Page 146 - Mechanical Engineer's Data Handbook

THERMODYNAMICS AND HEAT TRANSFER 135

A, =area of receiving body (mz)
e, =emissivity of radiating body (= 1 for black body)
e, =emissivity of surroundings
e = emissivity of intermediate wall
u = Stefan-Boltzmann constant
(=5.67 x lO-’Wm-’ K-4 )
f= interchange factor
PCd F = geometric factor
Reynold’s number Re = -
P h, = heat transfer coefficient for radiation
Nusselt number Nu = 0.0243Re0.8Pr0,4 (W m-’K-’)
= 0.02Re0.’ for gases
Heat radiated from a body to surroundings
For non-circular pipes use:
q=oe,(T:- T:)A, (watts)
4 x Area of cross-section
d= Taking into account emissivity of surroundings
Inside perimeter
Heat transferred q = hAB, q=o(e,T:-e,T:)A1 (watts)
where : em = -
01 -02
e
In 2
v2
and O1 and B2 are the temperature differences at each
end of a plate or tube between fluid and surface. 0, is
called the ‘logarithmic mean temperature difference’.

3.14. IO Evaluation of Nu, Re and Pr
The fluid properties must be evaluated for a suitable
mean temperature. If the temperature difference be-
tween the bulk of the fluid and the solid surface is Interchange factor f
small, use the ‘mean bulk temperature’of the fluid, e.g.
the mean of inlet and outlet temperatures for flow in a
pipe. If the difference is large, use the ‘mean film This takes into account the shape, size and relative
temperature’ t, = (Mean bulk temperature + Surface positions of bodies.
temperature)/2. (1) Large parallel planes: f= el%
e, +e,-e,e,
3.14. I I Radiation of heat H
Radiated heat is electromagnetic radiation like light,
radiowaves, etc., and does not require a medium for its
propagation. The energy emitted from a hot body is
proportional to the fourth power of its absolute
temperature.

Symbols used: 9
q =radiated energy flow (watts)
T, =temperature of radiating body (K)
T2 =temperature of surroundings (K)
A, =area of radiating body (mZ)