Effect of the surfactant on complex multi-phase annular flows

In the modern world, the scale of industrial production within all sectors has reached unprecedented levels due to ever-growing demand and consumption of various products. A vast majority of industrial processes exploits the benefits brought by the multiphase flows whose complex dynamics are governed by the concoction of fundamental physics. Probing the details of such flows, experimentally and/or theoretically provides an ability to develop and optimise the needs of industrial applications. Yet, this progression is gradual as it depends on the advancement of measurement technologies that enable the investigation of the complex behaviour and topologies of many different possible combinations of phases utilised in industry. Use of the novel optical diagnostic techniques coupled with bespoke capacitance probes in the present study enables us to explore uncharted territory of two-phase gas-liquid annular flows in vertically orientated pipes. In the present study, a recently developed variant of laser-induced fluorescence (LIF) technique, termed structured-planar laser-induced fluorescence (S-PLIF), is used which allows us to eliminate biases commonly encountered during film-thickness measurements of gas-liquid flows due to refraction and reflection of the light at the interface. In parallel, a bespoke capacitance probe is also employed which permits us to conduct film thickness measurements with high temporal resolution along the perimeter of the pipe. Simultaneous application of these two measurement techniques provides an opportunity to study the subtle differences found in thin annular film structures caused not only by the function of liquid and gas flow rates, but also by the surface-active agents which are widely known to cause drastic changes in flow behaviour due to surface tension gradients. The flow characteristics are studied in terms of mean film thickness, roughness, probability density functions, time-scales of the flows, and gas entrainment in the liquid film. The analysis of the data reveals important changes in the flow characteristics due to the presence of soluble surfactant. Firstly, it is observed that surfactant promotes thinning of annular films at nearly all flow conditions investigated herein, hinting at its influence on the turbulence within the bulk flow. The behaviour of interfacial waves was also found to be notably altered by the surfactant where the film roughness and the time-scale of the waves increase in gas-sheared film flows with low to moderate turbulence and low gas entrainment. This corresponds to flows not in the `regular wave' regime. A decrease in both characteristics then follows upon an increase in turbulence to a sufficiently high level of the two phases. The high gas-shear rate not only limits the highest attainable wave amplitude downstream, but also results in high agitation of air and water phases, and thus, high gas and liquid entrainment. Ultimately, this smooths the base film populated with small-amplitude waves and substantially reduces the amplitude of large interfacial waves. Generally, good agreement with relevant literature correlations is found. The estimated time-scales of the wave dynamics and Marangoni flow showed that the surfactant plays an increasingly important role on waves with lower amplitudes. The sizes of the bubbles entrained in the surfactant-free liquid film are found to exhibit log-normal distributions that become flatter with a decrease in the gas Reynolds number, while this distribution is maintained for surfactant-laden flows. On the other hand, wider distributions in the bubble sizes are found for the surfactant-laden flows at the highest gas-shear rate for all liquid Reynolds numbers. The normalised location of the bubbles (quantified as the relative entrainment depth, i.e., distance of the bubble from the local air-water film height in the wall-normal direction divided by the local film thickness) follows a Gaussian distribution, where the majority of the bubbles accumulate in the middle of the thin film. Understanding the need for further development of the multiphase flows that involves the use of surfactants, motivated us to develop a method to prepare water soluble fluorescent surfactant, which is described in the present work. Furthermore, a detailed characterisation of the fluorescent surfactant is also provided, which may encourage further experimental and modelling investigations of the relevant surfactant-laden multiphase dynamics found in small- and large-industrial scale applications.Open Acces