Periodic heat transfer in directly opposed free and forced convection flow

Abstract

An experimental study of the heat transfer from small circular cylinders placed horizontal to a downward flowing air stream is reported. Based on heat-transfer measurements and flow visualization, a model for directly opposed free and forced convection was developed. Three modes of flow were observed. For very low velocities the free convection, buoyant plume dominates the heat transfer. At a “lower critical” Reynolds number, when the free and forced convections are of the same order of magnitude, a well defined periodic heat transfer was obtained. The periodic heat transfer was due to the build-up of the buoyant forces to a magnitude where they overcame the downward force of the air flow. At an “upper critical” Reynolds number the periodic heat transfer abruptly ceases. For velocities greater than the upper critical limit the forces due to the air flow dominate. A potential like, laminar sheet forms, as a shroud around the thermal layer of the hot cylinder. The average heat transfer from the cylinder decreases with increasing Reynolds number for both the case of dominant free convection and the periodic heat-transfer regime. The minimum value of the heat transfer occurred at the upper critical Reynolds number.