Boiling heat transfer during single nanofluid drop impacts onto a hot wall

Abstract

Experiments were carried out to explore boiling heat transfer during successive impacts of single nanofluid drops onto a hot stainless steel plate. Nucleate boiling heat transfer and critical heat flux were improved significantly when nanometer-sized titanium-dioxide particles were dispersed in water drops. In contrast, colloidal dispersion of the nanoparticles degraded the heat transfer when the plate temperature was too high. A thin nanoparticle layer was formed on the plate during the nucleate boiling of nanofluid drops to improve the surface wettability. Observation of the impact process revealed that droplet spreading area at low plate temperatures was wider for the nanofluid drops. An increase in the liquid–solid contact area was expected to be a primary cause of the nucleate boiling heat transfer improvement. At high plate temperatures, phase change caused immediately after the drop impact appeared more significant for the nanofluid drops. It was considered that the significant vaporization in the initial stage inhibited the liquid–solid contact in the later stage to degrade the overall heat transfer.