bluuebunny/crossref_metadata_2025 huggingface dataset:
Two-Phase Flow Patterns and Flow-Pattern Maps: Fundamentals and Applications
Authors: Cheng Lixin , Ribatski Gherhardt , Thome John R.
Published: September 2008

A comprehensive review of the studies of gas-liquid two-phase flow patterns and flow-pattern maps at adiabatic and diabatic conditions is presented in this paper. Especially, besides other situations, this review addresses the studies on microscale channels, which are of great interest in recent years. First, a fundamental knowledge of two-phase flow patterns and their application background is briefly introduced. The features of two-phase flow patterns and flow-pattern maps at adiabatic and diabatic conditions are reviewed, including recent studies for ammonia, new refrigerants, and CO2. Then, fundamental studies of gas-liquid flow patterns and flow-pattern maps are presented. In the experimental context, studies of flow patterns and flow-pattern maps in macro- and microscale channels, across tube bundles, at diabatic and adiabatic conditions, under microgravity and in complex channels are summarized. In addition, studies on highly viscous Newtonian fluids (non-Newtonian fluids are beyond the scope of this review) are also mentioned. In the theoretical context, modeling of flow-regime transitions, specific flow patterns, stability, and interfacial shear is reviewed. Next, flow-pattern-based heat transfer and pressure drop models and heat transfer models for specific flow patterns such as slug flow and annular flow are reviewed. Based on this review, recommendations for future research directions have been given.
Numerical Experiments on Solid Handling Pump
Authors: Gupta Amit , Kshirsagar J. T.
Published: September 2008

Solid handling pumps are used in managing sewage and other solid materials that could flow along with the water or any other fluid. The design of solid handling pump components is also deviating significantly from clear water pump components. The flow passages need to be maintained to allow solid particles of specified sizes smoothly without any obstruction. This aspect of different design from conventional design causes performance deterioration of the pump. The laboratory testing of the solid handling pumps is normally carried out with clear water and a judgment is used based on experience about the deterioration in performance caused by solid particles (size and number of particles). The actual performance measurement with varying particle sizes and its cluster density is very rarely available. A case of solid handling pump is taken up for detail investigation using numerical approach. The preliminary analysis has been carried out with solid handling impeller with clear water as medium of flow. The solid particles are then injected with different sizes and number of solid. Both Eulerian and Langrangian multiphase flow (MPF) approach have been used to study the hydraulic performance deterioration and particle trajectory in the working pump. The paper describes the approaches used and discusses the results considering generally accepted behavior of solid handing pumps.
On 2D Liquid Sheet Atomization
Authors: Couderc Frederic , Estivalezes Jean-Luc
Published: September 2008

Liquid sheet atomization by coflowing air flows appears in a broade range of industrial process, but still remains not well understood. This paper is devoted to the numerical investigation of the air-assisted disintegration of a planar liquid sheet by two parallel air streams flowing on both sides. To do that, a DNS solver for two-phase incompressible viscous flows with interface capturing feature for non miscible fluids has been developped and validated [1]. The interface is captured by a Level-Set method, which has become very popular during the last ten years. However, unlike classical approaches, stress tensor jump conditions across the interface are explicitly taken into account without introducing any smoothing. Although the physical phenomenon is tridimensional, experimental studies show that the initial stage of the liquid sheet oscillations is mainly two-dimensional which justifies the two dimensional simulations done in this paper. We present a first two-dimensional spatial simulation which shows the gas flow dynamics in interaction with the liquid sheet oscillations. By separation of the air boundary layer behind the liquid sheet at its maximum amplitude location, vortical structures are created and evolve in time with the frequency of the liquid sheet global oscillation. We investigate the effects of the main flow parameters such as outer air velocity, air boundary layer thickness on the main characteristics of the flow and the global oscillation frequency. The first result from our study concerning the frequency oscillation shows a linear variation of the frequency with air velocity. This is in complete agreement with experimental results of [2], whereas inviscid linear stability theory predicts a quadratic evolution. Evidence from those results shows that two-dimensional spatial simulations can provided relevant information on the early stage of liquid sheet atomization.
Experimental Investigations on the Effect of Swirl and Bluff Body Wake in Circular Jets
Authors: Senthil R. , Mahalakshmi N. V.
Published: September 2008

Swirl with bluff body wake in jets finds application in gas turbine engines, turbo machinery etc. to strengthen the recirculation region, improve combustion efficiency, enhance flame stabilization, improve fuel air mixing and blow off limits and lower pollutant formation. This paper discusses the combined effect of swirl and rotating bluff body wake in circular jets. The flow pattern of the swirl jet has been studied in the near vicinity of the jet exit for different inlet high Reynolds numbers (only for which the recirculation effect is beneficial), critical swirl angle (30 degree-similar to what is employed in industrial furnaces and burners), and a constant blockage ratio of the bluff body at eight different axial positions in the recirculation zone. Reynolds stresses, mean axial and tangential velocities, turbulent intensities, turbulent kinetic energy are measured and compared for the bluff body rotating and non-rotating cases. A DANTEC Dynamics make constant temperature anemometer with X-probe has been used to measure mean and turbulence parameters.
Assessment Measures for LES Applications
Authors: Celik I. , Klein M. , Janicka J.
Published: September 2008

Anticipating that Large Eddy Simulations will increasingly become the future engineering tool for research, development and design, it is deemed necessary to formulate some quality assessment measures that can be used to judge the resolution of turbulent scales and the accuracy of predictions. In this context some new and refined measures are proposed above and beyond those already published by the authors in the common literature. These new measures involve (a) fraction of total turbulent kinetic energy, (b) relative grid size with respect to Kolmogorov or Taylor scales, (c) relative effective sub-grid/numerical viscosity with respect to molecular viscosity, and (d) some property related to power spectra of turbulent kinetic energy. In addition, an attempt is made to segregate the contributions from numerical and modeling errors. Proposed measures are applied to various benchmark cases, and validated against fully resolved LES and/or DNS whenever possible. Along the same line of thinking, the authors present a perspective for verification of under-resolved direct numerical simulations.
A Parallelized, Dynamic Solution Adaptive, Multi-Grid, Overset Chimera Unstructured Grid Solver
Authors: Zhao Xiang , Zhang Sijun
Published: September 2008

This paper presents a parallelized, dynamic solution adaptive, multi-grid, overset Chimera unstructured finite volume solver for all-speed flows. The methods and schemes applicable for all speeds were implemented in the general-purpose CFD computer code. The hanging node solution adaptation, parallel computing algorithm using MPI libraries, overset chimera grid to allow multiple moving bodies calculation and algebraic multi-grid speeding-up convergence method were all facilitated in the solver. With the enhancement of higher order scheme data reconstruction, higher order upwind-biased differencing, multi-pressure-correction, and Bi-CGSTAB and GMRES matrix solvers, the solver can efficiently and accurately handle flow problems in several cases including benchmark and practical.
Transport of Particulates in a Rotating Internal Cooling Ribbed Duct
Authors: Shah Anant , Tafti Danesh K.
Published: September 2008

A ribbed square duct (P/e = 10, e/Dh = 0.10, and rotating with an angular velocity ωz) subjected to sand ingestion is studied using an Eulerian-Lagrangian framework. Particle sizes of 10μm and 50μm with response times (normalized by friction velocity and hydraulic diameter) of 0.06875 and 1.71875 respectively are considered. The calculations are performed for a nominal bulk Reynolds number of 20,000 and a Rotation number of 0.35 (based on hydraulic diameter and mean velocity) under fully-developed conditions. It is found that at any given instant in time about 67% of the total number of 10 micron particles are concentrated in the vicinity (within 0.05 Dh) of the duct surfaces, compared to 99% of the 50 micron particles. Both the particle sizes show preferential concentration near the smooth walls. The 10 micron particles, which are more sensitive to changes in flow, exhibit selective concentration on the trailing wall as compared to negligible concentration of the 50 micron particles. Both the particle sizes show negligible accumulation on the leading wall due to the action of Coriolis forces which push the particles towards the trailing wall. At the side walls of the duct, the 10 micron particles exhibit a high potential to erode the region in the vicinity of the rib due to secondary flow impingement. At the ribbed walls, while the 10 micron particles exhibit a fairly uniform but low propensity for erosion, the 50 micron particles show a much higher tendency to erode the surface. The trailing wall rib face facing the flow is by far the most susceptible to erosion and deposition for all particle sizes. In comparison to the front surface, the top and back surfaces of the rib do not exhibit a large propensity to be eroded. The potential for erosion in the rotating duct at the trailing wall and rib is at least an order of magnitude larger than in a stationary duct, whereas the leading wall has little or no erosion.
Contact Line Dynamics in Liquid-Vapor Flows Using Lattice Boltzmann Method
Authors: Li Shi-Ming , Tafti Danesh K.
Published: September 2008

A mean-field free-energy lattice Boltzmann method (LBM) is applied to simulate moving contact line dynamics. It is found that the common bounceback boundary condition leads to an unphysical velocity at the solid wall in the presence of surface forces. The magnitude of the unphysical velocity is shown proportional to the local force term. The velocity-pressure boundary condition is generalized to solve the problem of the unphysical velocity. The simulation results are compared with three different theories for moving contact lines, including a hydrodynamic theory, a molecular kinetic theory, and a linear cosine law of moving contact angle versus capillary number. It is shown that the current LBM can be used to replace the three theories in handling moving contact line problems.
Two-Fluid Large-Eddy Simulation Approach for Gas-Particle Turbulent Flows Using Equilibrium Assumption
Authors: Shotorban Babak , Balachandar S.
Published: September 2008

This article illustrates a two-fluid large-eddy simulation (LES) approach for gas-particle turbulent flows. The equilibrium assumption in which the velocity of particles is approximated in terms of the velocity and acceleration of the gas phase, is made for the development of gas-particle LES formulation in this study. A filtered Eulerian velocity field is defined for particles and expressed in terms of the temporal and spatial derivatives of the gas-phase filtered velocity field. Also, filtered particle concentration defined in the Eulerian framework is governed by a transport equation with a closure problem resulted from filtering the particle concentration nonlinear convection term and in the form of subgrid-scale particle flux. A Smagorinsky kind of formulation is used to model the subgrid-scale particle flux and close the transport equation of the filtered particle concentration. The developed gas-particle LES formulation is implemented in a homogeneous shear turbulence configuration and results are discussed. It is shown that the equilibrium assumption is valid for sufficiently small particle time constants through conducting the direct numerical simulation of the same configuration.
CFD-Analysis of the Flow Consisting of Two Mass Flows With Different Temperatures and/or Two Phases in a Turbocompressor
Authors: Brune Kai-Henning , Stoffel Bernd , Matyschok Berthold , Mu¨nz Stefan
Published: September 2008

In this paper, numerical results of the mixing behavior of two mass flows having different temperatures in an elbow, which is located upstream of a turbocharger compressor, are presented. The background for these investigations are the thermal problems and the problems due to droplet impacts, which arise as a result of Low Pressure Exhaust Gas Recirculation for a turbocharger compressor. In these investigations, the temperature distributions resulting at the inlet of the compressor are mainly of interest. Of further interest are the trajectories of possibly arising water droplets in the inflow of the turbocharger compressor. In the first step different injection configurations are calculated without the compressor in order to get a first impression of the mixing behavior of two mass flows in an elbow. Afterwards three of the injection geometries are calculated including the compressor to see the influence of the compressor on the mixing behavior. The results show a strong influence of the direction of the injection and of the relative position of the injection to the elbow unit. Subsequently, visualization tests are made to qualitatively check the mixing behavior of the different elbow geometries.
Calculation of Droplet Size Distribution in Condensing Flow
Authors: IJzermans Rutger H. A. , Hagmeijer Rob , Sidin Ryan S. R.
Published: September 2008

Condensing flows can be found in a large variety of industrial machinery such as steam turbines and supersonic gas conditioners. In many of these applications, it is very important to predict the droplet size distributions accurately. In the present research, the droplet size distribution in condensing flows is investigated numerically. We consider condensing flows with droplets that nucleate and grow, but do not slip with respect to the surrounding gas phase. To compute the coupling between the condensed phase and the carrier flow, one could solve the general dynamic equation and the fluid dynamics equations simultaneously. In order to reduce the overall computational effort of this procedure by roughly an order of magnitude, we use an alternative procedure, in which the general dynamic equation is initially replaced by moment equations complemented with a closure assumption. This closure assumption is based on Hill’s approximation of the droplet growth law. The method thus obtained, the so-called Method of Moments, is assumed to approximately accommodate the thermodynamic effects of condensation, such as the temperature, pressure and velocity field of the carrier flow. We use the Method of Moments as a basis for the calculation of the droplet size distribution function. We propose to solve the general dynamic equation a posteriori along a number of selected fluid trajectories, keeping the flow field fixed. This procedure, called Phase Path Analysis [1], leads to accurate size distribution estimates, at a far lower computational cost than solving the general dynamic equation and the fluid dynamics equations simultaneously. In the present paper, we investigate the effect of a variation in the liquid mass density on the droplet size distribution, using the proposed method. In case of a varying liquid mass density, both the equation for the dropltet growth rate and the moment equations are modified. This modified form coincides with the usual form of the moment equations in the event that the variation in liquid density is negligible. This research is relevant for condensation in flows where large temperature differences may occur which lead to significant variations in the liquid mass density. We show that the implementation of a variable liquid mass density in the Method of Moments and the Phase Path Analysis results in a higher extremum in the droplet size distribution, whereas the skewed shape of the distribution function is nearly similar to that obtained in the constant liquid density case.
Lagrangian Scalar Tracking for Laminar Micromixing at High Schmidt Numbers
Authors: Gobert Christian , Schwertfirm Florian , Manhart Michael
Published: September 2008

In many mixing processes the Schmidt number can easily reach very high values. In such cases the computation of the scalar field by Eulerian methods causes extremely high memory requirements. In the present study we circumvent this problem by adopting a Lagrangian particle tracking method to compute the scalar field in a laminar T-mixer configuration. The flow field is computed by direct numerical simulation (DNS). The movements of representative molecules are determined by the Langevin equation, describing convection by the flow-field and diffusion due to Brownian motion. The scalar field is computed by evaluation of the particle distribution. The particle density required for calculation of resolved concentration fields increases with increasing Schmidt number. For stationary flow regimes, the effective particle density could be increased via sampling in time. In the unsteady case, the effective density can be augmented by parallelization over the particles. No model is required in this approach, and concentration fields for very high Schmidt numbers can be computed. With this method is was possible to compute the fully resolved concentration field in a T-shaped micromixer at Schmidt number 3571 and Reynolds numbers 186 (steady) and 240 (unsteady). Schlu¨ter et. al. [CIT, 76(11), 2004] examined the stationary configuration experimentally. The agreement between numerical and experimental results is excellent. The developed method provides a possibility to compute the fully resolved scalar field in laminar flow regimes at a wide range of Schmidt numbers. It seems that in the cases under consideration Euler methods cannot provide such results. Therefore the proposed method represents a new basis for the prediction of mixing and the development of mixing models for high Schmidt number flows.
Near-Wall Velocity and Bubble Characteristics in Microbubble Laden Flows
Authors: Morse Daniel R. , Shepard Thomas G. , Liburdy James A.
Published: September 2008

Microbubble drag reduction has been observed in high Reynolds number turbulent flows. The interaction of microbubbles with the viscous sublayer seems to be of interest. In this study a microchannel on the order of 100 microns was used to simulate the shear rate of the near-wall region of a high Re flow. The interaction of the fluid flow with microbubbles at low void fraction in a microchannel was studied. The microbubble sizes ranged from approximately 10–50 microns. The liquid phase velocities were obtained by PIV measurement techniques. Electrolysis was used to generate bubbles within the channel and microbubble velocities in the flow were determined using separate cross correlation calculations. Simultaneous comparisons are made between the image-averaged bubble velocity and the image-averaged fluid velocity. Image processing techniques were utilized to both remove bubbles and decrease noise in the image. Results are shown comparing the fluid only velocity profile with the two-phase velocity profiles at three flow rates and two bubble generation cases. Results presented include the phase velocity differences, bubble size and bubble separation distances for three flow rates and three different bubble generation levels. It is seen that flow rates within the microchannel significantly reduce the average bubble size.
Merging Near-Wall RANS Models With LES for Separating and Reattaching Flows
Authors: Jakirlic´ Suad , Kniesner Bjo¨rn , Sˇaric´ Sanjin , Hanjalic´ Kemal
Published: September 2008

A method of coupling a low-Reynolds-number k–ε RANS (Reynolds-Averaged Navier-Stokes) model with Large-Eddy Simulation (LES) in a two-layer Hybrid LES/RANS (HLR) scheme is proposed in the present work. The RANS model covers the near-wall region and the LES model the remainder of the flow domain. Two different subgrid-scale (SGS) models in LES were considered, the Smagorinsky model and the one-equation model for the residual kinetic energy (Yoshizawa and Horiuti, 1985), combined with two versions of the RANS ε equation, one governing the “isotropic” (ε˜; Chien, 1982) and the other the “homogeneous” dissipation rate (εh; Jakirlic and Hanjalic, 2002). Both fixed and self-adjusting interface locations were considered. The exchange of the variables across the interface was adjusted by smoothing the turbulence viscosity either by adjusting the RANS model parameters, such as Cμ (Temmerman et al., 2005), or by applying an additional forcing at the interface using a method of digital-filter-based generation of inflow data for spatially developing DNS and LES due to Klein et al. (2003). The feasibility of the method was illustrated against the available DNS, fine- and coarse grid LES, DES (Detached Eddy Simulation) and experiments in turbulent flow over a backward-facing step at a low (Yoshioka et al., 2001) and a high Re number (Vogel and Eaton, 1985), periodic flow over a series of 2-D hills (Fro¨hlich et al., 2005) and in a high-Re flow over a 2-D, wall-mounted hump (Greenblat et al, 2004). Prior to these computations, the method was validated in a fully-developed channel flow at a moderate Reynolds number Rem ≈ 24000 (Abe et al., 2004).
IceCube CFD Drilling Model
Authors: Martin Richard A. , Thompson Tim , Ansari Naseem , Guetari Chokri
Published: September 2008

A numerical model of a hot water drill used to produce deep holes in clear ice at the South Pole for the IceCube neutrino observatory program scheduled for completion in 2010 has been developed. The model was built using the ANSYS commercial computational fluid dynamics (CFD) code, ANSYS CFX. This drill model is helping us to understand the water/ice melting process near the bottom of the drill hole, and to evaluate the influence of nozzle size, spray angle, water flow rate, and water temperature on the drill hole shape, and on drilling speed. The basis for the model is ANSYS CFX, which has multi-phase, conjugate heat transfer capabilities. The model utilizes a multi-phase approach, and simulates motion of the drill with respect to the ice. The sensitivity of model predictions to mesh resolution, turbulence model, and interfacial heat transfer coefficients, area, and drag coefficient was studied, and the results were used to determine preferred values in each case. This multi-phase model was selected after evaluating an equilibrium model and obtaining results showing not completely satisfactory comparisons to experimental data from the South Pole. Computations at a drill depth of 1292 m allowed validation of code results using actual field data obtained during the 2004–2005 IceCube drilling season at Antarctica. A series of steady-state runs using two drill sizes, two drill speeds, and one spray angle were performed for conditions at 1292 m to determine if a smaller nozzle orifice would enable faster drilling, preferably by a factor of two. The model predicted a drill hole diameter of from 18 in to 25 in and an up-flow water temperature of from 20°C to 28°C. The drill hole diameter is consistent with values measured at the IceCube site, but the water temperature is about 10°C low. No evidence of the nozzle tip impacting the bottom of the hole was found in the drill speed range 3.5 ft/min to 7 ft/min. A nozzle spray angle of 25 degrees was found to make little difference in hole depth or diameter. Reducing the nozzle diameter from 1 in to 0.75 in at the same water volumetric flow rate resulted in an increase in the drill hole depth by from 16% to 20%. The latter result implies that faster drilling is expected when using a smaller size orifice with zero degree spray angle. The IceCube drill model is now available to determine the effects of key variables, to evaluate the performance of new nozzle designs, and to specify drill speed versus depth. Recommendations specific to faster drilling speeds resulted in a near doubling of actual speed during the 2005–2006 season at the South Pole.
Critical Mass-Flow in Orifice-Nozzles at the Disintegration of Superheated Liquids
Authors: Wirth K.-E. , Rossmeissl M.
Published: September 2008

Flashing atomization is initiated when the superheated liquid is exposed to a sudden pressure drop below saturation vapor pressure at the prevailing temperature. Superheated atomization is used to enhance atomization where conventional pressurized liquid injection cannot achieve the desired fine sprays. Essentially flashing is initiated by in situ bubble nucleation followed by bubble growth. If the internal flow conditions are such that stable cavitation bubbles can form, then such bubbles promote the atomization of the base liquid already in the capillary of the nozzle. Bubble creation via cavitation can be achieved by changing the nozzle geometry such as the orifice length-to-diameter ratio, the inlet corner radius, the surface conditions of the nozzle material and the flow velocity in the orifice. In case of superheated atomization, the mass flow and thus the flow velocity in the orifice can just be indirectly set by the combination of superheat temperature upstream the nozzle and the difference between high pressure level upstream and ambient pressure downstream the orifice. Consequently the internal geometry and the material of the nozzle are of vital impact on the atomization process of the ejected liquid.
Installation Effects in Wind Ductwork
Authors: Lu¨ Yu-Kun , Zhang Jian
Published: September 2008

The installation effects of different installation forms in ductwork are simulated by FLUENT in this paper. The results show that the pressure losses have some relations with the distances between different components. With the increasing of the distances between the two components: the pressure loss will reduce firstly in the combination of bend and reducer, then increase continuously; the pressure loss will increase firstly in the combination of bend and increaser, then reduce continuously; the pressure loss increases firstly in the combination of tee and bend, then it reduce continuously, and the main reason of pressure loss generated in the combination of gate valve and bend is relative installation position. So, Installing components reasonable can save energy in system running.
Case Study of Impeller Profile With Suitable Rotative Guidevane
Authors: Tsugawa Takuji
Published: September 2008

In the previous paper, the optimum meridian profile of multi-stage impeller with guidevane was obtained by means of diffusion factor using twelve shape factors. The coefficient of peripheral absolute velocity at impeller inlet kCu1c means the effect of multi-stage impeller with guidevane. In this paper, the optimum meridian profile of multi-stage or single-stage impeller with rotative guidevane was obtained by means of fifteen shape factors. The fifteen shape factors mean impeller inlet relative flow angle β1, impeller turning angle Δβ, impeller axial velocity ratio kc12 = Cm2/Cm1, impeller diameter ratio kd12 = D1c/D2c, impeller outlet hub-tip ratio ν2, impeller tip solidity σt(imp), impeller mid span solidity σc(imp), impeller hub solidity σh(imp), guidevane tip solidity σt(gv), guidevane mid span solidity σc(gv), guidevane hub solidity σh(gv), guidevane axial velocity ratio kc34 = Cm4/Cm3, guidevane diameter ratio kd34 = D3c/D4c and rotational speed ratio Rn = ngv/nimp. The suitable rotational speed ratio means suitable rotational speed of guidevane. The axial velocity ratio and diameter ratio of guidevane means the optimum meridian profile of guidevane outlet. In the 15th dimensional optimum method, the hydraulic efficiency and suction specific speed are calculated by diffusion factor. This method is also used for the modification of multi-stage or single-stage present impellers and guidevanes.
The Effects of the Boundary Stencils on the Field Spatial Resolution When Using Compact Finite Differences
Authors: Jordan Stephen A.
Published: September 2008

When establishing the spatial resolution character of a composite compact finite differencing template for high-order field solutions, the stencils selected at non-periodic boundaries are commonly treated independent of the interior scheme. This position quantifies a false influence of the boundary scheme on the resultant interior dispersive and dissipative consequences of the compound template. Of the three ingredients inherent in the composite template, only its numerical accuracy and global stability have been properly treated in a coupled fashion. Herein, we present a companion means for quantifying the resultant spatial resolution properties. Compact boundary stencils with free parameters to minimize the field dispersion (or phase error) and dissipation are included in the proposed procedure. Application of the couples templates to Burgers equation at the non-periodic boundary showed significant differences in the predictive error.
Application of CFD to the Design of Centrifugal Pumps
Authors: Yedidiah S.
Published: September 2008

This paper presents results of tests performed on side-suction inlet-nozzles of centrifugal pumps. These results demonstrate, that there exists a need to revise our approach towards the adaptation of CFD to the design of centrifugal pumps. Also, it quotes a number of additional publications which discuss other aspects of the existing problem. The subject matter presented in this paper relates directly to the design of centrifugal pumps. However, there are indications, that similar problems may also exist in relation to the design of other fluids machines.
Modification of the Classical Theory of the Virtual Mass of an Accelerating Spherical Particle
Authors: Kendoush Abdullah Abbas
Published: September 2008

A semi-analytical solution to the virtual mass of a spherical particle accelerating at high speed is obtained. Boundary layer separation coupled with potential flow was assumed around the solid sphere. The new solution of the virtual mass coefficient CV converges to the original solution of 0.5 upon removing the separation. The author showed earlier (Kendoush, J. Appl. Mech. 72(801)2005) that the virtual mass coefficient could reach a value of five for a rotating sphere in fluids.
Effect of Divergence Angle on Flow Through Inlet Section of Diffuser of a Gas Turbine Combustion Chamber: The CFD Approach
Authors: Kulshreshtha Digvijay B. , Channiwala S. A. , Chaudhary Jitendra , Lakdawala Zoeb , Solanki Hitesh , Dikshit Saurabh
Published: September 2008

In the combustor inlet diffuser section of gas turbine engine, high-velocity air from compressor flows into the diffuser, where a considerable portion of the inlet velocity head PT3 − PS3 is converted to static pressure (PS) before the airflow enters the combustor. Modern high through-flow turbine engine compressors are highly loaded and usually have high inlet Mach numbers. With high compressor exit Mach numbers, the velocity head at the compressor exit station may be as high as 10% of the total pressure. The function of the diffuser is to recover a large proportion of this energy. Otherwise, the resulting higher total pressure loss would result in a significantly higher level of engine specific fuel consumption. The diffuser performance must also be sensitive to inlet velocity profiles and geometrical variations of the combustor relative to the location of the pre-diffuser exit flow path. Low diffuser pressure losses with high Mach numbers are more rapidly achieved with increasing length. However, diffuser length must be short to minimize engine length and weight. A good diffuser design should have a well considered balance between the confliction requirements for low pressure losses and short engine lengths. The present paper describes the effect of divergence angle on diffuser performance for gas turbine combustion chamber using Computational Fluid Dynamic Approach. The flow through the diffuser is numerically solved for divergence angles ranging from 5 to 25°. The flow separation and formation of wake regions are studied.
Convective Heat Transfer Intensification in Laminar Duct Flow
Authors: Olayiwola B. O. , Walzel P.
Published: September 2008

An experimental investigation was carried out to study the influence of pulsation and special surface geometry on the convective heat transfer in laminar flow. The experiments were performed using a glycerol-water mixture of 23 wt% glycerol. Ethanol was used as a coolant. The amplitude of pulsation was between 0.37 and 0.91 mm and the frequency range was 26.7 to 42.7 Hz. The mean flow Reynolds number range was between 50 and 1143. All the geometrical parameters of the channel such as the relative fin spacing and relative fin thickness were constant. The enhancement factor E, i.e. the ratio of heat transfer coefficient due to pulsation compared to steady flow diminishes at low Pe. A maximum E was observed in medium ranges of Pe and small Pe. The amount of heat transferred from the working fluid also depends on κ value. So far, a maximum heat transfer enhancement of E = 2.5 at κ = 3 and Pe = 2750 was obtained. The enhancement factor also increases with increasing pulsation amplitude.
A Two Phase Model of a Phosphoric Acid Doped PEM Fuel Cell
Authors: Cheddie Denver F. , Munroe Norman D. H.
Published: September 2008

A two-phase model of an intermediate temperature (120–200 °C) proton exchange membrane (PEM) fuel cell is presented. This model accounts for two phase effects due to gas solubility in the phosphoric acid/PBI electrolyte, and considers aqueous phase electrochemical reactions. It accounts for all polarization and transport phenomena, and shows a good fit with experimental data in the temperature range (150–190 °C). This paper investigates catalyst utilization in intermediate temperature PEM fuel cells with phosphoric acid doped membranes. Simulations show that, under normal operation, 1–2% of the catalyst is utilized at both electrodes. Strategies are suggested to help reduce the cost of producing power from such systems.
A Numerical Study of Turbulent Flow in Helical Static Mixers
Authors: Rahmani Ramin K. , Ayasoufi Anahita , Keith Theo G.
Published: September 2008

Many processing applications call for the addition of small quantities of chemicals to working fluid. Hence, fluid mixing plays a critical role in the success or failure of these processes. An optimal combination of turbulent dispersion down to eddies of the Kolmogoroff scale and molecular diffusion would yield fast mixing on a molecular scale which in turn favors the desired reactions. Helical static mixers can be used for those applications. The range of practical flow Reynolds numbers for these mixers in industry is usually from very small (Re ∼ 0) to moderate values (Re ∼ 5000). In this study, a helical static mixer is investigated numerically using Lagrangian methods to characterize mixer performance under turbulent flow regime conditions. A numerical simulation of turbulent flows in helical static mixers is employed. The model solves the three-dimensional, Reynolds-averaged Navier-Stokes equations, closed with the Spalart-Allmaras turbulence model, using a second-order-accurate finite-volume numerical method. Numerical simulations are carried out for a six-element mixer, and the computed results are analyzed to elucidate the complex, three-dimensional features of the flow. Using a variety of predictive tools, mixing results are obtained and the performance of static mixer under turbulent flow condition is studied.
Oscillating Pipe Flow: High-Resolution Simulation of Nonlinear Mechanisms
Authors: Ghasemi Arash
Published: September 2008

A new perspective suitable for understanding the details of nonlinear pumping (formation of traveling shocks) inside a pressurized cavity is constructed in this paper. Full compressible axisymmetric three-dimensional Navier-Stokes equations are used as the starting point to cover all complexities of the problem that exceedingly increase for particular ranges of Mach, Reynolds and Prandtl numbers. Then a very high-order numerical method is introduced to preserve the user-defined order of accuracy for practical simulations. For removal of spurious waves, higher-order compact filters are derived. All equations are marched in time using the classical Runge-Kutta algorithm which is appropriate for problems involving fine-scale temporal fluctuations. As the most important part of simulation, Navier-Stokes Characteristic Boundary Conditions are used for accurate calculation of wave reflection specially at singular points, i.e., corner points and points across the axis of symmetry. A simultaneous characteristic-decomposition is devised in this paper which completely resolves stability problems arising from problem-dependent treatment of corner points. Numerical experiments are performed for high-Reynolds laminar flows inside the shock region to determine the effect of frequency change on both shock formation (stationary flow) and transient solution. The current approach which favorably compares to the previous experimental data, may be used as a robust tool for understanding the less-understood problem of shock/Stokes-Layer interaction and its consequences on transition to turbulence in Oscillating Pipe Flow.
Mixed Convection Heat Transfer From a Square Cylinder to Power-Law Fluids in Cross-Flow
Authors: Anjaiah N. , Dhiman A. K. , Chhabra R. P.
Published: September 2008

Laminar mixed convection flow and heat transfer to power-law fluids from a square cylinder has been analyzed numerically in the steady flow regime. The full momentum and energy equations along with the Boussinesq approximation have been solved by using a SMAC implicit finite difference method implemented on an uniform staggered grid arrangement for the range of Reynolds number 5 to 40, power-law index 0.6 to 1.4, Prandtl number 1 to 10 and Richardson number 0 to 0.5 in both bounded and unbounded flow configurations. The wall effects have been studied for a fixed blockage ratio of 1/15. The effects of buoyancy on the flow and heat transfer characteristics of power-law fluids have been elucidated. It is found that the mixed convection can initiate an asymmetry in the flow and temperature fields even within the steady flow regime. The variation of drag coefficients, and of the Nusselt number have been reported for a range of values of the Reynolds number, Prandtl number and Richardson number for both shear thickening and shear thinning fluids.
PIV Investigation of Flow Behind Surface Mounted Detached Square Cylinder
Authors: Panigrahi P. K.
Published: September 2008

The flow field behind surface mounted detached square ribs over an approaching flat plate turbulent boundary layer has been studied. The Reynolds number based on the rib height has been set equal to 11075. The ratio of gap size from the flat plate surface to the square rib size has been varied between 0.2 and 1.0. The ratio of the approaching boundary layer thickness to the rib height is equal to 0.2. The PIV (2-component and stereo) technique in both stream wise and cross-stream measurement planes have been implemented. The PIV data has been acquired at two different resolutions. The high resolution measurements have been used to show the flow field at immediate downstream of the detached ribs. The oil flow visualization study has been carried out to relate the surface flow patterns to that of the flow structures. The mean and rms velocity field, average stream wise and span wise vorticity field, turbulent energy production and stream traces have been reported. The invariant of the velocity gradient tensor has been calculated to distinguish between the rotational and shear contribution of the vorticity field. The recirculation bubbles with foci like structure behind the detached ribs are displaced upward and its size drops with an increase in the gap size. The flow below the detached rib is film like flow for lower gap size leading to significant near wall modification of the flow structures. For higher gap size, the viscous effect predominates in the near wall region. The stream traces in the cross stream plane show additional node-saddle patterns in the near wall region indicating greater near wall flow structures and hence better mixing. The turbulence intensity, vorticity and velocity gradient tensor invariant results confirm the efficacy of the detached rib with smaller gap to cylinder size as an effective passive flow control tool for near wall mixing enhancement.
PIV Measurements of the Cross-Flow Velocity Field in the Near Wake of a Generic Pickup Truck
Authors: Khalighi Bahram
Published: September 2008

The cross-flow field (flow in planes normal to the direction of motion) in the near wake of a generic pickup truck is investigated experimentally using Particle Image Velocimetry (PIV). The PIV measurements of the velocity field normal to the free-stream direction are carried out at four stream-wise locations behind the cab and the tailgate. The PIV data are processed to obtain the instantaneous velocity field, the mean and the turbulence properties of the flow. The instantaneous results in the near wake of the cab show various vortical structures. The mean velocity data shows that the flow moves from the sides toward the center of the bed near the tailgate. The velocity data in the near wake of the tailgate shows a pair of counter-rotating vortices that induces a downwash velocity field at the symmetry plane. This downwash promotes an attached flow behind the tailgate, thus generating a pressure recovery which leads to reductions in the total drag.
Detached Eddy Simulation for Model Hydro Turbine
Authors: Wu Xiaojing , Liu Shuhong , Wu Yulin
Published: September 2008

In this paper, detached eddy simulation method is applied to the numerical simulation for whole passage of a model hydro turbine. The method combines the strong points of Reynolds-averaged Navier-Stokes and Large eddy simulation. In this model, Spalart–Allmaras turbulent model is improved, which reduces to a RANS formulation near a solid surface and to a subgrid model away from the wall. The hexahedron type mesh is used to divide the model, which can decrease the mesh scale and computation cost. In this paper, a unsteady turbulent simulation is done for model hydro turbine with this viscous model. The internal flow, vortex motion and pressure fluctuation inside hydro turbine can be studied from the result, which are also compared with the experiment data. It can be seen that this method can describe the complex flow of the turbine well while the mesh density is not very high.
Modeling of Dam-Break Flows With Conservative Level Set Method
Authors: Sˇtrubelj Luka , Tiselj Iztok
Published: September 2008

Conservative level set method, introduced by E. Olsson in 2005 is discussed. Calculations of dam-break on dry and wet surface were performed. The method was found to be appropriate for modeling of the dam-break flows.
Numerical Performances of Explicit Cartesian Methods for Compressible Moving-Boundary Flow Problems
Authors: Hsu Kwen
Published: September 2008

Cartesian method, which in some places is mentioned as Volume-Tracking Method, is one of the popular methods used in simulating transient flow problems involving complex moving boundaries. It has the advantage of being time-saving, efficient and robust for certain types of fluid-structure interaction problems. This method is featured by a Cartesian background Euler mesh discretizing the flow domain and a moving surface cutting through it. The most critical operation of this method is treating the cells cut by the moving boundaries accurately and stably. When the Cartesian methods are applied, the temporal discretization of the governing equations of the flow can be either implicit or explicit. For simulations cases in which time-accurately capturing wave propagation or flow evolution is essential, explicit approach still plays an important role among the researchers and currently available simulation codes. The current study is focused on the numerical performances of the explicit type of Cartesian methods when applied on the compressible flow cases. The accuracy of the simulation results, stability and grid-convergence problems resulted from a moving, impermeable boundary cutting through the background mesh are addressed. Example problems include the one-dimensional piston problem and the expanding sphere flow problems. In one case the sphere expands supersonically thus a spherical shock is generated. In another case it expands at a subsonic speed and works as a monople impulse noise source. To the best knowledge of the author, the problem of expanding-sphere generated acoustic impulse has not been reported anywhere else. Simple theoretical analyses are included and results of numerical experiments are reported.
Velocity Distributions in (r,θ) Directions for Laminar Flow of a Film Around Horizontal Circular Tube
Authors: Abyaneh M. H. J. , Saidi M. H.
Published: September 2008

Velocity distributions in (r,θ) directions are evaluated by solving simultaneous simplified Navier-Stokes equations (NSE) and continuity equation (CE) in polar coordinate. The analysis is based on steady state laminar flow of thin falling liquid film on a horizontal circular tube, for cases in which traction on the film surface is considered negligible. It is a common geometry for part of engineering problems such as evaporator, condenser, absorber, generator of absorption chillers and other similar units in mechanical and chemical engineering. Knowledge of the velocity profiles is usually needed for: 1- solving governing energy and species equations 2- estimating the average and film surface velocity, and 3- evaluating film thickness distribution and its gradient. Two models of velocity distributions are considered, namely actual model and simplified model. Models are compared not only with each other but also with semi actual model in (x,y) coordinate given in the literatures. The average and film surface velocity profiles and film thickness distribution for these models have been shown in various conditions. The results clearly show that the larger flow rates and/or smaller tube diameter increases the calculation error.
Global Laser Rangefinder Profilometry: Initial Test and Uncertainty Analysis
Authors: Carneal Jason B. , Atsavapranee Paisan
Published: September 2008

Global Laser Rangefinder Profilometry (GLRP) is a novel optical technique for instantaneous measurement of complex three-dimensional surfaces. A functional GLRP system has been constructed in the Maneuvering and Seakeeping Basin (MASK) at the Naval Surface Warfare Center, Carderock Division (NSWCCD). The system is capable of measuring surface height displacements over 800 measurement points at 30 Hz. The MASK GLRP system was used to measure the surface profiles of large waves produced by wave-makers in the MASK and bow waves generated by a surface ship remote-controlled model (RCM). Several large wave measurements were performed at various wave heights and compared to sonic probe measurements. The large wave measurements were found to be consistent with sonic probe measurements to within 5%. The results from the large wave measurements and RCM model bow wave measurements are presented and discussed. Data was collected in calm water to quantify sources of error, including optical jitter. The random error of the GLRP system is estimated at approximately 1.6 mm. The purpose of this work was to test the ability of the GLRP system for use in tests commonly performed at NSWCCD.