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We investigate drop break-up morphology, occurrence, time and size distribution, through large ensembles of high-fidelity direct-numerical simulations of drops in homogeneous isotropic turbulence, spanning a wide range of parameters in terms of the Weber number $We$ , viscosity ratio between the drop and the carrier flow $\mu _r=\mu _d/\mu _l$ , where d is the drop diameter, and Reynolds ($Re$) number. For $\mu _r \leq 20$ , we find a nearly constant critical $We$ , while it increases with $\mu _r$ (and $Re$) when $\mu _r > 20$ , and the transition can be described in terms of a drop Reynolds number. The break-up time is delayed when $\mu _r$ increases and is a function of distance to criticality. The first break-up child-size distributions for $\mu _r \leq 20$ transition from M to U shape when the distance to criticality is increased. At high $\mu _r$ , the shape of the distribution is modified. The first break-up child-size distribution gives only limited information on the fragmentation dynamics, as the subsequent break-up sequence is controlled by the drop geometry and viscosity. At high $We$ , a $d^{-3/2}$ size distribution is observed for $\mu _r \leq 20$ , which can be explained by capillary-driven processes, while for $\mu _r > 20$ , almost all drops formed by the fragmentation process are at the smallest scale, controlled by the diameter of the very extended filament, which exhibits a snake-like shape prior to break-up. [ABSTRACT FROM AUTHOR]

The conditional velocity time averages (⟨Ui|ξ⟩) and conditional mixture fraction time averages (⟨Φ|ωi⟩) were computed based on the Eulerian approach from the experimental data measured in a macroscale multi-inlet vortex chemical reactor. The conditioning events were determined by equally sized intervals of the sample space variable for the mixture fraction (ξ) and the velocity vector (ωi). The experimental data, which consisted of instantaneous velocities and concentration fields for two Reynolds numbers (Re = 3250 and 8125), were acquired using the simultaneous stereoscopic particle image velocimetry (stereo-PIV) and planar laser induced fluorescence techniques. Two mathematical models, the linear approximation and probability density function (PDF) gradient diffusion, were validated by experimental results. The results of the velocity conditioned on the mixture fraction demonstrated that the linear model works well in a low turbulence region away from the reactor center. Near the reactor center, high velocity gradients coupled with low concentration gradients reduce the accuracy of the linear model predictions. Nevertheless, an excellent agreement was found for the conditional events within ±2Φrms (mixture fraction root mean square). Due to lower concentration gradient in the tangential direction, the linear model better predicted the tangential velocity component for all locations investigated. The PDF model with an isotropic turbulent diffusivity performed inadequately for the tangential and axial velocity components. A modified version of the PDF model that considers the three components of the turbulent diffusivity produced a better agreement with the experimental data especially in the spiral arms regions of significant concentration gradients. Furthermore, the mixture fraction conditioned on the velocity vector components showed a more linear behavior near the reactor center, where the PDF of the mixture fraction is a Gaussian distribution. As the concentration gradients became prominent away from the reactor, ⟨Φ|ωi⟩ also deviated from the linear pattern. This was especially remarkable for the mixture fraction conditioned on the tangential velocity. The overall prediction of ⟨Φ|ωi⟩ improves at higher Reynolds number as the fluid mixing is enhanced. [ABSTRACT FROM AUTHOR]

The main goal of this work is to explore the closures used in two-fluid models to represent the interphase forces between particles and high-speed flows. To this aim, a hyperbolic two-fluid model for high-speed, monodisperse, gas–particle flow is employed to study the interaction of a thin, moderately dense (volume fraction α p < 0. 2) particle curtain impacted by an incident shock with Mach number M s. Mimicking the experimental setup, the numerical setup consists of a one-dimensional shock tube with a thin particle curtain in the driven section. This allows to validate the two-fluid model against recent experimental campaigns exploring a wide range of particle diameters, material densities, volume fractions, curtain widths and shock speeds. In general, the two-fluid model allows to reproduce the experimental data where the highest discrepancy is obtained in the configurations with the smallest M s. Attention is drawn to the particle-Mach-number (M p) dependence of the drag and added-mass coefficients, which have not yet been explored extensively in the literature. Also, the two-fluid model based on kinetic theory includes a particle pressure accounting for particle–fluid–particle (pfp) interactions. Thus, a parametric study is presented to evaluate the impact of the drag coefficient, the added-mass coefficient, and the magnitude of the pfp pressure. The complete drag model accounting for particle Reynolds number R e p , M p and α p is more accurate than previous drag models depending only on R e p and α p. Due the high particle-to-gas density ratio, the added-mass model has only a minor impact on the results. The magnitude of the pfp pressure has a significant impact on the spread of the curtain due to the high slip velocity. [Display omitted] • Assessment of a hyperbolic compressible two-fluid model is presented using experimental data for shock–particle interactions. • Drag modeling with Mach dependency is taken from very recent simulation data and compared to existing model without Mach effects. • Added-mass modeling from the two-fluid model is also explored. • The pfp pressure term arising from kinetic theory in the two-fluid model ensures hyperbolicity and has a visible effect on the curtain dynamics. [ABSTRACT FROM AUTHOR]

Turbulent wall-bounded flows exhibit a wide range of regimes with significant interaction between scales. The fluid dynamics associated with single-phase channel flows is predominantly characterized by the Reynolds number. Meanwhile, vastly different behaviour exists in particle-laden channel flows, even at a fixed Reynolds number. Vertical turbulent channel flows seeded with a low concentration of inertial particles are known to exhibit segregation in the particle distribution without significant modification to the underlying turbulent kinetic energy (TKE). At moderate (but still low) concentrations, enhancement or attenuation of fluid-phase TKE results from increased dissipation and wakes past individual particles. Recent studies have shown that denser suspensions significantly alter the two-phase dynamics, where the majority of TKE is generated by interphase coupling (i.e. drag) between the carrier gas and clusters of particles that fall near the channel wall. In the present study, a series of simulations of vertical particle-laden channel flows with increasing mass loading is conducted to analyse the transition from the dilute limit where classical mean-shear production is primarily responsible for generating fluid-phase TKE to high-mass-loading suspensions dominated by drag production. Eulerian–Lagrangian simulations are performed for a wide range of particle loadings at two values of the Stokes number, and the corresponding two-phase energy balances are reported to identify the mechanisms responsible for the observed transition. [ABSTRACT FROM AUTHOR]

Turbulent mixing in the confined swirling flow of a multi-inlet vortex reactor (MIVR) was investigated using planar laser induced fluorescence (PLIF). The investigated Reynolds numbers based on the bulk inlet velocity ranged from 3290 to 8225, and the Schmidt number of the passive scalar was 1250. Measurements were taken in the MIVR at three different heights (¼, ½, and ¾ planes). The mixing characteristics and performance of the MIVR were investigated using instantaneous PLIF fields and pointwise statistics such as mixture fraction mean, variance, and one-point concentration probability density function. It was found that the scalar is stretched along velocity streamlines, forming a spiral mixing pattern in the free-vortex region. In the forced-vortex region, mixing intensifies as the turbulent fluctuations increase significantly there. The mixing mechanisms in the MIVR were revealed by identifying specific segregation zones. At Re = 8225 the mixing in the free-vortex region was dominated by both large-scale structures and turbulent diffusion, while in the forced-vortex region mixing is dominated by turbulent diffusion. © 2016 American Institute of Chemical Engineers AIChE J, 63: 2409-2419, 2017 [ABSTRACT FROM AUTHOR]

The multi-inlet vortex reactor (MIVR) is used for flash nanoprecipitation to manufacture functional nanoparticles. A validated computational fluid dynamics model is needed for the design, scale-up, and optimization of the MIVR. Unfortunately, available Reynolds-averaged Navier-Stokes methods are unable to accurately model the highly swirling flow in the MIVR. Large-eddy simulations (LES) are also problematic, as excessively fine grids are required to accurately model this flow. These dilemmas led to the application of the dynamic delayed detached eddy simulation (DDES) method to the MIVR. In the dynamic DDES model, the eddy viscosity has a form similar to the Smagorinsky sub-grid viscosity in LES, which allows the implementation of a dynamic procedure to determine its model coefficient. Simulation results using the dynamic DDES model are found to match well with experimental data in terms of mean velocity and turbulence intensity, suggesting that the dynamic DDES model is a good option for modeling the turbulent swirling flow in the MIVR. © 2016 American Institute of Chemical Engineers AIChE J, 62: 2570-2578, 2016 [ABSTRACT FROM AUTHOR]

The velocity field in a macroscale multi-inlet vortex reactor (MIVR) used in "flash nanoprecipitation (FNP)" process for producing functional nanoparticles was investigated using stereoscopic particle image velocimetry (SPIV). Based on the experimental data, a simple model was proposed to describe the average velocity field within the reactor. In the model, the axial and azimuthal velocities could be well described by the combination of two coflowing Batchelor vortices. In this model, six dimensionless coefficients are identified by nonlinear curve fitting, and their dependence on Reynolds number can be linearly described. This simple model is able to accurately predict the mean velocity field within the confined turbulent swirling flow based purely on Reynolds number. [ABSTRACT FROM AUTHOR]

A technique for visualizing and quantifying reactive mixing for laminar and turbulent flow in a microscale chemical reactor using confocal-based microscopic laser induced fluorescence (confocal μ-LIF) was demonstrated in a microscale multi-inlet vortex nanoprecipitation reactor. Unlike passive scalar μ-LIF, the reactive μ-LIF technique is able to visualize and quantify micromixing effects. The confocal imaging results indicated that the flow in the reactor was laminar and steady for inlet Reynolds numbers of 10, 53, and 93. Mixing and reaction were incomplete at each of these Reynolds numbers. The results also suggested that although mixing by diffusion was enhanced near the midplane of the reactor at Rej = 53 and 93 due to very thin bands of acidic and basic fluid forming as the fluid spiraled towards the center of the reactor, near the top, and bottom walls of the reactor, the lower velocities due to fluid friction with the walls hindered the formation of these thin bands, and, thus, resulted in large regions of unmixed and unreacted fluid. At Rej = 240, the flow was turbulent and unsteady. The mixing and reaction processes were still found to be incomplete even at this highest Reynolds number. At the reactor midplane, the flow images at Rej = 240 showed unmixed base fluid near the center of the reactor, suggesting that just as in the Rej = 53 and 93 cases, lower velocities near the top and bottom walls of the reactor hinder the mixing and rection of the acidic and basic streams. Ensemble averages of line-scan profiles for the Rej = 240 were then calculated to provide statistical quantification of the microscale mixing in the reactor. These results further demonstrate that even at this highest Reynolds number investigated, mixing and reaction are incomplete. Visualization and quantification of micromixing using this reactive μ-LIF technique can prove useful in the validation of computational fluid dynamics models of micromixing within microscale chemical reactors. [ABSTRACT FROM AUTHOR]

The microscale multi-inlet vortex reactor (MIVR) is designed for use in Flash NanoPrecipitation (FNP), a promising technique for producing nanoparticles within small particle size distribution. Fluid mixing is crucial in the FNP process, and due to mixing's strong dependence upon fluid kinematics, investigating velocity and turbulence within the reactor is crucial to optimizing reactor design. To this end, microscopic particle image velocimetry has been used to investigate flow within the MIVR. Three Reynolds numbers are studied, namely, Rej = 53, 93 and 240. At Rej = 53, the flow is laminar and steady. Due to the strong viscous effects at this Reynolds number, distinct flow patterns are observed at different distances from the reactor top and bottom walls. The viscous effects also retard the tangential motions within the reactor, resulting in a weaker vortex than appears at the higher Reynolds numbers. As the Reynolds number is increased to 93, the flow becomes more homogeneous over the depth of the reactor due to weaker viscous effects, yet the flow is still steady. The diminishing effects of viscosity also result in a stronger vortex. At the highest Reynolds number investigated, the flow is turbulent. Turbulent statistics including tangential and radial velocity fluctuations and Reynolds shear stresses are analyzed for this case in addition to the mean velocity field. The tangential motions of the flow are strongest at Rej = 240. Both the tangential and radial velocity fluctuations increase as the flow spirals toward the center of the reactor. The magnitudes of the tangential and radial velocity fluctuations are similar, suggesting that the turbulence is locally isotropic. [ABSTRACT FROM AUTHOR]

It is well known that bubble-column flow-regime predictions of the Eulerian two-fluid model depend strongly on the model formulation. However, there is no general consensus on what force models must be included to match experiments for even the simplest flows. The flow simulated in this work corresponds to the bubble-column experiments of Harteveld with the additional assumptions of: (i) uniform-size (non-coalescing) bubbles, (ii) spatially uniform bubble injection, and (iii) non-deforming (spherical) bubbles. As in Monahan et al.'s 2005 article, the complete set of interphase force models includes drag, added-mass, lift, rotation, and strain. The simulation results are presented in the form of flow maps parameterized by the bubble Reynolds number and the average void fraction. As expected, the flow maps show a strong dependence on the model parameters; however, qualitatively only two types of flow maps are observed: (1)flows that transition from the homogeneous to the heterogeneous regime at relatively low average void fractions and (2) flows that transition at relatively high average void fractions (0.4-0.6) as are observed in the experiments. Remarkably, type-2 flow maps are only observed for two-fluid models that include the complete set of forces. [ABSTRACT FROM AUTHOR]

The velocity and concentration fields in a liquid-phase confined planar-jet reactor were measured using particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF). Measurements were taken at downstream distances from the jet splitter plates of 0, 1, 4.5, 7.5, 12, and 15 jet widths for a Reynolds number of 50,000 based on the hydraulic diameter of the test section. The velocity and concentration field data were analyzed for such flow statistics as mean velocity, Reynolds stresses, turbulent kinetic energy, and scalar mean and variance. The turbulence dissipation rate was also estimated based on a large-eddy PIV approach using the strain-rate tensors computed from velocity fields and the subgrid scale (SGS) stress obtained from the Smagorinsky model. Computational fluid dynamics (CFD) models, including a two-layer k-ε turbulence model, gradient-diffusion models, and a scalar dissipation rate model, were validated against experimental data collected from this facility. The experimental and computational results were found to be in good agreement. [ABSTRACT FROM AUTHOR]

Highlights • A transitional delayed detached eddy simulation model was developed for use in microscale vortex reactors. • Simulation results were compared to microscopic particle image velocimetry and laser induced fluorescence experiments. • Excellent agreement was observed between simulation and experiment. • The model accurately simulated both the laminar and turbulent flow regimes in the reactor. Abstract The objective of the presented work is to verify a delayed detached eddy simulation (DDES) model for simulating transitional swirling flow in a micro-scale multi-inlet vortex reactor (MIVR). The DDES model is a k- ω based turbulence model with a low Reynolds number correction applied to the standard k- ω model such that the Reynolds-averaged Navier-Stokes (RANS) component of the DDES model is able to account for low Reynolds number flow. By limiting the dissipation rate in the k-equation, the large-eddy simulation (LES) part of the DDES model behaves similarly to a one-equation sub-grid model. The turbulent Reynolds number is redefined to represent both modeled and resolved turbulence level so that underestimation of the RANS length scale in the LES range can be reduced. Applying the DDES model to simulate both laminar and transitional flow in the micro-scale MIVR produces an accurate prediction of mean velocity and turbulent intensity compared with experimental data. It is demonstrated that the proposed DDES model is capable of simulating transitional flow in the complex geometry of the micro-scale MIVR. These simulation results also help to understand the flow and mixing patterns in the micro-scale MIVR and provide guidances to optimize the reactor for the application of producing functional nanoparticles. [ABSTRACT FROM AUTHOR]

The microscale multi-inlet vortex reactor (MIVR) has been developed for use in flash nanoprecipitation, a technique to generate functional nanoparticles. A scaled-up MIVR is motivated by the desire for a higher output of nanoparticles than the microscale reactor can provide. As the first step of this scaling process, the flow characteristics in a macro-scale MIVR have been investigated by stereoscopic particle image velocimetry. The studied Reynolds numbers based on the inlet geometry range from 3290 to 8225, resulting in a turbulent swirling flow within the reactor. The flow in the mixing chamber is found to be unstable with a wandering vortex center. The vortex wandering is constrained to a small area near the center of the reactor and has little effect on the mean velocity field. However, the measured turbulence kinetic energy and Reynolds stresses are found to be sensitive to the vortex wandering. The flow characteristics of the macro-scale MIVR are compared with the microscale MIVR in terms of swirl ratio and micromixing time. It is found that the swirl ratio and micromixing time of the flow increases as the MIVR is scaled up, indicating a flow with stronger swirl yet less mixing effectiveness in the scaled-up reactor. [ABSTRACT FROM AUTHOR]

Abstract: This work focuses on the prediction of the turbulent flow in a three-dimensionial Confined Impinging Jets Reactor with a cylindrical reaction chamber by using Large Eddy Simulation. Three-dimensional unsteady simulations with different sub-grid scale models, numerical schemes and boundary conditions were performed for various flow rates, covering different flow regimes. First, a qualitative analysis of the flow field was carried out and then predictions of the mean and fluctuating velocities were compared with micro Particle Image Velocimetry data. Good agreement was found both for the mean velocity components and the fluctuations. For low to moderate Reynolds numbers the sub-grid scale model was found not to be very relevant, since small scales are of less importance, as long as scalar transport and chemical reaction are not in play. An important finding is the good prediction of the high velocity fluctuations detected in particular at higher Reynolds number due to the natural instability of the system, strongly enforced by the jets unsteadiness. [Copyright &y& Elsevier]

Abstract: A combined particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF) system was employed to investigate the turbulent mixing in a confined liquid-phase rectangular-wake flow with a Reynolds number based on hydraulic diameter of 37,500 and a Schmidt number of 1250. The simultaneous velocity and concentration field data were analyzed for flow statistics such as turbulent fluxes, turbulent viscosity and diffusivity, and turbulent Schimdt number. The streamwise and transverse turbulent fluxes were found to be of the same magnitude. The turbulent flux vector was not aligned with the mean concentration gradient. The turbulent Schimdt number was about 0.8. The spatial correlations of turbulent fluxes and concentration fluctuations were evaluated. In the correlation field, there were a positive and a negative vertically oriented elliptical correlation region, which were symmetric around the basis point. The correlation region was a horizontally oriented ellipse with negative values of the correlation coefficient. The correlation field of was also an ellipse with a horizontal major axis. The behavior of large-scale structures in both the velocity and concentration field was studied using linear stochastic estimation with a defined event of concentration fluctuation. Vortex streets were observed in the estimated velocity fields. The streamwise growth of the structure size increased linearly initially but then grew more slowly. [Copyright &y& Elsevier]

Abstract: Rapid precipitation of both organic and inorganic compounds at high supersaturation requires homogenous mixing to control the particle size distribution. We present the design and characterization of a new multi-inlet vortex mixer (MIVM). The four-stream MIVM allows control of both the supersaturation and the final solvent quality by varying stream velocities. The design also enables the separation of reactive components prior to mixing. Finally, the design enables mixing of streams of unequal volumetric flows, which is not possible with alternate confined impinging jet mixing geometries. We characterize the mixing performance of the MIVM using competitive fast reactions (the so-called “Bourne reactions”). Adequate micromixing is obtained with a suitably defined Reynolds number when . The experimental results are compared to computational fluid dynamics (CFD) simulations of the fluid mechanics and parallel reactions in the MIVM. Excellent correspondence is found between the simulation and the experimental results with no adjustable parameters. The CFD simulations provide a powerful tool for the optimization of these complex mixing geometries. [Copyright &y& Elsevier]