Numerical Evaluation of Surface Roughness Effects on Film-Cooling Performance in a Laidback Fan-Shaped Hole

Gas turbines are widely used in various industrial applications particularly in the aero-propulsion of aircraft engines. To improve the efficiency and thermal performance of gas turbines, a working fluid with a higher temperature at the inlet of the gas turbine engine is required and the higher inlet temperature often exceeds the melting temperature of the material used in the turbine blades. Film-cooling approach as an external cooling method is applied to solve this issue, in which the coolant is injected through the cooling holes located on the blade surfaces to create a layer of coolant over the blades and protect the blades from the hot mainstream flow. Fan-shaped holes are recently utilized instead of the cylindrical holes to reduce the momentum of the coolant jet and improve the lateral spreading of the coolant which results in a more uniform coolant layer on the blade surfaces.

There are several numerical and experimental studies regarding the geometrical parameters of the fan-shaped holes which have significant effects on the film-cooling effectiveness such as injection angle, metering length, lateral and forward expansion angles, etc. However, the influence of surface roughness of the cooling holes on the film cooling performance has not been investigated numerically. Over the last decade, LES has been extensively used for complex flow configurations, as it has been proven to precisely and reliably predict highly unsteady vortical and anisotropic turbulence flows. The high-resolution predictions of the resolved length and time scales of the flow field led to LES as a CFD tool for more comprehensive investigations of the appropriate mean-flow quantities and instantaneous quantities for film-cooling prediction.

In this paper, the numerical simulation was conducted in order to predict the influence of surface roughness on the film-cooling process of a cross-flow through a baseline 7-7-7 laidback fan-shaped hole. Therefore, the three-dimensional compressible LES approach in a fan-shape hole for a flat plate was conducted using commercial software ANSYS CFX V19.3. The computational results were validated by the measurements in terms of the velocity field, as well as the film-cooling effectiveness on the flat plate for different blowing ratios. Equivalent sand grain roughness was applied in the numerical simulations and compared with the experimental data. The computational data showed that the surface roughness increased the velocity through the fan-shaped hole, which results in an increase in the mixing of the coolant and the mainstream flows.