© 2021 Elsevier Masson SASIn this paper, effect of the heaters' clearance on mixed convective cooling performance in a discretely heated horizontal duct is examined. Tests are carried out for a constant value of the modified Grashof number (GrDh*=1.15×107), and for Reynolds numbers (ReDh) between 792 and 3,962. In the analysis, surface to surface radiation and wall conduction are coupled with convective heat transfer. Experiments involve measurements of surface temperatures and air velocity while ANSYS Fluent is used to conduct numerical simulations. Variations of the local surface temperatures, hot spot temperature, global conductance (C) and effective Nusselt number (Nueff) with Reynolds number (ReDh) are presented. Finally, it is disclosed that up to 10.77% reduction in the hot spot temperature and an improvement of up to 20.84% in the system's global conductance can be achieved with an increase in the clearance.