Quiz 11 Flashcards
Is a pyrolysis model a part of the CFD model? Motivate your answer.
Not explicitly, the pyrolysis model is only needs the incident energy on a surface and converts that into volatiles being released from the surface, the CFD then takes care of the heat transfer process and combustion process.
What are the four different flame spread models that Phil mentions?
o Highly detailed, local pyrolysis model ▪ Physical / chemical kinetic processes ▪ Multi-component species ▪ Heat and mass transfer o Arrhenius based, local model ▪ Surface devolatalisation described by global model ▪ Solid material represented by virgin and char ▪ Heat and mass transfer o Empirically correlated ▪ Cone calorimeter or equivalent ▪ Heat transfer o Simple heat of gasification
Which pyrolysis models does FDS have?
o Prescribed
o Arrhenius
o Kind of simple heat of gasification (liquid)
Shortly describe how a simple heat of gasification model would work.
o Time to ignition and subsequent mass loss rate correlated by total incident energy
Describe the three main steps in a Cone Calorimeter Flame Spread Model.
o Accumulate incident heat flux until ignition criteria met
o Select heat release rate curve based on current incident flux
o Volatile release prescribed by empirical curve
What makes flame spread hard to simulate?
o Wide range of real-world materials
o Non-homogenous combinations
o Often unknown properties
How does FDS treat liquid fuels (describe the general approach)?
o The rate at which liquid fuel evaporates when burning is a function of the liquid temperature and the concentration of fuel vapor above the pool surface. According to the Clausius-Clapeyron relation, the volume fraction of the fuel vapor above the surface is a function of the liquid boiling temperature.
o The evaporation rate of the fuel is governed by Stefan diffusion.
o For simplicity, the liquid fuel itself is treated like a thermally-thick solid for the purpose of computing the heat conduction. There is no computation of the convection of the liquid within the pool.
What command must you use to make burnt fuel disappear in FDS?
BURN_AWAY=.TRUE.
