Characteristics of Elongated Vapour Cloud Explosions

The purpose of this paper is to characterize elongated vapour cloud explosions (VCEs) and to identify the resultant difference in blast wave shape relative to those predicted by well-known VCE blast prediction methodologies (e.g., BST, TNO, etc.), the blast curves for which are normally based on an assumption that the flammable gas cloud is hemispherical and located at grade level. BakerRisk’s Blast Wave Target Interaction (BWTI™) computational fluid dynamics (CFD) code was used to investigate the VCE blast loads resulting from elongated flammable gas clouds. The BWTI™ predictions were first validated against the recent Buncefield JIP test data. A blast wave diagram was developed to illustrate typical blast waves generated during the initial flame acceleration process and the subsequent flame propagation. It was found that the aspect ratio of the elongated cloud/congestion volume (i.e., ratio of length to width to height) has a significant impact on the resultant blast wave shape. The blast wave shape in elongated VCEs is was found to be controlled by the negative phase, which is generated by the primary blast wave during the initial acceleration phase in which an effective spherical (or cylindrical) flame front still exists. This primary negative phase plays a key role in the follow-on flame acceleration process, which either attains a steady flame speed or experiences a deflagration-to-detonation transition (DDT).