Slug Motion Research Articles

ECT Studies of Gas−Solid Fluidized Beds of Different Diameters

The bed dynamic behavior in three gas−solid fluidized beds, 0.05, 0.1, and 0.3 m in diameters, is studied using the electrical capacitance tomography (ECT). The ECT employs the neural network multicriteria optimization image reconstruction technique. The time-averaged solids concentrations obtained by the ECT are generally in good agreement with those obtained by the optical fiber probe. The 0.1 m ID fluidized bed exhibits the spiral motion of rising bubbles in the bubbling regime. However, for the 0.3 m ID fluidized bed, more than one spiral motion of bubble swarms is observed in the bubbling regime. The small fluidized bed exhibits the round-nosed slug motion. The ECT studies reveal a radial symmetry of the time-averaged solids concentration distribution for the turbulent regime. Furthermore, the ECT provides the time-averaged and transient properties of the bubble/void phase and the emulsion phase. For a large fluidized bed, the standard deviations of the fluctuations of the cross-sectional averaged so...

Experimental and Numerical Analysis of Transient Liquid Slug Motion in a Voided Line

In this paper, the hydrodynamics of a single transient slug in a voided line was investigated numerically and experimentally. In the experiments, the liquid slugs of various lengths were propelled into an inclined smooth steel pipe under several different driving air pressures. The pipe segment terminates in an open ended elbow; the pressure time histories are recorded at this elbow. The recorded peak pressures are correlated to the tank pressures and pipe and slug geometry. Dimensionless parameters are developed to present the experimental data. Moreover, the flow is investigated numerically using several Godunov type schemes, namely, basic Godunov Scheme, Total Variation Diminishing (TVD), based Weighted Average Flux (WAF) method and two different Monotone Upstream Schemes for Conservation Laws (MUSCL) methods. These schemes employ Godunov’s approach facilitating exact and Harten, Lax and van Leer’s modified approximate solver (HLLC) type solution methods solving the Riemann problem of gas dynamics equations. Furthermore, the recorded peak pressures at the elbow are compared with the results of the numerical analysis and results of two earlier studies.

A Model forDictyosteliumSlug Movement

Dictyosteliumslug movement results from the coordinated movement of its 105constituent cells. We have shown experimentally that cells in the tip of the slug show a rotational cell movement while the cells in the back of the slug move periodically forward (Siegert & Weijer, 1992). We have put forward the hypothesis that cell movement in slugs is controlled by chemotaxis to scroll waves generated in the tip which convert to twisted scroll or planar waves in the back of the slug (Bretschneideret al., 1995). The coordinated movement of all individual cells in response to these waves could then result in forward movement of the slug. We now test this hypothesis by extending our model for mound formation (Bretschneideret al., 1997) to include two cell types with different signalling and movement properties. All cells are able to relay cAMP and move chemotactically in response to cAMP gradients. Cells interact by adhesion, pressure and friction with neighbouring cells and the extracellular matrix. The model can generate stable scroll waves propagating from the tip to the back of a slug which coordinate forward cell movement and result in slug migration. We use the model to investigate the influence of cell type specific differences in excitability, adhesion and cell interactions on slug motion.

Analysis of Liquid Slug Motion in a Voided Line

A quasi-two-dimensional two-phase flow cylindrical model of slug motion in a voided line is developed that can reasonably predict the change of flow pattern of the slug, air entrainment, “holdup” and the distribution of axial velocity. However, when using the theory of incompressible momentum transfer to estimate the pressure-time history of slug at the elbow, the calculated results are not in good agreement with those of the experiments. Further analysis of the experimental results indicate that an acoustic, or waterhammerlike response may occur immediately upon impact of the high-speed slug with the elbow, and subsequently, the waveform exhibits momentum transfer due to the acceleration of the slug at the elbow.

LIQUID SLUG MOTION IN A VOIDED LINE

Transient individual liquid slug motion in a voided line is investigated both numerically and experimentally. In a series of experiments, slugs of various lengths are propelled into an empty pipe under varying air pressures controlled at an upstream reservoir. The pressure–time histories created by the impacting slugs at an elbow located at the end of the pipe reach are measured, and the peak pressures are in reasonable agreement with results reported in an earlier study. An analytical model is developed to predict the slug acceleration and its impulse at the elbow. Comparisons between measured and predicted parameters reveal the random nature of the slug motion and subsequent impact.

Local differential pressure analysis in a slugging bed using deterministic chaos theory

Local instantaneous pressure signals obtained in a slugging bed have been analyzed using both classical and advanced signal analysis methods. Dual static pressure probes (DSPP) have been used in prior research to collect the local axial differential pressure signals, which can deliver the necessary information about the presence of the bubbles or slugs in the bed. For the data considered in this work, nylon spheres with a diameter of 0.3 cm were used as a fluidization material. The data were captured at a frequency of 100 Hz. Evaluation of the dominant slug frequency was completed using the autocorrelation coefficient and power spectrum. The pseudo-phase space and mutual information function were used as a preliminary analysis for deterministic chaos in the experimental data. Since the mutual information does not demand any particular functional relationships between the data points, it is a better method (when compared with the autocorrelation coefficient) for measuring the predictability of nonlinear systems. The correlation integral was used to evaluate the fractal dimension, embedding dimension and the degrees of freedom of the system. One may characterize both slugs and particle motion in the system with the correlation coefficient if the dominant steady slug motion is mixed with the irregular particle motion. The Kolmogorov entropy, which is a quantitative measure of the amount of information lost or gained by a system as it evolves, is evaluated via the correlation integral to determine the multi-dimensional nature of the signal. Both the Kolmogorov entropy and the correlation coefficient showed similar trends for the changes in the system parameters.

Dynamics and control of a spacecraft with a moving pulsating ball in a spherical cavity

A model with two interacting bodies, for a spacecraft with liquid, has been developed. The spacecraft without liquid is the ‘tank’ and is characterized by its inertial properties and the location and size of a spherical cavity. In the cavity is located the ‘slug’, a sphere of uniform density with a variable radius. At the point of contact between slug and tank the interaction force and torque are prescribed as a function of liquid properties and state variables. The model is named SMS, for Sloshsat Motion Simulator. Its initial objective is to support the development of control laws for the Sloshsat spacecraft, presently under development and scheduled to be launched in 1998. Sloshsat is to perform a series of experiments for the validation of CFD models of spacecraft with onboard liquid. The SMS parameters are to be predicted by CFD simulations and finally to be validated by Sloshsat results. The special feature of SMS is the variable size of the slug. It provides a degree of freedom for the modelling of the variable shape of onboard liquid. The dynamic behaviour of SMS is illustrated and discussed for some particular cases, including a PAM-D nutation model. Control of SMS is evaluated for commanded uniform rotation about its intermediate principal axis and stabilized by a cold-gas jet system with 12 nozzles. The cavity centre is on the axis of rotation but not at the centre of mass of the tank. Although the tank is easily stabilized by its reaction control system, a very small friction between slug and tank causes angular momentum to accumulate in the slug motion and results in large oscillations of Size.

Hydrodynamics of a transient slug motion in an empty pipe system

Hydrodynamics of a transient slug motion in an empty pipe system

Computation of solid circulation rates in a fluidized bed from tracer particle concentration distributions

AbstractExperimentally measured profiles of tracer particle concentration inside a fluidized bed were employed to calculate rates of bed‐solid circulation induced by the presence of a central jet and by slug motion. Lagrangian minimization and direct pattern search techniques were used to determine the solids transfer rates, which should minimize the error in the tracer particle mass balance while satisfying the bed solid mass balance. Time‐average steady‐state mixing of bed solids and upstream weighting of tracer concentrations were employed in the formulation of the minimization scheme. Quantitative information on solid circulation for three nominal jet velocities of 52, 37 and 25 m/s is provided. The computed solid circulation rates show that there is a region of high circulation around the jet and a region of lower circulation induced by the slug motion.

Fate of solids fed pneumatically through a jet into a fluidized bed

AbstractSolid tracer particles were fed pneumatically through a jet into a fluidized bed to simulate the feeding of solids via a pneumatic transport line into a fluidized‐bed reactor operating in the slugging‐bed mode. The fluidized bed was defluidized instantaneously at different times after the initiation of the tracer particle injection. The bed was then sampled layer by layer to provide the radial and axial concentration profiles of the tracer. Regular and high‐speed movies (1,000 frames per second) were taken to study the operation of the fluidized bed and the phenomena of the gas‐solid two‐phase jet.Experimental results on solid mixing, jet constriction and slugging frequencies, slugging bed height, slug length, jet penetration, and jet half‐angle at three nominal jet velocities of 52, 37, and 25 m/s and corresponding solids loadings are presented. Additional experimental results on jet constriction and slugging frequencies, and slug volume (axial slug size) obtained for a wider range of jet velocities confirm the hydrodynamic trends observed during the tracer particle injection experiments. The results indicate that solids mixing increases, and well‐mixed conditions are reached earlier, with an increase in jet injection velocity. The obtained mixing times were correlated successfully in terms of the excess gas velocity. The experimental data on jet penetration and slug motion were satisfactorily correlated by modified versions of existing theoretical relations. The modifications included the effect of the injected solids on jet penetration and jet half‐angle and also the effect of our semicircular column geometry and single wall‐slug configuration on the observed slug motion.

Deposition of aerosol particles and flow resistance in mathematical and experimental airway models.

Aerosol deposition and flow resistance in obstructed airways were determined from five mathematical and experimental airway models. The first three models were theoretical and based upon Weibel's symmetrical lung model with 1) uniform reduction of airway diameter in various groups of airway generations; 2) obstruction of a few major airways such that a severe uneven flow distribution occurs in the lung; 3) focal constriction of selected large airways. In model 3, an empirical formula was utilized to assess deposition and resistance in the constricted airways. The remaining two models were tested experimentally; 4) oscillation of a compliant wall in a straight tube and 5) two-phase gas-liquid flow utilizing human sputum in a rigid branching tube. In models 1, 2, and 3, airway resistance increased to a greater extent than did the increase of aerosol deposition except when small airways were obstructed in model 1. Here, the increase of aerosol deposition was slightly higher than the rise in airway resistance. A sharp increase of aerosol deposition with a minimal increase of flow resistance was demonstrated in models 4 and 5. These data indicate that aerosol deposition may be a more sensitive indicator of airway abnormalities than overall airway resistance in small airways obstruction, during oscillation of large and medium airway walls, and when excessive secretions within the airways move with a wave or slug motion.