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• 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]

Steam condensation containing non-condensable gases is the main method of heat removal under severe nuclear accidents. To investigate the condensation characteristic, several numerical simulation studies have been conducted. Previous numerical models generally assume that the condensate film is thin or even negligible owing to the existence of non-condensable gases. However, the condensation rate improved rapidly as steam concentration rises. As the condensation rate rises, the velocity and thickness of the film will also sharply increase, thereby affecting the condensation process. In this case, the film flow will transition to a turbulent regime. To consummate the condensation model, a two-phase condensation model with predictable turbulent film was proposed. The model can predict the condensation performance well in a broad range, with deviation generally within 20%. In addition, the prediction correlation and contour diagram were obtained to identify the thermal resistance ratio and the flow regime of the film according to the proposed model. • A two-phase condensation model with predictable turbulent films is proposed. • The mechanism of the film effect on the condensation process is revealed. • The distribution of fluid film thermal resistance ratio is given. • The discriminant method of fluid film flow regime is obtained. [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]

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]

The high efficiency heat extraction from the containment in severe accident is of utmost importance for nuclear power plants, and the Passive Containment Heat Removal System (PCS) is utilized for removing heat efficiently in third generation nuclear units. In order to enhance external steam-air condensation heat transfer, previous studies have primarily focused on specific structures, rib tubes, S-shaped tubes, or inclined tubes bundles, for instance. In the present study, a novel structure of helical coiled tube was designed and its external heat transfer characteristics were obtained through numerical simulation. The effects of coiled pitch, radial helical pitch, tube diameters and heights, two tube arrangements of vertical and horizontal on heat transfer characteristics were elucidated. It can be concluded that radial helical pitch and coiled pitch jointly impact the heat transfer, a proper large radial helical pitch and narrow coiled pitch can result in an approximate 9% enhancement in condensation heat transfer coefficient (h a), and the utilization of narrowed pipe diameters and longer tube heights facilitates the augmentation of heat transfer. Moreover, for vertical arrangement, h a can be enhanced over 15% by rising tube numbers, for horizontal arrangement, h a primarily influenced by tube pitch, the heat transfer is deteriorated by narrow tube pitch, but improved by widening them. • A novel effect that influences the external condensation heat transfer was proposed. • Derived condensation behavior during steam-air external condensation on helical coiled tube. • Effects of various parameters on heat transfer characteristics outside helical coiled tube were elucidated. • A helical tube bundle structure exhibiting exceptional heat transfer performance was obtained. [ABSTRACT FROM AUTHOR]

• The heat transfer coefficient exhibits non-monotonic behavior concerning the inclination angle, particularly at low vapor quality, where an optimum coefficient is observed around ±25°. • An inclination factor has been introduced to account for the impact of the inclination angle on condensation heat transfer in inclined tubes. • A new model for condensation heat transfer coefficients in downward inclination is proposed. Validation using 579 data points reveals absolute and mean absolute deviations of 0.74 % and 14.15 %, respectively. Existing prediction models for condensation heat transfer coefficients in an inclined tube with various orientations are insufficient. This study aims to address this limitation by conducting experiments to analyze steam condensation at low mass fluxes and develop a generally applicable condensation prediction model for an inclined tube. The study employs a stainless steel tube with an inner diameter of 25 mm and an effective heat transfer length of 1050 mm. Experimental investigations are conducted at a pressure of 0.2 MPa, with mass fluxes ranging from 20 to 50 kg/m2s, vapor qualities ranging from 0.2 to 0.85, and inclination angles between -40 and 40° The study examines five flow patterns: smooth stratified, wavy-stratified, intermittent, wavy-annular, and annular flow. Findings demonstrate that the inclination angle significantly influences the observed flow patterns. Upward inclination leads to increased gas-liquid interface fluctuation and intermittent flow region, while downward inclination diminishes smooth stratified flow and reduces the liquid film thickness. The heat transfer coefficient exhibits non-monotonic behavior concerning the inclination angle, particularly at low vapor quality, where an optimum coefficient is observed around ±25° An inclination factor has been introduced to account for the impact of the inclination angle on condensation heat transfer in inclined tubes. The current literature and experimental data in this study are utilized to propose an inclination factor correlation applicable to downward inclination. The inclination factor correlation considers factors such as gas-liquid shear, gravity, mass flux, vapor quality, surface tension effects, and tube diameter. By combining the inclination factor with a classical prediction model, a new model for condensation heat transfer coefficients in downward inclination is proposed. Validation using 579 data points reveals absolute and mean absolute deviations of 0.74 % and 14.15 %, respectively. The model applies to a range of parameters, including F between 0.19 and 6.55, X tt between 0.046 and 1.46, Eo between 10.55 and 28.83, and tube diameters between 8.3 and 25 mm, encompassing inclination angles from 0 to 60° in downward inclination. This model offers valuable insights for the design of downward inclined heat exchangers. [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]

As an efficient heat transfer method, in-tube condensation heat transfer is widely used in various fields, including passive residual heat removal systems (PRHRS) of floating power plants under ocean conditions. The experimental investigation aimed to study the condensation heat transfer characteristics of steam in a 25 mm inner diameter horizontal tube with a heat transfer length of 1050 mm under rolling motion. The study used a mass flux of 30 kg/(m2·s) and investigated average vapor qualities, rolling amplitude, and rolling period ranging from 0.32 to 0.79, 0 °–20°, and 10 s–20 s, respectively. The experimental results show that the condensation flow patterns under rolling conditions are more complex than those under non-rolling motion due to the influence of the rolling period and rolling amplitude. The instantaneous heat transfer coefficient (IHTC) changes periodically due to the rolling motion and has the same change period. The pulsation amplitude of the IHTC increases with increasing rolling amplitude but weakens as the vapor quality increases. The effect of the rolling period on the IHTC is much less than that of the rolling amplitude. At low vapor quality, the time-averaged heat transfer coefficient (THTC) under the rolling motion condition increases with the rolling amplitude and period, and the maximum enhancement of THTC is 44%. As the vapor quality increases, the THTC under rolling conditions is the same as that under non-rolling conditions. These findings provide important insights into the condensation heat transfer characteristics of vapor in a horizontal tube under rolling motion, which can help improve the design and operation of in-tube condensation heat transfer systems in various fields. [ABSTRACT FROM AUTHOR]

• The experimental pressure range (0.15 ~ 1.6 MPa) and subcooling range (28 ~ 122 °C) are relatively large. • The effect of tube diameter and inclination angle on the condensation with non-condensable gases was analyzed. • An experimental correlation containing the effect of tube diameter and inclination angle was obtained. Steam-air condensation is one of the important physical phenomena after reactor accidents, which has direct impacts on the discharge of containment residual heat after the accident. Previously, many researchers have proposed experimental correlations for condensation of non-condensable gas, which include the influence of pressure, concentration, and wall subcooling. However, there are relatively few studies on the influence of tube diameter and inclination angle. In order to investigate the heat transfer characteristics of the condenser at different tube diameters (19 mm, 15 mm, 12 mm) and inclination angles (0 ~ 90°), experiments were carried out in the pressure range of 0.15 ~ 1.6 MPa, air mass fraction of 14 ~ 87%, and wall subcooling of 28 ~ 122 °C. It shows that the condensation heat transfer coefficient (HTC) increases with the decrease of the tube diameter and inclination angle. The condensation HTC with a tube diameter of 12 mm and an inclination angle of 0°(Horizontally) is around 32 ~ 38% larger than that of the heat transfer tube with a tube diameter of 19 mm and an inclination angle of 90°(Vertically). Based on a total of 2276 data points from the present work and related experiments, an experimental correlation containing the effect of tube diameter and inclination angle is proposed, and its deviation is within ± 15% with 95% confidence. [ABSTRACT FROM AUTHOR]