Since Nukiyama's pioneering experiment and findings in the 1930s many researchers have further investigated the boiling process and developed empirical or semi-empirical methods to predict the boiling heat transfer coefficient. However, several details of the local heat transport phenomena are still not fully understood. The heat transfer from the heated wall to the bulk fluid is extremely complex and many aspects are involved: nucleation, dynamic wetting characteristics, surface tension and other fluid properties, gravitation, bulk subcooling, etc… Only in the recent years experimental methods reached spatial and temporal resolutions that allow observing the boiling process on the scales of these individual aspects and local dynamic phenomena in the two-phase boundary layer. Similarly, numerical methods reached a level of robustness and resolution that allows studying the influence of these aspects and phenomena in a computer simulation. This work will give an overview on recent developments and modern experimental and numerical methods nucleate boiling heat transfer research, Results from combined generic experiments and numerical simulations are presented. They give a deep insight into the local and transient transport phenomena in the two-phase boundary layer.

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Variable Conductance Heat Pipes (VCHP) are used as a cooling device which can passively keep the evaporating temperature by enclosing non-condensable gas (NCG) with refrigerant. The VCHP has often a bend section between evaporating and condensing area, since the condensation section should be placed vertically above the evaporation section. It is known that the bend may disturb a liquid flow to the evaporation section. The effect of the VCHP orientation, filling ratio of the refrigerant, amount of the charged NCG have been examined. A thin plate was inserted to more the effects of the bend. In this study, the refrigerant behaviors in the VCHP with the bend were diagnosed based on flow visualization by neutron radiography. It was observed that liquid plugs formed at the bend and suppressed the condensed liquid supply to the evaporating section without the thin plate. It was also shown that the thin plate inserted in the bend was effective to avoid the liquid plug formation.

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