Integrating aerodynamic loss into film cooling performance evaluation from a thermodynamic cycle perspective: a case study in compound angle design

Film cooling is widely employed for thermal protection, but it inevitably reduces the performance of the thermodynamic cycle. The performance penalty arises not only because the cooling air bypasses the heating process, but also because of the aerodynamic loss generated during mixing with the mainstream. Extensive efforts have been made to reduce cooling air consumption by improving film cooling effectiveness, while accepting an aerodynamic loss increase as the side effect. However, current research seldom discusses the tradeoff between the reduced air bleeding and increased aerodynamic loss. This paper introduces a cycle-based evaluation for film cooling performance incorporating aerodynamic loss and demonstrates that minimizing aerodynamic loss, rather than simply reducing air bleeding, should be the primary criterion for film cooling design. A semi-theoretical model is developed, combining cycle analysis, coolant bleeding, and aerodynamic loss calculation. A comparative study is conducted between the optimal designs obtained under the conventional criterion and the proposed cycle-based criterion, using the compound angle as a case study. The results show that the influence of compound angle on normalized cycle efficiency reaches 1.10 %. The maximum normalized cycle efficiency occurs when the aerodynamic loss is minimized, corresponding to a compound angle of 0°. This value is 0.43 % higher than that obtained at the compound angle with the minimum bleeding ratio. These findings highlight the importance of incorporating aerodynamic loss in film cooling design.