Graduate Thesis Or Dissertation
 

A study of opposing mixed convection in the GRTS and in downward pipe flows using the FLUENT CFD code

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/bz60d160m

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  • Downward flow opposing mixed convection conditions have been studied using the FLUENT CFD code. This work was comprised of two primary objectives. The first was to study core channel flow reversal susceptibility in the Gas Reactor Test Section (GRTS) at low flow conditions. The second objective was the development of a general stability correlation for opposing mixed convection conditions in downward pipe flows as a function of pipe radius, gas material, inlet velocity and elevated wall temperatures. The traditional criterion that buoyancy forces must be considered in mixed convection flows when the Gr/Re² value is on the order of 1.0 was used to identify low flow conditions in the GRTS when core channel flow reversal may occur. The corresponding GRTS power levels and flowrates were studied using FLUENT. An initial FLUENT model was used to model the solid core GRTS components to obtain the individual coolant channel wall temperatures for each axial level of each coolant channel of the GRTS associated with the low flow conditions susceptible to flow reversal. These coolant channel wall temperatures were then used as boundary conditions in a more refined second FLUENT model containing just the fluid regions of the GRTS that was used to obtain the flow characteristics. It was found that flow reversal did not occur in the GRTS even when the traditional criterion predicted that buoyancy forces could not be neglected. When FLUENT was used to study flow reversal for downward opposing mixed convection as a function of pipe radius, gas material, inlet velocity and elevated wall temperatures, it was found that while the traditional criterion that buoyancy forces must be considered when the Gr/Re² value is on the order of 1.0 is a good estimate, it is insufficient to give a good prediction for when flow reversal occurs. The ability of the flow to reverse was largely dependent on pipe radius and gas material. This thesis presents two correlations, one for pipe radii below 2 cm, and one for pipe radii 2 cm and above, predicting when flow reversal will occur as a function of Grashof and Reynolds numbers.
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