4 Transport Phenomena Flashcards
What happen during the diffusion of momentum/viscous motion
The transport occurs in the direction normal to the plates in which there is no motion.
What’s the Stefan flow?
a transport phenomenon concerning the movement of a chemical species by a flowing fluid (typically in the gas phase) that is induced to flow by the production or removal of the species at an interface
Which molecule collision scenarios exist?
- head-on collison
- off-center collision
- limiting off-center collision
What’s the mean free path
The distance a molecule traveled before th next collision occurs
l = travel distance / collision number
Which transport phenomena coefficients are pressure dependent?
- Diffusion coefficient D
- kinematic viscosity v
- thermal diffusivities Alpha
Which transport phenomena coefficients are temperature dependent?
- viscosity coefficient (mü)
- thermal conductivity coefficient lambda
- product of density and diffusion coefficient
What’s thermal diffusivity?
It measures the rate of transfer of heat of a material from the hot end to the cold end.
What’s the thermal conductivity of a material?
A measure of its ability to conduct heat
What does the Schmidt number indicate?
Influence of viscosity to mass diffusion
What does the Prandtl number indicate
Influence of viscosity o thermal diffusion
What does the Lewis number indicate?
Influence of thermal diffusion to mass diffusion
Which second order diffusion effect exist?
- Dufour effect
- Soret effect
What’s the Dufour effect?
When heat diffution occurs in the presence of a concentration gradient
What’s the Soret effect?
When mass diffusion occurs in the presence of a temperature gradient
Which importance do second order diffusion have?
Usually small and negligible
Soret effect is not negligible for H2 and soot
When is thermal diffusion important?
for light species (H, H2, He) at low temperatures
Under which circumstances can species diffusion also occur?
- presence of body forces
- pressure
These effects are also usually small unless these forces are extremely large
At which temperature are diffusion coefficent measurements mostly done?
At room temperature
What’s a constitutive relation?
A relation between two physial quantities that is specific to a material or substance
and approximates the response of that material to external stimuli.
What does our simplified system consist of?
- unsteady terms describing the time variation of the system
- diffusion therms
describing strong temperature & concentration gradients - convection terms
describing fluid mechanical aspects of the system - chemical reaction terms
What do we neglect in simplified systems?
- body forces
- radiation on heat transfer
- second order diffusion
- viscous heating
What are coupling functions?
They make it easier to express and solve equations describing the relation between
energy and mass conservation
What is assumed when using coupling functions?
- Le = 1
- constant D
What is assumed when using local coupling functions?
- Le is not 1
- D is not constant
