Active clearance control systems in the high-pressure turbine section of commercial aircraft engines are used to close the gap that exists between the turbine blades and shroud, minimizing leakage flow over the blade tip. Reducing this leakage flow will benefit specific fuel consumption, potentially saving the airline industry millions of dollars every year and lowering environmentally harmful nitrous oxide (NOx) emissions, and it will also benefit the exhaust gas temperature overshoot that occurs during takeoff, improving the on-wing life of hot-section components and the time between overhauls. Current clearance control systems onboard modern engines only yield modest benefits because they rely on slow-response case cooling to match the thermal deflections of high-pressure turbine components. Significant gains may be realized by employing an active control system consisting of clearance probes and fast-response actuators to maintain tight clearances throughout the entirety of flight.
HTSMA active clearance control actuator concept. CDP, compressor discharge pressure.
Long description of figure 1.
| Actuator |
Approximate system weight, lb |
Rate capability, in./sec |
Maximum temperature, °F |
Power consumption |
|---|---|---|---|---|
| Shape-memory alloy | 0.9 | ~0.1 | 930 | Low |
| Piezoceramic (PZT) | 43 | 200 | 400 | Moderate |
| Servohydraulic | 58 | 0.5 | 600 | High |
In-house researchers at the NASA Glenn Research Center are developing a high-temperature shape-memory alloy (HTSMA) actuator that is demonstrated to have the physical robustness necessary to withstand the extreme temperatures of the high-pressure turbine. Shape-memory alloys are promising actuation candidates because they have energy densities an order of magnitude higher than other candidate actuators, an attractive characteristic in meeting the stringent weight and size goals of a flight-worthy actuator package. They are also preferred because they can potentially operate without the addition of bulky power amplifiers, thereby minimizing energy consumption and weight of the actuation system. A viable HTSMA actuator concept was developed for application in the Active Clearance Control Test Rig at Glenn. The concept consists of several parallel HTSMA wires (Ni30Pt20Ti50) that longitudinally expand or contract with changes in temperature, moving the shroud inward or outward relative to the blades. For implementation in a high-bypass turbofan engine, each actuator must displace at least 0.033 in. against large time-varying loads up to 2200 lbf and wide variations in the ambient temperature.
For evaluation of the HTSMA concept, a high-fidelity model of the actuation system was developed, the design was optimized, and the actuators were simulated to verify controllability across the engine’s operating envelope. Transient simulations in a testbed engine simulation (see the graphs) revealed that a design consisting of ten 0.09-in.-diameter wires can provide rub-free tracking during rapid operating point transitions. It also showed that only a small amount of bleed coolant air from the fan is necessary for actuation and that a linear proportional-integral-derivative control law with anti-windup protection is sufficient for precise shroud control with the nonlinear actuator across the operating envelope.
High-pressure turbine component deformation and simulated active clearance control scenarios during takeoff.
Long description of figure 2.
Shape-memory alloys, along with conventional servohydraulics and piezoelectric stacks are part of a broad feasibility study of active clearance control actuators. Servohydraulic actuators are a low-risk conventional technology, but resulting designs are high in weight and require large hydraulic power draw to operate. On the other hand, the two smart material actuators--piezoelectrics and shape-memory alloys--are high-reward technologies with greater energy densities but are considered to be of lower technological maturity and, therefore, higher-risk. The ultimate goal of this ongoing effort is to design, build, and demonstrate conventional and smart material actuation devices on the Active Clearance Control Test Rig.
This in-house study was conducted by Glenn’s Controls and Dynamics Branch and the QSS Group, Inc., in collaboration with Glenn’s Mechanical Components and Advanced Metallics Branches.
DeCastro, J.; and Melcher, K.: A Study on the Requirements for Fast Active Turbine Tip Clearance Control Systems. AIAA-2004-4176, 2004.
DeCastro, J.; Melcher, K.; and Noebe, R.: System-Level Design of a Shape Memory Alloy Actuator for Active Clearance Control in the High Pressure Turbine. AIAA-2005-3988, 2005.
QSS Group, Inc., contact: Jonathan A. DeCastro, 216-433-3946, Jonathan.A.DeCastro@nasa.govLast updated: October 12, 2006
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