|M.Sc Student||Blayer Yuval|
|Subject||Effect of Phasing on Mass Streaming in an Acoustic|
|Department||Department of Applied Mathematics||Supervisor||Professor Yehuda Agnon|
For an acoustic wave oscillating near a solid boundary, a temperature gradient will develop along the direction of propagation. This effect, known as 'thermoacoustic streaming', results from a non-zero heat flux, a time-averaged product of two zero-averaged oscillating quantities (velocity and temperature). Thermoacoustic heat pumps use acoustic waves to drive this streaming effect in order to pump heat from a cooled source to hot sink (similarly to other types of heat-pumps e.g. air-conditioning or refrigerators). An analogous 'mass streaming' effect has been recently discovered, in which the acoustic field near a sorbing, solid boundary, generates a net flux of one (or more) species in a gas mixture. In the latter process, the solid and the fluid exchange mass, as well as heat, creating a mass pumping effect.
In this study, the effect of acoustic phasing - the phase difference between pressure and velocity oscillations - on the mass pumping is investigated. A versatile device, capable of generating the entire range of acoustic phasing, was constructed. A porous medium, referred to as a “stack”, is placed inside the acoustic field in order to create good contact between the sorbent and the gas mixture. Once applied, the acoustic field creates a mass flux along the stack, parallel to the propagation axis.
Two humidity sensors measured the difference in water vapor concentration, in an air-vapor mixture, between both ends of the stack, as a measure of the mass streaming magnitude. At long times with respect to the initiation of the applied acoustic field, the system stabilizes as it reaches a constant humidity gradient, sustained by a balance of the mass ’pumping’ and ’destructive’ fluxes, e.g. molecular diffusion and Taylor-Aris dispersion, that generate a flux down the concentration gradient.
Experiments performed by applying multi-phase acoustic field to various types of stacks, differ in characteristic pore radius, and measure the concentration delta as a function of configuration. Measurements compared to numerical simulations of a complementary model show good agreement. Experimental and model results reveals an interesting relation between acoustic phasing and a key dimensionless parameter, representing the ratio between the acoustic and the diffusive time scales. According to this relation, mass streaming in a travelling wave field, represented by zero phase angle, increases for denser types of stacks. When increasing the pore radius of the stack, favorable phasing shifts toward 90 degrees, a standing wave phasing. An idealized analysis, suggesting a physical justification for the relation between phase and stack density is proposed.