MEASUREMENT OF MIXED-CONVECTION HEAT TRANSFER FROM AN ARRAY OF DISCRETE SOURCES IN A HORIZONTAL RECTANGULAR CHANNEL WITH AND WITHOUT SURFACE AUGMENTATION

Experiments have been performed with water and FC-77 to investigate mixed-convection heat transfer from a four-row, in-line array of 12 square heat sources that are flush mounted to the lower wall of a horizontal, rectangular channel. The experimental data encompass heat transfer regimes characterized by pure natural convection, mixed convection, laminar forced convection and the initiation of transition to turbulence. The variation of the row-average Nusselt number with Reynolds number exhibits a minimum, suggesting that, due to buoyancy-induced flow, heat transfer may be enhanced and pumping power requirements reduced by reducing the flow rate. Experiments have also been performed with FC-77 and a heat source array whose surface area is augmented by the use of longitudinal fins. Conditions for the first heater row are forced convection dominated, while conditions for rows 2-4 are characterized by natural convection (row 4 only), mixed convection and transition to turbulence. In contrast to variations for the flush-mounted sources, row-average Nusselt numbers for the finned sources decrease monotonicalty with decreasing Reynolds number for rows 1-3 and are nearly independent of Reynolds number for row 4. For both the unfinned and finned arrays, increasing the Rayteigh number enhances heat transfer and increases the Reynolds number range for which enhancement occurs. Appropriate scaling parameters are introduced for both geometric configurations to characterize mixed-convection heat transfer from an array of discrete sources.