(549a) On the Frequency of Large Waves in Vertical Gas-Liquid Annular Flows

A flow of gas in the core of a vertical pipe and of liquid in the annulus between the pipe wall and the gas phase is referred to as an annular flow. Annular flows can be found in both upwards (when both phases travel against gravity) and downwards pipe orientations. Both upwards and downwards annular flows are of high industrial importance; they can be found in the oil-and-gas industry (e.g. raisers, transfer pipelines and gas-liquid oil wells) or various industrial units, such as condensers, evaporators, distillation columns and chemical reactors.

One of the most important features of annular flow is the presence of large waves (often referred to as disturbance waves in the literature). These do not only carry a high fraction of the liquid phase being transported, but are also considered crucial for the process of liquid entrainment into the gas phase. These waves are usually characterised in terms of their amplitude (relative to mean film thickness), specific wave length (which is the distance between two successive crossings of the mean film thickness) and interfacial frequency.

In this paper results will be presented from an extensive effort to characterise the statistics of the interfacial frequency of these large waves, in both upwards and downwards annular flows using conductivity-probe and Laser-Induced Fluorescence (LIF) techniques. The two different measurement methods are used for the quantification of the interfacial wave frequency: (i) from direct counting of the waves having a thickness of 2-5x the mean film thickness; and (ii) by carrying out a power spectral density analysis from which the frequency at the maximum power is identified. Importantly, our results indicate that the large amplitude waves are best identified by direct wave counting, as opposed to the analysis of the power spectral density that, traditionally in the literature, is the most widely accepted method for this purpose.