COMPARISON OF MODELS TO PREDICT LOW ENGINE ORDER EXCITATION IN A HIGH PRESSURE TURBINE STAGE

Markus Jocker,1 Alexandros Kessar,1 Torsten H. Fransson,1 Gerhard Kahl,2 Hans-Jurgen Rehder3

1 Royal Institute of Technology S-100 44 Stockholm, Sweden markus@egi. kth. se alex@egi. kth. se fransson@egi. kth. se

2

MTU Aero Engines D-80995 Munich, Germany Gerhard. Kahl@muc. mtu. de

3

German Aerospace Center D-37073 Gottingen, Germany hans-juergen. rehder@dlr. de

Abstract The paper compares three numerical strategies to predict the aerodynamic ro­tor excitation sources of "Low Engine Order" (LEO) in a high-pressure turbine stage. Main focus is laid on methods to compute the stator exit fbw. The aim is to evaluate computationally cheap approaches to avoid modeling the whole circumference of the stator. A single passage viscous strategy, a single pas­sage inviscid linear blade movement strategy, and a viscous multi-passage sector strategy are compared and evaluated. The assessment of the prediction quality is made by comparison of the computed stator exit few to experimental data. The main result is that only the global behavior of the stator exit flaw is estimated right, both the level and amplitude of Mach number and pressure are computed with poor agreement to experiments. Future evaluations of the resulting rotor excitation pressure are needed to estimate the level of necessary agreement to give acceptable predictions of the low engine order forced response.

Keywords: Forced Response, Low Engine Order Excitation, High Pressure Turbine, Stator

Non-Uniformity

145

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

Unsteady Aerodynamics, Aeroacoustics and Aeroelasticity of Turbomachines, 145-159. © 2006 Springer. Printed in the Netherlands.

Nomendatrue
A	Area	[m2]
h/H	Relative blade height	[-]
m	Passage number	[-]
M	Mach number	[-]
N	Number of stator passages	[-]
фиогш	Circumferential coordinate normalized with nominal pitch an-	[-]
	gle
Subscripts
ax	Axial
t	Throat
t	Total value
Abbreviations
2D	Two-dimensional (constant radius)
CFD	Computational Fluid Dynamics
DLR	German Aerospace Center
(L)EO	(Low) Engine Order
L2F	Laser Two Focus Anemometry
SP	Single passage model
MP	Multi passage model

1. Introduction

Forced vibrations are characterized by aerodynamic excitation sources, which are ft>w disturbances acting periodically on the blades and originate from up­stream and/or downstream obstacles. The time-periodic excitation is in all cases caused by the relative rotational motion of excitation source and the ex­cited structure, which leads to excitation frequencies multiples of the rotation frequency. A common way to identify forced response regions of a blade row is the "Campbell Diagram”, a typical example for a turbine rotor is shown in Fig. 1, which is a key plot in the unsteady design process. The diagram shows crossings of excitation frequencies due to upstream and downstream vanes, as well as burner cans, with the eigenfrequencies of the blades. At these cross­ings, the risk of resonant excitation of the structure exists. Practically, in high- pressure turbines, vane passing does not excite the 1st fhx mode because of its low eigenfrequency (typical frequencies correspond to 8 to 10 excitations per revolution in the operating range). To the original figure presented in Ref. [1], an additional line was added indicating another excitation of low frequency, named the "Low Engine Order" (LEO) excitation. Such can be caused by non­uniformities on the stator blade row due to manufacturing variations and wear (for example vane erosion or burnout). The excitations can induce vibrations in fundamental blade modes such as the first bending or torsion mode, charac­terized by low frequency and possibly high vibration amplitudes. The severity is increased due to occurrence at high load operating conditions.

Figure 1. Example of a typical turbine Campbell diagram indicating Low Engine Order resonance

During the last decade intensive research and development activities have focused on the computation of wake passing frequency excitation in turboma­chinery stages. The goal was to benchmark the capabilities of computational tools for further application during engine design. Furthermore, the investi­gation of unsteady fbw physics was addressed aiming at widening the un­derstanding of the mechanisms of aerodynamic blade vibration excitation, for example in [2].

Only little work has been published on the computation and understanding of low engine order excitations. These are more difficult to predict than vane passing excitations due to the following reasons:

■ The frequency and magnitude of the low engine order excitation is not known a priori, because it is caused by unknown variations in the vane geometry due to manufacturing tolerances or wear on the vanes during operation.

■ The periodicity of the excitation is often not known a priori. The spatial periodicity of the stator exit flow must be assumed to be over the whole circumference.

For a low-pressure turbine rotor, [3] investigated the 2nd engine order tem­perature variations, which emanated from the combustor. The excitation was modeled with help of measured data. A full annulus analysis was presented by [4] indicating LEO excitations due to throat width variations in the stator. [5] showed a systematic study of low engine order forced response considering the effects of throat width variation in the stator and temperature distortion. A typical sector of the stage was modeled to compute the aerodynamic excita­tion. In [6], linear inviscid ft>w and non-linear viscous ft>w models for the un­steady flow in the ADLARF fan rotor are compared, LEO boundary conditions were derived from experiments. All these investigations derived the unsteady boundary conditions either from full annulus or sector vane computations, or calculated them from measured data.

This paper investigates the LEO excitation in a high-pressure turbine with transonic flow, for which a companion experimental program prescribes the excitation source. The objective is to assess different modeling options to compute the unsteady inlet boundary conditions due to a given distortion in the stator geometry. Instead of modeling the coupled stator-rotor domain, the computationally cheaper but more modeling intensive approach of de-coupling the stator and rotor computation is chosen, because the overall goal is to find approaches suitable for design. The stator fbw analysis is the first critical step in the de-coupled approach. Unsteady pressure response on the rotor can presently not be evaluated due to the lack of experimental data.

The stator configuration is given by a sinusoidal variation of stator throat area, both a 5th engine order variation and a 7th engine order variation is avail­able with two amplitudes of throat area variation each 2% and 4%. To limit the study on the modeling evaluation, the 5 th engine order case with 4% varia­tion was chosen (see also Fig. 2). Three modeling strategies are evaluated and compared.