bluuebunny/crossref_metadata_2025 huggingface dataset:
Effects of Ice Formation on the Flowfield of an Aircraft Engine Inlet
Authors: Taghavi Ray R. , Jin Wonjin
Published: July 2009

The effects of typical rime and glaze ice on the performance of the M2129 S-duct inlet are computationally investigated using the steady-state RANS solution. The glaze ice accretion produces a substantial degradation of the inlet performance due to its obstructive shape to the in-flow, while the effect of the rime ice is not significant. Compared to the clean inlet, the secondary flow region at the engine face of the duct inlet is increased by 600 percent for the glaze iced inlet. Total pressure recoveries at the engine face for the rime and glaze ice case are 98.8 and 95.8 percent, respectively. Also, the glaze ice causes 26 percent reduction in the mass flow rate at the engine face. In addition, the adverse effects on the performance of the inlet are enhanced by an increase in freestream Mach numbers due to the stronger and more extensive shock formations in the inlet flow. With increasing free stream Mach numbers from M∞ = 0.13 to 0.85, total pressure recovery decreases from 0.985 to 0.62 with the glaze ice accretion. And the level of the mass flow rate with the glaze ice accretion is 76 percent of that in ice-free condition at M∞ = 0.13; however, it decreases to 68 percent at M∞ = 0.475.
Laminar-Turbulent Transition Modeling Strategies for Thermally Protected Airfoils
Authors: Tobaldini Neto Luiz , de Mattos Pimenta Marcos , da Silva Guilherme Araujo Lima
Published: July 2009

The present article discusses some of the transition modeling strategies typically applied by the industry. Transition effects on the heat transfer characteristics and calculation for airfoil anti-ice protection systems build the framework of the developments of interest. Results of the studied models are presented for flat plate data taken from the ERCOFTAC experiments.
The Similarity of Ducted Jets With Low and High Pressure Gradients
Authors: Arima Marcos Noboru , Pimenta Marcos de Mattos , da Silva Guilherme Araujo Lima
Published: July 2009

The objective of the present work is to study what type of effects the dimensionless jet parameters really consider. To do it, three classical dimensionless jet parameters are redeveloped using an unified methodology. This methodology is based on the integral balances of mass and momentum. The momentum balance terms are classified as inertial or pressure terms and as flux or source terms. This redevelopment enlighten the meaning of the dimensionless jet parameters and allows the definition of a new set of parameters. A scaling methodology is presented to compare the dimensionless jet parameters adequacy in scaling center line jet velocity and jet width at different operational conditions. Two regimes are distinguished: low pressure gradient and high pressure gradient. The scaling for low pressure gradient is based on two criteria: the diffusivity ratio proportionality and the linear expansion rate of the jet. The diffusivity is modelled using the Prandtl’s mixing length model and a dimensional analysis based only on inertial momentum terms. The scaling for high pressure gradient is based on jet velocities and widths scaled simultaneously by pressure and inertial momentum terms. The application of this methodology to jet literature data shows jet similarity for low and high pressure gradients. However, some drawbacks are identified in scaling the jet width at high pressure gradients.
A New Methodology for the Evaluation of the Maneuvering Characteristics of Surface Ship Models
Authors: Park Joel T. , Hayden David D. , Kim Joseph H. , Melendez Mark P.
Published: July 2009

An Arc Second Indoor Global Positioning System (IGPS) has been applied in the evaluation of the maneuvering characteristics of surface ships. The system consists of a series of laser transmitters located around the Maneuvering and Seakeeping (MASK) Basin at the David Taylor Model Basin (DTMB) in West Bethesda, Maryland. Receivers for the location of the model within the MASK are installed on a free-running surface ship model. Since the system provides position of the model within the basin as a function of time, velocity is also computed from finite differences of position and time. The system has been effective in providing velocity calibration data versus propeller shaft speed in straight-line runs. The characteristics of circle maneuvers have also been determined from the system such as the steady turn radius as a function of approach velocity and rudder angle, velocity in a turn, and the drift angle. Quantitative results are provided for a surface ship model in circle maneuvers and velocity calibration from straight-line runs. The typical Froude number range for a test is typically 0.1 < Fr < 0.4 with an uncertainty of ±0.0048 at the 95% confidence level.
Numerical Study of Dead-End Micro Polymer Electrolyte Membrane Fuel Cell
Authors: Wu Yan Ling , Poh Hee Joo , Lum Kah Wai , Xing Xiu Qing
Published: July 2009

In this paper, 3D full size simulation on single cell dead-end micro PEMFC is carried out using Computational Fuel Cell Dynamics (CFDC) analysis. The active area in this Micro PEMFC is about 10 cm2, producing 1A of current under standard condition (25 °C and 1 atm). The dead end anode configuration is achieved by increasing the air flow rate well above stoichometric at the cathode to obtain the complete depletion of hydrogen at anode exit. It is also assumed that the presence of water is only in the vapor phase. Different types (single serpentine and triple serpentine) of gas channel design in dead-end anode are studied and results are compared. The polarization curves for both designs as well as contour plots for the different cell region are presented.
Development of Microcapsules Including a Gas Bubble for Shock Wave Based Drug Delivery
Authors: Tamagawa Masaaki , Ishimatsu Norikazu
Published: July 2009

This paper describes the trial of making microcapsules including a bubble for shock wave drug delivery systems, evaluation of their mechanical properties and development of new driving mechanics of the microcapsules.
Predicting the Two-Phase Flow Patterns Implementing the Froude Number Correlation
Authors: Rahman Mohammad A. , Heidrick Ted , Fleck Brian A.
Published: July 2009

In this study, two-phase air/water horizontal flow characteristics with different conduit geometries were investigated. After mixing in a horizontal feeding conduit (100 cm long and 2.54 cm ID) the two-phase, air/water fluid passes through an industrial nozzle, which generates spray. Three different novel geometries were investigated in this study along with the existing design. Flow stability and different flow regimes (dispersed, intermittent and slug) for these three types of geometries were investigated. It is observed that when the input liquid content (εL) decreased to 0.5 (high ALR), the two-phase flow became unstable. It is concluded that the novel geometries are capable of producing more stable and dispersed two-phase flow pattern and spray in an industrial nozzle due to enhanced turbulent mixing.
Multi Scale Modeling for Granular Flows
Authors: Zhao Xiang , Zhang Sijun
Published: July 2009

A numerical method has been developed to predict granular flows. This work involves the combination of the Discrete Element Method (DEM) to describe the discrete flow of the granular solids and Computational Fluid Dynamics (CFD) to describe the continuum flow of the interstitial fluid. The numerical models can satisfactorily describe granular materials for all flow regimes, quantify fluid-particle and particle-particle interactions and their effects on the granular materials via detailed micro-dynamic analysis and finally provide the macroscopic continuum equations with constitutive equations such as stress, strain and other physical quantities describing the state of the system based on a newly developed micro-macro averaging procedure.
The Rise of Complexity in Describing Fluid Flow in Rotating Cavities
Authors: Will Bjo¨rn-Christian , Benra Friedrich-Karl
Published: July 2009

This paper is concerned with rotor-stator cavities in fluid machinery and contains a general survey explaining the fundamental effects and geometries. Starting with the free disk model the basic flow structure is shown. An attempt to derive the frictional moment for the free disk, using the Eulerian-equation of turbomachines, leads to the conclusion that 1D coherences are not sufficient to describe this type of flow. Next, a disk rotating in an enclosed housing is considered with respect to the appearance of a rotating fluid core, which mainly influences the static pressure distribution on the walls and the frictional resistance. Furthermore the inclusion of a superposed through-flow leads to a more realistic model that could be used as an example in the gap between an impeller and the stationary casing of a pump. In fact the core rotation is strongly influenced by the properties and direction of the incoming through-flow. As a consequence the pressure distribution is determined by the resulting core rotation, normally expressed in terms of the core rotation coefficient. For instance the pressure distribution acting on the back area of the impeller is a main part of the resulting axial force. The rise of complexity can be illustrated by considering the number of describing dimensionless parameters. The main parameters are derived using dimension analysis and their relevance is shown with respect to the flow physics.
Delay of Unsteady Flow in a Radial Diffuser
Authors: Ahmed Saad A.
Published: July 2009

The operation of centrifugal compressor systems is limited at low-mass flow rates by fluid flow instabilities leading to rotating stall or surge. These instabilities limit the flow range in which the compressor can operate. They also lower the performance and efficiency of the compressor. Experiments were conducted to investigate a model of radial vaneless diffuser at stall as well as stall-free operating conditions. The speed of the impeller was kept constant at 2000 RPM, while the mass flow rate was reduced gradually to scan the steady and unsteady operating conditions of the compressor. The flow rate through the compressor was gradually decreased until flow instability is initiated at the diffuser. The flow rate was further reduced to study the characteristics of rotating stall. These measurements were reported for diffuser diameter ratios, Do/Di, of 2.0 with diffuser width ratio, b/Di, of 0.055. At lower flow rates than the critical, the rotating stall pattern with one stall cell was dominant over the pattern with two cells. In addition, the instability in the diffuser was successfully delayed to a lower flow coefficient when rough surfaces were attached to one or both sides of the diffuser with the lowest values achieved by attaching the rough surface to the shroud. Results show that the roughness has no significant effect on stall cell characteristics.
Numerical Modeling of Turbulent Surface Wave Motion Using a Coupled Boundary Element-Finite Difference Technique
Authors: Jamali Mirmosadegh
Published: July 2009

In this paper an effective numerical technique is presented to model turbulent motion of a standing surface wave in a tank. The equations of motion for turbulent boundary layers at the solid surfaces are coupled with the potential flow in the bulk of the fluid, and a mixed BEM-finite difference technique is used to obtain the wave and boundary layer characteristics. A mixing-length theory is used for turbulence modeling. The results are compared with previous experimental data. Although the technique is presented for a standing surface wave, it can be easily applied to other free surface problems.
Numerical Analysis of a Hybrid Heat Sink Using Phase Change Material: Application to Cooling of Electronic Components
Authors: Faraji Mustapha , El Qarnia Hamid
Published: July 2009

The aim of the present work is to study the thermal performance of a hybrid heat sink used for cooling management of protruding substrate-mounted electronic chips. The power generated in electronic chips is dissipated in phase change material (PCM n-ecosane with melting temperature Tm = 36°C) that filled a rectangular enclosure. The advantage of using this cooling strategy is that the PCMs are able to absorb a high amount of heat generated by electronic component (EC) without acting the fan, during the charging process (melting of the PCM). A (2D) mathematical model was developed in order to analyze and optimize a heat sink. The governing equations for masse, momentum and energy transport were developed and discretised by using the volume control approach. The resulting algebraic equations were next solved iteratively by using TDMA algorithm. Numerical investigations were conducted in order to optimize the thermal performance of the heat sink. The optimization involves determination of the key parameters of the heat sink that maximize the time required by the base of the electronic component to reach a critical temperature.
On the Critical Flows of Pressurized Pipe Systems
Authors: Chung Moon-Sun , Lee Sung-Jae
Published: July 2009

Two-phase critical flows through some breaks of pressurized pipe systems are calculated by an improved critical flow model. These phenomena have many difficulties in predicting the two-phase critical flow rates at the break points mainly due to the inaccuracies of the critical flow model. Further, the case of critical flow through the small bottom break of horizontal pipe, the accuracy of a quality prediction model is also important as well as the critical flow model itself. In this study, an improved critical flow model is introduced which is based on a new sound speed criterion derived from the hyperbolic two-fluid model for non-equilibrium flow and this model is implemented to a system analysis code. Following to the vertically upward flows with quality variation and Marviken tests as benchmark problems, the small bottom break of a pressurized horizontal pipe is calculated and discussed in some details. The assessment results on the critical flow test through a small bottom break in a horizontal pipe show the importance of the accuracy of the critical flow model as well as that of the quality prediction model to reduce the relative errors.
Computational Modeling of the Cathode Catalyst Layer of a PEMFC
Authors: Kamarajugadda Sai , Mazumder Sandip
Published: July 2009

The effect of the cathode catalyst layer’s structure and composition on the overall performance of a polymer electrolyte fuel cell (PEMFC) is investigated numerically. The starting point of the sub-grid scale catalyst layer model is the well-known flooded agglomerate concept. The proposed model addresses the effects of Nafion loading, platinum loading, platinum/carbon ratio, agglomerate size and cathode layer thickness. The sub-grid scale model is first validated against experimental data and previously published results, and then embedded within a two-dimensional validated computational fluid dynamics code that can predict the overall performance of the fuel cell. The integrated model is used to explore a wide range of the compositional and structural parameter space, mentioned earlier. In each case, the model is able to correctly predict the trends observed by past experimental studies. The studies show that the presence of an optimal performance with varying Nafion content in the cathode is more due to the local agglomerate-level mass transport and conductivity losses in the polymer coating around the agglomerates than due to the amount of Nafion within the agglomerate. It was also found that platinum mass loading needs to be at a moderate level in order to optimize fuel cell performance. Cathode tortuosity has a significant effect on fuel cell performance at low values of agglomerate radius.
Natural Convective Heat Transfer From an Isothermal Vertical Square Cylinder Mounted on a Flat Adiabatic Base
Authors: Oosthuizen Patrick H.
Published: July 2009

Natural convective heat transfer from an isothermal vertical cylinder with a square cross-section which has an exposed horizontal top surface has been numerically studied. The exposed upper surface is maintained at the same temperature as the vertical walls of the cylinder. The cylinder is mounted on a flat horizontal adiabatic base plate. The interest in this situation stems from the fact that it is an approximate model of some electrical component cooling situations. The flow has been assumed to be steady and laminar and it has been assumed that the fluid properties are constant except for the density change with temperature which gives rise to the buoyancy forces, this having been treated by using the Boussinesq approach. The solution has been obtained by numerically solving the three-dimensional governing equations, these equations being written in terms of dimensionless variables. The numerical solution has been obtained using a commercial finite element method based code, FIDAP. The solution has the following parameters: the Rayleigh number, Ra, based on the height of the heated cylinder, h, and the overall temperature difference Tw − Tf, the dimensionless size of the square cross-section of the cylinder surface, W = w/h, w being the size of the cross-section, and the Prandtl number, Pr. Because of the applications that motivated this study, results have only been obtained for Pr = 0.7. A wide range of the other governing parameters has been considered. The conditions under which the heat transfer from the exposed upper surface can be neglected compared to that from the vertical side walls in the evaluation of the mean Nusselt number for the entire cylinder have been explored.
The Effect of Obstacle Geometry and Position on Thermal Stratification in Solar Hot Water Storage Tanks
Authors: Altuntop Necdet , Ozceyhan Veysel , Tekin Yusuf , Gunes Sibel
Published: July 2009

In this study the effect of obstacle geometry and its position on thermal stratification in solar powered domestic hot water storage tanks are numerically investigated. The goal of this study is to obtain higher thermal stratification and supply hot water for usage as long as possible. The temperature distributions are presented for three different obstacle geometries (1, 2 and 3) and six different distances (f = 0.3, 0.4, 0.5, 0.6, 0.7 and 0.8 mm) from the bottom of the hot water storage tank. The numerical method is validated using both experimental and numerical results available in the literature. It is observed from the results that the thermal stratification increases with the increasing obstacle distance from the bottom of the hot water storage tank for obstacle 1 and 3. The obstacle 2 provides less thermal stratification than the obstacles 1 and 3. As a result, in a duration of 30 minutes, the obstacle 3 provides the best thermal stratification for the distance of f = 0.8 mm from the bottom of the hot water storage tank.
Heat Transfer Enhancement in Channel Flow Using an Inclined Square Cylinder
Authors: Yoon Dong-Hyeog , Yang Kyung-Soo , Choi Choon-Bum
Published: July 2009

The large-scale vortices shed from a cylindrical object as a vortex generator can be used to enhance heat transfer in a heat exchanger [1]. Furthermore, the large-scale vortices induce secondary vortices on the walls of a heat exchanger, which also play a positive role in heat transfer enhancement.
Numerical Prediction of Flow and Heat Transfer Rates in Metal Based Microchannels Using Lattice Boltzmann Method
Authors: Parida Pritish R. , Ekkad Srinath V.
Published: July 2009

Metal-based Microchannel Heat Exchangers (MHEs) are of current interest due to the combination of high heat transfer performance and improved mechanical integrity. In the present work, a simple two-dimensional thermal lattice Boltzmann model without viscous heat dissipation and pressure compressible work has been developed to simulate the heat transfer phenomenon in Cu- and Al-based micro-channels. A 2D fluid-solid conjugate heat transfer problem is solved using LBM and Fluent. For the Cu specimen, the height of the channel considered was 204 μm and the top and bottom wall thickness was taken to be same as the channel height. The LBM results were compared with 3D and 2D fluent models. The study also compares the numerically computed velocity profile with the analytical results and compares the Nusselt number values predicted by LBM and Fluent with the experimental data. Owing to the simplicity of the thermal LB model, promising results were obtained from the LBM predictions.
Carbon Monoxide Transport Through Water Curtain
Authors: Chow W. K. , Ip Mabel K. K.
Published: July 2009

Drencher water systems are commonly installed across openings for substituting fire resisting constructions in the Far East such as Hong Kong. There were studies on blocking heat and smoke, but not yet on toxic gases. Spreading of toxic gas through the water curtain discharged from the system will be studied by taking carbon monoxide as an indicator to evaluate the system performance in a fire. Blocking of carbon monoxide was demonstrated in a two-room structure with a ‘fire’ side and a ‘protected’ side. A water curtain with common design data on operating pressures and flow rates was discharged in the opening between them. A propanol pool fire of 600 mm diameter and heat release rate 165 kW was burnt in the ‘fire’ room. Carbon monoxide concentration was measured in the ‘protected’ room. It was observed that water curtain cannot block the spreading of carbon monoxide. The concentrations in the protected room were roughly the same with or without discharging the curtain. The reason is because of the large porosity inside the water curtain as proposed earlier.
Heat Transfer Enhancement in a Tube Using Triangular Ribs
Authors: Ozceyhan Veysel , Gunes Sibel
Published: July 2009

A numerical study was undertaken for investigating the heat transfer enhancement in a tube with triangular cross sectioned ribs. The spacing between the ribs were kept constant as a distance of tube diameter, D. Three different rib thicknesses were considered for numerical analyses. Uniform heat flux was applied to the external surface of the tube and air was selected as working fluid. Numerical calculations were performed with FLUENT 6.1.22 code, in the range of Reynolds number 8000–36000. The results obtained from a smooth tube and rib inserted tube were compared with those from the experimental studies in literature in order to validate the numerical method. The variation of Nusselt number, friction factor and overall enhancement ratios for the tube with triangular cross sectioned ribs were presented. Consequently, a maximum gain of 1.34 on overall enhancement ratio is obtained for S/D = 0.75.
CFD Approach Analysis of Chemical Reactions Coupled Convective Heat Transfer in Reformer Ducts
Authors: Yuan Jinliang , Yang Guogang , Sunde´n Bengt
Published: July 2009

Thermo-mechanical failure of components in a compact steam reformer is a major obstacle to bring this technology to real-life applications. The probability of material degradation and failure depends strongly on the convective heat transfer in the fuel gas flow duct and local temperature distribution in multifunctional materials. It is of significant importance to accurately predict the convective heat transfer coupled with catalytic reactions within the reformer components. In this paper, the simulation and analysis of combined chemical reactions and transport processes are conducted for a duct relevant for compact design steam reformer, which consists of a porous layer for the catalytic reforming reactions of methane, the fuel gas flow duct and solid plates. A fully three-dimensional computational fluid dynamics (CFD) approach is applied to calculate transport processes and effects of thermal conductivities of the involved multi-functional materials on convective heat transfer/temperature distributions, in terms of interface temperature gradients/heat fluxes and Nusselt numbers. The steam reformer conditions such as mass balances associated with the reactions and gas permeation to/from the porous anode are implemented in the calculation. The results show that the classic thermal boundary conditions (either constant heat flux or temperature, or combined one) may not be applicable for the interfaces between the fuel flow duct and solid plate/porous layer.
Enhancement of Natural Convection Heat Transfer Rate in Internal Compressible Flows by Inserting Stationary Inserts
Authors: Tanbour Emad Y. , Rahmani Ramin K.
Published: July 2009

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 device in the natural convection systems with compressible flow. Generally, a stationary heat transfer enhancement insert consists of a number of equal motionless units, placed on the inside of a pipe or channel 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 of 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 (CFD) tools to study insert performance. This paper presents the outcomes of the numerical studies by the authors on an industrial stationary heat transfer enhancement insert and illustrates how a heat transfer enhancement insert can improve the heat transfer in a buoyancy driven compressible flow. The numerical predictions were validated using experimental data. Using different measuring tools, the global performance of the insert and the impact of the geometrical parameters are studied in order to choose the most effective design for a given application.
Transient Human Thermal Comfort Response in Convective and Radiative Environments
Authors: Al-Othmani Mohamad , Ghaddar Nesreen , Ghali Kamel
Published: July 2009

In this work, human transient thermal responses and comfort are studied in non-uniform radiant heating and convective heating environments. The focus was on a change from walking activity of human in outdoor cold environment at high clothing insulation to warm indoor environment at sedentary activity level associated with lower clothing insulation. A transient multi-segmented bioheat model sensitive to radiant asymmetry is used to compare how fast the human body approaches steady state thermal conditions in both radiative and convective warm environments. A space thermal model is integrated with the bioheat model to predict the transient changes in skin and core temperature of a person subject to change in metabolic rate and clothing insulation when entering conditioned indoor space. It was found that overall thermal comfort and neutrality were reached in 6.2 minutes in the radiative environment compared to 9.24 minutes in convective environment. The local thermal comfort of various body segments differed in their response to the convective system where it took more than 19 minutes for extremities to reach local comfort unlike the radiative system where thermal comfort was attained within 7 minutes.
Transient Damped Wave Conduction and Relaxation in Human Skin and Thermal Wear During Winter
Authors: Sharma Kal Renganathan
Published: July 2009

Damped wave conduction and relaxation in the human skin layer and thermal fabric layer are modeled with a temperature dependent heat source in the human tissue layer. Steady state temperature profiles are derived from the Fourier heat conduction equation. The general solution for the temperature is assumed to be a sum of the transient temperature and steady state temperature. This makes the boundary conditions in space for the skin and fabric layers homogeneous for the transient temperarature. The hyperbolic PDE is solved for by the method of separation of variables. The use of final condition in time in addition to the initial temperature condition leads to bounded infinite Fourier series solutions. These solutions are bounded and does not violate second law of thermodynamics. The model can be used to interpret experimental observations of maximum heat flux that is a parameter of the warm/cool feeling of human skin in winter. For large relaxation times of human skin tissue, τrs>(1+U*)2(b−a)216π2αs, the transient temperature can be expected to undergo oscillations. These oscillations will be supercritical and grow with time for strong heat sources, U* > 1 and may be subcritical damped oscillatory for weak heat sources, U* < 1. For large thermal relaxation times of thermal fabric material, τrf>a24π2αs, the transient temperature in the thermal fabric layer may be expected to be subcritical damped oscillatory.
Scaling of Passive Condenser System Separate Effect Facility
Authors: Revankar Shripad T. , Oh Seungmin , Zhou Wenzhong , Henderson Gavin
Published: July 2009

The Passive Containment Cooling System (PCCS) of the Simplified Boiling Water Reactor (SBWR) is a passive condenser system designed to remove energy from the containment for long term cooling period after a postulated reactor accident. Depending on pressure condition and noncondensable (NC) gas fraction in drywell (DW) and suppression pool (SP), three different modes are possible in the PCCS operation namely the forced flow, cyclic venting and complete condensation modes. The prototype SBWR has total of six condenser units with each units consist of hundreds of condenser tubes. Simulation of such prototype system is very expensive and complex Hence a scaling analysis is used in designing an experimental model for the prototype PCCS condenser system. The motive for scaling is to achieve a homologous relationship between an experiment and the prototype which it represents. A scaling method for separate effect test facility is first presented. The design of the scaled test facility for PCCS condenser is then given. Data from the test facility are presented and scaling approach to relate the scaled test facility data to prototype is discussed.
Chemical Kinetics Parameter Estimation for Ammonia Borane Hydrolysis
Authors: Basu Sumit , Zheng Yuan , Gore Jay P.
Published: July 2009

Onboard hydrogen storage is an enabling factor in the development of fuel cell powered passenger cars. Ammonia borane (AB) hydrolysis is one of the potential technologies for onboard hydrogen storage. In this study, kinetics of catalyzed ammonia borane hydrolysis using ruthenium-supported-on-carbon has been measured. For reacting flows, chemical kinetics determines the rates of heat generation and species production or consumption in the overall energy and mass balances respectively. Kinetic measurements under isothermal conditions provide critical data for the design of hydrolysis reactors. It is, however, not always possible to eliminate the effects of internal diffusion in a heterogeneous chemical reaction. In such cases, the reaction efficiency (η), which depends on the effective liquid phase diffusivity (Deff) in the catalyst medium, should be determined. Determination of intrinsic kinetic parameters using apparent kinetics data is, thus, a challenge. In this study, the change in AB concentration (CAB) with reaction time (t) has been directly measured. It was observed that the AB hydrolysis reaction had orders between zero and one in a temperature range of 26°C to 55°C. A unified Langmuir-Hinshelwood (LH) model has been adopted to describe the reaction kinetics. The intrinsic kinetic parameters (A, Ea, ΔHads, K0) as well as Deff need to be estimated by inverse analysis of the measured CAB vs t data. Conventionally, kinetic parameters are determined using linear fitting. Sometimes, however, it is impossible to converge to a unique value by using the linear fitting approach as there are several values providing regression coefficients greater than 0.99. In this study, the multiple-variable inverse problem has been solved using a nonlinear fitting algorithm based on Powell’s conjugate-gradient error minimization. This algorithm minimizes errors without using derivatives. As a result, the uncertainties in the kinetic parameter estimation have been significantly reduced by the new approach.
An Unstructured Reacting Flow Solver With Coupled Implicit Solution of the Species Conservation Equations
Authors: Kumar Ankan , Mazumder Sandip
Published: July 2009

Many reacting flow applications mandate coupled solution of the species conservation equations. A low-memory coupled solver was developed to solve the species transport equations on an unstructured mesh with implicit spatial as well as species-to-species coupling. First, the computational domain was decomposed into sub-domains comprised of geometrically contiguous cells—a process termed internal domain decomposition (IDD). This was done using the binary spatial partitioning (BSP) algorithm. Following this step, for each sub-domain, the discretized equations were developed using the finite-volume method, written in block implicit form, and solved using an iterative solver based on Krylov sub-space iterations, i.e., the Generalized Minimum Residual (GMRES) solver. Overall (outer) iterations were then performed to treat explicitness at sub-domain interfaces and non-linearities in the governing equations. The solver is demonstrated for a laminar ethane-air flame calculation with five species and a single reaction step, and for a catalytic methane-air combustion case with 19 species and 22 reaction steps. It was found that the best performance is manifested for sub-domain size of about 1000 cells, the exact number depending on the problem at hand. The overall gain in computational efficiency was found to be a factor of 2–5 over the block Gauss-Seidel procedure.
Parametric Control of a Two-Phase Thermal Management System for Space Applications
Authors: Brooks M. J. , Nortje L. C. , Lear W. E. , Sherif S. A.
Published: July 2009

As spacecraft increase in complexity, greater power is required to drive their onboard systems. The resulting generation of waste heat demands efficient thermal control, especially for electrical components emitting heat at high flux densities. Weislogel proposed a passive two-phase heat transport system for space application, driven by constant volume boilers, called the pulse thermal loop (PTL). This paper describes four methods of operating a PTL using real-time pressure data as the control parameter. Preliminary results are presented from an experimental loop using R-134a as the working fluid. Control is exercised through algorithm-based schemes implemented in LabVIEW. Results suggest that stable operation of the loop is best achieved by actuating flow control valves in response to a preset pressure difference between the boilers. Control schemes based on absolute pressure, set pulse frequency, and a combination of absolute and differential pressures are also described. Performance data are presented, and some of the challenges faced during PTL operation are discussed, including start-up and asymmetrical pressurization of the boilers.
Flow Boiling Patterns of Liquid Nitrogen in Micro-Tubes
Authors: Zhang P. , Fu X. , Wang R. Z.
Published: July 2009

Application of liquid nitrogen to cooling is widely used in such fields as cooling of the high temperature superconducting devices, cryosurgery and so on. In order to have a full understanding of the flow and heat transfer characteristics of liquid nitrogen in micro-tube, high-speed digital photography was employed to acquire the typical flow boiling patterns of liquid nitrogen in micro-tubes in the experiments. The main flow patterns were bubbly flow, slug flow, churn flow and annular flow. And the confined bubbly flow and mist flow were also observed. These flow patterns were characterized on the flow regime maps. And the surface tension force and the size of the tube diameter were found to be the major factors affecting the flow pattern transitions.
The Effect of Magnetic Field on Buoyancy-Driven Convection in a Differentially Heated Square Cavity With Two Insulated Baffles Attached to Its Isothermal Walls
Authors: Ghassemi M. , Pirmohammadi M. , Sheikhzadeh G. A.
Published: July 2009

Steady, laminar, natural-convection flow in the presence of a magnetic field in a differentially heated square cavity which two insulated horizontal baffles attached to its vertical walls is considered. The vertical walls are at different temperatures while the horizontal walls are adiabatic. In our formulation of governing equations, mass, momentum, energy and induction equations are applied to the cavity. To solve the governing differential equations a finite volume code based on Patankar’s SIMPLER method is utilized. Numerical predictions are obtained for various Rayleigh number (Ra), Hartmann number (Ha) and baffles position at the Prandtl number Pr = 0.733. At low Rayleigh number regime with weak magnetic field, a circulating flow is formed in the cavity. When the magnetic field is relatively strengthened, the thermal field resembles that of a conductive distribution, and the fluid in much of the interior is nearly stagnant. Further, when the magnetic field is weak and the Rayleigh number is high, the convection is dominant and vertical temperature stratification is predominant in the core region. However, for sufficiently large Ha, the convection is suppressed and the temperature stratification in the core region diminishes. The numerical results show that the effect of the magnetic field is to decrease the rate of convective heat transfer. The average Nusselt number decreases as Hartmann number increases.
Reduced Order Modeling of Light Scattering by a Cloud of Particles
Authors: Hall James B. , Moore Travis J. , Jones Matthew R.
Published: July 2009

It is often desirable to unobtrusively determine physical characteristics of a cloud of particles. Measurements of the radiation scattered by the cloud contain information regarding properties such as refractive index of the particles, the volume fraction of the particles and the number density of the particles. Inference of these properties from light scattering measurements requires the solution of an ‘inverse problem’. Methods for solving this type of inverse problem often require the repeated solution of the associated forward problem. Therefore, techniques capable of reducing the computational requirements of the forward problem are of interest.
Effect of Upstream Step on Flat Plate Film Cooling Effectiveness Using PSP
Authors: Rallabandi Akhilesh P. , Grizzle Joshua , Han Je-Chin
Published: July 2009

The effect of a step positioned upstream of a row of film cooling holes on the film cooling effectiveness is studied systematically using the steady state Pressure Sensitive Paint (PSP) technique. The upstream step effect is studied on four separate hole geometries: simple angled (axial angle 30°) and compound angled (axial angle 30°, compound angle 45°) cylindrical and fan-shaped film cooling holes. Each plate considered has 7 holes, each hole 4mm in diameter. The plates with cylindrical holes have a spacing of 3 diameters (12mm) between the centers of two consecutive holes, while the fan-shaped holes have a spacing of 3.75d (15mm). Three different step heights (12.5%d, 25%d and 37.5%d) are studied. Also studied is the effect of the width of the step; the distance of the step upstream of the hole and the positioning of the step downstream of the film-cooling hole. Four separate blowing ratios are reported for all tests: M = 0.3, M = 0.6, M = 1.0 and M = 1.5. All studies have been conducted with a mainstream of 25m/s velocity at an ambient temperature of 22C. Results indicate an increase in film-cooling effectiveness in the region near the hole due to the upstream step for all the plates considered. This increase due to the step is found to be most significant in the case of compound angled cylindrical holes and least significant in the case of simple angled fan-shaped holes.
Edge Detection Based on Wavelet Analysis With Gaussian Filter and Application in Multiphase Flow
Authors: Chen Bin , Guo Fude , Yang Yahui , Ning Tao
Published: July 2009

In this paper an edge detection algorithm based on wavelet transform with Gaussian filter was proposed. In this algorithm original images are firstly converted into gray images and then each pixel was analyzed using wavelet transform to find the local maximum of the gray gradient of each pixel along the phase angle direction and compared with a given threshold value, through which real edge can be kept and fake ones will be eliminated. In the computation of local maximum, the gray gradients computed in eight directions, which can improve precision of edge detection. After the investigation of influence of filter length, scale and threshold value on the edge detection and validation by comparison with N.L. Fena´ndez-Garci´a’s Minimean and Minimax methods for 100 real color images, we applied this algorithm to bubbly gas-liquid two-phase flow. Compared with Canny operator, present result is more close to real bubble shape and can distinguish very close bubbles, which indicates the algorithm is effective and can be used to study the interaction between different phases and flow field in multiphase flow.
Numerical Computation of Compressible Laminar Flow With Heat Transfer in the Entrance Region of a Pipe
Authors: Ziaei Rad M. , Nouri-Broujerdi A. , Seume J.
Published: July 2009

The authors’ research work on pressure drop along gas transmission pipelines raised questions regarding the development length of the corresponding compressible flow and the effect of heat transfer in the entrance region on the pressure drop along the whole length of the pipe. In this paper, laminar, viscous, compressible flow in the entrance region of a pipe is investigated numerically in two dimensions. The numerical procedure is a finite-volume based finite-element method applied on unstructured grids. This combination together with a new method applied for boundary conditions allows accurate computation of the variables in the entrance region. The method is applied to some incompressible cases in order to verify the results. The results are confirmed by previous numerical and experimental research on the developing length in incompressible flow.
An Investigation of Thermal and Velocity Fields for a Confined Jet Over the RE Range of 1,000-24,000
Authors: Jeffers N. , Punch J. , Walsh E.
Published: July 2009

Contemporary electronic systems currently generate high heat fluxes at component level. Impingement cooling is an effective way to generate high heat transfer coefficients in order to meet thermal constraints. This paper investigates the heat transfer and hydrodynamic characteristics of a confined impinging liquid jet with a nozzle-to-plate spacing (H/D) ratio of 0.5. A custom measurement facility was created to infer local heat transfer rates from infra-red images of a jet impinging on a 12.5μm thick stainless steel foil configured to generate uniform heat flux. Particle-Image Velocimetry (PIV) was performed in order to obtain quantitative velocity data within the jet. A series of experiments were run for Reynolds numbers (Re) in the range of 1,000–24,000 for a jet of 8 mm diameter (D). For Re > 4,000, the local heat transfer rate — in terms of Nusselt number (Nu) as a function of dimensionless radius (r/D) — had a plateau section between 0 < r/D < 0.6 followed by a peak at r/D ∼ 1.35. For higher Re the Nu peak exceeds that of the plateau section. For Re < 4,000, a plateau section exists between 0 < r/D < 0.4 followed by a shoulder located between 1 < r/D < 1.4. The PIV data for Re > 4,000 showed a strong vortex in the area of the secondary peak in Nu which was not present in the lower Re range. This phenomenon — the local peaks of heat transfer rate — has been previously reported in the literature with a degree of uncertainty as to the related fluid mechanics. This paper contributes to an understanding of the fluidic phenomenon responsible for the distribution of heat transfer rate in confined jets.
Modeling of Two-Phase Flow Structure Evolution in Subcooled Nucleate Convective Boiling With Coupling of Bubble-Tracking and Two-Fluid Models
Authors: Kljenak Ivo , Koncˇar Bosˇtjan , Sˇtrubelj Luka , Mavko Borut
Published: July 2009

A model of subcooled nucleate boiling flow in a vertical channel at low-pressure conditions is proposed. The model consists of a three-dimensional bubble-tracking model and a two-dimensional two-fluid model which are coupled off-line. By taking into account dynamic phenomena (liquid flow, bubble motion and interaction) and thermal phenomena (liquid heating, bubble nucleation and condensation), the model is able to simulate the gradual evolution of void fraction profiles along the boiling channel. The model is assessed by simulating experiments that were performed at Purdue University (USA) at atmospheric pressure in a vertical annulus with a central heating rod.
An Investigation of the Physical Mechanism of Heat Transfer Augmentation in Stagnating Flows Subject to Freestream Turbulence
Authors: Gifford Andrew R. , Diller Thomas E. , Vlachos Pavlos P.
Published: July 2009

Experiments have been performed in a water tunnel facility to examine the physical mechanism of heat transfer augmentation by freestream turbulence in classical Hiemenz flow. A unique experimental approach to studying the problem is developed and demonstrated herein. Time-Resolved Digital Particle Image Velocimetry (TRDPIV) and a new variety of thin film heat flux sensor called the Heat Flux Array (HFA) are used simultaneously to measure the spatio-temporal influence of coherent structures on the heat transfer coefficient as they approach and interact with the stagnation region. Velocity measurements of grid generated freestream turbulence are first performed to quantify the turbulence intensity, integral length scale, and isotropy of the flow. Laminar flow and heat transfer at low levels of freestream turbulence (Tux ≅ 0.5–1.0%) are then examined to provide baseline flow characteristics and heat transfer coefficient. Similar experiments using the turbulence grid are then performed to examine the effects of turbulence with mean turbulence intensity, Tux ≅ 5.5%, and integral length scale, Λx ∼ 3.25 cm. At a mean Reynolds number of ReD = 21,000 an average increase in the mean heat transfer coefficient of 43% above the laminar level was observed. To better understand the mechanism of this augmentation, flow structures in the stagnation region are identified using a coherent structure identification scheme and tracked in time using a customized tracking algorithm. Tracking these structures reveals a complex flow field in the vicinity of the stagnation region. Filaments of vorticity from the freestream are amplified near the plate surface leading to the formation of counterrotating vortex pairs and single sweeping vortex structures. By comparing the transient heat flux measurements with the tracked vortex structures it is clear that heat transfer augmentation is due primarily to amplification of stream-wise vorticity and subsequent vortex formation near the surface. The vortex strength, length scale, and distance from the stagnation plate are key parameters affecting augmentation. Finally, a mechanistic model is examined which captures the physical interaction near the wall. Model results agree well with measured heat transfer augmentation.