Modeling heat transfer in dilute two-phase flows using the Mesoscopic Eulerian Formalism

In dilute two-phase flows, accurate prediction of the temperature of the dis- persed phase can be of paramount importance. Indeed, processes such as evaporation or chemical reactions are strongly non-linear functions of heat transfer between the carrier and dispersed phases. This study is devoted to the validation of an Eulerian description of the dispersed phase –the Meso- scopic Eulerian Formalism (MEF)– in the case of non-isothermal flows. Di- rect numerical simulations using the MEF are compared to a reference La- grangian simulation for a two-dimensional non-isothermal turbulent jet laden with solid particles. The objectives of this paper are (1) to study the influ- ence of the thermal inertia of particles on their temperature distribution and (2) conduct an a posteriori validation of the MEF, which was recently ex- tended to non-isothermal flows. The focus is on the influence of additional terms in the MEF governing equations, namely heat fluxes arising from the Random Uncorrelated Motion (RUM). Results show that mean and rms of particle temperature are strongly dependent of the thermal Stokes number. The mean temperature is satisfactorily predicted by the MEF, comparing to the Lagrangian reference. Under the conditions of the present study, the RUM heat fluxes have a marginal influence on the mean particle tempera- ture. However, a significant impact was observed on the magnitude of particle temperature fluctuations. Neglecting the RUM heat fluxes leads to erroneous results while the Lagrangian statistics are recovered when it is accounted for in the regimes of low to moderate thermal Stokes number

Simulation of particle flow in an inertial particle separator with an Eulerian velocity re-associated two-node quadrature-based method of moments

This paper presents research into practical simulations of particle flow in inertial particle separators (IPS) for helicopters and tilt rotor aircraft. The flow field of the carrier gas is predicted by means of the two-equation k-ϵ turbulence model. An Eulerian methodology is used to trace the particle trajectories of foreign particles such as droplets, ice and sand. To predict the characteristics of particle wall bouncing in dilute particle flow, the velocity re-associated two-node quadrature-based method of moments (VR-QMOM) is used. The particle distribution in the IPS is predicted for various particle sizes and these are compared with results from a Lagrangian particle tracking method. The particle-wall interactions and the separation efficiencies are studied for solid particles bouncing off perfectly elastic walls and an IPS shell coated with the M246 alloy which changes the coefficients of restitution. The simulated separation efficiencies predicted by the Eulerian method are compared with the simulation using the Lagrangian method over a range of particle sizes. The VR-QMOM method is seen to reproduce the particle bouncing and trajectory crossing behavior and to agree well with the Lagrangian method for predicted separation efficiencies. The new VR-QMOM method is shown to be an accurate and convenient alternative to established Lagrangian approaches

Modeling of Gas-Solid Flows in Industry with Computational Fluid Dynamics Tools: An Assessment of Modeling Methodologies and Computational Challenges

The modeling work and simulation results contained within this thesis come from two different applications that together emphasize multiple of the challenges currently faced by researchers in the field of numerical modeling of gas-solids flows with computational fluid dynamics (CFD) tools, and highlight avenues of potential resolutions to these challenges. In the first body of work, the MFiX CFD suite, developed by the Department of Energy’s National Energy Technology Laboratory (DOE NETL), was utilized to model and simulate several experimental conditions of a fluidized bed (in 2D), and a hopper (in 3D), where solid-solid collision effects play a dominant role. Of concern in these studies are the physical prediction capabilities and associated computational costs of the three different multiphase frameworks available in MFiX: the Discrete Element Model (DEM), the Two Fluid Model (TFM), and the newer hybrid Multiphase Particle in Cell Model (MPIC). Initial selection of an appropriate multiphase modeling framework that satisfies the level of detail and computational cost constraints associated with the problem at hand, is crucial to the successful use of CFD tools in industry. The DEM and TFM frameworks were deemed to be the most accurate in simulating transient pressure profiles in the fluidized bed scenario compared to experimental measurements. TFM framework also proved to be 35% faster. While the MPIC framework was on average 90% faster than the DEM framework, it failed to produce reasonable predictions of physical flow behaviors. An additional motivation behind this research was to test and explore further reductions in computational costs offered by a recently developed interface of MFiX with the linear solver library, PETSc. Using previously identified numerical strategies in PETSc to solve the pressure equations, a more robust solver convergence behavior than the native pressure solver package was achieved across all three frameworks. Most notably, it enabled the use of larger and fewer time steps in the DEM framework, resulting in a 4-20% reduction in overall solve time to simulate 20 seconds of fluidized bed flow. Despite the significant reduction in computational time, simulation accuracy in terms of predicting the average pressure drop was slightly diminished using the PETSc solver in the DEM framework. Simulations of pure granular flow in a hopper revealed that while the TFM framework experienced difficulties converging the solids pressure term, it was still capable of predicting mass discharge rates that were very similar to those of the DEM framework, but at a comparatively lower computational cost. Again, the MPIC framework predictions differed significantly from the DEM results which are considered the benchmark/gold standard for modeling granular multiphase flows. Thus, despite the significant computational advantages of the MPIC framework over the other two, proper caution needs to be exercised when utilizing it to simulate densely packed solid flows. In the second body of work, a collection of CFD models and simulations were developed using the ANSYS Fluent DPM framework to simulate air combustion of three different coal types (Powder River Basin (PRB), Illinois #6, and Sufco 2) from select experiments conducted on a pilot-scale combustor from the University of Utah. The objective of this study was to investigate the sensitivity of ash deposition behaviors to select modeling parameters, with the aim of formulating a particle capture model. Ash formation and deposition is a complex physio-chemical process that negatively affects boiler operation and predicting ash deposit growth rates with CFD modeling techniques is extremely challenging. Many previous attempts by others neglect the importance of adequately resolving the particle size distribution and using an adequate spatial resolution near the heat transfer surface. Combustion modeling methodologies were validated against experimental measurements of flue gas ash concentrations and reactor profiles of temperature, and estimates of velocity. Simulation predictions were deemed to be in satisfactory agreement with experimental measurements. Temperature and velocity profiles were only mildly influenced by the resolutions of both the particle size distribution model and the near-boundary spatial mesh. Simulation predictions for impaction rates on a collector probe boundary were large in comparison to measured values of deposition rates, enforcing the importance of capture efficiency in the effort to accurately predict ash deposit growth rates. Impaction rates also proved to be moderately sensitive to the number of bins used to resolve the particle size distribution, and the degree of this sensitivity was unique to each coal type further emphasizing the challenges in universally modeling combustion and ash deposition across fuel sources. Impaction rates increased significantly when employing a more refined near-boundary mesh which highlights the importance of spatial resolution modeling parameters in successful ash deposition simulation efforts. Additionally, a Weber number criteria capture method was tested across all three coal types and critical Weber numbers were identified in each case which were significantly different between coal types. These values were found to be much smaller than 1 which signifies the importance of considering attraction forces between the particles and the deposition surface. Predicted deposition rates when applying the critical Weber numbers as capture criteria agreed well with measured values, but demonstrated sensitivity to the number of bins in the particle size distribution model. In the PRB and Illinois coal cases, a mere 10% adjustment in the critical Weber cutoff value resulted in a roughly 30% difference in predicted deposition rates, demonstrating that this method of modeling particle capture is not universal and should be used with caution. Results from this work demonstrate that ash deposition processes are still not fully understood, and the reinforces the need for more collaborative efforts between CFD modelers and experimentalists

ASHEE: a compressible, equilibrium-Eulerian model for volcanic ash plumes

A new fluid-dynamic model is developed to numerically simulate the non-equilibrium dynamics of polydisperse gas-particle mixtures forming volcanic plumes. Starting from the three-dimensional N-phase Eulerian transport equations for a mixture of gases and solid particles, we adopt an asymptotic expansion strategy to derive a compressible version of the first-order non-equilibrium model, valid for low concentration regimes and small particles Stokes $St<0.2$. When $St < 0.001$ the model reduces to the dusty-gas one. The new model is significantly faster than the Eulerian model while retaining the capability to describe gas-particle non-equilibrium. Direct numerical simulation accurately reproduce the dynamics of isotropic turbulence in subsonic regime. For gas-particle mixtures, it describes the main features of density fluctuations and the preferential concentration of particles by turbulence, verifying the model reliability and suitability for the simulation of high-Reynolds number and high-temperature regimes. On the other hand, Large-Eddy Numerical Simulations of forced plumes are able to reproduce their observed averaged and instantaneous properties. The self-similar radial profile and the development of large-scale structures are reproduced, including the rate of entrainment of atmospheric air. Application to the Large-Eddy Simulation of the injection of the eruptive mixture in a stratified atmosphere describes some of important features of turbulent volcanic plumes, including air entrainment, buoyancy reversal, and maximum plume height. Coarse particles partially decouple from the gas within eddies, modifying the turbulent structure, and preferentially concentrate at the eddy periphery, eventually being lost from the plume margins due to the gravity. By these mechanisms, gas-particle non-equilibrium is able to influence the large-scale behavior of volcanic plumes.Comment: 29 pages, 22 figure

Numerical Simulation of Three-Phase Flows in the Inverse Fluidized bed

The inverse three-phase fluidized bed has excellent potentials to be used in chemical, biochemical, petrochemical and food industries because of its high contact efficiency among each phase which leads to a good mass and heat transfer. The understanding of the hydrodynamics and flow structures in inverse three-phase fluidized beds is important for the design and scale up purposes. A CFD model based on the Eulerian-Eulerian (E-E) approach coupled with the kinetic theory of the granular flow is successfully developed to simulate an inverse three-phase fluidization system. The proposed CFD model for the inverse three-phase fluidization system is validated by comparing the numerical results with the experimental data. Investigations on the hydrodynamics and flow structures in the inverse three-phase fluidized bed under a batch liquid mode are conducted by numerical studies. The development of the fluidization processes and the general gas-liquid-solids flow structures under different operating conditions are further studied by the proposed three-phase E-E CFD model. Parametric studies including different inlet superficial gas velocities, particle densities, and solids loadings are investigated numerically. The numerical results show a general non-uniform radial flow structure in the inverse three-phase fluidized bed. It is also found that the particle distribution profiles in the axial direction relate to the solids loading, particle density and inlet superficial gas velocity. The existences of the liquid and solids recirculation inside the inverse three-phase fluidized bed are also noticed under the batch liquid mode. Moreover, the proposed CFD model for the inverse three-phase fluidized bed is further modified by adjusting the bubble size. The modified CFD model takes the bubble size effects into account and performs better on estimating the average gas holdup. In addition, a correlation between the bubble size and the superficial gas velocity, gas holdup and physical properties of the liquid and solid phases is proposed based on the numerical results. The predicted bubble size and the gas holdup in the inverse three-phase fluidized beds under a batch mode using the proposed correlation agree well with the experimental data. Therefore, the proposed three-phase E-E CFD model incorporated with the bubble size adjustment can be used to predict the performance of the inverse three-phase fluidization system more accurately

Mixing and Demixing Processes in Multiphase Flows With Application to Propulsion Systems

A workshop on transport processes in multiphase flow was held at the Marshall Space Flight Center on February 25 and 26, 1988. The program, abstracts and text of the presentations at this workshop are presented. The objective of the workshop was to enhance our understanding of mass, momentum, and energy transport processes in laminar and turbulent multiphase shear flows in combustion and propulsion environments