International Journal of Thermal Sciences 153 (2020) 106347 Available online 7 March 2020 1290-0729/© 2020 Elsevier Masson SAS. All rights reserved. Nanofuids stability effect on a thermosyphon thermal performance Karen Cacua a, b, * , Robison Buitrago-Sierra a , Elizabeth Pab� on b , Anderson Gallego a , Camilo Zapata a , Bernardo Herrera a a Instituto Tecnol� ogico Metropolitano, Advanced Materials and Energy Group, Faculty of Engineering, Calle 54A No 30-01, Medellín, Colombia b Universidad Nacional de Colombia, Sede Medellín, Advanced Material Science Group, Carrera 65 #59A-110, Medellín, Colombia A R T I C L E INFO Keywords: Thermosyphon Nanofuids stability Alumina nanofuids Surfactant ABSTRACT Nanofuids stability on rest is important to characterize the nanofuids thermophysical properties before being used on different thermal systems. However, this stability can be modifed during devices operation because of different thermal loads, fuid movements and phase changes. Particularly, in Two Phase Closed-Thermosyphon (TPCT), nanoparticle deposition on the evaporator surface is one of the most important issues that affects their thermal performance, this deposition indicates a lost in the nanofuids stability during operation and also a modifcation in their thermophysical properties. In addition, surfactants are commonly employed to improve nanoparticles dispersion and nanofuids stability and its effect on the TPCT thermal performance has been little studied. Surfactant presence modifes the surface tension of the base fuid, affecting the boiling heat transfer in the TPCT evaporator, which contributes also with the thermal performance variations. In this study, the stability of Al 2 O 3 -water nanofuids was evaluated on rest exploiting different surfactants. After that, nanofuids with high and low stability were used as working fuid in a thermosyphon in order to evaluate the effect of stability and the presence of surfactant on the TPCT thermal performance. Stability after several operation cycles was also studied and results show that the nanofuid with Sodium Dodecylbenzene Sulfonate SDBS at critical micelle concen- tration as surfactant was stable after several operation cycles. On the contrary, nanoparticles were completed sedimented after operation for nanofuids without surfactant and with Cetyl Trimethyl Ammonium Bromide CTAB. Decrease in the thermal resistance was up to 24% for the different nanofuids (stable and unstable) and surfactant solutions in comparison with water, but no effects of nanofuid stability on rest in the fnal thermal performance were not found. Moreover, interesting phenomena during operation were observed such as a dry path with water and high bubble formation with surfactant presence. 1. Introduction Nanofuids were presented more than two decades ago as alternative fuids to improve the thermal effciency of thermal devices. Nanofuids in most cases improve heat transfer, which reduce the volume of heat exchangers, saving energy, consequently water consumption and in- dustrial waste [1]. The enhancements of heat transfer with nanofuids have been explained through mechanisms such as collision between wall and particles, collision between particles, Brownian motion of nano- particles, migration of particles associated with higher volume fraction of nanoparticles and reduction in thickness of boundary layer and par- ticle clustering [2]. However, stability problems have been extensively remarked as the main challenge to resolve to use them at real scale [1–13], specially at elevated temperature [14,15]. Nanoparticles agglomeration and sedimentation produce variations in thermal properties over time, additionally, surfactant presence in some nanofuids cause effects in different heat transfer mechanisms which modifes the thermal behavior of nanofuids used as working fuids. Despite of the number of researches about using nanofuids in heat transfer applications, there is not a standard for preparing nano- fuid. No specifc nanomaterials morphology, concentration or surfac- tants have been established to achieve the highest possible thermal conductivity, insignifcant increase in viscosity and long term stability of the nanofuid [16]. One of the devices where the nanofuids have been used as working are the Two-Phased Closed Thermosyphon (TPCT). This is a special but simple type of gravity-assisted wickless heat pipe, that comprises an evaporator, an adiabatic section and a condenser, where the evaporator * Corresponding author. Instituto Tecnol� ogico Metropolitano, Advanced Materials and Energy Group, Faculty of Engineering, Calle 54A No 30-01, Medellín, Colombia. E-mail address: karencacua@itm.edu.co (K. Cacua). Contents lists available at ScienceDirect International Journal of Thermal Sciences journal homepage: http://www.elsevier.com/locate/ijts https://doi.org/10.1016/j.ijthermalsci.2020.106347 Received 15 July 2019; Received in revised form 24 February 2020; Accepted 25 February 2020