bluuebunny/crossref_metadata_2025 huggingface dataset:
Pool Boiling Mechanism of HFE-7100
Authors: Moghaddam Saeed , Kiger Kenneth T.
Published: March 2009

To study the pool boiling mechanism of HFE-7100, a micro array of forty-four Resistance Temperature Detectors (RTD’s) covering a 1mm in diameter circular area was microfabricated around a single cylindrical cavity on a thin silicon membrane. Constant heat flux was applied to the surface using a thin film heater microfabricated on the backside of the membrane. Images of the bubbles and the temperature of the heated wall underneath and around the bubble were recorded during the pool boiling process. Using the images of the bubbles, their volume, velocity, and frequency of departure was calculated. The acquired experimental data provided the fundamental parameters required for evaluating several boiling models whose development was based on the bubble diameter, frequency of departure, and velocity. For the conditions of this experiment, it seems that the current data can be best explained by transient heat conduction to the liquid adjacent to the heated wall and subsequent pumping of the superheated liquid by the bubbles. However, more experimental data in different conditions are required before solid conclusions can be reached. Details of the experimental results, models, and comparison between the two are presented in this paper.
A Study of an Oscillating Corner Meniscus With Phase Change Using Image Analyzing Interferometry
Authors: Panchamgam Sashidhar S. , Gokhale Shripad J. , Plawsky Joel L. , DasGupta Sunando , Wayner Peter C.
Published: March 2009

The thickness and curvature profiles in the contact line region of a moving evaporating thin liquid film of pentane on a quartz substrate were measured for the thickness region, δ < 2.5 microns. The critical region, δ < 0.1 microns, was emphasized. The profiles were obtained using image analyzing interferometry and an improved data analysis procedure. The precursor adsorbed film, the thickness, the curvature, and interfacial slope (variation of the local “apparent contact angle”) profiles were consistent with previous models based on interfacial concepts. Isothermal equilibrium conditions were used to evaluate the Hamaker constant in-situ and to verify the accuracy of the procedures. The profiles give fundamental insights into the phenomena of phase change, pressure gradient, fluid flow, spreading, and the physics of interfacial phenomena in the contact line region. The experimental results demonstrate explicitly for the first time, with microscopic detail, that the disjoining pressure controls fluid flow within an evaporating completely wetting thin curved film and the stability of the thin film. The change in the thickness of the adsorbed film with time is demonstrated for the first time.
Detached Eddy Simulation of Turbulent Flow and Heat Transfer in a Ribbed Duct
Authors: Viswanathan Aroon K. , Tafti Danesh K.
Published: March 2009

Numerical predictions of a hydrodynamic and thermally developed turbulent flow are presented for a stationary duct with square ribs aligned normal to the main flow direction. The rib height to channel hydraulic diameter (e/Dh) is 0.1, the rib pitch to rib height (P/e) is 10 and the calculations have been carried out for a bulk Reynolds number of 20,000. Detached Eddy Simulation (DES) has been used to compute the flowfield and the heat transfer. DES calculations are carried out on a 963 grid, a 643 grid and a 483 grid to study the effect of grid resolution. Based on the agreement with earlier LES computations and experimental data the 643 grid is observed to be suitable for the DES computation. DES and RANS calculations carried out on the 643 grid are compared to LES calculations on 963/1283 grids and experimental measurements. The flow and heat transfer characteristics for the DES cases compare well with the LES results and the experiments. The average friction and the augmentation ratios are consistent with experimental results, predicting values within 15% of the measured quantities, at a cost lower than the LES calculations. RANS fails to capture some key features of the flow.
Numerical Simulations of Dynamics of a Tunnel Fire
Authors: Pathak Kedar , Aung Kendrick
Published: March 2009

Study of fire in a tunnel is very important for fire safety. Increasing concerns over terrorism put a lot of focus on the fires in tunnels as they are used extensively in mass transit systems all over the world. A lot of experiments have been carried out to study the fire hazard, smoke movement, and the effects of ventilation on fire behavior. In this paper, dynamics of a ventilated tunnel fire have been simulated using Computational Fluid Dynamics (CFD) Software, CFX 5.6, from Ansys Inc. Simulations considers different models of turbulence and radiation heat transfer. Combustion of methane is modeled using the chemical reaction schemes available in the CFX software. Two turbulent models, k–ε and Shear Stress Transport, are considered. Radiant heat exchange between the species is modeled using P1 model available in CFX 5.6. The results of the simulation have highlighted the effects of ventilation on the fire and movement of harmful gases such as carbon monoxide and nitrogen oxide. Comparison of simulated temperature fields and flame shape with the experimental data has shown good agreement.
The Possibility of Using the Ground as a Seasonal Heat Storage: The Numerical Study
Authors: Olszewski Pawel
Published: March 2009

Humankind can effectively utilize only part of the solar energy reaching a surface of the Earth. It is due to the low density of the solar radiation and its unfavorable distribution. The majority of solar energy falls to the low latitude countries, where space-heating requirements are marginal. In these countries the solar heat is used for preparing water for washing or cleaning purposes, and this process works in one, or — maximum — a few daily cycles. In countries located at higher latitudes, where space heating is necessary in cold months, the current solar energy is insufficient to meet the space heating demand. The heat storage in deep layer of the ground is the one of possible way for solution of this problem. During the heating season, energy storage is discharged supplying the heat pomp cooperating with domestic heating system and during the summer months the storage can be charged by fluid heated in solar collectors. The main aim of presented research was analysis of using the ground layer as a heat storage system in the countries located in higher latitudes. The first variable taken into consideration was the output temperature of water leaving the solar collectors. The temperature distribution in the ground depends on the inlet water temperature, primary heated in the solar collectors, and forced into vertical boreholes. The temperature field in the ground was calculated using the duFort-Frankel finite-difference numerical method. A numerical code for 3D time dependent storage simulation has been created. The next step of analysis was calculation of waters’ temperature at the borehole output during cold months when the ground storage is discharged. This water works as a low-temperature reservoir of the heat pomp supplying the dwelling heating system. The solution of the problem is focused on an optimization of all parameters for the most efficient utilization of energy stored in the ground. The numerical genetic algorithms are scheduled to use to achieve this target.
Mixing in Large Scale Tanks: Part III — Predicting Slurry Pump Performance
Authors: Leishear Robert A. , Dimenna Richard A. , Stefanko David B. , Lee Si Y.
Published: March 2009

This third in a series of four papers (Parts I–IV) presents the equations used for the initial evaluation of a pump’s ability to suspend solids and extends those equations to establish the minimum local velocity required to suspend those solids. This minimum velocity was used in a finite difference model in Part I to predict the ability of a pump to suspend, or slurry, solids that had settled on the bottom of a nuclear waste tank. To slurry waste, the Advanced Design Mixer Pump (ADMP) discharges a fluid jet that impinges on, shears, and then suspends the waste. Prior to the pump’s installation in a waste tank, the local velocity at a point in the flow required to suspend solids was found from available equations, material properties, and empirical data for similar pumps. Also, the computational fluids dynamics (CFD) model was validated in Part II by comparing it to flow rates measured in a full scale test facility where the ADMP was operated. The CFD fluid model could then be used to predict flow rates throughout the actual waste tank where the pump was to be installed, and the ability of the pump to adequately slurry the waste could be shown. All that needed to be done was to compare the local velocity of the fluid required to shear the waste into suspension to the velocities modeled throughout the waste tank. In short, this paper validates the theoretical and experimental basis for the derivation of a minimum velocity required for the flow stream to shear the waste into suspension. The final installment to this series of papers (Part IV) validates the application of the CFD model, by concluding that a nuclear waste tank is effectively cleaned to the wall throughout most of tank, using the ADMP.
Experimental Investigation of the Flow Field of a Ceiling Fan
Authors: Jain Ankur , Upadhyay Rochan Raj , Chandra Samarth , Saini Manish , Kale Sunil
Published: March 2009

A ceiling fan is the predominating comfort provider in tropical regions worldwide. It consists of an assembly of an electric motor with 3–4 blades suspended from the ceiling of a room. Despite its simplicity and widespread use, the flow induced by a ceiling fan in a closed room has not been investigated, and sub-optimal designs are in wide use. There is vast potential for energy conservation and improved comfort by developing optimized fan designs. This work develops a fundamental understanding of the flow characteristics of a ceiling operating inside a closed room. Using smoke from thick incense sticks, the flow field created by the ceiling fan is visualized. In most regions, the flow is periodic and three-dimensional. Vortices are seen to be attached to the blade tip and hub, which reduces downward flow and increases energy consumption. Only the middle 75% of blade actually pushes the air downwards, and the comfort region is limited to a cylinder directly under the blades; velocities in this region were measured with a vane anemometer. Winglets and spikes attached to the blade tip disrupted the tip vortex, and increased downflow by about 13% without any increase in power consumption.
Artificial Neural Network-Based Flow Regime Classification Techniques for Gas-Liquid-Fiber Three-Phase Flows
Authors: Xie T. , Ghiaasiaan S. M.
Published: March 2009

The feasibility of a transportable artificial neural network (ANN)–based technique for the classification of flow regimes in three phase gas/liquid/pulp fiber systems, using pressure signals as input, was demonstrated in this study. Both supervised and unsupervised neural network models were applied for implementing regime classification. Data obtained in a vertical column (1.8m high and 5.08cm in diameter) were used, and a supervised ANN was designed and successfully tested that used some characteristics of the power density spectrum of the recorded signals of a pressure sensor as input. The developed ANN showed encouraging transportability. An ANN-based method was also developed for adjusting the processed signals of one sensor before feeding them as input to an ANN that had been trained based on data from another similar sensor. The method further improved transportability. The objectivity of the experimentally-identified flow regimes and their transition conditions was verified by the application of a Kohonen self-organizing neural network.
Mixing in Large Scale Tanks: Part I — Flow Modeling of Turbulent Mixing Jets
Authors: Lee Si Young , Dimenna Robert A. , Leishear Richard A. , Stefanko David B.
Published: March 2009

Flow evolution models were developed to evaluate the performance of the new advanced design mixer pump (ADMP) for sludge mixing and removal operations in one of the large-scale Savannah River Site (SRS) waste tanks, Tank 18. This paper is the first in a series of four that describe the computational model and its validation, the experiment facility and the flow measurements used to provide the validation data, the extension of the computational results to real tank conditions through the use of existing sludge suspension data, and finally, the sludge removal results from actual Tank 18 operations using the new ADMP. A computational fluid dynamics (CFD) approach was used to simulate the sludge removal operations. The models employed a three-dimensional representation of the tank with a two-equation turbulence model, since this approach was verified by both test and literature data. The discharge of the ADMP was modeled as oppositely directed hydraulic jets submerged at the center of the 85-ft diameter tank, with pump suction taken from below. The calculations were based on prototypic tank geometry and nominal operating conditions. In the analysis, the magnitude of the local velocity was used as a measure of slurrying and suspension capability. The computational results showed that normal operations in Tank 18 with the ADMP mixer and a 70-in liquid level would provide adequate sludge removal in most regions of the tank. The exception was the region within about 1.2 ft of the tank wall, based on an historical minimum velocity required to suspend sludge. Sensitivity results showed that a higher tank liquid level and a lower elevation of pump nozzle would result in better performance in suspending and removing the sludge. These results were consistent with experimental observations.
Mixed Convection in a Large Area Atmospheric Pressure CVD Reactor
Authors: Vanka S. P. , Luo Gang , Glumac Nick G.
Published: March 2009

This paper investigates a new CVD reactor geometry to deposit uniform films on large area substrates at atmospheric pressure. The reactor employs a stagnation flow geometry with an impinging central jet. The substrate is also rotated in order to get film uniformity. Calculations have been performed for a wide range of parameters to investigate the effects of inlet flow rates, substrate rotation, and height of the reactor chamber. It is seen that for some combinations of the parameters the flow above the wafer is unsteady. The unsteadiness of the flow is subsequent to the sharp turning at the conclusion of the inlet nozzle. The sudden expansion makes the shear layer unsteady much in the same way as the flow in a sudden expansion. Thus, the effect of rounded corners on damping such instabilities of the shear layers is explored. By employing the rounded corners, we have been able to reduce the RMS non-uniformity to about 1% at atmospheric pressure on 30 cm wafer. The impinging jet geometry can therefore be used for the deposition of many thin solid films without the penalty of a vacuum system and associated equipment costs.
The Effect of Conjugate Heat Transfer on Film Cooling Effectiveness
Authors: Silieti Mahmood , Divo Eduardo , Kassab Alain J.
Published: March 2009

We investigate the numerical prediction of film cooling effectiveness of a two-dimensional gas turbine endwall for the cases of conjugate and adiabatic heat transfer models. Further, the consequence of various turbulence models employed in the computation are investigated by considering various turbulence models: ‘RNG’ k-ε model, Realizable k-ε model, Standard k-ω model, ‘SST’ k-ω model, and ‘RSM’ model. The computed flow field and surface temperature profiles along with the film effectiveness for one and two cooling slots at different injection angles and blowing ratio of one are presented. The results show the strong effect of the conjugate heat transfer on the film effectiveness compared to the adiabatic and analytically derived formulae and show that turbulence model used significantly affects the film effectiveness prediction when separation occurs in the film hole and some level of jet lift-off is present.
Calculation of Flow in Mixing Vessels With Various Turbulence Models
Authors: Basara Branislav , Alajbegovic Ales , Beader Decan
Published: March 2009

The paper presents calculations of flow in a mixing vessel stirred by a six-blade Rushton impeller. Mathematical model used in computations is based on the ensemble averaged conservation equations. An efficient finite-volume method based on unstructured grids with rotating sliding parts composed of arbitrary polyhedral elements is used together with various turbulence models. Besides the standard k-ε model which served as a reference, k-ε-v2 model (Durbin, 1995) and the recently proposed hybrid EVM/RSM turbulence model (Basara & Jakirlic, 2003) were used in the calculations. The main aim of the paper is to investigate if more advanced turbulence models are needed for this type of CFD applications. The results are compared with the available experimental data.
Fluid Flow and Heat Transfer in a Lid-Driven Cavity Due to an Oscillatory Thin Fin: Periodic State
Authors: Shi Xundan , Khodadadi J. M.
Published: March 2009

A computational study of periodic laminar flow and heat transfer within a lid-driven square cavity due to an oscillating thin fin is presented. The lid moves from left to right and a thin fin positioned perpendicular to the right stationary wall oscillates in the horizontal direction. The length of the fin varies sinusoidally with its mean length and amplitude equal to 10 and 5 percent of the side of the cavity, respectively. Two Reynolds numbers of 100 and 1000 with a Pr = 1 fluid were considered. For a given convection time scale (tconv), fin’s oscillation periods (τ) were selected in order to cover both slow (τ/tconv>1) and fast (τ/tconv<1) oscillation regimes, covering a Strouhal number range of 0.005 to 0.5. The number of the cycles needed to reach the periodic state for the flow (Nf) and thermal (Nt) fields increases as the fin oscillates faster with Nf < Nt. The periodic flow field for the case with Re = 1000 and TR = 10 is distinguished by the creation, lateral motion and subsequent wall impingement of a CCW rotating vortex within the lower half of the cavity. Periodic flow and thermal fields of the other nine cases studied were not as varied. Phase diagrams of the stream function and temperature vs. fin’s length clearly exhibit the synchronous behavior of the system. Amplitude of fluctuations of the kinetic energy and temperature are very intense near the fin. As the fin oscillates slower, a greater portion of the cavity exhibits intense fluctuations. For slow to moderate oscillations, the maximum value of Kamp is observed to be greater for Re = 1000 in comparison to Re = 100. For fast oscillations, this behavior is reversed. The maximum values of the amplitude of fluctuations of temperature increase monotonically as the fin oscillates slower. The maximum values of θamp are greater for Re = 1000 compared to Re = 100. The amplitude of fluctuations of the mean Nusselt number on four walls increase as the fin oscillates slower.
Effects of Hole Geometry on Film Coverage and Cooling Capability
Authors: Liu J. T. , Peng X. F.
Published: March 2009

A numerical simulation was conducted to investigate the film effectiveness of film cooling using a single hole with two types of geometry: cylindrical hole with constant cross section and shaped hole with conically widened exit. The film cooling jet was injected through a 30° inclined hole to the surface and with lateral directions of 0°, 45° and 90°, for the blowing rates of 0.5, 1.0 and 2.0, respectively. The film effectiveness analyzing method was discussed based on the simulation. An effort is performed to form a more comprehensive evaluation technology with the definition of three parameters, film coverage, average cooling capability and uniformity of film. The results indicate that the film quality of compound angle injection depends on the equilibrium between the lateral and axial momentum components of coolant jet, and that the film protects the surface effectively at moderate blowing rate. The use of a shaped hole shows noticeable advantage.
Mesh-Generation Parameters Influence on Centrifugal Compressor Simulation for Design Optimization
Authors: Dufour Guillaume , Carbonneau Xavier , Arbez Pierre , Cazalbou Jean-Bernard , Chassaing Patrick
Published: March 2009

As advocated by Denton [1], three-dimensional CFD calculations are more and more used in the process of designing centrifugal compressors. Therefore, numerical accuracy is an issue if valuable design decisions are to be based on CFD results. This paper aims at correlating mesh-parameters and calculated flow quantities. The design-of-experiment method (DOE) is used to identify which mesh-parameters influence any given flow quantity. The emphasis is put on global performances (mass-flow rate, pressure-ratio and efficiency), and on local features (friction co-efficients and local velocity-profiles). The outcome of the study is the connection between mesh-parameters and specific flow-properties. This can be further used to derive numerical bench-marks, such as reported by Dufour et al. [2] for the tip-clearance flow.
An Adaptive Wavelet Method for the Incompressible Navier-Stokes Equations in Complex Domains
Authors: Wirasaet Damrongsak , Paolucci Samuel
Published: March 2009

An adaptive wavelet-based method provides an alternative means to refine grids according to local demands of the physical solution. One of the prominent challenges of such a method is the application to problems defined on complex domains. In the case of incompressible flow, the application to problems with complicated domains is made possible by the use of the Navier-Stokes/Brinkman equations. These equations take into account solid obstacles by adding a penalized velocity term in the momentum equation. In this study, an adaptive wavelet collocation method, based on interpolating wavelets, is first applied to a benchmark problem defined on a simple domain to demonstrate the accuracy and efficiency of the method. Then the penalty technique is used to simulate flows over obstacles. The numerical results are compared with those obtained by other computational approaches as well as with experiments.
A Fast Multi-Level Boundary Element Algorithm for Stokes Flows in Two-Dimensions
Authors: Dargush G. F. , Grigoriev M. M.
Published: March 2009

Recently, we have developed multi-level boundary element methods (MLBEM) for the solution of the Laplace and Helmholtz equations that involve asymptotically decaying non-oscillatory and oscillatory singular kernels, respectively. The accuracy and efficiency of the fast boundary element methods for steady-state heat diffusion and accoustics problems have been investigated for square domains. The current work extends the MLBEM methodology to the solution of Stokes equation in more complex two dimensional domains. The performance of the fast boundary element method for the Stokes flows is first investigated for a model problem in a unit square. Then, we study the performance of the MLBEM algorithm in a C-shaped domain.
Hot-Film Measurement of Natural Convection in Liquid Gallium With and Without Magnetic Fields
Authors: Xu B. , Li B. Q. , Stock D. E.
Published: March 2009

The velocity and temperature fields induced by natural convection in liquid gallium were measured. Measurements were taken with and without an external magnetic field applied to the liquid gallium. The velocity field was measured with a hot-film anemometer and the temperature field with a thermocouple. The hot film was calibrated over a narrow range of temperatures in a rotating turntable filled with liquid gallium. The external magnetic field damped both the velocity and temperature fields compared to similar conditions when no external magnetic field was present. The experimental results compared reasonably well with previous numerical predictions.
Numerical Analysis of the Turbulent Flow Field Applied to Thermal Spallation Drilling
Authors: Nieckele Angela O. , Figueira da Silva Luis Fernando , Pla´cido Joa˜o Carlos R.
Published: March 2009

Thermal spallation is a possible drilling technique which consists of using hot supersonic jets as heat source to perforate hard rocks at high rates. This work presents a numerical analysis of a typical spallation drilling configuration, by the finite volume method. The time-averaged conservation equations of mass, momentum and energy are solved to determine the turbulent compressible gas phase flow field. Turbulence is predicted by the classical high Reynolds number κ-ε model, as well as with a low Reynolds number κ-ε model. The influence of the jet Reynolds number is investigated. Special attention is given to the rock surface temperature, since its accurate determination is required to predict spallation rates under field-drilling conditions.
Three-Dimensional Transient Finite Element Analysis for Microstructure Formation and Residual Stresses in Laser-Aided DMD Process
Authors: Ghosh S. , Choi J.
Published: March 2009

Despite immense advances in Laser-Aided Direct Material Deposition process, many issues concerning the adverse effects of process parameters on the stability of variety of properties and the integrity of microstructure have been reported. Macroscopic aspects are important in predicting macroscopic defects or optimizing process conditions, while microstructural features such as phase appearance, morphology, grain size, spacing, or micro-defects are certainly no less important in determining the ultimate properties of the solidified product. Traditional solidification theories as applied to castings or related processes are inappropriate in describing solidification in high-energy beam processes involving significantly greater cooling rates. Due to the complexity and nonlinearity of this process, analytical solutions can rarely address the practical manufacturing process. This paper is an attempt towards a methodology of finite element analysis for the prediction of solidification microstructure and macroscopic as well as microscopic thermal residual stresses in this process. The computer simulation which is based on metallo-thermomechanical theory and finite element analysis for coupled temperature, solidification, phase transformation and stress/strain fields can prove to be a very useful tool in predicting the material behavior and optimizing the process parameters to obtain the best material properties. This model would reduce long and cumbersome experimental routes to predict the material behavior under similar loading conditions.
Maximum Power Extraction From a Hot Stream in the Presence of Phase Change Under Limiting Collecting Temperatures
Authors: Ordonez Juan C. , Chen Sheng
Published: March 2009

In this paper we consider the fundamental problem of maximizing the power extraction from a hot stream when the collecting stream experiences a phase change and there are limits imposed by the materials on the operating temperatures. It constitutes an extension of [4] where it was pointed out the existence of an optimal mass flow rate ratio of the hot stream to the collecting stream. In this work, we study the effects of the restrictions imposed by limiting temperatures on the spatial configuration, power extraction and the optimal matching of the two streams. An optimal hot-stream-to-collecting-stream mass flow rate ratio can be found when the collecting stream experiences a phase change while in contact with the hottest section of the hot stream. Associated to the optimal mass flow rate ratio there is also an optimal heat exchanger area allocation. The effects of several operating parameters on the optimal configuration are documented. This paper constitutes an illustration of how thermodynamic optimization leads to the discovery of system structure (constructal theory [1]).
Effect of Spray Angle in Spray Cooling Thermal Management of Electronics
Authors: Schwarzkopf J. , Cader T. , Okamoto K. , Li B. Q. , Ramaprian B.
Published: March 2009

The paper presents an experimental and numerical study of the effect of spray angle on spray cooling when applied in the thermal management of electronics. A thermal test chip provided the heated target, and was cooled by a single pressure swirl atomizer. A perfluorocarbon (PF5060) was employed as the coolant. The coolant was subcooled to a fixed level of 26° C, and was sprayed directly onto the heated target at a fixed flow rate of 22 ml/min. The spray angle was varied between 0 and 60 degrees, and the outlet of the atomizer was located at a fixed radius of 1.4 cm from the heated target. The model of Mudawar and Estes (1996) was also modified to account for the effect of spray angle, then used to assist in interpretation of the experimental data. In an effort to estimate the heat transfer characteristics, an inverse heat transfer algorithm is developed. A direct finite element model is applied with estimated heat flux distributions to simulate the thermal field in the test microchip for various cooling conditions. Experimental results are presented for a number of cases and compared with the model’s predictions. The experimental data and model both showed that cooling capability dropped off when spray angle exceeded 50 degrees.
Numerical Modeling of Flow in a Large Quench Tank
Authors: Kernazhitskiy Sergey , Recktenwald Gerald
Published: March 2009

A computational fluid dynamics (CFD) model is developed to simulate isothermal flow in a large quench tank used for heat treating of steel. As a surrogate for a full simulation of quenching, the isothermal model enables a computational economical comparison of many different design configurations of the quench tank. The model includes most of the geometric complexity of the tank including the skip, the heavy beams used to support it, deflector baffles used to control the flow, and the inlet ducting. Partial factorial screening studies are conducted to identify the most important design variables. The second phase of the project involves examining four new conceptual designs for the quench tank. The outcome of the second phase was the identification of a promising new design that could be realized by the addition of ducting to the existing design. The contributions of this research include a better understanding of the parameters affecting large scale batch quench processes and the development of new directions for quench tank design.
Simulation of Flow Around a Static and Oscillating in Pitch NACA 0015 Airfoil Using URANS and DES
Authors: Szydlowski J. , Costes M.
Published: March 2009

This paper presents numerical simulations of the flow around a NACA 0015 airfoil at static and dynamic stall. The treatment of these configurations is a very challenging task for CFD applications. The turbulent flow around the static and in pitch oscillation airfoil is computed using different approaches: 2D RANS, 3D RANS and DES methodologies and with finer and finer meshes in order to try to reach a space converged solution. The main conclusion of the paper is that the prediction of static and all the more dynamic stall is not mature with present modeling capabilities.
A Periodically Perturbed Backward-Facing Step Flow by Means of LES, DES and T-RANS: An Example of Flow Separation Control
Authors: Sˇaric´ Sanjin , Jakirlic´ Suad , Tropea Cameron
Published: March 2009

Turbulent flow over a backward-facing step perturbed periodically by an alternating blowing/suction through a thin slit situated at the step edge was studied computationally using the LES (Large Eddy Simulation), DES (Dettached Eddy Simulation) and T-RANS (Transient Reynolds-Averaged Navier-Stokes) techniques. The flow configuration considered (ReH = UcH/ν = 3700) has been investigated experimentally by Yoshioka et al. (2001). The periodical blowing/suction with zero mass flux is governed by a sinusoidal law: ve = 0.3Ucsin(2πfet), Uc being the centerline velocity in the inlet channel. Perturbation frequencies fe corresponding to the Strouhal numbers St = 0.08, 0.19 and 0.30 were investigated (St = feH/Uc). The experimental observation, that the perturbation frequency St = 0.19 represents the most effective case, that is the case with the minimum reattachment length, was confirmed by all computational methods applied. However, the closest agreement with experiment (the reattachment length reduction of 28.3% compared to the unperturbed case) was obtained with the LES (24.5%) and DES (35%) methods whereas the T-RANS computations show a weak sensitivity to the perturbation: 5.9% when using the Spalart-Allmaras model and 12.9% using the k–ω SST model.
Experimental Analysis of an Air-to-Air Heat Exchanger for Use in a Refrigeration Brayton Cycle
Authors: Kargar Alireza , Hosni Mohammad H. , Eckels Steve , Gielda Tomas
Published: March 2009

The refrigeration Brayton cycle, which has been used extensively in various industries, has an excellent potential for use in automotive air conditioning applications. However, the air-cycle system has a couple of drawbacks including fog generation and low cycle efficiency. In this research project, an air-to-air heat exchanger called a ‘mixer’ is designed and used at the outlet of a refrigeration Brayton cycle. The primary function of the mixer is to remove moisture from the secondary warm airflow into the system. Successful moisture removal from the secondary airflow results in achieving the second function of fog dissipation from the primary cold airflow. In order for the system to perform appropriately, the moisture removal rate must be kept at the highest possible rate. The experimental results from this research project reveal that to enhance moisture removal rate, one may either increase the primary cold airflow rate, decrease the secondary warm airflow rate, or the combination of the above airflow adjustments. Furthermore, based on experimental results, one may speculate that there is an optimum point in decreasing the secondary airflow rate. However, in increasing the primary airflow rate, one must be aware of the pressure drop through the cold side of the mixer as the higher pressure drop results in higher power consumption for the Brayton cycle. It is important to point out that appropriate levels of the primary and secondary airflows impacts the mixer effectiveness, and that for a constant cold airflow rate, decreasing the warm airflow rate below the cold airflow rate results in higher effectiveness.
The Effect of Martian Dust on Radiator Performance
Authors: Hollingsworth D. Keith , Witte Larry C. , Hinke Jaime , Hurlbert Kathryn
Published: March 2009

Experiments were performed in which the effective emittance of three types of radiator coatings was measured as Martian dust simulant was added to the radiator surface. The apparatus consisted of multiple radiator coupons on which Carbondale Red Clay dust was deposited. The coupons were powered by electric heaters, using a guard-heating configuration to achieve the accuracy required for acceptable emittance calculations. The apparatus was contained in a vacuum chamber that featured a liquid-nitrogen cooled shroud that simulated the Martian sky temperature. Radiator temperatures ranged from 250 to 350 K with sky temperatures from 185 to 248 K. Results show that as dust was added to the radiator surfaces, the effective emittance of the high-emittance coatings decreased from near 0.9 to a value of about 0.5. A low-emittance control surface, polished aluminum, demonstrated a rise in effective emittance for thin dust layers, and then a decline as the dust layer thickened. This behavior is attributed to the conductive resistance caused by the dust layer.
NUWC-Russia-UK Collaborative Research on Compliant Coatings
Authors: Bandyopadhyay Promode R. , Henoch Charles , Hrubes J. Dana
Published: March 2009

We report the results from a collaborative effort between NUWC, Russia and UK on the development of compliant coatings for undersea application of drag reduction and turbulence control. The coatings were manufactured in Novosibirsk in Russia. The coated cylindrical models were shipped to NUWC and to the University of Nottingham in the UK. Some reduction in drag and wall-pressure fluctuation spectra has been observed on a small cylinder model. Further confirmatory tests are recommended with a large diameter model.
Wall-Layer Scale Electromagnetic Turbulence Control in an Axisymmetric Body
Authors: Bandyopadhyay Promode R. , Castano John M. , Thivierge Daniel P.
Published: March 2009

The progress made with the control of turbulence in a boundary layer developing over a small axisymmetric body in saltwater at moderate Reynolds numbers is briefly described. A resonance-interference mechanism of control by means of a small periodic Lorenz force confined to the near-wall region, designed to overcome the issue of low efficiency of electromagnetic turbulence control in general, is attempted to alter surface normal turbulence near-wall. At a low momentum thickness Reynolds number of 2300, drag is reduced by 15–25% at a freestream speed of 5.12 m/s with an efficiency of 2–3.4%. Bi-polar pulsing succeeds in lowering surface-normal turbulence intensity near wall. It also makes wall pressure fluctuations less spiky. Positive uni-polar pulsing is found to weaken the sources of wall-pressure fluctuations residing in the logarithmic region of the boundary layer. Further confirmatory work is needed with robust electrodes and drag measurements on a large diameter axisymmetric body.
A Procedure to Establish Inflow Conditions for LES of Spatially Developing Turbulent Boundary Layers
Authors: Liu Kunlun , Pletcher Richard H.
Published: March 2009

Inlet conditions have a dramatic effect on numerical simulations of turbulent boundary layers due to the strong convective characteristic of flows with a displacement thickness Reynolds number as high as 2000. The fluid structures present in the inlet flow will be transported downstream, but some information may be modified or lost during this transport process. This numerical receptivity phenomenon may reduce the performance of numerical calculations and increase the difficulty in establishing the downstream turbulence. The interaction between the inlet conditions and the numerical scheme may generate an inlet buffer zone adjacent to the inlet. This buffer zone costs in terms of CPU resources. The purpose of this paper is to present an inlet condition generator to achieve very good downstream profiles and reduce the extent of the buffer zone. In this method, the recycle plane is dynamically selected according to the downstream instantaneous field. Different adjustment methods are applied to the inner, outer and free stream part based on their different similarity laws. With this inlet generation technique, the inlet buffer zone is very thin. The numerical results compare well with current experiments and DNS results.
Analysis of Cavitating and Non-Cavitating Transient Flow in Pipelines Connected by Junctions
Authors: Bartolini C. M. , Caresana F. , Pelagalli L.
Published: March 2009

The paper deals with the development of experimental benches designed with the aim of producing test cases of transients flow through different types of junctions both in non-cavitating and cavitating conditions. The non-cavitating test cases consist in a rapid depressurization of a reservoir to which a pipe network containing the junction is connected while in the cavitating test cases the same pipe network is discharged towards an open-air reservoir by means of an ad hoc very rapid opening valve apparatus. First of all the experimental data allowed for a deeper insight on the subject from a phenomenological point of view. Then the recorded pressure time histories have been used to validate the numerical results of a code which solves the flow by means of a 1D model suitable to describe unsteady friction and cavitating phenomena. In the code the discrete gas-cavity model (DGCM) introduced by Provoost and Wylie is integrated in the method of characteristics and the junctions are modeled using a standard constant pressure approach improved with the gas cavity model.
Tracking of Nanoparticles Using Evanescent Wave Microscopy
Authors: Banerjee Arindam , Kihm Kenneth D.
Published: March 2009

A novel three-dimensional tracking technique of nanoparticles in nanometer spatial resolution using a Ratiometric Total Internal Reflection Fluorescence Microscopy (R-TIRFM) is presented. Evanescent waves from the total internal reflection of an argon-ion laser (488 nm) are used to generate a thin sliced illumination field with its effective visualization range of 544-nm, equivalent to twice of the penetration depth of the evanescent wave field. Fluorescence-coated polystyrene spheres of 500-nm diameter (SG = 1.05) are used as tracers subjected to the Brownian diffusive motions. A ratiometric analysis of the fluorescence particle images together with a neural network particle-pair identification algorithm is used to track the tracer particle locations across multiple image frames in full three-dimensional ways. The techniques are used to examine the Brownian diffusive motion of nanoparticles as they approach the very near-wall region within a few hundred nanometers of a glass-water interface. The experimental results show that the measured Brownian diffusion coefficient is in good agreement with the theoretical hindered diffusion coefficient near a wall.
Nano-PTV Using Confocal Microscopy
Authors: Park Jae S. , Choi Chang K. , Kihm Kenneth D.
Published: March 2009

A conventional microscopy and confocal microscopy was used to enable nano-particle tracking using a three-dimensional (3-D) diffraction image pattern caused by an optical aberration. The Point Spread Function (PSF) of a particle image is known as a function of a defocus distance from a focal plane. Using the PSF of nano-particles, particle’s x and y positions as well as z-position can be obtained. A dry objective lens (40X, 0.75NA) is used to detect diffraction patterns of 500-nm fluorescent micro-spheres suspended in water solution inside a 100-μm square micro-tube. The purposes of this presented study are to investigate experimentally the 3-D PSF of fluorescent nano-particle in thick specimen and to develop the 3-D particle tracking technique using 3-D diffraction images of conventional microscopy and CLSM.
Experimental Study on Water Mist Suppression of Poly(Methyl Methacrylate) PMMA Fires
Authors: Yao B. , Qin J. , Chow W. K.
Published: March 2009

Suppression of Poly(methyl methacrylate) PMMA fires by water mist will be studied in this paper. A simple test is developed to study the critical water flow rate under different radiant heat flux. The test is found to be suitable for studying the extinguishment effects of fine water droplets involving oxygen displacement, gas phase and fuel surface cooling. Water mist is generated by a single pressure nozzle, with the water mist characteristics measured by the Laser Doppler Velocimetry or the Adaptive Phase Doppler Velocimetry System (LDV/APV system). The interaction between water mist and the PMMA flame will be studied in a confined space with ventilation control in a cone calorimeter. The heat release rate, oxygen, carbon dioxide and carbon monoxide concentrations, and other important parameters of the interaction under various conditions are measured. It is found that discharging adequate amount of water mist would suppress the diffusion flame in the confined space. Reignition might occur once water mist stopped discharging to the fuel surface. Higher heat release rate and more smoke and toxic gases were produced than from those in first ignition.
Double Laser Crystallization (DLC) of Pre-Patterned Amorphous Silicon Film and Amorphous Silicon Film With a Patterned Assisting SiO2 Layer
Authors: Xu Li , Chung Jaewon , Grigoropoulos Costas P.
Published: March 2009

In this study, pre-patterned amorphous silicon films are crystallized by the double laser crystallization (DLC) technique. Temperature distribution upon laser irradiation is modified by patterning the a-Si film, thus controlling the crystal growth. Patterns with a flattened concave feature is found to be favorable for large crystal growth with high localization, yielding grains with the size of 1.5 μm × 4 μm. As an alternate method (to pre-patterning the amorphous silicon film) for obtaining large crystal growth, double laser crystallization of amorphous silicon film with patterned SiO2 cap layer is proposed. The SiO2 layer assists the lateral growth of the crystals by acting as a thermal reservoir and slowing down the cooling rate, and additionally helps reduce the roughness of the polycrystalline surface to about 3nm (R.M.S.). With this alternate method, the grain width is increased from 0.5 μm to 1.5μm.
Stochastic Heat Transfer in Fins and Transient Cooling Using Polynomial Chaos and Wick Products
Authors: Emery A. F. , Bardot D.
Published: March 2009

Stochastic heat transfer problems are often solved using a perturbation approach which yields estimates of mean values and standard deviations for properties and boundary conditions that are random variables. Methods based on polynomial chaos and Wick products can be used when the randomness is a random field or white noise to describe specific realizations and to determine the statistics of the response. Polynomial chaos is best suited for problems in which the properties are strongly correlated, while the Wick product approach is most effective for variables containing white noise components. A transient lumped capacitance cooling problem and a one-dimensional fin are analyzed by both methods to demonstrate their usefulness.
Micro- and Nanoscale Phonon Heat Transport in Silicon
Authors: Ju Y.
Published: March 2009

Micro- and nanoscale energy transport in semiconductors is one of the critical research areas for emerging nano-electronics. Key features of phonon dispersion curves are re-examined, which motivates the use of phonon density of states obtained from ab initio calculations as a basis for constructing a semi-phenomenological thermal conductivity model. Thermal conductivity data on silicon nanowires are analyzed to identify dominant phonon modes. The consistency of the present thermal conductivity model is examined by comparing its prediction with the thermal conductivity data from bulk germanium samples with controlled amount of point defects. The thermal conductivity modeling study provides input parameters for a two-fluid phonon transport model for silicon and related semiconductors, which can play an important role in computer aided design of nanoelectronic devices and simulation of ultra-fast phenomena.
Vehicle Thermal Management Simulation Using a Rapid Omni-Tree Based Adaptive Cartesian Mesh Generation Methodology
Authors: Srinivasan Kumar , Wang Z. J. , Yuan Wei , Sun Richard
Published: March 2009

CFD simulation of vehicle under-hood and under-body poses several challenges. Specifically, the complexity of the geometry involved makes the use of traditional mesh generation approaches, based on the boundary-to-interior methodology, impractical and time consuming. The current work presents the use of an interior-to-boundary method wherein the need for creating a ‘water-tight’ surface mesh is not a pre-requisite for volume mesh generation. The application of the new method is demonstrated for an actual passenger vehicle under-hood model with nearly a hundred components. Coupled radiation/convection simulations are performed to obtain the complete airflow and thermal map of the engine compartment. Results are validated with test data. The new method results in significant gains in efficiency over traditional approaches allowing the simulation tool to be used effectively in the vehicle development process.