Lecture 9
Dr. Srinath V. Ekkad
Chapter 6  Convection
Boundary
Layer Features
Boundary Layers: Physical Features
•
Velocity Boundary Layer
–
A consequence of viscous effects
associated with relative motion
between a fluid and a surface.
–
A region of the flow characterized by
shear stresses and velocity gradients.
–
A region between the surface
and the free stream whose
thickness
increases in
the flow direction.
δ
–
Why does
increase in the flow direction?
δ
–
Manifested by a
surface shear
stress
that provides a drag
force,
.
s
τ
D
F
–
How does
vary in the flow
direction?
Why?
s
τ
Boundary Layer Features
(cont.)
•
Thermal Boundary Layer
–
A consequence of heat transfer
between the surface and fluid.
–
A region of the flow characterized
by temperature gradients and heat
fluxes.
–
A region between the surface and
the free stream whose
thickness
increases in the flow direction.
t
δ
–
Why does
increase in the
flow direction?
t
δ
–
Manifested by a
surface heat
flux
and a
convection heat
transfer coefficient
h
.
s
q
F
–
If
is constant, how do
and
h
vary in the flow direction?
(
29
s
T
T
F

s
q
F
Local and Average
Coefficients
Distinction between
Local
and
Average Heat Transfer Coefficients
•
Local Heat Flux and Coefficient
:
•
Average Heat Flux and Coefficient for a Uniform Surface Temperature
:
•
For a
flat plate in parallel flow
:
Transition
Boundary Layer Transition
•
How would you characterize conditions in the
laminar region
of boundary layer
development?
In the turbulent region
?
•
What conditions are associated with
transition
from laminar to turbulent flow?
•
Why is the Reynolds number an appropriate parameter for quantifying transition
from laminar to turbulent flow?
•
Transition criterion
for a flat plate in parallel flow:
Transition (cont.)
What may be said about transition if Re
L
< Re
x,c
?
If Re
L
> Re
x,c
?
•
Effect of transition on boundary layer thickness and local convection coefficient:
Why does transition provide a significant increase in the boundary layer thickness?