bluuebunny/crossref_metadata_2025 huggingface dataset:
Performance Comparison of Gas Turbine Cycle Combined With Supercritical CO2 Recompression and Regenerative Cycle
Authors: Han Yuqi , Zhuge Weilin , Zhang Yangjun , Chen Haoxiang
Published: October 2018

With the aim to recover waste heat from a specific micro gas turbine (MGT), and improve the thermal efficiency and the system compactness, simulation models of regenerative gas turbine cycle combined with supercritical CO2 recompression cycle and supercritical CO2 regenerative cycle respectively are developed. The influence of the introduction of the gas turbine recuperator with three cycle coupling methods on the thermal efficiency of the system is discussed. Compare to the micro gas turbine system combined with supercritical CO2 regenerative cycle, the improved system can increase the thermal efficiency and the output power by 3.32 percent point and 10.54% respectively. The impact on system performance of cycle parameters, including split ratio, the maximum temperature of the bottoming cycle, the recuperator effectiveness of the bottoming cycle and the hot side outlet temperature of the intermediate heat exchanger have been analyzed and optimized. From the viewpoints of the thermal efficiency and the heat transfer area, performance comparison between two bottoming cycles with different coupling methods is done. The multi-objective optimization study shows that the regenerative gas turbine cycle coupled in series with supercritical CO2 recompression cycle performs better than that coupled in parallel with supercritical CO2 regenerative cycle in terms of thermal efficiency.
Synthetic Jet Flow Control Optimization on SD7003 Airfoil at Low Reynolds Number
Authors: Kamari Djavad , Tadjfar Mehran
Published: October 2018

Synthetic jet crossing the boundary layer has been widely implemented on the airfoil’s top surface to control the flow field. Introducing a genetic algorithm coupled with artificial neural network (ANN) was used in this study to find optimum values for design parameters. Optimization was done for SD7003 airfoil at Reynolds number of 60,000 and angles of attack of 13° and 16°. URANS equations were employed to solve the flow field and k–ω SST was used as the turbulence model. The synthetic jets were implemented tangential to boundary layer (TBL). It was found that at optimum values of design parameters a significant improvement in aerodynamic coefficients by increasing lift and reducing drag can be achieved. Drag force reduction was achieved by reducing pressure drag at post stall and a significant reduction of separation zone.
Effect of Fan Speed on Air Flow Through a Green Wall Module
Authors: Abdo Peter , Huynh B. P. , Avakian Vahik
Published: October 2018

Green or living walls are active bio-filters developed to enhance air quality. Often, these walls form the base from which plants are grown; and the plant-wall system helps to remove both gaseous and particulate air pollutants. They can be classified as passive or active systems. The active systems are designed with ventilators which force air through the substrate and plant rooting system, therefore the air is purified and filtered through a bio-filtration process which also acts as a natural cooling system. A fan positioned at a central opening on the module’s back face drives air through the medium-plant-roots mix and then onward through the plants’ canopy; and these would help to remove both gaseous and particulate pollutants from the air. Pressure drop across the module, air flow distribution through it as well as the total flow rate have been obtained. The effect of different fan speeds on the total air flow and on its distribution through the module is investigated in this study in order to optimize the energy consumption of the fans whilst maintaining the modules biofiltration efficiency.
Re-Entry Jet and Shock Induced Cavity Shedding in Cloud Cavitating Flow Around an Axisymmetric Projectile
Authors: Wang Yiwei , Ye Bingsheng , Wang Jingzhu , Huang Chenguang
Published: October 2018

Shedding is one of the most important expressions of the instability of cavitating flow. Most previous research works were focused on the shedding mechanism induced by the re-entry jet. Shock induced shedding on a wedge is identified recently by using time resolved X-ray densitometry which attracted lots of attention. In the present paper, cavitation dynamics around an axisymmetric body are investigated. Both shock propagation and re-entry jet as inducing factors of shedding are observed in different cycles in a single experiment. Relevant numerical simulations are carried out based on a fully compressible approach under the framework of the open-source code OpenFOAM. Numerical and experimental results agree well with each other. Results indicate shedding is induced by the re-entry jet in the first cycle. Re-entry jet occurs and cut the cavity off on the should which induces the shedding of cloud cavity in the first cycle. However in the second and subsequent cycles, shocks are generated by the collapse of shedding cavities and propagate to the cavity closure and induces stronger re-entry jet. Its effect on cavity instability is indirect which still needs a strong re-entry jet as the medium media.
Bubble Formation From Porous Plates in Liquid Cross-Flow
Authors: Shepard Thomas , Ruud Eric , Kinane Henry , Law Deify , Ordahl Kohl
Published: October 2018

Controlling bubble diameter and bubble size distribution is important for a variety of applications and active fields of research. In this study the formation of bubbles from porous plates in a liquid cross-flow is examined experimentally. By injecting air through porous plates of various media grades (0.2 to 100) into liquid flows in rectangular channels of varying aspect ratio (1–10) and gas/liquid flow rates the impact of the various factors is presented. Image processing techniques were used to measure bubble diameters and capture their formation from the porous plates. Mean bubble diameters ranged from 0.06–1.21 mm. The present work expands upon the work of [1] and further identifies the relative importance of wall shear stress, air injector pore size and gas to liquid mass flow ratio on bubble size and size distribution.
Assessment of Cavitation Models in the Prediction of Cavitation in Nozzle Flow
Authors: Villafranco Dorien O. , Do Huy K. , Grace Sheryl M. , Ryan Emily M. , Holt R. Glynn
Published: October 2018

Cavitation inside fuel injector nozzles has been linked not only to erosion of the solid surface, but also to improved spray atomization. To quantify the effects of the resulting occurrences, the prediction of cavitation through computational modeling is vital. Homogeneous mixture methods (HMM) make use of a variety of cavitation sub-models such as those developed by Kunz, Merkle, and Schnerr-Sauer, to describe the phase change from liquid to vapor and vice-versa in the fluid system. The aforementioned cavitation models all have several free-tuning parameters which have been shown to affect the resulting prediction for vapor volume fraction.

The goal of the current work is to provide an assessment of the Kunz and Schnerr-Sauer cavitation models. Validation data have been obtained via experiments which employ both acoustic techniques (passive cavitation detection, or PCD) and optical techniques (optical cavitation detection, or OCD). The experiments provide quantitative information on cavitation inception and qualitative information as to overall vapor fraction as a function of flow rate, and nozzle geometry. It is shown that inception is fairly well captured but the amount of vapor predicted is far too low. A sensitivity analysis on the tuning parameters in the cavitation models leads to some explainable trends, however, several parameter sweeps results in outlier predictions. Recommendations for their usability and suggestions for improvement are presented.
Fluid Flow and Heat Transfer Behavior for Turbulent Flows in a Tube With Vortex Generator Pairs for an Efficient Heat Exchanger
Authors: Islam Md. , Chong Z. , Bojanampati S.
Published: October 2018

Various technologies have been developed to enhance flow mixing and heat transfer in order to develop an efficient compact heat exchanging devices. Vortex generators/turbulent promoters generate the vortices which reduce the boundary layer thickness and introduce the better mixing of the fluid to enhance the heat transfer. In this research experimental investigations have been carried out to study the effect of delta winglet vortex generator pairs on heat transfer and flow behavior.

To generate longitudinal vortex flow, two pairs of the delta winglet vortex generators (DWVG) with the length of 10mm and winglet-pitch to tube-diameter ratio (PR = 4.8) are mounted on the inner wall of a circular tube. The DWVG pairs with two different winglet-height to tube-diameter ratios (Blockage ratio, BR = 0.1 and 0.2), three attack angles (α = 10°, 20°, 30°) and three spacings between leading edges (S = 10, 15 and 20mm) are studied. The experiments were conducted with DWVGs pairs for the air flow range of Reynolds numbers 5000–25000. The influence of the DWVGs on heat transfer and pressure drop was investigated in terms of the Nusselt number and friction factor. The experimental results indicate that DWVG pair in a tube results in a considerable enhancement in Nusselt number (Nu) with some pressure penalty. It is found that DWVG increases Nu up to 85% over the smooth tube. It is also observed that Nusselt number increases with Re, blockage ratio and attack angle. Friction factor decreases with Re but increases with blockage ratio, spacing and attack angle. And 30° DWVG pair with S = 20mm, BR = 0.2 gets the highest friction factor. The Highest thermal performance enhancement (TPE) was noticed for α = 10°, S = 20mm, BR = 0.2 for turbulent flows. To obtain qualitative information on the flow behavior and vortex structures, flow was visualized by laser sheet using smoke as a tracer supplied at the entrance of the test section. The generation and development of longitudinal vortices influenced by DWVG pairs were clearly observed.
Effects of Fluid Viscosity and Impingement Angle on Submerged Jet Flowfields
Authors: Karimi Soroor , Fulton Matthew , Shirazi Siamack A. , McLaury Brenton
Published: October 2018

Many researchers have utilized submerged jet impingement geometry to study solid particle erosion/corrosion. However, only a few studies have investigated changing impingement angle and fluid viscosity. In this study, Particle Image Velocimetry (PIV) experiments were conducted using 14 micron glass spheres for direct impingement geometry at viscosities of 1, 14, and 55 cP. These viscosities correspond to Reynolds numbers of approximately 57000, 4000, and 1000, respectively. It was observed that by increasing the viscosity the flow exiting the nozzle transitioned from extremely turbulent to laminar flow. The data indicated fully turbulent flow at the outlet for viscosities of 1 and 14 cP. In the case of 55 cP flow, the flow exiting the nozzle became laminar contributing to a higher maximum velocity in 55 cP flow. Experiments at these viscosities were also conducted at impingement angles of 90, 75, and 45 degrees to investigate the effects of the impinging jet angle on a flat plate. Additionally, a series of Computational Fluid Dynamics (CFD) simulations of the flowfield were performed to compare with the experimental data collected in this paper.
Using CFD Simulations to Improve the Air-Cooled Steam Condenser Performance in Severe Windy Conditions via Proper Tuning of Blades Pitch Angles
Authors: Darbandi Masoud , Khorshidi Behzadi Hamid Reza , Farhangmehr Vahid , Schneider Gerry E.
Published: October 2018

The use of air-cooled steam condenser (ACSC) in thermal power plants has become so normal since a few decades ago. It is because there are so many valuable advantages with the ACSC implementation, e.g., little dependency on water consumption and benefiting from the forced convection heat transfer instead of the natural one to condense the steam. However, the thermal performance of an ACSC can be readily defected by the ambient wind; specifically, when the ambient temperature is high. This research work benefits from the computational fluid dynamics tool to study the details of ACSC’s thermal performance in such undesirable ambient windy conditions. Furthermore, this work suggests an effective remedy to increase the heat rate from the proposed ACSC. Evidently, the flow rate of cold air through the heat exchangers of proposed ACSC has direct influence in heat transfer rate from the heat exchangers of ACSC. One remedy to achieve higher cold air flow rates through these heat exchangers is to improve the design of its fans or blowers. However, for an ACSC already in service, one should look for other cost-effective remedies. So, if one wishes to improve the performance of those fans without changing their design one should pay attention to some other simple ways with little costs to implement them. This work suggests to tune up the pitch angles of blades of ACSC’s fans properly. The details of implementing this remedy are presented in this paper.
Assessing the Value and Balance of In-Class Demos and Videos in Fluid Mechanics Lecture
Authors: Shepard Thomas G.
Published: October 2018

The use of in-class demonstrations and videos in an introductory fluid mechanics can have many positive outcomes in regards to student learning and engagement. However, the face-to-face class time an instructor has during lecture is a valuable commodity which can disappear all too quickly given the amount of topics to be covered, example problems, exams, etc. Thus, there is a balance to be struck amongst the various in-class activities, which must also factor in the amount of preparation time demanded of the instructor. This paper examines the utility of in-class demonstrations and videos using student surveys and feedback from both the instructor and students. Survey results reveal that students see the benefits of videos and in-class demonstrations differently and generally agree on the balance to be struck in the how class time is allocated. The results further reveal how students view in-class time in light of the opportunity to have a flipped, or partially flipped course whereby students watch lecture content outside of class. Student recall for relevant fluid mechanic concepts highlighted during demos is discussed. Additionally, the paper describes some of the specific fluid mechanics demonstrations and videos while providing references to other resources.
Theoretical and Numerical Analysis of the Pressure Distribution and Discharge Velocity in Flows Under Sluice Gates
Authors: Epple Philipp , Steppert Michael , Steber Michael , Malcherek Andreas
Published: October 2018

The flow under sluice gates is well known in open channel hydraulics. There are theoretical, semi-empirical and empirical equations to determine the flow rate under a sluice gate Most of these formulas are based on the Bernoulli equation applied at the inflow cross section and in the vena contracta behind the gate. In 2017 Malcherek [1] showed that it is also possible to apply the integral momentum balance to the sluice gate. When assuming hydrostatic pressure distributions in the inflow cross section and on the weir’s plate then the simple formula VA=(3/4) g h0=0.6122 g h0 is obtained, which is in perfect agreement with the classical vena contracta theory for small opening ratios h0/a. In the outflow cross section under the gate the bottom pressure was assumed to be the mean of the hydrostatic bottom pressure before and behind the sluice gate. In this paper Malcherek’s momentum balance theory will be investigated in further detail with numerical CFD RANS computations of the free surface flow below sluice gate. The exact pressure distributions on the bottom as well as on the gate were obtained for different openings ratios and flow conditions at the sluice gate in a systematic parameter study. These pressure distributions have been introduced into the integral momentum equation and the discharge velocity as well as the flow rate at the sluice gate were investigated and compared with the pure numerical results. These results were also compared with the theoretical and empirical approaches of the literature and a detailed analysis is given.
Coherent Structures and Correlation Fields in the Mixing Transition of a Turbulent Round Free Jet
Authors: Krohn Benedikt , Qin Sunming , Petrov Victor , Manera Annalisa
Published: October 2018

Turbulent free jets attracted the focus of many scientists within the past century regarding the understanding of mass- and momentum transport in the turbulent shear field, especially in the near-field and the self-similar region. Recent investigations attempt to understand the intermediate fields, called the mixing transition or ‘the route to self-similarity’. An apparent gap is recognized in light of this mixing transition, with two main conjectures being put forth. Firstly the flow will always asymptotically reach a fully self-similar state if boundary conditions permit. The second proposes partial and local self-similarity within the mixing transition. We address the later with an experimental investigation of the intermediate field turbulence dynamics in a non-confined free jet with a nozzle diameter of 12.7 mm and an outer scale Reynolds number of 15,000. High speed Particle Image Velocimetry (PIV) is used to record the velocity fields with a final spatial resolution of 194 × 194 μm2. The analysis focuses on higher order moments and two-point correlations of velocity variances in space and time. We observed local self-similarity in the measured correlation fields. Coherent structures are present within the near-field where the turbulent energy spectrum cascades along a dissipative slope. Towards the transition region, the spectrum smoothly transforms to a viscous cascade, as it is commonly observed in the self-similar region.
Numerical Analysis of Droplet Impact on a Smooth Slippery Surface
Authors: Yeganehdoust F. , Karimfazli I. , Dolatabadi A.
Published: October 2018

Spontaneous bouncing of a droplet that impacts a surface is a mechanism that occurs for the moderate range of droplet impact velocities and is caused by the formation of a stable air layer (cushion) between the droplet and the surface. This bouncing behavior is more pronounced on Lubricant Impregnated Surfaces (LISs) inspired by the natural non-wetting surface of the pitcher plant, which relies on the stable formation of a thin lubricant film across its surface.

In this study, we performed modeling of the water-oil–air interfacial surfaces using the volume of fluid (VOF) methodology to simulate the impact of a water droplet onto a lubricant smooth surface with an oil as the lubricant. To resolve the effects of the air surrounding the droplet, computational cells were extensively small to capture the presence of the sub-micron layer of air trapped underneath the droplet during the impact.

The model was able to capture the initiation and subsequent effect of the air cushion on the droplet hydrodynamics. We found that the stability of the air cushion and the impact dynamics are independent of the oil viscosity for specific thicknesses of lubricant layers, whereas the impact conditions such as velocity and droplet properties played a significant role on the outcome of droplet impact. Hence, the dynamics of a droplet falling on a specific thickness of oil film was influenced by the squeezed air trapped between the two immiscible fluid (water and oil). In addition, the formation of high pressure dimple region was evident, which in some cases lead to entrapment of the air bubble. Finally, we validated the results with the existing experimental data in the literature.
A Comparison of Strong and Weak Coupling Schemes for Computational Aeroelasticity in OpenFOAM
Authors: Seraj Sabet , Fereidooni Amin , Grewal Anant
Published: October 2018

Two coupling schemes for fluid-structure interaction using the OpenFOAM structural solver sixDoF Rigid Body Motion are developed. The first scheme is developed by modifying the baseline leapfrog weak coupling scheme to minimize the lag between the fluid and structural solvers. The second is a strong coupling scheme based on the Crank-Nicolson method. The two newly implemented schemes and the baseline are compared through the aeroelastic simulation of a NACA 64A010 airfoil and the Benchmark Supercritical Wing. The aeroelastic solutions obtained using the newly implemented schemes exhibit significantly lower sensitivity to changes in time step size compared to the baseline weak coupling scheme. The modified weak coupling and strong coupling schemes perform comparably for the cases studied.
Computational Modeling of Flow Rate Measurements Using an Orifice Flow Meter
Authors: Xu YiQin , Coxe Daniel , Peet Yulia , Lee Taewoo
Published: October 2018

This study is concerned with understanding and improvement of mass flow rate measurement uncertainty and errors encountered at low flow rates and start-up in commercially available flow rate measurement devices, such as orifice flow meters. The flow through a typical cylindrical flange-tapped orifice flow meter is modeled computationally so the actual mass flow rate is known a-priori. Empirical predictions from the reading of “virtual” pressure sensors are compared with the actual flow rate and the measurement errors are quantified and analyzed. Commercial code ANSYS-Fluent is compared in this study to the in-house high-fidelity spectral-element solver Nek5000, so that conclusions about the applicability of a commercial code to the calculations of measurement uncertainty in the orifice flow meters can be made.
A Wall Film Model for Membrane Fouling
Authors: Mamouri Sina Jahangiri , Tarabara Volodymyr V. , Bénard André
Published: October 2018

Deoiling of produced or impaired waters associated with oil and gas production represents a significant challenge for many companies. Centrifugation, air flotation, and hydrocyclone separation are the current methods of oil removal from produced water [1], however the efficiency of these methods decreases dramatically for droplets smaller than approximately 15–20 μm. More effective separation of oil-water mixtures into water and oil phases has the potential to both decrease the environmental footprint of the oil and gas industry and improve human well-being in regions such as the Gulf of Mexico.

New membrane separation processes and design of systems with advanced flow management offer tremendous potential for improving oil-water separation efficacy. However, fouling is a major challenge in membrane separation [2]. In this study, the behavior of oil droplets and their interaction with crossflow filtration (CFF) membranes (including membrane fouling) is studied using computational fluid dynamics (CFD) simulations. A model for film formation on a membrane surface is proposed for the first time to simulate film formation on membrane surfaces. The bulk multiphase flow is modeled using an Eulerian-Eulerian multiphase flow model. A wall film is developed from mass and momentum balances [3] and implemented to model droplet deposition and membrane surface blockage. The model is used to predict film formation and subsequent membrane fouling, and allow to estimate the actual permeate flux. The results are validated using available experimental data.
Experimental Investigation of Solid Particle Erosion in Successive Elbows in Gas Dominated Flows
Authors: Asgharpour Alireza , Zahedi Peyman , Khanouki Hadi Arabnejad , Shirazi Siamack A. , McLaury Brenton S.
Published: October 2018

When a fluid containing erosive particles flows through two or more elbows mounted in series with short distances between them, the downstream elbows which are installed after the first elbow are exposed to erosion from a flow that is not fully developed. Particle velocities and concentration distributions change due to change of flow direction within the first elbow which may cause the characteristics of erosive wear in the second elbow to be different than the first. In the present study, the erosion magnitude and pattern in two 4-inch standard successive elbows installed with 13D distance from each other are examined experimentally. Utilizing a state-of-the-art ultrasonic technique, erosion measurements are performed in gas-sand and gas-liquid-sand annular flows to investigate the effects of particle size and flow rates. A paint removal study is also performed to capture the erosion patterns in two elbows for different flow conditions. Using two clear elbows, a flow visualization is performed which shows the quality of phase distribution in the two elbows in series in annular and stratified flows. Erosion measurements show the results in the second elbow for this geometry and these flow conditions are less than or in a few cases nearly equal to the first elbow. The location of maximum erosion for all the cases considered is around 45 and 65 degrees from the inlet of the elbow in the first and second elbows, respectively. Two areas are identified in erosion patterns of both elbows. One high intensity particle impact region which is affected primarily by first impact of particles, and another pattern is formed due to impacting particles that have rebounded from the first impact.
Influence of Typical Subgrade Structures on Aerodynamic Characteristics of High Speed Trains in Cross Wind Conditions
Authors: Sun Zhenxu , Yang Guowei
Published: October 2018

Due to geographical and environmental constraints, highspeed railways use a variety of subgrade structures such as ground, embankments with different height, viaducts, etc. When trains run on embankments and viaducts, the flow around the car body is more complex than the ground. Under the action of crosswind, there are obvious differences in the cross-wind aerodynamic characteristics of high-speed trains on different subgrade structures. The unreasonable subgrade structure will affect the cross-wind safety of the train. At the same time, the structure of the train is complex, the bogie and pantograph have an important role on the flow field characteristics of the train, and the over simplified profile of the short train cannot accurately reflect the true aerodynamic characteristics of the train. In the present paper, in order to study the influence of typical subgrade structure on the aerodynamic characteristics of high speed trains, a real high-speed train with 9 carriages at the speed of 200 km/h was taken for case study, and the details of windshields, bogies and pantographs were taken into consideration. The cross wind velocities were chosen as 20, 30, 35 and 40 m/s. The aerodynamics performance of the highspeed train under the four conditions of plane ground, 3m-embankment, 6m-embankment and viaduct were simulated and compared, and the differences and regularities in the aerodynamic characteristics under cross wind conditions on different subgrade were analyzed. The results provide a reference for train safety control on complex subgrade structures under cross wind condition.
Effect of Fluid Properties on Slug Dissipation in Enlarged Impacting Tee
Authors: Mohammadikharkeshi Mobina , Dabirian Ramin , Shoham Ovadia , Mohan Ram S.
Published: October 2018

An experimental and theoretical investigation is conducted on the effect of inlet slug length and fluid properties on slug dissipation in an enlarged impacting tee-junction (EIT). The EIT is the building block of a multiphase manifold. Prediction of the slug dissipation length in the EIT provides a guideline for the optimum diameter and length of a manifold. A multiphase flow loop is utilized to investigate slug dissipation in an EIT. The EIT inlet pipe is inclined slightly upward at 5 degrees, in which stationary slugs of different lengths are formed. The generated slugs are pushed into the EIT branches by a pre-determined gas flow rate. Over 80 experimental runs are conducted with superficial gas velocities between 3 to 9 m/s and inlet slug lengths between 40d to 90d, for both air-water and air-oil flows. The experimental data confirm that increasing the superficial gas velocity, as well as the slug body length in the inlet pipe lead to increasing the slug dissipation length in the EIT branches. Furthermore, the data demonstrate that higher slug dissipation length is obtained with water as compared to oil. A mechanistic model is developed for the prediction of slug dissipation in an EIT. A comparison between the developed mechanistic model predictions and the experimental data show a discrepancy less than 20%.
Effects of the Forcing Frequency on Pulsating Impinging Jet Behavior and the Boundary Layer on the Target Curved Wall
Authors: Kharoua N. , Khezzar L. , Nemouchi Z.
Published: October 2018

In the present work, time-dependent responses of Nusselt number, friction coefficient and pressure profiles to the passage of groups of coherent structures along a curved impingement wall, is considered. It is meant to replicate a more realistic picture of the flow. The jet considered belongs to heating applications where the jet flow temperature is higher than that of the impingement wall.

The flow was simulated using Large Eddy Simulation with the Dynamic Smagorinsky sub-grid-scale model. The plane jet was forced at frequencies increasing gradually to a maximum of 2200 Hz with an amplitude equal to 30% of the mean jet velocity. The computational domain was divided into 16.5 million hexahedral computational cells whose resolution was assessed based on the turbulence scales.

It was found that for low forcing frequencies (e.g., 200Hz), coherent forced primary vortices induced by the pulsations are separated by less organized vortices naturally induced similar to those of the unforced jet. It could be seen that the natural vortices have moderate effects on the boundary layer development on the impingement surface starting at relatively short distances from the stagnation point compared to the forced vortices. Increasing the forcing frequency to 1000Hz reduces the distance separating successive forced vortices causing the pairing phenomenon to occur at a certain distance along the target wall. Increasing the forcing frequency further to 2200Hz makes the pairing phenomenon followed by vortex breakdown to occur at shorter distances along the target wall.

The smaller forcing frequencies yield large and strong distant vortices which affect the dynamical field noticeably in conjunction with an important deterioration of heat transfer due to their strong mixing effect and entrainment of cold air from the surroundings. On the other hand, high frequencies generate smaller vortices which are relatively close to each other. Thus, they have a weaker effect allowing the growth of the boundary layer on the target wall up to a distance equal to four times the jet-exit width where the minimum heat transfer is observed. In fact, the small successive vortices form a sort of shield preventing the cold air from the surroundings to reach the target wall until their breakdown.
CFD Characterization of a Wet Foam Flow Rheological Behavior
Authors: Dallagi Heni , Gheith Ramla , Al Saabi Ahmad , Faille Christine , Augustin Wolfgang , Benezech Thierry , Aloui Fethi
Published: October 2018

In some industrial processes, aqueous foams flow presents an important phase of the process, whereas, they cause pressure drop when designing and dimensioning systems. Identifying the different rheological parameters of foam flow is an interest key to understanding the interfacial phenomena. Actually, the difficulty to model the rheological parameters of foam flow is a major challenge.

In this study, we present a robust model to describe the foam fluid inside horizontal channels by the reverse approach of a numerical simulation (Computational Fluid Dynamics: CFD), based on the behavior laws of the Herschel-Bulkley type, for the non-Newtonian fluids. This reverse method starts from experimental (deduced from Particle Image Velocimetry (PIV) technique) results of the previous experimental work of Chovet (2015). The pressure losses measurements near-wall velocity fields, velocity profiles and the wall shear stress evolution including the void fraction from 55% to 85%, are considered in order to identify the different parameters of the developed model to determine the nature of the flow, the foams rheological behavior and the foam flow regime along the length of the channel.

The numerical study (CFD) is applied for two conditions: the first one for a wet foam flow with a void fraction of 70% and a foam flow velocity of 2cm/s (one-dimensional regime) and the second one, for a foam quality of 55% and a flow rate of 6cm/s. The numerical evolutions are identical to experimental ones for these same conditions. Therefore, we can conclude that the Herschel-Bulkley rheological model can correctly describe the aqueous foams fix behavior.
Interactions of Flow Structure With Nano- and Micro-Particles in Turbulent Channel Flows
Authors: Mofakham Amir A. , Ahmadi Goodarz , McLaughlin John
Published: October 2018

This study is concerned with the effects of the flow structures including the near-wall coherent eddies in turbulent channel flows on the dispersion and deposition of nano- and micro-particles. A pseudo-spectral computational code was used for direct numerical simulations (DNS) of the Navier-Stokes equations and the corresponding time histories of the instantaneous fluid velocities were evaluated. Under the oneway coupling assumption, the trajectories of a wide range of particle sizes from 10 nm to 80 μm with dimensionless relaxation time of 2.2e−6 to 142 were obtained by solving the particle equation of motion including Stokes drag and Brownian excitations. Dispersion and deposition of particles in the turbulent flow were evaluated and the effects of turbulence structure on different size particles were studied. The simulation results showed that the concentration distribution of small particles that behave like fluid tracer particles were quite random. However, the preferential concentrations appeared as the dimensionless relaxation time increased to 2–20. In particular, the influence of coherent structures in the near-wall regions was clearly detectable on the concentration distribution of particles, as well as, in their deposition pattern. For τ+ = 20 particles due to the increase of relaxation time and inertia of particles, the small-scale turbulent features were filtered out and only the effect of large-scale turbulent eddies could be identified. For τ+ = 2–20 particles, the ensemble/time average of the position of the deposited particles showed specific spacing which was comparable to the size of the near-wall coherent structures.
Design of Turbo-Compression System for HALE UAV Propulsion System
Authors: Kang Young Seok , Rhee Dong Ho , Lim Byeung Jun , Jun Sangook , Park Tae Choon , Lee Yang Ji , Jun Yong Min
Published: October 2018

A turbo-compression system design and its performance analysis procedure for a high altitude long endurance UAV (HALE UAV), of which cruising altitude is within the stratosphere, is presented. To fly at a relatively low speed for a long time and to make engine performance less sensitive to flight altitude, a hydrogen fueled internal combustion engine was chosen for a propulsion system. To utilize an internal combustion engine as a propulsion system at a high altitude, a proper inlet pressure boost system such as a series of turbochargers is required. Hydrogen is highly reactive gas and sometimes backfiring or preignition may occur due to its low ignition energy at stoichiometric ratio. Therefore, fuel to air ratio should be reduced as low as 0.6 to avoid such phenomena. Then rarefied ambient intake air pressure should be boosted up to 1.7 bar to produce required power from the lean burn engine. To gain high pressure ratio from the turbo compression system, at least three stage serial turbocharger with proper intercooler system at each compressor exhaust is required. To analyze multi-stage turbocharger performance at the cruising altitude, an explicit one-dimensional analysis method has been established mainly by matching required power between compressors and turbines. Each compressor performances were corrected according to Reynolds number at a given flight altitude. Compressor efficiency and surge margin deteriorate as the operating altitude increases. Then compressor efficiencies were reflected as functions of flight altitude and corresponding Reynolds number. Once operating points of each turbocharger was determined, then adequate turbochargers were searched for from commercially available models based on performance analysis results. Also, adequate water to air intercoolers were chosen for the turbo-compression system to secure flexibility of placing main components inside the engine bay as well as to obtain high heat exchange efficiency of the heat exchangers. Based on the designed turbo-compression system, technical demonstration test of the turbo-compression system inside altitude environment test chamber in Korea Aerospace Research Institute is planned. Altitude condition in stratosphere is simulated mainly with two stage centrifugal compressor and additional fan will be used to fine control the flight altitude. The turbo compression system will be controlled with a single waste gate located just downstream of the engine to secure simple controllability of the turbo compression system. The test results will validate main components as well as system layout design methods and give more reliable control schedule of the turbo compressions system according to the flight altitude.
Two-Phase Flow Visualization for a Hybrid Heat Sink
Authors: Rahman Danish , Almomani Ahmad , Hassan Ibrahim , Al-Hamidi Yasser , Rahman Aziz
Published: October 2018

This paper aimed to study two-phase flow under adiabatic conditions through the process of flow visualization. This was done through the use of a test section with a cross flow and a jet impingement (swirl jet). The flow regimes under different air-water flow rates were determined using a high-speed camera that recorded digital videos. For each of the flow rates the pressure differential between the inlet and the outlets were measured. Through the pressure drop it is proposed that the types of flow regimes may later be able to be predicted. Nine air-water flow rates were considered to collect data and generate a flow map for the impingement jet and cross flow. The major observed flow regimes within the crossflow and impingement jet followed the predicted trend with bubbly and plug flow in the former, and slug flow in the latter. It was further observed that increasing the air flow rate increased the likelihood of bubbly and plug flow in both the cross-flow and impingement jet. In the cross flow, a lower air flow rate resulted in bubbly flow while within the impingement jet, a lower air flow rate resulted in slug flow.
Particle Image Velocimetry Measurements of Flow Inside a Hydrocyclone With an Air Core
Authors: Shingote Chinmay , Furlan John , Kadambi Jaikrishnan , Visintainer Robert , Ke Renjie , Garman Mohamed
Published: October 2018

Hydrocyclone separators are widely used in the oil and mining industries to sort, classify and separate solid particles or liquid droplets within liquid suspensions. In this study, the liquid-air flow inside a mining hydrocyclone is investigated using particle image velocimetry (PIV). Previous work completed by Ke (2016), using the same technique and experimental setup as the current study, mainly focused on the flow velocities in the radial-tangential plane within a hydrocyclone running with an air core. Due to optical limitations, the velocity measurements in the radial-tangential plane were limited to the inlet head. Previous work by Chinmay et al (FEDSM 2017) was also performed which concentrated mainly on the dimensions of the air core.

The study revealed several physical phenomena and flow patterns of the multi-phase flow in a hydrocyclone. An air core is generated along the central axis of the hydrocyclone, as the underflow is open to atmosphere. In this study, the flow field in the radial-axial (r-z) plane has been investigated, and measurements have been obtained in the conical regions outside of the inlet head including the center (axially) of the cyclone.

Particle Imaging Velocimetry (PIV) method was used with a laser as the light source in order to obtain experimental data for the purpose of developing and validating computational models of hydrocyclone flows. Refractive index matching technique was used, with the hydrocyclone model being made of clear acrylic material. The test liquid used in the experiments was sodium iodide aqueous solution (63.3% NaI by weight), in order to achieve a refractive index match between the curved inner cyclone walls and test fluid. The seed particles used in the experiments were 10 μm silver coated hollow glass spheres which were introduced into the flow by global seeding. Three fields of view (FOV) were investigated in the r-z plane of the hydrocyclone. Two dimensional velocity vector maps were obtained in each of the fields of view. Additionally, a SONARtrac unit was installed in the overflow piping in order to measure the % air content of the cyclone overflow. The PIV and % air content measurements will be presented and discussed.
Particle-Imaging Velocimetry and Pressure Measurements of the Turbulent Flows Inside the Intake Duct
Authors: Zhang Ruizhi , Huang Renfang , Zhu Lei , Dai Yuanxing , Zhou Jiajian , Luo Xianwu
Published: October 2018

The present study mainly treats the flow measurement for the intake duct of a water-jet propulsion equipment using stereoscopic particle image velocimetry (SPIV) technique. To obtain clear flow field information in the intake duct, the authors have developed a suitable SPIV calibration method and used a time filter to preprocess images. The velocity distributions in an intake duct is investigated at the flow rate ranging from 0–35 m3/h by an independently designed test rig, which is composed of an open circulating water tank, a pump, a control valve and other measuring sensors. The pressure transducers are also used to measure the pressure fluctuations at two typical positions near the lip of the intake duct. The velocity distributions at the midplane of the intake duct depict that there is a jet flow due to the presence of duct lip, and a strong separation flow is induced between the jet and the duct wall. The experimental results are useful to give a clear insight into the flow behaviors inside the intake, and show the reason of the flow distortion at the outlet of intake duct. Moreover, some guidance can be provided for the application of SPIV in the intake duct.
A Test of Steady State Erosion Models
Authors: Pease Leonard F. , Fuher Arich J. L. , Bamberger Judith Ann , Minette Michael J.
Published: October 2018

Here we ask the question: how well is the erosion of particle beds in vessels with curved bottoms at industrial scale flow rates represented by models of radial wall jets traversing flat surfaces using the critical shear stress for erosion from the Shields diagram? This mathematical construction has been used successfully to predict the functional forms for the extent of erosion with time using two dimensionless fitting parameters (Pease, et al., 2017). However, the direct prediction of the curves without fitting and scaling has not been tested quantitatively. Here we evaluate the radial wall jet models of Poreh, et al., (1967) and Rajaratnam (1976) and the expressions for the Shields diagram by Paphitis (2001) and Cao, Pender, and Meng (2006). The use of two models for each element accounts for uncertainty in model selection. The data selected to benchmark these models was obtained in a geometrically scaled version of an industrial scale mixing vessel with 12 jets arrayed in a double ring configuration (Meyer, et al., 2012). These particular jets were operated continuously with observations at steady-state, providing a direct comparison between the long-time erosion fronts and these proposed long-time solutions (i.e., where the applied shear stress equals the critical shear stress for erosion) without interference from transients or parameters that affect transients (e.g., the particle bed thickness). We find experimentally that the extent of the erosion depends significantly on the vessel curvature. Even so, we also find that all of these formulations significantly over predict the extent of erosion observed experimentally. A discussion of model features that may be modified to revise the theory into quantitative agreement is presented.
Investigation of the Causes of Static Instability in Externally Pressurized Gas Journal Bearings
Authors: Lawrence Tom M. , Kemple Marvin D.
Published: October 2018

Externally pressurized air journal bearings (air journal EPB’s) are lubrication-free, non-contact bearings that utilize air as the lubrication film. Unlike non-pressurized journal bearings that rely on rotation for hydrodynamic forces, the bearings generate a significant load bearing, restorative, hydrodynamic force independent of rotation. Empirical tests were performed measuring this restorative force and the mass flow out of the bearing-gap as a function of the eccentric displacement of the shaft from the bushing. Two different external pressurization configurations were examined. In one, the bearing-gap was pressurized via two rows of six feedholes located near the ends of the bearings (FH bearings). In the second, the bearing-gap was pressurized via porous liners (PL bearings). The so-called pressure compensation of the bearings was varied by altering the feedhole diameters for the FH bearings and changing the permeability of the PL bearings. The bearing clearance was varied by using shafts of varying OD’s. CFD calculations were then performed to simulate the empirical testing and were found to be in good agreement. The CFD analysis was then expanded over a wide design space for both the FH and PL bearings. The expectation was that the restorative force would continuously increase as the shaft was eccentrically displaced from the bushing reaching its maximum when the shaft was “grounded” so that the shaft contacted the bushing. However, the expanded CFD analysis showed a surprise result as it indicated that there were regions in the design space of the FH bearings where the maximum restorative force occurred at an eccentric displacement that placed the shaft near the bushing but did not ground it. Further eccentric displacement caused a decline rather than the expected increase in the restorative hydrodynamic force indicating a region of negative bearing stiffness. A reference was found that observed bearing load fall off (negative stiffness) with heavily loaded bearings (operating at high eccentricities) similar to our CFD findings. This area of operation was termed the “static instability region”. This paper presents further CFD analysis of the pressure wave in the bearing-gap at varying eccentricities. These waves are graphically dissected to show the characteristics that cause negative stiffness. The results indicate that PL bearings have no static instability and that the area of instability for FH bearings can be predicted.