Modelling and validation of atmospheric expansion and near-field dispersion for pressurised vapour or two-phase releases

The consequence modelling package Phast includes discharge models for vessel orifice releases. These models first calculate the depressurisation between the stagnation and orifice conditions and subsequently impose the 'ATmospheric EXpansion model' ATEX for modelling the expansion from orifice conditions to the final conditions at atmospheric pressure. The latter post-expansion conditions are used as the source term for the Phast 'Unified Dispersion Model' UDM. The current paper summarises the results of a literature review on atmospheric expansion modelling and provides recommendations on selection of ATEX model equations to ensure a most accurate prediction for the near-field UDM jet dispersion against available experimental data. First, the correctness of the numerical solution to the ATEX equations has been verified analytically and the importance of non-ideal gas effects is investigated. Secondly, both ATEX expansion options have been applied to known available experimental data for orifice releases. For these experimental data it was confirmed that the ATEX conservation-of-momentum option without a velocity cap provides overall more accurate concentration predictions than the isentropic assumption. However the existing default 'minimum thermodynamic change' option was found to mostly impose conservation of entropy (velocity cap not applicable) for two-phase releases and conservation of momentum (velocity cap applicable) for sonic gas jets. Rainout calculations for flashing two-phase releases are currently always based on the isentropic assumption, which is inconsistent with the recommended conservation of momentum; a further investigation is recommended. (C) 2017 Elsevier Ltd. All rights reserved.