Heat Transfer to Turbulent Boundary Layers With Variable Free-Stream Velocity

Abstract

Local heat-transfer coefficients have been determined for the turbulent boundary layer of a two-dimensional subsonic air flow over the surface of a flat plate, with the air velocity variable over the plate length. The boundary layer, formed with almost constant velocity, was then subjected to a strong acceleration which produced a three to fourfold increase in the free-stream velocity, and after this maximum there was a gradual deceleration of the flow. Maximum velocities ranged from 200 to 900 fps. The use of the results, obtained for a particular kind of variation of velocity, is primarily as a check of methods of predicting heat transfer under circumstances of this type. It is shown that adequate predictions can be made from an analysis based on constant properties, and that the most important element of the analysis is an account of the thermal energy in the boundary layer at any point along the plate. Results are given also for constant velocity along the plate, both for a completely turbulent flow and for turbulent flow occurring after natural transition. The heat-transfer coefficients so obtained for the turbulent boundary layer agree best with the prediction of the von Karman analogy.