6.1 A typical scenario
6.2 The physical and mechanical processes considered
6.3 How PHOENICS accounts for the processes
6.4 Some typical findings
Let it be supposed that, possibly as a result of an explosion,
followed by rupture of gas and oil pipes, a fire starts, and spreads
through an oil platform.
Let it be further supposed that fire-fighting devices are activated.
How well can the spread of the flame, and the success of the efforts
to contain and extinguish it, be predicted by CFD techniques in
general and by EXPLOITS in particular?
Will it be possible, with the aid of EXPLOITS, to improve the design
of the fire-fighting measures?
These are the questions which will now be addressed.
Whereas the gas-dispersion process is slow, and may be treated as a
steady-state one, while on the other hand the explosion and blast
phenomena are extremely rapid, the fire-spread process proceeds at
an intermediate rate.
Explosion and blast occupy milli-seconds, whereas fires may last for
hours.
Crucial to the development of the fire are the rates of supply of
fresh gas and oil to the conflagration. These are to be regarded as
INPUTS to the CFD calculation.
However, the rate at which the liquid fuel vaporises, so entering
the gaseous phase in which alone significant exothermic reaction can
occur, is an OUTPUT of that calculation; for it depends on the flow
of heat and fluids.
It is for this reason that RADIATIVE heat transfer assumes an
importance in fire-spread simulation which it possesses in none of
the other three hazards which EXPLOITS deals with.
Heat radiated from the flame to the liquid-fuel surface, and to the
metal surfaces oveer which it flows, substantially controls the rate
at which the oil vaporises,
It is essential, therefore, to include radiative transfer in the
simulation procedure,
This entails that one must calculate not only the distributions
of gas composition and temperature throughout and surrounding the
platform, but also the SOOT concentration at each point; for that
is what mainly affects the radiation.
To calculate the soot concentration, it is necessary to have
quantitative data on the kinetics of the relevant chemical
reactions; and these must be used in a manner which takes account of
the fluctuations of gas condition resulting from turbulence.
The chemical-kinetic data are not yet very securely established.
Therefore, influenced also by the fact that fuel-supply conditions
are themselves never much better than guesses, the modeller of fires
may well decide NOT to use precious human and computer time on soot
kinetics,
If he does take a short cut, it will usually be by guessing the
emissivity of the flame; but certainly not by neglecting radiation.
Fires may be extinguished by the injection of foam, or of sprays of
water, the effects of which therefore require to be simulated.
Both foam and sprays necssitate the activation of two-phase-
simulation capabilities, in the first case without, and the second
case with, relative motion between the liquid and the gaseous
materials.
In both cases, the latent heat of vaporisation of the liquid has
to be taken into account.
PHOENICS possesses (and therefore EXPLOITS does also) all the
necessary equipment for simulating the processes and phenomena just
described.
Of particular importance is the new IMMERSOL technique, which makes
it possible, without inordinate expense, to handle the radiative,
convective and in-solid-conductive processes simultaneously.
The multi-fluid model of PHOENICS, in particular, makes it possible
to account for the influence of turbulent fluctuations on smoke
production, as the following gas-turbine related picture shows.
The multi-fluid model predicts a different soot distribution from
that which a conventional (ie single-fluid) nodel would predict; and
there is every reason to presume that the former is the more
correct.
Soot-concentration distributions compared;
multi-fluid above; single-fluid below.
The differences are not great in this case; but sometimes they can be significant.
PHOENICS is well-supplied, as has aready been mentioned, with means
of simulating two-phase phenomena, having been indeed the first
general-purpose code to do so, in the early 1980's.
To illustrate this, it may suffice to show one more picture from the
spray-drier sequence introduced above.
This now follows.
6.1 A typical scenario
6.2 The physical and mechanical processes considered
(a) Fuel-supply rates
(b) Radiation
(c) Chemical kinetics
(d) Water-spray and foam effects
6.3 How PHOENICS accounts for the processes
Two-phase effects