name the three modes of heat trany.
Answers
Answer:Basic Modes of Heat
Transfer
Abstract:
Heat transfer can occur in three different modes:
conduction, convection and radiation. The
purpose of this application note is not to
bombard the reader with a ton of equations.
However, a fixture designer must be well aware
of the controlling parameters in the equations in
each heat transfer mode. Understanding and
utilizing the key parameters in the equations will
determine the success of the thermal design,
hence the performance, cost and reliability of the
fixture.
Conduction:
Conduction is the most common method of heat
transfer from a heat source, such as an LED and
a driver, to the heatsink in LED fixtures.
Therefore, it is important to go through this
concept and the related equation in more detail.
Thermal conduction happens when two solid
materials that are at two different temperatures
are in direct contact, e.g. an Aluminum heatsink
and a driver. Please see Figure 1 for an example
of a driver making contact with a heatsink. For all
practical purposes, metals are used for
heatsinking purposes since they have very good
thermal conductivity. Please see Table 1 for
thermal conductivity of multiple common
materials. For more details on thermal
conductivity discussion please refer to the
“Practical Thermal Design Considerations”
application note.
Material
Thermal Conductivity
W/(m K)
Normalize
d to Al
Normalized
to Air
Air, atmosphere
(gas) 0.024 8541.7 1.0
Aluminum 205 1.0 8541.7
Copper 401 0.5 16708.3
Gold 310 0.7 12916.7
Iron 80 2.6 3333.3
Silver 429 0.5 17875.0
Zinc Zn 116 1.8 4833.3
Plastics,
foamed 0.03 6833.3 1.3
Plywood 0.13 1576.9 5.4
Polycarbonate 0.19 1078.9 7.9
Table 1. Thermal Conductivity of Some Common
Materials
Aluminum is the most commonly used metal for
the heatsink and mechanical body or chassis of
an LED based fixture because of its lower cost
compared to other higher heat conductive
materials such as copper and silver. Although
some high thermal conductivity plastic material is
also used for this purpose, metals like aluminum,
and to a much lesser degree copper, are used in
the overwhelming majority of LED lighting
applications. Various grades of aluminum will
have somewhat different thermal conductivity.
Figure 1. Conduction Mode Heat Transfer: Driver
Making Direct Contact with the Heatsink
Explanation: