Droplets in annular-dispersed gas-liquid pipe-flows

Abstract

Annular-dispersed gas-liquid pipe-flows are commonly encountered in many industrial applications, and have already been studied for many decades. However, due to the great complexity of this type of flow, there are still many phenomena that are poorly understood. The aim of this thesis is to shed more light on some of these processes involving the dispersed-phase of an annular-dispersed flow. One specific topic we investigated is the occurance of flooding, and the role of the dispersed-phase in this. The flooding phenomenon is related to the liquid-loading phenomenon that is of crucial importance in the exploitation of most gas-wells.Our approach has been to perform both experimental and computational studies, using some experimental results as an input for the simulations. With the experimental work the dispersed-phase in the core of an annular-dispersed air-water flow is studied using Phase Doppler Anemometry (PDA): the pipe has a diameter of 5 cm, and a length of about 240 pipe diameters. In a vertical-upward flow the influence of the droplets on the flow reversal phenomenon, which marks the onset to churn-annular flow, is investigated. It is shown that the dispersed-phase is not directly causing the flow reversal, since all detected droplets move cocurrent with the gas-flow. However, by affecting the film thickness distribution, it can influence it indirectly. The measurements also show that the gravity is negligible with respect to the strong axial acceleration of the individual droplets, making the inclination of the pipe to the horizontal irrelevant with respect to this. The statistics of the PDA-measurements have been used as an input for our simulations, mimicking the atomisation process of an actual annular-dispersed flow as realistically as possible. The computations are performed with a finite-volume in-house LES-code. Wall-functions are implemented to allow for rough walls, and to make high Reynolds-number computations feasible. The dispersed-phase is treated using point-particles that are individually tracked. Both mono-and poly-dispersions are used in the computations. The acceleration of a dispersed-phase in a high-velocity gas-flow seems to either act as added wall-roughness, increasing locally the turbulence intensity, or slow-down the mean gas-flow, decreasing locally the turbulence intensity. It is shown that particles in a small intermediate size-range have the largest overall acceleration, and hence are most effective in increasing the total pressure-gradient; this is also observed with the experimental results. The overall acceleration of a particle can be understood by considering the relevant time-scales involved: the particle relaxation-time, the particle residence-time, and the time-scale of the large-scale turbulence. In an actual horizontal annular-dispersed pipe-flow, in general, the liquid film at the bottom of the pipe is thicker, and thus the gas-liquid interface will most likely show a circumferential variation in waviness, i.e. a variation of the roughness. Also, the dispersed-phase concentration will be largest in the bottom region of the pipe due to the gravitational settling. Both the variation of wall-roughness and the non-homogeneous distribution of the dispersed-phase are shown to generate a secondary flow: a mean flow in the cross-section of the pipe, usually manifested as multiple counter-rotating cells. This secondary flow is shown to affect the circumferential variation of the deposition of the dispersed-phase, and may increase the concentration of the dispersed-phase in the core of the flow.