Heat Transfer

426 Heat Transfer: Thermal Management of Electronics

16.1.3 so n i c li m i T

The maximum velocity of a compressible uid passing through a converging–

diverging nozzle is equal to the speed of sound in that uid, and happens at the throat

of the nozzle. The vapor ow in the evaporator of a heat pipe is similar to the ow

in a converging–diverging nozzle. The vapor velocity increases along the evaporator

and reaches a maximum, at the end of the evaporator section, that can not be higher

than the local speed of sound. This ow condition is called the sonic limit and the

associated heat transfer limit is given by [3]:



Q

ch A

s

fg v

=

+

ρ

γ

00

2( )