A numerical study of periodic slug flow at zero gravity and normal gravity

This paper reports numerical simulations of slug flow at zero and normal gravity. The particular experimental results chosen for validation were obtained at microgravity under conditions which resulted in evenly-spaced and evenly-sized Taylor bubbles facilitating a simulation with periodic boundary conditions. The numerical technique was a free-surface method which explicitly tracked the motion of the gas-liquid interface using a volume-of-fluid specification and a finite volume discretisation of the solution domain. The large scale features of the bubble such as the classic bullet-shaped nose were well predicted by the model. Unsteady features of the bubble shape such as waves in the film and fluctuations of the bottom surface were also predicted but are harder to compare quantitatively to the experiments. The velocity field predictions reveal several interesting features of the flow. When viewed by an observer moving with the bubbles, the liquid slug is dominated by a large recirculating region with the flow travelling from the leading to the trailing bubble along the tube centreline. In this frame of reference, the near-wall region features a jet of fluid issuing from the film of the leading bubble which entrains fluid in the slug. As the film of the trailing bubble begins to form, the entrained fluid must be ejected since the flowrate in the film of each bubble must be the same. It appears to be this process that drives the main recirculation.