bluuebunny/crossref_metadata_2025 huggingface dataset:
Flow Stability of Laminar Supercritical CO2 Sudden Expansion Flow With Field Synergy Principle
Authors: Deng Bi-Li , Zhang Xin-Rong
Published: March 2011

Forced convection of two-dimensional supercritical CO2 flow in plane symmetric sudden expansion duct is investigated numerically in this paper. Simulations were conducted under three wall boundary conditions, i.e., heating, cooling and isothermal, at low Reynolds numbers. Critical Reynolds numbers above which flow asymmetric occurs were examined under each condition. Results of the isothermal case agree well with previous numerical results. However, Compared with the isothermal case, smaller critical Reynolds numbers are found under both of heating and cooling conditions. In addition, the critical Reynolds numbers decrease with the increasing of heating/cooling intensity each under these two conditions. Furthermore, a new concept from the viewpoint of field synergy is introduced to illustrate this phenomenon of the reduction of flow stability.
Fluid Motion and Heat Transfer of Boiling With Impinging Flow in a Mini-Tube With Nonlinear Thermocapillary Solutions
Authors: Ono Naoki , Hamaoka Atsushi , Otsubo Yuta
Published: March 2011

Boiling heat transfer with impinging flow can be an effective way for cooling a small heated area such as CPUs and laser emitting devices. In the phenomena the movement of liquid layer on the heated surface strongly affects the detachment of boiling bubbles and the heat flux. In this study, nonlinear thermocapillary solutions such as button aqueous solutions were applied to this type of boiling with impinging flow aiming to promote heat transfer. These solutions have special characteristics that the surface tension increases as the temperature is raised over some temperature. It is expected that this tendency about the surface tension will promote the wetting of the heated surface and the detachment of boiling bubbles. In the experiment, T-shaped mini tubes were built with quartz tubes and used for flow boiling. The inner diameter of the tube was 2 mm and the outer diameter was 4 mm. The liquid flow impinged at the junction point where small area was heated by using a conducting thin film coated at the outer surface of the tube. The test fluids were butanol aqueous solution and pure water. The flow rate of the liquid was the order of 1 ml/min, the concentration of the butanol aqueous solution was 7.15 wt %. The liquid motion was observed by CCD video camera system. It was found from the experiment that the motion of the liquid layer of the butanol solution at the impinging area was very different from that of pure water. The layer of the butanol solution tended to extend to the hotter part of the heated area. In another experiment for precisely fixing the imposed heat flux value, T-shaped mini channel with small copper surface installed for heating the fluid was prepared. The cross section of the channel was rectangular shape of 3 mm × 3 mm, and the entire channel was made of insulating polymer material. It was found that the heat transfer of the boiling with impinging flow in using butanol solution was more promoted than that in using pure water.
Direct and Indirect Methods for Calculating Thermal Emission From Layered Structures With Nonuniform Temperatures
Authors: Wang L. P. , Basu S. , Zhang Z. M.
Published: March 2011

The determination of emissivity of layered structures is critical in many applications, such as radiation thermometry, microelectronics, radiative cooling, and energy harvesting. Two different approaches, i.e., the “indirect” and “direct” methods, are commonly used for computing the emissivity of an object. For an opaque surface at a uniform temperature, the indirect method involves calculating the spectral directional-hemispherical reflectance to deduce the spectral directional emissivity based on Kirchhoff’s law. On the other hand, a few studies have used a combination of Maxwell’s equations with the fluctuation-dissipation theorem to directly calculate the emissivity. The present study aims at unifying the direct and indirect methods for calculating thermal emission from layered structures with a nonuniform temperature distribution. Formulations for both methods are given to illustrate the equivalence between the indirect and the direct methods. Thermal emission from an asymmetric Fabry-Perot resonance cavity with a nonuniform temperature distribution is taken as an example to show how to predict the intensity, emissivity, and the brightness temperature. The local density of states, however, can only be calculated using the direct method.
Numerical Simulation of Temperature Field in Multi-Pass Compound Groove Weld for High Strength Pipeline Steel X80
Authors: Tong Lige , Kang Qilan , Wang Li , Sui Yongli , Bai Shiwu
Published: March 2011

Welding is a process for joining similar metals. Heat input and interpass temperature are important technical parameters that affect the welding temperature distribution field. 2D simulation of multi-pass compound groove of welding process for X80 pipeline steel is carried out by finite element software ANSYS, in which the distribution of T8/5 of welding filling layers is analyzed. The results show that the effect of heat input to T8/5 is significant. The T8/5 becomes longer with the increase of the heat input. Because of the obvious preheat influence of the preceding pass to the following, the T8/5 becomes longer with the increase of the weld pass quantity. When interpass temperature is equal to preheat temperature, the difference of the T8/5 on filling layer is tended to be insignificant. The T8/5 will be located within a reasonable range by choosing the appropriate heat input and interpass temperature.
Analysis of Fouling Characteristic in Enhanced Tubes Using Multiple Analogies
Authors: Li Wei , Li Guanqiu , Zhang Zhengjiang , Xu Zhiming , Yang Shanrang
Published: March 2011

This paper provides a comprehensive analysis on cooling tower fouling data taken from seven 15.54 mm I.D. helically ribbed, copper tubes and a plain tube at Re = 16000. A new mathematical model has been developed. The mass transfer coefficient Km is calculated through three analogies, which are Prandtl analogy, Von-Karman analogy, and j factor analogy. Fouling deposition is assumed to be determined by two processes, which are corresponding to heat flux and fluid friction. Von-Karman analogy is proved the best analogy among the three. Series of semi-theoretical fouling correlations as a function of the product of area indexes and efficiency indexes were developed. They were applicable to different internally ribbed geometries. The correlations can be directly used to assess the fouling potential of enhanced tubes in actual cooling tower water situations.
A Transient Technique to Determine Thermal Conductivity and Thermal Contact Resistance of Porous Materials
Authors: De Jaeger Peter , T’Joen Christophe , Huisseune Henk , De Paepe Michel
Published: March 2011

The application of a transient technique for the measurement of effective thermal conductivity and thermal contact resistance of porous media is discussed. A sensitivity analysis has proven that direct measurement of thermal contact resistance from a single temperature recording is not feasible. It requires the measurement of at least one additional sample with different height. The estimation of effective thermal conductivity is done by solving the inverse heat conduction problem (IHCP). The direct problem is treated analytically by describing the system with a quadrupole formalism in Laplace domain. The inversion procedure was found to be computational expensive. For this reason, the analytical solution of a reference case was obtained and used to validate a finite difference scheme. The indirect problem of the IHCP is solved via the Levenberg-Marquardt algorithm. Preliminary results are shown to demonstrate the method. Future actions consist of calibrating the experimental setup, benchmark with known materials and report uncertainty.
Experimental Investigation of Emissivity of Aluminum Alloys and Application of Multispectral Radiation Thermometry
Authors: Wen Chang-Da
Published: March 2011

Experiments were first conducted to measure the spectral normal emissivity values of a variety of aluminum alloys at 600, 700, and 800 K. Multispectral radiation thermometry (MRT) using linear emissivity models (LEM) and log-linear emissivity models (LLE) were then applied to predict surface temperature. Results show that the spectral emissivity decreases with increasing wavelength and increases with increasing temperature. Alloy effect becomes evident at higher temperature. The surface oxidation becomes fully-developed after the first hour heating and results in constant emissivity. Half of temperature predictions by MRT emissivity models provide the absolute temperature error under 10% and a quarter if the results are under 5%. The better emissivity model to suitably represent the real surface emissivity behaviors the more accurate inferred temperature by MRT can be achieved. Increasing the order of emissivity model and increasing the number of wavelengths cannot improve temperature measurement accuracy. More accurate temperature measurement by MRT can be achieved at higher temperature. Overall, three emissivity models give good results most frequently and provide the best compensation for different alloys, the number of wavelengths, and temperatures.
Numerical Simulation of the Injection of Solution Sprays Into a Plasma Jet
Authors: Shan Y. G. , Hu Y. , Qi X.
Published: March 2011

Plasma spraying using solution precursors is a relative new thermal spray technology which enables to elaborate finely structured ceramic coatings with nano- and sub-micrometric features. This process involves the injection of a solution spray of ceramic salts into a plasma jet either as a liquid stream or gas atomized droplets. Solution droplets or the stream interacts with the plasma jet and break up into fine droplets. The solvent vaporizes very fast as the droplets travel downstream followed by precipitation and pyrolysis. Depending on the heating and trajectory history of droplets, different states of particles are formed and impact on the substrate to generate coatings. The deposition process and the properties of the coating are extremely sensitive to the process parameters, such as torch operating conditions, injection modes, injection parameters, and substrate temperatures. This paper describes a 3D model to simulate the transport phenomena and the trajectory and heating of the solution spray in a plasma jet. A simplified model is employed to simulate the evolution process and the formation of the solid particle from the solution droplet. The temperature and velocity fields of the jet are predicted. The effect of the injection angle, injection velocity, the torch operating power and the substrate position on the heating and trajectory of injected droplets is discussed. The particle/droplet size distributions on the substrate are predicted for different process parameters.
Cooling Characteristics of Ultrafine Cryoprobe Utilizing Convective Boiling Heat Transfer in Microchannel
Authors: Okajima Junnosuke , Maruyama Shigenao , Takeda Hiroki , Komiya Atsuki , Jeong Sangkwon
Published: March 2011

This paper describes a novel cooling system to be applied in cryosurgery. An ultrafine cryoprobe has been developed to treat small lesions which cannot be treated by conventional cryoprobes. The main problem of the ultrafine cryoprobe is the reduction of the heat transfer rate by the small flow rate due to the large pressure drop in a microchannel and the large ratio of the surface area to the volume. In order to overcome these problems, we utilized boiling heat transfer in a microchannel as the heat transfer mechanism in the ultrafine cryoprobe. The objectives of this paper are to develop an ultrafine cryoprobe and evaluate its cooling characteristics. The ultrafine cryoprobe has a co-axial double tube structure which consists of inner and outer stainless steel tubes. The outer and inner diameters of the outer tube are 0.55mm and 0.3mm, respectively. The outer and inner diameters of the inner tube are 0.15mm and 0.07mm, respectively. The inner tube serves as a capillary tube to change the refrigerant from liquid state to two-phase flow. Furthermore, two-phase flow passes through the annular passage between the inner and out tube. The hydraulic diameter of the annular passage is 0.15mm. Furthermore, HFC-23 (Boiling point is −82.1°C at 1atm) is used as the refrigerants. The temperature of the ultrafine cryoprobe was measured. The lowest temperatures were −45°C in the insulated condition and −35°C in the agar at 37°C (which simulates in vivo condition). Furthermore, the frozen region which is generated around the ultrafine cryoprobe was measured 5mm from the tip of cryoprobe at 120s, and resulted to be 3mm in diameter. Moreover, the change of the refrigerant state is calculated by using the energy conservation equation and the empirical correlations of two-phase pressure drop and boiling heat transfer. As a result, the refrigerant state in the ultrafine cryoprobe depends on the external heat flux. Finally, the required geometry of the ultrafine cryoprobe to make high cooling performance is evaluated.
Thermal Performance Improvement of a Heat Sink With Piezoelectric Vibrating Fins
Authors: Park Hee Seung , Kim Sung Jin
Published: March 2011

A heat sink with piezoelectric vibrating fins is developed through attaching piezoelectric actuators to the fins of a heat sink, and the heat transfer characteristics of the heat sink are experimentally investigated. Thermal performance improvement of the heat sink by the vibration of the fins is observed compared to the thermal performance of a natural convection heat sink with static fins under a fixed heat sink geometry condition. The thermal performance of the heat sink changes as the vibration amplitude of the fins or the vibration frequency of the fins changes. Particularly, if the vibration frequency of the fins matches up to the natural frequency of the fins, the vibration amplitude is significantly increased by resonance and the thermal performance also increases. The natural frequency of the fins changes with the heat sink temperature because the geometry of the fins changes and the properties of the fins change due to the temperature change.
Heat Transfer From Rotating Discs With Finite Thickness Subjected to Outer Air Streams
Authors: aus der Wiesche Stefan
Published: March 2011

The heat transfer from rotating discs in an outer air stream is of major importance for many technical applications. Experimentally determined heat transfer coefficients are presented for a large range of rotational and crossflow Reynolds numbers including also the effects of finite disc thickness and incidence to the uniform air stream. The extreme conditions of a rotating disc in still air and a stationary disc in an air crossflow are considered, too.
Optimization of the Capillary Tube for Domestic Refrigerator With Mixture of Propane and Isobutene as Refrigerant
Authors: Huan Zhongjie , Guo Chaohong
Published: March 2011

The hydrocarbon mixture of R290/R600a with mass ratio of 50/50 is regarded as a promising long-term refrigerant to replace R12 in domestic refrigerators since it can potentially provide higher COP than R12 system under the same operating conditions. In practical application, the length of the capillary tube should be matched with this alternative mixture; otherwise, the refrigerator system can probably consume more energy than the R12 system. In this paper, a simulation model of adiabatic capillary tube, in which some empirical coefficients are modified due to their inapplicability for R290/R600a, is established to illustrate the influences of the length of the capillary tube on the performance of the refrigerator. During the investigation a domestic refrigerator of 150L is used to test the thermodynamic cycle performances of R290/R600a (50/50) under conditions with different lengths of the capillary tube. The experimental results indicate that the R290/R600a (50/50) mixture with the optimized capillary length, which was obtained from simulation, has the higher energy efficiency and a lower discharge temperature than the original R12 system. The proposed mixture is proven to be a suitable long-term alternative in terms of its excellent thermodynamic cycle performances.
Experimental Study on the Jet Flow Issued From an Asymmetrical 2D Nozzle
Authors: Murayama Taisuke , Hirata Katsuya , Funaki Jiro
Published: March 2011

This research concerns the jet from an asymmetrical two-dimensional nozzle, especially about the influence of the lip length of the nozzle. The authors conduct experiments at a Reynolds number Re of 6000. The aspect ratio of the nozzle exit is fixed to 300. And, the lip length is 0, 2.0h, 3.3h and 5.0h, where h denotes the height of the nozzle exit. Using a hot-wire-anemometer, the authors show mean-velocity profiles at various downstream sections, in order to reveal fundamental characteristics of the jet in both the near and far downstreams. Furthermore, in the near downstream, the authors visualise the flow by smoke so as to get quantitative information using a PIV technique, and perform the dominant-frequency measurements.
Numerical Study of Flow and Cooling Characteristics of Impinging Liquid Jets on a Moving Plate
Authors: Son Sangil , Son Gihun , Park Ilseouk , Lee Piljong
Published: March 2011

Liquid jet impingement on a moving plate, which is applicable to cooling of hot plates in a steel-making process, is investigated numerically by solving the conservation equations of mass, momentum and energy in the liquid and gas phases. The free-surface or liquid-gas interface is tracked by an improved level-set method incorporating a sharp-interface technique for accurate imposition of stress and heat flux conditions on the liquid-gas interface. The level-set approach is combined with a non-equilibrium k-ε turbulence model. The computations are made for multiple jets as well as a single jet to investigate their flow and cooling characteristics. Also, the effects of moving velocity of plate, jet velocity and nozzle pitch on the interfacial motion and the associated flow and temperature fields are quantified.
Effect of Spacer Grids on the Local Heat Transfer
Authors: Cesna Benediktas , Uspuras Eugenijus
Published: March 2011

The paper presents the experimental data of the local heat transfer in the fuel rod assembly with honeycomb type spacer grids. It was established experimentally that the separation of the boundary layer behind the spacer grid increases the interchannel coefficient of transverse mixing of flow, and therefore, the local heat transfer. If the distance between the spacer grids is relatively small, they influence one another through the redistribution of mass flow in elementary channels. The local heat transfer behind the spacer grid varies with a distance from it. The local heat transfer can be determined accurate to within ± 3% using similar equations for any elementary cell.
Liquid-Solid Contact at Evaporation of Water Liquid Droplet Emulsified With Lubricant and Its Adhesion Situation
Authors: Nagai Niro , Matsumura Takenori , Yamaguchi Shingo , Maeda Yasuyuki , Ikebata Kazuhiko
Published: March 2011

In aluminum die casting process, water-soluble lubricant which is a kind of emulsion and consists of oil, surfactant and water are sprayed onto metal die surfaces, where the surface temperature is relatively high, over 300°C. When a lot of small liquid droplets of the water-soluble lubricant impact on a superheated surface, it is important to assure that liquid-solid contacts occur and oil component adhere onto the surface. However, many parameters such as liquid temperature, concentration, diameter and impact speed, affect this phenomena, so the phenomena themselves has not been totally revealed and it has not been fully established how to assure the liquid-solid contact and adhesion of oil component. Therefore, as a fundamental and experimental research, this paper aims to obtain essential information and data of liquid-solid contact of water-soluble lubricant and its adhesion situation in order for improving aluminum die-casting process. Firstly, evaporation curves, especially Leidenfrost temperature, of water droplets including water-soluble lubricant on metal surface were measured. Experimental parameters were as follows: temperature of droplet, 20 ∼ 80°C, concentration, 0 ∼ 50% corresponding to surface tension 20 ∼ 70mN/m, falling height of droplet, 1 ∼ 9cm, and two kinds of lubricant. The metal surface was made of SKD61, usually used as materials of die casting. Experimental results show that Leidenfrost temperature is not strongly affected by droplet velocity, but significantly affected by concentration and liquid temperature. It is worthy to note that there was found to be optimum temperature and concentration of droplet from view points of aluminum die casting. These results can be explained by effects of surface tension and interfacial temperature between solid and liquid. Secondly, visual observations of these phenomena by high-speed camera and measurements of electric resistance between droplet and metal surface were conducted. Surface chemical analysis was also performed by SEM/EDX after evaporation experiments for detecting carbon and silicone components on the surface to check adhesion situations of lubricant. The obtained results show that liquid-solid contact becomes gradually localized and limited as initial surface temperature increases. However, liquid-solid contact surely occurred at higher surface temperature than Leidenfrost temperature, 210 ∼ 270°C, and lubricant adhesion was clearly confirmed even if liquid-solid contact duration was limited to very small time scale. This result implies that oil component can adhere on die surface of higher temperature than Leidenfrost temperature.
Experiment Study on Ground Temperature Recovery and Operating Performance of Intermittent Heat Storage in GSHP System
Authors: Li Sufen , Dai Lanhua , Shang Yan
Published: March 2011

The ground source heat pump system (GSHP) continuous operation will result in heat accumulation of the soil around the underground heat exchangers leading the descend operation performance of heat pump system. Based on experiment of GSHP system in summer season, the temperature distribution of humid soil around the vertical boreholes, the power consumption of the heat pump unit, the water temperature in the inlet and outlet of the underground heat exchangers, the heat release rate of per unit length of the buried pipes in humid soil and the cooling coefficient of performance (COP) for the heat pump unit were acquired during the different intermittent heat storage modes. This study investigated the impact of soil temperature change around the borehole wall on the performance of heat pump operation, the influence of intermittent heat storage on the soil temperature near the borehole wall and the performance of heat pump unit, and the effect of intermittent heat storage process on the recovery rate of soil temperature. The results showed that proper control of the operation and interval time of GSHP can effectively improve the soil temperature field around the underground heat exchangers and enhance the performance of GSHP system, thus achieving high efficiency operation of heat pump units.
Optical Tomography With a Least Square Finite Element Formulation of the Collimated Irradiation in Frequency Domain
Authors: Balima Olivier , Boulanger Joan , Charette Andre´ , Marceau Daniel
Published: March 2011

This paper presents a numerical study of optical tomography in frequency domain for the reconstruction of optical properties of scattering and absorbing media with collimated irradiation light sources. The forward model is a least square finite element formulation of the collimated irradiation problem where the intensity is separated into its collimated and scattered parts. This model does not use any empirical stabilization and moreover the collimated source direction is taken into account. The inversion uses a gradient type minimization method where the gradient is computed through an adjoint formulation. Scaling is used to avoid numerical round errors, as the output readings at detectors are very low. Numerical reconstructions of optical properties of absorbing and scattering media with simulated data (noised and noise-free) are achieved in a complex geometry with satisfactory results. The results show that complex geometries are well handled with the proposed method.
Numerical Simulation of Impinging Jet Cooling
Authors: Mikhaylov Yury V. , Myagkov Leonid L. , Malastowski Nikolay S.
Published: March 2011

This paper presents a technique for heat transfer prediction of impinging jet cooling of internal combustion engine piston based on numerical simulation in ANSYS CFX. This technique implies the simulation of the hydrodynamics of the interaction between cooling oil jet and moving piston. The following steps were made in the stage of verification: the results of oil flow simulation in open channel were compared with corresponding experimental data; the interaction between the oil jet and piston underside was simulated and the results were qualitatively compared with the shots obtained in experimental study by means of hi-speed photography; the quantitative estimation of heat transfer computation results is made by comparing obtained temperature values with experimental data and shows satisfactory results agreement. The comparison of the piston temperature fields shows the efficiency of using impinging jet for piston cooling. In conclusion the further development trends of presented technique were suggested.
Particles and Droplets Coagulation and Clusters Formation in the Earth’s Atmosphere and in Technical Applications
Authors: Derevich I. V.
Published: March 2011

On the base of modern probability approach closed equation for probability density function of coordinates and velocities of two particles in turbulent flow is obtained. The system of equations for balance of mass, averaged velocities and intensities of turbulent chaotic motion of particles with account of correlated motion of particles are deduced. The closed expressions for intensity of relative chaotic motion between particles are obtained on the base of probability density function of particles displacement with correlation effects. Spectral presentation of second velocity moments of gas phase is used for calculation of intensity of particles relative chaotic motion. Boundary condition taking into account coefficients of new particle formation and momentum restitution during two particles collision is found. Formula for calculation of turbulent coagulation kernel of particles in gravity field is gain. Influence of cloud turbulence and turbulence in a pipe flow on intensity of droplets coagulation is studied. Strong effects of relative turbulent diffusion between droplets, droplets inertia and droplets gravitational settling on intensity of coagulation are found out. Connection between internal structure of turbulence type and coagulation rate is illustrated. The calculation results are compared with data of large eddy simulations. The results of calculation intensity of droplets relative motion in atmospheric conditions are presented.
Thermoelectricity at the Organic-Inorganic Interface
Authors: Yee Shannon , Malen Jonathan , Reddy Pramod , Segalman Rachel , Majumdar Arun
Published: March 2011

Electronic transport in molecular junctions has been studied through measurements of junction thermopower to evaluate the feasibility of thermoelectric (TE) energy generation using organic-inorganic hybrid materials. Energy transport and conversion in these junctions are heavily influenced by transport interactions at the metal-molecule interface. At this interface the discrete molecular orbitals overlap with continuum electronic states in the inorganic electrodes to create unique energy landscapes that cannot be realized in the organic or inorganic components alone. Over the past decade, scanning probe microscopes have been used to study the electronic conductance of single-molecule junctions[1–5]. Recently, we conducted measurements of junction thermopower using a modified scanning tunneling microscope (STM)[6]. Through our investigations, we have determined: (i) how the addition of molecular substituent groups can be used to predictably tune the TE properties of phenylenedithiol (PDT) junctions[7], (ii) how the length, molecular backbone, and end groups affect junction thermopower[8], and (iii) where electronic transport variations originate[9]. Furthermore, we have recently found that large (10 fold) TE enhancement can be achieved by effectively altering a (noble) metal junction using fullerenes (i.e., C60, PCBM, and C70). We associate the enhancement with the alignment of the frontier orbitals of the fullerene to the chemical potential of the inorganic electrodes. We further found that the thermopower can be predictably tuned by varying the work function of the contacts. This yields considerable promise for altering the surface states at interfaces for enhanced electronic and thermal transport. This paper highlights our work using thermopower as a probe for electronic transport, and reports preliminary results of TE conversion in fullerene-metal junctions.
Experimental Study of Heat Transfer in Horizontal Gas-Liquid Intermittent Flow
Authors: Noville Ivan , Bannwart Antonio C.
Published: March 2011

The present work is aimed at the experimental measurement of the two-phase heat transfer coefficient in the intermittent horizontal gas-liquid flow pattern, as well as its comparison with some existing correlations. The intermittent flow pattern is very common in oil production flow lines. In order to reach this objective an experimental apparatus was built, consisting of a 5 cm i.d., 6.0 m long pipe test section fed up with several air-water mixtures pre-heated at different inlet temperatures. Heat transfer was accomplished by surrounding the pipe with a cooling water jacket under various inlet flow rates and temperatures, flowing co-currently with the central pipe gas-liquid mixture. The heat transfer coefficient was experimentally determined and compared with four published correlations. Among then, a mechanistic model was selected for further improvement, with very satisfactory results.
Thermal Optimization of Plate-Fin Heat Sinks With Variable Fin Thickness
Authors: Kim Dong-Kwon , Jung Jaehoon , Kim Sung Jin
Published: March 2011

In the present paper, we conducted thermal optimization of plate-fin heat sink with fin thickness varying in the vertical direction. The model based on volume averaging theory (VAT) was used for this optimization. It is shown that the thermal resistance of plate-fin heat sink is reduced by allowing the fin thickness to increase in the vertical direction. In the case of water-cooled heat sink, the thermal resistance decreases up to about 20%. The amount of the reduction increases as either pumping power increases or the length of heat sink decreases.
Performance and Liquid Water Distribution in PEFCs With Different Anisotropic Fiber Directions of the GDL
Authors: Soe Naing Kyaw Swar , Tabe Yutaka , Chikahisa Takemi
Published: March 2011

To maintain proton exchange membrane fuel cells (PEFC) at high efficiencies without flooding, it is necessary to control the liquid water transport in the gas diffusion layer (GDL). This experimental study investigates the effects of the GDL fiber direction on the cell performance using an anisotropic GDL. The results of the experiments showed that the efficiency of the cell was much better when the fiber direction was perpendicular to the channel direction, and that the cell with perpendicular fibers was more tolerant to flooding than the cell with fibers parallel to the channel. To determine the mechanism that gives rise to the fiber direction effects, the liquid water behavior in the channel was observed through a glass window on the cathode side. Additionally, a small cell with a 2 cm2 active area was made, to investigate the water distribution inside the GDL by freezing the water and observing the ice distribution. These ice pictures showed that reactions are more active under channels than under ribs. This is because the accumulated water prevents reaction under the ribs, and indicates the importance of water removal from the rib zones.
Two Phase Gas-Liquid Bubbly Flow Modeling in Vertical Mini Pipe
Authors: Kebriaee M. H. , Karabi H. , Khorsandi S. , Saidi M. H.
Published: March 2011

Studies on two-phase flow in small scale pipes have become more important, because of the application of mini-scale devices in several engineering fields including, high heat-flux compact heat exchangers, and cooling systems of various types of equipment. In a mini pipe the behavior of two phase flow is not the same as flow in conventional pipes. The difference is caused by different effective forces; for e. g. inside a mini pipe capillary forces are more important in comparison with gravitational forces. This paper is devoted to numerical simulation of gas-liquid two phase flow in a vertical mini pipe. Prediction of bubble shape and the effects of gas and liquid velocities on flow characteristics are considered. Also simulation involves prediction of changes in average void fraction along pipe axis. Numerical simulations in this paper are performed by a designed and developed CFD package which is based on Eulerian-Eulerian approach. The governing equations which are solved in the CFD package are momentum, continuity and Fractional Volume of Fluid (VOF) function equations. The fluid is assumed to be viscous and incompressible. The pressure-velocity coupling is obtained using the SIMPLEC algorithm. The geometry, which have been studied in this paper, is a D = 1.02 mm pipe, with 500 mm height. Bubble shape and the distribution of void fraction in a mini pipe are related to many parameters such as: gas and liquid velocities, pressure losses and etc. Since these mechanisms vary over time, time-average value of void fraction is used. Comparisons between Numerical results and experimental work which performed by hibiki et al. [1] indicated good agreement. Also results have shown that the present model is capable to simulate the behavior of nitrogen-water two phase flow in a mini pipe with acceptable accuracy. Furthermore, the results indicates that average void fraction along the pipe axis is related to the height and nitrogen superficial velocity. Also it is observed that at constant nitrogen superficial velocities, average void fraction decreases with water superficial velocity increments.
Computational Models for Predicting Cooling Tower Fill Performance in Cross-Counterflow Configuration
Authors: Reuter Hanno C. R. , Kro¨ger Detlev G.
Published: March 2011

In cooling towers packed with trickle or splash fills, which have almost isotropic or anisotropic flow resistance, the air flow through the fill is oblique or in cross-counterflow to the water flow, particularly at the cooling tower inlet when the fill loss coefficient is small or when the fill hangs down into the air inlet region. This results that the fill Merkel number or transfer characteristic for cross-counter flow is between that of purely counter- and crossflow fills. When using CFD to model natural draught wet-cooling tower performance for isotropic fill resistance, two- or three-dimensional models are therefore required to determine fill performance. In this paper, the governing fundamental partial differential equations are derived in cylindrical and Cartesian co-ordinates to determine the cooling water temperature, water evaporation rate, air temperature and air humidity ratio in two-dimensional cross-counterflow fills for both saturated and supersaturated air. To solve these equations, a relation is proposed to determine Merkel numbers for oblique air flows by linear interpolation and extrapolation of purely cross- and counterflow Merkel numbers in terms of the air flow angle. This model is compared to analytical Merkel numbers obtained for different air flow angles using a single drop trajectory model. A linear upwind computational model and an Eulerian FLUENT® model are developed to evaluate fill performance characteristics from test data and to model fill performance in cooling towers respectively. The results of these two models are compared and verified with a FLUENT® Euler-Lagrange model.
A Method to Determine the Performance Characteristics of Cooling Tower Spray Zones
Authors: Reuter Hanno C. R. , Viljoen Dawie J. , Kro¨ger Detlev G.
Published: March 2011

Cooling tower spray zones play an important role in cooling tower performance. Ideally they must distribute the cooling water uniformly onto the fill and must produce small drops at minimal pressure head to maximise heat and mass transfer in the spray zone with minimal pumping power. Limited thermal performance characteristic data is found in literature for cooling tower spray zones, since it is virtually impossible to measure spray zone performance accurately. In this paper, the method used to model the performance of cooling tower spray zones and results obtained for a medium pressure swirl nozzle are presented. Water flow distribution and drop size distribution tests are conducted on cooling tower spray nozzles to investigate the effects of varying different operating parameters, such as air and water flow rates, and installation parameters, such as nozzle height, nozzle spacing and direction of spray, on performance. The suitability of superimposing single nozzle flow distribution data to obtain the water distribution for a grid of equally spaced nozzles with variable nozzle spacing is investigated. Furthermore, a single nozzle simulation model is developed and used to model single spray nozzles. The single nozzle model and the superposition model are subsequently used to obtain initial drop conditions to model the spray zone using the commercial CFD package FLUENT®. The proposed modelling approach allows for the evaluation and performance prediction of existing and new nozzle design configurations. Correlations are presented for the Merkel number and loss coefficient for the downspray nozzle investigated.
A Reconstruction Scheme Based on the Radiative Transfer Equation for Graded Index Semi-Transparent Media
Authors: Boulanger Joan , Balima Olivier , Charette Andre´
Published: March 2011

Graded refractive index media appear in numerous industrial applications such as non-isothermal flows, optics material processing, biological imaging. Refractive index gradient has been an early help for combusting flow visualisation. The numerical treatment of radiative transport is difficult in such media due to the curvature of rays, especially when the media are not optically thick. Computer-aided remote probing (inversion) is done today with the help of the diffusion approximation adapted to varying refractive index media but is unsuitable for thin media. Therefore, it is important to develop an approach allowing the use of the radiative transport equation which is the most complete formalism for radiative transfer to date and to couple it to reconstruction schemes. The aim of this study is to demonstrate the reconstruction of an arbitrary refractive index distribution from a least-squares gradient-based iterative inversion algorithm taking advantage of the full transient Radiative Transfer Equation (tRTE). The finite-difference discrete-ordinates method for the tRTE and its adjoint has been implemented, accounting for spatial changes in the distribution of the refractive index in a semi-transparent medium. A least-squares gradient-based iterative algorithm has been designed and elementary tests have been carried to demonstrate reconstruction possibilities.
A Strategy for Modeling Soot Formation and Radiative Transfer in Turbulent Flames
Authors: Dorey Luc-Henry , Tesse´ Lionel , Bertier Nicolas , Dupoirieux Francis
Published: March 2011

A strategy has been developed in order to compute unsteady convective and radiative heat transfers in an industrial combustion device. This paper presents the first validation step, which consists in modeling detailed combustion chemistry, soot formation and radiative transfer in a one-dimensional premixed laminar flame with industrial codes. A tabulation method was used to describe gas-phase chemistry; a semi-empirical acetylene-based two-equation model was adapted to predict soot formation and oxidation; and a Monte Carlo method was applied to evaluate radiative transfer in the flame. Applied on three different sets of conditions, the soot model gave results in the experimental uncertainty range of measurements. Radiation calculation showed that soot particles dominate radiative transfer.
Influence of Thermal Effects During Fast Filling Operations on Adsorption Capacity in a Hydrogen Cryo-Adsorption Storage Tank
Authors: Aleksic Petar , Næss Erling , Buenger Ulrich
Published: March 2011

Charging and discharging operations of on-board hydrogen adsorption storage systems involve exothermic and endothermic processes. Temperature elevations caused by the released heat of adsorption, compression work and thermal mass introduced from the inlet gas result with a reduction of the storage capacity. The main objective of this work was the investigation of the impact of temperature elevations on the decrease of adsorption storage capacity during high-pressure charging of a hydrogen cryo-adsorption storage tank. The experimental operating conditions were compatible with practical applications for hydrogen adsorption on-board storage systems. The analysis was conducted with two adsorbent classes: activated carbon (NORIT R0.8) and metal-organic-frameworks (Cu-BTC-1,3,5). Adsorption isotherms for hydrogen uptake for both adsorbents are measured by a gravimetric method and fitted in accordance to the Langmuir equation. The experimental study was carried out in a cylindrical tank with granular adsorbents in which the bed temperature was measured at various positions. A typical average temperature increase in the core of the storage column during hydrogen charging experiments with the CuBTC and NORIT R0.8 was 16.2 K and 20.4 K at 2 MPa respectively. Such temperature elevation results in a loss in the adsorption storage capacity of 14.75% for the system packed with the CuBTC adsorbent. Solutions for increased efficiency of the hydrogen cryo-adsorption storage tanks are proposed.
Evaporative Microchannel Cooling: An Atomistic Approach
Authors: Frijns A. J. H. , van den Akker E. A. T. , Hilbers P. A. J. , Stephan P. , van Steenhoven A. A.
Published: March 2011

Heat generation and temperature rise in electronic devices is a technical problem with increasing importance, since the number of transistors per surface area on integrated circuitries is rapidly increasing. If the heat cannot effectively be carried away damage in the circuitry may occur. Therefore enhanced and integrated cooling is needed. A promising technique is evaporative microchannel cooling. However, a major problem in modeling such micro-device is that the continuum approach starts to fail in the vapor phase and more detailed modeling becomes necessary. Since on these small scales the boundary and interface conditions are very important for the overall performance of the device, we choose the approach in which we start with understanding the essential physical phenomena at a molecular level. In this paper a detailed particle-based model is derived for these interactions: local interactions between the three phases are studied by molecular dynamics (MD) simulations in a detailed way. In this way physically and thermodynamically correct interface and boundary conditions (e.g. slip velocities and temperature jumps) are ensured. Finally, the enhanced heat transfer in the evaporative zone (Argon on a Calcium surface) is simulated by our molecular model and is compared to the results obtained by the continuum microregion model developed by P. Stephan et al. (Int. J. Heat Mass Transfer, 35, pp. 383–391, 1992).
Modeling a Loop Heat Pipe With a Capillary Wick in the Condenser
Authors: Kariya H. Arthur , Koveal Catherine , McCarthy Matthew , Brisson John G. , Wang Evelyn N.
Published: March 2011

We present a model to predict the behavior of a novel loop heat pipe with a capillary wick in the condenser. The heat pipe consists of a single evaporator and multiple vertically-stacked condensers interdigitated with radial-outflow fan blades of a blower. The resulting heat exchanger assembly maximizes heat transfer area by integrating the heat sink and blower together. As a result of the stacked design, the lowest condenser has the highest liquid pressure in the presence of gravity. The capillary pressure generated by the condenser wick is used to compensate for the difference in the liquid pressures among the condensers. This paper focuses on modeling and designing the heat pipe to form receding menisci in the condensers. The effect of the compensation chamber pressure and viscous losses on the meniscus orientation is examined. The results of the model show that an advancing meniscus is formed at lower heat inputs and a threshold heat input must be exceeded to form a receding meniscus. Guidelines for selecting appropriate wicks for the evaporator and condenser are reported.
Intermittent Flow Modeling: Part I—Hydrodynamic and Thermal Modeling of Steady, Intermittent Flows in Constant Area Ducts
Authors: Abraham J. P. , Sparrow E. M. , Tong J. C. K. , Minkowycz W. J.
Published: March 2011

A model for predicting fluid flow and convective heat transfer in all flow regimes has been implemented for steady mainflows in pipes and ducts of constant cross section. The key feature of the model is its capability to predict transitions between purely laminar and purely turbulent flow, while the latter flows are also predicted with high accuracy. The flow regime need not be specified in advance but is determined automatically as the flow evolves during its passage along the pipe or duct. Intermittently in the transition regime is fully accounted. It was shown that fully developed flows are necessarily restricted to either the laminar regime or the turbulent regime, but that a fully developed intermittent regime exists. The effects of the flow conditions at the inlet of the pipe or duct, velocity profile shape and turbulence intensity, on the subsequent transitions were quantified. To facilitate the heat transfer analysis, the turbulent-Prandtl-number concept, widely used to inter-relate the turbulent viscosity and thermal conductivity, was extended to encompass both intermittent and laminar flows. The presented results include all-flow-regime fully developed friction factors and fully developed Nusselt numbers. The locations where laminar-flow breakdown occurs and where fully developed begins are also presented.
Mechanism and Modeling of Rewetting Initiation of Hot Dry Surface in Saturated and Subcooled Film Boiling
Authors: Ohtake Hiroyasu , Kobayashi Shingo , Nagashima Nozomi , Koizumi Yasuo
Published: March 2011

The behavior of rewetting on a high superheated and dry surface, focusing on rewetting temperature just as collapse of saturated and subcooled film boiling was investigated experimentally and analytically. Saturated and subcooled pool film-boiling experiments were conducted by using a Silicon wafer with 20 mm length, 20 mm width and 0.5 mm thickness and pure water at atmospheric condition. Saturated and subcooled impingement jet experiments were also preformed. Then, the model analysis of rewetting initiation of a hot dry surface in saturated and subcooled film boiling was constructed by using hydrodynamics instability on liquid-vapor interface (Rayleigh-Taylor instability) and a heat conduction model (rewetting model under a falling film). Some physical parameters on Rayleigh-Taylor instability, for example released period of bubble from vapor film on a heater, generated volume rate of vapor and so on, were estimated by using Two-Phase Boundary Layer theory of saturated and subcooled film boiling. The present analytical results also showed that as the liquid subcooling was high, MHF temperature was higher. Namely, the predictions agreed with the present experimental results and Dhir-Purohit’s correlation. In addition, the present model of MHF temperature was developed by taking into account the dependence on thermal conductivity of wall of the MHF-temperature.