Numerical investigation of slug flow in pulsating heat pipes using an interface capturing approach

Abstract

The thermal performance of pulsating heat pipes is based on a self-sustained fluid motion in presence of phase change phenomena. To study this complex coupling of pressure and temperature, the volume-of-fluid method is a widely used approach to capture the phase distribution and interface dynamics, while the Lee model is employed to account for phase change. However, no consensus can be found in literature regarding the film resolution requirements, turbulence or an optimal choice of the mass transfer intensity parameter. In this study, the influence of these factors on pulsating heat pipe simulations is systematically investigated. A closed interface-tracking CFD-VOF method is presented, whereby phase change is only assumed to occur in computational cells within the phase boundaries. First, the model is successfully validated on the one-dimensional analytic sucking interface problem to proove the preservation of supersaturated states and the correct calculation of momentum and energy sources due to phase changes. Subsequently, as a basic reference for the flow in a pulsating heat pipe, the impact of model parameters on a Taylor bubble expanding due to evaporation within a micro-channel is investigated. It has been observed that both, the near wall film discretization and choice of the mass transfer intensity factor are significantly influencing the transferred latent heat and thus, the expansion velocity of the bubble. Finally, the model is extended to account for flow phenomena in an experimental two-turn pulsating heat pipe. The simulation results revealed that it is crucial to adjust the mass transfer intensity factor of condensation in order to achieve viable pressure levels within the closed system. The proposed model was demonstrated to be easily implemented, numerically efficient, and capable of performing in depth two-phase studies.