FDS grid resolution D* (Large Sprinklered Warehouse)

[FDSuser268]

Hi everyone,

For a large sprinklered warehouse, calculating the D* ratio using an unconstrained HRR (e.g., 100 MW) yields different results compared with using a sprinkler limited HRR (e.g., 20 MW). This suggests that one could reasonably perform two separate D* assessments: one for a sprinklered scenario and another for an unsprinklered (or pre sprinkler activation) scenario.

Given the building is a large warehouse (large footprint), the D* ratio based on the sprinkler capped HRR may align with a coarse mesh resolution; however, further refinement typically increases computational time substantially, which may be impractical for large footprint warehouses.

When the intent is to assess occupant tenability in an industrial fire scenario, a mesh sensitivity study using the 4–16 D* range (benchmark) can support the argument that the mesh is coarse (e.g., D* = 4), but refining it to something like D* = 8 may not be justified due to the scale of the warehouse and the resulting computational cost.

Regarding my first question: when calculating D*, is Q usually taken as the HRR at sprinkler activation (i.e., the design or sprinkler controlled HRR), or the unconstrained maximum HRR.

Is appropriate to argue that the mesh size is adequate even if D*/Δx = 2 for the sprinkler capped HRR, provided the mesh sensitivity study demonstrates that key tenability parameters remain stable and that the chosen resolution is resolved.

[drjfloyd]

The D*/dx metric is a means of comparing grid resolutions between different simulations. It first appears in FDS Guides after the validation effort with the US Nuclear Regulaotry Commission that resulted in NUREG 1824. The 4-16 range was a footnote that stated this was the range used FDS validation cases at that time. It was never a recommended range, and in current guides this footnote has been removed (in the current, far more extensive validaiton work, D*/dx ranges from 1 to 118). The way to demonstrate that a grid resolution is adequate is some form of grid sensitivity study. If you can show that for a specific scenario that some grid resolution is adequate, then you have a good argument to make to your AHJ that the resolution is OK to use.