Interfacial behaviour in stratified and stratifying annular flows

This thesis describes work which was focused on stratifying annular flow in horizontal tubes. Stratifying-annular flows in horizontal tubes are characterized by the tube being wetted with liquid over its whole periphery but with a tendency for more of the liquid to be present in the layer at the bottom of the tube. For such a condition to exist, there have to be some mechanisms by which the liquid is transported at the top of the pipe in opposition to the influence of gravity. Such flows are typically experienced in hydrocarbon recovery and in many other applications as for instance in gas-condensate lines. Unless the liquid phase (to which a corrosion inhibitor is often added) can adequately wet the top of the pipe, corrosion and ultimate pipe failure may occur in this region; this is a crucial problem for the petroleum industry. Three physical mechanisms have been identified for the transport of the liquid phase to the top of the tube, namely: droplet transport, wave spreading and mixing, and secondary flow. The secondary flow mechanism seems unlikely to contribute significantly and the wave spreading mechanism is only significant in smaller diameter tubes (typically 25 mm). For larger pipes, it is the droplet transport mechanism which is likely to occur; in this mechanism, droplets are entrained from the liquid layer at the bottom of the pipe and transported in the gas core at the top where they deposit to form a liquid film. Two mechanisms of droplet transport seem to be significant, namely ballistic transport in which larger droplets move in a direction governed by their initial release velocity and diffusional transport in which droplets move randomly under the influence of the gas flow turbulence. For pipes of medium diameter (for example in the 78 mm diameter pipe used in the experiments described here), ballistic droplet transport is likely to be the dominant mechanism but diffusional transport is expected to be dominant for large diameter pipes. The work described in the thesis comprised both experimental and computational studies. In the experimental work, a special visualization method (namely axial view photography) was employed to study the droplet entrainment and transport mechanisms. The distribution of liquid film flow rate around the pipe was determined using a film extractor device. The axial viewing system was used to investigate droplet entrainment in stratifying-annular air-water flows in a 0.078 m diameter horizontal pipe. The process of droplet entrainment was captured using a high speed cine camera. Both ligament and bag breakup mechanisms leading to droplet entrainment were identified. The creation of a ballistic droplet from a ligament was clearly observed. The film flow rate measurements were used in conjunction with previous measurements of droplet mass flux in the core in comparisons with a computational model which aimed to predict the transport of droplets using Reynolds Averaged Navier Stokes (RANS) modelling embodied in a commercial CFD code (CFX). In this model, the turbulent gas core was modelled and the motion of droplets emitted from the liquid layer at the bottom of the pipe was tracked in this turbulent field. Ultimately, the droplets are deposited on the tube wall and the film flow rate may be calculated using the predicted deposition rates as input data. The thesis closes with a description of a numerical experiment aimed at investigating the influence of the turbulence model on droplet transport. Specifically, comparisons were made between RANS and Large Eddy Simulation (LES) models. [For supplementary files please contact author]