|
A free membership is required to access uploaded content. Login or Register.
Transport Process
|
Uploaded: 6 years ago
Category: Chemical Engineering
Type: Lecture Notes
Rating:
N/A
|
Filename: 02.pptx
(522.48 kB)
Page Count: 20
Credit Cost: 2
Views: 117
Last Download: N/A
|
Transcript
Problem
From the given illustration below, calculate the steady-state shear stress ?yx and the shear rate dvx/dy when the distance between the plates is 0.500 cm and the viscosity of the fluid in-between is 1.77 cp.
Assumption
No-slip condition between the plates and the fluid
Fluid properties are constant all throughout
Plate and fluid motion is unidirectional only
No other external forces acting on the fluid
Both plates have the same uniform surface area
Solution
Viscosity of Fluids
As a fluid moves, shear stress develops.
Recall Physics 71 definition
“No-Slip” condition
The layer of fluid adjacent to the boundary surface has zero velocity relative to that surface.
Movement of a Fluid
What if the surface is stationary?
What if it is moving?
Movement of a Fluid
Velocity at wall should be zero
But we know that there is flow, i.e. nonzero velocity
Movement of a Fluid
LAMINAR FLOW
Movement of a Fluid
Viscosity of a Fluid
Viscosity of a Fluid
Viscosity (dynamic, µ)
Constant of proportionality
Resistance to flow
Gives rise to viscous forces that resist the relative movement of adjacent layers in the fluid
Viscosity of a Fluid
Newtonian Fluids
A linear relationship exists between the shear stress ? and the velocity gradient dv/dy.
GASES
Increases with increasing temperature
Independent of pressure (up to 1000 kPa)
At > 1000 kPa, viscosity increases with increasing pressure
LIQUIDS
Decreases with increasing temperature
Independent of pressure
Viscosity of Fluids
Outline
Molecular Transport Equations
Viscosity of Fluids
Fluid Flow
Fluid Flow
Fluid Flow
LAMINAR
Low velocity
No lateral mixing
TURBULENT
High velocity
Formation of eddies
Fluid Flow
Viscous forces are dominant
Occur at low velocities
Layers of fluid seem to slide by one another without eddies or swirls
No lateral mixing in the fluid
Fluid travels smoothly and in regular paths
Laminar Flow
Inertial forces are dominant
Fluid travels in random, chaotic paths
Heavy and lateral mixing occurs
Turbulent Flow
Ratio of inertial forces (?v2) on a fluid to the viscous forces (?v/D) acting on it.
Used to characterize different flow regimes
NRe < 2100 : laminar
2100 < NRe < 4000 : transition
NRe > 4000 : turbulent
Reynolds Number
|
|
Comments (0)
|
Post your homework questions and get free online help from our incredible volunteers
|