Session: Student Poster Competition

Submission Number: 177140

Geometric Effects of Hole Angles on the Similarity of Film Cooling Effectiveness Under Thermal Variations 

Film cooling is a key thermal protection technique for high-temperature components in aero-engine turbines. Its effectiveness depends on both the flow and thermal parameters and the geometric configuration of the cooling holes. Previous studies have often described the variation of film cooling effectiveness using a similarity criterion expressed as the product of the blowing ratio and the temperature ratio (MR multiplied by TR to the power of n). The exponent n generally exhibits a segmented variation, reflecting distinct behaviors of cooling effectiveness at different temperature ratios. However, these relations are typically derived under constant hole geometry. In practical turbine blades, the inclination and compound angles of cooling holes vary with the blade curvature and layout requirements, altering the jet trajectory and its interaction with the mainstream flow. Such geometric variations may compromise the universality of existing similarity laws. This study therefore examines how hole angle variations influence the similarity characteristics of film cooling effectiveness.

Three representative hole-angle configurations were selected, corresponding to inclination angles of 30 and 60 degrees and a compound angle of minus 30 degrees. Two typical temperature ratios were used for comparison. Numerical simulations were performed to obtain the distribution of film cooling effectiveness under different blowing ratios, and the deviation between results at various temperature ratios was used as a similarity indicator. Deviation contour maps, relationships between similarity index and deviation, and the distribution of low-deviation regions were analyzed together to identify how angle variation affects the similarity behavior. This approach accounts for both geometric effects on flow structure and thermal influences on similarity correction, providing a comprehensive understanding of the cooling mechanism.

The results show that as the inclination angle increases from 30 to 60 degrees, the deviation in cooling effectiveness becomes larger and the high-deviation region expands. The relationship between the similarity index and blowing ratio exhibits a clear segmented pattern. At low blowing ratios, deviations remain small and the similarity relation holds well, while at higher blowing ratios, deviations rise sharply and similarity decreases. This indicates that a larger inclination angle causes the jet to detach from the wall earlier, weakening the attachment of the cooling air and producing a discontinuous change in cooling behavior.

When a compound angle is introduced, the overall deviation is greatly reduced and the distribution becomes more concentrated. The similarity index curve becomes smoother and nearly continuous. In regions where deviations are below five percent, results under different temperature ratios coincide closely, showing that the compound angle improves similarity. This improvement occurs because the compound angle introduces both streamwise and lateral momentum components, enhancing wall coverage and mixing, and reducing the entrainment effects of the mainstream flow. As a result, the coolant distribution becomes more uniform and the cooling behavior more stable.

A comprehensive comparison reveals that for all hole-angle configurations, the minimum deviation appears when n is about 0.85. Under this condition, the low-deviation regions from different temperature ratios nearly overlap, forming a unified similarity relationship. Statistical analysis shows that most minimum deviations lie within the range of 0.8 to 0.9. This indicates that when n equals 0.85, the influence of coolant density and jet attachment achieves a balance at the momentum scale, allowing different geometric and thermal conditions to be described consistently by the same criterion.

In conclusion, this study clarifies the mechanism by which variations in hole angles influence the similarity of film cooling effectiveness. The findings refine the existing theory of cooling similarity and provide a quantitative reference for the optimization of cooling hole design and the prediction of thermal performance on complex turbine surfaces.

Presenting Author: Longyi Liu Beihang University

Presenting Author Biography: Longyi Liu received the B.S. degree in Flight Vehicle Propulsion Engineering from Beihang University, Beijing, China, in 2022. He is currently working toward the Ph.D. degree in Engineering Thermophysics with the School of Energy and Power Engineering, Beihang University, Beijing, China. His research interests include Similarity Theory, Double-Wall Cooling and Film Cooling.

Authors:

Longyi Liu Beihang University
Zixiang Tong Beihang University