Comprehensive parameter analyses on steam-air condensation at equal-height-difference natural circulation loop under large air mass fraction

As the development of small-sized reactors progresses, a passive containment cooling system (PCCS) named equal-height difference natural circulation loop (EHDNCS) has been developed. Experiments were carried out at elevated air mass fractions to assess the condensation heat transfer coefficient (HTC) based on the steam-air mixture pressure, air mass fraction, and wall subcooling. Research indicates that as the air mass fraction increases, the HTC for condensation tends to decrease. Conversely, a higher pressure of the steam-air mixture within the containment leads to an increase in HTC. The condensation HTC increases with increasing wall subcooling; however, when the air mass fraction exceeds 75 %, the condensation HTC remains nearly constant with changes in wall subcooling. A quantitative relationship between the condensation HTC and the height of the PCCS (sum of riser height and 0.5 times heat exchanger height) has been derived by analyzing the driving force and resistance of the cooling water flow within the natural circulation loop. The optimal conditions for adjusting the riser height and the initial riser-to-heat-exchanger height ratio have been determined to enhance the heat transfer performance of the PCCS. Drawing from experimental results, an empirical correlation equation has been formulated to estimate the condensation HTC for PCCS utilizing enhanced heat dissipation through natural circulation systems (EHDNCS), with a prediction deviation within ±31 %.