Session: 23-10 Labyrinth Seals

Paper Number: 152122

CFD Modeling of a High-Pressure Honeycomb Seal for Centrifugal Compressors 

The leakage flow between stator and rotor is one of the main sources of performance degradation in rotating machines like gas or steam turbines and compressors. In particular, the mixing between the leakage flow (secondary flow) and the main flow causes the reduction in the overall system efficiency. Seals of different types are used nowadays in a large variety of fluid machines to face this issue.

Among the different types of seals, labyrinth seals are one of the most applied in turbomachinery world, due to their capability to handle extreme operating conditions in terms of high temperature, rotational speeds and pressure ratios. Moreover, they also represent a good trade-off between leakage control and reduced costs, since they are easy to manufacture and do not necessitate demanding maintenance. However, the use of labyrinth seals may be limited by the dynamic instability induced in some operating conditions.

Lateral fluid forces acting on the rotor are generated in correspondence of the seal mainly due to the presence of a non-uniform circumferential pressure distribution. Consequently, sub-synchronous unstable vibrations may arise leading to dynamic instability issues, especially when the machine is close to its maximum power condition.

Seals-induced vibrations  may result in increased wear, reduced efficiency, and even catastrophic failures during service in extreme cases. For those reasons, the study of the dynamic behavior of seals becomes particularly significant in high-speed applications such as centrifugal compressors, where the overall stability of the system could be compromised by the significant forces acting on the shaft.

To overcome this problem, innovative types of seals such as honeycomb seals have been gradually developed starting from the second half of XX century to meet the specific turbomachinery requirements in terms of efficiency maximization, reduction in dynamic instability and better response against oxidation and wear.

From an engineering perspective, the dynamic interaction between the rotor and the fluid passing through the seal has been traditionally studied through the evaluation of the rotor-dynamic coefficients of stiffness and damping. These parameters are commonly computed considering a linear relationship between shaft displacement and forces exchanged with the fluid in the clearance.

Instead, the determination of accurate dynamic coefficients is of paramount importance in the design phase of rotating machines to predict the seals-induced instability and increase the reliability of the system. Finite Element Analysis (FEA) and Computational Fluid Dynamics (CFD) simulations have emerged as widely adopted tools for modeling the flow-seal interaction.

This paper is focused on the development of an optimized CFD model of a honeycomb seal for a centrifugal compressor and on the estimation of the dynamic coefficients from the developed CFD model. The numerical results are then validated with the experimental data from a Baker Huges test-rig where  the experimental data reproduce real operating conditions of a centrifugal compressor.

Presenting Author: Andrea Riva Politecnico di Milano

Presenting Author Biography: Andrea Riva is a PhD student at the Department of Mechanical Engineering of Politecnico di Milano. His research activity is focused on CFD modelling and experimental validation of turbomachinery components such as oil-film bearings and seals.

Authors:

Andrea Riva Politecnico di Milano
Edoardo Gheller Politecnico di Milano
Steven Chatterton Politecnico di Milano
Paolo Pennacchi Politecnico di Milano
Allegra Fabrizi Baker Hughes
Claudio Orazi Baker Hughes
Giuseppe Vannini Baker Hughes