bluuebunny/crossref_metadata_2025 huggingface dataset:
Mathematical Model of Heat Transfer in High-Porous Materials
Authors: Alifanov Oleg M. , Budnik Sergey A. , Nenarokomov Aleksey V. , Cherepanov Valery V.
Published: March 2011

The main purpose of this study was to confirm operability and effectiveness of the complex methods developed for theoretical prediction and experimental-computational determining some properties of modern highly porous materials, used as thermal protection shields in objects of space engineering, power engineering, etc. Them concern Fibrous Materials (FM), Open-Cell Reticulated Foams (OCRF) etc. The basis for the method proposed is in the combination of Direct Mathematical Simulation (DMS) for global structure of the complex irregular systems, which have the property of local regularity and Inverse Heat Transfer Problem (IHTP) methods. This approach makes possible to obtain and to predict such properties as radiation and molecular thermal conductivities, energy accommodation coefficient, complex refractive index, scattering indicatrix, scattering and absorption factors, etc. Such problems are of great practical importance in the study of material properties.
Study on Heat Transfer Mechanism of Isolated Bubble Nucleate Boiling With MEMS Sensors
Authors: Yabuki Tomohide , Nakabeppu Osamu
Published: March 2011

This paper describes an experimental investigation of heat transfer mechanism beneath isolated bubble during nucleate boiling with MEMS sensors having high temporal and spatial resolution in temperature measurement. The MEMS sensor fabricated for the boiling research includes eight thin film thermocouples and an electrolysis trigger on the topside of 20 × 20 mm2 silicon substrate and thin film heater on the backside. The electrolysis trigger initiates bubble growth by supplying hydrogen gasses as bubble nuclei with the electrolysis of the water by two electrodes. In the experiment, temperature fluctuation beneath an isolated bubble during saturated nucleate boiling of water was measured with the sensor. The measurement data presented strong evaporation and dry-out of the microlayer in the bubble growth phase and rewetting of the dry-out area in the bubble departure phase. Moreover, heat transfer induced by the boiling bubble was evaluated by computing local heat flux through a transient heat conduction simulation in the sensor substrate using the measured data as boundary condition. The heat transfer analysis shows that the local heat flux in the microlayer evaporation area has high value of the order of MW/m2, and the maximum value of about 2 MW/m2 is indicated near the center in an early phase of the bubble growth. On the other hand, the heat flux is very low of around zero at the dry-out area, where microlayer had disappeared completely, and slight increase was observed at the rewetting area. Total heat transferred from the surface reached to about half of latent heat in the bubble until the bubble departure. Finally, initial thickness of the microlayer under the bubble was estimated by integrating the derived local heat flux. As the result, it was distributed in a few μm within the measurement area.
Combined Heat Transfer of Natural Convection-Conduction and Surface Radiation in an Open Cavity Heated by Constant Flux
Authors: Wang ZhiYun , Yang Mo , Zhang YuWen , Li Ling
Published: March 2011

Combined heat transfer of natural convection-conduction and surface radiation in an open cavity heated by constant flux is studied in this paper. Flow model is laminar and SIMPLE algorithm and QUICK scheme are employed. The relevant parameters are as follows, Prandtl number is 0.7 and dimensionless solid thickness is 0.2, conductivity ratio rangs from 0 to 1000, Rayleigh number ranges from 103 to 109, surface emissivity ranges from 0 to 1. The numerical results shows secondary circular formed as an effect of radiation which increased the average Nusselt number about from 54.1% to 100.3%.
Numerical Analysis of Single Phase Multi Layered Micro-Channel Heat Sink With Inter-Connects Between Vertical Channels
Authors: Morshed A. K. M. M. , Khan Jamil A.
Published: March 2011

Micro-channels embedded in solid matrix have already proven to be a very efficient way of electronics cooling. Traditional micro-channel heat sinks consist of single layer of parallel channels. Although micro-channel heat sink can achieve very high heat flux, its pumping requirement for circulating liquid through the channel increases very sharply as the flow velocity increases. The pumping requirement can be reduced by stacking multi layers of micro-channels. By introducing multi layers of channels, the flow velocity through each channel is reduced for the same total mass flow rate of the coolant. A novel approach to take advantage of multi layered channel is proposed in this study where the vertical channels are interconnected to allow cross flow of the coolant. The cross-flow between the channels disrupts boundary layer enhancing heat removal capacity of the heat sink. A CFD model has been developed using commercially available software package FLUENT to evaluate overall thermal performance of multi layered micro-channel heat sink. A parametric study of the flow rates and the effect of the number of layers and interconnections have been performed. Significant reduction in thermal resistance has been observed for multiple layers, it is also observed that this reduction in thermal resistance is dependent on the thermal conductivity of the heat sink material.
Novel PCM and Jet Impingement Based Cooling Scheme for High Density Transient Heat Loads
Authors: Parida Pritish R. , Ekkad Srinath V. , Ngo Khai
Published: March 2011

Breakthroughs in the recent cutting-edge technologies have become increasingly dependent on the ability to safely dissipate large amount of heat from small areas. Improvements in cooling techniques are therefore required to avoid unacceptable temperature rise and at the same time maintain high efficiency. Jet impingement is one such cooling scheme which has been widely used to dissipate transient and steady concentrated heat loads. With constantly increasing transient cooling needs, conventional pin-fin cooling and conventional jet impingement cooling are not meeting the requirements. Considerable improvements are therefore required to meet such stringent requirements without any significant changes in the cooling system. A combined cooling scheme based on jet impingement and phase change materials (PCMs) is presented as one such alternative to existing cooling systems. A high heat storage capability of PCMs in combination with a high heat transfer rates from impingement cooling can help overcome the existing heat distribution and transient cooling problems in high heat flux dissipating devices. Preliminary conjugate CFD simulations show promising results. Additionally, experimental validation of the simulation predictions has also been performed. A reasonably good agreement was found between the predictions and experiments.
Experimental Study of Natural Convective Heat Transfer From an Inclined Isothermal Square Cylinder With an Exposed Top Surface Mounted on a Flat Adiabatic Base
Authors: Kalendar Abdulrahim , Oosthuizen Patrick H. , Alhadhrami Abdulrahman
Published: March 2011

Natural convective heat transfer rates from inclined cylinders with a square cross-section and which has an exposed top surface have been experimentally studied. When relatively small square cylinders with exposed top surfaces inclined at an angle to the vertical are used, the inclination angle to the vertical has, in general, a considerable effect on the magnitude of the mean heat transfer rate and on the nature of the flow over the surfaces that make up the cylinder. In the situation here considered the cylinder is mounted on a large flat essentially adiabatic surface with the other cylinder surfaces exposed to the surrounding air and with the cylinder, in general, inclined to the vertical at angles between vertically upwards and vertically downwards. The situation considered is an approximate model of that which occurs in some electrical and electronic component cooling problems. The cross-sectional size-to-height ratio of the square cylinders used in the present study was comparatively small, i.e. the square cylinders were short, the width, w, of the square cylinders being 25.4 mm and the width-to-height ratios of between 1 and 0.25 being used. One of the main aims of the present work was to determine how the cross-sectional size-to-height ratio of the square cylinder, i.e., w/h, influences the mean heat transfer rate from the cylinder at various angles of inclination between vertically upwards and vertically downwards. The heat transfer rates were determined by the transient method, this basically involving heating the model and then measuring its temperature-time variation while it cooled, the tests being carried out inside a large enclosure. Tests were carried out in air with all models at various angles of inclination to the vertical between vertically upwards and vertically downwards. The effects of w/h, Rayleigh number, Ra, and angle of inclination, φ, on the mean Nusselt number, Nu for the entire cylinder have thus been studied. The Rayleigh number, Ra, based on the cylinder height, h, was between approximately 1E4 and 5E6. The experimental results have been compared with the results obtained in an earlier numerical study.
Effect of Minichannel Cross Sectional Shape in Condensation
Authors: Del Col Davide , Bortolin Stefano , Cavallini Alberto , Matkovic Marko
Published: March 2011

The objective of this work is to present new condensation heat transfer coefficients measured inside a single square cross section minichannel, having a 1.18 m side length, and compare them to the ones measured in a circular minichannel. This subject is particularly interesting since most of the mini and microchannels used in practical applications have non circular cross sections. The test section used in the present work is obtained from a thick wall copper tube which is machined to draw a complex passage for the coolant. This experimental technique allows to measure directly the temperature in the tube wall and in the water channel. The heat flux is determined from the temperature profile of the coolant in the measuring sector. The experimental technique is checked in single phase runs which represent a severe testing condition due to the low values of heat flux. Tests have been performed with R134a at 40°C saturation temperature, at mass velocities ranging between 200 and 800 kg m−2s−1. As compared to the heat transfer coefficients measured in a circular minichannel, in the square minichannel the authors find that heat transfer is enhanced at the lowest values of mass velocity; this must be due to the effect of the surface tension. No heat transfer coefficient increase has been found at the highest values of the mass velocity where condensation seems to be shear stress dominated.
Experimental and Analytical Study on the Heat Transfer Characteristics in a Natural Circulation Loop for an Integral Type Reactor
Authors: Park Hyun-Sik , Choi Ki-Yong , Cho Seok , Yi Sung-Jae
Published: March 2011

The heat transfer characteristics in a natural circulation loop for an integral type reactor were experimentally investigated by using the VISTA facility and steady-state natural circulation flow rates were acquired for various core powers and feed water flow rates. The experimental data were compared with the predictions from existing correlations of Duffey et al. (1987) and Vijayan et al. (2002). It was shown that Duffey et al. (1987)’s correlation for a two-phase natural flow predicted the experimental data well. Also the experimental data on the natural circulation in the primary loop of the VISTA facility were analyzed by using a best-estimate system analysis code, MARS. The MARS code predicted the overall natural circulation flow characteristics reasonably.
An Assessment of Different Boundary Conditions in a Naturally Ventilated Venlo-Type Greenhouse
Authors: Kruger Sunita , Pretorius Leon
Published: March 2011

This paper presents a parametric study of the indoor climate of a four span greenhouse subjected to natural ventilation. The effect of different heat fluxes through the greenhouse covering on the airflow patterns as well as temperature and velocity distributions were determined. Appropriate effective heat flux boundary conditions were introduced in the CFD model to investigate temperature and velocity distributions at plant level. Initially, three different simulations were done to represent zero wind speed conditions. Secondly, a velocity of 1m/s was specified at the domain inlet boundary. Results indicated that for all cases, the velocity distribution was heterogeneous and quite high for wind still days around midday. Temperature distributions were more homogeneous, decreased with the presence of a wind. Results indicated that a parametric value of 20% of the maximum daily solar radiation approximates previously simulated wall temperatures. It was also concluded that design changes such as additional openings including side and/or more roof ventilators be utilized to enhance ventilation on wind still days, as well as the warmer parts of the day.
Phase Transition of Partially Miscible Solvents Induced by Heating Cycles: Application for the Remediation of Contaminated Sediments and Sludge
Authors: Ullmann Amos , Brauner Neima , Dahan Guy , Golan Tal , Ludmer Zvi
Published: March 2011

A novel process entitled “Sediments Remediation Phase Transition Extraction” (SR-PTE) is being developed for a simultaneous removal of both heavy metals and organic pollutants from contaminated sediments or sludge. The process uses partially miscible solvent mixtures, where one of the components is water. By heating the mixture above a certain temperature, a single phase is formed. This allows the organic solvent, containing an appropriate chelating agent, to penetrate the wetted sediment voids and efficiently extract simultaneously the organic and heavy metal pollutants. The phase separation, occurring during the cooling stage, is fast and allows the pollutants to propagate and concentrate in the lighter organic phase, leaving the sediments and the lower aqueous phase practically clean. The SR-PTE technology was tested on authentically polluted river sediments and on heavily contaminated sludge from a waste-water treatment plant. The extraction efficiency was found to improve by the phase transition cycle induced by temperature variation (about twice than that obtained when the extraction was carried out isothermally). Additionally, with the induced phase transition, the formation of stable emulsions is prevented albeit the presence of natural surfactants in the treated media. The process was tested on lab scale and bench-scale reactor. No significant effects of the process up-scaling from lab to bench scale were observed.
Modelling of Condensation in a Circular Minichannel by Means of the VOF Method
Authors: Da Riva E. , Del Col D. , Cavallini A.
Published: March 2011

3D steady-state Volume of Fluid (VOF) simulations of condensation of R134a inside a 1 mm i.d. circular minichannel are proposed. The minichannel is horizontally oriented and both the effects of gravity and surface tension are taken into account. A uniform interface temperature, as well as a uniform wall temperature, were fixed as boundary conditions in order to model the phase change process. Simulations were performed at mass fluxes G = 100 kg m−2s−1 and G = 400 kg m−2s−1. It has been assumed that the flow was turbulent inside the vapour core, while for the condensate film two different computational approaches have been considered. The first approach (i.e. “laminar liquid film”) corresponds to the assumption that the flow is laminar inside the liquid phase and turbulent inside the vapour phase. For the second approach (i.e. “turbulent liquid film”), instead, a low Reynolds form of a turbulence model has been used through both phases. The aforementioned approaches are compared to each other and the computed heat transfer coefficients are compared against experimental data by Matkovic et al.: data at low mass flux is well predicted by the “laminar liquid film” approach and overpredicted by “turbulent liquid film” approach, while data at high mass flux is underpredicted by the former approach and well predicted by the latter. The evolution of the vapour-liquid interface along the minichannel, as well as the velocity field, are reported. Besides, the computed cross sectional void fraction is compared against empirical correlations available in the literature.
On Uncertainty and Local Sensitivity Analysis for Steady-State Conjugate Heat Transfer Problems
Authors: Rauch Christian , Almbauer Raimund
Published: March 2011

In recent years, significant effort has been put into developing automated multi-physics simulation. The exchange of boundary conditions has lead to more realistic as well as more complex simulations with usually slower convergence rate when the coupling is being performed between two different codes. In this paper the equations of local sensitivities for element centered steady-state combined convection, conduction, and thermal radiation problems are being derived along with some implementation details. A numerical analysis on the stability of the solution matrix is being conducted. The relative importance of the heat transfer modes at various locations are investigated by their uncertainty factors and conclusions are being drawn.
Modeling of Heatmasstransfer in “Wet” and “Dry” Storages for Spent Nuclear Fuel
Authors: Fedorovich Evgeny D. , Karyakin Yuri E. , Mikhailov Vladimir E. , Astafieva Vera O. , Pletnev Alexander A.
Published: March 2011

There are considered the methods of mathematical and physical modeling of the heatmasstransfer in storage facilities of different types {water pools, “dry” cask type storages}. There are shown the computational results, obtained in application to the storage facilities, located at nuclear power plant territory. There are considered the technological operations, connected with the spent fuel transference from interim “wet” storage to a longer term storage in casks facility {cask’s loading with fuel, its drying under vacuum}. Special attention is devoted to an accident situation in “wet” storage — a loss of the coolant circulation through pool and following loss of water due evaporation. Problems of further investigations are formulated.
Heat Transfer of Supercritical Carbon Dioxide in Printed Circuit Heat Exchanger Geometries
Authors: Kruizenga Alan , Anderson Mark , Fatima Roma , Corradini Michael , Towne Aaron , Ranjan Devesh
Published: March 2011

The increasing importance of improving efficiency and reducing capital costs has lead to significant work studying advanced Brayton cycles for high temperature energy conversion. Using compact, highly efficient, diffusion-bonded heat exchangers for the recuperators, has been a noteworthy improvement in the design of advanced carbon dioxide Brayton Cycles. These heat exchangers will operate near the pseudocritical point of carbon dioxide, making use of the drastic variation of the thermo-physical properties. This paper focuses on the experimental measurements of heat transfer under cooling conditions, as well as pressure drop characteristics within a prototypic printed circuit heat exchanger. Studies utilize type-316 stainless steel, nine channel, semi-circular test section, and supercritical carbon dioxide serves as the working fluid throughout all experiments. The test section channels have a hydraulic diameter of 1.16mm and a length of 0.5m. The mini-channels are fabricated using current chemical etching technology, emulating techniques used in current diffusion bonded printed circuit heat exchanger manufacturing. Local heat transfer values were determined using measured wall temperatures and heat fluxes over a large set of experimental parameters that varied system pressure, inlet temperature, and mass flux. Experimentally determined heat transfer coefficients and pressure drop data are compared to correlations and earlier data available in literature. Modeling predictions using the CFD package FLUENT are included to supplement experimental data. All nine channels were modeled using known inlet conditions and measured wall temperatures as boundary conditions. The FLUENT results show excellent agreement in total power removal for the near pseudocritical region, as well as regions where carbon dioxide is a high or low density fluid.
Development of a Heat-Transfer Correlation for Supercritical Water Flowing in a Vertical Bare Tube
Authors: Mokry Sarah , Farah Amjad , King Krysten , Gupta Sahil , Pioro Igor , Kirillov Pavel
Published: March 2011

This paper presents an analysis of heat-transfer to SuperCritical Water (SCW) in bare vertical tubes. A large set of experimental data, obtained in Russia, was analyzed and a new heat-transfer correlation for SCW was developed. This experimental dataset was obtained within conditions similar to those for proposed SuperCritical Water-cooled nuclear Reactor (SCWR) concepts. Thus, the new correlation presented in this paper can be used for preliminary heat-transfer calculations in SCWR fuel channels. The experimental dataset was obtained for SCW flowing upward in a 4-m-long vertical bare tube. The data was collected at pressures of about 24 MPa for several combinations of wall and bulk-fluid temperatures that were below, at, or above the pseudocritical temperature. The values ranged for mass flux from 200–1500 kg/m2s, for heat flux up to 1250 kW/m2 and for inlet temperatures from 320 to 350°C. Previous studies have confirmed that there are three heat-transfer regimes for forced convective heat transfer to water flowing inside tubes at supercritical pressures: (1) Normal Heat-Transfer (NHT) regime; (2) Deteriorated Heat-Transfer (DHT) regime, characterized by lower than expected Heat Transfer Coefficients (HTCs) (i.e., higher than expected wall temperatures) than in the NHT regime; and (3) Improved Heat-Transfer (IHT) regime with higher-than-expected HTC values, and thus lower values of wall temperature within some part of a test section compared to those of the NHT regime. Also, previous studies have shown that the HTC values calculated with the Dittus-Boelter and Bishop et al. correlations deviate quite substantially from those obtained experimentally. In particular, the Dittus-Boelter correlation significantly over predicts the experimental data within the pseudocritical range. A new heat-transfer correlation for forced convective heat-transfer in the NHT regime to SCW in a bare vertical tube is presented in this paper. It has demonstrated a relatively good fit for HTC values (±25%) and for wall temperature calculations (±15%) for the analyzed dataset. This correlation can be used for supercritical water heat exchangers linked to indirect-cycle concepts and the co-generation of hydrogen, for future comparisons with other independent datasets, with bundle data, as the reference case, for the verification of computer codes for SCWR core thermalhydraulics and for the verification of scaling parameters between water and modeling fluids.
Unsteady Wake and Coolant Density Effects on Turbine Blade Film Cooling Using PSP Technique
Authors: Rallabandi Akhilesh P. , Li Shiou-Jiuan , Han Je-Chin
Published: March 2011

The effect of an unsteady stator wake (simulated by wake rods mounted on a spoke wheel wake generator) on the modeled rotor blade is studied using the Pressure Sensitive Paint (PSP) mass transfer analogy method. Emphasis of the current study is on the mid-span region of the blade. The flow is in the low Mach number (incompressible) regime. The suction (convex) side has simple angled cylindrical film-cooling holes; the pressure (concave) side has compound angled cylindrical film cooling holes. The blade also has radial shower-head leading edge film cooling holes. Strouhal numbers studied range from 0 to 0.36; the exit Reynolds Number based on the axial chord is 530,000. Blowing ratios range from 0.5 to 2.0 on the suction side; 0.5 to 4.0 on the pressure side. Density ratios studied range from 1.0 to 2.5, to simulate actual engine conditions. The convex suction surface experiences film-cooling jet lift-off at higher blowing ratios, resulting in low effectiveness values. The film coolant is found to reattach downstream on the concave pressure surface, increasing effectiveness at higher blowing ratios. Results show deterioration in film cooling effectiveness due to increased local turbulence caused by the unsteady wake, especially on the suction side. Results also show a monotonic increase in film-cooling effectiveness on increasing the coolant to mainstream density ratio.
An Integrated Nano-Structured Heat Spreader for High Heat Flux Electronic Systems
Authors: Bhopte Siddharth , Desu Seshu B. , Sammakia Bahgat
Published: March 2011

With the increase in power densities and decrease in chip and electronic package dimensions, their thermal management is a challenge and is a focus of several ongoing research efforts. To achieve the desired thermal management for optimal device operation, heat generated by the chip has to be effectively transferred to the ambient via several structures of the electronic package. Hence the need for development of high thermal conductivity structures is of prime importance. Heat spreaders quickly spread the heat generated by the chips over a larger area from where it is conducted to the ambient via heat sink. Heat-spreading research from a materials view point involves direct combination of high thermal conductivity materials within the microprocessor substrate. In this paper, a novel nano heat spreader design is proposed. Highly conductive graphene layer is integrated on a silicon carbide substrate on one side and thin films of carbon nano tubes as thermal interface material on the other side. Analytical solutions and case studies are presented to show that the proposed approach for the heat spreader design can yield very high effective thermal conductivity while remaining mechanically flexible, as required for reducing thermal stresses.
Thermal Performance of Carbon Nanotube Enhanced Vapor Chamber Wicks
Authors: Kim Sungwon S. , Weibel Justin A. , Fisher Timothy S. , Garimella Suresh V.
Published: March 2011

Vapor chambers are often used as spreaders to dissipate high heat fluxes by taking advantage of liquid-vapor phase change. Wicking of the working fluid in vapor chambers is accomplished through capillary action, which is strongly affected by the wick structure. Traditionally, copper meshes with micrometer-scale pore sizes have been used as wicking structures, but it is expected that heat fluxes in the next generation of high-power electronic devices will cause boiling in these devices and lead to dryout with conventional wick materials. With a goal of increasing maximum heat dissipation and reducing thermal resistance, a wick structure composed of both conventional copper mesh and carbon nanotubes has been developed and characterized. The high-permeability mesh provides for a low-resistance bulk flow path while the carbon nanotubes, with their high thermal conductivity and high surface area, modify the wick surface for enhanced capillary action. CNT-enhanced integrated wicks were fabricated by sintering a copper mesh on Cu-Mo-Cu substrates, on which CNTs were grown. A thin layer of copper was evaporated onto the CNTs to improve wicking and wettability with water, the working fluid of interest. Samples grown under varying degrees of positive bias voltage and varying thicknesses of post-CNT-growth copper evaporation were fabricated, so that the surface morphology of the samples could be varied. The resultant boiling curves and associated wick thermal resistances indicate that micro/nano integrated wicks fabricated with higher positive bias voltages during CNT synthesis, and thicker copper coatings, lead to improved thermal performance and lower wick thermal resistance. Notably, heat fluxes at the heater surface of greater than 500 W/cm2 were observed without reaching a critical heat flux condition.
Dropwise Condensation Life Testing of Self Assembled Monolayers
Authors: Bonner Richard W.
Published: March 2011

The increasing thermal demand of electronics devices has pushed the limits of current two-phase thermal technologies such as heat pipes and vapor chambers. The most obvious area for thermal improvement is centered around the high heat flux generating chips including improved evaporators, thermal interfaces, etc. However, heat fluxes in the sink/condensing regions have also risen as the size of electronics packages has decreased. One way to reduce the thermal resistance associated with condensation is to promote dropwise condensation. In previous work, the condensation performance improvement using self-assembled monolayer coated surfaces (to promote hydrophobicity) has been shown. However, the question of the life of the self-assembled monolayer coatings needs to be addressed before the technology is adopted, as this has plagued other dropwise condensation coatings in the past. Presented here is a general use of self-assembled monolayer coatings to promote dropwise condensation in electronics device applications, including a summary of recent work regarding dropwise condensation on gradient surfaces. Also presented is experimental data from a life test of self-assembled monolayers on copper and gold plated surfaces. In the life test, the surfaces have been continuously exposed to saturated steam at 60°C. Both surfaces have continued to promote dropwise condensation for over 9 months under conditions representative of heat pipe electronics cooling applications.
Pool Boiling of Mixtures for Electronics Thermal Management
Authors: Sathyanarayana Aravind , Joshi Yogendra , Im Yunhyeok
Published: March 2011

Electrical and chemical compatibility requirements of electronic components pose significant constraints on the choice of liquid coolants. These constraints have led to the use of fluoroinerts and Novec liquids as coolants, which are plagued by significantly lower thermal conductivity, specific heat, and latent heat of vaporization compared to water, and also a number of these chemicals have significant environmental impact. These factors necessitate the development of new heat transfer fluids with improved heat transfer properties and applicability. Mixture formulations provide an avenue for enhancing the properties of existing heat transfer fluids. These can be tuned for specific applications. Mixture formulations of Novec fluid (HFE 7200) with alcohols and ethers (HFE 7200 and methanol; HFE 7200 and ethoxybutane) are considered in this study. A 1 cm × 1 cm Silicon (Si) sample having copper nanowire arrays is used as the test surface for pool boiling. Experiments are done under saturated conditions and also at different sub-cooled conditions to investigate the thermal performance of these new fluid mixtures. Pool boiling heat transfer performance and the critical heat flux are measured for fluid mixtures and compared with the corresponding base fluid. From the pool boiling experiments, it was observed that adding methanol to pure HFE 7200 enhances the CHF of the resulting mixture and adding ethoxybutane to pure HFE 7200 reduces the incipience temperature for boiling.
Picosecond Transient Thermal Imaging Using a CCD Based Thermoreflectance System
Authors: Christofferson James , Yazawa Kazuaki , Shakouri Ali
Published: March 2011

A new Charge Coupled Device (CCD) based, full-field thermoreflectance thermal imaging technique is demonstrated with 800 picoseconds temporal resolution. Transient thermal images of pulsed heating in single interconnect vias of 350nm and 550nm in diameter are shown. The use of pulsed laser diodes and dedicated synchronization circuits can significantly lower the cost and the image acquisition time compared to the scanning pump-probe laser systems. Also the same set up can be used to study transient thermal phenomena in a wide dynamic range from sub nanoseconds to seconds.
Measurement of Heat Transfer and Pressure Drop During Condensation of Carbon Dioxide in Microscale Geometries
Authors: Fronk Brian M. , Garimella Srinivas
Published: March 2011

Heat transfer coefficients and pressure drops during condensation of carbon dioxide (CO2) are measured in small quality increments in microchannels of 100 < Dh < 200 μm. Channels are fabricated on a copper substrate by electroforming copper onto a mask patterned by X-ray lithography, and sealed by diffusion bonding. The test section is cooled by chilled water circulating at a high flow rate to ensure that the thermal resistance on the condensation heat transfer side dominates. A conjugate heat transfer analysis in conjunction with local pressure drop profiles allows driving temperature differences, heat transfer rates, and condensation heat transfer coefficients to be determined accurately. Heat transfer coefficients are measured for G = 600 kg m−2 s−1 for 0 < x < 1 and multiple saturation temperatures. Preliminary results for a 300 × 100 μm (15 channels) test section are presented. These data are used to evaluate the applicability of correlations developed for larger hydraulic diameters and different fluids for predicting condensation heat transfer and pressure drop of CO2.
Characterization of Carbon Nanotube Array Based Thermal Interface Material After Bonding Process
Authors: Chu Rong-Shiuan , Zhao Yang , Majumdar Arun
Published: March 2011

Vertically Aligned Carbon Nanotube (CNT) Arrays are promising to use as advanced thermal interface material. While possessing high thermal conductivity for an individual tube, carbon nanotube array based thermal interface materials (TIMs) fell short of expectations due to poor CNTs-target surface contacts. Investigations suggested that the overall resistance can be potentially reduced to less than 1 m2-K/MW by increasing the number of tubes to target surface contacts. This paper use chromium/gold/indium assisted thermal pressure-bonding to enhance contacts. A CNT array with 12.7% areal density was bonded to an experimental glass surface with 2-μm indium bonding layer and 10 nm-chromium/150 nm-gold adhesion layers under pressure of 196 KPa and temperature of 350 °C. Phase sensitive photothermal reflectance method was used for thermal measurement. The overall resistance, including CNTs-glass contact resistance and effective CNT array thermal resistance, is 1.1 m2-K/MW ± 27%. Although the contact resistance was reduced to 0.39 m2-K/MW ± 15%, the effective thermal conductivity of the post-bonded 80 μm long CNTs was 114 W/m-K ± 22%, which was lower than the expected lower bound of the thermal conductivity of 12.7% filled CNT array. It was suggested that the deformation of CNT array after mechanical bonding reduced its performance.
Numerical Simulation on Fluid Flow and Heat Transfer of Film Cooling With Backward Injection
Authors: Li Xianchang
Published: March 2011

Film cooling has been used in gas turbine engines for many years. In film cooling, the relatively cold air blankets the blade surface so that heat transfer between the hot working gas flow and blades is reduced. The main issues related to film cooling are its poor coverage, aerodynamic loss, and increase of heat transfer coefficient due to strong mixing. To overcome these problems, film cooling with backward injection is investigated in this paper, while the coolant air in the existing designs is injected with a forward angle in the mainstream direction. The focus of this paper is on the simple cylindrical hole with different blowing ratios and angles. Numerical simulation is carried out to explore the detailed flow structure and heat transfer behavior of the backward cooling scheme, and its performance is compared with that of the existing design.
Experimental Heat Transfer Enhancement in Single-Phase Liquid Microchannel Cooling With Cross-Flow Synthetic Jet
Authors: Fang Ruixian , Jiang Wei , Khan Jamil , Dougal Roger
Published: March 2011

The present study experimentally investigated a new hybrid cooling scheme by combination of a microchannel heat sink with a micro-synthetic jet actuator. The heat sink consisted of a single rectangular microchannel measured 550 μm wide, 500 μm deep and 26 mm long. The synthetic jet actuator with a 100 μm diameter orifice was placed right above the microchannel and 5 mm downstream from the channel inlet. Micro jet is synthesized from the fluid flowing through the microchannel. Periodic disturbance is generated when the synthetic jet interacts with the microchannel flow. Heat transfer performance is enhanced as local turbulence is generated and propagated downstream the microchannel. The scale and frequency of the disturbance can be controlled by changing the driving voltage and frequency of the piezoelectric driven synthetic jet actuator. The effects of synthetic jet on microchannel heat transfer performance were studied based on the microchannel flow Reynolds number, the jet operating voltage and frequency, respectively. It shows that the synthetic jet has a greater heat transfer enhancement for microchannel flow at lower Reynolds number. It also shows that the thermal effects of the synthetic jet are functions of the jet driving voltage and frequency. We obtained around 42% heat transfer enhancement for some test cases, whereas the pressure drop across the microchannel increases very slightly. The paper concludes that the synthetic jet can effectively enhance single-phase liquid microchannel heat transfer performance and would have more promising enhancements if multi-jets are applied along the microchannel.
Physics-of-Failure Design-for-Reliability of a Bidirectional Multi-Output DC-DC Power Converter
Authors: Jha Vibhash , McCluskey F. Patrick
Published: March 2011

The design of a prototype power converter was evaluated and improved using the physics-of-failure based design-for-reliability approach. Thermal simulation of the power module board with the heat sink has been carried out and these results have been used further for estimating the overall thermal cycle reliability. The solder joint reliability has been investigated using two different kinds of solders — SnAgCu (SAC305) and SnPb eutectic solder.
Simulation of Char Dust Combustion Inside a Pyroscrubber Downstream of a Petroleum Coke Calcining Kiln
Authors: Zhao Lei , Wang Ting
Published: March 2011

A pyroscrubber is a furnace used in the petroleum coke calcining industry to recover energy from the carbonaceous contents, including char dust and hydrocarbon volatiles of the exhaust gas from the calcination kiln. The combusted hot gases are used to generate steam and produce electricity, so it is important to optimize the pyroscrubber performance to produce high-grade combusted gases to generate steam but with minimal emissions. A previous study employed the locally-homogeneous flow (LHF) model to study rhe means to improve combustion efficiency and reduce emissions. In the LHF model, the inter-phase exchange rates of mass, momentum and energy are assumed to be infinitely fast, so the dispersed phase (char dust) can be simplified as the gas phase, and the complex two-phase flow is then treated as a single-phase flow. In this study, LHF model is replaced with a solid particle combustion model by incorporating both finite-rate heterogeneous and homogeneous combustion processes. Results reveal that the particle combustion model generates much higher local flame temperature (2200K) than in LHF model (1800K). All char particles are burned before or in the high-bay area. Total energy output of the case with particle combustion model is 92% of the LHF model. Furthermore, motivated by the potential energy saving from removing the air blower power supply, this study further investigates the possible benefit of running the pyroscrubber with the ventilation doors open. Three cases with different combinations of air injections and door opening have been studied. Results show that the gas flow is stably stratified with a large amount of the entrained cold air moving at the bottom of the chamber and the hot combusted gas moving through the top. With bottom doors completely open, sufficient air can be drawn into the pyroscrubber without the need of blowing air in, but the combustion gases will be overcooled making this practice unfavorable from the energy saving point of view.
Fluid Spread Model Validation for Emerging Liquid Tank Impact Predictive Methods
Authors: Brown Alexander L. , Wagner Gregory J.
Published: March 2011

Transportation accidents frequently involve liquids dispersing in the atmosphere. An example is that of aircraft impacts, which often result in spreading fuel and a subsequent fire. Predicting the resulting environment is of interest for design, safety, and forensic applications. This environment is challenging for many reasons, one among them being the disparate time and length scales that are necessary to resolve for an accurate physical representation of the problem. A recent computational method appropriate for this class of problems has been described for modeling the impact and subsequent liquid spread. Because the environment is difficult to instrument and costly to test, the existing validation data are of limited scope and quality. A comparatively well instrumented test involving a rocket propelled cylindrical tank of water was performed, the results of which are helpful to understand the adequacy of the modeling methods. Existing data include estimates of drop sizes at several locations, final liquid surface deposition mass integrated over surface area regions, and video evidence of liquid cloud spread distances. Comparisons are drawn between the experimental observations and the predicted results of the modeling methods to provide evidence regarding the accuracy of the methods.
Air Ingress Analysis: Computational Fluid Dynamics Models
Authors: Oh Chang H. , Kim Eung S.
Published: March 2011

Idaho National Laboratory (INL), under the auspices of the U.S. Department of Energy (DOE), is performing research and development that focuses on key phenomena important during potential scenarios that may occur in very high temperature reactors (VHTRs). Phenomena identification and ranking studies to date have ranked an air ingress event, following on the heels of a VHTR depressurization, as important with regard to core safety. Consequently, the development of advanced air-ingress-related models and verification and validation data are a very high priority. Following a loss of coolant and system depressurization incident, air will enter the core of the High Temperature Gas Cooled Reactor through the break, possibly causing oxidation of the core and reflector graphite structure. Simple core and plant models indicate that, under certain circumstances, the oxidation may proceed at an elevated rate with additional heat generated from the oxidation reaction itself. Under postulated conditions of fluid flow and temperature, excessive degradation of lower plenum graphite can lead to a loss of structural support. Excessive oxidation of core graphite can also lead to a release of fission products into the confinement, which could be detrimental to reactor safety. Computational fluid dynamics models developed in this study will improve our understanding of this phenomenon. This paper presents two-dimensional (2-D) and three-dimensional (3-D) computational fluid dynamic (CFD) results for the quantitative assessment of the air ingress phenomena. A portion of the results from density-driven stratified flow in the inlet pipe will be compared with the experimental results.
Three-Dimensional Numerical Simulation of Non-Newtonian Blood in Two Coronary Stents
Authors: Gori F. , Boghi A.
Published: March 2011

The objective of the present study is to carry out CFD simulations in a realistic 3D geometry of two coronary stents in physiological conditions and to assess the influence of the non-Newtonian behavior of blood, modeled as Casson fluid. The stents used are made of 12 rings and are similar to real coronary ones. Artery is modeled as a cylinder with rigid walls and the blood is assumed as incompressible non-Newtonian fluid in laminar flow. A commercial computational fluid dynamic code is used with a mesh composed of non uniform tetrahedrons. The simulations are performed in steady and unsteady state. Wall Shear Stress, as well as its time variations and the non-Newtonian behavior, are investigated in unsteady state. Results suggest that the Newtonian behavior is leading to an overestimation of the restenosis percent.
Turbulent Thermal Diffusion Over a Locally-Heated Two-Dimensional Hill
Authors: Houra T. , Nagano Y. , Tagawa M.
Published: March 2011

We measure flow and thermal fields over a locally heated two-dimensional hill. The heated sections on the wall are divided into upstream and downstream portions of the hill model. These sections are heated independently, yielding various thermal boundary conditions in contrast to the uniformly heated case. In the separated region formed behind the hill, it is found that the mean temperature profiles in the uniformly heated case are well decomposed into the separately heated cases. This is because the velocity fluctuation produced by the shear layer formed behind the hill is large, so the superposition of a passive scalar in the thermal field can be successfully realized. The rapid increase in the mean temperature near the uniformly heated wall should be due to the heat transfer near the leeward slope of the hill. On the other hand, the mean temperature distributions away from the wall are strongly affected by the turbulent thermal diffusion on the windward side of the hill.
Influence of Rheological Parameters on Turbulent Heat Transfer in Drag-Reducing Viscoelastic Channel Flow
Authors: Tsukahara Takahiro , Ishigami Takahiro , Kurano Junya , Kawaguchi Yasuo
Published: March 2011

Direct numerical simulations (DNS) of a drag-reducing viscoelastic turbulent channel flow with heat transfer had been carried out for three kinds of rheologically different fluids (e.g., different values of Weissenberg number). The molecular Prandtl number was set to be 0.1–2.0. A uniform heat-flux condition was employed as the thermal boundary condition. In this paper, we present various statistical turbulence quantities including the mean and fluctuating temperatures, the Nusselt number (Nu), and the cross-correlation coefficients and discuss about their dependence on the rheological parameters and the Prandtl-number dependency of the obtained drag-reduction rate and heat-transfer reduction rate.
Roughness Effect on the Heat Transfer Coefficient for Gaseous Flow in Microchannels
Authors: Turgay Metin B. , Yazicioglu Almila G. , Kakac Sadik
Published: March 2011

Effects of surface roughness, axial conduction, viscous dissipation, and rarefaction on heat transfer in a two–dimensional parallel plate microchannel with constant wall temperature are investigated numerically. Roughness is simulated by adding equilateral triangular obstructions with various heights on one of the plates. Air, with constant thermophysical properties, is chosen as the working fluid, and laminar, single-phase, developing flow in the slip flow regime at steady state is analyzed. Governing equations are solved by finite element method with tangential slip velocity and temperature jump boundary conditions to observe the rarefaction effect in the microchannel. Viscous dissipation effect is analyzed by changing the Brinkman number, and the axial conduction effect is analyzed by neglecting and including the corresponding term in the energy equation separately. Then, the effect of surface roughness on the Nusselt number is observed by comparing with the corresponding smooth channel results. It is found that Nusselt number decreases in the continuum case with the presence of surface roughness, while it increases with increasing roughness height in the slip flow regime, which is also more pronounced at low-rarefied flows (i.e., around Kn = 0.02). Moreover, the presence of axial conduction and viscous dissipation has increasing effects on heat transfer with increasing roughness height. Even in low velocity flows, roughness increases Nusselt number up to 33% when viscous dissipation is considered.