Recent heat pipe applications to heat removals in various thermal engineering fields requires to investigate more precisely thermal behavior in the heat pipe. For example, a startup problem was not important for relatively high vapor pressure working fluid such as water or normal fluids except for liquid metals like sodium or mercury which have low vapor pressure at normal ambient temperature. However, when the heat pipe is applied to a heat removal of an equipment undergoing a rapid change in temperature or heat load like a nuclear reactor, it is required to understand transient behavior at a startup time to make appropriate designs with sufficient thermal allowance.
The transient behavior relating to the startup of the heat pipe has been investigated by many researchers. But most of the investigations were relating to liquid metal heat pipe, or wick heat pipe. A few of them were relating to water or organic fluid thermosyphon heat pipe. But in those studies an evaporating section was loaded indirectly by a heater, therefore, a heat capacity and a time constant were so large that the transient behavior was not so different from the one at a steady state experiment.
The purpose of this study is to obtain understandings of fluid flow and heat transfer characteristics of the thermosyphon heat pipe in case of rapid transient operations.