bluuebunny/crossref_metadata_2025 huggingface dataset:
Numerical Simulation of the Flow Field in a Statically and Dynamically Eccentric Annular Seal With Non-Circular Whirl Orbits
Authors: Vijaykumar Anand , Morrison Gerald
Published: March 2011

The flow field in an annular seal is simulated for synchronous circular whirl orbits with 60Hz whirl frequency and a clearance/radius ratio of 0.0154 using the Fluent CFD code. Fluent’s Moving Reference Frame model (MRF) is used to render the flow quasi-steady by making transformations to a rotating frame. The computed flow fields for velocity, pressure and shear stress measurements are compared with the experimental data of Winslow (1994), Thames (1992) and Cusano (2006). The CFD predictions are found to be in good agreement with the experimental results. The present CFD methodology can be extended to other whirl frequencies and clearances. The dynamic wall pressure distributions in an annular seal for non-circular whirl orbits was obtained using CFD. The simulations were performed using a time dependant solver utilizing Fluent’s Dynamic Mesh model and User Defined Functions (UDFs). The wall pressure distributions obtained from the simulations are compared with data of Cusano (2006). The CFD simulations over predicted the pressure field when compared to experimental results however the general trends in pressure contours are similar. The flow field for varying rotor eccentricities are also studied by performing coordinate transformations and rendering the flow quasi-steady at set eccentricities using Fluent’s MRF model. The computed velocity and pressure fields are compared with the time dependant solution obtained using Fluent’s Dynamic Mesh model and UDFs for the same eccentricity. Good agreement in the velocity fields is obtained however the pressure fields require further investigation.
The Microfluidic Trapping of Antibody-Secreting Cells
Authors: Romanuik Sean F. , Grist Samantha M. , Haq Moeed , Gray Bonnie L. , Gulzar Naveed , Scott Jamie K.
Published: March 2011

Therapeutic antibodies (Abs) are a rapidly growing and economically promising biotechnological research area. Therapeutic Ab production typically involves screening large numbers of Ab-secreting cells (ASCs) in order to identify those producing Abs targeting a specific antigen (Ag) with the highest affinity; a process often requiring weeks to complete. We are contributing to a multidisciplinary project focused upon the development of an immunobiosensing array ultimately intended to directly monitor the Ag-specific Ab production by thousands of ASCs on a single slide in real-time. Each ASC shall be microfluidically guided and trapped near a surface plasmon (SP) resonant nanohole array sensor so as to detect the binding of secreted Abs to Ag immobilized onto the sensor’s surface. This paper presents the initial progress of our contribution to this project: the development of polymeric microfluidic devices to guide and trap large ASC populations within arrays of single-cell traps. More specifically, this paper presents several different polymer-based microfluidic trapping devices, based upon perfusive flow-through cell traps and microwells which trap settling cells, which have been evaluated using COMSOL® simulations and tested using microsphere- and cell-based flow experiments. Our initial results are promising, and verify the functionality of our microfluidic cell trap designs.
Optimization of Diffuser/Nozzle Elements for Rectification Valveless Micropumps
Authors: Azarbadegan A. , Cortes-Quiroz C. A. , Moeendarbary E. , Eames Ian
Published: March 2011

There has been a growing interest in understanding the flow behaviour inside diffuser/nozzle elements in order to identify performance characteristics of these elements for micropump applications. Flat-walled diffuser/nozzle element is the most commonly used type for valveless micropump applications due to its ease of fabrication and compact design. In this paper, we study generic flat-walled diffuser/nozzle elements and apply optimization techniques to explore how the pumping efficiency can be improved by changing geometry to provide higher rectification efficiency and lower pressure drop in rectification valveless micropumps. The primary motivation for this study is to evaluate the performance of flat-walled diffuser/nozzle elements based on geometry variations under several Reynolds numbers (Re). In this study we employ a design methodology for diffuser/nozzle elements that incorporates computational fluid dynamics (CFD) within an optimization methodology. To start the process a series of geometric parameters are selected including element neck width, depth, divergence angle, and entrance fillet radius. Then, the pressure drop and rectification property of an element are calculated as performance parameters, i.e., by varying the geometry it is desirable to maximise pressure rise and the rectification property of the element. Design of experiments (DOE) is employed to generate the experimental table which corresponds to different geometries representing the design space. These limited numbers of geometries generated by DOE are evaluated by using CFD to obtain corresponding performance parameters. By preparing all the design and performance parameters, Surrogate model (SM) technique is applied to obtain the relationship (approximation function) between design and performance parameters. Eventually, based on the developed approximation functions or response surfaces, a multi-objective genetic algorithm (MOGA) is employed to maximise pressure rise and rectification property of diffuser/nozzle element. This design methodology is a very powerful tool to design and optimise flat-walled diffuser/nozzle elements for micropump applications and can speed up the micropump design process significantly.
Review and Modeling of Two-Phase Frictional Pressure Gradient at Microgravity Conditions
Authors: Awad M. M. , Muzychka Y. S.
Published: March 2011

First, a detailed review of two-phase frictional pressure gradient at microgravity conditions is presented. Then, a simple semi-theoretical method for calculating two-phase frictional pressure gradient at microgravity conditions using asymptotic analysis is presented. Two-phase frictional pressure gradient is expressed in terms of the asymptotic single-phase frictional pressure gradients for liquid and gas flowing alone. In the present model, the two-phase frictional pressure gradient for x ≅ 0 is nearly identical to single-phase liquid frictional pressure gradient. Also, the two-phase frictional pressure gradient for x ≅ 1 is nearly identical to single-phase gas frictional pressure gradient. The proposed model can be transformed into either a two-phase frictional multiplier for liquid flowing alone (φl2) or two-phase frictional multiplier for gas flowing alone (φg2) as a function of the Lockhart-Martinelli parameter, X. Comparison of the asymptotic model with experimental data at microgravity conditions is presented.
Reduced-Order Fluidic Model for Flow Instabilities in Two-Phase Microfluidic Heat Exchangers
Authors: Miler Josef L. , Refai-Ahmed Gamal , Touzelbaev Maxat N. , David Milnes P. , Steinbrenner Julie E. , Goodson Kenneth E.
Published: March 2011

Two-phase microfluidic heat exchangers have the potential to provide high-heat flux cooling with lower thermal resistance and lower pumping power than single-phase heat exchangers. However, the process of phase change in two-phase heat exchangers can cause flow instabilities that lead to microchannel dryout and device failure [1–3]. Modeling these flow instabilities remains challenging because the key physics are highly coupled and occur over disparate time and length scales. This work introduces a new approach to capture transient thermal and fluidic transport with a reduced-order model consisting of fluidic, thermal, and phase-change submodels. The present study presents a reduced-order, transient, multichannel fluidic circuit submodel for integration into this proposed modeling approach. The fluidic submodel is applicable in flow regimes in which a thin liquid film exists around the bubble. Flow response to boiling is modeled considering bubble overpressure. An adaptive time step approach is used to treat the rapid flow response at short time scales after initial bubble vaporization. Using a seeded bubble technique for testing two-phase flow response, the model predicts a stability threshold at 0.015 W of localized superheating for two 100-micron square channels in parallel with a pump flow rate of 0.15 ml/min. Once integrated with the proposed reduced-order thermal and phase change models, this fluidic circuit model will yield criteria for stable two-phase heat exchanger operation considering factors such as pumping pressure, channel geometry, and applied heat flux that can be compared to experimental observations.
Validation of Lattice-Boltzmann Aerodynamics Simulation for Vehicle Lift Prediction
Authors: Duncan Bradley D. , Fischer Axel , Kandasamy Satheesh
Published: March 2011

Simulation tools are used in the design of vehicles to reduce the cost of development and to find robust engineering solutions earlier in the design process. Prediction of drag using aerodynamics simulation is critical for assessing aerodynamic efficiency of designs, including upper body shape, underbody surfaces, wheels and aerodynamic treatments such as spoilers, deflectors and underbody covers. The Lattice-Boltzmann Simulation approach has been used broadly to simulate both steady and unsteady flow regimes accurately and to provide robust prediction of drag. Beyond drag, other vehicle performance metrics are now predicted using this type of simulation such as, for example, wind noise levels, heat exchanger performance, brake cooling and thermal protection of sensitive components. In particular, aerodynamic lift is important for production vehicles for assessing handling attributes at high speed. In this paper, the validation of aerodynamics simulation for vehicle lift is examined and extended through a study of three detailed full-scale vehicles. For high-performance road vehicles the front- and rear-axle lift force, and the balance between them, are critical for driving dynamics for highway driving and must be considered along with the drag during development. Often a trade-off between lift and drag performance is required for a successful design. Furthermore, since the lift is highly dependent on the detailed pressure distribution in the underbody region and near the wheels, evaluation of lift should also account for on-road effects using rotating wheels and moving ground plane. In this study the drag, front lift and rear lift were evaluated using Lattice-Boltzmann Simulation and compared to full-scale wind-tunnel tests, using both static- and moving-ground configurations. Care was taken to include the effect of the floor boundary layer, suction system, moving belt and rotating tires, all of which are designed to emulate on-road conditions inside a wind-tunnel. The results show good prediction of both drag and lift performance, and provide confidence to extend the use of aerodynamic simulation for lift prediction earlier in the design process.
Heat Transfer Enhancement in Microchannel Flow: Presence of Microparticles in a Fluid
Authors: Alquaity Awad B. S. , Al-Dini Salem A. , Wang Evelyn N. , Shuja Shahzada Z. , Yilbas Bekir S. , Suabedissen Karl J.
Published: March 2011

In the present study, a numerical model was developed for laminar flow in a microchannel with a suspension of microsized phase change material (PCM) particles. In the model, the carrier fluid and the particles are simultaneously present, and the mass, momentum, and energy equations are solved for both the fluid and particles. The particles are injected into the fluid at the inlet at a temperature equal to the temperature of the carrier fluid. A constant heat flux is applied at the bottom wall. The temperature distribution and pressure drop in the microchannel flow were predicted for lauric acid microparticles in water with volume fractions ranging from 0 to 8%. The particles show heat transfer enhancements by decreasing the temperature distribution in the working fluid by 39% in a 1 mm long channel. Meanwhile, particle blockage in the flow passage was found to have a negligible effect on pressure drop in the range of volume fractions studied. This work is a first step towards providing insight into increasing heat transfer rates with phase change-based microparticles for applications in microchannel cooling and solar thermal systems.
Wind Tunnel Experiments on How a Porous Fence Affects Flow and Erosion on Sand Dune
Authors: Aoshima Daisuke , Nakamura Itsuki , Sakamoto Yusuke , Tsukahara Takahiro , Yamamoto Makoto , Kawaguchi Yasuo
Published: March 2011

For the purpose of combating desertification, it is important to understand mechanisms of the wind-blown sand movement, which is essentially a complicated two-phase flow phenomenon of sand particles and air. Therefore, we investigated the flow field around a model dune and the erosion process of the dune. In this study, we employed a porous fence, which was installed on the model dune, and examined its effect on the sand movement. The erosion process and its relationship with the turbulent intensity and the flow around the dune were discussed focusing on dependence of the flow field on the fence porosity. We tested four types of porous fences, which had different porosities: 0% (no permeability), 10%, 30%, and 50%. How a position of the fence affects suppression of the dune erosion was also examined. In the present experimental range, it can be concluded that the most effective fence position to suppress the sand movement should depend on porosity of the fence.
The LS-STAG Method for Viscous Incompressible Flows in Irregular Geometries: Basics of the Discretization and Application to Viscoelastic Flows
Authors: Botella Olivier , Cheny Yoann
Published: March 2011

The LS-STAG method is an immersed boundary method for viscous incompressible flows based on the staggered MAC arrangement for Cartesian grids, where the irregular boundary is sharply represented by its level-set function. The level-set function enables us to compute efficiently all relevant geometry parameters of the so-called “cut-cells”, i.e. the cells that are cut by the immersed boundary, reducing thus the bookkeeping associated to the handling of complex geometries. One of the main features of the LS-STAG method is the use of a consistent and unified discretization of the flow equations in both Cartesian and cut-cells, which has been obtained by enforcing the strict conservation of global invariants of the flow such as total mass, momentum and kinetic energy in the whole fluid domain. After a short discussion on the salient features of the LS-STAG method, we will present one of its most recent application: The computation of viscoelastic flows governed by the Oldroyd-B constitutive equation.
Transient Simulations of 3-D Supersonic Micronozzle Flow
Authors: Louisos W. L. , Hitt Darren L.
Published: March 2011

A numerical investigation of transient performance of 3D linear micronozzles has been performed. The baseline model for the study is derived from the NASA/Goddard Space Flight Center MEMS-based hydrogen peroxide micro-thruster prototype. The 3D micronozzles investigated here have depths of 25μm, 50μm, 100μm, and 150μm and employ expanders with a 30° half-angle. A hyperbolic-tangent actuation profile is used to model the opening of a microvalve in order to simulate start-up of the thruster. The inlet stagnation pressure when the valve is fully opened is 250kPa and generates a maximum throat Reynolds number of Remax ∼ 800. The complete actuation occurs over 0.55ms and is followed by 0.25ms of steady-state operation. The propulsion scheme employs 85% pure hydrogen peroxide as a monopropellant. Simulation results have been analyzed and thrust production as a function of time has been quantified along with the total impulse delivered. Micronozzle impulse efficiency has also been determined based on a theoretical maximum impulse achieved by a quasi-1D inviscid flow responding instantaneously to the actuation profile. It is found that both the flow and thrust exhibit a response ‘lag’ to the time-varying inlet pressure profile. Simulations are compared to previous 2D results and indicate that thrust per unit nozzle depth, impulse, and efficiency increase with nozzle depth and approach the 2D results for nozzle depths greater than 150μm.
Modeling and Experiments of a Resonant Sonochemical Reactor
Authors: Wang Yi-Chun , Yao Ming-Chung
Published: March 2011

This work aims at analyzing and realizing a horn-type sonochemical reactor which can be operated in a very low ultrasonic power density but results in a large volume of cavitation zones. The sonoreactor contains three main components, namely a Langevin-type piezoelectric transducer (20 kHz), a metal horn, and a circular cylindrical sonicated cell filled with tap water. In order to diminish the generation of cavitation bubbles near the horn-tip, an enlarged cone-shaped horn is designed to reduce the ultrasonic intensity at the irradiating surface and to get better distribution of energy in the sonicated cell. It is demonstrated both numerically and experimentally that the cell geometry and the horn position have prominent effects on the pressure distribution of the ultrasound in the cell. With appropriate choices of these parameters, the whole reactor works at a resonant state. Several acoustic resonance modes observed in the simulation are realized experimentally and used for generating a large volume of cavitation field.
Characterization of Switching Time and Cell Stress in a Gravity-Driven Microfluidic Cell Sorter Based on Hydrodynamic Switching
Authors: Grad Michael , Smilenov Lubomir , Brenner David , Attinger Daniel
Published: March 2011

In this work we describe the control and characterization of the switching time and hydrodynamic stress in a microfluidic cell sorter. The device was designed to sort small (<1000) populations of live cells in buffer solution labeled with standard bio-markers such as live dyes or green fluorescent protein (GFP). Sorting occurs through a hydrodynamic switching technique where high-speed solenoid valves control a sheath flow used to steer sorted cells away from the unsorted bulk population. The device is constructed from a reusable hard plastic polymethylmethacrylate (PMMA) chip machined with 127μm × 50μm microchannels and sealed with adhesive tape. Open reservoirs in the chip facilitate pipette access, standard microscope visualization, and a simple disassembly and cleaning procedure. The sorting frequency of this type of device is typically limited by the hydrodynamic switching time. Here, we present a theoretical and numerical analysis of the device switching time. These results show that the sorter switching time t is practically limited by the velocity of the flow and the characteristic length between inlet and outlet channels. We validate this theoretical result with experimental data obtained from flow visualizations, along with experiments conducted to evaluate the repeatability of the hydrodynamic switching scheme and the survival rate of sorted fibroblast cells Manually operated, the sorting frequencies were approximately 10 cells per minute, with switching time constants of approximately 130ms. Current throughput is limited by this switching time to approximately 450 cells per minute. Automation can increase the velocity and reduce the spacing between cells, thereby increasing throughput by at least an order of magnitude. The cell sorter was then tested by manually sorting 100 beads in 7 minutes, and 30 cells in less than 3 minutes, and was successfully used in the framework of a study on the bystander effect occurring during cell irradiation. Experiments with Trypan Blue dye verified that cell viability was maintained during the sorting process.
Best Estimate Plus Uncertainty Analysis of LBLOCA for a Pickering B CANDU Reactor
Authors: Blake Larry , Gavrus George , Vecchiarelli Jack , Stoklosa J.
Published: March 2011

The Pickering B Nuclear Generating Station consists of four CANDU reactors. These reactors are horizontal pressure tube, heavy water cooled and moderated reactors fuelled with natural uranium. Under a postulated large break loss of coolant accident (LOCA), positive reactivity results from coolant void formation. The transient is terminated by the operation of the safety systems within approximately 2 seconds of the start of the transient. The initial increase in reactor power, terminated by the action of the safety system, is termed the power pulse phase of the accident. In many instances the severity of an LBLOCA can be characterized by the adiabatic energy deposited to the fuel during this phase of the accident. Historically, Limit of Operating Envelope (LOE) calculations have been used to characterize the severity of the accident. LOE analyses are conservative analyses in which the key operational and safety related parameters are set to conservative or limiting values. Limit based analyses of this type result in calculated transient responses that will differ significantly from the actual expected response of the station. As well, while the results of limit calculations are conservative, safety margins and the degree of conservatism is generally not known. As a result of these factors, the use of Best Estimate Plus Uncertainty (BEPU) analyses in safety analyses for nuclear power plants has been increasing. In Canada, the nuclear industry has been pursuing best estimate analysis through the BEAU (Best Estimate Analysis and Uncertainty) methodology in order to obtain better characterization of the safety margins. This approach is generally consistent with those used internationally. Recently, a BEAU analysis of the Pickering B NGS was completed for the power pulse phase of a postulated Large Break LOCA. The analysis comprised identification of relevant phenomena through a Phenomena Identification and Ranking (PIRT) process, assessment of the code input uncertainties, sensitivity studies to quantify the significance of the input parameters, generation of a functional response surface and its validation, and determination of the safety margin. The results of the analysis clearly demonstrate that the Limit of Operating Envelope (LOE) results are significantly conservative relative to realistic analysis even when uncertainties are considered. In addition, the extensive sensitivity analysis performed to supplement the primary result provides insight into the primary contributors to the results.
Study the Influence of Position and the Angle of the Winglets on MITE Micro Air Vehicle
Authors: Ganji Babak , Sadr-Eshkevari Romina
Published: March 2011

In recent years, small aircraft has been thoroughly studied and superior designs have been extensively developed. The aerodynamic design of micro aerial vehicles (MAVs), the most important small aircrafts, in Low-Reynolds number flow (LRNF) has become one of the main concerns to the profession. LRNF is mostly influenced by the airfoil design. Similar to all aircrafts, vertical elevons and winglets play an important role in the aerodynamics of MAVs. On this basis, the present study aimed to assess the effect of lateral angle alterations of the two vertical winglets in the aerodynamics of micro tactical expendable (MITE) in LRNF. A finite element flow solver (FEFS) based on structured grid was employed for studying the aerodynamic characteristics of MITE. The findings of the present study suggest that with the gradual increase in cant angle φ, lift force decreases and drag force remains unchanged. Also with the increase of lateral angle θ, drag force increases significantly and negligible changes are observed in lift force. Vertical elevons play an important role in the control of MITE. Also the effect of Reynolds number on aerodynamic coefficients is discussed.
Application of Uncertainty Analysis in the Comparison of Void Fraction Calculations With Experiment
Authors: Pascoe Jason , Parlatan Yuksel , McLaughlin B. , Fung Sophia
Published: March 2011

Safety analysis computer codes are designed to simulate phenomena relevant to the assessment of normal and transient behaviour in nuclear power plants. In order to do so, models of relevant phenomena are developed and a set of such models constitutes a computer code. In accident or transient analysis the values of certain output parameters (margin parameters) are used to characterize the severity of the event. The accuracy of the computer code in calculating these margin parameters is usually obtained through validation and variation in the margin parameter is estimated through the propagation of variation in the code input. A method for estimating code uncertainty respect to a specific output parameter has been developed. The methodology has the following basic elements: (1) specification and ranking of phenomena that govern the behaviour of the output parameter for which an uncertainty range is required; (2) identification of models within the code that represent the relevant phenomena; (3) determination of the governing parameters for the phenomenological models and Identification of uncertainty ranges for the governing model parameters from validation or scientific basis, if available; (4) decomposition of the governing model parameters into related parameters; (5) identification of uncertainty ranges for the modelling parameters for use in Best Estimate Analysis; (6) design and execution of a case matrix; and (7) estimation of the code uncertainty through quantification of the variability in output parameters arising from uncertainty in modelling parameters. This methodology has been employed using simulations of Large Break Loss of Coolant Accident (LOCA) tests in the RD-14M test facility to calculate the uncertainty in the TUF thermal hydraulics code calculation of the coolant void fraction. The uncertainty has been estimated with and without plant parameters (parameters specific to the RD-14M test loop). The TUF coolant void fraction uncertainty without plant parameters was determined to be 0.08 while the uncertainty with plant parameters included was determined to be 0.11. The uncertainty value without plant parameters included is comparable to the uncertainty in the measurements (0.09). The uncertainty value with plant parameters included is larger than the variation in the bias (0.10) of the TUF calculation of void fraction. From these findings, it can be concluded that the estimated accuracy of the TUF code calculation of void fraction is consistent with the available experimental data.
Flow Visualization Techniques for the Evaluation of Non-Contact Trace Contraband Detectors
Authors: Staymates Matthew , Gillen Greg , Smith Wayne , Lareau Richard , Fletcher Robert
Published: March 2011

Efforts are underway in the Surface and Microanalysis Science Division at the National Institute of Standards and Technology to study trace aerodynamic sampling of contraband materials (explosives or narcotics) in non-contact trace detection systems. Trace detection systems are designed to screen people, personal items, and cargo for particles that have contaminated surfaces. In a typical implementation of people screening, a human subject walks into a confined space where they are interrogated by a series of pulsed air jets and are screened for contraband materials by a chemical analyzer. The screening process requires particle and vapor removal, transport, collection, desorption, and detection. Aerodynamic sampling is the critical front-end process for effective detection. In this paper, a number of visualization techniques are employed to study non-contact aerodynamic sampling in detail. Particle lift-off and removal is visualized using high-speed videography, transport of air and particles by laser light scattering, and desorption surface heating and cooling patterns by infrared thermography. These tools are used to identify sampling inefficiencies and may be used to study next-generation screening approaches for aerodynamic sampling of particles and vapors.
The Penetration of Submerged Round Turbulent Gas Jets in Water
Authors: Weiland Chris , Vlachos Pavlos
Published: March 2011

Direct measurements of the interfacial behavior of submerged high speed gas jets with speeds ranging from subsonic to supersonic Mach numbers were performed using high speed digital photography and shadowgraphs. The results indicate that the jets preferentially pinch-off near the axial position which in previous experimental work has been shown to correspond to the location of the maximum streamwise velocity turbulence fluctuations. Using the optical method presented in this paper, the data indicates that the electroresistivity probe technique used by past researchers to quantify the jet penetration into the ambient fluid biases the measurement by up to 30 diameters as the probe cannot identify true jet continuity as opposed to advecting bubbles. We introduce a theoretical jet penetration distance based on a simple force balance of the jet cross-section which compares reasonably well with the measured data. This theoretical jet penetration distance scales with the square of the Froude number and requires an estimation of the jet centerline properties as they evolve downstream of the orifice to accurately predict the pinch-off point. An experimental jet penetration distance is introduced and is defined as the 98.5% contour of the orifice attached gas jet presence over the measurement time.
Computational Investigation of Liquid Spray Dispersion Modification by Conical Nozzle Attachments
Authors: Pougatch Konstantin , Salcudean Martha
Published: March 2011

Liquid sprays characteristics such as the droplet size and the dispersion angle are determined by the atomizer design and the physical properties of the liquid and the surrounding gas. One of the options to change these characteristics is to attach a specially designed piece to the nozzle exit. While there can be a variety of shapes of such attachments, we chose a conical geometry to exploit its axial symmetry and, at the same time, obtain the results that can be generalized to other configurations. Thus, we investigate an addition of the conically shaped attachment to the premixed gas-assisted high-pressure atomizer with the previously developed numerical model. It is a two-fluid Eulerian-Eulerian model with a catastrophic phase inversion that was developed for compressible gas-liquid mixtures and can be applied to both the flow through the nozzle-atomizer and to the dispersion of the spray. The model also accounts for the break-up and coalescence effects of bubbles and droplets. Our investigation reveals that the conical nozzle attachments act as spray limiters by reducing the natural expansion angle of a spray. Also, the droplets produced by the nozzle with a conical addition tend to be larger than the ones obtained with a stand alone nozzle. The largest droplets were generated by the smallest attachment angle considered −10°. With the increase of the angle, the spraying characteristics become closer to the ones of the stand alone nozzle. It can be concluded that the conical shape of the attachments with a relatively small angle may be used when higher jet penetration and lower dispersion are desirable. The attachments with larger angles do not offer a substantial difference from the stand alone nozzle. Another important conclusion is that the dispersion of the jet is determined by the radial momentum transferred to the liquid before or immediately after the phase inversion takes place. Thus, for improved dispersion, the area where the atomization is taken place should not be restricted.
An Aero-Acoustic Performance Prediction Method of Sirocco Fan
Authors: Lee Chan , Kil Hyun Gwon
Published: March 2011

A design-analysis method of Sirocco fan is developed for constructing 3-D impeller and scroll geometries, and for predicting both the aerodynamic performance and the noise characteristics of the designed fan. The aerodynamic blading design of fan is conducted by blade angle, camber line determinations and airfoil thickness distribution, and then the scroll geometry of fan is designed by using logarithmic spiral. The aerodynamic performance of designed fan is predicted by the mean line analysis with flow blockage, slip and pressure loss correlations. Based on the predicted performance data, fan noise is predicted by two models for cutoff frequency and broadband noise sources. The present predictions for the performance and the noise level of actual fans are well agreed with measurement results.
Evaluation of Turbulence Models Using Large-Eddy Simulation Data
Authors: Raiesi Hassan , Piomelli Ugo , Pollard Andrew
Published: March 2011

The performance of some of the most commonly used eddy viscosity turbulence models to predict separated boundary layer flows in adverse pressure gradient has been evaluated against large eddy simulations. The LES results were used to assess the consistency of the different terms in the k–ε, ζ–f, k–ω and Spalart-Allmaras models. For the separated boundary layer, the eddy-viscosity assumption works well, and anisotropic effects are not significant. However, the near-wall treatment used in k–ε models was found to have a critical effect on the predictive accuracy of the flow (and, in particular, of separation and reattachment points). None of the wall treatments tested resulted in accurate prediction of the flow field.
The Histogram of Pressure Oscillations Amplitudes, in Lean Premixed Combustions, Caused by Thermo-Acoustic Instabilities
Authors: Mehdizadeh Nasser Seraj , Akbari Nozar
Published: March 2011

Lean premixed combustion is widely used in recent years as a method to achieve the environmental standards with regard to NOx emission. In spite of the mentioned advantage, premixed combustion systems, with equivalence ratios less than one, are susceptible to the combustion instability. To study the lean combustion instability, by experiments, one premixed combustion setup, equipped with reactant supplying system, is designed and manufactured in Amirkabir University of Technology. In this research, gaseous propane is introduced as fuel and several experiments are performed at nearly atmospheric pressure, with equivalence ratios within the range of 0.7 to 1.5. In this experiments fuel mass flow rate is varied between 2 and 4 gr/s. Unstable operating condition has been observed in combustion chamber when equivalence ratio is less than one. To distinguish the combustion instability for various operating conditions, probability density functions, spectral diagrams, and space distribution of pressure oscillations, along with Rayleigh Criterion, are utilized. Accordingly, effect of equivalence ratio on stabilizing the unstable combustion system is investigated. Moreover, convective delay time is calculated for all experiments and the results are compared with Rayleigh Criterion. This comparison has shown good agreement the experimental results and Rayleigh Criterion. Finally, stability limits are identified based on inlet mass flow rate and equivalence ratio.
Experimental Measurement of Mixing Lengths of Converging Particulate Flows in Microchannels
Authors: Nadler Michael , Hitt Darren L.
Published: March 2011

Laminar flow mixing remains an active area of research within the microfluidics community. Traditional mixing methods often rely upon turbulent flow for mixing which is generally not present on the micro-scale and so alternative approaches must be sought. In this work we investigate the potential for enhanced laminar mixing due to the addition of inert particles, of varying size and concentration, to converging flows of miscible fluids at a 3-way microchannel ‘Y’ junction. One inlet fluid is fluorescently-labeled to enable imaging of the converging fluid interface downstream of the junction. Using the techniques of fluorescent microscopic imaging and digital image analysis, the degree of mixing is quantified for varying flow states by the measurement of dye concentration (intensity) profiles at downstream locations. It is found that the addition of particulates to the flow does not noticeably enhance laminar mixing over that which would normally occur due to mass diffusion processes alone.
Experimental Study of Microbubble Formation in Micro Channel
Authors: John T. J. , Hegab H.
Published: March 2011

The formation of unconfined spherical microbubbles in a micro channel is investigated in this study. A liquid channel (primary channel) and a gas channel (secondary channel) which is perpendicular to each other form a T-Junction where the microbubbles are formed. Fused silica tubes of 20μm inner diameter and 360μm outer diameter is used as the secondary channel. The hydraulic diameter of the liquid channel is varied to study its effect on the bubble formation. Because of the small size of the gas orifice compared to the hydraulic diameter of the liquid channel the microbubbles formed are completely spherical, thus giving enough facility to study its formation and detachment criteria. The microbubble formation inside the micro channel is studied subjected to variable parameters such as liquid flow rate, gas flow rate, and the hydraulic diameter of the liquid channel. Silicon wafer subjected to dry etching techniques is used to make both the primary micro channels and channels which holds the fused silica tube (secondary channel). A glass piece is anodically bonded to the silicon wafer which seals both the channel from the top. Holes are drilled on to the glass piece to provide the inlet and outlet of the channels and the T-junction is continuously monitored using an Olympus i-speed camera capable of taking 10000 frames per second. Di ionized water is used as the liquid and nitrogen is used as the gas for bubble formation. The results obtained show that the bubble diameter decreases with the increase in the liquid flow rate, and decrease in hydraulic diameter of the liquid channel. The gas flow rate does not have any effect on the bubble diameter, as the gas flow rate was increased the frequency of the bubble formation increases.
S-Shaped Pin-Fins for Enhancement of Overall Performance of the Pin-Fin Heat Sink
Authors: John T. J. , Mathew B. , Hegab H.
Published: March 2011

In this paper the authors are studying the effect of introducing S-shaped pin-fin structures in a micro pin-fin heat sink to enhance the overall thermal performance of the heat sinks. For the purpose of evaluating the overall thermal performance of the heat sink a figure of merit (FOM) term comprising both thermal resistance and pumping power is introduced in this paper. An optimization study of the overall performance based on the pitch distance of the pin-fin structures both in the axial and the transverse direction, and based on the curvature at the ends of S-shape fins is also carried out in this paper. The value of the Reynolds number of liquid flow at the entrance of the heat sink is kept constant for the optimization purpose and the study is carried out over a range of Reynolds number from 50 to 500. All the optimization processes are carried out using computational fluid dynamics software CoventorWARE™. The models generated for the study consists of two sections, the substrate (silicon) and the fluid (water at 278K). The pin fins are 150 micrometers tall and the total structure is 500 micrometer thick and a uniform heat flux of 500KW is applied to the base of the model. The non dimensional thermal resistance and nondimensional pumping power calculated from the results is used in determining the FOM term. The study proved the superiority of the S-shaped pin-fin heat sinks over the conventional pin-fin heat sinks in terms of both FOM and flow distribution. S-shaped pin-fins with pointed tips provided the best performance compared to pin-fins with straight and circular tips.
Calibration of Triple-Wire Probes Using an Artificial Neural Network
Authors: Rostamy Noorallah , Akbari Soheil , Sumner David , Bergstrom Donald J.
Published: March 2011

Hot-wire anemometry is an established technique for velocity measurements in turbulent flows. Calibration of hot-wire probes is challenging due to the nonlinear relationship between the probe output voltage and the velocity, and the sensitivity to the temperature difference between the heated wire and the ambient flow. A triple-wire probe contains three mutually orthogonal wires that permit the three components of the local instantaneous velocity vector to be measured simultaneously. Calibration data reduction methods for multi-wire probes, based on variable-angle calibration techniques, may include curve-fits and direct-interpolation schemes. In the present study, a novel calibration data reduction method for a triple-wire probe is reported which uses an artificial neural network. Such a method has been successfully applied by other researchers for the calibration of seven-hole pressure probes. For the triple-wire probe, the neural network is used to produce a calibration relation between the three probe output voltages and the three components of the local velocity vector. Variable-angle calibration data were obtained for a triple-wire probe for velocity magnitudes from 5 to 40 m/s, yaw angles from −35° to +35°, and roll angles from 0° to 345°. A three-layer perceptron feed-forward network, using a Levenberg-Marquardt training algorithm, was applied to the calibration data, to map the mean voltages to the mean velocity components. The network was tested using an independent data set. The present results yielded standard errors of approximately ±0.38 m/s, ±0.25 m/s and ±0.26 m/s in the magnitudes of the streamwise, vertical, and cross-flow velocity components, respectively. The results showed that the present neural network model is not significantly sensitive to the size of the calibration data set, suggesting it may be a more convenient calibration data reduction method compared to other methods.
Sessile-Water-Droplet Contact Angle: the Effect of Adsorption
Authors: Ghasemi H. , Ward C. A.
Published: March 2011

A method has been recently proposed for determining the surface tension of solid-vapor interfaces. The proposed method was used in conjunction with Gibbsian thermodynamics to investigate both analytically and experimentally the possible role of line tension in determining the contact angle of sessile-water-droplets. After forming a sessile-water-droplet in a closed system, its contact angle was determined by measuring the curvature of three-phase contact line and the height of the axisymmetric droplet on its centerline. The total number of the moles in the closed system was determined from the minimum in the Helmholtz function. The total number of moles in the system was then changed to a new value and the system allowed to come to equilibrium again. The contact angle in the new equilibrium condition could be measured and predicted by taking the adsorption at the solid-liquid and solid-vapor interfaces into account but with line tension completely neglected. The predicted values of contact angle are in closed agreement with those measured indicating line tension plays no role in determining the contact angle of mm-sized water droplets on a polished Cu surface. The surface tension of the solid-vapor interface was approximately constant and equal to the surface tension of adsorbing fluid; that is, the Young equation could be simplified.
Long Term Dynamics of Water-Entry Cavity
Authors: La Foy Roderick R. , Jung Sunghwan , Vlachos Pavlos
Published: March 2011

Many engineering applications involve the motion of objects crossing a fluid interface. The dynamics of this process are often complicated due to the interplay of surface tension, gravity, and inertia. Nevertheless, a simple analysis using potential flow theory works well to predict the interfacial profile of the air cavity formed during an impact. Most current theories however, cannot predict the behavior of the air cavity after pinch off occurs. We therefore investigated the long term dynamics of water entry in both experiment and theory. It was found that shortly after pinch off the cavity dynamics become governed primarily by thermodynamic gas relations. The internal pressure slowly rises due to the cavity volume decreasing while the ambient liquid pressure quickly increases as a result of the descent of the projectile. This effect is incorporated into our model to correctly predict the cavity geometry.
A Novel Dynamics Lattice Boltzmann Method for Gas Flows
Authors: Hsu C. T. , Chiang S. W. , Sin K. F.
Published: March 2011

The lattice Boltzmann method (LBM), where discrete velocities are specifically assigned to ensure that a particle leaves one lattice node always resides on another lattice node, has been developed for decades as a powerful numerical tool to solve the Boltzmann equation for gas flows. The efficient implementation of LBM requires that the discrete velocities be isotropic and that the lattice nodes be homogeneous. These requirements restrict the applications of the currently-used LBM schemes to incompressible and isothermal flows. Such restrictions defy the original physics of Boltzmann equation. Much effort has been devoted in the past decades to remove these restrictions, but of less success. In this paper, a novel dynamic lattice Boltzmann method (DLBM) that is free of the incompressible and isothermal restrictions is proposed and developed to simulate gas flows. This is achieved through a coordinate transformation featured with Galilean translation and thermal normalization. The transformation renders the normalized Maxwell equilibrium distribution with directional isotropy and spatial homogeneity for the accurate and efficient implementation of the Gaussian-Hermite quadrature. The transformed Boltzmann equation contains additional terms due to local convection and acceleration. The velocity quadrature points in the new coordinate system are fixed while the correspondent points in the physical space change from time to time and from position to position. By this dynamic quadrature nature in the physical space, we term this new scheme as the dynamic quadrature scheme. The lattice Boltzmann method (LBM) with the dynamic quadrature scheme is named as the dynamic lattice Boltzmann method (DLBM). The transformed Boltzmann equation is then solved in the new coordinate system based on the fixed quadrature points. Validations of the DLBM have been carried with several benchmark problems. Cavity flows problem are used. Excellent agreements are obtained as compared with those obtained from the conventional schemes. Up to date, the DLBM algorithm can run up to Mach number at 0.3 without suffering from numerical instability. The application of the DLBM to the Rayleigh-Bernard thermal instability problem is illustrated, where the onset of 2D vortex rolls and 3D hexagonal cells are well-predicted and are in excellent agreement with the theory. In summary, a novel dynamic lattice Boltzmann method (DLBM) has been proposed with algorithm developed for numerical simulation of gas flows. This new DLBM has been demonstrated to have removed the incompressible and isothermal restrictions encountered by the traditional LBM.