ASSESSMENT OF SHAPED FILM HOLES UNDER CONJUGATE HEAT TRANSFER CONDITIONS

Optimized film hole shape is normally evaluated by its adiabatic film effectiveness. It is necessary to assess the actual effectiveness of a shaped film hole under a real condition of conjugate heat transfer (CHT) process. Based on a numerical CHT method, this paper compares the cooling performances of a round hole, a baseline fan-shaped hole, and a related optimized fan-shaped (OFS) hole. The difference in cooling effectiveness caused by hole configurations is investigated for varied Biot number, convective heat transfer coefficient ratio, and hole lateral pitch. A thermal decoupling method is used to separate the contributions of external film cooling, bore cooling, and internal cooling to overall cooling effectiveness (OCE) of different shaped holes. Results show that a fan-shaped hole contributes more to film cooling effectiveness than a round hole and raises significantly the overall cooling effectiveness. However, the impact of optimizing fan-shaped holes on enhancing overall cooling effectiveness is not as anticipated compared to the improvement in an adiabatic experiment, the improvements in area-averaged OCE remain below 5% under typical engine conditions (<i>Bi</i> = 0.3, <i>h<sub>e</sub>/h<sub>i</sub></i> = 0.5) at blowing ratios of 0.5, 1.5, and 2.5. The effect of hole configuration on overall cooling effectiveness declines due to the metal conductivity and the enhanced internal cooling. The benefit of reducing the lateral pitch surpasses the optimization of shaped hole. It confirms a conjecture that the array of hole placement is more valuable than the shape optimization of fan-shaped holes on a real turbine blade.