Entrainment Limitations in Thermosyphons and Heat Pipes

Abstract

The isothermal characteristics and high effective thermal conductivity of heat pipes and thermosyphons make them particularly useful in air to air and process to air heat recovery systems. Although previous investigations have developed successful techniques for predicting many of the transport limitations, entrainment remains the least understood. Current entrainment modeling techniques have resulted in a large range in the predicted axial heat flux required for the onset of entrainment. Included here is a review of the present analytical methods used to predict the liquid entrainment as a function of the pipe’s physical parameters and working fluid properties, for both thermosyphons and heat pipes. The results of the models are compared with existing experimental data in an effort to determine the accuracy of the predictive techniques. Using a sample copper/water thermosyphon and a similar screen wicked heat pipe, comparisons of the experimental entrainment limit and those predicted by seven thermosyphon and four heat pipe models were made. The results of this comparison can provide insight for designers developing heat pipe exchangers and will provide a basis for further understanding the phenomena which govern this limit.