The effect of ultrasound manipulation on boiling performance during immersion cooling

In eliminating the heating problem during operation in electronics, immersion cooling is an efficient thermal management method. This study investigates the influence of ultrasound on boiling heat transfer in immersion cooling using PF-5060 dielectric fluid. Flat and vertically oriented heaters were tested with and without ultrasound. The experiments were carried out for 5 cases and 3 protrusion amounts: embedded, 2 mm protruding, and 4 mm protruding, depending on the location of the heated surfaces on the Teflon plate. Ultrasound was applied with a frequency range of 9.2–24 kHz with the help of a probe immersed in the liquid. The results show that the superheat temperature is affected by the position of the active heat source relative to the liquid-free surface and each other. Increasing the amount of protrusion also increased the superheat temperature. However, in the boiling regime, ultrasound had a negligible effect on the heat transfer coefficient (HTC). This is related to the fact that even without ultrasound excitation, thermal bubbles formed during boiling create micro-convection, which enhances the mixing effect through unstable fluid movements. At the bottom heat source, with a 56 mm distance between the heaters, the maximum heat transfer coefficient was found to be 31.023 kWm⁻². In this study, ultrasound was applied perpendicular to the fluid flow direction and in the opposite direction of the buoyant forces. Therefore, one of the most important results of this study is that the force direction created in the flow field by ultrasound affected the HTC. In addition, a high-speed camera was used in this study, clearly showing the changes that occurred in the fluid regime.