Semi-analytical model for pool boiling of nanofluids

Abstract

Pool boiling characteristics of stable nanofluids are influenced by the relative size of the nanoparticles in the dispersed phase and the surface characteristics of the boiling region. This paper proposes a composite, semi-analytical model to study the surface particle interactions, incorporating various effects of particle deposition such as modifications in surface wettability, surface roughness and increased resistance in the path of heat transfer during pool boiling. A modified expression for nucleation site density incorporating the surface particle interaction parameter and wettability parameter has been presented. The present approach focuses on the individual effects of particle concentration, surface particle interactions and wall superheat on the boiling curves, thereby rendering a clear understanding of the mechanisms responsible for the boiling performance of nanofluids and their relative magnitudes of dominance at different experimental conditions. Rough and smooth boiling surfaces having arithmetic roughness values of 308nm and 53nm, respectively were studied and the corresponding values of surface particle interaction parameters were 6.16 and 1.06, respectively. Results indicate that the effects of changes in surface wettability are overcome by the changes in surface roughness. Boiling on nanoparticle-coated surfaces has been presented as a solution to counter the transient nature of nanofluid boiling while retaining the advantageous surface properties and accordingly, enhancements of up to 67% were obtained while boiling the basefluid on the nanoparticle-coated rough surface, as compared to boiling the basefluid on a clean, rough surface. The effects of boiling hysteresis in nanofluids were found to be quite significant for the rough surface and a higher heat transfer performance was obtained for an increasing heat flux boiling run as compared to the decreasing heat flux case. The heat transfer for the basefluid on the clean, smooth surface was lesser than that on the corresponding rough surface, on account of fewer nucleation sites on the smooth surface. Further, the heat transfer while boiling with nanofluid was deteriorated by as high as 30% while boiling with the nanofluid. The predictions of the developed model were compared with experimentally obtained boiling curves for nanofluids as well as for the basefluid on nanoparticle-coated surfaces.