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The effects of interficial shear between liquid film and flowing-steam cannot be neglected during the steam condensation process in vertical tubes, and the Nusselt condensation model should be modified. In this paper, the heat transfer characteristics of steam condensation in vertical tube were investigated by experiments. Both the Nusselt theoretical model and the modified model considering interficial shear effects were adopted and compared with the experimental measurements. The results show that interfacial shear stress have significant influence on the liquid film flow state and thin liquid film thickness, which further results in higher condensation heat transfer coefficient. As the modified model considers interficial shear correctly, and its calculation results agree well with the experimental results. [ABSTRACT FROM AUTHOR]

Temperature distribution in a parallel-plate heater with the cooling of pulsating flow is theoretically studied. The results indicate the temperature of the heated plate fluctuates periodically as the flow pulsates. Effects of the wall thickness and its thermal properties, pulsating period and amplitude are analyzed based on the theoretical result. The fluctuation amplitude and the phase-lag of the outer surface temperature are magnified by the increase of plate thickness and the decrease of thermal diffusivity and pulsating period, and vice versa. It should be noted that the temperature difference between the outer wall and the inner wall, and the heat difference between the heat source and transported by the fluid caused by the unsteady condition should not be ignored. The results of the present study can be used to obtain a precise temperature fluctuation and the variation of the heat flux in unsteady flow condition. [ABSTRACT FROM AUTHOR]

Boiling heat transfer equipment in a vessel can be affected by the additional force which is generated by the rolling, swing and heaving motion of the vessel. Bubble behavior is very important for the research of boiling phenomenon. Bubble behavior under rolling motion condition is experimentally studied by using a high speed camera. The experiment is conducted in a subcooled flow boiling rectangular channel, and the cross section size of the channel is 2 mm × 40 mm. Two types of bubbles with large discrepancies in sliding and condensation behaviors can be observed in the captured images. The first type bubbles disappear quickly after generation and the slide distance is only a few times of bubble maximum diameter, while the second type bubbles can survive a longer time after leaving the nucleation site and slide for a long distance with the flowing fluid. Bubble characteristics under rolling motion are separately studied for different type bubbles based on the above reasons. The results show that the lifetime, maximum diameter, nucleation frequency and sliding velocity of the first type bubble are periodically fluctuated and the period is same with the rolling motion. The fluctuation intensity of the bubble lifetime and maximum diameter can be enhanced by the increase of the rolling amplitude. The peak value of bubble lifetime, maximum diameter, and nucleation frequency appears when the rolling platform plate rolls to the maximum positive angle, while opposite trend can be observed in the variation of bubble sliding velocity. In view of the characteristics of the second type bubbles, lifetime and maximum diameter are not measured. And the variation of nucleation frequency and sliding velocity of the second type bubbles under the effect of rolling motion is same with the first type bubbles. Furthermore, the effects of additional force, variation of local pressure and flow rate oscillation on bubble behavior are analyzed. The results indicate that the fluctuations of the bubble parameters can be generated by the variation of local pressure caused by rolling motion even no influential flow rate fluctuation occurs. The effect of the acceleration variation vertical to the heated surface on bubble behavior is unclear and need more researches in the future work. [ABSTRACT FROM AUTHOR]

Abstract: An experimental investigation was conducted to study bubble sliding in subcooled flow boiling of water in a vertical narrow rectangular channel. Bubble behaviors were recorded using a high speed digital camera and an automatic digital image processing algorithm dealing with bubble sliding parameters (including bubble numbers, bubble velocities and bubble equivalent diameters) was proposed. The bubble image sequences were analyzed to obtain the distribution of bubble diameters and bubble velocities as well as their mean spatial-temporal value. Most of the bubbles in the experiments slide along the heated surface, but not lift off the surface, as is usual in a conventional channel. Two types of bubbles with different behaviors were observed in the experiments. The first type has a shorter lifetime and their diameters or volumes change a lot through rapid vaporizing and condensing. On the contrary, bubbles which were classified as the second type have a longer lifetime and their diameters or volumes change slowly. Bubble sliding velocity and bubble equivalent diameter distribution profiles were obtained based on the results of the automated analysis. The distribution of bubble sliding velocities and bubble equivalent diameters indicates that both of them cover a wide range. At last, mean bubble velocities, mean relative velocities between bubbles and local liquids and bubble densities under different working conditions were obtained to manifest the effects of mass flow rate, inlet subcooling and heat flux. [Copyright &y& Elsevier]

• A numerical analysis model of bubble growth was developed. • The growth characteristics of bubbles within the grid elements of petal-shaped fuel elements are investigated. • Development of a correlation formula for bubble growth prediction. Based on the VOF multiphase flow model and bubble growth model, a numerical analysis model of bubble growth of subcooled flow boiling in a petal-shaped fuel rod channel was established. The growth characteristics of a single bubble and the influence of thermal parameters were analyzed. Firstly, the growth rate, shape, and trajectory of the bubble were investigated under typical working conditions. Secondly, the influences of inlet subcooling, inlet velocity, and wall superheat on the bubble growth characteristics were clarified. Finally, the existing bubble growth rate prediction correlation was evaluated based on the results of numerical calculations. The results show that the bubble growth rate is fast at the early stage of bubble growth when the evaporation heat flow from the microlayer plays a major role. With the disappearance of microlayer evaporation, the bubble diameter increases slowly. The bubble diameter decreases continuously when the evaporation heat flow in the overheating layer is less than the condensation heat flow after the bubble lift-off. Compared to the rectangular channel, the bubble diameter in a petal-shaped fuel rod channel has a larger equivalent diameter and more obvious lateral movement. With the increase of inlet flow velocity and inlet subcooling, the total evaporation heat flow of the bubble decreases, while the condensation heat flow increases, which results in the decrease of bubble diameter. The influence of increasing wall superheat on bubble diameter is opposite to that of increasing inlet flow velocity and inlet subcooling. Based on the numerical results, a new prediction correlation for bubble growth rate in a petal-shaped fuel rod channel was developed, whose prediction error is within ±20 %. The related research results help to further reveal the subcooled boiling heat transfer mechanism in a petal-shaped fuel rod channel. [ABSTRACT FROM AUTHOR]

• The flow instability in the NC loop with the start-up stage of PCCS was observed. • Transient thermal-hydraulic characteristic vs operating parameters were discussed. • The empirical correlation proposed can accurately predict duration of pressure drop. Nuclear safety has attracted increasing global attention. Passive Containment Cooling System (PCCS) is one of the several passive safety systems designed to ensure the safety of nuclear power plants (NPPs). A large-scale test facility was built to simulate the PCCS for understanding the thermal–hydraulic characteristics with operating conditions in the start-up process. Parameter analysis is carried out to reveal the influences of various parameters on the thermal–hydraulic characteristics. Results show that the transient-state performance of the PCCS strongly depends on the initial containment pressure and air mass fraction. The air mass fraction has a greater effect on the transient heat transfer process. Meanwhile, the effect of cooling water temperature on the transient characteristics can be ignored. The response time, which is defined to characteristic the heat transfer ability of PCCS, is proposed. In addition, a non-dimensional empirical correlation for response time is developed to reveal the relationship between the heat transfer capacity of the natural circulation and forced circulation, while the deviation mostly within ± 20%. Studies on the relevant physical processes or phenomena are helpful for the safety analysis or accident study related with PCCS. [ABSTRACT FROM AUTHOR]

• The dominant oscillation frequency increased as the heaving period decreased. • The dominant oscillation frequency increased as the heaving amplitude increased. • The inertial and condensation forces played a major role during bubble necking. • The additional inertial forces and heat transfer were enhanced by heaving motion. • A correlation considering the effect of heaving conditions was proposed. With its advanced efficiency in heat transfer, the steam direct contact condensation method is now extensively employed in many areas. The pressure oscillation characteristics of unstable steam jets under heaving conditions were investigated numerically in this study. The dominant oscillation frequency gradually rose with the decrease of the heaving period and the increase of the heaving amplitude. The dominant oscillation frequency under the heaving condition was higher than that under the static condition, with a maximum increase of 27.4%. The inertia and condensation dominated the evolution of steam bubbles at the necking stage under heaving conditions. With the decrease of the heaving period and the increase of heaving amplitude, the additional force induced by heaving movement increased, contributing to the condensation heat transfer enhancement. A dimensionless correlation predicting the dominant oscillation frequency at a low flow rate under heaving conditions was fitted, and the maximum error was within ±6.2%. [ABSTRACT FROM AUTHOR]

The reactor containment integrity and the pressure inside the containment are the most concern in the practical running process. To evaluate the transient heat transfer, non-equilibrium fraction (NEF) is proposed to represent the completed degree of transient condensation. An empirical correlation for NEF is developed and the fit error is within ±25% for 97% of results. Based on this, a simplified calculation model for transient heat transfer rate is also developed, then the transient condensation heat transfer coefficient (HTC) is obtained to characterize the intensity of transient heat transfer. The effect of mixed gas pressure, air mass fraction and temperature of cooling water on NEF and transient condensation HTC are discussed. The result indicates that the initial air mass fraction and initial gas-mixture pressure play an important role in transient-state heat transfer, and the influence of initial cooling water temperature can be ignored. This study aims to improve the understanding of transient-state heat transfer and provide guidance to practical engineering. [ABSTRACT FROM AUTHOR]

The passive containment cooling system (PCCS) as a passive technology has been applied in Generation Ⅲ nuclear power plants. Experiments were conducted in a scale model to analyze the thermal stratification in containment during the containment pressure dropping. Results indicate that the obvious thermal stratification occurs in the containment during the transient-state heat transfer process but not during the steady-state heat transfer process. For the transient-state process, the steam-air mixture temperature increases by 2 °C for each 100 mm increase in the elevation direction of the pressure tank. The nonuniform distribution of air in containment causes thermal stratification, which is more pronounced for lower air mass fraction and higher initial containment pressure. The thermal stratification effect on the condensation heat transfer process is assessed via diffusion layer heat transfer and film heat transfer to evaluate its effect. The diffusion layer heat transfer coefficient (HTC) is strongly affected by thermal stratification, whereas no effect is observed on the film HTC. This leads to the total condensation HTC at the top of the heat exchanger is approximately 3–5 times than that at the bottom in the first 100 s of the containment pressure dropping process. To improve the overall heat transfer performance of PCCS, approaches that enhance the heat transfer capacity at the bottom of the heat exchanger should be considered for prioritization, especially the methods of steam diffusion heat transfer capacity. [ABSTRACT FROM AUTHOR]

• Coriolis force played a major role for dominant oscillation frequency under ocean condition. • Dominant oscillation frequency of steam submerged jet rose as rolling period decreased. • Dominant oscillation frequency of steam submerged jet rose as rolling angle increased. • A formula was used to obtain steam bubble oscillation frequency under rolling condition. Steam direct contact condensation has been widely applied in many fields because of its high heat and mass transfer efficiency. However, this process may cause damage to related devices due to its pressure oscillation, especially under ocean condition. The axial and radial components of the additional force and buoyancy force were analyzed in this numerical study. Results indicated that the Coriolis force played a more important role than other forces. The effects of rolling period and rolling angle on the dominant oscillation frequency were investigated. Under rolling condition, as the rolling period decreased and the rolling angle increased, the dominant oscillation frequency of steam submerged jet increased because of the enhanced Coriolis force, which promoted the necking of the bubble. Moreover, a correlation was proposed to predict the dominant oscillation frequency of steam submerged jet under rolling condition. The maximum deviation was less than ± 9.5%. [ABSTRACT FROM AUTHOR]