Engineering Principles

Heat Transfer: conduction, convection, radiation

Heat Transfer

EP 1112 Lecture 5 What is Heat Transfer?

Heat transfer is defined as the transmission of energy from one region to another as a result of a temperature difference between them.

EP 1112 Lecture 5

• Important to study heat transfer because the design of many components is influenced by heat transfer.

• Examples include: Boilers, Refrigerators, Heat Exchangers, Turbine Blades, Transformers, Bearings, Electrical Components, Engines, Machine Tools…

EP 1112 Lecture 5

• The 3 modes of Heat Transfer are:

Conduction Convection Radiation

EP 1112 Lecture 5

• Heat transfer can be:

Steady (does not change with time) Unsteady (changes with time) One dimensional (1D) Two dimensional (2D) Three dimensional (3D)

EP 1112 Lecture 5 Conduction

Conduction is a process by which heat flows from a region of higher temperature to a region of lower temperature within a medium (solid, liquid or gaseous) or between different mediums in direct physical contact. Energy is transmitted by direct molecular communication without appreciable displacement of the molecules.

EP 1112 Lecture 5 Convection

Convection is a process of energy transport by the combined action of heat conduction, energy storage and mixing motion.

Convection is important as the mechanism of energy transfer between solid and liquid or gas. For example, between a hot metal plate and the surrounding air.

EP 1112 Lecture 5

Transfer of energy by convection from surface to the surrounding fluid takes place by

(1) Heat conduction from the surface to

adjacent particles of the fluid. Internal energy

of particles of fluid increases.

(2) Fluid particles move away to regions of lower temperature where they will mix.

EP 1112 Lecture 5 Radiation

Radiation (or radiant heat transfer) is a process by which heat flows from a high temperature body to a lower temperature body when the bodies are separated in space (even in a vacuum).

Thermal Radiation refers to radiation in the range of wavelengths 0.1 μm to 100 μm.

EP 1112 Lecture 5

What is the mode of heat transfer for the following ?

(1) An object 10 metres away from a large oil fire which is being heated.

(2)A central heating radiator in a room.

(3)The heat flow in an engine spark plug.

EP 1112 Lecture 5

(1) Radiation

(2) Convection

(3) Conduction

EP 1112 Lecture 5 Basic Laws of Heat Transfer

Conduction
Q (T
− T )

For conduction, we have: ’ kA 2 1 t x

where Q = quantity of heat passing through (J) t = time (s) T2 = higher temperature (oC or K) T1 = lower temperature (oC or K) x = distance (m) k = thermal conductivity A = area (m2)

Question: what are the units of thermal conductivity?

EP 1112 Lecture 5

Units must balance