The Transpired Turbulent Boundary Layer in Various Pressure Gradients and the Blow-Off Condition

Abstract

An experimental study of the reduction in heat transfer to a transpiration-cooled flat surface subjected to pressure gradients (zero, negative, and positive) is presented for flow conditions similar to those encountered in gas turbines. The investigation is carried out for high injection rates and determines the blow-off conditions under which the boundary layer is lifted away from the wall by the transpired coolant. The study was conducted in a hot blow-down wind tunnel facility. The transient nature of the facility ensures that the wall remains isothermal. The Reynolds number, the ratio of the gas to wall temperatures, and the pressure gradient parameters K are chosen to be representative of the conditions found in advanced gas turbines. The effect of the pressure gradient was found to be small. However, a local strong acceleration can reduce the cooling effectiveness. The heat transfer rates or Stanton numbers on a solid surface downstream of a transpiration cooled wall are found to be sensitive to the upstream injection ratio (b) and to the pressure gradient parameter. The data indicate that the ratio of Stanton numbers with and without cooling is nonzero for values of the injection parameters larger than values obtained theoretically by Kutateladze. The predicted value of the critical injection ratio (bcr ) determined from this study agrees well with the experimental data of Liepmann and Laufer for a free mixing layer, which is similar to a transpired boundary layer near blow-off as pointed out by Coles.