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Projects / Programmes source: ARIS

Transient two-phase flows

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Periods
January 1, 2004 - December 31, 2008
Research activity

Code Science Field Subfield
2.13.00  Engineering sciences and technologies  Process engineering   

Code Science Field
T200  Technological sciences  Thermal engineering, applied thermodynamics 
T210  Technological sciences  Mechanical engineering, hydraulics, vacuum technology, vibration and acoustic engineering 
T160  Technological sciences  Nuclear engineering and technology 
T130  Technological sciences  Production technology 
T121  Technological sciences  Signal processing 
Keywords
Two-phase flow, bubbles, cavitation, transient phenomena, complex systems, experimental techniques, numerical simulations, cascade modelling
Evaluation (rules)
source: COBISS
Evaluation of bibliographic research performance indicators according to ARIS methodology
Citations Citations for bibliographic records in COBIB.SI that are linked to records in citation databases
source: WoS source: Scopus source: COBISS
Researchers (7)
no. Code Name and surname Research area Role Period No. of publications
1.  03923  PhD Anton Bergant  Process engineering  Researcher  2004 - 2008  392 
2.  05912  PhD Andrej Bombač  Process engineering  Researcher  2004 - 2008  222 
3.  15259  PhD Jure Mencinger  Computer intensive methods and applications  Researcher  2004 - 2008  43 
4.  04471  PhD Matjaž Perpar  Process engineering  Researcher  2004 - 2008  128 
5.  01371  PhD Zlatko Rek  Process engineering  Researcher  2004 - 2008  217 
6.  20661  Borut Stražišar  Manufacturing technologies and systems  Technical associate  2004 - 2008  24 
7.  03544  PhD Iztok Žun  Process engineering  Head  2004 - 2008  540 
Organisations (1)
no. Code Research organisation City Registration number No. of publications
1.  0782  University of Ljubljana, Faculty of Mechanical Engineering  Ljubljana  1627031  29,205 
Abstract
The primary drive in the technical advances of heat and mass transfer being sought is to develop a deeper understanding of the physics of multiphase flow, and having acquired that insight, to develop appropriate mathematical models to predict the phenomena involved. Next, some if not all of the models will need to be incorporated into appropriate software (namely CMFD) ready for testing and validation against experimental data. Research efforts will be particularly required on the interactions between the carrier phase turbulence and bubbles, between bubbles and boundaries (walls, free surface) and between bubbles (collisions and pair interactions, agglomerate formation and break-up, collective effects involving clusters of bubbles). There is a specific need for modelling of such meso-scale structures and for developing new measuring techniques adapted to such scales. In all these mechanisms, account should be made of interface deformations that have been scarcely considered so far, either due to limitations of the measuring techniques in experiments or due to limitations in terms of computational power in simulations. It is envisaged that the multiphase flows considered will not be restricted to simple geometry only but will also include more complex systems including a variety of boundary types, moving surfaces (such as stirrers) which are common in industry. The approach could lead to developments in low dimensional models suitable for active control schemes. Research actions are organised in three thematic addressing some of the key scientific issues of relevance in the process industry. These are: (a) Development of cascade modeling Our strategy distinguishes four steps to disassemble the complex system in different scales in order to obtain necessary information for multiscale modeling in the fifth stage. The steps are: (1) Structures identification which provides frozen details in spatial character, (2) Spatial relationships between frozen structures, (3) Time-space evolution of each structure, and (4) Statistical relevance. All four steps proved to serve well in cascade modelling that can be done in reassembling step (5) in order to mimic complex bubbly flows. One of the goals of the program anticipates further development of intelligent instrumentation which contributes towards system optimization. Successful scale separation is going to be accompanied by further development of coupled numerical modelling, like two-fluid modelling with incorporated bubble distribution function or VOF technique imbedded in bubble tracking scheme. (b) Cavitation phenomenon Despite of numerous literature on cavitation, the details that describe a particular cavitating flow pattern, like cloud cavitation, are not sufficient to provide information that is indispensable for multi-scale numerical simulation. The aim of this part of the project is to perform a slow down experiment with expanded bubbles to the size to be able to obtain the data on meso- as well as on micro-scales. A second complex treatment is going to be devoted to the transient flows in pipeline systems. (c) Two-phase flow in microchannels There is no doubt, that in the future the miniaturisation of technical equipment will be the most important task in engineering science. The rationalized material and energy consumption including the reduction in environmental pollution, are only some benefits of smaller components. It is even more important that a lot of applications are not conceivable without miniaturised systems, e.g. in medical technology or in space flight, where the size or the mass of the equipment is decisive. The following objectives are going to be considered: (1) Experimental investigation of two-phase flow structures in simple and complex mini- and micro-channels, (2) Numerical modelling of two-phase flow phenomena and (3) Interface sharpening numerical requirements due to surface interface discontinuity.
Significance for science
Research actions were organised in three thrust areas addressing some of the key scientific issues of relevance in the process industry. a) Development of cascade modeling: The essence of all multiphase flows is in interfacial area generation that bounds different phases and in the underlying processes of interfacial breakup and coalescence. This research continued an effort by LFDT to follow a new paradigm in multi-scale modeling of bubbly flow that enables to simulate evolution of interfacial area concentrations. b) Cavitation phenomena: b1)The following basic physical phenomena were studied for slot cavitation: transient characteristics of bubble structures, bubble tear-off, bubble breakup, bubble cluster formation, bubble cluster collapse, bubble cluster impact rate on solid surface, time scale estimate of bubble and bubble cluster life span. Within the 5th frame (PREVERO) the group contributed to completely new approach in cavitation modeling for the end user AVL List which was implemented into CFD code FIRE V8.5. b2)Hydrodynamic investigation of piping system devices and development of novel models for transient cavitating pipe flow were oriented towards industrial conditions c) Two phase flow in mini- and microchannels has been considered by experimental and numerical studies. Citations obtained in the field of bubbly flow prove the worldwide role of programme group. Most citations refer to the scientific contributions related to bubbly flow studies on micro scale, like transverse migration of single bubbles in shear flows, drag coefficients of single bubbles [COBISS.SI-ID 2576155], and particle tracking method for bubbly flow simulation [COBISS.SI-ID 2394651]. These are the subjects that are connected to the first program thematic. Promising citation index is also produced by paper [COBISS.SI-ID 9522203] that appeared on the third place among TOP 25 HOTTEST ARTICLES IN IJMF IN 2006. The paper deals with characterization of diabatic two-phase flows in microchannels (third program thematic), subject related to fundamental knowledge needed in MEMS and MECS technologies. Finally, the citations of gas-filled cavity structures in a vessel with dual-impellers paper refer to the second program thematic [COBISS.SI-ID 3648539]. This confirms the relevance of the proposed program in years 2004-2008. Top1% Most Cited Paper: Transverse migration of single bubbles in simple shear flow, Chem. Eng. Sci, 57, 1849-1858 2002, [COBISS.SI-ID 5085723]; contents belongs to the first program thematic; Number of citations 58 (According to Sciencewatch WoS, http://sciencewatch.com/about/met/thres-highlyctd/ engineering papers published in 2002 are ranked among Top 1% beyond threshold 46). This paper represents successful continuation of small bubble migration studies first introduced by Zun in 1980 [COBISS.SI-ID 2555163] to explaine the so-called shear-induced lift force. Trajectories of single air bubbles in simple shear flows of viscous liquids were measured in current study to provide an experimental database and to present an empirical correlation of a net transverse lift force CT. It was confirmed that CT for small bubbles is a function of the bubble Reynolds number Re, whereas CT for larger bubbles is well correlated with a modified Eotvos number Eo. The critical bubble diameter causing the radial void profile transition from wall peaking to core peaking in an air–water bubbly flow can be well predicted by the proposed CT correlation. The correlation is currently widely used in home made as well as comercial computer codes. The group is worldwide recognized by modeling of complex phenomena on multiple scales. The group is also recognized by taking part at international conference organizations and the role of programme chairman prof. Žun who received JSMF Award by the Japanese Society of Multiphase Flow.
Significance for the country
The research group has been well involved in European projects: Experimental and CFD technology for preventive reduction of Diesel engine emissions caused by cavitation erosion-PREVERO, EU Contract No. ENK6-CT-2002-00605 from 2002 to 2005, Heat and mass transfer in micro-channels, HPRN-CT-2002-00204 from 2002 to 2006, Surge-net, G1-RT-CT-2002-05069, from 2002 to 2005 and Dynamic behaviour of air valves, HPR1-CT-1999-00103, from 2002 to 2007. There were number of researchers and students from the EU countries according to these EU projects. The research group has been also well involved in industrial applications in the following companies: Litostroj Power, TE Šoštanj, AVL, Bosch, Lek, Energetika, Brinox and Gorenje. There are 2 adjunct researchers from industry permanently working at LFDT (from Litostroj Power and AVL). Three outstanding accomplishments after ARRS research registry are: 1 new product development which was released to the world market on September 2008 (GORENJE NGKA oven), 1 technology improvement (4th block electrostatic filter of Thermal Power Plant Šoštanj), and 1 existing product improvement (guard-gate in hydroelectric power plant Plave II). In the first application, the development of a large oven new generation in which heat is spread evenly throughout the oven so that one can simultaneously cook on three of the five levels. Programme group LFDT contributed by numerical simulation of temperature and velocity field over a two-phase boundary and oven optimization. In the second application, electrostatic filtering efficiency of flue gases of 275MW block power has been improved in Thermal Power Plant Šoštanj utilizing numerical simulation of velocity and pressure field. In the third application, the influence of the gate shape on the gate discharge and hydraulic force characteristics was investigated due to the insufficient knowledge of its role in hydroelectric power plants. New numerical model enabled dynamic analysis of guard-gate operation in hydroelectric power plant Plave II which is going to be significantly affected by a new construction of reversible plant Avče. Dissemination: Ecellent collaboration with the Faculty of Farmacy, Ljubljana University has been recognized by KRKA 2005 Award v for the best PhD Thesis of R. Dreu (co-supervisor I. Žun, supervisor S. Srčič, Faculty of Farmacy). The work is related to pelets coating in fluidized bed. Strong scientific collaboration has been either established or continued with EPFL Lausanne Switzerland, Kobe University Japan, University Adelaide Australia, Technical University Eindhoven and Delft University the Netherlands. Program subjects were embeded into 5 undergraduate and 3 post graduate courses at the University of Ljubljana, Microchannel short course at EPFL Lausanne, Switzerland and at HTRI in Dallas Texas, USA. Programm group was also a partner in IHP-RTN-00-2 training network.
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