Experimental and numerical investigations of combined free convection and radiation heat transfer in an upward-facing cylindrical cavity

Due to the emergence of second-stage concentrators and lens systems, heat transfer in upward-facing cavities becomes more and more important. In the first part of this paper, the impacts of heat flux, cavity tilt angle and surface heating condition on combined free convection and radiation heat transfer in an upward-facing cylindrical cavity subjected to constant heat flux were explored experimentally. Results suggest that the studied parameters seriously affect the free convection and radiation heat transfer in cavity. Compared with the monotonous variations of the cavity surfaces average temperature, as well as the free convection and radiation heat transfer Nusselt numbers Nuc and Nur with cavity tilt angle for downward-facing cavity, there exist extremums for upward-facing cavity. In view of this interesting result, three-dimensional numerical analysis validated by related experimental results was performed to further investigate the effect of cavity tilt angle from the perspective of physical mechanism. Taking heat flux and cavity tilt angle as independent parameters, empirical correlations of Nuc and Nur in three surface heating conditions were proposed separately. In addition, a novel correlation incorporating the effect of surface heating condition with considerable accuracy was firstly developed to facilitate the engineering application whose surface heating condition alters frequently.