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The aerodynamic losses in gas turbines are mainly caused by profile loss secondary flow, and tip leakage loss. This study focuses on tip leakage flow of high-pressure turbine stages. An annular turbine cascade was constructed with fixed blades on the casing, and the distance between blade tip and the hub was considered as tip clearance gap. The effect of endwall movement on loss mechanism was investigated by using experimental and numerical techniques. The measurements were obtained while the hub was fixed but the numerical calculations were carried out for both stationary and moving cascades. Upstream and downstream flows were measured by using a calibrated five-hole pressure probe. The steady incompressible turbulent flow was obtained by solving Reynolds averaged Navier-Stokes equations and by employing shear stress transport (SST)

The applications of gas turbine engines have growing importance in the fields of electric power generation as well as marine and aerospace applications. Gas turbine engines have large powers, and, therefore, increasing the efficiency of gas turbines reduces fuel consumption and increases the economic operation of these machines. In order to increase the efficiency, detailed and fundamental comprehension of loss mechanisms is necessary. In turbines, clearance gaps are necessary to avoid the contact between rotating blades and the fixed casing. Tip leakage flow is induced due to pressure difference between blade pressure and suction surfaces. The leakage flow interacts with the passage vortex and produces aerodynamic losses. Although tip clearance in low-pressure turbines is usually about 1% of the blade height, tip clearance loss increases the total flow losses in a turbine and contributes to the efficiency degradation of gas turbines. In high-pressure stages, tip clearance gap is typically about 6% of the blade height due to the reduction of the blade length [

In previous studies, experimental and numerical techniques were used to investigate the flow and loss mechanisms in turbine cascades. While experimental measurements give clear qualitative indications on loss levels, numerical studies provide detailed information on flow and loss mechanism in turbine blades. A number of previous experimental and numerical studies had been conducted to investigate tip leakage flow and loss mechanism using linear cascades [

Large tip clearance gaps typically 5% of the sblade height exist in the high-pressure stages of industrial gas turbines. Williams et al. [

Yaras et al. [

To the authors’ knowledge, the effect of large tip clearance gaps on the losses of annular turbine cascades has not been intensively studied before. The aim of this study is to investigate the effect of large tip clearance gap on loss generation downstream of an annular turbine cascade by using experimental measurements and numerical calculations. Moreover, the study aims to investigate the effect of rotational speed on tip leakage loss. The remainder of this paper is organized by introducing detailed information on the experimental procedure including setup and measurement technique in Section

Flow measurements were performed in the present study by using pneumatic pressure probes. The probes are reliable and robust tool for flow measurements in turbomachines. Three-hole probes are used for two-dimensional flow measurements while five-hole probes are used for the three-dimensional flow. In general, the smaller the probe used, the lower the blockage effect and the better the accuracy. Recently, many researchers used pressure probes to investigate flow losses in turbine cascades [

Figure

Five-hole probe.

The calibration was performed by placing the probe at a number of predefined angular settings in uniform flow with constant velocity magnitude and low turbulence intensity. After the calibration task, nondimensional calibration coefficients were obtained from the measured pressures at the probe sensing holes. A calibration map was produced by rotating the probe through a range of pitch and yaw angles. A calibration mechanism was used in this study to change pitch angle

Five-hole probe calibration maps.

During cascade measurements, the pressure signals obtained from the five-hole probe were measured and the pressure coefficients were determined. The pitch angle

A low-speed wind tunnel was used which is a blow-down facility of an annular turbine cascade (Figure

Specifications of the annular cascade.

Chord length, | 55.2 |
---|---|

Axial chord, | 34 |

Blade spacing at the tip, | 46.5 |

Blade spacing at the root, | 61.9 |

Inlet blade angle, | 76.1 |

Outlet blade angle, | 14.5 |

Blade turning angle, (°) | 89.4 |

Stagger angle, (°) | 39.9 |

Blade height, | 44.5 |

Tip clearance gap, | 4.5 |

Aspect ratio, | 0.81 |

Experimental set-up.

The velocity distribution of the inlet flow was measured upstream of the blades at a distance of an axial chord

Measurement mesh.

The three-dimensional flow was obtained by solving continuity and momentum equations. For turbulent flow calculations, eddyviscosity approach was used. It includes a number of classes of models which approximate the effect of the turbulence on the mean motion by modifying the coefficient of viscosity. The effective viscosity coefficient that was used in the computation of the flow field is the sum of the molecular viscosity

The governing equations for incompressible flow are given by

The SST

The flow was solved around a single blade considering periodic boundary conditions. The inlet plane was selected at a distance of 50 mm (1.47

Computational grid.

The flow was solved at blade Reynolds number of

The flow was solved by using the commercial CFD code Fluent. Pressure-velocity coupling was achieved by using SIMPLE algorithm as given by Patankar [

The three-dimensional flow downstream of the annular cascade was investigated on a plane which exists at a distance of 0.29

Mid-span deviation angle downstream of the blade.

Figure

Mid-span wake downstream of the blade.

The figure shows good agreement between the numerical calculations and the experimental measurements with the maximum loss near the blade trailing edge. This is attributed to the boundary layers which were developed on the blade pressure and suction surfaces and merged downstream of the blade trailing edge generating the wake.

The solution grid dependency was investigated by increasing the number of cells by 60% to obtain a mesh with a total number of cells of about 1100000. The cells were added mainly in the boundary layer and in the regions of high flow gradients. Figure

Total pressure loss coefficient through blade channel.

The figure indicates that there was no significant improvement in the solution by increasing the number of the cells. The mass-averaged total pressure loss coefficient on the downstream plane was changed by 0.8% when the number of cells was increased by 60% and therefore, the solution was considered grid independent.

The velocity on the downstream plane was represented in dimensionless form using the mass averaged velocity. Figure

Downstream dimensionless velocity.

Experimental measurement

Numerical calculations

Figure

Downstream total pressure coefficient.

Experimental measurement

Numerical calculations

Figure

Endwall visualization.

Pressure side

Suction side

Figure

Figure

Tip surface flow visualization.

Blade leading edge

Blade mid-chord

Stream lines.

Hub

Mid-clearance gap

Figure

Total pressure coefficient at different rotational speeds.

Rotational speed = 800 rpm

Rotational speed = 1600 rpm

Rotational speed = 3200 rpm

Mass-averaged total pressure loss coefficient versus rotational speed.

Leakage streamlines.

with rotation

without rotation

The three-dimensional steady turbulent flow was calculated through an annular turbine cascade with tip clearance and was measured by using five-hole probe experimental technique. Good agreements were obtained between experimental measurements and numerical calculations. The numerical model predicted the rolling vortex at the blade leading edge in the tip clearance gap. Tip vortices were also predicted, which agrees with previous publications. The calculation performed at different rotational speeds showed that increasing the rotational speed reduces the tip leakage vortex and reduces also the mass-averaged total pressure loss coefficient. The rotation initiates counterflow from the blade suction side to the blade pressure side, which reduces the tip leakage flow and the associated tip leakage vortex. This observation will be confirmed experimentally in a future work.

Constant

Chord length

Axial chord

Total pressure coefficient

Mass-averaged total pressure loss coefficient

Functions

Blade height

Turbulent kinetic energy

Probe calibration coefficients

Pressures measured at five-hole probe

Static pressure

Total pressure

Mean pressure

Production of

Blade spacing

Fluctuating velocity

Mean velocity

Mass-averaged velocity

Coordinate in the

distance through axial chord.

Pitch angle

Yaw angle

Inlet blade angle

Exit blade angle

Model coefficients

Model constant

Tip clearance gap

Kronecker second-order tensor

Coefficients

Molecular viscosity

Turbulent eddy viscosity

Specific turbulent dissipation rate

Absolute value of vorticity

Fluid density

Model coefficients.

Inlet

Exit

Tip or turbulent

Root.