Experimental and computational investigation on comparison of micro-scale open rotor and shrouded rotor hovering in ground effect

Various investigations on open rotor (OR) hovering in-ground effect (IGE) are carried out, but few papers report shrouded rotor (SR) hovering IGE. This paper compares aerodynamic performance and flowfield characteristics of OR and SR hovering IGE by both experimental measurements and computational fluid dynamic (CFD) simulations. Experimental results reveal that in IGE flight, the aerodynamic performance of SR is more sensitive than that of OR. And at constant power, SR offers more thrust than OR at the same ground distance. Ground has a great influence on thrust for OR below 2.2 rotor radius distance, while for SR it shows obvious effect below 1.5 rotor radius distance. It is also shown that normalized aerodynamic coefficients of OR and SR are independent on rotor speed. In addition, for OR the rotor thrust coefficient changes nearly linearly with the logarithmic distance from ground, while for SR it changes nonlinearly. Flowfield analysis by CFD shows that shroud changes the tip flow features and expands the slipstream area of SR. When ground distance gets small, back pressure below the rotor-disk plane increases, which is more obvious for SR than OR. Furthermore, shroud thrust of SR decreases because of tip leakage flow and flow separation.

Tip-Shroud Labyrinth Seal Effect on the Flutter Stability of Turbine Rotor Blades

The effect of the tip-shroud seal on the flutter onset of a shrouded turbine rotor blade, representative of a modern gas turbine, is numerically tested, and the contributions to the work per cycle of the aerofoil and the tip shroud are clearly identified. The numerical simulations are conducted using a linearized frequency-domain solver. The flutter stability of the shrouded rotor blade is evaluated for an edgewise mode and compared with the standard industrial approach of not including the tip-shroud cavity. It turns out that including the tip shroud significantly changes the stability prediction of the rotor blade. This is due to two facts. First, the amplitude of the unsteady pressure created in the inter-fin cavity due to the motion of the airfoil is much greater than that of the airfoil. The impact of this contribution increases with the frequency. Second, the effect of the outer shroud of the rotor blade, which usually is not included either in the simulations, has an opposite trend with the nodal diameter than the airfoil reducing the maximum and minimum damping. It is concluded that the combined effect of the seal and its platform tends to stabilize the edgewise mode of the rotor blade for all the examined nodal diameters and reduced frequencies. Finally, the numerical results are shown to be consistent with those obtained using an analytical simplified model to account for the effect of the labyrinth seals.

Tip-Shroud Labyrinth Seal Effect on the Flutter Stability of Turbine Rotor Blades

The effect of the tip-shroud seal on the flutter onset of a shrouded turbine rotor blade, representative of a modern gas turbine, is numerically tested and the contribution to the work-per-cycle of the aerofoil and the tip-shroud are clearly identified. The numerical simulations are conducted using a linearised frequency domain solver. The flutter stability of the shrouded rotor blade is evaluated for an edgewise mode and compared with the standard industrial approach of not including the tip-shroud cavity. It turns out that including the tip shroud significantly changes the stability prediction of the rotor blade. This is due to the fact that the amplitude of the unsteady pressure created in the inter-fin cavity, due to the motion of the airfoil, is much greater than that of the airfoil. It is concluded that the combined effect of the seal and its platform tends to stabilise the rotor blade for all the examined nodal diameters and reduced frequencies. Finally, the numerical results are shown to be consistent with those obtained using an analytical simplified model to account for the effect of the labyrinth seals.

A Comparison of Tip Gap Loss Sensitivity for Shrouded vs. Unshrouded Turbine Rotors

A numerical study was conducted to evaluate the loss sensitivity of shrouded vs. unshrouded turbine rotor blades. Accuracy is demonstrated with a series of grid independence studies. Application of the methods is performed through various studies related to the effects of shrouding a High-Pressure Turbine (HPT) rotor blade for a NASA-specified N+3 timeframe single-aisle aircraft engine at takeoff conditions. Flat, Recessed, and Shrouded rotor configurations are evaluated at tip clearances from 0.25% to 4% of blade span. Mach # distributions, near-tip blade loading, and other flow characteristics are examined. Plots of stage efficiency vs. tip clearance are presented, with trends compared to available experimental data. It is shown that for the imposed boundary conditions, the addition of a shroud improves stage efficiency and significantly reduces sensitivity to tip clearance at higher clearance fractions. A casing recess is also shown to slightly increase sensitivity to tip clearance for tip clearances greater than 0.5%. Total pressure loss profiles vs. blade span are also compared, providing insight into the mechanisms behind the performance of the three configurations.

Flow Interactions Between Shrouded Power Turbine and Nonaxisymmetric Exhaust Volute for Marine Gas Turbines

The marine gas turbine exhaust volute is an important component that connects a power turbine and an exhaust system, and it is of great importance to the overall performance of the gas turbine. Gases exhausted from the power turbine are expanded and deflected 90 degrees in the exhaust volute, and then discharge radially into the exhaust system. The flows in the power turbine and the nonaxisymmetric exhaust volute are closely coupled and inherently unsteady. The flow interactions between the power turbine and the exhaust volute have a significant influence on the shrouded rotor blade aerodynamic forces. However, the interactions have not been taken into account properly in current power turbine design approaches. The present study aims to investigate the flow interactions between the last stage of a shrouded power turbine and the nonaxisymmetric exhaust volute with struts. Special attention is given to the coupled aerodynamics and pressure response studies. This work was carried out using coupled computational fluid dynamics (CFD) simulations with the computational domain including a stator vane, 76 shrouded rotor blades, 9 struts and an exhaust volute. Three-dimensional (3D) unsteady and steady Reynolds-averaged Navier-Stokes (RANS) solutions in conjunction with a Spalart-Allmaras turbulence model are utilized to investigate the aerodynamic characteristics of shrouded rotors and an exhaust volute using a commercial CFD software ANSYS Fluent 14.0. The asymmetric flow fields are analyzed in detail; as are the unsteady pressures on the shrouded rotor blade. In addition, the unsteady total pressures at the volute outlet is also analyzed without consideration of the upstream turbine effects. Results show that the flows in the nonaxisymmetric exhaust volute are inherently unsteady; for the studied turbine-exhaust configuration the nonaxisymmetric back-pressure induced by the downstream volute leads to the local flow varying for each shrouded blade and low frequency fluctuations in the blade force. Detailed results from this investigation are presented and discussed in this paper.