The concurrent flow of gas/liquid in pipes poses a great challenge due to the difficulty associated with the flow of fluid. The flow is characterized by the existence of flow regimes which can be identified by the geometrical arrangement of the phases in a pipe, with Churn flow being the least understood flow pattern in vertical pipes because of the controversies associated with its existence, therefore making it difficult to be predicted. This work aims at investigating the behavior of air–silicone oil flows in vertical and horizontal pipes for effective gas–liquid transportation. To help predict the various flows that exist in these pipes, a drift-flux model was developed for the efficient calculation of void fraction. This model is often used to characterize and predict flow regimes for lots of geometries. The model was developed to calculate void fraction for the accurate prediction of flow regimes that were observed in this work. The various flow patterns in existence were identified, and the model generated for each of them by employing Zuber and Findlay‟s correlation. Afterwards, the parameters obtained from the drift-flux model, Co (distribution parameter) and Vd (drift velocity) were fitted as linear functions, and their values were obtained from the slope and the intercept respectively. The developed model had better results for the void fraction as compared to the existing correlations investigated.
CHAPTER 11.1 Introduction
The simultaneous flow of fluids is difficult in a safe and controlled way, with the exception that various behaviors of the flow can be predicted with adequate reliability. It deals with the concurrent flow of fluids within different phases (i.e. gas, liquid and solid) or the different chemical properties but in the same phase, for example gas-liquid, gas-solid, liquid-solid, liquid-liquid and gas-liquid-solid (Abdulkadir, 2015). Multiphase flows are encountered in industries like; the petroleum, chemical, and nuclear industries. The transportation of gas- liquid two-phase flow in the petroleum industry over long distance is quite common. This simultaneous flow is encountered in instances like the flow of oil from the reservoir to the separator, and to the process facilities. As pressure decreases, gas starts to evolve, thereby creating a two-phase flow in the pipeline. Various difficulties are encountered in the flow of these fluids, some of which are phase velocity differences and the existence of several flow regimes. These flow regimes include; bubbly, slug, churn, plug, and stratified, among many others. The existence of these flow regimes in transportation lines poses certain challenges to the industry because they increase the pressure drop, heat transfer, mass and corrosion rate in the pipeline. Since the accurate prediction of these flow patterns is essential to the success of designing multiphase flow systems in vertical and horizontal flows, there is therefore, need to investigate the behavior of the fluids in pipes of various inclinations, for effective transportation in the industry.
1.2 Critics of Churn Flow One may wish to know the reason for this particular subheading, the critics of churn flow. The topic is discussed because of the various schools of thoughts and ideologies from different research about the existence of churn flow regime. The question is, does Churn flow exist as a distinctive flow pattern or, it is just an extension of slug flow? To address this, this work is yet to find out as Mao and Dukler, and Hewitt and Jayanti presented different ideas about the existence of this particular flow.
According to Mao and Dukler (1993) in their paper, “The Myth of Churn Flow?” they presented evidence that proved that churn flow pattern is a simple and continuous extension of the condition of slug flow and that no transition actually existed. Therefore it is not a distinctive and separate flow pattern on its own. In view of this, they presented two different pieces of evidence to buttress their point. These are visual evidence and instrumental evidence where experiments were performed to support their findings.
1.2.1 Visual Evidence This evidence is no different from what other researchers observed in transparent pipes. Based on this, one can conclude that their observations proved the existence of churn flow as a unique and separate pattern that exists as a transition from, slug flow to annular flow.
According to Mao and Dukler, stable slug flow is an upward motion of a quasi-periodic arrangement of alternating Taylor bubbles and liquid slugs at a constant speed of propagation, and that the length of the Taylor bubbles and the liquid slugs remain the same as they rise. The velocity associated with the bubbles and liquid slugs is uniformly upward and the same as that which exists in front of both the slug and bubble. As the gas rate increases, the flow becomes chaotic, and the size of the liquid slug and Taylor bubbles increase forming lumps of bubbles as they move up and down the pipe. The flow then becomes oscillatory and displays irregular periods. Again, the main characteristic of slug flow is the falling of the liquid film around the Taylor bubbles and it disappears. All these observations show characteristic features of churn flow, thereby proving that it exists as a unique flow pattern……