Media thermal conductivity is important in various heat-transfer processes. Many
conventional fluid conductors suffered low conductivity and environmental issues. Therefore,
research was active in finding out alternative systems, mostly relying on aqueous liquids that are lowcost

and ecofriendly. After the emergence of carbon nanotubes (CNTs), with their many special
structural, electrical and thermal properties, they have been examined for many applications,
including heat-transfer processes. Adding CNTs to water yields CNT aqueous nanofluids that have
been widely investigated as heat-transfer media. The literature shows that CNT addition improves
water thermal conductivity and other water properties, such as viscosity, surface tension, freezing
point and boiling point. The literature also shows that nanofluid thermal conductivity improvement
is affected by CNT type and concentration, in addition to other factors such as surfactant addition. All
these subjects were widely described in literature, focusing on experimental, modelling and theoretical
accounts. Despite the wide literature, there exist inconsistencies and discrepancies between reports
that need to be justified. In addition to technical papers, many reviews were published on various
aspects of the subject including experimental results and mathematical modeling. However, the very
basic question here is as follows: Why does adding CNT to water affect its thermal conductivity? In
spite of the wide published literature, this issue was not targeted in a simple qualitative approach.
This review provides a clear understanding of how CNTs improve thermal conductivity of aqueous
nanofluids. A qualitative model is presented to explain mechanisms behind improvement as
presented in the literature. CNT type effects are discussed with other factors such as aspect ratio,
Reynold number, dispersion quality, composition, temperature and additives. CNT functionalization
is described. Relations to estimate nanofluid thermal conductivity are discussed. The model will help
specialists to tailor CNT aqueous nanofluid characteristics as desired by varying types and
concentrations of CNT and surfactant, and other factors. 

Title

Processes

Publisher

MDPI

Publisher Country

Switzerland

Indexing

Thomson Reuters

Impact Factor

3.5

Publication Type

Both (Printed and Online)

Volume

12

Year

2024

Pages

835