On the limitations of CFD modelling of flow boiling at high flow velocities and high heat fluxes

Končar, Boštjan (author), Tekavčič, Matej (author), Gajšek, Aljoša (author), Draksler, Martin (author), Fellinger, Joris (author), Richou, Marianne (author)

The ability of computational fluid dynamics (CFD) models to predict flow boiling at high heat flux and high flow velocity conditions has been investigated. High heat fluxes of about 10 MW/m$^2$ and high flow velocities of about 10 m/s typically appear in water cooling channels of divertor target elements in fusion reactors. In particular, the heat flux partitioning model used in the two-fluid CFD formulation was studied. CFD simulations of flow boiling in realistic divertor target cooling channels were performed and compared with conservative single-phase simulations. The predictive capability of CFD models for boiling was evaluated using experimental data, covering a wide range of flow velocities and heat fluxes. Existing CFD models correctly predicted void fraction and wall temperature at low flow velocities, but produced erroneous results at higher velocities (above 3 m/s) resulting in unphysical wall temperature overestimation. The study identified the wall heat flux partitioning model as the main contributor to the mispredictions. By analysing the effects of key boiling parameters, the limitations of the conventional RPI (Rensselaer Polytechnic Institute) model have been identified and a targeted modification is proposed to improve its performance under divertor-relevant conditions. The proposed modification serves as a demonstrative improvement and a basis for future mechanistic model development. The simulations and model analyses are performed within the framework of the ANSYS CFX code and the results are compared with flow boiling experiments in uniformly and top-heated flow channels.

• Language: English
• Keywords: flow boiling, heat flux partitioning, high flow velocities, wall temperature, CFD, void fraction
• Work type: Article
• Typology: 1.01 - Original Scientific Article
• Organization: FMF - Faculty of Mathematics and Physics
• Publication status: Published
• Publication version: Version of Record
• Year: 2026
• Number of pages: 21 str.
• Numbering: Vol. 245, art. 127707
• PID: 20.500.12556/RUL-182650
• UDC: 536
• ISSN on article: 1879-2189
• DOI: 10.1016/j.ijheatmasstransfer.2025.127707
• COBISS.SI-ID: 246364419
• Publication date in RUL: 20.05.2026

Record is a part of a journal

• Title: International journal of heat and mass transfer
• Shortened title: Int. j. heat mass transfer
• Publisher: Elsevier
• ISSN: 1879-2189
• COBISS.SI-ID: 23007493

Licences

• License: CC BY 4.0, Creative Commons Attribution 4.0 International
• Link: http://creativecommons.org/licenses/by/4.0/
• Description: This is the standard Creative Commons license that gives others maximum freedom to do what they want with the work as long as they credit the author.

Secondary language

• Language: Slovenian
• Keywords: razdelitev toplotnega toka, vrelišče toka, temperatura stene