Nanofluids for Boiling Heat Transfer Applications: from a Hopeless Task to New Research Opportunities

The enhancement of critical heat flux and heat transfer coefficient during nucleate boiling is crucial to the thermal management communities for the potential widespread impact of scientific breakthroughs on advanced cooling technologies and other applications. Since the early 1990s, nanofluids (i.e., functional liquid suspension containing particles smaller than 100 nm) have been proposed as a technique for the enhancement of nucleate boiling.

Unfortunately, years of published results on this topic have only led to contradictory results regarding the enhancement or deterioration of performance during nanofluid boiling. To date, nanofluids have not led to any practical or implementable boiling enhancements solutions owing to the primary drawback that the nanoparticles tend to settle out of suspension over time.

Let’s change the viewpoint for a moment: while nanoparticle deposition during boiling has stopped any possible application of nanofluids, on the same time, it has opened new research opportunities introducing a new concept of surface functionalization using a low-cost, easy-to-implement deposition technique.

On the basis of the selected nanoparticle and base fluid combination, this new deposition technique might be applied to realize controlled and stable coatings for many different applications not limited to heat transfer ones, such as anti-fouling and self-cleaning, anti-bacterial, anti-icing, anti-corrosion, etc. to be implemented in many different fields: advanced heat transfer, biomedical, oil and gas industry, aeronautical, etc.

This seminar will present the influence of nanofluids on boiling heat transfer and related research activities on surface functionalization via nanoparticle deposition through nanofluid boiling tests carried out at the Nano Heat Transfer Lab (NHT-Lab) of the University of Padova.

Complementary talk:

Flow and evaporation of liquids from nanofiber-coated substrates
Michael Heinz, SFB 1194, Project A04

The dynamic wetting and dewetting of surfaces by liquids are fundamental phenomena in many technical processes. Concerning applications like spray cooling, ink jet printing or drug delivery in medical or cosmetic sectors, porous wall structures are relevant.

Investigations showed that (nano-) structured coatings essentially affect the heat and mass transfer in wetting processes – compared to smooth surfaces liquids can imbibe into porous structures. However, the underlying phenomena are, to a large extend, unknown.

In this presentation, an overview on ongoing efforts to experimentally investigate the drop impact and evaporation from nanofiber coated substrates will be given. The general approach of future experiments and the progress in the preparation of nanofiber mats will be reviewed.

The diameter of electrospun nanofibers can vary between 100 nm and several micrometers depending on the polymer-solvent-system and several process parameters of the electrospinning procedure. Apart from diameter variations it is possible to manipulate the fiber orientation and surface porosity of the fibers.