A major desire for the Gas-cooled Fast Reactor (GFR) is increased reliance on passive emergency cooling using natural circulation of gas at elevated pressure. Since GFR cores have high power density and low thermal inertia, the decay heat removal (DHR) in depressurization accidents is a major challenge to be overcome. This is due to (1) gas has inherently inferior heat transport capabilities compared to liquid and (2) high surface heat flux of GFR relative to the high temperature gas-cooled thermal reactor (HTGR) which strongly affects the gas flow under natural circulation and places the flow into the mixed convection regime, which is not yet fully understood. One of the issues of mixed convection is that the transition from laminar to turbulent flow is not clearly defined in the existing literature. This paper reviews previous work on heat transfer mechanisms and flow characteristics of the mixed convection transitional regime, and shows that two transitional zones exist between laminar or laminar-like flow and fully turbulent flow for the upward heated case. A description of the MIT/INL experimental facility designed to obtain the data in these and other regions not covered in previous work is given. Finally, some computational results for transitional flow, using the commercial code FLUENT, are presented.
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