Spectral density analysis of the interface in stratified oil-water flows

In this work the wavy interface of stratified oil-water flows was investigated using wire conductance probes. The experiments were carried out in a 38 mm ID acrylic pipe using water and oil (Exxsol D140 oil: ρo = 830 kg m−3, μo = 0.0055 kg m−1 s−1) as test fluids. High-speed imaging revealed that almost two-dimensional interfacial waves develop at the inlet junction for input oil-to-water flow rate ratios different from one. Downstream the inlet section, however, the interface has a complex three dimensional structure with very small amplitude contributions. The structure of such interfaces can be properly investigated from the power spectrum of the conductance probe signal. A rigorous and detailed methodology is presented for estimating the power spectrum of the interface signal that is based on the Wiener-Khinchine theorem and makes extensive use of a Fast Fourier Transform (FFT) algorithm. Interface spectra were studied at two locations, close to the inlet of the test section and at 7 m downstream. The results showed that the waves at the inlet have a unique peak frequency of about 19 Hz and that, at the downstream location, this frequency is still present but has a smaller significance compared to that caused by the mechanical vibrations of the set up. This frequency was independent of the flow rates and could be a characteristic of the pair of the test fluids used rather than of the flow.