Two-phase annular flow

Abstract

This dissertation is concerned primarily with the problem of predicting the hold-up and pressure drop in annular flow when only the gas and liquid flowrates, the physical properties of the fluids and the dimensions of the system are known. A model of annular flow is presented, and the validity of the model is tested.As a preliminary to the development of the model, the flow is considered in three separate regions: • Flow in the liquid film is considered and the various velocity profiles used by previous workers in the field are discussed. A rigorous analysis of a film in laminar flow is presented.• Consideration of the interfacial phenomena is of particular importance since the high pressure gradients encountered in annular flow indicate intense interfacial activity. These phenomena are considered in two distinct systems; those associated with in wetted-wall columns and those associated with annular - flow. • The flow of gas in the central core is then considered. Methods by which the gas can the forces required to support the liquid film are discussed.A model, which assumes that the phenomena in counter-current flow has an analogy in co-current annular flow is presented. In the model, the operating conditions in annular flow are determined from the intersection of a curve (based on the force requirements of the liquid film) and a supply curve (based on the forces the gas core is able to supply). It is shown that a rapid increase in the interfacial activity (flooding) will result in stable intersections of the and demand curves for the co-current system. The point at which the rapid increase in interfacial activity occurs is called the of Interaction. It is suggested that the Point of Interaction occurs at the film thickness at which, for a given liquid and gas flow, the relative velocity between the gas and the liquid is equal to the relative velocity required to cause of a film of the same thickness in a tube of the same diameter. By using the theory of Jameson and Cetinbudaklar to predict the points, the point of interaction can be determined theoretically. The operating conditions in annular flow can thus be predicted from theoretical considerations only, and this is, we believe, the first model of annular flow which can be used to calculate the hold-up and pressure drop when only the fluid flowrates, their physical properties and the system dimensions are known, without relying upon a basic empirical relationship. Predictions of the model are tested by comparison with experimental results in the annular flow regime reported by previous workers and obtained as part of this work. The accuracy of the predictions is reasonable but a better understanding of the flooding phenomenon is required so that the direction of the curve after interaction can be determined exactly. The model is found to predict the correct tendencies under change of liquid and gas flowrates and to be applicable to co-current downwards annular flow and to counter-current annular flow. To be useful, the model must provide a means of determining whether annular flow does develop at the flowrates under consideration. Methods of determining the upper and lower limits of the annular flow regime are suggested. Experimental results in the annular flow regime and in the regime after in wetted-wall columns are obtained and discussed in terms of the model. Methods of increasing the overall accuracy of the predictions are discussed. These include determination of the true profile in the liquid film; allowing for the flow of liquid in waves and as droplets in the gas core; and obtaining a better understanding of the phenomenon so that the curve after can be determined accurately.