Quiz 7 Flashcards
What does it mean when a reaction is exothermic?
Exothermic reactions are reactions or processes that release energy, usually in the form of heat or light. In an exothermic reaction, energy is released because the total energy of the products is less than the total energy of the reactants.
What does a large Damköhler number imply?
Implies that chemistry able to fully consume the fresh mixture and attain an equilibrium mixture (reaction rate faster than convective mass transport rate)
What kind of flames are the most common one for a fire safety engineer?
Turbulent diffusion (non-premixed) flames
Where in a premixed flame is the radical concentration the highest?
In the early part of the reaction zone where the flame front starts.
What is the Karlovitz number used for?
Compares the characteristic time of flame displacement with the characteristic time of the smallest structures of the turbulence to determine how much the flame region is effected by turbulence. A large value means that turbulence is intense enough to affect the structure of the reaction (which implies that a well-stirred reactor model is appropriate).
Does one typically model the complete reaction kinetics when doing CFD for fire safety engineering purposes? If not, how is it done?
Not typically. For engineering purposes it is easier to use approximate global reaction where a simplified chemistry is used (using fuel + oxidant which produces products and any inert part is carried forward unaffected).
What combustion model does FDS use as standard?
o The reactor model reduces to a simple “mixed is burnt” approximation called the Eddy Dissipation Concept. By default, FDS uses a mixing-controlled combustion model, meaning that the reaction rate is infinite and limited only by species concentrations. However, FDS can also employ finite-rate reactions using an Arrhenius model.
o The default combustion model in FDS assumes that the reaction is mixing-controlled, meaning that the reaction rate is infinite and limited only by species concentrations.
Which two extinction models does FDS have?
In the simpler model, ’EXTINCTION 1’, only the cell temperature and oxygen concentration are considered because detailed thermophysical gas species properties are not invoked, as they are in the ’EXTINCTION 2’ model. The second optional extinction model in FDS, referred to as ’EXTINCTION 2’, considers both the oxygen and the fuel content of a given grid cell at the start of a time step. If the potential heat release from the reactants cannot raise the temperature of the cell above the empirically determined critical flame temperature, TCFT, combustion is suppressed.
In FDS, what is the difference between a primitive species and a lumped species?
Primitive gas species can be tracked individually while lumped species is a mixture of one or more primitives that are tracked together.
In FDS, what is EPUMO2 used for?
If the enthalpy of formation of the fuel and heat of combustion are not specified, for simple chemistry cases only, the heat of combustion is assumed to be based on the amount of energy released per unit mass of oxygen consumed by the reaction.
