JAXA Supercomputer System Annual Report February 2025-January 2026

Report Number: R25EACA66

Subject Category: JSS Inter-University Research

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• Responsible Representative: Daisuke Sasaki, Professor, Department of Aerospace Engineering, Osaka Metropolitan University
• Contact Information: Daisuke Sasaki(daisuke.sasaki@omu.ac.jp)
• Members: Yuki Kawamoto, Daisuke Sasaki, Mio Yukimitsu

Abstract

Heat pipes, which transport heat through the flow of a working fluid undergoing phase changes by evaporation and condensation, are installed not only in common electronic devices but also in equipment intended for use in space. Since the fluid is enclosed within a metal tube, it is difficult to obtain physical measurements other than the outer wall temperature. Therefore, computational fluid dynamics (CFD) is considered effective for understanding and predicting the internal flow. In this study, we aim to enable flow prediction inside heat pipes and further achieve highly efficient heat transport. In this fiscal year, we numerically reproduced gas-liquid two-phase flow with phase change and investigated the effects of pipe diameter on bubble behavior and heat transfer characteristics, particularly in the heated section.

Reference URL

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Reasons and benefits of using JAXA Supercomputer System

In phase change calculations, the accuracy of the temperature gradient formed near the gas-liquid interface is directly linked to the calculation of the amount of phase change, making it necessary to ensure high resolution near the interface. Therefore, large-scale parallel analysis on the JAXA supercomputer is essential.

Achievements of the Year

In this study, the diameter of the circular tube in a heat pipe was varied to evaluate differences in bubble growth and heat transfer characteristics depending on the tube diameter. Figure 1 shows the time history of the superheat distribution in circular tubes with diameters of 1, 2, and 4 mm. The white line represents the gas-liquid interface. For the 1 mm tube, the bubble developed while maintaining a spherical shape and transitioned into a Taylor bubble. For the 2 mm tube, the bubble first changed from a spherical to an ellipsoidal shape and then transitioned into a Taylor bubble. For the 4 mm tube, the bubble changed from a spherical shape to an ellipsoidal shape and then to a spherical-cap-like shape before transitioning into a Taylor bubble.Figure 2 shows the time history of the spatially averaged Nusselt number. From these results, it was observed that heat transfer tends to become stronger as the tube diameter increases.

Fig.1: Time history of superheat for each tube diameter:(a) tube diameter 1 mm (b) tube diameter 2 mm (c) tube diameter 4 mm

Fig.2: Spatially averaged Nusselt number for each tube diameter

Publications

- Oral Presentations

M. Yukimitsu, D. Sasaki, Y. Kawamoto, S. Takahashi, and H. Fuke, 'Effect of tube diameter on the growth of a single bubble and heat transfer in a circular tube,' Numerical Fluid Dynamics Symposium, Kitakyushu, December 2025 (in Japanese).

Usage of JSS

Computational Information

• Process Parallelization Methods: MPI
• Thread Parallelization Methods: OpenMP
• Number of Processes: 128 - 480
• Elapsed Time per Case: 200 Hour(s)

JSS3 Resources Used

Fraction of Usage in Total Resources*1(%): 0.06

Details

Please refer to System Configuration of JSS3 for the system configuration and major specifications of JSS3.

*1: Fraction of Usage in Total Resources: Weighted average of three resource types (Computing, File System, and Archiver).

*2: Fraction of Usage：Percentage of usage relative to each resource used in one year.

ISV Software Licenses Used

*2: Fraction of Usage：Percentage of usage relative to each resource used in one year.

JAXA Supercomputer System Annual Report February 2025-January 2026