Turbulent Fluidization Regime Research Articles

Heat transfer in a fluidized bed tubular solar receiver. On-sun experimental investigation

Using particles as heat transfer fluid in a solar receiver is an attractive way to increase the global efficiency of solar power plants. In the case of the particle-in-tube solar receiver concept, the fluidized particles circulate vertically in tubes thanks to a controlled pressure gradient and an additional air injection. Experiments are conducted with olivine particles in a one-tube mock-up at the 1 MW CNRS solar furnace (France). The tube of high aspect ratio (more than 3 m height over 48 mm internal diameter) is irradiated along a 1-m height with several solar flux configurations that correspond to realistic operation conditions of a solar thermal power plant. The novelties of this paper lie in extending the operating parameters compared to earlier studies and in linking the fluidization regimes to wall heat transfer. Slugging, turbulent fluidization and fast fluidization regimes are detected within the receiver tube. It is shown that the system can tolerate high solar flux densities, up to 800 kW/m2. The system proves to be highly controllable and flexible with respect to particle mass flux variation and transient operations. Particle temperature increase ranges from 100 to 650 °C depending on the particle mass flux. Maximum thermal efficiency and particle outlet temperature of respectively ∼ 60 % and 680 °C are obtained, proving that this technology can be combined with highly efficient thermodynamic cycles. Global wall-to-particle heat transfer coefficient varies strongly with particle mass flux. It reaches a quasi-plateau above 15 kg/(m2s) with a mean value of 1000 ± 200 W/(m2K) and peaks data up to 1500 W/(m2K). A dimensionless heat transfer index is derived to account for the effects of the particle mass flux on the experimental results. It highlights that both increasing the temperature and working in the turbulent fluidization regime result in the increase of the heat transfer between the tube walls and the particles.

Effect of temperature on the hydrodynamics of a fluidized bed circulating in a long tube for a solar energy harvesting application

The particle-in-tube solar receiver concept for solar towers uses fluidized particles as heat transfer fluids. The experiments are conducted with a single tube of aspect ratio H/D = 67 irradiated over a 1-m height using concentrated solar energy. Olivine particles of Geldart’s Group A are used. Fluidization regimes are identified thanks to pressure signal analyses, and regime maps are plotted depending on the temperature in the range 150–700 °C. Both the local slip Reynolds number and the particle temperature govern the regime transitions. The limits and size of the turbulent fluidization regime domain decrease with temperature. The particle volume fraction also decreases with temperature. Finally, the intensity of the wall-to-particle heat transfer is discussed as a function of the fluidization regimes. As an indicator of the heat transfer intensity, a dimensionless coefficient is derived. This coefficient increases with temperature and exhibits the highest values for the turbulent fluidization regime.

Experimental Study of an upflow Fluidized Bed: Identification of Fluidization Regimes

The concept of solar receiver using fluidized particles as heat transfer fluid is attractive from the point of view of its performance but also of the material used. In this concept, the receiver is composed of tubes subjected to concentrated solar radiation in which the fluidized particles circulate vertically. Circulation in the tubes, immersed in a “nurse” fluidized bed, is ensured thanks to a controlled pressure difference imposed on the latter and secondary aeration. This ventilation located at the bottom of the absorber tubes makes it possible to control the fluidization regimes. The latter strongly influence the parietal heat transfers and therefore the performance of the receiver. In order to better understand the conditions of appearance of these regimes and to better identify them, a study at room temperature was carried out with a tube 45 mm in internal diameter and 3.63 m in height. The tube is instrumented with several pressure sensors distributed over its height. More than 170 experiments have been performed exploring wide ranges of particle and aeration flow rates, with and without particle circulation. Signal processing methods, classically used in the scientific literature of fluidized beds, are applied. Combined together, these methods have enabled the identification of bubbling, pistoning (of the wall and axisymmetric), turbulent fluidization and rapid fluidization regimes. The pooling of all this information allows the establishment of a diagram of the fluidization regimes and their transition, showing that the local slip velocity is the key parameter governing the structure of the flow.

Catalyst-scale investigation of polydispersity effect on thermophysical properties in a commercial-scale catalytic MTO fluidized bed reactor

The methanol-to-olefin (MTO) process is commercialized worldwide, yet the understanding of in-furnace phenomena is still lacking. Accordingly, this work explores the multiphase flow and thermochemical characteristics in a commercial-scale MTO fluidized bed reactor using a three-dimensional multiphase particle-in-cell model. After model validations, the intrinsic relationship of solid transportation and thermochemical properties is elucidated together with the study of the polydispersity effect on reactor performance. The non-uniform gas-solid distribution in the reactor demonstrates the turbulent fluidization regime. Heat transfer coefficients (HTCs) of about 300 and 200 W/(m2·K) are observed in the medium and upper regions, respectively. The small velocity, particle Reynolds number, temperature, and HTC correspond to those large-size catalysts in the dense region where particles are nearly close packing with a large solid volume fraction. Increasing particle size distribution (PSD) width increases the catalyst HTCs and the mass fraction of C2H4, promoting reactor performance. Besides, increasing the PSD width enlarges gas viscosity, gas specific heat capacity, and gas thermal conductivity but decreases gas density.

Computational simulations of hydrodynamics of supercritical methanol fluid fluidized beds using a low density ratio-based kinetic theory of granular flow

Hydrodynamics of fluid and particles were simulated using a low density ratio-based kinetic theory of granular flow (KTGF) in supercritical methanol (ScM) fluidized beds (FB). Results indicated that the fluidization state of the methanol fluid and particle mixtures change progressively from particulate to aggregative fluidization. A transition exists with wavy-like flows near the bottom and churn-like flows at the upper part along bed height, unlike the homogeneous fluidization in atmospheric methanol fluid FB and turbulent fluidization regime with particle strands in ScM FB. The threshold to identify the occurrence of strands was proposed in terms of mean value and standard deviation of solid volume fractions. The frequencies of particle strands increased with increasing methanol fluid temperatures and pressures. The computed fluid volume fractions agreed with experimental data in a supercritical carbon dioxide fluid fluidized bed.

A hybrid mesoscale closure combining CFD and deep learning for coarse-grid prediction of gas-particle flow dynamics

This study develops filtered two-fluid model (fTFM) closures by coupling computational fluid dynamics (CFD) and deep learning algorithm (DL) for enabling coarse-grid simulations at reactor scales. Mesoscale drag, solids pressure and viscosity are modeled using an isotropic or anisotropic method. Subsequently, a priori analysis and a posteriori analysis of the present models along with other previously proposed closures are conducted. Comparison with the experimental data covering a broad range of operating conditions indicates that the mesoscale solids stress can be neglected in bubbling and turbulent fluidization regimes. However, the contribution of solids stress is clearly not insignificant at very low superficial gas velocities. Moreover, the drag model considering the anisotropy shows better prediction performance in the turbulent fluidization regime. In short, the present study develops and validates a DL-fTFM coupling algorithm applicable for gas-particle simulations.

CFD investigation of low-attrition air-reactor designs for the NETL chemical-looping combustion reactor

The flow dynamics and attrition characteristics of three air reactor designs are investigated using an Eulerian-Eulerian kinetic theory model. The hydrodynamics of these reactor designs are analyzed to identify the flow regimes in which the particles evolve. The investigated designs, which differ in the configuration of the secondary flow ports, exhibit bubbling and turbulent fluidization regimes in similar spatial regions. The most substantial differences in the hydrodynamics was in the turbulent fluidized regions near the secondary inlets. The mechanical stressing environments undergone by the particles were analyzed using the collision energy spectra, reconstructed using the local flow field properties (i.e. granular temperature and particle number density). The spectra show that the increase in the specific collision power is associated with the regions near the secondary injector ports, which are in a turbulent fluidization regime. A simple analysis relying upon a partitioning of the energy spectrum, showed that attrition rate may be reduced by about 34% over the original design.

Disclosing recurrence properties in fluidized beds

The time-efficient method of recurrence CFD (rCFD) uses the pseudo-periodic nature of a flow, extrapolating the passive transport quickly to infinity. In that scope, the recurrence/distance matrix for bubbling and turbulent fluidization regimes are investigated to find the recurrence properties. The results indicate the need for posterior spatial filtering in the turbulent regime which, in turn, reveals recurrent uniform superstructures with a clear fingerprint on the distance/recurrence matrix.

Numerical analysis of hydrodynamics and thermochemical property of biomass gasification in a pilot-scale circulating fluidized bed

This study presents a three-dimensional reactive multiphase particle-in-cell (MP-PIC) model to simulate the hydrodynamics and thermochemical property of the biomass gasification process in a pilot-scale circulating fluidized bed gasifier. The bed hydrodynamics and thermochemical characteristics are investigated, together with discussing the effects of operating conditions on the species distribution, lower heating value (LHV), and combustible gas concentration (CGC). The results show that solid particles present oscillation behaviors for the time elapsed at the turbulent fluidization regime. Solid flux and rising velocity have peaks of about 249 kg/(m2·s) and 6.2 m/s in the core region of the bed, respectively. Increasing the gasification temperature reduces the CGC from 15.85% to 15.16%. However, only the concentration of H2 increases with the enlargement of the steam biomass ratio. Comparatively, the equivalence ratio (ER) has the most direct impact on gasification efficiency, and the increase of the ER adversely affects the LHV and CGC. The effect of particle diameter on the LHV, the magnitude of which can cover a range from 3.87% to 3.98%, is negligible compared with the other operational parameters, at least for the specific cases in the present investigation.

Studies on the local flow characteristics and flow regime transitions in a square fluidized bed

Flow patterns seen in a fluidized bed of square cross-section during the transition from bubbling to turbulent fluidization were studied with differential pressure data and optical fiber probe signals. The transition velocity to turbulent flow regime (Uc), based on differential pressure analysis varies with measurement locations. Spatial distributions of the dilute and dense phases were illustrated by contour maps of the time averaged solids holdup (εs) and standard deviation (σs). A clear correlation between σs and εs of the local solids holdup signals was found and developed using a bimodal distribution analysis. Based on that, the relationship between local fluctuations and global regime transition was proposed. The transition to the turbulent regime is identified to the proportion of the σs = 0.16 contour line enclosure reaching higher than 0.5. A new regime map is proposed to identify the bubbling, turbulent and fast fluidization regimes and to delineate the regime transitions in terms of the relationship between εs and σs.

Photocatalytic degradation of n-hexane in a circulating fluidized bed: An investigation based on the freeboard entrainment model

The fluidization behaviour of a circulating fluidized bed (CFB) photoreactor was varied by applying different flow (Q) and solid feed rates (Fs), and the performance of the reactor for the degradation of the target pollutant n-hexane was compared for a wide range of fluidization regimes. The Freeboard Entrainment Model was applied to model and predict the values of solids hold-up (εs) in the experimental range evaluated. The optimum n-hexane degradation rate for the CFB was found to be 4.0 × 10−4 mol m-3 s-1 operating in turbulent fluidization regime with εs = 0.10 with the finer solid size distribution, demonstrating that high-velocity fluidization regimes can be used to enhance photocatalytic reaction rates based on better UV-photocatalyst contact.

Diagnosis of hydrodynamic regimes from large to micro-fluidized beds

Micro-fluidized bed (MFB) technology is a newly emerging technique that is receiving increasing interest for various industrial applications. MFB is generally characterized by the use of inner diameters (Dt) of a few millimeters, and a large specific contact surface area between the walls and the fluidization system. These specific features have the advantage of enabling ultra-fast heat dissipation for exothermic reactions, as well as isothermal conditions. However, the wall effect also clearly prevails, leading to complicated hydrodynamic phenomena. In addition, the study of MFB is challenging, as a consequence of the difficulties encountered in its characterization, due to strong probe interference effects. In order to understand the wall effect and to make use of suitable fluidization conditions for practical applications of MFB, the present study first establishes a diagnostic methodology, and then applies this to study the influence of a reduction in Dt from large to micro-fluidized bed scales. Experiments were carried out in six glass columns with Dt ranging between 4 and 100 mm, using spherical Geldart group B particles (glass beads). The methodology described here is based on the analysis of pressure fluctuation measurements, which are used to monitor fluidization simultaneously in the time and frequency domains. Fixed bed, pseudo-homogeneous fluidization, bubbling, slugging, and turbulent fluidization regimes are identified in MFBs. When Dt is reduced from large-to micro-fluidized bed scales, minimum fluidization, bubbling and slugging are delayed, whereas the onset of turbulent fluidization occurs more rapidly. The late stage of turbulent fluidization covers a relatively wide range of gas velocities, and is found to have homogeneous fluidization structures, which are of interest for practical MFB applications such as Fischer–Tropsch synthesis.

Design of a Fast Internal Circulating Fluidized-Bed Gasifier with a conical bed angle

The main purpose of a fast internal circulating fluidized bed gasifier is the steam reforming of solid organic matter, like biomass, to a nearly nitrogen-free syngas. The calorific value of this syngas is approximately three times higher than the gas from common air-driven gasifiers. This article deals with a study of the particle dynamics in a 1 MWt fast internal circulating fluidized bed plant and focuses on the design of the gasification reactor?s geometry. Superheated steam is used for the fluidization and gasification in the reactor. The gasification of solid fuels causes an increase in the volume flow of the fluidizing gas and at the same time also a change in the fluidization regime. Approaching a turbulent fluidization regime or even fast fluidization is not desirable. However, with the proper design of reactor, i. e., an appropriately conical bed angle, suitable gasification conditions in the form of a fluidizing regime can be achieved across the entire height of the bed. For the purposes of the experimental research, a semi-industrial unit was set-up. The process was designed and experimentally tested on a lab-scale, cold-flow model and scaled-up to a semi-industrial process. The guidelines for designing the geometry of the gasification reactor were set.

Assessment of mesoscale solid stress in coarse‐grid TFM simulation of Geldart A particles in all fluidization regimes

This study focused on assessing the effect of mesoscale solid stress in the coarse grid two‐fluid model (TFM) simulation of gas–solid fluidized beds of Geldart Group A particles over a broad range of flow regimes, including bubbling, turbulent, fast, and pneumatic transport fluidization regimes. Particularly, the impact of mesoscale solid pressure, mesoscale solid viscosity, and mesoscale solid stress anisotropy were investigated by comparing six different coarse‐grid TFM settings. Compared with the available experimental data, it is found that both the kinetic theory‐based TFM with only drag correction and the filtered TFM can predict the flow behavior in all fluidization regimes. Mesoscale solid pressure and viscosity have the opposite impact on flow hydrodynamics; they compete and offset each other, which confirms the assumption employed in many previous studies that the mesoscale solid stress could be neglected in coarse‐grid TFM simulation. Published 2018. This article is a U.S. Government work and is in the public domain in the USA. AIChE J, 64: 3565–3581, 2018

Development and validation of an enhanced filtered drag model for simulating gas-solid fluidization of Geldart A particles in all flow regimes

Coarse-grid two-fluid simulation of gas-solid fluidized bed reactors based on the kinetic theory of granular flow exhibits a significant dependence on drag models, especially for Geldart A particles. Many drag models are available in the literature, which have been reported to work for different systems. This study focused on the evaluation of an enhanced filtered drag model along with other different drag models derived from different methods for three-dimensional two-fluid model simulations of gas-solid fluidized beds of Geldart A particles covering a broad range of fluidization regimes, including bubbling fluidization, turbulent fluidization, fast fluidization, and dilute phase transport regimes. Eight drag models were selected, which included five heterogeneous drag models and three homogeneous drag models. Comparison with the available experimental data demonstrates the need for modification of homogeneous drag models to account for the effect of mesoscale structures (i.e., bubbles and clusters). The enhanced filtered drag model and energy-minimization multi-scale (EMMS) drag models were found to achieve superior predictions in all fluidization regimes, while the other drag models were only capable of predicting certain fluidization regimes. The results of this work provide a guideline for choosing appropriate drag models for simulating Geldart A particles and suggestions on developing more reliable and general drag models applicable in all flow regimes.

CPFD study of a full-loop three-dimensional pilot-scale circulating fluidized bed based on EMMS drag model

For the purpose of process optimization and scale-up, coupling energy-minimization multi-scale (EMMS) drag model with computational particle fluid dynamics (CPFD) is a good option for large scale three dimensional full-loop circulating fluidized bed (CFB) simulation. In this study, such integration is applied for a complex CFB with six cyclones, based on Barracuda platform, to investigate the gas-solids flow dynamic characteristics in scaled-up system, especially in the dense flow region, and to characterize different fluidization regimes. The simulation results are compared with those obtained from Wen-Yu model, in terms of axial pressure distribution, solid volume fraction, particle recirculation, etc., and are evaluated against available measurement data. The result demonstrates that EMMS drag model can predict the axial pressure distribution well, especially in the dense region, which gives an S-shape solid volume fraction in the axial direction and two-core-annulus structures (‘M’ shape) in the radial direction. Meanwhile, EMMS drag model provides smaller drag force between the gas phase and the solids that the particle recirculation fluxes inside and outside the riser is relatively smaller. With the increase of superficial gas velocity, three different fluidization regimes, including multiple bubble regime, exploding bubble regime and turbulent fluidization regime, are identified based on EMMS drag model. The results show that the dense region is clear at the multiple and exploding bubble regimes; while the particle volume fraction and mass flow rate in the riser top are larger than those in the middle at the turbulent fluidization regime, which gives a guidance that the riser height is limited that particles do not have enough space to distribute.

CFD simulation of CO2 sorption on K2CO3 solid sorbent in novel high flux circulating-turbulent fluidized bed riser: Parametric statistical experimental design study

In this study a high flux circulating-turbulent fluidized bed (CTFB) riser was confirmed to be advantageous for carbon dioxide (CO2) sorption on a potassium carbonate solid sorbent. The effect of various parameters on the CO2 removal level was evaluated using a statistical experimental design. The most appropriate fluidization regime was found to occur between the turbulent and fast fluidization regimes, which was shown to capture CO2 more efficiently than conventional fluidization regimes. The highest CO2 sorption level was 93.4% under optimized CTFB operating conditions. The important parameters for CO2 capture were the inlet gas velocity and the interactions between the CO2 concentration and the inlet gas velocity and water vapor concentration. The CTFB regime had a high and uniform solid particle distribution in both the axial and radial system directions and could transport the solid sorbent to the regeneration reactor. In addition, the process system continuity had a stronger effect on the CO2 removal level in the system than the process system mixing.

X-ray imaging for flow characterization and investigation of invasive probe interference in travelling fluidized bed

X-ray imaging was employed to visualize the internal flow structures of the travelling fluidized bed (TFB), operating in the bubbling and turbulent flow regimes. The degree of interference of a 4-mm diameter intrusive probe inserted in the gas-fluidized bed was investigated by comparing the time-average voidage in a region of the bed operated with and without the probe being present. Transition from bubbling to the turbulent fluidization regime was visually inspected from images obtained by the X-ray system and quantitatively characterized in terms of statistical parameters associated with grayscale intensity probability distribution functions (PDF). Voidage data obtained by different measurement techniques are compared with time-average voidages obtained by X-rays and borescopic imaging. The hydrodynamic data provided by this study extend the benchmark database produced from the experiments performed on the TFB for validation of CFD and other models.

Investigation of gas–solids flow characteristics in a circulating fluidized bed with annular combustion chamber by pressure measurements and CPFD simulation

For large scale circulating fluidized bed (CFB) boilers, a new type of annular combustion chamber was proposed to improve the secondary air penetration and provide adequate space to arrange the heat transfer surface. To investigate the gas–solids flow characteristics and optimize the structure of the annular combustion chamber, a “cold” test rig was built with a cross sectional area of 1.18m2 and a height of 8.0m. High frequency pressure transducers were used to obtain the pressure fluctuations characteristics. CFD simulation based on computational particle fluid dynamic (CPFD) was used to obtain pressure fluctuations and particles concentration. The time domain analysis, power spectrum analysis and Hilbert–Huang transform were applied to the experimental and simulation data. Different flow regimes including single bubble regime, exploding bubble regime and turbulent fluidization regime were identified. The flow characteristics were investigated in terms of standard deviation and higher-order moments, auto-correlation, power spectrum, and EMD energy entropy. The results show that the gas–solids flow in the bottom region of the annular combustion chamber was relatively uniform. The gas–solids flow in annular combustion chamber was proved similar to that in a conventional CFB riser.

Experimental study of the effect of friction phenomena on actual and calculated inventory in a small-scale CFB riser

Accurate information concerning riser inventory in a fluidized bed is required in some applications such as the calcium looping process, because it is related to the CO2 capture efficiency of the system. In a circulating fluidized bed (CFB), the riser inventory is normally calculated from the riser pressure drop; however, the friction and the acceleration phenomena may have a significant influence on the total riser pressure drop. Therefore, deviation may occur in the calculation from the actual mass. For this reason the magnitude of the friction and the acceleration pressure drop in the entire riser is studied in small-scale risers. Two series of studies were performed: the first one in a scaled cold model riser of the 10kWth facility, and the second one in the 10kWth fluidized bed riser under process conditions. The velocities were chosen to comply with the fluidization regimes suitable for the calcium looping process, namely, the turbulent and the fast. In cold-model experiments in a low-velocity turbulent fluidization regime, the actual weight (static pressure drop) of the particles is observed more than the weight calculated from a recorded pressure drop. This phenomenon is also repeated in pilot plant conditions. In the cold-model setup, the friction and acceleration pressure drop became apparent in the fast fluidization regime, and increased as the gas velocity rose. Within calcium looping conditions in the pilot plant operation, the static pressure drop was observed more than the recorded pressure drop. Therefore, as a conservative approach, the influence of friction pressure drop may be neglected while calculating the solid inventory of the riser. The concept of transit inventory is introduced as a fraction of total inventory, which lies in freefall zones of the CFB system. This fraction increases as gas velocity rises.