NUMERICAL ANALYSIS OF THE PHASE-CHANGE HEAT TRANSFER INSIDE A PULSATING HEAT PIPE WITH OVERCRITICAL NUMBER OF TURNS

摘要

The Pulsating Heat Pipe (PHP) is a promising device in the family of heat pipes. With no need for a wick, they exhibit a high heat transfer to weight ratio. Moreover, the wickless design removes limits commonly associated with conventional heat pipes, increasing the maximum power transfer per single heat pipe. These peculiarities make it an ideal candidate for many high power applications. Nonetheless, there is though only partial knowledge on the driving mechanism, which restricts prediction accuracy. Most Pulsating Heat Pipe studies rely on experiments to test configurations, while simulations usually depend on semi-empirical correlations or adaptations of reduced theoretical models. Experiments provide detailed data for a particular geometry in lab fixed conditions, but it offers limited flexibility to test alternative configurations. Semi-empirical models use previous experimental data to create non-dimensional formulations. Though approaching an increased set of conditions, correlations apply with reasonable accuracy only to a small range, outside of which the prediction ability progressively falls. High order numerical analysis such as Computational Fluid Dynamics (CFD) modeling could potentially provide full visualization, but due to the complex flow behavior, previous studies used this method only in simple configurations with a small number of turns. The present research will expand the potential of this modeling technique by presenting the CFD analysis of a complex Pulsating Heat Pipe configuration. The importance of this study lies in the fact that this configuration, with a number of turns greater than a critical parameter, shows a reduced sensitivity to gravity and is therefore particularly important for applications where restrictions on installations make the positioning sub-optimal. The research simulates using a CFD commercial software a two-dimensional Pulsating Heat Pipe with sixteen turns. The heat pipe, with a 2 mm internal diameter, is filled with water at 50% of mass. To visualize the oscillation pattern of liquid and vapor slugs and plugs inside the Pulsating Heat Pipe, the model performs a transient analysis on the device. A Volume of Fluid (VOF) solver for multiphase analysis, coupled with the Lee model for evaporation and condensation mass transfer, calculates the interactions between the liquid and the gas phase inside the tube. The study follows the geometric and operational conditions from previous experiments. The analysis regards a Pulsating Heat Pipe operating in a vertical position with the condenser section placed in the upper sector. During the initial operations, the system flow distribution fluctuates between different flow modes as the fluid slugs and plugs structure forms. After stabilizing the heat transfer results well agree with the tested values. Moreover, the increased resolution allows us to fully visualize the internal operation, retrieving additional information on the temperature and ratio of liquid and gas phase along the heat pipe.