Investigation Of Condensation Of Steam In The Presence Of Noncondensable Gases

Condensation of steam in the presence of noncondensable gases is an important heat transfer mode in the passive containment cooling system (PCCS). In the past decade, great efforts have been made on the investigation of this regime. However, the data are very limited due to the difficulty in the experiment technique. So far the mechanism and the effects of developing flow, condensate film roughness, property variation in the gas phase, system pressure, and sorts of noncondensable gases and its contents on condensing are not adequately understood. The correlations and analytical models developed for vapor flow in tubes are limited to their experimental conditions, and they cannot be used beyond their data base with confidence.The experiment of condensation of steam in the presence of air was performed in a single vertical tube in a closed system consisted of the condenser tube, boiler, condensate separator drum, and connection lines from the boiler to the condenser tube. The condenser tube was 55.0mm O.D., 49.0mm I.D., and 3.5m effective length. A 67.0mm I.D. concentric jacket pipe surrounded the test condenser. Steam was generated in the boiler, flowed upward to the inlet of connection pipes, then flowed downward into the condenser tube, the cooling water flowed countercurrently through the annulus.The experiment covered the range of the pressure 1.0~3.0bar, heating power 2.0 ~ 15. 0Kw, steam mass flow rate 0.001 ~0.004kg/s, air mass flow rate 0.0 ~ 0.00163kg/s, and cooling water temperature 20 - 60癈. Measurements were made of system pressure, mass flow rate of steam, air, and cooling water, temperatures of condensing flow and cooling flow, and the water level of boiler.The major phenomena observed in the present experiment are similar to those by other investigators. The mass flow rate of steam, the system pressure and air content are the most important factors affecting steam condensation. The condensing length increases with the boiler heating power and air content increasing, , while it decreases with the system pressure increasing. Under the normal pressure, the wall temperature and the centerline temperature along the condensing length nearly remain constant forpure steam, while they decrease continuously for steam/air mixture. However, when the steam condenses over, the temperatures drops rapidly.An analytical model for the forced condensation of steam in the presence of noncondensable gases was proposed based on the mass, momentum and energy conservation equations of condensing mixture flow, involving the analogy between heat and mass transfer in the gas/liquid interface. The effects of developing flow, condensate film roughness, property variation in the gas phase were considered. The present model predicted well the condensing data of METU-CTF, MIT, and our experiments.A two dimension analytical model for the natural condensation of steam in the presence of noncondensable gases was proposed based on the mass, momentum and energy conservation equations of condensing mixture flow. The effects of interfacial resistance, superheating, free convection due to both temperature and concentration gradients, mass diffusion and thermal diffusion, and variable properties in both the liquid and gas-vapor regions were considered. The present model predicted well the data of Tagami's experiment.