bluuebunny/crossref_metadata_2025 huggingface dataset:
Optimization of a Parallel Plate Heat Sink Using Volume Averaging Theory
Authors: Blake Benjamin A. , Catton Ivan
Published: April 2009

A parallel plate heat sink is optimized using a model based on the volume average theory (VAT). VAT is briefly developed and the numerical scheme is described. The numerical simulation is carried out in FORTRAN. The resulting VAT solutions are verified by comparison to experimental results via a Nusselt number correlation. The procedure for optimization is described and, as an example, a heat sink of a size appropriate for cooling a CPU is optimized for minimum thermal resistance, maximum effectiveness, and maximum heat transfer rate per unit volume. Seven parameters are included in the simulations: fin thickness, fin length, fin height, fin pitch to thickness ratio, base width, base thickness, and pore Reynold’s number. Three are chosen for optimization: fin height, fin pitch, and pore Reynolds number. The responses are optimized for an aluminum heat sink.
Thermal Management of Heat Generating Devices in Close Proximity on Printed Circuit Boards
Authors: Harvest Jared , Fleischer Amy S. , Weinstein Randy D.
Published: April 2009

This study uses experimentally validated computational fluid dynamics models to predict the behavior of the localized thermal interactions between adjacent components on vertically orientated circuit boards in natural convection. Using the developed models, the effects of power density, component proximity, component geometry, circuit board material, board packing density and board separation distance on the maintenance of optimal operating temperatures for all components are investigated. The device separation distance beyond which the components no longer thermally influence each other is identified and the influence of various parameters on this distance is studied. The parametric study is designed using Design of Experiments methodology to best interpret the interaction between parameters and can easily be applied to other packaging situations to allow designers to optimally place components on a circuit board in close proximity to minimize negative thermal interactions.
Nanofluids Suspensions: Possible Explanations for the Apparent Enhanced Effective Thermal Conductivity
Authors: Vadasz Peter
Published: April 2009

The spectacular heat transfer enhancement revealed experimentally in naofluids suspensions is being investigated theoretically at the macro-scale level aiming at explaining the possible mechanisms that lead to such impressive experimental results. In particular, the possibility that Dual-Phase-Lagging heat conduction in the heterogeneous medium (nanofluid suspension) could have been the source of the excessively improved effective thermal conductivity of the suspension is shown to provide a viable explanation. The investigation of alternative possibilities is needed however prior to reaching an ultimate conclusion.
Characterization of Optical Microcavity Whispering-Gallery-Mode Resonators
Authors: Quan Haiyong , Guo Zhixiong
Published: April 2009

In the past decade, optical microcavity whispering-gallery mode (WGM) resonators have received increasing attention for applications in optical communications and nanotechnology. In this paper, the theory for describing radiation transfer and heat transfer in the micro/nanoscale devices is presented first. Then the characteristics of waveguide-microdisk coupling WGM miniature resonators are numerically studied. Focus is placed on the parametric studies over a broad range of resonator configuration parameters including the microcavity size and the gap separating the microdisk and waveguide. The finite element method is used for solving Maxwell’s equations which govern the propagation of electromagnetic field and the radiation energy transport in the micro/nanoscale devices. The EM field and the radiation energy distributions are obtained and compared between the on-resonance and off-resonance cases. A very brilliant ring with strong EM field and high radiation intensity is found inward the periphery of the microdisk under resonances and high energy storage is achieved. The microdisk size affects significantly the resonant frequencies and their intervals. The scattering spectra for three different microcavity sizes are obtained. The gap obviously influences the qualify factor, the full width at half maximum, and the finesse of the resonant modes as well as the capability of energy storage.
On the Implementation of Stream-Wise Periodic Boundary Conditions
Authors: Beale Steven B.
Published: April 2009

Fully-developed periodic boundary conditions have frequently been employed to perform calculations on the performance of typical elements of heat exchangers. Many such calculations have been achieved by transforming the equations of motion to obtain a new set of state variables which are cyclic in the stream-wise direction. In others, primitive variables, based on substitution schemes are employed. In this paper; a review of existing procedures is provided, and a new method is proposed. The method is based on the use of primitive variables with periodic boundary conditions combined with the use of slip values. Either pressure difference or mass flow rate may be prescribed, and both constant wall temperature and constant heat flux wall conditions may be considered. The example of an offset-fin plate-fin heat exchanger is used to illustrate the application of the procedure. The scope and limitations of the method are discussed in detail, and the mathematical basis by which the method may be extended to the consideration of problems involving mass transfer, with associated continuity, momentum, and species source/sinks is proposed.
Industrial Applications of Thermal Devices With Meso-Scale Features
Authors: Kelly Kevin W. , McCandless Andrew , Marques Christophe , Turner Ryan A. , Luke Patrick , Motakef Shariar , Overholt Matthew R.
Published: April 2009

Two commercial applications are presented that are based on heat transfer augmentation through the use of micro scale geometries. First, we present a micro-channel cross flow heat exchanger, manufactured by a derivative of the LIGA micromachining process. Both the in-plane and cross-flow passages have characteristic widths which can be as low as 250 micrometers. The manufacturing process of the heat exchanger is described, and the scaling laws capturing various design parameters are discussed. Experimental results which validate these scaling laws are presented. A second product, the Micro Jet Cooling Array (MJCA), consists of an array of small diameter impinging microjets with jet diameters as low as 300 micrometers, and provides extremely high heat transfer coefficients over relatively large target areas. The return flow in the MJCA is based on a patent-pending process that essentially isolates the jets from each other. In this manner a large number of small diameter jets can be placed next to each other without the deleterious effect of (a) cross-washing of neighboring jets, and (b) jet-to-jet flow variations due to variations in the discharge pressure over the target. The manufacturing of the MJCA, the scaling laws, and related experimental results are presented.
Investigation of Lightpipe Volumetric Radiation Effects in RTP Thermometry
Authors: Frankman D. J. , Webb B. W. , Jones M. R.
Published: April 2009

A major obstacle to the widespread implementation of Rapid Thermal Processing (RTP) is the challenge of wafer temperature measurement. Frequently, lightpipe radiation thermometers are used to measure wafer temperatures in RTP reactors. While the lightpipe distorts the wafer temperature profile less than temperature measurement techniques which require physical contact, the presence of the lightpipe influences the wafer temperature profile. This paper presents the results of a theoretical study exploring that influence. The coupled radiation/conduction transport in the lightpipe enclosure is solved numerically. Radiation transfer in the system is modeled with varying levels of rigor, ranging from a simple volumetrically non-participating treatment to a full spectral solution of the Radiative Transfer Equation. The results reveal a rather significant effect of the lightpipe on the wafer temperature, which depends on the separation between the lightpipe tip and the wafer. The study illustrates clearly the need to model the lightpipe as a volumetrically participating, semitransparent medium, and further, the importance of accounting for spectral variation of the lightpipe properties in the prediction of the radiative transfer. Finally, two primary mechanisms are identified by which the lightpipe affects the wafer temperature distribution.
Optimization of Vortex Promoter Design Using a Dynamic Data Driven Approach
Authors: Icoz Tunc , Jaluria Yogesh
Published: April 2009

Thermal management of electronic equipment is one of the major technical problems in the development of electronic systems that would meet increasing future demands for speed and reliability. It is necessary to design cooling systems for removing the heat dissipated by the electronic components efficiently and with minimal cost. Vortex promoters have important implications in cooling systems for electronic devices, since these are used to enhance heat transfer from the heating elements. In this paper, an application of Dynamic Data Driven Optimization Methodology (DDDOM), which employs concurrent use of simulation and experiment, is presented for the design of the vortex promoter to maximize the heat removal rate from multiple protruding heat sources located in a channel, while keeping the pressure drop within reasonable limits. Concurrent use of computer simulation and experiment in real time is shown to be an effective tool for efficient engineering design and optimization. Numerical simulation can effectively be used for low flow rates and low heat inputs. However, with transition to oscillatory and turbulent flow at large values of these quantities, the problem becomes more involved and computational cost increases dramatically. Under these circumstances, experimental systems are used to determine the component temperatures for varying heat input and flow conditions. The design variables are taken as the Reynolds number and the shape and size of the vortex promoter. The problem is a multi-objective design optimization problem, where the objectives are maximizing the total heat transfer rate, as given by the Nusselt number, Nu, and minimizing the pressure drop, ΔP. This multi–objective problem is converted to a single-objective problem by combining the two objective functions of the form Nutota/ΔPb, where a and b are constants.
Effect of Inclination on Pool Boiling of FC-72 Dielectric Liquid on Porous Graphite
Authors: Parker Jack L. , El-Genk Mohamed S.
Published: April 2009

Saturation pool boiling experiments of FC-72 liquid on a flat, porous graphite and smooth copper surfaces measuring 10 × 10 mm investigated the effect of surface orientation on nucleate boiling and Critical Heat Flux (CHF). The inclination angle of the surface increased from 0° (upward-facing) to 60°, 90°, 120°, 150°, and 180° (downward facing). Results demonstrated significant increases in the nucleate boiling heat transfer coefficient and CHF on porous graphite, compared to those on copper. At low surface superheats, increasing the inclination angle increases the nucleate boiling heat transfer coefficient, which decreases with increased inclination angle at high surface superheats. These results and the measured decreases of CHF with increased inclination angle are consistent with those reported earlier by other investigators for dielectric and non-dielectric liquids. On smooth surfaces and micro-porous coatings, the reported fractional decreases in CHF with increased inclination angle are almost identical, but markedly larger than those measured in this work on porous graphite. On these surfaces the reported CHF in the downward-facing position (180° inclination) is ∼10–20% of that in the upward-facing position (0° inclination), compared to ∼53.3% on porous graphite. The CHF values of FC-72 liquid on porous graphite, which also decreased with increased inclination angle, are correlated using the general form suggested by Kutatelatze (1961) to within ± 5% of the experimental data.
Genetic-Programming-Based Symbolic Regression for Heat Transfer Correlations of a Compact Heat Exchanger
Authors: Cai Weihua , Sen Mihir , Yang K. T. , Pacheco-Vega Arturo
Published: April 2009

We describe a symbolic regression methodology based on genetic programming to find correlations that can be used to estimate the performance of compact heat exchangers. Genetic programming is an evolutionary search technique in which functions represented as parse trees evolve as the search proceeds. An advantage of this approach is that functional forms of the correlation need not be assumed. The algorithm performs symbolic regression by seeking both the functional structure of the correlation and the coefficients therein that enable the closest fit to experimental data. This search is conducted within a functional domain constructed from sets of operators and terminals that are used to build tree-structures representing functions. A penalty function is used to prevent large correlations. The methodology is tested using first artificial data from a one-dimensional function and later a set of published heat exchanger experiments. Comparison with published results from the same data show that symbolic-regression correlations are as good or better. The effect of the penalty parameters on the “best function” is also analyzed.
Numerical Study on Natural Convection and Radiation From a Square Disk
Authors: Willockx Arnout , Tjoen Christophe , Steeman Hendrik-Jan , De Paepe Michel
Published: April 2009

In this paper, a numerical study of natural convection from a disk is presented. The disk is placed vertically in a closed cavity (cylinder) and has a constant heat flux. Different numerical simulations of this test case are executed at various gravity accelerations (=g) inside the cavity. The accelerations are varied from 9.81 m/s2 to 53 m/s2. The Rayleigh number changes with these accelerations. The flow pattern and the temperature distribution inside the cavity are visualized. For natural convection inside a cavity, a vortex is expected in the air flow: a plume of warm rising air at the centre of the cylinder above the heated square disk and descending colder air at the walls of the cavity. The air velocity is higher in the central plume. At w = 9.81 m/s2, the maximum air velocity is 0.05 m/s. This velocity increases with increasing acceleration w till 0.6 m/s at w = 53 m/s2. At low w, the flow pattern exists of a stable vortex and thus a steady-state flow. At w = 15 m/s2, the vortex becomes more unstable and is swirling. At w = 27 m/s2, the vortex is even more swirling and the following periodical phenomenon takes place: first the vortex starts to dissolve in a small vortex at the top of the cylinder and a vortex below this around the square disk. Then, the lower vortex starts to increase again and the upper vortex is fading. So there is again one big vortex with a central, unstable plume that reaches the top of the cylinder. After a few seconds, the plume dissolves again. This phenomenon has no constant period. The higher w, the faster this phenomenon happens and thus the shorter the period. At w = 53m/s2, the vortex seems more turbulent than laminar, however the Rayleigh number is still in the laminar range (Ra<106). The numerical simulations are verified with existing correlations. There was a good agreement between correlations and numerical simulation.
A Molecular Dynamics Simulation Study on the Effect of Methanol on the Structural Characteristics of Lipid Bilayers
Authors: Pinisetty Dinesh , Moldovan Dorel , Devireddy Ram
Published: April 2009

To further our understanding on the water permeation process through a lipid bilayer membrane in the presence of a commonly used cryoprotective agent (methanol), we performed detailed molecular dynamics simulation (MD) studies. The MD simulations analyzed the influence of methanol (12 mol %) on the structure of a fully hydrated dipalmitoylphosphatidylcholine (DPPC) in the fluid phase and under equilibrium conditions at 323K.
Heterogeneous Nucleation With Artificial Cavities
Authors: Qi Yusen , Klausner James F.
Published: April 2009

Bubble incipience in artificial cavities manufactured from silicon has been studied using gas nucleation and pool boiling. Moderately wetting water and highly wetting ethanol have both been used as the bulk fluid with cylindrical cavities, as well as those with a triangle, square, and rectangle shape cross section. Nominal cavity sizes range from 8 to 60 μm. The incipience conditions observed for water using both gas nucleation and pool boiling suggest that bubble initiation originates from a concave meniscus. Cornwell’s contact angle hysteresis theory for vapor trapping cavities is used to explain the gas nucleation results. The pool boiling results are more difficult to explain. Using ethanol, cavities appeared to be completely flooded and were not activated using either gas nucleation or pool boiling. Using water and gas nucleation cavities were almost always activated, provided the incipience criterion was satisfied; in contrast cavities in pool boiling with water activated with different superheats during different experiments. The difference in incipience behavior between gas nucleation and pool boiling with water is explained based on vapor trapping and thermal suppression considerations. Based on limited experimental results, it appears that the backpressure does not influence gas bubble incipience, provided the pressure difference is the same. The experimental results presented affirm the theory of heterogeneous nucleation from vapor trapping cavities provided contact angle hysteresis and vapor trapping are fully accounted for. However, the results also suggest that the theoretical considerations required for a deterministic model for incipience from vapor trapping cavities during boiling is more complex than previously hypothesized.
Numerical and Experimental Investigation of Thermal Signatures of Buried Landmines in Dry Soil
Authors: Moukalled F. , Ghaddar N. , Kabbani H. , Khaled N. , Fawaz Z.
Published: April 2009

This paper reports a numerical and experimental investigation conducted to study the surface thermal signature of buried landmines. Numerical predictions are obtained by solving an unsteady three-dimensional energy balance model for heat transport in dry soil with a buried mine using the conservative finite-volume method. The model is validated by comparing generated results against published analytical and numerical data in addition to indoor measurements performed on dry soil inside an environmental chamber. The thermal signatures are observed while cooling takes place after exposing the soil surface to a radiant heat flux for a specified period. Transient temperature profiles produced numerically agree well with thermocouple measurements recorded at shallow soil depths and with surface IR images. The difference between predicted and measured surface temperatures is less than 0.4°C and the difference in thermal signature is less than 0.3°C. Sit in. The numerical model is also used to predict perturbations of the expected thermal signatures that are compared to the real (measured) ones from the IR images. The thermographic analysis shows good promise as a method for detecting shallowly buried land mines where not only the temperature difference or contrast images generated by the thermal signatures are matched between the IR images and the simulation images with high accuracy, but also the absolute temperatures for many images generated at discrete time intervals.
Coupling Radiation in Participating Media With Other Modes of Heat Transfer: Segregated Versus Coupled Solution
Authors: Mazumder Sandip
Published: April 2009

Traditionally, radiation in participating media is coupled to other modes of heat transfer using an iterative procedure in which the overall energy equation (EE) and the radiative transfer equation (RTE) are solved sequentially and repeatedly until both equations converge. While this explicit coupling approach is convenient from the point of view of computer code development, it is not necessarily the best approach for stability and convergence. A new numerical procedure is presented in which the EE and the RTE are implicitly coupled and solved simultaneously, rather than as segregated equations. Depending on the average optical thickness of the medium, it is found that the coupled solution approach results in convergence that is between two to one hundred times faster than the segregated solution approach. Several examples in one and two-dimensional media, both gray and nongray, are presented to corroborate this claim.
Molecular Dynamics Simulation of Thermal Transport at Nanometer Size Point Contacts on a Planar Silicon Substrate
Authors: Saha Sanjoy , Shi Li
Published: April 2009

A Non Equilibrium Molecular Dynamics (NEMD) simulation has been used to calculate the temperature distribution in the substrate side of a nanometer scale point contact on a planar silicon substrate with different doping concentrations and contact radii. The size of the non-uniform temperature zone was found to approach the average nearest-neighbor distance of impurity dopants when the contact radius was reduced below this distance. At a contact radius of 0.5 nm, the calculated spreading thermal resistance at the nano-point contact agrees with those obtained using two phonon transport models. At a contact radius between 1 nm and 6 nm, however, the spreading resistance from the NEMD is much larger than those from the two models that assume small deviation from the equilibrium distribution.
Simulation of Heat Transfer With LBM and Lagrangian Methods for Microfluidic Applications
Authors: Thummala Nishitha , Papavassiliou Dimitrios V.
Published: April 2009

This work presents a Lagrangian approach to simulate convective heat transfer in small scales. The fully developed flow field, simulated by a Lattice Boltzmann Method, is combined with Lagrangian tracking of thermal markers to determine the behavior of an instantaneous scalar line source located at the wall of a channel. The resulting probability density functions are used to calculate the behavior of continuous line sources of heat at the wall of the channel, as well as the temperature for the case of constant temperature or constant heat flux from the wall. This method is resourceful in terms of computational efficiency, in that it can be used to simulate various thermal boundary conditions and Prandtl number fluids with a single flow field resulting from a Lattice Boltzmann simulation.
Flow Bifurcations and Heat Transfer Enhancement in Asymmetric Grooved Channels
Authors: Guzma´n Amador M. , Villar Fernando A.
Published: April 2009

Numerical investigations of the flow bifurcations, transition scenario and heat transfer enhancement in asymmetric grooved channels are performed by direct numerical simulations of the mass, momentum and energy equations. The governing equations are solved for laminar and time-dependent transitional flow regimes by the spectral element method in a periodic computational domain with appropriated boundary conditions. Numerical results show a flow transition scenario with two Hopf bifurcations B1 and B2, occurring in critical Reynolds numbers Rec1 y Rec2, respectively. Fundamental frequencies ω1 and ω2, and super harmonic combinations of both develop as the Reynolds number increases from a laminar to higher transitional flow regime. Numerical calculations demonstrate that the time-average mean Nusselt number (the non-dimensional heat transfer rate), increases significantly as the flow passes from a laminar to a periodic—and then to a quasi-periodic flow regime. This increase is accompanied by a reasonable increase in both the friction factor and the pumping power. The obtained behavior is comparable to other geometries and configurations as well as to previously reported numerical results for the studied geometry. This numerical investigation shows a transition scenario at the onset of turbulence, similar to the Ruelle-Takens-Newhouse scenario, which has not been found or reported by other researchers using this geometry. The numerical simulation results also show the existence of a bifurcation scenario that develops a path-dependent flow and heat transport behavior. In the vicinity of the first Hopf flow bifurcation (and consequently, the critical Reynolds number Rec1), the resulting stable time periodic flow depends on both the initial flow conditions and the way in which the incremental process to higher flow regimes is carried out.
Microscale Boiling Heat Transfer Near the Meniscus
Authors: Haendler Brenda E. , Walther David C. , Liepmann Dorian , Pisano Albert P.
Published: April 2009

Results are presented experimentally measuring the localized temperature profile due to microscale boiling of a silicon-Pyrex bonded wafer with a 100 μm deep, 500 μm wide and six mm long microchannel. Experiments were performed using an infrared camera equipped with a magnifying lens. By using a camera, the dynamic temperature profile is shown from the inside channel all the way out to where the temperature of the wafer reaches the bulk temperature of the heating source. Temperature profiles are shown for both water and methanol as the working fluid applying between five and twenty degrees Celsius of superheat to the bulk wafer. Using these results, a discussion of the relevant heat transfer modes and nondimensional numbers is given to gain insight into the range of influence that phase change in a microchannel has on the temperature of the wafer. Additionally, discussion is given about modeling of microscale phase change using a commercial fluid dynamics software package. The importance of these results with respect to implementation into the fuel intake manifold for a micro engine based portable power system is also discussed.
Evaporation and Nucleate Boiling of an Individual Droplet on Surfaces
Authors: Wang X. D. , Lu G. , Peng X. F. , Wang B. X.
Published: April 2009

A visual study was conducted to investigate the evaporation and nucleate boiling of a water droplet on heated copper, aluminum, or stainless surfaces with temperature ranging from 50°C to 112°C. Using a high-speed video imaging system, the dynamical process of the evaporation of a droplet was recoded to measure the transient variation of its diameter, height, and contact angle. When the contact temperature was lower than the saturation temperature, the evaporation was in film evaporation regime, and the evaporation could be divided into two stages. When the surface temperature was higher than the saturation temperature, the nucleate boiling was observed. The dynamical behavior of nucleation, bubble dynamics droplet were detail observed and discussed. The linear relationships of the average heat flux vs. temperature of the heated surfaces were found to hold for both the film evaporation regime and nucleate boiling regime. The different slopes indicated their heat transfer mechanism was distinct, the heat flux decreased in the nucleate boiling regime more rapidly than in the film evaporation due to the strong interaction between the bubbles.
Thermal Contact Resistance and Thermal Conductivity of a Carbon Nanofiber
Authors: Yu Choongho , Shi Li , Cassell Alan M. , Cruden Brett A. , Ngo Quoc , Li Jun
Published: April 2009

We have used a microfabricated device to measure the thermal contact resistance between a 152-nm-diameter carbon nanofiber and the measurement device, and found that heat conduction models based on Fourier’s law can underestimate the thermal contact resistance at the nanoscale constriction if a large reduction in the effective thermal conductivity of the constriction is not taken into account. With the thermal contact resistance reduced by the deposition of a platinum layer on the contact, we observed that the thermal conductivity of the carbon nanofiber grown by using a plasma enhanced chemical vapor deposition method is comparable to that of graphite fibers grown by pyrolysis of natural gas prior to high-temperature heat treatment.
Feature Evolution During Sub 100NM Gap-Fill and Etch
Authors: Mungekar Hemant , Lee Young S. , Venkataraman Shankar
Published: April 2009

Inductively coupled plasma (ICP) reactors are being used at low gas pressure (<100mTorr) and high plasma density ([e] > 1013/cm2) processes in semiconductor fabrication. In these reactors plasma is generated by inductively coupled electric field while positive ions are accelerated anisotropically by applying a negative bias RF to the substrate. Semiconductor manufacturers face many challenges as wafer size increases while device geometries decrease. Two key challenges for both process design and electronics processing equipment design are (a) scale up of process from 200mm to 300mm diameter substrate, and (b) deposition and etching features with high aspect ratios. A unified phenomenological model to explain profile evolution trend as a function of aspect ratio for deposition (gap fill) and trench etch using ICP reactors is presented. Trends for feature evolution as a function of pressure for gap fill and trench etch are reviewed and explained. The article emphasizes importance of low pressure for sub-100nm gap-fill and trench-etch applications in ICP processing reactors.
Heat and Mass Transfer in an Industrial EFG Silicon Tube Growth System
Authors: Wang C. , Yang B. , Zhang H.
Published: April 2009

Progressively expanding of photovoltaic industry has caused worldwide silicon feedstock shortage. The fast growth of high quality thin tubes is critical to achieve high solar cell efficiency while reducing the consumption of raw materials. A previously developed comprehensive two-dimensional global model, has been used to predict electromagnetic induction heating and heat and mass transfer in the entire growth system. To achieve more accurate simulation results, a two dimensional local model is developed to simulate the temperature distribution along the silicon tube and temperature gradient at the solid-liquid interface, which are critical for stable growth, and residual stress. The radiation, convection and conduction heat transfer between silicon tube and environment and treatments of solid-liquid interface movement and solidification latent heat generation at the growth interface are discussed in this paper. Simulation results of the electromagnetic and temperature fields for a large diameter EFG is presented. A one-dimensional dynamic model is used to study the oscillation of silicon tube thickness under different conditions. Parametric studies have been performed to study the effects of pull rate and tube thickness on tube temperature distribution and tube quality. The effects of different carrying gas flow rates will also be investigated. The relationship between the temperature profile along the silicon tube and the silicon tube thickness, pull rate, and gas flow rate is established.
Analysis of Effect of Geometric Parameters on Performance of Solar Chimney Plants
Authors: Serag-Eldin M. A.
Published: April 2009

The paper presents a computational model for predicting the flow details within solar chimney plants. It comprises governing partial differential equations expressing conservation of mass, energy and momentum in addition to a two equation model of turbulence. Boundary-fitted-coordinates are employed to fit the wall shapes accurately. An actuator disc model is adopted to express the effect of an embedded wind turbine. The model is first applied to study the details of the flow within the collector-stack assembly. It is then applied to evaluate the effect of plant size and various geometric parameters on output.
A Theoretical Investigation Into the Optimal Longitudinal Profile of a Horizontal Pin Fin of Least Material Under the Influence of Natural Convection
Authors: Kobus Chris J.
Published: April 2009

Fundamental level research is presented in this paper discovering the optimum pin fin profile maximizing material utilization (maximum heat transfer with minimum volume) in the case of natural convection. Although this problem has been studied in the past, the current research allows for an axially variable convective heat transfer coefficient due to the axially variable diameter in such pin fins. The current model is compared with previous studies assuming a constant convective heat transfer coefficient. The comparison shows the prior solutions to be special cases of the model in the current research.
Determination of Laser Welding Process Parameters Through a Combined Neural Networks and Computational Fluid Dynamics Approach
Authors: Hauser Thomas , Hradisky Michal , Li Leijun
Published: April 2009

In this paper weld pool shapes of type 304 stainless steel obtained through computational fluid dynamics simulations (CFD) will be used to train artificial neural networks. The neural network is then used to determine the experimental parameters of the ND:YAG laser system on 304 steel by feeding it with the parametrized, experimentally obtained weld pool parameters. The output of the neural network are the process parameters like laser spot diameter and laser power on the surface of the weld pool. These parameters are very difficult to determine accurately through experiments but are necessary inputs for CFD simulations of the weld pool. Accurate CFD simulation is required for simulation and modeling of the solidified micro structure of the material.
Freezing Characteristics in Bio Porous Media
Authors: Ma Y. , Peng X. F. , Wu H. L.
Published: April 2009

An experimental investigation was conducted to visually observe the freezing characteristics of bio-solutions. The evolution of the crystals growth was discussed under different conditions. In the nets dipping in the salt solution, the growing direction and speed of crystals were partly influenced by the grid wires. When the concentration of solutions in the front area of the interface was changed, the interface would be greatly influenced, or the freezing characteristics in this area would be totally altered.
CFD Optimization Application on Airside Plate Fins of Condenser Coil of Gravity-Assisted Heat Pipe
Authors: Wang Jing , Wang Jianbing , Guo Liuyang , Wei Suili , Hu Dayong
Published: April 2009

The heat transfer and fluid flow characteristics of airside plate fins of the condenser coil of a gravity-assisted heat pipe are numerically predicted. Based on these CFD computations, the optimization of the airside structure of the heat pipe is carried out by studying the influence of different geometry design variables on a combined heat exchanger evaluation function K, which can be changed by an engineer according to his application experience. In this work, one hundred of DOE’s points are sampled across the design space of the heat pipe based on the Latin Hypercube method. The Least Squares Fitting method is used to fit these sampled points to the RSM of the design space. The Mixed Integer Optimization Algorithm is used to explore the maximum specially-defined K on the Response Surface; and the corresponding optimal size and shape of the heat pipe are finally obtained. The optimization procedure is performed automatically by employing the optimizer-Optimus, grid generator-Gambit, and CFD solver-Fluent.
Experimental and Numerical Study of Acoustic Streaming in a Cylindrical Enclosure
Authors: Lin Yiqiang , Farouk Bakhtier
Published: April 2009

Acoustic streaming motion generated by finite-amplitude resonant oscillations in an air-filled two-dimensional cylindrical enclosure was experimentally studied and numerically simulated. The oscillatory flow field in the enclosure was created by the vibration of one end of the cylindrical resonator (L = 325 mm, R = 12.5 mm). The frequency of the wall vibration was chosen as f = 1062 Hz, such that the corresponding wavelength is equal to the length of the resonator. A standing wave was then generated in the closed tube. In the experiment, the flow field was visually studied by a smoke generator, He-Ne Laser and a digital camera. The pressure wave in the axial points was measured by a piezoresistive pressure transducer (Endvco #4428A). To simulate the flow field, the full compressible form of the Navier-Stokes equations in cylindrical coordinates was considered and solved by a highly accurate flux-corrected transport algorithm for convection terms and a central differencing scheme for the viscous and diffusive terms. In both of the experimental and numerical studies, outer acoustic streaming due to interaction of acoustic waves with viscous boundary layers was observed, and the effects of sound field intensity on the formation of streaming structures were studied.
Measurement of the Thickness of the Liquid Microlayer Between a Sliding Bubble and a Heated Wall
Authors: Li Xin , Hollingsworth D. Keith , Witte Larry C.
Published: April 2009

A laser-based method has been developed to measure the thickness of the liquid microlayer between a cap-shaped sliding bubble and an inclined heated wall. Sliding vapor bubbles are known to create high heat transfer coefficients along the surfaces against which they slide. The details of this process remain unclear and depend on the evolution of the microlayer that forms between the bubble and the surface. Past experiments have used heat transfer measurements on uniform-heat-generation surfaces to infer the microlayer thickness through an energy balance. These studies have produced measurements of 20 to 100 μm for refrigerants and for water, but they have yet to be confirmed by a direct measurement that does not depend on a first-law closure. The results presented here are direct measurements of the microlayer thickness made from a reflectance-based fiber-optic laser probe. Details of the construction and calibration of the probe are presented. Data for saturated FC-87 and a uniform-temperature surface inclined at 2° to 15° from the horizontal are reported. Millimeter-sized spherical bubbles of FC-87 vapor are injected near the lower end of a uniformly heated aluminum plate. The bubbles grow rapidly and change from a spherical to an elliptical shape and finally to a cap-shape with a large section of the bubble surface sliding along the microlayer adjacent to the wall. This evolution is captured by high-speed (1000 frames/sec) images in plan and side views. These image sequences allow measurements of bubble speed, acceleration, and size, but they do not attempt to resolve the microlayer itself. The laser probe yielded microlayer thicknesses of 22 to 55 microns for the cap-shaped bubbles. Bubble Reynolds numbers range from 600 to 4800, Froude numbers are from 0.9 to 1.7, and Weber numbers are from 2.6 to 47.
Analysis of Radiation Heat Transfer in an Enclosure Fire Including the Effect of Scattering
Authors: Yuen W. W. , Chow W. K.
Published: April 2009

The need for an accurate simulation of the radiative heat transfer in a fire zone model is demonstrated. Results show that the lack of an accurate model of the relevant physics of radiative heat transfer can lead to uncertainty which can severely limit the usefulness of a fire zone model. An accurate numerical model of radiative exchange including the effect of scattering, is applied to simulate the effect of radiative heat transfer on fire growth. Typical conservation equations in a fire zone models are used.
Effect of Fiber Volume Fraction on Thermal Conductivity of a Woven-Mat Composite Lamina
Authors: Golestanian Hossein
Published: April 2009

Models are presented for the determination of thermal conductivity of a composite lamina with woven fiber mats. In analyzing the cure cycle of a composite part, the common practice has been to use weight-averaged thermal properties. The limitation of this approach becomes apparent when one finds that thermal conductivity calculated for fiberglass/epoxy composite is very close to thermal conductivity of carbon/epoxy composite. This happens for composite parts with the same fiber volume fraction. In weight-average formulations the effect of fiber thermal conductivity is overshadowed by the density of the constituents. To overcome this problem, one needs to take another approach. In this investigation finite element analysis is performed to determine thermal conductivities of fiberglass/epoxy and carbon/epoxy composite lamina. The resulting thermal conductivities are different for the two composite types. These results make more physical sense since thermal conductivity of carbon fiber mat is much higher than that of fiberglass mat.
Resin Velocity and Pressure Distribution in Resin Transfer Molding of a Composite Cylinder
Authors: Golestanian Hossein
Published: April 2009

Resin Transfer Molding (RTM) process in the manufacturing of a composite cylinder is investigated. Resin flow in the woven fiber mat is modeled as flow through porous media to determine resin velocity and pressure distribution along the part. Five-harness carbon and eight-harness fiberglass mats with epoxy resin composites are investigated. Fiber mat permeability for the two fiber types are determined experimentally. These values are then employed in numerical models to simulate the injection cycle of the RTM process. ANSYS finite element software is used to perform the analysis. The results indicate that resin velocity in fiberglass mats is almost six times the velocity in carbon fiber mats. This is due to the higher permeability of fiberglass mats. The sharp drop in the resin velocity into carbon fibers indicates that flow problems will exist in the manufacturing of large carbon/epoxy parts with RTM processes.
Assessment of Steady State PSP, TSP, and IR Measurement Techniques for Flat Plate Film Cooling
Authors: Wright Lesley M. , Gao Zhihong , Varvel Trent A. , Han Je-Chin
Published: April 2009

Several steady state measurement techniques are used to measure the film cooling effectiveness on a flat plate. Pressure sensitive paint (PSP), temperature sensitive paint (TSP), and infrared (IR) thermography are used to measure the film cooling effectiveness. To compare these measurement techniques, a single row of cylindrical holes, with a compound angle, are used. Seven holes (D = 4 mm) are equally spaced 12 mm apart, and the hole length-to-diameter ratio is 9.92. The axial angle (θ) of the holes is 30°, and the compound angle (β) is 45°. In addition to evaluating the various measurement techniques the effect of the coolant blowing ratio is considered; effectiveness measurements are taken for blowing ratios, M, of 0.4, 0.6, 1.2, and 1.8. The effect of mainstream turbulence intensity is considered with the addition of a turbulence grid to the low speed wind tunnel. Of the three steady state measurement techniques considered in this study, PSP demonstrates the most promise for the measurement of the film cooling effectiveness. Because PSP is a mass transfer technique, film effectiveness measurements can be readily obtained near the film cooling holes. Although the heat transfer techniques of TSP and IR thermography are more desirable than traditional thermocouples or liquid crystal thermography, the applicability of measurements near the holes is questionable due to conduction problems associated with steady state heat transfer techniques.
Thermal and Dynamics Analysis of Meniscus-Controlled Materials Processing
Authors: Yang B. , Zheng L. L.
Published: April 2009

Fundamental physical processes involved in meniscus-controlled materials processing include meniscus formation and dynamics, movement of solidification interface, and the interaction at the crystal-liquid-vapor tri-junction. Final product shape that can be grown by different techniques depends on the meniscus shape, heat transfer and solidification interface. The fluid flow and heat transfer in the melt and dynamics of meniscus are critical for determining the stable growth conditions for better quality of the grown crystals. In this paper, a theoretical thermal and dynamic model have been developed to describe the heat transfer and dynamics of meniscus and its interaction with solidification. A simplified form of the model will also be developed to allow the investigation of ribbon (or tube) growth that exhibits one-dimensional feature in the most regions. This model will be used to conduct parametric study, and the important process and geometry conditions will be investigated such as the crystal dimension, die-top height, pull rate, and die-top temperature. The dynamic response of meniscus to the potential perturbations during growth such as pull rate and die-top temperature variations, and misalignment between the die and silicon tube will be investigated extensively. From this study, an operating window for stable meniscus will be obtained, and growth procedure that leads to improving the grown crystal quality will be identified.
DNS Based Analysis and Modeling of the Turbulent Heat Transfer by Natural Convection in Liquid Lead-Bismuth
Authors: Otic´ I. , Gro¨tzbach G.
Published: April 2009

Results of a direct numerical simulation for Rayleigh-Be´nard convection with Pr = 0.025 are used to analyze the transport equations for the turbulent heat flux. These results show the importance of the pressure diffusion and of the pressure scrambling term in the budget of turbulent heat flux equations. Analysis using the two-point correlation method shows, that a suitable model for the pressure scrambling term may give good approximation of the total temperature pressure gradient correlation, if the flow field is locally dominated by small structures. DNS results show, that a standard pressure scrambling model predicts the total temperature pressure gradient correlation acceptably well for this type of flow. DNS based analysis of the standard pressure scrambling model indicates, that an application of the mixed time scale may improve the model and reduce the number of empirical coefficients.
Analysis of Hydrogen Permeation Through Sub-Micron-Thick Palladium Alloy Membranes
Authors: McLeod Logan S. , Degertekin F. Levent , Fedorov Andrei G.
Published: April 2009

Separation of hydrogen from the reaction products stream leaving fuel processor is an essential step prior to its introduction to the fuel cell. To this end, we are developing micromachined Pd/Ag alloy membranes for in-situ hydrogen separation suitable for integration with catalytic fuel reforming microreactors. In this work, we report an analysis of mass transport and kinetics of hydrogen permeation through a non-porous palladium membrane for the case of the sub-micron membrane thickness. A simplified model has been developed which divides the permeation into seven distinct regimes; gas phase mass transport to the surface, adsorption onto the surface, transition into the bulk material, solid-phase diffusion through the bulk material, transition to the effluent surface, desorption into the gas phase and diffusion away from the surface. Historically, this permeation process is limited by the bulk diffusion step and therefore membrane thickness controls permissible flux. Based on the present model applied to the sub-micron membrane, and utilizing accepted values from the literature, desorption of hydrogen from the Pd surface back into the gas phase has been identified as the rate-limiting step for this process below a ‘critical’ temperature. This result indicates that careful consideration of membrane packaging is critical for maximizing hydrogen flux for the sub-micron membrane and that further efforts to increase flux via thinner membranes would be futile.
Efficiency of a New Straight Cooling Fin
Authors: Hanin Leonid
Published: April 2009

In 1926 E. Schmidt [1] discovered his optimum parabolic straight cooling fin. The most critical premise used in his analysis was the “length-of-arc” assumption that consists of neglecting the curvature of the fin exposed surface. The problem of optimum straight cooling fin design without the “length-of-arc” assumption was solved in [2] which led to the discovery of the optimum circular fin. In the present work, thermal efficiency of this new optimum straight cooling fin is computed.