Study of Blade Tip Geometry in High Pressure Turbine Gas Turbine Engine - ANSYS Simulation

In high pressure turbine gas turbine engine, 30% of the losses is due to leakage flows over the blade tip. Tip geometry is known as one of the main factors affecting these flows. It is the aim of this project to study the behaviour of these flows over different designs of the tip. The project will be performed computationally using ANSYS software.

Objectives

This project aims at:

1. Creating 3D models for blade tips on CAD.
2. Conducting a CFD analysis on Ansys to determine the pressure around the blade tips.

A gas turbine blade is one of the components of a gas turbine. They are used to converting and extracting the wind energy into rotations which are then converted to mechanical power for the steam engines. (Goldstein, 1971).

Blades can be classified into three categories, the impulse, reaction and the impulse-reaction  gas turbine blades. (ffden, n.d.).

The flow of air around a turbine blade is shown below. The upper side of the turbine, experiences a negative pressure while the lower side will have a positive pressure as the fluid particles hit the surface almost perpendicularly. There is a supersonic flow region, with high turbulence behind the blade.

Research has shown that the shape of the blade tip influences the pressure drop at the blade. (Achary, 2017).

Blade Design

The blades were designed on Autodesk Inventor. the first blade was flat at the top without any fillet, or contour. The second blade was similar to the first profile but was filleted at the top. Another blade was modeled with a contour at the top. The three blade designs.

Boundaries

The boundary types were changed. The inlet boundary was set as a velocity inlet, while the outlet was set as a pressure outlet. The front surface was set as a symmetry plane to allow viewing of the interior of the design. Air was passed through the inlet at 100m/s.

Solutions

For the solutions, the pressure velocity coupling was set as the coupled scheme. For the methods, least square cell-based gradient, second order pressure and second order upwind momentum was used for the spatial discretization for the methods. The continuities were set to 1e-05. The solution was set to run at 200 iterations as one of the stopping criteria.

Residuals

The residuals showed that the solution converged in x, y and z continuity and the k omega. This is an indication that the mesh used was fine enough to cause convergence and that the models used are appropriate with the air velocity. The figure below shows the residuals plots.

Pressure contours

The maximum pressure was 1.42e+04 Pa at the back front side of the blade. At the blade tip, the minimum pressure was -5.59e+03 Pa near the leading edge. The pressure at the tip was 1.00e-03 Pa. on the lower side of the blade tip, there was a high pressure of around -7.64e+03 Pa. The maximum pressure of the front surface of the blade was 9.84e+03 Pa.

The maximum pressure of the front face was 1.57e+04 Pa. The tip region had a pressure of -2.86e+03 Pa. The minimum pressure was -1.03e+04 Pa on the lower side of the profile.

The profile had less negative pressure than the previous model.

The maximum pressure of the front face was 1.44e+04 Pa. The tip region had a pressure of 3.56e+03 Pa. The minimum pressure was -1.00e+04 Pa on the lower side of the profile.

The results showed that the blade whose tip had shard edges, profile 1, produced the smallest pressure of 9.84e+03 Pa. The blade with an island at the centre had the least negative pressures of -1.00e+04 Pa, followed by the tip with blade whose tip was filleted, with a minimum pressure of -1.03e+04 Pa.

Conclusion

In this work, three geometries of blade tips for use in a gas turbine were simulated on Ansys Fluent. At 100 m/s inlet speed, the tip with the sharpest edge produced the lowest pressure drop, followed by the blade whose tip had an extrusion at the centre and lastly the blade with filleted or rounded edges.

References

Achary, S., 2017. Turbine Blade Aerodynamics, s.l.: s.n.

ffden, n.d. Turbines and Compressors. [Online]
Available at: http://ffden-2.phys.uaf.edu/212_fall2003.web.dir/Oliver_Fleshman/turbinesandcompressors.html
[Accessed 13 March 2021].

Goldstein, R. J., 1971. Advances in Heat Transfer Volume. Science Direct.