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Abstract Previous investigations on steam condensation in the presence of air have revealed general heat transfer characteristics in vertical plates and single tubes. However, there remains limited knowledge regarding the thermal-hydraulic phenomena and heat transfer properties of tube bundles. In order to evaluate steam condensation on the external surfaces of tube bundles, STAR-CCM + code with a diffusion boundary layer steam condensation model was employed in this work. In the assessments, 3 × 3 tube bundles with various tube pitches (from 1.5d to 5d) were investigated, and the results were compared to those of a single tube. The data analyses mainly focused on three aspects: (i) global and local condensation phenomena in tube bundles; (ii) heat transfer characteristics in tube bundles; and (iii) influences of tube pitches and global parameters (such as pressure and air mass fraction) on the condensation heat transfer. The phenomena analyses indicate that the axial field profiles are mainly determined by the development of a concentration boundary layer, while the radial field profiles are mostly affected by the tube pitches. Local heat flux analyses indicate that the condensation heat flux exhibits a significant reduction downwards along the vertical tube. In comparison, the condensation heat flux fluctuates in the tube peripheral direction. In the latter case, heat transfer enhancement and inhibition regions are identified and classified. For the tube average heat transfer, only the central tube is significantly affected by the tube pitches, and the degradation may be more than 30% at a tube pitch of 1.5d. The tube pitch effect is almost independent of the bulk air mass fraction and changes slightly with an increase in the wall sub-cooling and bulk pressure. [ABSTRACT FROM AUTHOR]

Abstract Steam condensation in the presence of air is a relevant phenomenon in various industrial applications. Its heat transfer property has been broadly revealed by experimental and numerical investigations on vertical plates or single tubes. However, these results may not be directly applied to tube bundle cases, since there may exist mutual interactions among adjacent tubes. To have an insight on this problem, the present work conducted numerical simulations on various tube bundles with a tube pitch 1.5 times of the tube diameter. The cases evaluated were classified into three categories including the single row, double row, and triple row. In each category, structures with various tube columns were assessed. The results indicate that tube bundles will thicken the near-wall high concentration air layer, resulting in inhibited condensation heat transfer. This phenomenon is defined as the inhibition effect. On the other hand, a heat transfer enhancement effect caused by a strengthened natural circulation driven by the density difference between the mainstream and the high concentration air region is found and defined as the stack effect. The average condensation heat transfer for tube bundles is determined by the relative magnitude between the inhibition effect and the stack effect. The stack effect becomes much intensive with the increase of tube rows and columns, which can enhance condensation heat transfer. Typically, the results in the triple row categories show that the maximum average heat transfer coefficient can be 14% greater than that of the single tube. [ABSTRACT FROM AUTHOR]

• Experiments were conducted at pressures up to 1.6 MPa and subcooling up to 128 °C. • The air deterioration effect obviously weakens with the increase of pressure. • The power exponents of pressure and subcooling were clarified in wider ranges. • A more generalized experimental correlation was proposed based on 1500 data points. In loss of coolant accidents, steam condensation containing air is one of the most important thermal–hydraulic phenomena inside nuclear reactor containment. To evaluate the relation amongst heat transfer coefficients (HTCs) and thermal parameters, e.g., gas pressure, concentration and wall subcooling etc., several experimental correlations were fitted in previous studies. However, these experiments were performed at pressures lower than 0.6 MPa, thus the proposed correlations may not be suitable for integrated nuclear reactor containments with a design pressure greater than 1.0 MPa. To further evaluate this problem, the study carried out experiments at pressures from 0.15 to 1.6 MPa and wall subcooling from 20 to 128 °C. The effects of pressure and wall subcooling on tube external condensation HTCs were discussed. The results indicate that the non-condensable gas deterioration effect obviously weakens with the increase of pressure, and the power exponents of pressure, wall subcooling and mass fraction were clarified according to the broad-range experimental data. Based on the 1500 data collected from the present experiment and the Su, Fan, Dehbi experiments, a more generalized experimental correlation was proposed with deviations generally within ± 15%, which unified the data from different experiments in various parameter ranges. [ABSTRACT FROM AUTHOR]

Under the severe accident of the nuclear reactor, the strong coupling phenomenon exists between the transport and condensation of the steam-air-hydrogen multi-component gas in the containment atmosphere. Under this coupling effect, the flow patterns of mixture gas induced by condensation are critical to hydrogen accumulation risk assessment. The general flow characteristics of typical flow patterns were qualitatively predicted and confirmed in our previous work. However, the transient mass transfer process of multicomponent gases is complex, and flow pattern transitions are difficult to be captured. To further clarify these contents, experiments and corresponding numerical simulations are combined in the present work. The formation mechanism of the typical flow patterns is further revealed. The flow pattern transition boundaries of the mixture are quantified based on more data in confirmatory experiments. Experiments are performed under wider wall sub-cooling (Δ T = 60–110 °C) and wider range of steam concentration (X steam) from 16% to 90%, the volume concentration of helium in non-condensable gases (X He / X Non) ranges from 10% to 70%. Results show that the homogeneous downward flow is generally formed when X He / X Non < 40%. Under medium helium concentration (X He / X Non : 42%–55%), when X steam > 30%, the flow pattern of the mixture gas is the separation flow. Under high helium concentration (X He / X Non > 55%), the flow pattern of the mixture gas is the homogeneous upward flow when X steam > 20%. Furthermore, the empirical component correlations to classify the three flow patterns are proposed for the first time. Correlations and numerical simulation have good consistency in the prediction of flow patterns under most component conditions. • Transient mass transfer of multicomponent gases induced by condensation is discussed. • The formation mechanism of the typical flow patterns is further revealed. • Flow pattern transition boundaries of the mixture gas are quantified based on test. • Empirical component correlations to classify the typical flow patterns are proposed. • Correlations and numerical simulation results have good consistency. [ABSTRACT FROM AUTHOR]

• Assessments and evaluation of numerical model of steam condensation with non-condensable gases were carried out based on OpenFOAM. • A new correction of suction effect was proposed with consideration of suction region and surface curvature. • Validation based on TOSQAN, COAST, Su and Dehbi's facility was carried out with previous and new suction corrections. • Validation results of the new correction showed good agreement with experimental results with different curvatures. Steam condensation with non-condensable gases is one of the most important phenomena in containment of nuclear power plant (NPP) after accidents. With the application of passive containment cooling system (PCCS), steam is condensed on surfaces with different curvatures of PCCS and structures. Previous investigations indicated the need for suction corrections in the diffusion boundary layer model of steam condensation in the presence of non-condensable gases. By implanting different suction effect corrections in OpenFOAM, this study found that existing suction effect corrections cannot make good predictions for steam condensation with non-condensable gases on varied curvature surfaces, which resulted from cases lack of diversity. Thus, a new correction was proposed based on Gasthof number and previously corrections. This correction was validated by data sets from TOSQAN, COAST, Dehbi and Su's facilities. The relative deviations of 90.0 % simulation results based on the new correction from the experimental results were within 20.0 %. [ABSTRACT FROM AUTHOR]

Under severe accident conditions, in addition to steam and air, there are also other non-condensable gases inside the containment, such as CO 2 produced by the molten corium concrete interaction (MCCI). Due to the significantly higher molecular weight and density of CO 2 compared to steam and air, CO 2 will have a significantly different impact on the steam condensation process compared to air. However, few scholars have paid attention to the impact of CO 2 on the condensation process, and there is no empirical correlation applicable to this condition. Therefore, this article investigates the effect of different CO 2 concentrations on the steam condensation heat transfer coefficient (CHTC) in the range of system pressure from 0.2 to 1.3 MPa and subcooling from 30 to 130 °C through experimental methods. Meanwhile, based on relevant experimental data, a new correlation suitable for calculating the steam CHTC in the presence of CO 2 and air was fitted. The results indicate that under low steam concentration conditions, the addition of CO 2 will reduce CHTC, while under high steam concentration conditions, CO 2 will increase CHTC. The increase in system pressure will strengthen the condensation promotion effect of CO 2 and weaken its inhibitory effect on condensation. In addition, by comparing experimental data under different component conditions, it was found that the new empirical correlation has high computational accuracy, and the calculation results of this correlation can contain more than 90 % of the data within a 15 % error range. • The study examines the effect of denser CO 2 gas on condensation heat transfer in accident conditions. • Experiments were conducted to study how varying CO 2 concentrations impact the CHTC. • The article analyzes how CO 2 and hydrogen jointly alter steam's CHTC in non-condensable gas environments. • A precise experimental formula is derived for calculating steam's CHTC in air and CO 2 mixtures. [ABSTRACT FROM AUTHOR]

Under severe accident conditions, in addition to steam and air, there may also be hydrogen produced by the reaction of zirconium alloys and steam, CO 2 produced by the molten corium concrete interaction (MCCI) and other gases in the containment. These gases, whose properties are quite different from air, can significantly affect the steam condensation process outside the tube bundle. This paper aims to investigate the effects of non-condensable gas mixtures with varying component concentrations on the steam condensation process outside a 3 × 5 tube bundle using computational fluid dynamics (CFD) methods. The thermal conditions studied are set to a pressure of 0.4 MPa, a subcooling of 40 K, and a molar concentration of steam of 0.66. The results show that CO 2 has an acceleration effect on the flow field outside the tube bundle, which promotes the diffusion of steam to the inside of the bundle, thus increasing the condensation heat transfer coefficient (CHTC) of the bundle and decreasing the difference in CHTC between different heat transfer tubes inside the bundle. Hydrogen, on the other hand, inhibits the downward flow of the gas mixture, causing a decrease in CHTC. However, at high hydrogen concentration (X H2 >0.1), the gas mixture will form a buoyant flow upward, and then the CHTC of the tube bundle will be significantly increased by up to 177%. When the two gases act together, they will weaken each other's influence on the flow field, thus reducing the CHTC of the tube bundle. [ABSTRACT FROM AUTHOR]

• The strongly coupling relationship between curvature and inclination is discovered. • The inflection point of inclination effect on condensation characteristics is found. • The mechanism of coupling effect between inclination and curvature is revealed. • The condensation performance of various curvature and inclination is compared. Steam-air condensation is one of the critical thermal–hydraulic phenomena in containment under accident conditions. Currently, sufficient experimental research and numerical analysis have been conducted on steam–air condensation outside the tube/plate. However, few studies have investigated the influence of curvature and inclination on condensation characteristics, and the local phenomena and mechanisms that affect the condensation process have not yet been fully revealed. In addition, it is of vital importance to evaluate the coupling effect between curvature and inclination in practical applications. To investigate the above-mentioned effects, numerical simulations were conducted over a wide parameter range (P = 0.2 ∼ 1.6 MPa, ω a = 0.1 ∼ 0.9, ΔT = 20 ∼ 100 K), and the related mechanisms were analyzed in detail. The results showed that there is a strong coupling relationship between curvature effect and inclination effect and the inflection point in the influence of inclination effect on steam condensation characteristics is discovered. Compared to the traditional conclusion that condensation capability gradually increases with the decrease of inclination angle. The effect of inclination on the condensation characteristics can reverse in different curvature regions. When the curvature is greater than 5 m−1, the inclination angle is negatively correlated with the condensation capability. When the curvature is less than 5 m−1, the inclination angle is positively correlated with the condensation capability. Furthermore, the enhancement effect originate from the curvature effect will greatly decrease as the inclination angle increases. [ABSTRACT FROM AUTHOR]

• A coupled two-phase model for liquid film and steam-air condensation is proposed. • The film effect is proposed and the influence of the film effect is evaluated. • The various mechanisms of film thermal resistance and film flow are considered. • The distribution of the condensate film flow regime is obtained. Steam condensation containing air is a vital thermal-hydraulic phenomenon in containment under nuclear power plant severe accident sequences. This phenomenon has been extensively analyzed numerically. Due to the existence of air, the condensate film has been neglected in most studies and a single-phase model has been used for the numerical analysis. However, whether the condensate film can be ignored is still unknown under high pressure and high steam concentration. Due to the influence of condensate film is mostly ignored in the recent analysis of steam condensation containing air. To evaluate the influence of condensate film, the present work proposed a new coupled two-phase model for condensate film and steam condensation containing air based on diffusion boundary layer theory. In addition, the coupled two-phase model was validated with the COAST, Dehbi, Su, and Fan experimental results. The results indicated that the coupled two-phase model could predict the steam condensation heat transfer coefficient well within a wide range of parameters (tube diameter from 19 to 38 mm, tube length from 1 to 3.5 m, pressure from 0.15 to 1.6 MPa, air concentration from 10% to 80%, subcooling from 10 to 117 ℃), and the deviation is mostly within 20%. Moreover, the condensate film effect was proposed in this study and evaluated in detail based on the coupled two-phase model. The mechanism of the condensate film effect was considered, and the distribution of the film flow regime was obtained. [ABSTRACT FROM AUTHOR]

Steam condensation containing air is a ubiquitous phenomenon in practical industrial applications. It has been extensively investigated through numerical analysis and experimental studies. The conclusion that the high concentration air layer is the primary thermal resistance of the condensation process is widely accepted. Therefore, thinning the high concentration air layer is the key to enhancing condensation performance. For this purpose, applying the extraction method to remove the high concentration air layer near the heat exchanger can potentially be a direct and effective way to enhance condensation performance. To evaluate the extraction effect, this work conducted numerical simulations for a single tube that added extractors at P = 0.3 MPa, △T = 15 °C, and ω a = 0.56. In the discussions, the condensation performance with various extraction structure parameters (e.g., extraction height Z , extraction distance △L , extractor diameter D , number of extractors n) were investigated. The results indicated that the extractor could greatly enhance the condensation performance. For the single extractor case, the extractor (Z = 0.75 m, D = 50 mm, △L = 5 mm) has the optimal enhancement effect, which can reach 42.4%. For the multi-extractor case, the enhancement effect is optimal (76.3%) when the number of extractors n is 11. • The extraction effect is proposed and its influence on condensation is discussed. • The relationship between the extraction parameters is illustrated. • The mechanism of the extraction effect on the process of condensation is revealed. • The suggestion of optimal extraction parameters scheme is given. [ABSTRACT FROM AUTHOR]

• The effect of CO 2 on the steam condensation outside a vertical tube is numerically analyzed. • A CFD model to simulate the steam condensation in presence of CO 2 /air is validated by the experiments carried out on the COAST facility. • The steam concentration will influence of the effect of CO 2 on steam condensation heat transfer coefficient. Under severe accident conditions of a pressurized water reactor nuclear power plant, the reactor core may melt, and the molten corium even reacts with concrete in the containment, producing a large amount of CO 2 into the entire containment gas space. This may have an obvious impact on the heat transfer characteristics of steam condensation in the containment. However, there are few studies focused on this phenomenon. In this paper, a commercial CFD software, STAR-CCM+, is used to study the effect of CO 2 on steam condensation heat transfer. The calculations are based on 0.4 MPa gas pressure, 40 K wall sub-cooling, 0.66 or 0.2 steam mole fraction, and the CO 2 mole fraction varies from 0 to 0.34 or 0.8. The results show that the CO 2 demonstrates different influence laws at high and low steam concentrations, respectively. When the steam mole fraction is 0.66, as the proportion of CO 2 in the non-condensable gas increases, the condensation heat transfer coefficient (HTC) will increase, and the enhancement can reach 30%. When the steam mole fraction is 0.2, the condensation HTC will first decrease and then increase, as the proportion of CO 2 in the non-condensable gas increases. The effects of gas pressure and wall sub-cooling on the condensation HTC are also analyzed. The results show that the condensation HTC increases with the increase of the gas pressure and decreases with the increase of wall sub-cooling. [ABSTRACT FROM AUTHOR]

Steam condensation with existence of air is an important thermal hydraulic phenomenon in reactor containment. Previous investigations mostly focused on the surfaces of plates or tube bundles which are in parallel with the gas flow direction, and discussions on heat transfer surfaces with various inclination angles were merely restricted to plates. In addition, there is limited information about the condensation promotion effect caused by enlarged gas flow velocity at different tube inclination angles. To investigate the coupling effects of tube inclination and flow velocity, numerical simulations were carried on a 3 × 3 tube bundle. The local thermohydraulic profiles and heat transfer mechanism of steam condensation were analyzed carefully. The results indicate that the increase of inclination angle and flow velocity can dramatically promote condensation heat transfer. At the velocity of 0.3 m/s, the condensation heat transfer enhanced 327.8% as the tube bundle orientation changes from vertical to horizontal. At the inclination angle of 0°, the condensation heat transfer increased 223.2% as flow velocity increased from 0.3 m/s to 3 m/s. The inhibition effect and stack effect in tube bundle varied by the tube inclination and flow velocity. [Display omitted] • The inclined 3 × 3 bundle condensation was simulated at velocities up to 3.0 m/s. • The inclination leads to areas with different air mass fraction in the bundle. • Classic inhibition and stack effect vary from inclination angles and velocities. • The inclination and acceleration can enhance condensation heat transfer. [ABSTRACT FROM AUTHOR]

Steam condensation consisting of noncondensable gas has been broadly investigated on external surfaces of various single tube and plates. Previous numerical analyses revealed that the high concentration air layer (HCAL) is the main thermal resistance for condensation. Comparing with the single tube/plate, condensation for bundle can be more complex. The HCAL near the adjacent tube surfaces may interfere and then influencing flow and heat transfer characteristics in the bundle. To evaluate the bundle effect, this study conducted experiments based on a 3 × 3 tube bundle at a wider pressure range (0.15–1.6 MPa). The results indicate that the bundle effect can be divided into two sub-effects: the enhancement effect and the inhibition effect, and the enhancement effect can make the average heat transfer performance of the bundle better than that of the single tube. The parameter thresholds for the two sub-effects were determined according to the broaden parameter scope. Generally, at small subcooling and low pressures, the inhibition effect dominants, and the bundle condensation heat transfer coefficient (CHTC) can be 22% less than that of the single tube; at large subcooling and high pressures, the enhancement effect dominants, and the bundle CHTC can be 71% greater. • The 3 × 3 bundle condensation experiment was conducted at pressures up to 1.6 MPa. • The tube bundle both has enhancement and inhibition effects on condensation. • The enhancement effect will exceed the inhibition effect with pressure increases. • The bundle average condensation performance can be greater than that of single tube. [ABSTRACT FROM AUTHOR]