Session T28: Multiphase Flows: General

Statistical analysis of freezing stages in supercooled water microdroplets

The solidification of metastable liquids is a relatively unexplored process. An important example is the freezing of supercooled water microdroplets, which is relevant to atmospheric processes. We investigated freezing in 40 µm water droplets that were supercooled by evaporation in a vacuum chamber, a process that has many similarities to the freezing of water in clouds. Individual droplets froze after different times of flight due to the randomness of ice nucleation. A large number of drops were imaged optically to capture the dendritic growth of ice crystals and subsequent solidification processes up to the cracking and shattering of drops. More than thirteen thousand droplet images, recorded at several times of flight, were analyzed and classified into eight image types that correspond to different stages of freezing. As the time of flight increased, the distribution of image types evolved from primarily liquid drops to primarily shattered drops. Despite a substantial broadening caused by ice nucleation statistics, the change in distributions with time could be used to determine what physical freezing stages correspond to the observed image types, their temporal order, and their approximate duration. The homogeneous ice nucleation rate was estimated and found to be consistent with previous measurements.   

An Experimental Study to Characterize the Dynamic Wind-Driven Runback Process of Aircraft Deicing Fluids Pertinent to Aircraft Ground Anti-/De-icing

A comprehensive experimental campaign was conducted to characterize the transient runback process of the aircraft deicing fluids over a flat surface as driven by boundary layer airflows pertinent to aircraft ground anti-/de-icing. The experimental study is conducted by leveraging a low-speed wind tunnel available at Iowa State University to generate a boundary layer airflow over a flat test plate mounted horizontally along the bottom wall of the tunnel test section. Two kinds of most-commonly-used deicing fluids, i.e., Newtonian Type-I deicing fluids and non-Newtonian shear thinning Type-IV deicing fluid, are used as the working fluids for the experimental study. In addition to using a high-speed imaging system to record the dynamic wind-driven runback process of the deicing fluids, a novel digital image projection (DIP) system is also utilized to achieve non-intrusive, time-resolved measurements of film thickness distributions of the wind-driven deicing fluid flows over the test plate under different test conditions. A theoretic analysis was also performed to correlate to the experimental study in order to the gain further insight into the underlying physics for a better understanding of interactions among the multiphase flow system (i.e., airflow interacting with Newtonian Type-1 and non-Newtonian shear-thinning Type-IV fluids).   

An Experimental Study to Characterize the Dynamic Interactions of DBD Plasma Discharges with Water and Ice Pertinent to Aircraft Inflight Icing Mitigation

An experimental study was performed to characterize the Dielectric-Barrier-Discharge (DBD) plasma discharges interacting with a complex multiphase system (i.e., air, water and ice) associated with ice accretion process over airframe surfaces in the context of aircraft inflight icing mitigation. The experimental study was carried out in the Icing Research Tunnel available at Iowa State University (i.e., ISU-IRT) to generate typical aircraft icing conditions with adequate liquid water content (LWC) levels in the frozen cold incoming airflows.  An array of DBD actuators were embedded over the surface of an airfoil/wing model were supplied with high voltages in either alternating current for AC-DBD plasma actuation or nanosecond pulses for ns-DBD plasma actuation. During the experiments, in addition to using a high-resolution imaging system to record the dynamic anti-/de-icing operation over the airfoil/wing surface upon switching on the DBD plasma actuators, a high-speed Infrared (IR) thermal imaging system is also utilized to quantitatively map the temperature distributions over the surface of the airfoil/wing to characterize the effects of DBD plasma actuations on the coupled heat and mass transfer of the ice accretion process. The findings derived from the present study are very helpful to explore/optimize design paradigms for the development of novel plasma-based anti-/de-icing strategies tailored specifically for aircraft inflight icing mitigation to ensure safer and more efficient aircraft operation in atmospheric icing conditions.   

Annular Flow Film Thickness via Planar Laser-Induced Fluorescence

Annular flow is a multiphase flow regime of two-phase gas-liquid flow characterized by a thin liquid film layer on the pipe wall surrounding a fast-moving gas core. The film layer thickness significantly impacts several critical flow properties, but the literature is scarce compared to other flow regimes. In this experimental work, planar laser-induced fluorescence (PLIF) was used to study the top and bottom film thickness over a range of both liquid and gas flow rates. In addition, the measurements were compared to results from established horizontal flow data. The scarcity of literature on annular flow is especially true for the sensitivity of the film thickness to inclination angle. Therefore, annular flow film behaviors will also be potentially studied over a wide range of inclination angles, including both upward and downward vertical orientations.   

Two-phase flow simulation of the coolant onto simplified stator coil structure

Stator coils of automobiles in operation generate heat and are cooled by coolant poured from above. Since the behavior characteristic of the coolant poured on the coils is not clarified yet due to its complexity, the three-dimensional two-phase flow simulation is conducted to characterize the flow. In this study, the coils are modeled with horizontal rectangular pillar arrays where the poured coolant touches down. The two-phase flows are simulated by using the phase-field lattice Boltzmann method of which the solid-fluid boundary is improved. The effects of the governing parameters on the coolant behavior, such as the physical properties of the working fluid, the wettability, and the gap between the pillars are investigated. The results show that the coolant tends to spread across the pillars because of its high viscosity. Moreover, the liquid spreads extensively when the contact angle is small. It is found that the smaller gaps are preferable for spreading over a wider area. However, in case of too narrow gaps, the discharge efficiency decreased. We also conducted the simulation for the flow into the stacked pillars, and found the wetting area of the inner pillars becomes larger than that of the exposed pillars due to the capillary force.   

Displacement Flow in Penny-Shaped Cracks

We revisit the classical displacement flow problem in a growing penny-shaped fracture. When a single fluid is injected into a brittle material, the viscous dissipation in the fracking fluid and fracture energy required at the crack tip resist the propagation. Experimental studies have demonstrated the existence of two asymptotes of propagation. If viscous dissipation dominates the crack dynamics, the propagation is in the viscous regime. If the fracture toughness is the limiting process, the propagation is in the toughness regime. Here, we report an experimental study of injection in an oil-filled fracture in gelatine, a model material to study fractures in elastic-brittle materials. The initial oil-filled crack is penny-shaped, and upon injection of an immiscible liquid, the crack further expands by pushing the oil. We investigate the propagation of the fracture and the displacement of the oil/water interface in both the viscous and toughness regimes. Our experiments show that the propagation dynamics in this displacement configuration differ from the single-fluid injection. We explain the experimental results, compare the behavior in the two regimes, and provide insights into the underlying physics with scaling arguments.   

An examination of the one-fluid formulation for two-phase flows

For direct numerical simulations of two-phase flow, the established methodology is the application of the one-fluid formulation, which applies everywhere in the domain, including at the gas-liquid interface. The physical consistency of this formulation is examined by considering whether it satisfies the governing equations in the bulk region of the domain and the jump conditions at the interface. An analysis of various versions of the momentum and energy one-field formulations published in the literature reveals a frequent inconsistency, particularly for problems involving phase change. The issue is in part attributed to a lack of equivalence between conservative and non-conservative forms of the equation, although other issues are also uncovered. A proposed generalized one-field formulation is advanced that is able to exhibit the desired level of physical consistency by satisfying interfacial and bulk phase relations.    

Numerical studies of three-phase gas-liquid-liquid multiphase system

Direct numerical simulation (DNS) is used to examine the dynamics of a three-phase gas-liquid-liquid multiphase system. The simulation is done in a three-dimensional system with a fully periodic domain. The system is developed into a statistically steady state, which consists of a continuous liquid phase, buoyant gas bubbles and smaller heavy drops. The Reynolds numbers of the bubbles and the drops are moderate. The effect of bubble deformability is examined by changing its surface tension, while the drops stay nearly spherical. We compared the results of one bubble in a "unit cell" and eight freely interacting bubbles in a larger cell. The distribution of drops around one bubble in a "unit cell" is uneven and depends on the bubble deformability, but the distribution of drops around freely interacting bubbles is relatively uniform for selected parameters. We also examined the dependency of the slip velocities and the velocity fluctuations on the drop volume fraction and bubble deformability.   

A phase inversion problem with controlled thin sheet breakup

The phase inversion physical configuration has been proposed to study the atomization of multiphase flows in a controlled set-up with low sensitivity to the boundary and initial conditions. Macroscopic quantities such as the kinetic and potential energies show convergence upon mesh refinement, while the enstrophy and the droplet size distribution do not. One reason can be found in the breakup of thin structures. A well-known drawback of the volume of fluid (VOF) method is that the breakup of thin liquid films is mainly caused by numerical aspects rather than by physical ones. The rupture occurs when the thickness reaches the order of the grid size and by refining the grid the breakup events are delayed. In this work, we present a controlled topology change algorithm to overcome the grid dependency of the breakup. First, we identify the thin films or ligaments by computing quadratic moments of the VOF indicator function. Then, we induce the breakup by making holes in the films before the thickness reaches the grid size. We show for a phase inversion problem with moderate Re and We numbers that the convergence upon grid refinement of the droplets size distribution is improved.   

Adjoint-based optimisation of interfacial flows in the sharp interface limit. Application to the Stefan problem.

The flows encountered in energy conversion systems consist of a wealth of complex phenomena. While computational fluid dynamics has now become a present tool in describing and predicting such multi-physics flows, these state-of-the-art computational resources still provide limited insight towards a robust optimization framework. Targeted manipulation of such flows by enhanced designs or active control strategies is however crucial for improvements in performance and robustness and venturing beyond standard operating conditions. The transition from model-based numerical simulations to model-based optimal control requires an alternative approach, which allows access to inverse information. As far as two-phase flows are concerned, to date, inverse information has been extracted from simulations of simplified configurations with additional unrealistic assumptions, or from low-fidelity models. In this work, on the other hand, the one-fluid formulation is used, where the sharp interface representation is captured by a level set method. We focus our study on the Stefan problem, where the motion of the interface is a function of the jump in gradient of temperature. The gradient extraction strategy differentiate-then-discretize is then used to minimize tracking-type objective functions.   

Mitigation of pressure fluctuation in two-phase slug flow by rib geometry

Slug flow has received attention for industrial applications because it is one of the most prevalent and dangerous two-phase flows due to severe pressure fluctuations. The effects of rib elements installed at the inner surfaces of a channel on the flow structure and pressure fluctuations are experimentally investigated with a particular focus on how the rib elements alter slugging phenomena compared to a smooth channel. Pressure fluctuations for the slug in ribbed channels are weakened due to air bubble entrainment. Two distinct mechanisms of air bubble entrainment are suggested to accounts for different levels of mitigation of pressure fluctuation and the increase in flow randomness, which depend on the width-to-height ratio of the rib geometry.   

Micro-droplet nucleation through solvent exchange in turbulent buoyant jet.

Solvent exchange is a process involving mixing between a solute- containing good solvent and a poor one. The process creates oversaturation and thus the nucleation of the micro-sized solute droplets. Despite increasing interests and numerous efforts, such ternary systems on a macro-scale and in the turbulent regime have remained unexplored. We experimentally study the solvent exchange process by injecting mixtures of ethanol and trans-anethole into water, forming a turbulent buoyant jet with initial Reynolds numbers Re₀ = 555 and Re₀ = 1387, and two different compositions, namely ratios of ethanol:oil=100:1 and ethanol:oil=33:1, thus creating local oversaturation of the trans-anethole by turbulent entrainment. We measure the concentration of the nucleated droplets with a light attenuation technique and find the radial distribution to be sub-Gaussian. The evolution of the oversaturation flux reveals continuous droplet nucleation downstream and radially across the jet, in contrast to entrainment-based models, which we attribute to the limited mixing capacity of the jet. This work extends the understanding of solvent exchange into the turbulent regime for the first time, and brings in a novel type of flow, broadening the scope of multicomponent, multiphase turbulent jets with phase transition.