Abstract:
A family of dual-winglet rotor blades are designed to dissipate the low energy flow in the wake of a turbine rotor. In some embodiments a dual-winglet having a first winglet transitioning from the lift surface of a rotor blade and a second winglet transitioning from the pressure surface of the rotor blade creates two distinct streams in the wake of the rotor. In one embodiment the first winglet curving away from the lift surface turns the lift force toward the center of the rotor plane while a second, smaller, winglet curving away from the pressure surface of the rotor blade turns the lift force away from the center of the rotor plane. In other embodiments winglets create a virtual shroud that expands the wake to dissipate the low-energy flow in the turbine wake. In another embodiment a dual winglet combines the aforementioned mixing effect with the wake expansion effect.

Description:
PRIORITY CLAIM 
       [0001]    This application claims priority to provisional patent application No. 62/355,269 filed on Jun. 27, 2016. 
     
    
     TECHNICAL FIELD 
       [0002]    The present disclosure relates in general, to rotor blades for fluid turbines. Specifically, the present disclosure relates to increased power extraction by dissipating rotor wake pressure with rotor winglets. 
       BACKGROUND 
       [0003]    In general, horizontal axis fluid turbine rotor blades comprise two to five blades arranged evenly about a central axis and coupled to an electrical generation machine. 
         [0004]    Generally speaking, a fluid turbine structure with an open unshrouded rotor design captures energy from a fluid stream that is smaller in diameter than the rotor. In an open unshrouded rotor fluid turbine, as fluid flows from the upstream side of the rotor to the downstream side, the average axial fluid velocity remains constant as the flow passes through the rotor plane. Energy is extracted at the rotor resulting in a pressure drop on the downstream side of the rotor. The fluid directly downstream of the rotor is at sub-atmospheric pressure due to the energy extraction and the fluid directly upstream of the rotor is at greater than atmospheric pressure. The high pressure upstream of the rotor deflects some of the upstream air around the rotor. In other words, a portion of the fluid stream is diverted around the open rotor as if by an impediment. As the fluid stream is diverted around the open rotor, it expands, which is referred to as flow expansion at the rotor. Due to the flow expansion, the upstream area of the fluid flow is smaller than the area of the rotor. 
         [0005]    The Betz limit calculates the maximum power that can be extracted from a volume of moving fluid by an open blade, horizontal axial flow turbine (HAWT). The Betz limit is derived from fluid dynamic control-volume theory for flow passing through an open rotor, and by applying one-dimensional equations based on the principles of the conservation of mass, momentum and energy. According to the Betz limit, and independent of the design of the fluid turbine, a maximum of 16/27 of the total kinetic energy in a volume of moving fluid can be captured by an open-rotor turbine. Conventional turbines commonly produce 75% to 80% of the Betz limit. 
         [0006]    A fluid turbine power coefficient (Cp) is the power generated over the ideal power available by extracting all the wind kinetic energy approaching the rotor area. The Betz power coefficient of 16/27 is the maximum power generation possible based on the kinetic energy of the flow approaching the rotor, and the rotor area. For an open rotor, the rotor area used in the Betz Cp derivation is the system maximum flow area described by the diameter of the rotor blades. The maximum power generation occurs when the rotor flow velocity is the average of the upstream and downstream velocity. This is the only rotor velocity that allows the flow-field to be reversible, and the power extraction to be maximized. At this operating point, the rotor velocity is ⅔ the wind velocity, the wake velocity is ⅓ the wind velocity, and the rotor flow has a non-dimensional pressure coefficient of −⅓ at the rotor exit. The −⅓ pressure coefficient is a result of the rotor wake flow expanding out to twice the rotor exit area downstream of the rotor station. 
         [0007]    Induced drag is generated by a rotor blade due to the redirection of fluid during the generation of lift as a column of fluid flows through the rotor plane. The redirection of the fluid may include span-wise flow along the pressure side of the rotor blade along a radial direction toward the blade tip where the fluid then flows over to the opposite side of the blade. The fluid flow over the tips joins a chord-wise flow, otherwise referred to as bypass flow, forming rotor-tip vortices. The rotor-tip vortices mix with vortices shed from the trailing edge of the rotor blade to form the rotor wake. 
         [0008]    It is commonly known that the rotor wake affects the rotor intake. A column of fluid encounters a rotor as an impediment, in part, because a portion of the fluid flowing around the rotor expands in the wake of the rotor in a form referred to as the stream column. Fluid flowing around the rotor plane is referred to as the bypass flow. Bypass flow passes over the outer surface of the stream column. Since the stream column can be considered to be comprised of an infinite fore-body and an infinite after-body, the resulting pressure force on the stream column is zero (refer to D&#39;Alemberts paradox). Increasing lift over the rotor, and hence increasing the amount of energy extracted from the stream column, creates slower moving flow in the rotor wake, therefore, impeding flow through the rotor. This impediment increases the volume of the rotor wake. In other words, as more power is extracted at the rotor, the rotor stream column will expand and more fluid flow will bypass the rotor. As a result, maximum power is achieved from the two opposing effects of: increased power extraction resulting in relatively lower flow rates; and reduced power extraction resulting in relatively higher flow rates. 
         [0009]    Proposed solutions to the above mentioned paradox include: increasing the size of the wake area to allow for increased wake expansion; and injecting high-energy fluid into the rotor wake. Both solutions have been proven to allow for increased energy extraction at the rotor. 
         [0010]    Using idealized but broadly representative models, the power coefficient of a dual-tip rotor blade based upon rotor diameter is increased over a non-tip rotor blade by the ratio of the velocity at the location of the rotor blade, divided by the free-stream fluid velocity. This is measured as velocity (U) at the rotor blade plane (P) at a power extraction factor of zero (0), referred to as UP-0. Similarly, a rotor extracting power is measured as velocity (U) at the rotor blade plane (P) from minimum power production, up to a rated power (R), referred to as UP-R. 
         [0011]    A fluid power coefficient (Cp) is a function of wake velocity ratio and thrust coefficient (Ct). Thrust coefficient is the ratio of pressure drop across the rotor over the dynamic pressure of the wind flow, which represents a critical parameter for the design of the rotor. 
         [0012]    Providing an area for wake expansion in the down-stream region of the rotor plane results in a low exit-plane pressure coefficient (CTE) that allows for a relatively higher rotor-thrust coefficient. 
       SUMMARY 
       [0013]    Disclosed herein is an apparatus providing an annular formation of varying relative pressures, creating a mixing element on a rotor blade tip. A mixing element is designed to provide a spiral flow, following the wake of each rotor blade, of high-energy flow from ambient-flow upstream of the rotor that mixes with the rotor wake-flow downstream of the rotor. In combination, a rotor blade design and aerodynamic dual-tip diffusor and mixing element provide a system for increased energy output compared to rotors of identical total rotor diameter. A dual tip on each rotor blade is designed to take advantage of a high rotor thrust coefficient, providing reduced coefficient of pressure in the rotor-wake and a high flow stream for increased mixing of rotor-wake flow with bypass flow at the exit plane of the rotor. 
         [0014]    A horizontal-axis fluid rotor having multiple blades with high-energy bypass flow coupled with a dual-tip rotor blade design provide increased power generation compared to a flat or non-tip rotor blade design. The dual rotor blade tips form a high-energy mixer that injects a helix of high energy bypass flow into the helical exit stream that emanates from the trailing edge and of the rotor blades. 
         [0015]    As understood by one skilled in the art, the aerodynamic principles the present disclosure are not restricted to a specific fluid, and may apply to any fluid, defined as any liquid, gas or combination thereof and, therefore, includes water as well as air. In other words, the aerodynamic principles of a dual-tip wind rotor blade apply to hydrodynamic principles in a dual-tip water rotor blade. 
         [0016]    These and other non-limiting features or characteristics of the present disclosure will be further described below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    The following is a brief description of the drawings, which are presented for the purposes of illustrating the disclosure set forth herein and not for the purposes of limiting the same. Example embodiments of the present disclosure are further described with reference to the appended figures. It is to be noted that the various features and combinations of features described below and illustrated in the figures can be arranged and organized differently to result in embodiments which are still within the spirit and scope of the present disclosure. To assist those of ordinary skill in the art in making and using the disclosed systems, assemblies and methods, reference is made to the appended figures, wherein: 
           [0018]      FIG. 1  is front, perspective view of the present embodiment; 
           [0019]      FIG. 2  is a detail view of the dual-tip rotor of the present embodiment; 
           [0020]      FIG. 3  is a detail view of the dual-tip rotor of the present embodiment 
           [0021]      FIG. 4  is a diagram depicting the fluid-stream flow over the dual-tip rotor of the present embodiment; 
           [0022]      FIG. 5  is front, perspective view an iteration of the present embodiment; 
           [0023]      FIG. 6  is a detail view of the dual-tip rotor of the iteration of  FIG. 5 ; 
           [0024]      FIG. 3  is a detail view of the dual-tip rotor of the iteration of  FIG. 5 ; 
           [0025]      FIG. 8  is a diagram depicting the fluid-stream flow over the dual-tip rotor of the iteration of  FIG. 5 ; 
           [0026]      FIG. 9  is front, perspective view of an iteration of the embodiment; 
           [0027]      FIG. 10  is a detail view of the dual-tip rotor of the iteration of  FIG. 9 ; 
           [0028]      FIG. 11  is a diagram depicting the fluid-stream flow over the dual-tip rotor of the iteration of  FIG. 9 ; 
           [0029]      FIG. 12  is front, perspective view of an iteration of the embodiment; 
           [0030]      FIG. 13  is a detail view of the dual-tip rotor of the embodiment of  FIG. 12 ; 
           [0031]      FIG. 14  is a diagram depicting the fluid-stream flow over the dual-tip rotor of the embodiment of  FIG. 12 ; 
       
    
    
     DETAILED DESCRIPTION 
       [0032]    The example embodiments disclosed herein are illustrative of advantageous fluid rotor systems, and assemblies of the present disclosure and methods or techniques thereof. It should be understood, however, that the disclosed embodiments are merely examples of the present disclosure, which may be embodied in various forms. Therefore, details disclosed herein with reference to example fluid rotor systems or fabrication methods and associated processes or techniques of assembly and or use are not to be interpreted as limiting, but merely as the basis for teaching one skilled in the art how to make and use the advantageous fluid rotor systems of the present disclosure. 
         [0033]    A more complete understanding of the components, processes, and apparatuses disclosed herein can be obtained by reference to the accompanying figures. These figures are intended to demonstrate the present disclosure and are not intended to show relative sizes and dimensions or to limit the scope of the example embodiments. 
         [0034]    Although specific terms are used in the following description, these terms are intended to refer to particular structures in the drawings and are not intended to limit the scope of the present disclosure. It is to be understood that like numeric designations refer to components of like function. 
         [0035]    The term “rotor” or “rotor assembly” is used herein to refer to any assembly in which blades are attached to a shaft and able to rotate, allowing for the generation of power or energy from fluid rotating the blades. Example embodiments of the present disclosure disclose a fixed-blade rotor or a rotor assembly having blades that do not change configuration so as to alter their angle or attack, or pitch. 
         [0036]    In certain embodiments, the leading edge of a rotor assembly may be considered the front of the fluid rotor system, and the trailing edge of a rotor assembly may be considered the rear of the fluid rotor system. 
         [0037]      FIG. 1  presents a front perspective view of a rotor of the present disclosure having a dual-tip, also referred to as a dual winglet or double winglet. The rotor has a primary structure, otherwise referred to as the rotor shaft  110  that extends from the root  109  to the dual tip, having pressure-surface  111  and a lift surface  113  according to the shape of the airfoil cross section. The rotor blade  100  further comprises a pressure-surface winglet  114  and a lift-surface winglet  112 . The pressure-surface winglet  114  turns from the pressure-surface  111  to angle  118  that is between 15° and 35° with respect to a vertical centerline  142  that resides along the rotor shaft  110  and is proximal to the center of gravity of each cross section along the rotor shaft  110 . The lift-surface winglet  112  turns from the lift-surface  113  at angle  116  that is between 70° and 120° with respect to vertical centerline  142 . 
         [0038]      FIG. 2  presents a detailed view of the lift-surface winglet. The rotor shaft  110  is rotationally engaged at the root  109  with a nacelle  132  shown in section view against a horizontal center-line  134 . The lift-surface winglet  112  has a lift surface  124  and a pressure surface  126 . The lift surface  124  transitions from the lift surface  113  of the primary shaft  110 . The leading edge of the rotor shaft  110  transitions along the first winglet toward the trailing edge at an angle  131  that is between 20° and 60° with respect to the vertical centerline  142 . 
         [0039]      FIG. 3  presents a detailed view of the pressure-surface winglet. The rotor shaft  110  is rotationally engaged at the root  109  with a nacelle  132  shown in section view against a horizontal center-line  134 . The pressure-surface winglet  114  is an airfoil that has a lift surface  122  and a pressure surface  120 . The leading edge of the rotor shaft  110  transitions along the second winglet toward the trailing edge at an angle  133  that is between 20° and 60° with respect to the vertical centerline  142 . One skilled in the art will understand that the lift surfaces will create increased velocity and decreased pressure when compared to the increased pressure and decreased velocity in the flow over the pressure surface. 
         [0040]      FIG. 4  presents a side perspective view of a rotor of the present embodiment. A column of air  143  encounters the rotor and results in lift, represented by vectors  138  that change direction as the encounter the winglets. The lift side of each winglet divides the overall lift into two streams at the dual winglet. Stream  136  is created by the pressure-surface winglet  114  and stream  134  is created by the lift-surface winglet  112 . These two streams  136 ,  134 , are counter rotating vortices that mix free stream, or bypass, flow into the wake of the rotor. In other words, stream  136  mixes bypass flow from the column of moving air  143 , into the counter-rotating vortices  134 . 
         [0041]    The fluid power coefficient (Cp) as a function of wake velocity ratio and thrust coefficient (Ct) may be increased because of the low exit-plane pressure coefficient (CTE) that allows for a relatively higher rotor-thrust coefficient. The rotor design may take advantage of a highly cambered rotor shaft  110 , designed for a greater Cp without stalling as it would without the dual winglet. 
         [0042]    Referring to  FIG. 5 , the illustration depicts an iteration of a rotor blade design, the embodiment having a dual winglet on the tip of the rotor blade  200 . The rotor has a primary structure, otherwise referred to as the rotor shaft  240  that extends from the root  238  to the dual tip along a vertical centerline  242 . The shaft  240  has a pressure-surface  236  and a lift surface  234  according to the shape of the airfoil cross sections. The rotor blade  200  further comprises a winglet that meets the shaft  240  at the rotor tip which is furthest from the root  238 . A plane  245  is perpendicular to the vertical centerline  242  proximal to the rotor tip. An arcuate winglet adjoins the rotor shaft proximal to the tip. The winglet is tangent to the plane  245  where it adjoins the rotor shaft  240 . A first arcuate extension  212  extends downwind from the rotor shaft at an angle  232  that is generally between 5° and 65° with respect to a vertical centerline  242  and preferably between 30° and 60° with respect to the vertical centerline  242 . A second arcuate extension  214  extends upwind from the rotor shaft at an angle  232  that is generally between 5° and 65° with respect to a vertical centerline  242  and preferably between 30° and 60° with respect to the vertical centerline  242 . 
         [0043]    One skilled in the art understands that the winglet exists in the upwind area and the downwind area with respect to the centerline and that the airfoil cross sections at either end of the arcuate winglet may be similar to those that transition from the lift surface  234  and pressure surface  236  as illustrated in the aforementioned embodiment ( FIG. 4 ). 
         [0044]      FIG. 6  presents a detailed view of the first arcuate extension of the winglet of the iteration of the embodiment of  FIG. 5 . The rotor shaft  210  is rotationally engaged at the root  209  with a nacelle  232  shown in section view against a horizontal center-line  234 . The first arcuate extension of the winglet  212  has a lift surface  224  and a pressure surface  226 . The leading edge first arcuate extension of the winglet transitions along the first arcuate extension toward the trailing edge at an angle  233  that is between 20° and 75° with respect to the vertical centerline  242 . 
         [0045]      FIG. 7  presents a detailed view of the second arcuate extension of the winglet. The rotor shaft  210  is rotationally engaged at the root  209  with a nacelle  232  shown in section view against a horizontal center-line  234 . The second arcuate extension of the winglet  214  is an airfoil that has a lift surface  222  and a pressure surface  220 . The leading edge of the second arcuate extension of the winglet transitions along the second arcuate extension toward the trailing edge at an angle  235  that is between 20° and 75° with respect to the vertical centerline  242 . One skilled in the art will understand that the lift surfaces will create increased velocity and decreased pressure when compared to the increased pressure and decreased velocity in the flow over the pressure surface. 
         [0046]      FIG. 8  presents a side perspective view of a rotor of the iteration of the embodiment of  FIG. 5 . A column of moving air  243  encounters the rotor and results in lift, represented by vectors  238  that change direction as the encounter the arcuate extensions of the winglet. The lift side of each arcuate extension divides the overall lift into two streams. Stream  236  is created by the second arcuate extension  214  and stream  234  is created by the first arcuate extension  212 . These two streams  236 ,  234 , are counter rotating vortices that mix free stream, or bypass, flow into the wake of the rotor. In other words, stream  236  mixes bypass flow from the column of moving air  243 , into the counter-rotating vortices  234 . 
         [0047]      FIG. 9  presents a front perspective view of a rotor of an iteration of the present disclosure having a dual-tip, also referred to as a dual winglet or double winglet. The rotor has a primary structure, otherwise referred to as the rotor shaft  310  that extends from the root  309  to the dual tip, having pressure-surface  311  and a lift surface  313 , on the opposite side of the pressure-surface, according to the shape of the airfoil cross section. A centerline  342  resides along the center of the long axis of the rotor shaft  310 . The rotor blade  300  further comprises a first winglet  314  and a second winglet  312 . The first winglet  314  turns from the lift-surface  313  to angle  318  that is between 15° and 35° with respect to a plane that is perpendicular to a vertical centerline  342 . The second winglet  312  turns from the lift-surface  313  at angle  316  that is between 70° and 120° with respect to a plane that is perpendicular to a vertical centerline  342 . 
         [0048]      FIG. 10  presents a detailed view of the dual-winglet of the iteration of the embodiment  300  of  FIG. 9 . The rotor shaft  310  is engaged at the root  309 . The root  309  is rotationally engaged with a nacelle  332  shown in section view against a horizontal center-line  334 . The second winglet  312  has a lift surface  324  and a pressure surface  326 . The first winglet  314  is an airfoil that has a lift surface  322  and a pressure surface  320 . One skilled in the art will understand that the lift surfaces will create increase velocity and decreased pressure when compared to the decreased velocity and increased pressure in the flow over the pressure surface of the respective airfoils. 
         [0049]      FIG. 11  presents a side, section view of a rotor of the iteration of the embodiment  300 . A column of air  342  encounters the rotor and results in lift over the rotor shaft and thus rotation of the rotor about the nacelle  332 . A cross section of the flow over the dual winglet is described by flow vectors  328  and  330 . The highly cambered airfoils  314  and  312  create a virtual shroud, thus imitating the effect of a duct surrounding the rotor  310 . The resultant flow vectors  328  and  330  create a bell shaped area in the wake of the rotor plane. This expanding area creates a region for the low pressure in the wake of the turbine to expand, thus dissipating the wake flow, allowing the flow to return to ambient more rapidly than a rotor without the dual winglet and therefore allowing for a rotor with an airfoil cross section capable of greater energy extraction without stall. 
         [0050]      FIG. 12  presents a front perspective view of a rotor of the iteration of the embodiment of the disclosure having a dual-tip, also referred to as a dual winglet or double winglet. The rotor has a primary structure, otherwise referred to as the rotor shaft  410  that extends from the root  409  to the dual tip, having pressure-surface  411  and a lift surface  413  according to the shape of the airfoil cross section. A centerline  442  extends from the root through the tip along the rotor shaft  410 . The rotor blade  400  further comprises a pressure-surface winglet  414  and a lift-surface winglet  412 . The pressure-surface winglet  414  turns from the pressure-surface  411  to angle  418  that is between 15° and 35° with respect to the centerline  442 . The pressure-surface winglet  414  then turns downwind at angle  417  that is between 75° and 95° with respect to the vertical centerline  442 . The lift-surface winglet  412  turns from the lift-surface  413  at angle  416  that is between 70° and 120° with respect to the centerline  442 . 
         [0051]      FIG. 13  presents a detailed view of the wing-tips that make up the double winglet of the iteration of the embodiment  400  of  FIG. 12 . The rotor shaft  410  is engaged at the root  409 . The root  409  is rotationally engaged with a nacelle  432  shown in section view against a center-line  434 . The dual winglet is made up of a first winglet  412  and a second winglet  414 . The first winglet  412  has a lift surface  424  and a pressure surface  426 . The second winglet  414  is an airfoil that has a lift surface  422  and a pressure surface  420 . One skilled in the art will understand that the lift surfaces will create increase velocity and decreased pressure when compared to the decreased velocity and increased pressure in the flow over the pressure-surface of the respective airfoils. 
         [0052]      FIG. 14  presents a side perspective view of a rotor of the present embodiment. A column of air  442  encounters the rotor and results in lift, represented by vectors  438  that change direction as the encounter the winglets. The lift side of each winglet divides the overall lift into two streams at the dual winglet. Stream  436  is created by the second winglet  414  and stream  434  is created by the first winglet  412 . These two streams  436 ,  434 , cause counter rotating vortices that mix free stream, or bypass, flow into the wake of the rotor. In other words, stream  436  mixes bypass flow from the column of moving air  442 , into the counter-rotating vortices  434 . 
         [0053]    The winglet  414  turns both upstream and downstream. When turning upstream the counter rotating vortices  436  are created. The portion of the winglet  414  that turns downstream creates and expanding stream  450 . The expanding flow over the dual winglet is described by flow vector  450 . The highly cambered airfoils  414  and  412  create a virtual shroud, thus imitating the effect of a duct surrounding the rotor  410 . The resultant flow vector  450  creates a bell shaped area in the wake of the rotor plane. This expanding area creates a region for the low pressure in the wake of the turbine to expand, thus dissipating the wake flow, allowing the flow to return to ambient more rapidly than a rotor without the dual winglet and therefore allowing for a rotor with an airfoil cross section capable of greater energy extraction without stall. One skilled in the art understands the effects of wake mixing and wake expansion that are affected by the afore-described dual winglet. 
         [0054]    The present disclosure has been described with reference to example embodiments. Modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the present disclosure be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof. 
         [0055]    Although the systems and methods of the present disclosure have been described with reference to example embodiments thereof, the present disclosure is not limited to such example embodiments and or implementations. Rather, the systems and methods of the present disclosure are susceptible to many implementations and applications, as will be readily apparent to persons skilled in the art from the disclosure hereof. The present disclosure expressly encompasses such modifications, enhancements and or variations of the disclosed embodiments. Since many changes could be made in the above construction and many widely different embodiments of this disclosure could be made without departing from the scope thereof, it is intended that all matter contained in the drawings and specification shall be interpreted as illustrative and not in a limiting sense. Additional modifications, changes, and substitutions are intended in the foregoing disclosure. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the disclosure.