Optical measurements in evolving dispersed pipe flows

Optical laser-based techniques and an extensive data analysis methodology have been developed to acquire flow and separation characteristics of concentrated liquid–liquid dispersions. A helical static mixer was used at the inlet of an acrylic 4 m long horizontal pipe to actuate the dispersed flows at low mixture velocities. The organic (913 kg m−3, 0.0046 Pa s) and aqueous phases (1146 kg m−3, 0.0084 Pa s) were chosen to have matched refractive indices. Measurements were conducted at 15 and 135 equivalent pipe diameters downstream the inlet. Planar laser induced fluorescence (PLIF) measurements illustrated the flow structures and provided the local in-situ holdup profiles. It was found that along the pipe the drops segregate and in some cases coalesce either with other drops or with the corresponding continuous phase. A multi-level threshold algorithm was developed to measure the drop sizes from the PLIF images. The velocity profiles in the aqueous phase were measured with particle image velocimetry (PIV), while the settling velocities of the organic dispersed drops were acquired with particle tracking velocimetry (PTV). It was also possible to capture coalescence events of a drop with an interface over time and to acquire the instantaneous velocity and vorticity fields in the coalescing drop

Simplified mechanistic model for the separation of dispersed oil-water horizontal pipe flows

A mechanistic model that predicts the separation of oil-water dispersed horizontal pipe flows was investigated. Different droplet diameter averages were implemented in the model and the accuracy of the resulting predictions was assessed by comparing each case against experimental data. The experimental data used was obtained in a pilot scale two-phase flow facility using tap water and oil (828 kg m-3, 5.5 mPa s) as test fluids. The results show that the separation length is highly sensitive to the drop diameter, but further investigation is required to determine which drop diameter average produces more accurate predictions of the flow profile

A mechanistic model for the prediction of flow pattern transitions during separation of liquid-liquid pipe flows

A one-dimensional mechanistic model that predicts the flow pattern transitions during the separation of dispersed liquid-liquid flows in horizontal pipes was developed. The model is able to capture the evolution along the pipe of the four characteristic layers that develop from initially dispersed flows of either oil-in-water or water-in-oil at a range of mixture velocities: a pure water layer at the bottom, a settling (flotation/sedimentation) layer, a dense-packed zone, and a pure oil layer on the top. Coalescence correlations from literature were included in the model to predict the drop growth due to binary drop coalescence and the coalescence rate of drops with their corresponding interface. The model predictions on the evolution of the heights of the different layers were partly compared against available experimental data obtained in a pilot scale two-phase flow facility using tap water and oil (828 kg m−3, 5.5 mPa s) as test fluids. It was shown that the evolution of the four characteristic layers depends on the rates of drop settling and drop-interface coalescence. Oil-in-water dispersions separated faster than water-in-oil ones, while dispersions with smaller drop-sizes were more likely to exhibit depletion of the dense-packed zone

In-line monitoring of mixing performance for smart processes in tubular reactors

This work is focused on the experimental analysis of the fluid dynamics characteristics of a tubular reactor equipped with Kenics static mixers working under turbulent flow con-ditions, with the specific aim of demonstrating the advantages of in-line monitoring tools for continuous process applications. Electrical Resistance Tomography, pressure trans-ducers and Particle Image Velocimetry are employed to evaluate the mixing performance, the pressure drop and the flow field, respectively, considering the standard configuration of the mixers, consisting in mixing elements with alternating orientation, a single mixing element or multiple elements with the same orientation. The applicability of Electrical Resistance Tomography for offering insight into continuous reactors is assessed and the potential of monitoring the mixing performance inside the static mixers is shown. The experimental data suggest that alternatives to the standard element configurations might be adopted for optimizing the fluid mixing process, taking into account the mixing per-formances and the pressure drop, for which a novel correlation based on distributed and concentrated contributions is proposed

Application of acoustic techniques to fluid-particle systems – A review

Acoustic methods applied to opaque systems have attracted the attention of researchers in fluid mechanics. In particular, owing to their ability to characterise in real-time, non-transparent and highly concentrated fluid-particle systems, they have been applied to the study of complex multiphase flows such as fluidised beds. This paper gives an overview of the physical principles and typical challenges of ultrasound and acoustic emission AE methods when applied to fluid-particle systems. The principles of ultrasound imaging are explained first. The measurement techniques and signal processing methodologies for obtaining velocity profiles, size distribution of the dispersed phases, and solid volume fraction are then discussed. The techniques are based on the measurement of attenuation, sound speed, frequency shift, and transit time of the propagated sound wave. A description of the acoustic emission technique and applications to fluid-particle systems are then discussed. Finally, extensions and future opportunities of the acoustic techniques are presented

Vortex-induced waves and the mechanism of drop entrainment in transition from stratified to dispersed oil-water pipe flows

This dissertation presents new insights on flow pattern transition from stratified to non-stratified of two-phase oil and water flows in horizontal pipes. A novel approach is implemented to facilitate investigation of drop entrainment which identifies onset of the particular transition, where a cylindrical bluff body is located transverse to flow direction to induce instabilities in the form of vortex-induced interfacial waves in stratified flows. Numerical investigations of two-dimensional single-phase flows performed with CFD code FLUENT shows that vortex shedding frequency increases with decrease in the cylinder diameter while the size of vorticity region expands with increase in cylinder diameter. From the findings, two cylinder diameters, 2 mm and 8 mm are selected for experimental investigation in two-phase flows to generate vortex shedding frequency in the range of 1 to 100 Hz. Findings of high-speed visualization on the flow patterns and interfacial wave characteristics showed that higher instabilities were achieved with increasing cylinder diameter where the transition boundaries were shifted towards lower mixture velocities and waves with higher amplitude were produced. This is attributed to the size of vorticity regions, which are attached to the interface to actuate the vortex-induced waves as demonstrated by the particle image velocimetry (PIV) results. Variations of the vortex shedding behavior achieved by various cylinder diameters were found to be reflected on the resulting vortex-induced waves. The cylindrical bluff body approach is further implemented for the investigations of drop entrainment using a cylinder diameter that corresponds to gap ratio of 0.656 as it provides high instabilities at minimum wall effects. The use of simultaneous PLIF and PIV was introduced to visualize the wave’s evolution with high spatial and temporal resolution while obtaining the velocity field around the waves at the same time. Drop entrainment was identified to occur through detachment of drop from interfacial waves and is formulated into a phenomenological model developed based on force balance. Further analysis of the deformed wave dynamics during drop detachment shows relation to the input flowrate ratio, r

Flow pattern transitions in oil-water flows past a bluff body

In this Thesis a novel approach is followed to facilitate the experimental investigations on the flow pattern transitions from separated to dispersed flows using a cylindrical bluff body in horizontal oil-water flows. A transverse cylindrical rod is used as a bluff body which is placed under the interface of the two immiscible liquids and near the test section inlet to passively generate interfacial perturbations and breaking waves. This approach was inspired from the use of hydrofoils in ships that reduce frictional drag via increased air entrainment. Studies are carried out using two flow facilities and high speed imaging combined with laser based measurements are performed at two axial locations along the test section, immediately after the cylinder and at large distance away from the cylinder. The effect of a confined geometry on the characteristics of the von Karman vortices and on the general flow behaviour immediately downstream of the cylinder are investigated in single phase water flows. It is found that the 3D pipe geometry does not affect significantly the vortex shedding behind the cylinder at least in the central plane of the pipe. The frequencies of the vortex shedding were comparable to those from a cylinder in an unconfined liquid. The results from two phase flows reveal that the cylinder reduces the mixture velocity for the transition separated to dispersed flows. It also actuates interfacial waves that are found to be non-linear and convective. In many cases the waves have the same frequencies as the von Karman vortices depending on the submergence depth of the cylinder underneath the oil-water interface and on the Froude number of the water layer. The observations suggest that strongly non-linear waves are responsible for forming thin ligaments that eventually break up into droplets

Dynamics of spatially evolving dispersed flows

This dissertation provides a unique insight into the flow dynamics of evolving dispersed pipe flows. Kinetically unstable liquid-liquid dispersions are actuated in two horizontal flow loop systems. Novel conductivity and optical laser-based experimental methods are developed and applied at several axial locations capturing the flow characteristics and separation properties of the dispersions downstream the pipe with combined measurements of drop sizes, phase fractions and velocities. Flow pattern transitions are recorded for low mixture velocities as the dispersions flow. Drops segregate and coalesce forming a second continuous layer. Drop size measurements exhibit growth of the drops along the streamwise direction independent of the flow pattern, with larger drops recorded closer to the direction of buoyancy. A phenomenological model based on batch vessel settlers is modified and is found to predict well the axial evolution of the dispersions. Holdup and velocity measurements acquired from laser diagnostics are compared with CFD predictions obtained using a mixture approach implementing an effective viscosity model. Good comparisons are obtained by considering sedimentation, shear-induced diffusion and lift. The dispersions behave as suspensions of solid rigid spheres for the conditions investigated. Asymmetry in the velocity profiles is found for both experiments and simulations as the dispersions separate, with the maxima of the velocity located in the drop-free layer. Due to the prominent role of coalescence in the system, its dynamics are studied both during pipe flow and in a Hele-Shaw cell. For the former, high resolution velocity field measurements illustrate the vortices generated from the rupture point of the film inside a coalescing drop and its expanding neck until it fully merges with the bulk, being in agreement with scaling laws for immobile systems. The latter cases are used to investigate the effect of surface active agents and complex fluids. Surfactants are found to deform the interface, increase locally their concentration at the neck and change the propagation direction of the vortices. Xanthan gum addition in the coalescing phase slows down the neck expansion velocity and causes a spatial variation of the viscosity affecting the velocity field inside the drop