Measurements of thermoacoustic functions for single pores

A novel volume modulation technique is employed to measure the complex compressibility of a gas in a single pore. The complex compressibility depends on the geometry dependent thermal coupling between the gas and the pore walls, and is related to the thermoacoustic function fκ, or equivalently, F(λ). By measuring two different lengths of a given pore, end effects are eliminated, so that F(λ) corresponding to a uniform, infinite length pore is determined. Pores of circular, rectangular, coaxial, and hexagonal cross sections were investigated, as well as cylinders which contained wire mesh screens. A wide range of values for the ratio of the thermal penetration depth to pore size was achieved by working down to very low frequencies. Results for circular, rectangular, and coaxial pores are in excellent agreement with theory. Hexagonal pores show behavior which is very close to similar size circular pores. Qualitative aspects of the screen results can be understood by a comparison to the theory for parallel plates.