bluuebunny/crossref_metadata_2025 huggingface dataset:
Optimization of Longitudinal Rectangular Fins With Asymmetrical Boundary Conditions
Authors: Luna-Abad J. P. , Alhama Francisco
Published: March 2011

Longitudinal rectangular fins with asymmetrical boundary conditions and different temperatures at each base were been studied and optimized using relative inverse thermal admittance as performance coefficient. A Neutral Line was defined and determined for different temperature ratios at the bases. Using the network method for the numerical simulation, the optimization problem was solved in a classical way: starting with the fin volume, heat transfer coefficient and thermal conductivity, the optimal geometry or aspect ratio (length/thickness ratio) for dissipating maximum heat from the fin surface was determined. The results are illustrated through tables and universal graphics made with optimal points, which, are independent from the temperature ratio at the bases. Besides the optimal thickness the graphics also provide, the fin effectiveness and the transversal Biot number. Finally, the location of the Neutral Line is represented for the optimal geometry and temperature ratios. To illustrate the optimization process practical examples are shown.
A Functional Approach of the Experimental Device Transfer Function for Temperature Measurements in the UV-Visible Wavelengths With Multispectral Method
Authors: Pierre T. , Re´my B. , Degiovanni A.
Published: March 2011

The paper presented here concerns the temperature measurement in the ultraviolet-visible wavelengths. Emphasis is placed on the technique used to determine the temperature, the multi-spectral method. Using this spectral bandwidth is two-fold: on one hand according to the Planck’s law, the measurement sensitivity is very important, and on the other hand, as this interval is narrow, it is possible to make the assumption of constant emissivity. Previous works have shown that most of the error measurements come from the difficulty to know the transfer function of the experimental device. Thus it is proposed here to get rid of the difficulty to know every parameter involved in the transmission function that can be modelled by a quadratic function. Results are compared with experimental measurements and error calculi are also presented.
A Boundary Value Model for an Evaporating Meniscus
Authors: Chatterjee Arya , Plawsky Joel L. , Wayner Peter C.
Published: March 2011

The constrained vapor bubble (CVB) experiment is an experiment to study the effect of low Bond numbers on the microscopic and macroscopic transport in a heat pipe. The microscopic (∼30 μm) contact line region, where the solid, liquid and vapor phase meet, is of fundamental importance in this study of fluid flow and heat transfer. This region, while dominated by interfacial forces, is controlled by the boundary conditions set by physics at the macroscopic scale (∼1 mm) on one side and the microscopic scale (∼ 0.1 μm) on the other. Recent experimental investigation by our group has shown that an effective means of changing the microscopic boundary conditions (the wettability of the fluid) is by introducing surface roughness at the nanoscale to the solid surface. Here we attempt to examine some of the experimental results in the light of a model. The model solves a nonlinear, fourth order evolution equation for the film thickness. It also provides the contact angle, interface curvature and heat transfer profile in the contact line region. The model agrees well with the experimental data. The presence of hydrodynamic slip at the solid liquid interface seems to improve the agreement.
Mathematical Modeling of a Radiative Heat Transfer Process in the System of the Solar Energy Concentrator-Receiver for Space High-Temperature Solar Power Plant
Authors: Leonov Victor V.
Published: March 2011

Unlimited capacity, extraordinary constancy of emission characteristics and environmental purity enables solar energy to become a very promising and widely used power source in Solar System outer space development missions up to Martian orbit.
Heat and Mass Transfer Caused by a Laminar Channel Flow Equipped With a Synthetic Jet Array
Authors: Tra´vni´cˇek Zdeneˇk , Dancˇova´ Petra , Kordik Jozef , Vit Toma´sˇ , Pavelka Miroslav
Published: March 2011

Low-Reynolds-number laminar channel flow is used in various heat/mass transfer applications, such as cooling and mixing. A low Reynolds number implies a low intensity of heat/mass transfer processes, since they rely only on the gradient diffusion. To enhance these processes, an active flow control by means of synthetic (zero-net-mass-flux) jets is proposed. This arrangement can be promising foremost in microscale. The present study is experimental in which a Reynolds number range of 200–500 is investigated. Measurement was performed mainly in air as the working fluid by means of hot-wire anemometry and the naphthalene sublimation technique. PIV experiments in water are also discussed. The experiments were performed in macroscale at the channel cross-section (20×100)mm and (40×200)mm in air and water, respectively. The results show that the low Reynolds number channel flow can be actuated by an array of synthetic jets, operating near the resonance frequency. The control effect of actuation and the heat transfer enhancement was quantified. The stagnation Nusselt number was enhanced by 10–30 times in comparison with the non-actuated channel flow. The results indicate that the present arrangement can be a useful tool for heat transfer enhancement in various applications, e.g., cooling and mixing.
Intermittent Flow Modeling: Part 2—Time-Varying Flows and Flows in Variable Area Ducts
Authors: Abraham J. P. , Sparrow E. M. , Tong J. C. K. , Minkowycz W. J.
Published: March 2011

The all-flow-regime model of fluid flow, previously applied in [1] to flows with axially and temporally uniform Reynolds numbers, has been implemented here for flows in which the Reynolds number may either vary with time or along the length of a pipe. In the former situation, the timewise variations were driven by a harmonically oscillating inlet flow. These oscillations created a succession of flow-regime transitions encompassing purely laminar and purely turbulent flows as well as laminarizing and turbulentizing flows where intermittency prevailed. The period of the oscillations was increased parametrically until the quasi-steady regime was attained. The predicted quasi-steady friction factors were found to be in excellent agreement with those from a simple model under which the flow is assumed to pass through a sequence of instantaneous steady states. In the second category of non-constant-Reynolds-number flows, axial variations of a steady flow were created by means of a finite-length conical enlargement which connected a pair of pipes of constant but different diameters. The presence of the cross-sectional enlargement gives rise to a reduction of the Reynolds number that is proportional to the ratio of the diameters of the upstream and the downstream pipes. Depending on the magnitude of the upstream inlet Reynolds number, the downstream fully developed flow could variously be laminar, intermittent, or turbulent. The presence or absence of flow separation in the conical enlargement had a direct effect on the laminarization process. For both categories of non-constant-Reynolds-number flows, laminarization and turbulentization were quantified by the ratio of the rate of turbulence production to the rate of turbulence destruction.
Bubble and Film Dynamics During Condensation of Refrigerants in Minichannels
Authors: Keinath Brendon L. , Garimella Srinivas
Published: March 2011

Condensation of Refrigerant R404a in channels of diameter 0.5–3 mm is investigated. The specific focus of this work is to obtain quantitative information on flow mechanisms using image analysis techniques on high speed video. Experiments are conducted on condensing R404a at mass fluxes ranging from 200 to 800 kg m−2s−1 at vapor qualities of 0.05 to 0.95. The experiments are conducted at the high pressures representative of actual operation of air-conditioning and refrigeration equipment, instead of extrapolating findings for adiabatic air-water mixtures at atmospheric pressure to high pressure refrigerant condensation. Pressure drops are also measured as a function of geometry and operating conditions during the flow visualization experiments. Quantitative image analysis enables detailed computation of void fraction and vapor bubble parameters such as frequency, velocity, diameter and length with a high degree of repeatability and with low uncertainties. The effect of operating conditions on the flow patterns, void fractions, and vapor bubble parameters for saturation temperatures ranging from 30 to 60°C is documented. The resulting void fraction models provide closure for pressure drop and heat transfer models, which has thus far not been possible in the reported works in the literature.
Integrated Steam/Organic Rankine Cycle (ISORC) for Waste Heat Recovery in Distributed Generation and Combined Heat and Power Production
Authors: Chudnovsky Yaroslav , Gotovsky Mikhail , Arefiev Valentin , Greenman Mark , Fomin Victor , Khinkis Mark
Published: March 2011

Energy efficiency improvement and waste heat utilization in power generation and energy intensive industrial applications are in the main focus of the researchers and engineers nowadays. A great deal of experience was gained by the industrial leaders such as ORMAT, Siemens, Caterpillar, Turboden, and others. However, the commercially and semi-commercially available systems for waste heat utilization have certain restrictions that limit the utilization cycle efficiency to approximately 18%. The paper presents an innovative concept of waste heat utilization system that allows reaching the utilization cycle efficiency up to 28–30% employing low-boiling media such as butane, propane, pentane and others. Applying such a concept to Distributed Generation systems the overall energy efficiency could be boost up to 58–60% and further up to 90% in case of CHP production.
An Investigation of Flat-Plate Oscillating Heat Pipes
Authors: Cheng Peng , Thompson Scott , Boswell Joe , Ma Hongbin
Published: March 2011

The heat transfer performance of flat-plate oscillating heat pipes (FP-OHPs) was investigated experimentally and theoretically. Two layers of channels were created by machining grooves on both sides of copper plate, in order to increase the channel number per unit volume. The channels had rectangular cross-sections with hydraulic diameters ranging from 0.762 mm to 1.389 mm. Acetone, water and diamond/acetone, gold/water and diamond/water nanofluids were tested as working fluids. It was found that the FP-OHP’s thermal resistance depended on the power input and operating temperature. The FP-OHP charged with pure water achieved a thermal resistance of 0.078°C/W while removing 560 W with a heat flux of 86.8 W/cm2. The thermal resistance was further decreased when nanofluid was used as the working fluid. A mathematical model predicting the heat transfer performance was developed to predict the effects of channel dimension, heating mode, working fluid and operating temperature on the thermal performance of the FP-OHP. Results presented here will assist in optimization of the FP-OHP and provide a better understanding of heat transfer mechanisms occurring in an OHPs.
3-D Numerical Study on Shell-Side Comprehensive Performance of Shell-and-Tube Heat Exchanger With Combined Helical Baffles
Authors: Chen Guidong , Xu Jing , Zen Ming , Wang Qiuwang
Published: March 2011

In order to improve heat transfer performance of conventional segmental baffled shell-and-tube heat exchangers (STHXs), the shell-and-tube heat exchanger with combined helical baffles (CMH-STHX) were invented. In the present study, the CMH-STHX is compared with three other STHXs which were set up with continuous helical baffles (CH-STHX), discontinuous helical baffles (DCH-STHX) and segmental baffles (SG-STHX), by Computational Fluid Dynamics method. The numerical results show that, for the same mass flow rate at the shell side, the overall pressure drop of the CMH-STHX is about 50% and 40% lower than that of SG-STHX and CH-STHX. The heat transfer coefficient of the CMH-STHX is between those of CH-STHX and DCH-STHX and it is 6.3% lower than that of SG-STHX. The heat transfer coefficient under unit pressure drop h/Δp is introduced to evaluate the comprehensive performance of STHXs. The h/Δp of the CMH-STHX is 7.5%, 6.5% and 87.4% higher on average than those of the CH-SHTX, DCH-STHX and SG-STHX. Furthermore, the total heat transfer rate of CMH-STHX is about 25% higher than that of SG-STHX for the same total pressure drop of shell side. Supported by these results, the new heat exchanger (CMH-STHX) may be used to replace the conventional shell-and-tube heat exchanger in industrial applications.
Effect of Working Fluid on Pulsating Heat Pipe Thermal Performance
Authors: Fumoto Koji , Kawaji Masahiro , Kawanami Tsuyoshi
Published: March 2011

Pulsating heat pipes (PHPs) are complex heat transfer devices whose thermal performance is governed by a strong thermohydrodynamic coupling. Recently, PHPs have attracted attention as novel electronic cooling devices. In this study, we used a self-rewetting fluid and obtained new experimental results for the improvement of the heat transport efficiency in PHPs. In contrast to the case of common liquids, the surface tension of self-rewetting fluids increases with temperature. Because of the increase in the surface tension at high temperatures, these fluids tend to flow toward the dry spot appearing on a heated surface, and thus, the boiling heat transfer is improved. We constructed PHPs from multiport extruded aluminum tubes with square channels. The PHPs consisted of a heating section, an adiabatic section, and a condensation section with a heat sink. We investigated the effect of the type of working fluid and the fluid fill ratio on the device performance. The working fluids employed were a self-rewetting fluid, water, and ethanol. The thermophysical properties of the working fluid affected the device performance, which also depended strongly on the boundary conditions employed during the PHP operation. In particular, the use of a self-rewetting fluid in the PHPs helped enhance the heat transport efficiency to a considerable extent.
A Heterogeneous Multiscale Model for Interfacial Thermal Transport
Authors: Balasubramanian Ganesh , Kappiyoor Ravi , Puri Ishwar K.
Published: March 2011

While nonequilibrium molecular dynamics (MD) simulations are capable of producing accurate results for mesoscale models, the simulations for a few million molecules can take considerable duration even when run on multiple processors. However, the alternative continuum simulations, while faster, cannot account for any temperature discontinuities at interfaces, and as such, can be rather inaccurate. We present a solution to this by incorporating both models into a multiscale computational scheme, where MD simulations are run over regions where temperature discontinuities are expected, while SSPH simulations are carried out over the remainder of the domain. In order to validate our model, we investigate thermal transport across a Si-Ge nanoscale interface that is embedded within a mesoscale system using both the novel multiscale model and pure MD simulations. The results indicate that the output from the coupled MD-SSPH model is in good agreement with those of the pure MD simulation when the boundary temperatures are specified. However, as SSPH does not account for phonon scattering at nonperiodic reflective boundaries, the local temperatures obtained from the multiscale model tend to be higher than those for the pure MD simulation when boundary flux is specified.
Experimental Verification of Kirchhoff’s Thermal Radiation Law on Surfaces With Emittance Spectra Characterized by Optical Interference Phenomena
Authors: Makino Toshiro , Wakabayashi Hidenobu
Published: March 2011

This paper discusses Kirchhoff’s law on thermal radiation. The logic of derivation of the law is reconsidered, first. Then, spectra of normal emittance εN and normal incidence hemispherical reflectance RNH are measured on surfaces whose microstructure changes in a non-equilibrium experimental system to examine the validity of the complementary relation between εN and RNH, which is the suggestion of Kirchhoff’s law. As the results of the examination, it is illustrated experimentally on a variety of surfaces that the complementary relation is valid within an experimental uncertainty. Provided this conclusion is admitted, the followings are suggested. If a surface is designed so that it does not reflect a narrow spectral region of radiation to any direction and reflects the other spectral regions of radiation much over the hemisphere, then the surface can be a new spectrally-functional emitter of radiation. It is also suggested that thermal radiation emitted at a surface is considered as the emission of plane waves at the surface rather than as the emission of spherical waves by electric dipoles.
An Experimental Study of Two Phase Multiple Jet Cooling on Straight-Finned Surfaces Using a Dielectric Fluid
Authors: Chien Liang-Han , Chang Chin-Yao , Liu Han-Yang
Published: March 2011

In the present study, a jet cooling device for electronics was investigated using FC-72 as the working fluid. The system temperature of jet cooling device was set at 50°C. The jets were 5 mm above the test surface, and the heated area of the test surface was 12×12 mm2. The nozzle plate had 5 or 9 pores of 0.24 mm in diameter. The test surfaces included: a smooth surface and two straight-finned surfaces of either 400 or 800μm fin height, 200 or 400 μm fin thickness and gap width. The volume flow rate varied from 50 to 150 ml/min (Re = 1655 ∼ 1860). The results showed that the heat transfer performance increased with increasing flow rate, or increasing surface area enhancement ratio. The finned surface of 800μm fin height, 200μm fin thickness and gap width yielded the best performance, which is about 250% greater than the smooth surface at 150 ml/min. As a promising chip cooling solution, the thermal resistance of spray cooling on this straight-finned surface varied between 0.14 and 0.15 K/W in the range of 60 ∼ 121 W of heat input when the present jet cooling system operated at 150 ml/min. Correlations of multiple jets cooling on smooth and straight-finned surfaces have been proposed. For the data in the range of Re = 1655 ∼ 8960, Bo = 0.024 ∼ 0.389, surface area enhancement ratio (γA) = 2.07 ∼ 5.32, this correlation predicts 94% of the present data within ±20%.
Droplet Impingement and Wetting Hysteresis on Textured Hydrophobic Surfaces
Authors: Varanasi Kripa K. , Paxson Adam , Smyth Katherine , Kwon Hyuk-min , Deng Tao
Published: March 2011

We study the wetting energetics and wetting hysteresis of sessile and impacting water droplets on superhydrophobic surfaces as a function of surface texture and surface energy. Detailed experiments tracking contact line motion simultaneously with contact angle provides new insights on the wetting hysteresis, stick-slip behavior and dependence on contact line velocity. For sessile drops, we find three wetting regimes on these surfaces: equilibrium Cassie at small feature spacing, equilibrium Wenzel at large feature spacing, and an intermediate state at medium feature spacing. We observe minimum wetting hysteresis not on surfaces that exhibit Cassie wetting but rather on surfaces in the intermediate regime. We argue that droplets on these surfaces are metastable Cassie droplets whose internal Laplace pressure is insufficient to overcome the energy barrier required to homogeneously wet the surface. These metastable Cassie droplets show superior roll-off properties because the effective length of the contact line that is pinned to the surface is reduced. We develop a model that can predict the transition between the metastable Cassie and Wenzel regimes by comparing the Laplace pressure of the drop to the capillary pressure associated with the wetting-energy barrier of the textured surface. In the case of impacting droplets the water hammer and Bernoulli pressures must be compared with the capillary pressure. Experiments with impacting droplets show very good agreement with this simple pressure-balance model.
Bubble Growth During Nucleate Boiling in Microchannels
Authors: Christopher David M. , Lin Xipeng
Published: March 2011

The flow and heat transfer in microchannels has been of great interest for some years now due to the significantly higher heat transfer coefficients useful for enhancing the heat transfer in very small but high heat flux applications such as electronics cooling. Nucleate boiling heat transfer in microchannels is also of great interest for generating even higher heat transfer rates; however, numerous studies have shown that the bubble formation immediately fills the entire microchannel with vapor significantly reducing the heat transfer since the bubble size is normally of the same size as the microchannel. The bubble growth process is very fast and difficult to study experimentally, even with high speed cameras. This study numerically analyzes the flow and bubble growth in a microchannel for various conditions by solving the Navier-Stokes equations with the VOF model with an analytical microlayer model to provide the large amount of vapor produced by the curved region of the microlayer. As each bubble forms, the large pressure drop around the bubble causes the bubble to quickly break away from the nucleation site and move quickly downstream. The bubbles are quite small with the size depending on the bulk flow velocity, subcooling and the heating rate. The numerical results compare quite well with preliminary experimental observations of bubble growth on a microheater embedded in the channel wall for FC-72 flowing in a microchannel.
Analysis of Thermal Transport in Zinc Oxide Nanowires Using Molecular-Dynamics Simulations With the ReaxFF Reactive Force-Field
Authors: Bharathi Arvind Krishnasamy , van Duin Adri
Published: March 2011

The objective of this paper is to determine the thermal conductivity of Zinc Oxide nanowire by Steady State Non-equilibrium and Transient Non-equilibrium Molecular Dynamics (SS-NEMD and T-NEMD) simulations using the ReaxFF reactive force field [5]. While SS-NEMD uses an equilibrated system and statistical averaging; T-NEMD uses cooling/heating rates in order to calculate the conductivity. The validity of the methods is first verified using Argon as a test case. The thermal conductivity of Argon thus calculated is compared with those presented by Bhowmick and Shenoy [20]. We then study the effects of system size using SS-NEMD method while effects of periodic boundary conditions — 1D, 2D and bulk variation of conductivity with temperature are analyzed using T-NEMD simulations. The results obtained compare favorably with those measured experimentally [12, 13]. Thus the SS-NEMD and T-NEMD methods are alternatives to the traditional Green-Kubo approach. In conjunction with ReaxFF, they are computationally cheaper than the Green-Kubo method and can be used to determine the thermal conductivity of materials involved in surface chemistry reactions such as catalysis and sintering.
Enhanced Heat Transfer for Boiling of a Refrigerant on a Micro-Structured Cylindrical Surface and Effect of Saturation Temperature
Authors: Chen Tailian
Published: March 2011

Boiling on the outside surface of cylindrical tubes is an important heat transfer process widely used in industry applications. It is known that boiling heat transfer coefficient increases with increasing saturation temperature. However, a quantitative measure of saturation temperature effect on boiling heat transfer is not readily available, especially for boiling on surfaces of microstructures. This work was motivated by the need to predict evaporator performance in a chiller while taking into account the effect of saturation temperature on boiling heat transfer coefficient. Experiments of boiling of refrigerant R123 on the micro-structured outside surface of an evaporator tube have been performed at three saturation temperatures in the range of 4.4 to 17.8°C. Water flows inside the test tubes and provides heat to the refrigerant for boiling. In addition, experiments of R123 boiling on smooth cylindrical tubes have been performed at the saturation temperature 4.4°C to provide a baseline to quantify the enhancement in boiling heat transfer due to microstructures on the test tubes. For boiling on the micro-structured surface, the boiling heat transfer coefficient increases by nearly 15% for the temperature range considered in this work. Measurements also showed that heat transfer coefficient for boiling on the test tubes of micro-structures is 12.3 times higher than boiling on the smooth surface. The Cooper correlation over-predicted by 40% the boiling heat transfer coefficient on the smooth cylindrical surface, but significantly under-predicted the performance for boiling on the tubes of micro-structures. It is found that the prediction of Cooper correlation multiplied by an enhancement factor 7.9 has a good agreement with measured heat transfer coefficient for boiling on the tubes of micro-structures at all the three saturation temperatures. Visual observations indicated that bubble departure characteristics on the micro-structured surface are different from those on the smooth surface. In addition to promoted bubble nucleation by re-entrant cavities on the micro-structured surface, the different bubble departure characteristics also contribute to the enhancement of boiling performance.
Study on Biocorrosion Induced by Sulfate-Reducing Bacteria on Heat Exchanger Material in Cooling Water
Authors: Men Hong , Peng Yan , Zhang Jing , Yang Shanrang , Xu Zhiming
Published: March 2011

Corrosion associated with microorganisms has been recognized for over 50 years and yet the study of microbiologically influenced corrosion (MIC) is relatively new. MIC can occur in diverse environments. Industrial cooling water from rivers, lakes and sea water contain lots of microorganisms which are able to grow and multiply under certain conditions when pH, water temperature and sunlight etc are suitable. MIC is one of key cause of heat exchanger faults. MIC of heat exchanger materials in cooling waters has caused expensive unplanned outages, the need for local repairs and, in some cases, completes system replacement. Sulfate-reducing bacteria (SRB) are the main harmful bacteria in circulating cooling water. Under anaerobic conditions, SRB reproduce a lot to produce mucus, which speed up the formation of corrosion, erode the metal equipment, plug the pipeline, affect the efficiency of heat transfer, and bring a lot of inconvenience to the production. The corrosion behaviors of 304 stainless steel induced by SRB were studied by measuring the polarization curves, electrochemical impedance Spectrum, weight loss measurements of fore-and-aft biocorrosion, and electrochemical noise method. The electrochemical noise signal of 304 stainless steel corrosion were de-noised by using a wavelet threshold de-noising method, which made the quadratic biorthogonal spline wavelet as the mother wavelet and adopted an soft threshold processing function. The result showed that the slope of cathodic polarization curves measured included with SRB is lower than the one obtained without SRB, while the slope of anodic polarization curves is higher than it. It is concluded that the process of anode polarization was repressed at the presence of SRB. With the growth of the culture time, the value of electrochemical impedance without bacteria reduced at first, then rose, while with bacteria fell at all times. It indicated that SRB accelerated the corrosion of stainless steel. With the dipping time, a biofilm, under which corrosion products congregate to form local battery corrosion, was formed on the surface of stainless steel, so that the serious pitting corrosion is induced. The results from electrochemical noise method showed that the quadratic biorthogonal spline wavelet much smoother and it can remove the noise from the electrochemical noise effectively, and can effectively identify the location of the sudden changes in the signal and accurately reflect the useful information of the signal. The more useful information and data about biocorrosion induced by SRB are also gotten.
Mechanism of Critical Heat Flux for Convective Boiling of FC-72 in a Diverging Microchannel Heat Sink
Authors: Lu Chun Ting , Pan Chin
Published: March 2011

The present study explores experimentally the mechanism of critical heat flux (CHF) of FC-72 in a 10 parallel diverging microchannel heat sink with different distributions of artificial nucleation sites (ANS). The effects of mass flux and number of ANS with or without degassing on CHF of FC-72 are investigated. During CHF, flow visualization shows that the dryout of liquid film in annular flow appears near the outlet region with frequent rewetting of liquid film with slug bubble or rewetting of liquid column on the dryout surface. The results demonstrate that diverging microchannels with ANS distributed uniformly along the whole channel (Type-2 system) shows better flow boiling performance and higher CHF and can be recommended as a high-heat-flux microchannel heat sink. Moreover, five CHF correlations in the literature for flow boiling in microchannels are compared, which generally underpredict the CHF data of the present study. This suggests that the present diverging design with ANS enhances the CHF. The comparison also reveals that Bowers and Mudawar correlation predicts the present data best with MAE of 15.8% for the type-2 system.
The Effect of Liquid Film Evaporation on Flow Boiling Heat Transfer in a Micro Tube
Authors: Han Youngbae , Shikazono Naoki , Kasagi Nobuhide
Published: March 2011

Flow boiling in micro channels is attracting large attention since it leads to large heat transfer area per unit volume. Generated vapor bubbles in micro channels are elongated due to the restriction of channel wall, and thus slug flow becomes one of the main flow regimes. In slug flow, sequential bubbles are confined by the liquid slugs, and thin liquid film is formed between tube wall and bubble. Liquid film evaporation is one of the main heat transfer mechanisms in micro channels and liquid film thickness is a very important parameter to determine heat transfer coefficient. In the present study, liquid film thickness is measured under flow boiling condition and compared with the correlation proposed under adiabatic condition. The relationship between liquid film thickness and heat transfer coefficient is also investigated. Pyrex glass tube with inner diameter of D = 0.5 mm is used as a test tube. Working fluids are water and ethanol. Laser focus displacement meter is used to measure the liquid film thickness. Initial liquid film thickness under flow boiling condition can be predicted well by the correlation proposed under adiabatic condition. However, measured liquid film thickness becomes thinner than the predicted values in the cases of back flow and short slugs. These are considered to be due to the change of velocity profile in the liquid slug. Under flow boiling condition, liquid film profile fluctuates due to high vapor velocity and shows periodic pattern against time. Frequency of periodic pattern increases with heat flux. At low quality, heat transfer coefficients calculated from measured liquid film thickness show good accordance with heat transfer coefficients obtained directly from wall temperature measurements.
Spatio-Temporal Measurement of Convective Heat Transfer for the Separated and Reattaching Flow
Authors: Nakamura Hajime
Published: March 2011

Spatio-temporal distribution of convective heat transfer behind a backward-facing step was measured in order to explore the dynamic behavior of the heat transfer caused by flow separation and reattachment. This measurement was achieved by using a high-speed infrared thermograph which measured temperature fluctuation on a thin foil heated electrically. The attenuation due to both the thermal inertia and the lateral conduction was restored by solving heat conduction equations inside the test surface. As a result, it was clarified that the heat transfer enhancement in the flow reattaching region had a spot-like feature. Each spot spread with time and overlapped others, forming a complex feature in the spatio-temporal characteristics of the heat transfer. The characteristic spanwise wavelength of this structure was approximately 1.2 times the step height for Reynolds number of 2000 ≤ ReH ≤ 5500, regardless of the step height. The characteristic frequency of the fluctuation in the reattaching region was St = 0.04–0.08 although there was no clear periodicity. This fluctuation is likely to be related to the low-frequency unsteadiness observed in the literature.
Convective Air Heat Transfer Through 10 PPI Aluminum Foams
Authors: Mancin Simone , Zilio Claudio , Rossetto Luisa , Cavallini Alberto
Published: March 2011

This paper presents the experimental heat transfer coefficient and pressure drop measurements obtained during single phase air heat transfer through four 10 pores per inch (PPI) metal foams with different porosities and different foam core heights, 40 mm and 20 mm respectively. The specimens have been inserted in an open-circuit type wind channel with rectangular cross section. The effect of the foam core height on the heat transfer has been analysed by imposing three constant heat flow rates at the bottom plate of the samples: 250, 325 and 400 W and varying the air mass velocity between 0.005 and 0.025 kg m−2 s−1 at around atmospheric pressure. The temperatures of the base, of the upper plate and of the air, at the inlet and outlet of the test section, have been measured by means of several T-type calibrated thermocouples. The pressure drops during air flow through the samples are recorded by means of a high accuracy differential pressure transducer. The experimental results are presented in terms of heat transfer coefficient, normalised mean wall temperature, pressure gradient, permeability and inertia coefficient. The measured pressure gradients and heat transfer coefficients have been compared against the predictions of two different models recently developed by present authors.
Optimization of Natural Air Cooling in a Vertical Channel of Electronic Equipment
Authors: Nishino Yasushi , Ishizuka Masaru , Hatakeyama Tomoyuki , Nakagawa Shinji
Published: March 2011

The natural convection cooling capability in a compact item of electronic equipment was investigated quantitatively by experiment and numerical simulation with a simple channel model. The optimization of the channel sizes, especially the clearance between heated walls, was discussed. The channel model, which consists of a vertical duct of rectangular section, was applied as the experimental model of electronic equipment in this study. The channel model consists of two heated copper walls and two transparent acrylic walls. The clearance between the copper walls of the channel was varied from 5 mm to 15 mm. Temperature measurement on the copper wall surfaces and velocity measurement of natural air flow in the channel by using a particle image velocimetry (PIV) were conducted for a few clearances of the channel. Numerical simulation was also carried out, with a model following the experimental setup, for more detailed discussion of various clearances of the channel. The relationship between the clearance and the temperature rise of the walls or velocity profile was investigated. The correlation between the Rayleigh number and the Nusselt number was obtained from measured temperature. The natural cooling capability and the velocity profiles depend on the clearance between the copper walls. When the wall clearances are more than 15 mm, the cooling is not enhanced. On the other hand, in the case that the clearance becomes less than 7 mm, the cooling capability becomes significantly lower. Consequently, it is clarified that the clearance from 8 mm to 10 mm is the best size for natural cooling from the view point of the space and the capability.
Inhibition of Blockage by Ice Crystal in Water in a Micro-Channel by Using Antifreeze Protein
Authors: Hagiwara Yoshimichi , Ohnishi Yosuke , Yamamoto Daichi
Published: March 2011

Experiments have been conducted into the freezing of water flow and the aqueous solution flow of winter flounder antifreeze protein in a micro-channel of 0.15mm in height, 1.2mm in width and 21mm in length. The local temperature is measured with a sheathed thermocouple of 0.1mm in diameter. Nearly flat interfaces, parallel to the cooling sidewall of the channel, are observed in the case of pure water regardless of flow rate. On the other hand, serrated interfaces are observed in the case of protein solution flow regardless of flow rate. The decreasing rate of the temperature changes when the interface reaches the thermocouple. Around this instant, a slight increase in the temperature is observed due to supercooling release. In the case of local cooling, the interface becomes more serrated as the flow rate increases. This is because the interaction between the interface and the protein continuously approaching the interface due to the flow occurs more frequently with an increase in the flow rate.
A New Two-Dimensional CFD Model to Predict the Performance of Natural Draught Wet-Cooling Towers Packed With Trickle or Splash Fills
Authors: Reuter Hanno C. R. , Kro¨ger Detlev G.
Published: March 2011

In the design of a modern natural draught wet-cooling tower, structural and performance characteristics must be considered. Air flow distortions and resistances must be minimised to achieve optimal cooling which requires that the cooling towers must be modelled two-dimensionally and ultimately three-dimensionally to be optimized. It is found that CFD models in literature are limited to counterflow cooling towers packed with film fills which are porous in one direction only and generally have a high pressure drop, as well as purely crossflow cooling towers packed with splash fill, which simplifies the analysis considerably. Many counterflow cooling towers are however packed with trickle and splash fills which have anisotropic flow resistances, which means the fills are porous in all flow directions and thus air flow can be oblique through the fill, particularly near the cooling tower air inlet. This provides a challenge since available fill test facilities and subsequently fill performance characteristics are limited to purely counter- and crossflow configuration. This paper presents a CFD model to predict the performance of natural draught wet-cooling tower with any type of fill configuration, which can be used to investigate the effects of different atmospheric temperature distributions, air inlet and outlet geometries, air inlet heights, variations in radial water loading and fill depth, fill configurations, rain zone drop size distributions, and spray zone performance characteristics on cooling tower performance for optimization purposes. Furthermore the effects of damage or removal of fill in annular sections and boiler flue gas discharge in the centre of the tower can be investigated. The fill performance characteristics for oblique air flow are determined by linear interpolation between counter- and crossflow fill characteristics in terms of the air flow angle. The CFD results are validated by means of corresponding one-dimensional computational model data.
Chaotic Mixing: A Sure Way for Optimal Thermal Conditions?
Authors: Speetjens M. F. M.
Published: March 2011

Chaotic fluid mixing is generally considered to enhance fluid-wall heat transfer and thermal homogenisation in laminar flows. However, this essentially concerns the transient stage towards a fully-developed (thermally-homogeneous) asymptotic state and then specifically for high Pe´clet numbers Pe (convective heat transfer dominates). The role of chaos in the asymptotic state at lower Pe, relevant to continuously-operating compact devices as, for instance, micro-electronics cooling systems, remains largely unexplored to date. The present study seeks to gain first insight into this matter by the analysis of a representative model problem: heat transfer in the 2D time-periodic lid-driven cavity flow induced via non-adiabatic walls. The asymptotic time-periodic thermal state is investigated in terms of both the temperature field and the thermal transport routes. This combined Eulerian-Lagrangian approach enables fundamental investigation of the connection between heat transfer and chaotic mixing and its ramifications for temperature distributions and heat-transfer rates. The analysis exposes a very different role of chaos in that its effectiveness for thermal homogenisation and heat-transfer enhancement is in low-Pe asymptotic states marginal at best. Here chaos may in fact locally amplify temperature fluctuations and thus hamper instead of promote thermal homogeneity. These findings reveal that optimal thermal conditions are not always automatic with chaotic mixing and may depend on a more delicate interplay between flow and heat-transfer mechanisms.
Large-Eddy Simulation for Temperature Fluctuation of Mixing Hot and Cold Fluids in a Tee Junction
Authors: Lu T. , Zhu W. Y. , Wang K. S.
Published: March 2011

In the present work the temperature fluctuations in a mixing tee were simulated on FLUENT platform using the Large-eddy simulation (LES) turbulent flow model with the sub-grid scale (SGS) model of Smagorinsky-Lilly (SL). The temperature and velocity fields, the normalized mean and fluctuating temperatures and velocities were predicted and analyzed with consideration of buoyancy. The normalized mean and fluctuating temperatures were defined to describe the time-averaged temperature and the time-averaged temperature fluctuation intensity. The numerical results of the normalized mean and fluctuating temperatures were compared with those of the experimental ones published in previous literature, which shows that numerical results have good agreement with the experimental data. The temperature fluctuations and power spectrum densities (PSD) at the locations having the strongest temperature fluctuations both in tee junction and on the walls were analyzed to evaluate the potential of thermal fatigue. The LES flow simulation and power spectral analysis are helpful for the Integrity evaluation of the structures such as the tee junction, elbow, piping system to predict the temperature fluctuation and thermal stripping in a tee junction of mixing hot and cold fluids.
DNS of Flame-Wall Interaction and Heat Transfer in a Constant Volume Vessel
Authors: Tsunemi Akihiko , Horiko Yoshihiro , Shimura Masayasu , Fukushima Naoya , Yamamoto Seiji , Nagaoka Makoto , Shim Youngsam , Tanahashi Mamoru , Miyauchi Toshio
Published: March 2011

Direct numerical simulations of turbulent hydrogen/air and methane/air premixed flames in a rectangular constant volume vessel have been conducted with considering detailed kinetic mechanism to investigate flame behaviors and heat losses. For the hydrogen cases, since heat release rate increases with pressure rise due to dilatation during combustion in the constant vessel, heat flux on a wall also increases. For the methane cases, the pressure increase does not raise wall heat flux significantly because of the decrescence of heat release rate caused by thermo-chemical reaction near a wall. Pressure waves caused by wall reflection fluctuate flame propagation for the hydrogen flames. Flame displacement speed decreases remarkably at the moment when the pressure wave passes through flame fronts from unburnt side to burnt side. However, the turbulent burning velocity at that time does not decrease because of increases of fluid velocity normal to the flame fronts.
Effects of Ultrasonic Irradiation on Ice Formation in Biological Tissue
Authors: Tada Yukio , Satou Yoshiaki , Kurokawa Makoto , Takimoto Akira , Onishi Hajime
Published: March 2011

A method to actively controlling crystallization is one of promising technique for cryopreservation. The object of this paper is to study the effect of ultrasonic irradiation on ice formation during freezing of biological tissue with supercooling. In the experiments, agar gel was frozen under irradiation of ultrasound at frequency of 28kHz. The measurements of temperature and the microscopic observation of ice crystals using fluorescent indicator were carried out. Firstly, it was found that the supercooled state was released by applying ultrasound at high intensity level, since ultrasonic cavitation effects were augmented. On the other hand, a weakly delay in the nucleation temperature was induced by applying ultrasound at low intensity level. Secondly, ice structure size decreased with increasing ultrasonic power under given supercooling degree at nucleation stage. The effect of ultrasound was enhanced by using large supercooling condition. Thirdly, paying attention to the thermal effect of ultrasound as changing temperature profile, sound and heat conduction in agar gel immersed in low temperature solution were numerically calculated. It was suggested that ultrasound at high frequency is effective to enlarge supercooled region.
Incipient Flow Boiling in a Vertical Channel With a Wavy Wall
Authors: Netz T. , Shalem R. , Aharon J. , Ziskind G. , Letan R.
Published: March 2011

In the present study, incipient flow boiling of water is studied experimentally in a square-cross-section vertical channel. Water, preheated to 60–80 degrees Celsius, flows upwards. The channel has an electrically heated wall, where the heat fluxes can be as high as above one megawatt per square meter. The experiment is repeated for different water flow rates, and the maximum Reynolds number reached in the present study is 27,300. Boiling is observed and recorded using a high-speed digital video camera. The temperature field on the heated surface is measured with an infrared camera and a software is used to obtain quantitative temperature data. Thus, the recorded boiling images are analyzed in conjunction with the detailed temperature field. The dependence of incipient boiling on the flow and heat transfer parameters is established. For a flat wall, the results for various velocities and subcooling conditions agree well with the existing literature. Furthermore, three different wavy heated surfaces are explored, having the same pitch of 4mm but different amplitudes of 0.25mm, 0.5mm and 0.75mm. The effect of surface waviness on single-phase heat transfer and boiling incipience is shown. The differences in boiling incipience on various surfaces are elucidated, and the effect of wave amplitude on the results is discussed.
Non-Condensable Gases Effect on Heat Transfer Processes Between Condensing Steam and Boiling Water in Heat Exchanger With Multirow Horizontal Tube Bundle
Authors: Morozov A. V. , Remizov O. V. , Tsyganok A. A.
Published: March 2011

The experimental investigations of non-condensable gases effect on the steam condensation inside multirow horizontal tube bundle of heat exchanger under heat transfer to boiling water were carried out at the large-scale test facility in the Institute for Physics and Power Engineering (IPPE). The experiments were carried out for natural circulation conditions in primary and secondary circuits of the facility at primary circuit steam pressure of Ps1 = 0.34 MPa. The experimental heat exchanger’s tube bundle consists of 248 horizontal coiled tubes arranged in 62 rows. Each row consists of 4 stainless steel tubes of 16 mm in outer diameter, 1.5 mm in wall thickness and of 10.2 m in length. The experimental heat exchanger was equipped with more than 100 thermocouples enabling the temperatures of primary and secondary facility circuits to be controlled in both tube bundle and in the inter-tubular space. The non-condensable gases with different density — nitrogen and helium were used in the experiments. The volumetric content of gases in tube bundle amounted to ε = 0.49. The empirical correlation for the prediction of the relative heat transfer coefficient k/k0 = f (ε) for steam condensation in steam-gas mixture was obtained.