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### Guest Editorial

J. Heat Transfer. 2008;130(4):040301-040301-1. doi:10.1115/1.2818789.
FREE TO VIEW

It is with great pleasure that we present this special issue of the Journal of Heat Transfer, dedicated to Energy Nanotechnology. This focus area is the natural convergence of two subjects of tremendous and lasting importance. The ever-growing global demand for energy in both developing and industrialized nations is widely recognized as one of modern society’s greatest challenges. To have a positive worldwide impact, new energy technologies must not only have the potential to be large scale and cost effective but must also address concerns about national security and environmental issues such as global climate change. As we strive to improve all aspects of the energy cycle—from primary production and extraction to storage, transmission, utilization, and mitigation—our attention naturally turns to nanotechnology because its additional degrees of freedom offer great potential for innovative breakthroughs.

Topics: Nanotechnology
Commentary by Dr. Valentin Fuster

### Research Papers: Micro/Nanoscale Heat Transfer

J. Heat Transfer. 2008;130(4):042401-042401-6. doi:10.1115/1.2787020.

The heat transfer characteristics of silica $(SiO2)$ nanofluids at $0.5vol%$ concentration and particle sizes of $10nm$ and $20nm$ in pool boiling with a suspended heating Nichrome wire have been analyzed. The influence of acidity on heat transfer has been studied. The $pH$ value of the nanosuspensions is important from the point of view that it determines the stability of the particles and their mutual interactions toward the suspended heated wire. When there is no particle deposition on the wire, the nanofluid increases critical heat flux (CHF) by about 50% within the uncertainty limits regardless of $pH$ of the base fluid or particle size. The extent of oxidation on the wire impacts CHF, and is influenced by the chemical composition of nanofluids in buffer solutions. The boiling regime is further extended to higher heat flux when there is agglomeration on the wire. This agglomeration allows high heat transfer through interagglomerate pores, resulting in a nearly threefold increase in burnout heat flux. This deposition occurs for the charged $10nm$ silica particle. The chemical composition, oxidation, and packing of the particles within the deposition on the wire are shown to be the reasons for the extension of the boiling regime and the net enhancement of the burnout heat flux.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2008;130(4):042402-042402-11. doi:10.1115/1.2818751.

The combustion synthesis of carbon nanotubes is reviewed, examining their formation and control in diffusion flames. Much of the initial work in this area employed coflow diffusion flames and provided insight into carbon nanotube (CNT) formation. However, the inherent multidimensional nature of such coflow flames made the critical spatial location difficult to maintain. Among this early work, our UIC group demonstrated the superiority of the opposed flow diffusion flame configuration due to its uniform radial distribution that reduces such flow to a one-dimensional process. While a summary of the early coflow flame work is presented, the use of the opposed flow diffusion flame will be the focus of this review. The production of carbon nanostructures in the absence of a catalyst is discussed together with the range of morphology of nanostructures generated when a catalyst is employed. The important aspect of control of the growth and orientation of CNTs and generation of CNT arrays through the use of electric fields is examined as is the use of anodized aluminum oxide templates. Fruitful areas for further research such as the functional coating of CNTs with polymers and the application of these opposed flow flames to synthesis of other materials are discussed.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2008;130(4):042403-042403-11. doi:10.1115/1.2818760.

Microscale truss architectures provide high mechanical strength, light weight, and open porosity in polymer sheets. Liquid evaporation and transport of the resulting vapor through truss voids cool nearby surfaces. Thus, microtruss materials can simultaneously prevent mechanical and thermal damage. Assessment of promise requires quantitative understanding of vapor transport through microtruss pores for realistic heat loads and latent heat carriers. Pore size may complicate exegesis owing to vapor rarefaction or surface interactions. This paper quantifies the nonboiling evaporative cooling of a flat surface by water vapor transport through two different hydrophobic polymer membranes, $112–119μm$ (or $113–123μm$) thick, with microtruss-like architectures, i.e., straight-through pores of average diameter of $1.0–1.4μm$ (or $12.6–14.2μm$) and average overall porosity of 7.6% (or 9.9%). The surface, heated at $1350±20Wt∕m2$ to mimic human thermal load in a desert (daytime solar plus metabolic), was the bottom of a $3.1cm$ inside diameter, $24.9cm3$ cylindrical aluminum chamber capped by the membrane. Steady-state rates of water vapor transport through the membrane pores to ambient were measured by continuously weighing the evaporation chamber. The water vapor concentration at the membrane exit was maintained near zero by a cross flow of dry nitrogen $(velocity=2.8m∕s)$. Each truss material enabled $13–14°C$ evaporative cooling of the surface, roughly 40% of the maximum evaporative cooling attainable, i.e., with an uncapped chamber. Intrinsic pore diffusion coefficients for dilute water vapor $(<10.4mole%)$ in air ($P$ total $∼112,000Pa$) were deduced from the measured vapor fluxes by mathematically disaggregating the substantial mass transfer resistances of the boundary layers $(∼50%)$ and correcting for radial variations in upstream water vapor concentration. The diffusion coefficients for the $1.0–1.4μm$ pores (Knudsen number $∼0.1$) agree with literature for the water vapor-air mutual diffusion coefficient to within $±20%$, but for the nominally $12.6–14.2μm$ pores (Kn $∼0.01$), the diffusion coefficient values were smaller, possibly because considerable pore area resides in noncircular, i.e., narrow, wedge-shaped cross sections that impede diffusion owing to enhanced rarefaction. The present data, parameters, and mathematical models support the design and analysis of microtruss materials for thermal or simultaneous thermal-and-mechanical protection of microelectromechanical systems, nanoscale components, humans, and other macrosystems.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2008;130(4):042404-042404-8. doi:10.1115/1.2818763.

This paper examines the effects of rarefaction, dissipation, curvature, and accommodation coefficients on flow and heat transfer characteristics in rotating microdevices. The problem is modeled as a cylindrical Couette flow with a rotating shaft and stationary housing. The housing is maintained at uniform temperature while the rotating shaft is insulated. Thus, heat transfer is due to viscous dissipation only. An analytic solution is obtained for the temperature distribution in the gas filled concentric clearance between the rotating shaft and its stationary housing. The solution is valid in the slip flow and temperature jump domain defined by the Knudsen number range of $0.001. The important effect of the momentum accommodation coefficient on velocity reversal and its impact on heat transfer is determined. The Nusselt number was found to depend on four parameters: the momentum accommodation coefficient of the stationary surface $σuo$, Knudsen number Kn, ratio of housing to shaft radius $ro∕ri$, and the dimensionless group $[γ∕(γ+1)](2σto−1)∕(σtoPr)$. Results indicate that curvature, Knudsen number, and the accommodation coefficients have significant effects on temperature distribution, heat transfer, and Nusselt number.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2008;130(4):042405-042405-8. doi:10.1115/1.2818764.

To improve the thermal performance of phase change materials (PCMs), graphite nanofibers were embedded into a paraffin PCM. The thermal effects of graphite fiber loading levels $(0–5wt%)$ and graphite fiber type (herringbone, ribbon, or platelet) during the melting process were examined for a $131cm3$ volume system with power loads between $3W$ and $7W$$(1160–2710W∕m2)$. It was found that the maximum system temperature decreased as graphite fiber loading levels increased and that the results were fiber-structure dependent.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2008;130(4):042406-042406-13. doi:10.1115/1.2818768.

Nanofluids, i.e., liquids containing nanometer sized metallic or nonmetallic solid particles, show an increase in thermal conductivity compared to that of the pure liquid. In this paper, a simple model for predicting thermal conductivity of nanofluids based on Brownian motion of nanoparticles in the liquid is developed. A general expression for the effective thermal conductivity of a colloidal suspension is derived by using ensemble averaging under the assumption of small departures from equilibrium and the presence of pairwise additive interaction potential between the nanoparticles. The resulting expression for thermal conductivity enhancement is applied to the nanofluids with a polar base fluid, such as water or ethylene glycol, by assuming an effective double layer repulsive potential between pairs of nanoparticles. It is shown that the model predicts a particle size and temperature dependent thermal conductivity enhancement. The results of the calculation are compared with the experimental data for various nanofluids containing metallic and nonmetallic nanoparticles.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2008;130(4):042407-042407-7. doi:10.1115/1.2789719.

Nanofluids are being studied for their potential to enhance heat transfer, which could have a significant impact on energy generation and storage systems. However, only limited experimental data on metal and metal-oxide based nanofluids, showing enhancement of the thermal conductivity, are currently available. Moreover, the majority of the data currently available have been obtained using transient methods. Some controversy exists as to the validity of the measured enhancement and the possibility that this enhancement may be an artifact of the experimental methodology. In the current investigation, $Al2O3$∕water nanofluids with normal diameters of $47nm$ at different volume fractions (0.5%, 2%, 4%, and 6%) have been investigated, using two different methodologies: a transient hot-wire method and a steady-state cut-bar method. The comparison of the measured data obtained using these two different experimental systems at room temperature was conducted and the experimental data at higher temperatures were obtained with steady-state cut-bar method and compared with previously reported data obtained using a transient hot-wire method. The arguments that the methodology is the cause of the observed enhancement of nanofluids effective thermal conductivity are evaluated and resolved. It is clear from the results that at room temperature, both the steady-state cut-bar and transient hot-wire methods result in nearly identical values for the effective thermal conductivity of the nanofluids tested, while at higher temperatures, the onset of natural convection results in larger measured effective thermal conductivities for the hot-wire method than those obtained using the steady-state cut-bar method. The experimental data at room temperature were also compared with previously reported data at room temperature and current available theoretical models, and the deviations of experimental data from the predicted values are presented and discussed.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2008;130(4):042408-042408-5. doi:10.1115/1.2789721.

Thermal conductivity equations for the suspension of nanoparticles (nanofluids) have been derived from the kinetic theory of particles under relaxation time approximations. These equations, which take into account the microconvection caused by the particle Brownian motion, can be used to evaluate the contribution of particle Brownian motion to thermal transport in nanofluids. The relaxation time of the particle Brownian motion is found to be significantly affected by the long-time tail in Brownian motion, which indicates a surprising persistence of particle velocity. The long-time tail in Brownian motion could play a significant role in the enhanced thermal conductivity in nanofluids, as suggested by the comparison between the theoretical results and the experimental data for the $Al2O3$-in-water nanofluids.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2008;130(4):042409-042409-6. doi:10.1115/1.2789722.

A photoelectrochemical model for hydrogen production from water electrolysis using proton exchange membrane is proposed based on Butler-Volmer kinetics for electrodes and transport resistance in the polymer electrolyte. An equivalent electrical circuit analogy is proposed for the sequential kinetic and transport resistances. The model provides a relation between the applied terminal voltage of electrolysis cell and the current density in terms of Nernst potential, exchange current densities, and conductivity of polymer electrolyte. Effects of temperature on the voltage, power supply, and hydrogen production are examined with the developed model. Increasing temperature will reduce the required power supply and increase the hydrogen production. An increase of about 11% is achieved by varying the temperature from $30°Cto80°C$. The required power supply decreases as the illumination intensity becomes greater. The power supply due to the cathode overpotential does not change too much with the illumination intensity. Effects of the illumination intensity can be observed as the current density is relatively small for the examined illumination intensities.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2008;130(4):042410-042410-11. doi:10.1115/1.2818765.

This paper presents a Monte Carlo simulation scheme to study the phonon transport and the thermal conductivity of nanocomposites. Special attention has been paid to the implementation of periodic boundary condition in Monte Carlo simulation. The scheme is applied to study the thermal conductivity of silicon germanium (Si–Ge) nanocomposites, which are of great interest for high-efficiency thermoelectric material development. The Monte Carlo simulation was first validated by successfully reproducing the results of (two-dimensional) nanowire composites using the deterministic solution of the phonon Boltzmann transport equation reported earlier and the experimental thermal conductivity of bulk germanium, and then the validated simulation method was used to study (three-dimensional) nanoparticle composites, where Si nanoparticles are embedded in Ge host. The size effects of phonon transport in nanoparticle composites were studied, and the results show that the thermal conductivity of nanoparticle composites can be lower than that of the minimum alloy value, which is of great interest to thermoelectric energy conversion. It was also found that randomly distributed nanopaticles in nanocomposites rendered the thermal conductivity values close to that of periodic aligned patterns. We show that interfacial area per unit volume is a useful parameter to correlate the size effect of thermal conductivity in nanocomposites. The key for the thermal conductivity reduction is to have a high interface density where nanoparticle composites can have a much higher interface density than the simple 1D stacks, such as superlattices. Thus, nanocomposites further benefit the enhancement of thermoelectric performance in terms of thermal conductivity reduction. The thermal conductivity values calculated by this work qualitatively agrees with a recent experimental measurement of Si–Ge nanocomposites.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2008;130(4):042411-042411-9. doi:10.1115/1.2818771.

Heterogeneous bubble nucleation was studied on surfaces having nanometer scale asperities and indentations as well as different surface-fluid interaction energies. Nonequilibrium molecular dynamics simulations at constant normal stress and either temperature or heat flux were carried out for the Lennard–Jones fluid in contact with a Lennard–Jones solid. When surface defects were of the same size or smaller than the estimated critical nucleus (the smallest nucleus whose growth is energetically favored) size of $1000–2000Å3$, there was no difference between the defected surfaces and atomically smooth surfaces. On the other hand, surfaces with significantly larger indentations had nucleation rates that were about two orders of magnitude higher than the systems with small defects. Moreover, nucleation was localized in the large indentations. This localization was greatest under constant heat flux conditions and when the solid-fluid interactions were weak. The results suggest strategies for enhancing heterogeneous bubble nucleation rates as well as for controlling the location of nucleation events.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2008;130(4):042412-042412-7. doi:10.1115/1.2818775.

The turbulent convective heat transfer behavior of alumina $(Al2O3)$ and zirconia $(ZrO2)$ nanoparticle dispersions in water is investigated experimentally in a flow loop with a horizontal tube test section at various flow rates $(9000, temperatures $(21–76°C)$, heat fluxes (up to $∼190kW∕m2$), and particle concentrations ($0.9–3.6vol%$ and $0.2–0.9vol%$ for $Al2O3$ and $ZrO2$, respectively). The experimental data are compared to predictions made using the traditional single-phase convective heat transfer and viscous pressure loss correlations for fully developed turbulent flow, Dittus–Boelter, and Blasius/MacAdams, respectively. It is shown that if the measured temperature- and loading-dependent thermal conductivities and viscosities of the nanofluids are used in calculating the Reynolds, Prandtl, and Nusselt numbers, the existing correlations accurately reproduce the convective heat transfer and viscous pressure loss behavior in tubes. Therefore, no abnormal heat transfer enhancement was observed in this study.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2008;130(4):042413-042413-7. doi:10.1115/1.2818783.

High time-resolution flow field measurement in two microchannels with a complex shape is performed by a micro-digital-holographic particle-tracking velocimetry (micro-DHPTV). The first microchannel has a $Y$ junction that combines the flow of fluid from two inlets into one outlet. In this case, two laminar velocity profiles from the inlet regions merge into one laminar velocity profile. The second microchannel has a convergence region from where a fluid flows into a divergence region. At this region, two recirculation regions appear. Consequently, approximately 250 velocity vectors in both cases can be obtained instantaneously. For a microchannel with the convergence region, the two recirculation regions that appear at the divergence point are captured from a three-dimensional vector field, with which the axes of recircular vortices have some alignment. The reason why we can observe this phenomenon is that a three-dimensional velocity, including the depth direction, can be obtained by micro-DHPTV.

Commentary by Dr. Valentin Fuster

### Technical Briefs

J. Heat Transfer. 2008;130(4):044501-044501-3. doi:10.1115/1.2818787.

Many studies have shown that addition of nanosized particles to water enhances the critical heat flux (CHF) in pool boiling. The resulting colloidal dispersions are known in the literature as nanofluids. However, for most potential applications of nanofluids the situation of interest is flow boiling. This technical note presents first-of-a-kind data for flow boiling CHF in nanofluids. It is shown that a significant CHF enhancement (up to $∼30%$) can be achieved with as little as 0.01% by volume concentration of alumina nanoparticles in flow experiments at atmospheric pressure, low subcooling $(<20°C)$, and relatively high mass flux $(⩾1000kg∕m2s)$.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2008;130(4):044502-044502-3. doi:10.1115/1.2787026.

A generic and material-independent dry route based on electrostatic force directed assembly (ESFDA) is used to assemble various nanoparticles onto multiwalled carbon nanotubes (CNTs). Charged and nonagglomerated aerosol nanocrystals are first produced using a mini-arc plasma source and then delivered in an inert carrier gas to electrically biased CNTs. The electric field near the CNT is significantly enhanced, and the aerosol nanoparticles are attracted to the external surface of CNTs. For the first time, CNTs have been sequentially coated with nanoparticles of multiple materials to realize the multicomponent coating. High resolution transmission electron microscopy images show that the nonagglomerated entity of nanoparticles and the crystallinity of both nanoparticles and CNTs are preserved during the assembly. The ESFDA technique enables unique hybrid nanostructures attractive for various energy applications.

Commentary by Dr. Valentin Fuster
J. Heat Transfer. 2008;130(4):044503-044503-4. doi:10.1115/1.2818784.

The potential of converting heat energy into electrical energy using a previously reported waveguide-ballistic device is presented. The interactions between incident electromagnetic waves and free electrons in a metal waveguide are analyzed with respect to their transport through a high-frequency ballistic rectifier using finite element method simulation. It was determined that the resulting conversion efficiency to a dc potential is approximately 6%, yielding a power density on the order of $30W∕m2$.

Commentary by Dr. Valentin Fuster