Nanoparticle layer detachment and its influence on the heat transfer characteristics in saturated pool boiling of nanofluids Yosuke Watanabe, Koji Enoki, Tomio Okawa ⇑ Department of Mechanical and Intelligent Systems Engineering, The University of Electro-Communications, 1-5-1, Chofugaoka, Chofu-shi, Tokyo 182-8585, Japan article info Article history: Received 12 January 2018 Received in revised form 14 March 2018 Accepted 16 April 2018 Keywords: Saturated pool boiling Nanofluid Nanoparticle layer Peeling test Adhesion force Critical heat flux Heat transfer coefficient abstract In nucleate pool boiling of nanofluid, nanoparticles suspended in liquid are deposited to form nanopar- ticle layer on the heated surface. As a result, surface properties are changed and the critical heat flux (CHF) is usually enhanced. However, since adhesion of the nanoparticle layer to the heated surface is not necessarily strong, partial detachment of the nanoparticle layer frequently occurs during nucleate boiling. In this study, peeling test was conducted for the nanoparticle layer formed during nucleate boil- ing of the water-based nanofluids to measure the adhesion force. The material of the heated surface was copper, and TiO 2 , Al 2 O 3 and SiO 2 were used as the nanoparticle material. It was found that the adhesion force is highly dependent on the nanoparticle material; it was greatest for SiO 2 and weakest for TiO 2 in the present experiments. Then, saturated pool boiling curves were obtained using the damaged heated surfaces after the peeling. For the nanoparticle-layer coated surfaces without peeling, the heat transfer coefficient (HTC) was lower and CHF was higher than those for the bare surface. It was shown that with an increase in the mass of the nanoparticles removed by peeling, the decrease of HTC and the increase of CHF were mitigated. Even higher HTC values and a lower CHF value than those for the bare heated surface were measured for significantly damaged surfaces. Ó 2018 Elsevier Ltd. All rights reserved. 1. Introduction Nanofluid is the liquid in which nanometer-sized solid particles are suspended in a base liquid such as water or oil. It has been reported that in nucleate pool boiling in nanofluid, the heat trans- fer coefficient (HTC) and the critical heat flux (CHF) can signifi- cantly be different from those in the pure liquid containing no nanoparticles. Extensive reviews by several investigators showed that the HTC in nanofluids can be enhanced, deteriorated, or unchanged depending on the experimental conditions, whilst the CHF is usually enhanced greatly [1–9]. It is hence considered that boiling heat transfer of nanofluid can effectively be used for the cooling of high-power-density devices such as the next- generation CPU and the inverters for fuel-cell and electric vehicles [10,11]; it may also be applied to the emergency cooling of nuclear reactors [12,13]. During nucleate boiling in nanofluid, nanoparticles are depos- ited to form a thin layer on the heated surface. The formation of the nanoparticle layer is considered as a main mechanism causing the difference of the heat transfer characteristics since the surface properties such as the roughness, the wettability and the wickabil- ity are totally changed [14–18]. For instance, Kim et al. [19] measured the contact angle of the nanoparticle-deposited heated surface to report that the contact angle decreased noticeably after the nucleate boiling in nanofluid and strong correlation was pre- sent between the surface wettability and the boiling heat transfer. Kim and Kim [20] showed that the CHF in the nanofluid can be correlated using the contact angle and the capillarity of the heated surface. It should however be noted that adhesion of the nanopar- ticle layer to the heated surface is not necessarily strong. Okawa et al. [21] found partial detachment of the TiO 2 nanoparticle layer from the heated surface during nucleate boiling in distilled water. They reported that the CHF enhancement by the nanoparticle layer was reduced when the nanoparticle layer detachment took place. Zuhairi et al. [22] also found the partial detachment of the nanoparticle layer for the TiO 2 - and SiO 2 -water nanofluids. Peculiar time-variation of the wall superheat was observed for these heated surfaces. It was discussed that such time-variation was related to the nanoparticle layer detachment. These experimental results show that the nanoparticle layer detachment may occur in the nucleate boiling of nanofluid and it can influence the boiling heat transfer characteristics. Nanoparticles are not expensive and nanoparticle layer can easily be fabricated on the heated surface. It is hence expected that https://doi.org/10.1016/j.ijheatmasstransfer.2018.04.072 0017-9310/Ó 2018 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: okawa.tomio@uec.ac.jp (T. Okawa). International Journal of Heat and Mass Transfer 125 (2018) 171–178 Contents lists available at ScienceDirect International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt