Abstract
A detailed numerical analysis of secondary flows in a transonic turbine is presented in this paper. The turbine stage is optimized by mitigating secondary flow through the method of non-axisymmetric endwall design. An automated optimization platform of NUMECA/Design3D was coupled with Euranus as a flow solver for the numerical investigation. The contoured endwalls of the stator and the rotor hub were designed based on equidistant Bézier curves along the camber line in the blade channel. The initial design samples were ten times the number of the design variables, and were generated through the LHS method for database generation. The optimization of the endwalls was achieved by using a state-of-the-art multi-objective optimization algorithm, NSGA-II, connected with the BPNN to increase the isentropic efficiency and decrease the secondary kinetic energy, while the mass flow and the degree of reaction were constrained to remain on the datum value as in the original geometry. The individual optimization of the hub endwalls of the stator and the rotor produced an increase in the efficiency of 0.27% and 0.25%, respectively, resulting in a cumulative improvement of 0.46% in the efficiency. The increase in the performance was analyzed at part-load conditions, and it was further confirmed through unsteady simulations.
Highlights
These losses can be reduced by the active control method of boundary layer blowing [4], but the passive control method of three-dimensional non-axisymmetric endwall design is widely employed to manipulate the secondary flow in axial flow compressors and turbines
The optimization of transonic axial turbines has not been investigated much, and less literature is available on the topic of non-axisymmetric endwall contouring for transonic stages
Trent 500 engine was redesigned by applying non-axisymmetric endwalls to the vane and the blade, and this was tested in the presence of an upstream Trent 500 HP model turbine designed by Brennan et al The overall improvement in the efficiency increased to 0.9% due to the redesign of the high pressure and intermediate pressure stages [10]
Summary
“Of all the fluid-dynamic devices invented by the human race, axial flow turbomachines are probably the most complicated.” This was aphoristically observed and narrated by Bradshaw [1]. To exploit the potential for reducing secondary flow, a non-axisymmetric endwall was redesigned again for the rotor of a high-pressure axial turbine in a Durham linear cascade. Trent 500 engine was redesigned by applying non-axisymmetric endwalls to the vane and the blade, and this was tested in the presence of an upstream Trent 500 HP model turbine designed by Brennan et al The overall improvement in the efficiency increased to 0.9% due to the redesign of the high pressure and intermediate pressure stages [10]. An automatic optimization setup of IsightTM (Dassault System, Waltham, MA, USA) grounded on the B-splines surface technique coupled with Ansys CFX (13.0, ANSYS Inc., Canonsburg, PA, USA) was used to optimize the hub and shroud the endwalls of the 1st stator and the rotor hub of the LISA turbine This resulted in an increase in efficiency by 0.4% based on steady simulations and a mixing plane model, but the degree of reaction after the optimization was drastically increased to 12.5%, removing the benefits of actual performance enhancement [12].
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