bluuebunny/crossref_metadata_2025 huggingface dataset:
Analysis of ZnO Thin Film as Thermal Interface Material for High Power Light Emitting Diode Application
Authors: Shanmugan S. , Zeng Yin O. , Anithambigai P. , Mutharasu D.
Published: November 2015

All solid-state lighting products produce heat which should be removed by use of a heat sink. Since the two mating surfaces of light emitting diode (LED) package and heat sink are not flat, a thermal interface material (TIM) must be applied between them to fill the gaps resulting from their surface roughness and lack of coplanarity. The application of a traditional TIM may squeeze out when pressure is applied to join the surfaces and hence a short circuit may result. To avoid such a problem, a thin solid film based TIM has been suggested. In this study, a zinc oxide (ZnO) thin film was coated on Cu substrates and used as the TIM. The ZnO thin film coated substrates were used as heat sink purposes in this study. The prepared heat sink was tested with 3 W green LED and the observed results were compared with the results of same LED measured at bare and commercial thermal paste coated Cu substrates boundary conditions. The influence of interface material thickness on total thermal resistance (Rth-tot), rise in junction temperature (TJ), and optical properties of LED was analyzed. A noticeable reduction in Rth-tot (5.92 K/W) as well as TJ (ΔTJ = 11.83 °C) was observed for 800 nm ZnO thin film coated Cu substrates boundary conditions when compared with bare and thermal paste coated Cu substrates tested at 700 mA. Change in TJ influenced the thermal resistance of ZnO interface material. Improved lux level and decreased correlated color temperature (CCT) were also observed with ZnO coated Cu substrates.
Hydrodynamic Forces and Flow Structures in Flow Past a Cylinder Forced to Vibrate Transversely and Inline to a Steady Flow
Authors: Peppa S. , Kaiktsis L. , Triantafyllou G. S.
Published: November 2015

This paper reports computational results of forces and wake structure in two-dimensional flow past a circular cylinder forced to vibrate both transversely and inline to a uniform stream, following a figure-eight trajectory. For a flow stream from left to right, we distinguish between a counterclockwise mode and a clockwise mode, if the upper part of the trajectory is traversed counterclockwise or clockwise, respectively. The present computations correspond to a range of transverse oscillation frequencies close to the natural frequency of the Kármán vortex street and several oscillation amplitudes, both for counterclockwise motion and clockwise motion. The nondimensional forces and nondimensional power transfer from the fluid to the body are calculated. The results demonstrate a strong dependence of the forces and power transfer on the direction in which the figure-eight is traversed. In general, counterclockwise motion maintains positive power transfer at higher oscillation amplitudes. Flow visualizations show that the wakes are characterized by the presence of two single (2S) vortex shedding mode at low oscillation amplitudes and can attain more complex structures at higher amplitudes.
Homogeneous Vapor Nucleation of Electrolyte Solution Within a Nanopore
Authors: Oh Jaekyoon , Yoo Yungpil , Kwak Ho-Young
Published: November 2015

Homogeneous vapor nucleation of the electrolyte solution within a nanopore at its superheat limit was studied using the bubble nucleation model based on molecular interaction. The wall motion of the bubble that evolved from the evaporated electrolyte solution was obtained using the Keller-Miksis equation and the distribution of temperature inside the bubble was obtained by solving the continuity, momentum and energy equations for the vapor inside the bubble. Heat transfer at the interface was also considered in this study. The nucleation rate of the 3 M NaCl solution at 571 K is estimated to be approximately 0.15×1028 clusters/m3s. With this value of the nucleation rate, the complete evaporation time of the 50 nm radius of the electrolyte solution is approximately 0.60 ns. The calculated life time of the bubble that evolved from the evaporated solution, or the time duration for the growth and subsequent collapse of the bubble, is approximately 32 ns, which is close agreement with the observed result of 28 ns. The bubble reaches its maximum radius of 301 nm at 13.2 ns after the bubble evolution.
Simulation of Heat Transfer Across Rarefied Gas in Annular and Planar Geometries: Comparison of Navier-Stokes, S-Model and DSMC Methods Results
Authors: Maharjan Dilesh , Hadj-Nacer Mustafa , Ho Minh-Tuan , Stefanov Stefan K. , Graur Irina , Greiner Miles
Published: November 2015

Steady state heat transfer through a rarefied gas confined between two parallel plates or two coaxial cylinders maintained at different temperatures is investigated using the nonlinear S-model kinetic equation and the DSMC technique for a large range of gas rarefaction. The profiles of heat flux, density and temperature are reported for different values of gas rarefaction parameter and given values of temperature and aspect ratios. In the slip regime the results of the S-model and DSMC technique are compared to the simulations performed using the Lin and Willis temperature jump boundary conditions at the at the solid surface implemented in ANSYS/Fluent CFD simulations. The analytical expressions for density number, temperature and heat flux in the free molecular regimes are obtained for both parallel plates and coaxial cylinders geometries with hot and cold surfaces having different values of the thermal accommodation coefficient. The solutions of these analytical expressions are compared to the S-model kinetic equation and DSMC technique results in the free molecular regime.
Mass Flow Rate Measurements Through Metallic Microtubes in the Slip and Transition Regimes
Authors: Silva Ernane , Deschamps Cesar J. , Rojas-Cárdenas Marcos
Published: November 2015

The exchange of momentum and energy in gas flows through microchannels is significantly influenced by the gas-surface interaction. At this scale often the gas is rarefied and therefore non-equilibrium effects in the fluid flow can arise in a layer which extends for a distance equivalent to the mean free path from the walls. Typical examples of non-equilibrium phenomena for rarefied gas flows are slip at the wall, thermal transpiration and temperature jump at the wall. The aim of the present study is to experimentally investigate the non-equilibrium effects present in an isothermal pressure induced flow for a large range of rarefaction conditions. The isothermal slip at the wall is usually characterized by the tangential momentum accommodation coefficient (TMAC). This coefficient depends on the molecular nature of the gas and on the physical characteristics of the surface, such as material and roughness. In particular this paper explores the influence of the surface material on the TMAC through measurements of the mass flow rate in capillaries for the special case of nitrogen. Commercially available microtubes of three different metallic materials — stainless steel, copper, and brass — were considered in the analysis. Measurements were performed with a dynamic measurement technique based on the constant volume method and comprehend the transitional flow regime and most part of the slip regime. Theoretical results obtained from the solution of the Boltzmann equation via the BGK kinetic model, which is a simplified approximation for the collisional term, were compared to the experimental results.
A State-of-the-Art Review on Pool Boiling on Nanostructure Surfaces
Authors: Ignácio Isabela , Cardoso Elaine Maria , Gasche José Luiz , Ribatski Gherhardt
Published: November 2015

The differences in the heat transfer coefficient (HTC) and critical heat flux (CHF) behaviors between nanostructured and smooth surfaces are attributed to modifications on the surface wettability and capillarity effects through the porous matrix generated by the nanostructure layer. Both act in order of improving rewetting effects, explaining the CHF augmentation. The fact that the contact angle decreases is commonly considered to justify the HTC reduction for nanostructured surfaces.

In this context, this study presents a critical review of the literature concerning the boiling phenomena on nanostructures surfaces. Care is exercised in order of characterizing the nanostructuring methods and compare heat transfer results obtained under almost similar conditions by different authors. Heat transfer mechanisms pointed in the literature as responsible for the heat transfer behaviors are also contrasted.
Conjugate Heat Transfer Simulation in Gaseous Flow Micro Heat Exchanger
Authors: Croce Giulio , Coppola Michele A.
Published: November 2015

An extended numerical analysis is performed in order to characterize the combined effect of compressibility, rarefaction and conjugate heat transfer (CHT) in counter current and parallel flow micro heat exchanger. Relatively short microchannel geometries are considered, leading to more significant dependence on compressibility and rarefaction effects. A fully compressible numerical solver, coupled with proper slip flow and temperature jump boundary conditions, previously extensively used for CHT computation in microchannel heat sinks, is adopted: thus, viscous dissipation is always taken into account and a wide range of channel exit Mach numbers can be considered, keeping Knudsen number within the limits of slip flow. A comprehensive range of fluid/solid thermal conductivity ratios, pressure ratios, temperature difference and channel aspect ratios are considered, in order to identify the dominant effects, as well as the optimal fluid/solid conductivity ratio, as a function of the heat exchanger design and operating parameters. Results are described in terms of heat exchanger efficiency and local Nusselt number.
Numerical Simulation of Taylor Flow in Micro Circular Tubes
Authors: Zhang Jingzhi , Li Wei
Published: November 2015

Heat transfer and flow characteristics of Taylor flow in micro capillary tubes have been investigated numerically with the Volume of Fluid (VOF) method. A constant heat flux (32kwm−2) is adopted at the tube wall. All seven computational cases have the same Reynolds number (Re=280), Capillary number (Ca=0.006) and homogenous void fraction (β=0.51), while the inlet gas volume fraction varies from 0.2 to 0.8. The results indicate that liquid slug length (Ll), gas slug length (Lg) and cell length (Lc) vary with α, while liquid film thickness δ remains constant. The friction factor f of Taylor flow is higher than single phase flow. The simulation results agree well with the correlation proposed by Kreutzer et al.. The Local Nusselt number (Nux) gets its peak value at the liquid film region, where the temperature difference between wall temperature (Tw) and fluid bulk temperature (Tbx) is smallest. The average Nu (Nuav) is about 2.8 times of single phase. This means that Taylor bubble can enhance the heat transfer coefficient in micro capillary tubes.
A General Correlation for Condensation Heat Transfer in Micro-Fin for Herringbone and Dimple-Texture Tubes
Authors: Li Wei , Guo Si-pu , Zhou Xiao-peng , Kukulka David J. , Xu Jin-liang
Published: November 2015

An experimental investigation was performed to evaluate the condensation characteristics inside smooth, herringbone and dimple-textured (Vipertex 1EHT) tubes; with the same outer diameter (12.7 mm); using R22 and R410a refrigerants; for a mass flux that ranges from 81 to 178.5 kg/m 2 s. The condensation saturation temperature is 47°C; with an inlet quality of 0.8 and an outlet vapor quality of 0.2. Results indicate that the condensation heat transfer coefficient of the herringbone tube was approximately 3 times that of the smooth tube for R22; and has an enhancement heat transfer factor of 2.3 for R410a. The enhancement heat transfer coefficient multiplier for the textured dimple tube is approximately 2 times that of a smooth tube for R22; and 1.8 for R410a. Severalpreviously reported correlations are used to compare the heat transfer coefficient measurements in the plain tube; while a new equation is proposed to predict the heat transfer coefficient in the herringbone tube.
Experimental Investigation of Evaporation and Condensation in Herringbone, Smooth and EHT Tubes
Authors: Zhou Xiao-peng , Sun Jian-jun , Guo Si-pu , Zhichuan Sun , Li Wei
Published: November 2015

An experimental investigation was performed for evaporation and condensation characteristics inside smooth tube, herringbone tube and EHT tube with the same outer diameter 12.7 mm, refrigerant are R22 and R410a. Mass flux are 60–140 kg/m2s, 81–178.5 kg/m2s, for evaporation and condensation respectively. The evaporation saturation temperature is 6°C, with inlet and outlet vapor qualities of 0.1 and 0.9, respectively. The condensation saturation temperature is 47°C, with inlet and outlet vapor qualities of 0.8 and 0.2, respectively. EHT tube has best evaporating performance for both R22 and R410a. Herringbone tube is also batter than smooth tube. Evaporation heat transfer coefficient increases with mass flux increasing obviously. Pressure drop of R22 evaporation in EHT tube is the highest, herringbone tube is a little higher than in smooth tube. Herringbone tube has highest condensation heat transfer coefficient, about 3 and 2.3 times that of smooth tube for R22 and R410a respectively. EHT tube has heat transfer coefficient about 2 and 1.8 times that of smooth tube for R22 and R410a respectively. Condensation heat transfer coefficient increases with increasing of mass flux, but very slowly, R410a flow in micro-fin tube even decreases with mass flux increasing.
Experimental Research on Condensation Outside a Horizontal Tube at Low Mass Fluxes
Authors: Li Wei , Chen Xu
Published: November 2015

An experimental investigation of R410a condensation outside a horizontal herringbone tube and a smooth tube has been conducted. The herringbone tube has a fin root diameter of 11.43 mm, a helical angle of 21.3 °, 48 fins with a fin height of 0.262 mm and an apex angle of 36 °, while the smooth tube has an inner diameter of 11.43 mm. Experiments were taken at a constant saturation temperature of 45°C, an inlet vapor quality of 0.8 and an outlet vapor quality of 0.1. The mass velocity ranged from 5 kg/(m2.s) to 50 kg/(m2.s). The outside condensation heat transfer coefficients for the herringbone tube vary from 617.53 W/(m2.K) to 856.37 W/(m2.K), whereas the heat transfer coefficients for the smooth tube vary from 1066.29 W/(m2.K) to 1413.09 W/(m2.K), nearly 1.5 times higher than the data of the herringbone tube. At such a low mass velocity, the smooth tube seems superior to the herringbone tube, which has not been discovered yet. The cause of such phenomenon might consist in the surface tension which plays a vital role in the condensation process. Under a low mass velocity, the surface tension results in the retention of liquid on the lower part of the tube, which thickens the film on the tube and worsens the heat transfer. Several calculations were made to find a suitable correlation for this experiment, aiming to find the point where the herringbone tube starts to lose its enhancement function.
Heat Transfer Performance of Supercritical Carbon Dioxide in Microchannels
Authors: Yang Chien-Yuh , Liao Kun-Chieh
Published: November 2015

This paper provides an experimental investigation of heat transfer performance and pressure drop of supercritical carbon dioxide cooling in microchannel heat exchanger. An extruded flat aluminum tube with 37 parallel channels and each channel of 0.5 mm × 0.5 mm cross section was used as the test section. Super critical carbon dioxide at pressure of 7.5 MPa and inlet temperature varied from 55 to 25 °C was tested. The temperature drops of CO2 cooled inside the test section was controlled at 2, 4 and 8 °C separately for each test to investigate the effect of properties change on the friction and heat transfer performance at various temperature cooling ranges near the critical point.

The test results showed that while the test conditions were away from (approximately 5 °C higher or lower) the critical point, both heat transfer and pressure drop performance agreed very well with those predicted by convention correlations. However, while the test conditions near the critical point, the difference between the present test results and the prediction values is very high. From the experiment results of various temperature change range inside the test section, we can find that both heat transfer and pressure drop were strongly affected by the temperature cooling ranges near the critical point.

Since there is a drastic peak of the properties change near the critical point, neither fluid properties at the average temperature nor the average properties at the inlet and exit temperatures may appropriately present the actual properties change in the test process. If we use the properties integrated but not averaged from inlet to the exit temperatures, we may obtain the results that agree well with the values predicted by conventional correlations. The heat transfer and pressure drop performance of super critical carbon dioxide are indeed similar to these at normal conditions if its properties were appropriately evaluated.
Mitigation of Biofouling Through In-Plane Application of Weak DC Current in Presence of Antimicrobials
Authors: Maskarenj Marshal Shahu , Chavali Ravi , Mathur Ankita , Ghosh Prakash Chandra , Mitra Sushanta Kumar
Published: November 2015

Membrane Bio Reactor (MBR) technology is a promising alternative to municipal and industrial wastewater treatment owing to low sludge production and wide range of acceptable influents. Biofouling in MBRs hampers long term functionality of the system through reduction in permeate flux over time. Membrane biofouling could necessitate periodic membrane backwashing or even require membrane replacement, thus increasing operational cost for the systems. Microbe-secreted extracellular polymeric substance (EPS) forms a complex matrix on the surface; is persistent against physical removal and tends to resist high concentrations of antimicrobial agents, thus playing a major role in membrane biofouling. There is a need for developing methods towards efficient removal of biofoulants from surfaces. In tandem with low DC current, the synergistic effect of antimicrobial agents has been reported successful towards reducing biofilm formation leading to biofouling. This paper discusses the application of in-plane bioelectric effect as a solution to biofouling in MBRs; especially Microbial Fuel Cells and Microbial Desalination Cells towards harnessing in-situ current for tackling biofouling, thus facilitating longer system functionality.
Possibility and Mechanism Study of Liquid-Metal Based Micro Electroosmotic Flow Pumps for Long-Time Running Purpose
Authors: Gao Meng , Gui Lin
Published: November 2015

This work is a further study of our previous work on liquid-metal based micro electroosmotic flow pump. Injection of room temperature liquid metal (gallium alloy) into microchannels can provide a simple, rapid and low-cost technique for electrode fabrication of electroosmotic flow pumps. In the micro electroosmotic flow pump, the electrode channels are fabricated symmetrically to both sides of the pumping channel in the same horizontal level. In the micropump, PDMS was used to fabricate the microfluidic chip and the liquid metal channel was separated from the pumping channel by a PDMS gap (≤40μm). Although the PDMS is insulative, small current was still found when voltage was applied on the electrodes and the electrical field successfully drove the fluid in the pumping channel. This liquid-metal based micropump can be very easy for fabrication and integration. This study is focused on the possibility and mechanism study of this liquid-metal based EOF pump to see if it can be used for long-time running. The experimental study shows that the pump works very stable and perfect for long-time running applications such as implantable medical devices.
The Influence of Electroosmotic Flow on the Von Kármán Vortex Street in the Wake of a Cylinder Located in a Microchannel
Authors: Scholz Mathias , Barz Dominik P. J.
Published: November 2015

The von Kármán vortex street is a flow instability that is observed in the wake of a blunt body if a certain (cylinder) Reynolds number is exceeded. It is one of the classical problems in fluid mechanics and a vast amount of research has been dedicated to the investigation of the fundamentals of this phenomenon. The present study is concerned with the numerical simulation of the flow in a microchannel having a cylinder located in its channel center. A pressure driven flow is induced in the channel described by the channel Reynolds number. The cylinder is subjected to an externally-applied electric field that causes electroosmosis in the electrical double layer which is present around the cylinder surface. In this setup, two distinctions to the classical von Kármán vortex street can be noted. On the one hand, the presence of the microchannel walls confines the flow field in lateral direction. On the other hand, the electroosmotic slip velocity impacts the flow topology in the vicinity of the cylinder and, thus, may have an impact on the formation and the periodic nature of the von Kármán vortex street. Various numerical simulations are performed to investigate the influence of the cylinder-diameter-to-channel-width ratio and the direction of the electrical field.
Parametric Study of Dielectrophoretic Interactive Motion of Particles
Authors: Hossan Mohammad Robiul , Benton Matthew J. , Dutta Prashanta , Dillon Robert
Published: November 2015

Dielectrophoresis (DEP) has become one of the most popular mechanisms for label free particle manipulations and transport in microfluidics. The efficacy of this mechanism is greatly dependent on the understanding and control of DEP interactive motion among particles. In this study, we performed a systematic investigation to understand the effect of particles size and electrical properties on DC DEP interactions among particles using in-house hybrid immersed boundary – immersed interface numerical method. Immersed boundary method is employed to predict flow field and immersed interface method is used to simulate electric field. The numerical model utilizes Maxwell’s stress tensor to obtain DEP forces, while solving transient Navier-Stokes equation it determines the hydrodynamic interaction between each of the particles and the fluid containing them. By varying the number of particles as well as the particles’ size, electrical properties and initial orientations, a number of possibilities were considered. Results indicate that the particles with similar electrical conductivities attract each other and tend to align themselves parallel to the external electric field regardless of sizes. If electrical conductivity of particles is lower than that of the fluid medium then the particle-particle interactions is caused by the negative DEP. If electrical conductivity of particles is higher than that of the fluid medium then the interactive motions of particle is attributed to the positive DEP. On the other hand, electrically dissimilar particles still attract each other but tend to align perpendicular to the electric field. Both negative and positive DEP contributes in interactions between electrically dissimilar particles. Numerical simulation also shows that the identical sized particles move at the same speed during interaction. In contrast, smaller particles moves faster than the larger particle during the interactions. This study explains the effect of size and electrical properties on DEP interactive motions of particles and can be utilized to design microfluidic devices for DEP particle manipulations.
Experimental Investigation on Flow Instability in a Single Vertical Microtube
Authors: You Qian , Hassan Ibrahim , Kadem Lyes
Published: November 2015

The experiments are conducted to study the flow boiling instability in a single microtube with 0.889 mm hydraulic diameter in vertical upward and downward flow directions (VU and VD). The subcooled dielectric liquid FC-72 is driven at mass fluxes varying from 700 to 1400 kg/m2·s, and the heat flux uniformly applied on the microtube surface is up to 9.6 W/cm2. The onsets of flow oscillations (OFIs) in both flow directions are observed. Their oscillation types and characteristics are presented as well. The effects of mass flux and heat flux on flow instability in vertical flow directions are discussed. The results show that as the mass flux increases, the OFI occurrence is postponed, and the compounded oscillation types (Ledinegg, pressure drop and density wave oscillations) turn to pressure drop type dominant. At low mass fluxes, the OFI appears earlier in VD than in VU due to the buoyancy force impeded the bubble discharging. As the mass flux increases, the OFI appearance in VD is close to the ones in VU and its flow oscillations tend to be re-stabilized. After OFIs appeared at a given mass flux, with more heat flux added, the density wave oscillation type in VU becomes more active. However, at a constant mass flux, as the heat flux increases, the flow instability in VD becomes “stable” which may be due to the rapid flow pattern change, and this kind of “stable” is not expected because the local dryout may accompany. Hence, the microtube with vertical upward flow direction (VU) performs better from flow boiling instability point of view.
Start-Up and Transient Behavior of an Oscillating Heat Pipe
Authors: Paudel Sagar Babu , Michna Gregory J.
Published: November 2015

Microelectronics devices require relatively low and uniform temperatures for maximum reliability and oscillating heat pipes (OHPs) may prove to be extremely well suited to this application. However, in some operating conditions, temperature overshoot has been observed at startup, which could lead to premature electronics failure. In this paper, the effect of various OHP operating parameters such as working fluid (deionized water, methanol, and acetone), inclination angle (vertical, 45°, and horizontal), fill ratio (50%, 60%, 70%, and 80%), and heat load (10W, 20W, and 30W) on start-up performance of a 20-turn OHP were investigated. This information could be used by a designer to optimize an OHP system to reduce the risk of start-up evaporator temperature overshoot before the system begins cooling. All working fluids performed best at high power. Water, for most cases, did not exhibit overshoot, but it performed well only in the vertical orientation with high fill ratios. However, methanol and acetone performed best at low fill ratios irrespective of inclination angle. Low power (10 W) was not sufficient to initiate oscillations when the fill ratio was high for these fluids, and overshoot occurred for both methanol and acetone at different conditions. Methanol, in general, did not overshoot either at low power and low fill ratio or high power and high fill ratio. However, acetone showed overshoot only at high heat load regardless of fill ratio and inclination angle. Start up behavior was somewhat unpredictable, with repeated test runs showing different results despite identical operating parameters.
Electrostatic Preclusion of Droplet Adhesion in a Microchannel
Authors: Hui Jonathan C. , Huang Peter
Published: November 2015

In many multiphase fluidic processes, such as in petroleum extraction and biochemical analysis involving microscale conduits, the lodging of immiscible droplets often leads to disastrous flow blockage. Without a thin-film lubrication layer surrounding the adhered droplets, a significantly higher threshold pressure gradient is required to reinitiate bulk flows. In this work, we investigate the surface tension-driven thin-film drainage process that leads to droplet adhesion and study how electrostatic repulsion between a charged droplet interface and a charged conduit wall can prevent direct contact between the two. We report on our multiphysics computational results of an oversized gas droplet in a water-filled flow microchannel under the influence of surface tension and interfacial electrostatic forces.
Experimental Investigation of a Cost Effective Skived Microchannel Cold Plate for Mass Production
Authors: Steinke Mark E. , Kamath Vinod
Published: November 2015

A liquid cold plate that utilizes skived microchannels has been developed to gain the benefits of direct liquid cooling, but minimize the expensive cost of such cold plates. The construction, application, and experimental results of the skived cold plate will be presented. Skiving is a mechanical process that cuts thin layers of material. It is an established process for making air cooled heat sinks. In this application, the fin field is skived and placed inside a housing that allows for liquid flow through the resulting fins.

The design boundary conditions and parameters will be described and performance per cost metric will be presented and used to evaluate future optimization possibilities. The objective of the present work was to minimize the thermal resistance while maintaining a low manufacturing cost. The design goal was to produce a cold plate that had sufficient thermal performance and the ability to be mass produced at a reasonable cost.

The resulting cold plate would also need to support warm water cooling of microprocessors. Warm water is a working fluid that has not been chilled below ambient temperatures. Therefore, the water temperature could be up to 45 degrees Celsius. The cold plate had a thermal resistance less than 0.3 °Ccm2/W. The pressure drop was minimized to lower the required pumping power and was less than 6 kPa at 1.0 liter per minute. Using a skiving process, it is possible to develop a cold plate that delivers good thermal performance and maintains a low production cost.
Exploration of Phase Change Within a Capillary Flow Driven Square Micro-Channel Using Lattice Boltzmann Method and Experimental Boundary Conditions
Authors: Borquist E. , Smith G. , Weiss L.
Published: November 2015

Previously published research examined the overall efficiency of heat transfer through a copper plated micro-channel heat exchanger. However, since the device is sealed and composed entirely of copper, understanding the phase change, temperature field, and density field of the working fluid is difficult empirically. Given that the efficiency was shown to be greatly increased by the working fluid phase change, this understanding within the device is important to designing devices of greater efficiency and different working fluids. One method of determining device and component performance is numerical modeling of the system.

Fluids that undergo phase change have long frustrated those attempting to successfully numerically model systems with acceptable stability. Over the past twenty years, the lattice Boltzmann method (LBM) has transformed the simulation of multicomponent and multiphase flows. Particularly with multiphase flows, the LBM “naturally” morphs the phase change interface throughout the model without excessive computational complexity. The relative ease with which LBM has been applied to some multicomponent/multiphase systems inspired the use of LBM to track phase change within the previously recorded experimental boundary conditions for the copper plated heat exchanger.

In this paper, the LBM was used to simulate the evaporation and condensation of HFE-7200 within a capillary flow driven square micro-channel heat exchanger (MHE). All initial and boundary conditions for the simulation are exactly those conditions at which the empirical data was measured. These include temperature and heat flux measurements entering and leaving the MHE. Working fluid parameters and characteristics were given by the manufacturer or measured during experimental work. Once the lattice size, initial conditions, and boundary conditions were input into MATLAB®, the simulations indicated that the working fluid was successfully evaporating and condensing which, coupled with the capillary driven flow, allowed the system to provide excellent heat transfer characteristics without the use of any external work mechanism.

Results indicated successive instances of stratified flow along the channel length. Micro-channel flow occurring due to capillary action instead of external work mechanisms made differences in flow patterns negligible. Coupled with the experimentally measured thermal characteristics, this allowed simulations to develop a regular pattern of phase interface tracking. The agreement of multiple simulations with previously recorded experimental data has yielded a system where transport properties are understood and recognized as the primary reasons for such excellent energy transport in the device.
Adsorption-Based Thermal Rectifier
Authors: Avanessian Tadeh , Hwang Gisuk
Published: November 2015

Controlling thermal energy transport (thermal diode) for the desired direction is crucial to improve the efficiency of thermal energy transport, conversion, and storage systems as electrical diodes significantly impact on modern electronic systems. The degree of thermal rectification is measured by the difference between the heat transfer rate in favorable and unfavorable directions to the heat transfer rate in the unfavorable direction. A gas-filled, nano-gap structure with two different surface coatings is considered to design the thermal rectifier. In such a structure where the characteristic length scale is similar to the order of the mean free path of the fluid particles (Knudsen flow regime), the effective thermal conductivity is dominantly controlled by the gas-surface interaction, i.e., thermal accommodation coefficient. For the thermal rectification, the adsorption-based, nonlinear thermal accommodation coefficient change is a key design parameter. Here, these are examined using the kinetic theory for various pressure and temperature ranges. Optimal material selections are also discussed.
Study of the Transient Electroosmotic Flow of Maxwell Fluids in Square Cross-Section Microchannels
Authors: Jiménez Edson M. , Escandón Juan P. , Bautista Oscar E.
Published: November 2015

Several kinds of fluids with non-Newtonian behavior are manipulated in microfluidic devices for medical, chemical and biological applications. This work presents an analytical solution for the transient electroosmotic flow of Maxwell fluids in square cross-section microchannels. The appropriate combination of the momentum equation with the rheological Maxwell model derives in a mathematical model based in a hyperbolic partial differential equation, that permits to determine the velocity profile. The flow field is solved using the Green’s functions for the steady-state regime, and the method of separation of variables for the transient phenomenon in the electroosmotic flow. Taking in to account the normalized form of the governing equations, we predict the influence of the main dimensionless parameters on the velocity profiles. The results show an oscillatory behavior in the transient stage of the fluid flow, which is directly controlled by the dimensionless relaxation time, this parameter is an indicator of the competition between elastic and viscous effects. Hence, this investigation about the characteristics of the fluid rheology on the fluid velocity of the transient electroosmotic flow are discussed in order to contribute to the understanding the different tasks and design of microfluidic devices.
Experimental Determination of Entropy Generation Rate Using Particle Image Velocimetry
Authors: Stewart James , Ring Brock , Worthen Rodney , Tucker Brody , Lemley Evan C.
Published: November 2015

As Particle Image Velocimetry (PIV) matures, new techniques have been developed for flow analysis that exploit PIV. One such recent use is in determining the underlying mechanisms of energy losses in fluidic systems. Often times, a First Law of Thermodynamics (FLT) approach is taken to determine the energy losses in a given fluidic system. However, a Second Law of Thermodynamics (SLT) approach allows for much greater detail of the energy losses to be determined. This paper will characterize the use of experimental PIV in conjunction with the SLT at the University of Central Oklahoma. A PIV apparatus can be used to observe the flow field in a given region of interest (ROI) of a fluidic system. This paper focuses on the use of PIV in conjunction with the SLT to determine the viscous dissipation rate and entropy generation rate of specified ROIs for dividing laminar flow in square ducts. Computational Fluid Dynamics (CFD) software is employed for simulation and experimental verification. A range of Reynolds numbers in the laminar regime (1–100) are used as a basis for determining volumetric flow rates through the system that are commonly found in micro-scale applications. The ROIs investigated here cover simple flow fields, where the results are compared to known analytical solutions derived for fully-developed flow in square ducts. The ROI is then focused on areas of greater interest that are internal and adjacent to a dividing flow geometry. CFD results have been used to calculate the entropy generation rate in a given dividing flow geometry and then compared to experimental results. Experimentally-based maps of viscous dissipation rate and entropy generation rate of the system are derived for analysis of sources of entropy generation.
Measurement of Liquid Film Thickness in Slug Flow of Air and Viscous Liquid in a Microchannel
Authors: Fung Lok , Kawaji Masahiro
Published: November 2015

Liquid film thickness data in slug flow in a 320 μm diameter capillary tube have been obtained and are compared with existing data and correlations. Solutions of glycerol in water at varying concentrations between 50 and 70% were injected into the capillary tube along with air, at ambient temperature. The thickness of the liquid film was measured using a laser confocal displacement sensor. Gas slug velocity data were obtained from high speed video images recorded at 40,000 frames per second. As liquid viscosity and hence capillary number was reduced, the film thickness around the gas slugs in the capillary tube decreased as expected. The liquid film thickness data were slightly underpredicted by existing correlations.
Metal Surface Wettability Manipulation by Nanoparticle Deposition During Nanofluid Boiling
Authors: Zhang Feini , Jacobi Anthony M.
Published: November 2015

Surface wettability of materials is important in heat transfer and thermal processes at micro-scale. This paper presents the manipulation of metal surface wettability by nanofluid boiling nanoparticle deposition. As confirmed by microscopy, particles can be deposited on metal surfaces by boiling in nanoparticle suspension, which significantly enhanced the surface wettabiliy relative to that of its original condition. The change in wettability is coupled to boiling conditions, such as nanoparticle concentration, heat flux, boiling duration, substrate roughness and so on. It has been observed that the higher the concentration of nanoparticles in the liquid during the boiling deposition process, the more pronounced the impact on wetting. Hence, surface wettability can be manipulated by controlling the nanoparticle concentration during the nanofluid boiling nanoparticle deposition (NBND) process. Such method can potentially be applied to enhance the heat transfer performance in thermal devices.
Modeling of a Non-Periodic Boundary Condition With Entrance and Exit in Dissipative Particle Dynamics
Authors: Johansson Erik O. , Yamada Toru , Yuan Jinliang , Sundén Bengt
Published: November 2015

Dissipative particle dynamics (DPD) have been widely used for the simulations of dynamics of both simple and complex fluids at nano/micro scales. In these simulations, periodic boundaries are usually employed in the main flow direction and the characterization of the flow and heat transfer is based on fully developed conditions. In the real nano/micro-fluidic devices, however, there are entrances and exits and the flow and temperature fields are not the same at different positions, making the periodic boundary conditions ill-suited due to problems with conservation of energy and momentum. This is the motivation of the present study to generate the non-periodic boundary condition having an entrance and an exit in the the DPD system and study the heat transfer characteristics in the entrance region. In this study, the entrance and exit regions are modelled for simulations of the flow in a parallel-plate channel based on the available methodology originally introduced for molecular dynamics. In this methodology, a body force acts on the DPD particles at the entrance region of the solution domain to generate the entrance region. This is region is so-called pump region. Also, a region to initiate the DPDe temperature was located followed by the pump region. Forced convection heat transfer of water flowing through a parallel-plate channel with constant wall temperature was simulated using this method. The simulations were implemented for different body forces in the pump region. The results were evaluated in terms of velocity, temperature and number density distributions in the channel and showed the effects of the compressibility of the DPD fluid and random movement (or Brownian motion). In addition, the Reynolds and Nusselt numbers were calculated to investigate their effects on the heat transfer characteristics at the entrance region.
Pressure Drop for Single- and Two-Phase Flows Through a Return Bend in Horizontal Rectangular Microchannel and Minichannel
Authors: Kawahara Akimaro , Sadatomi Michio , Miyagawa Shinichi , Mansour Mohamed H.
Published: November 2015

In this paper, single-phase liquid and two-phase gas-liquid pressure drop data through 180° return bends have been obtained for horizontal rectangular micro-channel and mini-channel. To investigate the size effects of the test channels, the hydraulic diameters were 0.25 mm and 3 mm respectively as the micro-channel and the mini-channel. The curvature radii of the bends were 0.500 mm and 0.875 mm for the micro-channel, while 6 mm for the mini-channel. To know liquid properties effects, distilled water, surfactant and glycerin aqueous solutions, ethanol and HFE (hydrofluoroether)-7200 were used as the test liquid, while nitrogen gas and air as the test gas. Pressure distributions upstream and downstream tangents of the bend were measured for the single-phase and the two-phase flows. From the pressure distribution data, the bend pressure loss was determined. By analyzing the present data, the bend loss coefficient for single-phase flow in both micro- and mini-channels could be correlated with Dean number. On the other side, the total bend pressure loss for two-phase flows were correlated by using an approach of Padilla et al., in which the total pressure loss is the sum of two pressure drop components, i.e., frictional pressure drop and singular pressure drop. The approach was found to be applicable to the present data for the micro- and the mini-channels if the frictional pressure drop was calculated by Lockhart-Martinelli method with Mishima & Hibiki’s correlation and Kawahara et al.’s correlation and the singular pressure drop was calculated by a newly developed empirical correlation.
Response Surface Optimization of Critical Structure Parameters of a Micropump Based on Synthetic Jet
Authors: He Xiuhua , Zhang Xitong , Huang Feng , Zhu Jiawei
Published: November 2015

Valveless micropump based on synthetic jet possesses larger flowrate and more successively outflow than valveless micropump with diffuser/nozzle elements. In order to optimize the critical structure parameters including the height of the pump chamber and the outlet diameter, a valveless piezoelectric micropump based on synthetic jet and for transporting liquid was designed and studied. A three-dimensional numerical simulation was done to obtain the flow feature and the performance of the micropumps with different structure parameters. The velocity boundary condition was applied to simulate the oscillation of the piezoelectric diaphragm and the SST flow model was utilized as the maximum Reynolds number was 800. Based on the simulation, quadratic linear regression was used in the response surface optimization. The response surface illustrates the region of the parameters leading to the largest flowrate. The results reveal that the optimal region is from 4.6 mm to 6.4 mm for the outlet diameter and from 7.2 mm to 8.8 mm for the height of the pump chamber. The maximum flowrate of the micropump can achieve 33.41 mL/min when the maximum Reynolds number is 800. It suggests that the interation exists of the height of the pump chamber and the outlet diameter. And the micropump with a higher pump chamber requires a larger outlet diameter.
Experimental demonstration of single electron transistors featuring SiO2 plasma-enhanced atomic layer deposition in Ni-SiO2-Ni tunnel junctions
Authors: Karbasian Golnaz , McConnell Michael S. , Orlov Alexei O. , Rouvimov Sergei , Snider Gregory L.
Published: November 2015

The authors report the use of plasma-enhanced atomic layer deposition (PEALD) to fabricate single-electron transistors (SETs) featuring ultrathin (≈1 nm) tunnel-transparent SiO2 in Ni-SiO2-Ni tunnel junctions. They show that, as a result of the O2 plasma steps in PEALD of SiO2, the top surface of the underlying Ni electrode is oxidized. Additionally, the bottom surface of the upper Ni layer is also oxidized where it is in contact with the deposited SiO2, most likely as a result of oxygen-containing species on the surface of the SiO2. Due to the presence of these surface parasitic layers of NiO, which exhibit features typical of thermally activated transport, the resistance of Ni-SiO2-Ni tunnel junctions is drastically increased. Moreover, the transport mechanism is changed from quantum tunneling through the dielectric barrier to one consistent with thermally activated resistors in series with tunnel junctions. The reduction of NiO to Ni is therefore required to restore the metal-insulator-metal (MIM) structure of the junctions. Rapid thermal annealing in a forming gas ambient at elevated temperatures is presented as a technique to reduce both parasitic oxide layers. This method is of great interest for devices that rely on MIM tunnel junctions with ultrathin barriers. Using this technique, the authors successfully fabricated MIM SETs with minimal trace of parasitic NiO component. They demonstrate that the properties of the tunnel barrier in nanoscale tunnel junctions (with <10−15 m2 in area) can be evaluated by electrical characterization of SETs.
Modeling of local dielectric charging induced by line scan during SEM observation
Authors: Cheng Zh. H. , Koyama H. , Kimura Y. , Shinada H. , Komuro O.
Published: November 2015

Evolution in emission of secondary electrons and backscattered electrons from thermally oxidized silicon sample during a single line-scan of a focused electron beam was measured. The influence of charging-up of the sample surface on the electron emission was analytically modeled. The detected current Ic of these electrons decreased sharply at the initial scanning stage and turned steady. The ratio of Ic to incident primary-beam current (Ip) at the steady state could be expressed in terms of “scanning line density” (λin), i.e., the number of incident electrons per unit scan distance. It was found that the ratio of Ic/Ip increased monotonously with decreasing λin. Furthermore, no difference in the change of the Ic/Ip at the steady states was observed with interval, i.e., the time between consecutive single-line scans at the same position, ranging from 10 μs to 500 ms. This result indicates that attenuation of the charge at the sample surface during the single line-scan is neglectable. An analytical model was proposed for correlating Ic/Ip, the potential at the irradiated position (Vs) and λin. The calculated Ic/Ip coincided quantitatively with the experimentally determined ratio. Besides, the analytical model showed that higher magnitude and less variation in Ic/Ip during the single line-scan can be expected with lower λin, which was experimentally verified. The parameter λin can be equally controlled through Ip and the line-scan speed v.