The decay of swirl in horizontal dispersed oil-water flow

This paper reports experimental work to investigate the characteristics of swirl in dispersed liquid-liquid flow in a horizontal pipe. In this work the focus was on determining the pressure gradient in a horizontal 25.4 mm (nominal one-inch) tube and its spatial evolution in swirling liquid-liquid flow. Experimental results are presented for Kerosene (EXXOL D80) and tap water at room temperature. The swirl was introduced by Kenic helical static mixer elements at the pipe inlet. The pressure gradient was measured for different numbers of helical elements at three different positions (near the beginning, middle and end of the test section) for single-phase flows of water and oil and also for dispersed flows of oil and water. A cross-shaped de-swirl device was used to distinguish effects due to swirl from those due to the additional mixing and turbulence induced by the mixer elements. A gamma densitometer system was applied for measuring the chordal volume fraction distribution across the tube and to obtain tomographic results for phase distribution across the tube both with and without helical elements. It was concluded that the introduction of helical elements at the tube inlet has a considerable influence on the flow behaviour and produces a swirl flow in the pipe whose intensity decays with distance from the inlet. The rate of decay depends on the type of dispersion (oil or water continuous). The influence of mixing phenomena on pressure gradient is manifested in several ways, including a sharpening of the pressure gradient peak in dispersed flow and the occurrence of a secondary pressure gradient maximum over a range of conditions. Furthermore, the results obtained with the gamma densitometer indicate that the helical element is an efficient means of mixing. It can generate a mixing intensity equivalent to a turbulent flow at higher mixture velocity.