Transient propagation behavior of two-phase flow equations

The capability of published two-phase flow equation sets to predict transient propagation behavior has been studied numerically. The equation sets are those cited by Wallis for separated flow and extensions of those used by Rudinger and Chang for dispersed flow. The primary difference between these two sets is that in the set cited by Wallis, the pressure gradient appearing in each momentum equation is weighted by the phase volume fraction, whereas in the extended Rudinger--Chang set, the pressure gradient appears only in the ''continuous'' phase. The original Rudinger--Chang set had to be modified because it can adequately describe only the transient flow of very dilute suspensions of solids in air. This numerical study shows that pressure pulses propagate at essentially the sound speeds obtained from characteristics analysis for the equation sets investigated. Comparisons of numerical results with experimental air-water pressure propagation data show that only the modified Rudinger--Chang equation set exhibits propagation behavior in good agreement with the experimental observations at low (less than 10 percent) void fractions. None of the equation sets adequately predict the experimental pressure propagation rates in the range of void fractions from 10 to 60 percent where the flow regime was observed to change from bubbly to full slug flow. The modified Rudinger-- Chang set explains an apparent discontinuity in the experimental pressure wave propagation speed at a void fraction of 50 percent if the entire pressure gradient is assumed to be carried by the liquid for less than 50 percent voids and by the vapor for greater than 50 percent voids; however, the magnitude of the calculated discontinuity is greater than the experimental discontinuity. (auth

State-of-the-art review of computational fluid dynamics modeling for fluid-solids systems

As the result of 15 years of research (50 staff years of effort) Argonne National Laboratory (ANL), through its involvement in fluidized-bed combustion, magnetohydrodynamics, and a variety of environmental programs, has produced extensive computational fluid dynamics (CFD) software and models to predict the multiphase hydrodynamic and reactive behavior of fluid-solids motions and interactions in complex fluidized-bed reactors (FBRS) and slurry systems. This has resulted in the FLUFIX, IRF, and SLUFIX computer programs. These programs are based on fluid-solids hydrodynamic models and can predict information important to the designer of atmospheric or pressurized bubbling and circulating FBR, fluid catalytic cracking (FCC) and slurry units to guarantee optimum efficiency with minimum release of pollutants into the environment. This latter issue will become of paramount importance with the enactment of the Clean Air Act Amendment (CAAA) of 1995. Solids motion is also the key to understanding erosion processes. Erosion rates in FBRs and pneumatic and slurry components are computed by ANL`s EROSION code to predict the potential metal wastage of FBR walls, intervals, feed distributors, and cyclones. Only the FLUFIX and IRF codes will be reviewed in the paper together with highlights of the validations because of length limitations. It is envisioned that one day, these codes with user-friendly pre and post-processor software and tailored for massively parallel multiprocessor shared memory computational platforms will be used by industry and researchers to assist in reducing and/or eliminating the environmental and economic barriers which limit full consideration of coal, shale and biomass as energy sources, to retain energy security, and to remediate waste and ecological problems

Thermo-hydrodynamic. Design of fluidized bed combustors : estimating metal wastage

Thermo-Hydrodynamic Design of Fluidized Bed Combustors: Estimating Metal Wastage is a unique volume that finds that the most sensitive parameters affecting metal wastage are superficial fluidizing velocity, particle diameter, and particle sphericity.  Gross consistencies between disparate data sources using different techniques were found when the erosion rates are compared on the same basis using the concept of renormalization.  The simplified mechanistic models and correlations, when validated, can be used to renormalize any experimental data so they can be compared on a consistent basis using a master equation

Analysis of liquid-solids suspension velocities and concentrations obtained by NMR imaging

COMMIX-M, a three-dimensional transient and steady-state computer program written in Cartesian and cylindrical coordinates, has been developed by Argonne National Laboratory. This computer program is capable of analyzing multiphase flow and heat transfer and utilizes the separate phases model wherein each phase has its own mass, momentum, and energy equations. This computer program is in its early stages of development for application to test various interphase interaction models and to predict design and processing of dense fluid-solids suspension systems. COMMIX-M contains preliminary constitutive relationships for interfacial drag, solids viscosities and stresses to describe the solids rheology, and shear lift forces from the literature. Also included is a solids partial slip boundary condition to allow non-zero tangential velocity at the tube walls. Analyses of some of the steady-state, fully-developed isothermal carrier fluid velocity and solids concentration data of Altobelli et al. and Sinton and Chow are presented. These experimental data obtained using three-dimensional time-of-flight nuclear magnetic (NMR) imaging techniques were carefully performed and represent some of the best available open literature data of their kind. NMR imaging offers powerful techniques to non-intrusively determine three-dimensional time-dependent velocity and concentration fields to assist development and validation of the constitutive models and the computer programs describing concentrated suspensions. Analyses of these NMR data, together with comparisons of computed and measured concentration and velocity profiles provide some insights into the mechanisms governing the observed phenomena. Recommendations for future research are given. To the authors' knowledge, these are the first such comparisons of theory and experiment

Experimental and CFD analyses of bubble parameters in a variable-thickness fluidized bed

International audienceBubble characteristics in a variable-thickness fluidized bed containing nine tubes were experimentally investigated by analyzing absolute and differential pressure fluctuations. The latter were obtained from vertically aligned probes traversing the bed interior for three bed thicknesses: thin, square, and full. The important bubble parameters, namely, frequencies, effective diameters, and velocities, were determined by analyzing autocorrelations and cross-correlations obtained from these differential pressure signals for the thin and square beds. Wall effects were assessed by comparing the pressure fluctuations as the bed thickness was increased from thin to square. It was found that bubbles move faster within and above the tube bank than below it. This behavior was also found to be more pronounced in the wall regions of the full bed, which might explain why some commercial fluidized-bed combustors experience unusual metal wastage near their tube supports. Although bubble sizes consistently agreed between thin and square beds, bubble velocity reduction was observed for the thin bed. The experimental thin-bed differential pressure measurements were analyzed using a two-phase computational fluid dynamics (CFD) hydrodynamic model. Excellent agreement was obtained between the experimental results and predictions from our hydrodynamic model for autocorrelations, cross-correlations, power spectral densities, and bubble parameters