The contact angle of nanofluids as thermophysical property

Droplet volume and temperature affect contact angle significantly. Phase change heat transfer processes of nanofluids – suspensions containing nanometre-sized particles – can only be modelled properly by understanding these effects. The approach proposed here considers the limiting contact angle of a droplet asymptotically approaching zero-volume as a thermophysical property to characterise nanofluids positioned on a certain substrate under a certain atmosphere. Graphene oxide, alumina, and gold nanoparticles are suspended in deionised water. Within the framework of a round robin test carried out by nine independent European institutes the contact angle of these suspensions on a stainless steel solid substrate is measured with high accuracy. No dependence of nanofluids contact angle of sessile droplets on the measurement device is found. However, the measurements reveal clear differences of the contact angle of nanofluids compared to the pure base fluid. Physically founded correlations of the contact angle in dependency of droplet temperature and volume are obtained from the data. Extrapolating these functions to zero droplet volume delivers the searched limiting contact angle depending only on the temperature. It is for the first time, that this specific parameter, is understood as a characteristic material property of nanofluid droplets placed on a certain substrate under a certain atmosphere. Together with the surface tension it provides the foundation of proper modelling phase change heat transfer processes of nanofluids

Simulation of natural convective boundary layer flow of a nanofluid past a convectively heated inclined plate in the presence of magnetic field

AbstractThis paper deals with the numerical simulation of transient magnetohydrodynamics natural convective boundary layer flow of a nanofluid over an inclined plate. In the modeling of nanofluids, dynamic effects including the Brownian motion and thermophoresis are taken into account. Numerical solutions have been computed via the Galerkin-finite element method. The effects of angle of inclination, buoyancy-ratio parameter, Brownian motion, thermophoresis and magnetic field are taken into account and controlled by non-dimensional parameters. To compute the rate of convergence and error of the computed numerical solution, the double mesh principle is used. Similarity solutions are calculated and presented graphically for non-dimensional velocity, temperature, local rate of heat and mass transfer with pertinent parameters. The modified Nusselt number decreases with increasing inclination angle, buoyancy-ratio parameter, Brownian motion and thermophoresis parameter, whereas it increases with increasing Prandtl number. Validation of the results is achieved with earlier results for forced convective flow and non-magnetic studies. Such problems have several applications in engineering and petroleum industries such as electroplating, chemical processing of heavy metals and solar water heaters. External magnetic fields play an important role in electrical power generation, inclination/acceleration sensors, fine-tuning of the final materials to industrial specification because of their controlling behaviour on the flow characteristics of nanofluids

State of the art of heat transfer of heat pipes and thermosyphons employing nanofluids as working fluid

The paper presents an overview on recent research concerning heat transfer using two-phase devices, namely thermosyphons, heat pipes, and pulsating heat pipes using nanofluids as working fluids in order to emphasise their potential in heat transfer. Relevant experiments performed by the authors are presented in order to support the conclusions, together with some theoretical considerations about the heat transfer using nanofluids, their selection and application, and the effects of nanoparticle deposition due to nanofluid boiling. As shown, there is a significant potential of using nanofluids in two-phase heat transfer devices, and their success in improvement of the thermal performance of these devices depends on the transport of nanoparticles, which can occur if the vapour has the potential to transport the nanoparticles with it

NanoRound: A benchmark study on the numerical approach in nanofluids' simulation

Numerical simulation of nanofluid flows is of maximum importance for a large area of applications, especially in the solar energy technology. Even though a lot of numerical studies are available in the open literature, there is still a large debate in regard to the most appropriate approach when dealing with nanofluids. Plus, a precise simulation of the thermal fluid-solid system encompasses a profound understanding of the fundamental physical phenomena that appear in the nanofluid flow. In this idea, a number of simplifications and approaches are considered, and the aim of this benchmark study is to shed some light in the most suitable CFD approach when dealing with nanofluid flow. Finally, different approaches were considered by different research groups with relevant experience in CFD and are discussed accordingly and in connection with an experimental case that was chosen as a comparison. The current benchmark study was projected to be an ample reference for investigators interested in dealing with the numerical study of the nanofluids’ flow

The contact angle of nanofluids as thermophysical property

Droplet volume and temperature affect contact angle significantly. Phase change heat transfer processes of nanofluids – suspensions containing nanometre-sized particles – can only be modelled properly by understanding these effects. The approach proposed here considers the limiting contact angle of a droplet asymptotically approaching zero-volume as a thermophysical property to characterise nanofluids positioned on a certain substrate under a certain atmosphere. Graphene oxide, alumina, and gold nanoparticles are suspended in deionised water. Within the framework of a round robin test carried out by nine independent European institutes the contact angle of these suspensions on a stainless steel solid substrate is measured with high accuracy. No dependence of nanofluids contact angle of sessile droplets on the measurement device is found. However, the measurements reveal clear differences of the contact angle of nanofluids compared to the pure base fluid. Physically founded correlations of the contact angle in dependency of droplet temperature and volume are obtained from the data. Extrapolating these functions to zero droplet volume delivers the searched limiting contact angle depending only on the temperature. It is for the first time, that this specific parameter, is understood as a characteristic material property of nanofluid droplets placed on a certain substrate under a certain atmosphere. Together with the surface tension it provides the foundation of proper modelling phase change heat transfer processes of nanofluids

The contact angle of nanofluids as thermophysical property

none23siDroplet volume and temperature affect contact angle significantly. Phase change heat transfer processes of nanofluids – suspensions containing nanometre-sized particles – can only be modelled properly by understanding these effects. The approach proposed here considers the limiting contact angle of a droplet asymptotically approaching zero-volume as a thermophysical property to characterise nanofluids positioned on a certain substrate under a certain atmosphere. Graphene oxide, alumina, and gold nanoparticles are suspended in deionised water. Within the framework of a round robin test carried out by nine independent European institutes the contact angle of these suspensions on a stainless steel solid substrate is measured with high accuracy. No dependence of nanofluids contact angle of sessile droplets on the measurement device is found. However, the measurements reveal clear differences of the contact angle of nanofluids compared to the pure base fluid. Physically founded correlations of the contact angle in dependency of droplet temperature and volume are obtained from the data. Extrapolating these functions to zero droplet volume delivers the searched limiting contact angle depending only on the temperature. It is for the first time, that this specific parameter, is understood as a characteristic material property of nanofluid droplets placed on a certain substrate under a certain atmosphere. Together with the surface tension it provides the foundation of proper modelling phase change heat transfer processes of nanofluids.mixedM. Hernaiz,V. Alonso,P. Estellé,Z. Wu,B. Sundén,L. Doretti,S. Mancin,N. Çobanoğlu,Z.H. Karadeniz,N. Garmendia,M. Lasheras-Zubiate,L. Hernández-López,R. Mondragón,R. Martínez-Cuenca,S. Barison,A. Kujawska,A. Turgut,A. Amigo,G. Huminic,A. Huminic,M.-R. Kalus,K.-G. Schroth,M.H. BuschmannHernaiz, M.; Alonso, V.; Estellé, P.; Wu, Z.; Sundén, B.; Doretti, L.; Mancin, S.; Çobanoğlu, N.; Karadeniz, Z. H.; Garmendia, N.; Lasheras-Zubiate, M.; Hernández-López, L.; Mondragón, R.; Martínez-Cuenca, R.; Barison, S.; Kujawska, A.; Turgut, A.; Amigo, A.; Huminic, G.; Huminic, A.; Kalus, M. -R.; Schroth, K. -G.; Buschmann, M. H

Publication IV Characterization of bulk AlN crystals with positron annihilation spectroscopy Characterization of bulk AlN crystals with positron annihilation spectroscopy

a b s t r a c t We have applied positron annihilation spectroscopy to study in-grown vacancy defects in bulk aluminium nitride (AlN) crystals grown by physical vapor transport. We interpret the lowest lifetime value of about 155 ps, measured at low temperatures, to represent the annihilations from the free state of the positron in the crystal lattice. The increased lifetime at high temperatures is an indication of positrons annihilating as trapped at vacancy defects, and a second lifetime component could be separated from the lifetime spectra at temperatures above 400 K. The same lifetime component non-open volume defects acting as shallow hydrogenic traps for positrons were detected, with concentrations of about 10 18 cm À3 or higher. These defects are the dominant negative acceptor defects in these samples