Prashant Gunjal

This paper presents a computational study of some boundary collocation solution methods for the Laplace equation in cylindrical coordinates with axisymmetry. The methods compared are (i) the direct boundary element method (BEM), (ii) the method of fundamental solutions (MFS) with fixed sources and (iii) the Trefftz method. Relative accuracy of these methods are compared for two test problems. The first problem is a simple problem of heat transfer through a cylindrical rod which is a standard benchmark problem in this field. The second problem deals with heat transfer in silicon melt for Czochralski (CZ) process which involves a singularity in the boundary conditions at the corner of the crystal–melt interface. All the three methods indicated above are highly successful for the simple (first) problem with MFS and Trefftz being simpler to implement than the BEM. However, the Trefftz method was not effective for the second problem due to the boundary singularity and the MFS showed oscillations near the singularity point. Hence the use of higher order non-conforming elements with accurate Gauss–Kronrod integration schemes in the direct BEM method was investigated. It was found that the boundary singularity does not deteriorate the accuracy of the results if this improved numerical integration procedure is used in the direct BEM. Hence higher order elements with Gauss–Kronrod integration schemes can be used for the solution of many free interface problems encountered in crystal growth.

Hydrodynamics of trickle-bed reactors involve complex interactions of gas and liquid phases with packed solids. Such complex interactions manifest in different flow regimes occurring in trickle-bed reactors. Knowledge of prevailing flow regime, pressure drop, and liquid ...

The process of spreading/recoiling of a liquid drop after collision with a flat solid surface was experimentally and computationally studied to identify the key issues in spreading of a liquid drop on a solid surface. The long-term objective of this study is to gain an insight in the phenomenon of wetting of solid particles in the trickle-bed reactors. Interaction of a falling liquid drop with a solid surface (impact, spreading, recoiling, and bouncing) was studied using a high-speed digital camera. Experimental data on dynamics of a drop impact on flat surfaces (glass and Teflon) are reported over a range of Reynolds numbers (550–2500) and Weber numbers (2–20). A computational fluid dynamics (CFD) model, based on the volume of fluid (VOF) approach, was used to simulate drop dynamics on the flat surfaces. The experimental results were compared with the CFD simulations. Simulations showed reasonably good agreement with the experimental data. A VOF-based computational model was able to capture key features of the interaction of a liquid drop with solid surfaces. The CFD simulations provide information about finer details of drop interaction with the solid surface. Information about gas–liquid and liquid–solid drag obtained from VOF simulations would be useful for CFD modeling of trickle-bed reactors. © 2004 American Institute of Chemical Engineers AIChE J, 51: 59–78, 2005

Packed-bed reactors are widely used in petrochemical, fine chemical, and pharmaceutical industries. Detailed knowledge of interstitial flow in the void space of such packed-bed reactors is essential for understanding the heat and mass transfer characteristics. In this paper, fluid flow through the array of spheres was studied using the unit-cell approach, in which different periodically repeating arrangements of particles such as simple cubical, 1-D rhombohedral, 3-D rhombohedral, and face-centered cubical geometries were considered. Single-phase flow through these geometries was simulated using computational fluid dynamics (CFD). The model was first validated by comparing predicted results with published experimental and computational results. The validated model was further used to study the effect of particle arrangement/orientation on velocity distribution and heat transfer characteristics. The simulated results were also used to understand and to quantify relative contributions of surface drag and form drag in overall resistance to the flow through packed-bed reactors. The model and the results presented here would be useful in elucidating the role of microscopic flow structure on mixing and other transport processes occurring in packed-bed reactors. © 2005 American Institute of Chemical Engineers AIChE J, 51: 365–378, 2005

Hydrodynamics and mixing in trickle bed reactors (TBR) are governed by bed porosity, capillary forces, wetting and non-uniform distribution at the inlet. In this work, experiments were carried out to study pressure drop, liquid hold-up and residence time distribution for pre-wetted and non-wetted bed. CFD models were developed, in which bed heterogeneity and capillary models were incorporated. Mixing and RTD were simulated using the CFD model. This study was further extended to study the flow mal-distribution in TBR. Based on these results, current capabilities and limitations of the CFD model were discussed. The model and the results discussed here would be useful to extend application of CFD models for simulating mixing in TBR.L'hydrodynamique et le mélange dans les réacteurs à lits ruisselants (TBR) sont gouvernés par la porosité du lit, les forces capillaires, le mouillage et la distribution non uniforme à l'entrée. Dans ce travail, on a mené des expériences afin d'étudier la perte de charge, la rétention de liquide et la distribution de temps de séjour pour des lits pré-mouillés et non mouillés. Des modèles de CFD ont été mis au point, dans lesquels l'hétérogénéité des lits et des modèles capillaires sont introduits. Le mélange et la RTD ont été simulés à l'aide du modèle de CFD. Cette étude a été généralisée pour étudier la mauvaise distribution de l'écoulement en TBR. D'après ces résultats, les capacités et limites actuelles du modèle de CFD sont examinées. Le modèle et les résultats analysés ici pourraient être utiles pour étendre l'application des modèles de CFD à la simulation du mélange en TBR.