This was a keynote lecture before the retirement of Prof. Žun as the co-chairman of the European Japanese Two Phase Flow Group. On this special occasion he reviewed some of the key points that he started working on 40 years ago and have created very strong bonds with his Japanese friends. The joint results have proven to be crucial in the field. The development of the basic conceptual viewpoints that were opened at that time on the transient characteristics of turbulent bubbly flow in a vertical duct are reviewed with respect to the first archival papers on void fraction profiles in bubbly flow by Serizawa et al. in 1975, transverse lift force acting on the bubble by Žun in 1980, and momentum and heat transfer in bubble flow by Sato et al. in 1981. Later, Serizawa and Kataoka (1994) assumed that interactions between the bubble wake and the liquid shear field might be the reason for the observed lateral segregation of deformed bubbles in the opposite direction as expected from the classical lift force. Based on numerical investigations, Tomiyama et al. (1995) confirmed this presumption and related the inversion of the direction of the bubble migration to the presence of a slanted wake behind the deformed bubble. Bubble wake drift is the consequence of a significant interaction between the bubble interface and the surrounding liquid and is dependent upon the bubble size (Tomiyama et al. 2002a). There are unresolved questions regarding the initial conditions that were first pointed out by Tomiyama et al. (2002b). Additional problems arise when one wants to deal with complex multiscale phenomena. Complex bubbly flow is comprised of different scales, which makes it almost impossible to consider all simultaneous interactions at the same time (Zun 2002). In contrast to what the papers refer as the dynamic view point in two-fluid models, in most cases the inverse dynamic methods are being used, which automatically determine the force functions required to accomplish a stated goal. In extreme cases this may lead towards empirical fitting, which may drift us away from realistic multiscale physics. However, rapid progress in computer performance and corresponding numerical methods gives us a good prospect of realizing detailed numerical simulation of the two-phase flow. Among those, the interface tracking scheme implemented with non-uniform subcell scheme provided considerable improvement in our understanding of single bubble behaviour (Hayashi and Tomiyama 2007, Zun et al. 2012). Recently, progress has been made in the case of interfacial breakup in a manifold, which leads us towards the prediction of initial conditions based on prescribed mixing at continuous flow rates of both phases (Gregorc andZun 2013). By only changing the prescribed flowrates, different two-phase flow patterns can be simulated. At the end of the presentation, very recent results of numerical predictions obtained at LFDT based on the high performance computing of gas jet breakup in a highly turbulent liquid cross flow will be shown for the first time. The results are validated by a corresponding experiment and show the simulation capabilities of coupled LES and VOF numerical schemes. Altogether, ten experimental set-ups developed at the Laboratory for Fluid Dynamics and Thermodynamics (LFDT) are presented and discussed in terms of physical and numerical modeling of bubbly flow. The following inlet configurations are included: (I) localized, forced inlets by jets or nozzles, (II) highly distributed “inlets”, including porous mixer, sparger, and vapor generation at cavitation nucleation sites, (III) volume mixing by suddenly imposed body forces.
B.01 Organiser of a scientific meeting
COBISS.SI-ID: 14351387To better predict transitions between the various two-phase flow patterns, it is necessary to update our way of thinking from one-of-a-kind flow pattern maps of limited applicability to a generalized approach that includes multiscale information about the flow itself. This would require combined numerical and experimental approach to identify and understand the governing phenomena. In our previous work published in 2013 (Chem Eng Sci 102:106-120), a comparison of the distribution of bubbles with equivalent diameter revealed the inlet mixer's strong impact on bubble size and bubble distribution along the horizontal mini-tube of 1.2 mm ID and thus, the bubble to slug flow transition. In order to further clarify the effect of inlet conditions on flow patterns we performed a systematic study on tubes with 10, 6, 3, 1.2, 1 and 0.8 mm ID. Focus was on description of inlet conditions on resulting flow patterns evolution downstream the tube at different air and water flow rates at ambient pressure and temperature. Series of numerical simulations were performed to check the numerical capabilities for corresponding flow pattern prediction utilizing VOF in commercial code ANSYS Fluent. Three major flow patterns are considered: bubbly flow, slug flow and semi-annular flow. Flow pattern development length due to hydrodynamic effects and unavoidable numerical transients is discussed in details.
B.03 Paper at an international scientific conference
COBISS.SI-ID: 14684187Final report of feasibility studies of numerical modelling and understanding of physical processes of a gas jet injection into the transverse liquid flow. The evaluation of the prediction capabilities in terms of inlet conditions was done for: penetration distance of the gas jet in the liquid vein, stability of the gas jet and sensitivity of the gas jet shape on liquid inlet conditions. Numerical model was validated by experiment.
F.02 Acquisition of new scientific knowledge
COBISS.SI-ID: 14536731The subject of the invention is a process device for coating particles that falls within the field of chemical and pharmaceutical technology. It represents an improvement on the process equipment for coating particles by spraying from the bottom and works on the principle of fluidization technology. The process device for coating particles, according to the invention which has within the wall (1), one or more units placed, for which each unit consists of a swirl flow generator (4) with a perforated plate (3) and a draft tube (5) where centrally through the swirl flow generator (4) a single or multi-phase spraying nozzle (6) with a coating dispersion inflow (7) and inflow (8) of compressed air is installed, in which the swirl flow generator (4) has outward open and at an angle with regard to the vertical installed grooves (18). The process device for coating particles in the second implementation case, has within the wall (1), one or more units placed, for which each unit consists of a draft tube (5) and a single or multi-phase spraying nozzle (6), which is mounted centrally with regard to the draft tube (5) and penetrates the perforated plate (3a). The perforated plate (3a), which is in the outer area, between the wall of the draft tube (5), and the wall (1) of the device, in cross-section straight or curved and has a characteristic distribution of the size of round orifices with the largest cross sections in the area of the ground floor projection of the draft tube (5) and smaller orifices in the remaining part of the plate. The device for coating shall have along the periphery of the wall (1) and at the perforated plate (3a) the ring (21 ) with installed inflow slots (27) that are pressure powered from a common hollow ring (20) with a connector (22) for compressed air. The device for coating shall have along the periphery of the wall (1) above the perforated plate (3a) a radially installed circumferential inflow slot (24) and otherwise inclined to the horizontal plane, the slot (24) is pressure powered through the hollow ring (23) with a pressure connection (25).
F.08 Development and manufacture of a prototype
COBISS.SI-ID: 4066161Hydraulic transient event generates high-pressure and low-pressure waves that propagate in liquid-filled pipes at an approximate velocity of sound. Uncontrolled pressure waves may lead to operational difficulties in the system or even damage the system components. This phenomena should be carefully investigated during pipeline filling and emptying in hydropower and water supply systems in which we should provide adequate air release and air admission. This prevents formation of trapped gas pockets and pipeline rupture. Several original experimental runs have been carried out in pipeline apparatus in order to investigate maximum and minimum pressures in the system at different initial conditions, vaporous and gaseous cavitation phenomena during transient event and influence of unsteady skin friction on amplitude, shape and timing of the pressure pulse. The measured results have been used for numerical discrete gas cavity model validations. There is a good agreement between the computed and measured results. The fluid-structure interaction effects have been almost negligible; the forces acting on the pipeline supports have been within the prescribed limits.
F.01 Acquisition of new practical knowledge, information and skills
COBISS.SI-ID: 15147035