Eric Curtis

The effect of stator-rotor hub sealing flow on the mainstream aerodynamics of a turbine

An investigation into the effect of stator-rotor hub gap sealing flow on turbine performance is presented. Efficiency measurements and rotor exit area traverse data from a low speed research turbine are reported. Tests carried out over a range of sealing flow conditions show that the turbine efficiency decreases with increasing sealant flow rate but that this penalty is reduced by swirling the sealant flow. Results from time-accurate and steady-state simulations using a three-dimensional multi-block RANS solver are presented with particular emphasis paid to the mechanisms of loss production. The contributions toward entropy generation of the mixing of the sealant fluid with the mainstream flow and of the perturbed rotor secondary flows are assessed. The importance of unsteady stator wake/sealant flow interactions is also highlighted.Copyright

The Influence of Shroud and Cavity Geometry on Turbine Performance: An Experimental and Computational Study— Part II: Exit Cavity Geometry

Imperfections in the turbine annulus geometry, caused by the presence of the shroud and associated cavity, have a significant influence on the aerodynamics of the main passage flow path. In this paper, the datum shroud geometry, representative of steam turbine industrial practice, was systematically varied and numerically tested. The study was carried out using a three-dimensional multiblock solver, which modeled the flow in a 1.5 stage turbine. The following geometry parameters were varied: inlet and exit cavity length, shroud overhang upstream of the rotor leading edge and downstream of the trailing edge, shroud thickness for fixed casing geometry and shroud cavity depth, and shroud cavity depth for the fixed shroud thickness. The aim of this study was to investigate the influence of the above geometric modifications on mainstream aerodynamics and to obtain a map of the possible turbine efficiency changes caused by different shroud geometries. The paper then focuses on the influence of different leakage flow fractions on the mainstream aerodynamics. This work highlighted the main mechanisms through which leakage flow affects the mainstream flow and how the two interact for different geometrical variations and leakage flow mass fractions.

Controlling Tip Leakage Flow Over a Shrouded Turbine Rotor Using an Air-Curtain

This paper describes an experimental and computational investigation into the performance of an air-curtain seal used to control the leakage flow over the tip shroud of a turbine rotor. The results show that a seal of this type has the potential to reduce or eliminate shroud leakage whilst having a practical level of clearance between the stationary and moving components. The experimental measurements were undertaken using a single-stage low-speed air turbine equipped with a continuous circumferential nozzle in the casing to deliver an axisymmetric jet into the cavity over the rotor shroud. The jet was angled at 45° to the axial direction so that its momentum opposed the shroud leakage flow. In this arrangement the air-curtain was able to sustain the pressure difference between the inlet and outlet of the rotor blade row without any leakage. The test facility had comprehensive instrumentation for obtaining accurate measurements of turbine efficiency that were corrected for the externally supplied additional flow required for the air-curtain. Measurements were obtained for a range of jet flows and show the change in the turbine efficiency as the jet flow is increased. The measurements have been compared with calculations.Copyright

Reducing the Perfomance Penalty Due to Turbine Inter-Platform Gaps

Individual nozzle guide vanes (NGV’s) in modern aero engines are often cast as a single unit with integral hub and casing endwalls. When in operation there is a leakage flow through the chord-wise inter-platform gaps. A previous paper [1] has shown that these gaps can result in a stage efficiency penalty of 0.5% – 1.5% depending upon how well they are sealed. In this paper, numerical calculations are used to re-design the inter-platform gaps with the aim of minimizing their effect on the mainstream aerodynamics and hence reduce the efficiency penalty. Experiments using a full scale, low speed model turbine and an improved gap-design show that significant performance improvements are possible regardless of how well the gap is sealed.Copyright

The Effect of Clearance on Shrouded and Unshrouded Turbines at Two Levels of Reaction

In this paper, the effect of seal clearance on the efficiency of a turbine with a shrouded rotor is compared with the effect of the tip clearance when the same turbine has an unshrouded rotor. The shrouded versus unshrouded comparison was undertaken for two turbine stage designs one having 50% reaction the other having 24% reaction. Measurements for a range of clearances, including very small clearances, showed three important phenomena. Firstly, as the clearance is reduced, there is a “break-even clearance” at which both the shrouded turbine and the unshrouded turbine have the same efficiency. If the clearance is reduced further, the unshrouded turbine performs better than the shrouded turbine, with the difference at zero clearance termed the “offset loss”. This is contrary to the traditional assumption that both shrouded and unshrouded turbines have the same efficiency at zero clearance. The physics of the break-even clearance and the offset loss are discussed. Secondly, the use of a lower reaction had the effect of reducing the tip leakage efficiency penalty for both the shrouded and the unshrouded turbines. In order to understand the effect of reaction on the tip leakage, an analytical model was used and it was found that the tip leakage efficiency penalty should be understood as the dissipated kinetic energy rather than either the tip leakage mass flow rate or the tip leakage loss coefficient. Thirdly, it was also observed that, at a fixed flow coefficient, the fractional change in the output power with clearance was approximately twice the fractional change in efficiency with clearance. This was explained by using an analytical model.Copyright

Improving the Performance of a Turbine With Low Aspect Ratio Stators by Aft-Loading

Experimental data and numerical simulations are presented from a research turbine with low aspect ratio nozzle guide vanes (NGVs). The combined effects of mechanical and aerodynamic constraints on the NGV create very strong secondary flows. This paper describes three designs of NGV that have been tested in the turbine, using the same rotor row in each case. NGV 2 used three-dimensional design techniques in an attempt to improve the performance of the datum NGV 1 blade but succeeded only in creating an intense vortex shed from the trailing edge (as previously reported) and lowering the measured stage efficiency by 1.1% points. NGV 3 was produced to avoid the “shed vortex” while adopting a highly aft-loaded surface pressure distribution to reduce the influence of the secondary flows. The stage with NGV 3 had an efficiency 0.5% points greater than that with NGV 1. Detailed comparisons between experiment and CFD, including predicted entropy generation rates, are used to highlight the areas where the loss reduction has occurred and hence to quantify the effects of employing highly aft-loaded NGVs.Copyright

The Interaction of Turbine Inter-Platform Leakage Flow With the Mainstream Flow

Individual nozzle guide vanes (NGVs) in modern aero engines are often cast as a single piece with integral hub and casing endwalls. When in operation there is a leakage flow through the chord-wise inter-platform gaps. An investigation into the effect of this leakage flow on turbine performance is presented. Efficiency measurements and NGV exit area traverse data from a low speed research turbine are reported. Tests show that this leakage flow can have a significant impact on turbine performance, but that below a threshold leakage fraction this penalty does not rise with increasing leakage flow rate. The effect of various seal clearances are also investigated. Results from steady-state simulations using a three-dimensional multiblock RANS solver are presented with particular emphasis paid to the physics of the mainstream/leakage interaction and the loss generation.Copyright

Aerodynamic Design of High End Wall Angle Turbine Stages—Part II: Experimental Verification

An experimental investigation of a turbine stage featuring very high end wall angles is presented. The initial turbine design did not achieve a satisfactory performance and the difference between the design predictions and the test results was traced to a large separated region on the rear suction-surface. To improve the agreement between computational fluid dynamics (CFD) and experiment, it was found necessary to modify the turbulence modeling employed. The modified CFD code was then used to redesign the vane, and the changes made are described. When tested, the performance of the redesigned vane was found to have much closer agreement with the predictions than the initial vane. Finally, the flowfield and performance of the redesigned stage are compared to a similar turbine, designed to perform the same duty, which lies in an annulus of moderate end wall angles. A reduction in stage efficiency of at least 2.4% was estimated for the very high end wall angle design.

Aerodynamic Design of High Endwall Angle Turbine Stages: Part 2 — Experimental Verification

An experimental investigation of a turbine stage featuring very high endwall angles is presented. The initial turbine design did not achieve a satisfactory performance and the difference between the design predictions and the test results was traced to a large separated region on the rear suction-surface. To improve the agreement between CFD and experiment, it was found necessary to modify the turbulence modelling employed. The modified CFD code was then used to redesign the vane, and the changes made are described. When tested, the performance of the redesigned vane was found to have much closer agreement with the predictions than the initial vane. Finally, the flowfield and performance of the redesigned stage are compared to a similar turbine, designed to perform the same duty, which lies in an annulus of moderate endwall angles. A reduction in stage efficiency of at least 2.4% was estimated for the very high endwall angle design.Copyright

Aerodynamic Design of High Endwall Angle Turbine Stages: Part 1—Methodology Development

A design methodology is presented for turbines in an annulus with high endwall angles. Such stages occur where large radial offsets between the stage inlet and stage outlet are required, for example in the first stage of modern low pressure turbines, and are becoming more prevalent as bypass ratios increase. The turbine vanes operate within s-shaped ducts which result in meridional curvature being of a similar magnitude to the blade-to-blade curvature. Through a systematic series of idealised computational cases, the importance of two aspects of vane design are shown. First, the region of peak endwall meridional curvature is best located with the vane row. Second, the vane should be leant so as to minimise spanwise variations in surface pressure — this condition is termed ‘ideal lean’. This design philosophy is applied to the first stage of a low pressure turbine with high endwall angles.© 2012 ASME