bluuebunny/crossref_metadata_2025 huggingface dataset:
Comparison of Vacuum Chamber Tested Biporous Wicks With Thermal Ground Plane Testing
Authors: Reilly Sean W. , Amouzegar Ladan , Catton Ivan
Published: March 2011

Investigation of bi-porous wicks has yielded an effective method for increasing surface heat transfer when the heat flux is high. It was further found that addition of a mono-porous layer on the heated surface significantly reduced the heated wall surface temperature. These bi-layer wicks were designed for use in 3″×5″ heat spreading devices called Thermal Ground Planes (TGP) in order to transfer heat from a 1 cm2 source. In this work we will investigate the performance of a biporous wick with a monoporous layer in various test set-ups to show the versatility of this heat pipe-substrate. Tests were performed at UCLA and at Advanced Cooling Technologies (ACT) to investigate the wick. Experiments at UCLA were conducted in a vacuum chamber setup to isolate the performance of the wick whereas at ACT the wick lined the evaporator side of a TGP. In order to more closely simulate the operating conditions in a TGP and characterize the vapor spacing parameter, some tests at UCLA were performed with a restrictor plate above the wick similar to the space above the wick in the TGP. The data collected using both these experiments showed similar trends of performance as a function of the spacing above the wick. The motivation of this paper is then to validate that the two testing methods provide similar results while independently addressing different parameters.
Analysis and Simulation of Heat Transfer in a Superhydrophobic Microchannel
Authors: Enright Ryan , Hodes Marc , Salamon Todd R. , Muzychka Yuri
Published: March 2011

The transport behavior of a superhydrophobic Hele-Shaw channel subject to arbitrary velocity slip, temperature slip, and constant heat flux boundary conditions is analyzed, resulting in a general expression for the Nusselt number. The results of a scaling analysis and numerical simulation are then presented characterizing the thermal behavior of an idealized pillar-structured superhydrophobic surface in the low pillar concentration limit that treats the trapped gas phase as adiabatic. When thermal behavior is uncoupled from the flow, the temperature slip length is shown to follow the same φs−1/2 dependency on pillar solid fraction as the velocity slip length. Further analysis and simulation including the effects of Marangoni stress, so that the thermal and flow fields are no longer decoupled, yields a further geometric scaling parameter. It is demonstrated that the apparent slip length may be increased against an adverse channel temperature gradient due to the local non-equilibrium of temperature in the vicinity of each pillar.
A Planar Labyrinth Micromixer
Authors: Cogswell Jeremy T. , Li Peng , Faghri Mohammad
Published: March 2011

Rapid mixing of two fluids in microchannels has posed an important challenge to the development of many integrated lab-on-a-chip systems. In this paper, we present a planar labyrinth micromixer (PLM) to achieve rapid and passive mixing by taking advantage of a synergistic combination of the Dean vortices in curved channels, a series of perturbation to the fluids from the sharp turns, and an expansion and contraction of the flow field via a circular chamber. The PLM is constructed in a single soft lithography step and the labyrinth has a footprint of 7.32 mm × 7.32 mm. Experiments using fluorescein isothiocyanate solutions and deionized water demonstrate that the design achieves fast and uniform mixing within 9.8 s to 32 ms for Reynolds numbers between 2.5 and 30. Compared to the mixing in the prevalent serpentine design, our design results in 38% and 79% improvements on the mixing efficiency at Re = 5 and Re = 30 respectively. An inverse relationship between mixing length and mass transfer Pe´clet number (Pe) is observed, which is superior to the logarithmic dependence of mixing length on Pe in chaotic mixers. Having a simple planar structure, the PLM can be easily integrated into lab-on-a-chip devices where passive mixing is needed.
A Comparison Study of Heat Transfer Through Electron Beam Physical Vapor Deposition (EBPVD) and Air Plasma Sprayed (APS) Coated Gas Turbine Blades
Authors: Akwaboa Stephen , Mensah Patrick F.
Published: March 2011

Thermal barrier coatings (TBCs) are applied to blades, vanes, combustion chamber walls, and exhaust nozzles in gas turbines not only to limit the heat transfer through the coatings but also to protect the metallic parts from the harsh oxidizing and corrosive thermal environment. There is a growing interest in operating these hot gas path (HGP) components at optimal conditions which has resulted in a continuous increase of the turbine inlet temperatures (TITs). This has resulted in the increase of heat load on the turbine components especially in the high pressure side of the turbine necessitating the need to protect the HGP components from the heat of the exhaust gases using novel TBC such as electron beam physical vapor deposition thermal barrier coatings (EBPVD TBCs) and Air Plasma Sprayed thermal barrier coatings (APS TBCs). This study focuses on the estimation of temperature distribution in the turbine metal substrate (IN738) and coating materials (EBPVD TBC and APS TBC) subjected to isothermal conditions (1573 K) around the turbine blade. The heat conduction in the turbine blade and TBC systems necessary for the evaluation of substrate thermal loads are assessed. The steady state 2D heat diffusion in the turbine blade is modeled using ANSYS FLUENT computational fluid dynamics (CFD) commercial package. Heat transfer by radiation is fully accounted for by solving the radiative transport equation (RTE) using the discrete ordinate method. The results show that APS TBCs are better heat flux suppressors than EBPVD TBCs due to differences in the morphology of the porosity present within the TBC layer. Increased temperature drops across the TBC leads to temperature reductions at the TGO/bond coat interface which slows the rate of the thermally induced failure mechanisms such as CTE mismatch strain in the TGO layer, growth rate of TGO, and impurity diffusion within the bond coat.
Heat Sink Optimization: A Multi-Parameter Optimization Problem
Authors: Hansen Nicholas , Catton Ivan , Zhou Feng
Published: March 2011

Earlier efforts to optimize a simple pin fin heat sink only considered in line or rotated square configurations, a six or seven parameter problem. In this work, a staggered non-square array, two pitch dimensions, with non-circular pin cross-section are added to the heat sink description. The effects of optimizing fin eccentricity and pitch have both been shown to enhance the convective heat transfer as well as to increase the required pumping power thereby contributing to the performance. To optimize, the full conjugate problem must be solved. Volume averaging theory (VAT) is used to rigorously cast the point-wise conservation of energy, momentum and mass equations into a form that represents the thermal and hydraulic properties of the channel morphology. With a VAT based model, multiple heat sinks can be analyzed quickly while being able to change all dimensions. Using commercial software, S-Matrix Corporation’s DOE Fusion, a multi-parameter pin fin heat sink is optimized.
Obtaining Closure for Heat Exchanger Modeling Based on Volume Averaging Theory (VAT)
Authors: Zhou Feng , Hansen Nicholas , Catton Ivan
Published: March 2011

Volume Averaging Theory (VAT) has been used to rigorously cast the point-wise conservation of energy, momentum and mass equations into a form that represents the thermal and hydraulic properties of heat exchanger channel morphology. At the lower level, the media is described by a representative elementary volume (REV). Closure terms in the VAT equations are related to a local friction factor and a heat transfer coefficient of the REV. The terms in the closure expressions are complex and are evaluated using scaling suggested by VAT from either experimental data or the output of a CFD code. The VAT equations for a finned tube heat exchanger are given and that the key parameters can be obtained by suitable scaling is demonstrated.
Enhanced Heat Transfer for Various Gases Flowing Over a Twisted Heater Due to Exponentially Increasing Heat Input
Authors: Shibahara Makoto , Liu Qiusheng , Fukuda Katsuya
Published: March 2011

Forced convection transient heat transfer coefficients were measured for various gases (helium, nitrogen, argon and carbon dioxide gas) flowing over a twisted heater due to exponentially increasing heat input (Q0exp(t/τ)). The platinum ribbon with a thickness of 0.1 mm and a width of 4.0 mm was used as the test heater. It was twisted at the center of the heater with an angle of 45 and 90 degrees with respect to the upper part of the heater. The heat generation rate was exponentially increased with a function of Q0exp(t/τ). The gas flow velocities ranged from 1 to 10 m/s, the gas temperatures ranged from 313 to 353 K, and the periods of heat generation rate ranged from 45 ms to 17 s. The surface temperature difference and heat flux increase exponentially as the heat generation rate increases with exponential function. The heat transfer coefficients for twisted heater were compared with those of a plate heater. They are 13 ∼ 28% higher than those of the plate one. The geometric effect (twisted effect) of heater in this study shows an enhancement on the heat transfer coefficient. This is because the heat transfer coefficients are affected by the change in the flow due to swirling flow on the twisted heater. And also, it was understood that heat transfer coefficient increase with the angle of twisted heater due to swirl motion and raised turbulence intensity. Empirical correlations for quasi-steady-state heat transfer and transient one were obtained based on the experimental data.
Resistance Network Analysis of Airflow and Heat Transfer in a Thin Electronic Equipment Enclosure With a Localized Finned Heat Sink
Authors: Fukue Takashi , Ishizuka Masaru , Nakagawa Shinji , Hatakeyama Tomoyuki , Nakayama Wataru
Published: March 2011

In recent years, thermal design of electrical equipment becomes importance and fast thermal design is required due to the fast development of electrical devices. We have proposed the flow and thermal resistance network analysis (coupled network analysis) as a fast thermal design method for electrical equipment. In this paper, we described analytical accuracy of the coupled network analysis of thin electronic equipment including the finned heat sink. We especially focused on the prediction of thermal performance on heatsink by using the coupled network analysis. For considering the accuracy of the coupled network analysis, we compared the results of the coupled network analysis with those of CFD analysis and the experiment. The results showed that the coupled network analysis can predict accurate thermal performance of heat sink and moreover accurate temperature distribution of electrical equipment.
Optimization of a Phase Change Thermal Storage Unit
Authors: Bonadies Monica F. , Ricklick Mark , Kapat J. S.
Published: March 2011

When collecting the energy of the sun for domestic use, several options exist, one being the use of evacuated tube collectors with internal heat pipes. This study proposes a system integrating these collectors with a storage unit using the phase change of paraffin wax to store energy. The storage unit makes use of a finned heat exchanger, with paraffin wax on the shell side and glycol on the tube side as the heat transfer fluid. The heat exchanger is embedded within the storage paraffin wax with a volume of 2 ft3. The heat exchanger also includes a separate loop for water to flow through and receive thermal energy from the melted wax. Although the wax has the benefit of being inexpensive and nontoxic, it has the problem of low thermal conductivity. Therefore, the heat exchanger has large copper fins brazed to it to extend areas of high thermal conductivity into the wax reservoir. The unit used in this study contains 14 fins. The use of fins will help to speed up the melting of the wax while solar energy is collected, since there is more heat transfer area. When most of the wax is melted, heat can be exchanged to water for domestic use. To determine the benefit of the fins, wax and working fluid temperature data will be taken from a constructed thermal energy storage unit, and then it is used to verify a finite-difference analytical model of the thermal operating characteristics. The maximum operating temperature of the glycol/water mix heat transfer fluid was approximately 65° C when the fluid flowed at 1 gallon per minute. The storage unit was able to store melted wax overnight with a 2–3°C temperature drop with the ambient temperature approximately at 30°C. City water at approximately 3 gpm was used to test the freezing side. The one dimensional model proved useful in predicting the heat storage mode of the system but had some error in predicting the heat release mode of the unit. The model also points to the fact that there are several considerations to be taken when simulating phase change energy storage processes.
An Experimental Study of Condensate Retention on the Air Side of Flat-Tube Heat Exchangers
Authors: Gupta Deep , Park Young-Gil , Jacobi Anthony M.
Published: March 2011

Condensate on the air-side surface of flat-tube air-cooling heat exchangers can result in several adverse effects on the overall thermal hydraulic performance. In order to enhance the performance of compact heat exchangers under wet-surface conditions, promoting the drainage of condensed water is critical. Drainage channels on tube walls might provide a robust solution to promote condensate drainage from compact heat exchangers. A quantitative and qualitative evaluation of the effectiveness of drainage channels on the condensate retention characteristics of compact flat-tube louver-fin heat exchangers is presented. Various geometries of drainage channels are engraved on the surrogate tubes in contact with fin stock. Two fin designs are tested using dynamic dip testing method and a small wind tunnel. The data from the quantitative results and the visualization tests clearly show that the drainage channels reduce the condensate retention for one of the fin designs. The results also provide a comparison between drainage channels based on their geometry and dimensions.
Cold-Start Characteristics of Polymer Electrolyte Fuel Cells: A Simplified Analysis
Authors: Wang Yun , Mishler Jeff , Mukherjee Partha P. , Mukundan Rangachary , Borup Rodney L.
Published: March 2011

In this paper, we investigate the electrochemical reaction kinetics, species transport, and solid water dynamics in a polymer electrolyte fuel cell (PEFC) during cold start. A simplified analysis is developed to enable the evaluation of the impact of ice volume fraction on cell performance during cold-start. Supporting neutron imaging data are also provided to reveal the real-time water evolution. Temperature-dependent voltage changes due to the reaction kinetics and ohmic loss are also analyzed based on the ionic conductivity of the membrane at subfreezing temperature. The analysis is valuable for the fundamental study of PEFC cold-start.
Dynamic Characteristics of Polymer Electrolyte Fuel Cell and Hydrogen Tank
Authors: Wang Yun
Published: March 2011

In this paper, we develop 3D dynamic models for polymer electrolyte fuel cells (PEFCs) and hydrogen tanks, respectively. The PEFC model considers the key components of a single PEFC and couples the various mechanisms that govern fuel cell transient including the electrochemical double-layer behavior, species transport, heat transfer, liquid water dynamics, and membrane water uptake. The hydrogen tank model includes a 3D description of the hydrogen discharging kinetics coupled with mass/heat transport in a LaNi5–based hydrogen tank. Efforts are made to discuss the dynamic characteristics of the PEFC and hydrogen tank together with the possible coupling of the two systems. Local electrochemical and hydride reaction rates, transport processes and associated limiting factors are investigated.
Through-Plane Water Distribution in a Polymer Electrolyte Fuel Cell at Various Operating Temperatures
Authors: Wang Yun , Chen Ken S.
Published: March 2011

A multi-dimensional mathematical model is formulated for simulating the transport and electrochemical reaction phenomena in a polymer electrolyte fuel cell (PEFC). The model describes the two-phase flows, electrochemical reaction kinetics, species transport, and heat transfer, as well as their intrinsic couplings within a PEFC. Two-dimensional model predictions are computed for the two typical operating temperatures at 40 and 80 °C. Computed results reveal that liquid water level may be lower at the higher temperature operation due to water vapor phase diffusion. Detailed water and temperature distributions are displayed to explain the water and heat transport and their interaction. The computed water-content profiles are compared with available experimental data obtained by neutron imaging.
Self Heating Distribution in Arrayed Power Devices as a Function of External Package Temperature
Authors: Maize Kerry , Shakouri Ali
Published: March 2011

Thermoreflectance thermal images of a biased high power silicon transistor array reveal nonuniform self heating patterns with strong dependence on external die temperature. Good correlation is shown between the measured self heating distribution and simulations of the power and temperature distribution throughout the arrayed power device at three different external die temperatures: TE = 296, 333, and 373K. Methods that rely on purely electrical measurements to extract device thermal parameters generally assume uniform temperature and bias across the full device. Two-dimension temperature and power distribution maps provide important information about spatial self heating nonuniformity throughout the device, which can lead to more accurate assessment of device thermal parameters.
Experimental Investigation of Single-Phase Convective Heat Transfer of Nanofluids in a Minichannel
Authors: Liu Dong , Yu Leyuan
Published: March 2011

Nanofluids have been proposed as a promising candidate for advanced heat transfer fluids in a variety of important engineering applications. A consensus is now lacking on if and how the dispersed nanoparticles alter the thermal transport in convective flows. An experimental investigation was conducted to study single-phase forced convection of Al2O3-water nanofluid in a circular minichannel. The friction factor and convection heat transfer coefficients were measured for nanofluids of various volume concentrations (up to 5%) and were compared to these of the base fluid. The Reynolds number varied from 600 to 4500, covering the laminar, transition and early fully developed turbulent regions. It was found that the nanofluids exhibit pronounced entrance region behaviors in the laminar region. In the transition and turbulent regions, the onset of transition to turbulence is delayed in nanofluids. Further, both the friction factor and convective heat transfer coefficient are below these of water at the same Re in the transition flow. Once fully developed turbulence is established, the difference in the flow and heat transfer of nanofluids and water will diminish. A scaling analysis showed these behaviors may be attributed to the variation in the relative size of nanoparticle with respect to the turbulent microscales. This work suggests that the particle-fluid interaction has a significant impact on the flow physics of nanofluids, especially in the transition and turbulent regions. Consequently, nanofluids should be used in either the laminar flow or fully developed turbulent flow at sufficiently high Re in order to yield enhanced heat transfer performance.
A Numerical Simulation Study on the Effects of Crucible Rotation and Magnetic Fields in Growth of SiGe by the Travelling Heater Method
Authors: Takagi Youhei , Ishida Atsuko , Okano Yasunori , Dost Sadik
Published: March 2011

A numerical simulation study was carried out to shed light on the effects of applied crucible rotation and static magnetic field during the THM (travelling heater method) growth of bulk SiGe single crystals. The simulation results show that the application of crucible rotation weakens the radial silicon concentration gradient due to the effect of centrifugal force. The effects of applied static magnetic field direction and strength on the concentration field in the melt were also studied. It was found that the simultaneous application of crucible rotation and static magnetic field is best to grow large crystals with uniform composition. An optimum combination of crucible rotation rates and applied magnetic field strengths is determined.
Hybrid Superlattice Coolers for Dynamic Thermal Management of Microprocessor Hotspots
Authors: Sahu Vivek , Joshi Yogendra K. , Fedorov Andrei G.
Published: March 2011

A hybrid cooling scheme for thermal management of hotspots (300–500 W/cm2) in the presence of low background heat flux (100 W/cm2 over 1 cm2) is being investigated. It uses superlattice coolers (SLCs) to remove ultra high power density hotspot and microchannel heat sink for lower background heat flux. In this paper, transient response of the SLC for hotspot removal is studied. The effect of contact resistance, chip thickness, and hotspot size on the performance of the hybrid cooling scheme is also investigated.
A Skewed Two-Dimensional Probability Density Function for Methane-Air Turbulent Diffusion Flames
Authors: Abou-Ellail Mohsen , Amano Ryo S. , Elhaw Samer , Beshay Karam , Kayed Hatem
Published: March 2011

The present paper describes a mathematical model for turbulent methane-air jet diffusion flames. The mathematical model solves density-weighted governing equations for momentum, mass continuity, turbulent kinetic energy and its dissipation rate. The combustion model solves density-weighted transport equations for the mixture fraction “f”, its variance “g” and its skewness “s”. These variables are used to compute one part of the probability density function (PDF) in mixture fraction domain. The second part of the PDF is computed from the numerical solutions of the mixture fraction dissipation rate “χ” and its variance χ˜″2. The resulting two-dimensional PDF is defined in the mixture-fraction-scalar-dissipation-rate 2D space. The flamelet combustion sub-model is used to compute the mean flame temperature, density and species mass fractions. The flamelet model provides instantaneous state relationships for the stretched flamelets up to the extinction limit. The mean flame properties are computed through the integration of the stretched flamelet state relationships over the two-dimensional PDF. The present 2D probability density function model can predict rim-attached flames as well as unstable lifted flames. This is because the flamelet model provides information on the flame instability arising from the stretching effects of highspeed flowing gases. The new two-dimensional probability density function is used to predict the flame properties of a free jet methane-air flame for which experimental data exists.
Temperature Measurement With Radiation Correction for Very High Temperature Gas
Authors: Kim Chan Soo , Hong Sung-Deok , Seo Dong-Un , Kim Yong-Wan
Published: March 2011

When a thermocouple is placed in a high temperature gas-flow stream, the measured temperature could be biased from the true gas temperature due to a large radiation heat transfer from a thermocouple surface to its surroundings. In this study, two thermocouples of unequal diameters of 1/8 inch and 1/16 inch were used to correct the radiation effect. This method is called the reduced radiation error (RRE). The test results show that the radiation on the thermocouple sheath tubes is the source of the error in the gas temperature measurement, and the RRE method is very effective for radiation correction under negligible conditions with the conductive dissipation through the sheath tubes.
Estimation of Location and Size of Defects in a Solid Body via Inverse Heat Conduction Problem
Authors: Fazeli Hamid , Mirzaei Masoud , Forooghi Pourya
Published: March 2011

In the present paper, geometry specification in the inverse heat conduction problems (IHCP) is applied to detect the location and size of defect in a solid body. A crack or cavity is modeled as a supper-elliptic geometry whose parameters are estimated with inverse heat conduction problems. Both inverse analysis and gradient-based optimization method have been applied to an inverse algorithm that iteratively estimates the defect shape parameters. The inverse analysis is based on recording temperatures data on outer surface of solid domain that determines the objective function in the inverse algorithm with estimated and calculated temperatures. The employed gradient-based optimization method is constructed with the adjoint and sensitivity equations that are used to calculate the gradient of the objective function and the search step size, respectively. An unstructured grid is used and the computational domain is discretized with triangular elements. The finite element method (FEM) is employed to discretize the equations in analysis plus sensitivity and adjoint equations. The effects of different Biot numbers and noisy temperature data are investigated on inverse algorithm and the size and location of crack and complex cavity are calculated. Results show that the estimated shape has good agreement with exact shape of defect. The decrease of noisy data has improve the convergence rate and the increase of Biot numbers causes the estimated shape has the best accuracy in all cases.
Fluid Flow and Boiling Heat Transfer in Mini Channels
Authors: Venkatesan M. , Aravinthan M. , Das Sarit K. , Balakrishnan A. R.
Published: March 2011

Two phase flows in mini channels occur in many industrial applications such as electronic cooling, compact heat exchangers, compact refrigeration systems and in micro propulsion devices. Due to its significance, research on two phase flow in mini channels has become attractive. However, in recent times a controversy exists whether flow in minichannel is different from macro flow because there are still substantial disagreements among various experimental results. In the present study an experimental investigation is carried out for fluid flow and boiling heat transfer characteristics of mini channels with tube diameters ranging from 1–3mm. The tubes were made of SS with water as the working fluid. The variation in friction factor and Nusselt number with decrease in tube diameter for single phase flow was systematically studied. The point of Onset of Nucelate Boiling (ONB) was identified based on wall temperature profile. The effect of heat flux and mass flux on two phase pressure drop with three different tube diameters during sub cooled boiling were investigated. The results reveal that there is an unmistakable effect of tube diameter on fluid friction and onset of boiling during sub cooled boiling in tubes of mini channel dimensions.
A Method of Concurrent Thermographic-Photographic Visualization of Flow Boiling in a Minichannel
Authors: Ozer Arif B. , Oncel Ahmet F. , Hollingsworth D. Keith , Witte Larry C.
Published: March 2011

A method is developed to capture the distribution of surface temperature while simultaneously imaging the bubble motions in diabatic flow boiling in a horizontal minichannel. Liquid crystal thermography is used to obtain highly resolved surface temperature measurements on the uniformly heated upper surface of the channel. High-speed images of the flow field are acquired simultaneously and are overlaid with the thermal images. The local surface temperature and heat transfer coefficient can be analyzed with the knowledge of the nucleation site density and location, and bubble motion and size evolution. The horizontal channel is 1.2 mm high × 23 mm wide × 357 mm long, and the working fluids are Novec 649 and R-11. Optical access is through a machined glass plate which forms the bottom of the channel. The top surface is an electrically heated 76 μm-thick Hastelloy foil held in place by a water-cooled aluminum and glass frame. The heat loss resulting from this construction is computed using a conduction model in Fluent. The model is driven by temperature measurements on the foil, glass plate and aluminum frame. This model produces a corrected value for the local surface heat flux and enables the computation of the bulk fluid temperature and heat transfer coefficient along the channel. The streamwise evolution of the heat transfer coefficient for single-phase laminar flow is compared to theoretical values for a uniform-flux boundary condition. Examples of the use of the facility for visualizing subcooled two-phase flows are presented. These examples include measurements of the surface temperature distribution around active nucleation sites and the construction of boiling curves for locations along the test surface. Points on the curve can be associated with specific image sequences so that the role of mechanisms such as nucleation and the sliding of confined bubbles may be discerned.
Experimental Study of a Two-Phase Thermosyphon With Porous Graphite Foam Insert
Authors: Leong K. C. , Jin L. W. , Pranoto I. , Li H. Y. , Chai J. C.
Published: March 2011

This paper presents an experimental study of heat transfer in a pool boiling evaporator with porous insert. Porous graphite foams of different structures were tested with FC-72 and HFE-7000 coolants with the objective of maximizing the heat transfer in a pool boiling configuration. A two-phase thermosyphon facility was developed to investigate the system performance using graphite foams of block and fin structures. The effects of foam configuration, working fluid type and coolant filling volume on heater surface temperature and superheat were analyzed. The results showed that coolant filling volume has negligible effect on the cooling performance. On the other hand, the thermosyphon performance is significantly affected by the coolant properties and the configuration of the porous graphite foam. A comparison of the Bond numbers obtained for FC-72 and HFE-7000 indicates that the bubbles have to overcome higher surface tension forces before departing the foam surface in HFE-7000. Meanwhile, the effect of foam configuration on the boiling heat transfer performance implies that a properly designed geometry of porous graphite foam will lead to significant enhancement of the evaporation process in a thermosyphon system.
Effects of Thermal Boundary Conditions on Entropy Generation During Natural Convection in a Differentially Heated Square Cavity
Authors: Kaluri Ram Satish , Basak Tanmay , Balakrishnan A. R.
Published: March 2011

Natural convection is a widely occurring phenomena which has important applications in material processing, energy storage devices, electronic cooling, building ventilation etc. The concept of ‘entropy generation minimization’, which is a thermodynamic approach for optimization, may be very useful in designing efficient thermal systems. In the current study, entropy generation in steady laminar natural convection flow in a square cavity is studied with following isothermal boundary conditions: (1) Bottom wall is uniformly heated (2) Bottom wall is sinusoidally heated. The side walls are maintained cold and the top wall is maintained adiabatic. The thermal boundary condition in non-uniform heating case (case 2) is such that the dimensionless average temperature of the bottom wall is equal to that of uniform heating case (case 1). The prime objective of this work is to investigate the influence of uniform and non-uniform heating on entropy generation. The governing mass, momentum and energy equations are solved using Galerkin finite element method. Streamlines, isotherms, contour maps of entropy generation due to heat transfer and fluid friction are studied for Pr = 0.01 (molten metals) and 7 (water) in range of Ra = 103–105. Detailed analysis on the effect of uniform and non-uniform thermal boundary conditions on entropy generation due to heat transfer and fluid friction has been presented. Also, the average Bejan’s number which indicates the relative dominance of entropy generation due to heat transfer or fluid friction and the total entropy generation are studied for each case.
Heat and Mass Transport in Carbon Nantubes
Authors: Shiomi Junichiro , Carlborg Carl Fredrik , Maruyama Shigeo
Published: March 2011

We have investigated heat and mass transport in single-walled carbon nanotubes (SWNTs) using molecular dynamics methods. Particular attention was paid on the non-equilibrium dynamics at the interface between SWNT and other materials, which strongly manifests in nanoscale. In the first part, we have investigated the heat transport through the interface between SWNTs and surrounding argon matrices in liquid and solid phases. By analyzing the energy relaxation from SWNT to the matrices using non-stationary molecular dynamics simulations, elastic and inelastic thermal energy transports across the interface were separately quantified. The result reveals that the elastic interaction transports energy much faster than the inelastic one, but carries much smaller energy due to slow intra-SWNT phonon relaxation. In the second part, we have investigated a possibility to utilize nonequilibrium thermal interface to transport water through an SWNT. By applying the longitudinal temperature gradient to the SWNT, it is demonstrated that the water cluster is efficiently driven at average acceleration proportional to the temperature gradient. However, the transport simulations with a junction of two different SWNTs suggest that an angstrom diameter difference may result in a significant drag for small diameter SWNTs.
Vertical Tube Passive Condenser Under Secondary Pool Water Level Transient
Authors: Revankar Shripad T. , Zhou Wenzhong
Published: March 2011

An experimental work was carried out on a passive containment cooling system (PCCS) test facility where the effect of PCCS pool water level change on the PCCS heat transfer characteristics was investigated. The specific design of condensing tube was based on scaling analysis from the PCCS design of Economic Simplified Boiling Water Reactor (ESBWR). The annulus between the primary condensing tube and the secondary boiling tube is filled with water and serves as water pool. During the test, steam generated in the pool is discharged through three steam exit nozzles located symmetrically at the top of the secondary boiling tube. Transient tests carried out with secondary pool water level change show that the system pressure for complete condensation mode increases with decrease in water level, however rate of condensation is almost constant. However, if the PCCS is operated in through flow mode the system pressure (primary side pressure) is constant, however, the condensate rate decreases indicating that some of the steam does not condense.
Joule Heating Induced Temperature Gradient Focusing for Microfluidic Concentration of Samples
Authors: Ge Zhengwei , Yang Chun
Published: March 2011

Microfluidic concentration of sample species is achieved using the temperature gradient focusing (TGF) in a microchannel with a step change in the cross-section under a pure direct current (DC) field or a combined alternating current (AC) and DC electric field. Experiments were carried out to study the effects of applied voltage, buffer concentration and channel size on sample concentration in the TGF processes. These effects were analyzed and summarized using a dimensionless Joule number that is introduced in this study. In addition, Joule number effect in the Poly-dimethylsiloxane (PDMS)/PDMS microdevice was compared with the PDMS/Glass microdevice. A more than 450-fold concentration enhancement was obtained within 75 seconds in the PDMS/PDMS microdevice. Results also showed that the high frequency AC electric field which contributes to produce the temperature gradient and reduces the required DC voltage for the sample concentration. The lower DC voltage has generated slower electroosmotic flow (EOF), which reduces the backpressure effect associated with the finite reservoir size. Finally, a more than 2500-fold concentration enhancement was obtained within 14 minutes in the PDMS/PDMS microdevice, which was a great achievement in this TGF technique using inherent Joule heating effects.
Assessment of a Steam Generator Circulation Ratio by a Thermal Balance Based on External Wall Temperature Measurements
Authors: Deotto Geoffrey , Brunin Olivier , Nicoli Alexandre , David Franck
Published: March 2011

During operation, sludge steadily appears at a slow pace on the secondary side of nuclear power plant steam generators. This leads to clogging of the tube bundle support plates, and consequently to a change in the thermal-hydraulic flow conditions. The circulation ratio of a steam generator is defined as the ratio between the total flowrate circulating in the riser and the steam flowrate at the outlet of the steam generator. This is a good indicator of the hydraulic pressure losses in the circulation loop. In particular, the increase in hydraulic resistance due to the tube support plate clogging leads to a drop in this parameter. For this reason, in order to check that clogging does not reach too high a level, the circulation ratio is regularly evaluated on steam generators of French nuclear power plants, and then compared to established safety limits. The purpose of this paper is to present an accurate method to determine the circulation ratio of a steam generator based on temperature measurements taken around the wall of the steam generator. This method consists of carrying out a thermal balance of the flow circulating in the downcomer. In order to accomplish this, the temperature of the water circulating in the downcomer is evaluated using thermocouple belts put on the external wall of the appliance. However, additional hypotheses in the calculation method are considered in order to take into account for the heat transfer between hot water inside the downcomer and the sensors. The steam generator circulation loop and the clogging of the tube support plates are presented in §1. Then §2 and §3 describe in detail the method and the associated hypotheses as well as the required instrumentation. Finally, §4 presents an application of this method to real cases of clogged steam generators.
Assessment of Components of the Subcooled Boiling Model for CFD Simulations
Authors: Prabhudharwadkar Deoras , Lopez de Bertodano Martin A. , Buchanan John , Vaidheeswaran Avinash
Published: March 2011

This paper describes the details of validation of heat and mass transfer models used for subcooled boiling simulation with a CFD two-fluid model. This research was focused on assessment of the wall heat flux partitioning model using the state-of-the-art multidimensional experimental data available in the literature. Various constitutive relations used to close the vapor generation rate at the heated wall were studied and the best suited combination of these was obtained. The current study was restricted to vertical flows through pipe and annulus geometries. Three data sets from the literature were considered: first with R12 at about 26 bar pressure, second with water at atmospheric pressure and third with R113 at 2.69 bar pressure. In these data sets, the bubble diameter distribution across the ducts was measured. Bubble diameter estimation brings in the largest uncertainty in the two-fluid model predictions and hence using the data with known bubble sizes allowed to focus on assessment of other parameters which use constitutive relations to model vapor generation rate, e.g. bubble nucleation site density and bubble departure frequency at the wall. The simulations were carried out using the CFD code CFX-12. The R12 data used here corresponds to fluid-vapor density ratio which is equivalent to that of water-steam at 150 bar. Therefore the density ratio varies over two orders of magnitude. The surface tension also varies over a wide range from 0.0017 to 0.057 N/m. The ratio of the flow channel hydraulic diameter to the bubble diameter in these simulations varied between 4 and 40. The two-fluid model was modified, for cases involving bubble sizes too large to be represented using the continuum assumption, in order to obtain satisfactory results. Hence, the model that has been developed for this study is applicable for a wide range of physical conditions and bubble sizes.
Study on Flow Boiling Heat Transfer of Carbon Dioxide With PAG-Type Lubricating Oil in Pre-Dryout Region Inside Horizontal Tube
Authors: Dang Chaobin , Li Minxia , Hihara Eiji
Published: March 2011

In this study, the boiling heat transfer coefficients of carbon dioxide with a PAG-type lubricating oil entrained from 0 to 5 wt% in a horizontally placed smooth tube with an inner diameter of 2 mm were experimentally investigated under the following operating conditions: mass fluxes from 170 to 320 kg/m2s, heat fluxes from 4.5 to 36 kW/m2, and a saturation temperature of 15 °C. The results show that for a low oil concentration of approximately 0.5% to 1%, no further deterioration of the heat transfer coefficient was observed at higher oil concentrations in spite of a significant decrement of the heat transfer coefficient compared to that under an oil-free condition. The heat flux still had a positive influence on the heat transfer coefficient in low quality regions. However, no obvious influence was observed in high quality regions, which implies that nucleate boiling dominates in the low quality region whereas it is suppressed in the high quality regions. Unlike the mass flux under an oil-free condition, mass flux has a significant influence on the heat transfer coefficient, with a maximum increase of 50% in the heat transfer. On the basis of our experimental measurements of the flow boiling heat transfer of carbon dioxide under wide experimental conditions, a flow boiling heat transfer model for horizontal tubes has been proposed for a mixture of CO2 and polyalkylene glycol (PAG oil) in the pre-dryout region, with consideration of the thermodynamic properties of the mixture. The surface tension and viscosity of the mixture were particularly taken into account. New factors were introduced into the correlation to reflect the suppressive effects of the mass flux and the oil on both the nucleate boiling. It is shown that the calculated results can depict the influence of the mass flux and the heat flux on both nucleate boiling and convection boiling.
Experimental Study on Temperature Fluctuation Phenomena in a Closed Branch Pipe Connecting to High Velocity and High Temperature Flow in a Main Pipe
Authors: Takenaka N. , Hosokawa S. , Saito A. , Oumaya T. , Nakamura A. , Miyoshi K.
Published: March 2011

Flow structures and temperature fluctuations in a closed branch pipe connected to a high velocity and high temperature flow in a main pipe were investigated experimentally. A straight pipe and a pipe with a bend were tested for uniform and non-uniform temperature conditions. Visualization of the flow patterns, measurement of the flow velocity and measurement of the liquid and wall temperature distributions were conducted. The flow pattern near the stagnant point in the branch pipe was a spiral flow. The time averaged velocity distributions were linear and the structure of the spiral flow was a simple forced vortex with a vertical circulation. The spiral flow velocity was fluctuated with a long period of several tens seconds. It was shown that large liquid temperature fluctuations were initiated when the spiral flow penetrated into a thermal stratified layer in the bend. The liquid temperature fluctuations caused the wall temperature fluctuations and two types of the wall temperature fluctuations were observed.
Numerical and Experimental Studies of Turbulent Heat Transfer in Inclined Pin-Fin Channels With a Wavy Endwall
Authors: Oda Yutaka , Takeishi Kenichiro , Miyake Yoshiaki , Moriai Hideki , Motoda Yusuke
Published: March 2011

Endwall heat transfer characteristics of inclined pin-fin channels have been investigated to improve the cooling efficiency of gas turbine’s combustor liners. The time-mean local Nusselt number profiles were obtained in high spatial resolution by naphthalene sublimation technique based on the heat/mass transfer analogy and large eddy simulations. Inclined pin-fin channels with a flat or wavy endwall were tested and compared with each other. As a result, the time-averaged heat transfer coefficient of the wavy endwall was found to be higher than that of the flat endwall due to flow separation and reattachment from the hill of wavy endwall. Surprisingly, the wavy endwall case shows less or at least the same pressure loss compared to the flat endwall case. Therefore, it can be concluded that wavy endwalls can realize better heat transfer with less pressure loss as long as the aim consists in enhancing the endwall heat transfer in inclined pin-fin channels.
Investigation of the Frost Growth on a Parallel Flow Evaporator
Authors: Wu Jianghong , Ouyang Guang
Published: March 2011

The frost of the evaporator can greatly affect the performance of the air source heat pump. When humid air comes in contact with a cold surface maintained below the freezing temperature, a frost layer begins to grow. This paper experimentally studied the frosting process of a folded-louvered-fin, parallel flow evaporator in a heat pump central air-conditioning system under three conditions, in which three different diameter capillaries were used. The surface temperature distribution of the evaporator was measured by sixteen thermocouples which buried on the leeward side and mesoscale frost formation processes on its partial surface for three different test conditions were observed using a CCD camera. The experimental result showed that the surface temperature distribution of the parallel flow evaporator was non-uniform and frost formation generally began on some partial surface of the louvered fins where the surface temperature was lowest after the heat pump system had starting up for six minutes under condition B and C, while the fins began to frost after eight minutes under condition A. The non-uniform surface temperature distribution which was caused by the unequal distribution of the refrigerant flux in the microchannels of flat tubes led to uneven distribution of frost, furthermore, incomplete evaporation of refrigerant in some tubes may be a critical factor of folded-louvered-fins’ easily frost under relative high environmental temperature condition. The ice crystals distributions and frost thickness in frost period can be obtained by the digital image processing method in which the initial pictures can be converted into binary image. The results showed that in a thin layer near to fins’ surface, ice crystals were comparable high dense in the frost growing and full grown period of frost layer; the ice crystals rate was reduced in approximate linear curves with the increasing of the frost thickness (frost height), where the decreasing rate in the frost full grown period was slower than that in the frost growing period (In 0.1 mm frost thickness, the ice crystals rate had decreased to 58% in the frost growing period, while in the frost full grown period the ice crystals rate was 90%; and in 0.25 mm frost thickness, the ice crystals rate were 0%, 45% respectively.). It also can be found that the frost thickness increased as the time increasing and then finally reached 0.3mm, 0.35 mm, and 0.32mm respectively at three conditions. The frost thickness had reached 0.1mm, 0.05mm under condition B and C respectively in 6 minutes, however, concerning condition A, the frost began to grow after 8 minutes and frost thickness arrived only 0.05mm in 14 minutes.