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.
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.
A large eddy simulation formulation was proposed for spray combustion in gas turbine combustion chambers, using a FGM combustion model coupled with detailed chemical reaction mechanisms. The solver ...was developed using the open-source toolkit—OpenFOAM. The two-way coupling between spray particles and gas field was modeled by a Eulerian–Lagrangian method. The enthalpy loss resulting from spray evaporation was also considered in the FGM combustion model. The solver was first validated against the experimental data of a model combustion chamber. The predictions of mean and fluctuation velocities were in good agreement with the experimental values for both cold and reacting flows, and the flame shapes were also nicely reproduced. It was revealed that there existed local extinctions at the swirler exit, leading to a lift-off flame. The validated solver was then applied to a realistic combustor of an aeroengine. It was found that different from the diffusive nature of the flame in the model combustor, the spray combustion in the realistic combustor is of partially premixed nature. The predicted pressure loss and the exit temperature profile agreed rather well with the measurements, demonstrating the capability of the present solver to simulate realistic combustors with complex geometries.
•Extended enthalpy dimension of FGM model, established a LES solver for spray flame.•Verified the solver by experiment, the local extinction was captured in stable flame.•Obtained fine flow field structure and predicted RTDF of a realistic combustor.•Provided an effective tool for gas turbine combustor design.
•The effectiveness of trees on dispersing road traffic emissions is assessed.•CFD simulations of NOx and PM2.5 is performed using the OpenFOAM software.•Deposition of PM2.5 on trees is modelled.•The ...aerodynamic effects are more important than deposition.•On average, trees increase pollution, but reduce pollution for winds parallel to street.
This paper discusses the combined influence of building morphology and trees on air pollutant concentrations in the Marylebone neighbourhood (central London). Computational Fluid Dynamics (CFD) simulations are performed with OpenFOAM using the k-ε model. Aerodynamic and deposition effects of Platanus acerifolia trees are considered. While aerodynamic effects are treated as typically done in the literature, i.e. as a porous media, for the deposition an enhanced model with an additional sink term was implemented. CFD results are compared with UK AURN (Automatic Urban and Rural Network) station concentrations. Several meteorological conditions are analysed based on London City Airport weather station data, with attention to prevailing winds.
CFD simulations show that trees trap air pollution by up to about 7% at the Marylebone monitoring station in the spring, autumn and summer seasons, suggesting that the aerodynamic effects are similar over the different leaf seasons. Aerodynamic effects are more important at lower wind speeds causing little turbulent dispersion. Deposition effects are found to be 4 times less important with reductions of up to about 2%, with more deposition in summer due to a greater leaf area density. Furthermore, for winds parallel to Marylebone Road, the aerodynamic effects decrease concentrations suggesting that in such cases trees could be considered as a mitigation measures. This is different from perpendicular winds for which trees exacerbate trapping, as found in previous studies. The analysis of concentration levels obtained from CFD simulations across the whole street confirms a beneficial aerodynamic dispersive effect of trees of 0.7% in summer time for all wind directions averaged at a wind speed of 5m/s (yearly average wind speed observed in the area). Results highlight the need to account for both aerodynamic and dispersion effects of trees in CFD modelling to achieve a comprehensive evaluation and help city planners with a sustainable design of trees in urban environments.
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.
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.
•A turbulent single-phase attenuating liquid sheet has been studied•Three modes of breakup are identified: aerodynamic, perforation, and bag breakups•CFD and experimental results are presented and ...agree well•Formation of a bag and nucleation of a hole are studied•The dynamic of hole growth is analyzed and equations are suggested
We present a combined experimental-numerical study of the primary breakup of a single-phase attenuating liquid sheet emerging from a flat fan nozzle. An experimental setup has been built to produce flat fan liquid sheets/sprays at near industrial conditions, i.e. high pressure and turbulent. The process of sheet disintegration, for a transitional and a turbulent sheet, is visualized using high-speed shadowgraphy. The shadowgrams resolve features of the primary breakup at very fine spatial and temporal resolutions, revealing new information about attenuating liquid sheets. Three breakup mechanisms are identified in the central region of the sheet; two of these, aerodynamic instability and disruption by perforations, are well known. The third mechanism, bag breakup, appears not to have been previously reported in the context of flat fan sprays. Highly resolved Computational Fluid Dynamics (CFD) is employed to model the phenomena of sheet breakup. Features of the attenuating liquid sheet are computed and compared with experimental measurements where strong agreement is found. Breakup mechanisms similar to those observed experimentally are resolved in the CFD simulations, confirming the ability of the simulation methodology to extend studies to scales that are not achievable via laboratory experiments. The mechanisms of holes nucleation and growth are studied, we find that hole nucleation in a turbulent single-phase liquid sheet is due to the growth of local surface disturbance which is initiated by nozzle turbulence.
A series of numerical simulations were performed to investigate the influences of storage vessels shapes on sloshing dynamics under horizontal excitation by employing the open source code OpenFOAM, ...which has been extensively validated by experimental data for the sloshing flow problem. The results show that the membrane liquefied natural gas (LNG) tanks are subject to lower impact pressure than the cylindrical, rectangular and spherical tanks with the same volume of liquid and the overall tank dimensions, as the slope at the storage vessels bottom changes the flow direction of the liquid and therefore reduces the impact on the vertical wall. In the cylindrical and spherical tanks, higher impact pressure was found on the wall directly opposite to the excitation direction and the maximum impact point will shift away from the external excitation direction as the wave breaks up violently until a quasi-steady state of the sloshing wave rotating along the side wall is reached. The curved surface of the spherical tank could also help reduce the impact pressure when compared with the cylindrical tank.
•An OpenFOAM based numerical model is developed for predicting sloshing in tanks of different shapes.•The sloshing pressure in four types of vessels with the same volume of liquid is studied.•The LNG tanks were proved to be subject to lower impact pressure than the cylindrical, rectangular and spherical tanks.•Three-dimensional characteristics of sloshing in cylindrical and spherical tanks were analyzed.
The DCRFoam solver (density-based compressible solver) built on the OpenFOAM platform is used to simulate the reflection and diffraction processes that occur when detonation waves collide with ...various objects. Static stoichiometric hydrogen–oxygen mixtures diluted with 70% Ar are used to form stable detonation waves with large cells, with initial conditions of 6.67 kPa pressure and 298 K temperature. The diameters of the cylindrical obstacle range from 6 mm to 22 mm, with x = 230 mm, x = 244 mm, and x = 257 mm being the chosen position. Cylindrical, square, triangular, and inverted triangular obstacles are used, and the quenched detonation re-initiation processes behind them are investigated. In the detonation diffraction process, four triple points exist at the same time due to the effect of cylindrical obstacles of smaller diameters. The re-initiation distance of the detonation wave increases with the increase of cylindrical obstacle diameter. Both the Mach reflection angle and the decoupled angle decrease as the diameter increases. When the location of the cylindrical obstacles is changed, the detonation wave dashes into the obstacles with its different front structures, it is easier to realize the detonation re-initiation when the weak incident shock at the front of a detonation wave strikes the obstacles, and the re-initiation distance decreases by 17.1% when compared with the longest re-initiation distance. The detonation re-initiation distance is shortest under the action of cylindrical obstacles, however the quenched detonation cannot be re-initiated when the inverted triangle and square obstacles are used. The suppression effects of inverted triangle and square obstacles on detonation waves are more evident.
•Diffraction and re-initiation of detonations with large cell were simulated.•The DCRFoam solver built on the OpenFOAM platform was used.•Detonation waves with different front structures strike obstacles were studied.•Obstacles with different shapes and sizes were employed in this study.
•A LES model for the analysis of a natural circulation loop in presence of distributed heating is developed.•The model takes into account the fluid and the solid regions, heat generation and ...conduction in the pipes.•The approach is able to reproduce stable and unstable transients of DYNASTY.•New information gathered as stratification and counter-current flows occurring during flow reversal.•LES suitable to overcome RANS limitations in the stability and dynamic analysis of natural circulation systems.
Natural circulation is exploited in nuclear systems to passively remove power in case of accident scenarios. In this regard, the DYNASTY experimental facility at Politecnico di Milano has been setup to increase the knowledge on single-phase, buoyancy-driven systems in the presence of distributed heating. In this paper, the development of a computational fluid dynamics (CFD) model of DYNASTY is presented, focusing on the capability of CFD to assess the dynamic behavior of the facility. The large eddy simulation (LES) model takes into account both the fluid and the solid regions, with heat generation and 3D heat conduction resolved in the pipe walls. The study, conducted using OpenFOAM, shows (i) the capability of reproducing stable and unstable transients of DYNASTY, (ii) new observations on the features of flow reversals during unstable transients, (iii) the suitability and the advantages of LES for the prediction of the specific features of natural circulation systems.
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