bluuebunny/crossref_metadata_2025 huggingface dataset:
Convective Heat Transfer in Graphite Foam Heat Sinks With Baffle and Stagger Structures
Authors: Leong K. C. , Li H. Y. , Jin L. W. , Chai J. C.
Published: March 2011

Highly conductive porous media have recently been considered for enhanced cooling applications due to their large internal contact surface area, which promotes convection at the pore level. In this paper, graphite foams that possess high thermal conductivity but low permeability are investigated for convection heat transfer enhancement using air as coolant. Two novel heat sink structures are designed to reduce the fluid pressure drop. Both experimental and numerical approaches are adopted in the study. The experimental data show that the designed structures significantly reduce flow resistance in graphite foams while maintaining relatively good heat removal performance. The numerical results obtained based on the local thermal nonequilibrium model are validated by experimental data and show that the inlet air flow partially penetrates the structured foam walls, while the remaining air flows tortuously through slots in the structure. Flow mixing, which is absent in the block graphite foam, is observed in the freestream area inside the designed structure. It can be concluded that graphite foams with appropriately designed structures can be applied as air-cooled heat sinks in thermal management applications.
Heat Transfer Performance of Aluminum Foams
Authors: Mancin Simone , Zilio Claudio , Rossetto Luisa , Cavallini Alberto
Published: March 2011

Because of their interesting heat transfer and mechanical properties, metal foams have been proposed for several different applications, thermal and structural. This paper aims at pointing out the effective thermal fluid dynamic behavior of these new enhanced surfaces, which present high heat transfer area per unit of volume at the expense of high pressure drop. The paper presents the experimental heat transfer and pressure drop measurements relative to air flowing in forced convection through four different aluminum foams, when electrically heated. The tested aluminum foams present 5, 10, 20 and 40 PPI (pores per inch), porosity around 0.92–0.93, and 0.02 m of foam core height. The experimental heat transfer coefficients and pressure drops have been obtained by varying the air mass flow rate and the electrical power, which has been set at 25.0 kW m−2, 32.5 kW m−2, and 40.0 kW m−2. The results have been compared against those measured for 40 mm high samples, in order to study the effects of the foam core height on the heat transfer. Moreover, predictions from two recent models are compared with heat transfer coefficient and pressure drop experimental data. The predictions are in good agreement with experimental data.
Effects of Ultrasounds on the Heat Transfer Enhancement From a Circular Cylinder to Distilled Water in Subcooled Boiling
Authors: Baffigi Federica , Bartoli Carlo
Published: March 2011

The main aim of this work is to investigate experimentally the influence of ultrasonic waves, on the heat transfer enhancement, from a stainless steel circular cylinder to distilled water, in subcooled boiling conditions. This study has carried on for a few years at the Department of Energetics “L.Poggi.” The effect was observed since the 1960s: Different authors had investigated the cooling effect due to the ultrasonic waves at different heat transfer regimes, especially from a thin platinum wire to water. They had found out that the highest heat transfer coefficient enhancement was in subcooled boiling conditions. So this paper has the purpose to clarify the physical phenomenon and optimize a large range of variables involved in the mechanism. It reports the experimental results obtained with ultrasound at the frequency of 38 kHz, at two different subcooling degrees, ΔTsub=25°C and 35°C. The heat fluxes applied on the cylinder, the ultrasonic generator power Pgen, and also the placement of the heater inside the ultrasonic generator tank were varied. The ultrasonic waves seem to be very useful for a practical application in the last generation electronic components’ cooling: They need dissipating huge heat fluxes and avoiding high temperatures (≈150°C), after that they could damage themselves.
Multiscale Uncertainty Quantification Based on a Generalized Hidden Markov Model
Authors: Wang Yan
Published: March 2011

Variability is the inherent randomness in systems, whereas incertitude is due to lack of knowledge. In this paper, a generalized hidden Markov model (GHMM) is proposed to quantify aleatory and epistemic uncertainties simultaneously in multiscale system analysis. The GHMM is based on a new imprecise probability theory that has the form of generalized interval. The new interval probability resembles the precise probability and has a similar calculus structure. The proposed GHMM allows us to quantify cross-scale dependency and information loss between scales. Based on a generalized interval Bayes’ rule, three cross-scale information assimilation approaches that incorporate uncertainty propagation are also developed.
Time-Dependent Reliability Analysis for Function Generator Mechanisms
Authors: Zhang Junfu , Du Xiaoping
Published: March 2011

A function generator mechanism links its motion output and motion input with a desired functional relationship. The probability of realizing such functional relationship is the kinematic reliability. The time-dependent kinematic reliability is desired because it provides the reliability over the time interval where the functional relationship is defined. But the methodologies of time-dependent reliability are currently lacking for function generator mechanisms. We propose a mean value first-passage method for time-dependent reliability analysis. With the assumption of normality for random dimension variables with small variances, the motion error becomes a nonstationary Gaussian process. We at first derive analytical equations for upcrossing and downcrossing rates and then develop a numerical procedure that integrates the two rates to obtain the kinematic reliability. A four-bar function generator is used as an example. The proposed method is accurate and efficient for normally distributed dimension variables with small variances.
An Experiment for the Study of Free-Flying Supercavitating Projectiles
Authors: Cameron Peter J. K. , Rogers Peter H. , Doane John W. , Gifford David H.
Published: March 2011

Applications and research utilizing supercavitation for high-speed underwater flight has motivated study of the phenomenon. In this work, a small scale laboratory experiment for studying supercavitating projectiles has been designed, built, and tested. Similar existing experimental work has been documented in literature but using large, elaborate facilities, or has been presented with ambiguous conclusions from test results. The projectiles were 63.5 mm in length and traveled at speeds on the order of 145 m/s. Measurement techniques are discussed and used to record projectile speed, supercavity dimensions, and target impact location. Experimental observations are compared with a six degrees-of-freedom dynamics simulation based on theoretical models presented in literature for predicting supercavity shape and hydrodynamic forces on the supercavitating projectile during flight. Experimental observations are discussed qualitatively, along with quantitative statistics of the measurements made. Successful operation of the experiment has been demonstrated and verified by agreement with theoretical models.
Theoretic Study of Efficiency of Two-DOFs of Epicyclic Gear Transmission via Virtual Power
Authors: Chen Chao , Liang Teck Teh
Published: March 2011

Epicyclic gear train is a fundamental form of mechanical transmission with broad applications. Efficiency study of these trains is critical to design, optimization, and operation. It is known that the efficiencies of these systems are highly related to the internal power flows. We apply the concept of virtual power to find analytical expression of the efficiency of a two degrees of freedom train, with associated applicable ranges. The results are verified by an example.
Two-Phase Micro-Channel Heat Sinks: Theory, Applications and Limitations
Authors: Mudawar Issam
Published: March 2011

Boiling water in small channels that are formed along turbine blades has been examined since the 1970s as a means to dissipating large amounts of heat. Later, similar geometries could be found in cooling systems for computers, fusion reactors, rocket nozzles, avionics, hybrid vehicle power electronics, and space systems. This paper addresses (a) the implementation of two-phase micro-channel heat sinks in these applications, (b) the fluid physics and limitations of boiling in small passages, and effective tools for predicting the thermal performance of heat sinks, and (c) means to enhance this performance. It is shown that despite many hundreds of publications attempting to predict the performance of two-phase micro-channel heat sinks, there are only a handful of predictive tools that can tackle broad ranges of geometrical and operating parameters or different fluids. Development of these tools is complicated by a lack of reliable databases and the drastic differences in boiling behavior of different fluids in small passages. For example, flow boiling of certain fluids in very small diameter channels may be no different than in macro-channels. Conversely, other fluids may exhibit considerable ‘confinement’ even in seemingly large diameter channels. It is shown that cutting-edge heat transfer enhancement techniques, such as the use of nano-fluids and carbon nanotube coatings, with proven merits to single-phase macro systems, may not offer similar advantages to microchannel heat sinks. Better performance may be achieved by careful optimization of the heat sink’s geometrical parameters and by adapting a new class of hybrid cooling schemes that combine the benefits of micro-channel flow with those of jet impingement.
Heat Transfer and Fluid Flow Characteristics Over Streamlined Symmetrical Body Positioned at Unfavorable Angles to the Flow
Authors: Nabhan Mohamed B. W.
Published: March 2011

Heat transfer rates and friction variations are experimentally presented for a horizontal pipe with the insertion of an airfoil made from aluminum alloy 6010-T4 of average roughness 0.4 μ m. The test foil is tested at two temperature conditions, cold and hot. Fully developed laminar airflow is maintained in the test duct section, which is connected to a sub-sonic wind tunnel. The airflow characteristics is determined at different flow speed and foil tilt positions. The heat transfer rate, drag force coefficient, and Reynolds number results show a strong dependence on the test foil angle positions. It’s found that the larger foil tilt angle position, the larger is the drag coefficient and the higher is the heat transfer rate. Infra red thermograph and computer simulation using Fluent code 6.3.26 are used to support the experimental work.
Phonon Scattering in Bi2Te3/Sb2Te3 Superlattice
Authors: Wang Yaguo , Xu Xianfan , Venkatasubramanian Rama
Published: March 2011

Thermoelectric materials are characterized with the figure of merit, ZT = S2σT/κ, where T is the temperature, S the Seebeck coefficient, σ the electrical conductivity and κ the thermal conductivity. Many researches have been focused on reducing lattice thermal conductivity through increasing phonon scattering at interfaces. Thin-film superlattices are one of the promising candidates for high ZT thermoelectric materials. Several theoretical models have been used to explain the large ZT in superlattice. One comes from the extra scattering channels at interfaces introduced by the hetero-structure. Another is a result of quantum confinement effect which reduces the phonon group velocity propagating perpendicularly through the superlattice layers through flattening the dispersion curve of acoustic phonons. In this work, ultrafast time-resolved measurements were conducted on Bi2Te3, Sb2Te3 and Bi2Te3/Sb2Te3 superlattice (SL) films to detect coherent acoustic phonons in these materials. Scattering of these phonons is revealed in the Bi2Te3/Sb2Te3 SLs, which comes from the interfaces of the hetero-structure in SL. Also, a decrease of acoustic phonon velocity resulted from folding and flattening of phonons branches is observed. Results show that both interface scattering and the reduced phonon velocity contribute to suppressing the heat transfer process.
Common Currency for System Integration of High Intensity Energy Subsystems
Authors: Pratt David M. , Moorhouse David J.
Published: March 2011

Aerospace vehicle design has progressed in an evolutionary manner, with certain discrete changes such as turbine engines replacing propellers for higher speeds. The evolution has worked very well for commercial aircraft because the major components can be optimized independently. This is not true for many military configurations which require a more integrated approach. In addition, the introduction of aspects for which there is no pre-existing database requires special attention. Examples of subsystem that have no pre-existing data base include directed energy weapons (DEW) such as high power microwaves (HPM) and high energy lasers (HEL). These devices are inefficient, therefore a large portion of the energy required to operate the device is converted to waste heat and must be transferred to a suitable heat sink. For HPM, the average heat load during one ‘shot’ is on the same order as traditional subsystems and thus designing a thermal management system is possible. The challenge is transferring the heat from the HPM device to a heat sink. The power density of each shot could be hundreds of megawatts. This heat must be transferred from the HPM beam dump to a sink. The heat transfer must occur at a rate that will support shots in the 10–100Hz range. For HEL systems, in addition to the high intensity, there are substantial system level thermal loads required to provide an ‘infinite magazine.’ Present models are inadequate to analyze these problems, current systems are unable to sustain the energy dissipation required and the high intensity heat fluxes applied over a very short duration phenomenon is not well understood. These are examples of potential future vehicle integration challenges. This paper addresses these and other subsystems integration challenges using a common currency for vehicle optimization. Exergy, entropy generation minimization, and energy optimization are examples of methodologies that can enable the creation of energy optimized systems. These approaches allow the manipulation of fundamental equations governing thermodynamics, heat transfer, and fluid mechanics to produce minimized irreversibilities at the vehicle, subsystem and device levels using a common currency. Applying these techniques to design for aircraft system-level energy efficiency would identify not only which subsystems are inefficient but also those that are close to their maximum theoretical efficiency while addressing diverse system interaction and optimal subsystem integration. Such analyses would obviously guide researchers and designers to the areas having the highest payoff and enable departures from the evolutionary process and create a breakthrough design.
Analysis of Heat Transfer Characteristics in Liquefaction Processes of CBM With High Nitrogen Content
Authors: Gao Ting , Lin Wensheng , Gu Anzhong
Published: March 2011

Liquefaction is suggested to be one of the best ways to utilize coalbed methane (CBM). Different from ordinary natural gas, CBM usually contains a high proportion of nitrogen, so the liquefaction system performance, especially the heat transfer characteristics, is different from that of the ordinary natural gas liquefaction processes, and will change along with the nitrogen content of CBM feed gas. Four liquefaction processes for nitrogen/methane mixture are evaluated. For the composition with nitrogen ranging between 0 and 70% and with the methane recovery rate of 95% as the restricted index, the heat transfer characteristics for the heat exchanger systems of these four processes are investigated and compared on the basis of the optimization results given by the previous studies. This investigation provides a direction for improvement of the liquefaction system performance, and gives reference for the selection of the liquefaction process for CBM with different nitrogen content.
Tissue-Light Interaction During Monitoring of Thermal Lesion Using Quantum Dot Mediated Fluorescence Thermometry
Authors: Hanson Willard , Abdollahzadeh Najma , Han Bumsoo
Published: March 2011

Thermal therapy, destroying tumor in situ by localized heating, is emerging as one of the treatment options for benign and localized tumors. Despite many advantages of thermal therapy, its clinical application is still limited due to the lack of a reliable intraoperative monitoring technique of the thermal lesion. To address this challenge, an intraoperative thermometry technique has been proposed using the temperature-dependent fluorescence of quantum dots (QDs). Its feasibility is recently demonstrated by monitoring the spatiotemporal temperature during gold nanoshell-mediated heating. In the present study, the effects of tissue-light interaction on the QD-mediated thermometry were investigated both experimentally and theoretically so that the technique can be extended to in vivo applications. As for experimental investigation, the QD fluorescence through tissue phantom was characterized with varying the thickness of the phantom over a temperature range relevant to thermal therapy. The results showed that the QD fluorescence through tissue phantom was still linearly correlated to the local temperature, but the slope of the correlations decreased with the phantom thickness. As for theoretical investigation, the radiative transfer equation was reduced to the diffusion approximation, and the QD fluorescence through tissue phantom was predicted by numerically solving the diffusion approximation. The results confirmed that the diffusion approximation could describe the tissue-light interaction for the QD-mediated thermometry but further research is still required to improve the accuracy of the prediction.
Schmidt-Boelter Heat Flux Gage Sensitivity Coefficients in Radiative and Convective Environments
Authors: Nakos James T. , Brown Alexander L.
Published: March 2011

Commercial Schmidt-Boelter heat flux gages are always calibrated by using a radiative heat flux source where convection is minimized. This is because one can establish a reliable link to a National Institute of Standards and Technology (NIST) calibration standard. To the authors’ knowledge, no NIST traceable link exists for convective heat flux calibration. When heat flux gages are used in typical applications, convection is often not negligible. It has been common practice to assume that the sensitivity coefficient supplied by the manufacturer also applies for convective environments. This assumption is believed to be incorrect. If incorrect, this would result in uncertainties larger than typically reported (e.g., ±3%). This paper analyzes the heat transfer from an idealized Schmidt-Boelter heat flux gage. The analysis shows that the theoretical sensitivity coefficients in purely radiative and convective environments are not the same and, in fact, differ by the emissivity of the gage surface. The implication of this difference is that the accuracy specification supplied by the manufacturer (typically ± 3%) is not correct for measurement applications where convection is not negligible.
Modelling of Combined Heat and Mass Transfer of Water Droplets in Thermal Technology Equipment
Authors: Miliauskas Gintautas , Sinkunas Stasys , Norvaisiene Kristina , Sinkunas Kestutis
Published: March 2011

Water droplet evaporation process is numerically modelled under various heat and mass transfer conditions. Regularities of heat transfer process interaction are examined. Modelling in this work was performed using the combined analytical–numerical method to investigate heat and mass transfer in the two-phase droplets-gas flow system. The influence of forced liquid circulation on the thermal state of droplets is taken into account by the effective coefficient of thermal conductivity. Calculating the rate of droplet evaporation and the intensity of convective heating, the influence of the Stefan’s hydrodynamic flow is taken into account. Balancing energy fluxes in the droplet to within one hundredth of a percent and using the fastest sinking the droplet surface temperature is determined.
Surrogate-Based Modeling and Dimension-Reduction Techniques for Thermo-Fluid and Energy Systems
Authors: Cho Young-Chang , Du Wenbo , Gupta Amit , Tseng Chien-Chou , Sastry Ann Marie , Shyy Wei
Published: March 2011

Successful modeling and/or design of thermo-fluid and energy systems often requires one to address the impact of multiple “design variables” on the prescribed outcome. There are often multiple, competing objectives based on which we assess the outcome of optimization. Since accurate, high fidelity models are typically time consuming and computationally expensive, comprehensive evaluations can be conducted only if an efficient framework is available. Furthermore, informed decisions of model/hardware’s overall performance rely on an adequate understanding of the global, not local, sensitivity of the individual design variables on the objectives. The surrogate-based approach, which involves approximating the objectives as continuous functions of design variables from limited data, offers a rational framework to reduce the number of important input variables, i.e., the dimension of a design or modeling space. In this paper, we discuss the fundamental issues that arise in surrogate-based analysis and optimization, highlighting concepts, methods, techniques, as well as practical implications. To aid the discussions of the issues involved, we will summarize recent efforts in investigating cryogenic cavitating flows, active flow control based on dielectric barrier discharge concepts, and Li-ion batteries.
An Experimental Study on Properties of Local Burning Velocity for Hydrogen Added Hydrocarbon Premixed Turbulent Flames
Authors: Nakahara Masaya , Murakami Koichi , Hashimoto Jun , Ishihara Atsushi
Published: March 2011

This study is performed to investigate directly the local flame properties of turbulent propagating flames at the same weak turbulence condition (u′/SL0 = 1.4), in order to clarify basically the influence of the addition of hydrogen to methane or propane mixtures on its local burning velocity. The mixtures having nearly the same laminar burning velocity with different rates of addition of hydrogen δH are prepared. A two-dimensional sequential laser tomography technique is used to obtain the relationship between the flame shape and the flame displacement. The local flame displacement velocity SF is quantitatively obtained as the key parameters of the turbulent combustion. Additionally, the Markstein number Ma was obtained from outwardly propagating spherical laminar flames, in order to examine the effects of positive stretch and curvature on burning velocity. It was found that the trends of the mean values of measured SF with respect to δH, the total equivalence ratio Φ and fuel types corresponded well its turbulent burning velocity. The trend of the obtained Ma could explain the local burning velocity of turbulent flames only qualitatively. Based on the Ma, the local burning velocity at the part of turbulent flames with positive stretch and curvature, SLt, is estimated quantitatively. As a result, a quantitative relationship between the estimated SLt and the SF at positive stretch and curvature of turbulent flames could be observed for mixtures with increasing the Lewis number.
Pool Boiling Heat Transfer of Highly Wetting Fluids on Smooth Metallic Surfaces
Authors: Bon Bradley , Klausner James
Published: March 2011

The nucleate pool boiling heat transfer characteristics of smooth metallic surfaces were investigated using two highly wetting fluids. Perfluoro-n-hexane and n-hexane were chosen as the test fluids. Two single crystal metal surfaces (Copper and Aluminum) of two different crystal plane orientations in addition to two polycrystalline metal surfaces (Nickel and Titanium) were studied. Three different orientations of a non-metal surface (Silicon) were also studied. Heat transfer is seen to increase with increasing thermal conductivity for both single crystal and polycrystalline surfaces. For the single crystal metal surfaces, heat transfer slightly increases with decreasing planar density. For the silicon surfaces heat transfer increases with increasing planar density. Film boiling at incipience was observed for some of the surfaces. A plausible criterion for predicting the onset of film boiling at incipience is discussed.
Thermal Properties of Polyurethane Shape Memory Polymer Filled With Magnetite Particles
Authors: Weidenfeller Bernd , Anhalt Mathias , Marquardt Hauke , Schilling Frank R. , Razzaq Muhammad Y. , Frormann Lars
Published: March 2011

Temperature dependent thermal diffusivity (295K ≤ T ≤ 375K), specific heat capacity (290K ≤ T ≤ 380K) and thermal conductivity (300K ≤ T ≤ 340K) were measured on extrusion compounded and injection molded polyurethane shape memory polymers filled with different volume fractions (0%, 10%, 20%, 30%, 40%) of magnetite particles (10μm, 50μm and 150μm). With increasing particle content thermal diffusivity arises from α(PU + 0% Fe3O4) ≈ 0.13mm2/s to α(PU + 40% Fe3O4) ≈ 0.31mm2/s whereas d = 10μm particle sizes lead to higher values than larger particle sizes. Values measured for 150μm large particles are lying between values of composites with 10μm and 50μm particle sizes in the whole investigated temperature range. For higher filler contents differences in thermal diffusivity between composites of different particle sizes disappear. Thermal diffusivity decreases with increasing temperature, while thermal conductivity is increasing from λ(PU+0% Fe3O4) ≈ 0.2W/mK to λ(PU+40% Fe3O4) ≈ 0.6W/mK. Corresponding to glass transition temperatures of the polymer, the specific heat capacity shows a rise between 300K and 320K and a decrease between 350K and 370K.
Lower Limit of Homogeneous Nucleation Boiling Explosion in Water During Linear Boundary Heating and Contact With High Temperature Surface
Authors: Hasan Mohammad Nasim , Monde Masanori , Mitsutake Yuichi
Published: March 2011

The lower limit for the occurrence of homogeneous nucleation boiling explosion during water heating at atmospheric pressure has been determined by applying a new theoretical model proposed by the authors. Two different cases of liquid heating have been considered for the study of homogeneous nucleation boiling explosion. In one case, the liquid on the surface is linearly heated at a rate of 10 K/s to 109 K/s. In another case, the liquid suddenly contacts with a high temperature surface such as in case of quenching with jet impingement or droplet. For the linear boundary heating case, the liquid temperature limit at which homogeneous boiling explosion occurs without any cavity or surface effect, essentially corresponds to a value of 302 °C even though the surface is heated very slowly. On the other hand, during water contact with hot surfaces, the occurrence of the homogeneous boiling explosion within a characteristic time period of 1 millisecond is obtained at a maximum liquid temperature of 303 °C for a limiting steady state boundary temperature of about 304 °C. From the definition of the steady-state interface boundary temperature of two 1-D semi-infinite body contact, the lower limiting surface temperatures for the occurrence of the homogeneous nucleation boiling explosion have been determined for water contact with various solid surfaces with different liquid initial temperatures ranging from 0 °C to 100 °C. The effects of the parametric variation in the boundary heating conditions on various characteristics of the homogeneous boiling explosion such as liquid temperature and time of boiling explosion, heat-flux across the liquid-vapor interface at the boiling explosion etc. are also determined and discussed in context with other results available in the literature.
Correlation Between Thermal Diffusivity and Dynamic Mechanical Properties of Soft Magnetic Particle Filled Thermoplastic Composites
Authors: Kirchberg S. , Anhalt M. , Weidenfeller B.
Published: March 2011

Measurements of thermal diffusivity α by laser flash method (LFA) and storage modulus E′ by dynamic mechanical analysis (DMA) have been performed on polypropylene-iron silicon (PP/FeSi6.8) composites with filler particle content from 10 vol.-% to 60 vol.-% at temperatures from 300 K to 415 K and 200 K to 425 K, respectively. The thermal diffusivity induces a decline with increased temperature for all examined materials. The drop in thermal diffusivity versus temperature corresponds with the filler fraction. Observing the behavior of the storage modulus of FeSi6.8 filled PP one can show that the larger the filler fraction of particles in the polymer the stronger is the decrease of storage modulus versus temperature. The inflection point of the dynamic mechanical measurement curve at about 360 K corresponds very well with starting increase of thermal conductivity. The achieved results can be explained by the relation of the velocity v of phonons and its free path length l to the thermal diffusivity α according to α = (1/3)vl (Einstein approximation). Moreover, v is correlated to the bulk modulus K (and specific density ρ) via v ≈ (K/ρ)0.5.
Numerical Optimization of the Casting of Ceramic Material EUCOR
Authors: Kavicka Frantisek , Dobrovska Jana , Stransky Karel , Sekanina Bohumil , Stetina Josef
Published: March 2011

Corundo-baddeleyit material (CBM) – EUCOR – is a heat- and wear-resistant material even at extreme temperatures. This article introduces a numerical model of solidification and cooling of this material in a non-metallic mold. The model is capable of determining the total solidification time of the casting and also the place of the casting which solidifies last. Furthermore, it is possible to calculate the temperature gradient at any point and time, and also determine the local solidification time and the solidification interval of any point. The local solidification time is one of the input parameters for the cooperating model of chemical heterogeneity. This second model and its application on EUCOR samples prove that the applied method of measuring the chemical heterogeneity provides the detailed quantitative information on the material structure and makes it possible to analyze the solidification process. The analysis of this process entails statistical processing of the measurement results of the heterogeneity of the EUCOR components and performs the correlation of individual components during solidification. The verification of both numerical models was conducted on a real cast 350 × 200 × 400 mm block.
Direct Simulation of the Nonlinear Boltzmann Transport Equation for Phonons
Authors: Masao Yusuke , Matsumoto Mitsuhiro
Published: March 2011

In order to solve a Boltzmann transport equation (BTE) of phonons for investigating heat conduction in non-metallic solids, we propose to employ a DSMC (direct simulation Monte Carlo) scheme to simulate dynamics of phonons in analogy with rarefied gas. In this paper, we describe the DSMC scheme for phonon dynamics and present some results with our prototype codes for a face-centered cubic model. The dynamics of phonons with two branches of acoustic modes is discussed, in the case where the distribution of phonons in strong nonequilibrium situation is driven into equilibrium.
Extension of the Full Cavitation Model for Cryogenic Fluids
Authors: Yang H. Q.
Published: March 2011

The initial cavitation model, originally reported in 2000, has been successfully implemented within several CFD codes (CFD-ACE+, Fluent, ...) and applied throughout a variety of industrial applications. The objective of this study was to extend this model into cryogenic fluids and demonstrate its capability to simulate cryogenic propellant fed turbo pumps associated with liquid rocket engines. In the modeling approach described here the energy equation was modified to include the phase change effects due to cavitation. This necessitated variable physical properties, including vapor pressure, liquid density, vapor density, thermal conductivity, specific heat, fluid viscosity, surface tension, and evaporation heat for the cryogenic fluids. These properties were extracted from NIST tables which were implemented into the CFD-ACE+ computer code. Customized solution algorithm treatments were necessary to ensure robust numerical convergence as a result of the large variations in vapor pressure with respect to temperature typically present in cryogenic fluid pumping and diffusion phenomena. The Hord’s experiment was chosen for the validation as a result of its complete and comprehensive measurement set availability in the open literature. The calculated pressure and temperature profiles along a hydro foil, at different flow conditions for both nitrogen and hydrogen fluids, were compared with experimental measurements. Without any modification to the empirical constants, originally calibrated with water, the predicted results matched remarkably well with the experimental data. This effort has established a new standard compared to previous predictions for liquid nitrogen. For liquid hydrogen flows a significant improvement in the temperature recovery region compared to previous predictions is described and provides for an excellent baseline effort to improve upon.
Characterization of Hybrid-Wicked Copper Heat Pipe
Authors: Dai Xianming , Tran Levey , Yang Fanghao , Shi Bo , Yang Ronggui , Lee Y. C. , Li Chen
Published: March 2011

Thermal management of high power electronics is becoming a critical issue as the power density of semiconductors increasing. The flat heat pipe (FHP) is widely used in the electronic cooling because it is possible to interface with flat electronics packages without additional conductive and interface resistances. The heat flux of the next generation electronics may exceed 100 W/cm2, which is significantly beyond the cooling capabilities of commercially available FHP today. A novel micro scale hybrid wick was developed in this study to improve the effective thermal conductivity and working heat flux of FHP. The hybrid wick consists of multilayer of sintered copper woven meshes to promote the capillary pressure and microchannels underneath to reduce the flow resistance. The analysis indicates that the effective thermal conductivity and the capillary limit of flat heat pipe (FHPs) with this novel micro scale hybrid wicking structure can be significantly enhanced as compared to the reported FHPs. In this paper, the design of this innovative micro scale hybrid wick is illustrated. The fabrication and charging processes are also outlined. The preliminary experimental results show that the effective thermal conductivity can approach 12,270 W/(m·K), which is more than 30 times better than pure copper at approximate 91.3 W input heat.
Raman and Infrared Thermometry for Microsystems
Authors: Phinney Leslie M. , Lu Wei-Yang , Serrano Justin R.
Published: March 2011

This paper compares measurements made by Raman and infrared thermometry on a SOI (silicon on insulator) bent-beam thermal microactuator. Both techniques are noncontact and used to experimentally measure temperatures along the legs and on the shuttle of the thermal microactuators. Raman thermometry offers micron spatial resolution and measurement uncertainties of ±10 K; however, typical data collection times are a minute per location leading to measurement times on the order of hours for a complete temperature profile. Infrared thermometry obtains a full-field measurement so the data collection time is much shorter; however, the spatial resolution is lower and calibrating the system for quantitative measurements is challenging. By obtaining thermal profiles on the same SOI thermal microactuator, the relative strengths and weaknesses of the two techniques are assessed.
Excitation of Single Phonon Modes in Nanoscale Waveguides
Authors: Cheney Drew A. , Lukes Jennifer R.
Published: March 2011

We present a new computational method that excites guided phonon modes in nanoscale waveguides at a specific frequency and wavenumber. The method uses nonequilibrium molecular dynamics and Fourier analysis of particle displacements to extract mode shapes from single frequency excitations consisting of superposed spatial modes. These mode shapes are used to excite the waveguide inlet boundary so that single phonon modes are generated in the structure. Mode shapes and phonon spectra for a silicon planar waveguide with rigid wall boundaries are calculated to demonstrate the viability of the technique. This method improves upon molecular dynamics techniques that activate all possible phonon modes and are thus not able to isolate the contribution of any single phonon excitation. Application of our method will enable the computational investigation of single phonon mode propagation in nanostructures of varying geometry.
Neutron Imaging Feasibility of Liquid Metal Coolant Behaviors Inside a High-Temperature Alloy Heat Pipe
Authors: Kihm K. D. , Hussey D. , Pratt D. M. , Swanson A. D.
Published: March 2011

Experimental study has been conducted for multiscale characterization of liquid metal (Na) transport within a heat pipe using a neutron imaging technique. Feasibility of nonintrusive imaging of liquid sodium contained inside a nickel alloy heat pipe has been examined for the imaging resolution, exposure time requirements, background noises, and other basic considerations associated with neutron beam scattering. Preliminary results of neutron imaging show fairly acceptable feasibility of neutron imaging of liquid sodium contained inside an alloy jacket. Furthermore, challenges are identified to enhance the spatial and thermal resolutions in order to achieve more physically demanding imaging of the liquid metal thermal transport behaviors.
Effect of the Bottom and Top Configurations on Pool Film Boiling Around a Vertical Finite-Length Cylinder
Authors: Momoki Satoru , Araki Kenichi , Shigechi Toru , Yamada Takashi , Toyoda Kaoru , Yamaguchi Tomohiko , Odgerel Jambal
Published: March 2011

The bottom configuration of a vertical finite-length cylinder is an important factor to examine the convective heat transfer by film boiling around a vertical finite-length cylinder, as the vapor generated under the bottom surface grows thicker during flowing upward along the vertical lateral surface and finally leaves the top surface as bubbles. In this study, four types of silver cylinder with a vertical lateral length equal to the diameter of 32mm were prepared for the possible combinations of bottom and top configurations: with a flat bottom and a flat top, with a flat bottom and a curved top, with a curved bottom and a flat top, and with a curved bottom and a curved top, where “flat” refers to “horizontal” and “curved” to “convex hemispherical”. Quenching experiments have been carried out for the test cylinders for saturated and subcooled water at atmospheric pressure. The initial temperature in the measurement is 600 °C. Boiling curves were obtained from the cooling curves measured using a K-type thermocouple inserted near the center on the axis of the test cylinder and the film boiling process was observed by still and high speed video cameras. The following results were obtained from the experiments using four types of test cylinder. 1. For saturated water, the test cylinders are entirely covered with a thick continuous vapor film, however, the effect of bottom configuration on film boiling heat transfer is appeared within 18% in terms of the wall heat flux averaged over the entire surface depending on the vapor fluid flow on the bottom and vertical lateral surfaces. 2. For the cylinders with a flat bottom surface, the wall heat flux averaged over the entire surface increases significantly with an increase in liquid sub cooling. This is attributed to that the convective heat transfer and the surface area ratio on the vertical lateral surface are predominant and govern the total heat transfer. 3. The effects of the cylinder top configurations on the film boiling heat transfer are small as the heat transfer on the top surface is small compared with that on the vertical lateral surface. 4. The differences between film boiling characteristics due to the bottom and top configurations are explained by examining the average heat transfer coefficient composed of the heat transfer coefficient and the surface area ratio on each surface. 5. The minimum wall superheat corresponding to the vapor-film-collapse is almost constant at 133K for four types of test cylinder in saturated water. In subcooled water, the minimum wall superheat for the cylinders with a flat bottom surface is larger than that for the cases with a convex hemispherical bottom surface.
Modified Magnesium Hydride and Calcium Borohydride for High-Capacity Thermal Energy Storage
Authors: Amama Placidus B. , Spowart Jonathan E. , Voevodin Andrey A. , Fisher Timothy S.
Published: March 2011

MgH2 and Ca(BH4)2 are potential thermal energy storage (TES) materials that possess extraordinarily high inherent thermal energy densities of up to 2 MJ/kg. However, the high desorption temperatures at atmospheric pressure [>300°C for Ca(BH4)2, >400°C for MgH2] coupled with slow kinetics represent significant challenges for their use in TES. In order to address these challenges, the present work focuses on the development of new modification approaches based on nanostructuring via high-energy vibratory ball milling and catalytic enhancement using pure Ni and Ni alloys. Our work reveals that high-energy vibrating-mill technique with ball-to-powder weight ratio as low as 13:1can produce MgH2 powders with nanocrystallites after 2h of milling. MgH2 milled with Ni (5 wt%) and Ni5Zr2 (5 wt%) catalysts for 2 h showed apparent activation energies, EA of 81 and 79 kJ/mol, respectively, corresponding to ∼50% decrease in EA and ∼100°C decrease in the decomposition temperature (Tdec). On the other hand, the decomposition reaction of Ca(BH4)2 does not seem to be catalyzed by the Ni-based catalysts tested.
Estimating Density Reduction and Phonon Localization From Optical Thermal Conductivity Measurements of Porous Silica and Aerogel Thin Films
Authors: Hopkins Patrick E. , Kaehr Bryan J. , Dunphy Darren , Brinker C. Jeffrey
Published: March 2011

In this work, we measure the thermal conductivity of mesoporous silica and aerogel thin-films using a non-destructive optical technique: time domain thermoreflectance (TDTR). Due to the rough surfaces of the optically transparent silica-based films, we evaporate an Al film on a glass cover slide and fabricate the silica structures directly on the Al film, providing a “probe-through-the-glass” configuration for TDTR measurements. This allows the thermal conductivity of mesoporous silica and aerogel thin films to be measured with traditional TDTR analyses. As the thermoreflectance response is highly dependent on the thermal effusivity of the porous structures, we estimate the density of the films by varying the heat capacity in our analysis. This density determination assumes that the solid matrix in the silica structure has the thermal conductivity as bulk SiO2, which is valid if all the lattice vibrations are localized, consistent with the minimum thermal conductivity concept. We independently determine the density of the porous silica films with nitrogen sorption measurements of thin films using a surface acoustic wave (SAW) technique. The difference between the determined from the SAW technique and that estimated by the TDTR effusivity analysis lends insight into the relative contributions of localized and propagating modes to thermal transport.
Simulation of Heat and Mass Transfer Involving Vapor Condensation in the Presence of Non-Condensable Gases in Plane Channels
Authors: Saraireh Mohammad , Thorpe Graham , Li Jun-De
Published: March 2011

Results from computational fluid dynamics (CFD) simulations of heat and mass transfer involving the condensation of vapor in the presence of non-condensable gases in plane channels are presented. The simulations were carried out using FLUENT®. Convective heat and mass transfer and vapor condensation at a constant wall temperature were first investigated with the aim of comparing the CFD results with well established correlations. CFD simulations of heat and mass transfer and water vapor condensation in the presence of non-condensable air were then carried out for constant heat transfer coefficients for the condensation wall and coolant with different mass fractions of water vapor and inlet velocities. The predictions obtained from this are compared with experimental data and reasonable agreement has been found for the condensation rates of water vapor and heat flux. Finally, the condensation of the water vapor was simulated in a heat exchanger including both the cooling water and vapor-air mixture channels separated by solid walls. This simulation is close to reality and no assumptions are required for the temperature or heat transfer coefficient at the condensing wall. The difficulties of simultaneously simulating a gas mixture and liquid flowing in separate channels using commercially available CFD software are discussed and strategies to overcome these difficulties are outlined. Preliminary results from this third simulation will also be presented and compared with available experimental results.
Field Experiment of Top-Heat Thermosyphon
Authors: Hagino Naoto , Yandri Erkata , Yoshida Hiro
Published: March 2011

Three types of thermosyphon were tested. Type-1 has a horizontal condenser with a 536ml volume buffer chamber. The vertical heat transfer height was 3m. In type-2, condenser and connecting pipe were tilted by 10° to the ground with 3776ml buffer chamber. In type-3, only condenser was tilted and the others were the same as type-2. In type-2 and -3, the vertical heat transfer height was 4 m. The maximum coefficient of the heat transfer was 60% for type-1 and 40% for type-2 and -3. Detailed difference in performance of type-1, -2, and -3 were not elucidated because of the limited field data. However, the present results indicated the promising capability of thermosyphon without using circulation pumps. Optimizing the performance of the thermosyphon was left for the future study.
Flushing of Non-Condensable Gases by Saturated Steam
Authors: Patel Rajesh , Patel Raivat , Zhu Chao , Lin Chao-Hsin
Published: March 2011

This paper presents a study of flushing non-condensable (NC) gases out of a chamber by a saturated steam flow. During the flushing and mixing process, significant heat transfer occurs among the NC gases, steam and the chamber wall, with a coupled steam condensation. The flushing effectiveness hence is strongly dependent upon the mixing, condensation characteristics, the steam feeding rate as well as the thermal capacity of wall. The objective of this study is to explore modeling approaches on such a process which would be applied to assist the optimization of the process design and operation. An experimental system has been developed to provide a set of data for model validations. A simple mechanistic model based has also been developed to show the “equilibrium-based” flushing characteristics. However, to account for finite rate of heat and mass transfer and non-uniform mixing, a more complicated full-field computational fluid dynamics modeling and simulation must be involved. The typical boundary conditions in most commercial CFD codes (such as FLUENT) cannot be directly applied to the flushing processes due to the coupled surface condensation. Hence, in this paper, we have also proposed the condensation-based boundary conditions for the CFD simulations. Full-field CFD simulations with those boundary conditions are being investigated.