Bubble Dynamics in Pool Boiling on Nanoparticle-Coated Surfaces

Abstract

This work examines bubble dynamics for saturated nucleate boiling at atmospheric pressure for pure water on a 0.625mm-diameter zirconium wire with nanoparticle surface coating using a high-speed camera. Pool boiling experiments were carried out on the surface-modified wires fabricated by the electrophoretic deposition process using 60- to 80-nm titanium and 60- to 80-nm titania, respectively. Critical heat flux (CHF) enhancements for pool boiling were observed up to 27% for the titanium coatings and 65% for the titania coatings compared to bare zirconium wires. At 300 kW/m(2) surface heat flux, the nucleate site density on the titanium coating and the titania coating was observed to be 47% and 45% less than the bare wire. In terms of bubble departure volume, each coating had 16% and 29% smaller volume than the bare surface, respectively. In addition, the average bubble departure frequency of each coating was 43% and 101% higher than that of the bare case. The nanoparticle surface coatings enhanced the natural and forced convection contribution to the total heat flux, whereas the latent heat contribution was reduced. Water spreading experiments on the nanoparticle-coated surfaces were carried out to investigate the surface characteristics. The significant water spreading on Ti- and TiO2 nanoparticle-coated surface compared to on a bare Zircaloy-4 surface suggests enhanced lateral liquid transport to bubble nucleation sites due to the coating capillarity and wettability. We hypothesize that this mechanism plays important roles for boiling behavior as a function of input power and for CHF improvement.