bluuebunny/crossref_metadata_2025 huggingface dataset:
Development of an Operation Point Detection System for Centrifugal Pumps by Classifying the Time Signal of a Single Vibration Sensor
Authors: Licht Christian , Böhle Martin
Published: December 2014

In industry it is very common that pumps are not operating in their optimal operation point. The consequences are too expensive operation costs and higher loads on individual components of the entire plant. For this reason a method to detect the current operation point of a centrifugal pump has been developed.

The object is to introduce this method during the operation and without changing anything in the system. This method focuses on the monitoring of the vibrations of different operation points of the pump curve through the analysis of time signals of a single vibration acceleration sensor. To identify the current operation point the vibrations of an unknown point are compared to the vibrations of the known points. Extensive laboratory tests have been conducted for this contribution. For implementation of these investigations a test loop was designed. In the first step different time signals from several pump curves have been recorded. The recorded time signals are reduced to a few succinct values, by means of the method of time signal analyses. To classify these values the so called support vector machine is used.
Inflow Conditions and the Flow Behavior of a Biofluid in a Separated Flow Region
Authors: Hammad Khaled J.
Published: December 2014

Inflow conditions influence on the flow behavior of human blood in a separated and reattached flow region is investigated. Hemorheological data that account for the yield stress and shear-thinning non-Newtonian characteristics of blood are used. The governing mass and momentum conservation equations along with the Herschel-Bulkley constitutive equation are solved numerically using a finite-difference scheme. Two inflow velocity profiles are considered, uniform and fully-developed ones. A parametric study is performed to reveal the impact of inflow velocity profile, upstream flow restriction, and rheology on the recirculation strength and reattachment characteristics of the flow field. Uniform inflow conditions result in larger relative recirculation intensity, in comparison with the fully-developed ones, only for a moderate upstream flow restriction. The separated flow region size in the case of a fully-developed inflow is always smaller than the one observed for uniform inflow. Larger separated flow regions with stronger flow recirculation, are predicted by the Newtonian model in comparison with the yield shear-thinning model for all studied inflow and upstream restriction conditions. The separated flow region size displays a strong dependency on the inflow velocity profile and upstream flow restriction, in comparison with the observed dependency on the used hemorheological model.
Analysis of the Interplay Between Sedimentation and Thermophoresis in the Presence of Convection in Colloidal Suspensions
Authors: DarAssi Mahmoud , Hadji Layachi
Published: December 2014

We report results of an analytical investigation of linear convection in a nanofluid. We consider a colloidal suspension of solid particles in a Rayleigh-Bénard geometry set-up. The analysis is confined to the mass-dominated convection regime so that results are obtained through analytical means. Our findings depict the dependence of the critical conditions for convection onset as function of several parameters. Thus, the influence of several factors, such as the particle size, the mean volume fraction of particles, the thermophoretic force as well as the sedimentation force, on the instability onset is quantified.
Two-Phase Pressure Drop Prediction in Wet Gas Flow Through V-Cone Meter
Authors: He Denghui , Bai Bofeng
Published: December 2014

The pressure drop is considerably significant for the differential pressure meter to measure the flow rate of the two-phase flow. Little is known about the pressure drop characteristics of the V-Cone meter when it is used to measure the wet gas flow. The objective of this paper is to investigate the two-phase pressure drop of the V-Cone meter and develop a correlation for predicting its pressure drop. A V-Cone meter with the equivalent diameter ratio of 0.55 was investigated experimentally. The experimental fluid was air and water. The test pressure ranged from 0.1 MPa to 0.4 MPa, and the gas and liquid mass flow rate ranged from 100 Nm3/h to 500 Nm3/h and from 0.05 m3/h to 2.2 m3/h, respectively. The experimental results showed that the existing correlations, which are developed for the orifice plate meter and the Venturi meter, are not applicable for the V-Cone meter to predict the pressure drop. The two-phase mass flow coefficient, K, was used to develop the two-phase pressure drop correlation. The influences of the Lockhart-Martinelli parameter, the gas densiometric Froude number and the operating pressure on K were investigated. The new pressure drop correlation can accurately predict the pressure drop of the V-Cone meter for the wet gas. The relative error of the pressure drop is less than ± 9.0% at the 95.1% confidence level and the average relative error is 3.88%. The pressure drop prediction correlation provides a reference for developing the correlation of the wet gas measurement.
Analyze of the Size of Secondary Droplets due to Film Separation at the Blade Trailing Edge
Authors: Dohmen Hans Josef , Benra Friedrich-Karl , Schepers Cornelius , Schuster Sebastian
Published: December 2014

In many turbo machinery applications the working fluid is a mixture of a gas and a liquid. In such two-phase flows the liquid phase can be assumed to be a dispersed droplet flow. As the fluid passes the blades, a part of the droplets is deposited on the blades and forms a thin liquid film. At the trailing edge the thin liquid film separates and forms secondary droplets.

In this paper the ability of numerical investigations to study the size of secondary droplets is analyzed. In a first step a program solving the Thin Film Equations is used to analyze the film flow on the inlet guide vanes of an axial compressor. Secondly a CFD study with a Free Surface Model is performed to analyze the film stripping process. The ability of the Free Surface model to investigate film separation is validated against third party experimental results and the pro and contras of the Free-Surface model are presented. In the last step the stability of the formed droplets is investigated and the final droplet size is calculated.
A CFD Study of Existing Drag Models for Geldart A Particles in Bubbling Fluidized Beds
Authors: Estejab Bahareh , Battaglia Francine
Published: December 2014

In this study, seven drag models are examined to determine how they affect fluidization behavior of Geldart A particles of biomass and coal. Notwithstanding the notable number of numerical studies to find the best drag model for larger particles, there is a dearth of information related to drag models for finer Geldart A particles. Additionally, to our knowledge, these drag models have not been tested with a binary mixture of Geldart A particles. Computational fluid dynamics was used to model the gas and solid phases in an Eulerian-Eulerain approach to simulate the particle-particle interactions of coal-biomass mixtures and compare the predictions with experimental data. In spite of the previous findings that bode badly for using predominately Geldart B drag models for fine particles, the results of our study reveal that if static regions of mass in the fluidized beds are considered, these drag models work well with Geldart A particles. It was found that the seven drag models could be divided into two categories based on their performance. One category included the Gidaspow family of drag models (Gidaspow, Gidaspow-Blend, and Wen-Yu) and the Syamlal-O’Brien drag model; these models closely predicted the experiments for single solids phase fluidization. For binary mixtures, however, the other drag model group (BVK, HYS, Koch and Hill) yielded better predictions.
Analysis of New Multi Cellular Vortex Model Relevance to Atmospheric and Aerodynamic Vortices
Authors: Eshita Israt Jahan , Vatistas Georgios H.
Published: December 2014

In the present paper, we are presenting a new mathematical vortex model, which is capable of simulating single and multi-celled steady, incompressible, intense vortices. The solution is obtained using the MATLAB and Maple 14 software. The new methodology is shown to fairly correlate the actual data of naturally occurring and industrial vortices.
Experimental and Numerical Studies of Pump Transient Characteristics During Stopping Period
Authors: Wu Peng , Wu Dazhuan , Wang Leqin
Published: December 2014

Experimental and numerical studies were carried out to study the transient characteristics of the centrifugal pump during stopping periods. Different stopping schemes were realized by changing the rotational inertia of the flywheels. Transient revolution rotational speed, flow-rate, total pressure rise and torque were measured under different rotational inertia. Experimental results of different operating conditions were compared, and transient hydrodynamics performances of the centrifugal pump model were analyzed. In order to provide boundary conditions for numerical simulations, the revolution curves of the flow-rate and the rotational speed were polynomial fitted. Three dimensional unsteady incompressible viscous flows during stopping periods were studied by using DES model in FLUENT. Results show that the hydrodynamic characteristics of numeric and experiment agree well. The transient effect is not so evident, and the quasi-steady assumption is acceptable during most part of coastdown process. The pump characteristics are further explained by analyzing the relative velocity on the middle stream surfaces. At the end of the stopping period, the transient vortex evolution between blades is the main reason why the transient curves deviates the steady curve. The studies can help understand the operating characteristics of centrifugal pump when power failure accident occurs.
Experimental Modeling of Heat Transfer in a Continuous Casting Mould Model
Authors: Kalter R. , Righolt B. W. , Kenjereš S. , Kleijn C. R. , Tummers M. J.
Published: December 2014

Temperature distributions in a thin continuous casting mould model have been studied experimentally, using water as a working fluid. The mould model consists of two narrow walls and two broad walls. One of the broad walls of the mould model was cooled with cooling water of a fixed temperature. Inflow of two turbulent jets with a constant high temperature was from a bifurcated nozzle, submerged to a depth of 0.1 m below the air/water interface.

The temperature drop over the mould was measured as a function of the temperature difference between the liquid flowing into the mould and the cooling water temperature. From these measurements the overall heat transfer coefficient and heat transfer coefficient due to convection in the mould were calculated.

Temperature distributions at the cooled wall have been measured using thermochromic liquid crystal sheets, which have a specific color depending on the temperature. The shear layers of the two jets hit the cooled wall, leading to hot spot formation. The jets show a self-sustained oscillating behavior, leading to a non stationary temperature distribution at the cooled wall. Between the jets and the air/water interface, recirculation zones occur where the liquid cools down significantly, leading to large wall temperature differences in the mould.
Flow Over a Blunt Disk With Incidence: Location of Stagnation Point and Flow Separation
Authors: Helcig Christian , Uhkoetter Stephan , aus der Wiesche Stefan
Published: December 2014

The flow over a blunt disk placed in a stream of air was investigated by means of detailed velocity measurements employing Laser-Doppler-Anemometry (LDA). Special attention was spent to the effect of incidence. This parameter governed the location of the stagnation point and the length of the separation bubble. For a parallel disk with finite thickness, a large separation bubble was formed at the leading edge, followed by reattached regions of the turbulent flow. With increasing incidence, the length of the separation bubble significantly decreased, and the stagnation point moved from the blunt side to the leading edge. For angle of incidence higher than a special transition value, a stagnation flow over the disk surfaces resulted without significant turbulence. In case of a heated disk, the corresponding transition of the mean convective heat transfer coefficient as function of incidence was very sharp and discontinuous.
Digital Image Processing Algorithm for Determination and Measurement of In-Focus Spherical Bubbles
Authors: Xu Chong , Shepard Thomas
Published: December 2014

A novel image processing algorithm has been designed in MATLAB to analyze digital images of spherical bubbles which are backlit. The algorithm possess a series of steps to determine which bubbles are in focus, remove out of focus bubbles and record measurements of bubble size and location. Specifically, the algorithm relies on self-adaptive user determined thresholds, gradient detection, correlation determination, and multiple confirmation strategies. The results are capable of handling variations in bubble appearance due to inconsistent background and overlapping. This paper describes the details of the algorithm and its use in recording bubble measurements from images of varying bubble size and bubble number density. Results from the algorithm as compared with bubble measurements from the same image sets determined via the manual line drawing method.
Investigation of the Self-Priming Process of Self-Priming Pump Under Gas-Liquid Two-Phase Condition
Authors: Li Guidong , Wang Yang , Yin Gang , Cui Yurui , Liang Qihong
Published: December 2014

The self-priming process of self-priming pump is gas-liquid two-phase flow process with complex internal transient flow. The effect of gas-liquid mixing and separating performances will play a crucial role during self-priming process. In order to study the self-priming process and improve the self-priming performance, the model named JETST-100 was selected. Based on Eulerian-Eulerian multiphase flow model, transient numerical simulation of the gas-liquid mixing and separating phenomena on the pump chamber was carried out by using CFX software. The distributions of velocity vector, contours of gas volume fraction and liquid velocity of the pump or jet aerator, and the change of the gas volume fraction by monitoring points on the jet aerator were obtained. Test and simulation results show that the gas-liquid two-phase flow from the guide vane will form a larger velocity circulation, so that the gas-liquid separation is not sufficient. And the volume fraction of liquid inside pump chamber decrease with a large amount of water enters the outlet pipe. It is found that adding the baffle plate can prevent the generation of circulation on guide vane back, the gas-phase volume fraction of jet aerator inlet and nozzle outlet decreases, improve the self-priming performance of the pump.
Development of High-Efficiency Half-Ducted Propeller Fan for Packaged Air Conditioners
Authors: Iwase Taku , Kishitani Tetsushi
Published: December 2014

We developed a high-efficiency half-ducted propeller fan to reduce the electric power consumption of the outdoor unit of a packaged air conditioner by using a design tool combining computational fluid dynamics (CFD) with multi-objective optimization techniques based on a genetic algorithm (GA). The baseline fan was a half-ducted propeller fan with three blades of a currently available product. Blade shape was defined using 16 design variables including inlet and outlet blade angles, setting angles, blade length, sweep angles, dihedral angles, and so on. An in-house program was used to automatically generate the grids for CFD calculation. The objective functions were static pressure efficiency and fan noise level for optimization. The fan noise was calculated with an aerodynamic noise prediction model that used the relative inlet and outlet velocities of the fan blades from the CFD results. We found there was a trade-off relationship between the static pressure efficiency and the fan noise. We then selected the optimized fan that had the same noise level as the baseline fan but with an improved static pressure efficiency. The blade tip of the optimized fan was curled toward the suction side direction. Finally, we confirmed through experiments that the static pressure efficiency of the optimized fan was increased by 1.6% compared to the baseline fan.
Optimization of Axial Thrust Balancing Swirl Breakers in a Centrifugal Pump Using Stochastic Methods
Authors: Lefor Dominik , Kowalski Jan , Kutschelis Boris , Herbers Thomas , Mailach Ronald
Published: December 2014

Hydraulic axial thrust balancing in single-stage radial pumps is a frequently applied procedure to reduce the bearing load to a reasonable level. It leads to considerable efficiency loss, which gives reason to investigate the optimization potential of the common balancing methods. This paper’s focus is on so-called casing ribs, which are used in the default design of an examined industrial pump. Radial vanes on the casing wall of the front impeller side chamber work as swirl breakers by decreasing rotating flow whereby the static pressure at the shroud increases and counteracts the resulting axial thrust. The objective is to retain the reduction of axial thrust and to improve the internal efficiency simultaneously.

Therefore a CFD model of the industrial radial pump is created with Ansys CFX. Sufficient numerical quality is ensured whereby consistency is verified by a mesh study. The model is validated by integral values of the characteristic curve and axial thrust measurements as well as by experimental transient static pressure measurement at different locations of the pump flow. Probes are placed in the suction port, the volute and the impeller side chambers, where most balancing methods are implemented.

Since the side chamber contains a complex flow, the effect of geometry changes is hard to predict. For this reason a stochastically based sensitivity analysis using a comprehensively parameterized geometry of the front side chamber domain with the included casing ribs is carried out. For this purpose 110 design points are calculated and evaluated with support of the software Optislang. Correlations of parameters are suggested and important parameters regarding the objective are identified. Some reasonable model simplifications are conducted to reduce the computational time. According to the acquired findings a local optimization is executed using the best sample of the sensitivity analysis as start design. An evolutionary algorithm method determines a best design with an efficiency improvement of 0.26 percentage points. It is discussed in detail conclusively.
A Numercial Investigation of the Skin Friction Coefficient and Nusselt Number in a Laminar Wall Jet That Evolves From a Parabolic Velocity Profile to its Self-Similar Behavior
Authors: Issa Johnny , Ortega Alfonso
Published: December 2014

A finite volume numerical approach is used to study the steady, laminar, plane wall jet that evolves from a parabolic velocity profile with uniform temperature to its self-similar behavior downstream of the jet exit. A variety of Reynolds numbers ranging between 50 and 250 is considered in this numerical investigation. The working fluids are air and water with constant physical properties corresponding to Prandtl number of 0.712 and 7 at ambient conditions. In these types of flows, a developing region over which the flow converges to its self-similar behavior is observed in the vicinity of the jet exit. The location of the dimensionless virtual origin, which is of main importance in determining the flow field in the self-similar region, is carefully studied and correlated as a function of Reynolds number. The local skin friction coefficient is observed to converge to the analytical self-similar solution at downstream locations. Given that an analytical solution for the thermal behavior of this problem doesn’t exist in either the developing or self-similar regions, the thermal solution of this problem is studied for isothermal and uniform heat flux boundary conditions at the wall. The idea of a dimensionless thermal virtual origin is introduced and correlated as a function of Reynolds number. The Nusselt number dependence on Prandtl number, Reynolds number and the downstream location are obtained for both thermal boundary conditions at the wall.
Effect of Fluid Properties on Droplet Generation in a Microfluidic T-Junction
Authors: Loizou Katerina , Wong Voon-Loong , Thielemans Wim , Hewakandamby Buddhika
Published: December 2014

Over the last decade, significant work has been performed in an attempt to quantify the effect of different parameters such as flowrate, geometrical and fluid characteristics on the droplet break up mechanism in microfluidic T-Junctions. This demand is dictated by the need of tight control of the size and dispersity of the droplets generated in such geometries. Even though several researchers have investigated the effect of viscosity ratio on both the droplet break up mechanism as well as on the regime transition, fluid properties have not been included in most scaling laws. It is therefore evident that the contribution of fluid properties has not been quantified thoroughly. In the present work, the effect of fluid properties on the volume of droplets generated in a microfluidic T-junction is investigated. The main aim of this work is to examine the influence of viscosity of both the dispersed and continuous phase as well as the effect of interfacial tension on the size of droplet generated along with the break up mechanism. Three different oils have been utilised as continuous phase in this work to enable investigation of the effect of viscosity of the continuous phase with experiments performed at constant Capillary numbers. Various glycerol weight percentages have been employed to vary the viscosity of the dispersed phase fluid (water). Lastly, the effect of interfacial tension has been explored using two of the oils at constant μcUc (viscous force term). High speed imaging has been utilised to visualise and measure the volume of the resulting droplets. The viscosity ratio (viscosity of dispersed phase over viscosity of continuous phase) between the two phases appears to affect the droplet generation mechanism, especially for the highest viscosity ratio employed (mineral oil-water system) where the system behaves in a noticeably different way. Influence of interfacial tension is also noticeable even though less evident. In terms of the effect of viscosity of dispersed phase on the droplet generation a small difference on the volume of the droplets generated in olive oil glycerol systems is also reported. In an attempt to enumerate the effect of fluid properties on the droplet generation mechanism in a microfluidic T-junction, this paper will present supporting evidence in detail on the above and a comparison of the findings with the existing theories.
Experimental Study of Reynolds Number Effects on Three-Dimensional Offset Jets
Authors: Durand Zacharie M. J. , Clark Shawn P. , Tachie Mark F. , Malenchak Jarrod , Muluye Getnet
Published: December 2014

The effect of Reynolds number on three-dimensional offset jets was investigated in this study. An acoustic Doppler velocimeter simultaneously measured all three components of velocity, U, V and W, and turbulence intensity, urms, vrms, and wrms, and all three Reynolds shear stresses, uv, uw, and vw. Turbulent kinetic energy, k, was calculated with all three values of turbulence intensities. Flow measurements were performed at Reynolds numbers of 34,000, 53,000 and 86,000.

Results of this experimental study indicate the wall-normal location of maximum mean velocity and jet spread to be independent of Reynolds number. The effects on maximum mean velocity decay are reduced with increasing Reynolds number. Profiles of mean velocities, U, V and W, turbulence intensities, urms, vrms, and wrms, and turbulent kinetic energy, k, show independence of Reynolds number. Reynolds shear stress uv was independent of Reynolds number while the magnitude of uw was reduced at higher Reynolds number.
CFD Study of Hydrodynamics and Separation Performance of a Novel Crossflow Filtration Hydrocyclone (CFFH)
Authors: Motin Abdul , Tarabara Volodymyr V. , Bénard André
Published: December 2014

This research addresses various hydrodynamic aspects and the separation performance of a novel cross-flow filtration hydrocyclone (CFFH) using computational fluid dynamics. A CFFH is a device that combines the desirable attributes of a cross-flow filter and a vortex separator into one unit to separate oil from water. The velocity and pressure fields within the CFFH are estimated by numerically solving the filtered Navier-Stokes equations (by using a Large Eddy Simulation (LES) approach). The Lagrangian approach is employed for investigating the trajectories of dispersed droplets based on a stochastic tracking method called the Discrete Phase Model (DPM). The mixture theory with the Algebraic Slip Model (ASM) is also used to compute the dispersed phase fluid mechanics and for comparing with results obtained from the DPM. In addition, a comparison between the statistically steady state results obtained by the LES with the Wall Adaptive Local Eddy-Viscosity (WALE) subgrid scale model and the Reynolds Average Navier-Stokes (RANS) closed with the Reynolds Stress Model (RSM) is performed for evaluating their capabilities with regards to the flow field within the CFFH and the impact of the filter medium. Effects of the Reynolds number, the permeability of the porous filter, and droplet size on the internal hydrodynamics and separation performance of the CFFH are investigated. Results indicate that for low feed concentration of the dispersed phase, separation efficiency obtained based on multiphase and discrete phase simulations is almost the same. Higher Reynolds number flow simulations exhibit an unstable core and thereby numerous recirculation zones in the flow field are observed. Improved separation efficiency is observed at a lower Reynolds number and for a lower permeability of the porous filter.
Parametric Study of Churn Flow in Large Diameter Pipes
Authors: Hewakandamby Buddhika N. , Kanu Aime U. , Azzopardi Barry J. , Kouba Gene
Published: December 2014

Two phase flow in vertical risers are common place in oil and gas industry and many other process industries. Depending on the flow rates of the phases, there could be several flow patterns could exist inside the riser. These could vary from bubbly flow to annular flow with slug and churn flow in between. When the liquid phase flow rate is higher the bubbly flow exists while the annular flow is dominated by higher gas flow rate that forms a distinct gas core in the middle of the vertical riser. Of these flow regimes, churn flow is of particular interest as it is not well understood. The paper will report findings of an experimental campaign investigating the development of churn flow.

Experiments were carried out in a closed loop flow facility with a 127 mm ID, 11 m long vertical test section. The maximum flow rates achievable in the system were 17 and 1.2 m/s for gas and liquid phases respectively. Compressed air was used as the gas phase while water and water/glycerol mixtures were used as the liquid phase. The mixtures of water and glycerol were used to investigate the influence of the viscosity on the flow regime investigated. The flow was investigated using a Wire Mesh Sensor (WMS), an intrusive measurement device that can map the cross sectional distribution of phases. Void fraction measurements were made at several axial locations for a number of flow rate combinations from onset of churn flow until it turns into annular flow.

A region in flow rates where large liquid ligaments (wisps) suspended in the gas core was found and the breakup mechanism has been observed. Furthermore, huge waves were observed in this region. Analysis of results shows that the frequency of both huge waves and the wisps entrained in the gas core increase along the axial distance. The changes to the flow behaviour with the increase of viscosity and other findings will be presented in detail in the paper.
Effects of Polymer Additives on Intermittency in Forced Homogeneous Isotropic Turbulence
Authors: Wang Lu , Wang Yue , Cai Wei-Hua , Li Feng-Chen
Published: December 2014

The drag-reducing phenomenon in forced homogeneous isotropic turbulence (FHIT) with polymer additives was realized in large eddy simulation (LES) results, which causes the variation of turbulent characteristics. Study on intermittency of turbulence is of great importance in investigating turbulent drag-reduction mechanism, because the intermittency has close relationship with coherent structures (CSs) and the transfer of energy in turbulent flows. In the present work, the influences of polymers on intermittency in FHIT were analyzed in detail by extracting CSs, researching the flatness factor based on high-order correlation function of velocity derivative and wavelet transform, surveying local intermittence measure, and discussing four rotational invariants consisting of velocity-strain tensor and vorticity tensor. From the viewpoint of the results, it can be perceived that the intermittency occurs in both the Newtonian fluid and polymer solution flows; moreover polymer additives behave inhibitive effect on the intermittency in turbulent drag-reducing flows.
Selection of Surface Reflectivity for a Solar Cavity Receiver
Authors: Tu Nan , Wei Jinjia , Fang Jiabin
Published: December 2014

The thermal performance of a saturated steam solar cavity receiver was numerically studied. In order to improve the uniformity of heat flux and wall temperatures on the boiling tubes, the proper selection method of the surface reflectivity for the boiling tubes and the other cavity walls was found. The uniform surface reflectivity of the boiling tubes can only obtain the highly non-uniform heat flux and wall temperature distributions. When the surface reflectivity of the boiling tubes is selected according to the non-uniform distribution of heat flux, the wall temperatures and the heat flux on the boiling tubes appear much more uniform. And when the surface reflectivity of the other cavity walls is selected as high as possible, the thermal efficiency of the receiver would be improved. Meanwhile, the increasing surface reflectivity of the cavity walls has slight effect on the heat flux and wall temperature distributions of the boiling tubes.
An Extensive Study on LES, RANS and Hybrid RANS/LES Simulation of a Narrow-Gap Open Taylor-Couette Flow
Authors: Friess Christophe , Poncet Sébastien , Viazzo Stéphane
Published: December 2014

The present paper concerns a numerical benchmark of various turbulence modelings, from RANS to LES, applied to Taylor-Couette-Poiseuille flows in a narrow gap cavity for six different combinations of rotational and axial Reynolds numbers. Two sets of refined Large-Eddy Simulation results, using the WALE and the Dynamic Smagorinsky subgrid scale models available within an in-house code based on high-order compact schemes, hold for reference data. The efficiency of a RANS model, the Elliptic Blending Reynolds Stress Model (EB-RSM) [1], and a hybrid RANS/LES method, the so-called “Equivalent DES” [2], both run with Code Saturne, is then questioned. Thin coherent structures appearing as negative (resp. positive) spiral rolls are obtained by the LES but also the hybrid RANS/LES along the rotor (resp. stator) sides. More quantitatively, the hybrid RANS/LES does not improve the predictions of the EB-RSM for both the mean and turbulent fields, stressing the need for further theoretical development.
Hybrid RANS–LES Simulation of Wingtip Vortex Dynamics
Authors: Kolomenskiy Dmitry , Paoli Roberto , Boussuge Jean-François
Published: December 2014

This paper presents a feasibility study of a hybrid RANS–LES approach to numerical simulation of aircraft wing-tip vortices. A NACA 0012 wing is considered for which earlier published experimental and numerical data are available. Mesh sensitivity tests of our RANS solver and comparisons between two different turbulence models indicate that the RANS approach adequately describes the flow upstream from the trailing edge, but overestimates the rate of decay of the wing-tip vortex. A hybrid RANS–LES method is presented that results in a better agreement with the wind tunnel experiment, hence this approach is suggested for numerical simulation of the wake of an airliner.
Chaotic Heat Transfer in a Laminar Pulsating Flow With Constant Wall Temperature
Authors: Karami Mohammad , Jarrahi Mojtaba , Habibi Zahra , Shirani Ebrahim , Peerhossaini Hassan
Published: December 2014

The correlation between heat transfer enhancement and secondary flow structures in laminar flows through a chaotic heat exchanger is discussed. The geometry consists of three bends; the angle between curvature planes of successive bends is 90°. Numerical simulations are performed for both steady and pulsating flows when the walls are subjected to a constant temperature. The temperature profiles and secondary flow patterns at the exit of bends are compared in order to characterize the flow. Simulations are carried out for the Reynolds numbers range 300≤Re≤800, velocity amplitude ratios (the ratio of the peak oscillatory velocity component to the mean flow velocity) 1≤β≤2.5, and wall temperatures 310 ≤ Tw(K) ≤ 360. The results show that in the steady flow, heat transfer enhancement occurs with increasing Reynolds number and wall temperature. However, heating homogenization becomes almost independent of Reynolds number when homoclinic connections exist in the flow. Moreover, at high values of wall temperature, heat transfer enhancement is greater than mixing improvement due to the presence of homoclinic connections. In the pulsating flow, Nusselt number improves with β, and β≥2 is a sufficient condition for heat transfer enhancement. The formation and development of homoclinic connections are correlated with the heating homogenization.
Turbulent Flows Over Rough Forward Facing Steps
Authors: Shao Weijie , Agelin-Chaab Martin
Published: December 2014

This paper reports an investigation of the effects of rough forward facing steps on turbulent flows. The surfaces of the rough steps were covered with sandpapers. A particle image velocimetry technique was used to conduct measurements at the mid-plane of the test section and at several locations downstream to 68 step heights. A Reynolds number of Reh = 4800 and δ/h = 4.7 were employed, where h is the mean step height and δ is the incoming boundary layer thickness. The results indicate that mean reattachment length decreases with increasing roughness. In addition, the effect of the step roughness decreases with downstream distance. The proper orthogonal decomposition results showed that the step roughness affects even the large scale structures. Furthermore, the reconstructed turbulence quantities suggest that the step roughness suppresses the large scale turbulence.
Modeling on Hydrodynamic Coupled FCC Reaction in Gas-Solid Riser Reactor
Authors: Zhang Bo , He Pengfei , Zhu Chao
Published: December 2014

The fluid catalytic cracking (FCC) riser reactor consists of a bottom section of liquid feed injection and vaporization and an upward straight riser of vapor-catalysts transport and reaction. The product yield, obtained at the top of riser, is an accumulative result of liquid feed injection, vaporization by liquid contacting with hot catalysts, and subsequent catalytic cracking of feed vapor while being transported concurrently with catalysts through the riser. The FCC process involves not only these sequential sub-processes but also complicated coupling among multiphase fluid hydrodynamics, heat and mass transfer between phases, and catalytic kinetic reactions of vapor components in each sub-process. It is essential to build up a model covering all sub-processes/mechanisms mentioned above through riser reactor and giving prompt results, especially for real-time online optimization of industrial operation. This paper aims to develop a parametric model, integrated from bottom feed nozzle to top exit of riser, that can quickly predict both hydrodynamic and kinetic characteristics throughout the riser as well as various parametric effects on production yield and selectivity. Highlights of modeling contributions in this integrated model include a mechanistic and spatial-structural model of multiple-nozzle feeding with strong interactions not only among sprays themselves but also with cross-flowing steam and catalysts, a heat transfer model between gaseous and catalyst phases, and a more-rigorously derived model of reactant conservation in the multiphase flow transport. The convective nature dominating the nozzle feeding, riser transport and kinetic reactions allows us to simplify the governing equations in this integrated model to a set of coupled first-order ordinary differential equations whose solutions can be obtained quickly via Runge-Kutta algorithm. Compared to the published plant data, the predicted VGO conversion and gasoline yield from the proposed model shows a much better agreement to those from previous parametric models, which suggests the newly-added sub-models of previously overlooked mechanisms can be quite important. Some parametric effects, such as the effect of catalyst-to-oil ratio and catalyst inlet temperature, on production yield and selectivity are further predicted. The results show that a higher CTO or catalyst temperature normally leads to higher cracking conversion, higher gasoline production and lower coke content. However, a very high inlet temperature of catalysts does cause over-cracking and lower the gasoline selectivity.
Prediction of the Damaging Flow-Induced Vibrations in Tubular Heat Exchangers With Triangular Arrays
Authors: Khulief Yehia A. , Bashmal Salem A. , Said Sayed A. , Al-Otaibi Dhawi A. , Mansour Khalid M.
Published: December 2014

The prediction of flow rates at which the vibration-induced instability takes place in tubular heat exchangers due to cross-flow is of major importance to the performance and service life of such equipment. In this paper, the semi-analytical model developed in [1] for square tube arrays was extended and utilized to study the triangular tube patterns. A laboratory test rig with instrumented test section is used to measure the fluidelastic coefficients to be used for tuning the mathematical model. The test section can be made of any bundle pattern. In this study, two test sections were constructed for both the normal triangular and the rotated triangular tube arrays. The developed scheme is utilized in predicting the onset of flow-induced instability in the two triangular tube arrays. The results are compared to those obtained for two other bundle configurations; namely the square and rotated square arrays reported in [1]. The results of the four different tube patterns are viewed in the light of TEMA predictions. The comparison demonstrated that TEMA guidelines are more conservative in all configurations considered.
Study of Time-Resolved Vortex Structure of In-Cylinder Engine Flow Fields Using Proper Orthogonal Decomposition Technique
Authors: Zhuang Hanyang , Hung David L. S. , Chen Hao
Published: December 2014

The structure of in-cylinder flow field makes significant impacts on the processes of spray injection, air-fuel interactions, and flame development in internal combustion engines. In this study, the implementation of time-resolved Particle Image Velocimetry (PIV) in an optical engine is presented. Images at different crank angles have been taken using a high-speed double-pulsed laser and a high-speed camera with seeding particles mixed with the intake air. This study is focused on measuring the flow fields along the swirl plane at 30 mm below the injector tip under different intake air swirl ratios. A simple algorithm is presented to identify the vortex structure and to track the location and motion of vortex center at different crank angles. Proper Orthogonal Decomposition (POD) has been used to extract the ensemble and variation information of the vortex structure. Experimental results reveal that strong cycle-to-cycle variations exist in almost all test conditions. The vortex center is difficult to identify since multiple, but small scale, vortices exist during the early stage of the intake stroke. However, during the compression stroke when only one vortex center exists in most cycles, the motion of vortex center is found to be quite similar at different intake swirl ratios and engine speeds. This is due to the dominant driving force exerted by the piston’s upward motion on the in-cylinder air.
A Numerical Analysis on the Correlation Between Impact Conditions of Blood Drop on the Wall and its Resulting Bloodstain Pattern
Authors: Lee Geonkang , Hur Nahmkeon , Seo Young-Il
Published: December 2014

At the crime scene in case of homicide, spattered bloodstains at the incident site are important evidence. The patterns of the bloodstain on the floors and walls are determined by the impact conditions of blood drop such as drop size, impact angle and velocity. By analyzing the bloodstain pattern, one can retrace the origin of blood drops to reconstruct the crime scene. In the present study, motions of blood drop are analyzed to figure out the correlation between impact conditions of blood drop and bloodstain patterns. Two phase interfacial flows of blood drop impacting the wall are numerically simulated with the VOF method. To get the accurate results of bloodstains on the wall, non-Newtonian fluid viscosity and dynamic contact angle are used as the blood properties. By using these methods, patterns of bloodstains created from the blood drop are predicted for various Reynolds and Weber numbers.
Flow Characteristics Downstream of Swirl Flow Generators
Authors: Nakazawa Takanori , Kudo Masanori , Nishibe Koichi , Sato Kotaro
Published: December 2014

An attempt was made, through experiments and numerical simulations, to clarify the mechanisms associated with the flow characteristics downstream of both inlet guide vanes (IGVs) with different vane geometries and circulation control wings (CCWs) with different jet sheet momentum coefficients. The novelty of the present study lies in the proposal of a swirl flow generator which can control the flow angle without changing geometrically. The main objectives were: (1) to revalidate the numerical analysis method employed in the present study, which was used to assess the flow characteristics downstream of the IGVs and (2) to suppress flow oscillations through tangential blowing. Characteristic flow patterns downstream of flat-plate vanes and circular cylinders with tangential blowing, with identical exit flow angles, are presented, and the unsteady flow characteristics downstream of the IGVs are compared with those of the CCWs. Furthermore, the influence of the radius ratio of the device outlet to the vane exit on the unsteady flow characteristics downstream of the IGVs and CCWs is discussed based on the measured pressure and velocity fluctuations.
Numerical Simulation of Pressure Vibrations in a Francis Turbine Draft Tube With Air Admission
Authors: Huang Renfang , Yu An , Luo Xianwu , Ji Bin , Xu Hongyuan
Published: December 2014

The pressure vibrations in a draft tube are harmful for the stable operation for a Francis turbine at part load conditions. In this paper, air admission is proposed to depress those pressure vibrations. The unsteady flow in a Francis turbine, whose hydraulic performance has been tested experimentally, is simulated at a part load operation condition. The flow simulation is conducted using RANS methods coupling with SST k-ω turbulence model. The results indicate the pressure vibrations in the turbine are reasonably predicted by the present numerical method. Based on the calculations, the dominant pressure vibration component for a hydro turbine operated at part-load condition is caused by the vortex rope in draft tube, and its frequency is near 0.2 times of the runner rotation frequency. The frequencies of pressure vibration do not change by air admission, and the pressure vibration amplitude decreases with the air admission. Further, the depression effect for pressure vibration would be improved if air admission is from the crown holes instead of the spindle hole. The results also indicate that the turbine hydraulic efficiency changes periodically with the pressure vibration induced by vortex rope in turbine draft tube, would be degraded with air admission from the spindle hole, and improved with air admission from the crown holes. With the increase of air admission, the turbine hydraulic efficiency would improve. The present research will be helpful for the safe operation of Francis turbines.
Effect of Liquid Properties on Thermal Mixing in a T-Junction
Authors: Kashinsky O. N. , Lobanov P. D. , Kurdyumov A. S. , Pribaturin N. A.
Published: December 2014

Results of experimental study of mixing of liquids with different temperatures in a T-junction are presented. Experiments were performed with liquids which have significantly different physical properties. The liquid with higher temperature was injected through the branch of the T-junction. The test section was made of thin wall stainless tubes. The distribution of wall temperature over the surface was measured using a high speed infrared camera. Both time averaged and fluctuational characteristics of the temperature field were obtained from infrared image processing.

The structure of temperature field inside the channel was measured by a microthermocouple. It was mounted on the traversing unit which allowed its translation along the channel diameter. Measurements were made over different diameters in the same cross section. This allowed to construct the three-dimensional structure of the temperature field.

Results obtained provide experimental data necessary for validation of thermo hydraulic codes for the design of power equipment.
Unsteady Experimental Characterization of the Natural Wake of a Squareback Ahmed Model
Authors: Volpe Raffaele , Devinant Philippe , Kourta Azeddine
Published: December 2014

The increasing demand for such vehicles as station wagons or minivans has focused aerodynamic research for drag reduction on squareback models of which, the one proposed by Ahmed et al. [1] is one of the most used. The presented study aims at finely characterize the natural wake of a 1:1 squareback Ahmed body with experimental measurements, in order to draw possible strategies for drag reduction. Our analysis focused both on the detection of the mean wake topology, by means of 2D PIV as well as parietal pressure measurements, and on the study of the spectral fluctuations of velocity and pressures. The experimental results exhibit a left/right bistable behavior for the wake, highly sensitive to the incoming wind yaw angle, so that, most generally, the mean aerodynamic field shows an asymmetric behavior due to the predominance of one of the two stable positions. The characterization of the mean wake by averaging the fields on the whole acquisition time can then be biased and show an asymmetrical wake. Then, as far as the symmetric average field is of interest, it is relevant to study separately each topology using conditional averaging, and then build a symmetrical field by averaging the two bistable phases. The analysis shows that the well-known symmetrical wake and the corresponding rear pressure field derive from a random rotation of the toric recirculation bubble, which can induce instantaneous pressure losses of about 25%. The bistability does not affect the spectral energy distribution. Our results show a spectral activity mostly at low frequencies. In particular, we found in measurements along the vertical symmetry plane a peak at StH = 0.07, indicating the presence of the “bubble pumping” phenomenon, whereas pressure measurements at vehicle mid-height indicate the existence of vortex shedding.