1. CONVECTION HEAT TRANSFER
P M V Subbarao
Professor
Mechanical Engineering Department
IIT Delhi
A Controllable Characteristic of fluids……

2. Introduction
• Convection involves the transfer of heat by the motion and
mixing of "macroscopic" portions of a fluid
• This macroscopic motion is same as flow of a fluid past a solid
boundary.
• The term natural convection is used if this motion and mixing
is caused by density variations resulting from temperature
differences within the fluid.
• The term forced convection is used if this motion and mixing
is caused by an outside force, such as a pump.
• Heat transfer by convection is more difficult to analyze than
heat transfer by conduction .
• No single property of the heat transfer medium, such as
thermal conductivity, can be defined to describe the
mechanism.

3. • Heat transfer by convection varies from situation to
situation (upon the fluid flow conditions), and it is
frequently coupled with the mode of fluid flow.
• In practice, analysis of heat transfer by convection is
treated empirically (by direct observation).
• Convection heat transfer is treated empirically because of
the factors that affect the stagnant film thickness:
• Fluid velocity
• Fluid viscosity
• Heat flux
• Surface roughness
• Type of flow (single-phase/two-phase)

4. •
•
•
•
•
•
Convection involves the transfer of heat between a surface at a given
temperature (Ts) and fluid at a bulk temperature (Tb).
The exact definition of the bulk temperature (Tb) varies depending on
the details of the situation.
For flow adjacent to a hot or cold surface, T b is the temperature of the
fluid "far" from the surface.
For boiling or condensation, Tb is the saturation temperature of the
fluid.
For flow in a pipe, Tb is the average temperature measured at a
particular cross-section of the pipe.
Newton’s law of cooling suggests a basic relationship for heat transfer
by convection:
Q = hA( Ts − Tb )
h is called as Convection Heat Transfer Coefficient, W/m 2K

5. Realization of Newton’s Law Cooling
• A general heat transfer surface may not be isothermal !?!
• Fluid temperature will vary from inlet to exit !?!?!
• The local velocity of flow will also vary from inlet to
exit ?!?!
• How to use Newton’s Law in a Real life?

6. Local Convection Heat Transfer
Consider convection heat transfer as a fluid passes over a surface
of arbitrary shape:
Apply Newton’s law cooling to a local differential element with
length dx.
q = h( Ts − T∞ )
''
Ts > T∞
h is called as Local Convection Heat Transfer Coefficient, W/m2K

7. The total heat transfer rate q is
q = ∫ q '' dAs = havg AS ∆Tavg
As
Where, havg is the average convection heat transfer coefficient for
the entire surface.
1
q '' dAs
∫
As As
havg =
∆Tavg
where
1
∆Tavg =
∫ ( Ts − T∞ ) dAs
As As
q '' dAs
∫
Therefore
havg =
As
∫ (T
s
A
− T∞ ) dAs
How to Evaluate q’’?

8. Fundamental Aspects of Convection
• How to evaluate q’’ ?
• How the heat is transferred from solid to the first layer of
the fluid?
• Is this a part of convection?
• What is the temperature of the fluid particle, just adjacent
to the wall?
• What part of the fluid domain is under pure convection
transfer?

9. Concept of Solid Fluid Interaction : Maxwell’s Theory
• Perfectly
smooth surface (ideal surface)
Real surface
U2′
U1
U1′
U2
U2′
U′
U
Φ
Φ
Φ′
Specular reflection
Diffuse reflection
• The convective heat transfer is defined for a combined solid
and fluid system.
• The fluid packets close to a solid wall attain a zero relative
velocity close to the solid wall : Momentum Boundary Layer.

10. • The fluid packets close to a solid wall come to thermal
equilibrium with the wall.
• The fluid particles will exchange maximum possible
energy flux with the solid wall.
• A Zero temperature difference exists between wall and
fluid packets at the wall.
• A small layer of fluid particles close the the wall come to
Mechanical, Thermal and Chemical Equilibrium With
solid wall.
• Fundamentally this fluid layer is in Thermodynamic
Equilibrium with the solid wall.