The structure of turbulent flow in vertical upwards annular air water two-phase flow was examined. Experiments were carried out in a 32 mm internal diameter tube using laser Doppler anemometry. Simultaneous measurements of the two velocity components and the Reynolds stress were obtained by the use of two colours (blue and green) of a 50 mW argon ion laser. The gas core was seeded by polystyrene particles of 1 um diameter which were believed to follow the gas turbulent fluctuations. The characteristics of the signal were used to discriminate these tracer particles from the water droplets. The gas velocity profiles were shown to be more peaked at the centre of the tube than those observed in turbulent single phase flow. Comparative analysis with other data suggested that both interfacial roughness and, particularly, the momentum interchange between the droplets and the gas core, are the most important factors affecting the gas velocity profile in annular flow. Turbulent fluctuations of the gas velocity were found to be significantly higher than those typical of single phase flow, for similar gas Reynolds numbers. The interfacial shear, droplet size and concentration and the presence of disturbance waves at the interface were identified as being the most important factors affecting the gas turbulence in annular flow. A model was developed to predict the axial component of the turbulent fluctuations at the centre of the tube. The turbulence transport properties were observed to differ from those typical of single phase flow: i.e., higher production of turbulent energy (associated with higher anisotropy ratios), higher turbulence length scales and comparativelly lower dissipation ratios. Extrapolation of the mixing length theory to annular flow appeared to be inappropriate. Droplet size measurements showed that the gas velocity and the droplet concentration are the most important parameters affecting droplet size. At low droplet concentrations (where the gas-droplet interaction is more important than that between the droplets), a modified Weber number based on the homogeneous gas core momentum describes the maximum droplet diameter. At high droplet concentrations, the data suggests that coalescence is the dominant factor. Droplet velocity was found to be related to the size of the droplets: i.e., large droplets travel slower than small ones. The difference in velocity between large and small droplets was found to depend on the liquid and gas flow rates. This observation is related to conditions where droplet coalescence occurs. The effect of inserts on droplet size and the entrained fraction was examined. Disturbances in the channel geometry were found to affect the mean droplet size due to the creation of a new droplet population. The entrained fraction of liquid downstream of the insert was also affected. A model was formulated to describe the liquid interchange in the presence of a vertical plate.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:570318 |
| Date | January 1988 |
| Creators | Teixeira, Jose Carlos Fernandes |
| Publisher | University of Birmingham |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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