Mahmut Faruk Aksit

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.

Brush Seal Temperature Distribution Analysis

Brush seals are designed to survive transient rotor rubs. Inherent brush seal flexibility reduces frictional heat generation. However, high surface speeds combined with thin rotor sections may result in local hot spots. Considering large surface area and accelerated oxidation rates, frictional heat at bristle tips is another major concern especially in challenging high-temperature applications. This study investigates temperature distribution in a brush seal as a function of frictional heat generation at bristle tips. The two-dimensional axisymmetric computational fluid dynamics (CFD) analysis includes the permeable bristle pack as a porous medium allowing fluid flow throughout the bristle matrix. In addition to effective flow resistance coefficients, isotropic effective thermal conductivity as a function of temperature is defined for the bristle pack. Employing a fin approach for a single bristle, a theoretical analysis has been developed after outlining the brush seal heat transfer mechanism. Theoretical and CFD analysis results are compared. To ensure coverage for various seal designs and operating conditions, several frictional heat input cases corresponding to different seal stiffness values have been studied. Frictional heat generation is outlined to introduce a practical heat flux input into the analysis model. Effect of seal stiffness on nominal bristle tip temperature has been evaluated. Analyses show a steep temperature rise close to bristle tips that diminishes further away. Heat flux conducted through the bristles dissipates into the flow by a strong convection at the fence-height region.

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.

Effects of Geometry on Brush Seal Pressure and Flow Fields—Part I: Front Plate Configurations

Pressure and flow fields lay at the basis of such common phenomena affecting brush seal performance as bristle flutter, blow-down, hang-up, hysteresis, pressure stiffening, wear, and leakage. Over the past two decades of brush seal evolution, manufacturers and researchers have applied many geometric configurations to the front and backing plates of a standard brush seal in order to control the flow field and consequent seal performance. The number of studies evaluating the effect of geometric configurations on the brush seal flow field remains limited in spite of the high number of filed patent disclosures. This study presents a numerical analysis of brush seal pressure and flow fields with regard to common conceptual front plate configurations. A CFD model has been employed to calculate pressure and flow fields in the seal domain. The model incorporates a bulk porous medium approach for the bristle pack. The effectiveness of various conceptual geometries has been outlined in terms of flow field formation. Results disclose unique effects of geometry on pressure and flow fields such that a longer front plate drives outward radial flow while playing a protective role against upstream cavity disturbances. Findings also indicate that variations in front plate geometry do not directly affect leakage performance. A long front plate or damper shim considerably changes the flow field while at the same time having limited effect on the pressure field. Moreover, a strong suction towards the clearance enhances inward radial flow in clearance operation.

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.

Wear of Brush Seals: Background and New Modeling Approach

The brush seal, with superior leakage performance, is emerging as a new sealing technology to effectively control cooling and leakage flows in gas turbine engines. Because the bristles slide against the rotor surface, wear at the contact becomes a major concern as it determines the life and efficiency of the seal. To optimize seal life and efficiency, an in-depth study of the factors causing the seal stiffness is needed, and a good choice of materials must be made. This work investigates some of the past research on brush seal wear. Although considerable research has been done on material selection and tribopairs, the brief survey reveals the lack of reliable analyses to evaluate contact loads and to address heat transfer issues. The complicated nature of bristle behavior under various combinations of pressure load and rotor interference requires computer analysis to study the details that may not be available through analytical formulations. In an effort to meet this need, the present work includes a pre...

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.

A hardware test setup for grid connected and island operation of micro hydro power generation systems

This paper presents a micro hydro power generation hardware setup, established via a research project at Gebze Institute of Technology (GIT), Turkey. This project is founded by the government and aims to increase utilization of potential for small, local hydro power generation. The hardware setup consists of a newly designed cross flow type hydro turbine, two different generators (asynchronous and synchronous up to 20 kW power range) and all equipment necessary for grid connected operation and islanding. The setup can be used for two purposes: primarily performance tests of newly designed micro hydro turbines, and research and development studies to provide new regulations and technical guidelines regarding to energy management and grid connected and island (stand-alone) operation which concern for distributed generation versatility. The hardware setup offers a similar environment to that of real site which can easily be adapted to the clients test and turbine evaluation requirements.

Analysis of Brush Seal Bristle Stresses With Pressure-Friction Coupling

With their superior leakage performance brush seals are used in many demanding sealing applications. In recent years, they found ever increasing use in ground based large industrial gas and steam turbines replacing labyrinth seals. As the applications become more demanding, seal designs are pushed to their limits. The knowledge of brush seal bristle stresses is essential to determine seal pressure carrying capability and amount of creep for high temperature applications. Seal manufacturers continue to rely on their experience and simple models based on beam theory in order to develop the required robust seal designs. Although some analytical formulations are developed over the years, pressure-stiffness coupling and its effects on bristle stresses deserve further study due to complicated frictional bristle interactions. In order to explore brush seal bristle stresses with frictional effects, this paper presents a study using a 3-D finite element analysis. A maximum bristle stress relation is derived based on statistically designed experiments. Model accuracy is determined through verification simulations. A discussion on the effects of design and loading parameters on maximum stress is also included.Copyright