ENHANCED BOILING HEAT TRANSFER BY SUBMERGED ULTRASONIC VIBRATIONS

This paper describes a new two-phase cooling heat transfer cell based on a submerged vibration-induced bubble ejection process in which small vapor bubbles attached to a solid surface are dislodged and propelled into the cooler bulk liquid. This ejection technique involves forcibly removing the attached vapor bubbles with a submerged pressure difference generated by a vibrating piezoelectric diaphragm operating at ultrasonic frequencies. The piezoelectric driver induces pressure oscillations in the liquid near the heated surface, resulting in vapor bubble and liquid instabilities. These pressure differences generated by the piezoelectric driver operating at resonance enhance boiling heat transfer by removing attached vapor bubbles that insulate the surface. A small-scale vibration-induced bubble ejection module that produced a pressure difference at a heated surface using an ultrasonic piezoelectric actuator was used in this initial study. The initial experimental data that was obtained include the cooling capabilities of the cell. The efficacy of this cooling approach was tested on a calibrated heater that dissipated 107 W/cm at 120oC in the absence of the jet using natural convection. When the jet was on, the heat flux increased to 191 W/cm (i.e., an improvement of 78%) at the same surface temperature.