### 10.1.1. Setup

The choice of a steady-state or transient simulation depends on the type of simulation you want to analyze. For example, an analysis using a steady-state simulation is often satisfactory for monitoring global quantities. A transient simulation can be used to observe transient effects, such as recirculation zones.

Most bubble columns use two fluids: one continuous fluid and one dispersed fluid. The model is typically used in the continuous fluid, and the dispersed phase zero equation is used for the dispersed phase.

Non-drag forces become less significant with increasing size of the bubble column. For smaller columns, non-drag forces may be significant.

The Grace drag model is recommended, especially for modeling air/water.

A degassing boundary condition is generally employed at the top of the bubble column. The degassing boundary behaves as an outlet boundary to the dispersed phase, but as a wall to the continuous phase.

A reasonable estimate of the time scale is given by a factor of the length of the domain divided by the velocity scale (for example, 0.5 * L/U).

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