Applied Thermal Engineering 182 (2021) 116086 Available online 21 September 2020 1359-4311/© 2020 Elsevier Ltd. All rights reserved. Assessment of TiO 2 water-based nanofuids with two distinct morphologies in a U type evacuated tube solar collector Seyed Mohammad Sadegh Hosseini a , Mohammad Shafey Dehaj b a Department of Chemical Engineering, Vali-e-Asr University of Rafsanjan, Rafsanjan, Iran b Department of Mechanical Engineering, Vali-e-Asr University of Rafsanjan, Rafsanjan, Iran A R T I C L E INFO Keywords: TiO 2 nanowires TiO 2 nanoparticles U type evacuated tube solar collector Thermal performance ABSTRACT In the present study two samples of water based nanofuids having suspended TiO 2 species with spherical and wire-like geometry (TiO 2 NPs-nanofuid, TiO 2 NWs-nanofuid) were prepared and their thermophysical prop- erties were experimentally determined. The results showed that adding TiO 2 nanowires and TiO 2 nanoparticles to the base fuid could respectively improve the thermal conductivity up to 12.4% and 5.4%. TiO 2 NWs-nanofuid because of the particular confguration of the wire-like suspended species displayed better thermal properties. A thermal system included a U type evacuated tube solar collector, a circulating refrigerator bath, digital thermometers and a peristaltic pump was erected, in which, the working fuid fowed through the collector under a laminar regime in a closed loop. Several fow rates of 0.1, 0.2, 0.3, 0.4, and 0.5 L/min were considered for the collector testing. The collector performance results indicated that both the absorbed energy parameter F R (τ⍺) and the removal energy parameter F R U L are signifcantly infuenced using the nanofuids. Increasing fow rate makes the nano- fuids more effcient due to the heat transfer coeffcient enhancement. At 0.5 L/min, TiO 2 NWs-nanofuid and TiO 2 NPs-nanofuid could respectively enhance the maximum collector effciency (η 0 ) up to 21.1% and 12.2%. Thermal-hydraulic results proved that TiO 2 NWs-nanofuid, despite higher pressure drop due to its higher viscosity, was a better choice as compared to TiO 2 NPs-nanofuid for working in the solar collector. In this su- perior fuid the enhancement in pressure drop as a penalty was not notable as compared to improvement in the heat transfer coeffcient. 1. Introduction Nanofuids are homogenous and stable colloidal dispersions created generally by adding small particles, with at least one nanometer dimension, of metal, metal oxide or carbon nanomaterials with different morphologies into a base fuid [1]. It can be seen that in the past two decades the number of publications by retrieving the key word “nano- fuids” in the title (from the database of Web of Science) has grown up with an exponential rate [2]. The main topics of nanofuids in the literature are focused on the preparation and stability; nanoparticle type; base fuid type; thermophysical properties; heat transfer and fow characteristics; and thermal application [2]. Nanofuids are signifcantly used instead of conventional working fuids in cooling and heating op- erations such as heat exchangers, solar thermal collectors, nuclear re- actors, electronic systems, and boiling and condensation for increasing convective heat transfer rate and enhancing their thermal and economical performances [3,4]. This prominent characteristic of the nanofuids is due to the several factors, including higher specifc surface area comparing to the base fuid, Brownian motion of nanoparticles which provides more interaction in the fuid, interfacial layers which play the role of a thermal bridge between liquid bulk and solid particles for enhancing heat transfer, and particle clustering and shape of the particle [5]. In addition to experimental studies, recent advances in modeling and simulation of nanofuid fows lead to the fnding of the nanoparticles motions in a liquid and estimation of nanofuid thermo- physical properties [6]. Over the past few years, in the pursuit of improving the effciency and also due to the widespread use of the thermal solar collectors, improvement of their saving energy has been analyzed from different points of view, as well by using different working nanofuids [7]. It can be concluded that, compared to the common base fuids, the nanofuids could improve the heat transfer rate and collector effciency whereas nanofuids concentration and nanofuids volume fow rate are the main E-mail address: chehosseini@gmail.com (S.M.S. Hosseini). Contents lists available at ScienceDirect Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng https://doi.org/10.1016/j.applthermaleng.2020.116086 Received 11 January 2020; Received in revised form 20 June 2020; Accepted 15 September 2020