Cooling by impinging droplets has been the subject of several studies for decades and still is, and, in the last few years, the potential heat transfer enhancement obtained thanks to nanofluids’ use has received increased interest. Indeed, the use of high thermal conductivity fluids, such as nanofluids’, is considered today as a possible way to strongly enhance this heat transfer process. This enhancement is related to several physical mechanisms. It is linked to the nanofluids’ rheology, their degree of stabilization, and how the presence of the nanoparticles impact the droplet/substrate dynamics. Although there are several articles on droplet impact dynamics and nanofluid heat transfer enhancement, there is a lack of review studies that couple these two topics. As such, this review aims to provide an analysis of the available literature dedicated to the dynamics between a single nanofluid droplet and a hot substrate, and the consequent enhancement or reduction of heat transfer. Finally, we also conduct a review of the available publications on nanofluids spray cooling. Although using nanofluids in spray cooling may seem a promising option, the few works present in the literature are not yet conclusive, and the mechanism of enhancement needs to be clarified.

The measurement of particle size distribution (PSD) in nanofluids presents a challenge, especially when it requires to be conducted in-situ and real-time.Our work aims to enhance the capabilities of Light Extinction Spectroscopy (LES) technique for concentration and volumetric PSD (vPSD) determination of nanofluids. To reach this goal, numerical simulations are performed to verify robustness of LES data inversion algorithm and to identify the most relevant uncertainty sources. Via sensitivity analyses, we inspect the LES sensitivity 1) to complex refractive index spectra of the particle, 2) to system noise level.Since inherited noise has adverse effects on inversion stability, we characterize the system noise profile and embed it into numerical simulations. Before processing LES data, we check optical thickness and signal-to-noise ratio (SNR) profiles to adjust sample concentration. Besides, we limit wavelength spectrum to shorter wavelengths (202-484 nm), though it can be extended to UV-NIR.The experiments are carried out on well-dispersed water based nanofluids containing Polystyrene particles with median diameters of 120 and 300 nm. The results are compared with supplier's data, 3D-DLS measurements, and SEM images. After post-processing, we realize that LES results can still be greatly improved with an optimization algorithm on particle complex refractive indices.

In situ temperature measurement for closed systems is always challenging due to various physical limitations. With the aim to visualize the heat transfer phenomena in situ continuously with a longer time scale, octylamine substituted CsPbBr3 perovskite quantum dots (QDs) are synthesized, leading to its thermo-sensitity being doubled. An optical-based 2D temperature sensor based on these ligand-modified QDs is developed, targeting microreactors and other circumstances where conventional thermal sensing protocols are not applicable. The developed method has the advantages of repeatability, long lifetime, low cost, fast response time, and broader applicability since it does not require IR transparent substrates. On top of these advantages, the proposed method also provides satisfactory spatial and temporal resolution, which is typically limited by the camera’s specification rather than the materials.

No abstract