Modified shear stress transport model with curvature correction for the prediction of swirling flow in a cyclone separator

The paper investigates the confined swirling flow in a cyclone. The numerical simulations are performed using a proposed eddy viscosity turbulence model, which accounts for the effects of the streamline curvature and rotation. This distinguishes the current model from the conventional Eddy Viscosity Models (EVMs) that are known to fail to predict the Rankine vortex in swirling flows. Although computationally more expensive approaches, the Reynolds Stress Model (RSM) and Large Eddy Simulation (LES), have demonstrated a high capability of dealing with such flows, these techniques are often unsuited for use in complex design studies where computational speed and robustness are key factors. In the present approach, the Shear Stress Transport with Curvature Correction (SSTCC) turbulence model is modified by the introduction of the Richardson number to account for the rotation and curvature effects. The numerical predictions were validated using experimental results and also compared to the data obtained using the RSM model and various EVMs without the proposed modifications. The investigations started with a benchmark case of a flow through a channel duct with a U-turn, after which more challenging simulations of a high swirling flow within a cyclone separator device were performed. The results show that the proposed model is competitive in terms of accuracy when compared to RSM and proves to be superior to the RSM model in terms of computational cost. Furthermore, it is found that the proposed model preserves the ability to represent the Rankine vortex profile at different longitudinal levels of the cyclone. It is also more efficient in terms of the computational cost than the SSTCC model without the introduced modifications

Influence of wall friction on flow regimes and scale-up of counter-current swirl spray dryers

The structure of the vortex flow in swirl spray dryers is investigated after having fouled the walls with deposits typical of detergent manufacture. The range of Re and swirl intensity Ω characteristic of industry are studied using three counter-current units of varying scale and design. The friction with the deposits increases the flow turbulence kinetic energy and causes a drastic attenuation of the swirl and as a result, the vortex breaks down in the chamber forming recirculation regions (i.e. areas of reverse flow). Three flow regimes (1) no recirculation, (2) central and (3) annular recirculation have been identified depending on the swirl intensity. New control and scale up strategies are proposed for swirl dryers based in predicting the decay and the flow regime using the unit geometry (i.e. initial swirl intensity Ωi) and experimental decay rates function of the coverage and thickness of deposits. The impact in design and numerical modelling must be stressed. Adequate prediction of the swirl is vital to study fouling and recirculation, which surely play an important part in the dispersion and aggregation of the solid phase. Current models have no means to replicate these phenomena, and yet, in this case neglecting the deposits and assuming smooth walls would result in (a) over-prediction of swirl velocity up to 40-186% (b) under-prediction of turbulent kinetic energy up to 67-85% and (c) failure to recognise recirculation areas

AN INVESTIGATION OF A NOVEL GAS-SOLID SEPARATOR FOR DOWNER REACTORS

Rapid gas-solid separation is a critical stage in many industrial applications, such as fluid k *- lytic cracking (FCC), heavy oil upgrading, and biomass pyrolysis. In these applications, act gases must be separated quickly and efficiently from catalyst or heat-bearing particles to srminate cracking reactions. Several rapid separation devices have been proposed to achieve sse demands. However, most proposed designs were intended for FCC processes. Very few gas-solid separators have been proposed specifically for biomass pyrolysis. A novel gas-solid separator for biomass pyrolysis in a downer reactor is investigated in this jesis. An experimental study is performed to identify important separator geometry and operating conditions. Computational fluid dynamics (CFD) is used to gain insight into the two- shase flow structure in the separator. The numerical results are coupled with an original ^experimental technique for measuring the particle-wall restitution coefficient to select !appropriate materials for the separator’s internal surface

Gas-liquid two-phase flows in double inlet cyclones for natural gas separation

The gas-liquid two-phase flow within a double inlet cyclone for natural gas separation was numerically simulated using the discrete phase model. The numerical approach was validated with the experimental data, and the comparison results agreed well with each other. The simulation results showed that the strong swirling flow produced a high centrifugal force to remove the particles from the gas mixture. The larger particles moved downward on the internal surface and were removed due to the outer vortex near the wall. Most of the tiny particles went into the inner vortex zones and escaped from the up-outlet. The swirling flow was concentric due to the design of the double inlet for the cyclonic separator, which greatly improved the separating efficiency. The separating efficiency was greater than 90% with the particle diameter of more than 100 μm

Hydrodynamic Simulation of Cyclone Separators

Cyclone separators are commonly used for separating dispersed solid particles from gas phase. These devices have simple construction; are relatively inexpensive to fabricate and operate with moderate pressure losses. Therefore, they are widely used in many engineering processes such as dryers, reactors, advanced coal utilization such as pressurized and circulating fluidized bed combustion and particularly for removal of catalyst from gases in petroleum refinery such as in fluid catalytic cracker (FCC). Despite its simple operation, the fluid dynamics and flow structures in a cyclone separator are very complex. The driving force for particle separation in a cyclone separator is the strong swirling turbulent flow. The gas and the solid particles enter through a tangential inlet at the upper part of the cyclone. The tangential inlet produces a swirling motion of gas, which pushes the particles to the cyclone wall and then both phases swirl down over the cyclone wall. The solid particles leave the cyclone through a duct at the base of the apex of the inverted cone while the gas swirls upward in the middle of the cone and leaves the cyclone from the vortex finder. The swirling motion provides a centrifugal force to the particles while turbulence disperses the particles in the gas phase which increases the possibility of the particle entrainment. Therefore, the performance of a cyclone separator is determined by the turbulence characteristics and particle-particle interaction.Full Tex

Computational Fluid Dynamic Studies of Vortex Amplifier Design for the Nuclear Industry—II. Transient Conditions

In this paper computational fluid dynamics (CFD) techniques have been used to investigate the effect of changes to the geometry of a vortex amplifier (VXA) in the context of glovebox operations in the nuclear industry. These investigations were required because of anomalous behavior identified when, for operational reasons, a long-established VXA design was reduced in scale. The study simulates the transient aspects of two effects: back-flow into the glovebox through the VXA supply ports, and the precessing vortex core in the amplifier outlet. A temporal convergence error study indicates that there is little to be gained from reducing the time step duration below 0.1 ms. Based upon this criterion, the results of the simulation show that the percentage imbalance in the domain was well below the required figure of 1, and imbalances for momentum in all three axes were all below measurable values. Furthermore, there was no conclusive evidence of periodicity in the flow perturbations at the glovebox boundary, although good evidence of periodicity in the device itself and in the outlet pipe was seen. Under all conditions the modified geometry performed better than the control geometry with regard to aggregate reversed supply flow. The control geometry exhibited aggregate nonaxisymmetric supply port back-flow for almost all of the simulated period, unlike the alternative geometry for which the flow through the supply ports was positive, although still nonaxisymmetric, for most of the period. The simulations show how transient flow structures in the supply ports can cause flow to be reversed in individual ports, whereas aggregate flow through the device remains positive. Similar to the supply ports, flow through the outlet of the VXA under high swirl conditions is also nonaxisymmetric. A time-dependent reverse flow region was observed in both the outlet and the diffuser. It is possible that small vortices in the outlet, coupled with the larger vortex in the chamber, are responsible for the oscillations, which cause the shift in the axis of the precessing vortex core (and ultimately in the variations of mass flow in the individual supply ports). Field trials show that the modified geometry reduces the back-flow of oxygen into the glovebox by as much as 78. At purge rates of 0.65 m 3h the modified geometry was found to be less effective, the rate of leakage from the VXA increasing by 16-20. Despite this reduced performance, leakage from the modified geometry was still 63 less than the control geometry. © 2012 American Society of Mechanical Engineers

Modeling of Multiphase Flow in Hydrocyclone and Validation

Cyclone separators have exist since the 1800's and are still widely used in many industries. Although hydrocyclone are geometrically simple, the physics describing the flow and separation processes which occur in them is complex. Over the decades many researchers have studied these devises and have developed a number of theories and empirical models for design purposes. In practice, most cyclones are design using some type of empirical information. Physical prototypes are then built, tested and tuned until an acceptable level of performance is obtained. Recent advancement in numerical methods and in the performance capabilities of moderately priced computers have opened the possibility of developing computer-based methods, which can be effectively used for hydrocyclone design study. This is where this project play part, a study of multiphase flow in hydrocyclone with different configuration of parameters are manipulate using computer model will be proposed. In this model, the mixture of multiphase flow model is used to simulate the internal three-dimensional flow field of the hydrocyclone using computational fluid dynamics (CFD) method. AutoCAD and NUMECA FINE/Open are used as a medium to design and simulate the CFD model of hydrocyclone in this project to obtain optimum design configuration. The outcome of research is very helpful to explain the separation process and to optimize the hydrocyclone design. This study provides the potential to produce hydrocyclone designs with the required performance characteristics more quickly and more economically than older methods which use experimental design approach exclusively

Computational fluid dynamics analysis of a novel axial flow hydrocyclone

A comparative CFD investigation was carried out for a mini axial, a large axial and a large reverse flow hydrocyclone. The diameters selected were 5 mm and 75 mm for the mini and large hydrocyclones, respectively. The simulations were conducted using a large eddy simulation (LES) turbulence model with various subgrid scale models, and the results of the reverse flow hydrocyclone were validated against published LDV data. The numerical results confirmed that the LES-Smagorinsky model provides good prediction relative to the other subgrid scale models studied, giving an error percentage of 0.45% for the water split ratio. Numerical investigation of mini axial and reverse flow hydrocyclones were performed. The Lagrangian discrete phase model (DPM) was used to track the soda-lime glass particles released from the inlet surface. The soda-lime glass particles had a density of 2520 kg/m3 with a particle diameter range of 10 µm to 150 µm. The results indicate that the axial flow hydrocyclone gave a lower pressure drop and a higher cut size than the reverse flow hydrocyclone for inlet velocities ranging from 1-10 m/s. Thus, the axial flow hydrocyclone is an effective particle separator. The effect of inlet dimensions, vortex finder diameter and length on the performance and flow pattern was then investigated. Thirteen mini axial hydrocyclones separators were investigated for a fixed inlet velocity of 2 m/s. The simulations showed that changing the vortex finder diameter and length had a more pronounced effect on the separation efficiency and velocity profiles than changing the inlet dimensions. Decreasing the diameter of vortex finder translates to higher separation efficiency at the cost of higher pressure drop. The results showed that lengthening the vortex finder increases the separation efficiency but decreases the cut size as the vortex strength decreases with vortex finder length. The axial flow hydrocyclone can be a serious competitor to the reverse flow hydrocyclone for industrial use. A comprehensive study of 75 mm axial flow hydrocyclone enormously improved the understanding of the possibility of its use for industrial applications. A comparison of the axial and reverse flow hydrocyclones of 75 mm diameter showed that the cut size of the axial flow hydrocyclone was larger than the reverse flow hydrocyclone at particle concentrations of 4.88% and 10.47%. However, the pressure drop was significantly lower for axial flow hydrocyclone. The CFD-based investigation showed that the axial flow hydrocyclone could be successfully used to classify particles in the industry with a substantially lower pressure drop and pumping energy requirement

Experimental and simulation studies on performance of a compact gas/liquid separation system

The need of exploiting the offshore oil reserves and reducing the equipment costs becomes the motivation for developing new compact separation techniques. In the past years, the development of compact separators has almost solely focused on the cyclonic type separators made of pipes, because of their simple construction, relatively low cost of manufacturing and being able to withstand high pressures. Considerable effort has been put into the separator test program and qualification, and consequently notable advances in the compact separation technique have been made. However the application has been held back due to lacking of reliable predicting and design tools. The objectives of this study were threefold. Firstly, an experimental study was carried out aiming at understanding the separation process and flow behaviours in a compact separator, named Pipe-SEP, operating at high inlet gas volume fraction (GVF). Secondly it is to gain insight of the gas and liquid droplet flow in the compact separator by means of Computational Fluid Dynamics (CFD) simulations. Last but not least, the understanding and insight gained above were used to develop a comprehensive performance predictive model, based on which, a reliable optimizing design procedure is suggested. An experimental study was carried out to test a 150-mm Pipe-SEP prototype with a water-air mixture. Three distinct flow regimes inside the Pipe-SEP were identified, namely swirled, agitated, and gas blow-by. The transition of the flow regimes was found to be affected by inlet flow characteristics, mixture properties, geometry of the separator, and downstream conditions. A predictive model capable of predicting the transition of flow regimes and the separation efficiency was developed. A comparison between the predicted result and experiment data demonstrated that the model could serve as a design tool to support decision-making in early design stages ... [cont.]