Page 12 - Fundamentals of The Finite Element Method for Heat and Fluid Flow

INTRODUCTION
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Table 1.1
in W/mK at 20 C
◦
Thermal conductivity
Material Typical values of thermal conductivity of some materials
Metals:
Pure silver 410
Pure copper 385
Pure aluminium 200
Pure iron 73
Alloys:
Stainless steel (18% Cr, 8% Ni) 16
Aluminium alloy (4.5% Cr) 168
Non metals:
Plastics 0.6
Wood 0.2

Liquid:
Water 0.6

Gases:
Dry air 0.025 (at atmospheric pressure)

Table 1.2 Typical values of heat
2
transfer coefficient in W/m K
Gases (stagnant) 15
Gases (flowing) 15–250
Liquids (stagnant) 100
Liquids (flowing) 100–2000
Boiling liquids 2000–35,000
Condensing vapours 2000–25,000

where is the radiative property of the surface and is referred to as the emissivity. The net
radiant energy exchange between any two surfaces 1 and 2 is given by
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4
Q = F F G σA 1 (T − T ) (1.5)
1 2
where F is a factor that takes into account the nature of the two radiating surfaces; F G is
a factor that takes into account the geometric orientation of the two radiating surfaces and
A 1 is the area of surface 1.
When a heat transfer surface, at temperature T 1 , is completely enclosed by a much
larger surface at temperature T 2 , the net radiant exchange can be calculated by
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4
Q = qA = 1 σA 1 (T − T ) (1.6)
1
1
2