This work reports an experimental study of convective heat transfer of aqueous alumina nanofluids in a horizontal microchannel under laminar flow condition. The variation of local heat transfer coefficients, in both entrance and developed flow regime, is obtained as a function of axial distance. The heat transfer coefficient of nanofluids is found to be dependent upon not only nanoparticle concentration but also mass flow rate. Different to the behavior in conventional-sized channels, the major heat transfer coefficient enhancement is observed in fully developed region in microchannels. Discussions of the results suggest that the heterogeneous nature of nanoparticle flow should be considered.

The current study investigates the flow of nanofluids in microchannels experimentally and numerically. For this purpose, two microchannels of hydraulic diameters of 211 and 300 μm are used with alumina(45nm)-water nanofluids. The nanofluids with the concentrations 0.25, 0.50 and 1 vol% are used to observe the effect of volume fraction in the present analysis. With regard to the numerical simulation of nanofluids in microchannels, two approaches have been chosen in the current work. First one considers the nanofluids as single phase fluid and applies the mixture rule for evaluating properties for the simulation. The second type of modeling is done using the discrete phase approach which involves Eulerian-Lagrangian considerations. The fluid phase is assumed to be continuous and governed by Navier-Stokes equation. The movement of discrete nanoparticles is determined by the Newton’s second law which takes into account the body force, hydrodynamic forces, the Brownian and thermophoresis forces. The predictions are validated against experimental results obtained for nanofluid flow in a chemically etched silicon wafer channel. It is found that the discrete phase modeling is more accurate with regard to the prediction of nanofluids behavior in microchannels, as compared to the single phase model. The results also show the non-uniformity of nanoparticle distribution across the channel cross-section. This non-uniformity in distribution can be attributed to the shear induced particle migration. This can also be the reason for the difference in pressure drop and heat transfer from the single phase model. The pressure drop with 0.25 and 0.5 vol% of alumina is more or less same as that of water and thus, makes it a suitable cooling liquid. However, an enhancement in heat transfer is observed from the computational predictions.

Controlling the detailed structures of single-walled carbon nanotubes (SWNTs) is imperative for realizing many SWNT applications, and understanding the SWNT growth mechanism is important to improve the growth techniques. In the present study, we performed SWNT growth by a catalytic chemical vapor deposition (CVD) method in wide temperature and pressure ranges, using a high-vacuum CVD chamber. We focused on low CVD gas pressure and low temperature conditions and investigated the SWNT growth mechanism. SWNTs were synthesized by using ethanol gas as the carbon source. As the catalyst, Co and Mo metal nano-particles deposited onto silicon substrates were used. SWNTs were grown via the reaction between ethanol gas and the catalytic metal nano-particles at high temperature. The ethanol gas pressure ranged from 10 −3 Pa to 10 2 Pa, and the CVD temperature ranged from 400 to 900 °C. The yield of SWNTs was assumed to be proportional to the G-band intensity, which was measured by Raman scattering spectroscopy. SWNT samples were observed by scanning electron microscopy and transmission electron microscopy. An optimum CVD temperature existed for each ethanol gas pressure, and decreased with decreasing ethanol gas pressure. Moreover, SWNTs were grown even at 500 °C, when the ethanol gas pressure was low (less than 10 −2 Pa). In this study, the minimum temperature and pressure at which SWNTs could be grown were 450 °C and 10 −3 Pa. At low temperature and low CVD gas pressure, the activity of the catalyst and the growth rate of SWNTs were low, while the catalyst lifetime was long.

An experimental investigation has been carried out to study the heat transfer characteristics of CuO-Base oil nanofluid flow inside horizontal flattened tubes under constant heat flux. The nanofluid flowing inside the tube is heated by an electrical heating coil wrapped around it. The convective heat transfer coefficients of nanofluids are obtained for laminar fully developed flow inside round and flattened tubes. The effect of different parameters such as Reynolds number, flattened tube internal height, nanoparticles concentration and heat flux on heat transfer coefficient is studied. Observations show that the heat transfer performance is improved as the tube profile is flattened. The heat transfer coefficient is increased by using nanofluid instead of base fluid. Also, it can be concluded that decreasing the internal height of the flattened tubes and increasing the concentration of nanoparticles both contribute to the enhancement of heat transfer coefficient.

Semiconductor alloys with epitaxially embedded nanoparticles have been shown to be very promising materials for thermoelectric energy conversion applications. In this work, we report on thermal conductivity characterization of two classes of p-type nanoparticle-in-alloy composite materials: compensated InGaAs semiconductor matrix with randomly distributed ErAs nanoparticles, and GaSb and its alloys with embedded ErSb nanoparticles. The three omega method is used to measure thermal conductivity of all materials. It is shown that thermal conductivity of compensated p-type ErAs:InGaAs is comparable to the n-type ErAs:InGaAs and it reduces with the increase in the erbium concentration. ErSb:GaSb nanocomposites are intrinsically p-type and show a thermal conductivity substantially lower than the pure GaSb compound. By comparing nanostructured samples from alloyed (InGaSb) and unalloyed (GaSb) matrix materials, we show that alloying is complimentary to the role of the nanostructure in reducing thermal conductivity. We also discuss Boltzmann transport modeling that indicates an optimum nanocrystal size, and the prospects for further reductions in the lattice thermal conductivity.

Analyzing trajectories of particles in monolith filters is important for predicting the capture efficiency and improving the design of this class of filters. Modeling and simulations of the particle trajectories are carried out to evaluate the probability of capture by the filter’s front surface and filter channel’s inner wall. Due to Brownian motion and electrostatic attraction, the particles exhibit a random walk and their trajectories deviate from the streamlines of the fluid flow. Particle trajectories are computed by the integration of Newton’s second law, where the electrostatic force, the Brownian motion force resulting from random collisions of the particle with air molecules, and the drag force from the surrounding fluid are all taken into account. A computer simulation for computing the particle trajectories and evaluating the probability of particle capture by the filter was developed. For this model, both flow field and electric field must be provided. The electric charge was assumed to be uniformly distributed along the edge of the channels of the filter and calculated numerically. The flow field is difficult to obtain due to the complex geometry of the model. The commercial CFD package ANSYS CFX [1] is used to compute the flow field. The resulting velocity flow field is then used to evaluate the drag force on the particles. We assume a one-way coupling between the fluid flow and the particle motion. Although there can be over one million uniformly distributed channels per square centimeter in the monolith filter, for simulation purposes, a single unit cell which models only one channel is used. The single unit model effectively describes the behavior of particles outside and inside the channels of monolith filter. The effects of different forces and different particle sizes were analyzed to investigate which factors affect the capture efficiency.

In this study, the effect of adding nanodiamond powder as an additive to engine oil on laminar flow heat transfer enhancement and pressure drop increasing is experimentally investigated. The plain and microfin tubes were used as the test sections and were heated by an electrical coil heater to produce constant heat fluxes. Thermal conductivity and heat capacity of nanofluids were measured for different volume fractions and temperatures. Convection heat transfer coefficients and Nusselt numbers of nanofluids were obtained for different nanoparticle concentrations as well as various Peclet and Reynolds numbers. Experimental results show the enhancement of heat transfer due to the nanoparticles presence. Furthermore, the effect of particle concentration on pressure drop was studied for different heat fluxes. Finally, the performance evaluation of both nanofluid and microfin tube from the point view of heat transfer enhancement and pressure drop increasing is done.

The application of nanofluids in real systems could lead to smaller, more compact heat exchangers and reductions in material cost. However, few studies have been conducted which have carefully measured the thermo-physical properties and thermal performance of these fluids as well as examine the system-level effects of using these fluids in traditional cooling systems. In this work, dilute suspensions of 10 nm aluminum oxide nanoparticles in propanol (0.5 wt%, 1 wt%, and 3 wt%) were investigated. Changes in density, specific heat, and thermal conductivity with particle concentration were measured and found to be linear whereas changes in viscosity were nonlinear and increased sharply with particle loading. Nanofluid heat transfer performance data were generally commensurate with that measured for the baseline. For the 1 wt% concentration, a small but significant enhancement in the heat transfer coefficient was recorded for 1800 < Re > 2800 which is attributed to an earlier transition to turbulent flow. In the case of high particle loading (i.e. 3 wt%), the thermal performance was observed to deteriorate with respect to the baseline case. Discoloration of the fluid was also observed after being cycled at high flow rates and increased temperature.

Nanofluids have been proposed as a promising candidate for advanced heat transfer fluids in a variety of important engineering applications. A consensus is now lacking on if and how the dispersed nanoparticles alter the thermal transport in convective flows. An experimental investigation was conducted to study single-phase forced convection of Al 2 O 3 -water nanofluid in a circular minichannel. The friction factor and convection heat transfer coefficients were measured for nanofluids of various volume concentrations (up to 5%) and were compared to these of the base fluid. The Reynolds number varied from 600 to 4500, covering the laminar, transition and early fully developed turbulent regions. It was found that the nanofluids exhibit pronounced entrance region behaviors in the laminar region. In the transition and turbulent regions, the onset of transition to turbulence is delayed in nanofluids. Further, both the friction factor and convective heat transfer coefficient are below these of water at the same Re in the transition flow. Once fully developed turbulence is established, the difference in the flow and heat transfer of nanofluids and water will diminish. A scaling analysis showed these behaviors may be attributed to the variation in the relative size of nanoparticle with respect to the turbulent microscales. This work suggests that the particle-fluid interaction has a significant impact on the flow physics of nanofluids, especially in the transition and turbulent regions. Consequently, nanofluids should be used in either the laminar flow or fully developed turbulent flow at sufficiently high Re in order to yield enhanced heat transfer performance.

This paper reports the room temperature cross-plane thermal conductivity of pure silica zeolite (PSZ) MEL and MFI thin films. PSZ MEL thin films were prepared by spin coating a suspension of MEL nanoparticles in 1-butanol solution onto silicon substrates followed by calcination and vapor-phase silylation with trimethylchlorosilane. The mass fraction of nanoparticles within the suspension varied from 16 to 55%. This was achieved by varying the crystallization time of the suspension. The thin films consisted of crystalline MEL nanoparticles embedded in a non-uniform and highly porous silica matrix. They featured porosity, relative crystallinity and MEL nanoparticles size ranging from 40 to 59%, 23 to 47% and 55 to 80 nm, respectively. PSZ MFI thin films were made by in-situ crystallization, were b-oriented, fully crystalline and had a 33% porosity. Thermal conductivity of the PSZ thin films was measured at room temperature using the 3 ω method. The cross-plane thermal conductivity of the MEL thin films remained constant around 1.02 ± 0.10 Wm −1 K −1 despite increases in (i) relative crystallinity, (ii) nanoparticle size and (iii) yield as the nanoparticle crystallization time increased. Indeed, the effect of increases in these parameters on the thermal conductivity was compensated by the simultaneous increase in porosity. PSZ MFI thin films were found to have the same thermal conductivity as MEL thin films even though they had smaller porosity. Finally, the average thermal conductivity of the PSZ films was three to five times larger than that reported for amorphous sol-gel mesoporous silica thin films with similar porosity and dielectric constant.

In this study, laminar and turbulent forced convection flows of a nanofluid consisting of water and Al 2 O 3 in a horizontal smooth tube with constant wall temperature are investigated numerically. Studies that are related to the subject in the literature are reviewed. The determination of the nanofluid properties is calculated by means of the correlations of Palm et al. Two-dimensional elliptical governing equations are used to study the hydrodynamics and thermal behaviors of the nanofluid flow. A single-phase model is employed with either constant or temperature dependent properties. The investigation is performed for a constant particle size. The velocity and temperature vectors are presented in the entrance and fully developed region. Effects of nanoparticles concentration and Reynolds number on shear stress and pressure drop are presented. The Nusselt numbers and heat transfer coefficients of nanofluids are obtained for different nanoparticle concentrations. Numerical results show the heat transfer enhancement due to presence of the nanoparticles in the fluid. Heat transfer coefficient increases with increasing the particle volume concentration and also increasing wall shear stress values.

Applications of nanoparticles in the bio-medical field like nano-medicine, molecular imaging probes, fluorescence marker, gene carriers, are developing quickly owing to the unique characteristics of nanoparticles. Among these applications, the interaction of nano-particles with the living cells is of critical importance. The complex chemical properties and biological activities of the particles bring about undesirable cytotoxic potentials and special cell internalization. According to previous studies, the cell uptake kinetics of nanoparticles mainly depend on the concentration difference between extracellular and intracellular nanoparticles, the surface electric charge of the nanoparticle, and the active transport of the cell. For example, Ginzburg’s thermodynamic simulation and Park’s three-dimensional phase-field model quantitatively explain the transitions in membrane morphology after exposure to nanoparticles with different surface charge, respectively. However, recent studies have shown that the gold nanoparticles coated with hydrophilic and hydrophobic functional groups with the same concentration but in different orders, completely exhibit quite different intrusion ability at 4°C when the active transport of the cell is greatly inhibited. The results suggest that the interaction energy of nanoparticles and cell membranes may be another driving force for the nanopartcles’ mass transfer across the cell membrane. Thus, in this paper, the interaction energy of the differently coated nanoparticles (P) with cell membrane (M) in water (W) is studied theoretically and results are used to explain the former experimental findings.

Magnetic nanoparticles have been used in clinical and animal studies to generate localized heating for tumor treatments when the particles are subjected to an external alternating magnetic field. One approach to deliver the nanoparticles is via directly injecting the nanoparticles in the extracellular space of the tumor. Its advantage is that multiple-site injections can be exploited to cover the entire target region in the case of an irregularly shaped tumor. Currently since most tissue is opaque, the detailed information of the nanoparticle spreading after the injection can not be visualized directly and it is often quantified by indirect methods such as temperature measurements to inversely determine the distribution. In this study, we use a high-resolution microCT imaging system to investigate the nanoparticle concentration distribution in a tissue equivalent agarose gel. The preliminary results are promising to obtain a 3-D distribution of the ferrofluid in tissue. The local density variations induced by the nanoparticles in the vicinity of the injection site can be detected and analyzed by the microCT system. Experiments are performed to study how the injection amount, gel concentration, and nanoparticle concentration in the ferrofluid affect nanoparticle spreading in the gel. The obtained quantified information is important for future studies of temperature elevations in opaque tumor to design optimized treatment protocols.

This work reported an experimental study of flow boiling of pure water and aqueous alumina nanofluids in a 0.5 millimetre narrow channel. An open-ended stainless steel microchannel system is constructed where liquids are supplied by a syringe pump and heated by a variable DC power source, with synchronized measurement of the surface temperature distribution along the channel and pressure fluctuations associated with bubbles. The effect of nanoparticle concentration, inlet subcooling and mass flow rate and on critical heat flux in a microchannel is investigated. A modest increase in CHF is observed for nanofluids, being higher at higher particle concentrations and higher inlet subcoolings. Regular fluctuations in temperature and pressure signals suggest a cyclic boiling events occurring in the narrow channel; the large pressure fluctuation, coupling with the surface temperature and inlet temperature fluctuations, would affect flow boiling mechanisms in narrow channels.

Conversion of solar energy into heat is a common way to explore the clean and renewable energy source. Generally, a spectrally selective coating should be deposited to the solar-absorbing surface of the solar collector in order to reduce thermal loss emitted away from the surface in the form of thermal radiation. Nanofluids can be filled into a flat plate solar collector to absorb the solar radiation directly. The absorbance of the nanofluid in the solar spectrum may be tuned by various nanoparticles of different materials, sizes and mass fractions. In this paper, a theoretical investigation is carried out to model absorption, scattering and extinction of solar radiation within the nanofluid and to model the heat transfer within the liquid film in the direct solar absorption collector. The predicted extinction coefficient is much lower than the measured value in visible spectrum for the SiO 2 -water nanofluid. The calculated average temperature increase in the outlet of the collector using the measured radiative properties agrees with the measured value while that applying the predicted radiative properties is significantly lower. Nevertheless, the agreement in the predicted and the measured extinction coefficients of the TiO 2 -water nanofluid is reasonably good within the solar spectrum. The calculated average temperature increase is close to the measurement value. This work is helpful to understand radiative transport and heat transfer of direct solar absorption collectors using nanofluids.

Solar collectors with surface receivers have low overall energy conversion efficiencies due to large emissive losses at high temperatures. Alternatively, volumetric receivers promise increased performance because solar radiation can be transferred into a fluid medium, which subsequently reduces the concentrated heat at the surface. Nanofluid-based direct solar receivers, where nanoparticles in a liquid medium can scatter and absorb solar radiation, have recently received interest to efficiently distribute and store the thermal energy. We present a combined modeling and experimental study to investigate the efficiency of fluid-based solar receivers seeded with carbon-coated absorbing nanoparticles. A simple one-dimensional transient heat transfer model was developed to compare idealized surface receivers to idealized volumetric receivers. In the limit of idealized behavior, volumetric receivers were shown to be more efficient than selective-surfaces for high concentration levels ( C > 100) and/or tall receiver designs ( H > 10 cm). Furthermore, volumetric receivers allowed the working range of the fluid to be extended to higher temperatures while maintaining relatively high receiver efficiency. We also designed and custom built a cylindrical receiver to experimentally demonstrate the concept of nanofluid-based receivers and validate the modeling efforts. We explored the effect of particle characteristics such as size, distribution, and material properties, as well as collector parameters such as absorbing depth and level of solar concentration. The work offers design guidelines for the development of efficient volumetric receivers for future solar thermal energy conversion systems.

The aim of this study is to investigate the enhancement of thermal properties of various high temperature nanofluids for solar thermal energy storage application. In concentrating solar power (CSP) systems, the thermo-physical properties of the heat transfer fluids (HTF) and the thermal energy storage (TES) materials are key to enhancing the overall system efficiency. Molten salts, such as alkali nitrates, alkali carbonates, or eutectics are considered as alternatives to conventional HTF to extend the capabilities of CSP. However, there is limited usage of molten salt eutectics as the HTF material, since the heat capacity of the molten salts are lower than that of conventional HTF. Nanofluid is a mixture of a solvent and nanoparticles. Well dispersed nanoparticles can be used to enhance thermo-physical properties of HTF. In this study, silica (SiO 2 ) and alumina (Al 2 O 3 ) nanoparticles as well as carbon nanotubes (CNT) were dispersed into a molten salt and a commercially available HTF. The specific heat capacity of the nanofluids were measured and applicability of such nanofluid materials for solar thermal storage applications were explored. Measurements performed using the carbonate eutectics and commercial HTF that are doped with inorganic and organic nano-particles show specific heat capacity enhancements exceeding 5–20% at concentrations of 0.05% to 2.0% by weight. Dimensional analyses and computer simulations were performed to predict the enhancement of thermal properties of the nanofluids. The computational studies were performed using Molecular Dynamics (MD) simulations.

The presented work provides new perspectives on the origin of dropwise condensation of steam on ion implanted metallic surfaces. For the experimental study metallic samples made of different materials implanted with various ion species were used. Condensation experiments with saturated steam and surface analysis via contact angle and surface free energy measurements, atomic force microscopy and scanning electron microscopy were performed. It could be shown that available explanation approaches for the appearance of dropwise condensation based on a reduction of the metal surface free energy by ion implantation have to be discarded. Our results rather suggest a nucleation mechanism proceeding on surfaces featuring nano-scale surface roughness and chemical inhomogeneity. These characteristics seem to be induced by nano-particles precipitated due to supersaturation of the metal substrate with the doping elements. On unimplanted high-alloyed materials like stainless steel, dropwise condensation was found to develop spontaneously because of strong oxidation effects. Obviously the latter naturally produce surface modifications which are similar to those obtained artificially by ion implantation.