62 research outputs found
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Numerical investigation of heavy fuel droplet-particle collisions in the injection zone of a Fluid Catalytic Cracking reactor, Part I: Numerical model and 2D simulations
The present paper investigates the collisions between heavy gasoil droplets and solid catalytic particles taking place at conditions realized in Fluid Catalytic Cracking reactors (FCC). The computational model utilizes the Navier-Stokes equations along with the energy conservation and transport of species equations. The VOF methodology is used in order to track the liquid-gas interface, while a dynamic local grid refinement technique is adopted, so that high accuracy is achieved with a relative low computational cost. Phase-change phenomena (evaporation of the heavy gasoil droplet), as well as catalytic cracking surface reactions are taken into account. Physical properties of heavy and light molecular weight hydrocarbons are modelled by representative single component species, while a 2-lump scheme is proposed for the catalytic cracking reactions. The numerical model is firstly validated for the case of a single liquid droplet evaporation inside a hot gaseous medium and impingement onto a flat wall for droplet heating and film boiling conditions. Afterwards, it is utilized for the prediction of single droplet-catalyst collisions inside the FCC injection zone. The numerical results indicate that droplets of similar size to the catalytic particles tend to be levitated more easily by hot catalysts, thus resulting in higher cracking reaction rates/cracking product yield, and limited possibility for liquid pore blocking. For larger sized droplets, the corresponding results indicate that the production of cracking products is not favored, while solid-liquid contact increases. Hotter catalysts promote catalytic cracking reactions and droplet levitation over the catalytic particle, owed to the formation of a thin vapour layer between the liquid and the solid particle
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Numerical investigation of heavy fuel droplet-particle collisions in the injection zone of a Fluid Catalytic Cracking reactor, part II: 3D simulations
This study investigates the collisions between heavy gasoil droplets and solid catalytic particles taking place at conditions realized in Fluid Catalytic Cracking reactors (FCC). The computational model utilizes the Navier-Stokes equations along with the energy conservation and transport of species equations. The VOF methodology is used in order to track the liquid-gas interface, coupled with a dynamic local grid refinement technique in order to minimize the computational cost. Phase-change phenomena, as well as catalytic cracking surface reactions (2-lump scheme) are taken into account. In this paper, the numerical model is extended to investigate the droplet-particle collision process in three dimensions. In order to save computational resources, only half of the droplet is investigated, by imposing symmetry conditions. Firstly, single droplet-catalyst collisions are simulated and compared against the corresponding ones provided by 2D axisymmetric simulations and afterwards, the model is applied for the characterization of the collision dynamics between a single droplet and a particle cluster, i.e. a realistic 3D particle configuration. As the droplet flows through the space between the catalytic particles, important phenomena are observed, such as a) drop levitation due to the formed vapour layer and b) a thin liquid sheet formation, both of which affect the rate of gasoline production, as well as predictions for liquid pore blocking mechanism; a phenomenon frequently observed industrially. Results indicate that gasoline production decreases when the collision target is a particle cluster, instead of same number (as many as in the cluster) single catalysts, as the corresponding contact area decreases
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Aerodynamic breakup of an n-decane droplet in a high temperature gas environment
The aerodynamic droplet breakup under the influence of heating and evaporation is studied numerically by solving the Navier-Stokes, energy and transport of species conservation equations; the VOF methodology is utilized in order to capture the liquid-air interphase. The conditions examined refer to an n-decane droplet with Weber numbers in the range 15â90 and gas phase temperatures in the range 600â1000 K at atmospheric pressure. To assess the effect of heating, the same cases are also examined under isothermal conditions and assuming constant physical properties of the liquid and surrounding air. Under non-isothermal conditions, the surface tension coefficient decreases due to the droplet heat-up and promotes breakup. This is more evident for the cases of lower Weber number and higher gas phase temperature. The present results are also compared against previously published ones for a more volatile n-heptane droplet and reveal that fuels with a lower volatility are more prone to breakup. A 0-D model accounting for the temporal variation of the heat/mass transfer numbers is proposed, able to predict with sufficient accuracy the thermal behavior of the deformed droplet
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A numerical study on droplet-particle collision dynamics
The impact of liquid droplets onto spherical stationary solid particles under isothermal conditions is simulated. The CFD model solves the Navier-Stokes equations in three dimensions and employs the Volume of Fluid Method (VOF) coupled with an adaptive local grid refinement technique able to track the liquid-gas interface. A fast-marching algorithm suitable for the quick computation of distance functions required during the grid refinement in large 3-D computational domains is proposed. The numerical model is validated against experimental data for the case of a water droplet impact onto a spherical particle at low We number and room temperature conditions. Following that, a parametric study is undertaken examining (a) the effect of Weber number (= Ďu2Do/Ď) in the range of 8 to 80 and (b) the droplet to particle size ratio ranging in-between 0.31 and 1.24, on the impact outcome. This has resulted to the identification of two distinct regimes that form during droplet-particle collisions: the partial/full rebound and the coating regimes; the latter results to the disintegration of secondary satellite droplets from elongated expanding liquid ligaments forming behind the particle. Additionally, the temporal evolution of variables of interest, such as the maximum dimensionless liquid film thickness and the average wetting coverage of the solid particle by the liquid, have been quantified. The present study assists the understanding of the physical processes governing the impact of liquids onto solid spherical surfaces occurring in industrial applications, including fluid catalytic cracking (FCC) reactors
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Development of a two-phase flow model for the investigation of collisions between heavy gasoil droplets and catalytic particles in Fluid Catalytic Cracking Reactors
The goal of this work is to study computationally the flow induced by the collision between a single gasoil droplet and a spherical catalytic particle under realistic Fluid Catalytic Cracking (FCC) conditions. FCC reactors are found in the fossil fuel refineries and are used to upgrade heavy fuel (gas oil) to lighter products (gasoline or LPG), which are industrially more important. Gasoil is injected in the reactor and atomizes; the produced droplets vaporize intensely and come in contact with the hot fluidized solid catalysts. The âcrackingâ reactions accommodated at the particle porous surface (ex. zeolite) result in the decomposition of gasoil to lighter products.
The two-phase flow model developed solves the incompressible Navier-Stokes equations for mass and momentum, along with the energy conservation equation. The VOF methodology is used to track the liquid-gas interface, while a dynamic local grid refinement technique is adopted, so that high accuracy is achieved with a relative low computational cost. A local evaporation model coupled with the additional solution of the species transport equation is utilized to consider phase change. Cracking surface reactions are taken into account via a simplified 2-lump scheme.
The model is successfully validated in fundamental droplet dynamics flow conditions, such as droplet acceleration, droplet impingement onto flat and solid surfaces under isothermal conditions and droplet evaporation. Insights into these phenomena provide important information that are missing from experimental measurements. The numerical novelties of the current work include the implementation of a new Wetting Force Model to simulate drop-solid interaction, as well as the proposition of a sharpening scheme for the volume fraction field, to suppress diffusion.
Concerning FCC collisions, the numerical model is able to reproduce both the hydrodynamics (drop deformation, spreading, breakup), as well as the chemical products (gasoil converted to gasoline). It is found that droplets of similar size to the catalytic particles tend to be levitated more easily by hot catalysts, thus resulting in higher cracking reaction rates/cracking product yield, and limited possibility for liquid pore blocking. For larger sized droplets, solid-liquid contact increases.
The main ambition of the current Thesis, which is to combine the droplet hydrodynamics with the chemical reactions acts as a novel step towards the understanding of such micro-scale physical phenomena that are difficult to capture/measure in experimental apparatus. This fundamental numerical tool can provide insight to the spray system strategy of an FCC reactor for a wide range of operating conditions
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Numerical investigation of the aerodynamic breakup of a parallel moving droplet cluster
The present work examines numerically the aerodynamic breakup of a cluster of Diesel droplets moving in parallel with respect to the gas flow. Two- and three-dimensional simulations of the incompressible Navier-Stokes equations together with the VOF method are performed for Weber (We) numbers in the range of 5 up to 60 and non-dimensional distance between the droplets (H/D0) ranging from 1.25 to 20. The numerical results indicate that the proximity of droplets affects their breakup for distances H/D0â¤5. For low droplet proximity distances (H/D0â¤2.5), the droplets experience the so-called shuttlecock breakup mode, which has been also identified for droplets in tandem formations in a previous authorsâ work and is characterized by an oblique peripheral stretching of the droplet. With decreasing H/D0 the breakup initiation time decreases, while the drag coefficient increases relative to that of isolated droplets. When the distance between the droplets is low enough (H/D0<1.5), this can result in critical We number, i.e. minimum We number leading to breakup, lower than that of an isolated droplet at the same conditions
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On viscoelastic cavitating flows: A numerical study
The effect of viscoelasticity on turbulent cavitating flow inside a nozzle is simulated for Phan-Thien-Tanner (PTT) fluids. Two different flow configurations are used to show the effect of viscoelasticity on different cavitation mechanisms, namely, cloud cavitation inside a step nozzle and string cavitation in an injector nozzle. In incipient cavitation condition in the step nozzle, small-scale flow features including cavitating microvortices in the shear layer are suppressed by viscoelasticity. Flow turbulence and mixing are weaker compared to the Newtonian fluid, resulting in suppression of microcavities shedding from the cavitation cloud. Moreover, mass flow rate fluctuations and cavity shedding frequency are reduced by the stabilizing effect of viscoelasticity. Time averaged values of the liquid volume fraction show that cavitation formation is strongly suppressed in the PTT viscoelastic fluid, and the cavity cloud is pushed away from the nozzle wall. In the injector nozzle, a developed cloud cavity covers the nozzle top surface, while a vortex-induced string cavity emerges from the turbulent flow inside the sac volume. Similar to the step nozzle case, viscoelasticity reduces the vapor volume fraction in the cloud region. However, formation of the streamwise string cavity is stimulated as turbulence is suppressed inside the sac volume and the nozzle orifice. Vortical perturbations in the vicinity of the vortex are damped, allowing more vapor to develop in the string cavity region. The results indicate that the effect of viscoelasticity on cavitation depends on the alignment of the cavitating vortices with respect to the main flow direction
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Numerical investigation of droplet impingement onto hydrophobic and super-hydrophobic solid surfaces. The effect of Weber number and wettability
In this study, a new model for the wetting interaction between a liquid droplet and a solid surface is presented. Based on this model, a force which acts on the contact line is incorporated as a source term in the Navier-Stokes momentum equation. The advantage of the new model in comparison with the widely-used Brackbillâs model is that the contact angle is not inserted as a boundary condition, but is derived by the induced fluid flow and the adhesion physics of the liquid-surface combination. For the interface tracking, the Volume of Fluid (V.O.F) method is used, accompanied by an automatic local grid refinement technique in order to minimize the arithmetic diffusion of volume fraction and thus acquire more representative physical results. The new model is validated against experimental data for low and moderate We numbers both for hydrophilic and superhydrophobic surfaces. Results of the model are also compared against the standard Brackbillâs model for the implementation of the wetting force. The apparent contact angle during droplet spreading and recoiling is plotted in order to gain insight on the dynamic angle temporal evolution during the impingement process
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VOF simulations of the contact angle dynamics during the drop spreading: Standard models and a new wetting force model
Introduction
In this study,a novel numerical implementation for the adhesion of liquid droplets impacting normally on solid dry surfaces is presented. The advantage of this new approach, compared to the majority of existing models, is that the dynamic contact angle forming during the surface wetting process is not inserted as a boundary condition, but is derived implicitly by the induced fluid flow characteristics (interface shape) and the adhesion physics of the gas-liquid-surface interface (triple line), starting only from the advancing and receding equilibrium contact angles. These angles are required in order to define the wetting properties of liquid phases when interacting with a solid surface.
Methodology
The physical model is implemented as a source term in the momentum equation of a Navier-Stokes CFD flow solver as an "adhesion-like" force which acts at the triple-phase contact line as a result of capillary interactions between the liquid drop and the solid substrate. The numerical simulations capture the liquid-air interface movement by considering the volume of fluid (VOF) method and utilizing an automatic local grid refinement technique in order to increase the accuracy of the predictions at the area of interest, and simultaneously minimize numerical diffusion of the interface.
Results
The proposed model is validated against previously reported experimental data of normal impingement of water droplets on dry surfaces at room temperature. A wide range of impact velocities, i.e. Weber numbers from as low as 0.2 up to 117, both for hydrophilic (θadv = 10° - 70°) and hydrophobic (θadv = 105° - 120°) surfaces, has been examined. Predictions include in addition to droplet spreading dynamics, the estimation of the dynamic contact angle; the latter is found in reasonable agreement against available experimental measurements.
Conclusion
It is thus concluded that theimplementation of this model is an effective approach for overcoming the need of a pre-defined dynamic contact angle law, frequently adopted as an approximate boundary condition for such simulations. Clearly, this model is mostly influential during the spreading phase for the cases of low We number impacts (We <80) since for high impact velocities, inertia dominates significantly over capillary forces in the initial phase of spreading
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Numerical investigation of the aerodynamic breakup of droplets in tandem
The present work examines the aerodynamic breakup of four liquid droplets in tandem formation at Diesel engine conditions using the Volume of Fluid (VOF) method. The examined Weber (We) numbers range from 15 up to 64 and the non-dimensional distances between the droplet centres (L/D0) vary from 1.25 up to 20. Focus is given on the breakup process of the third droplet of the row, which is regarded as a ârepresentative chain dropletâ; its development is compared against that of an isolated droplet at the same flow conditions. It is found that for small distances and depending on the We number, the obtained shapes and breakup modes between the droplets are different, with the representative chain droplet experiencing a new breakup mode in the multi-mode regime, termed as âshuttlecockâ. This is characterized by an oblique peripheral stretching of the droplet caused by the acting of pressure forces at an off-centre region. Moreover, the drag coefficient and liquid surface area of the representative chain droplet are lower than the corresponding ones of the isolated droplet, while the breakup initiation time is longer and the minimum We number required for breakup (critical We) is higher; correlations are provided to quantify the effect of droplet distance on the aforementioned quantities. Generally, the droplet proximity becomes important for L/D0<âŻ9. Finally, the predicted drag coefficient is utilised in a simplified 0-D model that is capable of estimating the temporal evolution of droplet velocity of the representative chain droplet
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