Classifications

• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
  • F01D—NON-POSITIVE DISPLACEMENT MACHINES OR ENGINES, e.g. STEAM TURBINES
  • F01D11/00—Preventing or minimising internal leakage of working-fluid, e.g. between stages
  • F01D11/02—Preventing or minimising internal leakage of working-fluid, e.g. between stages by non-contact sealings, e.g. of labyrinth type
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
  • F01D—NON-POSITIVE DISPLACEMENT MACHINES OR ENGINES, e.g. STEAM TURBINES
  • F01D11/00—Preventing or minimising internal leakage of working-fluid, e.g. between stages
  • F01D11/08—Preventing or minimising internal leakage of working-fluid, e.g. between stages for sealing space between rotor blade tips and stator
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F04—POSITIVE - DISPLACEMENT MACHINES FOR LIQUIDS; PUMPS FOR LIQUIDS OR ELASTIC FLUIDS
  • F04D—NON-POSITIVE-DISPLACEMENT PUMPS
  • F04D29/00—Details, component parts, or accessories
  • F04D29/40—Casings; Connections of working fluid
  • F04D29/52—Casings; Connections of working fluid for axial pumps
  • F04D29/522—Casings; Connections of working fluid for axial pumps especially adapted for elastic fluid pumps
  • F04D29/526—Details of the casing section radially opposing blade tips
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F04—POSITIVE - DISPLACEMENT MACHINES FOR LIQUIDS; PUMPS FOR LIQUIDS OR ELASTIC FLUIDS
  • F04D—NON-POSITIVE-DISPLACEMENT PUMPS
  • F04D29/00—Details, component parts, or accessories
  • F04D29/66—Combating cavitation, whirls, noise, vibration or the like; Balancing
  • F04D29/68—Combating cavitation, whirls, noise, vibration or the like; Balancing by influencing boundary layers
  • F04D29/681—Combating cavitation, whirls, noise, vibration or the like; Balancing by influencing boundary layers especially adapted for elastic fluid pumps
  • F04D29/685—Inducing localised fluid recirculation in the stator-rotor interface

Definitions

• This invention relates to axial flow rotary machines, and more particularly to a casing extending circumferentially about a compression stage for a gas turbine engine.
• An axial flow, rotary machine such as a gas turbine engine for powering aircraft, has a compression section having a plurality of compression stages. As the working medium gases are drawn into the engine, the gases are compressed to raise the temperature and pressure of the gases. The gases are burned with fuel and expanded through a turbine to develop useful power and work. The work is transferred from the turbine section to the compression section via a rotor assembly. The rotor assembly in the compression section does work on the incoming gases to compress the gases.
• a compression section is the fan section of a high bypass ratio turbofan engine.
• the fan section may have a diameter of six to eight feet.
• a plurality of fan blades extend radially outward from the rotor assembly across the flowpath for working medium gases.
• the fan blades may be rotated at speeds in excess of 2,500 revolutions per minute about an axis of rotation. The rotating fan blades drive the gases rearwardly, compressing the working medium gases as the gases enter the fan section of the engine.
• Each fan blade has a tip.
• the tip is shrouded by the non-rotating adjacent casing; the fan blades do not have a shroud at the tip that rotates with the blades.
• a clearance is provided between the tips and the adjacent casing to accommodate transient movement of the blade outward with respect to the casing.
• This region is commonly referred to as the end wall region.
• the transient movement may occur in response to maneuver loads and the normal growth of the blade in response to rotational forces acting on the blade.
• the end wall region experiences different aerodynamic conditions than does the remainder of the compression stage. In the end wall region, the aerodynamic interaction between the tip of the rotor blade and the adjacent wall causes boundary layer effects and drag effects. These effects make it more difficult to force the flow rearwardly.
• the recess is provided with circumferentially extending grooves ( Figure 3).
• the grooved construction provides an increase in surge margin over a smooth wall
• Figure 6 curve C with a decrease in aerodynamic efficiency.
• the wall might be provided with a plurality of axially extending skewed chambers (Figure 2) or a combination of both a circumferentially extending grooves and axially extending skewed chambers (Figure 4).
• Figure 4 construction shows the greatest increase in surge margin with the smallest decrease in aerodynamic efficiency over a smooth wall.
• the axially skewed chambers decrease efficiency because the rotor blades use some of the work transferred to them to pump and re-pump the working medium gases in the axially skewed chamber.
• Pumping increases the level of pressure attainable in the end wall region in response to a sudden back pressure.
• pumping is the mechanism by which the axially extending chambers increase surge margin.
• the circumferentially grooved construction has a moderate gain in surge margin and a moderate loss in the aerodynamic efficiency in comparison to the smooth wall.
• This invention is predicated upon the recognition that interrupting circumferential grooves with a critical number of dams related to the number of adjacent fan blades results in an improvement in surge margin with a negligible effect on aerodynamic efficiency.
• the number of circumferentially spaced dams lies in a range which is a function of the number of blades of the array of rotor blades; and, the circumferentially spaced grooves are disposed radially outwardly only of the mid-chord region of the rotor blade to increase the level of the effect of the grooves and dams on the boundary layer and decrease the impact on the aerodynamic efficiency.
• a primary feature of the present invention is a casing radially outw.ard of an array of rotor blades in a compression section.
• the casing has a plurality of circumferentially extending grooves which are each circumferentially interrupted by a plurality of dams.
• the dams are circumferentially spaced and extend in grooves having a depth which twice the axial width of the groove.
• the lands extending between adjacent grooves are half the width of the axial width of the grooves.
• the number of dams lies in a range between three-fourths the number of rotor blades and one and one-half times the number of rotor blades.
• the circumferential length of the dams is approximately equal to the width.
• the number of dams in each groove is equal to the number of rotor blades and the dams are equally spaced about the circumference of the groove.
• the grooves are disposed in a groove region which extends over the mid-chord region of the blade such that the groove region and the adjacent surface of the case outwardly of the blades are at the same radial height.
• the adjacent surface of the case may be either recessed with respect to the inner surface of the casing or at the same radial height as the inner surface of the casing.
• Still another advantage of the present invention is the incremental increase in surge margin for a given decrease in aerodynamic efficiency which results from use of the circumferentially interrupted grooves whether the tip of the fan blade is at the same radial height as the remainder of the casing or recessed with respect to the surface of the casing.
• Figure 1 is a perspective view of a turbofan gas turbine engine showing a casing which circumscribes the tips of the fan blades.
• Figure 2 is a developed view (looking outwardly) of a portion of the fan casing shown in Figure 1, illustrating the relationship of circumferentially grooves which are interrupted by circumferentially spaced dams.
• Figure 3 is a developed view corresponding to the view shown in Figure 2.
• Figure 4 is an alternate embodiment of the construction shown in Figure 2 in which the grooves are staggered circumferentially.
• Figure 5 is a side-elevation view taken along the lines 3-3 of Figure 2.
• Figure 6 is a graphical representation of the change in percentage points of surge margin and aerodynamic efficiency for the construction shown in Figure 3 to the number of circumferentially spaced dams in the circumferential grooves expressed as a function of the number JVof blades in the array of rotor blades.
• Figure 1 is a perspective view of a turbofan gas turbine engine 10 of the type used to power commercial airliners.
• the engine includes a core section 12 and a fan section 14.
• a plurality of unshrouded fan blades 16 extend radially outward from a rotor 18.
• the rotor is disposed about an axis of rotation -4.
• Each fan blade has an airfoil 22 having a tip 24.
• the tip does not have a rotating shroud, but may have a part span shroud.
• the fan blade has a platform 26 which adapts the fan blade to engage an attachment slot in the rotor assembly.
• the length of the fan blade may exceed two feet and in the particular embodiment shown, is approximately twenty-seven (27) inches long as measured from the platform.
• the outer diameter of the tip of the fan blade is about ninety-four (94) inches as measured from the axis of rotation A.
• Figure 2 is an undeveloped view and Figure 3 is a developed view of the casing 34 looking outwardly toward the inner surface 36 of the casing.
• a plurality of grooves 38 extends circumferentially about the interior of the casing.
• a plurality of dams 42 are disposed in each groove. Each dam extends axially across the groove and radially in the groove to circumferentially interrupt the groove.
• Each dam is spaced from the adjacent dam by a pitch distance P.
• the pitch distance is the distance from a point on one dam to the corresponding point on an adjacent dam.
• the pitch distance P is equal to the circumference of the inner surface divided by the number of dams.
• the number of dams is equal, for example, to the number of blades JVof the array of rotor blades.
• Two rotor blade tips 24, each associated with an adjacent fan blade 16, are shown in Figure 2 in full and in Figure 3 in phantom.
• the rotor blades sweep upwardly (Figure 2) and from right to left ( Figure 3) under operative conditions as shown by the arrow V which indicates the direction of motion of the fan blades.
• Each fan blade has a leading edge 44 and a trailing edge 46.
• a suction sidewall 48 on one side of the blade extends from the leading edge to the trailing edge.
• a pressure sidewall 52 on the other side of the blade extends from the leading edge to the trailing edge.
• the fan blade has an axial length L as measured parallel to the axis of rotation.
• the fan blade has a leading edge region 54 which extends for the first twenty percent (20%) of the axial length L of the blade and a trailing edge region 56 which extends for the last twenty percent (20%) of the axial length L of the fan blade.
• the fan blade has a mid-chord region 58 which extends between the leading edge region and the trailing edge region.
• the casing has a groove region 62 which extends circumferentially at a location which is radially outwardly of the mid-chord region of the blade and contains the grooves.
• Figure 4 is a view corresponding to the view shown in Figure 3 of an alternate embodiment of the casing shown in Figure 3.
• the circumferential grooves 38 are interrupted by dams 42 which are staggered circumferentially with respect to the adjacent dam.
• FIG 5 is an enlarged, simplified cross-sectional view of the outer casing 36 and fan blade tip 24 shown in Figure 1, Figure 2, and Figure 3.
• the inner surface 36 of the casing outwardly of the fan blade tip is at the same radial location as the adjacent structure.
• the fan blade 16 may rub against the inner (abradable seal) surface 36, cutting a recess 64 which would result in a relationship to the inner surface as shown by the phantom lines.
• the surface 36 outwardly of the fan blade tip is at a different radial location than the inner surface of the adjacent structure.
• the recess may be deliberately formed in the outer case as shown, for example, in U.S. Patent No.: 4,239,452 discussed above.
• Figure 5 shows the plurality of grooves 38.
• each groove 38 has an axial width W, and a radial depth D as measured from the inner surface of the casing outwardly of the rotor blade.
• the ratio of the radial depth D to the axial width Wis greater than or equal to one.
• Each groove is spaced axially from the adjacent groove by a distance equal to one half of the axial width W, leaving a land 66 therebetween.
• a plurality of dams 42 at each groove extend axially from the land to circumferentially interrupt the full depth of the groove.
• the circumferential length L c of the dam is equal to the axial width W of the groove 38.
• Figure 6 shows the relationship of the change in aerodynamic efficiency (curve -E) and change in surge margin (curve S) for the interrupted groove 38 construction as a function of the number of dams (interruptions) 42 and pitch P.
• the number of dams is expressed in terms of the number of fan blades JV in the array of fan blades. The changes are measured in comparison to a base line construction which does not have dams that interrupt the circumferential continuity of each groove.
• the relationships shown in Figure 6 are based on an analysis of empirical data.
• dams 42 increases the surge margin with little or no decrease in the aerodynamic efficiency.
• the surge margin increases by approximately four points with no apparent decrease in aerodynamic efficiency.
• the quantity of dams increases, there is a slight decrease in aerodynamic efficiency until the quantity of dams is about equal to one and one-half times the number of rotor blades. Beyond that point, the surge margin does not measurably increase and aerodynamic efficiency continues to decrease. Accordingly, the optimum design range for the quantity of dams is believed to be equal to three-fourths to one and one-half times the number of rotor blades.
• the pitch P (spacing) between the dams for this range increases from about two-thirds of the circumference of the inner wall divided by the number NOf rotor blades to one and one-third times the circumference of the inner wall divided by the number JVof rotor blades.
• the low velocity gases in the boundary layer can cause significant losses because the boundary layer may separate from the wall 36.
• High velocity gases in the circumferentially extending grooves 38 are driven inwardly by the dams into the low energy boundary layer. These high- energy gases act to energize the boundary layer and avoid separation of the boundary layer. This avoids concomitantly the losses associated with separation of the boundary layer and results in an aerodynamic efficiency which remains constant even as the circumferential grooves provide an improvement in surge margin. Accordingly, adding the circumferentially spaced dams to the circumferential grooves allows the surge margin to increase without an unacceptable decrease in aerodynamic efficiency of the array provided the quantity of dams equals a number of fan blades within the optimum design range.
• the high velocities in the grooves are made possible by the pitch P of the dams which provides sufficient length of uninterrupted groove for the velocities to develop. In embodiments having too many dams, the high velocities never develop. In embodiments having too few dams, the high velocities develop but are not injected with desirable frequency into the boundary layer.

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• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• General Engineering & Computer Science (AREA)
• Structures Of Non-Positive Displacement Pumps (AREA)

Priority Applications (3)

Applications Claiming Priority (2)

Publications (1)

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Family Applications (1)

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Cited By (17)

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Citations (5)

• 1995
  • 1995-05-24 EP EP95922886A patent/EP0774050B1/de not_active Expired - Lifetime
  • 1995-05-24 DE DE69508256T patent/DE69508256T2/de not_active Expired - Fee Related
  • 1995-05-24 WO PCT/US1995/006582 patent/WO1995034745A1/en active IP Right Grant
  • 1995-05-24 JP JP50219196A patent/JP3640396B2/ja not_active Expired - Fee Related
  • 1995-06-14 TW TW084106058A patent/TW268070B/zh active

Patent Citations (5)

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