Data processing: vehicles – navigation – and relative location – Vehicle control – guidance – operation – or indication – With indicator or control of power plant
Reexamination Certificate
2001-01-04
2001-08-07
Zanelli, Michael J. (Department: 3661)
Data processing: vehicles, navigation, and relative location
Vehicle control, guidance, operation, or indication
With indicator or control of power plant
C701S029000
Reexamination Certificate
active
06272422
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates to rotor assemblies and liners within a gas turbine engine, and more particularly to radial clearance control between a rotor assembly and a liner disposed radially outside the rotor assembly.
2. Background Information
A gas turbine engine includes a fan section, a compressor section, a combustor section, and a turbine section disposed along a longitudinal axis. Air enters the engine through the fan section, passes through the compressor and into the combustor where fuel is mixed with the air and combusted. The combustion products, and any uncombusted air and/or fuel subsequently pass into the turbine and exit the engine through a nozzle. Collectively, the air and combustion products may be referred to as core gas, and the path through the fan, compressor, combustor, turbine, and nozzle referred to as the core gas path.
The fan, compressor and turbine sections include a plurality of rotor stages separated by stator sections. Each rotor stage includes a rotor assembly surrounded by a shroud. The rotor assembly includes a plurality of rotor blades attached to and circumferentially distributed around a disk. Radially outside of the rotor stage, the shroud defines the outer radial boundary of the gas path through that rotor stage. The outer radial surface of each rotor blade (i.e., the “blade tip”) is positioned in close proximity to the inner radial surface of the shroud. The design clearance between the blade tips and the shroud is a predetermined value, chosen to minimize efficiency losses attributable to core gas passing between the blade tip and the shroud, while at the same time avoiding interference with the shroud. The actual clearance between the blade tips and the shroud will vary during operation of the engine.
What is needed is a method and an apparatus for controlling the actual clearance between a rotor stage and a shroud within a gas turbine engine, one that can predict instantaneous clearance values as a function of time, and one that can determine instantaneous clearance values under steady-state and transient conditions.
DISCLOSURE OF THE INVENTION
It is therefore, an object of the present invention to provide an apparatus and a method for predicting the actual clearance between a rotor stage and a shroud within a gas turbine engine, one that can predict instantaneous clearance values as a function of time, and one that can determine that instantaneous clearance values under transient conditions.
The actual clearance between a rotor assembly and shroud at any point in time is a function of: 1) the design dearance; 2) the current operating conditions of the engine; 3) the amount of wear within the shroud and rotor assembly; and 4) certain thermal and mechanical properties of the shroud and rotor assembly. The current operating conditions refers to the current status of the engine and the environment in which it is operating. An engine operating in a steady-state mode is one in which the operating environment and power settings have been stable long enough for the various components within the engine to have reached a substantially stable temperature. An engine operating in a transient mode is one in which the operating environment and power settings have recently changed and the various components within the engine have not yet reached a substantially stable temperature. The thermal and mechanical properties of the shroud and rotor assembly include, but are not limited to, the thermal time constants (&tgr;) and a coefficients of expansion associated with the rotor disk, the rotor blades, and the shroud. The thermal time constant (&tgr;) is a value that reflects the rate at which an element (e.g., the rotor disk, rotor blades, or shroud) changes temperature. The coefficient of expansion reflects the rate at which an element (e.g., the rotor disk, rotor blades, or shroud) changes physical size in response to a thermal change. The differences in the thermal time constant and the coefficient of expansion between the rotor disk, rotor blades, and the shroud are attributable to the elements being comprised of different materials and/or having different physical geometries.
Under steady-state conditions, the clearance between the rotor blades and the shroud is substantially constant because there is no appreciable thermal expansion (negative or positive) within the disk, blades, and/or shroud. Under transient conditions, the clearance between the rotor blades and the shroud fluctuates predominantly because of the different thermal properties of the components that create the clearance. An engine operating at a first power setting that is rapidly changed to a significantly different power setting will, for example, experience a rapid change in rotor speed and a rapid change in core gas temperature. The rapid change in temperature will cause reactions of different magnitude in the disk, blades and shroud because of their different thermal properties. For example, the amount of time it takes the disk to become steady-state at the new core gas temperature is likely to be substantially more that it take the shroud or blades to become steady-state because of the mass of the disk. As a result, if the engine is operating at a low power setting and the power setting is substantially increased, the shroud is likely to radially expand at a faster rate than the disk thereby increasing the clearance between the rotor blades and the shroud until the disk reaches a steady-state condition at the new core gas temperature. Conversely, if the engine is operating at a high power setting and the power setting is rapidly decreased, the shroud is likely to radially contract at a faster rate than the disk thereby decreasing the gap between the rotor blades and the shroud until the disk reaches a steady-state condition at the new core gas temperature.
The graph shown in
FIG. 1
includes three curves indicative of engine parameters before, during and after a rapid transition from an idle power engine operating condition to a partial power engine operating condition. A first curve
122
represents the magnitude of the rotor speed (N
2
). A second curve
124
represents the magnitude of the instantaneous clearance between the rotor blades and the shroud. A third curve
126
represents the steady-state clearance between the rotor blades and the shroud. During the time interval T
0
, the engine is stable at an idle operating condition and the rotor assembly and the shroud are at thermal equilibrium. During this time, the instantaneous clearance is equal to the steady-state clearance. In a brief subsequent time period T
1
, the engine power setting is rapidly increased from the idle operating condition to the partial power engine operating condition. The change in power setting causes the rotational speed of the rotor assembly to increase (see curve
122
) and a radial expansion of the rotor assembly. As a result, the instantaneous clearance and the steady-state clearance both decrease due to mechanical growth of the rotor assembly. The increase in the engine power setting also causes an increase in the core gas temperature, and consequent heat transfer to and thermal expansion of the rotor assembly and shroud. Note that the curve depicting the reference steady-state clearance shows an initial greater decrease in gap because it assumes that the components (disk, blades, shroud) have changed temperature instantaneously. The difference between the instantaneous clearance curve
124
and the reference steady-state clearance curve
126
is predominantly a function of the mismatch between the thermal time constants of the rotor assembly and the shroud and the consequent thermal expansion of the same. In the time period T
2
, the operating conditions of the engine (e.g., the power setting, altitude, etc.) remain constant, although the clearance is in a transient mode. After the power setting of the engine was changed rapidly from idle to partial power, the temperature of the core gas also changed rapidly, becoming steady-state wi
Irwin Craig W.
Khalid Syed J.
United Technologies Corporation
Zanelli Michael J.
LandOfFree
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