This paper describes the effect of the state of the inlet boundary layer (laminar or turbulent) on the structure of the endwall flow on two different profiles of low-pressure (LP) turbine blades (solid thin and hollow thick). At present the state of the endwall boundary layer at the inlet of a real LP turbine is not known. The intention of this paper is to show that, for different designs of LP turbine, the state of the inlet boundary layer affects the performance of the blade in very different ways. The testing was completed at low speed in a linear cascade using area traversing, flow visualization and static pressure measurements. The paper shows that, for a laminar inlet boundary layer the two profiles have a similar loss distribution and structure of endwall flow. However, for a turbulent inlet boundary layer the two profiles are shown to differ significantly in both the total loss and endwall flow structure. The pressure side separation bubble on the solid thin profile is shown to interact with the passage vortex, causing a higher endwall loss than that measured on the hollow thick profile.

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The influence of surface roughness on the profile and end-wall total pressure losses in Low Pressure Turbines was investigated experimentally in a turbine high-speed rig. The rig consisted of a rotor-stator configuration. Both rows of airfoils are high lift, high aspect ratio and high turning blades that are characteristic of state of the art Low Pressure Turbines. The stator airfoils (both vanes and platforms) were casted and afterwards they were barreled to improve their surface finish up to 1.73 μm Ra. Then they were assembled in the rig and tested. The stator was traversed upstream and downstream with miniature pneumatic probes to obtain total pressure, flow angle and static pressure flow fields. Once this test was completed the rig was disassembled and the stator airfoils were polished to achieve a roughness size of 0.72 μm Ra, characteristic of Low Pressure Turbine polished airfoils. Once again, the stators were assembled in the rig and tested to carry out a back-to-back comparison between the two different surface roughnesses. The total pressure profile and end-wall losses were measured for a wide range of Reynolds numbers, extending from 8×104 to 2.4×105, based on suction surface length (Res∼1.5 ReCx) and exit Mach number of 0.61. Experimental results are presented and compared in terms of area average, radial pitchwise average distributions and exit plane contours of total pressure losses, flow angles and helicity. The results agree with previous studies of roughness in Turbines, a beneficial effect of surface roughness was found at very low Reynolds numbers, in stagnation pressure losses.

The effect of airfoil clocking (stator-stator interaction) on efficiency and noise of low pressure turbines (LPT) was investigated experimentally in a multistage turbine high-speed rig. The rig consisted of three stages of a state-of-the-art LPT. The stages were characterized by a very high wall-slope angle, reverse cut-off design, very high lift, and very high aspect ratio airfoils. The rig had identical blade count for the second and third stators. The circumferential position of the second stator was individually adjusted with respect to the third stator. Eight different circumferential clocking locations over one pitch were back-to-back tested. The rig was heavily instrumented with miniature five hole probes, hot wires, hot films, total pressure and temperature rakes, pressure tappings on the airfoil surface, two array of Kulites in a rotatory module, etc. Every clocking location was tested with the same instrumentation and at the same operating conditions with the intention of determining the impact of the clocking on the overall efficiency and noise. Due to the large amount of data, the results of this test will be reported in several papers. The present paper contains the impact on the overall efficiency, radial traverses, static pressure fields on the airfoils and averaged sound pressure levels in the duct. The comparison of the results suggests that the efficiency is weakly affected by clocking; however the effect on noise is noticeable for some acoustic tones at certain operating conditions.

This paper describes a new flow mechanism for the reduction of secondary flows in Low Pressure Turbines using the benefit of contoured endwalls. The extensive application of contoured endwalls in recent years has provided a deeper understanding of the physical phenomenon that governs the reduction of secondary flows. Based on this understanding, the endwall geometry of a linear cascade of solid-thin profiles typical of Low Pressure Turbines has been redesigned. Experimental data are presented for the validation of this new solution. Based on these data, a reduction of 72% in the SKEH and 20% in the mixed-out endwall losses can be obtained. CFD simulations are also presented to illustrate the effect of the new endwall on the secondary flows. Furthermore, an explanation of the flow mechanism that governs the reduction of the SKEH and the losses is given.