bluuebunny/crossref_metadata_2025 huggingface dataset:
Thermal Property Measurements of Carbon Nanotube Forest Synthesized by Thermal CVD Process
Authors: Cai Qingjun , Chen Bing-chung , Zhao Yuan , Mack Julia , Ma Yanbao , Chen Chung-lung , Wang Hengzhi , Ren Zhifeng
Published: June 2010

Carbon nanotube (CNT) forest/cluster synthesized by a thermal CVD process has millimeter growth height, large porosity and nano level pore size, plus high thermal conductivity of individual CNT, thus it is potentially a good wick structure material in developing micro heat transfer devices. However, thermal properties, including effective thermal conductivity (ETC) of a bulky CNT layer, may not be as good as the common metallic wick materials. In this paper, a Netzsch DSC 404 C Pegasus is used for measurement of the CNT heat capacity. CNT volume density is obtained by measuring the ratio of a bulky CNT weight and volume. Both the laser flash and 3-omega measurement methods are employed to measure ETC for CNT wick structures synthesized by the thermal CVD processes. For the laser flash method, measurement deviations caused by reflective silicon and thin substrate are corrected by surface treatment and increased sample thickness. Measurement results of the laser flash indicate that a 600μm thick CNT layer has ETC varying from 0.7–1.2W/m.K. For the 3-omega approach, the measurement system is validated on a quartz substrate. However, the test results yield larger ETC on 250μm CNT samples. Geometric and one dimensional thermal conduction analysis indicate that the bulky CNT thermal properties are tied to CNT synthesis processes. ETC of bulky CNT layer can be enhanced by straightening CNT growth and increasing CNT growth volume density.
Investigations of High Heat Flux Cooling Using Carbon Nanotube Bi-Wick Structure and Integrated Platinum Thermometer/Heater
Authors: Cai Qingjun , Zhao Yuan , Tsai Chialun , Chen Chung-lung
Published: June 2010

With the increase of power consumption in compact electronic devices, passive heat transfer cooling technologies with high heat flux characteristics are highly desired in microelectronics industries. Carbon nanotube (CNT) cluster/forest has high effective thermal conductivity, nano pore size and large porosity, which can be used as wick structure in a heat pipe heatspreader and provides high capillary force for high heat flux thermal management. In this research, investigations of high heat flux cooling of the CNT bi-wick structure are associated with the development of a reliable thermometer and high performance/interface free heater. A 100nm thick and 600μm wide Z-shaped platinum wire resistor is fabricated on the backside of a CNT sample substrate to heat a 2×2mm2 wick area. As a heater, it provides direct heating effect without thermal interface and is capable of over 800°C high temperature operation. As a thermometer, reliable temperature measurement is achieved by calibrating the resistance variation with temperature after the annealing process is applied. The CNT sample substrate is silicon. The backside of the silicon substrate is thermally oxidized to create a 2μm thick and pinhole-free SiO2 layer so that the platinum heater and thermometer can survive from the server CNT growth environments and without any electrical leakage. For high heat flux cooling, the CNT bi-wick structure is composed of 250μm tall, 100μm wide stripe-like CNT clusters and 50μm empty space. Using 1×1cm2 CNT bi-wick samples, experiments are completed in both the open and saturated environments. Testing results of CNT bi-wick structure demonstrate 600W/cm2 heat transfer capacity and good thermal & mass transport characteristics in the nano level porous media.
One-Dimensional Simulation of a Stirling Three-Stage Pulse-Tube Refrigerator
Authors: Etaati M. A. , Mattheij R. M. M. , Tijsseling A. S. , de Waele A. T. A. M.
Published: June 2010

A one-dimensional mathematical model is derived for a three-stage pulse-tube refrigerator (PTR) that is based on the conservation laws and the ideal gas law. The three-stage PTR is regarded as three separate single-stage PTRs that are coupled via proper junction conditions. At the junctions there are six fluid flow possibilities each defining its own boundary conditions for the adjacent domains. Each single stage cools down the gas in the regenerator to a lower temperature such that the system reaches its lowest temperature at the cold end of the third stage. The velocity and pressure amplitudes are decreasing towards the higher stages and there is an essential phase difference between them at different positions. The system of coupled PTRs is solved simultaneously first for the temperatures and then for the velocities and the regenerator pressures. The final result is a robust and accurate simulation tool for the analysis of multi-stage PTR performance.
Natural Convective Heat Transfer From an Array of Square Vertical Plates Mounted on a Large Plane Vertical Adiabatic Surface and With One High Heat Flux Element
Authors: Oosthuizen Patrick H. , Paul Jane T.
Published: June 2010

An array of nine square heated elements mounted in a square three-by-three pattern with no gap between the elements on a large vertical adiabatic surface with natural convective flow over the elements has been considered. Each of the elements has a uniform heat flux over its surface, the heat fluxes over eight of the elements being the same and the heat flux over the ninth element being higher than that over the other eight elements. The basic aim of the study was to determine the effect the position of the higher heat flux element on the mean temperatures of the other eight elements. The situation considered is an approximate model of situations that can arise in electronic cooling. The flow has been assumed to be steady and laminar and it has been assumed that the fluid properties are constant except for the density change with temperature which gives rise to the buoyancy forces, this having been treated by using the Boussinesq approach. The solution has been obtained by numerically solving the full three-dimensional form of the governing equations, these equations being written in terms of dimensionless variables using the commercial cfd code FLUENT. The solution has the heat flux Rayleigh number, the Prandtl number, the ratio of the heat flux over the high heat flux element to the heat flux over the other eight elements, and the position of the high heat flux element as parameters. Because of the application that motivated this work results have only been obtained for Pr = 0.7. Results have been obtained for a wide range of values of the other input parameters and the effect of these parameter values on the mean surface temperatures of each of the elements has been studied.
Natural Convective Heat Transfer From an Inclined Isothermal Square Cylinder With an Exposed Top Surface Mounted on a Flat Adiabatic Base
Authors: Kalendar Abdulrahim , Oosthuizen Patrick H.
Published: June 2010

Natural convective heat transfer from an isothermal inclined cylinder with a square cross-section and which has an exposed top surface and is, in general, at an angle to the vertical has been numerically studied. The cylinder is mounted on a flat adiabatic base plate, the cylinder being normal to the base plate. The situation considered is an approximate model of that which occurs in some electrical and electronic component cooling problems. The flow has been assumed to be steady and laminar and it has been assumed that the fluid properties are constant except for the density change with temperature which gives rise to the buoyancy forces, this having been treated by using the Boussinesq approach. The solution has been obtained by numerically solving the governing equations, these equations being written in terms of dimensionless variables using the height, h, of the cylinder as the length scale and Tw – TF as the temperature scale, TF being the undisturbed fluid temperature far from the cylinder and Tw being the uniform surface temperature of the cylinder. These dimensionless governing equations subject to the boundary conditions have been solved using the commercial cfd solver, FLUENT. The flow has been assumed to be symmetrical about the vertical center-plane through the cylinder. The solution has been used to derive the values of the mean Nusselt number for the cylinder, Nu. The solution has the following parameters: the Rayleigh number, Ra, the dimensionless cylinder width, i.e., the ratio of the width to the height of the heated cylinder, W = w/h, the Prandtl number, Pr, and the angle of inclination of the cylinder relative to the vertical, φ. Results have only been obtained for Pr = 0.7. Values of φ between 0° and 180° and a wide range of Ra and W have been considered. The effects of W, Ra, and φ on the mean Nusselt number, Nu, for the entire cylinder and for the mean Nusselt numbers for the various surfaces that make up the cylinder have been examined.
Measurement of the Thermal Conductivity of a Water-Based Single-Wall Carbon Nanotube Colloidal Suspension With a Modified 3-ω Method
Authors: Choi Tae Y. , Maneshian Mohammad H. , Kang Boseon
Published: June 2010

A modified 3-ω method applied to a suspended platinum microwire was employed to measure the thermal conductivity and convective heat transfer coefficient of water-based single-walled carbon nanotubes (CNT) solution, and an expression for calculating the convective heat transfer coefficient in a free convective fluid was introduced. The measurement technique was validated for three model systems including vacuum, air, and deionized water. It is found that there is excellent agreement of these three model systems with theoretical predictions. In addition, the frequency dependence on the third harmonic response measured in deionized water reveals existence of a very low working frequency below 60 mHz. The thermal conductivity and convective heat transfer coefficient of a nanofluid (water-based single wall CNTs colloidal suspension) were determined to be 0.73±0.013 W/m·K and 14900±260 W/m2·K respectively, which corresponds to enhancement of 19.4% in thermal conductivity and 18.9% in convective heat transfer as compared to water.
A Meshless Finite Difference Method for Conjugate Heat Conduction Problems
Authors: Varanasi Chandrashekhar , Murthy Jayathi Y. , Mathur Sanjay
Published: June 2010

In recent years, there has been a great deal of interest in developing meshless methods for computational fluid dynamics (CFD) applications. In this paper, a meshless finite difference method is developed for solving conjugate heat transfer problems in complex geometries. Traditional finite difference methods (FDMs) have been restricted to an orthogonal or a body-fitted distribution of points. However, the Taylor series upon which the FDM is based is valid at any location in the neighborhood of the point about which the expansion is carried out. Exploiting this fact, and starting with an unstructured distribution of mesh points, derivatives are evaluated using a weighted least squares procedure. The system of equations that results from this discretization can be represented by a sparse matrix. This system is solved with an algebraic multigrid (AMG) solver. The implementation of Neumann, Dirichlet and mixed boundary conditions within this framework is described, as well as the handling of conjugate heat transfer. The method is verified through application to classical heat conduction problems with known analytical solutions. It is then applied to the solution of conjugate heat transfer problems in complex geometries, and the solutions so obtained are compared with more conventional unstructured finite volume methods. Metrics for accuracy are provided and future extensions are discussed.
Thermodynamics-Based Object-Oriented Environmental Design of an Industrial Air Conditioner
Authors: Shah Amip , Lugo Wilfredo , Patel Chandrakant
Published: June 2010

Continued efforts to reduce the environmental impact of products and services are leading to the increased prevalence of life-cycle assessment (LCA) during the design phase. A key challenge in traditional process-based LCA is the validation of vast life-cycle inventory data, which easily numbers in the tens of thousands of data points. In this paper, we utilize an ‘object-based’ approach from software engineering to manage the large amount of data and combine this approach with simple thermodynamic principles, thus significantly reducing the chance of error in LCA inputs. The approach is demonstrated for the test case of a typical industrial air-conditioning unit. We find that implementation of the object-based approach identifies numerous potential errors in the life-cycle inventory data. Thus, the proposed approach enables the designer to perform a more robust environmental analysis of a given system. This facilitates a better identification and prioritization of opportunities for reducing the environmental footprint across the system life-cycle.
Modeling of Pressure Evolution During Nanosecond Laser Ablation of Metal Films
Authors: Hendijanifard Mohammad , Willis David A.
Published: June 2010

Nanosecond laser ablation is studied using a theoretical model combined with experimental data from laser ablation of metal films. The purpose of the research is to obtain the recoil pressure boundary condition resulting from explosive phase change. The ablation experiments are performed using a Nd:YAG laser of 1064 nm wavelength and 7 ns pulse width at full width half maximum. Three samples, 200 and 1000 nm aluminum films and 1000 nm nickel films, are used in the experiments. The transient shock wave positions are obtained by a time-resolved shadowgraph technique. A N2-laser pumped dye laser with 3 ns pulse width is used as an illumination source and is synchronized with the ablation laser to obtain the transient shock wave position with nanosecond resolution. The transient shock position is used in a model for finding the shock wave speed as well as the pressure, temperature, and velocity just behind the shock wave. A power law is used for fitting curves on the experimentally obtained shock wave position. Knowing the shock wave position, the normal shock equations are used to calculate the thermo-fluid properties behind the shock wave. The solutions are compared with the Taylor-Sedov solution for spherical shocks and the reason for the deviation is described. The thermo-fluid property results show similar trends for all tested samples. The results show that the Taylor-Sedov solution under-estimates the pressure behind the shock wave when compared to the normal shock results.
Theoretical Modeling for Impingement Cooling of Large Scale Coils of Aluminium
Authors: Smith Graham D. J.
Published: June 2010

The use of a cylindrical array of air jets to cool a 4.1 tonne coil of Aluminium alloy is shown, theoretically, to improve the free convection cooling time over the 673K (752°F) to 323K (122°F) range from 49hrs to roughly 4hrs. The transient theory developed to model this process includes both radiation and convection heat loss in a numerical solution which is applicable to large coils of aluminium typically produced on a hot strip mill. In situations where liquid spray chambers are not chosen as an alternative cooling method, the results indicate that air impingement jets are a viable technique for cooling unscheduled coil batches which require rapid handling to meet market demands. When comparing the various combinations of surfaces of the coil that may be cooled, impingement cooling the entire surface area (cooling time 3.75hrs) offers little advantage over neglecting to cool the coil’s hollow inner core (cooling time 4.28hrs). However, impingement cooling only the external and internal cylindrical areas (cooling time 5 hrs) represents possibly the easier method of cooling to apply in industry yet only extends the cooling time by 1.25hrs. Predictions for such coils of 6mm thick sheet over this 350K (662°F) range, suggest that cooling times of roughly 6hrs may be possible in practice.
Microscale Heat Transfer Measurements During Subcooled Pool Boiling of Pentane
Authors: Delgoshaei Payam , Kim Jungho
Published: June 2010

Measurements of space and time resolved subcooled pool boiling of pentane in earth gravity environments were made using a microscale heater array. Data from individual heater elements in the array were synchronized with bottom and side view images from two highspeed cameras. The bubble growth was primarily due to energy transfer from the superheated liquid layer. Transient conduction and/or microconvection was found to be the dominant heat transfer mechanism. A composite model consisting of microlayer evaporation and transient conduction was developed and compared with the experimental data.
Optimal Design of a JT Cryocooler With Stochastic Constraints—Part I: Formulation
Authors: Lloyd George M.
Published: June 2010

Cyrocoolers are notorious for being difficult to design and optimize. Reasons for this include subsystem complexity, large unknowns associated with material and transport parameters, and high sensitivity to manufacturing tolerances. The purpose of this paper is to address this topic by incorporating design uncertainty itself as a constraint during the optimization of a Joule-Thomson sorption cryocooler. In our method a Markov Chain Monte Carlo sampler is used as the means to develop a suitable ensemble from a practical set of computational results which circumscribe the power/efficiency characteristics of a cryocooler as a function of several dimensionless stochastic optimization parameters. The ensemble is used to estimate the covariance structure of the design uncertainty, which is then projected into the best low rank subspace where tests of hypothesis under the dominant generalized parameters can be formulated; growth in fluctuations of the generalized parameters along optimization trajectories becomes clearly evident and quantifiable. The method results in a classical power/efficiency diagram, with the addition of quantified design uncertainty. The utility of these diagrams is that they enable rapid-prototyping efforts to target the best cooler design that is most likely to function as expected either for model validation or production. This paper will present the methodogy and a comprehensive computation model of a JT metal hydride cryocooler and demonstrate its application.
A Method for Rapid Estimation of Fluid Temperature Drop in Duct Flows
Authors: Krishnan A. S. , Srinivasan K.
Published: June 2010

Owing to the persistent interest in the approximate methods, the order of magnitude analysis is revisited in the present work and a method is proposed for the quick estimation of drop in temperature of a fluid flowing through a circular duct. The principle involves judicious guessing of temperature drop for one of the investigated cases, while the temperature estimates for rest of the cases are deduced from the initial guess. A problem of duct flow with widely varying inlet temperature and mass flow rate conditions, typical of a high altitude simulation ground test facilities for scramjet combustors, re-entry vehicles, etc., are considered which vary from 400 K to 1200 K with corresponding mass flow rates of 300 kg/s to 0.5 kg/s. The results of this quick estimation method portray remarkable agreement with the exit temperatures of the fluid as predicted by solving the problem using transient quasi-one dimensional codes and two dimensional CFD techniques over the entire operating regime and hence provide a conservative estimate of vital parameters for any design exercise as well as to rate an existing system at off-design conditions. The significant advantages of the proposed method over the others are viz. (a) simplicity (b) lesser computational effort and (c) reasonable accuracy, will be elucidated in detail in this article.
Rheological Characteristics of Nanoparticle Compound Microencapsulated Phase Change Material Suspension
Authors: Wang Liang , Lin Guiping , Ding Yulong
Published: June 2010

Microencapsulated phase change material (MPCM) suspensions have large specific heat due to the latent heat of the phase change material and enhance the convective heat transfer consequently. However low thermal conductivity of the phase change material diminishes the heat transfer performance of the MPCM suspensions. To improve the thermal conductivity of the MPCM suspensions, TiO2 nanoparticles were added into the MPCM suspensions to formulate a novel thermal fluid—nanoparticle compound microencapsulated phase change material suspensions. In this paper, the rheological characteristics and shear viscosities of such slurries using a Bolin CVO rheometer (Malvern Instruments) over a range of shear rate (5–500s−1), MPCM concentration (0–20wt%) and TiO2 nanoparticle concentration 0.5wt% at temperature (20°C–40°C). The result shows that the viscosities of NCMPCM suspensions are almost independent of the shear rate, indicating Newtonian fluid under the conditions of this work and the viscosities depend strongly on temperature which fits well with the VTF function. Based on the effective volume fraction method and Vand equation, two methods that predict the viscosity of nanoparticle compound microencapsulated phase change material suspensions was analyzed and the result shows that the prediction data the effective volume fraction method fit the measurements well.
Simulation of Whispering-Gallery Mode Microsensing in a Microfluidic System
Authors: Huang Lei , Guo Zhixiong
Published: June 2010

Label-free detection using a whispering-gallery mode biosensor in a micro fluidic channel is simulated. The analyte transport in the solution is controlled by applied electric potentials and diffusion. The finite element method is employed for solving the charged species transportation equations, the Poisson equation, the equations of conservation of momentum and energy, and the Helmholtz equations for electromagnetic waves. The adsorption process of analyte on the micro resonator surface is monitored by the resonance wavelength shift in the sensor. Shift caused by temperature variation due to Joule heating is found to be negligible compared to that induced by analyte deposition. The deposition induced shifts behave in a manner similar to Langmuir-like adsorption kinetics. A linear correlation between the frequency shift and the analyte concentration in the solution is obtained. The applied voltage is found to affect the adsorption capability; and thus, the sensor sensitivity. Detection of very low concentration to the sub-ppm level using the sensor is demonstrated.
Transient Energy and Heat Transport in Metals
Authors: Ezzahri Y. , Shakouri A.
Published: June 2010

A recently developed Shastry formalism for energy transport is used to analyze the temporal behavior of the energy and heat transport in metals. Comparison with Cattaneo’s equation is performed. Both models show the transition between ballistic and diffusive regimes. Furthermore, because the new model considers the discrete character of the lattice, it highlights some new phenomena such as oscillations in the energy transport at very short time scales. The energy relaxation of the conduction band electrons in metals is considered to be governed by the electron-phonon scattering, and the scattering time is taken to be averaged over the Fermi surface. Using the new formalism, one can quantify the transfer from ballistic modes to diffusive ones as energy propagates in the material and it is transformed into heat. While the diffusive contribution shows an almost exponentially decaying behavior with time, the non-diffusive part shows a damped oscillating behavior. The origin of this oscillation will be discussed as well as the effect of temperature on the dynamics of the energy modes transport.
Multi-Level Heatsink Optimization
Authors: Hansen Nicholas , Catton Ivan
Published: June 2010

A heatsink is a hierarchical multilevel device with many parameters required for its description. Volume Averaging theory (VAT) is used to used to rigorously cast the point wise conservation of energy, momentum and mass equations into a form that represents the thermal and hydraulic properties of the micro channel (porous media) morphology and to describe the hierarchical nature of the heatsink. Closure for the upper level is obtained using VAT to describe the lower level and a simple plane fin with inner surface augmentation is chosen to demonstrate the process. The lower level augmentation chosen is an in line array of dimples with two parameters, pitch and diameter, needed for their description, along with a scale roughened surface with either forward or downward scales. The mid-level description requires fin pitch, base and tip thickness, shape, and height and the upper-level requires overall heatsink height, width and length, and baseplate thickness. A common size heatsink (4 inch width and length and 1 inch overall height) with a required heat removal of one kilowatt, an inlet temperature of 20°C, and a maximum allowable center temperature of 65°C is used to demonstrate an approach to heatsink optimization. This is an 8 parameter problem. Significant improvement in performance (heat removal/pumping power) and reduction in thermal resistance are shown to be achievable.
Numerical Evaluation of 2D and 3D Steady and Unsteady Flows of a Pair of Opposing Confined Impinging Air Jets
Authors: Chiriac Victor , Rosales Jorge L.
Published: June 2010

The steady and unsteady laminar flow and heat transfer characteristics for a pair of opposing confined impinging slot jets in 2D and 3D were evaluated numerically at two Reynolds numbers. The present study continues the authors’ earlier work [1] and identifies the main similarities and differences arising from the expansion to the third dimension. At lower Reynolds number jet (Re = 300), the flow interaction produces a symmetric, steady flow hydrodynamic pattern with the jets being deflected laterally for the 2D flow. At Re = 300, the 3D slot jet produces almost the same values as the 2D case, yet the flow is slightly asymmetrical and unsteady. However, by further increasing the Reynolds number to 750, a complex and highly unsteady flow develops for both 2D and 3D simulations. The symmetry of both the 2D and 3D flows is disrupted and the resulting complex flow patterns reveal the vortex pairing effects, leading to the jet “buckling and sweeping” motion, enabling the enhanced local heat transfer. The convective heat transfer coefficients and the unsteady flow development between the jets are thoroughly investigated, with the flow unsteadiness also characterized by analyzing the stagnation point displacement on the channel walls. The comparison between the 2D and 3D flow patterns indicate that the 3D opposite jets enhance the unsteady effects compared to the 2D unsteady opposite jets. The complex vortex patterns resulting from the unsteady jets interaction, as well as the velocity, vorticity and temperature fields for both 2D and 3D cases are thoroughly evaluated. The comparison between the 2D and 3D impinging air jets is documented and the impact on chip/microelectronics cooling is highlighted.
Simulated Thermal Performance of Triple Vacuum Glazing
Authors: Fang Yueping , Hyde Trevor J. , Hewitt Neil
Published: June 2010

The simulated triple vacuum glazing consists of three, 4 mm thick glass panes with two vacuum gaps with each internal glass surface coated with a low emittance coating. The two vacuum gaps are sealed by an indium based sealant and separated by a stainless steel pillar array with a height of 0.12 mm and a pillar diameter of 0.3 mm spaced at 25 mm. Both solder glass and indium based sealants have been successfully applied in vacuum glazing previously. The thermal performance of the triple vacuum glazing was simulated using a finite volume model. The simulation results show that although the thermal conductivity of solder glass (1 W.m−1.K−1) and indium (83.7 W.m−1.K−1) are very different, the increase in heat transmission of triple vacuum glazing with a 10mm frame rebate resulting from the use of an indium edge seal compared to a solder glass edge seal was 0.48%. Increasing both edge seal widths from 3 mm to 10 mm led to a 24.7% increase in heat transmission of the triple vacuum glazing without a frame and an 18.3% increase for a glazing with a 10 mm frame rebate depth. Increasing the rebate depth in a solid wood frame from 0 to 15 mm decreased the heat transmission of the triple vacuum glazing by 32.9%. The heat transmission of a simulated 0.5 m by 0.5 m triple vacuum glazing was 32.2% greater than that of 1 m by 1 m triple vacuum glazing.
Liquid Jet Impingement Without and With Heat Transfer
Authors: Jafar F. A. , Thorpe G. R. , Turan O¨. F.
Published: June 2010

Equipment used to cool horticultural produce often involves three-phase porous media. The flow field and heat transfer processes that occur in such equipment are generally quantified by means of empirical relationships amongst dimensionless groups. This work represents a first step towards the goal of harnessing the power of computational fluid dynamics (CFD) to better understand the heat transfer process that occur in beds of irrigated horticultural produce. The primary objective of the present study is to use numerical predictions towards reducing energy and cooling water requirement in cooling horticultural produce. In this paper, flow and heat transfer predictions are presented of a single slot liquid jet on flat and curved surfaces using a CFD code (FLUENT) for 2-D configurations. The effects of Reynolds number, nozzle to plate spacing, nozzle width and target surface configuration have been studied. Reynolds numbers of 250, 500, 700, 1800 and 1900 are studied where the liquid medium is water. Here, the Reynolds number is defined in terms of the hydraulic nozzle diameter, inlet jet velocity and fluid kinematic viscosity. The results show that Reynolds numbers, nozzle to plate spacing and nozzle width have a significant effect on the flow filed and heat transfer characteristics; whereas the target surface configuration at stagnation area has no substantial impact. The use of a numerical tool has enabled detailed investigation of these characteristics, which have not been available in the literature previously.
Effect of Biological Macro-Molecular Flow on the Hot Air Drying of Astragalus Slices
Authors: Yang Junhong , Di Qianqian , Zhao Jun , Wang Liqiu
Published: June 2010

Astragalus slice is one species of stem and root medicinal herb with the widely curative effects, also a special and typical plant porous material, and the drying operation is one of important processing technologies in its storage and further practical application. By using scanning electronic microscope (SEM), we mainly characterize the micrographics of parenchyma cell, trachea and aperture of Astragalus slices dried by hot air at 60°C, with the aim to discuss the relationships among the microstructure of sample, macro-drying property and drying mechanism in micro-scale. Results show that in hot air dried sample, the distribution status of cytoplasm inside parenchyma cells appears obvious different with that of untreated sample, behaving agglomeration and leaving wide space with cell wall. Furthermore, we find that in comparison with the untreated sample, the shape of aperture in hot air dried sample basically maintains un-changed, but most of apertures are fully filled with some perceived substance. It is demonstrated that the flow phenomenon of cytoplasm occurs inside matrix accompanied by the moisture diffusion in the mode of liquid water, particularly biological macro-molecular, which directly influence the permeability of moisture transfer path inside matrix and the quality of dried products. Hence, this study has significant meanings to develop the optimized drying technology of plant porous material focused on micro-mechanism and the quality of dried products.
Study the Shielding Gas Effect on the Metal Transfer and Weld Pool Dynamics in GMAW
Authors: Rao Z. H. , Hu J. , Liao S. M. , Tsai H. L.
Published: June 2010

This paper studied the influences of shielding gas compositions on the transport phenomena in the metal domain during gas metal arc welding (GMAW). A comprehensive model was developed to simulate the time-dependent processes of the electrode melting; the droplet formation, detachment, transfer and impingement onto the workpiece; the weld pool dynamics and bead formation and their transient coupling with the arc plasma. The transient melt-flow velocity and temperature distributions in the metal shielded by pure argon and argon-helium mixtures with various mixing ratios are presented. It is predicted that the increase of helium content and the resulting arc contraction induce an upward electromagnetic force at the bottom of the droplet to sustain the droplet at the electrode tip. As a result, the more oblate droplet and the longer droplet formation time are produced. The behaviors of the predicted droplet shape and detachment frequency are in agreement with the published results. It is also found that, under the identical energy input, the weld bead has a shallower penetration depth and broader bead width when helium content increases.
Preliminary Experiments With an Underexpanded Gas Jet Into Water
Authors: Uchida Mitsunori , Someya Satoshi , Okamoto Koji , Ohshima Hiroyuki
Published: June 2010

When a heat exchanger in a Fast Breeder Reactor cracks, highly pressurized water or steam escapes into the surrounding liquid sodium. A sodium-water reaction then occurs, forming disodium oxide (and hydrogen gas). It can cause secondary damages to the heat exchangers by the reaction heat and erosion corrosion. The released flow of steam from the cracks of the heat exchanger is an underexpanded jet because the ambient pressure outside the tubes is lower than the critical pressure. When the pressure of a jet released at high pressure cannot be reduced to the low pressure of the ambient fluid, the flow is said to be underexpanded. Because this expansion causes a reduction of pressure and the pressure is lower than the critical pressure, the velocity of the flow can reach supersonic speed. Several studies have examined the underexpansion of the gas-gas phase. However, there have been few studies on the underexpansion of gas-liquid two-phase flows. The flow characteristics of the gas-liquid two-phase flow differ from the gas-gas flow because breakups of the bubbles appear in the gas-liquid two-phase flow. Therefore, in this study qualitative measurement was carried out for the purpose of revealing the flow with the underexpanded gas jet injected into water. The gas jet distance L and the expansion angle θ were then obtained from averaged image of a high-speed camera. L and θ increased approximately linearly with increasing pressure. The entrainment velocity and the velocity of entrained water droplets into the gas jet were obtained by PIV. Images of unstable expansion near the jet nozzle were captured for the first time.
Heat and Mass Transfer and Kinetic Processes Modeling in a Methane Steam Conversion Facility
Authors: Restrepo Gustavo A. , Krylov Andrey N. , Sergievskiy Eduard D.
Published: June 2010

This paper deals with one of the most interesting methods of lowering heat losses: thermochemical recuperation (TCR) of waste heat present in high-temperature flue gases via methane steam reforming. An earlier proposed process flow diagram for TCR in a glass production facility was revised and a mathematical model was developed to predict the heat and mass transfer phenomena that take place in the catalytic chemical reactor included in that process. Comparison of data obtained by the new proposed model and published data confirmed the feasibility of using it in the investigation and synthesis of TCR processes. Obtained results also permit to find optimal temperature profile in order to increase the hydrogen rate formation.
Simulation of H2-Air Turbulent Diffusion Flame by the Combustion Model Using Chemical Equilibrium Combined With the Eddy Dissipation Concept
Authors: Fukumoto Kazui , Ogami Yoshifumi
Published: June 2010

This research aims at developing a turbulent diffusion combustion model based on the chemical equilibrium method and chemical kinetics for simplifying complex chemical mechanisms. This paper presents a combustion model based on the chemical equilibrium method and the eddy dissipation concept (CE-EDC model); the CE-EDC model is validated by simulating a H2-air turbulent diffusion flame. In this model, the reaction rate of fuels and intermediate species is estimated by using the equations of the EDC model. Further, the reacted fuels and intermediate species are assumed to be in chemical equilibrium; the amount of the other species is determined from the amount of the reacted fuels, intermediate species, and air as reactants by using the Gibbs free energy minimization method. An advantage of the CE-EDC model is that the amount of the combustion products can be determined without using detailed chemical mechanisms. The results obtained by using this model were in good agreement with the experimental and computational data obtained by using the EDC model. Using this model, the amount of combustion products can be calculated without using detailed chemical mechanisms. Further, the accuracy of this model is same as that of the EDC model.
Study of Smoke Stratification in Atrium Halls Using Scale Model
Authors: Fong N. K. , Ko C. H.
Published: June 2010

Nowadays, atrium building is very popular because it can provide extensity and attraction to the users even if they are inside the enclosed environment. Similar to other buildings, fire safety is one of the major concerns especially the atrium are linked to shopping arcades. The major challenge is to control the smoke movement in the case of fire and maintain a stable smoke layer clear height to allow sufficient time for the occupants to evacuate from the building. Therefore, an efficient smoke management system (SMS) is necessary. For the SMS to function properly, the smoke behaviour inside the atrium must be studied. One of the phenomena affecting the operation of the SMS is smoke stratification. That is, due to the vertical temperature gradient inside the atrium, a thermally stratified environment is formed and smoke will not be able to reach the smoke detectors/smoke outlets in the ceiling. In the past decade, various studies were conducted to study the smoke filling process in the atrium. Only a few studies were carried out to study smoke stratification in atrium. This paper attempted to study the factors leading to the development of thermally stratified environment in an atrium and the formation of smoke stratification under the ceiling space of an atrium building using scale model. These factors included the temperature of the smoke plume, the air temperature under the ceiling, the configuration of roof ceiling and the ambient air temperature. Two types of ceiling configurations such as a cuboid and a triangular prism are used. Data concerning the ceiling air temperature, smoke plume temperature, effect of different ceiling configuration and maximum smoke layer height in a thermally stratified environment are collected. Comparisons are conducted with the calculated results from National Fire Protection Association (NFPA) 92B equations. With all these information, better design criteria of smoke detection system, SMS in an atrium building can be developed. Finally, the experimental results can be used to investigate the discrepancies between the experimental measurement and the calculated results from NFPA 92B equations. Put abstract text here.
Effect of Initial Velocity Field on Smoke Diffusion Characteristic in Subway Tunnel
Authors: Yang Hui , Jia Li , Yang Lixin
Published: June 2010

In this paper, piston wind effect on smoke diffusion characteristic in subway tunnel is studied by using three-dimensional transient computational fluid dynamics (CFD) method. In the first simulation case, fire disaster is simulated with homogeneous resting initial field condition. In the second simulation case, the train’s decelerating process till stopping in the tunnel is simulated for getting three-dimensional tunnel air velocity field distribution. Then the final heterogeneous air velocity field when the train stops in the tunnel is taken as initial field condition and the same fire scenario as the first case is simulated again. The data obtained under both initial conditions are compared by detecting people evacuation safety and the influence of initial air velocity field is analyzed. The results show that the inertial air velocity field caused by train’s movement has significant influence on smoke diffusion at the first few minutes of fire disaster, which is the key time for people’s evacuation. The adopted method in this paper and the simulation result could be used in establishing more effective subway fire evacuation plan.
Computational Study of Conjugate Heat Transfer in T-Junctions
Authors: Kuhn Simon , Nicˇeno Bojan , Prasser Horst-Michael
Published: June 2010

Thermal fatigue is a relevant problem in the context of life-time extension of nuclear power plants (NPP). In many piping systems in NPPs hot and cold water is mixed, which leads to high temperature fluctuations in the region close to the solid wall and resulting thermal loads on the pipe walls that can cause fatigue. One of the relevant geometric test cases for thermal fatigue is the mixing in T-junctions. In this study we apply large–eddy simulations (LES) to the mixing of hot and cold water in a T-junction. We perform a set of simulations by using different formulations of the LES subgrid scale model, i.e. standard Smagorinsky and dynamic procedure, to identify the influence of the modelled subgrid scales on the simulation results. The results exhibit a large difference between the models, which is caused by the use of turbulent viscosity wall–damping functions when applying the standard model.
Numerical Analysis of a Metal Hydride Reactor With Embedded Heat Pipes to Enhance Heat Transfer Characteristics
Authors: Boo Joon Hong , Park Young Hark , Katsuta Masafumi , Bae Sang Chul
Published: June 2010

Numerical analysis was conducted for a heat pipe application in a metal hydride (MH) reactor for hydrogen gas storage. The hydriding and dehydriding characteristics of MH strongly depend on temperature and pressure. Due to its extremely low thermal conductivity however, it is very difficult to control the temperature of MH, especially when it is of vast bulk as in an MH reactor. This study deals with heat pipes embedded into the MH to increase the effective thermal conductivity of the system and thus to enhance the thermal control characteristics. The existing model was a brine-tube type MH reactor having cylindrical container with outer diameter of 76 mm and length of 1 m, which was partially filled with 8 to 10 kg of MH material. The hydriding and dehydriding processes occur at 10°C and 80°C, respectively. The heat-pipe type reactor model replaced the brine tubes and channels with copper-water heat pipes of the same dimensions. Three-dimensional numerical analysis predicted that the heat-pipe type MH reactor model enhanced thermal performance with faster response to the change of boundary conditions and higher degree of isothermal characteristics. Discussion is presented based on the numerical results of the two models compared with experimental results.
A Hybrid Solution Methodology for Efficient Solving of Radiative Heat Transfer in Non Gray Medium Using WSSG Model
Authors: Yadav Rakesh , Verma Atul K. , Bessette Didier
Published: June 2010

The weighted sum of gray gases (WSGG) method has been used extensively for radiation modeling of non gray medium gases. The radiative transfer equation (RTE) for each gray gas can be solved using the Discrete Ordinate (DO) model, which is sufficiently accurate over wide range of optical thickness, but it is computationally expensive. The P1 approximation is an efficient alternative to solve the RTE, but applicable to optically thick medium only. In the present work, a hybrid solution methodology to solve RTE in a non gray medium using WSGG model is proposed. Results for the case of four gray gases are presented. The philosophy is to achieve the optimum performance using combination of the P1 and DO model with respect to the optical thickness of the gray gases being solved with minimal loss in accuracy. In this approach, the transparent and/or optically thin gases are solved using the DO model while the P1 approximation is used for the optically thick gases. This combination reduces the total number of radiation intensity equation to be solved very significantly. A study of accuracy and computational cost for the hybrid solution methodology has been done for a variety of test cases in order of increasing complexity. It has been found that the hybrid solution methodology, in which RTE for two gray gases are solved by DO model and the remaining two gray gases by P1 approximation, provided similar accuracy with significant speedup compared to the conventional approach in which RTE for all gray gases are solved with DO model. Another variant of the hybrid approach, in which DO model is used only for the transparent gas, is also considered and found to be sufficiently accurate for most of the cases.
Simulation of Laminar Stratified Flow Boiling of Liquid in a Horizontal Tube by the Coupled Map Lattice Model
Authors: Chakraborty Indrajit , Ghoshdastidar P. S. , Biswas G.
Published: June 2010

A new heat transfer model for stratified flow boiling in a horizontal tube is proposed in this present study. In recent years, the subject of nonlinear dynamics has progressed and various tools of analysis have been proposed for complex systems. Coupled Map Lattice (CML) method is one such tool which makes it possible to simulate complex systems and to capture the qualitative nature of the phenomenon. In this work, steady stratified flow boiling of water is simulated qualitatively by using the CML model for laminar, hydrodynamically and thermally developing flow and heat transfer in a horizontal tube. The liquid enters in a constant wall temperature tube (Tw*>Tsat* at Pentrance*) in a subcooled or saturated condition. The present modeling by CML is based on the assumption that the flow boiling is governed by nucleation from cavities on the heated surface, migration of vapor into the core, forced convection and phase change in the bulk. The macroscopic variable chosen is temperature. The influences of mass flow rate, inlet subcooling and wall temperature have been studied. The results of the computations provide information on the effect of aforementioned parameters on the heat transfer coefficient and void fraction. The results show that the CML has been able to model flow boiling in a realistic manner.
Comparison of Analytical and Computational Thermal Models for Gas Metal Arc Braze Welding
Authors: Shah Sanjiv E. , Lee Jason C. , Rios-Perez Carlos , Seepersad Carolyn Conner
Published: June 2010

Analytical and computational models are constructed for predicting the temperature distribution in a workpiece during gas metal arc braze-welding (GMABW). Specifically, the weld zone is modeled with Rosenthal’s analytical model, a finite difference model (FDM) and a finite volume computational model, constructed in the FLUENT® software package. Each model relates controllable braze-welding process parameters, such as traverse speed and applied voltage, to the temperature field that develops during the braze-welding process. Model-based predictions are validated by comparison with experimental data obtained from braze-welded specimens of C22000 commercial bronze alloy (CuZn90/10%wt). Temperature data are collected during the braze-welding process via thermocouples and infrared pyrometers aimed at the top surface of the specimen. Recommendations are made regarding the range of applicability and limitations of the models. In addition, opportunities are discussed for applying these models as part of an automated control framework for GMABW.