Van der Meulen, Gerrit Pieter
(2012)
Churn-annular gas-liquid flows in large diameter vertical pipes.
PhD thesis, University of Nottingham.
Abstract
This thesis presents an investigation on the churn to annular flow pattern boundary in an 11 m tall, 127 mm id vertical riser. Experimental data on film thickness, pressure drop and drop size and velocity was analysed and interpreted. Entrained fraction, interfacial and wall shear stresses and the interfacial friction were calculated from the experimental data.
A new churn-annular flow transition boundary was derived based on trends, and in particular changes of slopes, observed in film thickness, pressure drop and structure velocity data. This is supported by observations made using high speed photography. Minima in slopes are found in plots of film thickness and pressure gradient with gas superficial velocity at low liquid flow rates. These minima are however not clearly visible at higher liquid flow rates in the data obtained.
Dimensional analysis of this transition boundary and those proposed by other workers, using Froude and Weber numbers, resulted in a closer agreement between transition boundaries then was achieved till present. The correlation found, which describes the boundary, performs well for different pipe diameters, fluid properties and experimental flow conditions.
It was observed that existing models for the calculation of interfacial and wall shear stresses, including the interfacial friction factor, do not perform well in churn type flows. Their performance and agreement with the present data at high gas flow rates, thus annular flow, was however better. This indicated that these models do not take some of the characteristics of the flow into account, e.g., gas core density. It was found that the latter parameter plays an important role in churn flow since the gas core density increases steeply with decreasing gas flow rate. New relationships for these parameters are suggested for a more accurate prediction in large diameter pipes.
The diameter, velocity, and entrained fraction of drops show similar trends to that of the liquid film thickness and pressure drop. The velocity and the entrained fractions show most profound information. The entrained fraction increases in churn flow with gas flow rate. It then shows a steep decrease in a transitional area. In this area it may be that the entrained fraction is more contained in large waves and wisps than in drops. At higher gas flow rates, the entrained fraction increases again, as is well reported by other researchers. Here the breakup and atomisation of large waves and wisps play an important role.
From comparison between drop fractions deposited by diffusion and direct impaction in the CFD and experimental results, there is evidence that in large diameter pipes a third deposition mechanism applies: Transitional deposition. Analysis shows that transitional impaction occurs at medium sized drops at medium gas flow rates. Around these conditions, large waves are present in the flow as described above. The third deposition mechanism probably occurs when the majority of entrained liquid is carried in large waves and wisps as reported above. Therefore, at the transition from churn to annular flow in large diameter vertical pipes, the behaviour of the flow is not typical to that observed in smaller diameter pipes.
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