WO1998030802A1

WO1998030802A1 - Enhancement of turbomachines and compressors by fluid removal

Info

Publication number: WO1998030802A1
Application number: PCT/US1997/023703
Authority: WO
Inventors: Jack L. Kerrebrock
Original assignee: Massachusetts Institute Of Technology
Priority date: 1997-01-13
Filing date: 1997-12-22
Publication date: 1998-07-16

Abstract

Improved performance of turbomachines and compressors in the pressure ratio attainable by a turbomachine having a given blade speed and number of stages of compression and an increase in the thermodynamic efficiency of the compression process is accomplished by removing compression fluid that has suffered viscous interaction with the bounding surfaces of the compressor. Removal of the fluid in a state of viscous interaction (boundary layer) must be effected at all locations in the compressor where it may limit diffusion to achieve optimum performance. Several embodiments of a structural nature are taught.

Description

ENHANCEMENT OF TURBOMACHINES AND COMPRESSORS BY FLUID REMOVAL

Background of the Invention: Field of the Invention

The invention relates to the field of turbomachines and compressors. More particularly, the invention relates to improving the pressure ratio obtainable by a turbomachine or compressor having a given blade speed and number of stages of compression and to increasing the thermodynamic efficiency of the turbomachine or compressor.

Prior Art

Thought about fluid dynamics and invention pertaining thereto has existed for a substantial period of time. So too has man's interest in creating power persevered. One of the arts in which substantial and powerful thought has been devoted is that of compressors and turbomachines. One of the most important areas driving such research is aeronautics and astronautics for both the commercial interests of high speed transportation and military interests for defense and the exploration of space. Some important issues with respect to the advance of compressors and turbomachines is the pressure ratio attainable and the efficiency of the machines. Reissue Patent to E. A. Stalker 23,108 discloses the provision of slots located well rearward on the blade to increase the effectiveness of the blade. This is taught in order to control the boundary layer on the blades of blowers and compressors to better enable the machine to run at lower than optimal speeds. J. R. Irwin, U.S. 2,720,356 imposes continuous boundary layer control for compressors by moving the boundary layer through porous surfaces. The teaching recommends to then reintroduce the viscous interactive flow to the main flow of the compressor at a later stage.

U.S. Patent 2,749,025 to Stalker focuses primarily on providing blades of later stages in a compressor with progressively larger radii rounded leading edges. This reduces losses associated with the flow angle into these blades which would normally be experienced at below optimum speeds. The substantially semi-circular nose cross- section is professed to be able to smooth the flow and avoid burbling when the approach vectors are far from optimum. A further step to assist the machine in these conditions is to remove the boundary layer in this area.

U.S. Patent 3,694,102 to Conrad teaches use of suction slots in stator blades to prevent separation of the boundary layer in supersonic blading. Conrad, however, fails to recognize the benefit of removing the boundary layer permanently from a compressor. This is evidenced by equating bleeding of the boundary layer to atmosphere to reintroducing the boundary layer into the compressor at another stage.

U.S. Patent 3,993,414 to Meauze discloses an axial supersonic compressor comprising a casing and a hub rotating in the casing and carrying blades. On each of the suction surfaces of the blades is formed a zone in which the curvative changes and which corresponds to a shock wave. A channel formed in each blade and opening in the zone is connected to a boundary layer aspiration means.

U.S. 3,897,168 to Amos and U.S. 4,595,339 to Naudet both disclose the recapture of energy from a withdrawn boundary layer to avoid losses.

None of the prior art discussed provides insight to the thermodynamic benefits of fluid removal from the flow path. In fact, many of skill in the prior art believed that reintroducing the fluid of the viscous interaction to the flow path at another compressor stage was beneficial to the functioning of the machine.

Summary of the Invention:

The above-discussed and other drawbacks and deficiencies of the prior art are overcome or alleviated by the teachings of the present invention.

By providing structure capable of removing the boundary layer of fluid in a turbomachine or compressor anywhere in the machine where viscous interaction limits the diffusion in the flow passages, the pressure ratio attainable for a given machine and the thermodynamic efficiency thereof are greatly enhanced. Implementation of fluid removal is accomplished by employing a variety of removal structures within the machine either alone or in combination depending upon the areas affected by viscous interaction and the desired improvement of the system. As will be recognized by one of ordinary skill in the art, the areas of viscous interaction (or boundary layer) cause the flow to fail to follow the surfaces of the machine. This contributes to further entropy in the system and thus loss of efficiency and of output of the machine. The present invention employs scoops, slots, porous surfaces and/or other equivalent means to remove the boundary layer and a passage through the blade to transport the fluid to an end use thereof. Whether the boundary layer fluid is removed to the internal cavity of the blade or to channels in the outer housing the fluid is employed in some way and is not reintroduced into the compressor flow path. This minimizes losses and can aid in cooling, operating accessories, etc. In the case of the fluid entering the space within a hollow blade, the fluid may be expressed outwardly or inwardly with differing effects on the machine.

As indicated above, optimum benefits are achieved by removing the boundary layer anywhere in the machine where viscous interactions tend to promote separation of the fluid. Some of the locations (not an exhaustive list) in which such boundary layer removal is beneficial are at a location on the blade near the trailing edge on the convex or suction side; on the casing; ahead of a rotor or a stator; on the hub; ahead of any shock impingement area and at blade tips (to avoid vortex blockage).

The above-discussed and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description and drawings.

Brief Description of the Drawings:

Referring now to the drawings wherein like elements are numbered alike in the several FIGURES:

FIGURE 1 is a thermodynamic representation of the effect of high-entropy fluid removal on compression efficiency; FIGURE 2 is a graph plotting fractional reduction in work (or fractional increase in efficiency) per fraction of fluid removed;

FIGURE 3 is a perspective schematic view of a scooped blade embodiment of the invention;

FIGURE 4 is a graphic representation of the pressure distribution on a compressor blade;

FIGURE 5 is a schematic representation of a shock wave impingement on a blade row and the removal of boundary layer by scoop;

FIGURE 6 is an axial schematic view of a Tip Vortex Blockage; FIGURE 7 is a schematic view of a removal location for boundary control to prevent Tip Vortex Blockages;

FIGURE 8 is a schematic perspective view of a scoop blade embodiment of the invention;

FIGURE 9 is a schematic perspective view of a slot blade embodiment of the invention; FIGURE 10 is a schematic perspective view of a porous surface blade embodiment of the invention;

FIGURE 11 is a graph of the variation with radius of ratio of blade-relative stagnation pressure to passage pressure; FIGURE 12 is an axial view of a shroud embodiment of the invention; and FIGURE 13 is a tangentive view of FIGURE 12.

Detailed Description of the Preferred Embodiment:

It is important to note at the outset that the inventors hereof recommend employing means to remove the boundary layer at all areas of potential separation to provide optimum performance, however it should also be noted that incremental gains are obtained with each removal area.

With respect to efficiency of compressors and other turbomachines, it is conventional in fluid-thermodynamic discussions of compression and expansion processes to represent the deviation of the process by an increase in the entropy of the fluid, denoted S. The entropy is related to the pressure and temperature, for a thermally and calorically perfect gas, by the relation

where the subscripts 1 and 2 denote the beginning and end states of the fluid undergoing the process. The compression process may then be represented by a trace on temperature entropy coordinates. Such a representation of the processes under discussion is shown in FIGURE 1. State 1 is at P,, T,. S- is the beginning of the compression process and the desired end pressure is P₃. For purposes of this discussion, the fluid is assumed to be removed from the flow path at the pressure P₂, which may have any value between the inlet and delivery pressures. A conventional compression process is represented by the full-line trace from points 1 to 3, which shows the entropy increase due to viscous effects that results from mixing of the high-entropy fluid in the boundary layers with the remainder of the flow. With fluid removal, the high entropy fluid, at state 6, is separated from the remainder of the flow, at state 4, and removed from the flow path. The fluid remaining in the flow path then has the entropy corresponding to point 4, lower than it would have at point 2 if the high entropy fluid of the boundary layer were reintroduced into the flow path as was the conventional way. After the removal, the high-entropy fluid is expanded to recover its available energy, while the remainder is compressed to the desired end state at P₃. Since its entropy is lower at the end state than for a conventional process, the compression work is less, as represented by the fact that T₅ < T₃.

The fractional reduction in compression work per unit of delivered fluid is given by the relation

where the subscript b refers to "with bleed" i.e. with fluid removal, while nb is without fluid removal. M is the relative Mach number of the flow to the surface at which removal is done, and η_p is the Polytropic efficiency of the overall compression process. This result shows that the gain in efficiency due to fluid removal increases with increasing M and depends on the overall compression ratio and the compression ratio at the point of removal. As an example, this latter dependence is illustrated for M=l .5 in FIGURE 2. It shows that the gain in efficiency is about one half percent for each percent of (high entropy) fluid removal.

While efficiency is always important in an environment of costly energy, of even more importance is that the invention enables a higher pressure ratio or pressure rise for given machine parameters. Therefore smaller, lighter machines may be employed where only larger, heavier machines have been indicated in the past. This is clearly a significant benefit regardless of the application. Moreover, when coupled with the largest most powerful compressors and turbo machines the invention allows them to produce at an unprecedented level. In order to provide one of ordinary skill in the art a full understanding of the invention, four points of boundary layer removal and transport methods are discussed hereunder. These are to be understood to be examples and do not limit the areas in which the present invention is employable and/or is beneficial. Referring to FIGURES 3 and 4, a section of a blade 50 is schematically illustrated wherein a hollow core 52 is accessed through a scoop 54 (it should be noted that the scoop can also take the form of a slot or a porous structure or any equivalent structure capable of removing the boundary layer). The blade is, for most of its design parameters, conventional, having a convex or suction side 56 and a concave or pressure side 58. The convex side, of course, tends toward the upstream end of the machine while the concave side tends toward the downstream end of the machine. These design parameters cause air on the intake (convex) side to move more quickly and have a lower pressure while the convex side moves less quickly and has a higher pressure. As the compression fluid moves toward the trailing area 59 of the blade on the convex side, however, the pressure of the fluid rises rapidly to meet the pressure coming off the trailing area of the concave side. The rapid pressure rise causes separation of the boundary layer. This leads to increased entropy and reduced deflection. It is, therefore, beneficial to remove the boundary layer at a location just ahead of the expected separation. This creates a thinner boundary layer and higher wall shear stress thus increasing the attainable pressure rise.

The location of boundary layer removal for optimum performance is just ahead of or in the region of most rapid pressure rise.

As will be appreciated by those skilled in the art, compressors and other turbomachines can be transonic such that tips of the rotor blades exceed the speed of sound while the hub ends of the blades are subsonic. Machines subjected to this condition suffer from shock impingement on the blades' surfaces that generates a sudden pressure rise in the immediate vicinity of the impingement. The pressure rise can cause the boundary layer to separate which is known from the foregoing to be counterproductive to both efficiency and attainable pressure ratio. To alleviate the separation, the boundary layer immediately upstream of the shock impingement location is removed (See FIGURE 5). By removing the boundary layer 60 upstream of the shock impingement 62 the boundary layer thickness at shock impingement is minimized. Thus separation and increased entropy are avoided and the flow follows surfaces as intended.

Another area of the compressor which traditionally has been a limiting factor on attainable diffusion and thus performance of the machine is the viscous interaction or boundary layer on the cylindrical outer housing of the machine. As hereinbefore stated, sudden or rapid pressure increases in relatively small areas cause separation of the flow from the boundary layer in that area and contribute to greater entropy /less diffusion of the system. Blades passing closely over discrete areas of the outer housing cause shock pressure changes and the attendant separation. Removing the boundary layer on the housing immediately upstream of the close gap area of the rotating blades or the stator blades alleviates the problem. Removal of the boundary layer according to the invention is also beneficial to negate the phenomenon known as Tip Vortex Blockage which is itself, again, an increase in entropy and decrease in diffusion, thus limiting effectiveness of the machine. Tip Vortex Blockage is illustrated schematically in FIGURE 6; the solution in FIGURE 7. As will be appreciated following perusal of FIGURE 6, the narrow gap between blade tips 70 and casing 72 and the pressure differential of the high and low pressure sides of the rotor, a jet of fluid 69 issues from the clearance and tends to roll into a vortex 71 with its axis aligned to the main flow direction. The vortex accelerates the main flow, reducing its diffusion and thus reducing efficiency and output.

The blockage is avoided by placing a flow removal port 74 in the suction surface of the blade near the trailing edge 76 thereof, thereby negating the effect of the vortex.

All of the removals of the boundary layer taught hereinabove can be accomplished by providing a scoop (most preferred) (see FIGURES 3, 5 and 8) a slot (see FIGURE 9) and perforated structure (see FIGURE 10) regardless of where in the machine the viscous interaction is being removed.

As one of skill in the art will recognize, although removing the viscous layer produces gains from the reduction of separation, there are losses associated therewith due to the removal of fluid upon which work has been expended. In order to alleviate the losses experienced, the inventors hereof have devised particular transport parameters and paths for the fluid. By transporting the boundary layer in certain ways to certain places, much of the work done on that fluid can be recaptured.

Each of the exemplified means for removing the boundary layer preferably lead to a radially oriented passage that carries the flow to either the root or the tip of the blade. In the most preferred embodiment a single radially oriented passage is provided which communicates with the boundary layer catching structure. While it may appear that pressure would build in the passage and prevent flow thereinto of the boundary layer, the concept is enabled by the matching of the pressure variation in the passage, due to centrifugal gradient, to the variation of the stagnation pressure relative to the moving blade. The scoop configuration is most preferred because it recovers in the capture fluid, some of the stagnation pressure of that fluid relative to the blade. In the case of rotating blades, this relative stagnation pressure increases with radius because of the increasing tangential speed of the blade. Thus, the stagnation pressure approximately matches the variation of pressure in the radial passage. The variation of the ratio of the stagnation pressure to the passage pressure with radius is shown in FIGURE 11 for the situation where the axial Mach number in the compressor is M_x = 0.5 and the tip tangential Mach number of the rotating blades at their tip is M_τ= 1.5.

Calculation of the parameters is accomplished by the equation:

passage

where r_τ is the tip radius of the compressor and the pressure ratio is set at unity at that radius. This shows that the stagnation pressure differs from the pressure in the passage by only a small fraction over the radial extent of the blade, so that a single passage suffices for fluid removal at all radii. As stated above, transport may be toward the root or the tip of the blade.

Transport to the root and through the hub of the blades provides the significant advantage that part of the energy expended to bring the fluid to blade speed can be recaptured by channeling that energy back into the rotor. Collected boundary layer fluid is then most preferably directed to other areas of the machine and not reintroduced to the flow. Such fluid may be used for cooling or running accessory equipment.

Where the viscous fluid is transported outwardly it can be discharged into a manifold defined by shrouds at the tips of the blades which maintains the thermodynamic advantage of avoiding reintroduction of the removed fluid to the flow. The embodiment is, however, limited to relatively low speed machines because of additional loading on components caused by a shroud clearance seal which rubs against the housing of the machine. Referring to FIGURES 12 and 13, axial and tangential views of the embodiment are provided. Blades 100 each include a peripheral shroud 102 and a clearance seal 104 which, as can be best observed in FIGURE 13, contacts outer housing or casing 106. Clearly these seals 104 create a radial force in the rotor blades. At high speeds the force may be sufficient to cause catastrophic damage to the blades. Thus, slower blade speeds are indicated. FIGURE 13 also provides a good view of the movement of the collected boundary fluid 107 through conduit 108 into manifold area 110 defined by shroud 102, casing 106 and seals 104. Fluid escapes from manifold area 110 through ports 112 of which there are at least one and preferably many. Withdrawn fluid is employed for sundry things but is not returned to the compressor flow path.

In an alternate embodiment of outward transport, the fluid is merely discharged to the clearance space and allowed to create a pressure wall which assists in preventing pressurized fluid from the pressure side from migrating back to the suction side and helps alleviate Tip Vortex Blockages. Those of skill in the art will recognize the benefit of the arrangement but will also note that the more important teaching herein is to avoid reintroduction of the viscous fluid to the flow. Thus, this embodiment is not as favored as the foregoing. It should be understood that the terms "immediately upstream" and "just ahead" of or "upstream of a condition causing a separation" are intended to convey that the boundary layer should be removed or lessened in thickness close enough to the separation causing phenomenon to prevent that occurrence. It may not be necessary to remove the layer precisely before that phenomenon to avoid separation. And while precise removal is optimal, avoidance of separation is paramount and provides the benefits of the invention.

While the preferred embodiments have been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustration and not limitation.

What is claimed is:

Claims

CLAIM 1. An improved compressor comprising: a) a housing within which are a plurality of rotating blade rows and stationary blade rows each having a plurality of blades; b) at least one of said blades in any of said blade rows having a boundary layer collector; c) at least one fluid passage in fluid communication with said collector for conveying boundary layer fluid to a location out of the flow path of the compressor.

CLAIM 2. An improved compressor as claimed in claim 1 wherein said at least one of said blades is a plurality of said blades.

CLAIM 3. An improved compressor as claimed in claim 1 wherein said at least one of said blades is all of said blades.

CLAIM 4. An improved compressor as claimed in claim 1 wherein said collector extends over the span of each of the blades having a said collector, said at least one passage being a single passage.

CLAIM 5. An improved compressor as claimed in claim 4 wherein said passage has a matched centrifugal pressure gradient variation to a variation of a stagnation pressure relative to moving blades in the rotating blade rows.

CLAIM 6. In a method for providing improved thermodynamic efficiency in a compressor, the improvement comprising: a) removing fluid that has undergone viscous interactions within said compressor to a location away from the main flow of fluid through said compressor to thereby improve the thermodynamic efficiency of the compressor.

CLAIM 7. An improved compressor as claimed in claim 4 wherein said collector is a slot.

CLAIM 8. An improved compressor as claimed in claim 4 wherein said collector is a scoop.

CLAIM 9. An improved compressor as claimed in claim 4 wherein said collector is a porous structure.

CLAIM 10. The method as claimed in claim 6 wherein the compressor includes a plurality of blades, at least one of which having a viscous interaction collector and wherein said step of removing viscous interactions is conducted through said collector over a span of said at least one blade said viscous interactions being conveyed through a single passage.

CLAIM 11. The method as claimed in claim 10 wherein said step of removing includes maintaining a centrifugal pressure gradient variation of said passage to a variation of a variation of a stagnation pressure relative to moving blades in a rotating blade row.

CLAIM 12. The method as claimed in claim 6 wherein said efficiency is improved by increasing the Mach number.

CLAIM 13. The method as claimed in claim 10 wherein said at least one blade having a viscous interaction collector is a plurality of said blades each having a viscous interaction collector.

CLAIM 14. The method as claimed in claim 13 wherein said plurality of blades is all of said blades.

CLAIM 15. An improved compressor comprising: a) a housing within which are a plurality of rotating blade rows and stationary blade rows each having a plurality of blades; b) at least one boundary layer collector; c) at least one fluid passage in fluid communication with said collector for conveying boundary layer fluid to a location out of the flow path of the compressor.

CLAIM 16. An improved compressor as claimed in claim 15 wherein said at least one collector is located just upstream of a point of separation of a fluid stream through said compressor.

CLAIM 17. An improved compressor as claimed in claim 15 wherein said at least one compressor is a plurality of collectors located just upstream of a point of separation of a fluid stream through said compressor.