Single-phase Computational Fluid Dynamics Research Articles

Comparative analysis of single and multiphase numerical frameworks for subcooled boiling flow in an internal combustion engine coolant jacket

Computational analysis of nucleate boiling occurring in liquid cooled applications, such as internal combustion engines is often implemented within a single phase Computational Fluid Dynamics (CFD) framework, owing to low vapor fractions involved. With increase in specific power and the resulting higher thermal loads, accounting for the presence of the vapor phase using a multiphase framework is required in certain conditions, despite the higher computational costs. While detailed resolution of the liquid and vapor phases in nucleate boiling using a two fluid model is excessively computationally expensive, the homogeneous mixture multiphase framework is a good compromise between resolution and computational cost. In this article a numerical wall boiling model is implemented within both, a single phase and the mixture multiphase frameworks. Results from the two approaches are compared with measurements in a channel flow. The results from both approaches are in good agreement with experiments. The single phase approximation is valid when the vapor generation is low. The sensitivity of the results to the computational grid is also discussed in detail. Further the two frameworks are used to simulate the heat transfer in the coolant jacket of a four-cylinder petrol engine. The results from the numerical simulations are compared with measurements. Both computational frameworks compare reasonably well with the measurements in terms of local metal temperature. However, the advantage of accounting for the vapor phase using the mixture multiphase framework is evident when the parameter related to vapor bubble interactions is analysed in detail. • The Blended Boiling Model is implemented within single phase and mixture multiphase frameworks in CFD. • Numerical results are validated with relevant experimental data in the literature. • The mixture multiphase framework is more accurate with increase in net vapor generation. • Excessive grid refinement leads to inaccurate results in both frameworks. • Heat transfer in passenger car engine is analysed using both frameworks and results are validated with test data.

Characterization of a small-scale crystallizer using CFD simulations and X-ray CT measurements

• Single and multiphase CFD of a small-scale crystallizer with two stirrer types. • Indications when each stirrer is preferably used based on hydrodynamics. • X-ray CT applied in mixing studies used for the validation of the CFD simulations. Although small-scale crystallizers have gained increasing importance in recent years, their hydrodynamic properties have not yet been thoroughly studied. The purpose of this work is to characterize an intensively stirred small-scale crystallizer and to analyze whether its fluid mechanics behavior deviates from being homogeneously well-mixed. To this aim, single and multiphase computational fluid dynamics (CFD) simulations have been conducted. Two types of stirrers, including one overhead stirrer (OS) and one magnetic stirrer (MS), were compared. The single-phase CFD simulations showed that the mixing characteristics of the two stirrers differ significantly, thereby leading to variations in the distribution of solids. The MS is an axial flow type impeller, which is better at handling solid suspensions than the OS. The simulations were complemented by X-ray computed tomography measurements, which confirmed that spatial variations in solid distribution occur and that assuming a homogeneous suspension might lead to errors when modeling crystallization processes.

Fluid-particle drag force measurements from particle-resolved CFD simulations of flow past random arrays of ellipsoidal particles

The lack of constitutive drag laws for non-spherical particles represents a current gap in simulating fluid-particle flows, even though such non-spherical shapes are encountered in numerous applications. Some previous studies have applied spherical drag models to non-spherical particles, while others have used ad-hoc methods to account for particle shapes. However, the present work demonstrates the need to develop dedicated non-spherical drag laws, given the limitations of these previous ad-hoc approaches. In the present study, single-phase computational fluid dynamics (CFD) simulations of fluid flow over random arrays of ellipsoids are used to directly calculate the fluid-particle drag forces. By varying the particle shape, orientation, solid fraction, and fluid-flow velocity (Reynolds number), this work probes the variables that would be important for future model development. A preliminary non-spherical drag model is proposed based on the CFD results, which illustrates the functional dependency of the non-spherical drag on particle aspect ratio and orientation.

Spectral Wave Explicit Navier-Stokes Equations for wave-structure interactions using two-phase Computational Fluid Dynamics solvers

This paper proposes an efficient potential and viscous flow decomposition method for wave-structure interaction simulation with single-phase wave models and two-phase Computational Fluid Dynamics (CFD) solvers. The potential part - represents the incident waves - is solved with spectral wave models; the viscous part - represents the complementary perturbation on the incident waves - is solved with the CFD solver. The decomposition strategy is called Spectral Wave Explicit Navier-Stokes Equations (SWENSE), originally proposed for single-phase CFD solvers ( Ferrant et al., 2003). Firstly, this paper presents a new two-phase SWENS Equations with interface capturing technique. To achieve this single-phase and two-phase decomposition, the incident fields are extended in the air with a density-weighted pressure. Secondly, an accurate and efficient interpolation method is proposed to transfer High Order Spectral (HOS) wave model's result on CFD mesh, which reduces drastically the divergence error of the interpolated velocity. Implemented within OpenFOAM, these methods are tested by three verification, validation, and application cases, considering incident wave propagation, high-order loads on a vertical cylinder in regular waves, and a Catenary Anchor Leg Mooring buoy in both regular and irregular waves. Speed-ups between 1.7 and 4.2 are achieved. The wave models and the interpolation method are released open-source to the public.

Shock waves in pulsed electrical discharges in liquids: numerical simulation and comparison to experiment

In this paper a computational model for the post-breakdown phase of an electrical discharge in liquids is presented and validated through comparison with data from a high voltage pulsed power discharge in a water-filled crucible. The numerical framework consists of an arc plasma channel modeled by equivalent resistor-inductor-capacitor circuit equations coupled to a computational fluid dynamics (CFD) model (based on OpenFOAM) that solves the compressible fluid equations for describing the shock waves associated with the rapid development of the plasma channel. The circuit model equations evolve the initial discharge plasma expansion which are then used to initialize the pressure pulse in the CFD model. Both single-phase (liquid) and two-phase (gas–liquid) CFD solvers were implemented and compared. The dynamics and magnitude of the simulated pressure pulse perturbations at the boundaries of the crucible were validated with the pressure sensor measurements made in an experiment of a pulsed electrical discharge in liquid water.

A comprehensive hydrodynamic analysis of a full-scale oxidation ditch using Population Balance Modelling in CFD simulation

This work exhibits the importance of the experimental validation when full-scale computational fluid dynamics (CFD) models are developed to provide a detailed analysis of the spatial variations in 3D of the fluid flow inside aerated tanks. Single-phase and two-phase CFD models were performed to study the fluid behaviour carefully by means of the velocity profiles and the aeration pattern in a full-scale oxidation ditch. Air hold-up, bubble size distribution and interfacial area density were calculated by polydisperse models where Population Balance Model (PBM) was governed by break-up and coalescence; the free-surface approach allowed the CFD model to describe the three-dimensional effect of bubbly plumes in large scales in detail. Tracer tests were carried out to obtain the flow pattern and the hydraulic distribution of the flow into two wastewater treatment lanes in order to define the boundary conditions for the model correctly. Despite the difficulty of performing velocity measurements of the fluid in 3D, with and without air bubbles, these provided essential information to validate the CFD model. From this analysis, several simulations were performed to improve the hydrodynamics and the operation of the process by relocating the propellers.

A pragmatic approach for simulating the fluid flow and heat transfer within industrial evaporator vessels

This article presents a pragmatic method for simulating the fluid flow and heat transfer in low-pressure, industrial evaporator systems. The method couples a single phase computational fluid dynamics model with an empirical correlation in order to account for the thermal effects of boiling. Due to the time-constraints often associated with industrial modelling, the method here is developed with the intention that rapid predictions of thermal and flow behaviour can be obtained to inform plant operators of the impact of changing evaporator process and operational configuration within the nuclear industry.The presented model results are compared against data obtained from an experimental rig of a scaled evaporator test section, based at Heriot-Watt University, UK. Tests operate at near-vacuum conditions (5 kPa) and experience a variety of heat transfer regimes, whereby portions of the rig are convectively heated and other regions undergo sub-cooled nucleate boiling.A mesh sensitivity study is performed in order to determine the optimal computational grid size, and demonstrate a sufficient level mesh independence, this is quantified using the GCI method. Additionally, a turbulence sensitivity study is carried out for a number of operational configurations. Finally, various boiling correlations are tested in order to determine the optimal correlation for the application here.Results in this work show that the modelling approach developed predicts temperatures in the various different regions of the rig well, also good agreement is achieved with the experimental data when using the Launder-Sharma K-Epsilon model (Launder and Sharma, 1974). In regions which were predicted to boil, it was found the Gorenflo nucleate boiling correlation (Gorenflo and Kenning, 2009) gave a good all-round approximation to the experimental temperatures.

Computational Fluid Dynamics Study of Serpentine Flow Field Proton Exchange Membrane Fuel Cell Performance

Fuel cell is an electro-chemical device that directly converts the chemical energy of fuel into usable electrical form without any toxic byproducts. Flow field plates are backbone of the fuel cell. It is the essential component of fuel cell with multifunctional character. In the present work, a complete three-dimensional steady-state isothermal single phase computational fluid dynamics model is proposed for a proton exchange membrane fuel cell with serpentine flow channel to analyze the species transport phenomenon. The numerical model was developed and analyzed using ANSYS FLUENT 15.0 and simulations were carried out to get the key parameters like variation of oxygen, hydrogen, liquid water activity, and water mass fraction in the flow channels, membrane water content, and membrane protonic conductivity. The numerical results show that, when the cell is operated at higher current densities, i.e. lower cell voltage, hydrogen and oxygen consumption is more as well as water generation is also more. The effect of cell operating temperature and reactants inlet humidity on cell performance was also studied. Finally, the polarization curve obtained was compared with the experimental results and it was found that the numerical results are having good agreement with the experimental results.

Effect of manifold flow configuration on two-phase ultra-high flux cooling

A two-phase, one-dimensional model for the pressure drop and flow distribution in a manifold-microchannel system was developed. The model was run with three different manifold configurations and trends were observed. First, the effect of mass flow rate was investigated in single-phase mode, and it was shown that while z-type manifolds provide superior flow distribution to that of c-type manifolds at low mass fluxes, at high mass fluxes c-type manifolds provide a superior flow distribution, due to the alignment of the inlet and outlet pressure gradients. In addition, due to the lack of an outlet pressure gradient, h-type manifolds were found to provide a more uniform flow distribution across the board. Next, the effects of low and high vapor qualities were investigated. In the end, it was shown that in z-type and c-type manifolds, the flow distribution worsens as vapor quality increases, while for h-type manifolds, the flow distribution improves. Accordingly, h-type manifolds are significantly simpler and less computationally expensive to design, requiring only single-phase computational fluid dynamics simulations, while z-type and c-type manifolds require both single-phase and two-phase computational fluid dynamics simulations.

Computational Fluid Dynamics (CFD)-Based Droplet Size Estimates in Emulsification Equipment

While academic literature shows steady progress in combining multi-phase computational fluid dynamics (CFD) and population balance modelling (PBM) of emulsification processes, the computational burden of this approach is still too large for routine use in industry. The challenge, thus, is to link a sufficiently detailed flow analysis to the droplet behavior in a way that is both physically relevant and computationally manageable. In this research article we propose the use of single-phase CFD to map out the local maximum stable droplet diameter within a given device, based on well-known academic droplet break-up studies in quasi-steady 2D linear flows. The results of the latter are represented by analytical correlations for the critical capillary number, which are valid across a wide viscosity ratio range. Additionally, we suggest a parameter to assess how good the assumption of quasi-steady 2D flow is locally. The approach is demonstrated for a common lab-scale rotor-stator device (Ultra-Turrax, IKA-Werke GmbH, Staufen, Germany). It is found to provide useful insights with minimal additional user coding and little increase in computational effort compared to the single-phase CFD simulations of the flow field, as such. Some suggestions for further development are briefly discussed.

A NUMERICAL STUDY OF A THREE-DIMENSIONAL PROTON EXCHANGE MEMBRANE FUEL CELL (PEMFC) WITH PARALLEL AND COUNTER FLOW GAS CHANNELS

Modeling the heat and mass transport in micro channel is being used extensively in researches and industrial applications. The aim is optimizing fuel cell designs before building a prototype for engineering application. In this study, numerical, three-dimensional, single phase computational fluid dynamics model of a proton exchange membrane fuel cell with both the gas distribution flow channels and the Membrane Electrode Assembly (MEA) has been developed. A single set of conservation equations which are valid for the flow channels, gas-diffusion electrodes, catalyst layers, and the membrane region, are solved by finite volume technique. The present simulated straight channels PEMFC model, accounts for major transport phenomena and the performance. Additionally, the effect of reversing the flow direction at cathode side has been investigated to study the fuel cell performance and species distribution. The results showed that, in the PEMFC with the counter flow channels, the output current density has been decreased and also the kind of species distributions has been influenced by this phenomenon. It is very important to model the back diffusion and electro-osmotic mass flux for determination of ionic conductivity of membrane which affects the performance of fuel cell. Finally, the numerical results are validated by available experimental data.

New method for predicting the incipient cavitation index by means of single-phase computational fluid dynamics model

Due to its serious consequences, cavitation in pipeline systems is one of the main concerns of engineers. In this context, the incipient cavitation index defined in many international standards such as those from International Society of Automation and International Electrotechnical Commission is a very important parameter as it accounts for the onset of cavitation. However, the standards define only the experimental determination of the incipient cavitation index, which is furthermore difficult to perform due to the considerable technical and economical burden of the experimental tests, above all in case of large-size systems. In this work, we propose a new method for predicting the incipient cavitation index by means of computational fluid dynamics, avoiding the need of making any experiment. The method is based on the generalized pressure criterion and requires only one Reynolds-averaged Navier–Stokes simulation of single-phase incompressible flow to provide estimates of the incipient cavitation index. The method is applied to predict the incipient cavitation index of multi-hole orifices with different geometrical characteristics, in terms of equivalent diameter ratio (0.40–0.70), relative thickness (0.73, 1.00), and number (13–52) and disposition of the holes. The experimental data revealed the reliability of the method. Its applicability was also confirmed for more complex geometries, and an application example regarding a control valve is briefly illustrated at the end of this article.

Numerical investigation of haemodynamics in a helical-type artery bypass graft using non-Newtonian multiphase model

The classic single-phase Newtonian blood flow model ignores the motion of red blood cells (RBCs) and their interaction with plasma. To address these issues, we adopted a multiphase non-Newtonian model to carry out a comparative study between a helical artery bypass graft (ABG) and a conventional ABG in which the blood flow is composed of plasma and RBCs. The investigation focused on the mechanism of RBC buildup in an ABG but the haemodynamic parameters obtained by single-phase and multiphase models were also compared. The aggregation of RBCs along the inside wall of a conventional ABG and at the heel of its distal anastomosis was predicted while a poor aggregation was observed along the helical ABG. In addition, RBCs were observed to gradually sediment along the gravity direction. However, the computed haemodynamic parameters by multiphase model qualitatively agreed well with those by single-phase model. It was concluded that (1) the single-phase computational fluid dynamics (CFD) is reasonable to do the computation of haemodynamic parameters in ABGs; (2) secondary flow does not definitely produce buildup of RBCs in the inside curvature, its configuration played an important role in the movement of RBCs and the dominating one-way rotating flow in a helical ABG guaranteed no buildup of RBCs on its inside wall and (3) gravity direction is important for the movement of RBCs which may help to explain why doing exercise is good for human health. This study helps to shed light on the migration of RBCs in ABGs, which cannot be explored by single-phase CFD models, and provides more understanding of the underlying flow mechanism for ABG failure.

Three-Dimensional Modeling and Development of the New Geometry PEM Fuel Cell

A complete three-dimensional and single phase computational fluid dynamics model for duct-shaped geometry proton exchange membrane (PEM) fuel cell was used to investigate the effect of using different connections between bipolar plate and gas diffusion layer on the performances, current density and gas concentration. The proposed model is a full cell model, which includes all the parts of the PEM fuel cell, flow channels, gas diffusion electrodes, catalyst layers and the membrane. Coupled transport and electrochemical kinetics equations are solved in a single domain; therefore, no interfacial boundary condition is required at the internal boundaries between cell components. This computational fluid dynamics code is used as the direct problem solver, which is used to simulate the two-dimensional mass, momentum and species transport phenomena as well as the electron- and proton-transfer process taking place in a PEMFC that cannot be investigated experimentally. The results show that the predicted polarization curves using this model are in good agreement with the experimental results. Also the results show that by increasing the number of connection between GDL and bipolar plate the performance of the fuel cell enhances and the current density at the cathode catalyst layer increases.

Three-dimensional computational fluid dynamic analysis of the conventional PEM fuel cell and investigation of prominent gas diffusion layers effect

A full three-dimensional, single phase computational fluid dynamics model of a proton exchange membrane fuel cell (PEMFC) with both the gas distribution flow channels and the membrane electrode assembly (MEA) has been developed. A single set of conservation equations which are valid for the flow channels, gas-diffusion electrodes, catalyst layers, and the membrane region are developed and numerically solved using a finite volume based computational fluid dynamics technique. In this research some parameters such as oxygen consumption, water production, velocity distribution, ohmic losses, liquid water activity and fuel cell performance for straight (base case) and prominent gas diffusion layers were investigated in more detail. The numerical simulations reveal that prominent gas diffusion layer improves the transport of the reactant gases through the porous layers; it is due to increase of the mentioned fuel cell efficiency, and prominent gas diffusion layers yield appreciably higher current density. Finally the numerical results of proposed CFD model (base case) are compared with the available experimental data that represent good agreement.

Single-phase computational fluid dynamics applicability for the study of three-dimensional flow in 5′ 5 rod bundles with spacer grids

Spacer grids play an important role in pressurized water reactor (PWR) fuel assembly in that they have significant influence on the thermal-hydraulic characteristics of the reactor core. But so far, the numerical studies are performed without regarding dimple and spring of spacer grids, just considering mixing vane. Moreover, these studies use k-ɛ turbulence model without considering the suitability of the other turbulence models upon the different spacer grids flow. A study is carried out to understand the 3-D single-phase flow in AFA-2G 5×5 rod bundles with spacer grids based on numerical method. In order to investigate the suitability of different turbulence models, k-ɛ model and k-ω model, the influence of different parts of spacer grid on the fluid flow is also predicted. By using second-order upwind scheme, hybrid grids technique, and improved SIMPLEC algorithm, the Reynolds averaged mass conservation and momentum conservation equations are solved, and the pressure and velocity field of flow are obtained. The numerical simulation results are compared with experiment results and the agreement is satisfactory. The simulation results show the influences of the spring, dimple and mixing vane, and the different characteristics of the k-ɛ model and k-ω model. Comparing with the experiment results, the simulation results suggest that the k-ω model is suitable for the simulation of the rod bundle flow with spacer grids; the spring and dimple are the main causes of the pressure loss in the spacer grid channel. The friction coefficient of the channel with spring and dimple is 1.5 times the coefficient of the channel with the vane. These results are beneficial to enhance the simulation ability of spacer grids flow and optimization design ability of spaces grid.

RANS modeling for flow in nuclear fuel bundle in pressurized water reactors (PWR)

This paper presents use of Reynolds-averaged Navier–Stokes (RANS) based turbulence model for single-phase CFD analysis of flow in pressurized water reactor (PWR) assemblies. An open source code called OpenFoam was used for computational fluid dynamics (CFD) study using computational meshes generated using Shari Harpoon. The PWR assembly design used in this analysis represents a 5 × 5 pin design including structural grid equipped with mixing vanes. The design specifications used in this study were obtained from the experimental setup at Texas A&M University and the results obtained are used to validate the CFD software, algorithm, and the turbulence model used in this analysis.

On the Establishment of a Web Portal for the Assessment of CFD in Reactor Safety Analysis

Within the framework of the responsibilities of the Organisation for Economic Co-operation and Development/Nuclear Energy Agency (NEA), three international writing groups (WGs) were set up in 2003 to produce state-of-the-art reports on specific aspects of the use of computational fluid dynamics (CFD) in addressing nuclear reactor safety (NRS) issues. Three reports followed on (a) the use of best practice guidelines in producing reliable computational results, (b) the status of the validation database for single-phase CFD applications to NRS, and (c) the extensions needed to the codes for application to two-phase NRS problems. The present paper summarizes the contents of the second of these documents and reports on the more recent activity of establishing a Web portal to act as a dynamic database for the material contained in the document. The Web pages can be reached via the NEA Web site and have been constructed using Wiki software, which enables users to readily find information they need in their respective application areas via active links, as well as to interactively support the maintenance of the site by contributing to the regular updating of the material. As a result of this endeavor, an up-to-date access point to the validation databases cataloged by the WG has been established, and a means has been provided for users to contribute with new material, thus keeping the information topical.

Elucidating Liquid Water Distribution and Removal in an Operating Proton Exchange Membrane Fuel Cell via Neutron Radiography

Neutron radiography was used to quantify the steady-state water content and its distribution in a 50 cm2 operating proton exchange membrane fuel cell. It was observed that the liquid water distribution near the corners of the gas-flow channels (GFCs) is influenced by the local gas-flow velocity as determined by the cathode stoichiometric flow ratio. At low velocity, the distribution of liquid water down the channel was found to be fairly uniform with only a slight reduction in liquid water content at the exit of the GFC corners. It was further observed that as the cathode gas-flow velocity is increased, a noticeable pattern develops in which liquid water is concentrated at the entrance to the GFC corners and becomes depleted in the corner and near the exit of the corner; liquid water content again increases further down the channel away from the corners. A single-phase computational fluid dynamics (CFD) model was developed and employed to help explain the observed water-distribution patterns. Flow-fields computed from our CFD model reveal recirculation regions in the GFC corners as well as in the areas of increased local gas-flow velocity, which help explain the experimentally observed liquid water distribution.

Application of Multiphase Computational Fluid Dynamics to Analyze Monocyte Adhesion

Study of the mechanisms of monocyte adhesion initiating atheroslerotic lesions has engaged investigators for decades. Single-phase computational fluid dynamics (CFD) analyses fail to account for particulate migration. Consequently, inconsistencies arise when correlating adhesion with wall shear stress (WSS). The purpose of this paper is to present, to our knowledge, the first computational analysis of in vitro U937 monocyte-like human cell adhesion data using a coupled multiphase CFD-population balance adhesion model. The CFD model incorporates multiphase non-Newtonian hemodynamic models to compute the spatial distributions of freely flowing monocytes and WSSs in control volumes adjacent to the wall. Measurements of monocyte adhesion onto an E-selectin-coated flow model that included an idealized stenosis and an abrupt expansion were available from the literature. In this study, we develop a new monolayer population balance adhesion model, based on the widely accepted mechanism of ligand-receptor binding, coupled to the CFD results. The monolayer population balance model accounts for the interactions of freely flowing, rolling, and adhering monocytes with surfaces via first-order reactions, transport of rolling cells in the monolayer, and the concept of a WSS detachment threshold, clearly evident in the adhesion experiments. The new paradigm of coupling the multiphase hemodynamic CFD model with the proposed adhesion model is illustrated by determining and interpreting the model parameters for experimental datasets having Reynolds numbers of 100 and 140. The coupled multiphase CFD adhesion model is able to simultaneously predict the spatial variations in freely flowing monocytes, their adherent number density, and carrier fluid WSSs adjacent to ligand-coated flow cell surfaces.