A Study of the Flow Pattern and Film Cooling Efficiency on a Flat Surface With Different-Shaped Holes

This paper aims to compare different hole geometries under identical conditions to acquire new and dependable data on film structure and heat transfer characteristics. The study involves five hole geometries: a cylindrical hole, a short fan-shaped hole, a laidback hole, a long fan-shaped hole, and a combined geometry. The experimental (particle image velocimetry) and numerical (large eddy simulation) results show that film separation behind the cylindrical hole occurs even at a blowing ratio of m = 1, whereas the long fan-shaped and combined holes provide a separation-free flow of the film at m = 1.5. They also avoid the formation of kidney-shaped vortices and the intense interaction between the jet and the main flow, which can lead to reduced thermal protection of the surface. An increase in cooling efficiency is directly proportional to the hole expansion ratio, and its maximum values are observed for the combined hole at m = 0.5, which is, on average, 3 to 5 times more efficient than the cylindrical hole. At higher blowing ratios (m = 1.5), both the long fan-shaped and combined holes demonstrate high efficiency, with spanwise-averaged values of η > 0.15 along the entire length of the model. An increase in cooling efficiency is associated with an increase in film flow instability. At the mode m = 1, the Strouhal number is equal to St = 0.46 for long fan-shaped and combined holes, 0.35 for the laidback hole, and 0.3 for the short fan-shaped geometry. This instability can contribute to a local increase in cooling efficiency, even if the average efficiency in other areas is not high.