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
