Simulations of acoustic streaming inside and surrounding thermoacoustic stacks

Abstract

Acoustic streaming generated inside and surrounding thermoacoustic engine stacks by standing waves in resonators is investigated numerically. The study is performed using the nonlinear 2-D code described recently by the authors [J. Acoust. Soc. Am. 111, 2076 (2002)]. The stack consists of parallel plates of negligible thickness. There is no restriction on plate separation, or on the length or position of the stack in the resonator. Temperature dependence of the fluid viscosity is taken into account. A temperature gradient imposed on the stack drives the sound field. Acoustic streaming patterns, investigated in terms of average mass flow velocity, are calculated by time averaging the momentum density. For typical plate separations on the order of the thermoviscous penetration depth, only the inner, boundary-layer streaming vortices exist between the plates. These inner vortices rotate in directions opposite those of the outer, more familiar Rayleigh streaming vortices encountered in wide channels (wider than about 10 penetration depths). The inner vortices extend beyond the ends of the stack out to distances on the order of the acoustic particle displacement. Flow patterns near the entrances to the stack are investigated as functions of channel width and acoustic amplitude [Work supported by ONR.]