Test Case 1: VKI U-Bend

Internal cooling channels of turbine blades are essential to enable high firing temperatures, and hence high efficiencies. Such cooling channels are characterized by multiple passages of relatively cool air through serpentine ducts. Among the salient features of these cooling passages, the U-bends that connect consecutive passages play a key role by turning the flow 180 degrees while still cooling the outer structure. The aim of this test case is to reduce the pressure loss associated to the flow turning by optimizing the shape of the bend.

The U-bend under study is one which has been investigated experimentally at the von Karman Insititute for Fluid Dynamics [1], [2]. The flow is incompressible and fully turbulent. The figure below shows the layout of the experimentally tested U-Bend, consisting of a straight inlet part, the bend formed of 2 circular arcs, and a straight oulet. This geometry serves as datum design to which relative improvements during the optimization need to be benchmarked. The optimal shape of the U-bend needs to remain within a bounding box described in the detailed document representing the available space in the turbine blade.

The datum design has been experimentally investigated both in terms of global pressure drop and detailed flow characteristics. The figure below shows the PIV results of the datum design in the midspan of the channel. A large separation zone is present and indicates the potential to improve the pressure loss. It is mendatory to first benchmark the CFD solver used within the optimization process with the available expreimental data.

Detailed description of the test case

A detailed description of the test case can be found here. It contains a description of the datum U-bend geometry, its boundary conditions and the experimental results for the validation of the used CFD solver. It further details the optimization test case by its geometrical design space and the objective to be minimized.

[1] T. Verstraete, F. Coletti, J. Bulle, T. V. der Wielen, and T. Arts. Optimization of a U-Bend for Minimal Pressure Loss in Internal Cooling Channels Part I: Numerical Method. ASME Journal of engineering for Gas Turbines and Power, 135:pp 051015, 2013.

[2] F. Coletti, T. Verstraete, J. Bulle, T. V. der Wielen, N. van den Berge, and T. Arts. Optimization of a U-Bend for Minimal Pressure Loss in Internal Cooling Channels – Part II: Experimental validation. ASME Journal of engineering for Gas Turbines and Power, 135:pp 051016, 2013.