EXPERIMENTAL-STUDY OF LAMINAR MIXED CONVECTION IN THE ENTRANCE REGION OF A HORIZONTAL SEMICIRCULAR DUCT

Combined free and forced convection is experimentally investigated for laminar water flow in the entrance region of a horizontal semicircular duct. With uniform heat input axially, measurements are made of axial and circumferential wall temperature variations, together with pressure drops across the heated section, in order to study the effects of the buoyancy-induced secondary flow. For mean Reynolds numbers ranging from 400 to 1600, a factor of over two for friction factor increase and a factor of over four for heat transfer enhancement are found at high heating rates (mean Rayleigh numbers up to 4.6 x 10(8)). Local and fully developed Nusselt numbers show substantial circumferential variations and increase with increasing heat flux level. Correlations are provided which disclose some key features of the laminar mixed convection regime.