טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentBlayer Yuval
SubjectEffect of Phasing on Mass Streaming in an Acoustic
Field
DepartmentDepartment of Applied Mathematics
Supervisor Professor Yehuda Agnon


Abstract

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.