Computational study on break-up mechanisms of isolated vapour slugs during saturated flow boiling conditions

a new generation of heat dissipation systems, based on the local phasechange of a working fluid. Efficient thermal control, especially in space applications, and the need to reduce mechanical elements (e.g. pumps, fans), has become of crucial importance. Two-phase closedloop wickless heat pipes, such as Thermosyphons (TS) or Pulsating Heat Pipes (PHP), can meet such requirements. In the present investigation, parametric numerical simulations are performed by means of an enhanced and validated Volume Of Fluid (VOF) based numerical simulation framework, that accounts for reduction of spurious currents and phase-change due to boiling/condensation, aiming to identify and quantify bubble break-up regimes in capillary two-phase flows. In this way, the breakup phenomena observed during microgravity experiments in mini-channel branches of a Hybrid Heat Pipe, can be further explained. Then different series of parametric numerical experiments of isolated vapour slugs within a mini-channel are performed, investigating the effect of various controlling parameters, on the vapour slug dynamics. A certain imposed pressure difference between the inlet and the outlet and a specific heat flux at the heated wall are used in each case. For the base case, the working fluid properties were selected as FC-72 (Perfluorohexane), vapour and liquid at their thermodynamic equilibrium point at 1 bar pressure, since this fluid was used in the parabolic flight campaign experiments. The overall simulation results reveal three prevailing break-up regimes: a “full break-up”, a “partial break-up” and a “no break-up” regime. It is characteristic that in all cases of “full break-up”, a liquid jet penetrates the vapour slug at its trailing edge, leading to the entrainment of a liquid droplet, which is breaking the liquid film between the vapour bubble and the wall of the channel, leading to the so called “full” break-up of the slug, into two subsequent bubbles. Hence, the peculiar phenomena of vapour slug break-up observed in the microgravity experiments are finally described and categorised according to the global hydrodynamic flow conditions as they are mapped using the parametric numerical simulations. Interestingly, the value of the applied heat flux, does not seem to influence the resulting break-up regime and its main characteristics.