Investigation Of Steam Condensation Heat Transfer With Non-condensable Gases Over A Vertical Tube External Surface

The passive containment cooling system?PCCS?is widely used in the advanced Generation III and III+nuclear reactor systems to maintain the integrity of the containment under long term post utmost accidents like the loss of coolant accident?LOCA?and the main steam line break accident?MSLB?.In the steam condensation process,the presence of large amounts of noncondensable gases?mainly air?will lead to the serious deterioration of heat transfer.Further research on steam condensation in the presence of air must be conducted.An experimental investigation has been conducted to evaluate the steam heat removal capacity with non-condensable gases?e.g.air,helium?over a vertical tube external surface.Under steam/air condition,condensation heat transfer coefficients have been obtained under the wall subcooling ranging from 27 to 70°C,total pressure ranging from 2.0×10⁵ Pa to 6.0×10⁵ Pa and air mass fraction ranging from 0.10 to 0.80.The experiments for the influence of the wall subcooling on the steam condensation heat transfer with a fixed pressure and air mass fraction have been performed.An empirical correlation for the heat transfer coefficient is developed.Under steam/air/helium?simulating hydrogen?condition,the effect of helium mole fraction in non-condensable gases on the heat transfer coefficient was investigated under the wall subcooling ranging from 30 to 62°C,total pressure ranging from 2.0×10⁵ Pa to 5.0×10⁵ Pa,air mass fraction ranging from 0.09 to 0.76 and helium mole fraction in non-condensable gases from 0.03 to 0.35.The empirical correlation that got from the helium experiments covered data with the range of±20%.There was not found the helium stratification under the experimental conditions.The effect of the wall subcooling on the steam condensation heat transfer coefficients has been researched by experiments when the pressure and the air mass fraction is stable.At the air/steam cases,condensation heat transfer coefficient has been obtained under the wall subcooling degree ranging from13 to 25°C,total pressure ranging from 0.4MPa to 0.6MPa and air mass fraction ranging from0.07 to 0.52.Under the same pressure and noncondensable gases mass fraction,the effect of wall subcooling on condensation heat transfer coefficient with noncondensable gases is stronger than that with pure steam.The empirical correlation is developed for the heat transfer coefficient which covered all data points within the range of±15%.Under air/helium/steam cases,the effect of helium on the heat transfer coefficient is investigated under the wall subcooling degree ranging from 18 to 27°C,total pressure ranging from 0.53MPa to 0.6MPa,steam mass fraction ranging from 0.6 to 0.92 and helium volume fraction in noncondensable gases keeping 0.3.None of the experimental conditions is found the helium stratification.The condensation heat transfer coefficient that got from steam/air/helium condition is about 20%lower than that got from air/steam cases.During steam condensation the presence of non-condensable gases is an important issue affecting the whole thermodynamic efficiency of the process.For this reason,many researchers have investigated it by theoretical or experimental methods.A theoretical model on steam condensation in presence of non-condensable gases over the vertical external surface based on the diffusion layer model is modified in the present paper.Based on previous authors'experience,the suction effect at the gas-liquid interface and other analogy drawbacks are identified and overcome by supplementing it with more detailed analysis as well as targeted experiments.The experimental data obtained for condensation,outside vertical tube with an external diameter of 38 mm,of air/steam and helium/air/steam mixture,have been used to verify the present heat and mass transfer analogy?HMTA?formulation.By comparing against different available experimental data and previous formulations,the HMTA formulation is demonstrated to be a accurate enough theoretical approximation.Condensation process of steam in the presence of air has been successfully modeled by applying a user defined function?UDF?added to the commercial computational fluid dynamics?CFD?package.Calculated profiles of temperature,air concentration,velocity components and condensation heat transfer coefficient are compared to experimental results.The simulation results indicate that there is a good agreement between the experimental results and the model predictions.It also shows that both the latent and the sensible HTC decreased with the increase of the air mass fraction,and the latent heat transfer is the dominant factor of the total condensation heat transfer when the air mass fraction less than 50%.Local latent HTC shows an upward tendency along the height direction of the heat transfer tube from bottom to top,with sensible HTC taken on an opposite trend.