COOREFLEX-turbo 3.2.6 - Leakage Performance and Rotordynamic Characteristics of Modern Dynamic Sealing Concepts with Leaf Seals

Motivation

Steam turbines, gas turbines and turbo compressors are highly developed components in mechanical engineering and essential in the energy sector. These turbomachines are not only deployed in industrial plants and conventional power stations but also in the context of renewable energies, like in biomass plants or geothermal and solar thermal power units. Therefore, the optimization of turbomachines keeps being a subject of intensive research and development.

One attractive way of increasing the efficiency is reducing internal leakages to not only avoid the loss of working fluid but also the emergence of undesired secondary flows. Using the example of steam turbines, leakages at shaft glands, guide vanes and runner blade shrouds comprise up to 30% of all internal losses.

Further development in dynamic sealing technology resulted in the emergence of sealing concepts with compliant elements. For example brush seals are already state of the art, clearly outperforming the classic labyrinth seal in terms of leakage. More advanced sealing concepts with compliant elements like various types of leaf seals and finger seals (see figures) offer additional advantages like further decreased leakage and/or longer lifetime. However, their characteristics are far less known and still being investigated. What matters is not only the leakage performance but also the rotordynamic behavior as sealing elements can have a stabilizing or destabilizing effect on the turbomachine as rotordynamic system.

Examples of advanced sealing concepts (l to r): Leaf seal [1], pressure activated leaf seal [2], non-contacting finger seal

Context

Improved sealing concepts form part of the AG Turbo joint research project COOREFLEX-turbo—Turbo Machines for Flexible Power Plants. AG Turbo is the German platform for pre-competitive, applied collaborative research on turbomachinery by industry, research centers and universities with financial support from the Federal Ministry for Economic Affairs and Energy (BMWi). The corresponding subproject COOREFLEX-turbo 3.2.6: Leakage Performance and Rotordynamic Characteristics of Modern Sealing Concepts with Leaf Seals is run at TUM Institute of Energy Systems in collaboration with GE Power.

Aims & Approach

The purpose of the project is to identify the most promising concept among the numerous different advanced sealing systems with a focus on leaf seals and its specification regarding leakage performance and rotordynamic characteristics.

For the experimental evaluation of turbomachine seals in realistic operating conditions the institute is equipped with two rotor test rigs. The static experiment allows the measurement of the leakage mass flow and the stiffness coefficients at different rotor excentricities. With the dynamic test rig the damping coefficients can be determined by exciting the rotordynamic system to its stability limit with the help of a magnetic actuator and the interacting aerodynamic forces within the seal. Both experiments can be run at different operating parameters with varying rotational speed, pressure drop and pre-swirl.

In parallel, model-based approaches for the analysis of sealings are developed with the aim to predict leakage as well as stiffness and damping coefficients theoretically. The focus is on three-dimensional numerical modeling using computational fluid dynamics techniques. The adequacy of the models is evaluated against experimental data.

Project partner: GE Power

Contact: Dipl.-Ing. Clemens Griebel

 

References:

[1] Jahn, I.H.J.; Gillespie, D.; Cooper, P. (2013): Hydrodynamic air-riding in leaf seals. GT2013-95585. In: ASME Turbo Expo 2013. San Antonio, Texas, USA.

[2] Grondahl, C. (2005): Pressure actuated leaf seals for improved turbine shaft sealing. AIAA 2005-3985. In: 41st AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit. Tucson, Arizona, USA.