UNSTEADY 3D NAVIER-STOKES CALCULATION OF A FILM-COOLED TURBINE STAGE WITH DISCRETE COOLING HOLE

Th. Hildebrandt, J. Ettrich

NUMECA Ingenieurburo, D-90530 Wendelstein Thomas. Hildebrandt@numeca. de

M. Kluge, M. Swoboda, A. Keskin, F. Haselbach, H.-P. Schiffer

ROLLS ROYCE Deutschland, Eschenweg 11, D-15287 Dahlewitz, Germany Marius. Swoboda@rolls-royce. com

Abstract Every modern high-pressure turbine needs a highly sophisticated cooling sys­tem. The most frequently used cooling method of to date is film cooling, char­acterized by a high degree of interaction between the main fbw and the cooling flaw. Therefore the effects of film cooling have to be taken into account in the aerodynamic design of film cooled high-pressure turbines.

Using modern commercial turbomachinery oriented CFD-methods, the mod­eling of film cooling holes can be achieved by various numerical methods of dif­ferent complexity. The so-called source term modeling is fast and easy to apply, but cannot provide very detailed ft>w information. In contrast, the discretization of every single cooling hole represents a very complex approach, but provides more in-depth information about the cooling pattern. The efforts of full-scale modeling need to be balanced against the more detailed and accurate results. In addition to the complex geometries of film cooled turbines, the flow phenomena are highly unsteady, thus requiring a CPU intensive time dependent numerical approach.

The present paper is focused on a detailed investigation of the unsteady flow field in a film cooled high-pressure turbine stage. An unsteady 3D Navier-Stokes calculation is applied to the entire stage configuration including a full discretiza­tion of all the cooling holes.

Nomenclature

M = Blowing rate

v = Velocity (m/s)

533

K. C. Hall et al. (eds.),

Unsteady Aerodynamics, Aeroacoustics and Aeroelasticity of Turbomachines, 533-549. © 2006 Springer. Printed in the Netherlands.

p = Pressure (Pa)

Ma = Mach Number

Re = Reynolds Number

p = Density (kg/m3)

Subscripts

c = cooling

2 = Inlet, exit conditions

t = total

is = isentropic

Abbreviations

NGV = Nozzle Guide Vane

1. Introduction

In order to obtain maximum thermodynamic cycle efficiency a high temper­ature level is required in the high pressure (HP) turbines of modern environ­mentally friendly gas turbines. The temperature level there is usually by far higher than the maximum allowable temperature of even the most advanced materials. Therefore, every modern HP turbine needs a sophisticated cooling system. From a variety of available cooling methods film cooling emerged as today’s standard cooling method. Relatively cool compressor air is injected through numerous holes and slots on the blade and endwall surfaces of a HP – turbine. Apart from the desired inflience of the injected cooling air on the heat transfer coefficients of the blade and endwall surfaces, the cooling jets have a considerable effect on the main fbw as well (Benz (1994), Hildebrandt et. al. (2001), Vogel (1997)). As a consequence, the effects of film cooling have to be taken into account in the aerodynamic design of a HP turbine.

Modern commercial Navier-Stokes solvers provide the designer in the turbo­machinery environment with a variety of options to simulate the flow inside the blade passage of a film-cooled turbine. The CFD modeling of film cooling holes can be achieved by various numerical methods of different complexity. The numerical technique of source term modeling is the fastest and least com­plex method to introduce the effects of film cooling into a 3D Navier-Stokes calculation of a turbine. This method is computationally least expensive and easy to apply, making it well suitable for the fast turn-around times, which are required in the modern design processes. The cooling ft>w is taken into ac­count by a distribution of various sources of mass, momentum and energy on the blade and endwall surfaces. In contrast, the full modeling of every single cooling hole represents the most complex approach. Using this method every cooling hole, including the cooling air plenum is discretized. Obviously, turn­
around times and engineering efforts are by far higher if compared to the source term method. The reward of applying this method to a film-cooled turbine is a high amount of very detailed ft>w information.

The complex ft)w phenomena of film cooling are apparently time dependent themselves, and additionally, highly inflienced by the unsteady rotor-stator in­teraction of the adjacent blade rows. The impinging wakes of a preceding blade row are periodically altering the local cooling efficiency along the blade sur­faces of the succeeding turbine rotor. Vice versa, the circumferentially chang­ing backpressure induced by a succeeding blade row can lead to considerable fluctuations in blade pressure distribution and shock location. The local blow­ing rate given by

is a function of the local velocity ratio, hence depending strongly on the pressure gradient between the plenum and the local ejection position on the blade surface. Therefore, a periodically flictuating blade pressure distribu­tion leads directly to an equivalently flictuating local film cooling efficiency. Therefore Unsteadiness is crucial if the focus is on very detailed cooling fbw phenomena.

The present paper is focused on a detailed investigation of an unsteady flow field in a film cooled high-pressure turbine stage. The flow is simulated using an unsteady 3D Navier-Stokes calculation of the entire turbine stage of a noz­zle guide vane and rotor configuration including a full modeling of all single cooling holes.