Patent Publication Number: US-10774659-B2

Title: Tip leakage flow directionality control

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of application Ser. No. 14/143,072, which was filed on Dec. 30, 2013, which is a continuation-in-part of U.S. patent application Ser. No. 13/540,752, filed Jul. 3, 2012, issued as U.S. Pat. No. 9,260,972. 
    
    
     BACKGROUND 
     The described subject matter relates generally to turbine engines and more specifically to cooling turbine blades. 
     Turbine engines provide efficient, reliable power for a wide range of applications in aviation, transportation and industrial power generation. Individual compressor and turbine section(s) may be subdivided into a number of stages, formed of alternating rows of rotor blade and stator vane airfoils. Due to various operational constraints, rotor blades and stator vanes are prone to leakage of compressed gases over their tips from a higher pressure surface to a lower pressure surface. Several attempts have been made to reduce these flows, but they cannot be completely eliminated due to clearance requirements over the airfoil tips which account for variations in both thermal and centrifugal growth of adjacent components. 
     SUMMARY 
     A gas turbine engine according to an exemplary aspect of the present disclosure includes, among other things, an airfoil having a tip wall that joins outer spanwise ends of a suction sidewall and a pressure sidewall, a tip leakage control channel recessed into an outer surface of the tip wall, a tip leakage control vane integrally formed on the tip wall adjacent to the tip leakage control channel and an air seal radially outward of the tip wall and positioned to minimize leakage at the tip wall. 
     In a further non-limiting embodiment of the foregoing gas turbine engine, the tip leakage control channel and the tip leakage control vane are curved. 
     In a further non-limiting embodiment of either of the foregoing gas turbine engines, the air seal is a grooved blade outer air seal (BOAS). 
     In a further non-limiting embodiment of any of the foregoing gas turbine engines, the grooved BOAS includes a plurality projections that extend inwardly from a radially inner surface of the grooved BOAS. 
     In a further non-limiting embodiment of any of the foregoing gas turbine engines, a plurality of grooves extend between adjacent ones of the plurality of projections to define a circuitous flow path at the radially inner surface. 
     In a further non-limiting embodiment of any of the foregoing gas turbine engines, the tip leakage control channel includes an inlet and an outlet recessed into the outer surface of the tip wall, the inlet beginning proximate a junction of the pressure sidewall and the tip wall, and the outlet terminating at a recessed portion of a junction of the tip wall and the suction sidewall. 
     In a further non-limiting embodiment of any of the foregoing gas turbine engines, the tip leakage control vane projects radially outward in a spanwise direction from a tip floor of the tip wall, a leading portion of the tip leakage control vane beginning proximate a junction of the pressure sidewall and the tip wall, and a trailing portion of the tip leakage control vane terminating proximate a junction of the suction sidewall and the tip wall. 
     In a further non-limiting embodiment of any of the foregoing gas turbine engines, a winglet extends from the tip leakage control vane at the suction sidewall. 
     In a further non-limiting embodiment of any of the foregoing gas turbine engines, a channel cooling aperture has a curvature. The channel cooling aperture feeds airflow to the tip leakage control channel from an internal cooling cavity. 
     In a further non-limiting embodiment of any of the foregoing gas turbine engines, the airfoil is a turbine airfoil. 
     A gas turbine engine according to an exemplary aspect of the present disclosure includes, among other things, an airfoil having a tip wall and a grooved blade outer air seal (BOAS) positioned radially outward of the tip wall and configured to minimize clearances at the tip wall. 
     In a further non-limiting embodiment of the foregoing gas turbine engine, the grooved BOAS includes a plurality projections that extend inwardly from a radially inner surface of the grooved BOAS. 
     In a further non-limiting embodiment of either of the foregoing gas turbine engines, a plurality of grooves extend between adjacent ones of the plurality of projections to define a circuitous flow path at the radially inner surface. 
     In a further non-limiting embodiment of any of the foregoing gas turbine engines, the plurality of projections extend inwardly at an angle relative to the radially inner surface. 
     In a further non-limiting embodiment of any of the foregoing gas turbine engines, a first projection of the plurality of projections extends at a first angle and a second projection of the plurality of projections extends at a second, different angle. 
     In a further non-limiting embodiment of any of the foregoing gas turbine engines, the airfoil includes a tip leakage control channel recessed into an outer surface of the tip wall. 
     A gas turbine engine method according to an exemplary aspect of the present disclosure includes, among other things, positioning a grooved blade outer air seal (BOAS) radially outward of a tip wall of an airfoil to define a circuitous flow path at a radially inner surface of the grooved BOAS. 
     In a further non-limiting embodiment of the foregoing method, the method includes capturing a first portion of a leakage flow in a tip leakage control channel formed into a radially outer surface of the tip wall. 
     In a further non-limiting embodiment of either of the foregoing methods, the method includes communicating a second portion of the leakage flow through the circuitous flow path. 
     In a further non-limiting embodiment of any of the foregoing methods, the method includes communicating a leakage flow along the circuitous flow path. 
     The embodiments, examples and alternatives of the preceding paragraphs, the claims, or the following descriptions and drawings including any of their various aspects or respective individual features, may be taken independently or in any combination. Features described in connection with one embodiment are applicable to all embodiments, unless such features are incompatible. 
     The various features and advantages of this disclosure will become apparent to those skilled in the art from the following detailed description. The drawings that accompany the detailed description can be briefly described as follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically depicts a cross-section of a turbofan gas turbine engine. 
         FIG. 2  shows a perspective of an exemplary gas turbine engine rotor blade. 
         FIG. 3A  is a top plan view of the rotor blade shown in  FIG. 2 . 
         FIG. 3B  is a partial cross-section of the rotor blade from  FIG. 3A . 
         FIG. 3C  is a graph showing relative pressures on opposing rotor blade surfaces. 
         FIG. 4A  depicts a vortex caused by leakage over the tip of the rotor blade. 
         FIG. 4B  shows a second view of the vortex caused by leakage between the tip and an adjacent seal surface. 
         FIG. 5A  is a magnified view of tip leakage control channels and vanes. 
         FIG. 5B  shows angles of tip leakage control channels and vanes from  FIG. 5A . 
         FIG. 5C  shows additional features to the tip leakage control channels and vanes of  FIG. 5A . 
         FIG. 6A  is a top plan view of a first alternative rotor blade tip wall configuration. 
         FIG. 6B  is a partial cross-section of the alternative rotor blade tip wall configuration shown in  FIG. 6A . 
         FIG. 7A  is a top plan view of a second alternative tip wall configuration. 
         FIG. 7B  is a partial cross-section of the alternative tip wall configuration shown in  FIG. 7A . 
         FIGS. 8A and 8B  illustrate another tip wall configuration. 
         FIGS. 9A, 9B and 9C  illustrate yet another tip wall configuration. 
         FIGS. 10A and 10B  illustrate a vortex caused by leakage between a tip wall and an adjacent seal surface. 
         FIGS. 11A and 11B  illustrate yet another tip wall configuration. 
         FIG. 12  illustrates another exemplary tip wall for an airfoil. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a cross-sectional view of gas turbine engine  10 , including low spool  12 , low pressure compressor (LPC)  14 , low pressure turbine (LPT)  16 , low pressure shaft  18 , high spool  20 , high pressure compressor (HPC)  22 , high pressure turbine (HPT)  24 , rotor blades  25 , high pressure shaft  26 , combustor  28 , nacelle  30 , propulsion fan  32 , fan shaft  34 , fan drive gear system  36 , planetary gear  38 , ring gear  40 , sun gear  42 , and fan exit guide vanes  44 . 
     In the example two-spool, high bypass turbofan configuration, low spool  12  includes low pressure compressor (LPC)  14  driven by low pressure turbine (LPT)  16  via low pressure shaft  18 . High spool  20  includes high pressure compressor (HPC)  22  driven by high pressure turbine (HPT)  24  via high pressure shaft  26 . Low pressure shaft  18  and high pressure shaft  26  are mounted coaxially and rotate at different speeds. The power core also includes combustor  28  arranged in flow series between the compressor and turbine sections. HPT  24  and LPT  16  can each include at least one stage of circumferentially distributed rotor blades  25 . More details of an example rotor blade  25  are described below. 
     Propulsion fan rotor  32  drives air through the bypass duct coaxially oriented between the engine core and nacelle  30 . Fan rotor (or other propulsion stage)  32  can be directly or indirectly rotationally coupled to low pressure shaft  18 . In advanced designs, fan drive gear system  36  couples fan shaft  34  to low spool  12 , with respective planetary, ring, and sun gear mechanisms  38 ,  40  and  42  providing independent fan speed control for reduced noise and improved operating efficiency. In more conventional turbofan designs, fan drive gear system  36  is omitted and fan  32  is driven directly as part of low spool  12 . Fan exit guide vanes (FEGVs)  44  are disposed between nacelle  30  and the engine core to reduce swirl and improve thrust performance through the bypass duct. In more compact engine designs, FEGV&#39;s may also be structural, providing combined flow turning and load bearing capabilities. 
     It will be recognized from the remainder of the description that the invention is by not limited to the example two-spool high bypass turbofan engine shown in  FIG. 1 . By way of further non-limiting examples, fan rotor  32  may additionally or alternatively include an unducted rotor, with turbine engine  10  thereby operating as a turboprop or unducted turbofan engine. Alternatively, fan rotor  32  may be absent, leaving nacelle  30  covering only the engine core, with turbine engine  10  thereby being configured as a turbojet or turboshaft engine. The described subject matter is also readily adaptable to other gas turbine engine components. While the working fluid is described here with respect to a combustion gas turbine, it will be appreciated that the described subject matter can be adapted to engines using other working fluids like steam. 
       FIG. 2  is a perspective view of turbomachine rotor blade  25 , and shows airfoil section  50 , pressure sidewall  52 , suction sidewall  54 , base  56 , leading edge  58 , trailing edge  60 , platform  62 , root  64 , tip wall  66 , tip shelf  68 , tip leakage control channels  70 , and tip leakage control vane  71 . 
     Rotor blade  25  includes airfoil  50  defined in part by pressure sidewall  52  (front) and suction sidewall  54  (back), each extending spanwise from base  56 , and chordwise between leading edge  58  and trailing edge  60 . Base  56  can include platform  62  and root  64 , which in this example of rotating blade  25 , secure airfoil  50  to a rotor disc (not shown). Tip wall  66  extends chordwise from leading edge  58  to trailing edge  60  to join respective outer spanwise ends of pressure sidewall  52  and suction sidewall  54 . Airfoil  50  can include one or more tip leakage control elements on or around tip wall  66 , such as tip shelf  68 , tip leakage control channel(s)  70 , and tip leakage control vane(s)  71 . While this example is shown as rotor blade  25 , airfoil  50  can alternatively define an aerodynamic section of a cantilevered stator vane, with attendant modifications made to the vane base for securing airfoil  50  to an outer circumferential casing. 
     In operation, pressurized gas flows generally chordwise along both sidewalls  52 ,  54  from leading edge  58  to trailing edge  60 . Airfoil  50  is provided with one or more elements on or around tip wall  66 , operating in conjunction with adjacent elements of engine  10  to reduce tip leakage losses. In this general example of blade  50 , airfoil  25  includes tip shelf  68  at the junction of pressure sidewall  52  and tip wall  66 . It will be recognized that in certain embodiments, tip shelf  68  may be omitted, leaving pressure sidewall  52  continuous up through its junction with tip wall  66 . Tip wall  66  can also include at least one tip leakage control channel  70  and/or leakage control vane  71 , such as is shown in the example embodiments below. With higher pressure differentials favored along the front-facing pressure sidewall  52 , some of the higher pressure gas flowing along suction sidewall  54  tends to leak over tip wall  66 . In traditional airfoil designs high pressure gradients from the pressure sidewall to the suction sidewall drives a leakage flow over the tip. This results in lost work extraction as the tangential momentum of the leakage flow is not changed by the airfoil, and higher aerodynamic losses as the leakage flow is reintroduced into the main passage flow. Both of these effects result in reduced efficiency. Tip leakage control channel(s)  70  and tip leakage control vane(s)  71  reduce some of the negative effects of this inevitable leakage flow. 
       FIG. 3A  is a top plan view of a first example embodiment of airfoil  50 , showing tip wall  66 .  FIG. 3B  is an upstream-facing cross-section taken along line  3 B- 3 B.  FIGS. 3A and 3B  include pressure sidewall  52 , suction sidewall  54 , leading edge  58 , trailing edge  60 , tip wall  66 , tip shelf  68 , tip leakage control channels  70 , tip leakage control vanes  71 , control channel inlets  72 , control channel outlets  74 , tip outer surface  76 , tip rib  78 , control vane leading portion  80 , control vane trailing portion  81 , control channel/vane sidewalls  82 A,  82 B, tip rib suction side  84 , control channel floors  86 , internal cooling cavity  88 , channel cooling apertures  90 , tip shelf cooling apertures  92 , and airfoil sidewall microcircuit  94 . 
     In this first example embodiment, tip wall  66  includes at least one curved tip leakage control channel  70  and at least one curved tip leakage control vane  71 . More than one leakage control channel  70  can be distributed across at least a chordwise portion of tip wall  66  extending between airfoil leading edge  58  and airfoil trailing edge  60 . Control channels  70  each include inlet  72  and outlet  74 , which may be recessed into or otherwise formed with radially outer surface  76  of tip wall  66 . One or more corresponding spanwise tip leakage control vanes  71 , can be formed with tip wall outer surface  76  between pairs of adjacent tip leakage control channels  70 . 
     Channel inlets  72 , generally disposed proximate the junction of pressure sidewall  52  and tip wall  66 . Inlets  72  can be offset widthwise from a junction in order to minimize effective tip clearance, and to block or trip some of the leakage flow G L  across wall  66 . The plurality of channel inlets  72  can be aligned along a chordwise path as shown. In certain embodiments, channel inlets are aligned with tip rib  78 , which can extend chordwise along at least a portion of the pressure side of tip wall  66  between airfoil leading edge  58  and airfoil trailing edge  60 . Tip rib  78  can be provided as part of outer surface  76  to clearance between radially adjacent engine components such as a casing, an air seal, or a rotor land (if configured as part of a stator vane). Control vanes  71  may have a leading portion  80  defined in part by chordwise-adjacent channel inlets  72 , and a trailing portion  81  defined in part by chordwise-adjacent channel outlets  74 . In this example, channels  70  are defined by sidewalls  82 A,  82 B, which can also respectively serve as control vane pressure sidewall  82 A and control vane suction sidewall  82 B. In certain embodiments, control vane leading portion  80  is contiguous with suction side  84  of tip rib  78  to further reduce leakage flow while directing the remainder into and through control channels  70 . Control vane trailing portions  81  may be contiguous with suction sidewall  52  between channel outlets  74  terminating at a recessed portion of the junction of suction sidewall  52  and tip wall  66 . 
     In this example, channel  70  has a box shaped cross-section, where adjacent channel sidewalls  82 A,  82 B extend substantially perpendicular to flat channel floor  86 . In this example, vanes  71  extend spanwise from adjacent control channel floors  86  recessed into tip wall  66  from outer surface  76 . In alternative embodiments, such as those shown in  FIGS. 6A and 6B , one or more vanes may extend spanwise from a lowered tip floor outer surface extending generally from airfoil leading edge  58  to trailing edge  60 . It will be appreciated that one or more channels  70  can additionally or alternatively have different cross-sections, and those cross-sections can vary between inlet  72  and outlet  74 . By way of one non-limiting example, one or more channels  70  can have a continuously curved sidewall to form a u-shaped cross-section. By way of other non-limiting examples, one or more channels  70  can have angled floors or sidewalls. In yet other examples, channel floor  86  can be omitted to form an upright v-shaped cross-section. Other irregular cross-sections are also possible for tailoring channels  70  and/or vanes  71  to different tip leakage profiles. 
     In certain embodiments, airfoil  50  is an internally cooled turbine blade, and includes at least one internal cooling cavity  88 . Cooling cavity  88  can be formed during investment casting of airfoil  50  using one or more shaped casting cores. The cores may be made from ceramics, refractory metals, or a combination thereof. An example of a combined ceramic and refractory metal casting core is described in commonly assigned U.S. Pat. No. 6,637,500 by Shah et al., which is herein incorporated by reference in its entirety. Other casting core technology may also be implemented. 
     One or more leakage control channels  70  can include at least one cooling aperture  90  in fluid communication with internal cavity  88  for cooling control channel  70 . While shown as a single round through hole drilled or cast into channel floor  86  proximate channel inlet  72 , aperture  90  may be one or more apertures  90 , at least some of which can have an alternative form or position tailored to the relative pressure profile over tip wall  66 . Aperture  90  may, for example, additionally or alternatively discharge coolant into channel  70  from tip rib suction side  84 , and/or channel sidewalls  82 A,  82 B. Tip shelf  68  can also additionally or alternatively include at least one pressure side cooling aperture  92  to direct coolant along the junction of pressure sidewall  52  and tip wall  66 . Apertures  92  thus may be diffusion holes or slots to create an effective wall of coolant along tip shelf  68  for impeding tip leakage. Apertures  92  can be in fluid communication with cooling cavity  88 , which may be the same cavity  88  feeding apertures  90 , or it may be a separate cooling cavity. In certain embodiments, pressure sidewall  52  and suction sidewall  54  can include one or more microcircuit cooling cavities  94  formed separate from or contiguous with internal cooling cavity  88 . Microcircuit(s)  94 , which may be formed in pressure sidewall  52  and/or suction sidewall  54  using at least one refractory metal casting core, can optionally be in fluid communication with apertures  92  or can alternatively feed coolant to dedicated apertures. 
       FIG. 3C  is a graph of relative pressures around tip wall  66 .  FIG. 3C  graphically shows the pressure relationship across tip wall  66  with vertical plotting of pressure ratio P s /P t  at relative chordwise positions x/x t . P s  is the localized static pressure and P t  is the localized total pressure. For simplicity, the scale x/x t  is measured linearly along chordwise tip rib suction side  84 , which also doubles in this example as an upstream wall of each channel inlet  72 . Other relative scales may be used giving slightly different relative pressure readings, but the relative pressure profiles will be substantially consistent across these different scales. 
     As is expected, pressure side (PS) and suction side (SS) readings of P s /P t  are equal when x/x t  is at point 0, corresponding to leading edge  58 . The pressure differential ΔP increases then decreases chordwise until being equal again when x/x t  reaches point 1, corresponding to trailing edge  60 . The exact pressure relationship along tip wall  66  will depend on operating conditions, sweep of the airfoil, relative curvatures of pressure sidewall  52  and suction sidewall  54 , among other factors. It can be seen that in the example turbine airfoil  50 , there is a fairly large ΔP range around the midchord region of the tip. Around midchord, the PS pressure has not yet fallen off, while the SS pressure drops to a minimum before recovering close to trailing edge  60 . Here, the maximum pressure differential ΔP max  between pressure side flow G P  and suction side flow G S  occurs just forward of midchord. 
     As seen in  FIG. 3A , tip wall  66  includes tip leakage control channels  70  and control vanes  71  at various relative chordwise positions x/x t . To illustrate operation of one such channel  70  disposed roughly midchord along tip wall  66 , FIG.  3 C shows approximations of the relative pressure differentials ΔP 1  and ΔP 2 , comparing leakage flow paths in a conventional blade tip versus a blade tip having at least one channel  70 . With a conventional tip, leakage flow G L  originating around point P 1  of pressure sidewall  52  will take the shortest path over the tip wall toward point S 1  roughly perpendicular to pressure sidewall  52  at that point. Leakage flow G L  thus collides roughly perpendicular with suction flow G S  around point S 1 . Since airfoil  50  is in relative rotational motion along with gas flows G P  and G S , and because pressure side gas flow G P  is necessarily slower than G S , leakage flow G L  between points P 1  and S 1  has virtually zero relative chordwise momentum, and high circumferential momentum, compared to substantial chordwise momentum of suction flow G S . 
     In contrast, tip leakage control channel  70  captures a localized portion of leakage flow G L  at inlet  72 , and redirects it through a curved portion of channel  70  toward airfoil trailing edge  60 . The redirected flow is ejected from the channel, entering the suction side gas stream proximate point S 2 , downstream of the normal point of entry S 1 . Since channel  70  can be recessed below the outermost surface of tip wall  66 , the flow enters the suction side gas stream below the junction of surface  76  and suction sidewall  54 . In this example, P s /P t  is actually greater at point S 2  than at point S 1 , reducing the magnitude of leakage based on a smaller pressure differential ΔP 2 . By redirecting the entry point of leakage flow G L  downstream toward point S 2 , channel  70  (and control vane  71 ) also imparts/converts a portion of the momentum into an increased chordwise component, which necessarily reduces the conflicting widthwise momentum component of the leakage flow perpendicular to suction side flow G S . This has two positive effects on the gas flows. 
     Redirecting leakage momentum downstream from point S 1  allows leakage flow G L  to more quickly integrate into G S , closer to suction sidewall  54 . Decreasing widthwise momentum and/or increasing the tangential momentum of leakage flow G L  entering suction side flow G S  reduces conflict and turbulence at the entry point(s) by permitting less penetration of leakage flow G L  to into the main flow path of suction gas flow G S . With a larger chordwise (tangential) momentum component aligned with flow G S , there also ends up being less boundary flow disturbance of suction gas flow G S , reducing flow separation around tip wall  66 . All of these increase efficiency by reducing the size and strength of resulting tip leakage vortices as shown in  FIGS. 4A and 4B . 
       FIG. 4A  shows tip leakage vortices exiting channel  70  and being integrated into suction gas flow G S , and includes pressure sidewall  52 , suction sidewall  54 , airfoil leading edge  58 , airfoil trailing edge  60 , tip wall  66 , tip shelf  68 , and tip leakage control channels  70 , and control channel cooling apertures  90 .  FIG. 4B  is a cross-section of tip wall  66  taken through channel  70  with air seal  96  radially adjacent tip wall  64  to minimize clearance at tip wall  66 . Optional internal details of the blade and air seal  96 , such as cavities and cooling apertures have been omitted for clarity and to better illustrate the effects of leakage control cavity  70 . 
     Air seal  96  cooperates with tip wall  66  to minimize clearance, and overall tip leakage therebetween. Air seal  96  can be any conventional or inventive blade outer air seal (BOAS) compatible with an unshrouded rotor blade. Air seal  96  may optionally include a sacrificial layer to reduce rubbing damage to tip rib  78 , or more generally to tip wall  66 , during maximum centrifugal and thermal expansion of airfoil  50  relative to the surrounding casing (not shown) onto which air seal  96  is mounted. 
     As explained above with respect to  FIG. 3C , large pressure differentials across tip wall  66  result in leakage flow G L  passing over tip shelf  68  and tip rib  78  with substantial initial widthwise momentum. Channels  70  and vanes  71  help capture a portion of that flow, and redirect it downstream as it passes over tip wall  66 , providing channel flow G C  with increased chordwise momentum compared to other leakage flow G L . The portion of leakage flow G L  flowing through channel  70  joins with suction gas flow G S  at a higher pressure downstream location. Entry flow G E  results in vortex V when joining suction gas flow G S . However, position of vortex V is smaller and closer to suction sidewall  54  than it would be absent leakage control channels  70  and vanes  71 . The magnitude of vortex V can also be reduced due to addition of chordwise momentum to entry flow G E  from channels  70  and vanes  71 . 
       FIG. 5A  shows two adjacent tip leakage control channels  70  bounding an intermediate tip leakage control vane  71 , and also includes pressure sidewall  52 , suction sidewall  54 , tip wall  66 , tip shelf  68 , tip leakage control channels  70 , tip leakage control vanes  71 , control channel inlets  72 , control channel outlets  74 , tip outer surface  76 , tip rib  78 , vane leading portion  80 , vane trailing portion  81 , control channel/vane sidewalls  82 A,  82 B, tip rib suction side  84 , and control channel floors  86 . Channel cooling apertures  90  and tip shelf cooling apertures  92  have been omitted for clarity. 
     As seen here, channel inlets  72  have first chordwise width W 1  proximate channel inlet  72 , and outlets have second chordwise width W 2  proximate channel outlet  74 . In certain embodiments, second chordwise width W 2  proximate outlet  74  is equal to or less than first chordwise width W 1 . In alternative embodiments, second width W 2  is greater than first width W 1  Similarly control vane  71  includes leading chordwise thickness t 1  proximate tip rib  78 , and trailing chordwise thickness t 2  proximate suction sidewall  54 . In certain embodiments, trailing chordwise thickness t 2  is equal to or less than leading chordwise width t 1 . In alternative embodiments, thickness t 1  is greater than thickness t 2 . Adjacent channels  70  can be separated by pitch P c , which is an average distance between the sidewalls  82 A,  82 B of adjacent channels  70 . P c  is shown as average separation because the inlet and outlet widths W 1 , W 2  of individual channels  70  may vary in the same channel  70  as well as between adjacent channels. In certain embodiments P c  is constant across at least a chordwise portion of tip wall  66 . Pitch P c  may vary elsewhere along tip wall  66  based on relative curvatures of channels  70  and vanes  71 , described below in  FIG. 5B . 
       FIG. 5B  shows the relative angles of channels  70  and vanes  71  shown in  FIG. 5A . Channel inlet  72  forms channel entrance interior angle α 1  relative to pressure sidewall  52 . Channel outlet  74  forms channel exit interior angle α 2  relative to suction sidewall  54 . In certain embodiments, angle α 2  is less than or equal to α 1 . The effective curvature radius of channel  70  is thus the same or greater than the local curvature radius of tip wall  66 . 
     In certain of those embodiments, channel entrance angle α 1  is between about 80° and about 95°. Channel entrance angle α 1  may be greater than 90° when leakage flow G L  cascading over the upper region of pressure sidewall  52  is expected to have a substantial chordwise flow component relative to the motion of airfoil  50  at the leakage point. When leakage flow G L  is expected to have substantially zero chordwise momentum around the leakage point, channel entrance angle α 1  may be less than or equal to about 90°. This may occur, for example, as a result of tip shelf  68  (and shelf cooling apertures  92 ) reducing net leakage flow G L . 
     Flow out of channel cooling apertures  90  (not shown in  FIG. 5B ) may also impart a counteracting momentum component to channel flow G C  once leakage flow G L  has entered inlet  72 . In certain embodiments, channel exit angle α 2  can be less than about 90° which can impart additional chordwise momentum to channel flow G C  exiting outlet  74 . Similarly, control vane leading portion  80  forms leading interior angle β 1  relative to pressure sidewall  52 , and control vane trailing portion  81  forms trailing interior angle β 2  relative to suction sidewall  54 . Angles β 1  and β 2  are measured around the respective chordwise midpoint of vane leading and trailing portions  80 ,  81 . In certain embodiments, angle β 2  is less than or equal to β 1 . 
       FIG. 5B  also illustrates another effect of tip leakage control channels  70  and vanes  71 . Since tip leakage flow typically enters channel  70  approximate perpendicular to the local junction with pressure sidewall  52 , channel gas flow will tangentially strike sidewall  82 A before being redirected. This can cause initial flow turbulence directly above channel inlet  72 , which introduces tip vortices into the clearance gap between tip wall  66  and air seal  96  (shown in  FIG. 4B ), blocking a portion of additional leakage flow G L  over tip wall  66 . These clearance vortices can be further enhanced by coolant flow directed outward from channel cooling apertures  92 . In addition, tangential contact of channel flow G C  with channel/vane sidewall  82 A imparts a measure of thrust onto vane  71 , allowing some work to be recovered from leakage flow. 
       FIG. 5C  is a side view of airfoil  50  with pressure sidewall  52 , suction sidewall  54 , tip wall  66 , tip shelf  68 , tip leakage control channel  70 , tip leakage control vane  71 , leakage control channel inlet  72 , leakage control channel outlet  74 , tip rib  78 , leakage control vane leading portion  80 , leakage control vane trailing portion  81 , leakage control vane sidewall  82 B, tip rib suction side wall  84 , and leakage control channel floor  86 . 
       FIG. 5C  shows the radial dimensions of leakage control channel and vane  70 ,  71 . As in  FIG. 4B , internal details of the blade such as cavities and cooling apertures have been omitted to show the remaining reference dimensions of leakage control channel  70  and vane  71 . It can be seen here that leakage control channel  70  can have first depth d 1  measured for example proximate inlet  72  and second depth d 2  proximate outlet  74 . The depths are generally the respective distances that channel floor  86  is recessed relative to an upper portion of tip wall  66 . This upper portion may be the top of tip rib  78  and/or control vane  71 , but in certain embodiments, first and second depths d 1 , d 2  may be measured from different upper surface. 
     Similarly, a first height h 1  of leakage control vane  71  is measured around its leading portion  80 , and a second height h 2  is measured around the trailing portion  81 . These heights h 1  and h 2  are typically determined relative to channel floor  86 . In certain embodiments, however, heights h 1  and h 2  can be determined relative to tip floor  76 . 
     Depending on pressure differentials along a particular airfoil  50  (e.g., as shown in  FIG. 3C ), dimensions, separation, and curvature of channels  70  and vanes  71  may be adapted to optimize redirection of leakage flow while still maintaining adequate tip cooling and material strength. Dimensions of channels  70  and vanes  71  will also affect the practical dimensions and angles that can be achieved. These relative dimensions and pitches can be adapted optimize performance based on expected or modeled pressure differentials at different locations around tip wall  66 . Relative curvatures can be defined first with dimensions resulting therefrom, or relative dimensions may be defined with resulting curvatures. The overall configuration of tip wall  66  can also be determined iteratively based on one or more constraints of the various widths, thicknesses, pitches, and angles. 
     For example, in locations where leakage is most likely, such as proximate midchord where pressure side pressures are highest, channels  70  may have a wider inlet W 1 . They can also be provided with a narrower outlet width W 2  relative to W 1 , which can increase the pressure and exit velocity of leakage flow G L  entering the suction gas stream G S . In other embodiments, at locations with lower relative suction side pressures, W 2  may be the same as or even greater than W 1  in order to more closely match the entry pressure and velocity. Further, the relative and absolute pressures, along with available tip wall surface area, will also determine the pitch P c  of channels  70 . As before, the midchord region of tip wall  66  may have smaller pitch values P c . It will be recognized that widths W 1  will generally vary inversely with thickness t 1  and vice versa. Similarly, widths W 2  generally vary inversely with thickness t 2  and vice versa. As also explained below, channel floor  86  may be sloped such that d 2  is less than d 1 . 
       FIG. 6A  shows tip wall  166 , which also includes pressure sidewall  52 , suction sidewall  54 , airfoil leading edge  58 , airfoil trailing edge  60 , tip shelf  168 , tip leakage control channel  170 , tip leakage control vanes  171 , control channel inlet  172 , control channel outlets  174 , tip floor  176 , tip rib  178 , control vane leading portions  180 , control vane trailing portions  181 , control channel/vane sidewalls  182 A,  182 B, tip rib suction side  184 , control channel floor  186 , channel cooling aperture  190 , tip shelf cooling apertures  192 , and airfoil sidewall microcircuit  194 .  FIG. 6B  is a cross-section taken across line  6 B- 6 B of  FIG. 6A . 
     Tip wall  166  is a first alternative embodiment of tip wall  66  described above. Similar to  FIG. 2 , pressure sidewall  52  and sidewall  54  each extend spanwise from airfoil base  56  between leading edge  58  and trailing edge  60 . Airfoil  50  also includes tip wall  166  extending chordwise from leading edge  58  to trailing edge  60 . In this first example alternative embodiment, at least one curved tip leakage control vane  171  projects radially outward in a spanwise direction from tip wall  166 . Control vanes  171  each include leading vane portion  180  and trailing portion  181  between adjacent sidewalls  182 A,  182 B. Control vane leading portion  180  begins proximate a junction of airfoil pressure sidewall  52  and tip wall  166 . Control vane trailing portion  181  terminates proximate a junction of airfoil suction sidewall  54  and tip wall  166 . 
     In this first alternative embodiment, rather than having channels recessed into a tip floor as shown in  FIGS. 3A and 3B , vanes  171  extend radially outward from a lower tip floor  176  extending at least partway between airfoil leading edge  58  and trailing edge  60 . More than one leakage control vane  171  can be distributed chordwise across at least a portion of tip floor  176 . A corresponding curved tip leakage control channel  170  can be defined at least in part by adjacent ones of the plurality of tip leakage control vanes  171 . As above, channel inlet  172  can be defined by adjacent control vane leading portions  180 , and channel outlet  174  can be defined by adjacent control vane trailing portions  181 . 
     Tip rib  178  can also project spanwise from at least a chordwise portion of tip wall  166 . In certain embodiments, tip rib  178  can extend at least partway between airfoil leading edge  58  and airfoil trailing edge  60  along pressure side of tip wall  166 , outward from tip floor  176 . In certain embodiments, such as is shown in  FIG. 6A , of the control vane leading portion  196  can be contiguous with tip rib suction side surface  184 . Tip shelf  168  with cooling apertures  192  (in fluid communication with internal cooling cavity  188  and/or microcircuits  194 ) can be recessed into a pressure side surface of tip rib  178 . Alternatively, the pressure side surface of tip rib  178  can be an extension of pressure sidewall  52 . 
     Similar to the illustrations shown in  FIGS. 5A and 5B , control vane  171  can have leading portion  180  forming leading interior angle β 1  relative to pressure sidewall  52 , and vane trailing portion forming interior angle β 2  relative to suction sidewall  54 . In certain embodiments, angle β 2  can be less than or equal to β 1 , which increases the chordwise component of leakage flow to reduce vortices and enhances work recovery from the captured leakage flow as described above with respect to  FIGS. 5A and 5B . 
       FIG. 7A  shows tip wall  266 , which also includes pressure sidewall  52 , suction sidewall  54 , airfoil leading edge  58 , airfoil trailing edge  60 , tip shelf  268 , ramped tip leakage control channel  270 , tip leakage control vanes  271 , control channel inlets  272 , control channel outlets  274 , tip floor  276 , tip rib  278 , control vane leading portions  280 , control vane trailing portions  281 , control channel/vane sidewalls  282 A,  282 B, tip rib suction side  284 , ramped control channel floor  286 , channel cooling aperture  290 , tip shelf cooling apertures  292 , and airfoil sidewall microcircuit  294 .  FIG. 6B  is a cross-section taken across line  6 B- 6 B of  FIG. 6A . 
     Tip wall  266  is a second alternative embodiment of tip wall  66  described above. Similar to  FIG. 2 , pressure sidewall  52  and sidewall  54  each extend spanwise from airfoil base  56  between leading edge  58  and trailing edge  60 . Airfoil  50  also includes tip wall  266  extending chordwise from leading edge  58  to trailing edge  60 . In this second example alternative embodiment, tip shelf  266  includes at least one curved and ramped tip leakage control channel  270 . 
     In this second example alternative embodiment, leakage control channel floor  286  is ramped upward, in contrast to the substantially flat channel floor  86  shown in  FIGS. 3A and 3B . Thus, control channel  270  is shallower at the suction side exit than at the pressure side inlet. As a result, leakage flow G L  is provided with increased pressure and exit velocity is decreased, allowing for more uniform leakage flow entering suction gas stream G S . 
     Leakage flow can be further controlled by widening control channel outlets  274 . As was shown in  FIGS. 5A and 5B , leakage control channels have an inlet width W 1  that may be the same as or differ from outlet width W 2 . In certain embodiments, including but not limited to the second alternative example embodiment of  FIGS. 7A and 7B , outlet width W 2  is greater than inlet width W 1  so as to further improve uniformity of the leakage flow entering suction gas stream G S . In certain of those embodiments, outlet width W 2  is approximately equal to the pitch P C  between adjacent control channels (see  FIG. 5A ). In such embodiments, as outlet width W 2  approaches the local channel pitch P C , the trailing chordwise thickness t 2  of adjacent leakage control vanes  271  will be less than leading control vane thickness t 1  and will approach zero at the junction of suction sidewall  54  and tip wall  266 . These shapes also contribute to reduced flow separation and tip vortices adjacent leakage control vanes  271 . 
       FIGS. 8A and 8B  illustrate a tip wall  366  of an airfoil  50 . The tip wall  366  is another alternative embodiment of the tip wall  66  described above. 
     The tip wall  366  includes a pressure sidewall  352 , a suction sidewall  354 , a tip shelf  368 , a tip leakage control channel  370  and a tip leakage control vane  371 . The tip leakage control channel  370  includes an inlet  372 , an outlet  374 , and a control channel floor  386 . A tip rib  378  includes a tip rib suction side  384  that establishes an endwall near the inlet  372  of the tip leakage control channel  370 . The control channel floor  386  of the tip leakage control channel  370  extends axially between tip leakage control vane sidewalls  382 A,  382 B (only sidewall  382 B shown in  FIG. 8A ). 
     In this embodiment, a plurality of radiused walls  369  are formed on the inner diameter corners of each tip leakage control vane  371 . For example, a radiused wall  369  may connect the tip rib suction side  384  to the control channel floor  386  of the tip leakage control channel  370 . The vane sidewalls  382 A and  382 B may also be connected to the control channel floor  386  via radiused walls  369  (best shown in  FIG. 8B ). 
     Among other benefits, the radiused walls  369  provide smooth surfaces for airflow to flow across as the airflow circulates through the tip leakage control channels  370 . The radiused walls  369  may additionally reduce the potential for cracking at sharp corners of the tip wall  366 . 
     The radiused walls  369  may include any radius. The radius may depend on design specific parameters, including but not limited to the cooling requirements of the airfoil  50 . 
     Other portions of the airfoil  50  may additionally or alternatively include radiused walls  369 . For example, as best shown in  FIG. 8A , a corner  367  defined between a sidewall  361  and an endwall  363  of a cooling cavity  388  formed inside the airfoil  50  may include a radiused wall  369 . This positioning of the radiused wall  369  may enable improved flow of airflow into a channel cooling aperture  390  that feeds the tip leakage control channel  370  from the cooling cavity  388 . 
     The locations of the radiused walls  369  shown in  FIGS. 8A and 8B  are intended as non-limiting. It should be appreciated that other locations of the tip wall  366  could benefit from radiused walls  369 . In addition, the use of radiused walls  369  may be combined with any other feature described in this disclosure without departing from the scope of the disclosure. 
       FIGS. 9A, 9B and 9C  illustrate yet another embodiment of a tip wall  466  of an airfoil  50 . The airfoil  50  includes a pressure sidewall  452 , a suction sidewall  454 , an airfoil leading edge  458 , an airfoil trailing edge  460 , a tip wall  466 , a tip shelf  468 , and tip leakage control channels  470  dispersed between tip leakage control vanes  471 . In one embodiment, the tip leakage control channels  470  and tip leakage control vanes  471  are curved. 
     In this embodiment, the tip wall  466  includes a winglet  401  that extends from the suction sidewall  454  of the airfoil  50 . In one non-limiting embodiment, the winglet  401  is formed at a junction between the suction sidewall  454  and the tip leakage control vane  471 . The winglets  401  can reduce the inducement of a vortex V that is results from a leakage flow G L  joining suction gas flow G S  near the suction sidewall  454 . 
     In a first embodiment, the winglet  401  may span the entire distance between the leading edge  458  and the trailing edge  460  of the airfoil  50  (see  FIG. 9A ). The winglet  401  is located adjacent to the suction sidewall  454 , in one embodiment. 
     In an alternative embodiment, each tip leakage control vane  471  includes a winglet  401  (see  FIG. 9B ). Each winglet  401  represents a discrete portion of the tip wall  466  that extends from each tip leakage control vane  471 . In other words, the winglets  401  may be localized features of each tip leakage control vane  471 . The winglets  401  may be formed on the control vane trailing portions  481  of the tip leakage control vanes  471 . 
     The winglets(s)  401  may be used either alone or in combination with any other tip wall feature described in this disclosure. By way of one non-limiting example, the winglet  401  could be used in combination with the tip wall  366  described above that includes one or more radiused walls  369 . 
       FIGS. 10A and 10B  illustrate yet another tip wall  566  of an airfoil  50 . In this embodiment, the tip wall  566  is positioned relative to an air seal  596 . The air seal  596  is positioned radially outward of the tip wall  566  to minimize clearances at the tip wall  566 . 
     In one embodiment, the air seal  596  is a grooved BOAS that includes a plurality of projections  505  that extend inwardly from a radially inner surface  507  of the air seal  596 . Grooves  509  extend between adjacent projections  505  and define pockets for circulating leakage airflow along a circuitous path. The “grooved” air seal  596  cooperates with the tip wall  566  to minimize clearances and overall tip leakage that may occur between the airfoil  50  and the air seal  596 . Although shown angled in this embodiment, the projections  505  and/or grooves  509  can be at any angle of 90° or less. In addition, each projection  505 /groove  509  may not necessarily extend at the same angle. Furthermore, although the projections  505 /grooves  509  are shown positioned axially relative to one another in  FIGS. 10A and 10B , these features could be circumferentially arranged. Other arrangements are also contemplated. 
     Referring to  FIG. 10B , the tip wall  566  could optionally include one or more winglets  501  (similar to those described in  FIGS. 9A, 9B and 9C ). The combination of a “grooved” air seal  596  and the winglets  501  desensitizes the mechanical tip clearance of the tip wall  566 . Other tip wall embodiments could alternatively be used in combination with a grooved BOAS. 
       FIGS. 11A and 11B  illustrate additional embodiments of a tip wall  666 . The tip wall  666  can include a pressure sidewall  652 , a suction sidewall  654 , a tip shelf  668 , a tip leakage control channel  670 , a tip leakage control vane  671 , a control channel floor  686 , an internal cooling cavity  688 , a channel cooling aperture  690 , and, optionally, a tip shelf cooling aperture  692  (shown omitted in  FIG. 11B ). 
     In this embodiment, the channel cooling aperture  690  includes a curvature  615 . The curvature  615  directs airflow at a specific angle into the tip leakage control channel  670  from the internal cooling cavity  688  located inside of the airfoil  50 . In the embodiment shown in  FIG. 11A , the channel cooling aperture  690  opens through the control channel floor  686  and divides the floor  686  into a stepped surface  617  in order to accommodate the curvature  615  of the channel cooling aperture  690 . The stepped surface  617  includes a first surface  619  that is elevated relative to a second surface  621 . The first surface  619  extends from a tip rib suction side  684  of a tip rib  678 , while the second surface  621  extends from the suction sidewall  654 . 
     Referring to  FIG. 11B , a channel cooling aperture  690 - 2  with curvature  615 - 2  extends further into the tip leakage control channel  670  than the FIG.  11 A embodiment. In one embodiment, the channel cooling aperture  690 - 2  extends to an outlet  674  of the tip leakage control channel  670  and is used in conjunction with a winglet  601  that extends from the tip leakage control vane  671  at the suction sidewall  654  of the tip wall  666 . The channel cooling apertures  690 ,  690 - 2  may be individual holes or could be continuations of the internal cooling cavity  688 , such as a slot, etc. 
     In another embodiment, a tip outer surface  676  of the tip wall  666  includes a first surface  623  and an angled surface  625 . The first surface  623  extends from the tip shelf  668  to the tip rib suction side  684  and may be at least partially flat. The angled surface  625  may extend radially inwardly from the first surface  623  into the tip leakage control channel  670 . The angled surface  625  establishes a relatively smooth surface for directing airflow into and through the tip leakage control channel  670  from a location outside of the airfoil  50 . 
     The angled surface  625  may be utilized in a tip wall configuration either alone or in combination with any other features. This disclosure is not limited to the exact configuration shown in  FIG. 11B , which shows, as a non-limiting embodiment, a tip wall that includes a combination of features including the angled surface  625 , winglet  601  and channel cooling aperture  690  with curvature  615 . 
       FIG. 12  illustrates yet another non-limiting embodiment of a tip wall  766 . The exemplary tip wall  766  can include a pressure sidewall  752 , a suction sidewall  754 , a tip shelf  768 , a tip leakage control channel  770 , a tip leakage control vane  771 , an internal cooling cavity  788 , and one or more airfoil sidewall microcircuits  794  (or skin cores/radial flow passage skin cores). 
     In this embodiment, the airfoil sidewall microcircuit  794  feeds airflow F directly into the tip leakage control channel  770 . The airfoil sidewall microcircuit  794  includes curved portions  799  that alter a flow direction of the airflow F within the airfoil sidewall microcircuit  794 . In one embodiment, one of the curved portions  799  is located near an inlet  772  of the tip leakage control channel  770 . In this way, the airflow F is aligned in the direction of the tip leakage control channel  770  as it is communicated into the inlet  772 . Put another way, the airflow F is communicated generally parallel to a control channel floor  786  of the tip leakage control channel  770  by incorporating the curved portions  799  into the airfoil sidewall microcircuit  794 . 
     Although the different non-limiting embodiments are illustrated as having specific components, the embodiments of this disclosure are not limited to those particular combinations. It is possible to use some other components or features from any of the non-limiting embodiments in combination with features or components from any of the other non-limiting embodiments. 
     It should be understood that like reference numerals identify corresponding or similar elements through the several drawings. It should also be understood that although a particular component arrangement is disclosed and illustrated in these exemplary embodiments, other arrangements can also benefit from the teachings of this disclosure. 
     The foregoing description shall be interpreted as illustrative and not in any limiting sense. A worker of ordinary skill in the art would understand that certain modifications could come within the scope of this disclosure. For these reasons, the following claims should be studied to determine the true scope and content of this disclosure.