Patent Application
20110236182
The invention relates to technology for reducing leakage of air or working gas through the clearance between a compressor or turbine blade tip and an adjacent shroud.
Turbine engines have circular arrays of airfoils on rotating disks in both the compressor section and the turbine section of the engine. The airfoils are radially oriented with respect to the rotation axis, and are closely surrounded by a shroud that defines an outer envelope for the flow path of the working gas (air or combustion gases). The greater the clearance between the blade tips and shroud, the less efficient is the conversion of energy between the working gas and the rotating disk, since some of the working gas leaks over the airfoil tip. This clearance varies under differing operating conditions such as engine startup due to differential thermal expansion, thus making it hard to control the leakage. Much effort has been made to minimize this clearance, including reducing it so much that the blade tips occasionally touch and abrade the shroud. Some designs dynamically control the shroud diameter during engine operation.
Plasma generators have been used on aerodynamic components of turbine engines to influence boundary layers in various ways. For example US patent publication 2009/0169356 describes generating plasma in the tip-to-shroud clearance for aerodynamic stabilization. US patent publication 2008/0089775 describes reducing the effective tip-to-shroud clearance by filling it with plasma. A type of plasma generator often used in these efforts is called a dielectric barrier plasma generator, in which one electrode is covered with insulation and a second electrode is exposed to the intervening gas. This type of plasma generator is described for example in U.S. Pat. No. 7,380,756, which describes that airflow is induced by the plasma in a direction from the exposed electrode to the covered electrode when an alternating voltage is applied between the electrodes.
The invention is explained in the following description in view of the drawings that show:
Shroud electrodes 68 are mounted in the shroud 36 facing the blade tip 31 and surrounding the path 33 of the blade tip. These electrodes 68 may be covered by an electrically insulating material 64, which may also serve as a thermal barrier for the shroud. Ceramic thermal barrier coatings are known, and some of them are electrically insulating, such as AL2O3. Each of the shroud electrodes 68 may be supplied with electricity via a conductor 70, which may be connected via a power bus 72 to a controller 74. The number of individually controllable power circuits CN depends on the ratio of blade electrodes B to shroud electrodes SE. CN=SE/B. If there is only one shroud electrode per blade electrode, and the shroud electrodes register in unison with the blade electrodes, then only one power circuit is needed, and all shroud electrodes are in a given shroud ring are controlled in unison.
The electrodes 54, 68 and conductors 60, 70 may be made of a high-temperature electrical conductor material such as iridium, platinum, yttrium, carbon fiber, graphite, tungsten, tungsten carbide, or others deposited by techniques known in the art. The conductors 70 in the shroud may be formed as power-conducting traces within the dielectric 64, similarly to conductors in printed circuit boards. The traces may lead to a connector 71 at some point on each shroud segment for connection to the power bus 72.
In the illustrated embodiment the blade tip electrode 54 is not covered by insulation, but is exposed to the clearance 42. As the blade tip 31 reaches each shroud electrode 68, the shroud electrode 68 and/or the blade tip electrode 54 is/are energized, producing a directed plasma 76 between the blade tip electrode 54 and the shroud electrode 68. This plasma induces a flow of ionized and neutral gas from the exposed electrode 54 to the covered electrode 68 as known in the art of dielectric barrier plasma generators. Such generators are described in U.S. Pat. No. 7,380,756, which is incorporated herein by reference, and are thus not further detailed here except as to enhancements.
The electrical potential delivered by the controller 74 may be in the form of alternating current (AC), also called a radio frequency (RF) voltage, as described in the incorporated U.S. Pat. No. 7,380,756 and others in the art. For example, if a turbine disk with 40 blades rotates at 3600 revolutions per minute (rpm), then 2400 activations or pulses of AC per second may be delivered by the controller 74 to each shroud electrode. If there are 40 shroud electrodes, then 2400 corresponding pulses of opposite phase may be delivered to each blade tip electrode 54. Alternately, either the shroud electrodes or the blade tip electrodes may provide a constant ground. Alternately the potential may be provided in pulses of DC current.
The number of shroud electrodes need not equal the number of blade electrodes. A formula for the activation frequency of a given shroud electrode in activations per second is B*RS/60, where B is the number of blades and RS is the disk rotation speed in rpm. For example, with 40 blades, a blade electrode in each blade tip, 80 shroud electrodes, and a rotation frequency of 3600 rpm, this calculation is 40*3600/60=2400 activations per second for each shroud electrode, regardless of the number of shroud electrodes. A formula for the activation frequency of a given blade electrode in activations per second is SE*RS/60, where SE is the number of shroud electrodes. In the above example this gives 80*3600/60=4800 activations per second for each blade electrode.
In the turbine section, activation may occur at a given rotational position where each blade electrode 54 reaches an offset position past a given shroud electrode 68 as shown in
In previous flow control applications, both electrodes are mounted on the same surface. This is seen for example in U.S. patent publication 2008/0089775, in which both electrodes are mounted on the shroud. In the present embodiment of
In order to time the electrical activations, the rotational position of the blades 30 may be provided to the controller 74 via a shaft timing sensor 78. Such timing devices are well-known, and may include a magnetic timing mark on the shaft 26 that is detected by a stationary coil or Hall-effect sensor 78 adjacent to the shaft. The controller may energize the electrical potential to each shroud electrode 68 in succession as the blade tip passes. It may also activate a corresponding opposite potential to the blade tip electrode. Alternately, either the blade electrode or the shroud electrodes may be grounded. Optionally the whole rotating frame may be grounded, which includes the shaft 26 and the attached disk 24 and blades 30. Such grounding allows the blade tips themselves to serve as ground electrodes.
Herein dielectric-covered electrodes 68 are shown in the shroud, and an exposed electrode 54 is shown in the blade tip. This can be reversed, with the exposed electrode 68 being in the shroud, and the covered electrode 54 in the blade tip. The timing of electrical activation depends on the direction of the plasma-induced gas flow between the electrodes, which in some embodiments is from the exposed electrode to the covered electrode per U.S. Pat. No. 7,380,756. When direct current is used, the induced gas flow follows the mobility drift of positive ions from positive to negative. In any case, activation occurs when the blade electrodes and respective shroud electrodes are adjacent and offset to produce a plasma-induced flow toward the pressure side of the airfoil. In one embodiment, the plasma may be generated by AC, and accelerated by a DC bias voltage between the shroud and blade electrode before and/or after the blade passes.
While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.
