A promising future for nanorefrigerants
A review article published in the IJR highlights the remarkable heat transfer properties of nanorefrigerants, which are leading to significant improvements in the performance of refrigeration systems.
“Nanofluids” is the term coined by S. Choi in 1995 to describe a new class of nanotechnology-based heat transfer fluids with augmented thermal properties, obtained by dispersing and stably suspending nanoparticles with typical dimensions on the order of 10nm. This concept was later extended to refrigerants and gave birth to the concept of nanorefrigerants. The International Journal of Refrigeration recently published an article1 that provides a comprehensive overview of experimental studies on thermophysical and rheological properties, boiling and condensation heat transfer, pressure drop characteristics, aggregation behaviour, and migration and degradation characteristics of various nanorefrigerants. It also reviews evaluations of the impact of nanorefrigerants on the thermodynamic and mechanical performance of refrigeration, air conditioning and heat pump systems.
In the article, the authors conclude that nanorefrigerants have a promising future due to their remarkable heat transfer characteristics in spite of some discrepancies in the experimental results.
The following points can be inferred from this review:
- The presence of nanoparticles has a significant impact on the thermophysical properties of refrigerants.
- The enhanced thermal conductivity of nanorefrigerants is partially responsible for enhanced boiling heat transfer characteristics.
- The heat transfer rate increases with increased nanoparticle concentration and decreased nanoparticle dimension.
- The most important factors that influence the flow boiling and condensation heat transfer in nanorefrigerants are vapour quality, heat flux, mass flux and particle concentration.
- The surfactant type and presence of lubricating oil inhibits the aggregation characteristics of nanoparticles in refrigerants, hence diminishing the agglomeration problems concerning their long term usage.
- The migration and degradation characteristics of nanoparticles have a crucial influence on the heat transfer performance of nanorefrigerants.
- Metal oxide nanoparticles (Al2O3, TiO2, and CuO) and allotropes of carbon (CNTs) can be considered as promising nano-additives to enhance the thermal performance of refrigerants/lubricants.
- The overall performance of HVAC systems is improved by the use of nanorefrigerants.
However, the authors think that many challenges still need to be addressed before nanorefrigerants can be used in diverse applications.
In particular, more experimental and quantum molecular dynamic studies are needed in order to have an atomic level understanding of the underlying mechanisms for thermal transport enhancement. More models are also required to predict the thermophysical properties of nanorefrigerants.
Furthermore, since most existing studies deal with HCFCs and HFCs as base refrigerants, studies of refrigerants with no ozone depletion potential and low global warming potential are needed.
1SANUKRISHNA S. S., PRAKASH M. J. An Overview of Experimental Studies on Nanorefrigerants: Recent Research, Development and Applications . International Journal of Refrigeration. Available in Fridoc.