Geometry and Explosions

Geometry has long been identified as of key importance to understanding the consequence of explosions. Flame speed and explosion pressure strongly depends on the gas cloud and the geometrical conditions.

How do we know this?

A number of large-scale gas-explosion experimental programmes took place in the 1980’s. One programme (GEP 80-86) investigated pressure development due to flame acceleration by obstacle generated turbulence which was identified as primarily responsible for the intensification of explosions occurring in the complex geometries, typically found on offshore platforms. 

The experimental data from the programme provided new insight about flame acceleration in complex geometries. It also provided data for the validation of a new CFD code, FLACS (Flame Acceleration Simulator) the forefather of Gexcon’s FLACS-CFD.

Size matters

The programme identified the importance of capturing small scale congestion within a CFD model. Gexcon’s best-practice guidance is that all geometrical details down to one inch play an important role in the development of an explosion and should be captured within a CFD model.

Arrangements matter too

How the geometry is arranged is also of critical importance. Lack of symmetry and the presence of non-uniform congestion can result in significant directional effects. If these aspects are not considered when modelling explosions, then resulting overpressure may be significantly under estimated or over predicted.

The two videos below show the impact of geometry on the development of a vapour cloud explosion. In both videos an identical mixture of propane-air was ignited in the centre of the cube and the blockage ratio was the same. The different in the resulting consequence can clearly be observed. Overpressure for the high-congestion case is over 100 times higher than the low-congestion case.

A low-congestion scenario (the blockage is achieved by a small number of ‘fat’ pipes),

A high-congestion scenario (the blockage is achieved by a large number of ‘thin’ pipes). 

FLACS-CFD (https://www.gexcon.com/products-services/flacs-software/) has been developed to account for the importance of these critical factors. It allows the user to perform:

1. Easy handling of multiple geometry import formats
2. Powerful geometry creation and clean up with CASD
3. Powerful visualisation within CASD and Flowvis (including lighting and textures)  

Geometry modelling in FLACS ‘86

Geometry modelling in FLACS-CFD 20