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Numerical study of flow boiling in micro/mini channels
KTH, School of Industrial Engineering and Management (ITM), Energy Technology. Kungliga Tekniska Högskolan.
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. , p. 74
Series
TRITA-REFR, ISSN 1102-0245 ; 17/02
Keywords [en]
numerical, boiling, micro channel, phase change.
National Category
Mechanical Engineering
Research subject
Energy Technology
Identifiers
URN: urn:nbn:se:kth:diva-204639ISBN: 978-91-7729-342-2 (print)OAI: oai:DiVA.org:kth-204639DiVA, id: diva2:1085907
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
List of papers
1. Numerical study of bubbles rising and merging during convective boiling in micro-channels
Open this publication in new window or tab >>Numerical study of bubbles rising and merging during convective boiling in micro-channels
2016 (English)In: Applied Thermal Engineering, ISSN 1359-4311, E-ISSN 1873-5606, Vol. 99, p. 1141-1151Article 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
Keywords
boiling, CFD, bubbles, numerical, microchannels
National Category
Energy Engineering
Research subject
Engineering Mechanics
Identifiers
urn:nbn:se:kth:diva-185700 (URN)10.1016/j.applthermaleng.2016.01.116 (DOI)2-s2.0-84958964200 (Scopus ID)
Projects
Effsys expanding
Funder
Swedish Energy Agency
Note

QC 20160429

Available from: 2016-04-25 Created: 2016-04-25 Last updated: 2017-11-30Bibliographically approved
2. A numerical study of the transition from slug to annular flow in micro-channel convective boiling
Open this publication in new window or tab >>A numerical study of the transition from slug to annular flow in micro-channel convective boiling
2017 (English)In: Applied Thermal Engineering, ISSN 1359-4311, E-ISSN 1873-5606, Vol. 112, p. 73-81Article in journal (Refereed) Published
Abstract [en]

A numerical study on the transition from slug flow (or elongated flow) to annular flow of convective boiling under high heat flux in a micro-channel with diameter of 0.4 mm is conducted. A constant velocity inlet boundary with mass flux 400 kg/m2 s, and heated wall with a constant heat flux (160, 80 kW/m2) are applied. A novel initialization method is proposed. Growth rate of the bubble and transition of the flow regime are well predicted by comparing with an experimental visualization. Effects of the transition are studied and findings are that this process disturbs thermal boundary layer which further enhances bubble evaporation.

Place, publisher, year, edition, pages
Elsevier, 2017
Keywords
Boiling, CFD, Heat transfer, Micro-channels, Transition
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-201005 (URN)10.1016/j.applthermaleng.2016.10.020 (DOI)000394831500008 ()2-s2.0-84992156947 (Scopus ID)
Note

QC 20170207

Available from: 2017-02-07 Created: 2017-02-07 Last updated: 2017-11-29Bibliographically approved
3. Numerical study of the interactions and merge of multiple bubbles during convective boiling in micro channels
Open this publication in new window or tab >>Numerical study of the interactions and merge of multiple bubbles during convective boiling in micro channels
2017 (English)In: International Communications in Heat and Mass Transfer, ISSN 0735-1933, E-ISSN 1879-0178, Vol. 81, p. 116-123Article in journal (Refereed) Published
Abstract [en]

Multi bubbles interaction and merger in a micro-channel flow boiling has been numerically studied. Effects of mass flux (56, 112, 200, and 335 kg/m2 ∗ s), wall heat flux (5, 10, and 15 kW/m2) and saturated temperature (300.15 and 303.15 K) are investigated. The coupled level set and volume of fluid (CLSVOF) method and non-equilibrium phase model are implemented to capture the two-phase interface, and the lateral merger process. It is found that the whole transition process can be divided to three sub-stages: sliding, merger, and post-merger. The evaporation rate is much higher in the first two stages due to the boundary layer effects in. Both the mass flux and heat flux affect bubble growth. Specifically, the bubble growth rate increase with the increase of heat flux, or the decrease of mass flux.

Place, publisher, year, edition, pages
Elsevier, 2017
Keywords
Bubbles, CFD, Flow boiling, Micro-channels
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-201011 (URN)10.1016/j.icheatmasstransfer.2016.12.011 (DOI)2-s2.0-85006515437 (Scopus ID)
Funder
Swedish Energy Agency
Note

QC 20170208

Available from: 2017-02-08 Created: 2017-02-07 Last updated: 2017-11-29Bibliographically approved
4. Simulation on the flow and heat transfer characteristics of confined bubbles in micro-channels
Open this publication in new window or tab >>Simulation on the flow and heat transfer characteristics of confined bubbles in micro-channels
2012 (English)In: ASME 2012 10th Int. Conf. on Nanochannels, Microchannels, and Minichannels Collocated with the ASME 2012 Heat Transfer Summer Conf. and the ASME 2012 Fluids Engineering Division Sum, ICNMM 2012, 2012, p. 63-70Conference paper, Published paper (Refereed)
Abstract [en]

3D simulations on confined bubbles in micro-channels with diameter of 1.24 mm were conducted. The working fluid is R134a with a mass flux range from 125kg/m2s to 375kg/m2s. The VOF model is chosen to capture the 2 phase interface while the geo-construction method was used to re-construct the 2-phase interface. A heated boundary wall with heat flux varying from 15kW/m2 to 102kW/m2 is supplied. The wall temperature was calculated. The effects of mass flux and heat flux are studied. The shape of the bubble was predicted by the simulation successfully and the results show that they are independent of the initial shape. Both thin film evaporation and micro convection enhance the heat transfer. However, the micro convection which is caused by bubble motion has greater contribution to the total heat transfer at the stage of bubble growth studied.

Keywords
3D simulations, Boundary walls, Bubble growth, Bubble motion, Flow and heat transfer, Initial shape, Thin film evaporation, Wall temperatures, Heat flux, Microchannels, Phase interfaces, Refrigerants, Heat transfer
National Category
Energy Engineering
Identifiers
urn:nbn:se:kth:diva-131334 (URN)10.1115/ICNMM2012-73108 (DOI)000335090900009 ()2-s2.0-84882351460 (Scopus ID)978-079184479-3 (ISBN)
Conference
ASME 2012 10th Int. Conf. on Nanochannels, Microchannels, and Minichannels, ICNMM 2012 Collocated with the ASME 2012 Heat Transfer Summer Conf. and the ASME 2012 Fluids Engineering Division Sum, ICNMM 2012; Rio Grande; Puerto Rico; 8 July 2012 through 12 July 2012
Note

QC 20131015

Available from: 2013-10-15 Created: 2013-10-14 Last updated: 2017-03-30Bibliographically approved

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