Keith A. Gillis
Michael R. Moldover
NIST, Thermophys. Div., Bldg. 221/Rm. A111, Gaithersburg, MD 20899
James B. Mehl
Univ. of Delaware, Newark, DE 19716-2570
An acoustic method is being developed for measurements of both the thermal and viscous diffusivities of gases using different sets of resonances of a single cavity. The ratio of the diffusivities, i.e., the Prandtl number, is a key parameter in determining the heat transfer from a solid to a moving fluid. Honeycomb inserts placed near the center (z=L/2) of a cylindrical cavity (length L) provide a large surface area with minimal blockage of longitudinal acoustic flow. Odd-order longitudinal modes, which have a velocity antinode at z=L/2, have a strong viscous interaction with the insert. For even-order modes, which have temperature antinodes at z=L/2, the thermal interaction dominates. Measurements of the resonance parameters of both types of modes are analyzed using a model which enables the viscous diffusivity to be determined from the odd modes and the thermal diffusivity from the even modes. Because the same insert is used for both the even and the odd modes, the ratio of the diffusivities is not sensitive to details of the geometry of the insert; thus, the Prandtl number can be determined more accurately than either diffusivity. [Work supported by ONR.]