Abstract:
Advancements in technology related to energy systems, such as heat exchangers, electronics,
and batteries, are associated with the generation of high heat fluxes which requires appropriate
thermal management. Presently, conventional thermal fluids have found limited application owing to
low thermal conductivity (TC). The need for more efficient fluids has become apparent leading to the
development of nanofluids as advanced thermal fluids. Nanofluid synthesis by suspending nano-size
materials into conventional thermal fluids to improve thermal properties has been extensively studied.
TC is a pivotal property to the utilization of nanofluids in various applications as it is strongly related
to improved efficiency and thermal performance. Numerous studies have been conducted on the
TC of nanofluids using diverse nanoparticles and base fluids. Different values of TC enhancement
have been recorded which depend on various factors, such as nanoparticles size, shape and type,
base fluid and surfactant type, temperature, etc. This paper attempts to conduct a state-of-the-art
review of the TC enhancement of metal oxide nanofluids owing to the wide attention, chemical
stability, low density, and oxidation resistance associated with this type of nanofluid. TC and TC
enhancements of metal oxide nanofluids are presented and discussed herein. The influence of several
parameters (temperature, volume/weight concentration, nano-size, sonication, shape, surfactants,
base fluids, alignment, TC measurement techniques, and mixing ratio (for hybrid nanofluid)) on the
TC of metal oil nanofluids have been reviewed. This paper serves as a frontier in the review of the
effect of alignment, electric field, and green nanofluid on TC. In addition, the mechanisms/physics
behind TC enhancement and techniques for TC measurement have been discussed. Results show
that the TC enhancement of metal oxide nanofluids is affected by the aforementioned parameters
with temperature and nanoparticle concentration contributing the most. TC of these nanofluids
is observed to be actively enhanced using electric and magnetic fields with the former requiring
more intense studies. The formulation of green nanofluids and base fluids as sustainable and future
thermal fluids is recommended.
