Search

Experimental research on flow characteristics of open natural circulation system was performed, to figure out the mechanism of the open natural circulation behaviors. The influence factors, such as the heating power, the inlet subcooled and the level of cooling tank on the flow characteristics of the system were examined. It was shown that within the scope of the experimental conditions, there are five flow types: single-phase stable flow, flash and geyser coexisting unstable flow, flash stable flow, flash unstable flow, and flash and boiling coexisting unstable flow. The geyser flow in flash and geyser coexisting unstable flow is different from classic geysers flow. The flow oscillation period and amplitude of the former are more regular, is a newly discovered flow pattern. By drawing the flow instability boundary diagram and sorting out the flow types, it is found that the two-phase unstable flow is mainly characterized by boiling and flash, which determine the behavior of open natural circulation respectively or jointly. Moreover, compared with full liquid level system, non-full liquid level system is more prone to boiling phenomenon, and the range of heat flux density and undercooling degree corresponding to unstable flow is larger.

Severe accident in a water-cooled nuclear power plant may lead to the production and release of hydrogen. If hydrogen forms flammable gas with air and steam in the containment, it may trigger more serious hydrogen explosion accident, threaten the integrity of the equipment and the containment. The behavioral characteristics of hydrogen under severe accident conditions become primarily responsible for the assessment of combustion risk. This review paper first identifies and describes various key physical phenomena associated to hydrogen behavior in the containment during a severe accident from four main aspects, including the source of hydrogen, hydrogen transport, combustion, and risk mitigation. The typical release process of hydrogen, the combustion limits of hydrogen and main risk mitigation measures are clarified comprehensively. Moreover, representative experimental facilities, related tests and key conclusions are introduced emphatically, which can provide specific guidance for future and ongoing related experimental research. Additionally, through the analysis of corresponding typical simulation studies based on LP method and CFD method, numerical methods suitable for various key phenomena are summarized and recommended. Currently, associated models realized in codes have limitations for predicting hydrogen behavior under certain conditions, which are mainly derived from the coupling effect of complex factors such as condensation, jets, and flame propagation, etc. The applicability and uncertainty of the models in these situations still need to be further evaluated and developed.

Aerosol is one of the main forms of fission products in nuclear reactor accidents, which are derived from direct leakage of solid fission products and homogeneous and heterogeneous nucleation of gaseous fission products. Pool scrubbing may reduce the leakage of radioactive aerosol, and it is vital for the prediction of radioactivity to accurately grasp the retention efficiency. In this paper, experimental research on aerosol retention characteristics in jet regime was carried out. Titanium dioxide and cesium iodide were used to simulate radioactive aerosols with different hydrophilicity, and the retention efficiency under different inlet pressure and aerosol size was obtained. The results show that the retention efficiency of aerosol increases significantly while the inlet pressure increases from 43 kPa to 110 kPa. While the inlet pressure is higher than 110 kPa, the retention efficiency still increases with the increase of inlet pressure, although the increasing trend is not obvious. The most easily permeable range of cesium iodide aerosol is smaller than that of titania aerosol, and the retention efficiency of cesium iodide aerosol is always higher than that of titania aerosol, which may be caused by hydrophilicity. However, with the increase of stokes number, the difference of retention efficiency between the two aerosols gradually decreases. The difference between different aerosols was analyzed in this paper and it was found that the influence of hydrophilicity was negatively correlated with stokes number. The results of this research can be used to assist the validation of pool scrubbing model.

As a new filter material, metal fiber has good high-temperature resistance, moisture resistance, and radiation resistance. To explore the influence of the fiber’s primary structural parameters on the fiber’s performance, this paper experimentally studied the relationship between the flow resistance, filtration efficiency, and dust holding capacity of the metal fiber filter material and the thickness and solid volume fraction (SVF). Under the condition of ensuring uniform deposition of NaCl aerosol, we found that the deposition law of NaCl polydisperse aerosol is the same as that of monodisperse aerosol. The results show that for the same fiber diameter, increasing the fiber’s thickness and SVF will increase the flow resistance of the fiber under the same dust holding capacity; that is, reducing the dust holding capacity of the fiber. In the depth filtration, the efficiency increases rapidly due to the aerosol deposition in the fiber, while the efficiency changes gently when the filter cake is covered on the filtration surface. At the same time, for each high filtration efficiency fiber, the variation characteristics of resistance with dust holding capacity in the deep filtration stage are the same. After the filter cake is completely covered on the filter surface, the growth trend of resistance is the same, independent of the fiber’s structural parameters.

As an important part of the ventilation system of nuclear facilities, the performance of filter is of great significance to the environment and personnel safety. With the operation of the filter, the filtration mode will change from depth filtration to surface filtration. In this paper, a numerical simulation method is developed for the unsteady deposition characteristics of particles in the surface filtration. The parcel injection model is applied to accelerate the formation and growth process of dust cake on the surface of fiber filter. A series of subroutines running in FLUENT environment are developed to enhance the capabilities of computational fluid dynamics code in ANSYS. The Brownian force is added to the DPM model to calculate the particle trajectory more accurately. The function that aerosol particles are intercepted and deposited on the surface of fiber filter to form dust cake is added. The porous media model is used to simulate the fiber filter and dust cake. The porosity and resistance coefficient change with the unsteady deposition of particles. Combined with the subroutine with FLUENT software, a numerical simulation method which can be used to predict the unsteady deposition characteristics of particles in the surface filtration is formed. The numerical simulation results are compared with the experimental results, and they are basically consistent. So the developed numerical simulation method can be used to predict the unsteady deposition characteristics of particles in the surface filtration, and can also provide a theoretical basis for the structural design of pleated filter.

Under the severe accident of the nuclear power plant, hydrogen-steam mixture gas will be released into the containment compartments. In this process, there is a strong coupling relationship between mixture gas diffusion and steam condensation on the compartment wall surface. Especially for local compartments with poor connectivity, steam condensation is likely to aggravate the risk of hydrogen accumulation and combustion. This coupling phenomenon, which has not been systematically studied in previous numerical simulations or experiments, is especially complex and needs to be further studied. Moreover, the applicability of mechanistic numerical models for predicting condensation-induced transient gas transport processes also should be further confirmed. In this study, the mechanistic numerical model was evaluated through the transient hydrogen mixing experiment of the THAI facility. Based on the validated model, the transient mass transfer process of the mixture gas induced by condensation in a typical single compartment during the steam-hydrogen release process is studied. Furthermore, the influence of release condition of gas source term, wall sub-cooling on the hydrogen accumulation phenomenon in the compartment was discussed. The conclusion is helpful to further understand the transport behavior of hydrogen-air-steam mixture gas in the local single compartment under the severe accident.

In order to study the influence of the water level of the water tank on the flow instability and heat transfer character istics,which is of the low-pressure two-phase open natural circulation system. The open natural circulation system was numerically simulated and experimentally verified. Numerical simulations are calculated by using a homogeneous flow model in this study, The flow rate and heat removal power of the system under different water level conditions were calculated and recorded, and the trend of changing with the water level was obtained. The results were then compared with the experimental results. It shows that the calculated open natural circulation flow and heat removal power results are consistent with the experimental. As the liquid level decreases, the flow state of the system gradually changes from the periodic flashing state inside the adiabatic section to the stable flashing operating state, and the heat exchange power also increases gradually. times. Therefore, in the low-pressure system, reducing the liquid level of the water tank can achieve a good effect of enhancing the flow stability of the low-pressure two-phase open natural circulation, and also has a good effect on the improvement of the heat removal power of the system.

The gravitational deposition and coagulation of aerosols were studied experimentally based on the COSTTHES (Containment Source-Term and Thermal Hydraulics Experiment System) facility and numerically investigated based on a continuous general dynamic equation; the experimental and the numerical results were in good agreement. In the present study, the applicability of the zero-dimensional model for deposition and coagulation of aerosols was verified. A considerable distortion can occur due to the coagulation of the aerosol, which significantly influenced the removal rate of the aerosol with a diameter greater than 0.76 µm, and the dynamic shape factor value of the aerosol used in the present experiment was obtained as 4. In the experimental and numerical studies, it was found that the combined effect of the gravitational deposition and coagulation of the aerosol can increase the number density of the aerosol with specific diameters, and the concentration-increment size would increase as settling continued.

Bubble scrubbing is recommended as a potential way to remove aerosol under accident conditions not only because of its sufficient specific transfer area and considerable residence time but also the feasibility of the concept. The purpose of this paper is to illuminate the effect of operating parameters such as liquid height, gas flow rate on micro-nano soluble aerosol deposition characteristic under bubble scrubbing condition. The results indicate that a strong hygroscopic growth phenomenon happened under bubble scrubbing condition. A direct effect is change in deposition mechanism and the deviation of MPPS. Liquid height can significantly improve the removal efficiency of micro-nano aerosol and present a more obvious influence on inertial-dominant aerosol than Brownian one. The increase of gas flow rate can weaken the deposition efficiency in range of 0–0.6 m3/h but will reverse to enhancement effect when exceeding 0.6 m3/h.

The transport phenomena in packed beds are sensitive to porosity variations. However, empirical correlations in the literature are not in agreement regarding the prediction of the mean porosity for packed beds, resulting in challenges for reactor design. In this study, the mean porosity variations in a densely packed bed with 1.2

The bubble growth in boiling water-cooled packed bed reactor were determined via experiments. Oxidized carbon steel balls were heated by electromagnetic induction method and packed in the glass tube with water as coolant. A high-speed camera was employed for the visualization study of bubble growth process. The effects of corner region, inlet temperature and mass flux on the bubble growth curve were analyzed, and bubble growth curve were proposed based on certain parameters. The results indicated that value predicted by bubble growth model is good agree with experimental data.

Bubble coalescence reduces specific area and weakens the work performance of bubble column. The bubble coalescence near gas sparger which is caused mainly by bubble growing is different from the ones occurring in major liquid. Bubble coalescence efficiency near gas sparger is influenced by many factors including sparger configuration, gas flow rate, bubble deformation, solution composition, etc. This work has conducted a set of visual experiments to study the coalescence characteristics near multi-orifice plate. The experiment parameters cover a wide range of conditions including large scope of gas flow rate, different kinds of solution and orifice configurations. The experimental results suggest that coalescence time is applicable to reflect the influence of the pitch of orifices and gas flow rate on bubble coalescence efficiency. As the number of orifices increases, bubble coalescence efficiency is reduced by the disturbance from the bubbles at adjacent orifices. A hindering coefficient is used to consider the hindering effect of additives on bubble coalescence efficiency. Finally a new calculation expression is established to predict bubble coalescence efficiency near multi-orifice plate whose fundamental form is based on the logistic curve of binary response. The calculated values that refer to this calculation expression are well consistent with the experimental results.

A series of experiments have been performed to investigate thermal stratification induced by steam-air mixture vertical injection, in the range of steam mass flux 3–30 kg/m2s, air mass fraction 0–5% and submergence depth 100–300 mm. Experimental results show that: thermal stratification occurs at the region where none of chugging-induced synthetic jets, steam-driven negative buoyancy jets and plume is strong enough. For pure steam experiments, thermal stratification occurs when the steam mass flux above 15 kg/m2s. The pool is mixed well firstly, then thermally stratified, and then re-mixed in sequence, as the water temperature increases. A little air can make thermal stratification and re-mixing occur ahead of time and more air can completely remove thermal stratification. Gas bypass induced by shallow submergence depth makes thermal stratification occur earlier and more severe. The submergence depth has little effect on thermal stratification in chugging regime.

Packed beds with large tube-to-particle diameter ratios (D/d) suffer from poor lateral heat transport. Consequently, for applications with internal heat generation, fluid with high temperature concentrates in the center of packed beds, which is unfavorable to the design of high efficiency reactors. To address this problem, a novel method is developed: a free channel—flow guiding conduit (FGC)—is constructed in the center of a packed bed using highly fluid-penetrable material. Therefore, in the presented work, CFD simulations are carried out to study the influences of FGC to a conventional packed bed on packing structure, fluid flow distribution as well as heat transfer. It shows that cold fluid with high velocities is guided directly into the original hot zones where fluid temperature is generally higher in a conventional packing. Fluid flow in the lateral direction can be promoted with the Venturi effect. Moreover, with the FGC in proper sizes, pressure drop can be reduced about 16–26%; heat transfer coefficients merely drop 4–6%. However, although favorable effects could be achieved using the FGC, it is also observed that if an oversized flow guiding conduit was imposed into a packed bed, loss of effective cooling for particles would be also resulted since excess fluid could bypass through the conduit. Consequently, unexpected fluid temperature distribution could get some recovery. These findings are helpful for understanding of transport phenomena in packed beds as well as the design of high efficiency reactors.

Thermal stratification induced by steam direct contact condensation is a significant technical concern to consider in the energy industry. Thirty experiments have been performed to investigate the effect of steam mass flux, bulk water temperature and air mass fraction on temperature distribution in the water pool. The condensation behavior and time evolution of temperature distribution are recorded and analyzed. Two condensation regimes of chugging and bubbling are observed. Three flow patterns, namely chugging-induced synthetic jets, steam-driven negative buoyancy jets and plume, are discussed. Three thermal patterns of mixing, stratification and re-mixing are identified. A small amount of air (1–2%) could make stratification pattern occur earlier and severer. The maximum vertical temperature difference is about 30 °C in pure steam experiments and 60 °C in mixture gas experiments. Re-mixing pattern occurs when the bulk water temperature is close to 80 °C. In addition, thermal penetration depth, the velocity of thermocline moving down and the thickness of thermocline are estimated through temperature data.

The channeling effect at the container wall results in difficulties for heat transfer in the central part of packed beds with small tube-to-particle diameter ratios. To overcome this shortcoming, a new kind of packed bed is developed with radial layered configuration: a fluid-penetrable wire mesh is used to divide the container into two parts in the radial direction, which allows particles of different sizes for regional loading. Therefore, following the general rule of fluid flow distribution in packed beds, small particles could be filled in the zone at the container wall to restrain the channeling effect and large particles in the center to promote local heat transfer capacity. In the present work, to facilitate the proper functionality of new equipment using the novel method, a CFD simulation combined with the DEM method for random packing generation is conducted to study the characteristics of the new packed bed on the packing structure, fluid flow, and particles-to-fluid convective heat transfer under a high fluid velocity regime (Re ∼ 104). It finds that the new packing method could exhibit a much regular structure than the conventional packed beds. And outstanding performance is observed in pressure drop reduction and heat transfer efficiency improvement. Moreover, the well-known equations for pressure drop and convective heat transfer predictions are found not applicable to the new bed and distinctive features of fluid velocity and temperature distribution are identified. These findings are helpful for the design of high efficiency reactors.

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.

Steam direct contact condensation in subcooled water is an inevitable process in pressure suppression system of advanced light water reactor. Ten pure steam experiments have been performed in the range of steam mass flux 5–80 kg/m2 s and water temperature 19–96 °C. The steam condensation behave, fluid oscillation and pressure oscillation are recorded and analyzed. Prominent small bubbles are observed on the bubble surface due to the Rayleigh-Taylor instability. Vortex ring appears after aspherical bubble collapses. The upper threshold of chugging is identified by temperature data acquired continuously. The amplitude and frequency of fluid oscillation are obtained and compared with previous correlations. Two typical pressure wave shapes of continuous oscillation and pulse oscillation are identified. The pulse oscillation is induced by steam bubble collapse and observed at low water temperature. The maximum pressure pulse peak is about 1 MPa measured in the nozzle and 0.1 MPa in the pool. The pressure oscillation intensity in the water pool increases first, then decreases rapidly, and then increases slowly. It has a maximum at water temperature about 55–75 °C and the related temperature increases with the increment of the steam mass flux. The pressure oscillation dominant frequency is analyzed and compared with available correlations.

The development of advanced packed beds has remarkable applications in the design of new equipment for related industries. Recently, a hybrid of the structured and random packed beds is proposed—the hollow structured packed bed (HSPB), which can be easily formed with ordered ring structures and a free channel along the centerline by the particle self-assembly feature in a random packed bed at 2

The multi-orifice plate gas sparger, mainly composed of a multi-orifice plate and a gas chamber, is one of the most common sparger facilities. The aeration performance of multi-orifice plate has a close relation with the multi-orifice plate configuration. In addition, the weeping phenomenon has a considerable influence on the gas chamber condition which affects the bubble detachment volume directly. This paper conducts a set of visual experiments to study the influence of multi-orifice configuration and gas chamber condition on the aeration performance of gas sparger. For multi-orifice plate, an improved theoretical model is proposed which considers the wave effect of the previous bubbles generated from adjacent orifices and the variance of the number of active bubbling orifice. A parameter is proposed to evaluate the aeration performance in order to overcome the difficulty caused by the randomness of bubble formation process. The experimental results suggest that the gas chamber filled with water is in favor of large bubble formation. The influence of the pitch of orifice on aeration performance can only be observed in high-restricted case. According to the theoretical model and experimental results, the influences of gas flow rate and the number of open orifices on the aeration performance are analyzed and a design criterion for the number of open orifice is proposed.

Present paper aims to experimentally investigate pressure oscillation induced by steam-air mixture gas sonic jets in subcooled water. Dynamic pressure signals and jet images are recorded and analyzed. Experimental results show that: the pressure oscillation dominant frequency decreases with the rise of water temperature, inlet pressure and air mass fraction. Air mass fraction has a complex effect on pressure oscillation intensity. When water temperature is less than 55 °C, as the air mass fraction increases, the pressure oscillation intensity increases first, and then decreases slightly, and then increases quickly. When water temperature is more than 55 °C, as the air mass fraction increases, the pressure oscillation intensity decreases first, and then increases quickly. The interfacial fluctuation height and pressure oscillation intensity dependency to the air mass fraction is the same near the nozzle exit. In addition, the validity of previous analysis model for steam sonic jet pressure oscillation dominant frequency is evaluated. A modified function considering the effect of air mass fraction is developed. The predicted results of steam-air mixture gas sonic jet pressure oscillation dominant frequency agree well with the experimental results. More than 98% of the data points are inside the error band of ±20%.

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.

Steam condensation in the presence of air is a vital heat transfer process in postulated loss of coolant accidents. To numerically evaluate local and global thermal hydraulic properties, several single-phase models have been developed in previous studies. However, potential mutual interactions between the liquid and gas phase (e.g. evaporation, radioactive material retention, etc.) impose requirements on two-phase models. Accordingly, the present work performed CFD evaluations on a two-phase boundary layer model (TPBL) which concerned the gas phase, liquid phase and their mutual interactions. Assessments were performed by comparing calculated results with the COPAIN and TOSQAN experimental data. The COPAIN cases show that predicted local and average heat fluxes are generally within 25% and 15% deviation, respectively. Detailed properties were obtained on the formation, distribution, and thermal resistance of liquid film as well as the model computational costs. In the TOSQAN cases, two steady states, including steam-air mixtures and steam-air-helium mixtures were considered. Via these cases, the applicability of the TPBL model in predicting local field profiles like velocity, temperature and concentration distribution was discussed. Results demonstrate that the calculated results overall agree well with the experimental ones. In general, the TPBL model is feasible in postulated accident analysis.

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%.

The flow patterns associated with boiling water-cooled packed bed reactors were investigated via experiments. A bed packed with spheres of different diameters, which were heated by means of electromagnetic induction, was investigated over a range of mass flux values and sphere diameters. Three flow boiling patterns occurred in the packed bed: bubble flow, slug flow, and annular flow. Furthermore, generalized flow pattern maps were plotted in terms of the mass flux versus the vapor quality. The effects of mass flux and sphere diameter on the flow region transition were analyzed, and criteria for transitions between flow patterns were proposed based on certain parameters. These parameters included the Reynolds number, ratio of the glass tube diameter to the sphere diameter, and vapor quality. The results indicated that more than 90% of the data are correctly described by the transition boundary criteria.

The pressure oscillation is an important characteristic of direct contact condensation of steam in subcooled water. Many experimental works have been performed on pure steam submerged jet condensation. However, the effect of non-condensable gas content on the characteristics of the pressure oscillation is not yet fully understood. So, present paper aims to investigate the effect of air mass fraction on the characteristics of the pressure oscillation. Experimental results show that: for pure steam jets, the pressure oscillation dominant frequency decreases with the rise of water temperature and nozzle diameter. While it increases with the rise of steam mass velocity, which is consistent with the most of previous research results. The pressure oscillation intensity increases with the rise of water temperature, steam mass velocity and nozzle diameter. For air-steam mixture gas jets, the effect of water temperature and steam mass velocity on pressure oscillation characteristics is the same as pure steam jets. The pressure oscillation dominant frequency rapidly decreases with the rise of air mass fraction. However, air mass fraction has a complex effect on pressure oscillation intensity. As the air mass fraction increases, the pressure oscillation intensity rapidly increases at first, then slowly decreases and then slowly increases. In addition, new correlations for pressure oscillation dominant frequency and intensity are developed. The predicted results agree well with the experimental results.

The present study numerically investigates the effect of the header match on the flow distribution in a central-type heat exchanger. Past studies have shown that the appropriate choice in two different header diameters (header match) will significantly help improve the flow distribution in the Z-type and U-type parallel heat exchangers. Under this circumstance, investigations are carried out on the central-type heat exchangers. According to our previous work on the effect of the geometric parameters on the flow distribution in a central-type exchanger (Zhou et al., 2017), three different types flow distribution have been pointed out. Considering the differences in characteristics among three types of the flow distribution. The investigations have been separately made on each type of flow distribution. The results indicate that for the different types of the flow distribution, the best choice of the cross-sectional area ratio of dividing header to combining header (DCR) is different. Besides, for all of three types of the flow distribution, when the DCR values less than 1, a great deal of flow maldistirbution will be brought. The present study is intended to figure out the effect of the header match on the flow distribution and provide constructive suggestions for the design of a central-type heat exchanger.

With the fast growth of 3D printing technologies, it is promising to apply 3D printed structures to nuclear power plant (NPP) devices for heat transfer enhancement. Since Inconel and steel are commonly used materials for NPP devices (e.g. reactor core components and steam generators), investigations on 3D printed structures made of these materials are of necessary. In the present work, an Inconel smooth plate and three 3D-printed test samples (grooves, pin-fins and cubes) with micro-porous structured surfaces were tested. The smooth plate is made of Inconel and the 3D printed samples are made of steel-bronze mixture. In tests, heat flux, surface temperature, pressure, etc. were obtained by a data acquisition system. In addition, videos were recorded by a NAC high speed camera for visualization study. Comparisons were made among various testing samples to find the optimum structure. Results indicate that all the 3D printed samples show better boiling heat transfer at low heat flux conditions compared to that of the Inconel plate. With the increase of heat flux, the degree of heat transfer enhancement weakens. Among the three 3D printed samples, the cubes sample demonstrates the best heat transfer performance and the pin-fins one shows the worst.

Bubble volume and bubble surface area are key parameters that affect the working performance of bubble columns. Due to their non-intrusive characteristics and high spatial resolution, image processing techniques can be used to measure bubble volume. However, bubbles in practical application are not always perfectly spherical or ellipsoidal. In homogenous flow regimes where bubble interaction is weak, the bubble geometry can be classified into three types: round cap, ellipsoidal, and disk-shaped. For large and severely distorted bubbles, it is difficult to employ existing image processing methods for measuring the bubble volume. This study describes an image analysis technique that was developed to obtain the volume and surface area of large and severely deformed bubbles. This proposed image processing method is based on a differential segmentation concept and possesses better adaptability because it considers both the bubble geometry and deflection angle of the bubble. The developed image processing technique is verified using the results of a bubble collection experiment. By making use of the developed image processing algorithm, the volume and surface area of discrete bubbles have been calculated. The relationship between bubble volume distribution and gas flow rate is discussed and the characteristics of bubble surface area variance are shown. Moreover, the influences of bubble volume and surface area on the bubble rise velocity are analyzed.

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.

Application research of two new liquid entrainment models for separately predicting the beginning of entrainment and branch entrainment quality is performed which is a continuation of previous experimental and theoretical studies on the entrainment at T-junction. The new onset entrainment model and branch quality model are developed on the basis of the scaling experiment for studying ADS-4 entrainment phenomenon in AP1000 plant, and the two new models along with previously published entrainment models are compared. The results show that previous models in the background of entrainment through a small break of primary pipe are not suitable for predicting the entrainment through a large size branch (e.g. ADS-4 pipe), while the two new models have a larger applicability. Also, the new entrainment models are written into the RELAP5 code, and then a typical 2-in SBLOCA transient in AP1000 is simulated separately by the modified RELAP5 and the original RELAP5 to further discuss the applicability of new models for the simulation of entrainment through ADS-4. The results show that the modified RELAP5 can get more reasonable simulation results. In view of original RELAP5 overestimated core coolant inventory, the modified RELAP5 can get a conservative core coolant inventory. Hence we can conclude that the new entrainment models are more suitable for the simulation of entrainment through ADS-4.

Experiments on single-phase flow through packed beds at 1.025 D / d D / d D / d > 2, more than one stable configuration can be found at the same aspect ratio. Hence, the influence of assembly method and its corresponding structure on the pressure drop are investigated. It is found that a minor change on the local structure could lead to notable pressure drop difference with very same porosity value in packed beds. And the pressure drop increase is observed as well with increasing mean porosity in the experiment. Therefore, the empirical correlations in literature fail to satisfy the experimental results. Furthermore, with proper assembly method, a highly order structure of packed bed can be achieved, which shows good performance on the pressure drop reduction. These experimental results would be useful for the optimum design in industrial applications as well as understanding the transport phenomena in packed beds.