2 COMBUSTION PROCESSES Flashcards
how much of the worldโs energy use does combustion account for?
~90%
is combustion an exothermic or endothermic reaction and why
EXOTHERMIC
as chemical energy is released
how is combustion a physical process?
it is the transportation of energy, mass and momentum
What does xi stand for
number of moles in a component
what is mole fraction
relative proportion of the molecules/atoms belonging to a component in a mixture
what is the difference between n, ni and N
n = total number of moles in system
ni = total number of moles in constituent
N = total number of molecules in system
what is AFR?
the ratio of mass of air in a system / mass of fuel in system
(usually based on mass)
What is equiv ratio?
Compares AFRstoich with AFRact.
indicates whether the system is fuel rich or lean
(aka Mixture Strength)
describe the flame and itโs characteristics when ๐ = 1
Stoich Combustion
- complete combustion
Neutral flame - no soot
describe the flame and itโs characteristics when ๐ < 1
LEAN mixture
ie lean combustion
Lean flame
- flame temp very
- excess oxidant makes for complete combustion
too much oxidant will chill the flame
describe the flame and itโs characteristics when ๐ > 1
RICH mixture
ie rich combustion
Rich flame
- soot usually forms
- incomplete combustion
- flame tip rises due to convection of soot
- flowing soot induces orange coloured flame
What are the 3 Ts of combustion?
TIME
TEMP
TURBULENCE
why is time significant in combustion?
need sufficient time for complete reaction.
Find this by calculating mean residence time (cheatsheet)
why is temp significant in combustion?
need correct temp to heat the fuel through decomposition to reach ignition point
if its too cold, the fuel will surround the air and never ignite
why is turbulence significant in combustion?
need enough turbulence to mix O2 and fuel completely
it induces the reaction by inducing the diffusion of reactants among each other
aka stirring
