Bruce M. Steinetz

Turbine Engine Clearance Control Systems: Current Practices and Future Directions

Improved blade tip sealing in the high pressure compressor (HPC) and high pressure turbine (HPT) can provide dramatic reductions in specific fuel consumption (SFC), time-on-wing, compressor stall margin, and engine efficiency as well as increased payload and mission range capabilities. Maintenance costs to overhaul large commercial gas turbine engines can easily exceed

High-Pressure-Turbine Clearance Control Systems: Current Practices and Future Directions

1M. Engine removal from service is primarily due to spent exhaust gas temperature (EGT) margin caused mainly by the deterioration of HPT components. Increased blade tip clearance is a major factor in hot section component degradation. As engine designs continue to push the performance envelope with fewer parts and the market drives manufacturers to increase service life, the need for advanced sealing continues to grow. A review of aero gas turbine engine HPT performance degradation and the mechanisms that promote these losses are discussed. Benefits to the HPT due to improved clearance management are identified. Past and present sealing technologies are presented along with specifications for next generation engine clearance control systems.

Sealing in Turbomachinery

Improved blade-tip sealing in a high-pressure compressor and high-pressure turbine can provide dramatic improvements in specific fuel consumption, time on wing, compressor stall margin, and engine efficiency as well as increased payload and mission range capabilities. Maintenance costs to overhaul large commercial gas turbine engines can easily exceed

Test Rig for Evaluating Active Turbine Blade Tip Clearance Control Concepts

1 million. Removal of engines from service is primarily due to the spent exhaust gas temperature margin caused mainly by the deterioration of high-pressure-turbine components. Increased blade-tip clearance is a major factor in hot-section component degradation. As engine designs continue to push the performance envelope with fewer parts and the market drives manufacturers to increase service life, the need for advanced sealing continues to grow. A review of aero-gas-turbine engine high-pressure-turbine performance degradation and the mechanisms that promote these losses are presented. Benefits to the high-pressure turbine due to improved clearance management are identified. Past and present sealing technologies are presented along with specifications for next-generation engine clearance control systems.

Characteristics of Elastomer Seals Exposed to Space Environments

Clearance control is of paramount importance to turbomachinery designers and is required to meet todays aggressive power output, efficiency, and operational life goals. Excessive clearances lead to losses in cycle efficiency, flow instabilities, and hot gas ingestion into disk cavities. Insufficient clearances limit coolant flows and cause interface rubbing, overheating downstream components and damaging interfaces, thus limiting component life. Designers have put renewed attention on clearance control, as it is often the most cost effective method to enhance system performance. Advanced concepts and proper material selection continue to play important roles in maintaining interface clearances to enable the system to meet design goals. This work presents an overview of turbomachinery sealing to control clearances. Areas covered include: characteristics of gas and steam turbine sealing applications and environments, benefits of sealing, types of standard static and dynamics seals, advanced seal designs, as well as life and limitations issues.

Performance of Subscale Docking Seals Under Simulated Temperature Conditions

ABSTRACT Improved blade tip sealing in the high pressure compressor and high pressure turbine can provide dramatic improvements in specific fuel consumption, time-on-wing, compressor stall margin and engine efficiency as well as increased payload and mission range capabilities of both military and commercial gas turbine engines. The design of a first generation mechanically actuated active clearance control system for turbine blade tip clearance management is presented along with the design of a bench top test rig in which the system is to be evaluated. The active clearance control system utilizes mechanically actuated seal carrier segments and clearance measurement feedback to provide fast and precise active clearance control throughout engine operation. The purpose of this active clearance control system is to improve upon current case cooling methods. These systems have relatively slow response and do not use clearance measurement, thereby forcing cold build clearances to set the minimum clearances at extreme operating conditions (e.g., takeoff, re-burst) and not allowing cruise clearances to be minimized due to the possibility of throttle transients (e.g., step change in altitude). The active turbine blade tip clearance control system design presented herein will be evaluated to ensure that proper response and positional accuracy is achievable under simulated high-pressure turbine conditions. The test rig will simulate proper seal carrier pressure and temperature loading as well as the magnitudes and rates of blade tip clearance changes of an actual gas turbine engine. The results of these evaluations will be presented in future works.

Pressure Balanced, Low Hysteresis, Finger Seal Test Results

Abstract A universal docking and berthing system is being developed by the National Aeronautics and Space Administration (NASA) to support all future space exploration missions to low-Earth orbit (LEO), to the Moon, and to Mars. The Low Impact Docking System (LIDS) is being designed to operate using a seal-on-seal configuration in numerous space environments, each having unique exposures to temperature, solar radiation, reactive elements, debris, and mission duration. As the LIDS seal is likely to be manufactured from an elastomeric material, performance evaluation of elastomers after exposure to atomic oxygen (AO) and ultraviolet radiation (UV) was conducted, of which the work presented herein was a part. Each of th e three candidate silicone elastomer compounds investigated, including Esterline ELA-SA-401, and Parker Hannifin S0383-70 and S0899-50, was characterized as a low outgassing compound, per ASTM E595, having percent total mass loss (TML) less than 1.0% and collected volatile condensable materials (CVCM) less than 0.1%. Each compound was compatible with the LIDS operating environment of –50 to 50 °C. The seal characteristics presented include compression set, elastomer-to-elastomer adhesion, and o-ring leakage rate. The ELA-SA-401 compound had the lowest variation in compression set with temperature. The S0383-70 compound exhibited the lowest compression set after exposure to AO and UV. The adhesion for all of the compounds was significantly reduced after exposure to AO and was further decreased after exposure to AO and UV. The leakage rates of o-ring specimens showed modest increases after exposure to AO. The leakage rates after exposure to AO and UV were increased by factors of up to 600 when compared to specimens in the as-received condition.

Effects of Compression, Staging, and Braid Angle on Braided Rope Seal Performance

Abstract A universal docking system is being developed by the National Aeronautics and Space Administration (NASA) to support future space exploration missions to low Earth orbit (LEO), to the moon, and to Mars. The candidate docking seals for the system are a composite design consisting of elastomer seal bulbs molded into the front and rear sides of a metal ring. The test specimens were sub-scale seals with two different elastomer cross-sections and a 12-in. outside diameter. The seal assemblies were mated in elastomer seal-on-metal plate and elastomer seal-on-elastomer seal configurations. The seals were manufactured from S0383-70 silicone elastomer compound. Nominal and off-nominal joint configurations were examined. Both the compression load required to mate the seals and the leak rate observed were recorded while the assemblies were subjected to representative docking system operating temperatures of –58, 73, and 122 °F (–50, 23, and 50 °C). Both the loads required to fully compress the seals and their leak rates were directly proportional to the test temperature.

Toward an Improved Hypersonic Engine Seal

Abstract : The finger seal is a revolutionary new technology in air to air sealing for secondary flow control and gas path sealing in gas turbine engines. Though the seal has been developed for gas turbines, it can be easily used in any machinery where a high pressure air cavity has to be sealed from a low pressure air cavity, for both static and rotating applications. This seal has demonstrated air leakage considerably less than a conventional labyrinth seal and costs considerably less than a brush seal. A low hysteresis finger seal design was successfully developed and tested in a seal rig at NASA Glenn Research Center. A total of thirteen configurations were tested to achieve the low hysteresis design. The best design is a pressure balanced finger seal with higher stiffness fingers. The low hysteresis seal design has undergone extensive rig testing to assess its hysteresis, leakage performance and life capabilities. The hysteresis, performance and endurance test results are presented. Based on this extensive testing, it is determined that the finger seal is ready for testing in an engine.

High-Temperature Braided Rope Seals for Static Sealing Applications

Future turbine engines and industrial systems will be operating at increased temperatures to achieve more demanding efficiency and performance goals. In the highest temperature sections of the engine new material systems such as ceramics and intermetallics are being considered to withstand the harsh thermal environment. Components constructed of these low expansion-rate materials experience thermal strains and a resulting reduction of life when rigidly attached to high expansion-rate, superalloy support structures. Seals are being designed to both seal and to serve as compliant mounts allowing for relative thermal growths between high temperature but brittle primary structures and the surrounding support structures. Previous seal research yielded several braided rope seal designs which demonstrated the ability to both seal and serve as a compliant mount. The hybrid seal was constructed of an all-ceramic (alumina-silica) core overbraided with a superalloy wire sheath (cobalt based superalloy). The all ceramic seal was constructed of an all-ceramic (alumina-silica) core overbraided with multiple ceramic (alumina-silica) sheath layers. Program goals for braided rope seals are to improve flow resistance and/or seal resilience. To that end, the current report studies the test results of: baseline and modified hybrid seals; two stage hybrid and two stage all-ceramic seal configurations; and single stage hybrid and single stage all-ceramic seal configurations for a range of seal crush conditions. Hybrid seal modifications include increasing the sheath braid angle and core coverage. For the same percent seal cross-sectional crush, results show that increasing the hybrid seal braid angle increased seal stiffness and seal unit load, resulting in flows approximately one third of the baseline hybrid seal flows. For both hybrid and all-ceramic seals, two stage seal configurations significantly outperformed single stage configurations. Two stage seal flows were at least 30% less than the single stage seal flows for the same seal crush. Furthermore, test results of single stage seals indicate that for both all-ceramic and hybrid seals, a specific seal crush condition exists at which minimum flows are achieved (i.e. increasing seal crush beyond a certain point does not result in better flow performance). Flow results are presented for a range of pressures and temperatures from ambient to 1300 F, before and after scrubbing. Compression tests results show that for both all-ceramic and hybrid seals, seal preload and stiffness increase with seal crush, but residual seal interference remains constant.