Investigations on the effect of foreign gas transpiration on a turbulent boundary layer

Abstract

Blowing through the wall is one of the most efficient methods to influence the characteristics of a turbulent boundary layer like heat transfer, skin friction, boundary layer profiles and flow separation. The most important technical application is cooling of a permeable wall by passing a coolant mass flow through the wall called transpiration cooling. The design of transpiration cooled devices requires the knowledge of empirical correlations describing the behavior of the hot gas boundary layer in the case of blowing with respect to skin friction (including flow separation) and heat transfer. Numerous publications are dealing v/ith blown boundary layers, but the special effect of blowing gas properties (foreign gas transpiration) mostly is not investigated. For this reason a hot air wind tunnel was built at the Technical University of Dresden. Using detailed measurements of velocity and temperature boundary layer profiles the influence of foreign gas transpiration on skin friction and heat transfer could be specified by means of CLAUSER'S method. Simple empirical correlations are created describing skin friction and heat transfer reduction depending on the blowing ratio, the temperature *Prof. Dr.-Ing. habil. 'Member AIAA Copyright © 2000 J. Meinert, J. Huhn (TU Dresden) and E. Serbest, O.J. Haidn (DLR Lampoldshausen). Published by the American Institute of Aeronautics and Astronautics, Inc. with permission. difference and especially the blowing gas properties. Several investigations deal with flow separation due to blowing, a critical blowing parameter being defined. The paper gives information on the test facility, measuring techniques, the evaluation of measured data and the generalization of experimental results. The empirical equations are also verified using a numerical calculating procedure. Measurements at the German Aerospace Center Lampoldshausen using an Hj/Oa-combustion chamber deliver experimental heat transfer results in the case of hot gas parameters according to practical applications. These test results give additional information on the heat transfer characteristics in the case of blowing parameters higher than the critical value (flow separation). All measurements are compared with empirical results to suggest simple and universal correlations to predict skin friction and heat transfer under different blowing conditions.