Mechanism of heat transport in nanofluids

We have calculated thermal conductivity of alumina nanofluids (with water and ethylene glycol as base fluids) using temperature as well as concentration-dependent viscosity, η. The temperature profile of η is obtained using Gaussian fit to the available experimental data. In the model, the interfacial resistance effects are incorporated through a phenomenological parameter α. The micro-convection of the alumina nanoparticle (diameter less than 100 nm) is included through Reynolds and Prandtl numbers. The model is further improved by explicitly incorporating the thermal conductivity of the nanolayer surrounding the nanoparticles. Using this improved model, thermal conductivity of copper nanofluid is calculated. These calculations capture the particle concentration-dependent thermal conductivity and predict the dependence of the thermal conductivity on the size of the nanoparticle. These studies are significant to understand the underlying processes of heat transport in nanofluids and are crucial to design superior coolants of next generation.     

Microfluidics analysis of nanoparticle mixing in a microchannel system

The microfluidics of controlled nanodrug delivery to living cells in a representative, partially heated microchannel was analyzed, using a validated computer model. The objective was to achieve uniform nanoparticle exit concentrations at a minimum microchannel length with the aid of simple static mixers, e.g., a multi-baffle-slit or perforated injection micro-mixer. A variable wall heat flux, which influences the local nanofluid properties and carrier-fluid velocities, was added to ensure that mixture delivery to the living cells occurred at the required (body) temperature of 37°C. The results show that both the baffle-slit micro-mixer and the perforated injection micro-mixer aid in decreasing the microchannel length while achieving uniform nanoparticle exit concentrations. The injection micro-mixer not only decreases best the system’s dimension, but also reduces the system power requirement. The baffle-slit micro-mixer also decreases the microchannel length; however, it may add to the power requirement. The imposed wall heat flux aids in enhanced nanoparticle and base-fluid mixing as well.     

Investigation on a spectral splitting photovoltaic/thermal hybrid system based on polypyrrole nanofluid: Preliminary test

The present work developed a spectral splitting hybrid photovoltaic/thermal (PV/T) system based on polypyrrole nanofluid. This hybrid PV/T system can overcome the limitation of temperature in traditional PV/T, and achieve a high-temperature thermal output. In this system, the polypyrrole nanofluid employed in the spectral splitting filter can absorb the solar radiation that can't be efficiently utilized by PV cell unit, and convert it into medium-temperature thermal energy. The principle and methodology of the experimental system design was discussed, and the effect of particle concentration on the performance of system was investigated as well. The present work not only verifies the application potential of polypyrrole nanofluid in spectral splitting PV/T system, but also obtains some important rules on the performance. The results indicate that the temperature of nanofluid and the PV efficiency of cell unit itself increases with the particle concentration, but the thermal efficiency decreases simultaneously. The maximum overall efficiency of this hybrid PV/T system with polypyrrole nanofluid filter was 25.2%, which was 13.3% higher than that without filter. More importantly, the medium-temperature thermal energy can be harvested in such a hybrid system. Furthermore, an optimal particle concentration can probably realize a higher overall efficiency.     

Thermal–hydraulic modeling of water/Al2O3 nanofluid as the coolant in annular fuels for a typical VVER-1000 core

In this paper, a thermal–hydraulic analysis of nanofluid as the coolant is performed in a typical VVER-1000 reactor with internally and externally cooled annular fuel. The fuel assembly for annular case with 8 × 8 arrays is considered for annular pin configuration. The considered nanofluid is a mixture composed of water and particles of Al₂O₃ with various volume percentages. The fuel rod is modeled using a CFD code. To validate the calculated results, the present results of solid fuel with nanofluid and pure water are compared with other studies which have been done with visual FORTRAN language, DRAGON/DONJON code, COBRA-EN code and the mentioned analytical approaches have been validated by comparing with the final safety analysis report (FSAR). The comparison of the calculated results shows that the results are in good agreement with other studies. Thus, the accuracy of the validation is satisfactory. Moreover, the temperature distributions of the fuel, clad and coolant are described for water/Al₂O₃ nanofluid in solid fuel and annular fuel. It is observed that as the concentration of Al₂O₃ nanoparticles increases, due to higher heat transfer coefficient of Al₂O₃ nanofluid, the temperature of the coolant is increased and the central fuel temperature is reduced. Thus, it improves margin from peak fuel temperature to melting. Finally, it is illustrated the use of the annular fuel instead of solid fuel in core of the reactor, security and efficiency of the nuclear power plant will be increased.     

Optimization of thermal performances and pressure drop of rectangular microchannel heat sink using aqueous carbon nanotubes based nanofluid

The present work focuses on analytical optimization of a rectangular microchannel heat sink using aqueous carbon nanotubes based nanofluid as coolant. The particles weight concentration used in this study is 0.01%. The density, the thermal conductivity and the rheological behavior of the nanofluid are experimentally investigated in order to evaluate the thermal resistance and the pumping power in microchannel under laminar flow. An analytical approach of optimization scheme was applied; it is compiled from a systematic thermal resistance model as an analysis method and the elitist non-dominated sorting genetic algorithm (NSGA2). The effects of the temperature, the channel aspect ratio, the channel wall ratio and the use of aqueous carbon nanotubes based nanofluid on the thermal resistance and the pumping power are investigated. The optimized results showed that use of the nanofluid as a working fluid reduce the total thermal resistance and can enhance significantly the thermal performances of the working fluid at high temperatures.     

H2O based different nanofluids with unsteady condition and an external magnetic field on permeable channel heat transfer

This paper investigates numerically the problem of unsteady magnetohydrodynamic nanofluid flow and heat transfer between parallel plates due to the normal motion of the porous upper plate. The governing equations are solved via the fourth-order Runge-Kutta method. Different kind of nanoparticles is examined. The effects of kind of nanoparticle, nanofluid volume fraction, expansion ratio, Hartmann number, Reynolds number on velocity and temperature profiles are considered. Also effect of different types of nanoparticles is examined. Results indicate that velocity decreases with increase of Hartmann number due to effect of Lorentz forces. Rate of heat transfer increase with increase of nanofluid volume fraction, Hartmann number and Reynolds number but it decreases with increase of expansion ratio. Also it can be found that choosing copper as a nanoparticle leads to highest enhancement.     

Spark Parameter Monitoring Feedback System for Preparation of Nanosilver Colloid in EDM

This study developed a system monitoring the electric discharge machine's (EDM) discharge energy and success rate to replace conventional oscilloscope observation. By using logic circuit, the signals are transmitted to the PC monitoring platform in order to display the discharge success times, discharge success rate, and electrode's consumption energy. The advantage of the proposed system is the capability to observe real-time discharges and record the experimental conditions, as well as optimize the discharge parameter settings. The experimental results suggest that, in the preparation of nanosilver colloid, the cost-performance of T_on–T_off at 10–100 µs is the optimal setting. The monitoring system also can take advantage of the discharge success rate to control the energy consumption of the electrode to obtain the standardization of products. The results suggest that, while discharge success rate, electrode's weight loss and wavelength of the absorption peak are considerably accurate, but concentration accuracy is relatively poor. The discharge success rate monitoring system is an innovative method that can help to realize electric discharge processing, optimize product quality, and it may be a powerful processing tool in the future.     

Measurement of thermal conductivity,viscosity and density of ionic liquid [EMIM][DEP]-based nanofluids☆

This article studied experimentally the effect of multi-wall carbon nanotubes(MWCNTs)on the thermo physical properties of ionic liquid-based nanofluids.The nanofluids were composed of ionic liquid,1-ethyl-3-methylimidazolium diethylphosphate [EMIM][DEP],or its aqueous solution[EMIM][DEP](1)+ H_2O(2)and MWCNTs without any surfactants.The thermal conductivity,viscosity and density of the nanofluids were measured experimentally.The effects of the mass fraction of MWCNTs,temperature and the mole fraction of water on the thermo physical properties of nanofluids were studied.Results show that the thermal conductivity of nanofluids increases within the range of 1.3%–9.7% compared to their base liquids,and have a well linear dependence on temperature.The viscosity and density of the nanofluids exhibit a remarkable increase compared with those of the base liquids.Finally,the correlation of the effective thermal conductivity and viscosity of the nanofluids was made using the models in the literatures.     

Role of nanoparticles on nanofluid boiling phenomenon: Nanoparticle deposition

Suspended nanoparticles inside the nanofluids can modify the characteristics of heated surfaces and the physical properties of the base liquids, offering a great opportunity to optimize boiling heat transfer. This paper reviews the mechanisms of nanoparticle deposition and the effects induced by deposited nanoparticles on surface roughness, force balance at the triple line, surface wettability, active nucleation site density, receding and advancing contact angles, boiling heat transfer coefficient and critical heat flux. Both enhancement and deterioration effects on boiling heat transfer coefficient and critical heat flux have been discussed. Most of the existing experimental data confirms the enhancement of critical heat flux using alumina nanofluid, however there is no consistency about its boiling heat transfer coefficient.     

Effect of alumina nanoparticles in the fluid on heat transfer in double-pipe heat exchanger system

This study was performed to investigate the convective heat transfer coefficient of nanofluids made of several alumina nanoparticles and transformer oil which flow through a double pipe heat exchanger system in the laminar flow regime. The nanofluids exhibited a considerable increase of heat transfer coefficients. Although the thermal conductivity of alumina is not high, it is much higher than that of the base fluids. The nanofluids tested displayed good thermal properties. One of the possible reasons for the enhancement on heat transfer of nanofluids can be explained by the high concentration of nanoparticles in the thermal boundary layer at the wall side through the migration of nanoparticles. To understand the enhancement of heat transfer of nanofluid, an experimental correlation was proposed for an alumina-transformer oil nanofluid system.