A state-of-the-art active clearance control (ACC) test rig has been fabricated and installed at the NASA Glenn Research Center (see the photograph). This rig is designed to evaluate the feasibility of implementing a fast-acting mechanical actuation system in the high-pressure turbine section of a modern jet engine to precisely adjust the clearance between the tips of rotating turbine blades and the sealing shroud that encases the rotor. Minimizing this clearance can reduce the spill of combustion gases over the ends of the turbine blades, thus enhancing both the efficiency and overall performance of the engine. Current engines cool the outer engine case flanges to decrease the clearance between the blade tips and the shroud. This system, however, is limited by slow response times and does not monitor the dynamic, asymmetric blade-tip-to-shroud clearance. For this reason, clearances must be set conservatively during engine buildup to avoid blade-tip to seal-shroud collisions during extreme engine condition transients (e.g., takeoff and reacceleration), thus foregoing the benefits of tighter tip clearances during cruise.
Reducing engine tip clearances by 0.010 in. would decrease specific fuel consumption by approximately 0.8 to 1 percent, significantly decrease nitrogen oxides (NOx), carbon monoxide, and carbon dioxide emissions, and lower exhaust gas temperature by approximately 10 °C. Additional benefits of tighter tip clearances include increased engine time-on-wing, decreased operating and maintenance costs, and increased mission range and payload capabilities.
Closeup of ACC rig showing internal components.
The new test rig installed at Glenn employs a fast-acting mechanical actuation system designed to improve on existing clearance-control methods (see the photograph). Simulated tip clearances in the test rig are tracked using capacitance-based proximity probes coupled with nine independent mechanical actuators through a closed-loop control scheme. The actuators position the segmented seal carriers to achieve the desired clearance levels on the basis of positional feedback from the proximity probes. The test rig simulates the geometry, temperature, and pressure differentials found in a high-pressure turbine section, allowing investigation of the candidate kinematic systems, seal concepts, and controls needed to implement the ACC system in an engine.
Room-temperature testing of the ACC rig showed that the kinematic system allowed seal carrier motion over a0.080-in. range, which easily encompasses the range needed for future turbine engine applications. The system leakage was tested at pressures up to 120 psig--comparable to pressure differentials anticipated in future engines. Seal effective leakage flow area was comparable to an engine industry reference level. Cyclic open-loop actuation of the seal carriers over a ±0.035-in. range (radially inward and outward) at 20 psig (the load limit of current stepper actuators) was used to examine system repeatability. This test showed that the carriers returned to their original locations after 20 cycles to within the 0.001-in. resolution of the gauge pin measurement technique used. Closed-loop actuation of the seal carriers using proximity probe feedback showed that the system could track a simulated engine flight clearance profile with better than 0.001-in. accuracy while satisfying required clearance change rates (see the graphs).
ACC test rig tracking of a simulated flight clearance profile at 20 psig from flight idle (time zero). Data includes the commanded clearance set point, three proximity probe readings measuring actual clearances, and a corresponding plot of error versus time (<0.001-in. error). The test was conducted at ambient temperature, and control was based on the minimum of the three probe readings. The negative position indicates radially outward movement.
Steinetz, Bruce M., et al.: Evaluation of an Active Clearance Control System Concept. NASA/TM-2005-213856 (AIAA-2005-3989), 2005. http://gltrs.grc.nasa.gov/cgi-bin/GLTRS/browse.pl?2005/TM-2005-213856.html
Find out more about this research at Glenn:
Turbine seals: http://www.grc.nasa.gov/WWW/TurbineSeal/
Structural seals and thermal barriers: http://www.grc.nasa.gov/WWW/structuralseal/
Mechanical Components Branch: http://www.grc.nasa.gov/WWW/5900/5950/
Dr. Bruce M. Steinetz, 216-433-3302, Bruce.M.Steinetz@nasa.gov
Authors: Dr. Bruce M. Steinetz, Dr. Scott B. Lattime, Shawn C. Taylor, Jonathan A. DeCastro, Jay J. Oswald, and Kevin J. Melcher
Headquarters program office: Aeronautics Research
Programs/Projects: UEET, Propulsion 21
Last updated: October 16, 2006
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