Numerical Simulations of Pulsating Heat Pipes, Part 1: Modeling

The pulsating nature of a closed loop pulsating heat pipe (CLPHP)makes the prediction of its thermal-hydraulic performance extremely challenging. Unfortunately, the design of CLPHPs is still quite an empirical process, requiring testing of many prototypes by cold plate fabricators and thus inhibiting their possibilities for application. This means that a reliable and robust CLPHP simulation code is needed to do the design and prediction of its thermal performance. In this study, an updated code based on a 1-D model that traces out the entire loop is presented, starting from its first version developed in 2015. The present code is based on the principles highlighted in the Three-Zone Model for flow boiling in a micro-channel (i.e. thin liquid film zone, liquid slug zone and dry-out zone), but applied to the oscillating flow of CLPHPs in both evaporating and condensing processes. The heat transfer processes between the wall and the fluid are modelled for both the liquid slugs (convective)and vapor plugs (film evaporation and condensation). The wall receives heat from the electronics to be cooled based on the boundary conditions applied locally and the wall conducts heat axially through a 1-D conduction scheme in addition to the two-phase pulsating mechanism. The present code can be classified as an engineering tool, being pragmatic and thus achieving accurate predictions with reasonably low computational times, as opposed to a research tool. More specifically, the kinetics of the flow are solved exclusively on the liquid slugs as the dynamics of the vapor and liquid film were assumed negligible, except for their changing lengths. The code also includes mechanisms related to bubble creation (new vapor plug formation by nucleation)and collapse (when two liquid slugs coalesce due to the vapor plug in between collapses).