1. 2nd class of Heat Transfer I
Thursday, January 16, 2014:
Convection Heat Transfer (Section 1-3);
Radiation Heat Transfer (Section 1-4)

2. Convection Heat Transfer
• Exchange of heat between a solid boundary
and a fluid in motion as a result of
temperature difference.
• Forced convection: motion of fluid effected by
an external device (e.g.. fan)
• Free (natural) convection: motion of fluid
effected by change of fluid density

3. Forced Convection Heat Transfer
q=hA(TW - Tinfinity )
where:
q is the heat transfer, in W
h is the convection heat transfer coefficient, in
W/m2 • °C
TW is the temperature of the wall (boundary), in °C
Tinfinity is the temperature of the moving fluid (°C)

4. Real World Examples of Forced and
Free Convection
• Forced: air moving over the fins of a
motorcycle engine
• Free: air moving due to a warm radiator in an
older house

5. Example Problem, Convection
Air at 20°C blows over a hot plate 50 by 75 cm
maintained at 250°C. The convection heat
transfer coefficient is 25 W/m2•°C. Calculate the
heat transfer.
q=hA(Tw – Tinfinity)= (25 W/m2•°C)(0.50 m)
(0.75 m) (250 °C – 20 °C)
q = 2156 W

6. Radiation Heat Transfer
• Energy exchanged between materials as result
of electromagnetic radiation transfer, as a
result of the materials having different
temperatures.

7. Real World Examples of Radiation Heat
Transfer
• Sun warming the planet Earth

8. Radiation Heat Transfer
q/A=σ(T1
4 - T2
4 )
where:
q is the radiation heat transfer, in W
A is the area under consideration
σ is the Stefan Boltzmann constant, W/m2 •K4
T1 is the temperature of surface 1
T2 is the temperature of surface 2

9. Example Problem, Radiation
Two infinite black plates at 800 °C and 300 °C
exchange heat by radiation. Calculate the heat
transfer by radiation.
q/A = σ (T1
4 - T2
4 )= 5.669 × 10-8 (W/m2 •K4)
(10734 – 5734)
= 69,031 W/m2 = 69.03 kW/m2

10. Questions ?
• Assignment: Read (for next class): 2-1, 2-2.
• Homework: Chapter 1, Problems 1, 2, 3, 5, 8,
12, 15, 22, 24, 27.
• That’s the end of lectures for Chapter 1
(Introduction).
• Start Chapter 2 (Steady-State Conduction-One
Dimension) next time.