Design of an Experimental Apparatus to Measure the Thermal Accommodation Coefficient Between Stainless Steel Surfaces and Rarefied Helium

The objective of this work is to design an experimental apparatus that can acquire data to benchmark rarefied gas heat transfer simulations, and determine the thermal accommodation coefficient at the interface between the solid surfaces and the gas. The design consists of an aluminum cylinder with an electric heater at its centerline, and within a stainless-steel sheath, centered inside a cylindrical pressure vessel whose temperature is controlled using an external water jacket. There is 0.47-cm-wide helium-filled gap between the inner cylinder and vessel wall. For a given heat generation rate, the temperature difference across this gap will increase as the gas pressure decreases due to ratification. Thermocouples will be bonded to the vessel’s outer surface, and the inner surface of the sheath that surrounds the heated aluminum cylinder. Two, two-dimensional computational meshes of the apparatus (one cross sectional and the other cross sectional is offset) and one three-dimensional computational mesh are constructed. These models include heat generation within the electric heater, conduction within the solid and gas-filled regions, and radiation heat transfer across the gas, and rarefied gas thermal resistances at the solid/gas interfaces. These simulations show that the difference between the thermocouple temperatures and the surfaces of the helium filled gap are small compared to the temperature across the gap. This will allow this apparatus design to be used to effectively benchmark the ANSYS/Fluent simulations, and determine the thermal accommodation coefficient.