Experiment of CHF enhancement by $Fe_3O_4$ nanoparticle coating in subcooled boiling region

Abstract

Nanofluids as a working fluid is desirable method to improve critical heat flux (CHF) in pool and flow boiling. In pool boiling, CHF enhancement by nanofluids have been identified from numerous experimental studies in various situations. In flow boiling, CHF enhancement by nanofluids also have been widely known. In this study, subcooled flow boiling CHF experiments were conducted with DI water on bare stainless steel and nanoparticle coated surface at atmospheric pressure. Iron oxide($Fe_3O_4$) nanofluids was coated on the tube surface in condition of 10 ppm volume, mass flux of $1,000 kg/m^{2} s$, inlet temperature of $100^\circ C$, heat flux of $2,500 kW/m^{2}$ and time of 30 min. Nanoparticles were deposited sufficiently on test sections during deposition process. Then, working fluid was changed to DI water. Experiment was conducted in mass flux from $1,000 kg/m^{2} s4 to $5,000 kg/m^{2} s$ and inlet temperature of 40, 60, $80^\circ C$. CHF results of bare stainless steel and nanoparticle coated surface was obtained in the subcooled boiling region. Overall trend of CHF enhancement decreased as exit quality decreased. In mass flux of 2,000, 3,000, $4,000 kg/m^{2} s$, the CHF enhancement ratio decreased and approached to zero as exit quality decreased. In mass flux of $1,000 kg/m^{2} s$, the CHF of nanoparticle coated surface on high subcooling condition was even lower than that of bare stainless steel surface. In mass flux of $5,000 kg/m^{2} s$, CHF enhancement ratio remained at least 20 % in high subcooling condition. In terms of mass flux, CHF enhancement was increased as mass flux increased. Static contact angle was measured in as-received surface, DI-water boiled surface, nanoparticle coated surface, and nanoparticle coated & DI-water boiled surface. Contact angle decreased significantly after the nanoparticle deposition process. Contact angle increased small after nanoparticle coated & DI-water experiment, consequentially, not much nanoparticle was detached by the high mass flux experiment. CHF enhancement was analyzed by using flow boiling CHF mechanism. There are three different DNB mechanism such as ‘Dryout under a vapor clot’, ‘Near-wall bubble crowding and vapor blanketing’ and ’Evaporation of liquid film surrounding a slug flow bubble’. Our experimental data was plotted to the G-x flow regime. In region of ‘Dryout under a vapor clot’, CHF enhancement ratio was small under 10%. In region of ‘Near-wall bubble crowding and vapor blanketing’, however, CHF enhancement ratio was high from 20% to 40%. Although exit quality is similar in subcooled boiling region, the CHF enhancement was different from the mechanism of DNB. In mechanism of ’Evaporation of liquid film surrounding a slug flow bubble’, CHF enhancement was relatively low from -10 % to 10 %. The each CHF mechanism was investigated with theoretical CHF model such as ‘critical bubbly layer model’ and ‘liquid sublayer dryout model’. CHF enhancement by nanoparticle occurred due to the improvement of wettability and rewetting characteristics of deposited surface. When the wettability is improved by nanoparticle coating, the surface properties and bubble characteristics like departure frequency, shape and size can be changed significantly. The active nucleation site density decrease as surface wettability increase. The nucleation site density also decreased as inlet subcooling increase. Therefore, the CHF enhancement could be decreased due to degradation of active nucleation site density as exit quality decreased in highly subcooled boiling region. With the improvement of surface wettability, the bubble departure diameter increase, whereas the bubble departure frequency decrease. In the ‘critical bubbly layer model’, the critical heat flux is a function of the mass flux into bubbly layer from liquid core and departure diameter. As the bubble departure diameter increase, the turbulent intensity increased, then turbulent velocity fluctuation increased, as a result, CHF is enhanced by turbulent fluctuation. In the ‘liquid sublayer dryout model’, CHF occur when the evaporation of the liquid sublayer is higher than the liquid supply to liquid sublayer, The liquid sublayer thickness is increased by wettability improvement. However, the bubble departure time is increased by wettability change. Therefore, CHF enhancement can be compensated. Finally, CHF mechanism of the evaporation of liquid film in slug flow can be explained with LFD-type CHF mechanism. Because the LFD in annular flow is not significantly affected by wettability improvement, CHF in slug flow also is not enhanced by the wettability.