Counter Current Flow Phenomenon & Pressure Drop Calculation in Annular Geometry.

Abstract

Liquid films falling under the influence of gravity are widely encountered in a variety of industrial two-phase flow applications (distillation columns, nuclear reactor cores, wetted walls and packed towers, heat pipes, vertical condensers etc.). The falling annular film represents a fundamental limiting case of the annular flow regime of two-phase gas-liquid flows. The maximum flow rates of gas and liquid phases which flow in opposite-directions (counter-current flow) are limited by a phenomenon known as a Counter-Current Flow Limitation (CCFL or flooding). In other words, flooding phenomenon is defined as the transition of part of liquid to a climbing film on increasing gas velocity.
The calculation of pressure drop in nuclear fuel bundles across its different components is very complex and is not discussed vastly in the literature. In general, spacers of various configurations like wire wrapped, honey comb, grid type are used in the fuel rod bundles to provide support to the fuel pins as well as facilitates in effective cooling of the fuel pins, but the pressure drop across it is appreciable.
This dissertation investigates the film thickness model in gas liquid two phase annular flow as well as pressure drop estimation in annular geometry such as fuel bundles and spacers. A mathematical model for the estimation of film thickness has been derived by the application of fundamental momentum equation and a numerical iterative technique with programming in MatLab has been adopted to estimate the film thickness in free falling film, a limiting case of gas liquid annular flow. An experimental test facility has been proposed to study and visualize the gas liquid interactions, flooding phenomenon and measurement of film thickness.
Spacer loss coefficient and subsequently, pressure drop in D5 fuel cluster have been estimated with the help of developed correlations based on flow area ratio. As this flow area ratio increases the spacer loss coefficient decrease & vice-versa for a given length of the spacer, but it increases with increase in the spacer length.
Pressure drop across spacer is noticeable and amounts to an appreciable percentage of the total pressure drop in the fuel cluster.