~ Pergamon Int. J. Multiphase Flow Vol. 23, No. 4, pp. 671-682, 1997 © 1997 Elsevier Science Ltd, All rights reserved Printed in Great Britain PII: S0301-9322(97)00009-8 0301-9322/97 $17.00 + 0.00 INTERMITTENT TRANSITION FROM BUBBLING TO JETTING REGIME IN GAS-LIQUID TWO PHASE FLOWS M. C. RUZICKA t, J. DRAHO~ ~, J. ZAHRADNIK ~ and N. H. THOMAS 2 qnstitute of Chemical Process Fundamentals, Academy of Sciences of the Czech Republic, Rozvojova 135, 16502 Prague, Czech Republic 2FAST Team, School of Chemical Engineering and FRED Ltd, Research Park, University of Birmingham, Birmingham B15 2TT, U.K. Received 18 November 1996; in revised form 5 March 1997) 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, l/f noise was revealed in the velocity spectrum. This kind of noise usually accompanies intermittent transitions. These results implied that liquid circulations with l/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. © 1997 Elsevier Science Ltd. Key Words: bubble formation, pressure fluctuations, liquid circulations, hydrodynamic interaction, regime transition, intermittency, 1If noise, Kolmogorov entropy, correlation dimension of attractor 1. INTRODUCTION The hydrodynamics of bubble columns is very complex. Despite long time efforts (e.g. Deckwer 1992; Kastanek et al. 1993), many problems have not been understood yet. Hydrodynamic interactions between the gas and liquid phases are of crucial importance. It is not clear how the dynamic complexity of the two-phase flow is built up from local interactions among bubbles, what is the character of induced liquid circulations and how they affect the formation of bubbles at the gas distributor, what is the origin of hydrodynamic regimes transitions, and where the stochasticity and behavioural unpredictability is coming from. These questions are difficult to answer even in the simple case of bubbling from a single orifice. The process of bubble formation was reviewed a decade ago by Tsuge (1986). Recently, the concept of nonlinear dynamic systems has been applied to study chaotic features of complex bubble formation dynamics (Tritton and Egdell 1993; Mittoni et al. 1995; Draho~ et al. 1996), but only at rather descriptive level. There are also studies on the effect of liquid motion on the bubble formation (Marshall et al. 1993; Oguz and Prosperetti 1993; Kim et al. 1994), but usually only a simple case of the effect of uniform and unidirectional stream of liquid on resulting bubble volume has been considered. The duality between bubbling and jetting regimes has been pointed out (Rabiger and Vogelpohl 1983, 1986), but the regimes transition itself has not been studied. Structural development of air-water plumes has been examined (Castillejos and Brimacombe 1987; Turkoglu and Farouk 1996), but there is a lack of information on far-field liquid motion induced by a buoyant gas jet in a bounded region. 671