This paper, as a second part of the study on the double flow control device (DFCD) which has been proven in Part I [1] to improve the flat plate film cooling considerably, describes an approach to optimize the device shape and arrangement using Taguchi method. The target cooling holes are conventional cylindrical ones of 3.0d pitch with 35 deg angle to the flat plate surface. The shape of the double flow control device to be optimized is based on the hemi-spheroid used in Part I.

The optimization process in this study is categorized as “static problem”, in which S/N ratios of “larger-the-better” characteristics are calculated for control parameters against their noise factor. The “larger-the-better” characteristics adopted in this study is the area averaged film effectiveness over the downstream region of the cooling hole. L18 orthogonal array is used to accommodate the experiment. Blowing ratios of the cooling air to the main flow used in this study are 0.5, 0.75 and 1.0, which are regarded as noise factor. Seven control parameters such as fillet radius, installation angle of the device are chosen and their effects on the film effectiveness are evaluated by the measurement as well as by RANS simulation. In this research, the optimization which used Taguchi method was at the same time carried out by an experiment and numerical simulation. From a comparison between the optimal parameter combinations attained from the measurement-based and CFD-based approaches, one can have an idea about the dependency of the optimal parameter combination on the characteristic evaluation approach. Additional investigation is also made on the effects of turbulence model upon the optimal parameter combination.

The flow fields in the downstream region of the optimal DFCD are observed using 3D Laser Doppler Velocimeter in order to understand how the device works on the ejected cooling air. In addition, Large-Eddy-Simulation (LES) is also executed in order to grasp unsteady flow structures created by the device and their interaction with the cooling air. It is found from the measurement as well as the LES analysis that the optimal DFCD generates comparatively large-scale longitudinal vortices, causing the drastic increase in film effectiveness.

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