An experimental study of interfacial dynamics during stratified oil-water draining from storage tank

Abstract

Oil-water stratification is an inevitable phenomenon in the petroleum industry. The present study describes stratified oil-water draining from a square tank through a 9 mm diameter drain pipe. The purpose of this study is to understand the flow physics of oil as well as air entrainment in drain pipe during drainage of water. In order to observe the influence of the viscosity of oil on the entrainment and flow structure, two different oil-water stratifications are selected. In these two cases, the viscosity ratios of oil to water are 1.64 and 55, respectively. While draining, the flow of single-phase liquid in the drainpipe is transformed to two-phase liquid-liquid flow and then three-phase flow. A pointed tip of oil is formed as the oil-water interface approaches the drainpipe inlet. Different flow patterns, namely bubbly flow, slug flow, churn flow, elongated slug flow, jet flow, and annular flow, are observed as air and oil enter the drain pipe. Reverse flow is observed in the case of a higher viscosity ratio. An empirical correlation based on non-dimensional numbers has been proposed for describing the condition of no air entrainment in the drain pipe. Optical probes are used successfully to detect the presence of several regimes of air-oil-water and oil-water in the drainpipe.