Lu Huilin

Hydrodynamics of binary fluidization in a riser: CFD simulation using two granular temperatures

A new computational fluid-dynamic (CFD) model with a separate granular temperature (2/3 random particle kinetic energy per unit of mass) equation for each phase or particle size was developed using constitutive equations derived earlier by Huilin, Gidaspow and Manger. In agreement with the experiment and model of Mathiesen, Solberg and Hjertager the new model computes the observed core-annular flow regime. It predicts the trends of the observed radial and axial particle diameter distributions. For elastic particles the computed particle velocity distributions are parabolic. They are close to the laminar type approximate analytical solution for flow in a pipe, where the mean velocity equals the inlet flux divided by the particle density and volume fraction. The computed turbulent intensity is lower for large particles than for small particles, as measured. This is in agreement with an approximate analytical solution for the granular temperature in the developed flow region of a riser for elastic particles. Computations show that for sufficiently inelastic particles the granular temperature in the center can be lower than near the wall resembling the measured particle fluctuating velocity distribution.

Hydrodynamic simulation of gas-solid flow in a riser using kinetic theory of granular flow

Abstract The dynamic behavior of gas–solids flow in a 6-m high riser was predicted using a transient two-dimensional (2D) hydrodynamic model based on the kinetic theory of granular flows. Instantaneous and local gas-particle velocity, void fraction and turbulent parameters were obtained. Predicted time-averaged particle concentrations and velocities reflect the classical core-annular flow structure in agreement with experimental measurements, in particular, with those reported by Miller and Gidaspow [AIChE J. 38 (1992) 1801]. Predicted instantaneous solids concentration frequencies compared well with the experimental data for various regions of the riser. Computed total granular temperature distributions in the riser qualitatively agree with experimental data. High thermal conductivities of fluidized powders (about 50 times that of the fluidizing gas) were estimated from the kinetic theory without adjusted parameters. Effects of initial conditions, inlet geometry, riser diameter and riser vertical inclination were assessed. Unexpected strong distortions of solids concentrations and vertical fluxes were predicted for small inclination angles of the order of 2°. Analysis of experimental data should, therefore, be carefully conducted to ensure that riser inclination is not too important over the length of the riser in order to eliminate potential artifacts due to this geometric parameter.

Size segregation of binary mixture of solids in bubbling fluidized beds

The fluidization behavior of binary mixture differing in size in the gas bubbling fluidized bed is experimentally and theoretically studied. The segregation phenomena are analyzed, and the relevancy of the pressure drop profile of binary mixture to the definition of its minimum fluidization velocity is discussed. The distributions of mass fraction of particles along the bed height are measured, and the profiles of the mean particle diameters of binary mixture are determined. A multi-fluid gas–solid flow model is presented where equations are derived from the kinetic theory of granular flows. Separate transport equations are constructed for each of the particle class, allowing for the interaction between size classes, as well as the momentum and energy are exchanged between the respective classes and the carrier gas. The segregations of the mean diameter are predicted. The numerical results are analyzed and compared with experimental data. D 2003 Elsevier B.V. All rights reserved.

Hydrodynamic modelling of binary mixture in a gas bubbling fluidized bed using the kinetic theory of granular flow

Abstract A multi-fluid Eularian CFD model with closure relationships according to the kinetic theory of granular flow has been applied to study the motions of particles in the gas bubbling fluidized bed with the binary mixtures. The mutual interactions between the gas and particles and the collisions among particles were taken into account. Simulated results shown that the hydrodynamics of gas bubbling fluidized bed related with the distribution of particle sizes and the amount of energy dissipated in particle–particle interaction. In order to obtain realistic bed dynamics from fundamental hydrodynamic models, it is important to correctly take the effect of particle size distribution and energy dissipation due to non-ideal particle–particle interactions into account.

Kinetic theory of fluidized binary granular mixtures with unequal granular temperature

We derive balance laws and constitutive relations for a binary granular mixture with unequal granular temperature. The complete pair distribution function for two colliding spheres was assumed as the product of Maxwellian velocity distribution for each species. The constitutive relations together with the transport equations form a model for binary granular mixture with unequal granular temperature.

Dimension measurements of hydrodynamic attractors in circulating fluidized beds

Abstract Experimental measurements of the correlation dimension, Kolmogorov entropy, and Lyapunov exponent of circulating fluidized bed (CFB) chaotic attractors were obtained by recording differential pressure and γ-ray porosity time series along the height of a cold experimental CFB operating with 75 μm diameter fluid catalytic cracking (FCC) catalyst. The attractor dimension did not vary with respect to the type of measurement taken. Both differential pressure and localized γ-ray densitometry measurements showed the existence of a low order hydrodynamic attractor, whose dimension varied between 1.5 and 2.0 over the range of gas velocities and solids fluxes studied. Differential pressure measurements indicated that the attractor dimension decreased slightly in the lower section of the CFB and at higher solids fluxes. Localized radial γ-ray bed porosity measurements indicate that the attractor dimension did not significantly vary across the bed cross-section but did show a tendency to be slightly lower near the riser wall.

Numerical Simulations of Hydrodynamic Behaviour in Spouted Beds

A hydrodynamic model of dense gas-solid flow was developed to predict the flow behaviour in spouted beds. Constitutive equations describing the particulate solids pressure, viscosity and elasticity moduli were implemented into a hydrodynamic simulation computer program. The resulting hydrodynamic simulations agree well with experimental measurements of time-averaged particle-and-gas velocity profiles, as well as with experimental solids concentration distributions obtained from three different articles issued from the fluidization literature.

Numerical Predication of Cracking Reaction of Particle Clusters in Fluid Catalytic Cracking Riser Reactors

Abstract Behavior of catalytic cracking reactions of particle cluster in fluid catalytic cracking (FCC) riser reactors was numerically analyzed using a four-lump mathematical model. Effects of the cluster porosity, inlet gas velocity and temperature, and coke deposition on cracking reactions of the cluster were investigated. Distributions of temperature, gases, and gasoline from both catalyst particle cluster and an isolated catalyst particle are presented. The reaction rates from vacuum gas oil (VGO) to gasoline, gas and coke of individual particle in the cluster are higher than those of the isolated particle, but it reverses for the reaction rates from gasoline to gas and coke. Less gasoline is produced by particle clustering. Simulated results show that the produced mass fluxes of gas and gasoline increase with the operating temperature and molar concentration of VGO, and decrease due to the formation of coke.

Numerical computation of a circulating fluidized bed combustor

A mathematical model to describe a circulating fluidized-bed combustor is presented. A modified two-phase model which was used in the bubbling fluidized-bed combustor is considered to simulate the dense zone of the bottom section. For the upper section of the bed the momentum and energy-balance equation are used to predict the temperature and velocity profiles for the gas and the particles. The model performs mass balances for the chemical gas species (O2, H2O, CO, CO2 and SO2) with consideration being given to the last for retention by limestone particles. The model is applied to typical conditions of a circulating atmospheric fluidized-bed boiler and the simulation results show the expected trends.

Chaotic behavior of local temperature fluctuations in a laboratory-scale circulating fluidized bed

Abstract Local instantaneous temperature signals obtained in a two-story gas–solids circulating fluidized bed have been analyzed using chaos theory. Dominant frequencies were assessed using spectral analysis. The pseudo-phase space was used as a preliminary analysis for deterministic chaos in the experimental data. The correlation dimension was used to evaluate the fractal dimension, embedding dimension and the degree of freedom of the system. In respect to heat transfer process, two distinct regimes of particular fractal dimensions were identified: a dilute-particle-controlled regime and a cluster-controlled regime.