Visualization and Measurement of Thermal Convection Flow in a Closed Duct Excited by Acoustic Standing Wave

This paper describes an experimental investigation on unsteady thermal convection flow in a horizontal closed-duct combined with acoustic standing wave induced in the duct. As the amplitude of acoustic standing wave increases, steady circulating streaming caused by nonlinear effect of finite amplitude oscillatory flow appears within every 1/4 wavelength section. It is called acoustic streaming. On the other hand, the thermal convection flow is characterized by Rayleigh number. In the range of high Rayleigh number, the convection current changes to unsteady and turbulent. When a strong acoustic standing wave is combined to the unsteady or turbulent convection flow, the flow pattern of the convection quickly changes to steady circulating flow similar to the acoustic streaming. PIV technique is applied to quantitative analyses of the resultant flow depending on mode of standing wave, sound pressure, and Rayleigh number. The results show that the resultant flow becomes unstable according to increase of Rayleigh number and at lower mode of oscillation.