O. Schennach R. Pecnik Institute for Thermal Turbomachinery and Machine Dynamics, Graz University of Technology, Graz 8010, Austria B. Paradiso Laboratorio di Fluidodinamica delle Macchine, Dipartimento di Energetica, Politecnico di Milano, 20133 Milano, Italy E. Göttlich A. Marn J. Woisetschläger e-mail: jakob.woisetschlaeger@tugraz.at Institute for Thermal Turbomachinery and Machine Dynamics, Graz University of Technology, Graz 8010, Austria The Effect of Vane Clocking on the Unsteady Flow Field in a One-and-a-Half Stage Transonic Turbine The current paper presents the results of numerical and experimental clocking investiga- tions performed in a high-pressure transonic turbine with a downstream vane row. The objective was a detailed analysis of shock and wake interactions in such a 1.5-stage machine while clocking the vanes. Therefore, a transient 3D Navier–Stokes calculation was done for two clocking positions, and the three-dimensional results are compared with laser-Doppler-velocimetry measurements at midspan. Additionally, the second vane was equipped with fast response pressure transducers to record the instantaneous surface pressure for 20 different clocking positions at midspan. DOI: 10.1115/1.2777199 Introduction To meet the objective of reduced costs and more compactness of turbomachinery, it is advantageous to reduce the number of stages resulting in high-pressure HP ratios and transonic condi- tions for these stages. To keep the efﬁciency at a high level, a detailed understanding of the unsteady ﬂow is necessary. The ﬂow unsteadiness in turbomachinery is highly related to the vane-rotor motion and the wake-wake interaction. Addition- ally, in HP low-aspect-ratio turbines the secondary ﬂows, strong potential ﬁelds and trailing edge shocks must be taken into ac- count. These effects cause a time-varying, nonuniform ﬂow ﬁeld downstream of the stage affecting the performance and boundary layer of the next vane row 1–3. Hummel 4 performed a two-dimensional numerical simula- tion of a single stage HP turbine and provided a guideline for positioning a second vane in order to minimize the effect of the trailing edge shock strength onto the second vane. The basic idea of clocking also known as indexing is to im- prove the overall efﬁciency by varying the circumferential and/or the axial position of adjacent vanes or blades. The most common method is to rotate the nozzle ring with respect to a downstream vane row, while the largest efﬁciency increase is achieved with equal blade counts. It is well known that a maximum of efﬁciency is achieved when the ﬁrst vane’s wake impinges the leading edge of the second vane. Much research work has been performed to investigate the in- ﬂuence of clocking in subsonic turbines. Experimental results re- ported by Huber et al. 5 for a two-stage turbine showed a 0.8% efﬁciency variation at midspan due to clocking of the second stage vane. Time accurate numerical studies by Arnone et al. 6 in a three-stage low-pressure LP turbine showed a 0.7% efﬁciency variation due to clocking. Furthermore, different investigated Rey- nolds numbers showed no major difference in the results. Rein- möller et al. 7 investigated the inﬂuence of clocking on the ﬂow ﬁeld between rotor and second vane as well as downstream the second vane using hot wire probes and pneumatic probes sup- ported by numerical simulations. The authors found 1% relative efﬁciency variation at midspan. Recent research work focuses on clocking effects in transonic turbines. Billiard et al. 8 focused on the heat transfer measure- ments on the second vane in a 1.5-stage HP turbine. It was found that clocking changes the mean levels of the heat transfer as well as the intensity and the trajectory of the ﬂuctuations. Gadea et al. 9 investigated the inﬂuence of clocking on the time resolved pressure ﬁeld of a second vane tested in a 1.5-stage HP turbine. It was shown that the optimum clocking position CP for aerody- namics is not the optimum for minimum unsteady forces. Halde- man et al. 10 performed aerodynamic measurements in 1.5-stage HP turbine indicating an overall efﬁciency increase of about 2%–3% using a variety of independent methods. The current paper focuses on the propagation of shock waves and their interactions at midspan of a highly loaded low-aspect- ratio transonic turbine stage with respect to different CPs. An unsteady three-dimensional Navier–Stokes calculation was used to discuss the interaction mechanism in detail. Two circumferen- tial measurement lines gained with laser-Doppler velocimetry LDV are used to validate the prediction. Furthermore, fast re- sponse surface-pressure measurements on the second vane were performed in order to get a better insight into the clocking phe- nomena. The experimental investigations were performed under engine representative conditions in a continuously running cold- ﬂow test facility of the Institute. Contributed by the International Gas Turbine Institute of ASME for publication in the JOURNAL OF TURBOMACHINERY. Manuscript received June 8, 2007; ﬁnal manuscript received June 20, 2007; published online May 7, 2008. Review conducted by David Wisler. Paper presented at the ASME Turbo Expo 2007: Land, Sea and Air GT2007, Montreal, Quebec, Canada, May 14–17, 2007. Paper No. GT2007-27848. Journal of Turbomachinery JULY 2008, Vol. 130 / 031022-1 Copyright © 2008 by ASME Downloaded From: http://turbomachinery.asmedigitalcollection.asme.org/ on 05/11/2016 Terms of Use: http://www.asme.org/about-asme/terms-of-use