Page 219 - Mechanical Engineers' Handbook (Volume 4)

208 Heat-Transfer Fundamentals

Figure 32 Equivalent thermal resistance of a heat pipe.

R wc The resistance of the liquid–wick combination at the condenser
R pc The radial resistance of the pipe wall at the condenser

Because of the comparative magnitudes of the resistance of the vapor space and the axial
resistances of the pipe wall and liquid–wick combinations, the axial resistance of both the
pipe wall and the liquid–wick combination may be treated as open circuits and neglected.
Also, because of the comparative resistances, the liquid–vapor interface resistances and the
axial vapor resistance can, in most situations, be assumed to be negligible. This leaves only
the pipe wall radial resistances and the liquid–wick resistances at both the evaporator and
condenser. The radial resistances at the pipe wall can be computed from Fourier’s law as

R pe
kA e
p
for flat plates, where
is the plate thickness and A is the evaporator area, or
e
ln(D /D )
R pe o i
2 Lk
ep
for cylindrical pipes, where L is the evaporator length. An expression for the equivalent
e
thermal resistance of the liquid–wick combination in circular pipes is
ln(D /D )
R we o i
2 Lk
e eff
where values for the effective conductivity, k , can be found in Table 28. The adiabatic
eff
vapor resistance, although usually negligible, can be found as
T (P P )
v
R va v,e v,c
hq
v ƒg