Experimental Study of Mixed Convection and Thermal Radiation Heat Transfer in Horizontal Rectangular Ducts with Different Heating Orientations

Abstract The current experimental work examines the combined effects of mixed convection and surface radiation on heating a horizontal rectangular duct from three distinct bottom orientations: CS1 (bottom), CS2 (top), and CS3 (side). The rectangular duct section consists of one heated isothermal wall, and the remaining walls are maintained under adiabatic conditions. The effects of Reynolds number, heat flow, surface emissivity, and heating orientation on the heated-wall temperature and total Nusselt number for convective and radiative heat transfer were systematically assessed. The findings show that forced-convection effects are strengthened as the Reynolds number increases from 978 to 1956, thereby significantly lowering the heated-wall temperature. The wall temperature increases, and the convective Nusselt number decreases by 5–13% as the applied heat flux increases from 644 to 1659 W/m². Among the three heating orientations, CS1 demonstrates the highest heat-transfer performance, followed by CS2 and CS3, respectively. Increasing the surface emissivity from 0.05 to 0.85 reduces the average wall temperature by approximately 15–16% and enhances the convective Nusselt number by 21–35%. The radiative Nusselt number rises significantly, indicating that surface radiation plays a major role in the total heat-transfer process. Stable thermal behaviour along the duct is shown by the axial profiles of the heat-transfer parameters being comparatively uniform. Overall, the results show that bottom heating (CS1) in conjunction with a high-emissivity surface offers the best heat-transfer performance and can be used successfully for heat dissipation and passive thermal management applications.