Simultaneous laser-induced fluorescence and capacitance probe measurement of downwards annular gas-liquid flows

Abstract This study focuses on the characterisation of downwards annular gas-liquid (air-water) flows, by employing a combination of advanced laser-based and capacitance-based measurement methods. A variant of laser-induced fluorescence (LIF), referred to as structured-planar laser-induced fluorescence (S-PLIF), eliminates biases commonly encountered during film-thickness measurements of gas-liquid flows, due to refraction and reflection of the light at the interface. A bespoke capacitance probe is also assembled to enable temporally resolved film-thickness measurements with high temporal resolution along the circumferential perimeter of the pipe. We compare the film mean thickness, roughness, and probability density functions obtained with each method. We find that both methods are able to measure time-averaged film thickness to within 20 % deviations from each other and from results obtained from the available literature. The resulting probe data suggest a biased (suppressed) standard deviation of the film thickness, which can be attributed to its working principle, i.e., measuring the film thickness averaged along the circumferential perimeter of the pipe. The auto-correlation functions of the time-traces provide an insight into the characteristic time-scales of the flows, which span a range from ∼ 10  ms for highly gas-sheared flows and increase to about 30 ms for the less turbulent falling films. The power spectral densities reveal modal frequencies that start from 2.5 Hz for falling films, and increase with the gas Reynolds number by almost an order of magnitude. The turbulent wave activity (slope in the power spectrum) reduces with a decrease in gas shear, and shows similarities to the decay of homogeneous and isotropic turbulence. The sizes of the bubbles entrained in the liquid film are measured from the S-PLIF images, and exhibit log-normal distribution that become flatter with a decrease in the gas Reynolds number. The normalised location of the bubbles (quantified as the relative entrainment depth, i.e., distance of the bubble from the wall over the local film thickness) follows a Gaussian distribution, with the peaks located between 0.5 and 0.6, indicating that the majority of the bubbles accumulate in the middle of the thin film.