Friday, September 5, 2008
1:00pm (334 JFB)
Title: No Title Submitted
This thesis presents studies of the behavior of miniature annular thermoacoustic prime movers and the imaging of the complex sound fields using Particle Image Velocimetry (PIV) inside the small acoustic wave-guides when driven by a temperature gradient.
Thermoacoustic engines operating in the standing wave mode are limited in their efficiency by a high degree of irreversibility that is inherent in how they work. Better performance can be achieved by using traveling waves in the thermoacoustic devices. This has led to the development of annular high frequency thermoacoustic prime movers consisting of a regenerator, which is a random stack of large surface area, in between a hot and cold heat exchanger, inside an annular wave-guide Sound is generated when heat is applied to the hot heat exchanger. The dev ice is an acoustic amplifier where sound is amplified by the regenerator; oscillations are sustained by positive feedback. Miniature devices were developed and studied with operating frequencies in the range of 2-4 kHz. This corresponds to an average ring circumference of 11 cm for the 3 kHz device, the resonator bore being 6 mm. A similar device of 11 mm bore, length of 18 cm was also investigated; its resonant frequency was 2 kHz. Sound power was extracted from the annular structure by an impedance-matching side arm and converted to electricity using a piezoelectric device. The nature of the acoustic wave generated by hear was investigated using a high speed PIV instrument. Although the acoustic device appears symmetric, its performance is characterized by a broken symmetry and by perturbations that exist in its structure. Effects of these are observed in the PIV imaging; velocity images show axial and radial components of the sound wave. Moreover, PIV studies show effects of streaming which affect the devices; efficiency. The efficiency is high, being ~40% of Carnot. This type of device shows much promise as a high efficiency energy converter; it can be further reduced in size for microcircuit applications.