Simulation Studies of Gas-Solid in the Riser of a Circulating Fluidized Bed

A numerical parametric study was performed on the influence of various riser exit geometries on the hydrodynamics of gas-solid two-phase flow in the riser of a Circulating Fluidized Bed (CFB). A Eulerian continuum formulation was applied to both phases. A two fluid framework has been used to simulate fully developed gas-solid flows in vertical riser. A two dimensional Computational Fluid Dynamics (CFD) model of gas-particle flow in the CFB has been investigated using the code FLUENT. The turbulence was modeled by a k-e turbulence model in the gas phase. The simulations were done using the geometrical configuration of a CFB test rig at the Universiti Teknologi Malaysia (UTM). The CFB riser column has 265 mm (width), 72 mm (depth) and 2.7 m height. The riser is made up of interchangeable Plexiglas columns. The computational model was used to simulate the riser over a wide range of operating and design parameters. In addition, several numerical experiments were carried out to understand the influence of riser end effects, particle size, gas solid velocity and solid volume fraction on the simulated flow characteristics. The CFD model with a k-e turbulence model for the gas phase and a fixed particle viscosity in the solids phase showed good mixing behaviour. These results were found to be useful in further development of modeling of gas solid flow in the riser

Reduced-order modelling of vortex-induced vibration of catenary riser

A new reduced-order model capable of analyzing the vortex-induced vibration of catenary riser in the ocean current has been developed. This semi analytical-numerical approach is versatile and allows for a significant reduction in computational effort for the analysis of fluid-riser interactions. The incoming current flow is assumed to be steady, uniform, unidirectional and perpendicular to the riser plane of initial equilibrium curvatures. The equations of riser 3-D motion are based on a pinned-pinned, tensioned-beam or flexural cable, modelling which accounts for overall effects of riser bending, extensibility, sag, inclination and structural nonlinearities. The unsteady hydrodynamic forces associated with cross-flow and in-line vibrations are modelled as distributed van der Pol wake oscillators. This hydrodynamic model has been modified in order to capture the effect of varying initial curvatures of the inclined flexible cylinder and to describe the space-time fluctuation of lift and drag forces. Depending on the vortex-excited in-plane/out-of-plane modes and system fluid-structure parameters, the parametric studies are carried out to determine the maximum response amplitudes of catenary risers, along with the occurrence of uni-modal lock-in phenomenon. The obtained results highlight the effect of initial curvatures and geometric nonlinearities on the nonlinear dynamics of riser undergoing vortex-induced vibration

Numerical and experimental comparisons of vortex-induced vibrations of marine risers in uniform/sheared currents

This paper presents a general theoretical reduced-order model capable of evaluating the multi-mode nonlinear dynamics of marine risers subject to uniform and sheared currents. The main objectives are to predict the vortex-induced vibration responses and parametrically compare between numerical and experimental results. The emphasis is placed on the analysis of cross-flow vibrations due to unsteady lift forces. The nonlinear equations governing riser axial/transversal motions are derived based on a top-tensioned beam model with typical pinned-pinned boundary conditions. The riser geometric nonlinearities owing to possible large dynamic displacements and multi-mode interactions are accounted for. To approximate the space-time varying lift force, the empirical hydrodynamic model, based on a nonlinear van der Pol wake oscillator with a distributed diffusive term, is used. A low-dimensional dynamic model and computationally-robust time-domain tool are then developed to evaluate the multi-mode fluid-riser interactions. These are very useful in dealing with large parametric studies involving varying system parameters

Vortex-induced vibration of catenary riser: reduced-order modeling and lock-in analysis using wake oscillator

A new reduced-order model capable of analyzing the vortex-induced vibration of catenary riser in the ocean current has been developed. This semi analytical-numerical approach is versatile and allows for a significant reduction in computational effort for the analysis of fluid-riser interactions. The incoming current flow is assumed to be steady, uniform, unidirectional and perpendicular to the riser plane of initial equilibrium curvatures

Pengaruh Perbedaan Diameter Saluran Penambah (Riser) 15 mm, 17 mm, 19 mm Terhadap Hasil Coran Eye Material Aluminium Dengan Media Cetakan Pasir

This research aim to know the chemical composition of aluminum material and the influence of the riser size to shrinkage, density, porosity defect, hardness and microstructure. The material of this research is aluminum from sparepart motor that is breakshoe used. In this research, the riser used are tubular with the variation of riser I height 60mm and diameter 15mm, variation of riser II height 60mm and diameter 17mm, variation of riser III height 60mm and diameter 19mm. Tests to be performed include testing depreciation, density, porosity observation, chemical composition, brinell hardness, and microstructure of material. The results of this study show the highest percentage shrinkage found on riser III variation is 2,91%, while riser variation I is 0,97% and the riser variation II is 1,94%. The highest density result found on the riser variation III of 2.83 and riser variation II of 2.76, the lowest density found on the riser variation I of 2.62. The result of chemical composition found chemical elements, namely Aluminum (Al) 86.79% as the main material, and Silicon (Si) 9.49%, so this material elements categorized as aluminum alloy silicon (Al-Si). The highest hardness price found on the riser variation III 76.71 BHN, while the riser variation II 71.81 BHN and the riser variation I 70.45 BHN

Residence time distribution measurements in an external-loop airlift reactor: Study of the hydrodynamics of the liquid circulation induced by the hydrogen bubbles

A detailed study of the residence time distribution (RTD) analysis of liquid phase has been performed in an external-loop airlift reactor of 20 L nominal volume, regarded as a global unit and discriminating its different sections (riser, gas–liquid separator and downcomer) using the tracer response technique. The reactor was used as an electrochemical reactor in order to carry out the electrocoagulation/electroflotation (EC/EF). The gas phase created in the riser is the hydrogen produced by water electrolysis.In order to use this reactor for a continuous EC/EF, hydrodynamic studies were carried out to control the operating conditions and to help modelling the electrocoagulation. Current density, position of the electrodes in the riser and the volumetric liquid flow (inlet flow) are the key parameters for the hydrodynamics. The experimental results revealed that both in the downcomer and the riser–separator zones, the flow model is axial dispersion. Interesting results were obtained: –The superficial liquid velocity (ULd) at the downcomer, decreased when the volume inlet flow increased (0<QL<2 L/min). –The Peclet number obtained in the downcomer was correlated to the current density and the electrodes position. –In the riser–separator zone the Peclet number decreased with the superficial liquid velocity in the riser indicating that the dispersion increased with an increase of turbulence created in the separator by an increase of liquid velocity. –The percentage of flow that quits the reactor without reacting increased when the main flow increased and the current intensity decreased. The global RTD can be reconstituted by the signal resulting from the junction and that from riser–separator and downcomer zone by using the convolution technique. The experimental results confirm this reconstitution. The experiments confirm also that the liquid crosses the reactor without achieving loops in the case of the continuous flow

Experiments and modelling of a draft tube airlift reactor operated at high gas throughputs

One-dimensional modelling of global hydrodynamics and mass transfer is developed for an annulus sparged draft tube airlift reactor operating at high gas throughputs. In a first part, a specific closure law for the mean slip velocity of bubbles in the riser is proposed according for, in one hand, the collective effects on bubble rise velocity and, in the other hand, the size of the liquid recirculation in the airlift riser. This global hydrodynamics model is found towel explain the global gas volume fraction measurements in the airlift riser for a wide range of superficial gas velocity (0.6 ≤ Jg ≥10 cm sˉ¹). In a second part, mass transfer in the airlift has been studied by using the gassing-out method and a dual-tip optical probe to measure the bubble size distributions. As for bubble columns, in such airlift, the volumetric mass transfer coefficient appears to be quite proportional to the gas superficial velocity. Finally, as in Colombet et al. (2011), mass transfer at the bubble scale seems to be weakly influenced by an increase of gas volume fraction

Remotely forced nearshore upwelling in southern California

[1] Alongshore winds in Baja California strongly influence nearshore temperatures hundreds of kilometers to the north at Point Loma, San Diego, California, on timescales of a week to a year. The time lag between wind and temperature is consistent with first mode coastal trapped wave phase speed. The nearshore cross-shelf circulation forced by the coastal trapped waves is, at least much of the year, oppositely directed at the surface and bottom. No relation is found between the winds and temperature for periods greater than a year. It is argued that similar results may be found elsewhere in the Southern California Bight. The relationship between stratification and bottom temperature varies over the 1.3 years of data, but for much of the time, warmer bottom waters are associated with even warmer surface waters and thus stronger stratification. The effects of the remotely forced cross-shelf exchange on coastal pollution, nutrient dynamics, and larval transport are briefly discussed