An aircraft turbine engine is exposed to high temperatures, high pressures, and high speeds, and case structure and rotating parts have different thermal and mechanical expansion and contraction. Most of turbine engines adapts ACC (Active clearance controls) system to manage thermal contraction of casing and control tip clearance automatically during operation. For example, case structure can be cooled and radially contracted by cooling air outside of casing structure and the system is working to minimize the gap and improve turbine performance. However, the current system does not account of blades tip loss progression caused by rub and oxidation. As engine deteriorates on wing, the mismatching error between tip clearance calculation and actual hardware condition is increased and engine efficiency is being worsen.
Real time measurement of tip clearances would be a crucial way to understand tip loss and adjust the gap to maintain turbine efficiency, but current technology of measuring sensors including laser, capacitive, eddy current, and microwave is not applicable for production engines due to technical limitations such as short life, high cost, design complexity, etc. Analytical method using engine parameters is currently being developed, but it has not been validated to apply for accuracy.
This study is to review radial tip clearances and pressure measurement in engine flight test and demonstrate its linear correlation of Pressure Efficiency against clearance changes during test. As a result, it proposes new methods for tip clearances evaluation based on pressure measurement and conversion curves. New methods include one-point, two-points and three-points (or multi-) pressure measurement to assess clearance changes for different operating conditions. The conversion curves would be developed during engines tests on different power levels and be applicable for production engines with low cost, longer life, efficiency improvement and so on.