Page 147 - Mechanical Engineer's Data Handbook

136 MECHANICAL ENGINEER'S DATA HANDBOOK

Parallel surfaces with intermediate wall

Let:
T= wall temperature
e-emissivity of wall

(2) Small body enclosed by another body: f=e,

(3) Large body (1) enclosed by body (2):
f= A e1ez
e2+L(e1-e1e2)
A2
(4) Concentric spheres and concentric infinite cylin-
ders:fas for (3)
(5) Parallel disks of different or same diameter: e
f=e,ez e2

Geometric factor F For side 1 : f, = e,e
e, +e-e,e
This takes into account the fact that not all radiation
reaches the second body. For side 2: f2= e2e
e, + e-e2e
(a) For cases (1) to (4) above, F= 1.
f T:+fZT':
(b) For case (5) with disks of radii r1 and r2 a distance Intermediate temperature: T4 =
x apart: fl +f2
q=f,aA(T:- T4)=f20A(T4- T:)

3.14. I2 Emissivity of surfaces
Heat radiated including f and F Emissivity depends not only on the material but also to
a large extent on the nature of the surface, being high
q=fFoA,(T:-T:) for a matt surface (e.g. 0.96 for matt black paint) and
low for a polished surface (e.g. 0.04 for polished
Heat transfer coeficient aluminium).
Therefore: h, =Po( T, + T2)( 7: + e)
4=hrA,(T,-T2)