Coupled hybrid modelling for fire engineering
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Date
28/11/2019Author
Ralph, Benjamin Michael
Metadata
Abstract
Due to time and cost constraints, fire engineers typically curtail the domain of analysis
when carrying out quantitative assessments using computational fluid dynamics
(CFD)-based fire modelling. This could embody unquantified hazard and is especially
critical when designing complex building with a shared ventilation system.
Secondarily,
prescriptive model solutions have been developed using engineering judgement and
empirical evidence. There is a risk that under-investigated mechanisms may lead to
unacceptable prescriptive solution risk levels.
The overarching thesis aim is to enable a more robust quantification of fire hazard
in complex buildings with a shared ventilation system. The objectives of the study are
to develop and evaluate a novel coupled hybrid model implementation and highlight
potential shortcomings of existing prescriptive design solutions for shared ventilation
systems.
This study develops a novel 3D-1D coupled hybrid modelling implementation within
Fire Dynamics Simulator (FDS). The new implementation addresses the pre-existing
limitation of time-dependent transport or storage within the 1D sub-domain and introduces
a novel fan model. A new experimental rig was created comprising two 1m3
boxes interconnected with shared mechanical ventilation. A propane burner was used
as a fire source, with propane flow rates ranging between 0:2 g=s to 0:45 g=s, in one of
the boxes. Variable speed controllers and dampers were used to alter the ventilation
with target free flow fan velocities of 1m=s to 3m=s.
The novel model implementation satisfactorily passed verification and presented
generally good agreement with the experimental results. Prediction of maximum temperature
in the fire and non-fire enclosures are typically within 40% and 5% respectively.
Prediction of ventilation duct velocity is typically within 5 - 25%. Model correction factors
of 1.0, 0.7, and 1.4 are proposed for enclosure temperature, in-duct temperatures,
and duct velocities respectively.
Experimental data demonstrate that empirical methods may not be suitable for
complex arrangements with shared ventilation because there is a stronger dependency
of fire hazard upon the ratio of heat losses to ventilation enthalpy advection when
compared to traditional arrangements. The data illustrate that remote area fire hazard
is very sensitive to the balance of the energy transfer rate (i.e. power) of the fire and the
ventilation system and that this relationship is non-linear; a correlation which would not
be well-captured using the typical modelling paradigm nor prescriptive design solutions.
The study concludes that the new coupled hybrid modelling implementation may
be used to analyse a total system with quantified uncertainties. Further development is
recommended for the new model implementation (e.g. conductive heat transfer within
the 1D sub-domain). Further experimentation is recommended to further inform prescriptive
design solutions for complex buildings with a shared ventilation system.