Investigations On Steam Condensation In The Presence Of Non-condensable Gases On External Surfaces Of Single Tubes/Tube Bundles

Steam condensation in the presence of non-condensable gas is one of the relevant thermal-hydraulic phenomena inside the containment under Loss of Coolant Accidents(LOCAs)or Main Steam Line Break accidents(MSLBs).It directly influences the heat removal efficiency of the Passive Containment Cooling System(PCCS)and then determines the pressure and structural integrity of the containment.Considering the non-condensable gas,e.g.air and hydrogen,existed inside the containment may affect the steam condensation heat transfer behavior,previous researchers have carried out some investigations via experimental,theoretical and numerical approaches.Since these studies are mostly performed under low-pressure conditions(0.1?0.5 MPa),the drawn conclusions may not be applicable to some small integrated/modular reactor containment with pressures ranging from 0.1 to 1.3 MPa.Besides,most published literature focuses on vertical single tubes/plates,yet none of them concern the effect of the bundle structure on steam condensation.This may not contribute to the optimization design of the PCCS heat exchanger.Finally,with the broader scope of the gas pressure and temperature,modifications for the diffusion boundary layer steam condensation model are needed in order to evaluate the suction effect and the thermal diffusion effect.Based on the above requirements,this study performed and completed the subsequent research contents:(1)To conduct experiments in a broader parameter range,a test facility for single tube/tube bundle steam condensation in the presence of non-condensable gas was built.It can provide working pressures of 0.1?1.6 MPa and working temperatures of 20?200 ?.Based on this facility,experiments were performed on single tubes at pressures from 0.1?1.6 MPa,wall subcooling from 27?97?,and air mass fractions from 0.1?0.8.Besides,the relationship between HTCs and affecting factors was evaluated in detail and an experimental correlation was fitted based on the experimental data of the present work and the previous studies.Results predicted by the correlation agree well with the experimental data,and it has advantages in the simple format and the larger parameter range.(2)A diffusion boundary layer steam condensation model was developed in the CFD software STAR-CCM+,and then it is fully validated by the present and previous experimental data.Results show that the original model has an obvious under-prediction of HTCs under high pressure and high steam mass fraction conditions and the deviations can be more than 50%.Corrections for the suction effect were made in the steam condensation model.Results predicted by the modified model match well with the experimental data and the deviations are mostly within±20%.In addition,thermal diffusion equations were embedded in the steam condensation model via user field functions so as to discuss the effect of temperature gradient on steam diffusion mass transfer.Results demonstrate that the thermal diffusion effect has remarkable influence at low wall sub-cooling and high gas superheat conditions,and its effect is negligible in the other conditions.(3)Based on the experimental and numerical results of the single tubes,experiments were firstly conducted for steam condensation in the presence of non-condensable gas on external surfaces of tube bundles.The effects of tube bundle on steam condensation in the presence of non-condensable gas were analyzed based on 3×3 tube bundles with tube pitches 2?4 times of the tube outer diameter.By classifying the bundle tubes and defining the bundle impact factor(BIF),the effects of tube rows/columns on the changes of the BIF were analyzed in detail.Results show that at certain parameter ranges of the bulk gas,the tube bundle can both enhance heat transfer and inhibit heat transfer.At low pressures,the condensation heat transfer enhances with the increase of the tube pitch;and at high pressures,the condensation heat transfer enhances with decrease with the tube pitch.The HTCs for tube bundles are greater than those of single tubes at the pressure of 1.6 MPa and the tube pitch of 2 times of tube outer diameter,which means tube bundle can enhance heat transfer.(4)In order to further recognize the mechanisms that influencing bundle condensation heat transfer,multi-dimensional simulations on bundle conditions were performed in the present work.By comparing the results of the tube bundles and single tube,the discrepancies of local field distribution and condensation heat transfer were revealed,and then two mechanisms for enhancing and inhibiting bundle condensation heat transfer were promoted:the air layer overlapping effect and the bundle suction effect.The numerical investigations in broader ranges of bundle rows/columns and tube pitches revealed that,the bundle enhancing heat transfer effect not only exists at high pressures,via a proper chose of tube pitch and row/column,the tube bundle can also have heat transfer enhancement effect under low-pressure conditions.