Forced convective heat transfer in uniformly heated horizontal tubes 1st report—Experimental study on the effect of buoyancy

Abstract

In forced convective heat transfer in a horizontal straight tube, buoyancy is likely to have a serious effect on the velocity and temperature fields. This report is devoted to clarifying the effect of buoyancy by means of accurate experiments on the fully developed flow of air in a uniformly heated horizontal tube. Velocity and temperature distributions are measured for large Re Ra and in laminar flow these distributions are shown to be essentially different from those known so far, for example, the velocity profile of Poiseuille flow. Nusselt numbers are calculated from the measured velocity and temperature distributions and shown to be about twice as large as those calculated by neglecting the effect of the secondary flow caused by buoyancy at Re Ra = 4 × 10 5. In turbulent flow, buoyancy has little effect on the velocity and temperature fields, but the critical Reynolds number is shown experimentally to be affected by the secondary flow.