Wettability effects of nanoparticles in pool boiling of a nanofluid

Boiling heat transfer of a nanofluid is a complex phenomenon having several attributes different from those of its base fluid. Although it is known that convective heat transfer coefficient of a nanofluid is enhanced relative to its base fluid owing to the increase of its thermal conductivities as well as Brownian motion of nanoparticles, the nucleate boiling heat transfer coefficient of a nanofluid has been found to increase, drop, or remain relatively unchanged compared with its base fluid. It has been speculated that the presence of nanoparticles in nanofluids have two effects on the boiling heat transfer process. Suspended nanoparticles can decrease surface tension of the nanofluid, reducing bubbles radii during nucleate boiling which results in enhancement of nucleate boiling heat transfer rate. On the other hand, the nanoparticles confined in the microlayer under the bubbles, can deposit on the heater surface after dry-out of the microlayer, causing changes of roughness and wettability of the heater surface.

Recently, X. Quan, D. Wang, and P. Cheng at Shanghai Jiao Tong University have carried out a series of experiments to study wettability effects of nanoparticles on boiling heat transfer. Since hydrophobic nanoparticles could aggregate easily in base fluids and thus would deposit on the heater surface rapidly, only nanofluids containing hydrophilic nanoparticles have been used in practical applications. In order to separate effect of the suspended nanoparticles’ wettability from the effect of morphology of nanoparticles’s deposition layer on boiling heat transfer, two sets of boiling experiments on nanofluids containing moderately hydrophilic nanoparticles or strongly hydrophilic nanoparticles were performed:  the first set of experiment was performed for boiling of moderately hydrophilic and strongly hydrophilic nanopartilces suspensions on a bare heater surface in comparison with boiling of pure water on a bare surface.  A visualization study of bubble dynamics in the selected nanofluid showed that the bubble size in the nanofluids containing moderately hydrophilic nanoparticles was smaller than those in its base fluid. The boiling curves for the first set of experiment show that the critical heat flux is substantially enhanced in the nanofluid having moderately hydrophilic nanoparticles. The second set of experiment was performed to study the effect of morphology of the deposited layer on the boiling of pure water. For this purpose, boiling experiments of pure water were performed on a strongly hydrophilic nanoparticles deposited heater, a moderately hydrophilic nanoparticle deposited heater and a bare heater.  It is found that morphologies of these nanoparticle deposition layers depend strongly on wettability of the nanoparticles, and different morphologies of nanoparticles deposition layers have different impacts on boiling heat transfer heat transfers. The results of the above research work are published in International Journal of Heat and Mass Transfer.

Inspired by the flotation theory, the authors proposed that moderately hydrophilic particles were adsorbed at bubble interfaces, preventing fluid drainage between bubble interfaces. As a result, bubble coalescence could be prevented by moderately hydrophilic nanoparticles. This led to a drop in bubble size, resulting in an increase in boiling heat flux and a drastic increase in critical heat flux. Also, moderately hydrophilic particle deposition layers are irregular and rough, thus, promoting bubble nucleation and enhancing nucleate boiling heat transfer. On the other hand, strongly hydrophilic particles were not adsorbed on bubbles interfaces, and therefore had no effects on bubble coalescence. Since strongly hydrophilic nanoparticle deposition layers are smooth, thick, and orderly, these strongly hydrophilic nanoparticle deposition layers deteriorate boiling heat transfer coefficient.

Based on the above consideration, it becomes evident that the nucleate boiling heat transfer coefficient of a nanofluid can be increase, decrease or remain to be relatively constant, depending on wettability of the suspended nanoparticles and their deposition layer on the heater. It is concluded that a nanofluid containing suspended moderately hydrophilic nanoparticles and their deposition layer on the heater surface can enhance boiling heat transfer and the critical heat flux substantially.