Boiling heat transfer during single nanofluid drop impacts onto a hot wall Tomio Okawa ⇑ , Kenta Nagano, Takahiro Hirano Department of Mechanical Engineering, Osaka University, 2-1 Yamadaoka, Suita-shi, Osaka 565-0871, Japan article info Article history: Received 14 May 2011 Received in revised form 26 August 2011 Accepted 26 August 2011 Available online 3 September 2011 Keywords: Nanofluids Drop impact Cooling Boiling heat transfer abstract Experiments were carried out to explore boiling heat transfer during successive impacts of single nano- fluid drops onto a hot stainless steel plate. Nucleate boiling heat transfer and critical heat flux were improved significantly when nanometer-sized titanium-dioxide particles were dispersed in water drops. In contrast, colloidal dispersion of the nanoparticles degraded the heat transfer when the plate temper- ature was too high. A thin nanoparticle layer was formed on the plate during the nucleate boiling of nano- fluid drops to improve the surface wettability. Observation of the impact process revealed that droplet spreading area at low plate temperatures was wider for the nanofluid drops. An increase in the liquid– solid contact area was expected to be a primary cause of the nucleate boiling heat transfer improvement. At high plate temperatures, phase change caused immediately after the drop impact appeared more sig- nificant for the nanofluid drops. It was considered that the significant vaporization in the initial stage inhibited the liquid–solid contact in the later stage to degrade the overall heat transfer. Ó 2011 Elsevier Inc. All rights reserved. 1. Introduction Nanofluid is the term used to describe the liquid that contains colloidal dispersion of nanometer-sized solid particles. Good heat transfer performance of nanofluids comparing with the base liquid has been reported in various types of boiling. As for the pool boiling in nanofluids, the critical heat flux is commonly improved, although conflicting results have been reported for the nucleate boiling heat transfer [1]. The change of heat transfer characteristics may be attributed to the deposition of nanoparticles on the heated surface during nucleate boiling, which modifies the surface properties including the wettability and the capillarity [2–4]. Since the contact angle usually decreases during the nucleate boiling in nanofluids, enhancement of the liquid contact with the heated surface would be one of the main reasons of the CHF improvement. It is therefore considered that the heat transfer characteristics in nanofluids are dependent on the amount of nanoparticles deposited on the heated surface. In fact, Okawa et al. [5] reported that the critical heat flux and nucleate boiling heat transfer are dependent on the boiling time after the dispersion of nanoparticles in the base liquid. It has been shown that the CHF is enhanced by the use of nano- fluid not only in the pool boiling but also in the flow boiling [6,7]. As in the case of pool boiling, nanoparticles are deposited on the heated surface to modify the surface properties. The heat transfer during quenching of high-temperature body is also influenced by the presence of nanoparticles. Kim et al. [8] studied the quenching of high-temperature metal spheres in water-based nanofluids to re- port that the critical heat flux and the temperature at the minimum heat flux increased when nanoparticles were accumulated on the sphere surface. Chun et al. [9] obtained the cooling curves during the quenching of a platinum wire in water and nanofluids. Although distinct difference was not found between the water and nanofluids, high cooling rate could be obtained when nanoparticle-coated wires were quenched with water. Spray cooling is another typical type of boiling heat transfer, and has a wide spectrum of industrial applications [10–13]. Several researchers applied the nanofluids to the cooling of high-tempera- ture body by droplets. Duursma et al. [14] measured the heat removal rate during nanofluid drop impacts onto a high-tempera- ture surface. Although the droplet behavior during the impact was considerably different between the pure liquid and nanofluid drops, no significant difference was found in the boiling curves. Sefiane and Bennacer [15] carried out the experiments to investi- gate the evaporation process of sessile droplets placed on a heated surface. It was reported that adding aluminum nanoparticles to pure ethanol led to a deterioration of the evaporation rate during the pinning phase. The above literature survey indicates that, although spray cool- ing is widely used to remove heat from a high-temperature body, studies concerning the heat transfer performance of nanofluids in this boiling situation are still limited. In the present study, single drops are let fall onto a hot wall successively to explore the depen- dence of the boiling heat transfer on the addition of nanoparticles to a base liquid. The initial plate temperature is set high enough to observe various boiling regimes from the film boiling to the nucle- ate boiling. Visualization of the impact process using a high-speed camera is also made to investigate the main reasons of different 0894-1777/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.expthermflusci.2011.08.007 ⇑ Corresponding author. Tel.: +81 6 6879 7257. E-mail address: t-okawa@mech.eng.osaka-u.ac.jp (T. Okawa). Experimental Thermal and Fluid Science 36 (2012) 78–85 Contents lists available at SciVerse ScienceDirect Experimental Thermal and Fluid Science journal homepage: www.elsevier.com/locate/etfs