Acoustic absorption measurements for characterization of gas mixing

Abstract

Controlling and/or monitoring the degree of mixing between constituents of a multicomponent media is a key problem in a variety of applications. Monitoring such mixing processes necessarily requires capabilities for quantification of the level of “mixedness.” However, quantification of molecular mixedness levels, as opposed to macroscale mixture uniformity, is difficult. This paper demonstrates the use of acoustic absorption measurements to characterize an average level of molecular mixedness between gases across the wave propagation path. This approach takes advantage of the fact that over a large frequency range, acoustic damping is dominated by vibrational relaxation processes. The vibrational relaxation frequency for a particular gas is often a strong function of the other species it is in molecular contact with. Thus, the relaxation frequency of each species in a multicomponent gas mixture varies with the level of molecular mixedness of the constituent species. This paper presents the results of example calculations and experiments demonstrating the feasibility of this approach and the significant sensitivity of acoustic absorption levels upon gas mixedness; e.g., measurements reported here show acoustic amplitude differences of up to a factor of 10 between identical gas mixtures whose only difference is the level of mixedness of their constituents.