bluuebunny/crossref_metadata_2025 huggingface dataset:
3-D Multiphase Flow Modeling of Bubble Growth and Burst in Spray Cooling Using Parallel Computing
Authors: Selvam R. Panneer , Johnston Joseph , Sarkar Suranjan
Published: April 2009

In this paper, we present an extension of the level set method from 2D into 3D for solving multiphase flow problems using distributed parallel computing. The model solves the incompressible Navier-Stokes equations to study the behavior of a bubble immersed in a thin liquid film at microscale as found in a spray cooling environment. Since modeling all aspects of spray cooling, including nucleation, bubble dynamics, droplet impact, convection and thin film evaporation is very difficult at this time; these phenomena have been divided and studied separately in order to study the heat transfer behavior of each phenomenon individually. We studied the droplet impact effect as seen in spray cooling by our 3D multiphase model in earlier studies. Through the 3D multiphase model this study simulates the dynamics of a nucleating bubble in a thin liquid film that merges with the ambient atmosphere above the film. In this study we did not consider the droplet impact effect to concentrate on the vapor bubble dynamics in thin liquid film and its effect on heat transfer. The effect of convective flow is not considered to keep the 3-D model simple. However the 2D model was modified to simulate the effect that a horizontal flow of constant velocity has on the growth and detachment of a nucleating bubble and discussed in the second part of the paper. This study illustrates the importance of considering the convective flow effect in our 3-D multiphase flow model in future with droplet impact for spray cooling modeling studies.
Evaluation of Condenser Transient Heat Transfer Behavior of the Small Scale Capillary-Pumped Loop
Authors: Lin Hung-Wen , Lin Yu-An , Lin Wei-Keng
Published: April 2009

A general method was developed to predict the condenser temperature at each power load. And a simulation program and infrared camera was used to study the flow characteristics. High density polyethylene porous material was added in the evaporator to CPL, the loop was designed on the ground with a horizontal position and scale down the whole device to the miniature size. From the experimental results, the CPL could remove heat 110W in steady state and keep the heat source temperature about 80°C. Finally, a good agreement between the simulation and experimental values has been achieved. Comparing with experiment and simulation results, the deviation values of the distributions of the condenser outlet temperature are less than 5%.
Experimental Verification of Kirchhoff’s Law on Thermal Radiation at an Electromagnetic Wave Level
Authors: Wakabayashi Hidenobu , Makino Toshiro
Published: April 2009

The Kirchhoff’s law on emission and absorption of thermal radiation is a law driven deductively for the energy equilibrium of thermal radiation. First, we consider on the basis of spectroscopic experiments and an electromagnetism whether the Kirchhoff’s law is valid for the spectra of emission and absorption of an electromagnetic wave which are characterized by the phase as well as the amplitude of the wave. The interference of radiation caused by the phase matching is a key for this problem. Second, we consider the possibility of systematic coherency of spherical waves emitted by a number of dipoles which are the sources of thermal radiation. This consideration is related to the validity of the assumption that emission sources of thermal radiation move randomly.
Effect of an Insulated Baffle on Pseudosteady-State Natural Convection Inside Spherical Containers
Authors: Duan Yuping , Hosseinizadeh S. F. , Khodadadi J. M.
Published: April 2009

The effect of an insulated thin baffle on pseudosteady-state natural convection within spherical containers is studied computationally. The computations are based on an iterative, finite-volume numerical procedure using primitive dependent variables, whereby the time-dependent, two-dimensional axisymmetric form of the governing continuity, momentum and energy equations are solved. Natural convection effect is modeled via the Boussinesq approximation. Parametric studies were performed for a Prandtl number of 0.7. For Rayleigh numbers of 104, 105, 106 and 107, baffles with 3 lengths positioned at 5 different locations were investigated. In effect, a parametric study involving 60 cases were performed. The computational results were benchmarked against previous data available in the literature by comparing the heat transfer correlations, temperature distribution and streamline patterns for cases with no baffle. In general, regardless of the presence of an insulated baffle, fluid that is heated adjacent to the surface of the sphere rises replacing the colder fluid which sinks downward. For high Ra number cases, the hot fluid at the bottom of the sphere is also observed to rise along the symmetry axis and encounter the sinking colder fluid. This behavior can lead to onset of oscillations in the temperature and flow fields. Due to blockage effect of an insulated thin baffle, multi-cell recirculating vortex structures are observed. The number and strength of these vortices depend on the position and length of the baffle. In the absence of heat transfer path through the insulated baffle, flow obstruction is the major feature of this problem. For the majority of the length and location combinations investigated, less heat is brought into the fluid thus lowering the time rate of rise of the bulk temperature. The extent of heat transfer modification depends on the Rayleigh number, length and location of the extended surface.
Upwind Solutions for Two-Phase Flow Based on a Hyperbolic Two-Dimensional Two-Fluid Model
Authors: Chung Moon-Sun , Kim Jong-Won
Published: April 2009

In this study, two-phase flow problems are numerically solved in two dimensions using a two-fluid model. This model includes the surface-tension terms incorporated in the momentum equations. Then the governing equations become hyperbolic type for which the upwind method like flux vector splitting (FVS) avails. For numerical tests, two-phase shock tube problem is solved to show the wave propagation characteristics. Cavity growth on the surface of a hemisphere headform is also calculated as a feasibility study. Advantages and capability of the present method are discussed in some detail.
Turbulent Forced Convection Correlation for the In-Tube Flow of Binary Gas Mixtures With 0.1<Pr<0.7
Authors: Diaz Gerardo , Campo Antonio
Published: April 2009

Turbulent forced convection correlations are available in the literature for gases (Pr ∼ 0.7), but the test data leave a gap in the range of Prandtl (Pr) number between 0.1 and 0.7 occupied by binary gas mixtures. In this paper we develop a turbulent forced convection correlation for the Nusselt (Nu) number of in-tube binary gas mixtures for the ranges of Reynolds (Re) number between 104 and 106 and Prandtl (Pr) number between 0.1 and 0.7. A fully connected back-propagation Artificial Neural Network (ANN) is used to learn the pattern of Nu as a function of Re and Pr. Available test data in the range of 0.001 < Pr < 0.1 and 0.7 < Pr < 1000 are provided to the ANN. The test data are separated in two sets to train and test the neural network. A training set with 80% of the data is used to predict a testing set with the remaining 20% of the data. After the network is trained, we make use of the excellent nonlinear interpolation capabilities of ANNs, to predict values of Nu for the sought range 0.1 < Pr < 0.7. These predictions are later used to generate a correlation that aptly covers the complete range of Prandtl numbers.
Spray Cooling Performance of Single and Multi-Jet Spray Nozzles Using Subcooled FC-72
Authors: Moreno Gilberto , You Seung M. , Steinthorsson Erlendur
Published: April 2009

In this study experiments were performed to evaluate the spray cooling performance of three different spray nozzles using gassy-subcooled (∼Tsub = 31°C) FC-72 as the working fluid. The three different nozzles tested can be characterized as a single hollow cone spray nozzle (Nozzle A), 2×2 jet array spray nozzle (Nozzle B) and a 4×4 jet array spray nozzle (Nozzle C). For all tests, a 10×10 mm polished (600 grit) copper surface was utilized as the heater and tests were carried out at near atmospheric pressure conditions. All three nozzles were tested at various flow rates and nozzle-to-heater distances and the results were compared. Results show that changing the nozzle-to-heater distance affects heat transfer rates more than critical heat flux (CHF). The spray boiling curves for all three nozzles were similar with Nozzle C, for some cases, demonstrating the highest heat transfer rates. The disparity in CHF values between the various nozzles was more apparent. Compared at an equivalent flow rate, Nozzle C consistently produced CHF values which were higher than those of the other nozzles. Some common trends observed for all nozzles are, increasing flow rate increases heat transfer rates and critical heat flux (CHF) but decreases nozzle efficiency.
Effect of Channel Geometry Variations on the Performance of a Microscale Bubble Absorber
Authors: Jenks Jeromy , Narayanan Vinod
Published: April 2009

The effect of microchannel geometry variations on heat transfer and absorption rate of an ammonia-water constrained thin film bubble absorber are presented. Experiments are performed at an absolute pressure of four bar and at a fixed inlet mass concentration of ammonia of 15 percent. The mass flow rate of the inlet weak solution are varied from 10 g/min to 30 g/min, and that of ammonia gas are varied from 1 g/min to 3 g/min. Five geometries, including two smooth-bottom-walled channels of differing depths, and three channels with structured bottom walls are considered. For identical rates of vapor absorption, the overall heat transfer coefficient for the 400 μm smooth microchannel is significantly larger than of the 150 μm smooth channel as well as that of the cross-ribbed and angled-cross-ribbed structured channel. The streamwise-finned channel exhibit high overall heat transfer coefficients for vapor flow rates of up to 2 g/min.
Numerical Simulation of Heat Transfer in a Microchannel Heat Sink With Micro Encapsulated Phase Change Material (MEPCM) Slurry
Authors: Kuravi Sarada , Kota Krishna M. , Du Jianhua , Chow Louis C. , Colvin David P.
Published: April 2009

A numerical model to investigate heat transfer characteristics of the MEPCM slurry flow inside a typical microchannel is presented in the paper. The temperature field inside the channel wall is solved three dimensionally and is coupled with the three dimensional velocity and temperature fields of the fluid. The effective specific heat method is used to model the heat sink (MEPCM) in the single phase fluid. The slurry is assumed to be homogeneous with temperature-independent properties. The effects of various parameters such as concentration of MEPCM particles and their size on the heat transfer characteristics of slurry flow are discussed. The results of the numerical model are validated with single phase fluid flow analysis and slurry flows in microchannels separately since no experiment has been performed till date on microsized channels (less than 500 microns in size) employing MEPCM slurry. The analysis would help in designing and optimizing future designs of MEPCM slurry flows in MMC heat sinks.
Wickability and Transient Cooling Capacity of Binary Mixtures
Authors: Salinas Mauricio A. , You S. M. , Short B. Elliott , Price Donald C.
Published: April 2009

Previously, aerospace and military, high heat-flux, electronics have been cooled with phase-change-modules composed of a metal-matrix saturated with a solid phase-change-material. This method requires the heat to be transferred from the source location, through the metal matrix, to the available phase-change-material. Empirical evaluations indicate that wick-based coldplates including non-metallic porous media, saturated with fluid may be much more effective. This wick-based method allows the capillary action of the wick to passively transport liquid from liquid-rich areas to the point of need; a much more efficient process. The optimization of a wick-based coldplate involves a careful balance between the wickability of the working fluid and its heat of vaporization (i.e. cooling capacity). The wickability and transient cooling capacity of both ethanol-water and methanol-water mixtures were empirically evaluated for the full mass fraction range. Testing procedures and results will be described in detail.
Numerical Investigation of Radiation Effects in Catalytic Combustion
Authors: Mazumder Sandip , Grimm Michael
Published: April 2009

In modeling catalytic combustion in a monolithic catalytic converter, it is generally assumed that the gas within the individual monolith channels does not interfere with thermal radiation. To date, no quantitative study has been undertaken to validate this assumption. Past studies for carbon monoxide combustion also appear to indicate that the emissivity of the washcoat has little effect on the thermal radiation field. In order to investigate these two issues, methane-air combustion on platinum is modeled inside a single channel of a monolith using a detailed surface reaction mechanism comprised of 24 reactions between 19 species. Radiation transport is modeled using the Discrete Ordinates Method and a gray formulation. Planck-mean absorption coefficients of the gases, calculated from the HITEMP and HITRAN databases, are used to investigate participating medium effects. All calculations were performed using the commercial CFD code, CFD-ACE+™, supplemented by user-subroutines for calculating the absorption coefficient of the gas mixture. Results show that the conversion percentages and temperature distributions are unaltered by the inclusion of participating medium radiation effects, verifying the commonly held belief, stated earlier. However, in strong contrast with carbon monoxide combustion, the emissivity of the washcoat was found to significantly affect flammability limits in the case of methane combustion—the flame being hotter and more stable for smaller values of emissivity.
Alumina Nanofluid for Spray Cooling Enhancement
Authors: Bansal Aditya , Pyrtle Frank
Published: April 2009

Nanofluids have been demonstrated as promising for heat transfer enhancement in forced convection and boiling applications. The addition of carbon, copper, and other high-thermal-conductivity nanoparticles to water, oil, ethylene glycol, and other fluids has been determined to increase the thermal conductivities of these fluids. The increased effective thermal conductivities of these fluids enhance their abilities to dissipate heat in such applications. The use of nanofluids for spray cooling is an extension of the application of nanofluids for enhancement of heat dissipation. In this investigation, experiments were performed to determine the level of heat transfer enhancement with the addition of alumina nanoparticles to the fluid. Using mass percentages of up to 0.5% alumina nanoparticles suspended in water, heat fluxes and surface temperatures were measured and compared. Compressed nitrogen was used to provide constant spray nozzle pressures to produce full-cone sprays in an open loop spray cooling system. Heat fluxes were measured for single-phase and evaporative spray cooling regimes.
Simulation of Unsteady Incompressible Viscous Flow Using Higher Order Implicit Runge-Kutta Methods: Staggered Grid
Authors: Ijaz Muhammad , Anand N. K.
Published: April 2009

A numerical method (SIMPLE DIRK Method) for transient incompressible viscous flow simulation is presented. The proposed method can be used to achieve arbitrarily high order of accuracy in time-discretization which is otherwise limited to second order in majority of the currently available simulation techniques. A special class of implicit Runge-Kutta methods is used for time discretization in conjunction with finite volume based SIMPLE algorithm. The algorithm was tested by solving for velocity field in a lid-driven square cavity. In the test case calculations, power law scheme of Patankar [2] was used for spatial discretization and time discretization was performed using a second-order implicit Runge-Kutta method. Time evolution of velocity profile along the cavity centerline was obtained from the proposed method and compared with that obtained from a commercial CFD software, FLUENT [3] using second-order implicit time discretization scheme. Steady state solution from the present method was compared with the benchmark numerical solution of Ghia et al. [4]. Good agreement of the second-order solutions of the proposed method with the second-order solutions of FLUENT [4] and Ghia et al. [4] concludes the feasibility of the proposed method.
Viscous Dissipation in Laminar Water Flows in Micro-Tubes
Authors: Yang In-Hwan , El-Genk Mohamed S.
Published: April 2009

Numerical calculations are performed to investigate the effect of viscous dissipation on the temperature rise and friction numbers for laminar water flows in micro-tubes. The calculated values are compared with those determined from reported experimental data for glass and diffused silica micro-tubes (D = 16 – 101 μm and L/D = 625 – 1479). The results confirm a definite slip at the wall with slip lengths of ∼ 0.7 μm and 1.0 μm, which decrease the friction number and the temperature rise in the micro-tubes, but their effect gradually diminishes as either D or L/D increases. The friction number decreases exponentially as D decreases and, to a lesser extent, as L/D increases. The effect of L/D on the friction number is insignificant for micro-tube diameters ≤ 20 μm. For D > 400 μm, the friction number approaches that of Hagen-Posieuille of 64 for macro-tubes when L/D > 1500, but approaches higher values at smaller L/D. The dimensionless analytical expression developed for calculating the friction number and the temperature rise for water flows in micro-tubes is in good agreement with both the numerical and experimental results.
Thermal Convection Bifurcation Within Isosceles Triangular Cavities
Authors: Ridouane El Hassan , Campo Antonio
Published: April 2009

Laminar thermal convection of air confined to an isosceles triangular cavity heated from the base and symmetrically cooled from the upper inclined walls has been investigated numerically. The system of transient conservation equations, subject to the proper boundary conditions, along with the equation of state assuming the air behaves as a perfect gas are solved with the finite volume method. In the conservation equations, the second-order-accurate QUICK scheme was used for the discretization of the convective terms and the SIMPLE scheme for the pressure-velocity coupling. The maximum height-to-base aspect ratio A is fixed at 0.5, while the Grashof number extends from a low Gr = 103 to a high Gr = 106. The influence of Gr on the flow and temperature patterns is analyzed and discussed for two opposing scenarios, one corresponds to increasing Gr and the other corresponds to decreasing Gr. It is found that two steady-state solutions are possible, excluding their solution images through a vertical mirror plane. The symmetrical solution prevails for relatively low Grashof numbers. However, as the Gr is gradually increased, a transition occurs at a critical value of Gr. Above this critical value of Gr, an asymmetrical solution exhibiting a pitchfork bifurcation arises and eventually becomes steady. The existing ranges of these unsteady and steady solutions are reported for the two opposing scenarios. Also, issues related to the observed hysteresis phenomenon are discussed in detail.
Treatment of Heterogeneous Electrocatalysis in Modeling Transport-Reaction Phenomena in PEFCs
Authors: Hasmady Saiful , Wacker Manuel Philipp , Fushinobu Kazuyoshi , Okazaki Ken
Published: April 2009

Thermal and water management is a critical issue in PEFCs. In this research, the thermal behavior of PEFC is focused. The objective is to understand the influence of heat on cell performance both by experiment and theoretical analysis, as well as improving cell performance and reliability. In order to investigate the theoretical behavior, especially in the catalyst layer where the electrochemical reactions occur, a detailed modeling of heterogeneous surface reaction coupled with reactant transport is needed. In this paper, a theoretical model that improves the dependency of the exchange current density with reactant concentrations by applying data from a known surface reaction steps found in catalysis is developed. It served as a preliminary step before the thermal-electrochemical behavior of a PEFC can be fully understood.
Turbulent Structure and Relaminalization of a Rotating Turbulent Channel Flow at High Rotaton Number
Authors: Sato Shinichiro , Obi Shinnosuke
Published: April 2009

The present study reports on a series of direct numerical simulation of turbulent rotating channel flows at rotation numbers higher than those previously reported. The bulk Reynolds number (Re) was set approximately constant at 5800, while the non-dimensional rotation number Ro was increased up to 5.79. The flow visualization showed that the structure of the enlarged eddies on the pressure side, and the orientation of the structure was inclined towards downstream. The investigation of the budget of Reynolds stress transport equation indicated that the pressure transport of Reynolds stress and re-distribution term were extraordinary large at higher Ro. This led the conventional decomposition of velocity-pressure correlation terms to being not appropriate in this flow. It is suggested that the Reynolds stress modeling should better target the correlation between velocity and pressure gradient, or pressure diffusion and re-distribution terms must be handled with the same efforts, which indeed increases the task of modeling.
Forced-Convection Heat Transfer Characteristic of Non-Newtonian Suspension
Authors: Yamada Masahiko , Nakajima Takeshi , Takama Natsuro , Nakamura Koh
Published: April 2009

It is well known that some water-suspension of minerals show a thixotropic characteristics from a viewpoint of rheology. It is usually considered that such rheological properties are due to the inner structure of suspensions. Such an inner structure of suspension may also exerts some effects on the thermophysical properties, and accordingly, on the heat transfer characteristics. In the present paper, heat transfer characteristics of non-Newtonian suspensions flow over a vertical heated plate have been investigated experimentally. Water suspensions of mineral powders such as bentonite and attapulgite were employed as sample fluids. Local heat transfer coefficients were determined under a variety of parameters such as concentration of suspension and flow velocity. Measurement results showed that the change of shear rate near the wall had some effects on the thermophysical properties of sample fluid, and that not only rheological properties of fluid but also its thermophysical properties near the heat transfer surface might exert a considerable effect on the heat transfer characteristics within the parameter range of the present study.
Atomistic Visualization of Ballistic Phonon Transport
Authors: Zuckerman Neil , Lukes Jennifer R.
Published: April 2009

Heat transfer in solid materials at short time scales, short length scales, and low temperatures is governed by the transport of ballistic phonons. In anisotropic crystals, the energy carried by these phonons is strongly channeled into well-defined directions in a phenomenon known as phonon focusing. Presented here is a new molecular dynamics simulation approach for visualizing acoustic phonon focusing in anisotropic crystals. An advantage of this approach over experimental phonon imaging techniques is that it allows examination of phonon propagation at selected modes and frequencies. The spatial, mode, and frequency dependence of ballistic energy transport gained with this approach will be useful for understanding heat transfer issues in high frequency electronics and short time scale laser-material interactions.
Study of Natural Convection in Inclined Square Enclosures With Uniform Heat Generation
Authors: Kulkarni Manish , Mitra Sushanta K. , Gaitonde U. N.
Published: April 2009

Two-dimensional laminar natural convection in an inclined square enclosure with uniform internal heat generation is studied here. The steady-state solutions are obtained for inclination angles of 45°, 30° and 15° and at Rayleigh number of 1.5 × 105. For these cases, the two counter-rotating rolls of fluid are present in the cavity. Streamlines, isotherms and heat transfer for these results are compared with the existing experimental results and are found to be in reasonably good agreement. It is found that the location of maximum non-dimensional temperature in the inclined cavity is higher than that for pure conduction case. The maximum non-dimensional temperature in the cavity decreases as the Rayleigh number increases. For Ra > 5 × 104, the maximum non-dimensional temperature in inclined cavity is almost independent of the inclination angle. It is also observed that the local Nusselt number at the top wall is greater than the pure conduction solution, whereas that for bottom wall it is lower than the Nusselt number for pure conduction. The effect of Rayleigh number and inclination angle on the local Nusselt number and modified local Nusselt number are also studied. For horizontal cavity, at Rayleigh number greater than or equal to 5 × 104, periodic solutions are obtained. In this case, two unstable secondary rolls are present near the center of top wall, in addition to the primary rolls. The secondary rolls are dissipated and recreated during one period of oscillation.
Modeling a Passive Cooling System for Photovoltaic Cells Under Concentration
Authors: Gray Allison , Boehm Robert , Stone Kenneth W.
Published: April 2009

Cooling of photovoltaic cells under high intensity solar irradiance is a major concern when designing concentrating photovoltaic systems. The cell temperature will increase if the waste heat is not removed and the cell voltage/power will decrease with increasing cell temperature. This paper presents an analysis of the passive cooling system on the Amonix high concentration photovoltaic system (HCPV). The concentrator geometry is described. A model of the HCPV passive cooling system was made using Gambit. Assumptions are discussed that were made to create the numerical model based on the actual system, the methods for drawing the model is discussed, and images of the model are shown. Fluent was used to compute the numerical results. In addition to the theoretical results that were computed, measurements were made on a system in the field. These data are compared to the theoretical data and differences are calculated. Theoretical conditions that were studied included uniform cell temperatures and worst case weather scenarios, i.e., no wind, high ambient conditions, and high solar irradiance. The performance of the Amonix high concentrating system could be improved if more waste heat were removed from the cell. Now that a theoretical model has been developed and verified, it will be used to investigate different designs and material for increasing the cooling of the system.
A Method of Measuring the Properties of Ash Deposits in a Coal Fired Reactor
Authors: Moore Travis J. , Jones Matthew R.
Published: April 2009

Coal is an important source of energy because of its potential for power generation and its abundance in the earth. A great deal of research is dedicated to developing cleaner and more efficient methods of generating power from coal. A major problem associated with almost all power generation processes involving coal is the formation of ash. Some of the ash that is formed is deposited on the tubes and walls of the combustion chambers. This accumulation of ash can significantly affect the thermal transport in the boiler. Modeling of the heat transfer occurring in a coal combustion process requires knowledge of the properties of these deposits. Accurate measurements of these properties will lead to better modeling capabilities and improved optimization of the design of coal fired reactors. Therefore, a method for accurate, in situ measurement of the emittance and thermal conductivity is highly desirable. This paper describes the development of a method for determining the total emittance and thermal conductivity of the deposited ash layers and analyzes the sensitivity of these properties to measurement errors.
Discussion on Luminescent Intensity of Pressure Sensitive Paint
Authors: Yamaguchi Hiroki , Matsuda Yu , Mori Hideo , Niimi Tomohide
Published: April 2009

The Pressure Sensitive Paint (PSP) is very useful and easy way to obtain the pressure distribution on surfaces. Therefore the PSP has been adopted to various flow fields to obtain pressure distributions, showing good agreement with other methods, such as a pressure tap. Many kinds of flow fields have been used in the manufacturing process, like small scale, low pressure and/or high speed flows; for example, semiconductor manufacturing processes where a mean free path of gas molecules is large, and micro-/nano-systems where a characteristic length is small. Recently the PSP is applied to these high Knudsen number flows. Usually it is considered that the luminescent of the PSP represents the surface pressure, but it is not clear yet that this “pressure” means “static” or “total” pressure. The PSP is considered to be in the boundary layer, leading to the local equilibrium of oxygen molecular density between in the gas phase and in the PSP is achieved. This becomes a large problem especially in the high Knudsen number regime. The mechanism of the PSP has been mainly discussed focused on the luminescence intensity and the oxygen quenching of luminescent molecules. Here, in this study, we tried to relate the flux of oxygen molecules and the surface pressure to the luminescence intensity of the PSP, and clarify the limit of application from the molecular kinetics point of view. The main target of this problem was in the high Knudsen number and the high Mach number flow fields.
Adsorption Performance of Round Fin and Tube Type Adsorber Employing Activated Carbon Fiber/Ethanol Pair
Authors: Kariya Keishi , Tateishi Ryo , Kuwahara Ken , Saha Bidyut Baran , Koyama Shigeru
Published: April 2009

The severity of the ozone layer destruction problem has been calling for rapid developments in environment friendly adsorption cooling systems. However, the widespread dissemination of adsorption system is hindered due to its poor performance, which mainly results from the inadequate design of adsorber/desorber heat exchanger. The present paper deals with the numerical investigation on the adsorption performance of a round fin-and-tube type adsorber/desorber heat exchanger using activated carbon fiber (ACF)-ethanol as adsorbent-refrigerant pair. The effects of the local heat and mass transfer as well as the fin geometry of the adsorber/desorber are accounted in the present two-dimensional modeling. Adsorption performance have been determined varying five parameters, such as the ACF bed apparent density, fin thickness, fin pitch, fin height and tube diameter along with evaporator and cooling water inlet temperatures. The results show that the adsorption performances increase by optimizing the tube diameter, fin height and fin pitch. It is also found that all parameters have some influence on the optimum adsorption cycle time and the optimum cycle time is found to be around 150 s.
Development of Thermal-Hydraulic Code Coupled With Heat Conduction in Structure for Thermal Striping Phenomena: Development of Calculation Module for Temperature Distribution in Structure
Authors: Tanaka Masa-Aki , Ohshima Hiroyuki
Published: April 2009

Thermal striping is one of the most important issues in terms of safety of Fast Breeder Reactors (FBRs). Thermal striping occurs where high temperature fluid mixes with low temperature one and it might cause structural damage. An analysis program (“MUGTHES”) for fluid-structure thermal interaction has been developed to evaluate the thermal striping phenomena and to establish the evaluation method. MUGTHES consists of two calculation modules for thermal-hydraulic analysis and for heat conduction analysis in structure and a conjugated heat transfer model that connects both modules. Thus, thermal-hydraulic field and temperature field in structure can be calculated simultaneously. Boundary Fitted Coordinate (BFC) system is employed to treat complex geometries in FBR plants. Governing equations in BFC system are discretized using the finite-volume approach to keep these conservation conditions in computation. In this paper, detailed description of the evaluation method of spatial differential terms in the equations was described. Numerical simulations were performed using fundamental problems related to one dimensional transient heat conduction in a cube and to radial transient heat conduction in a cylinder, in order to verify the discretization method in BFC system and the calculation method for thermal interaction between two calculation regions. In addition, numerical simulation of T-junction piping system was carried out to investigate characteristics of temperature fluctuation and to confirm the applicability of the program to a practical problem.
Numerical Simulation of Heat Pumping in a Pulse Tube Refrigerator
Authors: Koshimizu Takao , Kubota Hiromi , Takata Yasuyuki , Ito Takehiro
Published: April 2009

Numerical simulation of heat and fluid flow in a basic and an orifice pulse tube refrigerator have been performed to visualize heat pumping generated in the regenerator and the pulse tube, and to clarify the difference in heat pumping caused by the phase difference between pressure and displacement of gas. Common components of the regenerator and the pulse tube are used in the basic and the orifice pulse tube refrigerator. The flow in the tube is assumed to be one-dimensional and compressible. As governing equations, the continuity, momentum and energy equations are used in this study. From the temperature and velocity field obtained as a result of the simulation, the relation between the displacement and the temperature change of gas elements is visually clarified, and consequently it is found that the characteristic that the temperatures of gas elements are nearly higher than those of the regenerator material or the pulse-tube wall during compression and lower during expansion is very important for the heat pumping in basic and orifice pulse tube refrigerators. Furthermore, the behavior of heat pumping in the basic and the orifice pulse tube refrigerator is illustrated by analyzing the relation between the displacement of gas elements and heat quantity transferred to the wall from the gas elements, and the difference in heat pumping between the basic and the orifice pulse tube refrigerator is made clear.
Diameter and Length Effect on Diffusive-Ballistic Phonon Transport in a Carbon Nanotube
Authors: Shiomi Junichiro , Maruyama Shigeo
Published: April 2009

We report a non-equilibrium molecular dynamics (MD) study on heat conduction of finite-length single-walled carbon nanotubes (SWNTs). The length and diameter dependences of the thermal conductivity are quantified for a range of nanotube-lengths up to a micrometer at room temperature using two different temperature control techniques. A thorough investigation was carried out on the influence of intrinsic thermal boundary resistance between the temperature-controlled layers and the rest of the SWNT. The trend of length effect indicates a gradual transition from nearly pure ballistic phonon transport to diffusive-ballistic phonon transport. The nearly pure ballistic phonon transport was also confirmed by the minor diameter-dependence of thermal conductivity for short SWNTs. For longer SWNTs with stronger diffusive effect, the thermal conductivity is larger for SWNTs with smaller diameters.
The Effect of Water Content Diameter on the Structure of Light Oil-Water Emulsion Spray Flame
Authors: Fuchihata Manabu , Takeda Shuko , Ida Tamio
Published: April 2009

Microexplosions of light oil-water emulsified fuel droplets were successfully documented using a high-speed video camera with laser illumination. The local frequency of the explosion occurrence, temperature profile and exhaust gas emissions were measured in spray flames of water-in-oil type emulsion formed using an air-assist atomizer with a ring pilot burner. Those results indicate that the flame structure changes as the water droplet diameter in the emulsion fuel was varied, even if the fuel components and their fractions were same. When the fuel includes the water droplet, whose median diameter was about 75μm, HC and CO emission were reduced as compared to those for the fuel of smaller water droplet content. It is probable that if the water droplet diameter is uniform, avalanching microexplosions occur at certain local region in the flame, and the water content vaporizes almost at once and extinguishes the flame. It leads to HC and CO emission increase. When the water droplet diameters are large, atomizer atomizes those; therefore, emulsion droplets include various size of water droplet in the flame. Consequently, the avalanching microexplosion occurrence is avoided, and HC and CO emissions are reduced.
Electro-Thermal Analysis of Submicron Si MOSFET With Zoned Mesh Based on Semiconductor Physics Theory
Authors: Hatakeyama Tomoyuki , Fushinobu Kazuyoshi , Okazaki Ken
Published: April 2009

The results of electro-thermal analysis, which is widely known as hydrodynamic model, are strongly dependent on the mesh size of model. However, the theory and method of accurate mesh size have not been investigated. In this research, we focus on submicron Si MOSFET and show the mesh zoning method for electro-thermal analysis. First, we explain the mesh zoning method for vertical direction, i.e. the direction from the gate oxide to the bottom surface of MOSFET, which was derived from the theory of the semiconductor physics, because the calculation results were strongly dependent on the mesh size of vertical direction. Along the vertical direction, Debye length with consideration of submicron size is the most important length scale. This Debye length is not same to the conventional one. We briefly show the derivation procedure of Debye length considered submicron phenomena and the procedure of the mesh zoning method by using this Debye length. Then, we show the error of the calculation results depended on the mesh size for the lateral direction, i.e. direction from the source electrode to the drain electrode, and we investigate the most important part in MOSFET for precise calculation.
An Experimental Study of a Bubble-Driven Heat-Transport Device
Authors: Suzuki Osamu
Published: April 2009

We experimentally measured the heat-transport characteristics of a bubble-driven heat-transport device. The device consisted of a non-looped copper tube containing water. The tube was either meandered or spiraled to form tube bundles. The inner surface of the tube was smooth and its diameter small enough to enable the formation of vapor and liquid plugs in it. Two copper blocks were attached to the tube bundles, one as a heating block and the other as a cooling block. In the experiment, most of the wall temperatures measured on the tube fluctuated periodically at a quasi-steady state. Time-averaged temperature gradients between the heating and cooling sections of the device were constant. By increasing heater input from 300W to 350W, the amplitude of the temperature fluctuations decreased and the temperature gradients increased significantly. This behavior was regarded as a transition to critical heat transport condition. The effective thermal conductivity of the device was proportional to the heat-transport rate but did not depend on the formation of the tube bundle and the gravity effect. The temperature fluctuations had specific peak frequencies and a positive correlation was found between the frequency and effective thermal conductivity. These experimental results strongly suggest that the main heat-transport mechanism of the investigated device is based on the oscillation-induced transport of sensible heat.
Lattice Gas Analysis on Two-Phase Flow in Cathode of Polymer Electrolyte Fuel Cells
Authors: Tanigawa Hirofumi , Tsuruta Takaharu
Published: April 2009

A numerical simulation system for the analysis of the transport phenomena inside the cathode electrode has been developed. The Lattice Gas Automaton (LGA) method is used for two-dimensional simulations to study the gas and water behaviors in the different type of flow passages. Three kinds of flow passage, the serpentine-type, the straight-type and the column-type are selected and their performances are compared. In this simulation, we shall consider the reaction between the proton and the oxygen as well as the water behaviors inside the separator flow-channel. The protons are supplied randomly at a constant rate on the flow-path surface, and the water is considered to be produced when the oxygen encounters to the proton. The transients of the reaction rate corresponding to the power generation are counted and the relation between the cell performance and the two-phase flow behaviors is investigated. The influence of hydrophobic and hydrophilic electrode on the cell performance is also studied. It is found that the serpentine-type flow is effective for the electric generation performance but induces the larger pressure loss. For the serpentine-type separator the hydrophobic electrode is effective for reducing the plugging phenomenon.
Low-Grade Heat Source Utilization by Carbon Dioxide Transcritical Power Cycle
Authors: Chen Yang , Pridasawas Wimolsiri , Lundqvist Per
Published: April 2009

One way to reduce the fossil fuel consumption and mitigate environmental impact is to utilize low-grade heat sources for power production. In this paper, a transcritical carbon dioxide power cycle is analyzed for its potential in utilizing the low-grade heat sources. Solar thermal is selected as a representative of low-grade heat sources. TRNSYS 16 and Engineering Equation Solver (EES) are employed using co-solving technique to analyze the dynamic performance of the proposed system. Both daily performance and annual performance of the proposed system under Swedish climate conditions are simulated. The simulation results show that the proposed system can achieve 8% average thermal efficiency and consequently 2.43 kW average power production during the system working period on a randomly selected summer day with a 30 m2 solar collector. Over the whole year, the maximum daily power production is about 17 kWh and the maximum monthly power production is about 185 kWh.
Synthesis of Single Walled Carbon Nanotubes by Laser Vaporized Catalytic Chemical Vapor Deposition Technique
Authors: Yamamoto Soma , Tani Keita , Onaka Yasumori , Takata Yasuyuki , Suzuki Shinzo , Shibuta Yasushi , Maruyama Shigeo , Kohno Masamichi
Published: April 2009

SWNTs were synthesized by laser vaporized CCVD (Catalytic Chemical Vapor Deposition). The diameter distribution and the purity of SWNTs synthesized at different temperatures, laser intensities and catalysts were investigated by Raman spectroscopy. Both of them tended to shift towards a larger area as reacting temperature or laser intensity was increased. Ni, Co and Fe played a catalytic role, though Ag and Cu were less effective at our experimental conditions. In addition, the conventional laser oven technique and laser vaporized CCVD technique were also compared. The diameter distribution of SWNTs which were synthesized by the conventional laser oven technique was narrower than that of SWNTs synthesized by the laser vaporized CCVD technique.
Heat Transfer Measurements of Impinging Synthetic Air Jet
Authors: McGuinn Alan , O’Donovan Tadhg S. , Murray Darina B.
Published: April 2009

The implementation of synthetic jets for use in the cooling of electronics is a relatively new technology. It is well established that effective rates of cooling can be achieved using conventional steady flow impinging jets. However it has been shown that synthetic jets can deliver similar cooling effects without the need for an air supply system and therefore represent an extremely promising alternative for thermal management applications. A study has been undertaken of the heat transfer distribution to an impinging synthetic jet flow. The jet is directed at a heated copper plate, which approximates a uniform wall temperature. Nusselt number profiles generated by the synthetic jet for various Reynolds numbers and heights above the plate were obtained. Time varying velocity measurements were also carried out to provide information about the flow characteristics of the synthetic jet and to aid with evaluation of the heat transfer data. For continuous jets mean heat transfer distributions have been shown to have a direct relation to jet velocity profiles, however, for synthetic jets fluctuations in local heat flux illustrate a significant dependence on the driving frequency.
Influence of Near Hole Pressure Fluctuation on the Thermal Protection of a Film-Cooled Flat Plate
Authors: Burdet Andre´ , Abhari Reza S.
Published: April 2009

Pulsation of film cooling jets in turbines is driven by the near hole pressure fluctuation caused by deterministic interaction of stator/rotor blade rows. Jet pulsation is characterized by the coolant near hole reduced frequency ≅c and the pulsation amplitude coefficient ≅. Computational requirements for the utilization of a novel experimentally anchored near hole jet model for performing a computationally efficient parametric study of quasi-steady jet pulsations are introduced. Fluctuation of near hole pressure is simulated by setting a time-varying signal of static pressure for the outlet boundary condition of a film-cooled flat plate configuration. It is observed that the fluctuation of near hole pressure influences the blowing ratio hence the thermal protection downstream of the injection site. For a low mean blowing ratio (BR = 0.75), low-medium pulsation frequencies (≅c ≅ 0.10) are found to be slightly detrimental to the thermal protection versus a steady injection. On the contrary, for high pulsation frequencies (≅c ≅ 0.17), the thermal protection becomes better due to periodic jet disintegration into the wall surface caused by a higher level of transverse kinetic energy of the jet pulse. In addition, the overlapping of jet pulses appears to help the constant temporal spreading of coolant over the wall surface. For a higher mean blowing ratio (BR = 1.25), jet pulsation enhances jet lift off so that the thermal protection is in general worse compared to a steady injection.