Thermo-hydrodynamics of phase-change induced oscillating Taylor bubble flows
Résumé
This paper presents a synthesis of experimental results and theoretical analyses obtained in the frame of a joint collaborative research program between France in India, devoted to understanding of Taylor bubble/slug flows in mini channel geometries, under diabatic/adiabatic, oscillating/non oscillating flow conditions. This type of flow is especially encountered inside Pulsating Heat Pipes (PHP), in which the self-sustained thermally driven oscillating flow is generated by evaporation/condensation mass exchange process. Specialized and dedicated range of experiments was conducted using High Speed Videography (HSV), Particle Image Velocimetry (PIV) and Infra-Red Thermography (IRT). New understanding of the local thermo-hydrodynamics of Taylor bubbles and slugs was obtained owing to a specific experimental test bench in which the working fluid and the operating temperatures could be chosen by the user. The experimental results clearly showed that evaporation of the thin film is responsible for the oscillation patterns that are observed. In addition, experimental results obtained using a mechanical oscillation system in isothermal conditions show that classical correlations used to calculate pressure drops inside the liquid slug are quite satisfactory, in the case of this particular oscillating flow. PIV and IRT for non-boiling two-phase systems, both in adiabatic/diabatic conditions and oscillating/non-oscillating flows were also undertaken. Evaporation of the laid down liquid thin-film during motion of an isolated partially wetting liquid plug revealed that the hydrodynamic considerations of Taylor's law for predicting thin-film thickness is well able to predict the evaporative mass flux under diabatic conditions. Various flow aspects of liquid plugs were delineated. It is believed that such fundamental studies will provide the necessary framework for 'unit-cell' modelling approach of pulsating heat pipes.