Effects of the length scale of free-stream turbulence and cylinder size on heat transfer in laminar separated flows

Abstract

An experimental study is conducted to investigate the combined influence of length scale of free-stream turbulence, Λ, and cylinder diameter, D, on heat transfer from slightly heated circular cylinders in cross flow. By using a hot-wire anemometer, spectrum analysis of its signal with a Fourier analyzer is employed to investigate the separated shear layer formed behind the circular cylinder. Furthermore, a resistance thermometer is used to measure the time-averaged temperature behind the cylinder and the power-spectral density of the temperature fluctuation in the thermal boundary layer. The Nusselt number at the rear stagnation point varies with the relative scale Λ/D, although the Reynolds number is the same. Correspondingly, the laminar-to-turbulence transition region in the separated shear layer moves in the streamwise direction due to the change in the length scale ratio, and at the same time its variation gives a substantial increase in the power spectrum of the temperature fluctuation. It is disclosed that flow and heat transfer behind cylinders are affected by Λ/D, and are closely related with the streamwise movement of the transition region in the separated shear layer. A correlation equation including Λ/D as the parameter is derived to predict the heat transfer coefficient at the rear stagnation point.