Application of Experimental and Numerical Techniques to Microscale Devices

Two of the areas that have become relevant recently are the areas of mixing in micro-scale devices, and manufacturing of functional nanoparticles. MicroPIV experiments were performed on two different mixers, one a wide microchannel with the surface grooves, in the laminar regime, and the other, a confined impinging jets reactor, in the laminar and turbulent regimes. In the wide microchannel with surface grooves, microPIV data were collected at the interface and the midplane at the Reynolds numbers of 0.08, 0.8, and 8. The experiments were performed on three internal angles of the chevrons, namely 135°, 90°, and 45°. The normalized transverse velocity generated in the midplane due to the presence of the grooves, is the strongest for the internal angle of 135°, and in that, the normalized transverse velocity is maximum at the Reynolds numbers of 0.08 and 0.8. MicroPIV experiments were performed in a confined impinging jets reactors at Reynolds numbers of 200, 1000, and 1500. The data was collected in the midplane, and turbulent statistics were further computed. The high velocity jets impinge along the centerline of the reactor. Upon impinging, part of the fluid turns towards the top wall and the majority of it turn towards the outlet. This high velocity impingement causes and unstable zone called the impingement zone, which moves about the centerline line, causing the jets to flap back and forth. Spatial correlations were computed to get an estimate of the size of the coherent structures. Large eddy simulation was performed on the CIJR for the Reynolds numbers of 1000 and 1500, using OpenFOAM. The Reynolds number is based on the inlet jet hydraulic diameter. Excellent agreement was found with the experimental and simulation data. Turbulent reactive mixing in a rectangular microscale confined impinging-jets reactor (CIJR) was investigated using the pH indicator phenolphthalein in this study for three di_erent jet Reynolds numbers of 25, 1000 and 1500. Laminar flow regime was observed at Reynolds number of 25 whereas the flow was turbulent at Reynolds numbers of 1000 and 1500. An image processing technique was applied to instantaneous images to extract quantitative mixing data by identifying regions with pH ≥ 9.3 and regions with pH \u3c 9.3. The ensemble-averages were computed using these thresholded images to compare mixing performance between different Reynolds numbers. Finally, the spatial auto-correlation fields of the thresholded images fluctuations were evaluated, based on which large-scale turbulent structure were analyzed

Computational models for the simulation of turbulent poly-dispersed flows: Large Eddy Simulation and Quadrature-Based Moment Method

This work focuses on the development of efficient computational tools for the simulation of turbulent multiphase polydispersed flows. In terms of methodologies we focus here on the use of Large Eddy Simulation (LES) and Quadrature-Based Methods of Moments (QBMM). In terms of applications the work is finalised, in order to be applied in the future, to particle production processes (precipitation and crystallisation in particular). An important part of the work concerns the study of the flow field in a Confined Impinging Jets Reactor (CIJR), frequently used in particle production processes. The first part is limited to the comparison and analysis of micro Particle Image Velocimetry (μPIV) experiments, carried out in a previous work, and Direct Numerical Simulation (DNS), carried out in this thesis. In particular the effects of boundary and operating conditions are studied and the numerical simulations are used to understand the experimental predictions and demonstrate the importance of unavoidable fluctuations in the experimental inlets. This represents a preparatory work for the LES modelling of the CIJR. Before investigating the accuracy of LES predictions for this particular application, the model and the implementation are studied in a more general context, represented by a well-known test case such as the periodic turbulent channel flow: the LES model implementation in TransAT, the code used in this work, is compared with DNS data and with predictions of other codes. LES simulations for the CIJR, provided with the proper boundary conditions obtained by the previous DNS/μPIV study, are then performed and compared with experiments, validating the model in a more realistic test case. Since particle precipitation and crystallization often result in complex interactions between particles and the continuous phase, in the second part of the work particular attention has been paid in the modelling of the momentum transfer and the resulting velocity of the particles (relative to the fluid). In particular the possibility of describing poly-disperse fluid-solid systems with QBMM together with LES and Equilibrium Eulerian Model (EEM) is assessed. The study is performed by comparing our predictions with DNS Lagrangian data in the turbulent channel flow previously described, seeded with particles corresponding to a realistic Particle Size Distribution (PSD). The last part of the work deals with particle collisions, extending QBMM to the investigation of non-equilibrium flows governed by the Boltzmann Equation with a hard-sphere collision kernel. The evolution of the particle velocity distribution is predicted and compared with other methods for kinetic equations such as Lattice Boltzmann Method (LBM), Discrete Velocity Method (DVM) and Grad’s Moment Method (GM). The overall results of this thesis can be extended to a broad range of other applications of single-phase, dispersed multiphase and non-equilibrium flows

Doctor of Philosophy

dissertationThe development of models for use in computational fluid dynamics often rely on a set of assumptions, as resolving the full set of length and timescales is often unfeasible in terms of computational cost for applied flow regimes. Many models rely on statistical formulations as the method for development. In large-eddy simulation, the prescription of inlet conditions affect the overall outcome of the results. Two turbulent inlet methods are implemented into computational fluid dynamics code and simulations are run to compare results to a stationary inlet for an experimental coaxial jet flow configuration. Both of the methods rely on the recreation of first- and second-order statistics the mean and variance in the derivations. A flamelet model is presented that accounts for nonequilibrium effects in combustion systems. Subgrid effects of the model are accounted for with a presumed probability density function method for mixture fraction and scalar dissipation rates. The flamelet library approach is extended from past flamelet studies to include five independent variables: extent of reaction, mixture fraction, scalar dissipation rate, scalar variance, and heat loss. For dispersed multiphase flow simulations, transporting the moments of a population balance equation can often provide good results. Moments methods rely on tracking the statistical properties of a particle size distribution. Utilizing an Eulerian moment method provides a computationally cheaper way to track particles than using a Lagrangian method. The quadrature method of moments is used for the simulation of the particle size distribution of calcium carbonate precipitation. Two sets of simulations are run, the first set uses an idealized geometry with an increasing Reynolds number, the second set of simulations uses pilot scale reactors. The mixing rates that occur in each of these simulation sets affect the outcome of the particle size distributions of the precipitate particles. The conditional quadrature method of moments is used to simulate inert particles of nonnegligible Stokes number. These larger particles require using velocities as an internal coordinate. Simple cases are set up to show the implementation of the method showing proper behavior with particle trajectory crossing and wall interaction cases. A comparison of the method with experimental results of inert particle flow for monodisperse and polydisperse particle size distributions is shown for coaxial jet flow. The method is shown to be extendable to any arbitrary number of internal coordinates, which should make it useful in modeling of complex multivariate particle systems

A fundamental investigation of scaling up turbulent liquid-phase vortex reactor using experimentally validated CFD models

The production of uniform-sized nanoparticles has potential application in a wide variety of fields, but is still a challenge. One main reason that many lab-scale manufactured nanoparticles have not appeared in industry is because there is lack of control on physical properties and surface functionality of nanoparticles during massive production. Recently, a process called Flash Nanoprecipitation (FNP) has been developed to produce nanoparticles with controlled size and high drug-loading rate. In FNP, fast mixing is required to make sure that solvent and non-solvent mix homogeneously so that competitive precipitation of organics and polymer could result in functional nanoparticles with narrow size distribution. A multi-inlet vortex reactor (MIVR) has been developed to provide fast mixing for the FNP. The MIVR includes four inlets which are tangential to the mixing chamber of reactor. The MIVR has the operational advantage of providing different inlet-flow momentum and configurations compared to other reactors used in the FNP such as confined impinging jet reactor (CIJR). Former studies have already shown its ability of providing fast mixing and successfully producing functional nanoparticles in the FNP. However, until now all previous investigations about the MIVR only focused in its micro-scale (dimensions in millimetre). While the micro-scale MIVR does show great promise in the production of functional nanoparticles, the small dimensions and correspondingly small output of the micro-scale MIVR limit its usefulness to producing functional nanopraticles for applications requiring small production run such as high-value pharmaceutical agents. Some applications such as nanoparticle used in pesticides and cosmetics may require larger production run than the micro-scale MIVR can provide, making it economically unrealistic based on the relatively high capital and operating costs needed for a large number of reactors operating in parallel. For this reason, in the study we are interested in investigating the feasibility of scaling up the FNP process to a macro-scale MIVR capable of generating large quantities of functional nanoparticles, both rapidly and economically, and consequently developing experimentally verified computational fluid dynamics (CFD) models that can be used as design tools for further optimizing reactor design and operation parameters to produce customized functional nanoparticles. To accomplish this investigation, a macro-scale MIVR has been built with optical access. Non-intrusive, optical-based measurement techniques including particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF) were used to measure flow field and mixing, and related CFD models, specifically turbulence models were validated and developed for optimizing the MIVR and future model development of the FNP process

Turbulence in a microscale planar confined impinging-jets reactor

Confined impinging-jets reactors (CIJR) offer many advantages for rapid chemical processing at the microscale in applications such as precipitation and the production of organic nanoparticles. It has been demonstrated that computational fluid dynamics (CFD) is a promising tool for ‘‘experiment-free’’ design and scale-up of such reactors. However, validation of the CFD model used for the microscale turbulence applications requires detailed experimental data on the unsteady flow, the availability of which has until now been very limited. In this work, microscopic particle-image velocimetry (microPIV) techniques were employed to measure the instantaneous velocity field for various Reynolds numbers in a planar CIJR. In order to illustrate the validation procedure, the performance of a particular CFD model, the two-layer k–3 model, was evaluated by comparing the predicted flow field with the experimental data. To our knowledge, this study represents the first attempt to directly measure and quantify velocity and turbulence in a microreactor and to use the results to validate a CFD model for microscale turbulent flows

Process Simulation of Technical Precipitation Processes - The Influence of Mixing

This work develops and shows up methods to tackle multi-scale challenges in particle formation during precipitation crystallization. Firstly, molecular, micro- and meso-scale interactions in confined impinging jet mixers are investigated and simulatively predicted. Secondly, to build up on developed methods, macroscale as present for instance in stirred tank reactors is added to the considerations

Experimental and computational investigation of turbulent mixing in microscale reactors

Flash Nanoprecipitation (FNP) is a promising technique for mass production of nanoparticles for use in various areas. Mixing time is such a crucial factor that it affects the particle size distribution as well as the particle morphology. Turbulent mixing in microscale nanoprecipitation reactors, i.e., the planar conned impinging-jet reactor (CIJR) and the multi-inlet vortex reactor (MIVR), is therefore investigated by means of numerical simulations as well as experimental flow visualization methods. In the process of studying, the computational fluid dynamics (CFD) models are validated by comparing simulation results with experimental data. One of the experimental visualization techniques developed in this work uses the phenolphthalein as the tracer that characterizes the acid-base neutralization reaction. Mixing is qualitatively and, by applying a special image processing technique, also quantitatively evaluated. Coherent flow structures are also analyzed through spatial correlation and POD. For the MIVR, the microscopic particle velocimetry (micro-PIV or microPIV) is first employed to measure the velocity field. Results from Reynolds-averaged Navier-Stokes (RANS) simulations and large eddy simulations (LES) are compared to the micro-PIV results. Comparisons show LES is more suitable for simulating flow field in these reactors. In addition, another experimental method developed in this work is also applied to the MIVR, which couples the confocal laser scanning microscopy (CLSM) and the microscopic laser induced fluorescence (micro-LIF). More detailed and quantitatively accurate data are obtained for the CFD model validation. Passive scalar mixing and reactive mixing experiments are both accomplished to quantify the mixing at the maroscale and microscale respectively

Parametrical investigation for the optimization of spherical jet-stirred reactors design using large eddy simulations

Abstract Due to the importance of gas-phase chemical reaction kinetics in low-emission combustion, stirred tank reactors have been used for decades as an experimental tool to study high- and low-temperature oxidation. A Jet-Stirred Reactor (JSR) setup is valuable to determine the evolution of species mole fractions. For the accuracy of the experimental results, it is important that a JSR is designed such that the concentration field is as homogeneous as possible in order to avoid disturbance of the chemical kinetics. In this work, numerical simulations were performed to investigate the mixing in a JSR chamber. The turbulent structures inside the JSR and the nozzles are captured using Large Eddy Simulations. We conducted numerically a parametric study to evaluate the effects of thermodynamic conditions and geometrical parameters on the mixing characteristics. More specifically, the diameter of the spherical chamber is modified together with the diameter of the nozzles through which fresh gases are fed. The characterization of the gas flow inside a typical spherical JSR layout and results derived by the normalized standard deviation of a tracer mass fraction show that a reduction of the JSR diameter at high pressures improves the homogeneity. Further, we propose a new optimized configuration consisting of six nozzles pointing to the center of the reactor which provides a more uniform composition compared to the standard JSR design

Intensification of liquid mixing and local turbulence using a fractal injector with staggered conformation

Two self-similar, tree-like injectors of the same fractal dimension are compared, demonstrating that other geometric parameters besides dimension play a crucial role in determining mixing performance. In one injector, when viewed from the top, the conformation of branches is eclipsed; in the other one, it is staggered. The flow field and the fractal injector induced mixing performance are investigated through computational fluid dynamics (CFD) simulations. The finite rate/eddy dissipation model (FR/EDM) is modified for fast liquid-phase reactions involving local micromixing. Under the same operating conditions, flow field uniformity and micromixing are improved when a staggered fractal injector is used. This is because of enhanced jet entrainment and local turbulence around the spatially distributed nozzles. Compared with a traditional double-ring sparger, a larger reaction region volume and lower micromixing time are obtained with fractal injectors. Local turbulence around the spatially distributed nozzles in fractal injectors improves reaction efficiency