EFFECTS OF ULTRASONIC-WAVES ON NATURAL-CONVECTION, NUCLEATE BOILING, AND FILM BOILING HEAT-TRANSFER FROM A WIRE TO A SATURATED LIQUID

A series of experiments were carried out to make clear systematically the effects of ultrasonic waves on natural convection, nucleate boiling, and film boiling heat transfer from a heated 0.2 mm diameter platinum wire to saturated water or ethyl alcohol. The effects on the maximum and minimum heat flux points were also examined. The test wire is set in liquid above a 20 mm x 40 mm vibrating surface of an ultrasonic transducer whose resonance frequency is 28 kHz and maximum power is 33.6 W. A distinctive augmentation effect on heat transfer is observed in both the natural convection and film boiling heat transfer regions. Though no effect is observed in nucleate boiling of water, a small effect is attained in low heat flux nucleate boiling of ethyl alcohol. An increase of about 20% in the maximum heat flux is obtained in both liquids by applying ultrasonic waves. The minimum heat flux point is raised at higher values of both the degree of superheat and the heat flux. The profiles of sound pressure along the centerline of the ultrasonic field and those of the heat transfer coefficients in natural convection and in film boiling were measured and compared. Heat transfer coefficients attained under ultrasonic waves are shown to be depend largely on the distance from the vibrating surface to the test position. In general, the sound pressure profiles resemble a deformed sine wave and are roughly identical with those of the heat transfer coefficient as regards their wavelength and their maximum and minimum. It is considered that the sound pressure of ultrasonic waves may, directly or indirectly, have some connection with the mechanism of heat transfer augmentation.