Final Flashcards
What’s the Karlovitz number?
- non-dimensional stretch rate
- measure of the FLAME time in terms of the AERODYNAMIC time
Karlovitz number for laminar flames
Ka = l(T) / s(U)
Karlovitz number for premixed flames
Ka = turbulent strain rate / chemical reaction rate
How does the Karlovitz number change with the stretch rate?
Ka increases with increasing stretch rate
Whats the Lewis number?
Thermal diffusivity / mass diffusivity
Equation for the Lewis number
Le = Lambda / (DichteDc)
Properties of rich propane/air flame
Le <1
S < 0
open tip
dark tip
Properties of lean propane/air flame
Le > 1
S > 0
closed tip
bright tip
Properties of lean methane/air flame
Le <1
S < 0
open tip
dark tip
Properties of rich methane/air flame
Le > 1
S > 0
closed tip
bright tip
What’s the Damköhler number?
- non-dimensional
- relates CHEMICAL REACTION TIMESCALE to TRANSPORT PHENOMENA occuring in a system
Equation for Damköhler number
Da = char. Diffusion time / char. collision time
What’s the Mach number?
Measures the SPEED in relation to SOUND VELOCITY
Equation for Mach
Ma = v/c
What’s the Peclet number?
Measures the relative intensities of CONVECTIVE to DIFFUSIVE TRANSPORT
What’s the Reynolds number?
Measures the INERTIAL FORCE in relation to the VISCUOUS FORCE
Equation for Peclet number?
Pe = Re*Pr
Equation for Reynolds number
Re = (DichteuL) / Eta
What’s the Pradtl number
Measures the relative influence of VISCOSITY to THERMAL DIFFUSIVITY
Equation for Prandlt
Pr = v/Alpha = (Eta*c(P)) / Lambda
What’s FLAMMABILITY?
A mixture is said to be flammable if the resulting flame can propagate all the way to the top of the tube
What’s the FLAMMABILITY LIMIT?
Concentration limit beyond which flame propagation is not possible
Relations of lean flammability limits & unburned mixture temeprature
Increase of initial temperature = Decrease of lean FL
Flammability limit of hydrogen
Lean = 4% Rich = 75%
Influence of increase of mixture temperature on flammability limit
widens flammability limit (range)
Influence of increase of pressure on flammability limit of hydrogen
narrows lean & rich flammability limit
Influence of increase of pressure on flammability limit of hydrocarbons
narrows lean FL
widens rich FL
How does the flame temeprature change when flammability limit is approached?
It decreases
Advantages of Flammability limits
Useful concept to know what could cause a failure of flame propagation
Disadvatnages of flammability limits
Empirical observation without an established fundamental understanding
Mechanisms contributing to flammability limits
Chemical kinetics
- flammability limit is characterized by THERMAL EXTINCTION +
- CHAIN TERMINATION overwhelming chain branching
What’s extinction?
When leakage and the reduction in flame temperature becomes relatively severe, extinction occurs.
How to achieve extinction?
1 Remove a certain amount of chain-carrying or chain branching RADICALS
- introduce chemical INHIBITANTS
- Remove certain amount of HEAT
- reduce reaction rate (by cooling, decreasing reactant concentration, decreasing system pressure)
Does the disappearance of the flame imply extinction?
No