Diffusion Flashcards
What’s the Soret effect?
Mass/molecular diffusion caused by temperature gradient
What’s the Dufour effect?
Heat diffusion caused by concentration gradient
What does the Damköhler number show?
Flow/diffusion time (residence time) over the characteristic reaction time.
What does it mean when Le is not = 1?
Diffusion of heat and the various species occur with different diffusivities
Which are the characteristics of the element mass fraction Z(k)?
It is a conserved scalar, provided that the single diffusion coefficient D is used.
What is the advantage of using the element mass fraction Z(k) compared to the coupling function ß(i,j)?
It is independent of the specific chemical reactions that take place in the mixture
What is the advantage of using the coupling function ß(i,j) compared to the element mass fraction Z(k)?
The number of elements is usually much smaller than the number of species (L < N)
This implies that a solution of Z(k) is frequently not sufficient to solve for all the species concentration.
Derivatives of the coupling function formulation
- element conservation formulation
- mixture fraction formulation
Restrictions of the mixtrue fraction formulation
(Wrongful) assumptions of
- single diffusion coefficients
- unity Lewis number
What’s the collision Damköhler number for diffusive systems?
Characteristic Diffusion Time : Characteristic Collision time
What’s the collision Damköhler number for nondiffusive systems?
Characteristic Flow Time : Characteristic Collision time
What does the Arrhenius coefficient show?
The probability of reaction from a collision
When does Da -> 0 apply?
for chemically frozen region
When does Da -> infinity apply?
for equilibrium region
What’s the Damköhler number?
Characteristic flow time or diffusion time
/
characteristic reaction time
