About mechanism of circulation in bubble column

V.Yu.Velikodniy, V.P.Vorotilin, A.V.Eremeev, Yu.G.Yanovskiy

Applied Mechanics Institute of RAS

Leninsky prosp., 32a, GSP-1, 119991, Moscow, Russia

 

In paper the results of theoretical and experimental researches of gas-liquid flows in a field of gravity are submitted. The theoretical consideration of the problem is based on a new theory of buoyant turbulent jets. An experimental part of paper is devoted to questions of turbulent gas-liquid jet flow field visualization using microbubble of d≤10mcm and some dyeing substance. The purpose of the work is to describe mechanism and receive an estimation of velocity of large-scale circulation in bubble column arising due non-uniform supply of gas bubbles on its lower cross-section. Gas bubbling of liquid is used in various chemical engineering and safe guarding ecology processes connected with saturation of liquid phase by reactants passing in liquid from gas phase. In this work process of water purification on the base of ozone technology is considered. One of the basic devices on a line of water ozonization is the bubble column. In practice due to design and technological limitations the area of the disperser, from which the gas phase goes out, makes up a small part of all area of the column bottom. Raising bubbles of gas involve in motion so called joined mass of a liquid forming with it a turbulent jet of gas - liquid mixer. Submitted in work theory of turbulent jet flows based on a mechanism of capture (turbulization) of a outer laminar medium by large-scale portions on perturbed boundary between regions of laminar and turbulent flow. The existing approaches to the description of turbulent jets based on semiempirical models of turbulent viscosity this peculiarity of free turbulent flows in no way take into account. Presented theory based on the natural assumption that the external flow round the perturbed boundary of a turbulent jet happens in a break flow mode. Its characteristic feature is the quadratic law of flow resistance and formation of separated stagnant zones. From this was deduce correlation for velocity of capture (turbulization) of ambient liquid. This correlation considered as source of "turbulence" allows to write appropriate equation of mass balance of "turbulent" liquid which plays a role of closing equation for system of integral equations of balance of mass, momentum and composition of turbulent jet. For concrete experimental installation the estimations of parameters of jet flow and velocities of large-scale circulation are obtained. Results of experimental researches of development of gas-liquid jet on height of a column confirmed conclusions of the theory about capture and turbulization of an external laminar medium and stipulation by them large-scale circulation of liquid outside of zone of bubble flow. Conclusion remarks:

1.      Offered hydrodynamic model of bubble hydrobasin on the basis of representations about motion of mixture of liquid and gas bubbles as buoyant turbulent jet and confirmed by visual observations.

2.      Deduced closed set of equations describing motion of such jets and carried out the asymptotic analysis of its solution.

3.      Proposed mechanism of origin of large-scale circulation motions in a volume of hydrobasin deduced and obtained the estimations of its velocity.

4.      The complex vortex structures close to boundaries of a turbulent jet revealed experimentally.

5.      Revealed large-scale circulation stipulated by interaction of dispersal gas-liquid jet and walls of hydrobasin.