Introduction to
Heat Transfer
Outline
1.Heat Transfer Mechanisms
1.1. Conduction
1.2. Convection
1.3. Radiation
2. Conduction Heat Transfer
2.1. Series/Parallel Resistances
2.2. Geometric Considerations
Heat Transfer Mechanisms
Conduction
Transfer of energy of
motion between adjacent
molecules
May occur across solids,
liquids, and gases.
Heat Transfer Mechanisms
Conduction
Molecular
Vibration
Free Electron
Diffusion
(for solids)
*This is why metals
are good conductors.
Heat Transfer Mechanisms
Convection
Transfer by bulk transport and mixing of
macroscopic elements of warmer portions with
cooler portions of a gas or a liquid.
Heat Transfer Mechanisms
Convection
Forced Convection
Free/Natural Convection
Heat Transfer Mechanisms
Radiation
Transfer achieved by electromagnetic
waves and requires no physical
medium to propagate.
Heat Transfer Mechanisms
Identify the mechanisms:
Conduction Heat Transfer
Recall: Fourier’s Law of Heat Conduction
Where:
Q = heat flow rate
A = heat transfer area
k = thermal conductivity
T = temperature
𝑑𝑇
= temperature gradient
𝑑𝑦
ONE-DIMENSIONAL ONLY
𝑄
𝑑𝑇
= −𝑘
𝐴
𝑑𝑦
Conduction Heat Transfer
Recall: Fourier’s Law of Heat Conduction
Where:
q = heat flux
k = thermal conductivity
T = temperature
THREE-DIMENSIONAL
𝒒 = −𝑘𝛻𝑇
𝜕𝑇 𝜕𝑇 𝜕𝑇
𝛻𝑇 =
,
,
𝜕𝑥 𝜕𝑦 𝜕𝑧
Conduction Heat Transfer
Recall: Fourier’s Law of Heat Conduction
𝑄
𝑑𝑇
= −𝑘
𝐴
𝑑𝑦
Quantity
Q
A
k
T
SI Units
W
m2
W/m·K
K or °C
English Units
Btu/hr
ft2
Btu/hr·ft·°F
°F or °R
Conduction Heat Transfer
Useful Conversion Factors:
𝐵𝑡𝑢
𝑐𝑎𝑙
−3
1
= 4.1365 × 10
ℎ𝑟 𝑓𝑡 °𝐹
𝑠 𝑐𝑚 °𝐶
𝐵𝑡𝑢
W
1
= 1.73073
ℎ𝑟 𝑓𝑡 °𝐹
𝑚𝐾
𝐵𝑡𝑢
𝑊
1
= 3.1546 2
2
ℎ𝑟 𝑓𝑡
𝑚
𝐵𝑡𝑢
1
= 0.29307 𝑊
ℎ𝑟
Quantity
SI Units
English Units
Q
W
Btu/hr
A
m2
ft2
k
W/m·K
Btu/hr·ft·°F
T
K or °C
°F or °R
Conduction Heat Transfer
Thermal Conductivity, k
A measure of a material’s ability to
transfer thermal energy by conduction.
Thermal Diffusivity, 𝜶
A measure of a material’s ability to
respond to changes in its thermal
environment.
𝑘
𝛼=
𝜌𝑐𝑃
Conduction Heat Transfer
Thermal Conductivity
Conduction Heat Transfer
Thermal Conductivity
For most liquids, thermal
conductivity varies
moderately linear with
temperature:
𝑘 = 𝑎 + 𝑏𝑇
Conduction Heat Transfer
Thermal Conductivity
For most liquids, thermal
conductivity varies
moderately linear with
temperature:
Conduction Heat Transfer
Thermal Conductivity
For most liquids, thermal
conductivity varies
moderately linear with
temperature:
Conduction Heat Transfer
Common Insulators:
1. Cellular glass
2. Polyurethane
3. Polyisocyanurate
4. Polystyrene
5. Glass fiber
6. Calcium silicate
7. Mineral wool
Conduction Heat Transfer
Exercise:
1. Calculate the heat loss per m2 of surface area
for an insulating wall composed of 25.4-mm
thick fiber insulating board (k = 0.048 W/mK),
where the inside temperature is 352.7 K and the
outside temperature is 297.1 K.
Conduction Heat Transfer
Exercise:
2. A furnace wall is insulated by a material with thermal
conductivity expressed as
𝑘 = 0.08(1 + 0.003 𝑇)
where k is in Btu/hr-ft-°F and T is in °F. During
operation, the temperature inside the furnace reaches
1800°F, releasing heat at 3500 Btu/hr-ft2. Determine
the insulation thickness sufficient to bring the
temperature outside the wall to 300 °F.