When you set up explosion simulations, you must observe the FLACS grid guidelines to obtain a grid that gives accurate results but also keeps simulation time low.
The domain and grid for an explosion simulation in a simple 3D geometry.

Prerequisites

For a general definition of the concepts domain and grid refer to the article What is the domain and the grid?.

Guidelines for the domain definition

As a general rule in computational fluid dynamics, the boundaries of the simulation domain should be as far as reasonably possible from the critical geometry and expected flow features. For an explosion simulation in FLACS, you should as a minimum define a simulation domain that is large enough to comprise the explosion and blast wave propagation in the area of interest, including all relevant targets as shown in the picture above.

For unconfined and vented explosions, it will most often not be sufficient to just cover the expanded explosion products and targets in one direction within your explosion domain to avoid unwanted boundary condition effects (e.g. pressure reflections, as the pressure waves normally travel faster than the flame). For such scenarios, it is also important to have a sufficient distance to the z-boundary.
It is recommended to run at least one sensitivity study.

Guidelines for the grid

The guidelines for the grid settings in FLACS are based on experimental validation. You should use cubical cells (or aspect ratio deviation not more than 10%) in the core area where the explosion takes place and combustion gases expand. In addition, you should not stretch the grid in the areas where you are interested in monitoring the results. For example, in the picture above, the grid is maintained uniform to include all the targets of interest where results are monitored.

The guidelines for grid resolution around the gas cloud are as follows:

  • For an unconfined gas cloud you should have a minimum of 13 grid cells across the cloud in the direction(s) where both sides are unconfined.
  • If the cloud is confined on one side, you should use at least 10 grid cells in the direction orthogonal to the confinement.
  • For clouds confined on both sides (such as rooms filled with gas from wall to wall or floor to ceiling) must always be resolved by a minimum of 5-6 grid cells in the smallest direction.
  • For modelling internal explosions i.e. explosions within a vessel or connected vessel systems, you need to resolve the gas cloud within the vessel with at least 15 cells.

The number of grid cells described above is based on

  • the resolution of the region of high-congestion (for congestion-dominated scenarios) or the gas cloud dimension (whichever is smaller), and
  • the minimum dimension of congestion or cloud.

For highly reactive gases (e.g. hydrogen) it is recommended to use even finer grid sizes as the fast flames may generate pressure gradients in the flame front which also should be resolved. However, you should not use grid cells of 1-2 cm or less, because for very small cell sizes the sub-grid model for premixed combustion (explosions) is not applicable: in such cases the burning velocities tend to be severely over-predicted.

For more advanced settings it is recommended to consult the Grid Guidelines section in the FLACS User's Manual.