Numerical simulations of an experimental apparatus consisting of an 8×8 square array of heater rods within an aluminum enclosure are used to model a Boiling Water Reactor (BWR) fuel assembly. The total heat generation rate is varied from 100W to 500W and the enclosure wall temperature is set at 325°C. The void is filled with air and simulations are run at atmospheric pressure, low pressure (500 Pa) and without cover gas (total vacuum). Low pressure simulations at 500 Pa are run with two models, the first assumes a continuum heat transfer regime in the cover gas, and the second includes a non-continuum temperature jump on the surfaces of the rods and enclosure wall. A contact resistance is used to simulate a temperature jump on all inner surfaces of the model. Simulations show that natural convection does not have significant effects on heat transfer when the pressure is lowered to 500 Pa. In general, radiative heat transfer dominates over the conduction in the cover gas at a wall temperature of 325°C. Results show that a model using a total vacuum with only radiative heat transfer at the wall, predicts maximum rod to wall temperature difference as much as 19.1% higher than a model at atmospheric pressure, whereas a model at 500 Pa, that considers a slip flow regime with temperature jump at the wall, predicts temperatures that are 7.0% higher. A low pressure model at 500 Pa without a temperature jump does not result in any significant increase in temperatures. Temperatures from these simulations will be compared to future experiments at low pressures.

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