# Multiphase flows

The motion of dispersed objects (particles, droplets or bubbles) with a diameter larger than 10 micrometers is dominated by gravity, drag forces with the surrounding continuous medium (often air or water), and collisional contacts between the objects and with walls. Because such macroscopic objects collide inelastically, large-scale heterogeneous structures emerge. For many engineering applications it is important to be able to predict these dispersed flows through pipes, chutes, reactors and other vessels.

We are developing simulation models for macroscopic (non-Brownian) dispersed flows, where we use a multi-scale approach
to cover the entire range of length scales from the particle level to the level of industrial size reactors.
On the detailed level, we take into account the fundamental processes of momentum, heat and mass
transfer, whereas on the larger levels we use effective (coarse-grained) interactions and correlations
obtained on the detailed levels, often borrowing methods from statistical mechanics and soft matter science.
Because a model is only as good as its assumptions (*garbage in: garbage out*), an important part of our work
is the experimental validation of our models, through advanced non-invasive techniques such as Particle Image Velocimetry.

# Granular flow in rotating equipment

In exceptional cases, when granular particles are relatively large and the particle velocities are relatively low, the effect of the surrounding gas can be neglected. In one application, we have studied the mixing of granular materials in a rotating drum as a function of filling height and baffle size. We found that only relatively large, straight baffles perpendicular to the drum wall (67% of the drum radius) increase the mixing performance of the rotating drum.

More recently, we have studied the flow of granular particles down an inclined and rotating chute. The goal of this work is to study the influence of chute rotation on the flow behaviour and segregation in granular flows, and to obtain high quality experimental data with which discrete particle model simulations can be validated. The figures to the right show the experimental setup on a rotating table, and a snapshot (top view) of our simulations. We measure the surface particle velocity field through particle image velocimetry and particel tracking velocimetry. We find that, compared to a non-rotating chute, the surface flow velocity in a rotating chute is decreased initially due to compaction against the side walls (Coriolis effect) but ultimately increased due to the increasing influence of centrifugal forces.

- M.A.I. Schutyser, J.T. Padding
*et al.*,*Discrete particle simulations predicting mixing behavior of solid substrate particles in a rotating drum fermenter*, Biotechn. Bioeng.**75**, 666 (2001). - S. Shirsath, J.T. Padding, H.J.H. Clercx and J.A.M. Kuipers,
*Modelling of granular flows through inclined rotating chutes using a Discrete Particle Model*, proceedings of the CSIRO Ninth International Conference on Computational Fluid Dynamics in the Minerals and Process Industries, Melbourne (2012). - S.S. Shirsath, J.T. Padding, N.G. Deen, H.J.H. Clercx, and J.A.M. Kuipers,
*Experimental study of monodisperse granular flow through an inclined rotating chute*, Powder Technol.**246**, 235 (2013). - S.S. Shirsath, J.T. Padding, H.J.H. Clercx and J.A.M. Kuipers,
*Numerical investigation of monodisperse granular flow through an inclined rotating chute*, AIChE Journal**60**, 3424 (2014). - S.S. Shirsath, J.T. Padding, H.J.H. Clercx and J.A.M. Kuipers,
*Dynamics of granular flows down rotating semi-cylindrical chutes*, Procedia Engineering**102**, 731 (2015). - S.S. Shirsath, J.T. Padding, H.J.H. Clercx and J.A.M. Kuipers,
*Cross-validation of 3D particle tracking velocimetry for the study of granular flows down rotating chutes*, Chem. Eng. Sci.**134**, 312 (2015). - S.S. Shirsath, J.T. Padding, H.J.H. Clercx and J.A.M. Kuipers,
*Simulation study of the effect of wall roughness on the dynamics of granular flows in rotating semi-cylindrical chutes*, AIChE J.**61**, 2117 (2015). - S.S. Shirsath, J.T. Padding, H.J.H. Clercx and J.A.M. Kuipers,
*Dynamics of granular flows down rotating semi-cylindrical chutes*, in New Paradigm of Particle Science and Technology, Procedia Engineering**102**, 731-740 (2015). - L.J.H. Seelen, J.T. Padding and J.A.M. Kuipers,
*Improved quaternion based integration scheme for rigid body motion*, Acta Mechanica**227**, 3381 (2016). - L.J.H. Seelen, J.T. Padding and J.A.M. Kuipers,
*A granular Discrete Element Method for arbitrary convex particle shapes: method and packing generation*, submitted to Chem. Eng. Sci. (2018).

# Fluidized beds

In a fluidized bed, an uprising gas is flowing so fast that it causes the solid particles to "float". As a result, the particle emulsion has many properties of a liquid, including the formation of flow vortices and bubbles. Fluidized beds are used frequently in chemical reactors because they allow an intense contact between the gas (which may contain reactants) and the solid particles (which may be a growing product or contain catalysts).

In one line of research, we are focussing on developing large scale computational models for gas-solid fluidized beds. One direction is to use stochastic Lagrangian coarse-grained multi-particle collision techniques borrowed from soft matter simulations, see the section on colloids. Another direction is to use and improve the so-called Two-Fluid Model, in which the gas and solid phases are treated as two interpenetrating continua. The latter approach requires accurate expressions for the rheology of the solid phase, for which we are using the kinetic theory of granular flow. The picture above shows contour plots of the solids volume fraction at three successive times in a cylindrical fluidized bed, obtained from these TFM simulations.

In another line of resarch, we are focussing on accurate simulation methods to measure the viscosity of the granular particle emulsion and to model the influence of intruders (which may also be baffles or stirrers). The viscoscity is obtained both through Green-Kubo relations and more directly through measurement of the stress in Couette flow and measurement of the drag on a dragged sphere. We have developed a hybrid method in which granular particles are coupled with the surrounding gas through effective drag correlations, whereas large intruders are coupled more directly through immersed boundaries. The influence of an impacting intruder is first studied for a pre-fluidized bed, in which the gas flow has slowly been turned off again. The picture to the right shows the effect of a large spherical intruder impacting into a pre-fluidized bed of much smaller granular particles.

- V. Verma, N.G. Deen, J.T. Padding and J.A.M. Kuipers,
*Two-fluid modeling of three-dimensional cylindrical gas-solid fluidized beds using the kinetic theory of granular flow*, Chem. Eng. Sci.**102**, 227 (2013). - Y. Xu, J.T. Padding, M.A. van der Hoef and J.A.M. Kuipers,
*Detailed numerical simulation of an intruder moving through a granular bed using a hybrid Discrete Particle and Immersed Boundary (DP-IB) method*, Chem. Eng. Sci.**104**, 201 (2013). - V. Verma, J.T. Padding, N.G. Deen and J.A.M. Kuipers,
*Bubble formation at a central orifice in a gas-solid fluidized bed predicted by three-dimensional two-fluid model calculations*, Chem. Eng. J.**245**, 217 (2014). - V. Verma, J.T. Padding, N.G. Deen and J.A.M. Kuipers,
*Bubble dynamics in a 3-D gas-solid fluidized bed using ultrafast electron beam X-ray tomography and two-fluid model*, AIChE Journal**60**, 1632 (2014). - N.G. Deen, E.A.J.F. Peters, J.T. Padding and J.A.M. Kuipers,
*Review of Direct Numerical Simulation of Fluid-Particle Mass, Momentum and Heat Transfer in Dense Gas-Solid Flows*, submitted to Chem. Eng. Sci. (2013). - V. Verma, J.T. Padding, N.G. Deen, J.A.M. Kuipers, M. Bieberle, F. Barthel, M. Wagner and W. Hampel,
*Bubble characteristics in a 3-D gas-solid fluidized bed: predictions from ultra-fast X-ray tomography and two-fluid model*, proceedings of the 11th Circulating Fluidized Beds conference, Bejing, China (2014). - V. Verma, J.T. Padding, N.G. Deen and J.A.M. Kuipers,
*Effect of fluidized bed diameter on fluidization dynamics*, proceedings of the 11th Circulating Fluidized Beds conference, Bejing, China (2014). - V. Verma, J.T. Padding, N.G. Deen and J.A.M. Kuipers,
*CFD validations for 3D cylindrical gas-solid fluidized beds*, proceedings of the SINTEF 10th International Conference on Computational Fluid Dynamics in the Minerals and Process Industries, Trondheim (2014). - Y. Xu, J.T. Padding, M.A. van der Hoef and J.A.M. Kuipers,
*Numerical investigation of the vertical plunging force of a spherical intruder into a granular bed*, proceedings of the SINTEF 10th International Conference on Computational Fluid Dynamics in the Minerals and Process Industries, Trondheim (2014). - N.G. Deen, E.A.J.F. Peters, J.T. Padding and J.A.M. Kuipers,
*Review of Direct Numerical Simulation of Fluid-Particle Mass, Momentum and Heat Transfer in Dense Gas-Solid Flows*, Chem. Eng. Sci.**116**, 710 (2014). - V. Verma, J.T. Padding, N.G. Deen and J.A.M. Kuipers,
*Numerical Investigation on the Effect of Pressure on Fluidization in a 3-D Fluidized Bed*, Industrial & Engineering Chemistry Research**53**, 17487 (2014). - Y. Xu, J.T. Padding and J.A.M. Kuipers,
*Numerical investigation of the vertical plunging force of a spherical intruder into a prefluidized granular bed*, Phys. Rev. E**90**, 062203 (2014). - V. Sutkar, N.G. Deen, J.T. Padding, V. Salikov, B. Crueger, S. Antonyuk, S. Heinrich and J.A.M. Kuipers,
*A novel approach to determine wet restitution coefficients through a unified correlation and energy analysis*, AIChE Journal**61**, 769 (2015). - V. Verma, J.T. Padding, N.G. Deen and J.A.M. Kuipers,
*Effect of bed size on hydrodynamics in 3D gas-solid fluidized beds*, AIChE Journal**61**, 1492 (2015). - J.T. Padding, N.G. Deen, E.A.J.F. Peters and J.A.M. Kuipers,
*Euler-Lagrange modelling of the hydrodynamics of dense multiphase flows*, Advances in Chemical Engineering**46**, 137 (2015). - L. Yang, J.T. Padding and J.A.M. Kuipers,
*Comparison of a two-fluid model and an Euler-Lagrange model for simulation of dense gas-fluidized beds*, proceedings of the CSIRO 11th International Conference on Computational Fluid Dynamics in the Minerals and Process Industries, Melbourne (2015). - L. Yang, J.T. Padding and J.A.M. Kuipers,
*Modification of Kinetic Theory of Granular Flow for Frictional Spheres, Part I: Two-fluid model derivation and numerical implementation*, Chem. Eng. Sci.**152**, 767 (2016). - L. Yang, J.T. Padding and J.A.M. Kuipers,
*Modification of Kinetic Theory of Granular Flow for Frictional Spheres, Part II: Model validation*, Chem. Eng. Sci.**152**, 783 (2016). - Kay Buist, Luuk Seelen, Niels Deen, Johan Padding and Hans Kuipers,
*On an efficient hybrid soft and hard sphere collision integration scheme for DEM*, Chem. Eng. Sci.**153**, 363 (2016). - S.K.P. Sanjeevi, J.T. Padding and J.A.M. Kuipers,
*Direct numerical simulations of fluid drag forces of non-spherical particle*, proceedings of the CSIRO 11th International Conference on Computational Fluid Dynamics in the Minerals and Process Industries, Melbourne (2015). - L. Yang, J.T. Padding and J.A.M. Kuipers,
*Partial slip boundary conditions for collisional granular flows at flat frictional walls*, AIChE J.**63**, 1853 (2017). - L. Yang, J.T. Padding and J.A.M. Kuipers,
*Investigation of collisional parameters for rough spheres in fluidized beds*, Powder Technology**316**, 256 (2017). - S.K.P. Sanjeevi and J.T. Padding,
*On the orientational dependence of drag experienced by spheroids*, J. Fluid Mech.**820**, R1 (2017). - K.A. Buist, L.J.H. Seelen, N.G. Deen, J.T. Padding and J.A.M. Kuipers,
*Novel efficient hybrid-DEM collision integration scheme*, proceedings of the SINTEF 12th International Conference on Computational Fluid Dynamics in the Minerals and Process Industries, Trondheim (2017). - L. Yang, J.T. Padding and J.A.M. Kuipers,
*Experimental and numerical investigation of a bubbling dense solid-gas fluidized bed*proceedings of the SINTEF 12th International Conference on Computational Fluid Dynamics in the Minerals and Process Industries, Trondheim (2017). - I. Mema, V.V. Mahajan, B.W. Fitzgerald, H. Kuipers and J.T. Padding,
*Effect of lift force on dense gas-fluidized beds of non-spherical particles*, proceedings of the SINTEF 12th International Conference on Computational Fluid Dynamics in the Minerals and Process Industries, Trondheim (2017). - L. Yang, J.T. Padding and J.A.M. Kuipers,
*Three-dimensional fluidized beds with rough spheres: validation of a two fluid model by magnetic particle tracking and discrete particle simulations*, Chem. Eng. Sci.**174**, 238 (2017). - K.A. Buist, P. Jayaprakash, J.A.M. Kuipers, N.G. Deen and J.T. Padding,
*Magnetic particle tracking for nonspherical particles in a cylindrical fluidized bed*, AIChE Journal**63**, 5335 (2017). - V.V. Mahajan, J.T. Padding, T.M.J. Nijssen, K.A. Buist, and J.A.M. Kuipers,
*Non-spherical particles in a pseudo-2D fluidized bed: Experimental study*, AIChE Journal**64**, 1573 (2018). - L. Boer, K.A. Buist, N.G. Deen, J.T. Padding and J.A.M. Kuipers,
*Experimental study on orientation and de-mixing phenomena of elongated particles in gas-fluidized beds*, Powder Technology**329**, 332 (2018). - A. Zarghami and J.T. Padding,
*Drag, lift and torque acting on a two-dimensional non-spherical particle near a wall*, to appear in Advanced Powder Technology (2018). - S.K.P. Sanjeevi, A. Zarghami, and J.T. Padding,
*Choice of no-slip curved boundary condition for lattice Boltzmann simulations of high-Reynolds-number flows*, accepted for publication in Phys. Rev. E (2018). - L.J.H. Seelen, J.T. Padding and J.A.M. Kuipers,
*A granular Discrete Element Method for arbitrary convex particle shapes: method and packing generation*, submitted to Chem. Eng. Sci. (2018).

# Triboelectrification

Our goal is to develop new separation technologies for dry fractionation of food products. The dry fractionation is based on electrostatic charging of powders in a gas stream when they collide with the walls of a charging tube, and subsequent separation by an external electric field (see figure above). Our aim is to accurately model the dynamics and acquired charge of the particles in the charging tube, where triboelectric charging takes place when a particle collides with the metal walls. The challenge of this project is that we need to combine triboelectric charging of the particles (about which little is known) with electrostatic interactions and gas flow. The first results are encouraging.

- M. Korevaar, J.T. Padding, M.A. van der Hoef and J.A.M. Kuipers,
*Modeling of tribo-electrification of a pneumatically conveyed powder in a squared duct using DEM-CFD*, proceedings of the 2013 Annual Meeting of the Electrostatics Society of America. - M. Korevaar, J.T. Padding, M.A. van der Hoef and J.A.M. Kuipers,
*Integrated DEM-CFD modeling of the contact charging of pneumatically conveyed powders*, Powder Technology**258**, 144 (2014). - M.W. Korevaar, J.T. Padding, M.A. van der Hoef and J.A.M. Kuipers,
*DEM-CFD simulations and PIV/PTV experiments on charging of pneumatically conveyed powders*, proceedings of the SINTEF 10th International Conference on Computational Fluid Dynamics in the Minerals and Process Industries, Trondheim (2014). - M.W. Korevaar, J.T. Padding, N.G. Deen, J. Wang, M. de Wit, M.A.I. Schutyser and J.A.M. Kuipers,
*Hybrid PIV/PTV measurements of velocity and position distributions of gas-conveyed particles in small, narrow channels*, AIChE J.**61**, 3616 (2015).

# Spray-drying

Spray drying is used to convert liquid feed materials into a dry powder form, e.g. to produce milk powder. The liquid feed is sprayed (atomized) through high pressure nozzles in a spray chamber (see picture on the left) and the resulting droplets are mixed with hot gas to evaporate the liquid content.

The main objective of this work is to develop a reliable simulation tool that can predict the droplet and particle flow, agglomeration, and size distribution for a section of a large-scale spray dryer. To be able to handle many billions of droplets and particles, we have adapted a direct simulation monte carlo method, in which the droplets and particles collide stochastically according to predictions from kinetic theory. Full two-way coupling between the droplets/particles and the gas has been implemented. Collisions between droplets and/or particles lead to different outcomes, depending on dimensionless numbers (Weber number, Ohnesorge number and impact parameter). The figure below shows the simulation results for a spray of pure droplets.

- S.K. Pawar, J.T. Padding, N.G. Deen, J.A.M. Kuipers, A. Jongsma and F. Innings,
*Eulerian-Lagrangian modelling with stochastic approach for droplets-particles collision*, proceedings of the CSIRO Ninth International Conference on Computational Fluid Dynamics in the Minerals and Process Industries, Melbourne (2012). - S.K. Pawar, J.T. Padding, N.G. Deen, A. Jongsma, F. Innings and J.A.M. Kuipers,
*Lagrangian modelling of dilute granular flow-modified stochastic DSMC versus deterministic DPM*, Chem. Eng. Sci.**105**, 132 (2014). - Sandip Pawar, Ruud Abrahams, Niels Deen, Johan Padding, Gert-Jan van der Gulik, Alfred Jongsma, Fredrik Innings and Hans Kuipers,
*An Experimental Study of Dynamic Jet Behavior in a Scaled Cold Flow Spray Dryer Model Using PIV*, Can. J. Chem. Eng.**92**, 2013 (2014). - S. Pawar, J.T. Padding, N.G. Deen, A. Jongsma, F. Innings and J.A.M. Kuipers,
*Agglomeration study in the inlet section of a large scale spray dryer using stochastic euler-lagrange modelling*, proceedings of the SINTEF 10th International Conference on Computational Fluid Dynamics in the Minerals and Process Industries, Trondheim (2014). - S.K. Pawar, J.T. Padding, N.G. Deen, A. Jongsma, F. Innings and J.A.M. Kuipers,
*Drying kinetics of droplets containing solids using reaction engineering approach*, proceedings of the 19th International Drying Symposium, Lyon, France (2014). - S.K. Pawar, J.T. Padding, N.G. Deen, A. Jongsma, F. Innings and J.A.M. Kuipers,
*Numerical and Experimental Investigation of Induced Flow and Droplet-Droplet Interactions in a Liquid Spray*, Chem. Eng. Sci.**138**, 17 (2015). - S.K. Pawar, F. Henrikson, G. Finotello, J.T. Padding, N.G. Deen, A. Jongsma, F. Innings and J.A.M. Kuipers,
*An experimental study of droplet-particle collisions*, Powder Technology**300**, 157 (2016). - H. Patel, S. Das, J.A.M. Kuipers, J.T. Padding and E.A.J.F. Peters,
*A coupled VOF-IBM method for simulating 3D multiphase flows with contact line dynamics in complex geometries*, Chem. Eng. Sci.**166**, 28 (2017). - G. Finotello, J.T. Padding, N.G. Deen, A. Jongsma, F. Innings and J.A.M. Kuipers,
*Effect of viscosity on droplet-droplet collisional interaction*, Physics of Fluids**29**, 067102 (2017). - G. Finotello, R.F. Kooiman, J.T. Padding, K.A. Buist, A. Jongsma, F. Innings and J.A.M. Kuipers,
*The dynamics of milk droplet-droplet collisions*, Experiments in Fluids**59**, 17 (2018). - S.K. Pawar, J.T. Padding, N.G. Deen and J.A.M. Kuipers,
*A Novel Hybrid Computational Model Combining DSMC and a Kinetic Particle Model for Simulation of Spray Drying of Milk*, to be submitted (2018).

# Bubbly flows

Bubble column reactors are widely used as gas-liquid contactors in the chemical and energy industries. The bubble dynamics inside the column dictate the liquid phase flow. Modeling of such bubble columns is very challenging and typically empirical correlations are used even today.

The main objective of this work is to develop a stochastic Euler-Lagrange method that can predict the hydrodynamics of dense bubbly flows. For this we are adapting the direct simulation monte carlo approach to handle bubble collisions stochastically instead of deterministically, enhancing the speed of bubble collision calculations by a factor of 10 to 100 relative to more detailed discrete bubble model simulations, while still maintaining the same level of accuracy.

- S. Kamath, J.T. Padding, K.A. Buist and J.A.M. Kuipers
*Stochastic DSMC model for large scale dense bubbly flows*, proceedings of the SINTEF 12th International Conference on Computational Fluid Dynamics in the Minerals and Process Industries, Trondheim (2017). - S. Kamath, J.T. Padding, K.A. Buist and J.A.M. Kuipers,
*Stochastic DSMC method for dense bubbly flows: Methodology*, Chem. Eng. Sci.**176**, 454 (2018).

# collaboration

The research on granular flow was performed by my former PhD students Sushil Shirsath and Luuk Seelen. The research on granular viscosity and intruder impact was performed with my former PhD student Yupeng Xu. Within these projects we collaborated intensely with the Turbulence and Vortex Dynamics group (Prof. Herman Clercx) at Eindhoven University of Technology, with Prof. Detlef Lohse, Prof. Devaraj van der Meer and Dr. Martin van der Hoef of the Physics of Fluids group and with Prof. Stefan Luding and Dr. Anthony Thornton of the Multiscale Mechanics group, both at the University of Twente, and Prof. Onno Bokhove at University of Leeds. The research on fluidized beds of spherical particles was performed by my current and former PhD students Vikrant Verma and Lei Yang and was financially supported by an ERC-advanced grant of Prof. Hans Kuipers. The on-going research on fluidized beds of non-spherical particles, performed with my PhD students Sathish Sanjeevi, Vinay Mahajan and Ivan Mema, and my postdocs Vikrant Verma and Barry Fitzgerald, is financially supported by my ERC-consolidator grant and a number of NWO grants for computing time on the Dutch national supercomputer Cartesius, as well as a Prace grant for access to European supercomputers. The research on triboelectric charging was performed with my PhD student Martin Korevaar, in collaboration with Dr. Maarten Schutyser of Wageningen University, and is financially supported by an STW grant. The project on spray-drying was performed by our previous PhD student Sandip Pawar, continued by our current PhD student Giulia Finotello, and is financially supported by Tetrapak Heerenveen; we are indebted to intense collaborations with Dr. Alfred Jongsma and Dr. Fredrik Innings.