CHF and near-wall boiling behaviors in pool boiling of water on a heating surface coated with nanoparticles

Abstract

This study investigated the saturated pool boiling of water on a 12 mm diameter horizontal heating surface coated with titanium oxide (TiO2) nanoparticles under atmospheric pressure. The TiO2 coated surface was formed during nucleate boiling of TiO2 (25 nm diameter)-water-surfactant nanofluid on the plain heating surface at high heat flux conditions. It was found that the CHF of water boiling on the TiO2 coated surface increased up to about 1.8 times the CHF for the uncoated (plain) surface. To examine the mechanism of the CHF enhancement by TiO2 nanoparticle coating, liquid-vapor behaviors close to the heating surface were measured using a conductance probe with a tip diameter smaller than 5 μm. The probe signals and the void fraction distributions showed that there is little difference in the liquid-vapor structure in the boiling on the uncoated surface and on the TiO2 coated surface, that a liquid rich layer (a so called macrolayer) remains on the heating surface, and that in boiling on the TiO2 coated surface it does not dry out even at heat fluxes far higher than the CHF of the uncoated surface. The thickness of the macrolayer formed beneath large vapor masses was determined from the location where the probe signals corresponding to the large vapor masses disappear. It was found that the macrolayers formed on the TiO2 coated surface are thicker than those on the uncoated surface, and it is considered that this is the most likely to be a cause of the CHF enhancement with the TiO2 coated heating surface.

A comprehensive review on pool boiling of nanofluids

“…Meanwhile, the particle deposition can contribute to the emergence of new nucleation sites, yielding the improvement of HTC at the high particle concentrations. Sakashita [76] also reported a 1.8 times higher CHF than that of the bare surface for TiO 2 coated surface.…”

“…plays a critical role on the increment in CHF [76]. Namely, the presence of nanoparticle layer increases the trapping of liquid due to its porous structure and prevents the vapour layer formation on the surface.…”

Küresel Yüzeyler Etrafinda Nano Akişkanlarda Havuz Kaynama Isi Transferi̇ni̇n Deneysel İncelenmesi̇

“…Diğer taraftan, Şekil 6'da görüldüğü üzere, saf su ile karşılaştırıldığında, çekirdek kaynama ısı transferi üzerinde nano akışkanların önemli bir etkisinin olmadığı görülmektedir. Araştırmacılar, katı yüzey üzerinde biriken nano partiküllerin yüzeyin morfolojisini ve ıslatabilirliğini önemli ölçüde değiştirebileceği rapor etmişlerdir [12][13][14][15][16][17][18]. Film kaynama bölgesinde sadece birkaç mikron kalınlığındaki buhar filmi, böylesi bir partikül birikmesinden etkilenebilir [19].…”

Pressure effect on CHF enhancement in pool boiling of nanofluids

“…Kim and Kim [8] boiled different nanofluids with 0.2mm diameter horizontal wires to deposit layers of the nanoparticles on the wires and concluded that the CHF increase shown by the effect of the nanofluids is a result of both the increase in surface wettability and the capillarity in the porous layer of the deposited nanoparticles. Sakashita [9] measured liquid-vapor behaviors close to a heating surface using a conductance probe during pool boiling of water on an upwards-facing heating surface coated with TiO 2 nanoparticles, and specified the macrolayer thicknesses formed beneath vapor masses. Sakashita [9] established that the macrolayers formed on the TiO 2 coated surface are thicker than those on the uncoated (clean) surface, and concluded that this is most likely a main cause of the CHF enhancement with nanofluids.…”

“…Sakashita [9] measured liquid-vapor behaviors close to a heating surface using a conductance probe during pool boiling of water on an upwards-facing heating surface coated with TiO 2 nanoparticles, and specified the macrolayer thicknesses formed beneath vapor masses. Sakashita [9] established that the macrolayers formed on the TiO 2 coated surface are thicker than those on the uncoated (clean) surface, and concluded that this is most likely a main cause of the CHF enhancement with nanofluids. Kim et al [10] measured the CHF of water boiling on a 0.2mm horizontal wire on which a TiO 2 nanoparticle coated layer was previously deposited by previous boiling in nanofluid.…”