Abstract:
An air moving apparatus for generating cooling airflow is provided that includes a noise reduction system for reducing noise generated by a fan. The air moving apparatus includes a fan having a rotatable hub and a plurality of blades mounted to the hub for rotating about an axis of rotation to provide pressurized airflow. A sensor is situated on a surface of at least one fan blade for sensing airflow characteristics of the air flowing over the fan blade. An actuator, also situated on the surface of the fan blade, changes the characteristic of the airflow over the fan blade in response to the sensed airflow characteristic.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to an air moving apparatus and, more particularly to fans having low-noise characteristics and a method for actively optimizing such fan characteristics.  
         BACKGROUND OF THE INVENTION  
         [0002]    A wide variety of equipment and systems, such as portable and desktop computers, mainframe computers, communication infrastructure frames, automotive equipment, etc., include heat-generating components in their casings. As increasingly dense and higher performance electronics are packaged into smaller housings, the need for effective cooling systems is paramount to prevent failure of such sensitive electronics devices. One method used to remove heat from such equipment is to have an axial fan draw air from the exterior of the casing to blow cooling air over the heat-generating components. However, as the number of electronics devices in offices and households increase, so too does the number of cooling fans. As such, fan noise becomes significantly loud and undesirable.  
           [0003]    Noise reduction in fans generally is accomplished through either active and/or passive noise reduction techniques. In a passive noise reduction system, a fan may include a plurality of projections having a number of predetermined masses that are arranged at positions around the periphery of the blade. This results in creating an unstable mode for the fan. The unstable mode results in disruption of airflow over the blade, thereby resulting in less noise at the trailing edge. However, such a system requires the fan to rotate at a preset rotational speed for maximum effectiveness. Rotation of the fan at other than the preset speed results in decreased effectiveness of the noise reduction methods.  
           [0004]    An active noise reduction method includes a fan having a micro electro mechanical system that includes a thin silicon film forming an integrated circuit and an actuator connected to the circuit for generating vibrations. The fan reduces noise by causing the actuator to generate vibration that offsets or reduces unstable airflow along the blade body. However, the operation of the noise reduction system is less than optimal because the actuator and the sensing portion are configured as a closely spaced, or even single, device that is placed at one particular portion of the fan blade. Thus, the actuator and the sensing portion are separated by a negligible distance. As such, the system is unable to simultaneously sense the wake at the trailing edge of the blade and create turbulent flow at a predetermined point along the fan blade. 
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0005]    [0005]FIG. 1 is a side view of an airfoil illustrating the principles of vortex shedding;  
         [0006]    [0006]FIG. 2 is a perspective view of a fan having noise reduction capabilities in accordance with the invention;  
         [0007]    [0007]FIG. 3 is a side view of a fan blade of the fan of FIG. 2 having a sensor and actuator mounted thereon in accordance with the invention;  
         [0008]    [0008]FIG. 4 is a perspective view of the back side of the fan of FIG. 2 having a controller mounted thereon in accordance with the invention; and  
         [0009]    [0009]FIG. 5 is a flow diagram of the controller in operation in accordance with the invention.  
     
    
     DETAILED DESCRIPTION  
       [0010]    A known problem with axial fans relates to vortex shedding, which is the principle contributor of aero-acoustic noise in fan operation. Referring to FIG. 1, the mechanism of vortex shedding is shown. In a fan the direction of airflow  13  is partly over the surface of an axial fan blade  11  from the leading edge  16  to the trailing edge  19  of the airfoil of a pressure gradient. At the leading edge  16  of the airfoil and up to a certain distance along the blade  11 , the flow of air is laminar  18 . That is, there is smooth, uninterrupted flow of air over the surface contour  12  of the fan blade  11 . This air flow forms a boundary layer since the air flow has zero velocity right at the surface, and some distance out from the surface it flows at the same velocity as the local outside flow. If the boundary layer flows in parallel layers, with no energy transfer between layers, it is laminar. If there is energy transfer, airflow is no longer laminar, but turbulent  17 . All boundary layers start off as laminar. However, due to adverse pressure gradient surface roughness and other destabilizing influences, the airflow  13  begins to separate from the surface  12  of the airfoil blade  11  after a certain distance along the length of the airfoil blade  11 . As a result, the pressure and flow becomes more mixed and turbulent, with an increase in the radial or drag direction. The point at which the airflow becomes turbulent is known as a transition regime  15 .  
         [0011]    As air flows past the trailing edge  19  of the blade  11 , it generates a wake  23  behind the blade  11 . This is caused by the pressure gradient being in the opposite direction to the airflow. Therefore an eddy or air vortex  21  is created behind the trailing edge of the fan. A similar effect takes place with the airflow around the bottom side  14  of the fan blade  11 . These air vortices drop off the back of the fan blade creating the wake  23  behind the blade. This effect is known as vortex shedding. Vortex shedding  21  in this wake region  23  causes pressure fluctuation resulting in generation of acoustic waves and other unwanted vibration. These acoustic waves create noise when the fan is operating.  
         [0012]    Referring to FIG. 2, there is illustrated an air moving apparatus in the form of a tube-axial fan  37  in accordance with the present invention having increased noise reduction capabilities via the provided sensors  27  operating in concert with actuators  31  on the fan blade  25  of the fan  37 . The frequency of the oscillation of the actuator  31  for decreasing fan noise is dynamically determined from acoustic input received by the sensor  27  and actively adjusted by a controller  41  (FIG. 4) as desired for quiet operation. In this manner, the present fan  37  is particularly effective in those applications where the fan noise may be excessive, i.e. small casings enclosing high-density consumer electronics therein.  
         [0013]    The fan  37  includes a plurality of fan blades  25  extending generally radially outward from a hub  38 . Each fan blade  25  terminates at a tip end portion  28  thereof radially spaced from the hub  38  and has a leading edge  16  and a trailing edge  19  extending between the hub  38  and the tip end portion  28 . The fan is rotatively driven by an output shaft of a motor (not shown) that engages the center  39  of the hub  38 . The motor rotates the fan  37  about a central longitudinal axis that is defined by the receiving portion  39  of the fan  37 . This causes the fan blades  25  to draw air from an inlet side  26  of the fan  37  and to impart velocity to discharge the air from an outlet side  29  in the direction generally indicated by arrow  34 .  
         [0014]    Turning to FIG. 3, the fan blade  25  of the fan  37  in accordance with the present invention is shown in greater detail. The fan blade  25  has a bottom side  35  and a top side  33 . The top side  33  has mounted thereon a piezoelectric sensor element  27  made of thin organic polymer such as polyvinylidene fluoride (PVDF) or lead zirconate titanate (PZT). Using, for example, the PVDF piezoelectric sensor element  27  on the trailing edge  19  of the fan blade provides several significant advantages over sensors made of thin film silicon or the like. For example, the PVDF sensor material is an inexpensive thin plastic polymer sheet or film that has a thin electrically conductive nickel copper alloy deposited on each side. Electrical connections are made to the film using wires that may be attached to the conductive coating of the film using copper tape or conductive epoxy. The film itself may be cut to shape as needed and glued onto the appropriate location on the fan blade  25 . Thus, the advantages of using the PVDF sensor include its low cost and the ease in which the sensor may be configured for use in a variety of fan blade sizes.  
         [0015]    The sensor element  27  is attached on the trailing edge of the blade and senses pressure fluctuation and acoustic energy at the trailing edge of the blade  25 . Fluctuations in air pressure are detected by the sensor  27  when air pressure or sound waves, such as acoustical waves, cause the film to stretch and conduct electricity, thereby creating a closed circuit between the wires. The system of the present invention detects the closing and opening of the circuit to determine characteristics of the waves at the trailing edge of the blade  25 . Thus, the sensor is able to determine the presence of noise causing air waves.  
         [0016]    The top side of the fan blade  25  also has mounted thereon an actuator  31  made of piezoelectric element and a thin layer or fin  31  attached on the top surface. The actuator  31 , being also made of piezoelectric film, is made to vibrate, which in turn causes the fin  29  to vibrate as well. Applying and removing voltage to the film  29  causes the material to bend and then return to its original shape, thereby creating a vibration motion. Alternatively, two sheets of film may be joined together to form a bimorph. The sheets are arranged such that when voltage is applied to the bimorph, one film laminate lengthens while the other contracts. Voltage of the reverse polarity causes the bimorph to bend in the other direction. Thus, the vibration rate of the actuator is controlled in the first case by pulsing power to the film or in the second case by reversing the polarity of the voltage being supplied to the bimorph.  
         [0017]    As shown, the sensor element  27  and activator  29  are purposefully spaced apart. An advantage of such a configuration is the ability to detect noise in the area of the fan where most noise originates, i.e. the trailing edge, and to correct or eliminate the conditions that lead to the noise by creating turbulence in the laminar flow region. As such, fan noise caused by vortex shedding is reduced through the elimination of the shedding of vortices by deliberately converting laminar flow to turbulent flow.  
         [0018]    Referring to FIGS. 4 and 5, a controller  41  comprising a feedback control loop is shown mounted on the hub  43  on the reverse side of the fan  25 . The controller hardware may comprise a 16 bit analog-to-digital/digital-to-analog converter (ADC/DAC), such as the TMC320C62 digital signal processor (DSP), available from Texas Instruments Corporation.  
         [0019]    The controller  41  includes an adaptive controller  45  and an actuator controller  47  that is used for exciting the actuator by pulsing the voltage or controlling the voltage polarity. The feedback control loop of the controller  41  is mounted on the hub  43  of the fan  25  and receives power and signal from the rotating shaft of the fan. During operation of the fan  37 , the airflow over the fan blade  25  is laminar near the leading edge  16 , and changes to transition regime downstream. The transition of boundary lair from laminar regime occurs generally on the suction side (upper side)  33  of the airfoil blade  25 . Based on the acoustic feedback from the sensor  27  at the trailing edge, the actuator controller  47  causes excitation of the boundary layer at a particular predetermined frequency using the piezoelectric actuator  31  to vibrate the fin  29  at the appropriate frequency as determined by the adaptive controller  45 . Thus, the laminar airflow is converted to turbulent flow deliberately. Accordingly, the problems of noise associated with the transition to transitional flow and subsequent vortex generation is reduced  
         [0020]    Continuing to refer to FIG. 5, the control loop is shown in operation. As discussed above, the acoustic wave emitted from the blade  25  has a particular frequency spectrum. The sound pressure level at the trailing edge  27  is a function of the aerodynamic loading, speed, and the inlet turbulence level. The frequency spectrum also changes in a similar manner. Based on the acoustic input at the sensor  27 , the control circuit  41  (FIG. 4) determines the required frequency of the piezoelectric actuator  31 . In particular, the control loop determines the sound pressure level versus the frequency data from the sensor  27  input in narrow band over a period of time. The control loop then scales the data using a preset scale, such as A scale, of acoustinc averaging. From the scaled sound pressure data, the control loop determines the objectional frequency peaks, such as 1000 Hz, or any other objectionable frequency in the audible range of human hearing.  
         [0021]    The piezoelectric actuator  31  causes vibration on one end of the fin  29 . The fin  29  vibrates, generating pressure fluctuation on the surface of the airfoil blade  25 . The pressure fluctuation results in breakup of the attached laminar flow. This causes the laminar flow to transition to turbulent flow early and before reaching the trailing edge  19 , resulting in reduced or eliminated vortex shedding and correspondingly lowered noise levels. The amount of vibration required of the fin is adaptively determined by the controller  41 . In particular, the feedback control loop of the controller  41  determines frequency windows for generating correction signals. Depending on the level of turbulence generated the acoustic wave radiation at the trailing edge  19  changes.  
         [0022]    Based on the change of the acoustic wave radiation sensed by the piezoelectric sensor  27 , a control signal modifier or error signal  46  is generated. The generated error signal  46  is combined with the predefined actuator signal  49  to send a corrected signal  50  to the actuator  31 . The actuator control in the feedback loop creates the voltage signal to the actuator  31 . The resultant acoustic signal from this correction is again received from the sensor  31  and the above process is repeated until cancellations of the objectionable sound pressure peaks are eliminated. Thus, an active control loop is established. Accordingly, the control circuit automatically and dynamically establishes the appropriate signal for the actuator depending in the change in loading or any other parameter changes.  
         [0023]    While there have been illustrated and described particular embodiments of the present invention, it will be appreciated that numerous changes and modifications will occur to those skilled in the art, and it intended in the impendent claims to cover all those changes and modifications that fall within the true spirit and scope of the present invention.