Comparative Study of CFD and LedaFlow Models for Riser-Induced Slug Flow

The goal of this study is to compare mainstream Computational Fluid Dynamics (CFD) with the widely used 1D transient model LedaFlow in their ability to predict riser induced slug flow and to determine if it is relevant for the offshore oil and gas industry to consider making the switch from LedaFlow to CFD. Presently, the industry use relatively simple 1D-models, such as LedaFlow, to predict flow patterns in pipelines. The reduction in cost of computational power in recent years have made it relevant to compare the performance of these codes with high fidelity CFD simulations. A laboratory test facility was used to obtain data for pressure and mass flow rates for the two-phase flow of air and water. A benchmark case of slug flow served for evaluation of the numerical models. A 3D unsteady CFD simulation was performed based on Reynolds-Averaged Navier-Stokes (RANS) formulation and the Volume of Fluid (VOF) model using the open-source CFD code OpenFOAM. Unsteady simulations using the commercial 1D LedaFlow solver were performed using the same boundary conditions and fluid properties as the CFD simulation. Both the CFD and LedaFlow model underpredicted the experimentally determined slug frequency by 22% and 16% respectively. Both models predicted a classical blowout, in which the riser is completely evacuated of water, while only a partial evacuation of the riser was observed experimentally. The CFD model had a runtime of 57 h while the LedaFlow model had a runtime of 13 min. It can be concluded that the prediction capabilities of the CFD and LedaFlow models are similar for riser-induced slug flow while the CFD model is much more computational intensive

Study of drag and orientation of regular particles using stereo vision, Schlieren photography and digital image processing

© 2017 Elsevier B.V.A new experimental, image-based methodology suitable to track the changes in orientation of non-spherical particles and their influence on the drag coefficient as they settle in fluids is presented. Given the fact that non-spherical solids naturally develop variations in their angular orientation during the fall, none-intrusiveness of the technique of analysis is of paramount importance in order to preserve the particle/fluid interaction undisturbed. Three-dimensional quantitative data about the motion parameters is obtained through single-camera stereo vision whilst qualitative visualizations of the adjacent fluid patterns are achieved with Schlieren photography. The methodology was validated by comparing the magnitudes of the drag coefficient of a set of spherical particles at terminal velocity conditions against those estimated from drag correlations published in the literature. A noteworthy similarity was attained. During the fall of non-spherical solids, once the particle Reynolds number approximated 163 for disks, and 240 for cylinders, or exceeded those values, secondary motions composed by regular oscillations and tumbling were present. They altered the angular orientation of the particles with respect to the main motion direction and caused complete turbulent patterns in the surrounding flow, therefore affecting the instantaneous projected area, drag force, and coefficient of resistance. The impact of the changes in angular orientation onto the drag coefficient was shown graphically as a means for reinforcing existing numerical approaches, however, an explicit relation between both variables could not be observed

Large eddy simulation of inertial fiber deposition mechanisms in a vertical downward turbulent channel flow

The deposition pattern of elongated inertial fibers in a vertical downward turbulent channel flow is predicted using large eddy simulation and Lagrangian particle tracking. Three dominant fibers deposition mechanisms are observed, namely, diffusional deposition for small inertial fibers, free-flight deposition for large inertial fibers, and the interception mechanism for very elongated fibers. The fibers are found to exhibit orientation anisotropy at impact, which is strongly dependent on the fiber elongation. An increase in the fiber elongation increases the wall capture efficiency by the interception mechanism. The diffusional deposition mechanism is shown to dominate for fibers with large residence time, t⁺res, in the accumulation zone and small deposition velocities, v⁺z, while the free-flight mechanism governs deposition for fibers with small t⁺res and large v⁺z. This study describes how particles deposit on a surface and, ultimately for many practical applications, how such deposition may promote fouling

CFD-DEM Simulation of Turbulence Modulation in Horizontal Pneumatic Conveying

A study is presented to evaluate the capabilities of the standard k–ε turbulence model and the k–ε turbulence model with added source terms in predicting the experimentally measured turbulence modulation due to the presence of particles in horizontal pneumatic conveying, in the context of a CFD–DEM Eulerian–Lagrangian simulation. Experiments were performed using a 6.5-m long, 0.075-m diameter horizontal pipe in conjunction with a laser Doppler anemometry (LDA) system. Spherical glass beads with two sizes, 1.5 and 2 mm, were used. Simulations were performed using the commercial discrete element method software EDEM, coupled with the computational fluid dynamics package FLUENT. Hybrid source terms were added to the conventional k–ε turbulence model to take into account the influence of the dispersed phase on the carrier phase turbulence intensity. The simulation results showed that the turbulence modulation depends strongly on the model parameter Cε3. Both the standard k–ε turbulence model and the k–ε turbulence model with the hybrid source terms could predict the gas phase turbulence intensity trend only generally. A noticeable discrepancy in all cases between simulation and experimental results was observed, particularly for the regions close to the pipe wall. It was also observed that in some cases the addition of the source terms to the k–ε turbulence model did not improve the simulation results when compared with those of the standard k–ε turbulence model. Nonetheless, in the lower part of the pipe where particle loading was greater due to gravitational effects, the model with added source terms performed somewhat better

Numerical and experimental study of horizontal pneumatic transportation of spherical and low-aspect-ratio cylindrical particles

The work presented in this paper was carried out as part of the PARDEM project. The overall aim was to quantify the predictive capability of a coupled CFD-DEM approach to simulating the horizontal pneumatic conveying of spherical and low-aspect-ratio non-spherical particles. Carefully controlled experiments were carried out in a 6.5 m long, 0.075 m diameter horizontal conveying line with the aid of the laser Doppler anemometry (LDA). Three different sizes of spherical glass beads, ranging from 0.8 mm to 2 mm and low-aspect-ratio cylindrical shaped particle of size 1 × 1.5 mm were employed. Simulations of the experiments were performed using a two-way coupled computational fluid dynamics and discrete element method (CFD-DEM) implemented in the commercial software FLUENT-EDEM in an Eulerian–Lagrangian framework. Experimental and simulation results of gas and particle velocities for particle laden flow with spherical particles were compared, showing that the CFD-DEM method could capture the experimental trends. However, quantitative discrepancies between simulation and experimental results were observed. Further modelling of low-aspect-ratio cylindrical particles was conducted using a multi-sphere model to represent cylindrical particles in the DEM code. Drag equations were modified in the code to take the effect of particle shape into account. The simulation results of mean axial particle velocity agreed reasonably well with a maximum of 25% discrepancy when compared to experimental measurements using the LDA technique. The discrepancies between simulation and experimental results were attributed to the selected drag model, mesh size and lack of an appropriate mesh interpolation scheme in the selected code