Chemical and Biomolecular Engineering Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2751
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Item CFD INVESTIGATION OF A PULSE JET MIXED VESSEL WITH RANS, LES, AND LBM SIMULATION MODELS(2023) Kim, Jung; Calabrese, Richard V.; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Pulse Jet Mixed (PJM) vessels are used to process nuclear waste due to their maintenance free operation. In this study we model the turbulent velocity field in water during normal PJM operation to gain insight into vessel operations and to evolve a modeling strategy for process design and operator training. Three transient simulation models, developed using Large Eddy Simulation (LES), unsteady Reynolds-Averaged Navier-Stokes (RANS), and Lattice Boltzmann Method (LBM) techniques, are compared to velocity measurements acquired for 3 test scenarios at 3 locations in a pilot scale vessel at the US DOE National Energy Technology Laboratory (NETL). The LES and RANS simulations are performed in ANSYS Fluent, and the LBM simulations in M-STAR.The LES model well predicts the experimental data provided that the operational pressure profile within the individual pulse tubes is considered. While the RANS model failed to predict the data and exhibited significant differences from LES with respect to turbulence quantities, it is a useful comparison tool that can quickly predict averaged flow parameters. The LBM model’s rigid grid system is deemed unsuitable, as currently configured, for the NETL PJM vessel’s wide range of length scales and curved boundaries, resulting in the longest simulation time and least accurate velocity predictions. Predicted velocity and turbulence metrics are explored to better understand the strengths and failures of the three models. Because the LES model produced the most accurate predictions, it is exploited to generate animations and still images on various 2D planes that depict extremely complex flow patterns throughout the vessel with numerous local jets and mixing layer vortices The study concludes with recommendations for future research to improve the model development and validation strategy.Item Breakage of Single Droplets in 2-D Inertial Flows(2018) Ko, Derrick I.; Calabrese, Richard V; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Droplet break-up research has traditionally focused on droplets in: 1) generally uniform flow fields (constant strain rates or constant turbulence dissipation rates) that are easier to characterize and study, and in 2) highly complex flow fields generated by mixing devices in which the evolution of an entire droplet population with time is of interest. The current work adds to the existing body of knowledge by investigating the effect of short-term high-intensity deformation events on the break-up of single large droplets in both turbulent and inertial laminar flows. This approach consists of two components: high-speed imaging of droplets as they pass through a 2-D slit orifice and CFD simulations of the orifice flow field. The experimental trajectories of the droplets are combined with the CFD-generated flow field to determine the deformation history of the droplet prior to break-up. In turbulent 2-D orifice flows, droplets and bubbles on the order of the macroscale of turbulence were studied. For these large droplets and bubbles, it was found that the product of strain rate magnitude and un-deformed diameter (essentially the velocity difference across the droplet) was a more suitable velocity scale. A new form of locally-derived, trajectory-dependent Weber number, consisting of the maximum average strain rate magnitude over an exposure time equal to 0.04 multiplied by the Stokes particle relaxation time, was used to develop a break-up probability model that can be applied to the break-up of both liquid droplets and gas bubbles. The model adds weight to the break-up interpretation of Levich (1962); break-up occurs due to the internal pressure fluctuations overcoming the interfacial stresses of the deformed droplet. In inertial laminar 2-D orifice flows, the break-up of water droplets in oil was studied at two viscosity ratios. The recommended local Weber number was again based on the maximum average strain rate magnitude over a particular exposure time, but this exposure time was instead 8 multiplied by the oscillation time scale. Using the maximum length achieved by the droplet as the length scale was also found to better represent the potential for break-up. With these modifications, and an associated capillary number-based model for predicting the drop draw ratio, two local Weber number thresholds were defined. First, the threshold for break-up is at Welocal = 30. Second, the threshold for producing large daughter droplets, termed fracturing in this work, is at Welocal = 1,000. Between these thresholds, droplets may fracture or undergo a mechanism termed erosion in this work, where a small number of tiny droplets break off from the main body of the droplet. Both of these break-up types are based on an elongative end-pinching mechanism.Item COMPUTATIONAL FLUID DYNAMICS SIMULATIONS OF A PIPELINE ROTOR-STATOR MIXER(2017) Minnick, Benjamin Austin; Calabrese, Richard V; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Rotor-stator mixers provide high deformation rates to a limited volume, resulting in intensive mixing, milling, and/or dispersion/emulsification. CFD simulations of mixers provide flow field information that benefit designers and end users. This thesis focuses on transient three-dimensional simulations of the Greerco pipeline mixer, using ANSYS FLUENT. The modeled unit consists of two conical rotor-stator stages aligned for axial discharge flow. Flow and turbulence quantities are studied on a per stator slot and per rotor stage basis. Comparisons are made between the LES and RANS realizable k-ε model predictions at various mesh resolutions. Both simulations predict similar mean velocity, flow rate, and torque profiles. However, prediction of deformation rates and turbulence quantities, such as turbulent kinetic energy and its production and dissipation rates, show strong dependencies on mesh resolution and simulation method. The effect of operating conditions on power draw, throughput, and other quantities of practical utility are also discussed.Item Droplet Dynamics in Microfluidic Junctions(2012) Boruah, Navadeep; Dimitrakopoulos, Panagiotis; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The dynamics of droplets in confined microfluidic geometries is a problem of fundamental interest as such flow conditions occur in multiphase flows in porous media, biological systems, microfluidics and material science applications. In this thesis, we investigate computationally the dynamics of naturally buoyant droplets, with constant surface tension, in cross-junctions and T-junctions constructed from square microfluidic channels. A three-dimensional fully-implicit interfacial spectral boundary element method is employed to compute the interfacial dynamics of the droplets in the junctions and investigate the problem physics for a wide range of flow rates, viscosity ratios and droplet sizes. Our investigation reveals that as the flow rate or the droplet size increases, the droplets show a rich deformation behavior as they move inside the microfluidic devices. In the cross-junction, after obtaining a bullet-like shape before the flow intersection, the droplet become very slender inside the junction (to accommodate the intersecting flows), then it obtains an inverse-bullet shape as it exits the junction which reverts to a more pointed bullet-like shape far downstream. In the T-junction, the droplet obtains a skewed-bullet shape and a highly deformed slipper shape after entering the flows intersection. The viscosity ratio also has strong effects on the droplet deformation especially for high-viscosity droplets which do not have the time to accommodate the much slower deformation rate during their channel motion. Our results are in agreement with experimental findings, and provide physical insight on the confined droplet deformation.Item CFD SIMULATIONS FOR SCALE UP OF WET MILLING IN HIGH SHEAR MIXERS(2011) Yang, Meng; Calabrese, Richard V; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Rotor-stator mixers are widely used in the chemical and pharmaceutical process industries. Up to now, however, few papers discuss the mean flow and turbulence fields generated by them and their influence on final product quality. In this work, CFD results at different scales are used to aid in the scale up of crystal wet milling processes. CFD simulations were performed to simulate different scale mixers. In addition, wet milling studies were conducted at the bench scale to complement the CFD results and predict wet milling performance in larger scale mixers. The flow properties in a batch Silverson L4R rotor-stator mixer at 4000 and 6000 rpm were investigated. A hybrid technique was developed. The new method is computationally efficient compared with the standard sliding mesh method. Macro scale properties are predicted. The turbulent flow field and deformation rate field are compared and analyzed. After obtaining fully converged flow fields, one way coupled particle tracking calculations were performed using an efficient fast particle tracking code. Particles trajectories were recorded, and analyzed. To validate the simulated flow field, particle image velocimetry (PIV) experiments were conducted. CFD simulations of Silverson inline L4R (bench scale), 450LS (pilot scale) and 600LS (plant scale) mixers were conducted at constant tip speed to investigate the scale up effect. The macro scale properties werer predicted. The mean velocity, turbulent and deformation rate fields were investigated. The flow properties of the 450LS and 600LS mixers are quite similar, but they are significantly different from those of the L4R (bench scale) mixer. Therefore, it may be resonable to scale up from pilot scale to plant scale by the general accepted tip speed scale up criterion. However, considering tip speed alone may lead to a significant discrepancy between bench scale and larger scales. Bench scale wet milling experiment were performed at 4000, 6000 and 8000 rpm using sucrose and mannitol in the Silverson L4R inline mixer. The crystal size decreases with rotation rate at both free pumping conditions and constant flow rate conditions. To investigate the effect of flow rate, wet milling of granulated sucrose in the Silverson L4R inline mixer with constant rotor tip speed were performed at different flow rates. It is found that the crystal size increases with the flow rate.Item FLUID AND PARTICLE DYNAMICS IN AN AEROSOL VIRTUAL IMPACTOR(2004-05-03) Charrouf, Marwan; Calabrese, Richard V; Chemical EngineeringThe collection and characterization of chemical and biological aerosols is essential to many areas of particle research such as toxicological studies, pollutant sampling, and biohazard assessment. This work presents the simulation of a low cutpoint, high volume aerosol sampling device known as the "virtual impactor". A steady state, three dimensional RANS type calculation is done using the FLUENT(TM) computational fluid dynamics code to predict the turbulent flow field inside the device. Particle collection efficiency and wall losses are then obtained by solving the particle equation of motion governed by drag for mono-dispersed samples of spherical particles in the 0.1-0.4 micro-meter diameter range. Predictions of the mean fluid velocity field with the incompressible Reynolds stress model and the compressible k-epsilon turbulence model are relied upon for conducting particle tracking calculations. FORTRAN 90 computer code is developed to solve the particle equation of motion using an implicit second order accurate time integration scheme. In addition, a multi-variate, scattered point interpolation method is implemented to obtain the fluid velocity at a position away from an Eulerian mesh point. It is found that "adaptive" drag law models are necessary to correctly account for slip and compressibility. The results indicate the trends observed in the experiments, and a 50% cutpoint diameter between 0.250 and 0.275 micro-meter. Recommendations for improved modeling in future work are made.