Investigation of the thermal conductivity of propylene glycol nanofluids and comparison with correlations Jagannadha R. Satti, Debendra K. Das ⇑ , Dustin Ray Department of Mechanical Engineering, University of Alaska Fairbanks, P.O. Box 755905, Fairbanks, AK 99775-5905, USA article info Article history: Received 16 July 2016 Received in revised form 10 October 2016 Accepted 29 October 2016 Available online xxxx Keywords: Nanofluids Propylene glycol Particle size Thermal conductivity Volume concentration abstract Experimental study has been carried out to determine the thermal conductivity of five different nanoflu- ids containing aluminum oxide, copper oxide, zinc oxide, silicon dioxide and titanium dioxide nanopar- ticles dispersed in a base fluid of 60:40 (by mass) propylene glycol and water mixture. The effect of particle volumetric concentration up to 6% was studied with temperatures ranging from À30° to 90 °C. Experiments showed an increase in thermal conductivity of nanofluids with increasing concentration and temperature. The thermal conductivity of nanofluids showed a strong dependence on particle volu- metric concentration, particle size, properties of particles and the base fluid and temperature. Several existing theoretical models for thermal conductivity of nanofluids were compared with the experimental data, but they all showed disagreement. From comparisons, the most agreeable model was selected and a curve-fit constant was derived to match the data of propylene glycol nanofluids. This model expresses the thermal conductivity of nanofluids as a function of Brownian motion, Biot number, fluid temperature, particle volumetric concentration, and the properties of the nanoparticles and the base fluid. This model provided good agreement with 600 experimental data points obtained from five different nanofluids with an average absolute deviation of 1.79 percent. Because of the enhanced thermal conductivity with increasing temperature, nanofluids should be more beneficial at higher temperature applications. Ó 2016 Elsevier Ltd. All rights reserved. 1. Introduction Heat exchangers are used in a wide range of applications from food processing to residential heating to cooling automobile engines. While heat exchangers have been steadily improved through better materials and increased surface area, the heat transfer fluid remains unchanged. In cold regions of the world the heat transfer fluid is a glycol-water mixture to avoid freezing. Liquids have inherently low thermal conductivity compared to that of a solid and glycols have lower thermal conductivity than water. The thermal conductivity of a liquid can be increased by dispersing solid particles in that liquid. This concept of dispersing solid parti- cles in fluid has existed for years. Researchers first tried to use micro- and millimeter particles suspended in fluids, but encoun- tered problems such as sedimentation, clogging, erosion and high pumping power. With nanoparticles, several of the problems were resolved. Thus, a new class of heat transfer fluids evolved called nanofluids. Nanofluids are defined as suspensions of solid nanopar- ticles (less than 100 nm) in fluid. Nanofluids can consist of a variety of nanoparticles, such as metals (Al, Cu, Ag, Au), metal oxides (Al 2 O 3 , ZnO, CuO, TiO 2 ) and carbon-based materials (nanotubes, graphite, nanodiamonds). The nanoparticles are traditionally dis- persed in base fluids such as water (W), ethylene glycol (EG), propylene glycol (PG) and oils. As recommended in ASHRAE [1], in cold regions, it is a common practice to use a mixture of glycol and water for heating and cooling of buildings. The addition of ethylene or propylene glycol to water depresses the freezing point of mixture, but also decreases its thermal conductivity. Due to ethylene glycol’s toxicity, it is substituted by propylene glycol in residential buildings, even though propylene glycol has lower ther- mal conductivity than ethylene glycol. This weakness can be over- come by suspending nanoparticles in PG/W mixture to increase the thermal conductivity of the fluid. More than 50% of the fossil fuel consumed in cold regions like Alaska goes to building heating due to the long duration of the winter season. Therefore, PG/W nanofluids are attractive candidates to reduce fossil fuel burning in the cold regions including the sub-arctic and arctic regions. In spite of this promising application, there has been a lack of studies conducted on the thermal conductivity of PG/W based nanofluids. To fulfill this need, we have conducted measurements of the ther- mal conductivity of various nanoparticles (Al 2 O 3 , ZnO, CuO, SiO 2 http://dx.doi.org/10.1016/j.ijheatmasstransfer.2016.10.121 0017-9310/Ó 2016 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: dkdas@alaska.edu (D.K. Das). International Journal of Heat and Mass Transfer xxx (2016) xxx–xxx Contents lists available at ScienceDirect International Journal of Heat and Mass Transfer journal homepage: www.elsevier.com/locate/ijhmt Please cite this article in press as: J.R. Satti et al., Investigation of the thermal conductivity of propylene glycol nanofluids and comparison with correlations, Int. J. Heat Mass Transfer (2016), http://dx.doi.org/10.1016/j.ijheatmasstransfer.2016.10.121