bluuebunny/crossref_metadata_2025 huggingface dataset:
Heat Transfer and Critical Heat Flux of Subcooled Water Flow Boiling in a SUS304-Tube With Twisted-Tape Insert
Authors: Hata Koichi , Masuzaki Suguru
Published: March 2011

The subcooled boiling heat transfer (HT) and the steady-state critical heat fluxes (CHFs) in a short SUS304-tube with twisted-tape insert are systematically measured for mass velocities (G = 4016 to 13950 kg/m2s), inlet liquid temperatures (Tin = 285.82 to 363.96 K), outlet pressures (Pout = 764.76 to 889.02 kPa) and exponentially increasing heat input (Q = Q0 exp(t/τ), τ = 8.5 s) by the experimental water loop comprised of a multistage canned-type circulation pump controlled by an inverter. The SUS304 test tube of inner diameter (d = 6 mm), heated length (L = 59.5 mm), effective length (Leff = 49.1 mm), L/d (= 9.92), Leff/d (= 8.18) and wall thickness (δ = 0.5 mm) with average surface roughness (Ra = 3.89 μm) is used in this work. The SUS304 twisted tape with twist ratios, y [= H/d = (pitch of 180° rotation)/d], of 2.39, 3.39 and 4.45 are used. The relations between inner surface temperatures and heat fluxes for the SUS304-tubes with various twisted-tape inserts are clarified from non-boiling to CHF. The subcooled boiling heat transfers for SUS304-tubes with various twisted-tape inserts are compared with our empty SUS304-tube data and the values calculated by our and other workers’ correlations for the subcooled boiling heat transfer. The influences of the twisted-tape insert, the twist ratio and the swirl velocity on the subcooled boiling heat transfer and the CHFs are investigated into details and the widely and precisely predictable correlations of the subcooled boiling heat transfer and the CHFs for turbulent flow of water in the SUS304-tubes with twisted-tape inserts are given based on the experimental data. The correlations can describe the subcooled boiling heat transfer coefficients and the CHFs obtained in this work within −25 to +15% difference.
Thermodynamic Optimization of PEM Fuel Cell Stack Gas Channel for Optimal Thermal Performance
Authors: Obayopo Surajudeen O. , Bello-Ochende Tunde , Meyer Joshua P.
Published: March 2011

This study shows a procedure to optimize polymer exchange membrane (PEM) fuel cell gas channels in the systems bipolar plates with the aim of globally optimizing the overall system net power performance. Two geometrical gas flow channel configuration are considered, parallel channel and serpentine. Geometric optimization is carried out to determine the configuration which offers minimum pressure drop within the system and the maximum heat and mass transfer for optimal system performance. The systems are subjected to the constraints of total fixed volume and materials. The systems total and net power, efficiency and polarization curves are presented as a function of system temperature, pressure, geometry and operating parameters. Predicted results are compared successfully against past data involving channel flow profiles in gas channels. The configurations obtained from the geometric optimization reduce the overall pumping power requirement and subsequently cost for the supply of required fuel and oxidants to the fuel cell through the channels.
Generation and Propagation of Thermally Induced Acoustic Waves in Supercritical Fluids: Numerical and Experimental Results
Authors: Farouk Bakhtier , Lei Zhieheng
Published: March 2011

The behavior of thermally induced acoustic waves generated by the rapid heating of a bounding solid wall in a closed cylindrical chamber filled with supercritical carbon dioxide is investigated numerically and experimentally. A time-dependent one-dimensional problem is considered for the numerical simulations where the supercritical fluid is contained between two parallel plates. The NIST Reference Database 12 is used to obtain the property relations for supercritical carbon dioxide. The thermally induced pressure (acoustic) waves undergo repeated reflections at the two confining walls and gradually dissipate. The numerically predicted temperature of the bulk supercritical fluid is found to increase homogeneously (the so called piston effect) within the domain. The details of generation, propagation and dissipation of thermally induced acoustic waves in supercritical fluids are presented under different heating rates. In the experiments, a resistance-capacitance circuit is used to generate a rapid temperature increase in a thin metal foil located at one end of a closed cylindrical chamber. The time-dependent pressure variation in the chamber and the temperature history at the foil are recorded by a fast response measurement system. Both the experimental and numerical studies predict similar pressure wave shapes and profiles due to rapid heating of a wall.
From the Extremum Principle of Entransy Dissipation to Steady Balance Equations of Fluid Mechanics
Authors: Xu Mingtian , Cheng Lin
Published: March 2011

In the present work, an extremum principle of entransy dissipation is developed by taking into account the contribution of fluid friction. It is found that this extremum principle of entransy dissipation leads to the same balance equations as given in continuum mechanics for the steady heat conduction and shear flow in an incompressible fluid.
Effects of Surface Roughness and Oxide Layer on the Thermal Boundary Conductance at Aluminum/Silicon Interfaces
Authors: Hopkins Patrick E. , Phinney Leslie M. , Serrano Justin R. , Beechem Thomas E.
Published: March 2011

Thermal boundary resistance dominates the thermal resistance in nanosystems since material length scales are comparable to material mean free paths. The primary scattering mechanism in nanosystems is interface scattering, and the structure and composition around these interfaces can affect scattering rates and, therefore, device thermal resistances. In this work, the thermal boundary conductance (the inverse of the thermal boundary resistance) is measured using a pump-probe thermoreflectance technique on aluminum films grown on silicon substrates that are subjected to various pre-Al-deposition surface treatments. The Si surfaces are characterized with Atomic Force Microscopy (AFM) to determine mean surface roughness. The measured thermal boundary conductance decreases as Si surface roughness increases. In addition, stripping the native oxide layer on the surface of the Si substrate immediately prior to Al film deposition causes the thermal boundary conductance to increase. The measured data are then compared to an extension of the diffuse mismatch model that accounts for interfacial mixing and structure around the interface.
Band Models for Radiative Transfer in Non-LTE Diatomic Molecules of CO2-N2 Plasmas
Authors: Depraz Se´bastien , Rivie`re Philippe , Perrin Marie-Yvonne , Soufiani Anouar
Published: March 2011

A statistical narrow-band model is developed for the optically non-thin electronic systems of carbonaceous molecules in CO2-N2 plasmas and its accuracy is studied under equilibrium and non-equilibrium conditions. Line by line calculations are used to produce curves of growth of transmissivities from which band model parameters are calculated by least-square adjustments. The model is shown to provide quite accurate description of radiative properties and radiative intensities for Doppler, Lorentz, and Voigt line profiles, and for both local thermodynamic equilibrium and a multi-temperature description of the gas mixture thermodynamic state. The model is also suitable for a more general description of the gas thermodynamic state where the electronic state populations are arbitrary.
Influence of Ni-P Coating Microstructure on Condensation Heat Transfer
Authors: Cheng Lin , Cheng Yanhai , Xin Gongming , Zou Yong
Published: March 2011

The higher heat transfer performance due to dropwise condensation as compared with filmwise condensation has been of considerable interest to many researchers around the world. In this study, the effect of microstructure of Ni-P coating on condensation heat transfer performance was reported. The Ni-P deposits with various microstructures, such as nano-crystalline, amorphous and co-existence of both, could be obtained on the low carbon steel tube by adjusting the electroless plating processing parameters. The experimental results show that the steel tube with electroless Ni-P coating can greatly improve the performance of condensation heat transfer compared to un-coated steel tubes. The condensation state on tubes with Ni-P coating depends on the cooling water flow rate and steam temperature, it shows the co-existence state of both dropwise and filmwise on most working conditions. At the same time, the heat transfer efficiency of condensation is improved with the decreasing of the amounts of nanocrystalline phase in Ni-P deposit, because it can increase the free surface energy of surface. Therefore, increasing the degree of amorphous state of Ni-P coating will provide a better dropwise condensation effect.
Experimental Study on Flat Plate Pulsating Heat Pipe
Authors: Hu Chaofa , Jia Li
Published: March 2011

An experimental system of flat plate pulsating heat pipe was established and experimental research was carried out in this system to know the mechanism of heat transfer, start-up and operating characteristics. The factors, such as filling rate, heating power, heating method etc, which have great influence on the thermal performance of the plate pulsating heat pipe were discussed. The results indicate that heating power and filling rate are the important factors for the start-up of the plate pulsating heat pipe. Different filling rate needs different start-up power, and the start-up of the heat pipe in case of bottom heated is much easier than that of top heated. Increasing the heating power and enlarging the heating area can make the start-up easier. Heating power can also affect the start-up time of heat pipe under the condition of bottom heated, while it does not have some influence to the heat pipe of top heated. The thermal resistances of plate pulsating heat pipe are related with the heating power, and the higher the heating power is, the smaller the thermal resistances are. But the best filling rate which the heat pipe needs is different with different heating methods, and the performance of the heat pipe in the case of the bottom heated is better than the other.
Approximate Analytical Solutions for Marangoni Mixed Convection Boundary Layer
Authors: Zhang Yan , Zheng Liancun , Liu Jiemin
Published: March 2011

The paper deals with a steady coupled dissipative layer, called Marangoni mixed convection boundary layer, which can be formed along the interface of two immiscible fluids, in surface driven flows. The mixed convection boundary layer is generated besides the Marangoni convection effects induced flow over the surface due to an imposed temperature gradient, there are also buoyancy effects due to gravity and external pressure gradient effects. We shall use a model proposed by Chamkha wherein the Marangoni coupling condition has been included into the boundary conditions at the interface. The similarity equations are first determined, and the approximate analytical solutions are obtained by an efficient transformation, asymptotic expansion and Pade´ approximant technique. The features of the flow and temperature fields as well as the interface velocity and heat transfer at the interface are discussed for some values of the governing parameters. The associated fluid mechanics was analyzed in detail.
Heat Transfer and Pressure Drop Characteristics of Water in the Transitional Flow Regime of Horizontal Smooth Tubes at Constant Wall Temperature
Authors: Meyer J. P. , Du Preez A.
Published: March 2011

Owing to design limitations, heat exchangers are frequently forced to operate in the transitional flow regime, however, there exists no accurate measurements for both heat transfer and pressure drop in this region. In order to optimize a heat exchanger for maximum efficiency and minimum pressure losses, it is required to design it for the transitional flow regime. Therefore, the purpose of this study is to obtain accurate Nusselt numbers and pressure drop coefficients for water flowing through a horizontal smooth tube with a constant wall temperature. Heat transfer and pressure drop measurements were conducted on a 5.33 mm tube at Reynolds numbers ranging from 1 000 to 5 000.
Effect of Cell Size of a Honeycomb Porous Plate Attached to a Heated Surface on CHF in Saturated Pool Boiling
Authors: Mori Shoji , Shen Lujie , Okuyama Kunito
Published: March 2011

The critical heat flux (CHF) in a saturated pool boiling of water was investigated experimentally under the condition in which a honeycomb porous plate is attached to the heated surface. In a previous study, the CHF was shown experimentally to be approximately 2.5 times (approximately 2.51 MW/m2) compared to that of a plain surface for the case of a honeycomb porous plate with a vapor escape channel width of 1.4 mm and a channel height (plate thickness) of 1.0 mm (Mori and Okuyama (2009)). The enhancement is considered to result from the capillary supply of liquid onto the heated surface and the release of generated vapor through the channels. In the present paper, the vapor escape channel width was varied in the range of 1.4 mm to 7.9 mm, which was smaller than the Taylor instability wavelength (approximately 15.6 mm), and the effect of the channel width on the saturated pool boiling CHF of water has been investigated. The CHF values predicted by capillary limit models were compared with measured values. As a result, the main mechanisms for CHF enhancement using a honeycomb porous plate were shown to be due to liquid supply to the heated surface as a result of not only capillary suction but also the inflow of liquid through vapor escape channels from the top surface due to gravity. The ratio of the contribution in the mechanisms of the CHF enhancement was found to depend on the vapor escape channel widths. In particular, in the case of a larger cell width, the CHF was enhanced primarily due to the inflow of liquid through vapor escape channels from the top surface.
Heat Transfer Characteristics of Turbulent Gas Flow Through Micro-Tubes
Authors: Hong Chungpyo , Uchida Yuki , Yamamoto Takaharu , Asako Yutaka , Suzuki Koichi
Published: March 2011

This paper presents experimental results on heat transfer characteristics of turbulent gas flows though a micro-tube with constant wall temperature. The experiments were performed for nitrogen gas flows through a micro-tube with 242μm in diameter and 50 mm in length. The wall temperature was maintained at 5K, 20K and 30K higher than the inlet temperature by circulating water around the micro-tube, respectively. In order to measure heat transfer rate of gas flow through a micro-tube, the total temperature at a micro-tube exit was measured. The stagnation pressure was chosen in such a way that the Reynolds number ranges from 3000 to 12000. The outlet pressure was fixed at the atmospheric condition. The total temperature at the outlet, the inlet stagnation temperature, the mass flow rate, and the inlet pressure were measured. The heat transfer rates obtained by the present study are higher than those of the incompressible flow. This is due to the additional heat transfer near the micro-tube outlet caused by the energy conversion into kinetic energy. A correlation for the prediction of the heat transfer rate of the turbulent gas flow through a micro-tube was proposed.
Mass Nature of Heat and Its Applications IV: Thermal Wave and Periodic Temperature Oscillation in Metallic Films Heated by Ultra-Short Pulsed Lasers
Authors: Wang Haidong , Ma Weigang , Zhang Xing , Wang Wei
Published: March 2011

The ultra-fast heat transfer in metallic films has attracted great interest in modern femtosecond laser processing and metallic film manufacturing. Considering the unphysical infinite propagation speed of heat disturbances based on Fourier’s model, some hyperbolic heat transfer models have been developed in the past decades, leading to the character of thermal wave in metallic films under ultra-fast laser heating conditions. In this paper the thermomass model is applied to obtain the governing equation for heat conduction in the thin films under pulsed laser heating, which is a damped wave equation and identical with that based on the Hyperbolic Two-Step (HTS) model. The semi-implicit Crank-Nicholson scheme is used to solve governing equations. Numerical results show that there may be two kinds of temperature oscillations existed in metallic films heated by ultra-short pulsed lasers, and the thermally oscillating boundary condition usually dominates over that caused by thermal wave induced oscillation of the electron temperature, which is validated by the measurement of the temperature response at the rear surface using a femtosecond laser pump-probe system. The measured electron temperature curve agrees well with the theoretically predicted one.
Liquid Metal Heat Transfer Investigations Applied to Tokamak Reactor
Authors: Sviridov Valentin G. , Razuvanov Nikita G. , Ivochkin Yuriy P. , Listratov Yaroslav I. , Sviridov Evgeniy V. , Genin Leonid G. , Zhilin Vyatcheslav G. , Belyaev Ivan A.
Published: March 2011

The liquid metal (LM) investigations of hydrodynamics and heat transfer affected by longitudinal and transverse magnetic field (MF) were carried out. The current experiments program deals with temperature and velocity fields, local and average heat transfer intensities, temperature fluctuation intensities in a horizontal heated tube. The strong influence of thermo gravitational convection (TGC) in all regimes was observed in experiments applied to blanket and divertor of tokamak. Two unexpected troubles were discovered: zones of extremely low local heat transfer intensities in some regimes of MF and TGC joint influence; dangerous temperature low-frequent fluctuations with high amplitude near the tube wall in some MHD configurations with TGC affect. Numerical simulation of magneto hydrodynamic (MHD) heat transfer is working out simultaneously with experiments.
The Flow and Heat Transfer in a Packed Bed High Temperature Gas-Cooled Reactor
Authors: du Toit C. G. , Rousseau P. G.
Published: March 2011

Engineers are faced with two major challenges when carrying out the thermal-fluid design of a complex system consisting of many interacting components such as the PBMR power plant. The first challenge is to predict the performance of all the individual thermal-fluid components. The second challenge is to predict the performance of the integrated plant consisting of all its sub-systems. The complexity associated with the thermal-fluid design of complex systems requires the use of a variety of analysis techniques and simulation tools. These range from simple one-dimensional models that do not capture all the significant physical phenomena, to large-scale three-dimensional CFD codes that, for practical reasons, can not simulate the entire plant as a single integrated model. In the systems CFD approach a network code serves as the framework to link the models of the various components together and to control the solution. The models of the components can be of varying degrees of complexity. These can range from simple lumped models to complex fully three-dimensional CFD models. This paper gives a brief overview of the systems CFD (SCFD) approach and an overview of the SCFD model of the pebble bed nuclear reactor that was developed.
Experimental Investigation of a New Flat Plate Solar Heat Collector by Micro Heat Pipe Array
Authors: Zhao Yao-Hua , Zou Fei-Long , Diao Yan-Hua , Quan Zhen-Hua
Published: March 2011

The performance of a new flat plate solar heat collector with perfect combination of high efficiency and low cost is investigated experimentally. The new system described in this study uses a novel micro heat pipe array as a key component for the system. One such flat plate heat collector contains over 300 micro heat pipes per 1m2 and the hydraulic diameter of the micro heat pipes is 0.4–1.0mm. A detailed heat transfer experimental study is conducted during daylight hours over several months, focusing on the collector efficiency and overall efficiency of the system as well as total heat loss factor. The results show that the collector’s maximum instantaneous efficiency is up to 88%. Compared with conventional evacuated glass tube solar water heater, the system offers the additional benefits of high pressure resistance, low weight, good reliability and durability, easy integration into buildings and absence of freezing during winter months. Besides, compared with traditional flat-plate solar water system which is mainly sheet-and-tube concept, the system also shows many advantages: higher efficiency, much cheaper, absence of tube-bonding and freezing etc. Therefore, the system proposes a unique substitute to common solar water heating systems.
Numerical Simulation of Thermal Conductivity of Nanofluids
Authors: Fan Jing , Wang Liqiu
Published: March 2011

The recent first-principle model shows a dual-phase-lagging heat conduction in nanofluids at the macroscale. The macroscopic heat-conduction behavior and the thermal conductivity of nanofluids are determined by their molecular physics and microscale physics. We examine numerically effects of particle-fluid thermal conductivity ratio, particle volume fraction, shape, aggregation, and size distribution on macroscale thermal properties for nine types of nanofluids, without considering the interfacial thermal resistance and dynamic processes on particle-fluid interfaces and particle-particle contacting surfaces. The particle radius of gyration and non-dimensional particle-fluid interfacial area in the unit cell are two very important parameters in characterizing the effect of particles’ geometrical structures on thermal conductivity of nanofluids. Nanofluids containing cross-particle networks have conductivity which practically reaches the Hashin-Shtrikman bounds. Moreover, particle aggregation influences the effective thermal conductivity only when the distance between particles is less than the particle dimension. Uniformly-sized particles are desirable for the conductivity enhancement, although to a limited extent.
Experimental and Numerical Investigation of Flow of Nanofluids in Microchannels
Authors: Singh Pawan K. , Harikrishna P. V. , Sundararajan T. , Das Sarit K.
Published: March 2011

The current study investigates the flow of nanofluids in microchannels experimentally and numerically. For this purpose, two microchannels of hydraulic diameters of 211 and 300 μm are used with alumina(45nm)-water nanofluids. The nanofluids with the concentrations 0.25, 0.50 and 1 vol% are used to observe the effect of volume fraction in the present analysis. With regard to the numerical simulation of nanofluids in microchannels, two approaches have been chosen in the current work. First one considers the nanofluids as single phase fluid and applies the mixture rule for evaluating properties for the simulation. The second type of modeling is done using the discrete phase approach which involves Eulerian-Lagrangian considerations. The fluid phase is assumed to be continuous and governed by Navier-Stokes equation. The movement of discrete nanoparticles is determined by the Newton’s second law which takes into account the body force, hydrodynamic forces, the Brownian and thermophoresis forces. The predictions are validated against experimental results obtained for nanofluid flow in a chemically etched silicon wafer channel. It is found that the discrete phase modeling is more accurate with regard to the prediction of nanofluids behavior in microchannels, as compared to the single phase model. The results also show the non-uniformity of nanoparticle distribution across the channel cross-section. This non-uniformity in distribution can be attributed to the shear induced particle migration. This can also be the reason for the difference in pressure drop and heat transfer from the single phase model. The pressure drop with 0.25 and 0.5 vol% of alumina is more or less same as that of water and thus, makes it a suitable cooling liquid. However, an enhancement in heat transfer is observed from the computational predictions.
Performance of a Plate-Type Cross-Flow Enthalpy Exchanger Made of Paper
Authors: Lee Eul-Jong , Kim Nae-Hyun , Song Kil-Sup , Cha Sang-Jin , Lee Jung-Pyo
Published: March 2011

The effects of paper properties such as density, air permeability, moisture transmission rate on the thermal performance of plate-type enthalpy exchanger were experimentally investigated. Three enthalpy exchanger samples having different properties were made, and were tested according to the standard test procedure (KS B 6879). Effective efficiencies were defined, which accounted for the air leakage between supply and exhaust streams. Results showed that paper density affected the sensible heat transfer of the samples. Sensible heat transfer increased with density of the paper. It was also shown that moisture transmission rate alone was not a proper indicator for the efficiency of latent heat transfer. Air permeability should also be considered for adequate evaluation of the latent heat transfer. Best performance was obtained for the sample having highest paper density and moderate moisture transmission ratio.
Experimental Characterization of Single and Multiple Droplet Impingement on Surfaces Subject to Constant Heat Flux Conditions
Authors: Soriano Guillermo , Alvarado Jorge L. , Lin Yen Po
Published: March 2011

Spray cooling is one of the most promising technologies in applications which require large heat removal capacity in very small areas. Previous experimental studies have suggested that one of the main mechanisms of heat removal in spray cooling is forced convection with strong mixing due to droplet impingement. These mechanisms have not been completely understood mainly due to the large number of physical variables, and the inability to modulate and control variables such as droplet frequency and size. Our approach consists of minimizing the number of experimental variables by controlling variables such as droplet direction, velocity and diameter. An experimental study of single and multiple droplet impingements using HFE 7100 as the cooling fluid under constant heat flux conditions is presented. A monosized droplet train is produced using a piezoelectric droplet generator with the ability to adjust droplet frequency, diameter and velocity. In this study, heaters consisting of a layer of Indium Tin Oxide (ITO) as heating element, and silicon substrates are used. Film morphology was characterized using a Laser Induced Fluorescence (LIF) technique with a focus on the droplet impact zone by measuring variables such as film thickness and diameter of the impact zone. Infrared thermography was used to measure surface temperature at the liquid-solid interface. The IR thermography technique was also used to characterize temperature gradients at the droplet impact zone. The results and effects of droplet frequency, fluid flow rate, and fluid temperature on heat flux are also presented.
Frequency-Domain Thermoreflectance Technique for Measuring Thermal Conductivity and Interface Thermal Conductance of Thin Films
Authors: Zhu Jie , Tang Dawei , Wang Wei , Liu Jun , Yang Ronggui
Published: March 2011

The thermal conductivity of thin films and interface thermal conductance of dissimilar materials play a critical role in the functionality and the reliability of micro/nano-materials and devices. The transient thermoreflectance methods, including the time-domain thermoreflectance (TDTR) and the frequency-domain thermoreflectance (FDTR) techniques are excellent approaches for the challenging measurements of interface thermal conductance of dissimilar materials. A theoretical model is introduced to analyze the TDTR and FDTR signals in a tri-layer structure which consists of metal transducer, thin film, and substrate. Such a tri-layer structure represents typical sample geometry in the thermoreflectance measurements for the thermal conductivity and interface thermal conductance of thin films. The sensitivity of TDTR signals to the thermal conductivity of thin films is analyzed to show that the modulation frequency needs to be selected carefully for a high accuracy TDTR measurement. However, such a frequency selection is closely related to the unknown thermal properties and consequently hard to make before the measurement. Fortunately this limitation can be avoided in FDTR. Depending on the modulation frequency, the heat transport in such a tri-layer could be divided into three regimes based on the thickness of the film and the thermal penetration depth, the thermal conductivity of thin films and interface thermal conductance can be subsequently obtained by fitting different frequency regions of one FDTR measurement curve. FDTR measurements are then conducted along with the aforementioned analysis to obtain the thermal conductivity of SiO2 thin films and interface thermal conductance SiO2 and Si. FDTR measurement results agree well with the TDTR measurements, but promises to be a much easier implementation than TDTR measurements.
The Characteristics of Turbulent Heat Transfer in a Curved Rectangular Duct With Various Aspect Ratios
Authors: Choi Hang Seok , Park Tae Seon
Published: March 2011

The turbulent flow fields of a parallel plate or channel with spatially periodic condition have been widely investigated by many researchers. However the rectangular or square curved duct flow has not been fundamentally scrutinized in spite of its engineering significance, especially for cooling device. Hence, in the present study large eddy simulation is applied to the turbulent flow and heat transfer in a rectangular duct with 180° curved angle varying its aspect ratio. The turbulent flow and the thermal fields are calculated and the representative vortical motions generated by the secondary flow are investigated. From the results, the secondary flow has a great effect on the heat and momentum transport in the flow. With changing the aspect ratio, the effect of the geometrical variation to the secondary flow and its influence on the turbulent characteristics of the flow and heat transfer are studied.
Development and Validation of a Coal Combustion Model for Pulverised Coal Combustion
Authors: Chernetsky M. J. , Dekterev A. A.
Published: March 2011

To fully understand the processes of heat-and-mass transfer on the laboratory-scale and full-scale coal boilers, computer models are needed to develop, which can predict flow fields, heat transfer and the combustion of the coal particles with reasonable accuracy. In the work reported here, a comprehensive model for pulverized coal combustion has been presented. Attention has been given to the char burnout submodel, NOx formation sub-model and accurate calculation of the temperature of the particles. The model predictions have been compared with the experimental measurements of the laboratory-scale pulverized-coal combustion burner.
Numerical Modeling of Buoyancy-Induced Fluid Flow and Heat Transfer in a Staggered Tube Bank
Authors: Ivanov N. G. , Kirillov A. I. , Smirnov E. M. , Ris V. V.
Published: March 2011

Heat exchangers involving plain tubes in crossflow are widely employed in various industrial applications. Detailed studies have established relations between the heat transfer coefficient and the arrangement of tubes within the tube bank. Most of the data available in the literature concern with the tube bank heat transfer under forced convection conditions at relatively high Reynolds numbers, exceeding 103. However, the buoyancy effects become significant or even dominant under low-flow-rate conditions. Today reliable data for low-Reynolds-number regimes could be obtained using advanced Computational Fluid Dynamics techniques. The current contribution deals with a buoyancy-induced flow through a staggered tube bundle. Two-dimensional unsteady convection in a tube bank of 24 rows is computed. The tube walls are assumed to be at a uniform temperature. The buoyancy effects are described with the Boussinesq’s approximation. The computations have been performed for the Prandtl number of 6.09 and the Reynolds numbers in the range of 102-103. Development of unsteady flow and temperature fields along the flow path as well as variations of the heat transfer coefficients from tube to tube are analyzed. As a general result of the present parametric computations, the effects of the transverse and longitudinal pitches on the heat removal rate have been investigated.
Exploration of Experimental Techniques to Determine the Condensation Heat Flux in Microchannels and Minichannels
Authors: Derby Melanie M. , Lee Hee Joon , Craft Rose C. , Michna Gregory J. , Peles Yoav , Jensen Michael K.
Published: March 2011

This study seeks to analyze and explore experimental methods to study condensation heat transfer in micro- and mini-channel. Following, an experimental setup was built and initial results are presented. Several experimental techniques were reviewed, while two, thermoelectric coolers and a copper-heat-flux-sensor were analyzed in detail for condensation heat flux. It was concluded that thermoelectric coolers were not suitable as heat flux sensors for single-phase validation, but the copper-heat-flux-sensor was appropriate to measure heat transfer coefficients at the mini-scale. Condensation heat transfer coefficients were obtained experimentally in seven parallel square mini-channels of diameter 1mm. Existing condensation correlations were applied to these data; micro- and mini-scale correlations captured the appropriate trends, but the macro-scale Shah (1979) correlation fit the data best.
Microscale Heat Transfer Measurements During Subcooled Pool Boiling of Pentane: Effect of Bubble Dynamics
Authors: Delgoshaei Payam , Kim Jungho
Published: March 2011

Measurements of space and time resolved heat transfer during subcooled pool boiling of pentane in earth gravity were obtained using a microscale heater array. Data from individual heater elements in the array were synchronized with bottom and side view images from two high-speed cameras. The heat transfer mechanisms during bubble growth were found to be dependent on bubble dynamics and bubble growth time. Single phase heat transfer mechanisms (transient conduction and/or microconvection) were found to be dominant for single bubbles with short growth times. Two phase heat transfer mechanisms (microlayer evaporation and/or contact line evaporation) were found to be dominant for bubbles with longer growth times.
Parametric Model for the Analytical Determination of the Solidification and Cooling Times of Semi-Crystalline Polymers
Authors: Sobotka Vincent , Boyard Nicolas , Delaunay Didier
Published: March 2011

This work presents an efficient method to quickly calculate with good accuracy (about 5%) the solidification time of an injected semi-crystalline polymer slab. Under some hypotheses this polymer can be considered as a phase change material with a constant phase change temperature. We use a noteworthy property established on the ratio between the thickness of a solidifying phase change finite medium and the solidified thickness in a semi-infinite medium. The knowledge of this ratio enables to predict analytically the solidification time in a 1D finite medium. This ratio can be parameterized as a function of characteristic numbers in phase change problems: Stefan numbers and the ratio of thermal diffusivities of both phases. The results are compared with those given by a complete model integrating the physics of the coupling between heat transfer and crystallization kinetics. The solidification times computed from both models are very close, demonstrating the relevance of the simplified model. Finally, we also get a very good accuracy in calculating the total cooling time, from injection to ejection.
Use of Hot-Film Anemometry for Wall Shear Stress Measurements in Unsteady Flows
Authors: Ariyaratne C. , Wang F. , He S. , Vardy A. E.
Published: March 2011

Hot-wire and hot-film anemometry are widely used in steady flows for instantaneous velocity measurements, and their use has been extended to velocity and wall shear stress measurements in unsteady flows. The technique of hot-film anemometry relies on the Reynolds analogy which relates the diffusion of heat to the momentum exchange. The paper investigates the applicability of the analogy in linearly varying flows. The investigation is a combination of CFD analyses using the Transition SST model and experimental measurements. Results show that, in a linearly accelerating flow, while wall shear stress increases immediately upon the onset of acceleration, heat transfer indicates a relative lag in response. A quantitative analysis of the effects of flow parameters shows that the deviant behaviour is especially pronounced with increasing acceleration and/or reduced initial flow Reynolds number. The initial deviation can be predicted using a non-dimensional parameter based on turbulence timescales and acceleration rate, thereby providing a possible solution to correcting wall shear stress measurements using hot-film anemometry in fast accelerating flows.
Heater Size and Orientation Effect on Pool Boiling of FC-72
Authors: Raj Rishi , Kim Jungho , McQuillen John , Sheredy William , Booth Wendell , Charpie James , Eggers Jeffrey , Funk Gregory , Funk Justin , Valentine Russell
Published: March 2011

A recent study on pool boiling for upward facing square heaters reported two pool boiling regimes depending on the relative importance of buoyancy and surface tension forces. At higher gravity levels and/or with larger heaters when the ratio of heater size Lh (length of a side for a square heater) to capillary length Lc was greater than 2.1, boiling was buoyancy dominated and the heat transfer results were heater size independent. Boiling was surface tension dominated and heat transfer results were heater size dependent when Lh/Lc<2.1 (small heaters and/or low gravity conditions). This paper studies the effects of orientation on the balance between buoyancy and surface tension forces. The threshold value of Lh/Lc for transition between pool boiling regimes was found to be 1.8 for heaters oriented at 45°, 90°, and 135°.
Optimization of 3D Branching Networks of Microchannels for Microelectronic Device Cooling
Authors: Dulikravich George S. , Martin Thomas J.
Published: March 2011

The aim of this work is to present a methodology to develop cost-effective thermal management solutions for microelectronic devices, capable of removing maximum amount of heat and delivering maximally uniform surface temperature distributions. The topological and geometrical characteristics of multiple-story three-dimensional branching networks of microchannels were developed using multi-objective optimization. The design variables which will be subject to optimization in this analysis are the geometric parameters of the microchannel network, i.e. the number of network floors in a 3D network, the amount of branching levels per floor, the connectivity of the cooling channels, their cross-sectional areas and lengths. A conjugate heat transfer analysis software package (CHETSOLP) and an automatic 3D microchannel network generator (3DBNGEN) were developed and coupled with a multi-objective particle-swarm optimization (MOPSO) algorithm with a goal of creating a design tool for 3D networks of optimized coolant flow channels. Numerical algorithms in the conjugate heat transfer solution package include a quasi-1D thermo-fluid solver (COOLNET) and a 3D steady heat diffusion solver, which were validated against results from high-fidelity Navier-Stokes equations solver and analytical solutions for basic fluid dynamics test cases. The conjugate heat transfer solution is achieved by simultaneous prediction of the quasi-1D internal flow-field in the microchannel network and 3D internal temperature field in the solid substrate [1]. Minimization of the pumping power requirement and maximization of total heat removal subject to temperature uniformity (at the heated surface) were the objectives. Pareto-optimal solutions demonstrate that thermal loads of up to 400 W/cm2 can be managed with 3D multi-floor microchannel networks, with pumping power requirements that are up to 50% lower with respect to pumping power requirements in currently used high-performance cooling technologies, such as jet impingement and hybrid impingement-microchannel flow.
Large-Eddy Simulation of a Buoyant Plume Past a Bluff Body
Authors: Suto Hitoshi , Hattori Yasuo
Published: March 2011

A large-eddy simulation (LES) of a buoyant plume past a bluff body is performed. A round heat source is placed at the center of a horizontal flat wall, and a bluff body in the shape of a thick round plate is floating right above the heat source. The modified Rayleigh number based on the total heat input is set at 1.2×1010. On the basis of past studies, the Smagorinsky model is adopted as a subgrid-scale (SGS) model, and a partial slip boundary condition based on the wall law is applied to a horizontal flat wall and a disk surface. The validity of numerical results is ascertained by comparison with theoretical solution and experimental data. The blocking of upward flow and imparting turbulence through a bluff body vary the process of developing a buoyant plume, while properties of a fully developed plume rarely vary. With heat from a bluff body, another buoyant plume is formed near the center, piled with upward flow passing around the bluff body. Moreover, main positions of buoyant production of turbulent kinetic energy move a point from near the side of the bluff body to a point near the central axis. This affects the transition to a fully developed plume in turbulence statistics.
Nano-Structured Two-Phase Heat Spreader for Cooling Ultra-High Heat Flux Sources
Authors: Hashimoto Mitsuo , Kasai Hiroto , Usami Kazuma , Ryoson Hiroyuki , Yazawa Kazuaki , Weibel Justin A. , Garimella Suresh V.
Published: March 2011

A two-phase heat spreader has been developed for cooling high heat flux sources in high-power lasers, high-intensity light-emitting diodes, and semiconductor power devices. The heat spreader targets the passive cooling of heat sources with fluxes greater than 5 W/mm2 without requiring any active power consumption for the thermal solution. The prototype vapor chamber consists of an evaporator plate, a condenser plate and an adiabatic section, with water as the phase-change fluid. The custom-designed high heat flux source is composed of a platinum resistive heating pattern and a temperature sensor on an aluminum nitride substrate which is soldered to the outside of the evaporator. Experiments were performed with several different microstructures as evaporator surfaces under varying heat loads. The first microstructure investigated, a screen mesh, dissipated 2 W/mm2 of heat load but with an unacceptably high evaporator temperature. A sintered copper powder microstructure with particles of 50 μm mean diameter supported 8.5 W/mm2 without dryout. Four sets of particle diameters and different thicknesses for the sintered copper powder evaporators were tested. Additionally, some of the sintered structures were coated with multi-walled carbon nanotubes (CNT) that were rendered hydrophilic. Such nano-structured evaporators successfully showed a further reduction in thermal resistance of the vapor chamber.
Oscillatory Magnetothermal Convection of Non-Ferrous Fluid in a Vertical Cylindrical Enclosure With a Heated Rotating Disk at Bottom Center
Authors: Akamatsu Masato , Higano Mitsuo
Published: March 2011

The non-ferrous fluid is contained in a vertical cylindrical enclosure, which is cooled from both above and side isothermally. The lower end-plate has an isothermally heated disk in the center, while the outer part is thermally insulated. The magnetic gradient generated inside a bore space in a super-conducting magnet as a source of the magnetic force is replaced with that generated by the steady electric current circulating within a single circular electric coil. When the enclosure was installed inside a bore space with the magnetic gradient, our transient three-dimensional numerical computations for a non-ferrous fluid in the enclosure were showed the following numerical results. In the enclosure with the heated static disk at bottom center, when the heated static disk was fixed above the magnetic center, the magnetothermal convection with the symmetric velocity and temperature fields was induced by the magnetic force. The heat transfer rate of magnetothermal convection became larger than that of natural convection. On the other hand, when the heated static disk was fixed below the magnetic center, the magnetothermal convection with the asymmetric velocity and temperature fields was induced by the magnetic force. The heat transfer rate of magnetothermal convection became smaller than that of natural convection. These magnetothermal convections were independent of time. In the enclosure with the heated rotating disk at bottom center, when the heated rotating disk was fixed above the magnetic center, the symmetric magnetothermal convection without the time dependence was also computed. However, when the heated rotating disk was fixed below the magnetic center, the asymmetric magnetothermal convection with the time dependence was computed.
Two-Phase Flow on the Shell Side of a Shell and Tube Heat Exchanger
Authors: Sadikin Azmahani , McNeil David A. , Barmadouf Khalid H.
Published: March 2011

Two-phase flow on the shell side of a shell and tube heat exchanger is complex. Several studies have produced flow pattern maps that show surprising differences in flow regime boundaries for data sets that contain relatively small variations in fluid and flow properties. Despite this, correlations for void fraction and pressure drop are sufficiently accurate to allow the thermal-fluid design of heat exchangers to be completed. However, these correlations are based on experimental data taken from tube bundles containing tubes with diameters less than 20 mm. This study examines their applicability to tube bundles containing tubes with a diameter of 38 mm. Results for void fraction and pressure drop are presented for air-water flows near atmospheric pressure. The results were obtained for flows through a thin-slice, in-line tube bundle containing 10 rows. The tube bundle contained a central column of tubes with half tubes placed on the shell wall to simulate the presence of other columns. The tubes were 38 mm in diameter and 50 mm long with a pitch to diameter ratio of 1.32. Previous studies have shown that the void fraction in a shell-side, gas-liquid flow becomes constant after only a few rows. Thus, the void fraction was only measured at one location. A single-beam, gamma-ray densitometer was used to measure void fractions near row 7 in the maximum gap between the rows. Corresponding pressure drops were obtained between rows 3 and 10. Data are presented for a mass flux range of 25–688 kg/m2s and a gas mass fraction range of 0.0005–0.6. The measurements are shown to compare reasonably well with predictions from correlations available in the open literature. This shows that these methods can be used for tube-bundles containing larger diameter tubes. Some elements of a heat-exchanger design require a more complex analysis. For example, tube vibration calculations require the distribution of void and phase velocity along the tube length. Such analysis can be provided by multiphase computational fluid dynamic (CFD) simulations. CFD approaches to modelling these flows require empirical inputs for the drag coefficient and the force on the fluid by the tubes. These are deduced from the measured data. The wall forces are shown to scale well with increased tube diameter, however, caution is required when selecting the drag coefficients.