STUDY OF ENTRAINMENT MECHANISMS USING MULTI-SCALE SIMULATIONS AND OPTICAL SENSORS

Abstract

Combined experimental and numerical investigations have been performed to examine the entrainment dynamics of different kinds. In particular, three different interfacial disassociations are targeted, namely gas entrainment due to cuspoidal pull and free surface vortex, liquid droplet formation in gas core in annular flow along with the development of non-invasive measurement technique for liquid-liquid interfaces prior to the onset of entrainment under the well separated flow. A grid based volume of fluid framework is used to simulate free surface profiles under the presence of converging rotational fields. The external field is obtained by a pair of counter rotating cylindrical rolls. Upper rounded crests and downward facing cusp like structures are obtained as the consequences of rotational fields at lower and higher rotational fields, respectively. Entrainment of air in the form of thin filaments is established as the aftermath of cusp. The transition between the crest and cusp like interfaces is obtained in terms of dimensionless capillary number. The inter roll distance, their submergence below nominal interface, rotational and diametric asymmetry in applied fields are obtained as entrainment controlling parameters along with cylinder rotation. Entrained air filaments are found to follow the field of higher rotation. Combined experimental and numerical efforts have been made to understand free surface vortex and subsequent air entrainment in the form of discrete entities. The vortex is generated by submerged rotational fields with their axis normal to the gas-liquid interface. The rotation based Froude number and disc submergence ratio are obtained as the controlling parameters describing vortex dynamics. A generalized vortex profile is obtained based of the combined forced and free vortex analogies in the zones of differential field. A generalized logarithmic equation is obtained for penetration of vortex tip inside liquid prior to the occurrence of steady state. Asymmetric vortex core is observed with inclination of submerged fields and effect of liquid viscosity is demonstrated in the form of dimensionless Taylor number. Motion of buoyant particle is used to trace axial and radial circulations within the flow field. Detachment of discrete air entities from main vortex core is demonstrated for high Froude number and low disc submergence. Numerical simulations ii have also been used to find the physical reasoning behind vortex formation and genesis of discrete air volumes by plotting the velocity streams around the vortex core. Entrainment mechanisms are established for transition from annular to droplet flow using numerical simulations. Detachment of liquid lamella or droplets is obtained from the wall adhered liquid film using three mechanisms namely orificing, rolling and undercutting depending upon the mutual inertial strength of the phases. Orifice like interface profile is encountered due to the dominance of gravitational pull over the inertia, which is subsequently deformed by the rise in gaseous phase velocity caused by venturi effect. Rolling is depicted as deformation of interfacial wave by the fast moving gaseous phase leading towards the generation of liquid lamella. Undercutting is the characteristic at low gas velocities, which entails the deformation of interfacial wave in a direction opposite to the main stream flow and originates circulations for liquid detachment. Velocity streams around rolled and undercut waves depict the deposition and entrainment tendency of dislodged entities. An economical optical system has been proposed for reconstruction of the interface between stratified liquid layers prior to entrainment. The dimensionless optical signals have depicted exponential relationship with corresponding phase fraction and are used as basis for the reconstruction of arbitrary interface profiles. Different interface configurations varying from simplified linear to extremely random temporally evolving waves are reconstructed with good agreement under the flow and no flow conditions.