Mehmet Demiroglu

Fundamental Design Issues of Brush Seals for Industrial Applications

Advanced seals have been applied to numerous turbine machines over the last decade to improve the performance and output. Industrial experiences have shown that significant benefits can be attained if the seals are designed and applied properly. On the other hand, penalties can be expected if brush seals are not designed correctly. In recent years, attempts have been made to apply brush seals to more challenging locations with high speed (>400 m/s), high temperature (>650 °C), and discontinuous contact surfaces, such as blade tips in a turbine. Various failure modes of a brush seal can be activated under these conditions. It becomes crucial to understand the physical behavior of a brush seal under the operating conditions, and to be capable of quantifying seal life and performance as functions of both operating parameters and seal design parameters. Design criteria are required for different failure modes such as stress, fatigue, creep, wear, oxidation etc. This paper illustrates some of the most important brush seal design criteria and the trade-off of different design approaches.

Advanced Seals for Industrial Turbine Applications: Dynamic Seal Development

The ongoing need for higher performance industrial turbines has lead to extensive efforts to improve various components of gas turbines, steam turbines, compressors, and generators. One area being addressed is improved seals to reduce parasitic leakage flows. Major progress has been made to implement advanced dynamic seals into industrial turbines with resulting performance gains. Brush seals have significantly decreased labyrinth seal leakages in gas-turbine compressors and turbine interstages, steam-turbine interstage and end packings, industrial compressor shaft seals, and generator seals. Abradable seals are being developed for blade-tip locations in various turbine locations. The development and implementation of advanced seals in industrial turbines is summarized and with a focus on dynamic seals.

Advanced Seals for Industrial Turbine Applications: Design Approach and Static Seal Development

Changes in the market place are imposing increasing demands to improve efficiency (decreasing heat rate) and power output for both existing and new industrial turbines. The improvement is to be done while maintaining or decreasing emission levels. This demand has led to extensive efforts to improve the performance of the various components in industrial gas turbines, steam turbines, compressors, and generators. One of the critical areas being addressed is reducing the parasitic leakage flows through the various static and dynamic seals. Implementing advanced seals into industrial turbines has progressed well over the last several years, with significant operating performance gains achieved. Advanced static seals have been placed in gas-turbine hot gas-path junctions and steam-turbine packing ring segment end gaps. The status of efforts to develop and implement advanced static seals in industrial turbines is summarized. The design approach following design-for-six-sigma methodology is summarized, and the development efforts for each static seal type are presented.

Evaluation of Flow Behavior for Clearance Brush Seals

The industrial applications of brush seals have been increasing due to their superior sealing performance. Advances in the understanding of seal behavior have been pushing the design limits to higher-pressure load, temperature, surface speed, and rotor excursion levels. The highest sealing performance can be achieved when the bristle pack maintains contact with the rotor surface. However, due to many design and operational constraints, most seals operate with some clearance. This operating clearance cannot be avoided due to rotor runouts, transient operating conditions, or excessive bristle wear. In some applications, a minimum initial clearance is required to ensure a certain amount of flow rate for component cooling or purge flow. Typically, brush seal failure occurs in the form of degraded sealing performance due to increasing seal clearance. The seal performance is mainly characterized by the flow field in close vicinity of the bristle pack, through the seal-rotor clearance, and within the bristle pack. This work investigates the flow field for a brush seal operating with some bristle-rotor clearance. A nonlinear form of the momentum transport equation for a porous medium of the bristle pack has been solved by employing the computational fluid dynamics analysis. The results are compared with prior experimental data. The flow field for the clearance seal is observed to have different characteristics compared to that for the contact seal. Outlined as well are the flow features influencing the bristle dynamics.

An Investigation of Heat Generation Characteristics of Brush Seals

Brush seals are considered as a category of compliant seals, which tolerate a great high level of interference between the seal and the rotor or shaft. Their superior leakage characteristics have opened many application fields in the turbo-machinery world, ranging from industrial steam turbines to jet engines. However, brush seal designers have to find a trade-off between the lower parasitic leakage but higher heat generation properties of brush seals for given operation conditions. As brush seals can maintain contact with the rotor for a wide range of operating conditions, the contact force/pressure generated at the seal-rotor interface becomes an important design parameter for sustained seal performance and longevity of its service life. Furthermore, due to this contact force at the interface, frictional heat generation is inevitable and must be evaluated for various design and operating conditions. In this paper, frictional heat generation at the sealrotor interface is studied. To capture temperature rise at the interface, a thermal image of the seal and rotor is taken with an infrared camera under various operating conditions. The temperature map of the rotor is compared to results from thermal finite element analysis of the rotor to back calculate the heat flux to the rotor. A closed form equation for frictional heat generation is suggested as a function of seal design parameters, material properties, friction coefficient and empirical factors from testing.Copyright

NON-METALLIC BRUSH SEALS FOR GAS TURBINE BEARINGS

A non-metallic brush seal has been developed as an oil seal for use in turbomachinary. Traditionally labyrinth-type seals with larger clearances have been used in such applications. Labyrinth seals have higher leakage rates and can undergo excessive wear in case of rotor instability. Brush seals reduce leakage by up to an order of magnitude and provide compliance against rotor instabilities. Brush seals are compact and are much less prone to degradations associated with oil sealing. This paper describes the benefits and development of the non-metallic brush seals for oil sealing application.Copyright

EVALUATION OF BRUSH SEAL PERFORMANCE FOR OIL SEALING APPLICATIONS

Oil sealing at high speeds is one of the major problems engineers should address in turbomachinery design. High temperatures faced in oil sumps in aircraft engines, and large seal sizes typical in land based turbine applications further complicate the problem. Labyrinth seals can overcome problems faced with carbon seals in high temperature and large size applications. On the other hand, use of labyrinth seals may result in high leakage rates leading to increased oil consumption, unintended oil contamination in some flow cavities, early oil degradation or even fires in some cases. Successful engine secondary flow path applications of brush seals lead to questions of their applicability for oil sealing. Because brush seals are contact seals, oil temperature rise and coking become major issues in addition to leakage performance. This paper presents an investigative study of brush seal leakage and coking performance using common lube oil. Both metallic and non-metallic prototypes have been tested under static and dynamic conditions. It has been concluded that properly designed brush seals can achieve lower leakage rates than labyrinth seals without causing coking problems.

An Investigation of Tip Force Characteristics of Brush Seals

Thanks to their compliant nature and superior leakage performance over conventional labyrinth seals, brush seals found increasing use in turbomachinery. Utilizing high temperature super-alloy fibers and their compliance capability these seals maintain contact with the rotor for a wide range of operating conditions leaving minimal passage for parasitic leakage flow. Consequently, the contact force/pressure generated at seal rotor interface is of importance for sustained seal performance and longevity of its service life. Although some analytical and numerical models have been developed to estimate bristle tip pressures, they simply rely on linear beam equation calculations and other such assumptions for loading cases. In this paper, previously available analytical and/or numerical models for bristle tip force/pressure have been modified and enhanced. The nonlinear cantilever beam equation has been solved and results are compared to a linear cantilever beam equation solution to establish application boundaries for both methods. The results are also compared to experimental data. With the support of testing, an empirical model has been developed for tip forces under operating conditions.Copyright

A numerical study of brush seal leakage flow

With their increasing use in gas turbine engines, brush seals have drawn the recent attention of researchers. Among the challenging problems, perhaps leakage analysis is of primary importance in investigating sealing performance. This work presents a 2-D laminar flow analysis. Using a commercially available finite element analysis software FIDAPTM, the model predicts flow rate versus pressure drop for a given flow region. The brush seal flow domain is divided into representative smaller cells in which simulation takes place. The cells are chosen such that together they can represent whole seal. At the beginning, the model consists of a single cell of five bristles, one in the middle and four at each corner of a square. Then, other cells are added in order to extrapolate the results for a full seal. In parallel to the numerical work a simple analytical model has also been developed. This simplified solution of the 2D laminar Navier-Stokes equation predicts pressure drop across two bristles. Results from the numerical work are also compared to this analytical solution. They also show good agreement with experimental static leakage test data collected on a high-speed test rig. Currently at GE Corporate Research and Development Center Professor of Mechanical Engineering INTRODUCTION The brush seal consists of a set of fine diameter metallic or ceramic fibers densely packed between retaining and backing plates. As illustrated in Figure 1, the backing plate is positioned downstream of the bristles to provide mechanical support for the differential pressure loads. The circular seal is installed in a static member with bristles touching the rotor with an angle in the direction of the rotor rotation. In the case of rotor excursions, this cant angle helps reduce the contact loads allowing bristles to bend rather than buckle. The inherent flexibility enables the seal to survive large rotor excursions without sustaining any appreciable permanent damage. With thousands of flexible bristles moving around under various loads, modeling the leakage through bristles is a real challenge. There are many difficult issues that need to be addressed in a realistic leakage model. To name a few: • compliance (individual bristles move around) • hysteresis and pressure closure • hydrodynamic lift and bristle flutter • flow in axial, radial and tangential (swirl) directions • change in bristle density with changing interference and pressure closure • change in tip clearance with pressure closure and wear. American Institute of Aeronautics and Astronautics 1 R E T A I N I N G PLATE-

Stiffness Measurement for Pressure-Loaded Brush Seals

Brush seals are widely used as flexible seals for rotor-stator and stator-stator gaps in power generation turbo-machinery like steam turbines, gas turbines, generators and aircraft engines. Understanding the force interactions between a brush seal bristle pack and the rotor is important for avoiding overheating and rotor dynamic instabilities caused by excessive brush seal forces. Brush seal stiffness (i.e. brush seal force per unit circumferential length per unit incursion of the rotor) is usually measured and characterized at atmospheric pressure conditions. However, the inter-bristle forces, the blow-down forces and the friction forces between the backplate and the bristle pack change in the presence of a pressure loading, thereby changing the stiffness of the brush seal in the presence of this pressure loading. Furthermore, brush seals exhibit different hysteresis behavior under different pressure loading conditions. Understanding the increased brush seal stiffness and the increased hysteresis behavior of brush seals in the presence of a pressure loading is important for designing brush seals for higher pressure applications. In this article, we present the development of a test fixture for measuring the stiffness of brush seals subjected to a pressure loading. The fixture allows for measurement of the bristle pack forces in the presence of a pressure loading on the seal while the rotor is incrementally pushed (radially) into the bristle pack. Following the development of this test fixture, we present representative test results on three sample seals to show the trends in brush seal stiffness as the pressure loading is increased. Specifically, we study the effect of different brush seal design parameters on the stiffness of brush seals over a wide range of pressure loadings. These test data can be used for developing predictive models for brush seal stiffness under pressure loading. Furthermore, we demonstrate the utility of this fixture in studying the hysteresis exhibited by brush seals along with the importance of the backplate pressure balance feature present in several brush seal designs. The test results validate the bilinear force-displacement curves previously reported in the literature.Copyright