Green infrastructure can reduce PM2.5 traffic emissions on a city scale, by a combination of dispersion by trees and deposition on buildings, trees and grass. Simulations of PM2.5 concentrations were ...performed using a validated CFD model. A 2 × 2 km area has been reconstructed as a 3D representation of Leicester (UK) city centre which is on a scale larger than most of the other CFD studies. Combining both the effects of tree aerodynamics and the deposition capabilities of trees and grass is also something that has not yet been modelled at this scale. During summer time in Leicester City, the results show that the aerodynamic dispersive effect of trees on PM2.5 concentrations result in a 9.0% reduction. In contrast, a decrease of PM2.5, by 2.8% owing to deposition on trees (11.8 t year−1) and 0.6% owing to deposition on grass (2.5 t year−1), was also observed. Trees and grass are shown to have greater effects locally, as smaller decreases in PM2.5 were found when considering reduction across the whole boundary layer. Densely built areas like Leicester City centre have relatively less vegetation and subsequently have a smaller effect on PM2.5 concentration. It was found that particle deposition on buildings was negligible with less than 0.03%. An empirical equation was derived to describe the changes in PM2.5 based on ground surface fraction of trees and grass, and their deposition velocities.
•We model the effectiveness of trees and grass on traffic PM2.5 reduction.•City scale CFD simulations were performed under the OpenFOAM software.•Aerodynamics effect of tree prevails over deposition.•Tree are beneficial for wind speeds greater than 2 m s−1.•PM2.5 deposition on buildings is negligible with less than 0.03 %.
Liquid Metal Batteries (LMBs) are a promising grid-scale energy storage technology that offers low costs per kilowatt-hour, high energy and current densities, as well as low fade rates. The ...all-liquid composition of the batteries, as well as the presence of temperature gradients and electric and magnetic fields, result in the occurrence of multiple fluid phenomena. These can affect the hydrodynamic stability of the battery, thereby making their interactions critical to understand. In this work, the interaction of Rayleigh-Bénard convection and Electro-vortex flow is investigated as these types of flow will be present in Liquid Metal Batteries from laboratory to grid-scale. A single-layer electrode is simulated, and the computed results compared with experimental data from the literature. It was found that Rayleigh-Bénard convection is unsteady in the liquid metal electrode. The introduction of a 2 A current stabilises the convection cells, whilst the introduction of a 40 A current leads to the dominance of Electro-vortex flow at the central region of the electrode. The results in this work match experimental data closer than previously published models offering insight into the interaction between Rayleigh-Bénard convection and Electro-vortex flow in the anodes of discharging Liquid Metal Batteries.
•Development of a numerical model for Liquid Metal Batteries.•Coupled model of Rayleigh-Bénard convection and Electro-vortex flow.•At high currents, electro-vortex flow dominates under the current collector.•At low currents, Rayleigh-Bénard convection is stabilised by induction.
We devise a numerical method for passive advection of a surface, such as the interface between two incompressible fluids, across a computational mesh. The method is called isoAdvector, and is ...developed for general meshes consisting of arbitrary polyhedral cells. The algorithm is based on the volume of fluid (VOF) idea of calculating the volume of one of the fluids transported across the mesh faces during a time step. The novelty of the isoAdvector concept consists of two parts. First, we exploit an isosurface concept for modelling the interface inside cells in a geometric surface reconstruction step. Second, from the reconstructed surface, we model the motion of the face–interface intersection line for a general polygonal face to obtain the time evolution within a time step of the submerged face area. Integrating this submerged area over the time step leads to an accurate estimate for the total volume of fluid transported across the face. The method was tested on simple two-dimensional and three-dimensional interface advection problems on both structured and unstructured meshes. The results are very satisfactory in terms of volume conservation, boundedness, surface sharpness and efficiency. The isoAdvector method was implemented as an OpenFOAM® extension and is published as open source.
•CFD simulation of CO2 methanation in a shell-and-tube reactor was performed.•A conjugated heat transfer solver was developed using OpenFOAM.•The VOF model was implemented for phase change of the ...water on the shell side.•A large catalyst heat conductivity facilitates heat transfer to the shell side.•Using boiling water can effectively reduce the coolant flow rate.
This paper reports a computational fluid dynamics simulation of carbon dioxide (CO2) methanation via the Sabatier process in a shell-and-tube reactor, incorporating a water phase change simulation on the shell side using the volume of fluid (VOF) method. The essential heat removal factors and their mechanisms, such as the catalyst size, bed porosity, thermal conductivity, coolant flow rate, and coolant boiling, were primarily investigated. The computational code was a newly developed in-house version of ChtMultiRegionFOAM based on OpenFOAM v2006. This code can simulate catalyst-gas heat transfer, which is called the two-temperature model. The code was extended to new methanation and phase-change models based on the VOF method. The computational code was first validated using existing experimental data. After sufficient accuracy was confirmed, several parametric studies were conducted. Catalyst heat conductivity was the most influential factor among the catalyst-related parameters. The flow rate of the oil coolant significantly affected the cooling performance. Finally, the use of boiling water significantly reduced the coolant flow rate. Moreover, the cooling performance remained unchanged until the total heat release rate from the gas exceeded the total latent heat of the water.
CFD-based numerical wave tank (CNWT) models, are a useful tool for the analysis of wave energy converters (WECs). During the development of a CNWT, model validation is vital, to prove the accuracy of ...the numerical solution. This paper presents an extensive validation study of a CNWT model for the 1:5 scale Wavestar point-absorber device. The previous studies reported by Ransley et al. 1 and Windt et al. 2 are extended in this paper, by including cases in which the power-take off (PTO) system is included in the model. In this study, the PTO is represented as a linear spring-damper system, providing a good approximation to the full PTO dynamics. The spring stiffness and damping coefficients in the numerical PTO model are determined through a linear least squares fit of the experimental PTO position, velocity and force data. The numerical results for free surface elevation, PTO data (position, velocity, force), generated power and pressure on the WEC hull are shown to compare well with the experimental measurements.
•An extensive validation study of a CFD model for the 1:5 scale Wavestar point-absorber device is presented.•The validation presented in previous studies is extended by including cases with an active Power Take-Off (PTO) system.•The physical PTO system is accurately replicated through a linear spring-damper system in the numerical model.•The presented model of the Wavestar point-absorber is considered to be, in a general sense, validated.
Recent advances in the miniaturisation of plasma-based microelectromechanical systems have permitted their application in the space sector, such as flow control of flying vehicles and the potential ...usage in high precision attitude and orbit control systems for CubeSats. To determine its contribution in pressure, velocity, and temperature profiles for thrust analysis on micro-fluidic channels in space/vacuum environments, a novel DC micro-discharge code has been developed in the dsmcFoam+ solver in order to simulate the ionic effects on the gas flow during the breakdown voltage in microgaps.
The rarefied gas flow was solved using the direct simulation Monte Carlo (DSMC) method, with the Fowler–Nordheim equation to calculate the ionic current density during the vacuum field emissions effect for the micro-plasma simulation.
The incorporation of the Fowler–Nordheim model avoids the addition of an exponential increase in computational power requirements for a DSMC simulation. Instead of introducing new electrons and ions as equivalent particles into the system, the existing DSMC gas particles in the cells are accelerated. Through this, a computationally lightweight alternative for micro-scale plasma simulations utilising the dsmcFoam+ solver is achieved, which can be tailored to the thrust requirements of a particular mission.
Computational load imbalance is a well-known performance issue in multiprocessor reacting flow simulations utilizing directly integrated chemical kinetics. We introduce an open-source dynamic load ...balancing model named DLBFoam to address this issue within OpenFOAM, an open-source C++ library for Computational Fluid Dynamics (CFD). Due to the commonly applied operator splitting practice in reactive flow solvers, chemistry can be treated as an independent stiff ordinary differential equation (ODE) system within each computational cell. As a result of the highly non-linear characteristics of chemical kinetics, a large variation in the convergence rates of the ODE integrator may occur, leading to a high load imbalance across multiprocessor configurations. However, the independent nature of chemistry ODE systems leads to a problem that can be parallelized easily (called an embarrassingly parallel problem in the literature) during the flow solution. The presented model takes advantage of this feature and balances the chemistry load across available resources. Additionally, a reference mapping model is utilized to further speed-up the simulations. When DLBFoam it utilized with both these features enabled, a speed-up by a factor of 10 is reported for reactive flow benchmark cases. To the best of our knowledge, this model is the first open-source implementation of chemistry load balancing in the literature.
Program Title: DLBFoam
CPC Library link to program files:https://doi.org/10.17632/bb9zjfzcmm.1
Developer's repository link:https://github.com/blttkgl/DLBFoam
Licensing provisions: GPLv3
Programming language: C++
Nature of problem: Solution of chemical kinetics in parallel reacting flow solvers raises a computational imbalance across multiprocessor architectures. DLBFoam balances the load distribution evenly, providing significant speed-up in reacting CFD applications.
Solution method: The dynamic load balancing is implemented by distributing the point-wise chemistry problems from most loaded processes to less loaded ones using MPI communication protocol.
Additional comments including restrictions and unusual features: The present model is designed to work with the standard chemistry model class available in OpenFOAM (versions 7 and 8). For the time being, the model does not support derived combustion models such as “TDAC” and covers gas-phase reaction kinetics only. In addition, the boundary surface chemistry problems are neglected by the model.
Accurately simulating the flow and heat transfer in a compact assembly is a critical concern in the core design of the lead-cooled fast reactor. The key to numerical heat transfer lies in the ...effective closure of turbulent heat flux (THF) in the average energy equation of the low-Prandtl-number coolant. Thus, an advanced SST k-ω-kθ-ɛθ model suitable for turbulent heat transfer in liquid metals is developed within the OpenFOAM framework. This model is applied to optimize the assemblies of rod bundles with wire spacers and validated through the benchmark experiments. The impact of the numerical models and the simplified approach for the assemblies on heat transfer is assessed through cross-comparison. Exploration of fluid conditions (mass flow rate, Reynolds number, and Péclet number) and design parameters, including the pitch-to-diameter ratio P/D and the number of wire spacers, provides insights into their effects on heat transfer performance. Detailed analyses of thermal-hydraulic parameters, including temperature, mainstream velocity, and transverse flow, are leveraged to reveal the rotational flow and heat transfer pattern in the assemblies. A mode that can balance component heat transfer performance and pressure drop is evaluated by Nusselt number and friction factor. The applicability of scaled-down assemblies in research is demonstrated through a study of a complete 127-rod bundle.
•A novel SST k-ω-kθ-ɛθ model addresses heat transfer in low-Prandtl-number fluids.•Cross-comparison and validation of the numerical and geometric models are conducted.•Coolant in bundles with wire spacers exhibits rotational flow and heat transfer patterns.•Simultaneously increasing P/D and wire numbers optimizes the performance of the lead-cooled assembly.
The primary aim of this study is to develop a more efficient reaction mechanism for accurately simulating methane combustion, with a specific focus on ignition delay, laminar flame speed, and 2-D ...simulated flames, while also reducing computational time. Ten reduced reaction mechanisms for methane combustion were evaluated, with only one, "SK30," meeting the required accuracy standards. However, SK30 proved to be computationally intensive when simulating a 2-D premixed flame at a microscale. To address this challenge, a two-step reduction process was implemented. Firstly, an automated algorithm utilized direct relation graphs and sensitivity analysis with ignition delays as a reference to streamline the mechanism while maintaining accuracy. Subsequently, the second step involved identifying key reactions that had a more significant impact on flame speed than ignition delay through sensitivity analysis. Any missing reactions were then added judiciously, prioritizing the retrieval of the missing but important reactions to overall accuracy while minimizing computational cost. This process resulted in a novel mechanism comprising 25 species and 132 reactions for methane-air combustion. The validity of this mechanism was confirmed through comparison with a benchmark model, demonstrating satisfactory agreement in 1-D flame speed and 2-D premixed flame modeling. Most notably, the new mechanism substantially reduced processing time, achieving a 50 % speedup compared to SK30.
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