Keywords: CFD, two phase flow, droplet-wall interaction.

Two separate models for the simulation of two-phase flow were coupled to allow the simulation of applications with a free liquid surface and droplets interacting with this surface. The VOF model and the Lagrange droplet model, both available in FLUENT [1], are coupled by the modification of source terms. This new model is then applied to a simple problem and compared with the flow observation of a experimental set-up.

Computational fluid dynamics of two-phase flows are becoming increasingly interesting especially in process technology. Special flow examples from the steel industry concerning the behavior of droplets are:

• Cooling of hot surfaces using water spray
• Descaling hot strip in a mill
• Wipe-off of acid from a steel strip in a pickling line with a flat jet nozzle.

In all these examples the droplets from the nozzles are interacting with a wall or a liquid surface. Part of the droplets will be reflected and the rest will build a film on the wall or is absorbed by the liquid phase. Two types of models are available for the simulation of these flow problems.

In dispersed phases, particles or droplets are normally simulated by the use of a Lagrange-type model. The fluid phase is treated as a continuum by solving the time- averaged Navier-Stokes equations, while the dispersed phase is solved by tracking a large number of particles, bubbles or droplets through the calculated flow field. The dispersed phase can exchange momentum, mass, and energy with the fluid phase. Flow problems with a free surface can be simulated with special models where the surface between the two fluids is tracked. A single set of momentum equations is shared by the fluids, and the volume fraction of each of the fluids in each computational cell is computed throughout the domain.

To simulate the flow applications mentioned above, coupling of the two flow models is necessary.

The present work shows an attempt to couple the two two-phase models in order to simulate the wipe-off of acid from a steel strip at the exit of a pickling line.

Coupling of the Lagrange droplet model with the VOF free-surface model yields reasonable results as compared to an experimental test case. Since in the present experimental set-up the boundary conditions of the droplets leaving the nozzle orifice are only estimated and velocity profiles are not measured, no validation can be made.

1

"FLUENT 6 Users Guide", Fluent Inc., Lebanon, USA, 2001.

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