Abstract: Flow boiling heat transfer of nitrogen at high subcritical pressure conditions in a single vertical mini-channel with the diameter of 2.0 mm was experimentally investigated. The tested mass flux varied from 530 to 830 kg/(m²·s), the inlet pressure ranged from 630 to 1080 kPa, and the heat flux ranged from 0 to 223.2 kW/m². Effects of the mass flux and the inlet pressure on the nitrogen boiling curve were examined. Results showed that within the limited test conditions, the merging of three boiling curves indicates the dominance of nucleate boiling and the inlet pressure has a positive enhancement on heat transfer performance. Three heat transfer trends were identified with increasing heat flux. At low heat fluxes, the heat transfer coefficient increases first and then decreases with vapour quality. At intermediate heat fluxes, the heat transfer coefficient versus the vapour quality presents an inverted “U” shape. At high heat fluxes, a double valley shape was observed and the partial dry-out in intermittent flow and annular flow helps to interpret the phenomenon. The increasing inlet pressure increases the heat transfer coefficient over a wide range of vapour quality until the partial dry-out inception. The lower surface tension and lower latent heat of evaporation enhance the nucleate boiling for higher inlet pressure. A modified experimental correlation (mean absolute error (MAE)=19.3%) was proposed on the basis of the Tran correlation considering both the nucleate boiling and the partial dry-out heat transfer mechanism.

Key words: Mini-channel; Nitrogen; Flow boiling; Heat transfer; Regenerative cooling

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