Trailing Edge Thickness Impact on the Profile Losses of Highly Loaded Low Pressure Turbines Blades
Authors: Parra Jorge , Cadrecha David , González Ezequiel , Lázaro Benigno 
Published: September 2016

The losses breakdown of modern highly loaded low pressure turbines profiles shows that the trailing edge thickness can account for up to 20% of the overall profile loss depending on the thickness to pitch ratio highly affecting to the LPT overall performance. Additionally, this feature is of significant practical interest as the aerofoil mechanical behaviour and manufacturing costs are largely determined by the size of the trailing edge. Current trailing edge loss models are based on correlations derived from measurements on aerofoils very different with respect to the current state-of-the-art, they do not consider any effect of Reynolds number or lift coefficient, so it is questionable whether they are accurate enough for current applications and therefore an experimental validation campaign is required.

The aim of the present experimental investigation is to examine the influence of that geometrical parameter on the unsteady Reynolds lapse characterization by means of four different low speed linear cascades varying the thickness from 50% to 200% of a nominal case. Cascades A, B and C (with small, nominal and large thickness) meet the same lift coefficient reducing the back surface diffusion factor due to the different velocity at the trailing edge because of the blockage generated by the trailing edge thickness. Cascade B2, with nominal thickness, is modified to meet the same diffusion factor as Cascade A to decouple the effect of the diffusion factor from the effect of the trailing edge thickness. Total pressure probes, Laser-Doppler and hot wire anemometry are used to characterize the suction side boundary layer just upstream from the trailing edge as well as the near wake developing close to the trailing edge. Additional characterisations are conducted at half chord downstream from the cascade trailing edge to evaluate its loss coefficient. Upstream located moving bars are used to simulate the incoming wakes shed by one upstream blade row. The hot wire measurements performed slightly upstream from the profile trailing edge are post-processed locked to the passage of the moving bars. The resulting data are analysed to characterise the temporal modulation of the suction side boundary layer momentum thickness by the incoming wakes. The measurements indicate that both the time-mean value and the phase-averaged distribution of the boundary layer integral parameters are largely determined by the diffusion rate of the profile. On the other hand, a negligible effect of the trailing edge thickness is observed for the same diffusion rate. The measurements conducted downstream from the profile, both close to its trailing edge and half chord downstream, illustrate the role of the trailing edge thickness on the initial wake development. The data is recorded for 60s with a sampling rate of 25kHz obtaining between 150 and 650 phase-locked datasets depending on the Reynolds No.

Finally, the characterisation of the profile mix-out losses at the downstream plane is presented. The experimental results show that a significant reduction of losses can be achieved with thinner trailing edge, but, an increase in the number of aerofoils need to be allowed in order to get the full potential benefit of this strategy.