Wave behaviour in vertical multiphase flow
Creators: name: Zhao, Yujie
Publisher: name: Imperial College London
Year: 2014

description: The work described in this thesis was aimed at developing the understanding of two regimes in vertical gas-liquid flow in tubes, namely annular flow and churn flow. In annular flow, there is a continuous gas passage at the centre of the pipe with a film of liquid travelling upwards at the wall. Part of the liquid phase in annular flow may be entrained as droplets in the core gas flow. In churn flow there is also a gas core (which the present work has shown to be continuous) and a liquid film; however, the flow direction of the liquid in this film varies with time. Thus, the liquid flows upwards in large waves on the surface of the film; between the waves, the film may change direction and flow downwards towards the next wave. Such flows are extremely complex and there are aspects of their behaviour which are poorly understood. In the work described in this thesis, several areas have been studied. Disturbance waves are of central importance in annular flows. Such waves are characterised by their large amplitudes relative to the mean film thickness, their high translation velocities relative to the mean film speed, and their circumferential coherence (i.e. their “ring-like” structure when fully developed). An important part of the present work was concerned with the existence, development and translation of disturbance waves in upwards, gas-liquid annular flows. At very low liquid flow rates, disturbance waves are not formed (and, as other work has shown, the entrainment of droplets from the liquid film is negligible). In the present work, multiple conductance probe units have been employed to study the growth and development of disturbance waves. From the results, it is found that disturbance waves begin to appear and to start to achieve their circumferential coherence from lengths as short as 5 –10 pipe diameters downstream of the liquid injection location; this coherence gradually strengthens with increasing distance from the inlet. It is further shown that the spectral content of the entire interfacial wave activity shifts to lower frequencies with increasing axial 3 distance from the inlet, with the peak frequency levelling off after approximately 20 pipe diameters. Interestingly, on the other hand, the frequency of occurrence of the disturbance waves first increases away from the inlet as these waves form, reaches a maximum at a length between 7.5 and 15 pipe diameters depending on the flow conditions, and then decreases again. This trend becomes increasingly evident at higher gas and/or liquid flow-rates. Both wave frequency measures increase monotonically at higher gas and/or liquid flowrates. Important evidence regarding the mechanisms of disturbance waves and the associated droplet entrainment can be obtained by the axial view photography technique. This technique is described in Chapters 3 and 6. The technique was used to visualise the wave characteristics, in particular of the two entrainment mechanisms (bag break up and ligament break up mechanisms) proposed previously by Azzopardi (1983). The axial view photography technique provided visual evidence for the existence of the two mechanisms, although in contrast to Azzopardi’s findings, both break up mechanisms were observed to occur simultaneously. The axial view photography technique was also used in the present work to provide further insights into the inception of disturbance waves. It was found that the initiation mechanism for disturbance waves was the occurrence of a disturbance at a given location around the tube periphery. This is consistent with the idea of a link between turbulent burst phenomena and disturbance waves first proposed by Azzopardi and Martin (1986). The initial disturbance links up with similar disturbances to ultimately form the characteristic ring-like structure characteristic of fully developed disturbance waves. In churn flow the present work concentrated on three aspects: The use of axial view photography to explore the continuity of the gas core in churn flow. The development (in collaboration with two other research students –Deng Peng and Masroor Ahmad) of a correlation for entrainment rate and hence entrained fraction in churn flow. Measurements of 4 pressure gradient and holdup in churn flow, from which an average wall shear stress can be deduced. In the first task, it was shown (it is believed for the first time) that there is a continuous path for the gas phase near the tube axis. In churn flow the behaviour of entrained fraction is extremely complex and conventional methods for measuring it are no longer valid. Barbosa etal (2002) studied entrainment in churn flow using iso-kinetic sampling probes and the correlation referred to above was based on this data. The correlation has been widely used in predicting the entrained fraction at the transition between churn and annular flow. Since the direction of flow of the liquid film near the channel wall fluctuates, it is difficult to estimate the instantaneous value of wall shear stress. However, if measurements are made of total pressure gradient and liquid holdup, then the mean value of wall shear stress can be estimated. This procedure was pursued by Govan (1990) who used mechanically operated quick-closing valves to measure holdup. In the current work, a new measurement technique was utilised, namely quick closing pinch valve which offer a great accuracy and are easy to install. Pressure gradient and hold up data were collected over a wide range of gas and liquid flowrate. An averaged wall shear stress was then calculated based on these measurements. At higher liquid mass flow rates, the results were in good qualitative agreement with those of Govan (1990) but (at lower mass fluxes) anomalies occur which need further investigation.