|M.Sc Student||Weltsch Oren|
|Subject||Investigation of a Thermoacoustic Mass Streaming|
|Department||Department of Energy||Supervisor||Professor Dan Liberzon|
|Full Thesis text|
The thermoacoustic effect refers to the coupling between heat transfer and motion within an acoustic wave. Traditional thermoacoustics, developed during the past 30 years, refers to devices, e.g. heat-pumps or prime movers, that utilize the thermoacoustic effect. Here, an acoustic mass pumping effect, termed ‘Phase Exchange Thermoacoustics’ (PET), is investigated. PET relies on the coupling between mass transfer and motion in an acoustic wave; analogous to the heat transfer of traditional thermoacoustics, the mass transfer occurs through adsorption and desorption between the working gas mixture and an adsorbent-coated solid.
A theoretical model of the mass transfer process was developed, for a binary mixture, where only one of the mixture’s components is reactive, in the sense that it undergoes a reversible heterogeneous reaction. In order to gain accurate predictions from the model, reaction kinetics were included in the boundary conditions. It was found that for fast kinetics both the adsorption-desorption equilibrium coefficient (the Henry coefficient) and an oscillating Damköhler number, determine the reaction rate. An analytical solution was found for the reactive component’s distribution, from which an expression for selective mass streaming of the reactive component was derived. This was then used to calculate the gradient of the reactive component’s mass fraction, created and sustained by the acoustic wave, under a self-limiting condition. This condition occurs when acoustic pumping, the mechanism by which the gradient is created, is exactly balanced by diffusion and dispersion down the gradient.
In order to validate the developed model, an experimental system was designed and constructed, based on a standing wave acoustic resonator. Humid air was used as the binary gas working fluid and several solid materials were used, including plastic, ceramic (cordierite) and Zeolite 13X adsorbent. The experimental results showed that a difference of up to 37% in the vapor mass fraction could be created between both sides of the mass pump. The change in the vapor mass fraction as a function of the acoustic pressure amplitude followed the trend predicted by the model with varying accuracy (down to a minimum of ∼ 11% ). At lower acoustics pressure amplitudes the model overpredicted experimental results, presumably due to diffusion from the mass pump to the surrounding, not accounted for in the model. Measured performance utilizing various solid materials varied in accordance with the theoretical analysis of the boundary reaction kinetics.
The experimental and analytical results showed that the studied effect can be used for gas separation. Future work can include investigation of the benefits for traditional thermoacoustic heat pump under the presence of the studied mass streaming.