HEAT TRANSFER IN A CIRCULAR PIPE WITH ARTIFICIALLY PERTURBED LAMINAR AND TURBULENT AIR FLOW

Abstract

Artificially generated perturbation has been superposed on steady-state air flow before entering a pipe heated by about constant heat flux, to examine heat transfer under these circumstances. Periodic perturbation had a shape of half sine or square wave. Compared to unperturbed flow, unambiguous improvement of heat transfer throughout the tested range of Reynolds numbers was observed. The role of frequency and that of the waveform were manifested by the position and extension of the transient domain between laminar and turbulent flow, ,while there was no effect on the Nusselt number neither in laminar nor in turbulent domains, where, however, increase of the perturbation intensity entrained a uniform improvement of heat transfer at the same rate. Formulae haYe been presented for determining the critical Reynolds number and the Nusselt number.