Convective Heat Transfer

Contributed by:

Jonathan James Fri, Jan 21, 2022 08:12 PM UTC

Heat transfer through a plane slab, Natural and forced convection, Newton's law of cooling, Local and average heat transfer rate

1. Modes of Heat Transfer-1
P M V Subbarao
Professor
Mechanical Engineering Department
Change in Class Room : LH 519
Various Possibilities of a Natural
Happening …..

2. General Conduction Heat Transfer in a System
Conduction Heat Transfer from/to a System:
Heat flux due to conduction

q ' '  kT
  T ˆ T ˆj  T ˆ 
q ' '  k  i  k
 x y z 
   T T T  
Q q ' ' A  k  iˆ  ˆj  kˆ   A
 x y z 

3. Heat transfer through a plane slab
 T 
Q  kA 
 x 

4. HEAT TRANSFE IN TURNING
• Cooling of a workpiece on a lathe is affected not only by the
usual factors such as exposure of its surfaces and their
velocities, but also by the presence of the chuck and the jet
the cutting fluid splashing on the workpiece while moving
with the tool.
• The cooling effect of the surface area under the impinging
coolant jet is not uniform and therefore the heat transfer
coefficient in this zone should be treated as a distribution.
• The initial conditions of cooling at the beginning of the
pass, when the jet flashes also on the side face of the
workpiece, poses additional challenge in determination of
heat transfer coefficients.

5. Heat Convection
• Convection uses the motion of fluids to transfer heat.
• In a typical convective heat transfer, a hot surface heats the
surrounding fluid, which is then carried away by fluid
movement such as wind.
• The warm fluid is replaced by cooler fluid, which can
draw more heat away from the surface.
• Since the heated fluid is constantly replaced by cooler
fluid, the rate of heat transfer is enhanced.

6. Natural Convection
• Natural convection (or free convection) refers to a case
where the fluid movement is created by the warm fluid
itself.
• The density of fluid decrease as it is heated; thus, hot
fluids are lighter than cool fluids.
• Warm fluid surrounding a hot object rises, and is replaced
by cooler fluid.
• The result is a circulation of air above the warm surface

7. Forced Convection
• Forced convection uses external means of producing fluid
movement.
• Forced convection is what makes a windy, winter day feel
much colder than a calm day with same temperature.
• The heat loss from your body is increased due to the
constant replenishment of cold air by the wind.
• Natural wind and fans are the two most common sources
of forced convection.

8. Newton's Law of Cooling
• The rate at which a hot body cools to the temperature of its
surroundings is given by an empirical formula first discovered by
Sir ISAAC NEWTON
• Newton's law of cooling states, "For a body cooling in a draft, the
rate of heat loss is proportional to the difference in temperatures
between the body and its surroundings."
• Since the temperature change is proportional to the heat transfer.
dTbody
 Tbody  Tambient
dt
Energy transport due to two combined effects:
• diffusion: random molecular energy
• advection: bulk (macroscopic) fluid motion
The process of convection occurs between a moving fluid and
a surface at different temperatures.

9. Convection Heat Transfer
q ' '  Tsurface  Tambient 
q ' ' hTsurface  Tambient  Tsurface  Tambient
q ' ' hTambient  Tsurface  Tsurface  Tambient
where h is the local heat transfer coefficient.
Both the flux and transfer coefficient vary along the surface.

10. Local and Average Heat Transfer Rate
The local heat transfer rate is
Q hTsurface  Tambient dAs
The Total heat transfer rate is
q" dA  hT
As
s
As
surface  Tambient d As
q" dA s havg As Tsurface  Tambient 
As
Where h is average heat transfer coefficient:

11. 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
''
Therefore  dAs
q
As
havg 
T
A
s  T  dAs

12. Thermal Radiation
• Radiation does not require a medium to pass through; thus, it is
the only form of thermal energy travel present in vacuum.
• It uses electromagnetic (photons) means of transportation, which
travels at the speed of light.
• Thermal radiation is emitted by any matter with temperature
above 0 degree Kelvin (-273 °C).
• Radiative heat transfer occurs when the emitted radiation strikes
another body and is absorbed.
• The electromagnetic spectrum classifies radiation according to
wavelengths of the radiation.
• Main types of radiation are (from short to long wavelengths):
gamma rays, x-rays, ultraviolet (UV), visible light, infrared (IR),
microwaves, and radio waves.
• Radiation with shorter wavelengths are more energetic and
contains more heat.

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