Heat transfer enhancement in a loop thermosyphon using nanoparticles/ water nanofluid V. Kiseev, O. Sazhin ⇑ Ural Federal University, Lenin av. 51, Ekaterinburg 620000, Russia article info Article history: Received 18 July 2018 Received in revised form 24 October 2018 Accepted 22 November 2018 Keywords: Nanofluid Loop thermosyphon Heat transfer enhancement LED cooling device abstract This research focuses on two-phase thermal control systems, namely loop thermosyphons (LTS) filled with nanofluids and their use as LED cooling devices. The behavior of the fluid in the thermosyphons and the mechanisms explaining the possible impact of nanoparticles on the thermal properties of the working fluid, as well as the processes in the LTS, are addressed. Nanoparticle distribution in the nano- fluid, methods of preparing nanofluids and the nanofluid degradation processes (aging) are studied. The results are obtained from a set of experiments on thermosyphon characteristics depending on the ther- mophysical properties of the working fluid, filling volume, geometry and nanoparticle mass concentra- tions. The impact of nanofluids on the heat-transfer process occurring inside the thermosyphon is also studied. The results indicate the strong influence of nanoparticles on the thermal properties of the ther- mosyphons, with up to a 20–25% increase in the heat transfer coefficient. It is shown that this effect is due to the aggregation of nanoparticles and the formation of a micro/nano relief on the vaporization surface. Additionally, a method of calculating the hydrodynamic limit of the LTS is proposed, which allows for estimation of the maximum heat that can be transferred by means of the LTS. The nanofluids are shown to be effective means for enhancing heat transfer in two-phase thermal management systems. Ó 2018 Elsevier Ltd. All rights reserved. 1. Introduction Two-phase heat transfer systems are widely used in different technical devices, especially in spacecraft thermal control systems and electronic devices such as microprocessors, semiconductor power electronics, powerful modern laser sources, optoelectronic devices, and light-emitting diode (LED)-based lighting [1–3]. According to recent studies, the thermal performance of two- phase systems may be improved by using nanofluids, a new type of liquids that include metal or metal oxide nanoparticles. First proposed by Choi [4], the concept of nanofluid, which is in fact a suspension of nanoparticles, has been investigated by a great num- ber of researchers. For the time being, the focus is mainly on the phase-changing heat transfer of nanofluids, while previously they were analyzed for thermal conductivity [4–6] and single-phase flow heat transfer [7,8]. Of ever growing popularity is the applica- tion of nanofluids for the phase-changing heat transfer in heat pipes [9–30], specifically grooved heat pipes [11,12], wicked heat pipes [13,14], sintered heat pipes [15,16], loop heat pipes [17], oscillating heat pipes [18,19] and thermosyphons [20–30]. Xue et al. [20] studied the heat transfer characteristics of a ther- mosyphon charged with a carbon nanotube-water nanofluid. The experimental conditions included a nanoparticle mass concentra- tion of 1.3 wt%, a copper tube (thermosyphon) with an outer diam- eter (o/d) of 20 mm, and a charge ratio of 20%. The authors showed that the carbon nanotube nanofluid thermosyphon had a higher temperature in the evaporation section wall and increasing ther- mal resistance. Carbon nanotube-water nanofluid, in contrast with pure water, generally weakened the overall thermal performance of the thermosyphon. Khandekar et al. studied the overall thermal resistance of a closed two-phase thermosyphon based on water-based Al 2 O 3 (40–47 nm), CuO (8.6–13.5 nm), and nanofluids of laponite clay (disks 25 nm across and 1 nm thick) [21]. The thermosyphon, 720 mm in length and 16 mm in inner diameter, had a nanoparticle mass concentration of 1.0 wt%. The investigation revealed a lower thermal performance of all nanofluids compared to pure water. Specifically, increased wettability and entrapment of nanoparticles in the surface grooves decreased the Peclet number in the evapora- tor, thus deteriorating the overall thermal performance of the thermosyphon. Naphon et al. [22] studied the heat transfer characteristics of a thermosyphon charged with TiO 2 -water and TiO 2 -alcohol nanoflu- ids. The nanoparticle proportion by volume was 0.01%, 0.05%, 0.10% https://doi.org/10.1016/j.ijheatmasstransfer.2018.11.109 0017-9310/Ó 2018 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: oleg.sazhin@urfu.ru (O. Sazhin). International Journal of Heat and Mass Transfer 132 (2019) 557–564 Contents lists available at ScienceDirect International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt