Experiments on heat-stabilized laminar boundary layers in water

There has been much recent interest in the stabilization of water boundary layers by wall heating. Calculations based upon linear stability theory have predicted transition Reynolds numbers as high as 200 million for a zero pressure gradient boundary layer over a heated wall. The experiment described in this paper was intended to investigate these predictions. The test boundary layer develops on the inside surface of a cylindrical tube which is 0·1 m in diameter and 6·1 m in length. The displacement thickness is small relative to the tube radius under all conditions of interest. The tube is heated by electrical heaters on the outside wall. The location of transition is determined by flush-mounted hot-film probes, or by flow visualization at the tube exit. A transition Reynolds number of 15 million has been obtained without heat, which shows that free-stream turbulence and other perturbations are well controlled. A transition Reynolds number of 47 million has been obtained with an 8 °C wall overheat. However, as temperature is further increased there are no additional increases in transition Reynolds number, which is in contradiction to the theory. Several possible reasons for the discrepancy between theory and experiment have been investigated.