ASME Conference Presenter Attendance Policy and Archival Proceedings

2016;():V001T00A001. doi:10.1115/MNHMT2016-NS1.

This online compilation of papers from the ASME 2016 5th International Conference on Micro/Nanoscale Heat and Mass Transfer (MNHMT2016) represents the archival version of the Conference Proceedings. According to ASME’s conference presenter attendance policy, if a paper is not presented at the Conference, the paper will not be published in the official archival Proceedings, which are registered with the Library of Congress and are submitted for abstracting and indexing. The paper also will not be published in The ASME Digital Collection and may not be cited as a published paper.

Commentary by Dr. Valentin Fuster

Micro/Nanofluidics and Lab-on-a-Chip

2016;():V001T01A001. doi:10.1115/MNHMT2016-6421.

This paper presents the fluid flow in nanochannels with permeable walls using the molecular dynamics (MD) simulations. A three-dimensional Couette flow has been carried out to investigate the effect of the permeable surface on the fluid density distributions and the slip velocity. The ordering layer of molecules is constructed near the smooth surface but it was destroyed by the permeable ones resulting in the density drop in porous wall. The fluid density in porous wall is large under strong fluid-structure interaction (FSI) and it is decreased under weak FSI. The negative slip is observed for fluid flow past solid walls under strong FSI, no-slip under medium FSI and positive slip under weak FSI whatever it is smooth or porous. Moreover, the largest slip velocity and slip length occur on the smooth surface of solid wall. As predicted by Maxwell theory, the molecule is bounced back when it impinges on the smooth surface. The molecules, however, can reside in porous wall by replacing the molecules that are trapped in the pores. Moreover, the molecule can escape from the pore and enter the channel becoming a free molecule. After travelling for a period time in the channel, the molecule can enter the pore again. During the molecular movement, the momentum exchange has been implemented not only between fluid molecules and wall but also between the fluid molecules themselves in the pore, and the multi-collision between fluid molecules takes place. The reduced slip velocity at the porous wall results in the larger friction coefficient compared to the smooth surface wall. The molecular boundary condition predicted by Maxwell theory on the smooth surface is no longer valid for flow past the permeable surface, and a novel boundary condition should be introduced.

Commentary by Dr. Valentin Fuster
2016;():V001T01A002. doi:10.1115/MNHMT2016-6441.

We study the separation process of gaseous H2O/CH4 mixtures using nanoporous graphene membranes via molecular dynamics simulations. We run the simulation in an equilibrium system 10 times with different initial atomic velocities to overcome the inefficiency brought by the low pressure of the system. The results show that the H2O molecules can permeate the graphene membrane with a linearly time-dependent crossing number. The permeance of the H2O molecules reaches to 9.5×10−4 mol/m2sPa, far exceeding that of the polymer gas separation membranes. High selectivity of H2O over CH4 is also observed. In summary, this study demonstrates that the specific NPG cloud be adopted as an efficient membrane in natural gas dehydration.

Commentary by Dr. Valentin Fuster
2016;():V001T01A003. doi:10.1115/MNHMT2016-6446.

The wetting, spreading and drying of pure liquid and nanofluid sessile droplets on a patterned solid surface were investigated systematically in terms of liquid and surface property. The patterned nickel surface was characterized with diamond, circular, hexagon and rectangular pillars. The size ratio between interval and pillars varies from 1.0 to 5.0. The study was firstly carried out for the effect of pure water droplet size on liquid spreading and droplet evaporation process on diamond-shape micro structured substrate with LInterval/LPillar=1.0. Larger amount of liquid leads to a larger wetting area. With fixed substrate (diamond, LInterval/LPillar=1.0) and droplet size (1 μm), mixture of DI water and Ethanol (volume ratio varies from 0.5 to 2.0) was used for generating droplets with different surface tension and evaporation coefficient. Fingering shape would generate on the contact line. With higher concentration of ethanol, the fingering effect is stronger and appeared in a shorter time. The contact area shrinks when increase the size ratio of interval and pillar. This would reduce the length of the contact line, and thus slow down the liquid evaporation. The role of pillar shape was examined based on time for complete evaporation. The effect of surface material on evaporation process was conducted on nickel and PMMA substrate fabricated with the same design. Additionally, investigations were conducted with solutions consisted with nanoparticles and DI water. The mixture were made at different weight ration to achieve concentration of nanoparticles varies from 0.02% to 0.18% with an interval at 0.04%.

Commentary by Dr. Valentin Fuster
2016;():V001T01A004. doi:10.1115/MNHMT2016-6491.

The recently confirmed violation of the no-slip boundary condition in the flow of small-molecule liquids through microchannels and nanochannels has technological implications such as friction reduction. However, for significant friction reduction at low cost, the microchannel wall needs to be chemically inhomogeneous. The direct fluid dynamic consequence of this requirement is a spatial variation in the local degree of liquid slippage. In this work, the pressure-driven flow in a channel with periodically patterned slippage on the channel walls is studied using a spectrally accurate semi-analytical approach based on Fourier decomposition. The method puts no restrictions on the pitch (or wavelength) and amplitude of the pattern. The predicted effective slip length in the limits of small pattern amplitude and thick channels is found to be consistent with previously published results. The effective degree of slippage decreases with the patterning amplitude. Finer microchannels and longer pattern wavelengths promote slippage.

Commentary by Dr. Valentin Fuster
2016;():V001T01A005. doi:10.1115/MNHMT2016-6492.

In the present study, the interfacial dynamics of displacement of three dimensional spherical droplet on a rectangular microchannel wall considering wetting effects are studied. The two-phase lattice Boltzmann Shan-Chen model is used to explore the physics. The main focus of this study is to analyse the effect of wettability, low viscosity ratio and capillary number on the displacement of spherical droplet subjected to gravitational force. The hydrophobic and hydrophilic nature of wettabilities on wall surface are considered to study with capillary number, Ca=0.1, 0.35 and 0.66 and viscosity ratio, M ≤ 1. The results are presented in the form of temporal evolution of wetted length and wetted area for combined viscosity ratios and wettability scenario. In the present study, it is observed that in dynamic droplet displacement, the viscosity ratio and capillary number play a significant role. It is found that as viscosity ratio increases, both the wetted area and the wetted length increase and decrease in the case of hydrophilic and hydrophobic wettable wall respectively.

Commentary by Dr. Valentin Fuster
2016;():V001T01A006. doi:10.1115/MNHMT2016-6529.

Gravity-driven displacement of droplet on an inclined micro-grooved surface is studied using Pseudo-potential model of lattice Boltzmann method. To validate the numerical method, we find good agreement of the LB simulations with the pressure difference by Laplace’s law. The equilibrium contact angle of a droplet wetting on a smooth horizontal surface is studied as a function of the wettability, finding good agreement with an empirical scheme obtained with Young’s equation. The dynamic behavior of a droplet wetting on micro-grooved horizontal surface is found to be complex and greatly affected by the fraction of the grooved area and the groove width, the results indicate that the effect of grooves on contact angle is dependent on the fraction of the grooved area and the groove width has not a consistent effect on contact angles. For an inclined nonwetting micro-grooved surface, in given range, higher fraction of the grooved area and smaller groove width lead to greater benefit for droplet sliding down. What’s more, the results indicate that higher gravity value leads to a higher decrease of movement resistance of the droplet by decreasing the contact area between the droplet and solid surface.

Commentary by Dr. Valentin Fuster
2016;():V001T01A007. doi:10.1115/MNHMT2016-6562.

Morpholinos (MOs) are synthetic nucleic acids analogues with a non-charged backbone of morpholine rings. To enhance the MO-DNA hybridization assay speed, we propose the integration of a MO microarray with an ion concentration polarization (ICP) based microfluidic concentrator. The ICP concentrator collects target biomolecules from a ∼μL fluidic DNA sample and concentrates them electrokinetically into a ∼nL plug located in the vicinity of the MO probes. ICP preconcentration not only reduces the analyte diffusion length but also increases the binding reaction rate, and as a result, ICP-enhanced MO microarrays allow much faster hybridization than standard diffusion-limited MO microarrays.

Commentary by Dr. Valentin Fuster
2016;():V001T01A008. doi:10.1115/MNHMT2016-6580.

The use of liquids for the manipulation of light has many advantages over their conventional solid counterparts. With the emergence of microfluidic technologies to control fluid interfaces, various devices capable of replacing conventional optical components have been developed. Because liquids are intrinsically smooth and can change shape or form, they have been utilized for highly versatile components to manipulate light with high degrees of control using optofluidic technologies. Liquid lenses and beam steering devices are among the typical optofluidic devices that have gained much interest over recent years. In this work, we present high-performance tunable liquid prisms capable of wide beam steering of incoming light. By using the electrowetting phenomena, we are able to modulate the fluid-fluid interface at which beam steering occurs. Optical analyses were conducted to study the effect of liquid selection in the effectiveness of our prisms. Furthermore, the double-stacked prism configuration is proposed to achieve wide beam steering and its performance is compared with that of a single prism for different liquid selections. Finally, our analytical studies have been experimentally demonstrated. We successfully fabricated the tunable liquid prism filled with water and 1-bromonaphthalene (1-BN). Due to large refractive index difference between two liquids (nwater = 1.33 and n1−BN = 1.65 at λ = 532 nm), high-performance beam steering was enabled. With an apex angle of 25°, we were able to experimentally demonstrate a beam steering of β ≤ 8.82° with the single prism configuration. It was significantly improved up to β ≤ 17.04° for the double-stacked prism.

Topics: Prisms (Optics)
Commentary by Dr. Valentin Fuster
2016;():V001T01A009. doi:10.1115/MNHMT2016-6654.

The present work was movitated by an earlier study on pumps based on a Wankel geometry (Wankel pumps) which revealed a flow structure, rich in three dimensional vortices (including Taylor like counter-rotating ones), in the pump chamber, suggesting the possibility of a Wankel pump of also functioning as a mixer. To this end, numerical mixing experiments were run using the general scalar transport equation to model the evolution of species concentration. Interestingly, it was observed that species dispersion occurs predominantly sideways (laterally) and that vertical dispersion is almost non-existent. In other words, for binary mixing of two species with a Wankel pump, they must be introduced side by side to ensure effective mixing.

Commentary by Dr. Valentin Fuster
2016;():V001T01A010. doi:10.1115/MNHMT2016-6671.

The heat transfer performance of fluid flowing in a microchannel was experimentally studied, to meet the requirement of extremely high heat flux removal of microelectronic devices. There were 10 parallel microchannels with rectangular cross-section in the stainless steel plate, which was covered by a glass plate to observe the fluid flowing behavior, and another heating plate made of aluminum alloy was positioned behind the microchannel. Single phase heat transfer and fluid flow downstream the microchannel experiments were conducted with both deionized water and ethanol. Besides experiments, numerical models were also set up to make a comparison with experimental results.

It is found that the pressure drop increases rapidly with enlarging Reynolds number (200), especially for ethanol. With comparison, the flow resistance of pure water is smaller than ethanol. Results also show that the friction factor decreases with Reynolds number smaller than the critical value, while increases the velocity, the friction factor would like to keep little changed. We also find that the water friction factors obtained by CFD simulations in parallel microchannels are much larger than experiment results.

With heat flux added to the fluid, the heat transfer performance can be enhanced with larger Re number and the temperature rise could be weaken. Compared against ethanol, water performed much better for heat removal. However, with intensive heat flux, both water and ethanol couldn’t meet the requirement and the temperature at outlet would increase remarkably, extremely for ethanol. These findings would be helpful for thermal management design and optimization.

Commentary by Dr. Valentin Fuster
2016;():V001T01A011. doi:10.1115/MNHMT2016-6709.

In this paper, we present a novel design of bilayer polydimethylsiloxane (PDMS) microchannel formed by bifurcated junction, from which each curved branch lies on the upper and lower layer, respectively. With this 3D platform, we aim to investigate droplet formation and subsequent fission in a multiphase system using non-Newtonian fluids, which are ubiquitous in daily life and have been widely used in industrial applications including biomedical engineering, food production, personal care and cosmetics, and material synthesis. Numerical model has been established to characterize the non-Newtonian effect to droplet fission and associated breakup dynamics when droplet flows through 3D bifurcated junction, where droplets can deform significantly on account of the confining geometric boundaries, and the flow of the surrounding non-Newtonian liquid, both of which control the deformation and breakup of each mother droplet into two daughter droplets. Dispersions of sodium carboxymethyl cellulose in water, and dispersions of polyvinylchloride in dioctylphthalate have been used as model fluids in the study, with the former one possessing shear-thinning behaviour, while the latter one possessing shear-thickening behaviour. The understanding of the droplet fission in the novel microstructure will enable more versatile control over the emulsion formation when non-Newtonian fluids are involved. The model systems in the study can be further developed to investigate the mechanical property of emulsion templated particles such as drug encapsulated microcapsules when they flow through complex media structures, such as blood capillaries or the porous tissue structure, which feature with bifurcated junction.

Commentary by Dr. Valentin Fuster
2016;():V001T01A012. doi:10.1115/MNHMT2016-6717.

Plasma is a host of various analytes such as proteins, metabolites, circulating nucleic acids (CNAs), pathogens. The key process of plasma extraction is to eliminate the contamination from blood cells. Conventional methods, such as centrifugation and membrane filtration, are generally lab-intensive, time consuming and even dangerous. In this study, we report an integrated microfluidic device that combines inertial microfluidics and membrane filter. The integrated microfluidic device was evaluated by the diluted (x1/10, x1/20) whole blood, and the quality of the extracted blood plasma was tested. It was found that quality of extracted blood plasma from integrated device was equivalent to that obtained by the centrifugation. This study demonstrates a significant progress towards the practical application of inertial microfluidics with membrane filter for high-throughput and high efficient blood plasma extraction.

Commentary by Dr. Valentin Fuster


2016;():V001T02A001. doi:10.1115/MNHMT2016-6316.

Transient heat transfer during constrained melting of graphite-based solid-liquid phase change nanofluids in a spherical capsule was investigated experimentally. Nanofluids filled with self-prepared graphite nanosheets (GNSs) were prepared at various loadings up to 1% by weight, using a straight-chain saturated fatty alcohol, i.e., 1-dodecanol (C12H26O), with a nominal melting point of 22 °C as the base fluid. In-house measured thermal properties were adopted for data reduction, including thermal conductivity, dynamic viscosity, latent heat of fusion, specific heat capacity and density. A proper experimental approach depended on volume expansion was figured out to monitor the melting process of nano-enhanced phase change fluid in a spherical capsule indirectly and qualitatively characterize the process. A variety of boundary temperatures were also adopted to vary the intensity of natural convection. It was shown that under low boundary temperatures, a monotonous melting acceleration came into being while increasing the loading due to the monotonously increased thermal conductivity of the nanofluids. While increasing the boundary temperature leads to more intensive natural convection that in turn slowed down melting under the influence of nanoparticles because the contribution by natural convection is significantly suppressed by the dramatically grown dynamic viscosity, e.g., more than 60-fold increase at the loading of 1 wt.%. The melting rate is determined by the competition between the enhanced heat conduction and deteriorated natural convection.

Commentary by Dr. Valentin Fuster
2016;():V001T02A002. doi:10.1115/MNHMT2016-6339.

In present work, Al2O3/H2O nanofluid was prepared by ultrasonic oscillation. Furthermore, nanofluid flow boiling heat transfer in a vertical cube is experimentally studied, with 0.1% and 0.5% volume concentration and 20nm diameter. Some factors are under consideration, including heat flux on the heating surface (48∼289kW·m−2), pressure (0.2∼0.8MPa) and mass flow rate (400∼1100 kgm−2s−1). The results confirm that the flow boiling heat transfer of Al2O3/H2O nanofluid is improved mostly about 86% compared with pure water. And the average Nusselt number enhancement rate of nanofluid compared with deionized water is 35% in the range of this work. Moreover, the heat transfer capacity of nanofluid increase with the heat flux on the heating surface, pressure and the volume concentration of nanoparticle. It is proved that nanoparticle deposited on the heating surface by SEM observations, and TEM observations for nanoparticle confirm that nanoparticle have not obviously changed after boiling. In addition, the enhancement rate of nanofluid flow boiling heat transfer capacity increase with the pressure, and the influence of mass flow rate is negligible. In conclusion, this work is a supplement for nanofluid flow boiling heating transfer, especially for the influence of pressure.

Commentary by Dr. Valentin Fuster
2016;():V001T02A003. doi:10.1115/MNHMT2016-6340.

To compare and understand the laminar thermal-hydraulic performance of plate-fin channel with rectangle plain fin by using variable thermophysical properties of the most commonly used nanofluids (Al2O3-water), a three-dimensional numerical study is investigated by using the single-phase approach at a constant wall temperature boundary condition. Different models published in literatures are considered for the thermal conductivity and viscosity. On this basis, a parametric analysis is conducted to evaluate the effects of various pertinent parameters including nanoparticle volume fraction (0%–4%), Brownian motion of nanoparticle and Reynolds number (800–1500) on the heat transfer and flow characteristics of plain fin channel in detail. All the numerical results demonstrate that the addition of Al2O3 nanoparticle can enhance the heat transfer and flow pressure loss of base fluid because of the higher thermal conductivity and viscosity for nanofluids. And these enhancements are more obvious by increasing the volume fraction of nanoparticle, increasing Reynolds number, and considering the effects of nanoparticle Brownian motion. In addition, there are significantly differences in the thermal and flow fields for different nanofluids models at a fixed Reynolds number, which means that the effective theoretical formulas and empiric corrections for the nanofluids thrmophysical properties need to be studied extensively in the future.

Commentary by Dr. Valentin Fuster
2016;():V001T02A004. doi:10.1115/MNHMT2016-6343.

The solid-liquid phase transition process is of significant importance the widely usage of phase change material (PCM), including in thermal energy storage and maintaining working temperature. In this paper, a phase change lattice Boltzmann (LB) model has been established to investigate the effects of inclining angle on the melting process in a cavity filled with PCM, considering three kinds of heat flux distribution: uniform distribution, linear distribution and parabolic symmetry distribution. The simulations results show that for all the heat flux distributions, the slight clockwise rotation of cavity is able to accelerate the melting process. Furthermore, when more heat is transported into the cavity through the middle part (parabolic symmetry distribution) or bottom part (linear distribution), the effects of cavity clockwise rotation on temperature field are more than that of anticlockwise rotation.

Commentary by Dr. Valentin Fuster
2016;():V001T02A005. doi:10.1115/MNHMT2016-6393.

Employing nanofluids is an innovative way to enhance heat transfer in cooling system of internal combustion engine. the reasons for the significantly enhanced heat transfer properties of nanofluids are various. On one hand, the markedly increased thermal conductivity is the most apparent reason; on the other hand, the changed rheology properties of base fluid due to the disordered movements of countless nanoparticles is even more important. Because the size scale of nanoparticles is too small, in some cases of computational simulations nanofluids is simplified as single-phase fluids. However, the influence of nanoparticles for flow behaviors of base fluids distinctly should not be ignored. By means of molecular dynamics method, a nano-scale simulation on the rheology of nanofluids could be conducted, therefore the movements of nanoparticles could be directly observed, which is conducive to reveal the influence of movements of nanoparticles for rheology of nanofluids. The present work is intended to perform a molecular dynamic simulation on the rheology of water based nanofluids. By applying temperature difference, the velocity and temperature distribution of fluid zone are calculated to evaluate heat transfer through nanofluids. Moreover, the influence of temperature for the movements of nanoparticle is discussed.

Commentary by Dr. Valentin Fuster
2016;():V001T02A006. doi:10.1115/MNHMT2016-6400.

In this article, the heat transferring property of the copper-water nanofluids in self-exciting mode oscillating flow heat pipe under different laser heating power is experimented, as well as is compared with that of the distilled water medium in self-exciting mode oscillating flow heat pipe under same heating condition. The objective of this article is to provide the heat transfer characteristics of Cu-H2O nanofluids in self-exciting mode oscillating-flow heat pipe under different laser heating input, and to compare with the heat transfer characteristics of the same heat pipe with distilled water as working fluids.

The SEMOS HP used in this experiment is made of brass tube with 2mm interior diameter, which is consisted of 8 straight tubes with 4 turns’ evaporation section and 12 turns’ condensation section. The heat resource for the evaporation zone is eight channel quantum pitfall diode array semi-conductor laser heater with 940nm radiation wave length, while the radiation power of each channel is changeable within 0–50W and the facular size is 1×30mm2. The condensation section is set in a cooling water tank in which water is from another higher tank. The actual transferring rate could be calculated by the flow rate of the cooling water and the change of the temperature. The change of the temperature of the heat pipe wall is measured by those thermo-couple fixed in different section in the heat pipe and data is collected by a data acquisition. In the heat pipe the fluid filling rate is 43%, the pressure is 2.5×10−3Pa, and the heat pipe inclination angle is 55° while the size of the brass particle in the nanofluids is less than 60nm and volume proportion is 0.5%.

In this paper, the particularity of heat transfer rate of the SEMOS heat pipe with Cu-H2O fluid has been experimentally confirmed by changing the proportion of working fluid and Cu nonsocial particles in the heat pipe. By comparing the experimental result of these two different medium in the SEMOS HP, it is shown that the heat transferring rate with brass-water nanofluids as medium is much better than that with distilled water as medium under same volume proportion.

Commentary by Dr. Valentin Fuster
2016;():V001T02A007. doi:10.1115/MNHMT2016-6451.

The flow and heat transfer characteristics of nanofluids in the near-wall region were studied by non-equilibrium molecular dynamics simulation. The nanofluid model consisted of one spherical copper nanoparticle and argon atoms as base liquid. The effective thermal conductivity (ETC) of nanofluids and base fluid in shear flow fields were obtained. The ETC was increased with the increasing of shear velocity for both base fluid and nanofluids. The heat transfer enhancement of nanofluids in the shear flow field (v≠0) is better than that in the zero-shear flow field (v=0). By analyzing the flow characteristics we proved that the micro-motions of nanoparticles were another mechanism responsible for the heat transfer enhancement of nanofluids in the flow field. Based on the model built in the paper, we found that the thermal properties accounted for 52%–65% heat transfer enhancement and the contribution of micro-motions is 35%–48%.

Commentary by Dr. Valentin Fuster
2016;():V001T02A008. doi:10.1115/MNHMT2016-6477.

The effect of ethanol in the binary solution sessile droplet is investigated on the flow field, nanoparticle motion and nanoparticle deposition pattern. It is found that the droplets with ethanol exhibited three distinct flow regimes through the Particle Image Velocimetry (PIV) analysis on the flow field of droplets suspended with fluorescent microspheres. Regime I features furious flows and vortices which transport particles to the liquid-vapor interface and make them aggregate. In regime II, the aggregates of particles move towards the central area of the droplet dominated by Marangoni flow led by non-uniformity of ethanol along the droplet surface. As the droplet enters regime III, most ethanol has evaporated and it is dominated by the drying of the remaining water. The loading of ethanol in the solution prolongs the relative durations of regimes I and II, resulting in the variety of the final drying pattern of nanoparticles.

Commentary by Dr. Valentin Fuster
2016;():V001T02A009. doi:10.1115/MNHMT2016-6531.

The confined jet array impingement cooling using NEPCM (nano-encapsulated phase change material) slurry was investigated numerically using a homogeneous model based on effective heat capacity method. The nanofluids consists of the carrier fluid of polyalphaolefin (PAO) and the NEPCM particles of Polystyrene shell and paraffin core. The distributed slot jet array with the jet width W=100 μm, confinement height H=300 μm, jet-to-jet distance S=400 μm was investigated at first under different jet velocity, inlet temperature and NEPCM volumetric concentration. It was found that for a fixed jet velocity, there is an optimal NEPCM volumetric concentration and an optimal inlet temperature to achieve the maximum average heat transfer coefficient. The larger the jet velocity, the higher the optimal NEPCM concentration and the closer the optimal inlet temperature to the midpoint of melting temperature range of PCM where the peak of effective heat capacity achieves. The local heat transfer on the heating surface under the exit slot is the weakest, because of stagnant zone formed by the head-to-head collision of the two adjacent jets. The pressure drop and average heat transfer coefficient of six jet arrays with different H/W (=2, or 3) and S/W (=3, 4 or 5) were also compared.

Commentary by Dr. Valentin Fuster
2016;():V001T02A010. doi:10.1115/MNHMT2016-6574.

In this study, heat transfer characteristics of multi-walled carbon nanotube based nanofluids were investigated in horizontal microtubes with outer and inner diameters of ∼1067 and ∼889 μm, respectively. Carbon nanotubes (CNTs) with outer diameter of 10–20 nm and length of 1–2 micron as non-spherical nanoparticles were used for nanofluid preparation, where water was considered as basefluid. Nanofluid was characterized using the Scanning Electron Microscopy (SEM). According to obtained results, deposited CNTs have considerable effect on the convective heat transfer inside the microtube.

Commentary by Dr. Valentin Fuster
2016;():V001T02A011. doi:10.1115/MNHMT2016-6620.

Nanofluids are found to have good optical and thermal properties as direct sunlight absorbers in solar collectors. In this article, Co-H2O nanofluids were prepared through two-step method. The photothermal properties of nanofluids were investigated under different magnetic field intensity. The effect of Co-H2O nanofluids on the efficiency of a direct absorption solar collector was also investigated experimentally. The experimental results show that the applied magnetic field can enhance the solar absorption ability of Co-H2O nanofluids, and has an optimal magnetic field intensity 30Gs. The highest temperature of Co-H2O nanofluid (0.04wt%, 30Gs) is increased up to 39.5% compared with deionized water. The maximum efficiency of direct absorption solar collector with Co-H2O nanofluid under 30Gs magnetic field (0.1wt%, 30nm) is increased up to 51.70% and 13.24% compared with water and Co-H2O nanofluid without magnetic field, respectively. The results indicate that the magnetic field has the potential to effectively improve the solar absorption capabilities of direct absorption solar collectors with magnetic nanofluids.

Commentary by Dr. Valentin Fuster
2016;():V001T02A012. doi:10.1115/MNHMT2016-6630.

Absorption air conditioning system could be driven by low grade energy, such as solar energy and industrial exhaust heat, for the purposes of energy conservation and emission reduction. Its development is limited by its huge volume and high initial investment. The nanofluids, which possess the superior thermophysical properties, exhibit a great potential in enhancing heat and mass transfer performance. In this paper, nanofluids of H2O/LiBr with Fe3O4 nanoparticles were introduced into absorption air conditioning system. The effects of some parameters, such as the flow rate of H2O/LiBr nanofluids, nanoparticle size and mass fraction, on the falling film absorption were investigated. The H2O/LiBr nanofluids with Fe3O4 nanoparticle mass fractions of 0.01 wt%, 0.05 wt% and 0.1 wt%, and nanoparticle size of 20 nm, 50 nm and 100 nm were tested by experiment. The results imply that the water vapour absorption rate could be improved by adding nanoparticles to H2O/LiBr solution. The smaller the nanoparticle size, the greater enhancement of the heat and mass transfer performance. The absorption enhancement ratio increases sharply at first by increasing the nanoparticle mass fraction within a range of relatively low mass fraction, and then exhibits a slow growing even reducing trends with increasing the mass fraction further. For Fe3O4 nanoparticle mass fraction of 0.05wt% and nanoparticle size of 20nm, the maximum mass transfer enhancement ratio is achieved about 2.28 at the flow rate of 100 L·h−1.

Commentary by Dr. Valentin Fuster

Micro/Nanoscale Interfacial Transport Phenomena

2016;():V001T03A001. doi:10.1115/MNHMT2016-6314.

In this paper, the cross-plane thermal conductance σ of multi-layer graphene nanobundles (MLGNBs) is investigated using the non-equilibrium Green’s function method. For the normal MLGNBs, the σ has a positive dependence on the lateral area S due to more atoms involved in the heat transport in the larger S. However, the thermal conductance per unit area Λ is negative dependent on the S since high-frequency phonons contribute less to Λ with low transmission function and small number while the increased phonon branches are mainly located in the high-frequency range. Interestingly, as the S is larger than several square nanometers, the Λ converges to the macroscopic value, independently on the S. Then the staggered MLGNBs is investigated, the results show that increasing both staggering distance between neighboring graphene layers with each other and the graphene layer number in the central device can modulate the σ in a large scope due to the boundary scattering. Finally, in the MLGNBs junction, we found the variation of heat flux direction has an important effect on the σ while the layer number in the central device has weak effect on the cross-plane thermal transport. Our results help understand the cross-plane thermal transport of MLGNBs and provide a model to investigate the thermal property of layered material nanobundles.

Commentary by Dr. Valentin Fuster
2016;():V001T03A002. doi:10.1115/MNHMT2016-6318.

In this work, the interfacial thermal conductance across Cu/graphene/Cu interfaces is investigated using the density functional theory (DFT) and the nonequilibrium Green’s function (NEGF) method. In order to study how hydrogenation of graphene affects thermal transport behaviors at the interfaces of Cu/graphene/Cu, we also analyze the interfacial thermal conductance across Cu/hydrogenated-graphene/Cu (Cu/H-graphene/Cu) with both double-sided and single-sided hydrogenated graphene. Our results show that, the interfacial thermal conductance across Cu/H-graphene/Cu interfaces is almost twice of the value across Cu/graphene/Cu interfaces. For Cu/H-graphene/Cu with double-sided hydrogenated graphene (Cu/DH-graphene/Cu), the hydrogen atoms between graphene and Cu layers provide additional thermal transport channels. While for Cu/H-graphene/Cu with single-sided hydrogenated graphene (Cu/SH-graphene/Cu), the hydrogen atoms not only provide additional thermal transport channels at the hydrogenated side of graphene, but also reduce the equilibrium separation between graphene and Cu layers at the non-hydrogenated side of graphene due to the transfer of massive electrons, which enhances the interface coupling between graphene and Cu layers. The phonon transmission shows that both double-sided and single-sided hydrogenation of graphene can increase the heat transport across the interface. Our calculation indicates that the interfacial thermal conductance of Cu/graphene/Cu nanocomposition can be improved by hydrogenation.

Commentary by Dr. Valentin Fuster
2016;():V001T03A003. doi:10.1115/MNHMT2016-6402.

Despite that using surface-roughness-induced superhydrophobic surface as a solution for ice/snow accretion issues has achieved extensive progresses, its icephobicity breaks down in case of condensation frosting, while the high aspect ratio structure brings more concerns on its durability and sustainability. In this work we investigated condensate frosting on substrates fabricated with patterned micropillars having a small aspect ratio, and studied the freezing propagation with different pattern sizes. The results show that a coarse patterned substrate can effectively suppress the freeing propagation while a fine patterned one can drastically promote the freezing propagation. Frost coverage can also be reduced with proper pattern design. A theoretical model was developed to explain the mechanism of surface ice propagation, and agrees well in tendency with experiment measurements. The aim of this study is to provide some new insights on the influence of surface morphology on ice growth.

Commentary by Dr. Valentin Fuster
2016;():V001T03A004. doi:10.1115/MNHMT2016-6487.

Silicene, the silicon-based two-dimensional structure with honeycomb lattice, has been discovered to have tremendous application potential in fundamental industries. However, the thermal transport mechanism and thermal properties of silicene has not been fully explained. We report a possible way to control the thermal transport and thermal rectification in silicene nanosheets by designing distributions of a series of triangular cavities in this paper with the nonequilibrium molecular dynamic simulations. The cavities are arranged in a staggered way. The reflection of phonon at the vertex and the base of the triangular cavities are quite different. This difference is used to control the phonon transport in opposite directions and such an arrangement is expected to have very significant thermal rectification effect. The size of cavities, the distance between the triangular cavities and the distribution of cavities are investigated to observe the thermal rectification, which would benefit the design of an experiment that can clearly demonstrate thermal rectification.

Commentary by Dr. Valentin Fuster
2016;():V001T03A005. doi:10.1115/MNHMT2016-6508.

Chemical vapor deposited (CVD) graphene together with a superior gate dielectric such as Al2O3, is a promising combination for next-generation high-speed field effect transistors (FET). These high-speed devices are operated under high frequencies and will generate significant heat, requiring effective thermal management to ensure device stability and longevity. It is thus of importance to characterize the interfacial thermal resistance (ITR) between graphene/Al2O3 gate dielectric and graphene/metal contacts.

In this work, ITRs across the single-layer graphene/Al2O3 and the graphene/metal (Al, Ti, Au) interfaces were characterized from 100 K to 330 K using the differential 3ω method. Unlike previous works which mostly used exfoliated single or few-layer graphene, we used CVD large-scale graphene, which is most promising for FET fabrication due to cost and quality control, in the experiments. To ascertain the measured results and reduce uncertainty, different sandwich configurations including metal/graphene/metal, Al2O3/graphene/Al2O3 and metal/graphene/Al2O3 were used for the measurements. The effects of post annealing on different interfaces were also investigated.

Measurements of numerous samples showed an average ITR at 300K of 9×10−8 m2K/W for graphene/Al2O3, 6×10−8 m2K/W for graphene/Al, 5×10−8 m2K/W for graphene/Ti, and 7×10−8 m2K/W for graphene/Au interfaces. For the metal interfaces with graphene, the results are within the same order of magnitude as previous measurement results with graphite. However, ITR for graphene/Al2O3 is one order of magnitude higher than those reported for graphene/SiO2 interfaces. The measured ITRs for both metal and dielectric interfaces with graphene are almost temperature-independent from 100 K to 330 K, indicating that phonons are the major heat carrier. Annealing was found to have different effects on different interfaces. For graphene/Ti interfaces, ITR results measured before and after annealing consistently show a reduction of around 20%. However, such improvements on interfacial conductance were not observed for graphene/Al, graphene/Au and graphene/Al2O3 interfaces. The reduction of ITR of graphene/Ti interface is perceived to stem from the formation of Ti-C covalent bonds. However, neither the commonly used maximum transmission model nor the diffuse mismatch model explicitly considers bonding effects at the interface, which is why they poorly predict and explain all the aspects of the measurements. An improvement to the classic anisotropic DMM model was proposed by taking into account different bonding types and bonding area between graphene and Al2O3/metal layer, resulting in a better fitting with the experimental data.

Commentary by Dr. Valentin Fuster
2016;():V001T03A006. doi:10.1115/MNHMT2016-6533.

Head-on collisions of binary micro-droplets are of great interest in both academic research and engineering applications. Numerical simulation of this problem is challenging due to complex interfacial changes and large density ratio between different fluids. In this work, the recently proposed lattice Boltzmann flux solver (LBFS) is applied to study this problem. The LBFS is a finite volume method for the direct update of macroscopic flow variables at cell centers. The fluxes of the LBFS are reconstructed at each cell interface through lattice moments of density distribution functions (DDFs). As compared with conventional multiphase lattice Boltzmann method, the LBFS can be easily applied to study complex multiphase flows with large density ratio. In addition, external forces can be implemented more conveniently and the tie-up between the time step and mesh spacing is also removed. Moreover, it can deal with complex boundary conditions directly as those do in the conventional Navier-Stokes solvers.

At first, the reliability of the LBFS is validated by simulating a micro-droplet impacting on a dry surface at density ratio 832 (air to water). The obtained result agrees well with experimental measurement. After that, numerical simulations of head-on collisions of two micro droplets are carried out to examine different collisional behaviors in a wide range of Reynolds numbers and Weber numbers of 100 ≤ Re ≤ 2000 and 10 ≤ We ≤ 500. A phase diagram parameterized by these two control parameters is obtained to classify the outcomes of these collisions. It is shown that, at low Reynolds number (Re=100), two droplets will be coalescent into a bigger one for all considered Weber numbers. With the increase of the Reynolds number, separation of the collision into multiple droplets appears and the critical Weber number for separation is decreased. When the Reynolds number is sufficiently high, the critical Weber number for separation is between 20 and 25.

Commentary by Dr. Valentin Fuster
2016;():V001T03A007. doi:10.1115/MNHMT2016-6628.

Particulate fouling at elevated temperature is a crucial issue for microchannel heat exchangers. In this work, a microfluidic system is designed to experimentally study on the deposition of micro-particles suspended in microchannels, which simulates the working fluid in microscale heat exchangers. We have directly measured the deposition rate of microparticles and found that the number density of deposited particles was monotonically increased with solution temperature when constant flow rate of samples was maintained. Moreover, our results show that pulsatile flow, which was generated by a piezoelectric unit, could mitigate the particulate fouling in microchannels, and the deposition rate was decreased with increasing the frequency of pulsation within a low frequency region. Our findings are expected to gain better understanding of thermally driven particulate fouling as well as provide useful information for design and fabrication of microchannel heat exchangers.

Commentary by Dr. Valentin Fuster
2016;():V001T03A008. doi:10.1115/MNHMT2016-6646.

With the molecular dynamics simulations, we found that the nanoconstriction resistance arising from the single-constriction is inversely proportional to the constriction width, which can be well described by the two-dimensional ballistic resistance model we proposed. More importantly, after the nanoconstrictions are networked, the results elucidate a parallel relationship between ballistic resistances in parallel system, weather the constrictions are of equal width or not, and especially, a complicated superimposed effect of arrangement mode on ballistic resistances in series system, which could cause a decrease or further increase in the ballistic resistance. Thus, with the networked nanoconstrictions method, the thermal transport property of graphene could be tuned over a wider range. And we believe this route will effectively expand potential applications of two-dimensional graphene and also pave the way for three-dimensional materials in the future.

Topics: Graphene
Commentary by Dr. Valentin Fuster
2016;():V001T03A009. doi:10.1115/MNHMT2016-6714.

In microchannel flow boiling, bubble nucleation, growth and flow regime development are highly influenced by channel cross-section and physical phenomena underlying this mechanism are far from being well-established. Relative effects of different forces acting on wall-liquid and liquid-vapor interface of a confined bubble play an important role in heat transfer performances. Therefore, fundamental investigations are necessary to develop enhanced microchannel heat transfer surfaces. Force analysis of vapor bubble dynamics in flow boiling Silicon Nanowire (SiNW) microchannels has been performed based on theoretical, experimental and visualization studies. The relative effects of different forces on flow regime, instability and heat transfer performances of flow boiling in Silicon Nanowire microchannels have been identified. Inertia, surface tension, shear, buoyancy, and evaporation momentum forces have significant importance at liquid-vapor interface as discussed earlier by several authors. However, no comparative study has been done for different surface properties till date. Detailed analyses of these forces including contact angle and bubble flow boiling characteristics have been conducted in this study. A comparative study between Silicon Nanowire and Plainwall microchannels has been performed based on force analysis in the flow boiling microchannels. In addition, force analysis during instantaneous bubble growth stage has been performed. Compared to Plainwall microchannels, enhanced surface rewetting and critical heat flux (CHF) are owing to higher surface tension force at liquid-vapor interface and Capillary dominance resulting from Silicon Nanowires. Whereas, low Weber number in Silicon Nanowire helps maintaining uniform and stable thin film and improves heat transfer performances. Moreover, force analysis during instantaneous bubble growth shows the dominance of surface tension at bubble nucleation and slug/transitional flow which resulted higher heat transfer contact area, lower thermal resistance and higher thin film evaporation. Whereas, inertia force is dominant at annular flow and it helps in bubble removal process and rewetting.

Commentary by Dr. Valentin Fuster
2016;():V001T03A010. doi:10.1115/MNHMT2016-6722.

To micro-structures of porous materials, the capillary force which is deeply affected by wettability plays an important role. In this paper, directly oxidation method and functionalization by trichloro (1H, 1H, 2H, 2H-perfluorooctyl) - silane are used to modify metal mesh wettability and superhydrophilic and superhydrophobic copper meshes are fabricated. Super-hydrophilic mesh can block bubbles from flowing through, while the superhydrophobic mesh can hold a column of liquid by counteracting gravity which is defined as a self-compatibility of meshes in this paper. As reported in the previous studies, the mesh with micro-pores can modulate two phase flow pattern to enhance heat transfer. In the present study, the dynamic principle of bubbles as they flow through meshes with different wettabilities is studied. A mathematical model between the critical diameter and flow conditions is developed. A fundamental conclusion for the modulation theory of two phase flow in porous structures can be reached.

Commentary by Dr. Valentin Fuster
2016;():V001T03A011. doi:10.1115/MNHMT2016-6726.

This paper presents the effects of heat dissipation performance of pin fins with different heat sink structures. The heat dissipation performance of two types of pin fin arrays heat sink are compared through measuring their heat resistance and the average Nusselt number in different cooling water flow. The temperature of cpu chip is monitored to determine the temperature is in the normal range of working temperature. The cooling water flow is in the range of 0.02L/s to 0.15L/s. It’s found that the increase of pin fins in the corner region effectively reduce the temperature of heat sink and cpu chip. The new type of pin fin arrays increase convection heat transfer coefficient and reduce heat resistance of heat sink.

Commentary by Dr. Valentin Fuster
2016;():V001T03A012. doi:10.1115/MNHMT2016-6727.

The open system of visual loop heat pipe experimental rig driven by phase change of the refrigerant is established, which is used to research the effect of parameters of the volume, supplementary of refrigerant, properties of wick, height of evaporation cavity and heating power on the performance of this system quantitatively, also the heat transfer characteristics of refrigerant flow in the evaporator visually is studied. We observed and researched the whole process of system from the start up to the stable condition in the evaporator, the changes of refrigerant which is from boiling to the gas-liquid separation. From the experimental point of view, it provides a basis for the establishment of the closed system and for the creation of new mathematical model of the driving mechanism.

Topics: Design , Heat pipes
Commentary by Dr. Valentin Fuster
2016;():V001T03A013. doi:10.1115/MNHMT2016-6728.

For the state of condensation in tube, liquid condensate separation in middle process can prolong the state of steam entrance region of higher heat transfer coefficient. It is called short-tube effect theory. Combined with the traditional condenser, a shell and tube condenser was designed for experiment research in this paper, and compared with the traditional condenser by opening liquid distribution pipes arranged in both sides of condenser. The results showed that liquid distribution pipes with different diameter have different condensation effect. Under the same steam flow rate of inlet, liquid distribution pipes with different combination of diameter and number indicated that its coefficient of heat transfer are higher than the traditional heat transfer by 14.2%, 15.5% and 25.1%. This result illustrated that heat exchange efficiency of a shell and tube condenser with liquid distribution pipes is better than a traditional condenser.

Commentary by Dr. Valentin Fuster

Micro/Nanoscale Boiling and Condensation Heat Transfer

2016;():V001T04A001. doi:10.1115/MNHMT2016-6382.

Jumping-droplet enhanced condensation has recently attracted huge interest due to its remarkable potential of heat transfer performance enhancement, and studies have been done to design superhydrophobic surfaces with various surface morphologies. We fabricated a superhydrophobic micromesh-covered surface using a facile and scalable method. ESEM condensation experiment results show that droplets in pores formed by the mesh wires had faster growth rate in the upward direction than droplets on wires. This is mainly because of the confining role of the wires and higher heat transfer rate due to larger solid-liquid contact area. Also, these droplets always jumped at the size of pores (∼35 μm) when they coalesced with other droplets on wires. Moreover, droplets in pores were distorted by mesh wires, resulting in larger surface area. Theoretical predictions show, for a specific droplet radius, coalescence jumping of distorted droplets on the mesh-covered surface releases more excess surface free energy, and has larger jumping velocity than that of spherical droplets on the plate surface without mesh. This better performance was further validated by constant exposure of those two surfaces to electron beam during which work of adhesion was gradually increased. As expected, droplets on the mesh-covered surface coalesced and jumped while coalescing droplets on the plate surface could not as the exposure time increased. Our results offer new insights for designing hierarchical structured superhydrophobic surfaces to further enhance the performance of condensation heat transfer processes.

Topics: Condensation , Drops
Commentary by Dr. Valentin Fuster
2016;():V001T04A002. doi:10.1115/MNHMT2016-6436.

Single droplet based investigations have been performed for hundreds of years. However, in many industrial applications, such as printing, spray cooling and coating etc, numerous droplets will be produced. Droplet train, therefore, is a physical model to approach the complex situation. When the wall temperature is higher than the boiling point, the problem becomes even complex. The subcooling of the droplet, the superheat of the wall also influence the hydrodynamic pattern of the droplet impingement. The hydrodynamic behavior of the water droplet train impinging onto a hot surface (up to 220 °C) is investigated. A droplet train generator is employed to produce stable high velocity (around 6.35 to 19.13 m/s) droplet train (with a diameter around 0.1 mm) at the droplet frequency ranges from 27990 Hz to 55560 Hz. The hot surface is made by copper and heated with cartridge heaters. The effect of wall superheat on flow pattern is experimentally examined and reported. The results show that the wall temperature plays an significant role to the impingement. It influences the spreading speed, stable spreading diameter and splashing angle apparently.

Topics: Drops , Trains
Commentary by Dr. Valentin Fuster
2016;():V001T04A003. doi:10.1115/MNHMT2016-6465.

With the inspiration from electrowetting-controlled droplets, the potential advantages of electrowetting for bubble dynamics are investigated experimentally and numerically. In our experimental system, a 100 nanometer thin film gold metal was used as an electrode, and a 6.5 micrometer polydimethylsiloxane (PDMS) was spin-coated on the electrode acting both as an dielectric layer and hydrophobic surface. A two-phase model coupled with a electrostatics was used in our simulation work, where the body force due to the electric field acts as an external force. Our numerical results demonstrated that electrowetting can help the detachment of a small bubble by changing the apparent contact angle. Similar results were observed in our experiments that with electrowetting on dielectric, the contact angle of bubble on a hydrophobic surface will obviously decrease when a certain electrical field is applied either with a small size bubble (diameter around 1mm) or a relatively larger size bubble (diameter around 3 mm). When the applied voltage becomes high enough, both the experimental and numerical results demonstrate the characteristics of bubble detachment within a thin film liquid layer.

Commentary by Dr. Valentin Fuster
2016;():V001T04A004. doi:10.1115/MNHMT2016-6514.

Two-phase boiling in advanced microchannel heat sinks offers an efficient and attractive solution for heat dissipation of high-heat-flux devices. In this study, a type of reentrant copper microchannels was developed for heat sink cooling systems. It consisted of 14 parallel Ω-shaped reentrant copper microchannels with a hydraulic diameter of 781μm. Two-phase pressure drop characteristics were comprehensively accessed via flow boiling tests. Both deionized water and ethanol tests were conducted at inlet subcooling of 10°C and 40°C, mass fluxes of 125–300kg/m2·s, and a wide range of heat fluxes and vapor qualities. The effects of heat flux, mass flux, inlet subcoolings and coolants on the two-phase pressure drop were systematically explored. The results show that the two-phase pressure drop of reentrant copper microchannels generally increased with increasing heat fluxes and vapor qualities. The role of mass flux and inlet temperatures was dependent on the test coolant. The water tests presented smaller pressure drop than the ethanol ones. These results provide critical experimental information for the development of microchannel heat sink cooling systems, and are of considerable practical relevance.

Commentary by Dr. Valentin Fuster
2016;():V001T04A005. doi:10.1115/MNHMT2016-6544.

Boiling heat transfer is widely used in industry and aerospace, and it can be enhanced by surface structure treatment. Here, two types of Micro-Nano bi-porous copper surfaces (MNBPCS) were prepared by hydrogen bubble template method and then sintered in reducing atmosphere. The effect of surface morphology on the saturated pool boiling of ultrapure water was investigated. Results show that, both NMBPCS have superior heat transfer performance to the plain copper surface. When the heat flux is 100W/cm2, the wall superheat of the two MNBPCS are about 7 and 9 °C lower than the plain copper surface respective. When the heat flux is lower than 130W/cm2, the wall superheat of the mono-layer MNBPCS is lower than that of the multi-layer one, because the bubbles formed on the mono-layer MNBPCS can departure more easily than those on the multi-layer one. When the heat flux is higher than 130W/cm2, the multi-layer MNBPCS has lower wall superheat than that of the mono-layer one, own to its better liquid accommodation from the morphology structure. Significant hysteresis phenomenon was only found on the Multi-layer MNBPCS. Its wall superheat keeps almost the same at about 13°C for its bottom layer structure with smaller cave diameter, when the heat flux is higher than 75W/cm2. The CHF of each MNBPCS is higher than 200W/cm2, and the multi-layer one is higher than the mono-layer one own to its better liquid accommodation from the morphology structure.

Commentary by Dr. Valentin Fuster
2016;():V001T04A006. doi:10.1115/MNHMT2016-6566.

Understanding the condensation mechanism is crucial to enhance the heat transfer performance of numerous industrial applications such as power generations, fog harvesting, water desalination, cooling of nuclear reactor, and thermal management of electronic device. In the present study, simulations are performed to investigate the effect of surface wettability on droplet growth dynamics during dropwise condensation. To simulate droplet growth dynamics involving phase change heat transfer, thermal lattice Boltzmann method has been employed with two distribution function for fluid and temperature field. Simulations performed in this work are used to analyze the effect of surface wettability on nucleation time and the evolution of average droplet radius, height, base diameter, and contact angle of the droplet. It is observed that nucleation time increases exponentially with the contact angle. The growth rate of droplet is higher for smaller droplets compared to larger droplets.

Commentary by Dr. Valentin Fuster
2016;():V001T04A007. doi:10.1115/MNHMT2016-6596.

A simultaneous visualization and heat performance of oscillating heat pipes (OHPs) were performed. Experiments were performed under different surface wetting characteristics. Results showed that the start-up performance was improved on hydrophilic OHP as opposed to the copper OHP. A small bubble grew quickly and became a vapor plug in the evaporation section with hydrophilic surface. The process of vapor expansion and contraction accompanying liquid slug movement upward and backward continued to occur as a spring, and the OHPs started up. However, the hydrophobic OHP failed to start up. For the superhydrophobic OHP, nucleate boiling took place in the evaporation section, and the bubble expansion and contraction phenomenon were not observed. Heat transfer results showed that wall temperature fluctuations were observed at the start-up stage. The start-up time for the hydrophilic OHP was lowest and the amplitudes of temperature oscillations were increased in hydrophilic OHP compared to the copper OHP.

Topics: Wetting , Heat pipes
Commentary by Dr. Valentin Fuster
2016;():V001T04A008. doi:10.1115/MNHMT2016-6616.

Selective laser melting (SLM) is a promising manufacturing method which enables the production of complex structured components from base metal powders. With the development of SLM, the possibility of fabricating functional heat transfer devices such as heat pipes and heat sinks using this technique has also gained significant interest in the recent years. In this paper, the possibilities of producing microstructured surfaces using SLM to promote nucleate pool boiling heat transfer were explored. The SLM facility (SLM 250 HL by SLM Solutions GmbH) at the Future of Manufacturing Laboratory 1 of Singapore Centre for 3D Printing (SC3DP) in Nanyang Technological University (NTU), Singapore was employed for the fabrication of the surfaces. The machine is comprised of a Gaussian distributed Yb:YAG laser with maximum power of 400 W and laser beam spot size of 80 μm which melts and fuses the AlSi10Mg base powder of distribution size 20 μm to 63 μm layer-by-layer to develop three-dimensional structures.

In total, four 1 cm × 1 cm microstructured surfaces were produced; namely micro-cavity surface, micro-fin surface, micro-sized rectangular channel (MRC) surface and micro-sized square channel (MSC) surface. Saturated pool boiling experiments were conducted on these surfaces in a water-cooled thermosyphon with FC-72 as the coolant fluid under atmospheric condition. In comparison with a plain surface, the MRC and MSC surfaces exhibited marginal improvements in the average heat transfer coefficient whilst more significant enhancements of up to 51.2% were demonstrated with the micro-cavity and micro-fin surfaces. At low heat fluxes (< 7 W/cm2), minimal differences in heat transfer performances between the microstructured surfaces and plain surface were observed. For increased heat fluxes, incremental enhancements in the heat transfer coefficients were observed for the micro-cavity and micro-fin surfaces as compared to the plain surface. The highest enhancement in the heat transfer coefficient over the plain surface was determined to be 63.5% for the micro-fin surface at the heat flux of 17.9 W/cm2 and it was also observed that the heat transfer coefficient of micro-fin surface is consistently higher that of other microstructured surfaces for the range of heat fluxes tested. In addition, higher critical heat fluxes were also achieved with all microstructured surfaces as compared to the plain surface with the highest CHF of 46.2 W/cm2 for the micro-fin and MRC surface. Visual observations suggest that the enhancement in heat transfer from the microstructured surfaces is likely to be due to the increased bubble nucleation sites created from the extended surfaces and the artificial cavities. In summary, these results indicate the promising use of SLM to produce surface features that will enhance pool boiling heat transfer.

Commentary by Dr. Valentin Fuster
2016;():V001T04A009. doi:10.1115/MNHMT2016-6661.

Both theoretical and experimental research on CO2 physical characteristics and pool boiling heat transfer are taken in this paper. It is analyzed that CO2 pool boiling heat transfer outside the screwed and smooth tube based on experimental study. It is resulted with the relationship between heat transfer coefficient, evaporating pressure and the heat flux. It is also indicated the enhancement factors of three screwed tube compared with smooth tube. The results provides a basis for promoting CO2 heat transfer enhancement, and also provides a theoretical support in engineering.

Commentary by Dr. Valentin Fuster
2016;():V001T04A010. doi:10.1115/MNHMT2016-6712.

Flow boiling in an array of five parallel microchannels (W=200 μm, H=250 μm, L=10 mm) can be dramatically enhanced using self-excited and self-sustained high frequency two-phase oscillations induced by two-nozzle configuration. However, the effect of the two-phase oscillations is confined to the downstream of the microchannels. In this study, four-nozzle microchannel configuration is developed with an aim to extend mixing to the entire channel. Flow boiling in the four-nozzle microchannel is experimentally studied with deionized water over a mass flux range of 120 to 600 kg/m2 s. Overall average heat transfer coefficient (HTC) is significantly enhanced approximately 54.5% by extending the enhanced multi-channel mixing to the whole channel. It is equally important that the pressure drop can be further reduced by approximately 50%. Compared with previous two-nozzle design, four-nozzle configuration not only extends the mixing to the whole channel but also increase nucleation sites, which has been confirmed by visualization study to promote nucleation boiling.

Commentary by Dr. Valentin Fuster
2016;():V001T04A011. doi:10.1115/MNHMT2016-6713.

Flow boiling in Silicon Nanowire microchannel enhances heat transfer performance, CHF and reduces pressure drop compared to Plainwall microchannel. It is revealed by earlier studies that promoted nucleate boiling, liquid rewetting and enhanced thin film evaporation are the primary reasons behind these significant performance enchantments. Although flow regime plays a significant role to characterize the flow boiling Silicon Nanowire microchannel performances; surface characteristics, hydrodynamic phenomena, bubble contact angle and surface orientation are also some of the major influencing parameters in system performances. More importantly, effect of orientation (effect of gravity) draws a great attention in establishing the viability of flow boiling in microchannels in space applications. In this study, the effects of heating surface orientation in flow boiling Silicon Nanowire microchannels have been investigated to reveal the underlying heat transfer phenomena and also to discover the applicability of this system in space applications. Comparison between Nanowire and Plainwall microchannels have been performed by experimental and visual studies. Experiments were conducted in a forced convection loop with deionized water at mass flux range of 100kg/m2s – 600kg/m2s. Micro devices consist of five parallel straight microchannels with Nanowire and without Nanowire (Plainwall) (200μm × 250μm × 10mm) were used to investigate the effects of orientation. Two different orientations were used to perform the test: upward facing (0° Orientation) and downward facing (180° Orientation). Results for Plainwall show sensitivity to orientation and mass flux, whereas, little effects of mass flux and orientation have been observed for Nanowire configuration.

Commentary by Dr. Valentin Fuster

Micro/Nanoscale Thermal Radiation

2016;():V001T05A001. doi:10.1115/MNHMT2016-6337.

Based on the structure of metal parallel plates, this paper points out that thermal radiation in near field is greatly enhanced due to near-field effect, exceeding Planck’s blackbody radiation law. To study the effect of graphene on thermal radiation in near field, we add graphene layer into the structure. The result indicates that the graphene layer effectively suppresses the near-field thermal radiation between metal plates, having good ability of thermal insulation. In consideration of the thickness of 0.34nm of single-layer graphene, we can say graphene plays a very important role in controlling the near-field thermal radiation.

Topics: Metalwork , Graphene
Commentary by Dr. Valentin Fuster
2016;():V001T05A002. doi:10.1115/MNHMT2016-6352.

With the rapid development of the supersonic aircraft technology, tremendously, the aircraft Mach numbers get higher and higher, but on the other hand, the working condition become worse and worse. The photonic crystal material which is formed by the periodic micro/nanoscale structures can generate the photonic band gaps, and the photonic band gaps could reflect the energy of the electromagnetic wave effectively. Consequently, the photonic crystal material turns into the newly-developing hotspot on the field of thermal protection for the supersonic aircraft. In this paper, the aircraft states of Mach 6 are set as the target operating condition, and 5 optimum proposals are presented for the structures of typical photonic crystal material. The energy which gets into the body material is calculated; Based on the theory of the electromagnetic field, using the method of transmission matrix and Plane Wave Expansion (PWE), the characteristics of the photonic band gaps for one-and-three dimensional photonic crystals are calculated. Finally, the characteristics of the photonic band gaps are discussed, and optimal design for the performance of the photonic crystal material thermal protection are proposed.

Commentary by Dr. Valentin Fuster
2016;():V001T05A003. doi:10.1115/MNHMT2016-6420.

Thermochromic coating on yttria stabilized zirconia (YSZ) substrate was prepared by the sol-gel La0.825Sr0.175MnO3 nanoparticles and the binder composed of terpineol and ethyl cellulose. The surface morphology and the variable emittance properties of coating in the infrared range was evaluated from infrared reflectance spectra at various temperatures. LSMO nanoparticles with size 200 nm are obtained by the sol-gel process. The coating sample shows a thermochromic behavior with the rise of temperature. The emittance of coating increases from 0.62 at 173 K to 0.88 at 373 K. When the sample surface root-mean-square roughness is 122 nm, its emittance variation increases from 0.43 at 173 K to 0.73 at 373 K.

Commentary by Dr. Valentin Fuster
2016;():V001T05A004. doi:10.1115/MNHMT2016-6469.

The present study focuses on nanowire based metamaterials with excitation of magnetic polariton (MP) as selective solar absorbers. Finite-difference time-domain simulation is employed for numerically designing a broadband solar absorber made of lossy tungsten nanowires which exhibit spectral selectivity due to the excitation of MP. An inductor-capacitor circuit model of the nanowire array is developed in order to predict the resonance wavelengths of the MP harmonic modes. The effects of geometric parameters such as nanowire diameter, height, and array period are investigated and understood on tuning the magnetic polariton resonance and the resulting optical and radiative properties. In addition, the independence of incidence angles is demonstrated, which illustrates the potential applicability of the nanowire-based metamaterial as a high-efficiency wide-angle selective solar absorber. The results will facilitate the design of novel low-cost and high-efficiency materials for enhancing solar thermal energy harvesting and conversion.

Commentary by Dr. Valentin Fuster
2016;():V001T05A005. doi:10.1115/MNHMT2016-6471.

The photon transport and energy conversion of a near-field thermophotovoltaic (TPV) system with a selective emitter composed of alternate tungsten and alumina layers and a photovoltaic cell sandwiched by electrical contacts are theoretically investigated in this paper. Fluctuational electrodynamics along with the dyadic Green’s function for a multilayered structure is applied to calculate the spectral heat flux, and photocurrent generation and electrical power output are solved from the photon-coupled charge transport equations. The tungsten and alumina layer thicknesses are optimized to match the spectral heat flux with the bandgap of TPV cell. The spectral heat flux is much enhanced when plain tungsten emitter is replaced with the multilayer emitter due to the mechanism of surface plasmon polariton coupling in the tungsten thin film. In addition, the invalidity of effective medium theory to predict photon transport in the near field with multilayer emitters is discussed. Effects of a gold back reflector and indium tin oxide front coating with nanometer thickness, which could practically act as the electrodes to collect the photon-generated charges on the TPV cell, are explored. Conversion efficiency of 23.7% and electrical power output of 0.31 MW/m2 are achieved at 100 nm vacuum gap when the emitter and receiver are respectively at temperatures of 2000 K and 300 K.

Commentary by Dr. Valentin Fuster
2016;():V001T05A006. doi:10.1115/MNHMT2016-6510.

In this work, we investigated the reflection properties of artificial opals composed of submicron silica spheres with diverse structural parameters and under the effect of light in different states. Furthermore, the primary rules how the reflection properties of artificial opals convert as these factors changing have been revealed clearly. These factors can take effects in changing the shape, value, and position of the peak of the hemispherical reflectance of artificial opals. Then we got the distribution and propagation process of the Poynting vectors corresponding to the positions of the diffraction peak and the low reflectance in the artificial opals at normal and oblique incidence of P-polarization. Comparing with the theoretical interpretation which is a little complicated and nonobjective, this paper will provide a visual result to explain the reason why the structure has high reflectance in some spectral ranges.

Commentary by Dr. Valentin Fuster
2016;():V001T05A007. doi:10.1115/MNHMT2016-6551.

We present a 2D square loop-shaped nanostructure, which is made of a square loop aluminum array on Al2O3 spacer and Al substrate. High absorption peaks are obtained at 3.5μm and 9μm when the incident wave is vertically. In the design of dual-band or multi-band structure, the two high absorption bands are designed to stimulate the outer magnetic excitation of the first-order and the high-order magnetic resonance wavelength. For structure design with two absorption peaks or multiple absorption peaks, the expectation bands with high absorption would be obtained in the cooperation between first-order and higher-order magnetic resonance due to the outer structure. The main absorption peak due to the inner structure may be coupled the second absorption peak due to the outer structure. Then the absorption bandwidth could be broadened and the dual-band perfect absorption effect could be obtained in this loop-shaped structure.

Topics: Absorption
Commentary by Dr. Valentin Fuster
2016;():V001T05A008. doi:10.1115/MNHMT2016-6581.

For enhancement of absorption and transmission at a specified wavelength using magnetic polariton (MP) resonance, it is necessary to determine the accurate geometry parameters at the corresponding resonance condition. In this work, the feature of the geometry design problem is analyzed and a method is presented for accurately determining the geometry parameters for specified MP resonance mode, which combines the LC circuit model for MP and inverse technique. The LC circuit model is used to give an initial rough design of geometric parameters and parameters range for the inverse algorithm. The particle swarm optimization (PSO) algorithm is used to minimize the objective function and determine the optimized geometric parameters. The forward problem to evaluate the objective function is solved using rigorous method. The presented method is demonstrated to have good performance in the geometry design of MP resonance structure using several example cases.

Commentary by Dr. Valentin Fuster
2016;():V001T05A009. doi:10.1115/MNHMT2016-6584.

In this paper, simple selective solar absorbers with three layers are investigated, and their selective absorptivity spectra are quite appropriate for high performance solar absorbers. The simple solar absorber contains top ultrathin tungsten (W) layer, middle silica layers and W substrate. The thickness of silica can determine the location of absorptivity peak while the thickness of top W layer affects the intensity of absorptivity. Considering the total conversion efficiency, optimized thicknesses in solar absorbers are determined by genetic algorithm. This optimized thin film solar absorber keeps high absorptivities when incident direction varies from 0 degree to 60 degree in both TE and TM polarizations. Experiments validate the effectivity of thin film solar absorbers, and the deviation from simulations comes from increscent refractive index and surface non-uniform.

Commentary by Dr. Valentin Fuster
2016;():V001T05A010. doi:10.1115/MNHMT2016-6585.

Thermal barrier coatings provide excellent thermal insulation for metal components of gas turbines. Although the relationships between microstructures and mechanical properties as well as thermal conductivity of various TBCs have been extensively studied, there still exists a deficiency of a full understanding on microstructural-related thermal radiation transport inside them, which becomes more and more crucial for advanced gas turbine applications requiring higher operating temperatures. This study aims at presenting a microstructure-based numerical investigation on radiative transfer in the air plasma sprayed (APS) TBC.

In this study, the microstructures of APS TBCs are quantitatively reconstructed based on the Ultra-Small-Angle X-Ray Scattering (USAXS) measurement by the Stony Brook University group, in which the microscale interlamellar pores, intrasplat cracks and globular voids are regarded as oblate spheroids with different sizes and aspect ratios, with a specific distribution of orientations. This is a typical anisotropic medium, in which the physical properties vary with the observing direction. The anisotropic feature of radiative properties including the scattering coefficient and phase function is for the first time demonstrated using the discrete dipole approximation (DDA) method. A modified Monte Carlo method is proposed and implemented to solve the anisotropic radiative transfer problem in such medium. The spectral normal-hemispherical reflectance and transmittance of the coating are thus obtained and further compared with the experimental data from literature as well as our group to validate this numerical method. This work provides a versatile numerical framework for the study of the anisotropic radiative transfer mechanism in APS thermal barrier coatings based on microstructure charaterization.

Commentary by Dr. Valentin Fuster
2016;():V001T05A011. doi:10.1115/MNHMT2016-6632.

Near-field thermophotovoltaic (NFTPV) devices have received much attention lately as attractive energy harvesting systems, whereby a heated thermal emitter exchanges super-Planckian near-field radiation with a photovoltaic (PV) cell to generate electricity. This work describes the use of a grating structure to enhance the power throughput of NFTPV devices, while increasing thermal efficiency by ensuring that a large portion of the radiation entering the PV cell is above the bandgap. The device is modeled as a one-dimensional high-temperature tungsten grating on a tungsten substrate that radiates photons to a room-temperature In0.18Ga0.82Sb PV cell through a vacuum gap of several tens of nanometers. Scattering theory is used along with the rigorous coupled-wave analysis to calculate the radiation exchange between the grating emitter and the PV cell. A parametric study is performed by varying the grating depth, period, and ridge width in the range that can be fabricated using available fabrication technologies. By optimizing the grating parameters, it is found that the power output can be improved by 40% while increasing the energy efficiency by 6% as compared with the case of a flat tungsten emitter. Reasons for the enhancement are investigated and found to be due to the surface plasmon polariton resonance, which shifts towards lower frequencies. This work shows a possible way of improving NFTPV and sheds light on how grating structures interact with thermal radiation at the nanoscale.

Commentary by Dr. Valentin Fuster
2016;():V001T05A012. doi:10.1115/MNHMT2016-6647.

Nanofluids obtain high stability, improved heat transfer capability and excellent optical properties, the low-temperature nanofluid-based direct absorption solar collector (NDASC) has been previously investigated. However, the detailed radiation absorption and heat transfer mechanism for a NDASC with a solar concentrator operated on medium-temperature conditions were seldom researched. Therefore, this paper presents a numerical study on the solar collection characteristics of NDASC with a parabolic trough concentrator. CuO/oil nanofluids with various weight concentration from 0.05% to 0.1% were prepared, and used as working fluids of NDASCs, respectively. Using the developed heat transfer model, operating characteristics of NDASCs were simulated. Furthermore, the influences of weight concentration of nanofluids on the heat transfer characteristics in the NDASCs were analyzed and optimum weight concentration used for the designed NDASC obtained.

Commentary by Dr. Valentin Fuster
2016;():V001T05A013. doi:10.1115/MNHMT2016-6683.

Here, closed-end microcavity is proposed in which a semi-transparent metal film was formed atop microcavity. The structure shows weak angular dependence as well as quasi-monochromatic absorptance. Au is employed as material of the cavity walls and the covering thin film. Quasi-monochromatic absorption from the structure is observed in numerical simulation. High quality factor (Q factor) is obtained by strong confinement in the closed-end microcavity. Asymmetric and quasi-monochromatic absorption band with a Q factor of ∼28 at 1.85 μm was observed. This value was about 4-fold larger than that of the open-end microcavity. Additionally, the closed-end microcavity structure filled with SiO2 in cavity exhibits isotropic and quasi-monochromatic thermal radiation over a wide solid angle. This result suggests that both quasi-monochromatic and low-directivity absorptance can be realized by using this configuration.

Commentary by Dr. Valentin Fuster
2016;():V001T05A014. doi:10.1115/MNHMT2016-6692.

Solar-thermophotovoltaic system is expected to have high efficiency by converting wide spectral range solar energy into useful thermal radiation energy. However, the experimental STPV system shows much lower efficiency than theoretical one. To achieve high-efficiency, it is essential to obtain good spectrally matching between thermal radiation spectrum and PV cells spectral response. In this paper, the power generation tests using the whole configuration of the STPV system is described. The conversion efficiency of GaSb PV cell is estimated to be 20 to 23% against to the light intensity irradiated on the PV cell surface. The net system efficiency of 1.9% can be achieved. The application of thermal storage system to the STPV is also considered.

Topics: Solar energy
Commentary by Dr. Valentin Fuster
2016;():V001T05A015. doi:10.1115/MNHMT2016-6698.

Thermophotovoltaic (TPV) energy conversion enables millimeter scale power generation required for portable microelectronics, robotics, etc. In a TPV system, a heat source heats a selective emitter to incandescence, the radiation from which is incident on a low bandgap TPV cell. The selective emitter tailors the photonic density of states to produce spectrally confined selective emission of light matching the bandgap of the photovoltaic cell, enabling high heat-to-electricity conversion efficiency. The selective emitter requires: thermal stability at high-temperatures for long operational lifetimes, simple and relatively low-cost fabrication, as well as spectrally selective emission over a large uniform area. Generally, the selective emission can either originate from the natural material properties, such as in ytterbia or erbia emitters, or can be engineered through microstructuring. Our approach, the 2D photonic crystal fabricated in refractory metals, offers high spectral selectivity and high-temperature stability while being fabricated by standard semiconductor processes. In this work, we present a brief comparison of TPV system efficiencies using these different emitter technologies. We then focus on the design, fabrication, and characterization of our current 2D photonic crystal, which is a square lattice of cylindrical holes fabricated in a refractory metal substrate. The spectral performance and thermal stability of the fabricated photonic crystal thermal emitters are demonstrated and the efficiency gain of our model TPV system is characterized.

Commentary by Dr. Valentin Fuster
2016;():V001T05A016. doi:10.1115/MNHMT2016-6699.

The spectrally selective coating technology which can be applicable for solar-thermophotovoltaic (solar-TPV) generation systems is described in this paper. In solar-TPV system, the spectrally selective absorber plays a key role to obtain high-efficiency. The technologies of controlling thermal radiation spectrum at temperature over 1000°C, however, have not been established yet. We focus on metal-dielectric multi-layer coating. This selective coating shows enormously high absorptance at short wavelength range and sharp cutoff property. Thermal stability test confirms that this multi-layer structure can be one of the candidates for the selective coatings for solar-TPV systems.

Commentary by Dr. Valentin Fuster

Micro/Nanoscale Energy Devices and Systems

2016;():V001T06A001. doi:10.1115/MNHMT2016-6328.

Continuous water film formed on a hydrophilic or superhydrophilic surface can delay the formation of a vapor film in boiling and thus improve critical heat flux (CHF), therefore, the fabrication of hydrophilic or superhydrophilic surface is an efficient approach to enhance boiling heat transfer. In the present work, superhydrophilic TiO2 nanotube arrays (TiNAs) interfaces are fabricated by anodization in fluoride contained electrolyte, and the fluorine is found out to be the key factor affecting the wettability of TiNAs interfaces. After anodization, a stable oxy-fluoride layer was formed at the interface as form of -O-Ti-F, the fluorine atoms are linked to the interface as terminal groups. Due to the strong polarity of titanium oxy-fluorides, superhrophilic TiO2 nanotube arrays interface is obtained. Furthermore, we characterize the stability of titanium oxy-fluorides by storing. After store for 2 months, the inner titanium fluorides (TiF4) are lost due to its strong volatility. Fortunately, the content of titanium oxy-fluorides remains the same, and retain its remarkable superhydrophilic properties. It is potential to design energy-efficient devices ranging from boiling heat transfer to self-cleaning.

Topics: Nanotubes
Commentary by Dr. Valentin Fuster
2016;():V001T06A002. doi:10.1115/MNHMT2016-6329.

In this work, we first systematically investigate the ballistic transport properties of armchair WSe2 nanoribbons by using first-principles method. An enhancement in thermoelectric figure of merit (ZT) is discovered from monolayer to nanoribbons. To explore the origin of the enhancement mechanism, H-passication is introduced into the systems to make a comparison. The introduction of H-passivation stabilizes the dangling bonds at the ribbon edge and reduces the enhancement. It comfirms our suspect that the enhancement may be contributed from the disorder edge effect owing to the existence of dangling bonds. Our work provides instructional theoretical evidence for the application of armchair WSe2 nanoribbons as promising thermoelectric materials. The enhancement mechanism of disorder edge effect can also highlight the exploration of achieving outstanding thermoelectric materials.

Commentary by Dr. Valentin Fuster
2016;():V001T06A003. doi:10.1115/MNHMT2016-6357.

The utilization of solar energy in photovoltaics is limited due to the band gap of the materials. Hence, photovoltaic–thermoelectric hybrid system was proposed to utilize solar energy in the full spectrum of AM1.5G. On this basis, a novel design of GaAs solar cell is proposed in this paper for the full spectrum absorption in the cell structure, which consists of an ultra-thin GaAs layer with nanocones on the surface and a nanogrid–AZO–Ag back contact. The Finite Difference Time Domain method is used to analyze the full spectrum absorption features for TE and TM polarizations over the incident angles varying from 0° to 60°. The designed structure shows high absorption in the full spectrum. For GaAs layer, it is shown that the solar usable energy for GaAs solar cells in 300–900nm is absorbed by GaAs almost perfectly due to the anti–reflection property of the nanocone array. The absorbed energy in the back contact in the longer wavelengths over 900nm is due to the Fabry-Perot and the localized plasmonic resonances. The structure can collect full-spectrum incident photons efficiently in GaAs solar cells for the application of photovoltaic–thermoelectric hybrid system.

Commentary by Dr. Valentin Fuster
2016;():V001T06A004. doi:10.1115/MNHMT2016-6604.

We present the rarefaction effects on diffusive mass transport in micro- and nanoscales using the results of direct simulation Monte Carlo DSMC method. Unlike the previous investigations, the momentum and heat contributions are eliminated from the computations via uniform velocity, pressure, and temperature field considerations. The effects of global Knudsen number on the diffusion phenomenon are studied for the same Peclet number and a unique mixer shape. The results indicate that there is considerable weakening in diffusion mechanism for high Knudsen number cases. As a result, the non-dimensional diffusive mass fluxes would decrease and the non-dimensional mixing length would increase as the Knudsen number increases. The effective diffusion coefficient is calculated throughout the mixer using the diffusive mass fluxes and the species mass fraction gradients. It is observed that the effective diffusion coefficient can vary considerably as a result of local rarefaction variations. It reaches to the lowest value at the point of confluence, where the maximum mass fraction gradient magnitude would occur for the species. Moving away from this point, the local rarefaction effects would weaken and the effective diffusion coefficient would reinforce subsequently. All the presented results indicate that there would be a convergent to a limiting behavior, which corresponds to the continuum mass diffusion case. Despite this, the local rarefaction level decreases continuously. Unfortunately, because of a considerable increase in the statistical fluctuations at very low rarefaction levels, the simulations do not provide reliable results in the limit of continuum regime.

Commentary by Dr. Valentin Fuster
2016;():V001T06A005. doi:10.1115/MNHMT2016-6695.

The increasing power demands of portable electronics and micro robotics has driven recent interest in millimeter-scale microgenerators. Many technologies (fuel cells, Stirling, thermoelectric, etc.) that potentially enable a portable hydrocarbon microgenerator are under active investigation. Hydrocarbon fuels have specific energies fifty times those of batteries, thus even a relatively inefficient generator can exceed the specific energy of batteries. We proposed, designed, and demonstrated a first-of-a-kind millimeter-scale thermophotovoltaic (TPV) system with a photonic crystal emitter. In a TPV system, combustion heats an emitter to incandescence and the resulting thermal radiation is converted to electricity by photovoltaic cells. Our approach uses a moderate temperature (1000–1200°C) metallic microburner coupled to a high emissivity, high selectivity photonic crystal selective emitter and low bandgap PV cells. This approach is predicted to be capable of up to 30% efficient fuel-to-electricity conversion within a millimeter-scale form factor. We have performed a robust experimental demonstration that validates the theoretical framework and the key system components, and present our results in the context of a TPV microgenerator. Although considerable technological barriers need to be overcome to realize a TPV microgenerator, we predict that 700–900 Wh/kg is possible with the current technology.

Topics: Crystals
Commentary by Dr. Valentin Fuster

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