Prediction of coolant deflection tendencies in rotating film cooling

Film cooling is commonly used as an effective cooling technique to protect turbine blades from the thermal failure caused by operation in a high-temperature environment. The cooler air is injected into the high-temperature mainstream boundary layer on a blade surface and it generates a thin coolant film, which acts as a buffer to isolate the thermal loads effectively. In the present study, the classical boundary-layer theory is introduced into the analysis of the governing equations of film cooling under rotating operating conditions, and boundary-layer equations for film cooling are derived. Through simplification of the governing equations, it is demonstrated that four major governing parameters, the pressure gradient, viscosity, the Coriolis force and the centrifugal force, can significantly influence the coolant deflection phenomenon. Near the suction surface, the coolant deflects towards the high-radius locations noticeably. While near the pressure surface, the coolant can be driven to deflect centripetally and centrifugally according to the integrated effect of centrifugal force and Coriolis force. To judge the deflective direction of coolant flow near the pressure surface, a deflection ratio defined by the tangential to instantaneous ejection velocity components is proposed and validated numerically and experimentally. The experimental data and their comparison with the theoretical values indicate that this ratio provides an approach for distinguishing the coolant flow deflection in film cooling in regimes where the Coriolis and centrifugal forces are dominant in the flow field compared to the other forces. Use of the deflection ratio provides a design parameter for the arrangement of film-hole orientation on turbine blades.