1 . In the case of a second comparative axial flow fan F 3 , only a sweep angle is changed to 35° while the other design parameters have the same values as the present axial flow fan F 1 . In the case of third and fourth comparative axial flow fans F 4  and F 5 , only a pitch angle range is changed to 40˜27° and 46˜33°, respectively, while the other design parameters have the same values as the present axial flow fan F 1 . 
   A person skilled in the art will readily recognize from  FIG. 13  that the present axial flow fan F 1  is superior to the first through fourth comparative axial flow fans F 1  through F 4  in terms of volume flow rate and noise level. 
   When considering the case where the present axial flow fan F 1  and the fourth comparative axial flow fan F 4  are rotated at the same velocity of 2495 RPM, in the case of the fourth comparative axial flow fan F 4 , a volume flow rate is reduced by no less than 6% when compared to the present axial flow fan F 1 , whereby a cooling efficiency is deteriorated. Thus, it can be readily understood that the present axial flow fan F 1  reveals excellent operational characteristics. 
   As apparent from the above description, the cooling fan for a microwave oven, according to the present invention, provides advantages in that, since a magnetron and a high voltage transformer are disposed up and down in a electric device chamber and a mixed flow fan having a specified hub angle is arranged behind them, air sucked into the mixed flow fan in an axial direction can be directed to be evenly distributed over the magnetron and high voltage transformer, whereby the magnetron and high voltage transformer can be effectively cooled and thereby it is possible to decrease a size of the cooling fan. 
   Also, because the cooling fan according to the present invention comprises an axial flow fan having optimized design parameters such as a ratio of an outer diameter of the fan to a width of the electric device chamber defined in a cabinet, a hub ratio, a sweep angle, a pitch angle, a maximum camber rate, etc., a volume flow rate is increased to improve a cooling efficiency and a noise level is reduced. 
   Moreover, in the cooling fan according to the present invention, due to the fact that a blade tip of the axial flow fan is curved to have a predetermined curvature, it is possible to prevent a vortex flow band from being created on a negative-pressure acting surface of a blade, whereby a noise level can be further suppressed and a volume flow rate can be further increased. 
   In the drawings and specification, there have been disclosed typical preferred embodiments of the invention and, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention being set forth in the following claims.