Experimental investigation of transitional oil-water pipe flow

Abstract

In this thesis transitional oil-water pipe flow is experimentally studied. Here the word transitional relates to two main topics. First, the study focuses on the investigation of transitional flow patterns and resultant flow phenomena which neither are well described by stratified flow nor by homogeneously dispersed flow. Second, flow development, which can be of extensive length for oil-water flow, is investigated with help of consecutive measurement devices arranged along the test section. The experiments were conducted in two different multiphase flow laboratories. Tap water and different mineral oils with viscosities up to 120mPa*s were used as test fluids. The well flow loop at the Institute for Energy Technology (IFE) in Kjeller, Norway, provides a transparent 25m test section with inner diameter D = 100mm, which is equipped with advanced technology for flow visualization. Gamma densitometry and X-ray tomography were used to obtain detailed measurements of local phase fractions and cross-sectional phase fraction distributions. Three FBRM-probes were installed to investigate droplet size evolution. A static inlet mixer was installed to disturb the flow and enable investigating development of premixed flow. The Multiphase Flow Laboratory at the Norwegian University of Science and Technology (NTNU) provides a transparent test section which is easy to modify. A 50m long modification with a simple ball valve installed as adjustable inlet mixer was used to investigate flow development in terms of changing flow patterns and pressure gradients. Onset of dispersion at considerably lower mixture velocities compared to other studies without inlet mixing was found. Settling and inflow separation downstream of the mixing devices was observed. The flow development was further measured in terms of changing droplet sizes and pressure gradients. A rather dense packed droplet layer in the upper part of the pipe was characteristic for higher input water fractions. The occurrence of the dense packed layer always goes along with a significant increase of the pressure gradient. A simple model for predicting the pressure gradient in dense packed layer flow was proposed. The model considers the dense packed layer as independent phase with its own mixture properties. Model predictions are in good agreement with the measurements while the twofluid model for stratified flow and the homogeneous flow model fail. Furthermore, a tool for the prediction of flow development and development lengths downstream of a mixing device was developed based on simplified settling theory. Applying the tool together with the pressure gradient model allowed for qualitatively reproducing the observed flow development. Locally measured pressure gradient values along the test section could be reproduced with good agreement for low mixture velocities. For higher mixture velocities too fast separation was predicted, as the model does not consider turbulent mixing and droplet break-up