Modelling the effects of obstacles on dense gas dispersion in shallow layer models

摘要

A newly developed 2-d shallow layer model for dense gas dispersion in obstructed terrain is presented. The model solves the conservation equations of longitudinal and lateral momentum, dense gas mass and total mixture mass, averaged over the cloud height. Turbulence and diffusion are modelled by means of the entrainment velocity concept. The entrainment velocities are dependent on the ambient flow field and on the local cloud velocity. For model validation the Thorney Island 21 release was selected. In this field test 2000 m~3 of a mixture of Freon and Nitrogen with a relative density of 2 were instantaneously released. A semicircular fence 5 m high obstructed the flow 50 m from the release position. The model results are compared against the experimental in terms of the concentration time series, obtained in several positions upwind and downwind the obstacle. It is found that the predicted concentration time histories downwind the fence are in good agreement with the experiment both in the arrival time and in the order of magnitude for the sensors located on the fence axis of symmetry. Far from this axis the model underpredicts the cloud spreading.