Effect of acoustic streaming on the mass transfer from a sublimating sphere

The effect of the acoustic streaming on the mass transfer from the surface of a sphere positioned in an ultrasonic acoustic levitator is studied both experimentally and theoretically. Acoustic levitation using standing ultras onic waves is an experimental tool for studying the heat and mass transfer from small solid or liquid samples, because it allows an almost steady posi tioning of a sample at a fixed location in space. However, the levitator in troduces some difficulties. One of the main problems with acoustic levitati on is that an acoustic streaming is induced near the sample surface, which affects the heat and mass transfer rates, as characterized by increased Nus selt and Sherwood numbers. The transfer rates are not uniform along the sam ple surface, and the aim of the present study is to quantify the spatial Sh erwood number distribution over the surface of a sphere. The experiments ar e based on the measurement of the surface shape of a sphere layered with a solid substance as a function of time using a charge-coupled device (CCD) c amera with backlighting. The sphere used in this research is a glass sphere layered with a volatile solid substance (naphthalene or camphor). The loca l mass transfer from the surface both with and without an ultrasonic acoust ic field is investigated in order to evaluate the effect of the acoustic st reaming. The experimental results are compared with predictions following f rom the theory outlined [A. L. Yarin, M. Pfaffenlehner, and C. Tropea, J. F luid Mech. 356, 65 (1998); A. L. Yarin, G. Brenn, O. Kastner, D. Rensink, a nd C. Tropea, ibid. 399, 151 (1999)] which describes the acoustic field and the resulting acoustic streaming, and the mass transfer at the surface of particles and droplets located in an acoustic levitator. The results are al so compared with the experimental data and with the theoretical predictions of Burdukov and Nakoryakov [J. Appl. Mech. Tech. Phys. 6, 51 (1965)], whic h are valid only in the case of spherical particles much smaller than the s ound wavelength. Good agreement between experiment and the theory of Yarin is demonstrated. The time-averaged heat and mass transfer rates over a sphe re surface are greatest at the sphere's equator and least at its poles in t he experiment as predicted by the theory (the ultrasonic standing wave span s the vertical axis passing through the poles). The measured distribution o f the mass transfer rate over the sphere surface also agrees with the theor etical predictions, which shows that in strong acoustic fields sublimation (or evaporation) results from the acoustic streaming. (C) 2000 American Ins titute of Physics. [S1070-6631(00)02104-8].