The Influence of a Mainstream Thermal Boundary Layer on Film Effectiveness

Abstract

A theoretical and experimental investigation on the effect of a mainstream thermal boundary layer on adiabatic film effectiveness is presented. The theory is based on a simple model which accounts for mixing between the injected flow and a mainstream flow with a viscous and thermal boundary layer. In order to apply this theory, the adiabatic film effectiveness in a flow with uniform temperature must be known either from experiments or from another theory. Experiments are described for an injection geometry having a simple two-dimensional slot. These tests were conducted with an insulated lip having a lip-diameter to slot-height ratio of 0.6. The mainstream thermal boundary layer was produced by heating the surface in contact with the mainstream flow upstream of the slot. Velocity and temperature distributions were measured at various distances downstream of the slot, along with measurements of the adiabatic wall temperatures. All tests were performed at a secondary to mainstream mass flux ratio of 0.7, but with different amounts of mainstream heating. While a comparison between theoretical and experimental results shows a discrepancy near injection, the trend is correct, and the agreement downstream is good.