Numerical study on the motion and morphology of a Taylor bubble in a vertical capillary tube under oscillatory flow and vibration conditions

Oscillatory slug flow is the core flow pattern in heat transfer devices represented by pulsating heat pipes. These devices are inevitably exposed to vibrational environments in practical applications. To improve the robustness of pulsating heat pipes, the comprehensive effects of external vibrations on the hydrodynamics of oscillatory slug flow need to be examined. A single slug unit comprising a Taylor bubble and its adjacent liquid slug within a 2 mm vertical capillary was studied. The displacement, velocity, acceleration, expansion ratio, specific perimeter, length, and liquid film thickness of the bubble were numerically investigated. The results showed that external vibrations enhanced the bubble’s ascent in oscillatory flow. Fast Fourier transform analysis of the bubble velocity and acceleration indicated that vibrations introduced harmonic components to the bubble’s motion. In addition, vibrations caused periodic fluctuations in the size and morphology of the bubble. Vibrations also led to dispersed statistical distributions of bubble length and liquid film thickness. Further analysis demonstrated that the vibrations have different influence mechanisms in different regions. In the mainstream region, the vibrational energy overcame the fluid inertial effect and mainly caused fluctuations in the motion of the bubble. Near the inner wall, vibrations affected the velocity gradient distribution within the liquid film, leading to periodic changes in the wall viscous shear stress. These changes in viscous shear stress caused the vortices within the liquid film to be alternately strengthened and weakened, ultimately leadin g to the periodic variation of the bubble morphology.