EXPERIMENTAL AND NUMERICAL STUDY ON EFFECTS OF TURBULENCE PROMOTERS ON FLAT PLATE FILM COOLING

摘要

Turbulence promoters such as ribs inside turbine blade coolant channels are used to improve convective cooling but at the same time could influence external film cooling performance. The effects of rib orientation and rib position on film cooling performance are experimentally and numerically studied with a flat plate configuration in which external (main) flow and internal (secondary) flow are oriented perpendicular to each other. In the experiment, temperature fields are measured by thermo-couples varying blowing ratio at constant Reynolds number of main and secondary flows. To obtain detailed information about flow fields, Reynolds Averaged Navier Stokes (RANS) simulation and Detached Eddy Simulation (DES) are also performed using a commercial code Fluent. Temperature measured shows that rib orientation has a strong influence on film effectiveness. With forward-oriented ribs, higher film effectiveness is observed compared to the reference case without ribs. On the contrary with inverse-oriented ribs, lower film effectiveness is observed. The difference comes from the flow structure in the film cooling hole. With the forward-oriented ribs, straight stream lines are observed in the cooling hole, while with the inverse-oriented ribs, helical stream lines are observed. Due to the helical stream lines in the hole, ejection angle of the secondary flow to the main stream becomes large, resulting in so called lift-off and lower film effectiveness.