Contents lists available at ScienceDirect Carbohydrate Polymers journal homepage: www.elsevier.com/locate/carbpol Enhancement of nanofluid stability and critical heat flux in pool boiling with nanocellulose Won-Ki Hwang a,1 , Seunghwan Choy b,1 , Sub Lee Song a , Jaeyoung Lee a , Dong Soo Hwang b,c, ⁎ , Kwon-Yeong Lee a, ⁎⁎ a Handong Global University, 558, Handong-ro, Heunghae-eup, Buk-gu, Pohang, Gyeongbuk, 37554, Republic of Korea b Division of Integrative Bioscience and Biotechnology, Pohang University of Science and Technology (POSTECH), 77 Cheongam-ro, Nam-gu, Pohang, Gyeongbuk, 37673, Republic of Korea c Division of Environmental Science and Engineering, Pohang University of Science and Technology (POSTECH), 77 Cheongam-ro, Nam-gu, Pohang, Gyeongbuk, 37673, Republic of Korea ARTICLE INFO Keywords: Critical heat flux (CHF) Cellulose nanofiber Heat transfer Pool boiling Thermo-fluid engineering Nanofluid ABSTRACT A nanofluid, which is an aqueous fluid with nanoparticles, is an attractive medium for enhancing critical heat flux (CHF); however, its instability over a long period of time due to sedimentation and aggregation has impeded its successful application in industry. In this study, lightweight negatively charged TEMPO-oxidized cellulose nanofibers (CNFs) were utilized as a nano-sized substance in water and examined to enhance both the CHF performance and thermal stability of nanofluids. Owing to low density of the CNFs and long range repulsion between negatively charged CNFs, there were no aggregation and sedimentation of CNFs with multiple boiling/ cooling cycles. In addition, with CNF concentrations of 0.01, 0.03, 0.05, and 0.10 wt%, CHF enhancement in- creases of 40.7%, 45.1%, 54.9%, and 69.4%, respectively, were achieved over that of pure water. The present results demonstrated the great potential of CNFs as eco-friendly and cost-effective nano-substances that can overcome the instability of nanofluids. 1. Introduction Boiling is a key process in the field of thermal-fluid engineering, such as power generation, thermal management, chemical processing. Nanofluids, fluids with dilute suspensions of nano-structured sub- stances, have attracted interest in the field of thermal-fluid engineering over the last decade due to their high thermal conductivity, compared to base fluids such as water and oils (Choi & Eastman, 1995; Eastman, Choi, Li, Yu, & Thompson, 2001; Lee, Choi, Li, & Eastman, 1999). Nano- structured substances can be inorganic nanoparticles (Ag, Au, SiO 2 , Cu, TiO 2 , ZnO 2 , Al 2 O 3 )(Chu, Joung, Enright, Buie, & Wang, 2013; Jackson, Borgmeyer, Wilson, Chen, & Bryan, 2006; Kathiravan, Kumar, Gupta, & Chandra, 2010; Kathiravan, Kumar, Gupta, & Chandra, 2012; Kim, Bang, Buongiorno, & Hu, 2006; Kim, Bang, Buongiorno, & Hu, 2007; Vassallo, Kumar, & D’Amico, 2004), and organic nano-substances (carbon nanotube, graphene) (Park et al., 2010; Trisaksri & Wongwises, 2007). Since You, Kim, and Kim (2003) demonstrated the dramatic effect of nanofluid on critical heat flux (CHF) enhancement, many re- searchers have applied a variety of nanofluids to enhance the CHF of the base fluids. CHF describes the thermal limit of nucleate boiling, which is directly correlated with the highest heat flux. When the heat flux from the heating source reaches the CHF value, vapor bubbles suddenly cover the heating source, which impedes heat flux from the heating source to the liquid, and lead to a “boiling crisis”. This also leads to irreversible damage to the heating source (Keblinski, Eastman, & Cahill, 2005; Theofanous, Dinh, Tu, & Dinh, 2002). Therefore, na- nofluids have promising applications in the field of thermo-fluid en- gineering because they can achieve excellent CHF enhancement via a relatively simple method. Even in the presence of very low concentra- tions of nanoparticles (on the order of parts per million to parts per thousand), the CHF enhancement ratio usually achieved by nanofluids is on the order of several tens of percent (Ahn et al., 2010; Kamatchi, Venkatachalapathy, & Nithya, 2016). Various types of inorganic nanoparticles have been investigated https://doi.org/10.1016/j.carbpol.2019.03.023 Received 4 December 2018; Received in revised form 4 March 2019; Accepted 6 March 2019 ⁎ Corresponding author at: Division of Integrative Bioscience and Biotechnology, Pohang University of Science and Technology (POSTECH), 77 Cheongam-ro, Nam- gu, Pohang, Gyeongbuk, 37673, Republic of Korea. ⁎⁎ Corresponding author. E-mail addresses: dshwnag@postech.ac.kr (D.S. Hwang), kylee@handong.edu (K.-Y. Lee). 1 These authors equally contributed to this work. Carbohydrate Polymers 213 (2019) 393–402 Available online 07 March 2019 0144-8617/ © 2019 Published by Elsevier Ltd. T