Interfacial characteristics of stratified liquid–liquid flows using a conductance probe

In this paper, the characteristics of the interface in stratified oil–water flows and their changes during the transition to dual continuous flows were studied experimentally with a double-wire conductance probe. Experiments were carried out in an acrylic test section, 38 mm ID, using tap water and oil (ρ = 830 kg m⁻³ and μ = 0.0055 kg m⁻¹ s⁻¹) as test fluids. The boundaries between stratified and dual continuous flow were identified from high-speed imaging. A double-wire conductance probe, consisting of two 0.5 mm wires set 2 mm apart along a vertical pipe diameter, was used to obtain time records of the interface height in stratified flow. The probe was located either close to the test section inlet or at 7 m downstream the inlet, where the flow was fully developed. Data were collected for a period of 4 min at 256 Hz sampling frequency. A rigorous methodology was followed to treat the probe data and to estimate average parameters such as interface height with known accuracy and confidence intervals. The analysis ensured repeatability of the results. The procedure allowed accurate estimations of the power spectra of the probe signal and revealed the characteristic frequencies of the interface in stratified flow. It was found that the transition from stratified to dual continuous flow delayed to higher mixture velocities at input oil-to-water flow rate ratios, r, close to 1. At 7 m from the inlet, where the flow is fully developed, the interface was found to be fluctuating with three-dimensional characteristics for all conditions studied, while the oil-to-water velocity ratios, calculated from interfacial heights, were close to 1. The power spectra of the probe data showed peaks at low frequencies (1–3 Hz) that were attributed to the pumps. A range of high frequency contributions (between 10 and 40 Hz) appeared as the mixture velocity increased, which reflect the fluctuating nature of the interface. The relative intensity of these contributions increased with mixture velocity, and close to the transition to dual continuous flow, it became larger than that of the low-frequency contributions from the pumps. In contrast, close to the pipe inlet, for flow rate ratios different than one, waves appeared. These, however, died out further downstream.