In a paper published in the open-access journal Scientific Reports, researchers prepared carbon-based nanofluids (CbNFs) by using a two-step method and determined their thermal conductivity (λ) and diffusivity (α). These CbNFs were said to significantly enhance the characteristics of the base working fluids.
Study: Thermal conductivity and thermal diffusivity of fullerene-based nanofluids. Image Credit: Angel Soler Gollonet/Shutterstock.com
A highly stable fullerene (C60)-based NFs in the liquid phase was analyzed by using the current hot wire technique to measure λ at a temperature ranging from 254–323 kelvin.
Nanofluids in Heat Transfer and Cooling Technology
In recent years, CbNFs played an important role in the preparation of nanofluids (NFs) due to their excellent thermal properties as related to conventional liquids. NFs are nano-sized particles formed with conventional fluids (oil, water, glycerol, etc.). The molecular bond shared by CbNFs with BLs affects thermal and electrical conductivity and light absorption/emission features. Due to their excellent thermal properties, these materials are used for industrial purposes such as thermal storage and heat pipes.
Previous investigations by the researchers have shown improvement in the λ of NFs depending on the BFs used. The drawback of these NFs is that the value of λ is unstable and changes based on various preparation parameters such as base fluids (BFs), the concentration of NFs, and surfactants. According to the researchers, to attain the stable value of λ, surfactants are used in BFs to avoid cluster formation. One of the reasons for the apparent inconsistencies in numerous articles published on these measurements is the stability of NFs.
As a result, NF-based graphene nanomaterials and carbon nanotubes have been the focus compared to C60-based NFs. The current work focuses on a comprehensive analysis of the thermal properties of BFs.
Preparation of CbNFs
In the present study, the researchers used a two-step method to prepare NFs. The BLs used were 1,2,3,4 tetrahydronaphthalene (C10H12) and 1,2 dichlorobenzenes (C6H4Cl2). Later, dry C60 was added to the BLs with continuous stirring and then ultra-sonicated for successive 30-minute durations to eliminate any agglomerations.
The mixture was then allowed to sit overnight to ensure that the nanoparticles did not precipitate. It is important to note that no fullerene contamination was observed during the entire duration of the experiments. The λ and α values were measured using the transient multi-current hot wire experimental apparatus with a temperature range of 254-323 kelvin at atmospheric pressure. The electrical measurements were analyzed by using Keithley 2400 source meter, and the analysis data were correlated using proposed theoretical models.
Stable CbNFs Showed High Conductivity
Extremely steady-state CbNFs were synthesized without using any surfactant and two BFs, thus achieving high conductivity. The obtained conductivity value in this study is in very good agreement with the only reported value from 27 years ago. In the case of C6H4Cl2, there are no published data, so no comparison can be made. λ decreased with an increase in the concentration of C60 in the NFs. To clarify the abnormal improvement in NFs, experimental results were compared with those predicted by various theoretical thermal conductivity models. However, in this model, λ is overestimated at high concentrations of C60 in nanoparticles because the model doesn't account for λ.
Overall, the Brownian motion of C60 was not observed in any of the studied NFs. There was no discernible relationship between α and temperature for C10H12. When the temperature was raised from 254 to 323 kelvin, there was a minor drop in C6H4Cl2.
To summarize, the researchers of the present study investigated both λ and α of CbNFs with two base liquids - C10H12 and C6H4Cl2 - using transient multi-current hot wire at atmospheric pressure and temperatures ranging 254 to 323 kelvin. The enhancement of λ value using a small quantity of nano-additive is considered abnormal and contentious. It also noticed that the acquired values of 1,2,3,4-tetrahydronaphthalene are extremely close to those found in articles published more than 25 years ago, indicating the accuracy of the proposed hot-wire measurements.
The authors strongly believe that with the addition of C60 to the NFs, conductivity reduces. Also, there is a close agreement between both the achieved and expected results applying various thermal conductivity theoretical models. Further theoretical research is required to precisely expose the regulating mechanism(s) and explain the variation in thermal conductivity and diffusivity in this research work.
Reding, B., Khayet, M. (2022). Thermal conductivity and thermal diffusivity of fullerene-based nanofluids. Sci Rep 12, 9603 https://doi.org/10.1038/s41598-022-14204-y