Ability Of Computational Fluid Dynamics Research Articles

An engineering CFD model for fire spread on wood cribs for travelling fires

The temperature heterogeneity due to fire in large open-plan office compartments is closely associated with fire spread behaviour and has been historically limited to experimental investigations using timber cribs. This study explores the ability of Computational Fluid Dynamics (CFD) models, specifically the Fire Dynamics Simulator (FDS), to reproduce the results of full-scale tests involving fire spread over timber cribs for continuous fuel-beds. Mesh schemes are studied, with a fine mesh over the crib and 2 × 2 cells in the wood stick cross-section by default, this being relaxed in the surrounding regions to enhance computational efficiency. The simple pyrolysis model considers the charring phase and moisture. In application to the TRAFIR-Liège LB7 test, this calibrated “stick-by-stick” representation shows a good agreement for interrelated parameters of heat release rate, fire spread, gas phase temperature, and burn-away, a set of agreements which has not been demonstrated in previous studies. Fire spread shows relatively high sensitivities to: heat of combustion, ignition temperature, thermal inertia, radiation fraction, heat release rate per unit area, and the fuel load density. An approximately linear regression was found between the different fire modes and the thermal exposures, with “travelling” (and decaying) fires characterised by heat fluxes associated with the fire plume, while the growing fires were associated with proportionally higher heat fluxes on the horizontal surfaces of the sticks, in conditions where these receive more pre-heating. The trends in the overall HRR are more dependent on the fire spread rates than variations in the stick burning rates.

Mutual synchronization of self-excited acoustic oscillations in coupled thermoacoustic oscillators

Synchronization refers to the adjustment of rhythms of two nonidentical oscillators due to interaction. In this study, we report for the first time on the ability of computational fluid dynamics (CFD) to simulate the synchronization of self-sustained acoustic oscillations in two coupled thermoacoustic (TA) oscillators that interfere with each other through acoustic radiation. A bifurcation diagram containing both asynchronous and synchronous states is first mapped out by changing the end-to-end distance and the resonator length difference. It is found that the two TA oscillators are analogous to mutually coupled van der Pol oscillators with reactive coupling. Then, the dynamic characteristics of beating and periodic oscillations in asynchronous and synchronous states are analysed separately to facilitate the comprehension of the synchronization process. This research demonstrates that the CFD methodology provides a valuable numerical tool for studying the synchronization of thermally induced acoustic oscillations in TA systems.

Numerical investigation of transonic buffet on supercritical airfoil considering uncertainties in wind tunnel testing

To improve the predictive ability of computational fluid dynamics (CFD) on the transonic buffet phenomenon, NASA SC(2)-0714 supercritical airfoil is numerically investigated by noninstructive probabilistic collocation method for uncertainty quantification. Distributions of uncertain parameters are established according to the NASA wind tunnel report. The effects of the uncertainties on lift, drag, mean pressure and root-mean square pressure are discussed. To represent the stochastic solution, the mean and standard deviation of variation of flow quantities such as lift and drag coefficients are computed. Furthermore, mean pressure distribution and root-mean square pressure distribution from the upper surface are displayed with uncertainty bounds containing 95% of all possible values. It is shown that the most sensitive part of flow to uncertain parameters is near the shock wave motion region. Comparing uncertainty bounds with experimental data, numerical results are reliable to predict the reduced frequency and mean pressure distribution. However, for root-mean square pressure distribution, numerical results are higher than the experimental data in the trailing edge region.

CFD Simulations of Molten Salt Reactor Experiment Core

At present, no clear guidelines exist for modeling non-water-cooled small modular reactors (SMRs) despite the rising need for high-fidelity simulation tools to support regulators and the industry. Most SMR concepts currently under the Canadian prelicensing review adopted non-water-cooled–reactor technologies [molten salt reactor (MSR), gas-cooled reactor, and liquid metal–cooled reactor] that are new for Canada. There is a need for a modeling tool set that is broadly applicable for the assessment of advanced technologies used in SMRs. Computational fluid dynamics (CFD) can be used in performance evaluation and safety analysis of non-water-cooled SMRs for modeling three-dimensional (3-D) fluid flow and heat transfer in geometries of arbitrary complexity without resorting to geometry-specific empirical correlations. This study investigates the capabilities of existing models within a commercial CFD code to simulate the flow and heat transfer characteristics in a MSR configuration. The Oak Ridge National Laboratory (ORNL) Molten Salt Reactor Experiment (MSRE) configuration was simulated in this study using a stand-alone CFD approach, and CFD predictions were assessed with ORNL data. Intricate geometry details within the MSRE core were included in the computational model to study the associated geometric effects. The results obtained in this study showcased the ability of CFD to predict 3-D effects within the computational domain especially at the lower plenums. The predicted trends for the temperature rise in the fuel and moderator within the core were in good agreement with the ORNL data. The results presented in this paper constitute the first step in developing Canadian Nuclear Laboratories’ capability for CFD modeling of non-water SMRs.

Computational Fluid Dynamics Study of Separated Flow Over a Three-Dimensional Axisymmetric Hill

This paper investigates the ability of computational fluid dynamics (CFD) simulations to accurately predict the turbulent flow separating from a three-dimensional (3D) axisymmetric hill using a recently developed four-equation eddy-viscosity model (EVM). The four-equation model, denoted as k–kL–ω–v2, was developed to demonstrate physically accurate responses to flow transition, streamline curvature, and system rotation effects. The model was previously tested on several two-dimensional cases with results showing improvement in predictions when compared to other popularly available EVMs. In this paper, we present a more complex 3D application of the model. The test case is turbulent boundary layer flow with thickness δ over a hill of height 2δ mounted in an enclosed channel. The flow Reynolds number based on the hill height (ReH) is 1.3 × 105. For validation purposes, CFD simulation results obtained using the k–kL–ω–v2 model are compared with two other Reynolds-averaged Navier–Stokes (RANS) models (fully turbulent shear stress transport k–ω and transition-sensitive k–kL–ω) and with experimental data. Results obtained from the simulations in terms of mean flow statistics, pressure distribution, and turbulence characteristics are presented and discussed in detail. The results indicate that both the complex physics of flow transition and streamline curvature should be taken into account to significantly improve RANS-based CFD predictions for applications involving blunt or curved bodies in a low Re turbulent regime.

Numerical simulations of vented hydrogen deflagration in a medium-scale enclosure

A validation study was performed to investigate the ability of Computational Fluid Dynamics (CFD) models to predict hydrogen deflagrations in vented enclosures. The validation exercise was aimed at assessing the suitability of CFD as a reliable tool for explosion safety assessments and involved comparing CFD predictions with measurements from an experiment carried out by FM Global in a 64 m3 enclosure. The enclosure included a large square vent located in the center of one of its walls. The enclosure was filled with a homogenous hydrogen-air mixture of 18% v/v composition before ignition at its center. In this paper, CFD model predictions of the transient pressure and the flame speed are compared against experimental measurements. Additionally, peak overpressure predictions are compared against empirical correlations and the NFPA 68 vent sizing standard. The study focuses on the prediction of the first overpressure peak that is generated by external explosion. The agreement between the models’ predictions and experimental results is found to be satisfactory, which suggests that CFD models have the potential to predict explosion phenomena with reasonable accuracy. However, more extensive model validation and sensitivity studies are required before CFD models can be used with confidence in explosion safety assessments.

Effect of Scale on Hydrodynamics of Internal Gas-Lift Loop Reactor-Type Anaerobic Digester Using CFD

Abstract This work evaluates the ability of computational fluid dynamics (CFD) to simulate the flow and predict the hydrodynamics of internal gas-lift loop reactor (IGLR)-type anaerobic digester. In addition, it also analyzes if CFD can account for the effects of operating conditions, geometry as well as scale of the reactor. For this purpose, three-dimensional two-phase CFD simulations were performed using CFX for laboratory-scale and pilot-scale IGLR. The CFD predictions were evaluated against experimental data obtained from computer automated radioactive particle tracking (CARPT). The CFD predictions provided good qualitative but only reasonable quantitative comparison. After validation of CFD model, effect of gas flow rate, draft tube diameter, sparger geometry and reactor scale on flow pattern, liquid velocity and dead volume was investigated. Higher gas flow rates did not offer any significant advantage in increasing liquid circulation in the downcomer or decreasing the dead volume. Configuration with draft tube diameter half of tank diameter, equipped with cross sparger showed comparatively better liquid circulation than other configurations. For same superficial gas velocity, increasing the scale increases the magnitude of liquid velocity but fails to match the mixing intensity observed in laboratory scale. Different interphase forces, turbulence models and closures are also evaluated to improve the predictability of CFD models.

Comparison of numerical approaches to model FCC particles in gas–solid bubbling fluidized bed

A comparative study has been carried out to determine the ability of computational fluid dynamics (CFD) and computational particle fluid dynamics (CPFD) codes to predict the hydrodynamics of FCC particles in a bubbling fluidized bed. Simulations were conducted in both 2-dimensional (2-D) and 3-dimensional (3-D) CFD configuration and in a 3-D CPFD model. The new structure based force-balance (FB) model of Ahmadi Motlagh et al. (2014. Chem. Eng. Sci. 120, 22–36.) was incorporated in CFD to simulate the behavior of FCC in the bubbling fluidization regime. This model modifies the conventional drag correlation by considering the effect of interparticle forces on the formation of agglomerates inside the bed, updating the drag calculations by replacing the particle diameter in the Wen and Yu correlation by an agglomerate diameter. The effects of superficial gas velocity, particle diameter, particle-particle restitution coefficient and specularity coefficient on voidage are examined. The sensitivity of different gas–solid drag closures in CFD are tested and compared. Experimental time-average axial and radial voidage profiles were simulated with varying degrees of agreement with experimental data obtained using different advanced experimental techniques in the “Traveling fluidized bed” (Dubrawski et al., 2013. Powder Technol. 235, 203–220; Tebianian et al., 2014. Extending the comparison of voidage measurement and modeling techniques in fluidized beds. In: Proc. 11th International Conference on Fluidized Bed Technology, (Eds.), J. Li, X.J. Bao and W. Wang. Chemical Industry Press, Beijing 137–142.). Both the FB and CPFD models were successful in resolving key issues in 3-D, whereas 2-D models tended to seriously underpredict particle volume fraction, especially near the wall.

Experimental and numerical investigation of Helmholtz resonators and perforated liners as attenuation devices in industrial gas turbine combustors

This paper reports upon developments in the simulation of the passive control of combustion dynamics in industrial gas turbines using acoustic attenuation devices such as Helmholtz resonators and perforated liners.Combustion instability in gas turbine combustors may, if uncontrolled, lead to large-amplitude pressure fluctuations, with consequent serious mechanical problems in the gas turbine combustor system. Perforated combustor walls and Helmholtz resonators are two commonly used passive instability control devices. However, experimental design of the noise attenuation device is time-consuming and calls for expensive trial and error practice. Despite significant advances over recent decades, the ability of Computational Fluid Dynamics to predict the attenuation of pressure fluctuations by these instability control devices is still not well validated. In this paper, the attenuation of pressure fluctuations by a group of multi-perforated panel absorbers and Helmholtz resonators are investigated both by experiment and computational simulation. It is demonstrated that CFD can predict the noise attenuation from Helmholtz resonators with good accuracy. A porous material model is modified to represent a multi-perforated panel and this perforated wall representation approach is demonstrated to be able to accurately predict the pressure fluctuation attenuation effect of perforated panels. This work demonstrates the applicability of CFD in gas turbine combustion instability control device design.

Control of Tip Leakage Vortex Breakdown by Tip Injection in Unshrouded Turbines

Tip leakage flow is responsible for a number of aerodynamic losses in turbines, but the leakage loss mechanisms have not been fully elucidated yet. This paper computationally investigates the effects of tip injection on tip leakage vortex dynamics in an unshrouded turbine, in an attempt to seek new loss generation mechanisms in turbine tip clearance flows and to influence them by the injection. First, a linear cascade validation is introduced, and the computations are compared with the experimental data to identify the ability of computational fluid dynamics code to predict the tip leakage flow. The tip leakage vortex breakdown phenomenon and dynamics analysis are shown, and then the effects of tip injection on leakage vortex structure and vortex breakdown characteristics are investigated. Also, the effect of leakage vortex breakdown on aerodynamic performance of the turbine with injection is examined. Results indicate that the leakage vortex breakdown occurs when the pressure rise is large enough in the rear part of the blade suction side near the tip, and it is an important loss mechanism associated with tip leakage. Injection location has a different impact on the leakage vortex dynamics, dependent on the tip pressure field characteristics. With injection in the front part, the leakage vortex breakdown is delayed, where the turbine achieves the best performance improvement, and the vortex breakdown location moves downstream as the injection mass flow rate increases.

Numerical modelling of wind flow over a complex topography

Numerical modelling of wind flow over complex dune topography is an ambitious prospect. There is an increasing need to understand wind flow over complex topography for land planning purposes to enable prediction of sediment transport at a particular site. New surveying techniques permit the rapid development of digital terrain models, however a stumbling block is the ability of Computational Fluid Dynamics (CFD) to emulate the wind flow over such a landscape. To overcome these difficulties, it is important to establish the parameters within which such simulations can operate. This paper details an initial two-dimensional numerical model developed in order to test various modelling assumptions against experimental field wind data. Mason Bay, Stewart Island, New Zealand was chosen as an undisturbed but accessible experimental site with a prevalent on-shore wind perpendicular to a simple foredune and a complex down-wind parabolic dune system. A complex topographical two-dimensional model with vegetation represented as a roughness was compared against field data along a transect dissecting a dune system. This paper establishes that: ∗ Replicating the roughness patterns at the surface is important ∗ The inlet profile should be duplicated with care ∗ Modelling only a portion of the domain can have an effect on the flow patterns due to outflow effects ∗ There is a modelling decision to be made between the complexity of the topography and the sophistication of the turbulence model and degree to which vegetation and sand transportation are modelled. The long-term aim is to instil confidence in numerical techniques so that such technology can be used for predictive purposes.

Trailing vortices generated by a Rushton turbine: Assessment of URANS and large Eddy simulations

The discharge flow of a Rushton turbine is characterized by the formation of coherent vortex structures induced by the blade motion and called trailing vortices. The objective here is to assess the ability of computational fluid dynamics (CFD) to represent the trailing vortices and their relationship with turbulence properties. To this end, two simulations have been realized: an unsteady Reynolds-Averaged Navier-Stokes (URANS) simulation and a Large Eddy Simulation (LES) simulation. The trajectory of the trailing vortices predicted by the simulations has been compared with previous works. This comparison shows that the URANS simulation does not predict properly the trailing vortices while the LES results are very close to the experimental ones. As a consequence, the turbulence properties spatially correlated to the trailing vortices are well predicted by LES but not by URANS simulation.

Modeling hydrodynamics of gas–liquid airlift reactor

AbstractThis article deals with the physical modeling and numerical simulation of two‐phase bubbly flow in an airlift internal loop reactor. The objective is to show the ability of computational fluid dynamics (CFD) to correctly simulate hydrodynamics and axial dispersion in such a bubbly reactor. The modeling of two‐phase bubbly flow is based on the so‐called two‐fluid model derived from Reynolds‐averaged Navier–Stokes equations in two‐phase flow. From the local perspective, CFD leads to the distributions of phases, interfacial area, and velocity field in the whole volume of the airlift. Numerical simulations are discussed after comparison with experimental data. Sensitivity analysis is then presented to highlight the main parameters that must be taken into account in two‐fluid modeling, especially in terms of interfacial transfer of momentum and turbulence. Once local hydrodynamics has been discussed and validated, the axial dispersion is then addressed. The axial dispersion coefficient is estimated from simulation of transport equation of salt concentration. Given the time evolution of the concentration, measured by a “numerical” probe located in computed airlift reactor, it is possible to numerically estimate the axial dispersion in the airlift, in the same way as in the experiments. The axial dispersion coefficient determined after simulation is compared with the experimental one and the ability of CFD to simulate mixing time and axial dispersion is shown. In addition, a physical analysis of axial dispersion is proposed. © 2007 American Institute of Chemical Engineers AIChE J, 2007

Modeling of oxygen mass transfer in a gas–liquid airlift reactor

AbstractThis article focuses on the physical modeling and numerical simulation of mass transfer in two‐phase bubbly flow in an airlift internal loop reactor. The objective of this article is to show the ability of computational fluid dynamics (CFD) to correctly simulate mass transfer in such a bubbly reactor. The modeling of two‐phase bubbly flow is based on the so‐called two‐fluid model derived from Reynolds‐averaged Navier–Stokes equations in two‐phase flow. From the hydrodynamic perspective, the flow is steady state. Given the steady‐state distributions of phases, interfacial area, and velocity field in the whole volume of the airlift, mass transfer is computed and the evolution of oxygen concentration in the two phases is predicted. Numerical simulations are discussed after comparison to experimental data. The simulations are validated in terms of oxygen concentration in the liquid vs. time. Then, different points are discussed, in particular, the perfectly mixed reactor assumption in the liquid phase and the spatial and temporal heterogeneity of oxygen concentration in the gas arising from oxygen impoverishment in bubbles in the downcomer. This leads to heterogeneity of the transfer driving force between the gas and the liquid. Bubble age or residence time in the airlift loop reactor has been calculated to show the weak renewal of oxygen in bubbles in the downcomer. This discussion generates questions on the estimation of a global mass transfer coefficient from experiments in such a heterogeneous airlift reactor. © 2007 American Institute of Chemical Engineers AIChE J 2007

A rod-airfoil experiment as a benchmark for broadband noise modeling

A low Mach number rod-airfoil experiment is shown to be a good benchmark for numerical and theoretical broadband noise modeling. The benchmarking approach is applied to a sound computation from a 2D unsteady-Reynolds-averaged Navier–Stokes (U-RANS) flow field, where 3D effects are partially compensated for by a spanwise statistical model and by a 3D large eddy simulation. The experiment was conducted in the large anechoic wind tunnel of the Ecole Centrale de Lyon. Measurements taken included particle image velocity (PIV) around the airfoil, single hot wire, wall pressure coherence, and far field pressure. These measurements highlight the strong 3D effects responsible for spectral broadening around the rod vortex shedding frequency in the subcritical regime, and the dominance of the noise generated around the airfoil leading edge. The benchmarking approach is illustrated by two examples: the validation of a stochastical noise generation model applied to a 2D U-RANS computation; the assessment of a 3D LES computation using a new subgrid scale (SGS) model coupled to an advanced-time Ffowcs–Williams and Hawkings sound computation. In both cases, the ability of computational fluid dynamics to model the source mechanisms and of the CAA approach to predict the far field are assessed separately.

Modelling Sieve Tray Hydraulics Using Computational Fluid Dynamics

The ability of computational fluid dynamics (CFD) to model the complex two-phase hydrodynamics of sieve trays is examined. The key to a proper description of the flow is the estimation of the momentum exchange, or drag, coefficient between the gas and liquid phases. In the absence of sound theoretical models, empirical correlations for the average gas fraction on the tray, such as those of Bennet, Agrawal and Cook [AIChEJ, 29 (1983): 434], can be used to estimate the drag coefficient. Transient simulations of sieve trays of 0.3 and 0.9 m in diameter, operating in the bubbly flow regime, reveal the chaotic, three-dimensional character of the flow and the existence of circulation patterns in all three dimensions. The CFD simulations underline the limitations of simpler approaches wherein the flow is assumed to be two-dimensional or where the interaction of the liquid phase with the gas phase is either ignored completely or simplified greatly. The major advantage of the CFD approach is that geometry and scale effects are properly encapsulated and do not require further inputs. It is concluded that CFD can be a powerful investigative and design tool for sieve tray columns.

Application of Computational Fluid Dynamics to Multiphase Flow in Bubble Columns

The application of Computational Fluid Dynamics (CFD) to the simulation of bubble columns devices is discussed. A comparison between different modeling approaches has been carried out in order to understand which is the present ability of CFD codes to predict the flow in a gas–liquid system. The effect of different options of simulation, such as 2D and 3D grids with different density of cells, order of discretization, turbulence closure, has been studied for systems characterized by different values of gas hold-up. The analysis is restricted to bubbly flow conditions. Eulerian-Eulerian two fluid models have been used to describe both the time–dependent motion of the bubble plume and the time-averaged flow pattern in the column. The systems considered are those experimentally characterized by the group of Eigenberger (Becker et al., 1994), by Mudde et al. (1997), by the group of Dudukovic (Sanyal et al., 1999) and by Ho Yu and Kim (1991).

99/01409 Experimental and numerical studies on re burn jet penetration and mixing with application to boilers and municipal waste incinerators : Weber, R. et al. J. Inst. Energy, 1998, 71, (487), 94–109

Mixing of the reburn fuel with primary combustion-zone products is one of the most important parameters affecting NO x reduction. The mixing can be controlled by reburn jet parameters. The objective of the study is twofold: to assess the ability of Computational Fluid Dynamics (CFD) for predicting properties of the jets, and to develop a methodology for scaling the results obtained for isothermal flows into combusting flows in boilers and municipal-waste incinerators. To this end the fluid-flow experiments are performed using the IFRF stand for simulating flows in boilers and incinerators. The Laser Doppler technique is used to measure 3-dimensional flows, and the Laser Sheet technique (Mie scattering) is used to visualise the reburn jets, their trajectories and mixing. The CFD-computed time-mean 3-dimensional flow fields agreed well with the measured flow patterns. The predicted maximum penetration depth differed by 5-12% from the experimentally observed trajectories. Each of the eight reburn jets responded to the local flow conditions prevailing upstream of the injection position. Numerical simulations of the reburn-jet penetration depth and mixing required a good knowledge of the overall flow pattern within the boiler. This paper contains correlations for estimating the jet-penetration depth under isothermal and combustion conditions. It has been demonstrated that engineering correlations describing the jet trajectories in a cross-flow could not be applied to the boiler flow considered.