Experimental Benchmark of Simulations that Predict Temperatures of an 8x8 Array of Heater Rods within a Vessel Filled with Rarefied Helium Gas

A two-dimensional ANSYS/Fluent computational fluid dynamics model is constructed of an existing experiment that consists of a square 8x8 array of heater rods within a square cross section pressure vessel filled with helium. The model includes heat generation and conduction within the rods, conduction and radiation heat transfer across the helium between the rods and enclosure, and the effective thermal resistance at the gas/solid interfaces that is significant at low pressures when the gas is moderately rarified. This configuration is relevant to the vacuum drying process that is used when used nuclear fuel is transferred from underwater to dry storage. Simulations are performed for enclosure temperatures of 27°C and 427°C, and total rod axial heat generation rates between 160 W/m and 820 W/m. Moderately-Rarified simulations, which include an effective thermal resistance between the gas and solid surfaces, are performed for helium pressures of 100 and 400 Pa, thermal accommodation coefficients of 0.25 and 0.4, and a Lennard-Jones collision diameter of 1.9 angstroms. These simulations predict peak heater rod temperatures that are at least 5°C hotter than those predicted by a continuum model, which neglects the rarified gas thermal resistance, when the array heat generation rate is above 330 W/m. Simulations of earlier experiments in the same apparatus, with the helium pressure between 105 and 3x105 Pa, predicted peak rod temperatures that were within 5°C of measured values. The current simulation results indicate that the apparatus can be used with a high degree of certainty to benchmark Moderately-Rarified simulation results for rod axial heat generation rates above 330 W/m.