Vasques, João
(2018)
Investigation of interfacial gas-liquid flows using the brightness based laser induced fluorescence technique.
PhD thesis, University of Nottingham.
Abstract
In multiphase flows, certain flow regimes are characterized by the presence of waves that propagate along the interface between both phases, and they are hence known as interfacial flows. This thesis reports on the investigation of interfacial structures present in the slug and annular flow regimes in vertical gas-liquid flows. In order to achieve this, results were obtained using the Brightness Based Laser Induced Fluorescence (BBLIF) technique, which is a powerful visual diagnostic tool that allows for highly detailed spatial and temporal measurements of film flows over both the axial and longitudinal coordinates. The BBLIF was therefore employed in two different experimental campaigns.
In the first campaign, the dynamics of the falling film surrounding a Taylor bubble in slug flow conditions was investigated, in order to study its influence on the gas exchange process between the Taylor bubble and the wave region. This was achieved by employing a specially designed apparatus that held a Taylor bubble in a fixed position while simulating a downwards flow of liquid. It was observed that increases in the liquid flowrates lead to increases in the mean film thickness of the falling film, and on the relative amplitude, velocity and frequency of the waves present in it.
It was also shown that the wave behaviour is the driving force behind the entrainment events, as it was observed that the turbulence generated by the incoming waves led to perturbations in the skirt of the Taylor bubble, leading to gas entrainment. Two new mechanisms of entrainment have been identified and characterized for the first time. They consist of (i) entrainment through crater generation and (ii) entrainment through self enhancement of entrainment by action of re-coalescing bubbles, with both occurring simultaneously in slug flow conditions. It was concluded that the action of re-coalescing bubbles has been overly disregarded in terms of its impact on the entrainment behaviour.
In the second experimental campaign, the effect of flow orientation (in vertical flows) on the dynamics of the annular flow regime was investigated. This was conducted by obtaining measurements in upwards and downwards flows at two different axial locations. Close to the inlet, the mechanism of disturbance wave formation from the coalescence of small, high frequency wavelets has been observed and seen to be unaffected by the flow orientation at this stage. At 330 – 430 mm, where the flow is almost fully developed, it was observed that, with the exception of the lowest liquid Reynolds number, the frequency of the disturbance waves is higher for the downwards orientation, although they travel at the same velocity in both cases. Despite the higher number of waves present in downwards flow, the wave width is the same for both flow orientations, being equally reduced with increasing gas superficial velocity.
The behaviour of the ripple waves that cover the base film was also analysed. The analysis showed that there is an increase in the ratio between the ripple and disturbance wave velocities with increasing gas flowrate for the upwards case, whereas for the downwards case, the ratio is constant under the same conditions. This difference was linked to the differences in the base film thickness in both flow orientations due to gravitational action, being higher for the upwards case.
Lastly, the prediction of the disturbance wave velocity and frequency, together with the mean film thickness in the annular flow regime in the entrainment region was analysed. It was observed that existing models in the open literature for the prediction of these parameters do not perform well in the estimation of the data from the current experimental work. This was attributed to the fact that all the correlations reviewed were empirically determined from the particular set of results from which they were derived. In this thesis simplified monotonic relationships were established based on universal rules applicable to the annular flow regime. The major contribution of this work to the multiphase flow field is that these universal correlations have been validated for a large number of air-water systems and benchmarked against a multitude of systems that vary in terms of their duct properties (size, circular or rectangular geometry), flow orientation (vertically upwards and downwards, or horizontal), or flow conditions. This set of rules is of significant importance, since it allows for increased predictive accuracies during flow assurance design processes in multiphase flow systems.
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