
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]: