Development of a Novel Hybrid Field and Zone Fire Model
Dan Burton
2011
Abstract
This thesis describes the design and implementation of a novel hybrid
field/zone fire model, linking a fire field model to a zone model. This novel
concept was implemented using SMARTFIRE (a fire field model produced at the
University of Greenwich) and two different zone models (CFAST which is produced
by NIST and FSEG-ZONE which has been produced by the author during the course of
this work). The intention of the hybrid model is to reduce the amount of
computation incurred in using field models to simulate multi-compartment
geometries, and it will be implemented to allow users to employ the zone
component without having to make further technical considerations, in line with
the existing paradigm of the SMARTFIRE suite.
In using the hybrid model only the most important or complex parts of
the geometry are fully modelled using the field model. Other suitable
and less important parts of the geometry are modelled using the zone
model. From the field model’s perspective the zone model is represented
as an accurate pressure boundary condition. From the zone model’s
perspective the energy and mass fluxes crossing the interface between
the models are seen as point sources.
The models are fully coupled and iterate towards a solution ensuring
both global conservation along with conservation between the regions of
different computational method. By using this approach a significant
proportion of the computational cells can be replaced by a relatively
simple zone model, saving computational time. The hybrid model can be
used in a wide range of situations but will be especially applicable to
large geometries, such as hotels, prisons, factories or ships, where the
domain size typically proves to be extremely computationally expensive
for treatment using a field model. The capability to model such
geometries without the associated mesh overheads could eventually permit
simulations to be run in ‘faster-real-time’, allowing the spread of fire
and effluents to be modelled, along with a close coupling with
evacuation software, to provide a tool not just for research objectives,
but to allow real time incident management in emergency situations.
Initial ‘proof of concept’ work began with the development of one way
coupling regimes to demonstrate that a valid link between models could
allow communication and conservation of the respective variables. This
was extended to a two-way coupling regime using the CFAST zone model and
results of this implementation are presented. Fundamental differences
between the SMARTFIRE and CFAST models resulted in the development of
the FSEG-ZONE model to address several issues; this implementation and
numerous results are discussed at length. Finally, several additions
were made to the FSEG-ZONE model that are necessary for an accurate
consideration of fire simulations.
The test cases presented in this thesis show that a good agreement
with full-field results can be obtained through use of the hybrid model,
while the reduction in computational time realised is approximately
equivalent to the percentage of domain cells that are replaced by the
zone calculations of the hybrid model.
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