Turbulent Two-phase Flow Research Articles

Some Reasons for Nonmonotonic Variation of Descrete-Phase Concentration in a Turbulent Two-Phase Jet

Abstract—A mathematical model of a two-phase turbulent jet flow is presented, the averaged equations of which are written for each phase in Euler variables. Analysis of the results of calculations performed using this mathematical model made it possible to identify the origins of a nonmonotonic variation in the volume concentration of the dispersed phase along the axis of the two-phase jet. The influence of particle size, transverse velocity, particle concentration, and phase velocity slip in the initial section of the jet on the variation of the particle concentration in the jet is considered. It is shown that the listed factors cause a nonmonotonic variation in the particle volume concentration in a two-phase jet only when the value of the Stokes number based on the phase parameters in the initial section of the jet is greater than 0.14–0.15. At lower values of the Stokes number, when the particles can be regarded as a passive admixture, their volume fraction in the jet decreases monotonically along the jet axis, same as the impurity concentration in a gas jet of variable composition.

Thermal performance predictions for an HFE-7000 direct flow boiling cooled battery thermal management system for electric vehicles

In this paper, a novel battery thermal management system (BTMS) using the dielectric, non-flammable HFE-7000 refrigerant is proposed for electric vehicles (EVs). Its thermal performance is studied both numerically and experimentally. The refrigerant flows and boils on the battery wall surfaces, which lowers the thermal contact resistance as well as enhances the heat transfer process. Therefore, the thermal performance of the battery module is improved. The results indicate that forced convection heat transfer of the liquid refrigerant is dominating in the control of the temperature rise in the battery module. The maximum battery temperature drops to 35.10°C at 0.3ms-1 inlet velocity and a 5C discharge rate. In contrast, the temperature uniformity between individual battery cells primarily depends on the nucleate boiling heat absorption and local perturbation of the two-phase turbulent flow. A temperature difference of no more than 3.71°C can be observed at 5C discharge rate and 0.1ms- 1. In addition, good agreement was found between the numerical results and experimental data.

Simulation scaling studies of reactor core two-phase flow using direct numerical simulation

Tremendous growth in supercomputing power in recent years has resulted in the emergence of high-resolution flow analysis methods as an advanced research tool to evaluate single and two-phase flow behavior. In particular, unstructured mesh-based methods have been applied to analyze flows in complex reactor core geometries, including those of light water reactors (LWR). The finite-element based code, PHASTA, is utilized to perform large-scale simulations of two-phase bubbly flows in LWR geometries. Given the large computational cost of direct numerical simulation (DNS) coupled with interface tracking methods (ITM), typical domains encompass a portion of a single subchannel. In the presented research, the state-of-the-art analysis of turbulent two-phase flows in complex LWR subchannel geometries are demonstrated at both prototypical reactor parameters as well as scaled low pressure conditions. Three different cases are studied, a high-pressure simulation in prototypical reactor subchannel geometry, a low-pressure case in prototypical geometry and a final low-pressure case in a geometry scaled up to conserve the ratio between the bubble size and the domain pitch. Utilizing advanced statistical processing tools, these simulation conditions are compared to shed light on the relevancy of two-phase flow characteristics given the significant differences between LWR and low-pressure conditions. These findings can lead to the generation of useful guiding principles when researchers need to scale the two-phase flow behavior captured at low pressure and temperature conditions to those at reactor operating conditions.

Erosion of rectangular channel bend in two-phase natural gas particles turbulent flow

In this research, we study the effect of solid particles flow on the amount of erosion of the channel bends with different aspect ratios by using CFD. The results of this study determined the depth of erosion of the bends as well as the location of the erosion. The results indicate a relative reduction in erosion values in the low aspect ratios. Due to the shape and the type of Dean vortices created in the channel, the erosion is minimum for A.R. = 0.8 and 1.2 and is maximum for A.R. = 0.6, 1, and 1.4. Also, the obtained graphs show a relative reduction of about 25% of the erosion rate in the rectangular channels compared to circular duct. The location of the erosion and its amount obtained by simulation are compared with the available experimental results of a square bend that shows good agreement with the experimental data as a part of the evaluation.

Numerical simulation of flow boiling in an orthogonally rotating duct

Numerical predictions of flow boiling in a square tube rotating in vertical direction at a uniform rotating speed are presented. There-dimensional physical model and numerical model were established for simplified problem. Realizable k−e (RKE) turbulent model was employed to study vapor–liquid two-phase turbulent flow. Heat and mass transfer between vapor and liquid were solved based on volume of fluid multiphase flow model combined with user-defined function. The transient results show that wall superheat at onset of nucleate boiling (ONB) condition in rotating tube is obviously higher than that in stable tube. Based on Jens–Lottes formula of Bowring model, empirical formulas for wall superheat at ONB condition were modified with consideration of Rossby number. As the rotation speed increased, location of ONB shifts to the tube exit. Flow pattern in tube is determined by coupling effect of heating and rotation. Stability of flow pattern is gradually weakened during the increase in rotation speed. The numerical results are in fair agreement with two sets of experimental data for same physical model and working conditions.

A new approach to include surface tension in the subgrid eddy viscosity for the two-phase LES

Turbulent two-phase flows feature different mechanisms for production and dissipation of turbulent kinetic energy compared to the single-phase flows. However, this difference is usually neglected in developing eddy viscosity-based subgrid scale (SGS) models for the two-phase large eddy simulation (LES). In this study, a new approach is presented for the two-phase LES to include the surface tension, which is a production mechanism for the kinetic energy in the small scale motions, into the subgrid eddy viscosity model. We follow the Favre-filtered governing equations of interfacial flows based on the volume of fluid (VOF) approach and derive the transport equation for the turbulent kinetic energy to include the effect of surface tension. The original contribution of this study is to propose a new form for the eddy viscosity based on the mixing length assumption which includes an additional production mechanism of turbulent kinetic energy stemming from the interfacial work i.e. surface tension. The proposed model for eddy viscosity is employed to close all the SGS terms. The model performance is evaluated by means of the a-priori filtering of the fine grid simulation of phase inversion problem. To test the generality of the model at different physical conditions, two different density ratios were considered for the fine grid simulation. The results highlight a significant improvement of the eddy viscosity-based SGS models in prediction of the turbulent kinetic energy for the small unresolved scales particularly for the regions of low shear. Furthermore, the model appears to perform more accurately in the case of low density ratios. This study provides a proper perspective for future SGS models in the context of large eddy simulation of two-phase flows.

Experimental investigation of air–water turbulent swirling flow of relevance to phase separation equipment

Single and two-phase turbulent swirling flow inside a circular pipe in the presence of a conical bluff body is studied using high speed photography and laser-Doppler velocimetry. The geometrical configuration is relevant to liquid–gas separating equipment. Several flow regimes were identified and, in particular, the results show the presence of a spiral axial flow for low values of gas–liquid ratio (GLR). High speed photographs indicated that the flow consists of a gas core surrounded by a liquid annulus and forms a liquid–gas interface corrugations of capillary nature. The tip of the gas core becomes unstable and ejects gas bubbles in a spiraling stream. The velocity measurements indicated an increase of turbulent fluctuations in the liquid layer in the vicinity of the interface. The liquid layer maintained a Rankine vortex, while a weak forced vortex was obtained for higher GLR.

Spray characterization for engine combustion network Spray G injector using high-fidelity simulation with detailed injector geometry

This article presents a computational fluid dynamics study of the engine combustion network Spray G, focusing on the transient characteristics of the spray during the start of injection and the impacts of nozzle geometry details derived from the manufacturing process. The large-eddy-simulation method, coupled with the volume-of-fluid method, was used to model the high-speed turbulent two-phase flow. A moving-needle boundary condition was applied to capture the internal flow boundary condition accurately. The injector geometry was measured with micron-level resolution using X-ray tomographic imaging at the Advanced Photon Source at Argonne National Laboratory, providing detailed machining tolerance and defects from manufacturing and a realistic rough surface. For comparison, a nominal geometry and a modified geometry incorporating measured large-scale geometric features but no surface details were also used in the simulations. Spray characteristics such as mass flow rate, injection velocity, and Sauter mean diameter were analyzed. Significantly distinct spray characteristics in terms of injection velocity, spray morphology, and primary breakup mechanism were predicted using different nozzle geometries, which is mainly attributable to the realistic surface finish and manufacturing defects. The measured high-resolution geometry predicts a lower injection velocity, a wider-spreading spray, and an overall slower breakup rate with evident injector tip wetting compared to the ideally smooth nozzle boundary. This result implies that the manufacturing details of the injector, which are usually ignored in fuel injection studies, have a significant impact on the spray development process and should be taken into account for design optimization.

PIV Measurement and CFD Simulation of Liquid-Liquid Mixing in Mixer Settler with Rigid-Flexible Impeller

Abstract Particle image velocimetry (PIV) was used to measure the flow field in the mixing chamber of a mixer settler. An Eulerian-Eulerian approach, standard k-ε turbulence model, and multiple reference frames (MRF) technique were employed to simulate liquid-liquid two-phase flow, turbulent flow, and impeller rotation in the mixer settler, respectively. The effects of impeller type, impeller clearance and baffle configuration on the flow field were analyzed. Results showed that rigid-flexible impeller can spread the impeller energy more uniformly in the mixing chamber and increase the velocity of the continuous phase through the multi-body motion of flexible sheet compared with rigid impeller. The flow numbers of RF-RT, RF-PBT and RF-CBT systems were increased by 46 %, 54 % and 43 %, respectively, compared with RT, PBT and CBT systems. Meanwhile, the suction capacity of impeller was enhanced with a decreased impeller clearance, and the backflow was eliminated by the installation of baffles.

Performance analysis of axial and reverse flow cyclone separators

Performance characteristics of a novel cyclone with tangential inlet were presented in axial and reverse flow operation modes. 3-D and unsteady governing equations were used for the numerical solution of the two-phase turbulent flow in the cyclone separator. The Eulerian approach was used to solve the flow field, and the Reynolds Stress Model (RSM) with the scalable wall function was employed for the numerical study. The Lagrangian approach with the Discrete Phase Model was used to calculate the discrete phase by releasing particles from the inlet surface. CFD calculations were run for different geometric configurations to analyze the performance of the cyclones regarding pressure drop, cut-off diameter, and fractional efficiency. Axial and tangential velocity profiles are presented at the defined sections. The computational results of pressure drop, velocity field, and separation efficiency were also compared for the axial and reverse flow cyclones at the same flow rate. The results show that pressure drop and collection efficiency in reverse flow mode are higher than that of the axial flow operation. However, axial flow cyclones seem to be more efficient for small particles comparing to reverse flow cyclones.

Numerical simulation of solid-liquid mixing characteristics in a stirred tank with fractal impellers

The hydrodynamics of solid-liquid mixing process in a stirred tank with four pitched-blade impellers, fractal 1 impellers, and fractal 2 impellers were investigated using computational fluid dynamics (CFD) simulation. An Eulerian-Eulerian approach, standard k-ε turbulence model, and multiple reference frames (MRF) technique were employed to simulate the solid-liquid two-phase flow, turbulent flow, and impeller rotation, respectively. The effects of impeller speed, impeller type, impeller spacing, impeller blade tilt angle, impeller blade shape, solid particle size and initial solid particle loading on the solid particle suspension quality were investigated. Results showed that the homogenous degree of solid-liquid system increased with the increase of impeller speed. The impeller spacing of T5/6 and T and impeller blade tilt angle of 60° and 45° were appropriate for the solid-liquid suspension process. Fractal shape impeller was more efficient than jagged shape impeller in solid-liquid mixing process. Larger particle diameter and higher initial solid particle loading resulted in less homogenous distribution of solid particles. It was found that fractal impeller could improve the solid particle suspension quality compared with four pitched-blade impeller under the same power consumption, increasingly so with the fractal iteration number of fractal impeller. Moreover, fractal impeller reduced the size of impeller trailing vortex and consumed less power consumption compared with four pitched-blade impeller at the same impeller speed, and the more the number of fractal iteration, the higher the impeller energy utilization rate of fractal impeller.

Collision of Particles and Droplets in Turbulent Two-Phase Flows

The problems and features of an accounting of the collisions of particles (droplets) in turbulent two-phase flows are considered. The developed approaches for the determination of collision nuclei of monodisperse and bidisperse particles (droplets) in uniform isotropic turbulence, as well as under the combined action of turbulence, averaged velocity gradient, and gravity, are described. The results of experimental and theoretical calculations of the effect of collisions on the characteristics of two-phase jet flows and flows in channels are presented and analyzed.

Numerical Analysis of a Two-Phase Flow (Oil and Gas) in a Horizontal Separator used in Petroleum Projects

In the present paper, two-phase three-dimensional turbulent flow simulations are carried out by applying computational fluid dynamics (CFD) technique to the internal flow of a horizontal separator that is used in petroleum industry. Two different geometries are considered; the separator with a straight plate at the top and the separator with the straight plate on the side of the separator. Effects of the location, distance between the inlet of the separator and the diverter plate and inlet velocity on the separation efficiency are investigated by employing the standard k- ε turbulence model. For these purposes, three different distances between the straight diverter plate and the inlet to the separator (0.1 m, 0.15 m and 0.2 m) and four different velocities (0.25 m/s, 0.5 m/s, 0.75 m/s, and 1 m/s) are taken into account by means of Euler mixture model. It is revealed that the maximum separation efficiency is 99.772% when the mixture enters the separator from the top with the inlet velocity of 0.25 m/s and the plate is located 0.2 m away from the inlet section of the separator. An inverse correlation is detected between the inlet velocity and the efficiency of the separation since increasing the inlet velocity decreases the efficiency of the separation.

Modeling of bubble dynamics and heat transfer in polydispersed two-phase turbulent flow in a vertical pipe

The numerical results on the flow, bubble distributions and heat transfer in a bubbly polydispersed upward flow in a pipe are presented. The mathematical model is based on the Eulerian approach with considering the back effects of bubbles on the mean and turbulent characteristics of the carrier fluid phase. The model is coupled with the method of δ-function approximation. The model takes into account the interphase momentum transfer, bubble breakup and coalescence processes. The set of axisymmetrical RANS equations is used for modeling two-phase bubbly flows. Turbulence of the carrier fluid phase is predicted using the Reynolds stress transport model. The effect of variation in the gas volumetric flow rate ratio, inlet mean fluid temperature, and its velocity on the flow structure and heat transfer in the two-phase flow is analyzed. The addition of air bubbles results in a significant increase in the heat transfer rate (up to three times) and the effect augments by increasing the gas volumetric flow rate ratio. The numerical simulations show a good agreement with the experimental and numerical results of other authors.

Numerical Simulation of Bubble-Liquid Two-Phase Turbulent Flows in Shallow Bioreactor

An improved second-order moment bubble-liquid two-phase turbulent model is developed to predict the hydrodynamic characteristics of the shallow bioreactor using two height-to-diameter ratios of H/D = 1.4 and H/D = 2.9. The two-phase hydrodynamic parameters, the bubble normal and shear stress, the bubble energy dissipation rate, the bubble turbulent kinetic energy, etc. were numerically simulated. These parameters increased along with flow direction and constituted a threat to cells living at far distance away from the gas jetting inlet regions, rather than a finding of higher cell damage at near the jetting inlet region, as reported by Babosa et al. 2003. A new correlation named the turbulent energy production of bubble-liquid two-phase flow was proposed to successfully verify this experimental observation. A smaller H/D ratio makes more contributions to the generation of lower turbulent energy productions, which are in favor of the alleviation of cell damage. The extremely long and narrow shape of the bioreactor is deteriorative for cell living.

Liquid impingement cooling of cold plate heat sink with different fin configurations: High heat flux applications

The objective of this study is to investigate the cooling performance of the liquid jet impingement of a cold plate heat sink with different fin geometries using water and nanofluids as working fluids. In experiments, the monitored parameters mainly focus on the effects of different heat flux, the water mass flow rate, and heat source area on its thermal resistance. In the numerical study, a three-dimensional of a single-phase and two-phase Eulerian turbulent flow models are used to visualize the temperature and fluid flow behaviors of the cold plate heat sinks. The initial and the boundary conditions of the model based on the experimental conditions. It indicated from the experiments that the increasing of Reynolds number, heat source area, and power input results in decreased thermal resistance. Using nanofluids as working fluid gives the thermal resistance lower than de-ionized water as the working fluid. Additionally, the predicted results have verified with the measured data. It found that good agreement is obtained and gives an average error of 2.69%, 11.77% for CP base temperature and thermal resistance, respectively.

Near-wall dynamics of inertial particles in dilute turbulent channel flows

This investigation considers the effect of the Stokes number on the near-wall particle dynamics of two-phase (solid-fluid) turbulent channel flows. The spectral element method-based direct numerical simulation code Nek5000 is used to model the fluid phase at a shear Reynolds number, Reτ = 180. Dispersed particles are tracked using a Lagrangian approach with one-way coupling. Eulerian fluid and particle statistics are gathered and analyzed to determine the effect of the Stokes number, first on macroscopic statistics. Previous work of this nature indicates that mean streamwise particle velocities and root-mean-square velocity fluctuations are reduced in the bulk and increased very close to the wall, an effect which is stronger with increased particle Stokes number or inertial particles. This phenomenon has important consequences for mechanisms such as particle deposition and preferential concentration, and so for the first time, this work aims to elucidate the dynamics of this effect through rigorous analysis on various scales. An in-depth force analysis indicates the importance of the lift force, even at increased Stokes numbers, in predicting particle motion in the buffer layer and log-law regions. It is also observed that pressure gradient and virtual mass forces are significant close to the wall. Alongside bulk velocity and acceleration statistics, microscopic behavior is analyzed by considering region-based particle dynamics. Probability density functions are used to determine the effect of the Stokes number on particle motion in three near-wall regions, as well as within the bulk flow. It is observed that at higher Stokes numbers, the viscous sublayer contains particles with dynamic properties similar to those present in the buffer layer. This suggests rapid interlayer migration in the wall direction, causing increased particle turbulence intensities in near-wall regions. A local flow topology classification method is also used to correlate particle behavior with near-wall coherent turbulent structures, and a mechanism for particle sweep toward the wall is suggested. Finally, low-speed streak accumulation and interlayer particle fluxes are considered and the extent of mixing for low and high Stokes numbers is discussed.

A Review of Engine Fuel Injection Studies Using Synchrotron Radiation X-ray Imaging

Fuel spray characteristics directly determine the formation pattern and quality of the fuel/air mixture in an engine, and thus affect the combustion process. For this reason, the improvement and optimization of fuel injection systems is crucial to the development of engine technologies. The fuel jet breakup and atomization process is a complex liquid–gas two-phase turbulent flow system that has not yet been fully elucidated. Owing to the limitations of standard optical measurement techniques, the spray breakup mechanism and its interaction with the nozzle internal flow are still unclear. However, in recent years synchrotron radiation (SR) X-ray imaging technologies have been widely applied in engine fuel injection studies because of the higher energy and brilliance of third-generation SR light sources. This review provides a brief introduction to the development of SR technology and compares the critical parameters of the primary third-generation SR light sources available worldwide. The basic principles and applications of various X-ray imaging technologies with regard to nozzle internal structure measurements, visualization of in-nozzle flow characteristics and quantitative analyses of near-field spray transient dynamics are examined in detail.

Influence of electrolyte flow mode on characteristics of electrochemical machining with electrolyte suction tool

Abstract Electrochemical machining (ECM) is a promising nontraditional machining method for shape generation, which has many advantages such as capable of processing difficult-to-cut metallic materials without residual stress, good surface quality, high efficiency, no tool wear, etc. However, in many cases, problems of relatively poor machining accuracy and possible environmental risks hinder further application of this process. Hence an electrolyte suction tool has been proposed for ECM, which is expected to give a solution to these problems by restraining the electrolyte flow within the suction tool. In this paper, the effects of different electrolyte flow modes in suction tool on characteristics of ECM process were investigated both theoretically and experimentally. A multi-physics model coupling with a turbulent two-phase flow field and an electrical field was established for the ECM with the suction tool. A set of observational experiments and machining experiments were designed and conducted to clarify the characteristics of the electrolyte flow field and verify the results of simulation. The results show that under the inward flow mode (IFM), much surrounding air is sucked into the machining area and a relatively stable gas-liquid zone is formed, leading to poor machining characteristics. Meanwhile, the electrolyte is confined to a small area under the outward flow mode (OFM), which effectively improves both surface quality and machining accuracy of the ECM process.

Direct Numerical Simulation of Gas-Liquid Drag-Reducing Cavity Flow by the VOSET Method.

Drag reduction by polymer is an important energy-saving technology, which can reduce pumping pressure or promote the flow rate of the pipelines transporting fluid. It has been widely applied to single-phase pipelines, such as oil pipelining, district heating systems, and firefighting. However, the engineering application of the drag reduction technology in two-phase flow systems has not been reported. The reason is an unrevealed complex mechanism of two-phase drag reduction and lack of numerical tools for mechanism study. Therefore, we aim to propose governing equations and numerical methods of direct numerical simulation (DNS) for two-phase gas-liquid drag-reducing flow and try to explain the reason for the two-phase drag reduction. Efficient interface tracking method—coupled volume-of-fluid and level set (VOSET) and typical polymer constitutive model Giesekus are combined in the momentum equation of the two-phase turbulent flow. Interface smoothing for conformation tensor induced by polymer is used to ensure numerical stability of the DNS. Special features and corresponding explanations of the two-phase gas-liquid drag-reducing flow are found based on DNS results. High shear in a high Reynolds number flow depresses the efficiency of the gas-liquid drag reduction, while a high concentration of polymer promotes the efficiency. To guarantee efficient drag reduction, it is better to use a high concentration of polymer drag-reducing agents (DRAs) for high shear flow.