Determination the heat transfer characteristics of ultrasound assisted immersion cooling

In this Phd thesis, the effects of heat source location, protrusion amount, and heat flux on single and two-phase immersion cooling systems were experimentally investigated, with a focus on the ultrasound frequency. Four heat sources with equal power capacity were operated in pairs in 5 different combinations. The experimental study was conducted for three levels of protrusion: without protrusion, 2 mm protrusion, and 4 mm protrusion. The ultrasonic sound wave used in the experiments had a constant amplitude and ranged from 0 to 24 kHz. The heat flux applied to the heat sources was varied in the range of 4 kW/m² to 166.66 kW/m². PF-5060 dielectric fluid was used as the working fluid. According to the experimental results, it was observed that ultrasonic sound waves positively influenced heat transfer in single-phase natural convection. At low heat powers, the circulation of liquid molecules on the hot surfaces increased due to the propelling effect of the sound waves, resulting in an increase in the natural convection heat transfer coefficient of up to 107.49%. In all operating conditions, an increase in protrusion amount led to a decrease in the heat transfer coefficient due to the creation of additional hot surface. In the pool boiling regime, it was observed that the frequency of the ultrasonic sound wave did not have a distinctive effect compared to single-phase natural convection. The reason of this, even without ultrasound, the thermal bubbles formed during boiling was enhanced the mixing effect through unstable fluid motions caused by microconvection. On the other hand, it was concluded that the position of the active heat source relative to the free liquid surface and to each other affected the boiling temperature. The protrusion amount increased the boiling temperature by providing additional surfaces that hinder bubble formation and buoyancy forces.