A Review of the Advances and Challenges in Measuring the Thermal Conductivity of Nanofluids

Abstract

Fluids containing colloidal suspensions of nanometer-sized particles (nanofluids) have been extensively investigated in recent decades with promising results. Driven by the increase in the thermal conductivity of these new thermofluids, this topic has been growing in order to improve the thermal capacity of a series of applications in the thermal area. However, when it comes to measure nanofluids (NFs) thermal conductivity, experimental results need to be carefully analyzed. Hence, in this review work, the main traditional and new techniques used to measure thermal conductivity of the NFs are presented and analyzed. Moreover, the fundamental parameters that affect the measurements of the NFs' thermal conductivity, such as, temperature, concentration, preparation of NFs, characteristics and thermophysical properties of nanoparticles, are also discussed. In this review, the experimental methods are compared with the theoretical methods and, also, a comparison between experimental methods are made. Finally, it is expected that this review will provide a guidance to researchers interested in implementing and developing the most appropriate experimental protocol, with the aim of increasing the level of reliability of the equipment used to measure the NFs thermal conductivity.

Keywords: equipment for measuring the conductivity; nanofluids; nanoparticles; thermal conductivity; thermophysical properties.

Figure 1

Number of scientific articles presented in the ScienceDirect database from the year 2005 up to 2021.

Figure 2

Schematic diagram of the main techniques to measure the thermal conductivity of NFs.

Figure 3

Schematic representation of a TWH installation with a Wheatstone bridge (adapted from Roder [35]).

Figure 4

THW probe of tantalum wire with short and long wire placed on top of each other (adapted from Antoniadis et al. [39]).

Figure 5

Illustrative scheme of the steady-state parallel-plate method.

Figure 6

Laser flash measurement principle: an energy/laser pulse (red) heats the sample (gray) containing the nanofluid on the bottom side and a detector detects the temperature signal versus time on the top side (blue).

Figure 7

Schematic representation of the 3ω method.

Figure 8

Illustration of the transient plane source (TPS) technique: red arrows represent the direction of the heat flux (adapted from Lin et al. [80]).

Figure 9

Diagram of the thermal conductivity measurement system with temperature oscillation technique (adapted from Bhattacharya et al. (2004) [82]).

Figure 10

Illustration of the coaxial cylinder method used to measure the thermal conductivity of a liquid (adapted from Schiefelbein et al. (1998) [25]).

Figure 11

(a) TCi thermal conductivity analyzer (foreground), Tenney Jr. Thermal Chamber (background) source and (b) the MTPS sensor. Diameter of green surface is 17 mm. (adapted with permission from Harris et al. (2014) [88]).

Figure 12

(a) Schematic and equivalent thermal circuit of the heater and two semi-infinite mediums of the nanofluid and the substrate; Microfabricated heater device for measuring thermal conductivity of nanofluid and (b) cross-section of the heater on 2 mm thick quartz substrate (not to scale) (adapted from Oh et al. [93]).

Figure 13

A scheme of the experimental apparatus for the measurement of the thermal conductivity and heat capacity of NFs, where WL and RL represent the working and references line, respectively (adapted from López-Bueno et al. [24]).

Figure 14

Different features and properties of NPs and base fluid that influence thermal conductivity measurements.

Figure 15

Scheme adapted of diagram proposed by (Eastman et al. [100]) to explain excess thermal conductivity enhancement in NFs. Where κ is the thermal conductivity as a function of the packing fraction of the cluster $\mathsf{\Phi }$ (ratio of the volume of the solid particles in the cluster to the total effective volume of the cluster).

Figure 16

Thermal conductivity data obtained by Buonomo et al. [113] for the Al₂O₃-water using the flash and hot disk techniques.

Figure 17

Experimentally measured thermal conductivity of Al₂O₃ NFs in EG/Water in function of nanoparticle concentration compared to predictions of H-C model for corresponding particle shapes (adapted from Timofeeva et al. [123]).