PhD defense - T. Guedeney

Unsteady aerothermal analysis of a turbine stage with hot streaks migration: application to the mastery of blade performances

Delivered by Ecole Centrale de Lyon - MEGA
Speciality: Mechanics


November 29, 2012 - CERFACS and Ecole Centrale de Lyon

The flow unsteadiness in turbomachines is mainly due to the relative motion of rotors and stators. Despite the progress done over the last decades in Computational Fluid Dynamics (CFD), classical unsteady RANS techniques for multistage turbomachines are still expensive in CPU time, limiting their usefulness in industrial design. To reduce the cost of such unsteady flow simulations, a pseudo-spectral method (the so-called Time Spectral Method - TSM) has been proposed in the literature. It takes into account the flow time periodicity. Thanks to Fourier analysis, the unsteady Navier-Stokes equations can be read as 2N+1 steady equations coupled by a source term. This approach has already been implemented in the structured multiblock flow solver elsA. It efficently computes unsteady flows and shows major improvements regarding the computational time (to a factor up to ten). However this method is only able to compute a flow field composed of one frequency and its harmonics in each blade row. Thus, a work has been done to extend the method to the more general case in which the frequencies are coprime. This multifrequential approach (called Harmonic Balance Technique or HBT) has been implemented in elsA. However, contrary to the TSM, in which this source term is analytic, it turns algebraic with the HBT, leading to difficulties to compute the inverse Fourier matrix. Another difficulty lies in the fact that the HBT should be implemented with a phase-lagged conditions in order to reduce the computational domain to only one blade passage, regardless of the actual blade count. This means that the periodic phase-boundary conditions and rotor/stator interactions need a special treatment. In the first step, two rows (a rotor and a stator) are modeled with an injection boundary condition accounting for the inlet guide vane wakes. Therefore, two fundamentals frequencies are seen by the rotor. The influence of the frequency content in the rotor (the number of harmonics of the passing blade frequencies and their combinations). The validation of the results is done against classical time marching unsteady simulations. The method is then applied to several industrial compressors, namely the ECL4 transsonic compressor and the study compressor CREATE.

P. Ferrand	Research director CNRS	Referee
M. Manna	Professor University of Napoli	Referee
P. Cinnella	Professor ENSAM	Member
T. Obrecht	Engineer SNECMA Villaroche	Member
F. Sicot	CERFACS CFD	Member
G. Puigt	CERFACS CFD	Member
F. Leboeuf	Ecole Centrale de Lyon	Advisor

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