bluuebunny/crossref_metadata_2025 huggingface dataset:
Time Averaged and Fluctuating Heat Transfer Measurements in an Atomizing Mist Impinging Jet Nozzle
Authors: Lyons Oisn Fergal Peter , Persoons Tim , Murray Darina B.
Published: March 2011

Much is already known about the heat transfer characteristics of impinging air jets, and they are widely used in many engineering applications. There currently exist many correlations describing such characteristics. However, the complex internal structure of many nozzles can lead these to produce results which deviate from those predicted by correlations. One such nozzle is currently used in this research group to produce a water mist flow and this paper describes the experimental characteristics of its two phase behaviour.
CFD Model Validation and Prediction of Mist/Steam Cooling in a 180-Degree Bend Tubes
Authors: Dhanasekaran T. S. , Wang Ting
Published: March 2011

To achieve higher efficiency target of the advanced turbine systems, the closed-loop steam cooling scheme is employed to cool the airfoil. It is proven from the experimental results at laboratory working conditions that injecting mist into steam can significantly augment the heat transfer in the turbine blades with several fundamental studies. The mist cooling technique has to be tested at gas turbine working conditions before implementation. Realizing the fact that conducting experiment at gas turbine working condition would be expensive and time consuming, the computational simulation is performed to get a preliminary evaluation on the potential success of mist cooling at gas turbine working conditions. The present investigation aims at validating a CFD model against experimental results in a 180-degree tube bend and applying the model to predict the mist/steam cooling performance at gas turbine working conditions. The results show that the CFD model can predict the wall temperature within 8% of experimental steam-only flow and 16% of mist/steam flow condition. Five turbulence models have been employed and their results are compared. Inclusion of radiation into CFD model causes noticeable increase in accuracy of prediction. The reflect Discrete Phase Model (DPM) wall boundary condition predicts better than the wall-film boundary condition. The CFD simulation identifies that mist impingement over outer wall is the cause for maximum mist cooling enhancement at 45° of bend portion. The computed results also reveals the phenomenon of mist secondary flow interaction at bend portion, adding the mist cooling enhancement at the inner wall. The validated CFD simulation predicts that average of 100% mist cooling enhancement can be achieved by injecting 5% mist at elevated GT working condition.
Supersonic Two Phase Impinging Jet Heat Transfer
Authors: Parker Richard R. , Klausner James F. , Mei Renwei
Published: March 2011

The experimental heat transfer rates from a supersonic two-phase impinging air jet with disperse droplets is presented. The experimental configuration consists of an expanding disperse mixture of air and water through a converging-diverging nozzle, designed for Mach 3.26 with a liquid to air mass flow ratio ranging from 1.28 to 3.83%, impinging upon a thin film heater constructed of nichrome. The spatially varying heat transfer coefficient was measured, and peak values are on the order of 200,000 W/m2 − K. Two distinct regions of heat transfer are identified, one dominated by the jet impingement flow and another dominated by thin film heat transfer. The heat transfer coefficient of an impinging jet with dry air and no droplets was measured during the investigation as well. The heat transfer results are compared, and it is demonstrated that the addition of disperse water droplets to the jet significantly increases the heat removal capability of the jet as well as smoothing the spatial temperature distribution of the heater surface. As much as a ten fold increase in heat transfer rate is observed. These results demonstrate the usefulness of supersonic two-phase jets for high heat flux thermal management applications.
Pool Nucleate Boiling of Acetone and Acetone/Ethanol Mixture on Magnesium Alloy Surface
Authors: Yang X. , Yan Y. Y.
Published: March 2011

In this paper, the pool nucleate boiling performance of acetone and acetone/ethanol mixture on a magnesium alloy surface is studied experimentally. The pure acetone and the mixtures with two different mixing ratios are tested. A uniform heat flux is provided through the magnesium alloy tube to heat the mixtures around the magnesium tube surface. The general aim of this classic heat transfer experiment is to investigate the heat transfer performance of the binary fluids on the magnesium alloy surface, and the compatibility of the binary fluids with magnesium alloy. The results show that pure acetone presents better heat transfer performance than its mixtures. Compared two mixtures, the difference of 10% concentration of acetone (90% or 80%) do not result in a huge change of heat transfer coefficient. The scanning electronic microscope (SEM) observation suggested that either the pure acetone or acetone/ethanol mixtures could be used as working fluids of magnesium (Mg) alloy heat pipes.
Start Up and Transient Response to Power Input of a Capillary Assisted Thermosyphon for Shipboard Electronics Cooling
Authors: Tudor V. , Cerza M.
Published: March 2011

The future capabilities of naval ships will be directly related to the electronic components used in advanced radar systems, fire control systems, electric propulsion and even electric weapons. The next generation of naval warships will fall under the concept of an all electric ship, where turbines convert all the power produced by the engine into electricity. This electrical power can then be distributed given the ship’s mission and operating profile. The current need for advanced electronics cooling techniques is paramount since power dissipation levels are rapidly exceeding the capabilities of forced air convection cooling. This paper reports an experimental investigation of the start-up and transient response to heat load change of a capillary assisted thermosyphon (CAT) for the shipboard cooling of electronics components. The capillary assisted thermosyphon differs from a capillary pumped loop or loop heat pipe system in that the basic cooling-loop is based on a thermosyphon. The capillary assist comes from the fact that there is a wicking structure in the flat evaporator plate. The wicking structure allows uniformly spread of the working fluid across the flat plate evaporator in the areas under the heat sources as well as providing additional capillary pumping assist to the loop. A vertical flat plate, CAT evaporator was designed and tested under a fixed thermal sink temperature of 21°C. The condenser cold plate cooling water flow rate was fixed as 3.785 liters per minute (i.e. 1 gpm). The heat input varied from 250 to 1000W — evenly spread over the area of the evaporator. The CAT flat plate evaporator performed very well under this range of heat inputs, sink temperature, and cold plate flow rate. The main result obtained showed that the CAT loop reached steady state operation within 10 min. to 15 min. The average plate temperature did not exceed 70°C for the maximum heat input of 1000W. The CAT evaporator operating temperature increased with increasing heat input for all conditions tested and reached 60°C at 1000W. The continuous and stable operation of the CAT loop during start-up, steady-state and during transient/sudden heat input variations indicates that the CAT loop is a viable solution for high flux electronics components cooling.
Droplets on Micro Structured or Patterned Surfaces With Different Roughness Parameters
Authors: Yan Y. Y. , Zu Y. Q. , Tian C. Q. , Gao N.
Published: March 2011

Surface wetting properties play important roles in boiling and condensation. In this paper, the wetting behaviours of a single droplet on micro structured or patterned surfaces with different roughness parameters are investigated theoretically and numerically. A theoretical model is proposed to study wetting transitions. Comparison between results obtained by theoretical analysis with those of experiment indicates that the proposed model can give a better prediction of wetting transition. In addition, a numerical simulation based on lattice Boltzmann method (LBM) is performed to study surface wetting properties and also the evolution of droplet shapes, dynamic contact angle and corresponding velocity fields. In the simulation, the droplet size is comparable with the scale of micro posts on the surface.
About Stability of a Vapour Jet Condensing in a Micro Channel
Authors: Piaud Benjamin , Lavieille Pascal , Miscevic Marc
Published: March 2011

An unsteady model of condensation flow in capillary regime inside a cylindrical tube was developed, based on a two-fluid approach. The model takes into account the coupling between the liquid film zone (where the quality is low) and the nearly hemispherical meniscus which is present at the end of the condensation region. Numerically, a major difficulty is that this type of problem has a free boundary condition. Indeed the end location of the two-phase zone is the result of all the heat exchanges occurring upstream of this area. To overcome this difficulty a mathematical representation was specifically developed. The unsteady model presented is based on five dimensionless numbers characterizing this type of flow. A comparison between the results of this model and those of a previously developed stationary model is made. An excellent agreement is obtained. The presence of self-sustaining oscillations due to the intrinsic mechanisms of condensation are also obtained numerically. The variation of the Nusselt number with the boiling number is then determined and presented. These results complement the previous study by determining the frontier between the stable and unstable situations. The atypical behaviour of the Nusselt number preceding the onset of instability is also analyzed.
Theoretical Investigation on Effective Thermal Conductivity of a Disk-Shaped Composite System
Authors: Koito Yasushi , Tomimura Toshio , Torii Shuichi
Published: March 2011

This paper addresses the methodology to estimate the effective thermal conductivity of the wiring board, where the metal wiring network is very complicated and then the thermal conductivity of the metal wiring is more than 1000 times higher than that of the resign board. Based on the concept of analogy between the electric and the thermal resistance network, two types of estimation equations are derived by dividing the composite system parallel or perpendicular to the heated/cooled surface. When the ratio of higher to lower thermal conductivities is less than 10, the estimated values by these equations agree with each other. However, the difference is clearly found between them when the ratio is larger than 100. The estimated values are moreover compared with the exact solutions, which are obtained by numerical simulation of heat transfer using Microsoft Excel, and then the applicability of the present estimation methodology is discussed. It is found that the thermal resistance network obtained by dividing the composite system perpendicular to the heated/cooled surface is effective to estimate the effective thermal conductivity of the composite system.
Opposing Mixed Convection Heat Transfer in the Inclined Flat Channel in a Laminar-Turbulent Transition Region
Authors: Poskas Povilas , Poskas Robertas , Drumstas Giedrius
Published: March 2011

In this paper we present the results on experimental and numerical investigations of the local opposing mixed convection heat transfer in an inclined flat channel (φ = 60° from horizontal position) with symmetrical heating in the laminar-turbulent transition region. The experiments were performed in airflow (p = 0.2 and 0.4 MPa) in the range of Re from 1.5 · 103 to 5.3 · 104 and Grq up to 1.5 · 1010 and at the limiting condition qw1 ≈ qw2 ≈ const. The experimental data show similar tendencies in the heat transfer as it was revealed in vertical channel. It is already some difference in the local heat transfer for upper (stable density stratification) and bottom (unstable density stratification) walls. The data of the experimental investigations have been compared with the results of numerical modelling using Ansys Fluent 12.0 code. The modelling was performed using laminar and transition models for Rein = 3.1 · 103 and Grqin = 1.9 · 109 for the same conditions as during the experiment, i.e. with the same airflow pressure (0.4 MPa), velocity, and temperature at the inlet and heat flux at the walls of the flat channel with the same geometrical characteristics.
Experimental Investigation of Influence of Dissolved Salts and Surfactant on Heat Transfer in Atomized Spray Quenching of Metal
Authors: Alam Umair , Abd alrahman Khalid , Specht Eckehard
Published: March 2011

Spray quenching is widely used in industrial applications. In atomized spray quenching (ASQ), water and air are supplied to the nozzle at a certain flow rate and pressure to produce a full cone spray consisting of discrete droplets. Impingement density of spray i.e. coolant mass flow per unit area per second is considered to be the most influential parameter for heat transfer. Impingement density varies with radius and so as the heat flux. Water quality is altered by adding five different salts i.e. NaCl, Na2SO4, NaHCO3, Na2CO3, and MgSO4 in de-ionized water with various concentrations. On the other hand, a surfactant Ethoxylated ester, which is commonly added in cooling water in cast houses of metals, is added to pure water in different concentrations i.e 50, 100, 200 and 500ppm. A circular disc made of Nickel of thickness 2mm is heated to 600°C and sprayed on one side by atomized spray and the temperature distribution with respect to time is measured using Infrared camera on the other side of the disc. By this IR thermography, transient temperature measurement can be done within the window of 320×80 pixels with a minimum pixel real distance of 1mm on the sheet surface. Frequency of measurement is 150Hz. Since the temperature measurement and cooling sides are opposite at 2mm thickness apart, inverse heat conduction problem is solved by applying finite element method for calculating temperature and heat flux on the quenched side of metal sheet with respect to space and time. It has been observed that increasing the concentration of salts increase the leidenfrost point and shortens the film boiling regime. While addition of surfactants decrease the leidenfrost point and prolong the film boiling regime. Maximum heat flux position is considered as the wetting front position. There is an abrupt variation of heat flux at wetting front position due to the change of boiling phenomenon. Wetting front velocity has been compared for salt solutions, surfactant and de-ionized or pure water.
Modeling of Liquid Sprays and Thermal Mixing in Desuperheating Applications
Authors: Gao Jian , Trujillo Mario F.
Published: March 2011

Computational modeling is performed for liquid sprays and thermal mixing in desuperheating applications, in which water sprays are injected into a high-temperature steam crossflow (H2Ol-H2Og system) to reduce the enthalpy. The basis of the model is the standard RANS approach coupled to the Lagrangian treatment for statistical groups (parcels) representing the physical droplet population. A vaporization/boiling model is developed for the treatment of phase change under a single chemical species environment. Additionally, the best performing hybrid breakup models depicting the hydrodynamic breakup process governed by the Kelvin-Helmholtz and Rayleigh-Taylor instabilities, are used. The calculations are run for both air-H2Ol and H2Ol-H2Og systems. Under crossflow conditions, the calculations show that liquid sprays undergo sudden atomization near the injector tip, and have a much smaller characteristic droplet size compared to sprays injected into quiescent environments. This is attributed to the larger gas-droplet relative velocities drastically enhancing the Weber number. Additionally, a blockage effect of the spray on the crossflow is observed in the near-nozzle field, and the typical entrainment of small droplets into the wake is captured in the predictions. The region of maximum cooling coincides with the generation of the highest values of interfacial area produced by the sudden atomization in the near field.
Heat Transfer Mechanisms of Propane Boiling on Horizontal Steel Tubes With Smooth and Enhanced Surfaces
Authors: Luke A. , Mu¨ller Bjo¨rn C. F.
Published: March 2011

The trend towards a better understanding of the fundamentals of nucleate boiling in re-entrant cavities is supported by the variation of the heating surface’s characteristics and the identification of parameters influencing the heat transfer at enhanced tubes. The optimized surface of enhanced evaporator tubes supports the bubble formation by providing stable nucleation sites, which are cavities that trapped the necessary amount of vapor to generate the next bubble. The optimal size of the cavities for bubble formation depends on various thermodynamic properties of the fluid and the wall material. The knowledge of these physical mechanisms is important for the further optimization. The influence of micro- and macrostructures on the overall heat transfer coefficient is investigated with the refrigerant R134a and the hydrocarbon propane (R290) boiling in a wide range of reduced pressures (p* = ps/pc = 0.03 to 0.5) and heat fluxes (0.05 to 100 kW/m2). The measurements are carried out using a standard apparatus and a horizontally positioned, electrically heated surface with various wall materials. Two different materials — copper and mild steel — with the same surface preparation by polishing are investigated. Furthermore, heat transfer measurements are carried out on a plain mild steel tube and on an industrially manufactured surface of the GEWA-PB type. The polished surfaces demonstrate a deterministic microstructure, the roughness parameters depends strongly on the measurement direction. The heat transfer coefficient as function of the heat flux of the polished copper tube can be described by the correlation of the VDI Heat Atlas, while the mild steel surface differ from former investigations due to the deep re-entrant cavities remaining from the drawn surface. The onset of boiling is nearly the same of both materials because of these cavities on the mild steel surface. As presented in the recent years, the heat transfer of nucleate boiling at tubes with subsurface channels can be divided into different domains, each influenced by different parameters like wettability, the product of vapor density and evaporation enthalpy. The identification of parameters influencing the bubble formation is done by heat transfer measurements, high-speed-video recording and photographic documentation. The experimental results of this work are compared to results of the polished surfaces. The heat transfer coefficient increases drastically for the enhanced tube, especially for beginning nucleation. The same α-q-relationship as on plain tubes is observed for higher pressures and heat fluxes but for three times higher values of the heat transfer coefficient α.
Effective Thermal Conductivity of Six Sigma’s Copper Reinforced Column Grid Array Interconnect for Ceramic Microelectronic Packages
Authors: Eblen Mark
Published: March 2011

Thermal management of flip chip style integrated circuits often relies on thermal conduction through the ceramic package and high lead solder grid array leads into the printed wiring board as the primary path for heat removal. Thermal analysis of this package configuration requires accurate characterization of the sometimes geometrically complex package-to-board interface. Given the unique structure of the Six Sigma column grid array (CGA) interconnect, a detailed finite element submodel was used to numerically derive the effective thermal conductivity with comparisons to a conventional CGA interconnect. Once an effective thermal conductivity value is obtained, the entire interconnect layer can be represented as a fictitious cuboid layer for inclusion in a more traditional “closed-form” thermal resistance calculation. This method allows the package designer a quick and robust method to evaluate initial thermal design study tradeoffs.
Time-Resolved Heat Transfer and Fluid Dynamic Analysis of Natural Convection From Isothermal Horizontal Cylinders
Authors: Persoons Tim , Gorman Ian M. O. , Byrne Gerry , Murray Darina B.
Published: March 2011

This paper discusses the close coupling between fluid dynamics and local natural convection heat transfer rates from a pair of isothermally heated horizontal cylinders submerged in water. The presence of a second heated cylinder induces heat transfer enhancements of up to 10%, and strong fluctuations in local heat transfer rate. Therefore specific attention is focused on how the local heat transfer characteristics of the upper cylinder are affected by buoyancy induced fluid flow from the lower cylinder. The paper investigates a range of Rayleigh number between 2·106 and 6·106, and a vertical cylinder spacing between 2D and 4D. Simultaneous local heat flux measurements and flow velocity measurements using particle image velocimetry reveal oscillatory behaviour of the thermal plume, depending on operating conditions. A joint temporal analysis of the data has provided new insights into the governing mechanisms, which enables further optimisation of the heat transfer performance.
Heat Transfer From Liquid Nitrogen Flows in Smooth Pipes
Authors: De Giorgi Luigi , Bertola Volfango , Cafaro Emilio , Cima Carlo , De Salve Mario , Panella Bruno
Published: March 2011

Convective heat transfer for subcooled liquid nitrogen in a smooth horizontal pipe with internal sources is studied by analytical and numerical methods. For high Reynolds numbers the numerical results are in good agreement with standard heat transfer correlations. At smaller Reynolds numbers (<10,000), large circumferential and longitudinal temperature distributions can observed. The effect of localized heat sources on the heat transfer process is also investigated to simulate insulation failures in cryogenic pipelines. Results show that the presence of constant heat sources is detrimental to the heat transfer from both laminar and turbulent flows.
Simulation of Fluid Flow and Heat Transfer of Flat Plate Heat Sinks With Spiral Inserts
Authors: Subbuswamy Ganesh , Li Xianchang
Published: March 2011

High performance electronic chips are finding applications in almost all equipments in the modern world. However, these chips are coupled with potential overheating, which has been a serious concern for manufacturers as well as researchers ever. One of the best options for electronics cooling is to take advantage of heat sinks. Over years, many studies are focused on optimal designs of heat sinks, while some are also targeted at heat transfer enhancement. To explore how the heat sink performance can be further improved, this study integrates spiral tape inserts into the narrow channel of plate-finned heat sinks. Numerical simulation is carried out to examine the flow and heat transfer behavior of heat sinks with spiral inserts. Different parameters such as insert width, twist ratio, and the flow Reynolds number are investigated. It is observed that the inserts significantly increase the heat transfer rate with a penalty of higher pressure loss. In one of the cases studied, the heat transfer is increased by 368% with a rise of 810% in pressure drop. However, the inserts can produce a net benefit when the same pressure loss or fan power is considered. Therefore, the inserts can help make the heat sink more effective by reducing its size.
High-Flux Thin Film Evaporation on Nanostructured Surfaces
Authors: Xiao Rong , Chu Kuang-Han , Wang Evelyn N.
Published: March 2011

The heat generation rates of high performance electronics motivate the development of new thermal management solutions. Thin film evaporation with a jet impingement or spray system promise high heat fluxes up to 1000 W/cm2 with low thermal resistances. However, challenges with implementation currently limit the ability to reach the theoretical limits. In this work, we investigated the utilization of micro-/nanostructured surfaces to control the liquid film thickness and provide a sufficient liquid flow rate to achieve high heat removal rates. We developed a model to predict the propagation rates of the liquid film, which accounted for the curvature of the liquid meniscus. We also fabricated test devices with pillar diameters ranging from 500 nm to 10 μm, spacings of 3.5 μm to 10 μm, and heights of 5 μm to 15 μm, and validated the model with confocal microscopy and high speed imaging. Heaters and temperature sensors were also integrated onto the back side of the chip to investigate heat transfer performance. When heat was applied, the structures significantly enhanced the heat dissipation rates and reduced the thermal resistance. The heat dissipation rate was also found to be positively correlated to the propagation rate of the liquid film. However, surface fouling in the experiments led to challenges to maintain a stable liquid film, and decreased the heat removal capability. This work provides insights to designing micro-/nanostructured surfaces for thin film evaporation to meet the heat dissipation demands of future high performance electronic systems.
Thermal Management Considerations for Geographically Distributed Computing Infrastructures
Authors: Shah Amip , Bash Cullen , Arlitt Martin , Chen Yuan , Gmach Daniel , Sharma Ratnesh , Patel Chandrakant
Published: March 2011

This paper discusses an approach for optimizing the infrastructure thermal performance related to a geographically distributed computing service. Beginning by modeling the total energy costs associated with cooling a distributed environment, the cooling efficiency of a service is evaluated by superposing the piecewise IT workloads that may be delivered from various locations. We find that the total service-level thermal performance can be distinct from the facility- or infrastructure-level thermal performance, which requires a different global thermal management strategy relative to that of single-site environments. The approach is illustrated for a hypothetical example wherein a service is delivered from three different data centers in geographically diverse locations. Depending on the workload characteristics, the optimal distribution of resources across the data centers varies; but through dynamic resource allocation, it becomes possible to support the same service at increased energy efficiencies.
Active Flow Instability Control for Transient Two-Phase Electronics Cooling
Authors: Zhang Tie Jun , Peles Yoav , Wen John T. , Jensen Michael K.
Published: March 2011

Because of increasing power densities, refrigeration systems are being explored for two-phase cooling of ultra high power electronic components. Flow instabilities are potential problems in any two-phase refrigeration cooling system especially in transient applications. Oscillatory two-phase flow in a boiling channel can trigger transition to the critical heat flux (CHF). Active control methods can help better dynamic thermal management of electronic systems, even though transient two-phase boiling flow mechanisms are complicated. This paper presents a framework for the transient analysis and active control of pressure-drop flow instabilities under varying imposed heat loads. The first part of the paper is to study the external effects on boiling flow characteristics and the boiling oscillatory flow responses to transient heat load changes. Then based on the theoretical analysis of boiling flow oscillations, a set of active control schemes are developed and studied to suppress flow oscillations and, therefore, to increase the CHF. With the available control devices (i.e., inlet valve and supply pump), different active control schemes are studied to improve the transient two-phase cooling performance. Finally, a discussion is included to address potential future research.
Laminar Mixed Convection Pressure Drop in Vertical Ducts
Authors: Olsson Carl-Olof
Published: March 2011

The friction factor at laminar flow in vertical parallel plate ducts with uniform heat flux has been determined as function of Reynolds number and Rayleigh number using a numerical model. The velocity field and the wall shear stress are modified as compared to forced convection due to buoyancy effects when the Rayleigh number is increased, and for sufficiently large Rayleigh numbers the friction factor becomes increased. The duct height to width ratio has been 10, 20, and 50, and combinations of Reynolds number and Rayleigh number have been chosen in order to study mixed convection situations when the Reynolds number is larger as well as smaller as compared to free convection. The Prandtl number has been set to 0.72 corresponding to e.g. air and to 70 which is typical for e.g. some transformer oils, which are fluids that are frequently applied in buoyancy driven cooling applications.
Use of a Shroud and Baffle to Improve Natural Convection to Immersed Heat Exchangers
Authors: Boetcher S. K. S. , Kulacki F. A. , Davidson Jane H.
Published: March 2011

Optimizing heat transfer during the charge and discharge of thermal stores is crucial for high performance of solar thermal systems for domestic and commercial applications. This study models a sensible water storage tank for which charge and discharge are accomplished using a heat exchanger immersed in the storage fluid. The objective is to investigate the use of a baffle and shroud as a means to improve convective heat transfer and thermal stratification. The immersed heat exchanger is modeled as a two-dimensional isothermal cylinder which is situated near the top of a storage tank with adiabatic walls. Transient numerical simulations of the discharge process are obtained for 105 < RaD < 107. An adiabatic shroud and baffle whose geometry is parametrically varied is placed around and below the cylinder. Transient Nusselt numbers are calculated for different baffle-shroud geometries and Rayleigh numbers. Results indicate that a long baffle with a high shroud height is optimal.
Simulating the Heating Cycle of a Thin Film Photovoltaic Cell During Rapid Thermal Processing
Authors: El Hitti Georges , Mouawad Grace , El Khoury Khalil , Nemer Maroun
Published: March 2011

In this paper a detailed description of a modeling method for the heating cycle of a thin film Photovoltaic (PV) cell during Rapid Thermal Processing (RTP) is presented. The paper explores the challenges of numerically simulating the heating of the PV during RTP and proposes a comprehensive solution for solving these problems. In the proposed method, the thermal model is based on the Component Interaction Network (CIN) approach, while radiation is calculated by a zonal method based on the flux planes approximation and the Thin Layer Approximation (TLA) adapted for semitransparent sheets with small thickness. Various simulations using the model are performed and compared with experimental results. The comparisons demonstrate the efficiency of the modeling approach developed for PV cells in RTP chambers.
Micron-Scale Diagnostics for Through-Plane Transport Phenomena in Porous Electrodes
Authors: Hess Katherine C. , Epting William K. , Litster Shawn
Published: March 2011

This paper presents the development of a new method for characterizing the electrochemistry and transport phenomena in the porous electrodes of polymer electrolyte membrane fuel cells (PEMFCs). The new method uses a unique microstructured electrode scaffold (MES) that provide an architecture for obtaining measurements at discrete points through the thickness of an electrode. This paper reports on the design, fabrication and initial testing of an MES for measuring ionic potential across the thickness of a PEMFC’s cathode. The new fuel cell hardware and reference electrodes (REs), which gather electrolyte potential measurements through the thickness of the electrode via the MES, have been tested for accuracy and repeatability. The use of hydrogen oxidation reaction (HOR) REs versus oxygen reduction reaction (ORR) REs is analyzed and discussed. Polarization data was also gathered and the REs are used to separate the half-cell potentials. Finally, the preliminary fabrication of an MES and a micro-structural analysis are discussed.
AC Characterization of Heated Microcantilevers for Femtogram-Level Thermophysical Property Measurement
Authors: Gauffreau Andrew M. , DiPippo William , Park Keunhan
Published: March 2011

This article investigates the electrothermal properties and responses of a doped silicon microcantilever under periodic (ac) Joule heating. To this end, a numerical model using a finite element analysis (FEA) was implemented to compute the steady periodic solutions in the frequency domain. The FEA results agree well with the cantilever ac behavior that was not possible with a previous 1-D analysis. Based on the acquired ac characteristics of the heated cantilever, its feasibility as a probe for nanoscale property measurement was studied by comparing the 3ω voltage signals before and after sampling 449 fg of polyethylene directly on the cantilever. The 3ω signal of the sampled cantilever is considerably different from that of the bare cantilever, suggesting that sample properties can be determined using the processes discussed here. The heater size effect on the ac behavior of the cantilever was also investigated, confirming that the high frequency region is closely related with the thermal diffusion at the heater. The obtained results willexpand the characterization and functionality of microcantilevers leading to advancements in localized thermal analysis.
Effect of Reactant Preheating on the Stability of Non-Premixed Methane/Air Flames
Authors: Lamige Sylvain , Galizzi Ce´dric , Min Jiesheng , Perles Julien , Andre´ Fre´de´ric , Baillot Franc¸oise , Escudie´ Dany
Published: March 2011

This study details the influence of reactant temperature on the stability of non-premixed CH4/air co-flow jet flames. Flame characteristics have been studied for five temperature levels (from 295 K to 600 K). The hysteresis zone formed by the limits between attached and lifted flame translates towards higher methane jet velocities with an increase of initial temperature, independently of the air velocity range. Moreover, critical velocities vary linearly with initial temperature. In addition, attachment and lift-off heights have been obtained from CH* chemiluminescence visualization. Results point out that the attachment height decreases significantly with temperature. Observations also indicate that the intrinsic process of lifting is modified. Pre-lifting anchored flame local extinctions, not observed at room-temperature, appear at higher initial temperatures; their occurrence increases with temperature. The lift-off height of turbulent lifted flames is also reduced with temperature. Overall, results show that increasing local temperature in the stabilization zone enhances flame stability.
The Influence of Rheological Parameters in Wax Deposition in Channel Flow
Authors: Minchola L. R. , Azevedo L. F. A. , Nieckele A. O.
Published: March 2011

Wax deposition is a critical operational problem in crude oil transportation through pipelines in cold environments. Accurate prediction of the wax deposition is crucial for the efficient design of subsea lines. Wax deposition is a complex process for which the basic mechanisms are still not fully understood. Although Fick’s molecular diffusion model is considered by several authors as the leading deposition mechanism, it is shown that it does not represent well the wax deposition thickness, measured during the transient regime, in a simple experiment, in a rectangular channel, with a laboratory oil-wax mixture. Another important wax deposition mechanism identified is associated with the rheological properties of the fluid, since oil-paraffin mixtures shows a non-Newtonian behavior at temperatures below the fluid Wax Appearance Temperature. The mixture can be modeled as a Bingham fluid, with a dependence of the yield stress on wax concentration, temperature and rate of cooling. The present paper presents a numerical model for predicting wax deposition in channel flows considering the influence of rheological properties combined with a diffusion-based deposition mechanism. To determine the amount of deposit, the conservation equations of mass, momentum, energy and wax concentration in the mixture were numerically solved with the finite volume method. A nonorthogonal moving coordinate system that adapts to the wax interface deposit geometry was employed. The results demonstrated that additional deposition is obtained as a result of the non Newtonian behavior of the fluid. This trend is in agreement with experimental observation conducted in previous studies.
The Effect of Interstitial Layers on Thermal Boundary Conductance Between Lennard-Jones Crystals
Authors: English Timothy S. , Duda John C. , Jordan Donald A. , Norris Pamela M. , Zhigilei Leonid V.
Published: March 2011

Thermal transport at the interface between Lennard-Jones crystals is explored via non-equilibrium molecular dynamics simulations. The vibrational properties of each crystal are varied by changing the atomic mass of the crystal. By applying a constant thermal flux across the two-crystal composite system, a steady-state temperature gradient is established and thermal boundary conductance at the interface between the crystals is calculated via Fourier’s law. With the material properties of the two crystals fixed, thermal boundary conductance can be affected by an intermediate layer inserted between the two crystals. It is found that when the interstitial layer atomic mass is between those values of the crystals comprising the interface, interfacial transport is enhanced. This layer helps bridge the gap between the different vibrational spectra of the two materials, thus enhancing thermal transport, which is maximized when the interstitial layer atomic mass approaches the average mass of the two fixed crystals. The degree of enhancement depends on the vibrational mismatch between the interstitial layer and the crystals comprising the interface, and we report an increase in thermal boundary conductance of up to 50%.
Heat Transfer From a Wedge to Fluids at Any Prandtl Number Using the Asymptotic Model
Authors: Awad M. M.
Published: March 2011

Heat transfer from a wedge to fluids at any Prandtl number can be predicted using the asymptotic model. In the asymptotic model, the dependent parameter Nux/Rex1/2 has two asymptotes. The first asymptote is Nux/Rex1/2Pr→0 that corresponds to very small value of the independent parameter Pr. The second asymptote is Nux/Rex1/2Pr→∞ that corresponds to very large value of the independent parameter Pr. The proposed model uses a concave downward asymptotic correlation method to develop a robust compact model. The solution has two general cases. The first case is β ≠ −0.198838. The second case is the special case of separated wedge flow (β = −0.198838) where the surface shear stress is zero, but the heat transfer rate is not zero. The reason for this division is Nux/Rex1/2 ∼ Pr1/3 for Pr ≫ 1 in the first case while Nux/Rex1/2 ∼ Pr1/4 for Pr ≫ 1 in the second case. In the first case, there are only two common examples of the wedge flow in practice. The first common example is the flow over a flat plate at zero incidence with constant external velocity, known as Blasius flow and corresponds to β = 0. The second common example is the two-dimensional stagnation flow, known as Hiemenez flow and corresponds to β = 1 (wedge half-angle 90°). Using the methods discussed by Churchill and Usagi (1972, “General Expression for the Correlation of Rates of Transfer and Other Phenomena,” AIChE J., 18(6), pp. 1121–1128), the fitting parameter in the proposed model for both isothermal wedges and uniform-flux wedges can be determined.
Computational Investigation of Laminar Mixed Convection in a Vertical Pipe With Slurries of a Microencapsulated Phase-Change Material in Distilled Water
Authors: Scott David A. , Lamoureux Alexandre , Baliga B. Rabi
Published: March 2011

A computational investigation of steady, laminar, mixed convection in a vertical pipe, with essentially uniform heat flux imposed on its outer surface and slurries of a microencapsulated phase-change material (MCPCM) particles suspended in distilled water as the working fluid flowing upwards, is presented. The MCPCM particles considered here have a core of solid-liquid PCM contained in a thin solid shell. The mean effective diameter of these particles is 2.5μm; the melting of the core PCM takes place primarily in the temperature range 26°C to 30°C; and the latent heat of fusion of this PCM is 129.5kJ/kg. The total length of the pipe is 2.2479m; and its inside and outside diameters are 0.01257m and 0.01588m, respectively. The main contributions of this paper are the following: i) a homogenous mathematical model is shown to be applicable to the aforementioned mixed convection phenomena; ii) correlations for the effective properties of the MCPCM slurries and procedures for their implementation are presented; iii) difficulties with the standard definition of bulk temperature when the specific heat of the fluid changes significantly with temperature are elaborated; iv) a modified bulk temperature that overcomes these difficulties is proposed; v) a finite volume method (FVM) for the solution of the proposed mathematical model is described briefly; and vi) the numerical results are presented, compared to complementary experimental data, and discussed. These comparisons show that proposed model and FVM allow cost-effective computer simulations of the problems of interest.
Two-Dimensional Simulation of Magnetohydrodynamic Two-Phase Flow in Random Porous Media Using the Lattice Boltzmann Method
Authors: Hadavand Mahshid , Nabovati Aydin , Sousa Antonio C. M.
Published: March 2011

The present work employs the single relaxation time lattice Boltzmann method along with the pseudo potential model for the two-phase flow simulation of a ferrofluid in a random two-dimensional medium under the influence of a spatially variable external magnetic field. The magnetic field is created and controlled by placing a permanent magnet at the outlet end of the channel filled with a porous medium. The magnitude of the magnetic force acting on the ferrofluid is controlled by changing the distance of the magnet from the channel outlet. The spatially variable magnetic field strength was analytically calculated inside the channel using the available relations in the literature. The main goal of the present work is to qualitatively study the applicability of the single relaxation time (SRT) lattice Boltzmann method (LBM) to modelling flow of a ferrofluid and its steering into porous media. Penetration of the ferrofluid into the porous medium, which is initially filled with a fluid with no magnetic properties, was simulated in time. The simulation results for the flow front are presented and the effect of the magnetic field strength on the rate of flow penetration and front advancement was studied qualitatively. The LBM has proved to be a powerful tool for modelling flows, which involve multi-physics in complex geometries, when mesoscopic inter-particle forces and interaction with external complex forces have to be determined.
Heat Transfer During Gas Hydrate Film Formation on Gas-Liquid Interface
Authors: Liu Ni , Ouyang Xinping , Li Ju , Liu Daoping
Published: March 2011

Gas hydrates are solid, crystalline, ice-like compounds composed of water and guest molecules. The formation of gas hydrates is a complex process with heat and mass transfer in gas, liquid and solid. Increasing the hydrates formation rate and the storage capacity, reducing hydrate induction time are main technical barriers for the application of gas hydrate. A one-dimensional numerical model of heat transfer during gas hydrate film formation on gas-liquid interface is investigated by analyzing the process of static system. According to the rate of gas consumed, the relation between the thickness of hydrate film and time can be obtained. The temperature distribution of different phase in the system is analyzed and the effect of temperature distribution of water is confirmed. The result indicates that it is effective to accelerate the rate of hydrate formation by enhancing the heat transfer in water phase.
Fractal Dimension of Pore Size Distribution Inside Matrix of Plant Materials and Drying Behavior
Authors: Yang Junhong , Di Qianqian , Zhao Jun , Wang Liqiu
Published: March 2011

This paper studied the relationship between matrix microstructure of plant materials and drying behavior using Astragalus and Chinese Angelica root slices as examples. Astragalus and Chinese Angelica, two kinds of traditional Chinese Medicine Herb (CMH), are species of stem and root medicinal herb with the widely curative effects. Based on the measured results by using automatic surface area-porosity analyzer (at smaller scale ≤ 100nm), the volume fractal dimensions (DV) were estimated to characterize the pore size distribution inside matrix of samples dried by different drying technologies, including microwave drying at 200W and conventional hot air drying at 60 °C. In cases studied in this paper, the values of DV are well related with drying time. As compared to hot air dried sample, DV of microwave dried sample increases by 5.9% for Astragalus, while 7.7% for Chinese Angelica. In comparison with hot air drying, the microwave drying time is reduced to 1/6 (30min/180 min) for Astragalus, while 1/16 (50min/810min) for Chinese Angelica. Combined the observation of SEM images of sample, our findings is that, higher fractal dimension, less concentration of pore size distribution, better connectivity of pore or channel with multi-scale characteristics (aperture about 0.1μm–1μm, plasmodesma about 1nm–10nm, or 10Å–100Å), much loose distribution of cytoplasm inside parenchyma cells, better permeability of moisture transfer path during drying process, shorter drying time. Our results demonstrate that the volume fractal dimension (DV) of pore size distribution at smaller scale (≤ 100nm) could give a better understanding the mass transfer behavior of plant materials.
Conjugate Heat Transfer in Annular Laminar Film Condensation in Microchannels: Comparison of Numerical Model to Experimental Results
Authors: Nebuloni Stefano , Thome John R. , Del Col Davide
Published: March 2011

A comparison of the recently proposed numerical model for annular laminar film condensation heat transfer in microchannels of different internal shapes (circular, square, rectangular, etc.), including the effects of conjugate heat conduction in the channel walls, is made versus recent independent experimental results experiencing this effect. Notably, the thinning of the condensate film induced by surface tension due to gravity forces and shape of the surface, also known as the ‘Grigorig’ effect, has a strong consequence on the local heat transfer coefficient in condensation. The model, which is based on a finite volume formulation of the Navier-Stokes and energy equations for the liquid phase only, accounts for the contributions of the surface tension, axial shear stresses and gravitational forces and for the conjugate effects of axial and peripheral wall conduction and nonuniform heat flux. The model was previously validated versus experimental data available in the literature without accounting for conjugate effects, predicting microchannel heat transfer data to within 20% or better. Specifically, the updated version of the model includes the coupling between the thin film fluid dynamics, the heat transfer in the condensing fluid and the heat conduction in the channel wall. Since it is imperative to demonstrate that numerical heat transfer models are accurate and reliable, the present paper focuses on validating this new conjugate model versus recent actual experimental data for various small channels and test fluids experiencing this effect. The results are very encouraging and are presented here.