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Deposition Modeling from Multi-phase Dispersed Flow – a Boundary Layer Wall Function Approach
| Content Provider | Semantic Scholar |
|---|---|
| Author | Johnsen, Sverre G. Johansen, Stein Tore |
| Copyright Year | 2009 |
| Abstract | Modeling of mass transfer of particles to solid surfaces is a considerable challenge in most industrial processes, including heat exchanger applications. Using computational fluid dynamics (CFD) directly on these applications is extremely time consuming as we need an extremely fine grid to capture the details of the complex physical processes that dominate in the near-wall region. We propose a detailed boundary layer model, which can couple the physics in the wall region with the external flow. The developed mass-transfer wall function can be applied as a boundary condition for coarse grid CFD models. The boundary layer model incorporates gravity, turbulence and hydrodynamic lift and drag. In addition we include the effects of Brownian diffusion, thermophoresis, extended DLVO forces and the inter-particle collisions. Heat transfer, by the liquid and particulate phases, is coupled to the momentum equations. The one-dimensional boundary layer model is solved numerically in a fine grid that is capable of resolving the near wall XDLVO force length scales. When dispersed phase particles touch the wall they are considered deposited and removed from the flow. The effect of adhesion probability and particle re-entrainment is not considered. Thermodynamic and chemical effects, such as phase change or precipitation, are not included in this work. INTRODUCTION Fouling of solid surfaces exposed to fluids carrying particles, is a common and much investigated problem. Fouling is defined as accumulation of unwanted material on solid surfaces. The topic is of general interest in most process industries, including oil/gas, minerals, metals, cement, food, marine and fishing, and also areas like medicine and environment. Consequently, a vast amount of work has been done in this field. Sippola and Nazaroff (2002) and Guha (2008) cite an extensive list of published studies. They give comprehensive reviews of transport and deposition mechanisms of particles in gas and liquid flows, but neither of them includes near-wall XDLVO forces or granular stress effects. Johansen (1991a) describes the deposition of particles from a gas flow. Combined thermal-turbulent deposition was first time predicted by Johansen (1991b) (for gases) and for liquids by Adomeit and Renz (1996). In this paper, we develop a mathematical framework for solving the Navier-Stokes equations inside the boundary layer close to a solid surface, for a liquid phase carrying a dispersed particle phase. By numerical solution of the proposed transport equations for particle volume fraction, temperature and axial liquid velocity, we obtain the deposition mass transfer coefficient. The resulting mass transfer coefficient can be employed as a wall function for coarse grid CFD simulations. Our model includes the hydrodynamic forces, drag and lift, thermophoresis, turbulence, granular stress effects and near-wall XDLVO forces. First, we give a detailed overview of the model. Next, we employ the developed model to study how the deposition flux is affected by the different physical mechanisms involved, for different particle sizes. GOVERNING EQUATIONS We consider an Eulerian-Eulerian two-fluid model, consisting of an incompressible continuous liquid (l) phase and a mono-disperse incompressible inert particulate (p) phase (CaCO3), flowing close to a hot steel wall, as illustrated in Fig. 1. For simplicity, we will most of the time omit the p index when addressing particle properties. It is furthermore assumed steady turbulent flow, where the model Fig. 1 Schematic of the flow of a cold suspension close to a hot wall. The particles are affected by hydrostatic forces, inter particle interactions, thermal and particlewall forces. Proceedings of International Conference on Heat Exchanger Fouling and Cleaning VIII 2009 (Peer-reviewed) June 14-19, 2009, Schladming, Austria Editors: H. Müller-Steinhagen, M.R. Malayeri and A.P. Watkinson |
| File Format | PDF HTM / HTML |
| Alternate Webpage(s) | http://heatexchanger-fouling.com/papers/papers2009/34_Johnsen_F.pdf |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
