The effect of channel aspect ratio on flow boiling characteristics within rectangular micro-passages

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Abstract

In the present paper, a fundamental analysis on the effect of the channel aspect ratio on the bubble dynamics and heat transfer characteristics for the early transient stages of the bubble growth within confined microchannels of rectangular cross-section, under saturated flow boiling conditions, is conducted, utilising high resolution, 3D, transient, conjugate heat transfer simulations. The open-source toolbox OpenFOAM is applied for the simulations, utilising a custom, user-enhanced, diabatic Volume OF Fluid (VOF) solver. Two different series of numerical simulations are performed, focused on a single nucleation event from a single nucleation site and a single nucleation event from multiple, arbitrarily located, nucleation sites, respectively. In each series, three different values of channel aspect ratio are considered, corresponding to a narrow, a square, and a wide microchannel. For the first series, the simulations are performed for a low, a medium, and a high value of applied heat flux and mass flux. For the second series, only the lower values of applied heat flux and mass flux are used for each channel aspect ratio, since this constitutes the worst-case scenario from the overall heat transfer performance point of view, amongst the cases examined in the first series of simulations. The micro-passage aspect ratio has a significant effect in the generated bubble dynamics during the onset of the nucleate boiling regime, as the bubbles grow within the confined liquid crossflow. This alteration of the generated interfacial dynamics, in effect, regulates the size and position of the contact areas of the generated bubbles with the microchannel walls, with a direct effect in the individual contribution and therefore, the balance between the contact line and the liquid film evaporation mechanisms. Moreover, the work presents the quantification of the effect of the solid domain thermal inertia on the whole process and in particular on the local Nusselt numbers. It is evident that considering conjugate heat transfer in numerical simulations of flow boiling is compulsory in order to predict the physical processes in a correct form.
Original languageEnglish
Article number122201
Number of pages18
JournalInternational Journal of Heat and Mass Transfer
DOIs
Publication statusPublished - 20 Nov 2021

Bibliographical note

Funding Information:
This research was partially funded through the European Union's Horizon 2020 research and innovation programme (Marie Sk?odowska Curie grant agreement No. 801604), the European Space Agency (ESA MAP CORA projects TOPDESS and ENCOM4) and the UK's Engineering and Physical Science Research Council (grant EP/P013112/1). Dr. Andredaki would like also to thank the Advanced Engineering Centre of University of Brighton for the financial support through the Maintaining Continuity research grant scheme. Finally, Dr. Georgoulas would also like to thank University of Brighton specifically also for the financial support through the Rising Stars Initiative 2019?2020 Scheme.

Publisher Copyright:
© 2021 Elsevier Ltd

Keywords

  • Flow boiling
  • Microchannels
  • Multiphase flow
  • VOF
  • Conjugate heat transfer
  • Aspect ratio

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