M.C. Ruzicka

Homogeneous–heterogeneous regime transition in bubble columns

A simple physical model for homogeneous–heterogeneous regime transition in bubble columns is developed. The model is based on hydrodynamic coupling between gas and liquid phases. For the homogeneous regime, the coupling is made via bubble drift concept (Darwin, Proc. Camb. Phil. Soc. 49 (1953) 342). As a byproduct, a novel non-empirical formula for bubble slip velocity results, u/w=1−ae/(1−e). For the heterogeneous regime, the coupling is obtained in a simple formal way, recovering the classical result of Zuber and Findlay. The regime transition is considered as a smooth and gradual process characterized by a transition function. The model has five parameters: two terminal bubble velocities, bubble drift coefficient, Zuber–Findlay constant, and intermittency factor. All have clear physical meaning and are extractable from experimental data. The model gives formulas for the voidage-gas flow rate dependence separately for the homogeneous regime, heterogeneous regime and transition regime. The model gives a kinematic stability condition for the homogeneous regime and predicts the critical gas flow rate where the transition begins. It also predicts the maximum possible gas holdup in bubble columns. The model is verified by experiments with four different air–water bubble columns. Good agreement is found. Our results are compared with results of other authors. The model agrees with the drift-flux concept developed by Wallis (One-dimensional Two-phase, Flow, McGraw-Hill, New York, 1969) and with the stability theory of Shnip et al. (Int. J. Multiphase Flow 18 (1992) 705).

Effect of bubble column dimensions on flow regime transition

Abstract For the first time, detailed and consistent experimental data are presented on the effect of column size on the homogeneous–heterogeneous regime transition in bubble columns. Systematic measurements were performed in air–water bubble columns of diameters 0.14, 0.29 and 0.4 m and heights between 0.1 and 1.2 m . The critical point where the homogeneous regime looses stability and the transition begins was evaluated from the drift-flux plot. The results show that both the column height and width destabilize the homogeneous regime and advance the transition. Our data were compared with predictions of two available stability theories. Both the theories agree with the data relatively well. Our results also suggest that the column aspect ratio alone cannot replace the simultaneous effect of the column height and width.

Intermittent transition from bubbling to jetting regime in gas-liquid two phase flows

Abstract The transition from bubbling to jetting regime in nitrogen-water system was studied experimentally. The gas was introduced into a pool of stagnant liquid through a single orifice plate above a gas chamber. Two quantities were measured: pressure fluctuations in the gas chamber and velocity of liquid circulations near the orifice. Individual bubbles were formed at low gas flow rates (bubbling regime) while a continuous jet of gas was formed at high rates (jetting regime). The transition from bubbling to jetting regime (transition regime) displayed intermittent character. Jetting bursts of various length appeared at random in originally periodic pressure signal. The distribution of bubbling portion in the pressure signal was hyperbolical with exponent −1.33 indicating type III intermittency. Similar characteristic time scales were found in power spectra of both signals. 1/ f noise was revealed in the velocity spectrum. This kind of noise usually accompanies intermittent transitions. These results implied that liquid circulations with 1/ f noise induced by bubbles affected the bubble dynamics itself as a feed-back and caused the intermittent regime transition. The point of the regime transition was indicated by a sudden drop of Kolmogorov entropy, correlation dimension of the attractor, and Mann-Whitney statistic calculated from pressure signal. An explanation for this drop is suggested on the base of combination of properties of two attractors coexisting/competing within the intermittency range.

Structure of gas pressure signal at two-orifice bubbling from a common plenum

Bubble formation at two orifices with a common plenum was studied experimentally. Different modes of bubble formation were identified by means of analysis of gas pressure fluctuations measured in the plenum. Two synchronous regimes were found at low and high gas flow rates, separated by a wide range of asynchronous regimes. The effect of important operational parameters (barrier in the plenum and in the liquid, orifice spacing, water height, and column diameter) on the stability threshold of the lower synchronous regime was investigated.

Natural modes of multi-orifice bubbling from a common plenum

Abstract Bubble formation at multiple orifices with a common gas plenum was studied experimentally. Gas pressure fluctuations in the plenum were measured and different modes of bubble formation were identified. Main attention was paid to regular synchronous modes because of their importance in formation of uniform gas–liquid mixtures. Factors affecting the interactions among orifices on both the gas and liquid sides were identified.

Novel Desaturation Cell to Quantify Gushing Intensity: A Preliminary Study on Model Solutions

Beer gushing is an unwanted phenomenon that occurs when the beer foam spontaneously gushes out when a bottle or can is opened. Despite long-term research, gushing still has not been fully explained. To our knowledge, there is no simple, quick, and inexpensive method for reliably measuring gushing intensity. In this article, a desaturation cell enabling saturation, step-wise pressure drop, and image analysis of foam formation is presented. The gushing intensity was evaluated through foam formation rate and relative final foam volume. To study the physicochemical mechanism of gushing, the measurements were carried out in a simple gushing matrix made of water and bovine serum albumin (BSA), the aggregates or micelles of which serve as nucleation sites for bubble formation. The effect of hop bitter compounds (iso-α-acids and tetrahydro-iso-α-acids) and essential oils (linalool) in the BSA matrix was also tested. The hop bitter compounds increased the foam formation rate significantly while affecting the foam volume less. Linalool at low concentration caused an extremely high foam formation rate whereas, at higher concentration, did not affect the foaming of the BSA matrix.

Stromatactic patterns formation in geological sediments: field observations versus experiments

We demonstrate a novel purely hydrodynamic concept of formation of stromatactic cavities in geological sediments, originated by Hladil (2005a,b). First, the characteristic features of these cavities are described, as for their geometry and occurrence in the sedimentary rocks, and the several existing contemporary concepts of their formation are briefly reviewed. Then the new concept is introduced, and laboratory experiments described that were designed to validate it. Finally, the result obtained are presented and discussed, and the prospect for the future research is outlined. Note that the stromatactic patterns are three-dimensional cavities which are formed inside the rapidly thickening suspension/sediment. These are not the surface-related patterns like ripples or dunes.

Physicochemical approach to alkaline flocculation of Chlorella vulgaris induced by calcium phosphate precipitates

Alkaline flocculation has been studied due to its potential as a low-cost harvesting method for microalgae. However, surface properties (zeta potential, contact angles) as inputs into physicochemical interaction models have not yet been applied systematically. In this work, forced alkaline flocculation of the freshwater microalgae Chlorella vulgaris induced by calcium phosphate precipitates was studied as a model system. Response surface methodology was used to quantify the effect of independent variables (concentration of Ca2+ (0.5-0.5 mM) and PO43- (0.05-0.35 mM), pH (8-12) and ionic strength (1-19 mM)) on the zeta potential (ZP) of microalgae, and the turbidity (T) of inorganic precipitates. Flocculation tests and their modified versions were carried out. The flocculation efficiencies obtained were interpreted with respect to predictions of physicochemical interaction models. It was found that flocculation was possible under conditions where appropriate precipitates were formed in the presence of cells. Under these conditions, flocculation of negatively charged Chlorella vulgaris was induced not only by positively charged, but also by negatively charged calcium phosphate precipitates at an early phase of nucleation. The driving force for interactions between oppositely charged cells and precipitate particles was electrostatic attraction, while the attraction between equally charged entities may have resulted from a negative total balance of apolar (Lifsitz-van der Waals) and polar (acid-base) interactions. Medium components did not interfere with flocculation, while cellular organic matter decreased flocculation efficiency only to a very limited extent.