INFLUENCE OF THE ULTRASOUNDS ON THE HEAT TRANSFER IN SINGLE PHASE FREE CONVECTION AND IN SATURATED POOL BOILING

This paper collects the experimental results obtained in the Thermal Fluid Dynamics Lab at the Department of Energy and Engineering Systems at the University of Pisa, concerning the influence of ultrasounds in single phase free convection and in pool saturated boiling, at atmospheric pressure. The authors have just reported in previous papers their results about the enhancement induced by the presence of ultrasonic waves in subcooled boiling conditions. The ultrasounds are applied at the set frequency of 40 kHz, with a transducer output changing from 300 to 500 W, on a circular horizontal cylinder heated by Joule effect, immersed in distilled water. The tests in single phase free convection, without ultrasonic waves, are validated by means of the classical correlations reported in literature. Moreover, the authors managed to correlate the data also in presence of ultrasounds. The enhancement of the heat transfer coefficient is lower than the one obtained in subcooled boiling and with regards to the saturated pool boiling conditions, as far as for subcooling degrees very close to the saturation, the ultrasounds cause a decrease of the heat transfer. At the same time, the ultrasounds induce the critical heat flux (CHF) at higher heat flux compared with the case without ultrasounds. Also in saturated pool boiling, without ultrasonic waves, the experimental value of the CHF is compared with the one obtained by equations reported in the literature. This paper, joined to the others published by the authors, makes clear systematically the effects of ultrasounds on the heat transfer and shows as they could be very useful in the last generation electronic components cooling.