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Numerical study of bubbles rising and merging during convective boiling in micro-channels
KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.
KTH, School of Industrial Engineering and Management (ITM), Energy Technology, Applied Thermodynamics and Refrigeration.ORCID iD: 0000-0002-9902-2087
2016 (English)In: Applied Thermal Engineering, ISSN 1359-4311, E-ISSN 1873-5606, Vol. 99, 1141-1151 p.Article in journal, Editorial material (Refereed) Published
Abstract [en]

A three dimensional numerical study on bubble growth and merger in a micro-channel with diameter of 0.64 mm has been conducted. The working fluid is R134a and the wall material is steel. The inlet Reynolds number is set at 549 in order to keep the flow in laminar regime. Two different heat fluxes () are supplied to the wall to heat up the fluid. The coupled level set and volume of fluid (CLSVOF) method is used to capture the distorted two-phase interface. An evaporation model is also implemented through UDF (User defined function). The combination of these two methods has successfully eliminated spurious velocities which is a common problem in two phase flow simulation. The boiling and merger processes are well-predicted by the simulation. It is found that the whole process can be divided into three sub-stages: sliding, merger, and post-merger. The dynamics and heat transfer are found to be different in these stages. The evaporation rate is much higher in the first two stages due to the thermal boundary layer effects.

Place, publisher, year, edition, pages
Elsevier, 2016. Vol. 99, 1141-1151 p.
Keyword [en]
boiling, CFD, bubbles, numerical, microchannels
National Category
Energy Engineering
Research subject
Engineering Mechanics
Identifiers
URN: urn:nbn:se:kth:diva-185700DOI: 10.1016/j.applthermaleng.2016.01.116Scopus ID: 2-s2.0-84958964200OAI: oai:DiVA.org:kth-185700DiVA: diva2:922942
Projects
Effsys expanding
Funder
Swedish Energy Agency
Note

QC 20160429

Available from: 2016-04-25 Created: 2016-04-25 Last updated: 2017-03-30Bibliographically approved
In thesis
1. Numerical study of flow boiling in micro/mini channels
Open this publication in new window or tab >>Numerical study of flow boiling in micro/mini channels
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Boiling phenomena in micro scale has emerged as an interesting topic due to its complexity and increasing usage in micro electronic and mechanical systems (MEMS). Experimental visualization has discovered five main flow regimes: nucleate boiling, isolated bubbles, confine bubbly flow, elongated bubbly (or slug) flow, and annular flow. Two of these patterns (confine bubbles and slug flow) are rarely found in macro channels and are believed to have very different heat transfer mechanisms to that of nucleate boiling.

The development of a phenomenological model demands a deep understanding of each flow regime as well as the transition process between them. While studies in every individual flow pattern are available in literature, the mechanisms of transition processes between them remain mysterious. More specifically, how the isolated bubbles evolve into a confined bubbly flow, and how this further evolves into elongated bubbles and finally an annular flow. The effects of boundary conditions such as wall heat flux, surface tension, and interfacial velocity are unclear, too.

The aims of this thesis are to develop and validate a new numerical algorithm, perform a comprehensive numerical study on these transition processes, uncover the transition mechanisms and investigate effects of boundary and operating conditions.

Firstly, a sophisticated and robust numerical model is developed by combining a coupled level set method (CLSVOF) and a non-equilibrium phase change model, which enables an accurate capture of the two-phase interface, as well as the interface temperature.

Secondly, several flow regime transitions are studied in this thesis: nucleate bubbles to confined bubbly flow, multi confined bubbles moving consecutively in a micro channel, and slug to annular flow transition. Effects of surface tension, heat flux, mass flux, and fluid properties are examined. All these regimes are studied separately, which means an appropriate initial condition is needed for each regime. The author developed a simplified model based on energy balance to set the initial and boundary conditions.

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2017. 74 p.
Series
TRITA-REFR, ISSN 1102-0245 ; 17/02
Keyword
numerical, boiling, micro channel, phase change.
National Category
Mechanical Engineering
Research subject
Energy Technology
Identifiers
urn:nbn:se:kth:diva-204639 (URN)978-91-7729-342-2 (ISBN)
Public defence
2017-05-19, b1, Brinellvägen 23, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20170403

Available from: 2017-04-03 Created: 2017-03-30 Last updated: 2017-05-10Bibliographically approved

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