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Blade row interference and clocking effects in a one and a half stage turbine

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VKI PHDT 2007-02, Blade row interference and clocking effects in a one and a half stage turbine, ISBN 978-2-930389-24-9

Blade row interference and clocking effects in a one and a half

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  • Blade row interference and clocking effects in a one and a half
  • Blade row interference and clocking effects in a one and a half


Blade row interference and clocking effects in a one and a half stage turbine
By Nicolas Billiard, published in 2006,  ISBN 978-2-930389-24-9
PhD Thesis from the von Karman Institute/ Ecole Centrale de Lyon, France, Octobre 2006

Modern aero engines require higher power to weight ratio and lower specific fuel consumption. Each component of the gas turbine plays an important role and significant overall gains can be achieved only from the sum of small improvements on the different components. Moreover, life duration of these components is a crucial factor in the development of any engine used in transportation. The present work focuses on the high pressure turbine and in particular on the interactions between the first stage and the following stage. Blade row interferences (potential and viscous effects), which are penalizing the performance of the turbine, are source of in flow distortions for the following stages. Moreover, highly loaded HP turbine stages exhibit flows dominated by shock interactions. In addition, because it is located downstream of the combustion chamber, the high-pressure turbine operates in a harsh environment with turbine entry temperatures up to 2000 K, i.e. far above the melting point of the super-alloys used to manufacture the blades and other HP turbine components. For this reason, the high-pressure turbine parts are intensively cooled, which adds sources of distortion, and in some cases, the stator of the next stage also requires an efficient cooling scheme.

The present research, mostly experimental, consists in investigating and analyzing the aerodynamic and heat transfer in a one and half stage turbine. The turbine is tested under engine representative Reynolds number (106), Mach number (0.9), gas to wall temperature ratio (1.5) and gas to coolant temperature ratio (2.1). Pressure and heat flux measurements were performed in order to obtain both steady and unsteady components of the flow field. Previous investigations focused on the interaction between the inlet guide vane and the rotor of the high-pressure stage. The current work highlights the interaction between the high-pressure stage and the second stator. For this purpose, airfoils of the second stator are instrumented with pressure taps, fast response pressure transducers and thin-film gauges at 15, 50 and 85% span as well as at the hub and tip endwalls. Additionally, several types of probes monitor the second stator inlet and exit flow field (total pressure and temperature).

First, the effect of rotor blade passing events on the flow field of the second stator was quantified and explained. The second stator ingests a number of non-uniformities coming from the preceding stage like wakes, secondary flows, and a pitchwise static pressure gradient since the rotor exit Mach number is M = 0.9. One of the key results consists in the strong similarities between the pressure fluctuations and the unsteady heat transfer measured on the second stator. Concerning the stator-stator interaction, it appears that downstream of the first turbine stage there exist non-uniformities linked to the first stator. For this reason, the relative position of the second stator with respect to the first stator influences the second stator flow field. Both time-averaged and time-resolved flow field were investigated for 4 clocking positions. The positioning of the second stator has a significant effect on the rotor exit flow regime. Since the pitchwise variation of total pressure downstream of the rotor is linked to the inlet guide vane while the static pressure variation is caused by the vicinity of the second stator leading edge, clocking the second stator modifies the relative position of both quantities and thus the rotor exit Mach number. In addition, because the clocking affects the rotor exit Mach number, it has an impact on the flow time-resolved quantities measured around the second stator. Clocking 1 appears to have slightly lower static pressure fluctuations than the other clockings. Although the 15% span section experiences the smallest forces fluctuations for Clocking 2, it appears that globallyit is for Clocking 0 that the lowest level of force variation is applied on both a single blade and the entire second stator. Clocking has also a strong influence in terms of heat transfer rate, with a lower heat transfer rate for Clocking 1. Finally, combined measurements of static pressure and heat transfer on the second stator endwalls give indication of the impact of clocking on the secondary flows.

The understanding of the physical phenomena has been supported by CFD computations. A quasi-3D unsteady Navier Stokes solver developed at the University of Florence has been validated against experimental data in the stage alone configuration. The code was used to investigate in details the time-resolved flow field of the one and half stage turbine. Mechanisms of the first stator wake migration across the rotor blade row and its behaviour throughout the second stator have been put into evidence. For Clocking 1, experimentally the best clocking position on the point of view of the heat transfer and the unsteady force exerted on the second stator by the flow, this wake avenue impinges directly at the leading edge and accumulates with the boundary layer. For the other clockings, the same wake avenue mixes out inside the main flow, which results that the fluid momentum decays quickly in the passage.

The second part of the investigation was dedicated to observe the impact of the thickness of the turbine inlet boundary layer on the flow in the second stator. The so-called thick boundary layer has been generated by implementing an obstacle at both hub and tip endwalls at the inlet of the test section. This obstacle, a brass ring, generates a deficit in total pressure that corresponds to a thicker boundary layer. As a consequence, inlet total temperature as well as turbulence level is affected in the vicinity of the endwalls. Only small effects have been observed on the second stator pressure distribution. The most significant change concerns the attenuation of the static pressure fluctuations at 85% span because of the interaction of the thick boundary layer with the upstream rotor tip leakage flow. The steady heat transfer revealed large differences between the thin and the thick boundary layer configurations. However, no conclusions could be drawn because of a probable systematic error on the temperature measurements. The time-resolved heat transfer variations obtained at 85% span are coherent with the static pressure fluctuations measured, corresponding to a higher turbulence level. Finally, at the hub platform, it has been clearly identified that the thick boundary layer displaced downstream the attachment line. The heat transfer rate is also affected in the front of the passage.

The findings of this study may be used by designers to optimise the position of a second stator in terms of efficiency, blade force fluctuations or heat load minimization. It should be kept in mind that these three effects are unlikely to be achieved for the same clocking and with a maximum effect along the entire blade span.

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