Patent Application
20180010617
This disclosure relates generally to tip clearance control for turbomachines and more particularly to a device for controlling tip clearances of axial compressor rotor blades using low-alpha stator component structures.
A gas turbine typically includes an axial flow compressor, one or more combustors that are disposed downstream from the compressor, a turbine that is disposed downstream from the one or more combustors and a shaft that extends axially through the gas turbine. The compressor includes an outer casing and an inner casing that circumferentially surrounds at least a portion of the shaft. The compressor further includes alternating rows of compressor rotor blades and stator vanes that are disposed within the outer/inner casing. The compressor rotor blades are coupled to the shaft and extend radially outward towards the outer/inner casing. The stator vanes are arranged annularly around the shaft and extend radially inward from the outer/inner casing towards the shaft. A stage within the compressor generally comprises of one row of the compressor rotor blades and an axially adjacent row of the stator vanes.
During startup of the gas turbine engine, the operating temperature of both the rotor and stator assemblies increases up to a maximum anticipated level as the compressor and gas turbine engine reach a normal running speed and steady state condition. Over time, the increased operating temperature of the blades may cause the tips to weaken, fracture or even deteriorate at the distal ends, causing an inevitable increase in the annular space between the blade tips and casing (sometimes referred to as “sealing gap” or “clearance”). Any such increase in space between the blade tips and casing during normal operation translates into a reduction of both rotor and stator efficiency, which in turn decreases the overall compressor and engine efficiency.
In order to improve or at least maintain the continued efficiency of the compressor and gas turbine, the sealing gap, or clearance, between the rotor blade tips and casing of the compressor should remain as small as possible without adversely restricting gas flow or effecting free blade rotation during normal operating conditions. The efficiency of a compressor is adversely affected if it is operated with large clearances between the tips of the rotating blades and the attendant stationary components (i.e. shrouds). The requirement for tip clearances results from the fact that the rotating components, such as the blades and the wheel, increase in diameter considerably due to centrifugal stresses and thermal expansion while the stationary components, the shroud and casing, are subject to changes in dimension to a lesser degree.
During continuous operation of a compressor, the occurrence of a variety of operating conditions is encountered. These varying conditions may cause considerable variations in compressor tip clearance. For a particular set of operating conditions any desired running clearance between the rotating and stationary components can be obtained if the components are fabricated and assembled with an appropriate initial tip clearance, sometimes referred to a build clearance. However, the heavier rotating components of a compressor having a large mass are necessarily slow to respond to changes in operating conditions, thus requiring large initial tip clearances. The normal practice is to design the machine such that the desired clearance exists during maximum speed, steady-state (SS) operating conditions. As a consequence, however, during other periods of operation such as during transient operation, the clearance is less than the predetermined desired clearance.
Previous known means for reducing tip clearances, have involved shrouded blades, or abradable shrouds (casings) and coatings which are worn away by the blades as the rotating parts expand. These devices have not afforded a completely satisfactory solution to the problem of large tip clearances. The shrouded blades lead to a design which is inherently heavier and more difficult to manufacture than the unshrouded blade.
Another previous clearance control means used rotor and casing materials with large dimensional variability, caused by a relatively high coefficient of thermal expansion (CTE or α), resulting in rubbing and/or excessive tip clearance, both of which are detrimental to compressor performance and efficiency. This makes it difficult to manage clearances between the rotor tips and the inner casing without use of an active clearance control system. Many active clearance control systems, in order to help match the dimensions of the casing and the rotor, require use of cooling air, control valves, and actuators which adds complexity and reliability concerns.
Aspects and advantages of the disclosure will be set forth in part in the following description, or may be obvious from the description, or may be learned through practice of the disclosure.
A gas turbine engine is disclosed having a turbine, one or more hydrocarbon gas combustors and a compressor. The compressor has a rotor assembly with one or more rotor blade rows having circumferentially spaced-apart rotor blades, each blade extending radially outward from an inner wheel disk. The compressor also has a stator assembly with one or more stator vane rows having circumferentially spaced-apart stator vanes extending radially inward from an inner casing. Each stator vane row is positioned between adjacent rotor blade rows. The inner casing extends circumferentially around the rotor assembly to form a plurality of inner flow paths defined by the rotor blades cooperating with the stator vanes. The rotor blades exhibit a hot running rotor tip clearance and a cold build rotor tip clearance. The inner casing is constructed from at least one low-alpha metal alloy.
These and other features, aspects and advantages of the present disclosure will become better understood with reference to the following description and appended claims. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the disclosure and, together with the description, serve to explain the principles of the disclosure.
A full and enabling disclosure, including the best mode thereof, directed to one of ordinary skill in the art, is set forth in the specification, which makes reference to the appended figures, in which:
Repeat use of reference characters in the present specification and drawings is intended to represent the same or analogous features or elements of the present disclosure.
Reference now will be made in detail to embodiments of the disclosure, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation of the disclosure, not limitation of the disclosure. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present disclosure without departing from the scope or spirit of the disclosure. For instance, features illustrated or described as part of one embodiment can be used with another embodiment to yield a still further embodiment. Thus, it is intended that the present disclosure covers such modifications and variations as come within the scope of the appended claims and their equivalents.
Although exemplary embodiments of the present disclosure will be described generally in the context of an axial flow compressor used in an industrial gas turbine for purposes of illustration, one of ordinary skill in the art will readily appreciate that embodiments of the present disclosure may be applied to any device having a row of rotating blades that is positioned adjacent to a row of stationary or stator vanes and is not limited to an axial-flow compressor unless specifically recited in the claims. For example, the present disclosure may be incorporated into a compressor of a jet engine, a high speed ship engine, a small scale power station, or the like. In addition, the present disclosure may be incorporated into a compressor used in varied applications, such as large volume air separation plants, blast furnace applications, propane dehydrogenation, or the like.
As used herein, the term “radially” refers to the relative direction that is substantially perpendicular to an axial centerline of a particular component, and the term “axially” refers to the relative direction that is substantially parallel to an axial centerline of a particular component. Also as used herein, the term “low-alpha” refers a material exhibiting a property at or below a threshold value for the coefficient of linear thermal expansion (CTE). CTE is mathematically represented with the Greek letter alpha (α). CTE is defined herein as a material property indicative of the extent to which a material expands upon heating and is expressed as the fractional increase in length per unit rise in temperature. The term “low-alpha” refers to exhibiting a property where the coefficient of linear thermal expansion (CTE) is in the range of about 12 microns/meter/degrees Kelvin (μm/m-K) or less. The term “high-alpha” material is defined herein as a material exhibiting a property above about 12 microns/meter/degrees Kelvin (μm/m-K) coefficient of linear thermal expansion (CTE). The CTE property is essentially constant over the entire temperature range of about 20° C. to about 650° C., sometimes referred to as ‘mean’ or ‘average’ CTE.
Adequate clearance control during operation of a turbine can be accomplished by casings composed of a low-alpha metal alloy (having a low CTE), which in turn provide for larger cold build clearances. Many low-alpha metal alloys are inadequate since they are not strong enough at high operating temperatures to ensure safe operation. The need for higher strength at higher temperatures called for the use of nickel-based alloys and specialty steels, whose thermal conductivity is characteristically higher than that of previously used high-alpha metals. Some nickel-base alloys and specialty steels can provide adequate tip clearance control during maximum operating conditions and at part-power conditions, and can reduce the cold build clearances between the rotating and non-rotating structures.
Low-alpha metal alloys according to this disclosure can be implemented on a wide variety of rotating assemblies, particularly compressors that include a rotor rotating about a central longitudinal axis and a plurality of blades mounted to a wheel disk that extend radially outward. Most rotor assemblies also include an outer casing having a generally cylindrical shape and an inner casing spaced radially outwardly from the rotor and blades to define a narrow annular gap between the inner circumferential surface of the inner casing and end tips of the rotor blades.
Low-alpha metal alloys are used to construct the inner casing of the turbine and define a minimum annular gap (clearance) during thermal expansion of the rotor and the casing. The annular gap is referred to as tip clearance and is defined by the distance between the inner casing inner circumference and tips of the rotary blades. During periods of differential growth of the rotor (for example, due to the heat conducted up through the blades and rotor assembly as the engine and compressor reach nominal operating conditions), the casing will expand due to heat transfer from the compressed air and surrounding engine parts as the engine and compressor reach their normal operating speed.
Referring now to the drawings, wherein like numerals refer to like components,
In normal operation, air 36 is drawn into the inlet 32 of the compressor 12 and is progressively compressed to provide a compressed air 38 to the combustion section 14. The compressed air 38 is mixed with fuel in the combustor 34 to form a combustible mixture. The combustible mixture is burned in the combustor 34, thereby generating a hot gas 40 that flows from the combustor 34 across a row of turbine nozzles 42 and into the turbine section 16. The hot gas 38 rapidly expands as it flows across alternating stages of turbine blades 44 connected to the shaft 22 and the turbine nozzles 42. Thermal and/or kinetic energy is transferred from the hot gas 40 to each stage of the turbine blades 44, thereby causing the shaft 22 to rotate and produce mechanical work. The shaft 22 may be coupled to a load such as a generator (not shown) so as to produce electricity. In addition or in the alternative, the shaft 22 may drive the compressor section 12 of the gas turbine.
This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the disclosure is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they include structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.
