Experimental Study on Single-phase and Two-phase Heat Transfer in 5 × 5 Rod Bundle Under Bubbly Flow Conditions

Abstract

Understanding the complex thermal hydraulics of single-phase liquid flow and two-phase gas–liquid flows in rod bundles is essential for the efficiency and safety of reactors such as small modular reactors (SMRs) and pressurized water reactors (PWRs). To develop a better understanding of the convective heat transfer in normal operation conditions and during loss-of-coolant accidents, in this work, the local heat transfer coefficients were investigated for single-phase water flow at Re=3589.2,7178.4,11964 and for two-phase air–water flow under bubbly flow conditions. For both single-phase and two-phase flows, the heat transfer coefficients were obtained at eleven locations along one measurements line covering the near-wall regions, the subchannels, and the gaps between the rods, at three axial locations (z/DH=67.6, z/DH=85.8, and z/DH=103.1) and in the presence of MVSGs. The single-phase heat transfer was found to increase with the increase of Reynolds number, which had a more significant effect at Re≤7178.4. The two-phase heat transfer coefficients were increased by the increase of the superficial inlet gas velocity and decreased by the increase of the superficial inlet liquid velocity, due to the decrease of bubble sizes and void fraction. The split-type mixing vane spacer grid (MVSG) was found to significantly increase the heat transfer coefficients for both single-phase and two-phase flows. Furthermore, the effect of the MVSG is most pronounced for single-phase flow at high superficial inlet liquid velocities, which was found as the only case where the single-phase heat transfer coefficients are greater than the two-phase heat transfer coefficients. Furthermore, the MVSG was also found to reduce the effect of the buoyancy forces on the variation of the heat transfer coefficients in two-phase flow, which is directly related to the bubble dynamics, especially the void fraction and bubble sizes.

Department(s)

Chemical and Biochemical Engineering

Comments

Ministry of Education, Ethiopia, Grant None

Keywords and Phrases

Bubble dynamics; Convective heat transfer; Nuclear reactors; Rod bundle; Small modular reactors

International Standard Serial Number (ISSN)

1873-2100; 0306-4549

Document Type

Article - Journal

Document Version

Citation

File Type

text

Language(s)

English

Rights

© 2025 Elsevier, All rights reserved.

Publication Date

01 Dec 2025

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