AN EXPERIMENTAL STUDY OF THE COOLING EFFICIENCY ON CURVED SURFACES WITH DIFFERENT SHAPED HOLES

Abstract

Improving the film-cooling efficiency is one of the main goals of heat transfer in modern industrial applications. To follow the actual requirements to film-cooling systems, different hole geometries including fan-shaped, laidback, and complex shapes were earlier developed and studied. However, most appropriate studies were performed on flat surfaces whereas the actual geometry of protected walls has curved surfaces, e.g., gas turbine nozzle vanes. The present paper reports experimental results of comparative studies of the film-cooling efficiency for curved surfaces with five different hole geometries: cylindrical, short and long fan-shaped, laidback, and joint fan-shaped and laidback holes. Each experimental model had 11 holes located in a row perpendicular to the main stream direction and was studied using the IR-thermography technique. The experimental results show that the choice of the most efficient cooling hole shape also strongly depends on the blowing ratio value, which was changed in the range 0.5 ≤ m ≤ 1.5 during these studies. For cylindrical holes, the film separation takes place at m > 0.7. Meanwhile, short fan-shaped and laidback holes provide the film separation at m > 1. The highest values of the cooling efficiency were obtained for long fan-shaped holes in the blowing ratio range 0.5 ≤ m ≤ 1. Under mentioned conditions, long fan-shaped holes' efficiency is at least 35# higher compared to others. However, studies at m >1 showed that joint fanshaped and laidback holes provide maximal cooling efficiency due to the almost tangential-to-surface direction of the secondary air flow blown from these holes. The obtained results can be further applied in designing new cooling systems for the protection of the most thermally stressed elements of industrial applications.