Massively parallel LES of azimuthal thermo-acoustic instabilities in annular gas turbines

Increasingly stringent regulations and the need to tackle rising fuel prices have placed great emphasis on the design of aeronautical gas turbines, which are unfortunately more and more prone to combustion instabilities. In the particular field of annular combustion chambers, these instabilities often take the form of azimuthal modes. To predict these modes, one must compute the full combustion chamber, which remained out of reach until very recently and the development of massively parallel computers. In this article, full annular Large Eddy Simulations (LES) of two helicopter combustors, which differ only on the swirlers' design, are performed. In both computations, LES captures self-established rotating azimuthal modes. However, the two cases exhibit different thermo-acoustic responses and the resulting limit-cycles are different. With the first design, a self-excited strong instability develops, leading to pulsating flames and local flashback. In the second case, the flames are much less affected by the azimuthal mode and remain stable, allowing an acceptable operation. Hence, this study highlights the potential of LES for discriminating injection system designs. To cite this article: P. Wolf et al., C. R. Mecanique 337 (2009).     

The development of advanced computational methods for turbomachinery blade design

The paper describes the basic components of a turbomachinery blade design system in use within Rolls-Royce. A number of modelling aspects of the advanced computational methods in use and under development are reviewed together with areas for future research and development. A quasi-3D blade design system which is used for both compressors and turbines is described covering through-flow and blade-to-blade analysis. Various features of blade-to-blade analysis are discussed including the use of compatible design and analysis modes and coupled boundary layer analysis capable of handling attached and separated flow; examples are included to show capabilities. Advances being made in the development and application of Reynolds-averaged Navier–Stokes models are covered showing capabilities with regard to loss and heat transfer prediction. A fully coupled quasi-3D through-flow and blade-to-blade analysis system is described and results presented to show basic capabilities. The need for 3D flow analysis is discussed and the elements of a 3D blade design system presented showing how this links to the traditional quasi-3D system. Examples are given showing basic capabilities of the methods available and under development. Finally areas for future development are presented indicating the mathematical and numerical modelling problems to be addressed.     

Diagnosis of continuous rotor-stator rubbing in large scale turbine units using acoustic emissions

Continuous rubbing between the shaft and surrounding seals or end-glands of electricity generating turbine units can escalate into very severe vibration and costly rotor damage. Therefore such rotor-stator contacts require early diagnosis so as to minimize the financial consequences of any unplanned shutdowns. Acoustic emissions (AEs) or stress wave monitoring at the bearings has been identified as a sensitive non-destructive monitoring technique for such rub conditions [Electr. Eng. Jpn. 110(2) (1990); IEEE Proc. 6 (2000) 79; Hall and Mba, 14th International Congress on Condition Monitoring and Diagnostic Engineering Management (COMADEM’2001), Manchester, UK, 2001, p. 21]. However, experimental results from real turbines have been scarce. This paper presents a diagnosis of continuous rotor-stator rubbing in an operational 500 MW turbine unit via high frequency AE measurement within a 100 KHz-1 MHz ultrasonic band. As detailed by Sato [Electr. Eng. Jpn. 110(2) (1990)] and reported in this paper the onset of a continuous rub contact at a seal/gland was revealed by a sinusoidal modulation within the raw ‘rf’ AE response. By synchronous measurement at adjacent bearings, an estimation of the location of the rub was calculated using the phase delay between the adjacent AE modulations. Importantly, the AE diagnosis was closely corroborated by post-inspection of the turbine rotor.     

Conditions along the boundaries of bladed zones within the flow tracts of turbines

Hamilton's variational principle is applied to derive a system of conditions which expresses the balance of momentum and energy of an ideal gas across the selvadges

1 The reviewer of this paper confesses to be unfamiliar with the term 'selvadge(s)' which appears frequently herein. Clearly, however, it (variously) means 'boundary', 'edge', 'projection of edge line' (meridional or axial), 'locus of leading edges', etc.

of bladed zones within the flow tracts of turbines. This system provides the background for a correct formulation of optimal design problems for turbines and compressors. The exposition follows the model of a large number of blades when the basic equations can be averaged over the azimuthal co-ordinate. An analysis is given of the obtained conditions and a computational algorithm described.     

Analysis of laser drilled deep holes in stainless steel by superposed pulsed Nd:YAG laser radiation

Deep drilling of through holes in stainless steel (1.4301, sample thicknesses 5, 8 and 10 mm) has been performed with the superposed radiation of two pulsed Nd:YAG lasers with pulse duration of 0.5 ms superposed by 17 ns pulses. The drilling efficiency is improved by the spatially and temporally superposed radiation of the two lasers. The enhanced drilling speed and the larger reproducibility of the drilling time are explained by a modified formation of closures in the hole during percussion drilling which are recorded by high-speed photography. The metallographic hole analysis exhibits high-temperature oxidation marks. The development of these marks is described by a ray tracing of the incident beam within the hole and the resulting intensity distribution at the hole wall.