Abstract:
Turbulent buoyant jets are a major feature in fire hazards. The solution of the
Reynolds Averaged Navier-Stokes (RANS) equations through computational fluid
dynamic (CFD) techniques allow such flows to be simulated. The use of Reynolds
averaging requires an empirical model to close the set of equations, this is known as
the turbulence model. This thesis undertakes to investigate linear and nonlinear
approaches to turbulence modelling and to apply the knowledge gained to the
simulation of compartment fires. The principle contribution of this work is the reanalysis
of the standard k- ε turbulence model and the implementation and
application of more sophisticated models as applied to thermal plumes.
Validation in this work, of the standard k- ε model against the most recent
experimental data, counters the established view that the model is inadequate for the
simulation of buoyant flows. Examination of previous experimental data suggests
that the measurements were not taken in the self-similar region resulting in
misleading comparisons with published numerical solutions. This is a significant
conclusion that impacts of the general approach taken to modelling turbulence in
this field.
A number of methods for modelling the Reynolds stresses and the turbulent scalar
fluxes have been considered and, in some cases for the first time, are applied to nonisothermal
flows. The relative influence of each model has been assessed enabling
its performance to be gauged. The results from this have made a valuable
contribution to the knowledge in the field and have enabled the acquired experience
to be applied to the simulation of compartment fires.
The overall conclusion drawn from this thesis is that for the simulation of
compartment fires, the most appropriate approach with current computational
resources, is still the buoyancy corrected standard k- ε model. However, the
turbulence scalar flux should be modelled by the generalised gradient diffusion
hypothesis (GGDH) rather than the eddy-diffusivity assumption.
