Towards accurate three-dimensional simulation of dense multi-phase flows using cylindrical coordinates

Most industrial scale fluidized-bed reactors are cylindrical, and the cylindrical coordinate system is a natural choice for their CFD simulation. There are, however, subtle complexities associated with this choice when using the Two-Fluid Model. The center of the grid forms a computational “boundary” and requires special treatment. Conventionally, a free slip no-normal flow condition has been used which does not predict the hydrodynamics accurately even when predicted parameters are in good agreement with measurement. Another difficulty is posed by the extremely small cells near the grid center, especially when simulating small scale experiments. The presence of these small cells raises concerns over the applicability of the Two-Fluid Model and is known to result in slow simulation convergence. These issues are addressed in the present study and appropriate solutions are proposed including the centerline treatment and the use of a non-uniform grid. Finally, the study compares the Cartesian grid with the cylindrical grid for application to fluidization. It is shown that simulating a cylindrical bed using the cylindrical grid is not only more accurate but also more computationally efficient. The analysis presented along with the proven computational efficiency of the cylindrical grid is especially significant considering that modeling commercial scale reactors, with multiple solid phases and chemical reactions, not only will require accurate description of the fluidization process but will also be exceedingly expensive in terms of computational cost.BP (Firm

Mixing dynamics in bubbling fluidized beds

Solids mixing affects thermal and concentration gradients in fluidized bed reactors and is, therefore, critical to their performance. Despite substantial effort over the past decades, understanding of solids mixing continues to be lacking because of technical limitations of diagnostics in large pilot and commercial‐scale reactors. This study is focused on investigating mixing dynamics and their dependence on operating conditions using computational fluid dynamics simulations. Toward this end, fine‐grid 3D simulations are conducted for the bubbling fluidization of three distinct Geldart B particles (1.15 mm LLDPE, 0.50 mm glass, and 0.29 mm alumina) at superficial gas velocities U/Umf = 2–4 in a pilot‐scale 50 cm diameter bed. The Two‐Fluid Model (TFM) is employed to describe the solids motion efficiently while bubbles are detected and tracked using MS3DATA. Detailed statistics of the flow‐field in and around bubbles are computed and used to describe bubble‐induced solids micromixing: solids upflow driven in the nose and wake regions while downflow along the bubble walls. Further, within these regions, the hydrodynamics are dependent only on particle and bubble characteristics, and relatively independent of the global operating conditions. Based on this finding, a predictive mechanistic, analytical model is developed which integrates bubble‐induced micromixing contributions over their size and spatial distributions to describe the gross solids circulation within the fluidized bed. Finally, it is shown that solids mixing is affected adversely in the presence of gas bypass, or throughflow, particularly in the fluidization of heavier particles. This is because of inefficient gas solids contacting as 30–50% of the superficial gas flow escapes with 2–3× shorter residence time through the bed. This is one of the first large‐scale studies where both the gas (bubble) and solids motion, and their interaction, are investigated in detail and the developed framework is useful for predicting solids mixing in large‐scale reactors as well as for analyzing mixing dynamics in complex reactive particulate systems.British Petroleum CompanyNational Energy Technology Laboratory (U.S.)United States. Department of Energ

3D Eulerian modeling of thin rectangular gas-solid fluidized beds: Estimation of the specularity coefficient and its effects on bubbling dynamics and circulation times

This study aims at investigating the influence of the wall boundary conditions and specifically the specularity coefficient on the fluidization behavior of a thin rectangular fluidized bed by means of 3D numerical simulation employing an Eulerian description of the gas and the solid phases. Thin rectangular fluidized beds have been extensively used in the research literature since it is assumed that the flow behaves like a simpler two-dimensional flow and hence they offer validation data for 2D simulations. However, the effects of the front and the back walls are significant, influencing the sensitivity of the fluidization hydrodynamics to the third dimension whose consideration is thus necessary. In order to investigate the influence of the specularity coefficient, ϕ (a parameter controlling the momentum transfer from the particles to the wall), on the fluidization hydrodynamics, a parametric analysis is conducted and the response of the bubble dynamics, reflecting the gasmotion, and the circulation fluxes, displaying the solids motion, are examined in detail. The computational results are compared with available experimental data in order to determine the values of ϕ that lead to the accurate description of the fluidization hydrodynamics via a two-fold validation strategy which involves the calculation of the circulation time and the solids concentration maps. It is observed that the appropriate value of the specularity coefficient depends rather strongly on the superficial gas velocity of the bed.BP (Firm

Modeling of Biomass Char Gasification, Combustion, and Attrition Kinetics in Fluidized Beds

Char conversion is one of the most pivotal factors governing the effectiveness of fluidized bed gasification systems. Gasification-assisted attrition is a phenomenon whereby heterogeneous reactions progressively weaken a char’s structure throughout its lifetime leading to enhanced attrition and the production of a significant fraction of fines that exit the reactor unconverted. While this effect has been observed and measured experimentally, few models have been developed to quantitatively account for it, particularly for biomass chars. In this study, a transient gasification and combustion particle model is presented to describe primary fragmentation, attrition, and heterogeneous reactions of a single batch of particles. A conversion-dependent structural function is proposed to describe gasification-assisted attrition, and the model parameters are fitted to published experimental data from Ammendola, P.; Chirone, R.; Ruoppolo, G.; Scala, F. Proc. Combust. Inst. 2013, 34 (2), 2735–2740. The fragile structure of char derived from wood chips contributes to a higher initial attrition rate than char from wood pellets, but the hardness of both feedstocks is shown to deteriorate rapidly as they convert. A shrinking particle combustion model which accounts for variable feedstock properties is comprehensively presented and validated against the aforementioned data set. The combustion behaviors of both feedstocks are found to strongly depend on particle size/geometry because of significant mass transfer limitations. Using a residence time distribution approach, the model is extended to describe a continuously fed system in order to examine the sensitivity of steady-state outputs (conversion and residence time) to the operating temperature, pressure, and kinetics. As the temperature increases, the char reactivity also increases but the coupled and competing effect of gasification-assisted attrition acts to shorten the residence time of the char particles making complete char conversion very difficult even at 900 °C—the upper operating temperature limit for most single-stage fluidized bed gasification systems. Low operating temperatures result in longer average residence times and higher steady-state char inventories, and slower kinetics lowers the overall conversion. Because of inhibition effects, elevated operating pressures have a smaller impact on improving conversion compared to higher temperature. The steady model further provides a rigorous method for estimating the maximum stable biomass feeding rates as a function of relevant independent parameters including reactor temperature, pressure, volume, and feedstock characteristics.BP (Firm)United States. Department of Energy (National Energy Technology Laboratory Research Participation Program

Eulerian–Eulerian simulation of dense solid–gas cylindrical fluidized beds: Impact of wall boundary condition and drag model on fluidization

Modeling the hydrodynamics of dense-solid gas flows is strongly affected by the wall boundary condition and in particular, the specularity coefficient ϕ which characterizes the tangential momentum transfer from the particles to the wall. The focus of this study is to investigate the impact of ϕ on the fluidization hydrodynamics using a fully Eulerian description of the solid and gas phases in 3D cylindrical coordinates. In order to quantify this impact, tools for characterizing the bubbling dynamics and solids circulation are developed and applied to both lab-scale (diameters 10 cm and 14.5 cm) and pilot-scale (diameter 30 cm) cylindrical beds. Comparison of simulation predictions with experimental data for different fluidization regimes and particle properties suggests that values of ϕ in the range [0.01,0.3] are suitable for simulating most dense solid–gas flows of practical interest. It is also shown that for this range of ϕ, the fluidization hydrodynamics are not significantly dependent on the choice of ϕ especially as the bed diameter is increased. Additionally, 3D validation of the variable ϕ model by Li and Benyahia [1] shows the bubble diameter predictions to be in excellent agreement with experiment and the average value of ϕ predicted within the range [0.01,0.3]. Quantifying the impact of ϕ and establishing an appropriate range is not only important for accurate simulations at both lab and pilot scales but also validation of models and sub-models for a better understanding of the fluidization phenomenon. Finally, a comparison of the Gidaspow and Syamlal–O'Brien gas–solids drag model shows that the former is more applicable to homogeneous bubbling fluidization (U/U[subscript mf]< 4) while the latter is only suitable for high velocities (U/U[subscript mf]< 4) associated with larger bubbles and slugs.BP (Firm

A 2-D DNS study of the effects of nozzle geometry, ignition kernel placement and initial turbulence on prechamber ignition

A parametric direct numerical simulation study was conducted to investigate the effects of the initial flow field (quiescent or turbulent), nozzle inlet sharpness and width, main chamber composition (lean and stoichiometric), and ignition kernel placement in a two-dimensional prechamber (PC) ignition system. The strongly coupled operating and geometric parameters determine the time at which the flame exits the prechamber, the transient structure and penetration of the initially cold and subsequently hot reactive jet and their impingement on the lower main chamber (MC) wall, affecting the combustion mode and the fuel consumption rate. The temperature of the flame reaching and crossing the nozzle is affected by the flame exit time and is significantly lower than the adiabatic flame temperature of the planar flame, although no quenching is observed. Interaction with the flow field (strong small scale vortices for narrow and sharp entry nozzles, large vortices for wide nozzles) generated close to the exit increases the surface area of the flame and its interaction with the MC mixture. Jet penetration and impingement on the lower MC wall is determined by combustion in the PC and the flow field it generates in the main chamber. Impingement results in large scale vortical structures, which further contribute to the flame area increase and accelerate the consumption of the MC charge at later times. For the conditions studied, budget analysis shows that the main combustion mode is premixed deflagration with locally enhanced or reduced reactivity. Local flame–flame interactions which are more pronounced close to the nozzle exit and the lower MC wall can increase the propagation speed up to six times compared to the planar flame. The evolution of the probability density functions of different quantities is used to characterize the strongly transient process. © 2020 The Combustion Institute

Multiphase-flow Statistics using 3D Detection and Tracking Algorithm (MS3DATA): Methodology and application to large-scale fluidized beds

Bubble dynamics play a critical role in the hydrodynamics of fluidized beds and significantly affect reactor performance. In this study, MS3DATA (Multiphase-flow Statistics using 3D Detection And Tracking Algorithm) is developed, validated and applied to numerical simulations of large-scale fluidized beds. Using this algorithm, bubbles are detected using void fraction data from simulations and are completely characterized by their size, shape and location while their velocities are computed by tracking bubbles across successive time frames. A detailed analysis of 2D (across vertical sections) and 3D bubble statistics using 3D simulations of lab-scale (diameter 14.5 cm) and pilot-scale bed (diameter 30 cm) is presented and it is shown that the former (a) under-predicts sizes of larger bubbles, (b) cannot detect a large fraction of small bubbles (<3 cm) and (c) is unable to track the azimuthal motion of bubbles in the larger bed. The scalability of the algorithm is discussed by comparing the computational cost of computing bubble statistics on highly resolved grids. Even though 3D bubble detection is significantly more expensive than 2D detection, the cost is still negligible compared to the cost of accurate simulations. Besides application to fluidization simulation data of large fluidized beds, this algorithm can be easily extended to characterize bubbles, droplets and clusters in other areas of multiphase flows. Keywords: Multiphase flow; Fluidized bed; Eulerian simulations; Bubble dynamics; 3D statistics; Large-scale detection and trackin

Computational study of the Premixed Charge Compression Ignition combustion in a Rapid Compression Expansion Machine: Impact of multiple injection strategy on mixing, ignition and combustion processes

[EN] Combustion processes operating under the low-temperature (LTC) are a promising alternative for internal combustion engines, achieving high efficiency and low emissions within the legislative framework. Under this field of study, the concept of Premixed Charge Compression Ignition (PCCI) was introduced to control the combustion phase by varying the injection strategy. The present research, based on the aforementioned combustion strategy, employs Computational Fluid Dynamics (CFD) to study the mixing, ignition and initial combustion processes in a Rapid Compression and Expansion Machine (RCEM). The operating conditions of the PCCI strategy are achieved through the split injection technique, which separate the injection into several shots. In this case, the injection strategy consists of a long base injection and two very short, consecutive post injections delivered when the piston is close to the top dead center. Three different operating conditions have been considered which differ mainly in the duration and the starting time of the first injection. Large Eddy Simulations (LES) are selected to account for the effects of turbulence. Results are validated against experimental data, showing good agreement between both approaches. The effect of the first injection on the second one in multiple injection strategy is appreciated as the penetration of the subsequent fuel sprays is faster. Differences in the local mixture field and ignition dynamics are observed as a function of varying operating conditions. The ignition process is highly influenced by the operating strategies affecting the distribution of the mixture and thus the location of the low temperature oxidation. In the current computational study, in addition to the evaporating fuel scalar, the corresponding source term due to evaporation has also been provided to three passive transported scalars, namely three mixture fractions, one for each injection event. This allows for discerning the fuel stemming from each one of the pulses in the overall distribution, thus offering new insights into the effect of the injection strategy.Authors would like to thank the FVV (Forschungsvereinigung Verbrennungskraftmaschinen | Research Association for Combustion Engines, project "Partially premixed diesel combustion with multiple injections'', no. 1352) and from the Swiss Federal Office of Energy (grant no. Sl/501744-01). Additionally, the Ph.D. student Maria Martinez has been funded by a grant from the Government of Generalitat Valenciana with reference ACIF/2018/118 with financial support from The European Union and a grant for predoctoral stays out of the Comunitat Valenciana with reference BEFPI/2020/057.Martínez, M.; Altantzis, C.; Wright, YM.; Marti-Aldaravi, P.; Boulouchos, K. (2022). Computational study of the Premixed Charge Compression Ignition combustion in a Rapid Compression Expansion Machine: Impact of multiple injection strategy on mixing, ignition and combustion processes. Fuel. 318:1-16. https://doi.org/10.1016/j.fuel.2022.12338811631

Modeling of Biomass Char Gasification, Combustion, and Attrition Kinetics in Fluidized Beds

Char conversion is one of the most pivotal factors governing the effectiveness of fluidized bed gasification systems. Gasification-assisted attrition is a phenomenon whereby heterogeneous reactions progressively weaken a char’s structure throughout its lifetime leading to enhanced attrition and the production of a significant fraction of fines that exit the reactor unconverted. While this effect has been observed and measured experimentally, few models have been developed to quantitatively account for it, particularly for biomass chars. In this study, a transient gasification and combustion particle model is presented to describe primary fragmentation, attrition, and heterogeneous reactions of a single batch of particles. A conversion-dependent structural function is proposed to describe gasification-assisted attrition, and the model parameters are fitted to published experimental data from Ammendola, P.; Chirone, R.; Ruoppolo, G.; Scala, F. Proc. Combust. Inst. 2013, 34 (2), 2735–2740. The fragile structure of char derived from wood chips contributes to a higher initial attrition rate than char from wood pellets, but the hardness of both feedstocks is shown to deteriorate rapidly as they convert. A shrinking particle combustion model which accounts for variable feedstock properties is comprehensively presented and validated against the aforementioned data set. The combustion behaviors of both feedstocks are found to strongly depend on particle size/geometry because of significant mass transfer limitations. Using a residence time distribution approach, the model is extended to describe a continuously fed system in order to examine the sensitivity of steady-state outputs (conversion and residence time) to the operating temperature, pressure, and kinetics. As the temperature increases, the char reactivity also increases but the coupled and competing effect of gasification-assisted attrition acts to shorten the residence time of the char particles making complete char conversion very difficult even at 900 °C—the upper operating temperature limit for most single-stage fluidized bed gasification systems. Low operating temperatures result in longer average residence times and higher steady-state char inventories, and slower kinetics lowers the overall conversion. Because of inhibition effects, elevated operating pressures have a smaller impact on improving conversion compared to higher temperature. The steady model further provides a rigorous method for estimating the maximum stable biomass feeding rates as a function of relevant independent parameters including reactor temperature, pressure, volume, and feedstock characteristics.BP (Firm)United States. Department of Energy (National Energy Technology Laboratory Research Participation Program