Heavy gas dispersion over complex terrain
Authors
Hankin, Robin Keinion Stinson
Date
1997-07-25Awarding Institution
University of Cambridge
Author Affiliation
Department of Engineering
Qualification
Doctor of Philosophy (PhD)
Type
Thesis
Metadata
Show full item recordCitation
Hankin, R. K. S. (1997). Heavy gas dispersion over complex terrain (Doctoral thesis). https://doi.org/10.17863/CAM.14110
Description
Society benefits considerably from large scale industrial
activities. However, these activities can have undesirable side
effects which must be adequately controlled and monitored; one
motivation for this thesis is risk assessment. Industrial processes
often involve large amounts of hazardous, liquified gases. Accidental
release of such substances poses a threat to nearby populations and
the assessment of this risk is carried out in the UK by the Health and
Safety Executive.
Accidentally released gases are often denser than air and this thesis
addresses the physics of dense gas dispersion. As dense gas clouds
tend to adopt low-lying configurations, a shallow layer model is
indicated for simulating dense gas dispersion: a cloud is described in
terms of depth averaged quantities such as contaminant concentration.
If vertical accelerations are small, a hydrostatic pressure
distribution is appropriate and the shallow water equations may be
used.
This thesis presents a shallow layer model for dense gas dispersion.
The model is time dependent and two dimensional. This type of model
has been comparatively neglected up to the present but is useful
because it is capable of simulating the effect of complex terrain such
as valleys and mountain ranges.
The special physics of the leading edge is handled by augmenting the
shallow water equations: extra terms are added that account for the
interaction between the ambient fluid and the dense layer. In this
manner the front Froude number may be fixed.
The model has a number of free parameters, which have to be
empirically determined. The parameters used were either chosen on
theoretical grounds, or taken from earlier work.
A computational model has been developed to simulate the mathematical
model. This model differs from previous work in being time dependent
and fully two dimensional. The model uses the flux correction scheme
of Zalesak, generalized to account for complex terrain. The
computational model is checked against a number of theoretical
results.
The model is then used to simulate the large scale dense gas
dispersion field trials carried out at Thorney Island (instantaneous
and continuous), and its predictions are compared with the
experimental results. In general, there is no evidence to suggest
that changing the entrainment parameters would give better agreement.
Some assessment of the sensitivity of the model to the free parameters
is made.
Model predictions are shown to agree broadly with a number of integral
models whose parameters were based on these experiments; cloud
averaged concentrations as a function of downwind distance and time
were considered.
Model predictions are then compared with laboratory-scale experiments
in which dense gas was released over different slopes in a calm
ambient. Instantaneous and continuous releases were considered
A case study of a real hazard site, using the present model, is
presented.