The flow structure under mixed convection in a uniformly heated vertical pipe
Massachusetts Institute of Technology. Dept. of Nuclear Engineering.
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For decay heat removal systems in the conceptual Gas-cooled Fast Reactor (GFR) currently under development, passive emergency cooling using natural circulation of a gas at an elevated pressure is being considered. Since GFR cores have high power density and low thermal inertia, relative to the high temperature gas-cooled thermal reactor (HTGR), the decay heat removal (DHR) in depressurization accidents is a major challenge to be overcome. This is due to (1) a gas has inherently inferior heat transport capabilities compared to a liquid and (2) the high surface heat flux of the GFR strongly affects the gas flow under natural circulation. The high heat flux places the flow into a 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. Review of previous work on heat transfer mechanisms and flow characteristics of the mixed convection transitional regime shows that two transitional zones exist between laminar or laminar-like flow and fully turbulent flow for the upward heated case. Previous work has focused on liquids and thus is not applicable to gas mixed convection.(cont.) An experimental facility is designed to obtain the data in the regions not covered in previous work, using nitrogen, helium and carbon dioxide. The facility is expected to operate with heat fluxes up to 10kW/m2 and gas velocities up to 2.5m/s by natural circulation only. A velocity calibration method is designed in addition to the hotwire probe for velocity and temperature profiles measurement. Finally, computational simulations, using the commercial code FLUENT, are performed to select an appropriate turbulence model for investigating mixed convection transitional flow regimes. It was concluded that the basic models in FLUENT were not capable of predicting the transitional flow as the Launder-Sharma turbulence model does. Nevertheless, the advanced numerical algorithm and convenient post processor of FLUENT can still be utilized by using UDF to incorporate other turbulence models into the code.
Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 2005.Includes bibliographical references (p. 79-84).
DepartmentMassachusetts Institute of Technology. Dept. of Nuclear Engineering.
Massachusetts Institute of Technology