Natural convection heat transfer and intensification for a discrete heat source in a vertical annulus

The decay heat removal in advanced nuclear power plants encourages the use of natural convection cooling as a precaution during power outages. The ongoing designs of micro nuclear reactors institute an ambient air-cooled system via natural convection, which points to a localized heat source cooling in an open loop. The analyses presented in this paper address the problem of natural convection heat transfer of a heat source placed in an open-ended annular channel. Numerical simulations were carried out for various heat source lengths, moved along the inner cylinder of the annulus. Using the transition SST turbulence model, the influence of the annular gap size on heat transfer rates was investigated by adjusting the radius ratios between 3 and 5, while heat transfer enhancement was achieved by way of longitudinal fins. Results of heat transfer rates, local heat transfer characteristics, and mass flow rates are presented. The change in elevation of the heat source at L_c/b > 2.7 in the open system did not have a profound influence as indicated by the Rayleigh number buoyancy parameter and Nusselt numbers. However, the annular gap size was unequivocally the most influential geometrical parameter. Additionally, the average Nusselt numbers at any unfinned heated section length were adequately described by the correlation Nu_L = 0.959Nu_H(L/H)^0.855, for $0<L/H<1$. The number of fins and the fin height were the most important parameters for the finned system where case-specific gains of more than 50 % in average Nusselt number were obtained. The results of the present analyses provide invaluable information for the development of passively cooled systems utilizing ambient air.