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Cryogenic systems must be cooled rapidly from ambient to operating temperature and must also reject heat stably during operation. Polymer coatings are known to shorten cool-down by destabilizing the vapor film, but their behavior under operational heat load has rarely been examined. This study evaluates partial polyimide (PI) coatings in liquid nitrogen. Cylindrical specimens of SUS316L, copper, and Al6061-T6 (10 mm diameter, 100 mm length) coated with PI at thicknesses of 50–250 µm and coverages of 24–100% were cooled from ambient to saturation temperature. Surface heat flux and temperature were recovered from centerline temperature histories using Beck's sequential function specification method, and bubble behavior was captured at 1000 fps. Fully coated specimens raised the Leidenfrost point and shortened film boiling, reaching a cooling effectiveness of 2.0, whereas partially coated specimens achieved a maximum of 2.8 at 200 µm thickness and 52% coverage, consistent across all three metals. The boiling curves of the partially coated specimens exhibited two distinct heat-flux peaks, and visualization showed that bubbles generated on the coated surface disturb the vapor film on the bare surface early in cool-down, while bubbles from the bare surface later enhance cooling of the coated surface. This coupled interaction is defined here as bubble superposition. Operational heat load was then simulated with a GFRP-insulated copper block containing a cartridge heater. Fully coated surfaces exhibited up to 11 times higher thermal resistance than the bare surface, whereas partially coated surfaces maintained thermal resistance comparable to the bare surface, although burnout occurred at a lower heat load. Finally, a correlation relating cooling effectiveness to coating thickness, coverage, and substrate thermal effusivity was developed, predicting the measured values with R² = 0.877. Partial polymer coatings therefore mitigate the high thermal resistance associated with full coatings and are promising for cryogenic systems requiring both rapid cool-down and stable operation.
Acknowledgment: This research was supported by the National R&D Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT (RS-2026-25542124), and by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (RS-2025-25396743).
| Submitters Country | South korea |
|---|---|
| Are you a student? | Yes |
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