Gas mixing diagnostics using acoustic absorption measurements

Abstract

This paper demonstrates the use of acoustic absorption measurements to quantify the level of mixing between two gases. This work is motivated by the need for quantification of mixing effectiveness in a number of applications, such as in fuel/air preparation in combustion systems or various process control applications in chemical plants. The basic idea of the technique is to interrogate the medium with acoustic waves in the 20–200 kHz region where vibrational relaxation processes are the dominant factor affecting absorption. Our approach takes advantage of the fact that the 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 a dual component gas mixture can vary substantially with the level of molecular mixedness of the constituent species. We present experimental data illustrating the technique which show that the level of acoustic absorption varies by a factor of five, depending upon whether the constituent gases are unmixed or perfectly mixed. These values agree favorably with theoretical results generated from classical acoustic absorption equations and empirical values of relaxation frequencies.