Exploratory Work in Fiber Optic Brush Seals for Turbomachinery Health Prognostics

Brush seals are found in aircraft engines, compressors, steam turbines, and power generators. Their design enables excellent sealing while mitigating any adverse rotor dynamics. Brush seals rely heavily on a pressure differential across their bristle pack to force the bristles to press against the rotor. This phenomenon, commonly referred to as “blow-down,” enables brush seals to seal tightly at a variety of operating conditions and transient events. The phenomenon of blow-down is rather complex. There are frictional forces between each bristle and between the bristle tips and rotor, fluid flow through the bristle pack and beneath the bristle tips, dynamic excitation of individual fibers, and even dynamic excitation of the bristle pack as a whole. These factors are also functions of operating temperature, operating pressure, and rotor speed, thus making them extremely difficult to model for reliability and life predictions. To develop a better understanding of the physics behind blow-down as well as gain further insight into the general behavior of brush seals under different operating conditions, a brush seal incorporating fiber Bragg grating sensors was fabricated and experimentally tested at GE Global Research. Fiber Bragg grating technology and brush seal technology form a synergistic relationship. The fiber Bragg grating sensor cables can be inserted within the heart of a brush seal bristle pack to interrogate the seal’s health and operating environment without limiting or interfering with the sealing capability of the brush seal. As a consequence, thermal profiles and strain levels within a brush seal can be measured directly. Multifunctional fiber Bragg grating sensors are a platform for integral turbomachinery health prognostics. In this work, a Kevlar fiber brush seal was augmented with a series of fiber Bragg grating sensors designed to measure the thermal and the dynamic strain response of the seal. A series of tests were then performed to validate the proposed concept of using fiber Bragg grating sensors to interrogate the seal. The results of the experimental work performed indicate that the local thermal transients, bristle pack dynamics, and static strain can all be extracted using fiber Bragg grating sensors.