Experimental Study of Mixed Convection Heat Transfer to Thermally Developing Air Flow in a Horizontal Rectangular Duct

This paper describes the case of combined free and forced convection heat transfer to a thermally developing air flow in a horizontal rectangular duct which has been studied experimentally for laminar internal flow. The heated surfaces were subjected to a constant heat flux while the duct short sides were kept unheated, for three cases lower, upper side heated only and both sides heated with constant heat flux and for fiveduct angles of inclination 0° (horizontal). In the experimental study, the heat flux applied to the heated surfaces varied from 40 to 400 W/m2. This provides a modified Grashof number varied from 2.38*106 to 2.8*108 respectively, while the Reynolds number varied from 800 to 2100. Surface temperature, local heat transfer results reveal a significant effect of buoyancy force created by natural convection on the heat transfer process with the buoyancy makes the thermal boundary layer developed early in comparison with pure forced convection.