bluuebunny/crossref_metadata_2025 huggingface dataset:
Non-Gray Phonon Transport Using a Hybrid BTE-Fourier Solver
Authors: Loy James M. , Singh Dhruv , Murthy Jayathi Y.
Published: June 2010

Non-gray phonon transport solvers based on the Boltzmann transport equation (BTE) are frequently employed to simulate sub-micron thermal transport. Typical solution procedures using sequential solution schemes encounter numerical difficulties because of the large spread in scattering rates. For frequency bands with very low Knudsen numbers, strong coupling between the directional BTEs results in slow convergence for sequential solution procedures. In this paper, we present a hybrid BTE-Fourier model which addresses this issue. By establishing a phonon group cutoff (say Kn = 0.1), phonon bands with low Knudsen numbers are solved using a modified Fourier equation which includes a scattering term as well as corrections to account for boundary temperature slip. Phonon bands with high Knudsen numbers are solved using a BTE solver. Once the governing equations are solved for each phonon group, their energies are then summed to find the total lattice energy and correspondingly, the lattice temperature. An iterative procedure combining the lattice temperature determination and the solutions to the modified Fourier and BTE equations is developed. The procedure is shown to work well across a range of Knudsen numbers.
Fuel Flexible Biomass Reburn Technology
Authors: Xu Guang , Zhou Wei , Swanson Larry
Published: June 2010

Biomass reburn is a low NOx alternative to cofiring that effectively uses the high volatility and high char reactivity of biomass for NOx reduction. In this paper, computational fluid dynamics (CFD) and thermal modeling, and a NOx prediction model were used to evaluate the impacts of sawdust/coal reburn on the performance of a 250 MW opposed-fired boiler burning bituminous coal as the primary fuel. The results showed that the reburn system maintained overall boiler performance with a 50 – 70 °F reduction in the furnace exit gas temperature. Predicted losses in thermal efficiency were caused by the lower biomass fuel heating value (similar to biomass cofiring) and increase in unburned carbon. The higher unburned carbon emissions were attributed to an order of magnitude larger biomass mean particle size relative to bituminous coal. Thus, LOI emissions can be improved significantly by reducing the biomass mean particle size. The NOx predictions showed that for reburn rates above about 19%, adding dry sawdust biomass to a coal reburn system can improve NOx reduction; i.e., using pure dry sawdust as reburn fuel at 30% of the total heat input can lead to NOx levels about 30% less than those for pure coal reburn under for similar firing conditions.
A Numerical Study of the Thermal Performance of Two Stationary Insert Designs in Internal Compressible Flows
Authors: Tanbour Emad Y. , Rahmani Ramin K.
Published: June 2010

Enhancement of the natural and forced convection heat transfer has been the subject of numerous academic and industrial studies. Air blenders, mechanical agitators, and static mixers have been developed to increase the forced convection heat transfer rate in compressible and incompressible flows. Stationary inserts can be efficiently employed as heat transfer enhancement devices in the natural convection systems. Generally, a stationary heat transfer enhancement insert consists of a number of equal motionless segments, placed inside of a pipe in order to control flowing fluid streams. These devices have low maintenance and operating costs, low space requirements and no moving parts. A range of designs exists for a wide range of specific applications. The shape of the elements determines the character of the fluid motion and thus determines thermal effectiveness of the insert. There are several key parameters that may be considered in the design procedure of a heat transfer enhancement insert, which lead to significant differences in the performance of various designs. An ideal insert, for natural conventional heat transfer in compressible flow applications, provides a higher rate of heat transfer and a thermally homogenous fluid with minimized pressure drop and required space. To choose an insert for a given application or in order to design a new insert, besides experimentation, it is possible to use Computational Fluid Dynamics to study the insert performance. This paper presents the outcomes of the numerical studies on industrial stationary heat transfer enhancement inserts and illustrates how a heat transfer enhancement insert can improve the heat transfer in buoyancy driven compressible flows. Using different measuring tools, thermal performance of two different inserts (twisted and helix) are studied. It is shown that the helix design leads to a higher rate of heat transfer, while causes a lower pressure drop in the flowfield, suggesting the insert effectiveness is higher for the helix design, compared to a twisted plate.
Enhancement of Convective Heat Transfer in Internal Viscous Flows by Inserting Motionless Mixers
Authors: Rahmani Ramin K. , Ayasoufi Anahita , Keith Theo G.
Published: June 2010

In chemical processing industries, heating, cooling and other thermal processing of viscous fluids are an integral part of the unit operations. Enhancement of the natural and forced convection heat transfer rates has been the subject of numerous academic and industrial studies. Motionless mixers, also known as static mixers, are often used in continuous mixing, heat transfer, and chemical reactions applications. These mixers have low maintenance and operating costs, low space requirements, and have no moving parts. Heat exchangers equipped with mixing elements are especially well suited for heating or cooling highly viscous fluids. Shell and tube heat exchangers incorporate static mixing elements in the tubes to produce a heat transfer rate significantly higher than that of conventional heat exchangers. The mixing elements continuously create a new interface between the working fluid and tube wall, thereby producing a uniform heat history in the fluid. It is desired to employ motionless mixers in heat transfer applications to provide a high rate of heat transfer from a thermally homogenous fluid with low pressure drop. In the past, laboratory experimentation has been a fundamental part of the design process of a new static mixer for a given application as well as the selection of an existing static mixer. It is possible to use powerful computational fluid dynamics (CFD) tools to study the performance of these mixers without resorting to experimentation. In this paper, which is an extension to the previous work of the authors, the enhancement of performance of shell and tube heat exchangers by inserting motionless mixers (SMX and helical) is studied for creeping, laminar, and low-Re turbulent flows. It is shown that the studied mixers produced similar flow histories for the working fluid considered. Both SMX and helical mixers are able to increase thermal performance of heat exchangers. The SMX mixer manifests a higher performance in temperature blending and in heat transfer enhancement compared to the helical mixer. However, the pressure drop created by SMX elements, and consequently the required energy to maintain the flow in tube, is significantly higher.
Total Temperature Measurements of Gaseous Flow at Micro-Tube Outlet
Authors: Yamamoto Takaharu , Hong Chungpyo , Asako Yutaka , Suzuki Koichi
Published: June 2010

This paper presents experimental results on heat transfer characteristics of gaseous flow in a micro-tube with constant wall temperature. The experiment was performed for nitrogen gas flow through a micro-tube with 166 micro meters in diameter and 50mm in length. The wall temperature was maintained at 305K, 310K, 330K and 350K by circulating water around the micro-tube, respectively. The stagnation pressure is chosen in such a way that the exit Mach number ranges from 0.1 to 1.0. The outlet pressure was fixed at the atmospheric condition. The total temperature at the outlet, the inlet stagnation temperature, the mass flow rate, and the inlet pressure were measured. The numerical computations based on the Aribitary - Langrangian - Eulerian (ALE) method were also performed for the same cases of the experiment for validation of numerical computation. The both results are in excellent agreement. The total temperatures obtained by the present study are slightly higher than those of the incompressible flow. This is due to the additional heat transfer near the micro-tube outlet caused by the temperature decrease due to the energy conversion into the kinetic energy. A quantitative correlation for the prediction of the heat transfer rate of the gaseous flow in a micro-tube was proposed.
Subcooled Flow Boiling Heat Transfer of Nanofluids in a Microchannel
Authors: Vafaei Saeid , Wen Dongsheng
Published: June 2010

This work investigates the subcooled flow boiling of aqueous based nanofluids in a 510 μm single microchannel with a focus on the effect of nanoparticles on the critical heat flux (CHF). The surface temperature distribution along the pipe, the inlet and outlet pressures and temperatures are measured simultaneously for different concentrations of alumina nanofluids and dionized water. The experiment shows a remarkable increase ∼ 31% in the CHF under very low nanoparticle concentrations (∼0.1v%) and a nonlinear influence of nanoparticles on the subcooled boiling heat transfer.
Investigation of Airflow and Temperature Distribution in the Freezer Cabinet of a Domestic No-Frost Refrigerator
Authors: Nikbay Melike , Acikgoz M. Berkay , Kerpicci Husnu
Published: June 2010

Uniformity of temperature distribution in a loaded freezer cabinet is one of the most important factors affecting energy consumption of a refrigerator. Present study focuses on the airflow behavior and the temperature distribution inside the freezer compartment of a domestic no-frost refrigerator. Energy consumption increases in a freezer cabinet if the temperature difference between the warmest load package and average of all packages is high. The objective is to reduce the energy consumption by providing a uniform temperature distribution and also to keep the food fresh for a longer time. In this study, the air flow and heat transfer during on-time and off-time periods inside the freezer compartment is modeled by considering turbulent and laminar flow conditions in 3D transient CFD analyses. The initial and boundary conditions are provided from temperature controlled room and PIV measurements. The CFD analyses obtained are verified by experimental measurements.
Advanced Scaling Techniques for the Modeling of Materials Processing
Authors: Tello Karem , Duman Ustun , Mendez Patricio
Published: June 2010

The present work presents how scaling analysis can be applied into multiphysics and multicoupled problems related to welding processes. The formation of the weld pool surface depression in high current and velocity Gas Tungsten Arc Welding (GTAW) is dominated by the gas shear acting on the weld pool. Considering this dominant force the weld penetration was estimated and compared to experimental results. Plastic deformation and heat flow are coupled phenomena in Friction Stir Welding (FSW), the maximum temperature was estimated using scaling analysis and compared with experimental and numerical results reported in the literature. Although the simplicity of the scaling models, they are capable of capturing correct trends and order of magnitudes of the unknown estimations in a problem. Moreover, they are capable of determining the dominant forces that act on the process studied.
Simulation of Single Bubble Dynamics in Nucleate Pool Boiling Using a Conservative Level Set Method
Authors: Sajid Muhammad , Bennacer Rachid
Published: June 2010

Most energy conversion systems and cooling devices employ nucleate pool boiling because of its high efficiency of heat exchange. It is a liquid-vapor phase change process associated with ebullition, characterized by cyclic growth and departure of vapor bubbles from heated wall and greatly influenced by the bubble growth mechanism. Bubble dynamics is difficult to simulate due to the difficulty of tracking the liquid vapor interface without smearing it, the discontinuity in material properties due to high density ratio and the need to take surface tension into account that introduces a jump in the pressure field. This paper focuses on the accurate representation of surface tension effects on bubble dynamics in nucleate pool boiling. The complete Navier-Stokes equations are solved and liquid-vapor interface is captured using a conservative level-set technique, curvature of interface is computed using the level set function and surface tension forces are evaluated as a body force according to the continuum surface force method. This enables us to simulate flows with large density and viscosity differences, to capture the shape of the deforming interface of the bubble while maintaining good mass conservation. The ability of the model is demonstrated with the numerical example of a growing bubble.
Scavenging of Gaseous Pollutants by Falling Liquid Droplets in Inhomogeneous Atmosphere With Non-Uniform Temperature and Concentration Distributions
Authors: Elperin Tov , Krasovitov Boris , Fominykh Andrew
Published: June 2010

We analyze non-isothermal absorption of soluble atmospheric trace gases by the falling rain droplets with internal circulation which is caused by interfacial shear stresses. It is assumed that the concentration of soluble trace gases and temperature in the atmosphere varies in a vertical direction. In the analysis we accounted for the accumulation of the absorbate in the bulk of the falling rain droplet. The problem is solved in the approximation of a thin concentration and temperature boundary layers in the droplet and in the surrounding air. We assumed that the bulk of a droplet, beyond the diffusion boundary layer, is completely mixed and concentration of the absorbate and temperature are homogeneous and time-dependent in the bulk. By combining the generalized similarity transformation method with Duhamel’s theorem, the system of transient conjugate equations of convective diffusion and energy conservation for absorbate transport in liquid and gaseous phases with time-dependent boundary conditions is reduced to a system of linear convolution Volterra integral equations of the second kind which is solved numerically. It is shown that the non-uniform vertical distribution of absorbate and temperature in a gaseous phase strongly affects mass transfer during gas absorption by a falling droplet.
Convection Heat Transfer in Concentric Micro Annular Tubes With Constant Wall Temperature
Authors: Hong Chungpyo , Asako Yutaka , Suzuki Koichi
Published: June 2010

Heat transfer characteristics of gaseous flows in concentric micro annular tubes with constant wall temperature whose temperature is lower or higher than the inlet temperature were numerically investigated. The slip velocity, temperature jump and shear stress work were considered on the slip boundary. The numerical methodology was based on the Arbitrary-Lagrangian-Eulerian (ALE) method. The computations were performed for two thermal cases. This is, the wall temperature was constant at the outer wall and inner wall was adiabatic (Case i) and the wall temperature was constant at the inner wall and the outer wall was adiabatic (Case ii). The stagnation temperature was fixed at 300 K and the computations were done for the wall temperature which ranges from 250 K to 350 K. The outer tube radius ranged from 20 to 150 μm with the radius ratio 0.02, 0.05, 0.1, 0.25 and 0.5 and the ratio of length to hydraulic diameter was 100. The stagnation pressure was chosen in such a way that the exit Mach number ranged from 0.1 to 0.8. The outlet pressure was fixed at the atmospheric pressure. The heat transfer characteristics in concentric micro annular tubes were obtained. The bulk temperature and the total temperature are compared with those of both cooled and heated cases and also compared with those of the simultaneously developing incompressible flow obtained by SIMPLE algorithm. The results show that the compressible slip flow static bulk temperature along the length is different from that of incompressible flow. Therefore heat transfer characteristics of the gaseous flow are different from those of the liquid flow and also have different trends whether the wall temperature is lower or higher than the inlet temperature. A correlation for the prediction of the heat transfer rate of gas slip flow in concentric micro annular tubes is proposed.
An Experimental Investigation of Three-Dimensional Flat-Plate Oscillating Heat Pipe
Authors: Thompson Scott M. , Ma Hongbin , Winholtz Robert A. , Wilson Corey
Published: June 2010

An experimental investigation on the effects of condenser temperatures, heating modes and heat inputs on a miniature, three dimensional flat-plate oscillating heat pipe (3D FP-OHP) was conducted visually and thermally. The 3D FP-OHP was charged with acetone at a filling ratio of 0.80, had dimensions of 101.60 × 63.50 × 2.54 mm3, possessed 30 total turns, and had square channels on both sides of the device with a hydraulic diameter of 0.762 mm. Unlike traditional flat-plate designs, this new three-dimensional, compact design allows for multiple heating arrangements and higher heat fluxes. Transient and steady-state temperature measurements were collected at various heat inputs and the activation/start-up was clearly observed for both bottom and side heating modes during reception of its excitation power for this miniature 3D FP-OHP. The neutron imaging technology was simultaneously employed to observe the internal working fluid flow for all tests directly through the heat pipe’s copper wall. The activation was accompanied with a pronounced temperature field relaxation and the onset of chaotic thermal oscillations — all occurring with the same general oscillatory pattern at locations all around the 3D FP-OHP. Qualitative and quantitative analysis of these thermal oscillations, along with the presentation of the average temperature difference and thermal resistance, for all experimental conditions are provided. The novelty of the three-dimensional OHP design is its ability to still produce the oscillating motions of liquid plugs and vapor bubbles and, more importantly, its ability to remove higher heat fluxes.
Investigation of Flame Structure for Laminar Methane/Air Flame Impinging on a Flat Surface
Authors: Chander Subhash , Ray Anjan
Published: June 2010

A combined experimental and numerical study has been conducted to investigate the impinging flame structure. Inflame temperature profiles were obtained and compared with corresponding simulated profiles. For detailed understanding of flame structure numerical simulations were carried out using commercial CFD code FLUENT. Simulated temperature, heat flux and species profiles were analyzed. Further investigations were done by plotting streamlines, velocity magnitude profiles and species profiles. It has been seen that bulk of the combustion products were burnt rapidly in the narrow reaction zone at the tip of the flame. This was because of exponential relationship between the chemical reaction rate and temperature. Simulation results show high temperature in the region between the inner premixed and the outer non-premixed (diffusion) reaction zones. The burnt gas along the inner zone expands and molecules change their directions from initially parallel to diverging lines. Flow accelerated from stagnation point and attained maximum velocity at the start of wall-jet region.
Heat Transfer Enhancement by Irregular Acoustic Streaming in a Shallow Enclosure
Authors: Aktas Murat K. , Ozgumus Turkuler
Published: June 2010

The effects of classical and irregular acoustic streaming structures on convective heat transport in air-filled two dimensional rectangular enclosures are investigated numerically. The oscillatory fluid motion and the resulting streaming motion are driven by cyclic vibration of the enclosure left wall. The fully compressible form of the Navier – Stokes equations are employed to model the transport phenomenon in the enclosure. An explicit time-marching Flux-Corrected Transport (FCT) Algorithm is used to simulate the oscillatory flow field, streaming patterns and associated thermal convection in the enclosure. The vertical walls of enclosure are thermally insulated. The bottom wall is heated isothermally while the top wall is kept at the initial temperature. The transverse temperature gradients strongly affect the acoustic streaming velocities and structures. The irregular streaming significantly augments the heat transfer from the enclosure bottom wall.
Heat Transfer Analysis in Thermoacoustic Regenerators Using CFD Simulation
Authors: Zink Florian , Vipperman Jeffrey S. , Schaefer Laura A.
Published: June 2010

This work is a continuation of previously published work, which focused on a working simulation of a thermoacoustic engine using Fluent [1]. The present work utilizes the developed model to investigate four different resonator geometries and the effect of curvature on the pressure, velocity and temperature over the course of one period of oscillations. It is shown that all relevant flow variables are adversely affected, as the respective amplitudes decrease. As a cumulative parameter, the total energy transfer between the walls of the thermoacoustic stack and the working gas is also discussed. It is concluded that the energy transfer also decreases as a result of increasing curvature.
Experimental Study of Forced Convection Heat Transfer of Water in a Short Micro Duct at Turbulent Reynolds Number
Authors: Syed Iltesham Z. , Mukherjee Abhijit
Published: June 2010

The present study deals with experimental investigation of cooling of machining tools, by water flowing through a microduct at the tip of the tool. The microduct is of diameter of around 200μm and the flow takes place at turbulent Reynolds number. The outer wall temperature of microduct and the temperature of water at inlet and exit have been measured. The convective heat transfer coefficient is calculated at different wall temperatures and varying liquid mass flux. The experimental results shows that the average Nusselt numbers for the short micro-duct are higher than those predicted by conventional correlations for large diameter ducts. A correlation has been proposed to compute convective heat transfer during turbulent flow through a short microduct of a particular geometry for a range of Reynolds and Prandtl numbers.
Numerical Study of Natural Convection Around an Isolated Horizontal Cylinder and Determination of Critical Radius Effect With a Variable Heat Transfer Coefficient
Authors: Atayılmaz S¸. O¨zgu¨r , Demir Hakan , Agra O¨zden
Published: June 2010

Natural convection heat transfer from an insulated horizontal cylinder is studied analytically and numerically. Curved surfaces such as circular cylinder which has a radius smaller than a certain critical size, adding insulation to the surface increases the heat transfer form the surface. This phenomenon occurs if the effects of the increase of the outer surface area on the heat transfer are higher than the decrease by the total thermal resistance of the insulated cylinder. The critical radius is represented as a function of thermal conductivity of the object and convective heat transfer coefficient in the textbooks on heat transfer. This is only valid if both thermal conductivity and convective heat transfer coefficient are constant. In fact, the convective heat transfer coefficient varies with outer diameter of the cylinder while thermal conductivity can be taken as constant. Therefore, a numerical and an analytical study were performed in order to investigate the effects of variable heat transfer coefficient on determining the critical radius. For this aim an isolated horizontal cylinder having different insulation thickness and a constant thermal conductivity was modeled and solved numerically using FLUENT CFD software. Also the same problem was solved analytically and numerical and analytical results were compared. The variation of the total heat transfer from cylinder surface according to insulation thickness is obtained. It is found that the standard critical radius criterion led to significant errors compared to numerical results.
Mathematical Modeling of a Miniature Loop Heat Pipe With Two Evaporators and Two Condensers
Authors: Ku Jentung , Hoang Triem , O’Connell Tamara
Published: June 2010

A loop heat pipe (LHP) analytical model that simulates the steady state and transient thermal behaviors of LHPs with multiple evaporators and multiple condensers has recently been developed. It can be used as a stand-alone computer code or as a subroutine to general spacecraft thermal analyzers. Multi-evaporator and multi-condenser LHPs are more complex in their operation when compared to single-evaporator LHPs because of the thermal and fluid interactions among the evaporators, compensation chambers, and condensers. This analytical model has been used to simulate the thermal performance of a miniature loop heat pipe (MLHP) with two evaporators and two condensers in laboratory and thermal vacuum tests. In addition, the MLHP was tested in the laboratory under five different configurations where the relative elevations and tilts among loop components were varied so as to investigate the gravity effects on the loop performance and to verify the analytical model’s capability to predict such effects. The MLHP performance tests that were simulated included start-up, high power, heat transport limit, and heat load sharing between the two evaporators. In all tests that were modeled, the LHP analytical model accurately predicted the steady state and transient behaviors of the LHP. Furthermore, the model was run-time efficient and yielded stable solutions in all cases.
Improving Biporous Heat Transfer by Addition of Monoporous Interface Layer
Authors: Reilly Sean W. , Catton Ivan
Published: June 2010

Biporous evaporator wicks, generated by sintering copper particles into semi-uniform clusters, were demonstrated to achieve high flux, heat transfer performance for use in heat pipes by Semenic (2007). The effective thermal conductivity of thick biporous wicks at high heat fluxes was found to be reduced because the region next to the wall dried out prematurely allowing the wall interface temperature to rise well above the saturation temperature. The region above the dried out portion of the wick continued to work with the large pores between the clusters being primarily occupied with vapor and the small pores between the particles being occupied with the liquid. In this work, we report our efforts to reduce the size of the wall-wick interface dry-out region by sintering a thin layer of uniform size particles on the wall as originally suggested in a thesis by Seminic (2007). The boiling curve for this “double layer” wick diverges from a standard “single layer” biporous wick at the point of nucleation by reducing the wall temperature, and concurrently the overall temperature drop across the wick needed to drive a given heat flux. The temperature drop across the wick is reduced because the thin layer of particles between the biporous wick and the wall reduces the wall-wick interface resistance and also provides additional capillary channels underneath the biporous wick. Experimental data supports this hypothesis by showing a clear divergence between measured wall temperatures for the double layer wick from its single layer counterpart. The presumed point of nucleation in both wicks is similar, with the heat flux increasing much more rapidly than the liquid superheat and it is clear that this slope is much steeper for the double layer wick. This finding has great potential to expand the performance capabilities of heat pipes and vapor chambers because the new double layered wick can transfer more heat with less superheat thereby increasing the effective thermal conductivity of the wick and decreasing the wall-wick interface temperature for a given heat flux.
Burning Rates and Surface Characteristics of Hydrogen-Enriched Turbulent Lean Premixed Methane-Air Flames
Authors: Guo Hongsheng , Tayebi Badri , Galizzi Cedric , Escudie´ Dany
Published: June 2010

The burning rates and surface characteristics of hydrogen-enriched turbulent lean premixed methane-air flames were experimentally studied by laser tomography visualization method using a V-shaped flame configuration. Turbulent burning velocities were measured and the variation of flame surface characteristics due to hydrogen addition was analyzed. The results show that hydrogen addition causes an increase in turbulent burning velocity for lean CH4-air mixtures when the turbulent level in the unburned mixture is not changed. The increase rate of turbulent burning velocity is higher than that of the corresponding laminar burning velocity, suggesting that the increase in turbulent velocity due to hydrogen addition is caused by not only chemical kinetics effect, but also the variation in flame structure due to turbulence. The further analysis of flame surface characteristics and brush thickness indicate that hydrogen addition slightly decreases local flame surface density, but increases total flame surface area because of the increased flame brush thickness. As a result, turbulent burning velocity is intensified by the increase in total flame surface area and the increased laminar burning velocity, when hydrogen is added.
The Effect of Spatially Correlated Roughness and Boundary Conditions on the Conduction of Heat Through a Slab
Authors: Emery A. F. , Dillon H. , Mescher A. M.
Published: June 2010

The nominally one dimensional conduction of heat through a slab becomes two dimensional when one of the surfaces is rough or when the boundary conditions are spatially non-uniform. This paper develops the stochastic equations for a slab whose surface roughness or convective boundary condition is spatially correlated with correlation lengths ranging from 0 (white noise) to a length long in comparison to the slab thickness. The effect is described in terms of the standard deviation and the resulting spatial correlation of the heat flux as a function of depth into the slab. In contrast to the expectation that the effect is monotonic with respect to the correlation length, it is shown that the effect is maximized at an intermediate correlation length. It is also shown that roughness or a random convective heat transfer coefficient have essentially the same effects on the conducted heat, but that the combination results in a much deeper penetration than does each effect individually. In contrast to the usual methods of solving stochastic problems, both the case of a rough edge and a smooth edge with stochastic convective heat transfer coefficients can only be treated using direct Monte Carlo simulations.
Efficient Multiphase Cooling System for Large Area Using Multiple Jet/Spray
Authors: Sarkar Mita , Selvam R. Panneer , Kuss Mark , Balda Juan C.
Published: June 2010

Jet/spray cooling is a way of efficiently removing heat from a hot surface like in the back plate for high power electronic devices. In the present work, a close loop liquid jet/spray setup with a novel effluent/spent liquid removal system is established for large area cooling. A 4×4 nozzle array is used to generate the spray/jet targeting the heat source area of 4 (2×2) cm2. Flourinert FC-72™ is used as coolant liquid. The thermal performance data for multi-jet/spray cooling in a closed and confined system are obtained at various liquid temperatures and flow rates. The experimental work illustrates the multi-jet cooling system can reach critical heat fluxes up to 101 W/cm2. The results showed that the heat flux is increased with liquid sub-cooled temperature and flow rates in single phase as well as in phase change zone. It is further observed that the critical heat flux (CHF) increases with liquid sub-cooled temperature and flow rates.
Experimental and Numerical Study on the Cold Start Performance of a Single PEM Fuel Cell
Authors: Li Calvin H. , Beatty John , Durbin Sam , Hodgins Paul , Kwasniak Peter
Published: June 2010

A combined experimental and analytical investigation of single proton exchange membrane (PEM) fuel cells, during cold start has been conducted. The temperature influence on the performance of a single PEM fuel cell and the cold start failure of the PEM fuel cell was evaluated experimentally to determine the failure mechanisms and performance. The voltage, current and power characteristics were investigated as a function of the load, the hydrogen fuel flow rate, and the cell temperature. The characteristics of cold start for a single PEM fuel cell were analyzed, and the various failure mechanisms explored and characterized. In an effort to better understand the operational behavior and failure modes, a numerical simulation was also developed. The results of this analysis were then compared with the previously obtained experimental results and confirmed the accuracy of the failure mechanisms identified.
Thermal Conductivity Measurement of Cobalt-Iron Thin Films Using the Time-Resolved Thermoreflectance Technique
Authors: Jeong Taehee , Zhu Jian-Gang
Published: June 2010

Using the time–resolved thermoreflectance technique, the thermal conductivity of CoFe films are measured with various thicknesses and the results show a thickness-dependent thermal conductivity. In order to overcome the obstacle for the high thermal conductivity metal film measurement, a thermal barrier (SiNx) is added between the metal film and Si substrate.
Numerical Modeling of Liquid Drop Spreading Behavior on Inclined Surfaces
Authors: Park Young-Gil , Jacobi Anthony M.
Published: June 2010

A numerical study was conducted on the spreading behavior of liquid drops on flat solid surfaces. The model predicts the shape of liquid-vapor interface under static equilibrium using an unstructured surface grid composed of triangular elements. Incremental movement of base contour, i.e. solid-liquid-vapor contact line, is also captured such that the constrained boundary conditions, i.e. advancing and receding contact angles, can be satisfied. The numerical model is applied to a common experiment that studies the behavior of liquid drops on inclined surfaces, where the shape of the drops change in response to an alteration of total volume or gravitational direction. On a heterogeneous surface that has contact angle hysteresis, the shape of the base contour on the solid surface is not determined uniquely but rather dependent upon history. This study demonstrates such dependence by comparing the spreading of a liquid drop on a solid surface with different quasi-equilibrium paths.
Study of the Effect of Turbulence Promoters in Circular Cooling Channels
Authors: Uzarraga-Rodriguez N. Cristobal , Gallegos-Mun˜oz Armando , Rubio-Arana J. Cuauhtemoc , Campos-Amezcua Alfonso , Zdzislaw Mazur
Published: June 2010

A numerical analysis of a gas turbine first stage bucket with internal cooling (model MS7001E) is presented. The internal cooling system consists of 13 cylindrical channels with turbulent promoters (ribs), which are implemented in order to achieve temperature decrements inside the body blade. Three different geometrics (square, triangular and semi-circular cross-section) are studied. Each configuration is analyzed having full or half ribs. These are placed inside the cooling channels. The effects generated by the aspect ratio variation between rib pitch and rib height (P/e), for a constant aspect ratio given by ribs height and hydraulic diameter (e/Dh) are considered. The numerical simulation was developed using finite volume method, by means of commercial software based on computational fluid dynamics (CFD). Each one of the models generated for each study case was built in a 3D model, including the platform and airfoil of the blade. The models consider the effects generated by the hot combustion gases are flowing around the blade and the coolant flow is flowing inside the cooling channels. The study includes the solution of the conjugate heat transfer. The results show that the cooling channels with squared and triangular full-ribs present better cooling effects inside the body blade, reducing the temperature until 10°C at some point in the blade. However, these configurations produce a pressure drop from 3 to 4 times higher than cooling channels without ribs. The half ribs produce lesser temperature decrement, having smaller pressure drop. On other hand, the aspect ratio (P/e) has only effects on the pressure drop.
Characterization of Electronic Packages by Thermal Diffusion Tomography
Authors: Erturk Hakan
Published: June 2010

One of the most important functions of an electronic package is thermal management, as package is responsible from removing the heat generated by the transistors to ensure reliability. The quality of the package is very important for proper thermal management and it is important to have minimal flaws that increase thermal resistance of the package. Therefore, detection of flaws in the multi-layered package is critical during the assembly process development to monitor the package quality. This is achieved by techniques such as computerized tomography (CT) using x-rays, or scanning acoustic microscopy (SAM), all of which require very expensive equipment and significant processing time. Thermal diffusion tomography (TDT) can be used for detecting the flaws as a lower cost alternative to these imaging techniques. The feasibility of TDT as a fault detection technique for electronic packages with IR thermometry is considered in the current study. Two reconstruction algorithms considered; an iterative perturbation approach and Levenberg-Marquard method were found to be capable of detecting the flaws in the thermal interface layer.
Modelling Radiative Heat Transfer in Oxycoal Combustion
Authors: Porter Rachael , Pourkashanian Mohamed , Williams Alan , Smith David
Published: June 2010

The reduction of greenhouse gas emissions is essential to mitigate the impact of energy production from fossil fuels on the environment. Oxyfuel technology is a process developed to reduce emissions from power stations by removing nitrogen from air and burning the fossil fuels in a stream of pure oxygen. The remaining oxidiser is composed of recycled flue gas from the furnace to reduce temperatures. The product of this system is a flue gas with very high carbon dioxide concentration enabling more efficient capture and storage. Accurate modelling of oxyfuel is essential to gain better understanding of the combustion fundamentals and obtain accurate predictions of properties within the furnace that cannot be measured. Heat transfer to the furnace walls will be affected due to the different composition of the gases in the furnace. Carbon dioxide has higher heat capacity than nitrogen. Water vapour and carbon dioxide also exhibit absorption spectra of radiation in the infra-red region of the spectrum relating to wavelengths observed in combustion. Accurate CFD modelling of radiative heat transfer in oxyfuel combustion will require improvements to the radiative properties model to account for the spectral nature of radiation. In addition the impact of the solid fuel particles, soot and ash are considered. Several different radiative properties models have been tested to assess the impact on the predicted radiation and temperatures under air and oxy firing conditions. The results for radiation transferred to the walls are highly dependent upon the model chosen and the need for an accurate radiative properties model for oxyfuel firing, such as the full-spectrum k-distribution method is demonstrated.
Unsteady Natural Convection Heating of a Canned Non-Newtonian Liquid Food
Authors: Moraga Nelson O. , Silva Luis A. , Ortega Alfonso
Published: June 2010

This paper is concerned with the modeling of complex thermal-fluid phenomena that arises during the process of sterilization of canned food. In the sterilization process, food contained with a typical cylindrical can is subjected to a sudden change in temperature as the can is immersed in a steam bath at saturation temperatures of the order of 120 degrees C. Because of this transient heating event, complex transient buoyant motion is initiated in the can and therefore the heating of the food is as a result of diffusion and buoyant or natural convection processes. A Finite Volume method was used to investigate the unsteady two dimensional (axisymmetric) natural convection of a non-Newtonian liquid food during sterilization. The non-Newtonian liquid food was modeled using carboxy-methyl cellulose. Because its apparent viscosity depends on both temperature and non-linear velocity gradients, the resulting mathematical formulation is highly non-linear, thereby requiring a highly coupled implementation of the Finite Volume Method. Transient two dimensional velocity and temperature fields were obtained for different heating conditions and can aspect ratios. It was found that lower can aspect ratios and lower index n generally lead to shorter sterilization times.
Measurement of Unsteady Flow in Pump-Turbine for Hydropower Using PIV Method
Authors: Fukuda Nobuhiko , Someya Satoshi , Okamoto Koji
Published: June 2010

It is thought that the pressure fluctuation can occur due to the interaction between flow through guide vanes and flow into runner blades, resulting in a vibration of turbine and a blade cracking, in a hydraulic turbine operated in a wide range for flexible power demand. High accurate velocity measurement with high time/spatial resolution can help to clarify the mechanism of the interaction and to provide good experimental data for the validation of numerical procedure. So the aim of present study is to estimate the unstable velocity field quantitatively in the area between guide vanes and runner blades, using high time-resolved particle image velocimetry (PIV). Two types of velocity measurements were carried out, i.e., phase-locked measurement and high time sequential velocity measurement, in a pump-turbine model with 20 guide vanes and 6 runner blades. The characteristic of the flow field varied corresponding to the operating conditions such as flow rate and rotational speed. Opening angles of guide vanes were kept uniform. A clockwise vortex was generated at inside of the runner blade under smaller rotational speed. A counterclockwise vortex was separated at the backside of the runner blade under higher rotational speed. At any operating conditions, the velocity between guide vanes and runner blades oscillated periodically at the blade passing frequency.
Numerical Analysis on the Effects of Transversal Ribs on Forced Convection in Channels
Authors: Manca O. , Nardini S. , Ricci D. , Tamburrino S.
Published: June 2010

Heat transfer in fluids is very important in many industrial heating and cooling equipments. Convective heat transfer can be enhanced passively by changing flow geometry, boundary conditions or by increasing thermal conductivity of the fluid. Another possibility for increasing heat transfer with gas is to employ extended surfaces. When a fluid flows in a channel, transversal ribs can be used as fins and break the laminar sublayer creating local wall turbulence. However, as a consequence the presence of the ribs can significantly augment pressure drops. In this paper a numerical investigation is carried out on forced convection in channels heated by a constant heat flux. Also conductive effects are taken into account. The fluid is air and properties are assumed as function of temperature. Ribs of the same material of the channel walls are introduced and several arrangements are analyzed. The investigation is accomplished by means of the commercial code Fluent. A turbulence model is used. Results are presented in terms of temperature and velocity fields, average heat transfer coefficients, friction factor profiles and pressure drops. The aim of this study is to find arrangement of ribs such to give high heat transfer coefficients and low pressure drops. The maximum Nusselt number and friction factor have been detected for dimensionless pitches equal, respectively, to 12 and 10.
Two-Phase Flow With Surfactants in a Microchannel
Authors: Liu Dong , Sur Aritra
Published: June 2010

Addition of surfactants to liquids helps to eliminate intermittent two-phase flow patterns and alleviate flow instability. These features are very desirable for two-phase microfluidic applications. However, very little information is available on two-phase flow patterns of surfactant solution in the microchannels. The present paper reports a study of adiabatic two-phase flow with surfactants in a circular microchannel of a 180-μm diameter. Air-water mixtures with trace quantities of sodium dodecyl sulfate (SDS) were used in the experiments. The maximum superficial velocities measured were 4 m/s for the liquid and 65 m/s for the gas. High-speed photographic technique was employed to visualize various two-phase flow patterns and to identify the transition boundaries between different flow regimes. The results were compared to data obtained from air-water flow without surfactants. It was found that addition of surfactants brings in significant modification to the two-phase flow regimes as well as their transition characteristics in microchannels; in particular, slug flow is effectively suppressed.
Numerical Investigation on the Effect of Transversal Septa on Forced Convection in Circular Tubes
Authors: Manca O. , Nardini S. , Ricci D.
Published: June 2010

Conventional sources of energy have been depleting at an alarming rate, which makes future sustainable development of energy use very difficult. Thus, heat transfer enhancement technology plays an important role and it has been widely applied to many applications as in refrigeration, automotive, process industry, solar energy heater, etc. Convective heat transfer can be enhanced passively by changing flow geometry, boundary conditions or by increasing thermal conductivity of the fluid. Another possibility for increasing heat transfer with gas is to employ extended surfaces. In this paper a numerical investigation is carried out on forced convection in circular tubes with septa heated by constant fluxes and characterized by different shapes. When gas flows in a tube, septa with one or more openings can be used as fins. Furthermore, when the openings are arranged to give a spiral motion around the cylinder axis wall-fluid contact area increases. As a consequence the presence of the septa may significantly augment pressure drops. The fluid is air and properties are function of temperature. Septa of the same material of the tube are introduced and several shapes and arrangements are analyzed as well as different Reynolds numbers, baffle spacings and heat fluxes applied on the external surface. The investigation is accomplished by means of the commercial code Fluent. A k-e turbulence model is used with enhanced wall treatment options. Results are presented in terms of temperature and velocity fields, local and average heat transfer coefficients, friction factors and pressure drops for different values of heat flux, Reynolds numbers and baffle spacings. The aim of this study is to find the shape and arrangement of septa such to give high heat transfer coefficients and low pressure drops.