This document discusses the basic concepts and mechanisms of heat transfer, including conduction, convection, and radiation. It defines key terms like heat, thermal conductivity, specific heat, and convection heat transfer coefficient. Equations for one-dimensional steady-state heat conduction via Fourier's law, Newton's law of cooling, and the Stefan-Boltzmann law for radiation are presented. The relationships between thermal conductivity, thermal diffusivity, emissivity, absorptivity, and radiation heat transfer are also covered at a high level.

1. Basic concepts and
laws of heat transfer
analysis

2. Heat transfer
mechanisms
 Heat – thermal energy
 Heat as the form of energy that can be transferred from one
system to another as a result of temperature difference.
 Thermodynamic analysis – amount of heat transfer
 Heat transfer – rate of heat transfer
 Laws of heat transfer (rate energy transfer; direction of decreasing
temperature)
 Transfer of energy as heat – temperature gradient
 Modes of heat transfer:
- Conduction
- Convection
- Radiation

3. Conduction
HeatTransfer
 Conduction is the transfer of energy from the more energetic particles
of a substance to the adjacent less energetic ones as a result of
interactions between the particles.
 Conduction can take place in solids, liquids, or gases.
 Gases + liquids – collision + diffusion
 Solids – vibrations + free electrons
 𝑄𝑐𝑜𝑛𝑑 = −𝑘𝐴
𝑑𝑇
𝑑𝑥
= 𝑘𝐴
𝑇1−𝑇2
∆𝑥
(𝑊) - Fourier’s law of heat conduction
where: k – thermal conductivity; A – area;
𝑑𝑇
𝑑𝑥
- temperature gradient

4. Thermal
Conductivity
 Thermal conductivity, k is a measure of the material’s ability to
conduct heat.
 Units - 𝑊 𝑚 .𝑂
𝐶
 Good conductors
 Bad or poor heat conductors
 𝑘 =
𝐿
𝐴(𝑇1−𝑇2)
𝑄 𝐿 = ∆𝑥
 Specific heat, 𝐶𝑝is the measure of a material’s ability to store energy.
 Temperature is a measure of the kinetic energies of the particles such
as the molecules or atoms of a substance.
 The thermal conductivity of a substance is normally highest in the
solid phase and lowest in the gas phase.
 Unlike gases, the thermal conductivities of most liquids decrease with
increasing temperature, with water being a notable exception.

5. Thermal
Diffusivity
 Thermal diffusivity shows how fast heat diffuses through a
material. OR
 The ratio of the heat conducted through the material to the heat
stored per unit volume
 ∝ =
ℎ𝑒𝑎𝑡 𝑐𝑜𝑛𝑑𝑢𝑐𝑡𝑒𝑑
ℎ𝑒𝑎𝑡 𝑠𝑡𝑜𝑟𝑒𝑑
=
𝑘
𝜌𝐶𝑝
𝑚2
/𝑠
 Where: k – thermal conductivity; 𝜌𝐶𝑝 - heat capacity of a material
 Thermal conductivity vsThermal diffusivity

6. Convection
HeatTransfer
 Convection is the mode of energy transfer between a solid surface
and the adjacent liquid or gas that is in motion, and it involves the
combined effects of conduction and fluid motion.
 Types of convection: - forced and – natural/free convection
 𝑄𝑐𝑜𝑛𝑣 = ℎ𝐴𝑠 𝑇𝑠 − 𝑇∞ 𝑊 - Newton’s law of cooling
 Where: h – convection heat transfer coefficient in 𝑊 𝑚 .𝑂
𝐶; 𝐴𝑠 -
surface area; 𝑇𝑠 - surface temp; 𝑇∞ - temp of fluid far from surface
 h, depends on variables influencing convection like: surface
geometry, nature of fluid in motion, bulk fluid velocity

7. Radiation
HeatTransfer
 Radiation is the energy emitted by matter in the form of
electromagnetic waves (or photons) as a result of the changes in
the electronic configurations of the atoms or molecules.
 It doesn’t require an intervening medium, is fastest (at the speed of
light) and it suffers no attenuation in a vacuum.
 Surface phenomenon
 Maximum rate of radiation emitted from a surface;
𝑄𝑒𝑚𝑖𝑡,𝑚𝑎𝑥 = 𝜎𝐴𝑠𝑇𝑠
4
(𝑊) - Stefan-Boltzmann law
Where 𝜎 = 5.67 × 10−3 𝑊 𝑚 . 𝐾 - Stefan-Boltzmann constant
 Blackbody and Blackbody radiation
 Radiation emitted by a real surface: 𝑄𝑒𝑚𝑖𝑡 = 𝜀𝜎𝐴𝑠𝑇𝑠
4
(𝑊)
Where 𝜀 is emissivity of the surface.
 Absorptivity, ∝ of a surface

8. Radiation
HeatTransfer
2
 0 ≤ 𝜀 ≤ 1 ; 0 ≤ 𝛼 ≤ 1 - temp, wave length of the radiation
 Kirchhoff’s law of radiation (emissivity and absorptivity are equal)
 Rate at which a surface absorbs radiation: 𝑄𝑎𝑏𝑠𝑜𝑟𝑏𝑒𝑑 =∝ 𝑄𝑖𝑛𝑐𝑖𝑑𝑒𝑛𝑡
 Net radiation heat transfer, gaining or loosing energy by radiation
 Net rate of radiation heat transfer:
𝑄𝑟𝑎𝑑 = 𝜀𝜎𝐴𝑠 𝑇𝑠
4 − 𝑇𝑠𝑢𝑟𝑟
4 (𝑊)
 Radiation occurs parallel with conduction/convection – combined
heat transfer coefficient, ℎ𝑐𝑜𝑚𝑏𝑖𝑛𝑒𝑑
𝑄𝑡𝑜𝑡𝑎𝑙 = ℎ𝑐𝑜𝑚𝑏𝑖𝑛𝑒𝑑𝐴𝑠 𝑇𝑠 − 𝑇∞ (𝑊)
 Radiation vs conduction or natural convection vs forced
convection

9. Simultaneous
HeatTransfer
Mechanisms
 Not all three mechanisms of heat transfer can exist
simultaneously.
 Heat transfer is only by conduction in opaque solids, but by
conduction and radiation in semitransparent solids.
 A solid may involve heat transfer by convection and/or radiation
on its surfaces exposed to a fluid or other surfaces.
 Heat transfer is by conduction and possibly by radiation in a still
fluid and by convection and radiation in a flowing fluid.
 Heat transfer through a vacuum is by radiation only.

10. Ref:  Yunus A. Cengel, (2000). HeatTransfer,A PracticalApproach,
Second Edition.