bluuebunny/crossref_metadata_2025 huggingface dataset:
Synthesis and Characterization of Microencapsulated Phase Change Material of Magnesium Sulfate Heptahydrate/Urea Resin via Emulsion Polymerization Method
Authors: Liu Chenzhen , Ma Ling , Rao Zhonghao , Li Yimin
Published: March 2016

In this study, Microencapsulated phase change material (MicroPCM) was successfully synthesized by emulsion polymerization method, using magnesium sulfate heptahydrate (MSH) as core material and urea resin (UR) as shell material. The surface morphology of the MicroPCM was tested by scanning electron microscopy (SEM) and optical microscope (OM). The thermal property and particle size were investigated by differential scanning calorimetry (DSC) and laser particle size analyzer (LPA), respectively. The chemical structure of microcapsule was analyzed by Fourier-transform infrared spectroscopy (FTIR).
Effect of Surface Wettability and Gas/Liquid Velocity Ratio on Microscale Two-Phase Flow Patterns
Authors: Li Hongxia , Okaeme Charles C. , Yang Weilin , Zhang TieJun
Published: March 2016

Predicting and controlling the flow regime transition of multiphase fluids in microchannels is essential for various energy applications, such as flow boiling, de-emulsification and oil recovery processes. This in turn requires a better understanding of multiphase flow behaviors in microchannels with various channel surface wettability, fluid interfacial tension and flow rates. In this paper, experiments and Lattice Boltzmann method (LBM) simulations are carried out to study complicated multiphase flow at micro or meso scales. With the Shan-Chen multiphase LBM model, the flow pattern transitions of adiabatic two phase flow in a microchannel were investigated. The effects of surface wettability and liquid/gas velocity ratio on the flow regime transition were further studied. A series of two-phase flow experiments were conducted on a PDMS microfluidic device under different gas/oil velocity ratios. Under various surface wettability conditions, our simulation results agree well with the flow visualization experiments equipped with a high speed camera (HSC). Our finding shows that the cross-section meniscus curve width, corresponding to the shadow in the HSC photo, increases with decreasing contact angle, which was confirmed by the simulated liquid/gas distribution. Besides the influence of surface wettability, the role of gas/liquid velocity ratio on two-phase flow regime transition was discussed in detail. The proposed approach paves the way to probe complicated physics of multiphase flows in microporous media.
Heat Exchanger Improvement via Curved Microfluidic Channels: Part 1 — Impact of Cross-Sectional Geometry and Channel Design on Heat Transfer Enhancement
Authors: Marshall Samuel D. , Arayanarakool Rerngchai , Balasubramaniam Lakshmi , Li Bing , Lee Poh Seng , Chen Peter C. Y.
Published: March 2016

The efficiency of conventional heat exchangers is restricted by many factors, such as effectiveness of convective heat transfer and the cost of their operation. The current research deals with these issues by developing a novel method for building a lower-cost yet more efficient heat sink. This method involves using a specially designed curved microchannel to utilise the enhanced fluid mixing characteristics of Dean vortices, and thus transferring heat efficiently.

Numerical models have been employed to investigate the heat transfer enhancement of curved channels over straight equivalents, with the aim of optimising the heat exchanger design based on the parameters of maximising heat transfer whilst minimising pressure drop and unit cost. A range of cross-sectional geometries for the curved channels were compared, showing significantly higher Nusselt Numbers than equivalent straight channels throughout, and finding superior performance factors for square, circular and symmetrical trapezoidal profiles. Due the difficulty and expense in manufacturing circular microchannels, the relatively simple to fabricate square and symmetrical trapezoidal channels are put forward as the most advantageous designs. These results take into account both constant wall temperature and constant heat flux conditions. For a given set of channel dimensions, an optimal input flow rate condition is also determined.
Thermal Radiative Properties of (La,Sr)MnO3 Thermochromic Coating Prepared by Sol-Gel Process
Authors: Guo Li , Fan Desong , Li Qiang , Xuan Yimin
Published: March 2016

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.
Molecular Dynamics Study on Fluid Flow in Nanochannels With Permeable Walls
Authors: Xie Jian-Fei , Cao Bing-Yang
Published: March 2016

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.
Preparation and Photocatalysis of a Glass Coated With Nanometer TiO2 Codoped With N, F and Fe Elements
Authors: Ding Cong , Zhang Yufei , Chuan Chen , Liu Yanhua
Published: March 2016

Indoor air pollution seriously threats the life and health of human beings. The improvement of indoor air quality has become a focus that people pay more and more attentions to. The photocatalytic of pollutants based on TiO2 is a promising air purification technology. In order to overcome the disadvantages of nanometer powder TiO2 catalyst and to enhance the photocatalytic activity of TiO2, series of glass plates covered with doped-TiO2 were prepared and the photocatalysis them were studied.

The glass plates covered with TiO2 which was doped in advance with N, F, or/and Fe were prepared by a sol-gel method. The doping content of N, F, Fe and heat treatment temperature were determined using the orthogonal array of the Taguchi quality design. The prepared gel was coated on the glass by spin-coating method. The effects of doping level of N, F and Fe and heat treatment temperature on the photocatalytic capabilities were investigated. The photocatalytic capabilities of prepared glass plates were investigated by degrading the solution of methylene blue (MB,C16H18ClN3S). The results show that appropriate addition of N, F and Fe and temperaturae are effective for improving the photocatalytic activities of TiO2 under visible light. The optimal TiO2 was prepared under the condition that the doping amount of F element was 9at %, that of N is 7at %, and none of Fe under 400 °C calcination temperature. The degradation rate of the sample for methylene blue solution reaches 23.49% under visible light irradiation for 5 hours. The influence order of the factors was the calcination temperature > F > N > Fe.
Pressure Analysis of Airflow With Different Inlet Angle in the Oblique Rectangular Fin Channels
Authors: Liu Zhiying , Li Hui , Shi Lin , Zhang Yangjun , Li Ruixia , Fei Fan
Published: March 2016

Previous work about the oblique fin channels shows an abnormal variation on the static pressure drop. Thus numerical work was performed to investigate the influence of the lengthwise distance and the parallelogrammic fin section shape separately using two groups of fin mesh. The results showed that the influence of the parallelogrammic fin section shape was similar to the three-dimensional oblique fin channels. Besides, when air flow is askew to the fin, there is a vortex near the leeward side. The pressure had a rapid change in the entrance and vortex region and then decreased linearly along the flow length. The static pressure drop in the vortex region is determined by both the fin slope angle and the inlet angle. Thus the variation law of oblique fin channels is different from the straight fin channels.
Mode-Resolved Thermal Conductivity of Freestanding and Supported Bismuth Telluride Quintuple Layer
Authors: Shao Cheng , Bao Hua
Published: March 2016

The successful exfoliation of atomically-thin bismuth telluride quintuple layer (QL) attracts tremendous interest in investigating the electron and phonon transport properties in this quasi-two-dimensional material. While experimental results show that thermal conductivity is significantly reduced in Bi2Te3 QL compared to the bulk phase, the underlying mechanisms for the reduction is still unclear. Also in some measurements, the Bi2Te3 QL is usually supported on the substrate and the effect of the substrate on heat transfer in Bi2Te3 QL is unknown. In this work, we have performed molecular dynamics simulations and normal mode analysis to study the mode-wise phonon properties in freestanding and supported Bi2Te3 QL. We found that the existing of substrate will decrease the phonon relaxation times in Bi2Te3 QL in the full frequency range. Thermal conductivity accumulation function for both freestanding and supported Bi2Te3 QL are constructed and compared. We found that half of heat transfer in freestanding Bi2Te3 QL contributed from phonons with mean free paths larger than 16.5 nm, while in supported Bi2Te3 QL this value is reduced to 11 nm. In both cases phonons with MFPs in the range of 10–30 nm are the dominate heat carriers, which contribute to 55% and 53% of thermal conductivity in freestanding and supported cases.
Performance Analysis of Near-Field Thermophotovoltaic With a Multilayer Metallodielectric Emitter
Authors: Yang Y. , Chang J. Y. , Wang L. P.
Published: March 2016

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.
A Model of the Contact Thermal Resistance of Vertical Carbon Nanotube Arrays
Authors: Zhang Jin , Shi Bo
Published: March 2016

This paper developes a model to predict the thermal contact resistance of the vertically aligned carbon nanotubes (VACNTs). The model includes the effects of CNT array properties and surface roughness, with the aim of providing useful information for optimizing CNT array thermal contact resistance. The contact resistance is consisted of two parts: interfacial thermal resistance and constriction thermal resistance. The carbon nanotube (CNT) is treated as a thin elastic rod and macroscopic mechanical is used to calculate the mechanical properties of CNT. Greenwood-Williamson (GW) model is used to describe the roughness. The interfacial thermal resistance is calculated by molecular dynamics. The calculated values are in good agreement with experimental data. The interfacial thermal resistance is the domain major factor.
On the Operational Parameters Effects on Two-Phase Pressure Drop Characteristics of Reentrant Copper Microchannels
Authors: Deng Daxiang , Huang Qingsong , Xie Yanlin , Zhou Wei , Huang Xiang , Huang Yue
Published: March 2016

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.
Numerical Analysis of the Natural Convection in a Cylinder With an Internal Slotted Annulus
Authors: Shen C. Y. , Yang M. , Li L. , Zhang Y. W.
Published: March 2016

The heat dissipation of current busbur in power plant is one of the important issues in power transmission, usually through the cylinder slotted to strengthen heat dissipation. Natural convection in a cylinder with an internal slotted annulus is the computational model abstracted from it. Natural convection in a cylinder with an concentric slotted annulus is concerned. Attention is focused on the effects of different slotted sizes on natural convection. Numerical results showed that, the equivalent thermal conductivity increases with the increase of Rayleigh number. At high Ra, the system heat transfer exhibit rich nonlinear characteristics. When the slotted direction or the slotted degree changed, it would have an important impact on the flow and heat transfer in the system, and also influence the related nonlinear characteristics.
Tumor Shrinkage Study in Magnetic Nanoparticle Hyperthermia Based on Designed Heating Protocols
Authors: LeBrun Alexander , Ma Ronghui , Zhu Liang
Published: March 2016

Magnetic nanoparticle hyperthermia has attracted growing attentions recently due to its ability of confining nanoparticle-induced heating in targeted tumor region. Our recent studies have identified an injection strategy to achieve repeatable and controllable nanoparticle deposition patterns in PC3 tumors using microCT scans. Based on the injection strategy, simulation of temperature elevations in tumors is conducted to design heating protocols to induce irreversible thermal damage to the entire tumors. In this study, in vivo heating experiments are performed on PC3 tumors implanted on mice following the designed heating protocols. The tumors in the control group without heating triple their sizes over a period of eight weeks. The tumors in the heating groups are heated for either 25 minutes or 12 minutes, representing that the Arrhenius integral is equal to or larger than 4 or 1 in the entire tumors, respectively. The tumors in the heating group of 25 minutes disappear completely after the 3rd days, and the site maintains the disappearance for over eight weeks. The sizes of the tumors in the heating group of 12 minutes decrease in the first ten days, however, the tumors re-grow afterwards, and by the end of the 8th week, they are approximately 60% larger than their initial size. This study demonstrates the importance of imaging-based design for individualized treatment planning. The success of the designed heating protocol in complete damaging PC3 tumors validates the theoretical models used in planning the heating treatment in magnetic nanoparticle hyperthermia.
Accelerating the Mass Transport of DNA Biomolecules Onto DNA Microarray for Enhanced Detection by Electrokinetic Concentration in a Microfluidic Chip
Authors: Martins Diogo , Wei Xi , Levicky Rastislav , Song Yong-Ak
Published: March 2016

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.
An Experimental Study on Flow Boiling Characteristics of pHEMA Nano-Coated Surfaces in a Microchannel
Authors: Khalili Sadaghiani Abdolali , Şişman Yağmur , Özaydın İnce Gözde , Koşar Ali
Published: March 2016

In this study, the effect of pHEMA (Polyhydroxyethylmethacrylate) nanostructure coated surfaces on flow boiling was investigated in a rectangular microchannel. Experiments were conducted using deionized water as the working fluid to investigate flow boiling in a microchannel with dimensions of 14 cm length, 1.5 cm width, and 500 μm depth. The effect of pHEMA coatings (coated on 1.5 × 1.5 cm2 silicon plates) on heat transfer coefficients and flow patterns was assessed and supported using a high speed camera system. Although the contact angle decreases on nano-coated surfaces, due to surface porosity, boiling heat transfer coefficient increases. Furthermore, visualization results indicated that uncoated surfaces experienced a smaller nucleate boiling region. It was also observed that dryout occurs at higher heat fluxes for coated surfaces.
Accurate Geometry Design of Magnetic Polariton With Specified Resonance Wavelength: A Combined LC Circuit Model and Inverse Technique
Authors: Zhao Junming , Qiu Jun , Liu Linhua
Published: March 2016

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.
Nucleate Boiling Heat Transfer Enhancement Using Nanostructured Al-Alloy Plates
Authors: Şişman Yağmur , Khalili Sadaghiani Abdolali , Khedir Khedir R. , Karabacak Tansel , Koşar Ali
Published: March 2016

Bubble departure frequency and active nucleation site density are two main factors that affect the nucleate boiling heat transfer. The potential enhancement of boiling heat transfer can be accomplished by surface modification. This treatment can be realized with changing parameters such as porosity, tilting angle and cavity radius. In this study, effects of different nanostructured Aluminum-Alloy (Al-Alloy) 2024 sheets on subcooled boiling heat transfer are investigated. A simple and environmentally friendly technique is used in order to produce these plates that are immersed into boiling deionized water for 20, 60 and 120 minutes. To examine boiling heat transfer characteristics, nanostructured plates are placed inside a rectangular channel. The channel is heated through four cartridge heaters connected to a DC power supply while deionized water is pumped inside using a micro gear pump at constant mass fluxes of 50 kg/m2s, 75 kg/m2s and 125 kg/m2s. It was found that an increase in nano-structure height leads to higher boiling heat transfer coefficients. Furthermore, a high speed camera system was used to investigate flow patterns in the microchannel. Visualization results indicated that bubbles movde faster the nano-structure height increased.
The Effect of Inlet Turbulence Intensity on Nano-Particulate Soot Formation in Kerosene-Fueled Combustors
Authors: Darbandi Masoud , Ghafourizadeh Majid
Published: March 2016

In this work, we numerically study the effects of turbulence intensity at the fuel and oxidizer stream inlets on the soot aerosol nano-particles formation in a kerosene fuel-based combustor. In this regard, we study the turbulence intensity effects specifically on the thermal performance and nano-particulate soot aerosol emissions. To construct our computer model, we simulate the soot formation and oxidation using the Polycyclic Aromatic Hydrocarbons PAHs-inception and the hydroxyl concept, respectively. Additionally, the soot nucleation process is described using the phenyl route, in which the soot inception is described based on the formations of two-ringed and three-ringed aromatics from acetylene, benzene, and phenyl radical. We use the two-equation soot model in which the soot mass fraction and the soot number density transport equations are solved considering the evolutionary process of soot nanoparticles, where all the nucleation, coagulation, surface growth, and oxidation phenomena are suitable considered in calculations. For the combustion modeling part, we benefit from the flamelets library, i.e., a lookup table, considering a detailed chemical kinetic mechanism consisting of 121 species and 2613 elementary reactions and solve the transport equations for the mean mixture fraction and its variance. We take into account the turbulence-chemistry interaction using the presumed-shape probability density functions PDFs. We apply the two-equation high-Reynolds-number k-ε turbulence model with round-jet corrections and suitable wall functions in performing our turbulence modeling. Solving the transport equations of turbulence kinetic energy and its dissipation rate, the turbulence closure problem can be resolved suitably. Furthermore, we take into account the radiation heat transfer of soot and gases assuming optically-thin flame, in which the radiation heat transfer of the most important radiating species is determined locally through the emissions. To evaluate our numerical solutions, we first solve an available well-documented experimental test, which provides the details of a kerosene-fueled turbulent nonpremixed flame. Then, we compare the achieved flame structure, i.e., the distributions of mean mixture fraction, temperature, and soot volume fraction, with those measured in the experiment. Next, we change the turbulence intensities of the incoming fuel and oxidizer streams gradually. So, we become able to evaluate the effects of different turbulence intensities on the achieved temperature and soot aerosol concentrations. Our results show that using moderate turbulence intensities at both fuel and oxidizer stream inlets would effectively increase the maximum temperature inside the combustor and this would reduce the exhaust gases temperature. It also reduces the concentrations of soot in the combustor and its emission to the exhaust gases effectively.
Numerical Simulation of Flow and Heat Transfer in Rectangular Channel With Different Aspect Ratios
Authors: Wenhua Liu , Yang Mo , Ling Li , Liang Qiao , Zhang Yuwen
Published: March 2016

Turbulent flow and heat transfer in rectangular channel has an important significance in engineering. Conventional approach to caculate Nusselt number of rectangular channel approximately is to take the equivalent diameter as the characteristic length and use the classic circular channel turbulent heat transfer coefficient correlations. However, under these conditions, the caculation error of Nusselt number can reach to 14% and thus this approach can not substantially describe the variation of Nusselt number of rectangular cross-sections with different aspect ratios. Therefore, caculation by using equivalent diameter as the characteristic length in classic experiment formula needs to be corrected. Seven groups of rectangular channel models with different aspect ratios have been studied numerically in this paper. By using standard turbulence model, the flow and heat transfer law of air with varing properties has been studied in 4 different sets of conditions in Reynolds number. The simulation and experimental results are in good agreement. The simulation results show that with the increase of aspect ratio, the cross-sectional average Nusselt number increased, Nusselt number of circumferential wall distributed more evenly and the difference between the infinite plate channel and square channel went up to 25%. The effects of corner region and long\short sides on heat transfer have also been investigated in this paper. Results show that in rectangular channel, heat transfer in corner region is significantly weaker than it in other region. With the increase of aspect ratio, effect on the long side of heat transfer of the short side is gradually reduced, and then eventually eliminates completely in the infinite flat place. Based on the studies above, correction coefficient for rectangular channels with different aspect ratios has been proposed in this paper and the accuracy of the correction coefficient has been varified by numerical simulations. This can reflect the variation of Nusselt number under different aspect ratios more effectively and thus has current significance for project to calculate Nusselt number of heat transfer in rectangular channel.
An Experimental Investigation on Flow Behavior and Heat Transfer Affected by Roughness in the Circular Micro-Channels
Authors: Tian Yitu , Li Haiwang
Published: March 2016

Surface roughness is one of the most important factors to determine the flow and heat transfer characteristics of the microchannel. This paper experimentally and theoretically investigated the effects of surface roughness for the flow and heat transfer behavior within the circular microchannel. The stainless steel circular microchannels were fabricated by electrical spark-erosion perforating and drilling separately to control the relative roughness of the surface which is 1% for drilling method and 1.5% for electrical spark-erosion perforating method. Each test piece includes 44 identical circular microchannels in parallel with diameter of 0.4 mm. In the experiments, the air flowed through the circular microchannels with Reynolds number changing from 200 to 2600. The results showed that the surface roughness in microchannels has a remarkable effect on the performance of flow behavior and heat transfer within the circular microchannel. The values of Poiseuille number and Nusselt number are higher when the surface relative roughness is larger. At the same time, the flow behavior is inconsistent with the behavior within the macrochannel. For the flow behavior, Poiseuille number increases monotonously with the increase of Reynolds number, and is higher than the constant theoretical value. The Reynolds number for the transition from laminar to turbulent flow is between 1400 and 1600. For the heat transfer property, Nusselt number also increases as the increase of the Reynolds number.
Tuning the Thermal Transport Properties of Graphene via Nanoconstriction Network
Authors: Cao Bing-Yang , Yao Wen-Jun
Published: March 2016

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.
Experimental Research on CO2 Pool Boiling Heat Transfer Outside a Single Tube
Authors: Liu Shengchun , Dong Ziteng , Zhang Wenkai
Published: March 2016

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.
Experimental Investigation of the Air Flow Behavior and Heat Transfer Characteristics in Microchannels With Different Channel Lengths
Authors: Zhu Zhibing , Tao Zhi , Tian Yitu , Li Haiwang
Published: March 2016

This paper attempts to experimentally investigate the influence of channel lengths to the flow behavior and heat transfer characteristics in circular microchannels. The diameter of circular microchannels are 0.4mm and the lengths of them are 10mm and 20mm, respectively. All tests were performed with air. The experiments were completed with Reynolds number in the range of 300∼2700. Results of experiments show that the length of microchannels has remarkable effects on the performance of flow behavior and heat transfer characteristics. For the flow behavior, both the friction factor and Poiseuille number drop as the channel length increases, and all the experimental value are higher than the theoretical one. Moreover, the channel length doesn’t influent the value of critical Reynolds number. For the heat transfer characteristics, Nusselt number decreases with the increase of the channel length. The channel length also has a huge influence on the thermal performance. A better thermal performance is obtained in a shorter channel. The results also indicated that in both cases, the friction factor decreases with the increase of the Reynolds number. At the same time, the Poiseuille number increases when the Reynolds number keeps rising. This phenomenon is different from traditional theory that Poiseuille number is widely considered as a constant in laminar regime. It is also observed that the value of critical Reynolds number is between 1500 and 1700 in this paper, this value is lower than the value of theoretical critical Reynolds number of 2300. Nusselt number increases as the increase of the Reynolds numbers, however, the traditional theory considered that it is a constant in laminar regime.
The Simulations of Flow and Heat Over Microscale Sensors in Supersonic Rarefied Gas Flows Using DSMC
Authors: Darbandi Masoud , Mosayebi Ghasem
Published: March 2016

As the use of MEMS-based devices and systems are continuously increasing, the understanding of their correct characteristics becomes so serious for the related researches. In this study, the supersonic rarefied gas flow over microscale hotwires is investigated using the Direct Simulation Monte Carlo (DSMC) method. Indeed, the DSMC has been accepted as a powerful method to study the rarefied gas flow especially in transitional regime. Therefore, it can be considered as a reliable method to investigate the rarefied supersonic flow over microscale objects including the microscale hotwires. In this work, we study the effective parameters, which affect the performance of these sensors at constant sensor surface temperature conditions.

We use our developed DSMC code to perform our investigation. This code uses the DSMC algorithm to solve the rarefied gas flow on unstructured grid distributions. To validate our developed DSMC code, we solve the supersonic rarefied gas flow and heat transfer in microchannel considering different Knudsen number magnitudes. Comparing the achieved flow and heat transfer solutions with other available results and data reported on microchannel studies, we verify the accuracy of achieved results. Next we focus on hotwire sensor, which often consists of the combinations of different long narrow circular cylinders. We study the effects of grid resolution, time step size, and the number of simulated particles on the obtained results. We further study the effects of sensor temperature and sensor diameter on the sensor thermal performance. The achieved results indicate that the surface heat flux performs very similarly in different studied cases. For example, the achieved local Nusselt number distributions around the circular sensor show that the surface heat flux would gradually increase from the sensor stagnation point to its rear end as the temperature gradient increases. It reaches to a maximum magnitude and it then starts decreasing resulting in effective heat flux reduction. Finally, there is a low pressure zone at the rear side of cylinder, which is not considerably affected by the flow properties. The results also show that if the wire surface temperature increases, the Nusselt number would reduce. However, the amount of Nusselt Number reduction rate would decrease as the temperature increases. Furthermore, the results show that the Reynolds number decreases and the Knudsen number increases as the sensor diameter decreases, which is due to the transitional regime behavior. As is known, the flow at boundaries change the condition from the slip to transitional regime when the Knudsen number increases sufficiently; and the flow become rarefied. There is a reduction in the total heat flux rate as the sensor diameter is reduced.
A Solar Thermophotovoltaic System Using Spectrally Controlled Monolithic Planar Thermal Emitter/Absorber
Authors: Yugami Hiroo , Kohiyama Asaka , Shimizu Makoto , Iguchi Fumitada
Published: March 2016

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.
A Thermophotovoltaic System Using a Photonic Crystal Emitter
Authors: Chan Walker R. , Stelmakh Veronika , Soljacic Marin , Joannopoulos John D. , Celanovic Ivan , Waits Christopher M.
Published: March 2016

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.
Thermal Performance Analysis of Bi-Porous Metal Foam Heat Sink
Authors: Li Yongtong , Gong Liang , Xu Minghai , Joshi Yogendra
Published: March 2016

The three-dimensional forced convective heat transfer in a bi-porous metal foam heat sink is numerically investigated. Each of the metal foam layers has a distinct thickness, porosity, and pore density. The effects of these geometrical and morphological parameters on fluid flow and heat transfer are analyzed by employing the Forchheimer-Brinkman extended Darcy momentum equation and local thermal non-equilibrium energy equation. The numerical results show that the thermal resistance of the bi-porous metal foam heat sink is decreased with reduction in top layer metal foam porosity, as well as the bottom layer metal foam thickness, for a fixed bottom metal foam porosity of 0.9. The best thermal performance is achieved by employing a 30PPI metal foam at the bottom layer, and a 50PPI metal foam at the top layer. The optimal thickness of the bottom foam layer is about 1mm.
Droplet Fission in Non-Newtonian Multiphase System Using Bilayer Bifurcated Microchannel
Authors: Ren Yong , Koh Kai Seng
Published: March 2016

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.
Orientation Effects on Flow Boiling Silicon Nanowire Microchannels
Authors: Alam Tamanna , Li Wenming , Yang Fanghao , Khan Jamil , Li Chen
Published: March 2016

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.
Experimental Study of Nature-Inspired Enhanced Microscale Heat Transfer
Authors: Goh A. L. , Han B. , Ooi K. T.
Published: March 2016

Microscale heat transfer in macro geometry has been proven to yield comparable heat transfer performance to that of typical microchannels. This paper takes it a step further by looking at three passive heat transfer enhancement techniques to improve the heat transfer performance of the newly proposed system for single-phase liquid flow. The novelty of the study lies in that the enhancement features are designed based on inspiration from nature. Fish scale, Durian (a thorny tropical fruit), and Inverted Fish Scale enhancement profiles are considered. In this study, an annular microchannel is formed by securing a cylindrical insert of mean diameter 19.4 mm within a cylindrical pipe of internal diameter 20 mm. The enhancement features are introduced on the surface profile of the insert, while the heat is supplied to the flow via the cylindrical pipe of fixed surface area. Therefore, heat transfer is improved by increasing convective heat transfer coefficient, for a constant heat transfer area. The enhancement features serve to increase heat transfer coefficient by disturbing the flow and thermal boundary layers. Experiments were carried out to investigate the effect of the three enhancement profiles on the heat transfer and flow characteristics of the microscale flow. The extent of enhancement is computed with the Plain profile as benchmark. The constant parameters include microchannel length of 30 mm, mean hydraulic diameter of 600 μm, and heat input of 1000 W. Reynolds number range is 1,300 to 4,600, with water as working fluid. Results show that the Inverted Fish Scale profile doubles the Nusselt number as compared to the Plain profile. However, when friction factor increment is considered, Durian profile yields the best overall thermal performance, nearly 1.4 times better than the Plain profile. In the whole study, the maximum convective heat transfer coefficient achieved is 45.0 kW/m2·K, using Inverted Fish Scale profile at Reynolds number of 4,300. The pressure drop values of the system are all less than 3 bars, which may easily be achieved by a commercially available pump.
Optimization of a chlorine-based deep vertical etch of GaN demonstrating low damage and low roughness
Authors: Tahhan Maher , Nedy Joseph , Chan Silvia H. , Lund Cory , Li Haoran , Gupta Geetak , Keller Stacia , Mishra Umesh
Published: March 2016

The dry etching of GaN to form deep vertical structures is a critical step in many power device processes. To accomplish this, a chlorine and argon etch is investigated in detail to satisfy several criteria simultaneously such as surface roughness, crystal damage, and etch angle. Etch depths from 2 to 3.4 μm are shown in this paper. The authors investigate the formation of etch pits and its contributing factors. In addition, a nickel hard mask process is presented, with an investigation into the causes of micromasking and a pre-etch to prevent it. The authors show the results of optimized etch conditions resulting in a 2 μm deep, 0.831 nm rms roughness etch, with a 7.6° angle from vertical and low surface damage as measured by photoluminescence.
Observe Your Shadow
Authors: Rovšek Barbara
Published: March 2016

Observe Your Shadow was the title of an observational experiment that was, among others, conducted in the scope of the past year's (2014–2015) first Slovene science competition for elementary school pupils between the ages of 6 and 13. The main reason for establishing a new science competition was popularization of science and its experimental methods, particularly among elementary school students. Elementary school teachers are not generally specialists in science, but rather have (and should have) extremely wide scopes of interests and competencies. By providing them with ideas and instructions for science experiments, we aim to enrich regular school lessons. In the first year alone, the competition took place in over half of Slovene elementary schools, with a total of 9000 participating students. In this paper we shall report about pupils' responses to tasks related to one of the experiments, namely, observation of their shadows on a sunny day.