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Abstract The natural convection condensation heat transfer outside the vertical smooth tube and corrugated tube was experimentally investigated in pure steam and steam–air conditions, respectively. The aim of the research was to evaluate the heat removal capacity of the corrugated tube through comparing with the test results of the smooth tube. In the experiment, a corrugated tube was tested, the bulk temperature and the wall temperature of crest and trough were measured in detail. The result of the temperature measurement demonstrated that the difference of heat transfer characteristics at the crest and trough is not significant. In the pure steam condition, the influence of subcooling on condensation heat transfer coefficient of the corrugated tube is reduced. Comparing with the smooth tube, the condensation heat transfer coefficient of the corrugated tube has been increased for more than 10%. In the steam–air test condition, the condensation heat transfer coefficients have been obtained for the air mass fraction from 0.1 to 0.95 and the total pressure from 0.2 MPa to 0.5 MPa. The corrugated tube shows a slight enhancement with an average increase of condensing heat transfer coefficient about 10%. Highlights • Steam–air mixtures condensation outside a corrugated tube is conducted. • The difference of heat transfer characteristics at the crest and trough is compared. • The corrugated tube weakens the impact of subcooling on the coefficient of condensing heat transfer. • Compared with smooth tube, the corrugated tube enhances the steam-air condensation heat transfer process slightly. [ABSTRACT FROM AUTHOR]

In the past several decades, experimentalists have proposed a large number of correlations to estimate steam condensation rates in the presence of noncondensable gases within free convection regime. However, these correlations are in different forms, and even express conflicting dependencies of the condensation coefficient on some parameters. This study is to aim at developing a new correlation which can faithfully reflect the complex dependencies of condensation coefficient on pressure, air mass fraction and wall subcooling. Considering that more measured data can certainly help to analyze the relationship between condensation coefficient and these parameters, total 374 sets of experimental data have been obtained. It is found that there is a negative power function relation between condensation coefficient and wall subcooling, and the exponent varies with wall subcooling and total pressure. The new correlation, covering two orders of magnitude in the condensation heat transfer coefficient, is valid for pressure ranging from 0.2 to 0.5 MPa, air mass fraction ranging from 0.1 to 0.8 and wall subcooling ranging from 10 to 70 °C. Finally, we have made comparison with experimental data obtained by other investigators which shows that the new correlation is applicative for not only steam-air condensation but also steam-nitrogen and steam-argon condensation, and it can be extended to larger scope of engineering applications. [ABSTRACT FROM AUTHOR]

The development of the marine environment necessitates more rapid development of marine nuclear power plants, and the U-tube bundle heat exchangers, which exist in marine nuclear power plants, play a significant role in safety and economy. However, the radial non-uniform heating (NUH) brought on by the specific structure of the U-tube complicates the boiling heat transfer characteristics. Additionally, the environment of ocean rolling motion superimposed with NUH makes heat transfer more complex. The paper analyzes the time-averaged and transient heat transfer performance by contrasting the rolling UH experiments and rolling NUH experiments. The result demonstrates that the addition of NUH causes no impact on the influenced degree of rolling motion on the time-averaged temperature characteristics, which means a weakly coupled action of NUH and rolling motion. Based on the independence principle, considering the influencing factor of rolling motion and NUH, the developed correlation for calculating time-averaged heat transfer coefficient (HTC) can capture 96.67% of the experimental data within 15% of the error line, and the average absolute error is about 5.75%. For instantaneous fluctuated characteristics, the addition of NUH has a greater impact on the fluctuated range of water temperature than that of the wall temperature, and the fluctuation introduced by rolling motion is displayed based on the effect of NUH. Besides, the calculating method for determining the HTC fluctuation amplitude under uniform heating and rolling motion conditions also applies to situations in which NUH and rolling motion coexist, and the mean absolute error is around 13.80%, indicating that the coupled action of NUH and rolling motion is less obvious in forced circulation. The fluctuation amplitude of HTC depends more on the thermal parameters and rolling parameters. Finally, a novel computational logic approach was created and proposed to estimate transient HTC under conditions for NUH and rolling motion, which can anticipate the fluctuated amplitude and period of instantaneous HTC under corresponding conditions. • The coupled action of rolling motion and non-uniform heating are investigated. • A correlation applied for time-averaged rolling non-uniform heating is developed. • Fluid and wall temperature for transient rolling non-uniform heating are studied. • A transient rolling non-uniform heating heat transfer computing method is proposed. [ABSTRACT FROM AUTHOR]

Flow characteristics are relevant to the safe operation of natural circulation systems. Experimental research on flow characteristics in an open natural circulation loop was conducted. It was shown that flashing and geysering are the two fundamental phenomena influencing the flow characteristics; in addition, flashing and geysering interact reciprocally: geysering weakens the impact of flashing on the flow, while flashing can suppress geysering. Six circulation modes were identified based on the weight of flashing and geysering, as well as typical inlet flowrate curve and experimental observations. As the heating power varies, so do the weights of flashing and geysering, which result in variable flow behavior and the evolution of circulation modes. Based on the theoretical analysis and the calculation of average circulation flowrate by taking into account the effects of subcooling, flashing and heat dissipation, suggestions for the operating and improvement of the system were proposed. [ABSTRACT FROM AUTHOR]

A set of condensation experiments in the steam–air and pure steam conditions has been conducted to evaluate the heat removal capacity of a vertically pin-fin tube under free convection. Condensation heat transfer coefficient and heat transfer rate have been obtained. The bulk temperature, wall temperature and temperature near tube surface are measured in detail. As a reference, a smooth tube is tested under the same condition. For pure steam case, the pin-fin tube generates restrictions on the condensation heat transfer process in low wall subcooling region. With a higher wall subcooling, the pin-fin tube shows a larger condensation heat transfer coefficient than smooth tube does. For steam–air mixtures case, the result of the temperature measurement demonstrates that there is an air rich region near the pin-fin tube. Compared with the smooth tube, the pin-fin tube generates reduction effect on the condensation heat transfer process under low air mass fraction situation. As air mass fraction increasing, the condensation heat transfer coefficient of the pin-fin tube becomes larger than that of the smooth tube. [ABSTRACT FROM AUTHOR]

A set of condensation experiments in the pure steam and steam–air conditions have been conducted to evaluate the heat removal capacity of a vertically longitudinal finned tube in natural convection situation. The bulk temperature, wall temperature and temperature near the tube wall were measured in detail. The result of the temperature measurement demonstrated that there was an air rich region near the longitudinal finned tube while gas stratification phenomenon was not found in the mainstream. Condensation heat transfer coefficient and heat transfer rate have been obtained for air mass fraction ranging from 0.07 to 0.95, total pressure ranging from 0.2 MPa to 0.7 MPa. A smooth tube was also tested under the same condition as a reference. At pure steam case, the longitudinal finned tube played a positive effect on the condensation heat transfer, showing a growth of the condensation heat transfer coefficient by a factor of 3.86–4.89. At the steam–air case, the longitudinal finned tube generated restrictions on the condensation heat transfer process when air mass fraction was below 75%. However, the longitudinal finned tube enhanced the heat transfer process for the air mass fraction in excess of 75%. [ABSTRACT FROM AUTHOR]

• The time-averaged heat transfer characteristics under static and rolling motion conditions was compared. • The transient heat transfer coefficient under rolling motion was analyzed. • The difference of heat transfer characteristics between different flow channels was compared. • The effect of rolling parameters on heat transfer characteristics was analyzed. Rolling motion usually shows great influence on the dynamic and heat transfer characteristics in some marine nuclear power plants compared with land-based ones. The existence of two-phase fluid will aggravate the effect of rolling motion, indicating significant difference on the heat transfer mechanism. In present study, sub-cooled flow boiling in a vertically arranged tube bundle channels is experimentally investigated under various rolling motions. Results show that, when the rolling motion illustrates negligible effect on the mass flow rate, the influence of rolling motion can be attributed to the periodic change of the interface between hot and cold fluids as well as the circumferential transverse flow. A delay may occur on the fluid temperature since the rolling motion starts, while the delay of wall temperature variation is negligible. In addition, the farther away the flow channel is from the rolling axis, the more obvious the effect of the rolling motion is. Various rolling parameters including period and amplitude are considered. Note that the period of relevant parameters increases with after rising the rolling period, and the amplitude of fluid temperature increases with the increase of rolling frequency and amplitude. In comparison to the parameter of rolling amplitude, the rolling period displays a more critical role on the changes of wall temperature. [ABSTRACT FROM AUTHOR]

A vane-type separator has been successfully applied to gas-liquid and liquid-liquid separation. In order to evaluate the separation performance of the separator, a theoretical model for calculating the particle motion trajectory in the vane-type separator was developed. Firstly, the simplified steady incompressible Navier-Stokes equations were derived to acquire the equations of tangential velocity and axial velocity of continuous phase flow field. Then, the coefficients were determined by fitting the CFD results and it can be used to obtain the velocity at different swirler structure conditions. Further, the force analysis on the particle was conducted to develop the motion governing equations to predict the trajectory. The predicted separation lengths at different geometric swirl number conditions show a good agreement with the experimental results and its relative error is smaller than 3%. Besides, the effects of the key parameters including density difference, particle size and viscosity of the continuous phase on the separation length were investigated. • The velocity field in the swirl chamber was derived from Navier-Stokes equations. • A prediction model for the particle trajectory was developed. • The effects of the key parameters on the particle trajectory were studied. [ABSTRACT FROM AUTHOR]

U-tube steam generator, as one of the core equipment in nuclear power plants, has lateral non-uniform heating on the secondary side, which complicates the progress of heat transfer on the secondary side of the evaporator by causing fluid cross mixing. However, there is relatively little research on the properties of transverse non-uniform heat transfer in the secondary side tube bundle of the evaporator under natural circulation conditions with the free liquid surface. Therefore, to conduct comparative experiments and studies between uniform and non-uniform heating, an experimental setup with the natural circulation of the free liquid surface and the ability of transverse non-uniform heating was constructed, considering the characteristics mentioned above for the secondary side of the evaporator. The results indicate that higher proportions of gas-phase distribution are frequently found in the tube bundle on the high-power side, and corresponding positions usually have higher water temperature, heat transfer capacity, and heat transfer coefficient. Besides, four heat transfer correlations that are suitable for pool boiling are estimated, and the results show that the Rohsenow correlation has the best applicability to compute the heat transfer coefficient for uniform heating experiments while performing worse in non-uniform heating experiments. The main reason is the lack of consideration for the factors of fluid cross mixing in traditional correlations. • Fluid temperature distribution of transverse non-uniform heating are studied. • Local heat transfer of non-uniform heating conditions are compared. • Pool boiling heat transfer correlations under non-uniform heating are evaluated. • Error statistical analysis of prediciton is conducted to analyze the error sources. [ABSTRACT FROM AUTHOR]

In order to investigate the separation mechanism and provide some guidance for the separator design, the single-phase flow field of the bubble separator for the Molten Salt Reactor is simulated numerically by Reynolds Stress Model. The results show that a low-pressure and high-velocity region is induced under the obstruction of the guiding vanes in the flowing channel and the tangential velocity distribution resembles the Rankine vortex. Besides, the swirl number along the axial position increases first and then decreases due to the improvement of the first light phase outlet. Moreover, the effects of the key structure parameter of the generating swirler including deflection angle, the guiding vane number and the hub ratio on the flow field are investigated and their sensitivity analyses are conducted at the same time. It demonstrates that the deflection has the largest influence on the flow field and the hub ratio is the second. • The flow field of the bubble separator for MSR was simulated by CFD. • The swirl number in the swirl chamber increases first and then decreases gradually. • Sensitivity analysis of the structure parameters was conducted. • The deflection angle has the largest effect on flow field. [ABSTRACT FROM AUTHOR]

• Non-uniform heating tube bundle channel heat transfer experiments were conducted, and the ONB (Onset of Nucleate Boiling) data were obtained. • The existing correlation can not predict the wall superheat at ONB quite well under non-uniform heating. • The influence of Non-uniform heating on ONB has been theoretically analyzed. • A new correlation for ONB under non-uniform heating is developed. The transverse non-uniform heating tube bundle exists in steam generator and will affect the heat transfer and flow conditions in the shell side. It is important for the operational performance of the steam generator. In this study, a non-uniform heating tube bundle channel experimental device was set up. The channel consisted of two groups of heating tube bundles which were non-uniform heating in the transverse direction and uniform heating in the axial direction. Experiments and theoretical analysis were conducted to evaluate the influence of transverse non-uniform heating on the Onset of Nucleate Boiling (ONB). The locations of ONB during the experiments were determined from the heat transfer coefficient variations and the tube wall temperature variations. Experimental results at ONB were compared with existing well-known correlations under uniform heating conditions. And then under non-uniform heating conditions, the experimental wall superheat at ONB were furthermore compared with results predicted by the verified existing correlation. It was found that the correlation can't predict the experimental wall superheats well under transverse non-uniform heating conditions. Therefore, the influence of transverse non-uniform heating on ONB was theoretically analyzed. Correction factors were theoretically introduced and a new correlation was proposed. This new correlation was examined with experimental data and the result showed the correction factors need to be further studied. [ABSTRACT FROM AUTHOR]

• A large-scale thermal hydraulic test-bed applicable to SPLTHR is established. • The influence of filling ratio and initial vacuum degree are investigated. • The optimal filling ratio for the system falls within the range of 25% to 30%. • The system exhibits fastest start-up time when initial pressure is 6 kPa. As a specialized form of heat pipe, the separated heat pipe shows great potential in passive residual heat removal due to its high efficiency in transferring heat and flexible arrangement. A separated heat pipe system has been designed for the Swimming Pool-type Low-Temperature Heating Reactor (SPLTHR), and its heat transfer ability is crucial to ensure the safety of SPLTHR. To investigate the heat transfer performance of this system under various conditions, a large-scale thermal hydraulic test-bed applicable to SPLTHR has been established, utilizing water as its heat transfer medium and operating at sub-atmospheric pressures. The effect of filling ratio and initial vacuum degree on the start-up and heat transfer characteristics of the system is studied. The results show that the filling ratio has little effect on the heat transfer rate of the separated heat pipe within a certain range. The heat transfer is not optimized at either low or high filling ratios. The initial vacuum degree significantly impacts the system's start-up and heat transfer rate. Furthermore, the start-up and heat transfer characteristics of the system below the working temperature have been analyzed. This study has analyzed the influence of the initial parameters on the separated heat pipe, which provides data support for SPLTHR to use the separated heat pipe to export the residual heat. [ABSTRACT FROM AUTHOR]

Due to working at atmospheric pressure, main pump in pool-type heating reactor may undergo cavitation, which is harmful for its safe operation. In order to prevent cavitation happening after the temperature of primary circuit increases, it is vital to improve the anti-cavitation ability. Therefore, two anti-cavitation schemes using jetting device for main pump are proposed and studied in this paper. Firstly, a small-scale experimental system was constructed to compare the supercharging performance of the two schemes, which demonstrates that device performance is superior when excitation fluid pressurized by the auxiliary pump is used as primary fluid. Besides, a simulation model of the simplified jetting device for large-scale application in the superior scheme was established to study the effect of extraction ratio on flow field. The numerical results show the device efficiency curve approximates the Gaussian distribution. As the proportion of extraction fluid increases, on the one hand, the pressurization performance of the device will be more significant. On the other hand, complex eddies will be formed in the flow field due to boundary layer separation effect, which will increase hydraulic losses. Besides, loss caused by the jet collision with secondary fluid is also one of main losses. The optimization direction on the design of jetting device is to improve its efficiency and ensuring its supercharging performance. [ABSTRACT FROM AUTHOR]

The condensation enhancement research of pure steam and steam with non-condensable gas is of great significance for reactor safety. In this study, to evaluate the influence of chrome-plated tubes on condensation enhancement, pure steam and steam with air condensation heat transfer experiments are conducted, and the plating thicknesses of 1 μm (tube #1) and 10 μm (tube #2) are considered. The results show that dropwise condensation (DWC) occurred in the part of tube #1 surface, while tube #2 surface is filmwise condensation (FWC). For the pure steam condition, the steam condensation heat transfer coefficient (HTC) of tube #2 is higher than that of tube #1, and both are greater than the theoretical analysis of Nusselt. Thus, the condensation enhancement effect of the chrome-plated tubes is remarkable. For the steam with air condition, a new condensation HTC expression is proposed, which is related to gas pressure, wall subcooling, and air mass fraction. Compared with the widely used experimental correlations and the experimental data of polishing tubes, it is found that the condensation enhancement performance is not significant. This is because the dominant thermal resistance affecting heat transfer is the air layer at the gas-liquid interface. Thus, enhancing the disturbance of the gas layer is the key to the condensation enhancement of steam with non-condensable gas. [ABSTRACT FROM AUTHOR]

A full-scale passive residual heat removal system for 2 MW molten salt reactor has been designed and constructed to perform experimental studies. The present research aims at investigating the transient behaviors of natural circulation during the start-up process, as well as steady state characteristics of heat transport capacity and temperature distribution in the drain tank. It is seen that natural circulation will not initiate in the loop until boiling occurs in the heating section, which is resulted from the special structure of cooling thimble. In case of single-phase natural circulation in the cooling thimble, theoretical equations for predicting the flow rate are derived based on considerations of flow regime variation and heat exchange inside the loop. Six cooling thimbles have been used in the system. At normal operation temperature of 643 °C, each cooling thimble has a heat carrying capacity of 2665 W. It is found that for heat transfer from thimble tube to bayonet tube, radiation and conduction heat transfer dominate in steady state conditions and start-up transient, respectively. In addition, temperature distributions inside the drain tank suggest that molten salt may start to freeze even though the bulk temperature is much higher than the solidifying point. [ABSTRACT FROM AUTHOR]

The flashing instability induced water hammer phenomenon (FIIWH) is firstly found in an open natural circulation experimental facility, which is used to study the thermalhydraulics characteristic of natural circulation style PCS. The primary mechanism of the FIIWH is that, the periodic movement of flashing front in the upwards channel leads to the periodic violent changes of the local pressure and temperature at the underpart of upwards channel. However, the pressure change rate is more rapid than temperature, which causes the alternately transforms of water from superheated and subcooled at the underpart of upwards channel, and results in the periodic violent evaporation and steam condensation process there. Among them, the rapid condensation causes the sudden acceleration and collision of water columns and forms a transient high pressure peak i.e. the FIIWH. Besides, the horizontal tube at the underpart of upwards channel (pipeline structure) also makes an essential effect on the FIIWH. Furthermore, the influence factors on the region and intensity of FIIWH, e.g. the heating tube outlet temperature, inlet subcooling, loop structure and non-condensable gas in water are experimental studied in detail. The initial and terminal conditions of the FIIWH are also found out by experiments. And the contour map describing the scope and intensity of the FIIWH are drawn in this paper. Finally, a simplified numerical model is carried out to simulate the pressure peak of the FIIWH. The simulation result shows a good agreement with the experimental result on the low inlet subcooling conditions, while it has a disparity with the experiment result on the high inlet subcooling conditions. And the deviation reason and improvement orientation of the model are analyzed in this paper. [ABSTRACT FROM AUTHOR]

The Passive Residual Heat Removal Heat Exchanger (PRHR HX) is the key equipment of the Passive Residual Heat Removal System (PRHRS). In the Swimming Pool-type Low-Temperature Heating Reactor (SPLTHR), the reactor core pressure and outlet/inlet temperature are 0.32 MPa and 100/80 °C, respectively. In this case, the convection temperature difference of natural convection heat transfer outside PRHR HX is small, which affects the operation of PRHRS, so it is necessary to carry out related research. In present work, numerical and experimental study on the outside heat transfer characteristics of the tube bundle under the small convection temperature difference has been carried out. The results show that the high Reynold number k-ε turbulence model with enhanced wall treatment is suitable for simulating the outside natural convection of the tube bundle, and the simulation results are consistent with the experimental data. For fluid-solid-fluid coupled heat transfer process, the heat transfer temperature difference should be used as the parameter of Gr number. Within the boundary conditions of this study, the Nu numbers predicted by traditional correlations are 1.4–2 times lower than the experimental and the simulation results. According to the results of simulation and experiment, an outside natural convection heat transfer correlation of the tube bundle is proposed, and the error between the predicted value and the experimental data is within −3%–10%. The results of this study can provide a basis and reference for engineering design of the PRHRS. • Carried numerical and experimental researches on natural convection heat transfer outside the tube bundle under small temperature difference. • The calculation method for the Gr number outside the tube bundle in the process of fluid-solid-fluid heat transfer is given. • The experimental and simulation results of natural convection heat transfer outside the tube bundle under the small temperature difference are compared with the traditional correlation. • The correlation for predicting the natural convection heat transfer outside the tube bundle under the condition of small temperature difference is given. [ABSTRACT FROM AUTHOR]

• The heat transfer characteristics in non-uniform heating tube bundle channel were studied. • The heat transfer characteristics under uniform and non-uniform heating conditions were compared. • The applicability of Chen-type correlations for sub-cooled boiling was evaluated. • A new correlation suitable for non-uniform heating to predict the heat transfer coefficient was developed. Since the primary coolant temperature decreases along the U-tube, so the secondary side fluid in the steam generator tubes will receive non-uniform heating (NUH) from the primary side. Compared with uniform heating (UH), NUH has a great influence on the heat transfer characteristics of the secondary side, especially in terms of the sub-cooled flow boiling, during which the heat transfer mechanism changes. In this paper, sub-cooled flow boiling in a tube bundle channel under NUH and UH conditions are performed experimentally. The results show that NUH makes the onset of nucleate boiling (ONB) occur ahead of time, and it also leads to a greater radial inhomogeneity of fluid temperature field than UH. Four Chen-type correlations were evaluated, and it is found that the Gungor-Winterton correlation is most suitable for experiments under UH condition. However, the Gungor-Winterton correlation has a relatively large deviation under NUH condition, and it is also difficult to capture the predicted trend, which means that it is not applicable to NUH conditions. Therefore, an improved Gungor-Winterton correlation is proposed for NUH conditions. Its average relative error is 8.02%, and less than 2.18% of data has larger deviation than 20%. [ABSTRACT FROM AUTHOR]

• A compact axial separator with conical tube was studied. • The droplet trajectory comparison in the conical tube and equal-diameter tube was observed. • The effect of inlet oil fraction and liquid flow on separation efficiency was investigated. • The split ratio has considerable effect on separation performance. Centrifugal technology for two-phase flow separation is widely used in industrial fields. However, the liquid-liquid separation is more difficult than solid-liquid and gas-liquid separation due to its relative small density difference and emulsion caused by droplets breakup. And the current hydrocyclones for liquid-liquid separation have some shortcomings limiting further improvement of separation efficiency. Axial separators with static swirler emerged as an effective alternative method. In this paper, a compact axial separator with conical tube was introduced and investigated experimentally using oil as the dispersed phase and tap water as the continuous phase. A high-speed camera was used to verify the advantages of conical tube compared to a cylindric tube by oil droplets trajectories. Additionally, the separation performance and pressure drop on different inlet flow rate and oil fraction was studied. In the experiment, the split ratio was changed by controlling the valve in HPO pipe, and the results show that separation efficiency can be improved by increasing the split ratio to ensure the drain of oil core. [ABSTRACT FROM AUTHOR]

• The four flow modes were observed and analyzed of an EHDNCS. • The stability map of EHDNCS has been obtained based on 134 experimental cases. • The regions of stability and CIWH can be well divided by different equilibrium qualities. • Discrete and amplitude characteristics of the flow rate were analyzed for the four flow modes. • Increasing the inlet temperature and resistance is more easily induce flow instability of EHDNCS. As nuclear power plants tend to be miniaturized and integrated, equal-height-difference natural circulation systems (EHDNCSs) have potential applications in floating nuclear power plants. This work experimentally investigated the flow modes of an EHDNCS at low pressure. It has observed four quintessential flow modes in the experiments. The transient characteristics of each flow mode were analyzed by the visualization results. In addition, the dispersion and amplitude characteristics of flow rate were compared in different flow modes based on 134 experimental cases. The experimental results reveal that the flow oscillation is related to unsteady flow pattern for intermittent and condensation oscillations. The condensation induced water hammer (CIWH) is found in the condensation oscillation. According to the stability map, it is found that the regions of stability and CIWH can be well divided by equilibrium quality of −0.23% and 0.47%, and the unstable range is enlarged by increasing the inlet temperature and resistance. [ABSTRACT FROM AUTHOR]

Based on the residual heat removal system of molten salt reactor (MSR) in Oak Ridge National Laboratory, a set of water-cooled passive residual heat removal system (PRHRS) applied to 2 MW thorium based MSR is designed and built. During the preheating of the water-cooled PRHRS, the open and closed natural circulation are started respectively. Then, the thermal stratification effect in the water tank of the system and the overall heat transfer characteristics of the condenser are studied under different start-up modes, and the temperature distribution and heat transfer characteristics in the water tank under transient conditions are obtained. The results show that under open startup mode, the development degree of thermal stratification in water tank is hardly affected by the heating power, completely different from that in closed startup mode. And development law and overall heat transfer characteristics of condenser differed under both two startup mode. The results and characteristics could provide applicable and effective references for the actual operation and safety analysis of the PRHRS in the later commercial MSR. • A full-scale passive residual heat removal system for molten salt reactor was built. • The distribution of temperature in the water tank are studied. • The regulation of the degree of thermal stratification varying with power differs in the two startup mode. • A criterion to predict the development rate of thermal stratification in water tanks is proposed. • Overall and local heat transfer characteristics of the water tank are analyzed. [ABSTRACT FROM AUTHOR]

• Bubble resonance with 35 Hz is observed, which could intensify pressure oscillation intensity. • Air is present in dispersive small bubbles or a toroid after condensation. • The onset of chugging is given by correlations. • A new frequency correlation modified with probability is developed. An experimental study is conducted on the condensation behavior, onset of chugging, fluid oscillation, and pressure oscillation when a steam–air mixture is injected vertically into subcooled water at a low mass flux. Air is present in the form of dispersive small bubbles after condensation at a low water temperature and in the shape of a toroid at a high water temperature. Bubble resonance is first observed experimentally; a large bubble is almost stationary below the pipe outlet and air is sustainably released by small bubbles from the upper surface. The upper air mass fraction of chugging is obtained and compared with a transient conduction–diffusion model and momentum balance model. The dependence of different parameters on the amplitude, frequency, and probability of fluid oscillation is analyzed. A new frequency correlation modified with probability is developed and shows good agreement with the experimental data. During bubble resonance, the wave shape of pressure oscillation is harmonic with the frequency at about 35 Hz, and the intensity is larger than that under pure steam conditions. When bubble resonance does not occur, the presence of air can reduce the pressure oscillation intensity. [ABSTRACT FROM AUTHOR]

• Four typical interfacial shapes are observed. • The spreading rate of interfacial radius is in the range of 0.12–0.27. • The interfacial fluctuation height is proportional to the axial position with spreading coefficient 0.03 ± 0.008. • The maximum velocity and acceleration of interface fluctuation are estimated to be around 6 m/s and 20,000 m/s2, respectively. Interfacial characteristics of steam-air mixture sonic jets are recorded and processed using image processing technique. Interfacial shape, radius, fluctuation height, fluctuation velocity and fluctuation acceleration are analyzed. Four typical interfacial shapes including expansion-contraction, double expansion-contraction, expansion-contraction-divergence and expansion-divergence are observed and a three-dimensional interfacial shape map is developed. Gas dynamic effect is notable in the axial region of the near field. The interfacial radius increases linearly with the axial distance in the far field. The spreading rate increases with the rise of air mass fraction and its value is in the range of 0.12–0.27. The interfacial fluctuation height is proportional to the axial distance both in the near field and far field. The spreading coefficient is in the range of 0.03 ± 0.008. In general, both interfacial fluctuation velocity and acceleration increase with the axial distance. The growth rate decreases rapidly in the near field and then increases slightly again in the far field. The maximum velocity and acceleration of interface fluctuation are estimated to be around 6 m/s and 20,000 m/s2, respectively. [ABSTRACT FROM AUTHOR]