An Experimental Study of Temperature Separation in Swirling Flow

The objective of the present investigation is to substantiate the mechanism of acoustic streaming; this was carried out by conducting detailed internal surveys of a Ranque-Hilsch swirling flow test rig at the condition when the vortex whistle was attenuated or excited. This control of the vortex whistle was accomplished by two methods.

In the first method where the vortex whistle was attenuated by placing a restriction at the exhaust, a Rankine vortex was observed to form, accompanied by an almost uniform total temperature distribution in the radial direction except for a dip near the vortex core; when the vortex whistle emerged by the removal of the restriction, a forced vortex was formed, accompanied by an almost linear distribution of total temperature separation is indeed caused by acoustic streaming induced by the vortex whistle, which serves to deform a base Rankine vortex into a forced vortex.

In the second method where the vortex whistle was suppressed by the installation of an acoustic suppressor, at the time of sound suppression the total temperature at the centerline leapt upwards while the total temperature at the periphery took a sudden plunge. In spite of this, a result to be expected, the forced vortex formed before the sound suppression did not revert to a Rankine vortex. The cause of this unexpected result was traced to a contaminating effect of back flow entraining ambient air. Besides, the role of tuning mechanism of acoustic suppressor was identified to be that of suppressing the second harmonic of the vortex whistle rather than the previously-held fundamental frequency.

In addition, another phenomenon of intense heating caused by a pure tone sound, generically different from the vortex whistle was found; this may be related to Hartmann-Sprenger resonance tube effect.