Stiffness and Friction Characterization of Brush Seals
Ertuğrul Tolga Duran
Mechatronics, PhD. Dissertation, 2013
Assoc Prof. Mahmut F. Akşit (Thesis Supervisor), Assoc. Prof. Kemalettin Erbatur, Assoc. Prof. Ali Koşar, Prof. Dr. Yahya Doğu, Assoc. Prof. Hüseyin Kızıl
Date &Time: August, 13th, 2013 – 12:00
Place: FENS L055
Keywords: Brush Seal, Brush Seal Stiffness, Brush Seal Structural Analysis, Brush Seal Finite Element Analysis, Brush Seal Hysteresis, Bristle Stress, Brush Seal Friction Characterization, Pressure Stiffening
Improvements in steam and gas turbine technologies constantly drive harsher conditions, which result in thermodynamic cycles having higher pressure ratios and inlet temperature levels as well as increased leakage flows. Necessity of reducing the internal leakage flows and thermodynamic cycle losses while increasing the turbine overall efficiency puts greater importance on improvements in sealing technologies.
Labyrinth seals have been used almost since the invention of gas turbines and their technology is very well developed over the years. However, leakage rates are still not satisfactory and cannot meet the performance requirements of new generation turbines. Brush seals are fulfilling the leakage performance requirements and they can successfully compensate turbine transients at rotor rub conditions. Due to their superior and stable leakage performance and compact size, brush seals are replacing traditional labyrinth seals at critical sections of turbines.
A brush seal consists of a series of bristles which rub on the shaft surface. Since the bristles slide against the high-speed rotating shaft, friction and wear at the bristle tip contact becomes a major concern as it determines the life and efficiency of the seal and also rotor stability. Evaluating bristle tip contact forces and resulting stress levels at operating conditions is critical to optimize the seal performance and safety. A brief literature survey reveals the lack of test data and analysis methods for evaluating seal stiffness and stress levels at operating conditions. In an attempt to meet this need, custom test rig design and methodology has been developed to perform stiffness tests at turbine operating conditions. Analytical studies and finite element simulations have been performed for test seals and results have been correlated with the test data of this study. Furthermore, stiffness and friction characterization of brush seals has been conducted through correlated FE models, and MATLAB based code has been developed to generate brush seal FE models automatically for ABAQUS.
Custom test rig designs, analyses and test procedures to measure seal stiffness are presented in this study. Unlike previous tests in the literature, dynamic stiffness tests have been performed in addition to static tests with non-rotating rotor. Tests have been conducted at free-state and pressurized conditions, which helps to give better understanding of main brush seal phenomena in determining seal performance. Analytical studies, which are new to open literature, have also been developed in order to simulate the seal stiffness and bristle stress levels at operating conditions.
Considering the critical importance of contact loads on seal overall performance and system health, and due to complicated structure of brush seals, where bristles are contacting with each other as well as with backing plate and rotor, CAE analyses with high fidelity is required to simulate the test and turbine operating conditions. For this purpose, FE methodology has been developed for structural analyses of brush seals. 3D finite element models of test seals have been constructed and simulations have been performed for free-state and pressurized rotor-rub conditions. CAE model of brush seals includes rotor-bristle, bristle pack-backing plate and inter-bristle contacts with friction. Steady state simulations with non-rotating rotor and transient analyses with rotating rotor have been conducted, and extracted bristle tip force levels are correlated with the test results, all of which have been evaluated in this study. Analyses have been repeated by using Hexahedral and Space beam elements to determine optimum solution methodology. Seal stiffness hysteresis and pressure stiffening affects have been simulated and correlated with the tests. Inertial effects during dynamic tests have also been simulated through transient analyses and results show good agreement with the dynamic test data. Displacement and stress profiles obtained from correlated FE models give better understanding of brush seal behavior at turbine operating conditions.
Brush seal stiffness and friction characterization have been performed by using the correlated FE models. A series of seal structural analyses have been performed with different brush seal parameters and various friction coefficients. Brush seal characterization study has been conducted at steady-state conditions as well as at unpressurized and pressurized rotor-rub conditions. Bristle tip force and bristle stress levels have also been evaluated for characterization study.
A MATLAB code, which automatically generates the brush seal input file for ABAQUS simulations, has developed within the content of this study. The input file generator shortens the pre-processing time to less than a minute from 2-3 days.