Heat pipe startup transient measurements including internal visual observations

A specially designed heat pipe with a rectangular cross section, a flat mesh-type wick, and a glass wall through which internal mechanisms of heat pipe operation can be observed has been designed, constructed and used to observe the transient behavior of a heat pipe under startup conditions. Thermocouples located both on the outside of one metal wall and inside the vapor channel close to the glass wall are used to measure and record temperature profiles during startup. Startups to low powers are accomplished with no difficulty; however, startup to a higher power well below predicted steady-state limits results in boiling in the evaporator, followed by dryout and eventual failure.