A comparative study of internal thermal performance in film holes with modified flow passage configurations: Contracting, expanding, and slot-type designs

To improve coolant delivery and thermal management, the in-hole flow passages are often modified from the conventional cylindrical design. The resulting configuration changes can alter in-hole acceleration, separation, and secondary flow development, leading to distinct internal thermal performance. Local Nusselt number distributions were measured using a transient thermochromic liquid crystal technique and evaluated based on a one-dimensional semi-infinite conduction model. Numerical simulations were performed using the SST k-ω turbulence model within the RANS framework to investigate the underlying flow mechanisms. A reference cylindrical hole was examined together with contracting-type holes (C-hole I and II), expanding-type holes (E-hole I and II), and slot-type holes (slot hole and spanwise-expanded slot hole). The results indicated that contracting-type holes enhance heat transfer by accelerating in-hole flow and reducing inlet separation but at the expense of greater friction losses, with C-hole II showing a thermal performance factor below unity. Expanding-type holes promote vortex generation and wall shear, yielding more uniform heat transfer, and E-hole II shows the largest enhancement of 28%. Slot-type holes form symmetric inlet separation vortices, with spanwise expansion improving the mid-section heat transfer distribution but slightly reducing the overall Nusselt number compared with the standard slot hole. Overall, except for C-hole II, the modified configurations provide improved thermal performance with an acceptable increase in flow resistance.