Turbulent Boundary Layers with Foreign Gas Transpiration

Abstract

Blowing through the wall is one of the most efficient methods to influence the characteristics of a turbulent boundary layer such as heat transfer, skin friction, boundary-layer profiles, and flow separation. The most important technical application is to cool a permeable wall with a coolant mass flow through the wall, known as transpiration cooling. Designing transpiration-cooled devices requires the knowledge of empirical correlations for the calculation of skin friction and heat transfer in the case of blowing. The results of velocity and temperature boundary-layer profile measurements made at the Dresden University of Technology were used to characterize the influence of foreign gas transpiration on skin friction and heat transfer. Simple empirical correlations describing skin friction and heat transfer reduction as functions of the blowing ratio and especially the blowing gas properties were found. Further investigations deal with flow separation due to blowing, and a universal critical blowing parameter is defined. The empirical correlations have also been verified by means of a numerical calculating procedure. Measurements made at the German Aerospace Center in Lampoldshausen using an H2/O2 combustion chamber delivered experimental heat transfer results in the case of hot gas parameters according to practical applications.