Patent Publication Number: US-2017363212-A1

Title: Rotating seal and sealing element therefor

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to seals for fluid flow and more particularly to noncontact rotating seals. 
     Various types of turbomachinery, such as gas turbine engines, aircraft engines, and steam turbines are known and widely used for power generation, propulsion, and the like. The efficiency of the turbomachinery depends in part upon the clearances between the internal components and the leakage of primary and secondary fluids through these clearances. For example, large clearances may be intentionally allowed at certain rotor-stator interfaces to accommodate large, thermally or mechanically-induced, relative motions. Leakage of fluid through these gaps from regions of high pressure to regions of low pressure may result in poor efficiency for the turbomachinery. Such leakage may impact efficiency in that the leaked fluids fail to perform useful work. 
     Different types of sealing systems are used to minimize the leakage of fluid flowing through turbomachinery. For example, noncontact seals are often incorporated between two rotating elements or a rotating element and a stationary element. One common type of noncontact seal is a so-called “labyrinth seal” which includes a rotor having one or more thin annular flanges often referred to as “seal teeth”. 
     The labyrinth seal rotor rotates in close proximity to a stationary sealing element. In practical applications the stationary sealing element is usually made “abradable” or sacrificial relative to the seal teeth. In the event that the seal teeth contact the sealing element, any damage will occur preferentially to the sealing element, which is typically less expensive and/or easier to replace than the seal teeth. 
     One common type of abradable seal is a “honeycomb” seal comprising a cellular structure made from thin sheet material. One problem with existing honeycomb seals is that as clearances become small, bypass flow over the labyrinth seal teeth becomes a significant problem. 
     BRIEF SUMMARY OF THE INVENTION 
     this problem is addressed by the technology described herein, which provides a rotating seal assembly where a sealing element thereof includes layers of cells each having a relatively small radial height. 
     According to one aspect of the technology described herein, a sealing element apparatus for a rotating seal includes an array of cells defining an annular sealing surface, where the cells are divided into a plurality of layers extending parallel to the sealing surface. 
     According to another aspect of the technology described herein, a rotating seal apparatus includes: a sealing element comprising an array of cells defining an annular sealing surface, where the cells are divided into a plurality of layers extending parallel to the sealing surface; and a rotor disposed adjacent the sealing surface, the rotor comprising at least one annular seal tooth. 
     According to another aspect of the technology described herein, a rotating seal apparatus includes: a sealing element comprising a plurality of spaced-apart generally parallel walls, each wall having have a periodic shape comprising a series of peaks and valleys, the walls cooperatively defining an annular sealing surface. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures, in which: 
         FIG. 1  is a cross-sectional view of a labyrinth seal apparatus; 
         FIG. 2  is a plan view of a portion of a sealing element of  FIG. 1 ; 
         FIG. 3  is a plan view of a portion of an alternative sealing element; 
         FIG. 4  is a plan view of a portion of an alternative sealing element; 
         FIG. 5  is a plan view of a portion of an alternative sealing element; 
         FIG. 6  is a perspective view of a portion of an alternative sealing element in close proximity with a seal tooth; and 
         FIG. 7  is a perspective view of a portion of the sealing element shown in  FIG. 6 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views,  FIGS. 1 and 2  illustrate an exemplary rotating seal assembly  10  which includes a stator  12  positioned in close proximity to a rotor  14 . The stator  12  includes a sealing element  16  carried by a support  18 . In the illustrated example, the stator  12  is an annular structure which is positioned radially outboard of the rotor  14 , which in turn is mounted for rotation about an axis of rotation “A”. However, it will be appreciated that the rotor  14  could be positioned radially outboard of the stator  12 . Furthermore, it is possible that the rotating/stationary relationship of the rotor  14  and the stator  12  could be the inverse of that illustrated, or that both elements could be mounted for rotation. 
     It is noted that, as used herein, the term “axial” or “longitudinal” refers to a direction parallel to the axis A, while “radial” refers to a direction perpendicular to the axial direction, and “tangential” or “circumferential” refers to a direction mutually perpendicular to the axial and tangential directions. (See arrows “R”, and “T” in  FIGS. 1 and 2 ). These directional terms are used merely for convenience in description and do not require a particular orientation of the structures described thereby 
     The sealing element  16  is an annular structure defining a sealing surface  20  facing the rotor  14 . The sealing element  16  has an overall height “H 1 ” in the radial direction. 
     As seen in  FIGS. 1 and 2 , the sealing element  16  comprises an array of cells  22 . In the specific example illustrated, the cell array is a honeycomb structure comprising a repeating two-dimensional array of cells  22 , where each cell  22  is a regular hexagon defined by walls  24 . The walls  24  may be formed from thin sheet material, such as a metal alloy. This type of cellular structure is commonly referred to in the art as a “honeycomb seal”. In the illustrated example, the individual cells  22  have a width “W 1 ” measured in the axial direction (i.e. parallel to the axis A). As a nonlimiting example, the width W 1  may be approximately 1.5 mm (60 mils). It is noted that the U.S. customary term “mils” may be used herein to describe linear dimensions, wherein one mil is equal to 1/1000 of an inch. 
     The rotor  14  includes a body  26  with one or more annular seal teeth  28  extending radially outward therefrom. As used herein, the term “seal tooth” refers to a relatively thin annular structure or flange extending away from a body. In the illustrated example, each seal tooth  28  includes a radially outward facing end face  30  flanked by a pair of tapered side walls  32 . The end face  30  has a width “W 2 ” which is selected to suit a particular application. As an example, the width W 2  may be about 0.3 mm (10 mils) to about 2.0 mm (80 mils), or about half of the width W 1  of the cells  22  of the sealing element  16 . 
     When assembled in an engine, the stator  12  is positioned with the sealing surface  20  thereof in close proximity to the seal teeth  28  of the rotor  14 . A radial clearance “C” is present between the sealing surface  20  and the seal teeth  24 . The rotating seal assembly  10  is effective to reduce or prevent a leakage flow “F” between a first zone  34  located upstream of the rotating seal assembly  10  and a second zone  36  located downstream of the rotating seal assembly  10 . 
     The sealing element  16  is configured to act as a sacrificial element relative to the rotor  14 . Specifically, the material, dimensions, and cell configuration of the sealing element  16  are selected such that, in the event of the rotor  14  contacting the sealing element  16 , the rotor  14  can cut into or deform the sealing element  16  without causing damage to the rotor  14 . This sacrificial property may be referred to as the sealing element being “abradable”. This abradable property may be used to enhance sealing in multiple ways. For example, a nominal clearance between the sealing element  16  and the rotor  14  may be set very small with the expectation that the rotor  14  will cut into the sealing element  16  during initial operation, resulting effectively in a zero-clearance seal. Alternatively or in addition to this function, the abradable property allows the rotor  14  to temporarily deflect outwards during extreme or unusual operating conditions without permanent damage to itself. 
     in operation, the leakage flow F is made up of a leakage flow “L” passing through the radial clearance C in an axial direction, as well as a bypass flow “B” flowing in a path up and over the seal tooth  28 , into the honeycomb cell  22 , back down over the seal tooth  28 , and subsequently downstream. 
     In the prior art it is common for a seal assembly, such as the seal assembly  10  described above to operate with a radial clearance C on the order of about 0.76 mm (30 mils). Under these conditions, bypass flow B is not a significant portion of the leakage flow F. 
     However, for some applications it is desirable to configure the seal assembly  10  with a smaller radial clearance C in order to minimize the total leakage flow F. For example, the radial clearance C may be about 0.25 mm (10 mils). At such small clearances, the bypass flow B can be significant, for example 50% to 100% of the leakage flow L. 
     The bypass flow B can be reduced simply by reducing the overall height H 1  of the sealing element, with the effect of reducing a height of the cells  22 . However, this configuration would have the undesirable side effect of reducing the amount of space available for the seal teeth  28  to cut into the sealing element  16 . In some engine operating conditions, this could result in the seal teeth  28  contacting the support  18  and damaging the seal teeth  28 . 
     In order to reduce the bypass flow B while maintaining other desirable characteristics of the sealing element  16 , the sealing element may be divided into a plurality of layers. In the example illustrated in  FIG. 1 , the layers are defined by separators  38  extending transverse to the wall  24 . In this example, five separators  38  are shown which define five layers numbered  40 ,  42 ,  44 ,  46 , and  48  respectively. the layers  40 ,  42 ,  44 ,  46 , and  48  may be described as extending parallel to the sealing surface  20 . The number of layers may be varied to suit a particular application. The lowermost layer  40  is open at the sealing surface  20  and thus functions as a conventional honeycomb seal having a very small cell height “H 2 ”. The height H 2  may be selected to minimize the bypass flow B. For example, the height H 2  may be approximately 0.13 mm to 0.26 mm (5 mils to 10 mils). The remaining layers  42 ,  44 ,  46 , and  48  are closed off in the initial condition of the sealing element  16 . 
     The small cell height of the exposed layer  40  allows the rotating seal assembly  10  to operate with minimal bypass flow B. Due to the effectively smaller height of the cells  22 , the leakage air experiences higher flow resistance inside the cells  22  and hence the discharge coefficient (Cd) of the bypass leakage decreases. Testing has shown that this arrangement is effective to reduce the net leakage flow F through the rotating seal assembly  10 . 
     In the event that the rotor  14  should experience an excursion and cut into the innermost separator  38 , the cell height would still be effectively equal to the sum of the heights of the first and second layers  40  and  42 . 
     It is noted that the layers do not need to extend through the entire height H 1  of the sealing element  16  in order to provide the benefit of reduced bypass flow B. For example, the total height of all the layers  40 ,  42 ,  44 ,  46 , and  48  could be only about one-third of the overall height H 1 . This arrangement would minimize the number of layers required and thus the complexity of the sealing element  16 , while still ensuring that any increase in cell height would be minimized for any degree of abrasion. 
       FIG. 3  illustrates an example of an alternative sealing element  116  comprising an array of cells  122 . In the specific example illustrated, the cell array is a honeycomb structure comprising a repeating two-dimensional array of cells, where each cell  122  is a hexagon defined by walls  124 . The walls  124  may be formed from thin sheet material, such as a metal alloy. In the illustrated example, the individual cells  22  have a width “W 3 ” measured in the axial direction (i.e. parallel to the axis A), as well as a length “L 3 ” measured in the tangential direction (i.e. perpendicular to the axis A). As an example, the width W 3  may be significantly smaller than the cell width of a prior art honeycomb seal, for example approximately 0.76 mm (30 mils). Concurrently, the length L 3  may be on the order of three times the width W 3 . It is believed that this configuration is effective to reduce bypass flow as described above, while limiting the total amount of material in the walls  124  so as to minimize heat generation in the case of contact between a seal tooth and the sealing element  116 . Thus, the illustrated design is believed to be more effective than simply reducing all dimensions of the cells  122  equally. 
     The sealing element  116  may be used alone or in conjunction with the layered configuration described above. That is, one or more separators (not shown) may be provided extending transverse to the walls  124  for the purpose of dividing sealing element  116  into two or more layers each having a relatively short height. 
       FIG. 4  illustrates another alternative sealing element  216  similar in construction to the sealing element  116  described above and comprising an array of cells  222  defined by walls  224 . In the illustrated example, the individual cells  222  have a width “W 4 ” measured in the axial direction (i.e. parallel to the axis A), as well as a length “L 4 ” measured in the tangential direction (i.e. perpendicular to the axis A). These dimensions and their relative proportions may be the same as or similar to the respective width and length W 3 , L 3  of sealing element  116  described above. The cells  222  differ from the cells  222  only in that they are rectangular instead of hexagonal. 
     The sealing element  216  may be used alone or in conjunction with the layered configuration described above. That is, one or more separators (not shown) may be provided extending transverse to the walls  224  for the purpose of dividing sealing element  216  into two or more layers each having a relatively short height. 
       FIG. 5  illustrates another alternative sealing element  316  comprising an array of cells  322 . In the specific example illustrated, the cell array comprises a repeating two-dimensional array of cells, where each cell  322  is defined by walls  324 . The walls  324  may be formed from thin sheet material, such as a metal alloy. Each cell  322  as a shape which may be described as two rectangular elements  326 ,  328  respectively merged to form a single shape. The overall shape of the cell  322  may be described as a “staggered rectangle”, a “joggle”, or a shallow “S” shape. In the illustrated example, the individual cells  322  have a width “W 5 ” measured in the axial direction (i.e. parallel to the axis A), as well as a length “L 5 ” measured in the tangential direction (i.e. perpendicular to the axis A). As an example, the width W 5  may be significantly smaller than the cell width of a prior art honeycomb seal, for example approximately 0.76 mm (30 mils), or could be approximately the same as a cell width of a prior art honeycomb seal, for example approximately 1.5 mm (60 mils). Concurrently, the length L 5  may be on the order of 1 to 3 times the width W 3 . It is believed that this configuration is effective to reduce bypass flow as described above, while limiting the total amount of material in the walls  324  so as to minimize heat generation in the case of contact between a seal tooth in the sealing element  316 . 
     The sealing element  316  may be used alone or in conjunction with the layered configuration described above. That is, one or more separators (not shown) may be provided extending transverse to the walls  324  for the purpose of dividing sealing element  316  into two or more layers each having a relatively short height. 
       FIGS. 6 and 7  illustrate an example of an alternative sealing element  416 . More specifically, the sealing element  416  is an annular structure comprising a plurality of spaced-apart, generally parallel walls  424  abutting a support  418 . The walls  424  have a periodic or undulating shape comprising a series of peaks  450  and valleys  452  along a circumferential or tangential direction “T” which is perpendicular to the axis A. This type of shape may also be described as “wavy”. In the illustrated example, the shape comprises smooth curves, however other configurations such as sawtooth or square-wave may be used as well. The walls  424  may be made from an appropriate material such as a metal alloy. 
     The walls  424  define a sealing surface  420  facing a rotor  14 . Similar to the sealing element  16  described above, the sealing element  416  is configured to act as a sacrificial element relative to the rotor  14 . Specifically, the material, dimensions, and sell configuration of the sealing element  16  are selected such that, in the event of the rotor  14  contacting the sealing element  16 , the rotor  14  can cut into or deform the sealing element  16  without causing damage to the rotor  14 . This sacrificial property may be referred to as the sealing element  416  being “abradable”. 
     The wavy walls  424  of the sealing element  416  may be used alone or in conjunction with the layered configuration described above. That is, one or more separators (not shown) may be provided extending transverse to the walls  424  for the purpose of dividing sealing element  416  into two or more layers each having a relatively short height. 
     The sealing elements described above have several advantages over the prior art. In particular, they are believed to reduce bypass leakage flow and/or total leakage flow as compared to prior art honeycomb sealing elements, while still providing acceptable abrasion properties. 
     The foregoing has described a sealing element for a rotating seal. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. 
     Each feature disclosed in this specification (including any accompanying claims, abstract and drawings) may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features. 
     The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.