Nucleate Pool Boiling Heat Transfer in Wickless Heat Pipes (Two-Phase Closed Thermosyphons): A Critical Review of Correlations

With the objective of estimating the pool boiling resistance of two-phase closed thermosyphons (TPCTs), the work presented gives the state of the art of nucleate pool boiling correlations. A deep analysis of up-to-date equations reported for bubbles and nucleate boiling is carried out and recommendations are provided to select the most suitable and reliable mathematical models. After introducing the operation of thermosyphons, a section dedicated to bubbles provides basic knowledge on nucleation processes and bubble growth. The main boiling mechanisms occurring in thermosyphons with a filling ratio of 100%, i.e. pool boiling, are explained in a comprehensive way. The current state of the art of correlations predicting the number of active nucleation sites, the bubble departure diameters and bubble departure frequencies is presented. In addition, 27 correlations of nucleate pool boiling heat transfer coefficients are critically reviewed. For all sections, the advised correlations are reported in a clear and simple way using tables. The aim of this paper is to provide sufficient knowledge on two-phase heat transfer in TPCTs to facilitate the nucleate pool boiling heat transfer coefficient calculations for thermosyphon users. This paper can be taken as a starting point in the study of pool boiling in thermosyphons and heat pipes.