bluuebunny/crossref_metadata_2025 huggingface dataset:
Second Law and Energy Loss Analysis in a Turbulent Flow Through a Nozzle Type Duct
Authors: Javadinejhad S. R.
Published: April 2009

Amount of heat transfer is the primary concern in a heat exchanger design. The amount of energy that has been destroyed during the heat exchange process has been investigated by introducing a new dimensionless number. Analyises of simpler systems are often useful to understand more important features of complex pattern forming processes in various field of science and technology. The entropy generation have been studied by use of new dimensionless number . This number defined as the ratio of total energy loss to total heat transfer across the duct length. The temperature dependence on the viscosity is taken into consideration and results have been derived for various L/D ratio, nozzle angles and inlet temperature.
Effect of Conjugate Heat Transfer on MEMS-Based Thermal Shear Stress Sensor
Authors: Chao Jianghui , Shyy Wei , Thakur Siddharth S. , Sheplak Mark , Mei Renwei
Published: April 2009

The effect of conjugate heat transfer resulting from a Micro-electromechanical Systems (MEMS)-based thermal shear stress is investigated. Due to the length scale disparity and large solid-fluid thermal conductivity ratio, a two-level computation is used to examine the relevant physical mechanisms and their influences on wall shear stress. The substantial variations in transport properties between the fluid and solid phases and their interplay in regard to heat transfer and near-wall fluid flow structures are investigated. It is demonstrated that for the state-of-the-art sensor design, the buoyancy effect can noticeably affect the accuracy of the shear stress measurement.   This paper was also originally published as part of the Proceedings of the ASME 2005 Pacific Rim Technical Conference and Exhibition on Integration and Packaging of MEMS, NEMS, and Electronic Systems.
Remote Heat Pipe Based Heat Exchanger Performance in Notebook Cooling
Authors: Khandani Seyyed , Pokharna Himanshu , Machiroutu Sridhar , DiStefano Eric
Published: April 2009

Remote heat pipe based heat exchanger cooling systems are becoming increasingly popular in cooling of notebook computers. In such cooling systems, one or more heat pipes transfer the heat from the more populated area to a location with sufficient space allowing the use of a heat exchanger for removal of the heat from the system. In analsysis of such systems, the temperature drop in the condenser section of the heat pipe is assumed negligible due to the nature of the condensation process. However, in testing of various systems, non linear longitudinal temperature drops in the heat pipe in the range of 2 to 15 °C, for different processor power and heat exchanger airflow, have been measured. Such temperature drops could cause higher condenser thermal resistance and result in lower overall heat exchanger performance. In fact the application of the conventional method of estimating the thermal performance, which does not consider such a nonlinear temperature variations, results in inaccurate design of the cooling system and requires unnecessarily higher safety factors to compensate for this inaccuracy. To address the problem, this paper offers a new analytical approach for modeling the heat pipe based heat exchanger performance under various operating conditions. The method can be used with any arbitrary condenser temperature variations. The results of the model show significant increase in heat exchanger thermal resistance when considering a non linear condenser temperature drop. The experimental data also verifies the result of the model with sufficient accuracy and therefore validates the application of this model in estimating the performance of these systems.   This paper was also originally published as part of the Proceedings of the ASME 2005 Pacific Rim Technical Conference and Exhibition on Integration and Packaging of MEMS, NEMS, and Electronic Systems.
Effects of Internal Heat Generation on Solidification
Authors: Crepeau John C. , Siahpush Ali
Published: April 2009

We present solutions for solid-liquid phase change in materials that generate internal heat. This problem is solved for both cylindrical and semi-infinite geometries. The analysis assumes a temperature profile in the solid phase and constant temperature boundary conditions on the exposed surfaces. We derive differential equations governing the solidification thickness for both geometries as functions of the Stefan number and the internal heat generation (IHG). For the cylindrical geometry, the solidification layer obtains a steady-state value which is related to the inverse of the square root of the IHG. The solutions to the semi-infinite geometry problem show that when the surface is cooled to below the freezing point, a solidification layer forms along the edge and begins to grow until it reaches a maximum, then begins remelt.
Effect of Thermal Conductivity of an Impinging Premixed Butane/Air Circular Laminar Flame Jet System
Authors: Zhao Z. , Wong T. T. , Leung C. W.
Published: April 2009

Numerical simulations were performed to study the heat transfer characteristics of the impingement plate of an impinging flame jet system consisting of a premixed butane/air circular flame jet impinging vertically upward upon a horizontal rectangular plate at laminar flow condition. The study concentrated mainly on the effect of thermal conductivity of the impingement plate on its heat flux. A uniform temperature boundary condition was assumed at the water-side of the plate. In order to deal with the very complicated boundary conditions at the flame-side of the plate, experimental datum of the flame temperature were correlated using a multi-polynomial function. The heat flux distributions on the impingement plate, with the variations of Reynolds number (Re), were numerically simulated. Comparison between the experimental and numerical work has been made.
Influence of Partition Location on Natural Convection in a Partitioned Enclosure
Authors: Sankhavara C. D. , Shukla H. J.
Published: April 2009

In this study, Natural convection heat transfer and fluid flow of square partitioned enclosure with two partitions protruding centrally from the end walls of an enclosure have been analysed numerically using finite element method. The enclosure has opposite isothermal walls at different temperatures. The thickness and length of the partitions is fixed and equal to 1/10 and 1/4 of width of the enclosure respectively. Computation for Rayleigh number in the range of 104 to 106 has been conducted. The influence of different thermal boundary conditions at the end walls and at the partitions is included in the investigation. ‘Standard’ boundary conditions are introduced as more appropriate to simulate situations of practical engineering interest. To solve the relevant governing equations a segregated sequential solution algorithm is used after employing Boussinesq approximation. These equations after discretization were solved by using the tridiagonal matrix algorithm. Results clearly demonstrate that partition location has a significant effect on heat transfer.
Large Eddy Simulations of Staggered Parallel-Plate Flows
Authors: Pham M. V. , Plourde F. , Doan S. K.
Published: April 2009

Heat transfer enhancement is a subject of major concern in numerous fields of industry and research. Having received undivided attention over the years, it is still studied worldwide. Given the exponential growth of computing power, large-scale numerical simulations are growing steadily more realistic, and it is now possible to obtain accurate time-dependent solutions with far fewer preliminary assumptions about the problems. As a result, an increasingly wide range of physics is now open for exploration. More specifically, it is time to take full advantage of large eddy simulation technique so as to describe heat transfer in staggered parallel-plate flows. In fact, from simple theory through experimental results, it has been demonstrated that surface interruption enhances heat transfer. Staggered parallel-plate geometries are of great potential interest, and yet many numerical works dedicated to them have been tarnished by excessively simple assumptions. That is to say, numerical simulations have generally hypothesized lengthwise periodicity, even though flows are not periodic; moreover, the LES technique has not been employed with sufficient frequency. Actually, our primary objective is to analyze turbulent influence with regard to heat transfers in staggered parallel-plate fin geometries. In order to do so, we have developed a LES code, and numerical results are compared with regard to several grid mesh resolutions. We have focused mainly upon identification of turbulent structures and their role in heat transfer enhancement. Another key point involves the distinct roles of boundary restart and the vortex shedding mechanism on heat transfer and friction factor.
Numerical and Experimental Analysis for Large Radius Deformation of Stainless Steels by Laser Forming Process
Authors: Pennuto J. , Choi J.
Published: April 2009

In an effort to develop a process free of dedicated tooling, this research seeks to study large radius deformation by laser forming. Experimental testing was conducted to determine how the laser parameters affect the single pass output bend angle as well as the additive bend angle from successive parallel, evenly spaced laser irradiations. As an extension of the previous developments, this work seeks to develop a three-dimensional model to simulate the multi-scan laser process. It is of interest to determine how sophisticated a three-dimensional case is required for sufficient agreement to experimental data. The simulated results of bending angle are compared with experimental data and suggestions for future study include the implementation of phase transformation and microstructure data within the model to account for stress development resulting from phase transformation and grain growth.
Electronic Cooling Using Synthetic Jets
Authors: Pavlova Anna A. , Amitay Michael
Published: April 2009

Efficiency of synthetic jet impingement cooling and the mechanisms of heat removal from a constant heat flux surface were investigated experimentally. The effects of jet’s formation frequency and Reynolds number at different nozzle-to-surface distances were investigated and compared to steady jet cooling. It was found that synthetic jets are up to three times more effective than steady jets at the same Reynolds number. For smaller distances, high formation frequency (f = 1200 Hz) synthetic jets remove heat better than low frequency (f = 420 Hz) jets, whereas low frequency jets are more effective at larger distances, with an overlapping region. Using PIV, it was shown that at small distances between the synthetic jet and the heated surface, the higher formation frequency jet is associated with accumulation of vortices before they impinge on the surface. For the lower frequency jet, the wavelength between coherent structures is so large that vortex rings impinge on the surface separately.
Computation of Incompressible Three-Dimensional Mixed Convection Flows in Long Aspect Ratio Channels Using an Efficient Finite Difference Method for Vectorial Supercomputers
Authors: Nicolas Xavier , Xin Shihe
Published: April 2009

Based on Goda’s algorithm and second-order central finite differences, a very efficient vectorized code is tailored to solve 3D incompressible Navier-Stokes equations for mixed convection flows in high streamwise aspect ratio channels. The code takes advantage of incremental factorization method of ADI type, spectral decomposition of the ID Laplace operators and TDMA algorithm. It is validated through experiments of various Poiseuille-Rayleigh-Be´nard flows with steady longitudinal and unsteady transverse rolls.
Free Convection in a Rectagular Enclosure With Three Heated Sections on the Lower Surface and Isothermal Side and Top Surfaces
Authors: Oosthuizen Patrick H. , Paul Jane T.
Published: April 2009

The development of unsteady, three-dimensional free convective flow in a rectangular enclosure with multiple heated elements on the bottom horizontal surface has been numerically studied. The enclosure considered has rectangular horizontal lower and upper surfaces and rectangular vertical side surfaces. The horizontal width of enclosure is twice the vertical height of the enclosure while the longitudinal length of the enclosure is equal to the vertical height of the enclosure. There are three square symmetrically placed isothermal heated sections on the lower surface, the rest of this surface being adiabatic. The vertical side-walls and the horizontal rectangular upper surface of the enclosure are kept at a uniform low temperature. 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 unsteady, three-dimensional governing equations written in dimensionless form, the solution being obtained in terms of the vorticity vector and vector potential functions. The solution has the following parameters: the Rayleigh number, Ra, the Prandtl number, Pr, the dimensionless size, wH, of the square heated sections and the dimensionless distance between the heated sections on the lower surface, wS. Results have only been obtained for a Prandtl number of 0.7. In a given geometrical situation it was found in all cases that a steady flow exists at low Rayleigh numbers, that an unsteady flow develops at higher Rayleigh numbers and that the flow then again becomes steady at still higher Rayleigh numbers. The conditions under which unsteady flow develops and ceases and the variation of mean Nusselt number with Rayleigh number have been explored.
Exploration of Unsteady Spray Cooling for High Power Electronics at Microgravity Using NASA Glenn’s Drop Tower
Authors: Golliher Eric L. , Zivich Chad P. , Yao S. C.
Published: April 2009

At present, there is little understanding of the application of spray cooling to electronics in the microgravity environment. Typically in closed cycle terrestrial spray cooling systems, since not all of the liquid impinging on a hot substrate is evaporated, some residual liquid is separated from its vapor component by gravity and returned to the pump. This technique of phase separation is not available to spacecraft designers. Methods to predict spray cooling performance for ground systems do exist, but they are absent for the space environment. Particularly for NASA spacecraft, there is a need to design spacecraft that use high power laser systems and other systems that use evaporative spray cooling in microgravity. Such knowledge is very important for the performance and life of the device. Reliable analytical methods of predicting thermal response of a spray cooled substrate when considering a transient heat load, such as that found during start up and shut down of a space-based laser or other high heat flux electronics, do not exist. Our goal was to use NASA Glenn’s 2.2 second drop tower to investigate unsteady heat transfer at low Bond numbers and residual fluid behavior in spray cooling. The work contrasts other experiments aboard the NASA Glenn KC-135 low gravity aircraft [1]. Our future plans are to continue the experimental work and include the use of the NASA Glenn 5 second drop tower. This paper will report on some preliminary results of an interesting experimental study performed at NASA Glenn in the summer of 2004. The high speed camera and specially-designed “S.L.O.B.” drop rig provided video and data to assess the fluid management problems that arise in a microgravity spray environment, for both heated and unheated cases. The data show unexpected residual fluid management issues, such as the development of multiple spherical liquid globs, with apparent ordered and repeatable geometry, at the point of impact. The results of these experiments provide direction for further investigation in the future.
Measurement of Internal Temperature Distribution in PEMFCs by Nondestructive Inverse Method
Authors: Chang Mei-Hsia , Cheng Chin-Hsiang
Published: April 2009

A nondestructive inverse method is developed to determine the internal temperature distribution of PEMFCs. In this study, attention is focused on global measurement for the irregular temperature distribution at the interface between the carbon plate and the membrane electrode assembly (MEA) based on the measured temperature data on the outer surface of the end plate. In this report, a concept of point-by-point temperature prediction is adopted. This approach is particularly suitable for determining an irregular temperature distribution that is difficult to handle by the existing polynomial-function approach [1]. A number of test cases are considered in this study. Three irregular temperature functions are specified and regarded as exact temperature distributions for testing the performance of the approach. Influence of the uncertainties of the measured temperature data on the outer surface and the number of the temperature prediction points (NX×NY) on the end plate surface is evaluated. In addition, the effects of the initial guess on the uniqueness of the predictions are also investigated.
On the Effective Thermal Conductivity of Saturated Porous Media
Authors: Aichlmayr H. T. , Kulacki F. A.
Published: April 2009

Carefully quantified effective thermal conductivity measurements of saturated porous systems are reported. Solid-fluid systems considered include glass-water, glass-air, steel-water, and steel-air. These systems yield solid-fluid conductivity ratios of 1.08, 25.7, 102, and 2400, respectively. The solid phases consist of 3.96 mm glass spheres and 14 mm steel ball bearings, which give mean porosities of 0.365 and 0.403. The experimental method is based on the transient heating of a semi-infinite cylinder by a constant heat flux at the boundary. The data reduction technique is unique because it avoids determining the effective thermal diffusivity and quantifying the boundary heat flux. In addition, particular attention is paid to assessing experimental uncertainty. Consequently, this study provides data with a degree of precision not typically found the literature. A complete accounting of energy storage and transport in the transient system is conducted to complement the uncertainty analysis. A thorough literature review is also presented to facilitate a critique of the experimental results.
Transient Heat Transfer During Cryogenic Chilldown
Authors: Jackson Jelliffe , Liao Jun , Klausner James F. , Mei Renwei
Published: April 2009

Cryogenic fluids have found many practical applications in today’s world, from cooling superconducting magnets to fueling launch vehicles. In many of these applications the cryogenic fluid is initially introduced into piping systems that are in excess of 150 degrees Kelvin higher than the fluid. This leads to voracious evaporation of the fluid and significant pressure fluctuations, which is accompanied by thermal contraction of system components. This process is known as chilldown, and although it was first investigated more than 4 decades ago, very little data are available on the momentum and energy transport during this transient process. Consequently, the development of predictive models for the pressure drop and heat transfer coefficient has been hampered. In order to address this deficiency, an experimental facility has been constructed that enables the flow structure to be observed while temperatures and pressures at various locations are measured. This study focuses on the inverse numerical procedure used to extract the transient heat transfer coefficient information from the data collected; this information is then used to evaluate the performance of various correlations for heat transfer coefficient in the flow boiling regime. The method developed utilizes flow structure information and temperature measurements, in conjunction with numerical computations for the temperature field within the tube wall, to calculate the heat transfer coefficient. This approach allows the transition point between the film boiling regime and the nucleate boiling regime to be determined, and it also elucidates the variation of the heat transfer coefficient along the circumference of a horizontal tube, with the heat transfer on upper portion being significantly smaller than that at the bottom.
Comparison of LES and RANS Simulations of Turbulent Convection in a Structured Porous Media
Authors: Gullbrand J. , Wirtz R. A.
Published: April 2009

Numerical simulations are performed of the turbulent flow and thermal field in a structured porous media. The geometry of interest is an open lattice structure that is made up of mutually orthogonal, millimeter-scale, thermally conductive cylindrical elements. A large-eddy simulation (LES) is performed and the results are used as reference data to evaluate the performance of three commonly used Reynolds Averaged Navier-Stokes (RANS) models: the Spalart-Allmaras model (SAM), the k-ω model (KWM) and the shear stress transport k-ω model (KWM/SST). Mean velocities, turbulent kinetic energy profiles, and turbulent viscosity predictions are compared as well as friction factor, wall shear stress and Stanton number.
Effects of Inclination Angle on Falling Film Evaporation Using R-134a
Authors: Chien Liang-Han , Lin Hung-Ta
Published: April 2009

This manuscript discusses the effect of inclination angle and surface geometries on the falling film evaporation performance. Falling film evaporation experiments were conducted on a smooth plate and finned plates using refrigerant R-134a at 18 °C system temperature. The plate was inclined with angles between 10° and 40°, and the heat fluxes are between 36 and 73.5kW/m2. The local heat transfer coefficients are measured, and the falling film flow distribution is observed through a sight glass. The test results showed that the falling film evaporation heat transfer coefficient increases as the inclination angle increases or the heat flux increases. The finned surface yields better falling film evaporation heat transfer performance than the smooth surface. The ratio of the heat transfer coefficient of the finned surface versus the plain surface is between 2.5 and 12.4 folds. This ratio increases as the inclination angle increases. The visualization observation showed that bubble nucleation is more pronounced on the finned surface than the plain surface. The effect of boiling on falling film evaporation is discussed.
Model Based Predictive Control of Nucleate Boiling
Authors: Robertson Andrew , Hartland Jonathan
Published: April 2009

Conventional, liquid phase, forced-convection cooling systems enjoy elevated levels of heat transfer when operated in the Nucleate Boiling regime. Under such conditions, coolant flow rates can be greatly reduced leading to a significant reduction in pumping effort, a downsized cooling system and a subsequent increase in operational lifetime. In order to control heat transfer reliably, a closed-loop control system is necessary which requires a measurement or estimation of the instantaneous heat transfer coefficient. This work presents a control technique for Nucleate Boiling focused on automotive applications and based around a specially built laboratory test rig. The control system calculates the necessary coolant flow rate from an estimated heat flux. The simplicity of the control system is complemented by the complexity of estimating the instantaneous heat flux. Three techniques for estimating this heat flux are presented relying on measured system variables. Results for the third and most practical, technique are presented taken from the laboratory test rig.
Numerical Modeling of Solidification Process Using the Enthalpy-Porosity Technique
Authors: Kulkarni S. , Lin C. X. , Ghenai C.
Published: April 2009

The enthalpy-porosity technique is used in this study for numerical modeling of solidification of water and binary mixture of ammonium chloride and water (NH4Cl-H2O). A two-dimensional geometry was used to simulate the solidification process in the rectangular and trapezoidal cavities. The liquid fraction is computed based on an enthalpy balance. The mushy zone is modeled as pseudo-porous medium in which the porosity decreases from 1 (liquid) to 0 (solid) as the material solidifies. The solidification with single side cooled wall as well as both cooled side walls was investigated. The frozen layer thickness, the convective flow patterns, and the temperature distribution were obtained. The results obtained in the course of the study showed that the shape of the solid region and the convection pattern in the melt region compare well with experimental data obtained by Particle Image Velocimetry technique. The enthalpy-porosity model is predicting well the flow pattern and the thickness of the frozen layer.
Effects of Surface Roughness and Surfactant on Film Condensation Heat Transfer of Steam
Authors: Morrison Andrew T. , Kwark S. M. , You S. M.
Published: April 2009

The design of a two phase heat exchanger requires a fundamental knowledge of the parameters affecting film condensation. The interaction of roughened surfaces and change of surface energy by the addition of a surfactant to the system for quiescent steam was examined in this study. Three types of roughened surfaces were generated by roughening a polished surface with sandpaper, electroplating to obtain a thin porous coating of nodules onto the condenser surface, and soldering of porous aluminum foam to the condenser face. The addition of a nonionic surfactant, alcohol alkoxylate, to the system was evaluated for the roughened surfaces. For a Reynolds number less than 50, the roughened surfaces examined all degraded the condensation heat transfer. The addition of a surfactant to the system enhanced condensation heat transfer on polished surfaces almost two fold.
Condensation of Steam on Finned Surfaces With Addition of a Surfactant
Authors: Morrison Andrew T. , You S. M.
Published: April 2009

A fundamental knowledge of the parameters affecting film condensation is essential for the design of two phase heat exchangers. The current study examines the effect of extended surfaces and surface energy modifications and their interaction for condensation of steam in quiescent and vapor flow conditions. The enhancement of heat transfer for vertical, flat surfaces and two finned surfaces were compared for Reynolds numbers ranging from approximately 10 to 50. The addition of a nonionic surfactant, alcohol alkoxylate, to the system was evaluated for the same surfaces and vapor field conditions. Vapor flow of 0.25 m/s enhanced the heat transfer approximately 40%, while 0.5 m/s vapor velocity produced almost 100% increase in heat transfer. The addition of surfactant to the system produced small enhancement in heat transfer except in the case of condensate hold-up between the fins. In this case, the addition of surfactant increase the heat transfer an additional 25%, likely because the vapor flow and change of surface energy were sufficient to largely eliminate the hold-up of condensate between the fins.
Generalized CHF Correlation With Uniform Heat Flux for Annulus
Authors: Lee Sung-Woo , Kim Dong-Kook , Shim W. Jae-Woo
Published: April 2009

A new generalized CHF (Critical Heat Flux) correlation was suggested for water flow in uniformly heated Annulur tubes. The parametric ranges of experimental CHF data used for present analysis were as follows: 0.540< P (pressure) < 15.146 MPa, 197.77 < G (mass flux) < 653.52 kg/m2s, I.D (inner diameter) = 9.54 mm, O.D (outer diameter) = 19.4 mm, L (length) = 1.84 m, 75.48 < Inlet subcooling < 358.78 kJ/kg, 490.48 < qc (CHF) < 1775.89 KW/m2, and 0.120 < Xe (exit qualities) < 0.536. The new correlation was based on local condition hypothesis, and therefore consisted of the local variables such as tube diameter, pressure, mass flux of water and true mass fraction of steam. In addition, HBM (Heat Balance Method) that leads to the more accurate prediction of CHF than DSM (Direct Substitution Method) was used in developing the correlation. The new correlation was compared with 5 existing CHF correlations that showed comparatively good prediction in world literature. The new correlation predicted CHF better than the other ones with average error of −1.03% and root mean square error of 11.91%.
DNS of Three-Dimensional Unsteady Separated Flow and Heat Transfer in a Sudden-Expansion Channel
Authors: Yoshikawa Hiroyuki , Ishikawa Kimitake , Ota Terukazu
Published: April 2009

Numerical results of a three-dimensional unsteady separated flow and heat transfer in a sudden expansion rectangular channel are presented. A direct numerical simulation methodology was employed in the calculations using the finite difference method. Treated in the present study is a rectangular channel of aspect ratio AR = 4.0 and expansion ratio ER = 2.5 in a Reynolds number range from 200 to 1000. It is found that the flow becomes unsteady at Re = 400 and severely complicated at Re = 500 to 1000. The heat transfer characteristics are presented and discussed in relation to the flow ones.
DNS of Expansion Ratio Effects on Three-Dimensional Unsteady Separated Flow and Heat Transfer Around a Downward Step
Authors: Kito Aya , Sugawara Kazuaki , Yoshikawa Hiroyuki , Ota Terukazu
Published: April 2009

The direct numerical simulation methodology was employed to analyze the unsteady features of a three-dimensional separated flow and heat transfer around a downward step in a rectangular channel, and to clarify systematically the channel expansion ratio effects upon them. Numerical calculations were carried out using the finite difference method. The Reynolds number Re based on the mean velocity at inlet and the step height was varied from 300 to 1000. The channel expansion ratio ER is 1.5, 2.0 and 3.0 under a step aspect ratio of 36.0. It is found that the flow is steady upto Re = 500 but becomes sensibly unsteady at Re = 700 for all the three expansion ratios. In the case of ER = 2.0, the separated shear layer is most unstable. In the case of ER = 1.5, the longitudinal vortices formed near the side walls of channel are strongest. Nusselt number reaches its maximum in the reattachment flow region and also in the neighborhood of the side wall, and their locations depend greatly upon ER and Re.
Modelling of the Near-Wall Liquid Velocity Field in Subcooled Boiling Flow
Authors: Ramstorfer Franz , Breitscha¨del Bernd , Steiner Helfried , Brenn Gu¨nter
Published: April 2009

The subject of the present work is the modelling of the liquid streamwise flow velocity in the two-phase boundary layer in subcooled boiling flow under the influence of the vapor bubbles. Subcooled boiling flow experiments were carried out in a horizontal test channel in order to investigate the interaction between the bubbles and the liquid phase. The heater surface was located at the bottom of the test channel. The near-wall liquid flow velocity was measured using a two-component laser-Doppler anemometer. Based on the experimental data a model is proposed to describe the impact of the gaseous phase on the motion of the liquid in the subcooled boiling regime. It was observed that the axial velocity profiles near the wall follow a logarithmic law similar to that used in turbulent single-phase flow over rough surfaces. Based on this finding it is suggested to model the influence of the bubbles on the liquid flow analogously to the effect of a surface roughness. The correlation developed for an equivalent surface roughness associated with the bubbles yields good agreement of the modeled axial velocity profiles with the experimental data.
Design Considerations and Heat Transfer Enhancement of CFB Heat Exchangers for Flue Gas Heat Recovery
Authors: Jun Yong-Du , Lee Kum-Bae , Ko Seok-Bo , Islam Sheikh Zahidul
Published: April 2009

Now-a-day’s energy recovery process in the industry is a common practice for improving the production process while major concern goes to environment. The performance of the heat exchangers, used for the purpose of recovering energy, decreases continuously with time due to fouling depending on surface temperature, surface condition, construction material, fluid velocity, flow geometry and fluid composition. To overcome the fouling of fly ash on the heat transfer surface and erosion and periodical cleaning which are the major drawbacks in conventional heat exchangers for flue gas heat recovery, a no-distributor-circulating-fluidized-bed (NDCFB) heat exchanger with automatic particle controlling is devised. One of the main advantages of this model is the reduced pressure drop through the entire heat exchanger system, while increasing heat transfer performance. The research started with a single riser system with multiple down comers and multi-riser system is also studied. The heat transfer performance and pressure drop have been evaluated through experiments for these gas-to-water lab scale heat exchanger systems. However, due to the operational complexity, these two models are not readily applicable to real applications. As a derivation of the previous studies regarding the no-distributor CFB heat exchangers, third generation model of the heat exchanger is now under investigation.
A Total-Concentration Fixed-Grid Method for Two-Dimensional Diffusion-Controlled Wet Chemical Etching
Authors: Rath P. , Chai J. C. , Zheng H. Y. , Lam Y. C. , Murukeshan V. M.
Published: April 2009

This article presents a total concentration method for two-dimensional wet chemical etching. The proposed procedure is a fixed-grid approach. It is analogous to the enthalpy method used for modeling melting/solidification problems. The governing equation is formulated using the total concentration of the etchant. It includes the reacted and the unreacted concentrations of the etchant. The proposed governing equation includes the interface condition. The reacted concentration is used to capture the etchant-substrate interface implicitly. Since the grids are fixed, a diffusion problem remains a diffusion problem unlike the moving grid approach where the diffusion problem becomes the convection-diffusion problem due to the mesh velocity. For demonstration purposes, the finite volume method is used to solve for the transient concentration distribution of etchant. In this article, two-dimensional diffusion-controlled wet chemical etching processes are modeled. The results obtained from the proposed total concentration method are compared with available “analytic” solutions and solutions from moving-grid approach.
The Effects of Micro Scale Solute Redistribution on the Macro Scale Heat and Mass Transfer During Casting Processes
Authors: Nie Hong , Feng Yan-Hui , Zhang Xin-Xin
Published: April 2009

One-dimensional plate-like dendritic model is built to describe the microsegregation during binary alloy solidification with solid back-diffusion and complete diffusion in the liquid. The specialties of microsegregation for Al-Cu and Fe-C alloys are analyzed and compared by simulation. Moreover, the microsegregation model is developed to couple with the macro solidification model which simulates the heat and mass transfer with phase change for the new Inverse Casting Technology. The numerical results are proven to be in good agreement with experimental data. It shows that the microsegregation numerical model can reflect the micro mass transfer accurately and can be coupled reliably with macro heat and mass transfer model for solidification. In addition, both the micro and macro investigation shows that the solidification of Fe-C alloy progresses almost in equilibrium.
Hydrodynamic of Bubble Boiling Regime With Artificially Produced Nucleation Site
Authors: Basˇic´ Sanib , Marn Jure
Published: April 2009

In presented work a hydrodynamic of bubble boiling process in quiescent water has been investigated using PIV (Particle Image Velocimetry) experimental technique. Velocity distribution in narrow region over the artificially produced nucleation site that was active during the partial developed nucleate boiling has been presented in more details. Experimental methodology is based on specially arranged boiling measurement procedure and it includes a statistic cross-correlation function for the raw velocity vectors estimation and validation protocol where refined velocity filed of bubbling process is attained. Review of vertical velocity components in natural convection and bubble boiling regime at two selected temperatures along the boiling surface is given and some comparison is carried out. Vertical velocity profiles around the rising bubbles are presented in qualitative manner and vertical pumping characteristics of boiling process vs. natural convection are compared.
Modeling of Ultrashort Pulse-Train Laser Heating of Metal Films
Authors: Jiang L. , Tsai H. L.
Published: April 2009

Recently, a new laser micromachining technique using multiple ultrashort pulses with a picosecond-to-nanosecond separation as a train group has demonstrated some promising application potential, which also has high scientific merits in micro-/nano-scale ultrafast energy transport. However, the underlying physics of this technique remain poorly understood. In this study, the improved two-temperature model recently developed by the authors is used to analyze the pulse train technology, in which the full-run quantum treatments are used to calculate the significantly varying properties, including the electron heat capacity, electron relaxation time, electron conductivity, reflectivity, and absorption coefficient. This paper reports the effects of the pulse number per train, pulse separation, and repetition rate. Results from the present study have demonstrated that 1) it can increase the photon efficiency and further reduces the heat-affected zone; 2) the number of bursts required for melting decreases with the repetition rate, but at tens of MHz or higher, the repetition rate has a negligible impact; 3) the lattice temperature right after the thermolization time first increases to a peak and then decreases as the pulse separation increases.
Heat and Mass Transports in Porous Wicks Driven by a Gas-Phase Diffusion Flame
Authors: Raju Mandhapati P. , T’ien James S.
Published: April 2009

A one dimensional stagnation point diffusion flame stabilized next to a porous wick is studied using a numerical model. The bottom end of the one-dimensional wick is dipped inside a liquid fuel (ethanol) reservoir. The liquid is drawn towards the surface of the wick through capillary action against gravity. The model combines heat and mass transfer equations in the porous media with phase change and gas-phase combustion equations to investigate steady-state flow structure in the porous wick and flame characteristics in the gas phase. In one-dimensional system, the only steady solution in the porous wick that is stable is found to be in the funicular regime. There are two regions in the wick: a vapor-liquid two-phase region near the surface exposed to the flame and a purely liquid region deep inside the wick. The physics behind the two-phase flow driven by capillarity and evaporation has been studied in detail. The coupling between the flame and the porous transport involves three different length scales: flame standoff distance, wick height above the reservoir and capillary rise. Attempt is made to study the effect of the non-dimensional numbers that contains these scales. In the limit of fast chemical kinetics (large Damkohler number), the computed results depend only on two non-dimensional ratios: the ratio of wick height to capillary rise and the ratio of wick height to flame standoff distance. Thus, a simplified similitude has been identified.
Boiling of Water at Sub-Atmospheric Conditions With Enhanced Structures
Authors: Pal Aniruddha , Joshi Yogendra
Published: April 2009

Liquid cooling with phase change is a very attractive option for thermal management of electronics because of the very high heat transfer coefficients achievable. Two-phase liquid cooling can be implemented in a thermosyphon loop, which has an evaporator, where heat is absorbed from the source during boiling of the working fluid, and a condenser, where the absorbed heat is rejected. Water is a preferred working fluid for boiling heat transfer due to its excellent thermal properties. Using water at sub-atmospheric conditions helps in initiation of boiling at low temperatures, which is necessary for electronics cooling applications, often limiting the maximum temperature to 85°C for silicon devices. Past studies have also shown that using boiling enhancement structures improve heat transfer by lowering the incipience overshoot, increasing heat flux and reducing evaporator volume. However, detailed study on the effects of enhancement structures and sub-atmospheric saturation conditions on the boiling of water in a compact thermosyphon loop is lacking in the literature. The objective of this study is to understand the boiling phenomena under the above-mentioned conditions and to investigate their effectiveness in electronics cooling applications. Experiments were carried out in a thermosyphon setup at 9.7, 15 and 21 kPa saturation pressures for two different enhancement structure geometries at varying heat loads (1–170 W). The experimental investigation showed that very high heat fluxes (≥ 80 W/cm2) can be achieved by boiling at sub-atmospheric pressures with enhancement structures. It is observed that with decreasing system pressure, the surface temperature also decreased for all the heat loads. The surface temperatures attained were well below the acceptable value of 85° C for all the cases.
Application of Direct Newton’s Methods for Efficient Solution to Convective-Diffusive Transport Processes
Authors: Raju Mandhapati P.
Published: April 2009

Newton’s iterative technique is commonly used in solving a system of non-linear equations. The advantage of using Newton’s method is that it gives local quadratic convergence leading to high computational efficiency. Specifically, Newton’s method has been applied to finite volume formulation for convective-diffusive transport processes. A direct solution method is adopted. Development of sparse direct solvers has significantly reduced the computation time of direct solution methods. Here UMFPACK (Unsymmetric Multi-Frontal method), has been used to solve the resulting linear system obtained from Newton’s step. A simple damping strategy is applied to ensure the global convergence of the system of equations during the first few iterations. The efficiency of this method is compared to that of Picard’s iterative procedure and the SIMPLE procedure for convective-diffusive transport processes. A modified Newton technique is also analyzed which lead to significant reduction in total CPU time.
Aspects of Flow and Heat Transfer in Louver-Fin Round-Tube Heat Exchangers
Authors: Wu Hailing , Gong Ying , Zhu Xiaobo
Published: April 2009

Experimental and numerical investigations on flow and heat transfer were conducted for louver-fin round-tube two-row heat exchangers. The airflow velocity ranged from 1 m/s to 3 m/s. A three-dimensional numerical method was developed by modeling representative cell units with fluid-solid conjugated heat transfer. Results of three-dimensional numerical simulations were in good agreement with the experimental data. A stagnant flow region exists behind the round tubes, and results in diminished local convective heat transfer. For two-row heat exchangers operating at Reynolds number, Re<300, the first row dominates the heat transfer rate. With Re increasing, the heat transfer contribution of both rows tends to be more uniform. The flow pattern shows a recirculation region downstream of the heat exchanger at higher Re flows, which may be induced by a vortex-shedding instability from the tube and louver bank.
High Performance Thin Film Transistors (TFTs) of Polycrystalline Silicon Crystallized by the Double Laser Crystallization (DLC) Technique
Authors: Xu Li , Grigoropoulos Costas P.
Published: April 2009

Ultra-large grain poly-crystalline silicon has been formed by the double laser crystallization (DLC) method. In-situ images were captured to monitor the transient melting and solidification process in order to understand the crystallization induced by steep laser intensity gradients. SEM (scanning electron microscope) images of crystallized film after Secco etch revealed grain size up to 10μm. High performance thin film transistors (TFTs) were fabricated on the DLC-made poly-crystalline material. The highly localized crystal growth and well-defined orientation allowed precise definition of channels on large grains. The electrical performance of the fabricated devices was studied, indicating a field-effect mobility in the saturation range of undoped channel of 124 cm2/V.sec, threshold voltage of 0.2V and on-off current ratio of 1E8 for n-type devices.
Convective Boiling Heat Transfer and Pressure Drop Characteristics of R134a in a Microfinned Helically Coiled Tube
Authors: Cui Wenzhi , Li Longjian , Xin Mingdao , Chen Qinghua , Liao Quan , Jen Tien-Chien
Published: April 2009

The main purpose of this paper was to experimentally study the heat transfer and pressure drop characteristics of refrigerant R134a boiling inside a new geometry microfin helically coiled tube. Experiments were performed in a range of mass quality from 0.05 up to 0.9, mass velocity 70 ∼ 380 kg/m2s and heat flux 2.0 ∼ 21.8 kW/m2. The local and average convective boiling heat transfer coefficients were reported in this paper, which were found to be dependent on both of mass flux and heat flux. Compared with corresponding smooth helically coiled tube, the microfin helically coiled tube could enhance the convective boiling heat transfer very well. The enhancement factor was up to 2.2 with the variety of mass flux and heat flux. Heat transfer in annular flow was specially studied. A flow boiling heat transfer correlation was presented for the annular flow regime, which had a mean deviation of 9.1%. The frictional pressure drop values were obtained by subtracting acceleration pressure drop and gravitational pressure drop from the measured total pressure drop. The frictional pressure drop data can be well correlated by Lockhart-Martinelli parameter. Considering the corresponding flow regimes, i.e., stratified and annular flow, two frictional pressure drop correlations were proposed, and showed a good agreement with the respective experimental data.
Optimal Design for Thermal Performance of a Heat Sink Integrated With Thermoelectric Cooler
Authors: Wu M. C. , Peng C. H. , Lee C. Y. , Fang C. J. , Hung Y. H.
Published: April 2009

The demand for high execution speed and memory capacity for modern computers results in an increasing circuit density per unit chip and high power dissipation per unit volume. Consequently, traditional air cooling technology such as air-cooled heat sink is reaching the limits for electronic applications. Thermoelectric coolers are regarded as potential solutions for enhancing the performance of air-cooled heat sinks. In the present study, a semi-empirical method for exploring the thermal performance of a heat sink integrated with or without TEC has been successfully established. A concept of design of experiments (DOE) is applied, and a statistical method for sensitivity analysis of the influencing parameters is performed to determine the key factors that are critical to the design. By the statistical sensitivity analysis of ANOVA F-test for the temperature reduction (ΔTC−B) and COP of the TEC, the factor contributions of QP, Rext and I are 31.66%, 33.73%, 34.61% as well as 14.9%, 0%, 85.1%, respectively. By employing the gradient-based numerical optimization technique, a series of constrained optimal designs have been performed. Under the given constraints of COP≧2, the optimal value of ΔTC−B (3.3°C) is obtained with the corresponding Qp (31.99W) and Qte (16W). Comparisons between the results by the present optimal design and those obtained by the semi-empirical results have been made with a satisfactory agreement. The present optimal design shows that a heat sink integrated with TEC can extend the upper limits of thermal management for traditional air-cooled heat sinks.