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Multi-droplet motion in gas flow is a significant two-phase flow in nuclear power plant, chemical industry and environmental engineering. In contrast to droplet generation and collision in microscopic view, multi-droplet motion presents both macroscopic interaction with gas flow and microscopic single droplet motion characteristic. To simulate the multi-droplet motion, we presented the Euler grid approximation method to update the multi-droplet imformation in all nodes under each calculation step, and constructed a criterion about droplet velocity and direction to ensure droplet motion limited on grid nodes without loss of accuracy. The novelty of this method lies in its description of the virtual streamlines of multiple particles in the same state, as well as the discontinuous flow field composed of these virtual streamlines, furthermore, this method is independent of the time step and can simultaneously calculate droplet information of the entire field. Meanwhile, this paper focuses on the two-phase gas-liquid flow in the gravity separation space of the rotary vane separator, and the motion behavior of multiple droplets in the flow field is simulated based on the parameters of the actual steam-water separator.The result shows that the droplets tend to gather towards the center of the main stream, which is the focus area for droplet separation, and the position with higher density may be the point where the droplets collide. The model can be adopted to calculate the separation efficiency in the process of multi-droplet motion in the gravity separation space, rotary vane separator and other steam separation devices, and the results will help guide the optimal design of the separation device structure. [ABSTRACT FROM AUTHOR]

Abstract: Hydraulic Control Rod Drive Technology (HCRDT) is a newly invented patent of the Institute of Nuclear and New Energy Technology of Tsinghua University which owns HCRDT’s independent intellectual property rights while the integrated valve made up of three direct action solenoid valves is the key part of this technology. Therefore, the performance of the solenoid valve affects the integrated valve and the HCRDT directly. In this paper, firstly, we improve particle swarm optimization (PSO) by Cloud Model and the improved PSO is called CloudPSO. Then, the method is used to optimize the parameters of the direct action solenoid valve. At last, the results are verified by the experiments. The results prove that the method is feasible and accurate to obtain the key parameters of the solenoid valve in order to achieve the biggest electromagnetic force. Besides, the algorithm is analyzed. [Copyright &y& Elsevier]

Abstract: Control rod hydraulic drive mechanism (CRHDM) is a newly invented patent and Institute of Nuclear and New Energy Technology Tsinghua University owns CRHDM''s independent intellectual property rights. The Integrated valve which is made up of three direct action solenoid valves is the key part of this technology, so the performance of the solenoid valve directly affects the function of the integrated valve and the CRHDM. Based on the operation conditions of the Control Rod Hydraulic Drive System, the electromagnetic force of the direct action solenoid valve was analyzed by experiment and numerical method. The result shows that the electromagnetic force of the solenoid valve increases with the increasing current or the decreasing the length of the air gap. In addition, the electromagnetic force is saturated when the current goes up to the threshold, so the working input current should be the threshold. What is more, the electromagnetic force of the solenoid valve with the flat contact surface is larger than the one whose contact surface is cone. Finally, the operation time of the valve with the flat contact surface is shorter than the one with the cone contact surface on the condition that the length of the air gap is longer, but the result is opposite when the length of the air gap is shorter. In order to gain big electromagnetic force and short operation time, the structure of the solenoid valve is that the contact surface is flat with the seal ring. [Copyright &y& Elsevier]

Abstract: Control Rod Hydraulic Drive Mechanism (CRHDM) is a newly invented patent of the Institute of Nuclear and New Energy Technology Tsinghua University which owns CRHDM''s independent intellectual property rights while the integrated valve made up of three direct action solenoid valves is the key part of this mechanism. Therefore, the performance of the solenoid valve affects the integrated valve and the CRHDM directly. In this paper, we present a method to design the parameters of the direct action solenoid valve based on orthogonal experiment design, back propagation (BP) neural network and particle swarm optimization (PSO). The result proves that the method is feasible and accurate to design the parameters in order to obtain the biggest electromagnetic force. Besides, the result also shows that it is the current which influences the electromagnetic force of the direct action solenoid valve most. [Copyright &y& Elsevier]

Abstract: This paper discusses the behavior of the hydraulic control rod driving system (HCRDS) in cyclical swing. The first generation HCRDS with the hole–hole step cylinder, the important driving component, has been applied in 5MW nuclear heating reactor for several years and run well all along. The second generation with the chamfer-hole step cylinder has also been developed and studied. The HCRDS working principle, as well as the particular design of the hole–hole step cylinder, has been introduced in the paper. The control rod (CR) behavior when HCDRS vacillating in certain swing and cycle has been analyzed subsequently. The calculating result indicates that the CR can keep its balance in such unstable state and the operation of driving CR upwards or downwards can function properly likewise. In cyclical swing, the CR can drop down quickly and meet the reactor shut-down needs. The theoretical analysis proves the HCRDS reliable and safe, establishing basement for its further research and wide use. [Copyright &y& Elsevier]

Bubble lift-off diameter is characterized as the size of a bubble rising from a wall, which is vital in the boundary condition of heat transfer model and interfacial area transport equation. In this paper, mechanistic force analysis was conducted to explore a predictive model for bubble lift-off diameter in a vertical channel of subcooled boiling flow. Specifically, the component of growth force normal to the wall and the shear lift force lead to the lift-off of a bubble on the vertical surface. Through force analysis, we found that bubble lift-off diameter is arranged to be related to wall superheat, latent heat, liquid velocity, fluid properties, bulk liquid subcooling, etc. To account for the contribution of the influencing factors, the dimensionless bubble lift-off diameter was correlated with dimensionless parameters, including the Prandtl number, the Reynolds number, the Jacob number, and dimensionless subcooling. The proposed correlation was assessed according to experimental data and the predictions showed good agreement with the data. [ABSTRACT FROM AUTHOR]

• A new design of the capacitive control rods position indicator for SMRs is proposed in this paper. • The performance of the indicator is evaluated based on a theoretical model. • The maximum error of the indicator is ±0.05 mm. The position of control rods is one of the core parameters that can affect the power of nuclear reactors, and it is needed to be monitored by the position indicators. Based on the central shielding effect of the driving rod, the design of the coaxial capacitive position indicators is proposed in this paper. The theoretical rod position recognition model of the indicator is constructed by the conformal mapping, and it is verified by the numerical method. The results manifest that the performance of the indicator can be accurately analyzed by the theoretical model. The performance of the indicator is studied, and it is compared with that of the existing capacitive position indicator. The results show that compared with the existing design, the precision and the sensitivity of the new indicator can be enhanced by two orders of magnitude at the same time. [ABSTRACT FROM AUTHOR]

• The multi-droplets motion and phase change model was formulated. • The present model was utilized to simulate droplet spraying in the containment. • The parameters of the containment were presented against experimental results. • Influence of spray droplet parameters on the system performance was analyzed. The containment spray system is a vital part of the nuclear power plant to reduce the temperature and pressure within the containment during a hypothetical severe accident, which is bound up with the safety of the nuclear reactor. To investigate the performance of the containment spray system, the model coupling the multi-droplets motion and phase change model inducing the local gas parameters surrounding the droplets is formulated. The proposed model is utilized to simulate the droplet spraying process in the containment. The time evolution of various parameters of the containment spray system including the temperature, pressure, steam mass fraction, droplet sizes and trajectories were presented against the experimental results. Furthermore, the influence of the spray droplet parameters on the system performance was analyzed. The results indicate that the system performance is strongly dependent on the radii of the spray droplets, spray mass flow rate and the position distribution of the spraying nozzles, in the range of the temperature, pressure, droplet sizes, velocities and spray mass flow rate investigated. This study presents a droplet motion and phase change model that can be applied to droplet spray characteristics simulation, and performs a systematic analysis of the containment spray system performances. The results can be used to support the design and optimization of spray cooling systems in the containment and other applications. [ABSTRACT FROM AUTHOR]

Highlights • The force balance approach and correlation approach are conducted to predict bubble departure diameter. • A new semi-empirical correlation for bubble departure diameter is proposed in this paper. • The effects of dimensionless parameters on bubble departure diameters are analyzed. Abstract According to the force balance analysis on a bubble, the bubble departure diameter in horizontal subcooled flow boiling is mainly influenced by quasi-steady drag force, surface tension force and bubble growth force while an additional force, buoyancy force, plays an important role in vertical flow boiling. In this paper, the effects of these forces can be concluded by a series of dimensionless parameters including density ratio of vapor and liquid, Prandtl number, Jacob number and bubble Reynolds number. Based on the different forces, two different characteristic lengths are adopted to non-dimensionlize bubble departure diameters in horizontal flow boiling and vertical flow boiling, respectively. Finally, the semi-empirical correlations for bubble departure diameters in both horizontal and vertical subcooled flow boiling are proposed in this paper based on the force balance analysis and available experimental data from literature. The predicted results using the present correlations agree fairly well with the experimentally measured values with a mean relative error of 19.72%. [ABSTRACT FROM AUTHOR]

Control rod hydraulic drive system is a new type of internal control rod drive technology for the nuclear heating reactor. Control rod hydraulic decelerator is one of the major components of the CRHDS. The CRHD performs the rod drop speed control function to reduce the rod drop impact force and prevent the control rod cruciform blade from being deformed or damaged. The composition and working principle of the new CRHD are analyzed. The theoretical model of the rod drop dynamic process was developed and the flow field analysis of the CRHD flow channels was carried out to close the model. The full scale rod scram tests have been conducted to get the rod drop dynamic data. The theoretical results agree well with the experimental results, which verify its correctness. The rod drop theoretical model was then used to analyze the influence of the reactor coolant operating temperature and the hydraulic deceleration cylinder sidewall flow area on the dynamic characteristics of the new CRHD. The research results lay the base for the design and further optimization of the CRHD. [ABSTRACT FROM AUTHOR]

Capacitance sensors are frequently applied for dielectric objects measurements. However, the study on the ungrounded/grounded control rod position measurements with capacitance sensor is limited. In this connection, a system based on the helix-electrode capacitance sensor is applied to measure the rod position in NHR. Based on the variations of equivalent electrical permittivity with different control rod positions in NHR, the helix-electrode capacitance sensor has been successfully used for the ungrounded/grounded control rod position measurement due to its advantages of non-invasion, low-cost, high reliability and non-radiation. The finite element method is used to calculate the sensor capacitance and the sensor sensitivity is also presented. Then, the ungrounded/grounded control rod position experiment is carried out to measure the response of the helix-electrode capacitance sensor, in which a novel method is proposed to calibrate the helix-electrode capacitance sensor based on the grating linear displacement measuring probe. The capacitances obtained by simulations were compared with experiment results to verify the proposed models. The subsequent quantitative analyses on the data indicate the reliability and accuracy of the apparatus for monitoring control rod position. It is demonstrated that such a system shows promising applications for measurements of the control rod position in NHR. [ABSTRACT FROM AUTHOR]

The control rod position is an important factor that affects the power of nuclear reactors. The unique real rod position of nuclear reactors is indicated by the control rod position sensor, which is the key measuring equipment of nuclear reactors. The capacitance control rod position sensor (CCRPS) is designed for the 200 MW Nuclear Heating Reactor (NHR-200) developed by Tsinghua University. The CCRPS has the advantages of simple structure, high reliability, long life-time, and high precision. The research progresses of static performance of the CCRPS are reviewed in this paper, including the research method, error characteristics, and methods for enhancing the measuring precision. The static performance of various types of the CCRPS are summarized. The optimum design of the CCRPS is determined. As a result, it is hoped that the information presented in this paper can provide a foundation for the follow-up studies of the CCRPS. [ABSTRACT FROM AUTHOR]

• The step-up transient flow is the key physical process of Control rod hydraulic drive system (CRHDS). • The transient flow performance experiments of CRHDS are completed. • The step-up transient flow model is established, and the results are in good agreement with the experimental data. • The variations of key parameters and the mechanisms with different driving pressures at high temperature are analyzed. Control rod hydraulic drive system (CRHDS) has been invented for 200 MW nuclear heating reactor (NHR-200). Water hammer occurs and adversely affects the operation of CRHDS. The step-up transient flow model was built, and the mechanism of the flow process was analyzed. The transient flow performance experiments were performed and verified the theoretical model. Next, the model was applied to high temperature working conditions. The findings suggest that there are three water hammer phenomena in the step-up process, and the corresponding pressures decay quickly. The peak pressure of the second water hammer is the largest. In addition, the hydraulic cylinder acts as both a water hammer source and a fluid energy damper in the transient flow process. What's more, the variation laws of key parameters under different driving pressure conditions were obtained and studied. The research results provide a basis for the vibration reduction design and operation monitoring of CRHDS. [ABSTRACT FROM AUTHOR]

Jumping-droplet condensation pushes the boundary of condensation heat transfer by enabling microdroplet shedding via coalescence-induced droplet jumping. The latter is empowered by surface-to-kinetic energy conversion. Regardless of extensive studies of droplet jumping on ideally non-wetting surfaces, a quantitative description of droplet jumping from realistic surfaces remains a challenge due to limited insight into the complex energy conversion process that is strongly coupled with droplet–droplet and droplet–substrate interactions. Here, we use a three-dimensional (3D) pseudopotential multiphase multiple-relaxation-time lattice Boltzmann method (MRT-LBM) to simulate binary-droplet coalescence with various droplet sizes and surface wettability. Then, we developed a comprehensive and unified energy conversion model, derived by rigorously analyzing the dynamic droplet–surface interaction and quantifying the roles of droplet size scale, droplet size mismatch, and surface wettability. Our simulations capture coalescence and jumping dynamics of arbitrary-sized droplets on surfaces having various wettability and reveal the effect of droplet size and surface wettability. Validated by experiments, the energy model is then used to define the jumping/non-jumping boundaries for coalescing droplets on nanostructured surfaces. Our work demonstrates the key physics and a universal criterion governing self-propelled droplet shedding, key to the design of surfaces for enhanced condensation heat transfer, anti-frosting/icing, self-cleaning, and water/energy harvesting. [ABSTRACT FROM AUTHOR]

The steam-water separator is a crucial equipment supplying high-quality steam for the turbines in the AP1000 nuclear power station. It is comprised of the gravity separation space, primary separator (swirl-vane separator), secondary separator (wave-type vanes separator) and auxiliary equipment. In the present study, full-scale three-dimension modeling and simulation are conducted using the Euler-Lagrange approach based on the AP1000 steam generator design drawing to investigate the steam-water separating mechanism and the separation performance of the AP1000 separator. The details of steam-water separation efficiency and pressure drop are obtained in the gravity separation space, primary separator and secondary separator under various operating conditions. The numerical results indicate that the separation efficiency increases rapidly as the steam velocity increases and the relative humidity at the outlet of the AP1000 steam-water separator is less than 0.1% which satisfies the design standard of the AP1000 power station. In addition, the head cover and diffuser assembly structure into the upper of the primary separator greatly increases the steam-water separation efficiency though the pressure drop increases somehow as well, compared to the geometric model without head cover and diffuser assembly. Finally, the inlet velocity distribution of the secondary separator, resulting from the arrangement of the orifice plate in front of the secondary separator, affects the separation efficiency with ±9% discrepancy compared to the uniform inlet distribution. [ABSTRACT FROM AUTHOR]

When an entrained droplet travels through the steam-water separator of the steam generator in a nuclear power station, pressure decreases continuously along the droplet trajectories due to the local flow resistance and structure variation. This movement-induced pressure drop will result in droplet evaporation, which eventually affects the steam-water separation performance. To investigate the influence of the droplet motion on its phase change, a phase change model for single moving droplet is developed by combining static droplet phase change model and droplet motion model. The model well reproduces the droplet evaporation process, showing a fast evaporating stage followed by a thermal equilibrium evaporation stage. The discrepancies between the predicted results and the experimental measurements are within ±2%. Furthermore, the model is adopted in the Euler-Lagrange frame to obtain the phase change characteristics of droplets moving in the wave-type vanes. Based on the simulation, the effects of phase change on the droplet movement trajectory, radius, velocity, terminal position and separation efficiency are examined. In addition, the critical pressure difference that could affect the wave-type vanes separation efficiency about 0.01 % is proposed. The theoretical and numerical work can provide guidance to the design and optimization of the steam-water separating apparatuses as well as other applications where both of the droplet movement and phase change occur simultaneously. [ABSTRACT FROM AUTHOR]

The control rod hydraulic driving system (CRHDS) is a new type of built-in control rod drive technology and the IV is the key control component. The working principle of integrated valve (IV) is analyzed and the IV hydraulic experiment is conducted. There is transient flow phenomenon in the valve opening process. The theoretical model of IV opening process is established by the loop system control equations and boundary conditions. The valve opening boundary condition equation is established based on the IV three dimensional flow field analysis results and the dynamic analysis of the valve core movement. The model calculation results are in good agreement with the experimental results. On this basis, the model is used to analyze the transient flow under high temperature condition. The peak pressure head is consistent with the one under room temperature and the pressure fluctuation period is longer than the one under room temperature. Furthermore, the changing rule of pressure transients with the fluid and loop structure parameters is analyzed. The peak pressure increases with the flow rate and the peak pressure decreases with the increase of the valve opening time. The pressure fluctuation period increases with the loop pipe length and the fluctuation amplitude remains largely unchanged under different equilibrium pressure conditions. The research results lay the base for the vibration reduction analysis of the CRHDS. [ABSTRACT FROM AUTHOR]

The control rod hydraulic driving system (CRHDS) is a new type of built-in control rod drive technology. In the CRHDS the pulse flow from the pump into the hydraulic cylinder of the control rod hydraulic drive mechanism (CRHDM) is regulated by the integrated valve to perform the step motion of the reactor control rod. Transient flow occurs in the CRHDS during control rod step motion process which is studied by experiments. The time-history curves of flow rate, pressure and inner cylinder displacement were analyzed, and the results show that the water hammer pressure peak during the step-up motion is high, while there are no obvious pressure fluctuations in the corresponding step-down motion. In the step-up process, the pressure fluctuation amplitude increases with the increase of CRHDS driving pressure. The step-up time and the pressure increasing time before step-up decreases with the driving pressure. The step-up pressure increases with the driving pressure. In the step-down process, the step-down time, the step-down pressure and the pressure decreasing time before step-down do not change with the increase of the driving pressure. The experimental results lay the base for the working principle and vibration reduction analysis of the CRHDS and it’s also helpful for improvement of the working performance of the key facilities and instruments of the CRHDS loop. [ABSTRACT FROM AUTHOR]

• The control rod position measurement method of double capacitance values is proposed. • The structure of array electrode capacitance control rod position measurement sensor is proposed. • The static characteristic experiments and finite element model analyses are carried out. • The maximum measurement error can be limited to ± 4 mm within the measurement range of 840 mm. Control rod position measurement sensor is the key measurement component of the nuclear reactor. For the capacitance control rod position measurement sensor (CCRPMS), a new method of measuring the control rod position with double capacitance values is proposed. In order to realize this method, the array electrode CCRPMS is designed. For the array electrode CCRPMS, the static characteristic experiments are carried out, the finite element model is established. The relationship between the electrode arrangement method and the "eccentricity inconsistency" effect is analyzed. The function between double capacitance values with rod positions is fitted. For tilt states and different deflection angles of the measuring rod, the applicability of the function is evaluated. Static measurement errors of the sensor are systematically analyzed. Results show that the maximum measurement error of the array electrode CCRPMS can be limited within ±4 mm. [ABSTRACT FROM AUTHOR]

• CRHDS is a new type of built-in control drive technology. • The performance experiments of CRHDS are conducted to analyze the influence of characteristic parameters. • The process mechanisms are studied to lay the base for the equipment fault diagnosis and engineering application. • The direction and guidance are given for the design optimization of CRHDS. Control rod hydraulic drive system (CRHDS) is a new type of built-in control rod drive technology. The circuit flow resistance and driving pressure have significant effects on the stepping performance. The related experiments were conducted. Moreover, the mechanisms of step-up and step-down processes were analyzed and the performance characteristic parameters were obtained. Based on the experimental results, the variations of key parameters were analyzed. The analysis results show that the step-up and step-down processes can be divided into three stages, and displacement, cylinder pressure and flowrate correspond to each other. With the increase of circuit flow resistance, the key time parameters of step-up and step-down processes change first rapidly then slowly. With the increase of driving pressure, the step-up time decreases gradually, the step-down time changes little, and the time of pressure change increases slightly. The research results lay the base for the design and optimization for the CRHDS. [ABSTRACT FROM AUTHOR]

Control rod position sensors are the core measuring components of the control rod drive system. For the capacitance control rod position sensor with protection electrodes, the sensitivity of the sensor is obtained by experiments. The calculated model of the sensor is established, and it is verified by experimental results. Based on the calculated model, the regulation method of protection electrodes on the unevenness of the sensitivity field of the sensor is systematically studied. Performance analysis of the sensitivity field, the structure design, and error analysis of the sensor are carried out. The results show that the sensor sensitivity is 0.007 563 pF/mm and the maximum measuring error can be limited within ±0.5 mm, which can meet the actual use demands. The results provide a new technical scheme for the subsequent research of capacitance control rod position sensors. [ABSTRACT FROM AUTHOR]

• The experiments of capacitance rod position measurement sensor with and without protection electrodes are carried out. • The theoretical model of the sensor is established by using finite element method and verified. • The shielding effect on sensitivity and maximum eccentricity error of FHE-CRPMS-PE is analyzed. • The design of two-section FHE-CRPMS-PE is proposed and can realize control rod position measurement without losing step in NHR200. Capacitance rod position measurement sensor (CRPMS) is the key measurement equipment of control rod hydraulic drive system (CRHDS). For four-helix electrodes CRPMS with protection electrodes (FHE-CRPMS-PE), the static characteristic experiments are carried out and the sensitivity functions of FHE-CRPMS-PE are fitted. The shielding effect experiment is carried out. The computational model of FHE-CRPMS-PE is established by using finite element method, and the experimental results are in good agreement with the model calculation results. The maximum eccentricity error of FHE-CRPMS-PE is analyzed by using the computational model, the influence of shielding radius on sensitivity and maximum eccentricity error of FHE-CRPMS-PE is analyzed. The design of two-section FHE-CRPMS-PE is proposed. The static characteristic experiment and model calculation of the two-section FHE-CRPMS-PE are carried out. The results show that the two-section FHE-CRPMS-PE can meet the measurement error limit and realize control rod position measurement without losing steps. [ABSTRACT FROM AUTHOR]

• The measurement errors are classified, different solutions are proposed. • The multi-electrode capacitance rod position measurement sensor is designed. • The maximum measurement error of the sensor is 6.25 mm. Rod position measurement sensor is the key equipment of control rod stepping and scram measurement, its accuracy is directly related to the safety and reliability of NHR-200. The nonlinear errors of capacitance rod position measurement sensor (CRPMS) were classified, and the experiments of sensor edge effect and measuring rod eccentricity effect were carried out. The calculation model of CRPMS was established, the results of the model agree well with the experimental results. The relationship between eccentricity error and rod position depth was analyzed, and the design of multiple unequal length helix electrodes CRPMS based on the eccentricity error zeroing method was proposed. The static characteristic experiment of the CRPMS with suspending electrodes was carried out and the model was verified. The edge effect and eccentricity effect nonlinear errors were analyzed by using the model. The results show that the nonlinear errors can meet the requirement of rod position measurement accuracy. [ABSTRACT FROM AUTHOR]

• A new bubble-layer-based mechanistic model for the boiling annular-mist flow is established. • The accuracy of this model is verified with experimental data and simulation results of void fraction and CHF with high accuracy. • The variations of two-phase flow parameters in the bubble layer region and the core region are described separately, and the dryout point can be predicted. Accurate prediction of the boiling annular-mist flow as well as the occurrence of the dryout is of particular importance for the safe operation of heat systems. Therefore, this paper focuses on the analysis of the boiling annular-mist flow and aims to propose a mechanistic model to describe this boiling process. Firstly, the physical model is established which divides the flow field into the bubble layer region and the core region in the radial direction. The bubble layer serves as a pseudo boundary for generating bubbles, whose thickness is adopted as the bubble departure diameter. In addition, the annular-mist flow is divided into the core region evaporation flow section and the bubble layer evaporation flow section in the axial direction, based on the relationship between the thickness of the bubble layer and the film thickness. Then, a set of two-dimensional steady-state conservation equations is proposed, which considers the energy, mass and momentum exchanges in both the bubble layer region and the core region. The accuracy of this model is verified with the experimental data and simulation results of void fraction and CHF. Based on this model, the variations of two-phase flow parameters in the bubble layer region and the core region can be obtained separately, such as the velocity, void fraction and pressure. The dryout point can be predicted based on the variation of the film thickness. These findings promote the fundamental understanding of the thermal–hydraulic characteristics of the annular-mist flow. [ABSTRACT FROM AUTHOR]

As a new kind of built-in control rod drive technology, the control rod hydraulic drive system (CRHDS) is applied to the 200 MW nuclear heating reactor (NHR-200). The integrated valve (IV) is the key flow control component of CRHDS, realizing the stepping function of control rod. During operation, the rapid action of IV can trigger the water hammer, which can lead to system vibration, noise and instrument damage. Therefore, it is necessary to investigate the transient flow process. The composition and working principle of IV were presented, and the transient flow experiments were conducted. An equivalence method was proposed innovatively, and the transient flow model of IV was established, whose results were in good agreement with the experimental data. Finally, the model was applied to variable parameter conditions. The analysis results show that: IV can be simplified as flow capacitance, flow resistance, internal connecting pipe and closing valve. The water hammer pressure fluctuation amplitude decreases with the increase of valve closing time, cavity volume and system pipe diameter. The water hammer period increases with the increase of cavity volume and system pipe length. The research results provide guidance for the optimization and engineering application of CRHDS. • The integrated valve (IV) is the key flow control component of Control rod hydraulic drive system (CRHDS). • The transient flow performance experiments of IV are completed. • An equivalence method is proposed innovatively and verified by experiments. • The characteristic parameters and the mechanisms are analyzed. [ABSTRACT FROM AUTHOR]

• A capacitance rod position measurement sensor with protection electrode for measuring grounding metal rod is proposed. • The finite element model of the sensor is established, the static characteristic experiment of the sensor is carried out to verify the finite element model. • The sensor is optimized, the maximum eccentricity error can be reduced from 664.96 mm to 6.36 mm. • It can realize the rod position measurement without losing step in NHR. Control rod step and scram measurement is performed by control rod position measurement sensor. For winding wire capacitance rod position measurement sensor, the static experiments were carried out and the sensor sensitivity was obtained, the finite element model was established and the results of the model were verified by experiment. The influence of wire pitch, winding method and wire number on the eccentricity performance was analyzed, and the design method of installing grounding protection electrodes was proposed. The optimized combination of parameters of the sensor was obtained. The results show that increasing wire pitch, evenly arranging the wires into two electrodes when the wire pitch is fixed, and reducing the wire number are helpful to reduce the maximum eccentricity error. Furthermore, the maximum eccentricity error can be greatly reduced by adding protection electrodes, and the optimized design can meet the requirement of rod position measurement without losing step. [ABSTRACT FROM AUTHOR]

Bubbles dispersed in thin liquid layers are ubiquitous and play important roles in the heat and mass transfer in nature and industrial processes including the energy, chemical, and biology engineering; thus, understanding the dynamics of bubbles confined in a liquid layer remains an important topic in multiphase flows. Here, we report the dynamics of bubble formation from a submerged orifice in a thin liquid layer with a thickness comparable to the bubble size. First, four flow regimes, detachment, jetting bursting, non-jetting bursting, and hole-opened bursting, are observed experimentally and their typical features are analyzed. Then, the evolutions of bubble size at different regimes are studied, and a quasi-static force model is proposed to predict the bubble size, which agrees well with the experimental results. We demonstrate the role of the capillary force exerted by the liquid film in the bubble cap and bubble geometry confined in the liquid layer in modulating the bubble size. Finally, a regime map regarding the liquid layer thickness and surface tension is provided and the criteria between different regimes are discussed based on the bubble geometry analysis and energy balance. Our experimental investigation and theoretical analysis provide insights into the formation and bursting dynamics of bubbles confined in a liquid layer. [ABSTRACT FROM AUTHOR]

• Mechanisms of bubble shape deformation and drag coefficient were analyzed. • Previous correlations were evaluated based on data in a wide range of parameters. • Modified models were proposed with better accuracy and wider applicable range. In this paper, the dynamics and dominant parameters of bubble shape deformation and drag coefficient during bubble rising were theoretically analyzed based on fundamental insights into bubble motion. Modified models for the aspect ratio and drag coefficient applying to a wide range including saturate vapor-water conditions under high pressure (6.9–15.5 MPa) were finally proposed based on theoretical analyses and comprehensive evaluations on previous correlations of aspect ratio and drag coefficient. Results showed that the combination of Weber number and Morton number can reasonably characterize the bubble shape deformation, while the combination of Reynolds number, Eötvös number and Morton number can reasonably characterize the drag coefficient. The proposed aspect ratio correlation in this paper predicts 90% of the existing experimental and numerical data within ±20% with a relative mean error of 8.2% for bubble rising in both two-component systems at atmospheric pressure and mono-component vapor-water systems at 6.9–15.5 MPa, while the proposed drag coefficient correlation predicts 93.5% of the data within ±35% with a relative mean error of 13.8% covering a wide range of 10−3 ≤ Re ≤ 105, 10−2 ≤ Eo ≤ 103 and 10−14 ≤ Mo ≤ 107. [ABSTRACT FROM AUTHOR]

Subcooled boiling flow is widely used for heat and mass transfer in industries such as nuclear reactor, electronic equipment, and spacecraft, where the bubble dynamics is essential for understanding the gas-liquid flow behavior. Current studies focused on the gas void fraction or bubble size distribution at a global scale, but the dynamics of single bubbles are still obscure, which inhibits the fundamental understanding of gas-liquid flow and the precise prediction of heat transfer. Here, we develop a numerical method that captures the motion and condensation of bubbles in subcooled boiling flow using Euler-Lagrange method, with a proposed model for spatiotemporal lift-off bubble distribution on the heated wall as boundary condition. Our numerical method is validated by the agreement between the calculated gas void fraction distributions with the existing experimental data in vertical subcooled boiling flow. We further investigate the distribution of bubble position and size in the tube and find that the bubble size distribution shows different modes at different radial positions. Analysis of the single bubble dynamics shows that small and large bubbles suffer from centrifugal and centripetal lift forces, and thus tend to accumulate at a radial position near the wall and near the tube center, respectively, which reveals the mechanism of bubble size distribution. Besides, we find that the centripetal wall lubrication force is essential for the path instability of small bubbles near the wall. These findings promote the fundamental understanding of the gas-liquid phase distribution in the subcooled boiling flow and provide a numerical strategy for the precise prediction of mass and heat transfer in subcooled boiling flow. [ABSTRACT FROM AUTHOR]

Though bubbles are abundantly formed during the nucleate boiling in annular flows and affect the industrial performance during nucleation, departure, and bursting, the contribution of bubble dynamics in the heat and mass transfer in annular flows still needs to be quantified. Here, we used the semi-analytical film flow model to simulate the annular flow boiling in a vertical tube. We found the liquid film significantly affects the bubble departure diameter at wide working conditions when its thickness is smaller than the bubble's free departure size. A model for the departure diameter of the bubble considering the confinement of the thin liquid film was proposed, based on which theoretical models for the bubble-induced heat flux due to bubble formation and the bubble-induced droplet entrainment rate due to bubble bursting ejection were proposed. The contributions of the bubble-induced heat flux to the total (wall-to-liquid) heat transfer and the bubble-induced droplet entrainment rate to the total (liquid film-to-vapor core) mass transfer of annular flows can be up to 61% and 38% in the studied parameter range, respectively. The contribution of the bubble-induced heat flux to the total heat transfer increases with decreasing inlet mass flux and increasing tube diameter, wall heat flux, and pressure. The contribution of the bubble-induced droplet entrainment rate to the total mass transfer increases with decreasing inlet mass flux and increasing tube diameter. The proposed models of bubble-induced heat and mass transfer in annular flows may offer the first step to precisely control industrial performance by governing the bubble dynamics. • The contributions of bubble departure and bursting to the heat and mass transfer in annular flows were investigated. • Effects of the operating parameters on the bubble induced heat and mass transfer are quantified. • The contribution of the bubble departure-induced heat flux to the total heat transfer can be up to 61%. • The contribution of the bubble bursting-induced droplet entrainment rate to the total mass transfer can be up to 38%. [ABSTRACT FROM AUTHOR]

Highlights • The influence region concept of the evaporating droplet is put forward. • Numerical simulation of water droplet evaporation in hot air is conducted. • The dimensionless radius of influence region on Condition No. 2 is about 30. • The expression of influence region radius is obtained by curve fitting. Abstract The coupling mechanism between the evaporating droplet and surrounding gas is a challenging topic for heat and mass transfer of two phase flow. Generally, the mass, momentum and heat source terms are added to gas conservation equations to describe the action of the droplet on gas phase. However, usually, the source terms are only imposed to just the fluid mesh where the droplet is located, which can lead to the computation divergence. Therefore, this research tries to study the influence of the droplet on the fluid field to figure out the two-way coupling mechanism. The numerical simulation of the water droplet evaporation in hot air under is conducted based on VOF model, which reveals that only the specific fluid field region surrounding the droplet can be influenced by the droplet. The concept of influence region of the evaporating droplet is proposed based on theoretical analysis. Within influence region, parameters of the gas surrounding the droplet change dramatically due to the effects of the droplet. However, outside influence region, the effect can be neglected. In addition, numerous simulations are conducted to analyze the influence factors of influence region. The analysis reveals that the influence region has a radius about 30 times the droplet radius under condition No. 2. The influence region radius increases with increasing droplet radius, temperature difference and with decreasing operating pressure, the air humidity. Finally, the expression of the influence region radius is obtained by data fitting. [ABSTRACT FROM AUTHOR]

Highlights • PEEK are introduced as the control rod guide rod material. • Measurement resolution of the BCRMS is as high as ±7.09 mm. • The built models and BCRMS are proved to be an efficient solution to measure the control rod position in NHR-200. Abstract The accurate control rod position measurement is crucial to ensure the reactor safety and reliability. Based on the variations of equivalent electrical permittivity with different control rod positions in NHR-200, the built-in capacitance control rod position measurement system (BCRMS) has been successfully used for the PEEK material control rod guide rod due to its advantages of non-invasion, low-cost, high reliability and non-radiation. And a novel method is proposed to calibrate the built-in capacitance control rod position sensor (BCRS) based on the grating linear displacement measuring probe. The BRCMS was experimentally tested on the horizontal test facilities. In addition, Capacitances obtained by the built electric theoretical model and FEM analysis were compared with experiment results to verify the built models. Meanwhile, measurement resolution of the newly designed BCRMS is as high as ±7.09 mm. Errors caused by the eccentricity and sensor end effects are analyzed to be in the reasonable range. The built models and newly designed BCRMS are proved to be an effective solution to measure the control rod position in NHR-200. [ABSTRACT FROM AUTHOR]

Plug flow receives attention due to its advantages of low particle attrition, low pipeline wear and low energy consumption. A novel non-mechanical draft tube type feeder (DTF) for plug formation has been proposed in our previous study for vertical plug conveying of coarse particles. We further investigate the influence of key geometry parameter of the DTF on plug formation for different particle diameter. Furthermore, we attempt to extend the application of PIV technique to evaluate plug velocity in pneumatic plug conveying. Experiments for vertical plug formation are conducted with glass beads of three kinds of particle diameter (i.e. 6 mm, 4 mm and 2 mm). Plug formation characteristics are examined in terms of flow pattern in riser pipe, solids mass flowrate and pressure fluctuation. The experimental results show that four main features of plug flow pattern in the riser pipe are observed. Solids mass flowrate displays good linear relation to superficial gas velocity forthe same particle diameter in the plug flow regime, even with different relative entrainment height. Some interesting phenomena are observed for the pressure fluctuation. Plug velocity is successfully obtained by PIV technique with high temporal resolution. The present study provides a further understanding of the flow pattern and dynamic behavior of coarse particles in vertical plug formation with the draft tube type feeder. [ABSTRACT FROM AUTHOR]

Absorber sphere pneumatic conveying is a special application of pneumatic conveying technique in the pebble bed High Temperature Gas-Cooled Reactor (HTGR or HTR). Granular discharge through a vertical pipe with a bend outlet is one of the control modes to determine solid mass flowrate which is an important parameter for the design of absorber sphere pneumatic conveying. Granular discharge rate through the vertical pipe with a bend outlet in the small absorber sphere system are investigated by discrete element method simulation. The effect of geometry parameters on discharge rate, the discharge rate fluctuation in the vertical pipe, and the effect of friction on steady discharge rate ( W s ) are analyzed and discussed. The phenomena of discharge rate fluctuation in the vertical pipe are observed, which are mainly resulted from the evolution of the average downward granular velocity. The steady discharge rate decreases rapidly with sliding friction coefficient increasing from 0.125 to 0.5, and gradually saturates with the friction coefficient further increasing from 0.5 to 1. It is interesting that the linear relation between W s 2/5 and pipe internal diameter D with zero intercept are found for the vertical pipe discharge with a bend outlet, which is different from the orifice discharge through a hopper or silo with none-zero intercept. A correlation similar to Beverloo’s correlation is developed to predict the steady discharge rate through the vertical pipe with a bend outlet. These results are helpful for the design of sphere discharge rate through the vertical pipe with a bend outlet in the small absorber sphere system of the pebble bed HTGR. [ABSTRACT FROM AUTHOR]

The gas-solid hydrodynamics of coarse particles in a two-dimensional spouted bed (2DSB) are measured using 6 mm glass beads to obtain quantitative experimental results. A one-dimensional mathematical model based on gas-solid mass and momentum balance is used to characterize the spout-annulus interaction and to further interpret the experimental results. The superficial velocity and static bed height are in the range of 2.58 to 4.39 m/s and 8.0 to 13.3 cm, respectively. Pressure fluctuation periodicity of the bed pressure drop is characterized by power spectral density (PSD) analysis. The whole-field particle velocity profiles in 2DSB are obtained by high speed CCD camera and PIV technique. The results show that particle motion in the spout in the present 2DSB experiments is dominated by the balance of drag force, solid stress and gravitation. The existence of obvious solid stress from particle impaction and friction leads to energy dissipation, which limits the increase of vertical particle velocity along the spout axis ( v yc ) in the spout and results in a flat stage of v yc with slight fluctuation near the bottom of the fountain. The solid stress also contributes to the nearly flat peak of lateral profile of downward particle velocity in the annulus. These detailed results are helpful for understanding the gas hydrodynamics and particle motion behavior of coarse particles in 2DSB, and for verification of the extended CFD-DEM coupling method for particle size approaching fluid cell size. [ABSTRACT FROM AUTHOR]

Plug flow receives attention because of its advantage to reduce particle attrition, pipe wear and energy consumption. A novel non-mechanical draft tube type feeder (DTF) for plug formation is proposed in this study for vertical plug conveying of coarse particles. Particles in the feeder are entrained by gas stream through draft tube direct into the bottom of riser pipe. The plug formation experiments with coarse particles (glass beads particle diameter 4.0 mm) are conducted with this novel feeder. Plug formation characteristics are investigated by solids mass flowrate, high frequency pressure fluctuations and high-speed camera measurements. Power spectral density analysis is used to characterize frequency distribution of the high frequency pressure fluctuations. The experimental results show that natural plug formation is successfully achieved by the DTF for the superficial gas velocity u g in the range of 4.86 to 10.16 m/s. A “particle cloud” is initially formed by particles entrainment effect in the bottom of riser, then its porosity decreases and length increases by further particles entrainment and pickup to form a plug. The solids mass flowrate shows a good linear relation to the gas mass flowrate. The plug length shows to be stochastic in a certain range around 0.11 m, which is mainly attributed to the natural plug formation characteristics of the DTF. This study provides a natural plug formation feeder for vertical plug conveying of coarse particles. The feeder might be used for high temperature and/or high pressure operation environment in which high reliability is required. It might also be used to simplify the feeding device for vertical plug conveying of coarse particles. [ABSTRACT FROM AUTHOR]

An experimental visualization investigation is carried out to study the characteristics of flow boiling in a narrow channel represented by flow patterns, local void fraction variations and the net vapor generation (NVG) point. The experiment is conducted in a narrow rectangular channel with the size of 1 mm × 30 mm × 650 mm, which is heated by two sides using transparent ITO (Indium Tin Oxides) film for better visualization effect. In this study, it is found that the flow pattern transition boundary of heated vapor-water flow is consistent with several existing flow pattern transition lines for air-water flow experimental data and flow pattern transition criteria in narrow rectangular channel. Based on the experimental data of the void fraction and the thermodynamic equilibrium quality at the NVG point, assessments of the accuracy of existing models and correlations are performed. Finally, new empirical correlations for more accurate predictions for the void fraction and the NVG point are proposed based on the experimental results. The predictions using the present correlations agree well with the experimentally measured values with a mean absolute error (MAE) of 46.07% for the void fraction and 36.19% for the NVG point. • Double-side transparent ITO film heating method was used for better visualization. • Obtaining flow pattern, NVG point and local void faction through image processing. • Flow regime map for vapor-water flow in narrow channel is analyzed and compared. • Comprehensive assessments of existing correlations for NVG points and void faction. • New correlations for void fraction and NVG point with higher accuracy are proposed. [ABSTRACT FROM AUTHOR]

The application object of capacitance control rod position measurement sensor (CCRPMS) is the control rod hydraulic drive system (CRHDS). CRHDS can be applied to integrated water-cooled nuclear reactors. The design of straight electrodes CCRPMS with protection electrodes is proposed in this paper. The regulation method of protection electrodes on static characteristics of the sensor is studied. The structure optimization design of the sensor is conducted. Static calibration experiments are carried out for the sensor, which include 366 experimental points. The reference sensitivity function of the sensor is fitted by experimental results, and measurement errors of the sensor are analyzed. The results show that measurement errors can be limited within the range of ±4 mm, which can meet the accuracy requirements of CRHDS. • Straight electrodes capacitance control rod position measurement sensor with protection electrodes is proposed. • The static calibration experiments of the sensor are conducted. • The maximum measurement error of the sensor can be limited within ± 4 mm. [ABSTRACT FROM AUTHOR]

• Numerical simulation for microscale-confined condensation (MCC) was developed. • Simulation can resolve droplet nucleation, growth, coalescence, and departure. • MCC decouples nucleation barriers from droplet conduction resistance. • Heat transfer coefficient of MCC can be tuned by controlling the surface gap. • MCC outperforms state-of-art condensation by 250% at a steam temperature of 100 °C. Dropwise condensation has received extensive interest due to its potential to enhance the efficiency of power generation, water harvesting, and thermal management systems. The degree of heat transfer enhancement during dropwise condensation is hindered by the delicate balance between nucleation, droplet growth rate, and condensate shedding. Condensation between parallel surfaces having wetting contrast, termed microscale-confined condensation (MCC), presents a promising method to break the fundamental limits of dropwise condensation by decoupling droplet nucleation and growth from condensate removal. The key mechanisms governing MCC enable spontaneous condensate transfer from the condensing surface to a highly wetting absorbing surface to limit the maximum droplet size. Here, we developed and explored numerical simulations of MCC to elucidate the upper limits of the condensation heat transfer rate. To optimize MCC with respect to heat transfer, we examined the effects of surface gap or maximum droplet size, and condensing surface contact angle on droplet size distribution and overall condensation heat flux. We then developed a regime map in terms of surface wettability and shedding droplet size, and showed that smaller surface gaps and smaller condensing surface contact angles are preferential to achieve a higher heat flux in pure vapor condensation conditions. The optimized MCC outperforms state-of-art condensation including classic dropwise condensation and jumping-droplet condensation, with a 250% enhancement of heat transfer coefficient at a steam temperature of 100 °C. Through numerical simulations, our study not only calculates the upper bound of condensation heat transfer, but also provides guidelines for further enhancement of condensation via decoupling droplet shedding from nucleation. [Display omitted] [ABSTRACT FROM AUTHOR]

AP1000 (Advanced Passive 1000) moisture separator that consists of primary, gravity and secondary separators is a crucial device to eliminate droplets from steam and supply dry-saturated steam to turbines. In order to understand the working mechanism of AP1000 moisture separator, droplet-laden flows are simulated in moisture separators based on the Lagrangian–Eulerian approach. In terms of details, the actual droplets are represented by parcels whose equations of motion are cast in a set of ordinary differential equations (ODEs), and the steam phase is described by Reynolds-Averaged Navier–Stokes (RANS) equations. Particularly, the numerical stability of solving the ODEs by using a fourth-order Runge–Kutta scheme is analyzed by a theoretical method in order to increase the time step size to save the computational time. Further, a distinct algorithm is introduced to rise the execution speed of locating a droplet in the grid by about 6000 times. The results show that, firstly, the separation efficiency of primary separator is beyond 90%. According to the moisture distribution on the x – z plane, the swirl-vanes placed in the inner barrel indeed play an important role to eliminate droplets. Secondly, the gravity space has no separation function, but it can make droplets follow the steam without speed disparity. Thirdly, the separation efficiency of the secondary separator is about 25%. This low value is attributed to the fact that only the hooks at the first and third stages can trap droplets. [ABSTRACT FROM AUTHOR]

Moisture separators, such as cyclone and swirl-vane separators, are crucial devices that are used for eliminating droplets in flows, and supply dry-saturation steam for turbines. In this paper, droplet-laden flows were simulated in cyclone and swirl-vane separators based on Lagranian–Eulerian approach. With respect to details, the actual droplets are represented by some characteristic droplets, and the carrier phase is described by Reynolds-Averaged Navier–Stokes (RANS) equations. Our solving routine of the model was coupled with the commercial CFD (computational fluid dynamic) softwares, which were used to calculate the fluid properties of the carrier phase and store them on the mesh grids. However, the CFD softwares could hardly export the grids in an explicit data structure. Therefore, we carried out a data structure called k -d trees to rearrange the grids. Firstly, the results show that the executing speed of our new solver using k -d trees is thousands times larger than the old approach. Secondly, we computed the separation efficiencies of cyclone and swirl-vane separators. Thirdly, our modeling outcomes are verified against experimental data. Finally, the moisture distributions of the separators were analyzed. [ABSTRACT FROM AUTHOR]

• The interfacial parameters in boiling flow were measured by four-sensor conductivity probe under static and vibration conditions. • The time-averaged local parameters under vibration conditions were analyzed. • The instaneous change of local void fraction was analyzed by adopting the phase partitioning method. • The additional forces induced by mechanical vibration were calculated and compared with each other. • The effect of vibration frequency on local void fraction was studied. In this paper, the subcooled boiling flow experiments in a vertical annulus were conducted to study the effect of vibration on local interfacial characteristics. The local interfacial parameters including void fraction, bubble velocity, interfacial area concentration and Sauter mean diameter were measured by four-sensor conductivity probe. The time-averaged local parameters under vibration conditions were firstly measured and analyzed. It is found that the effects of vibration on time-averaged void fraction and interfacial area concentration are very limited. Therefore, we have developed the phase partitioning method to obtain the instantaneous variation of void fraction for further investigation. The instantaneous void fraction along the radial direction varies periodically with the vibration phase. The additional forces induced by vibration are analyzed and compared with each other. Among them, the tangential force is the dominant, which can account for the change tendency of void fraction under vibration. Besides, the variations of void fraction depend on different working conditions. The change tendencies of void fraction near the inner wall and outer wall are opposite in relatively low averaged void fraction regime while those in relatively high averaged void fraction regime are consistent. [ABSTRACT FROM AUTHOR]

• The established dynamic mathematical model of the CCRMS were verified. • The CCRMS is basically realized the dynamic process measurement function. • The dynamic characteristics of the CCRMS system were obtained. In this paper, the dynamic mathematical model of the capacitance control rod position measurement sensor (CCRMS) were established. The experimental research, numerical calculation, and model validation were carried out for the case of the velocity dynamic response calculation, scramming dynamic response calculation, scramming time test and scramming performance measurement test of the CCRMS. The results show that the validity of the proposed model is verified and the CCRMS is basically realized the dynamic process measurement function with one step display measurement requirement. Furthermore, the dynamic characteristics of the double-helix structure CCRMS system were obtained based on the proposed model and experimental research. Moreover, the dynamic measuring error of the CCRMS is less than ±1.20 mm, the delay time of the CCRMS is t = 3 τ which is less than 0.84 ms, and the dynamic response time of the capacitance measuring instrument is about 8 ms. [ABSTRACT FROM AUTHOR]

• The established model of the capacitance control rod position measurement sensor (CCRMS) were verified. • The CCRMS is basically realized the one-step display measurement requirement. • The static characteristics of the CCRMS were obtained based on the proposed model. The accurate and reliable measurement of the control rod position concerns the operation and safety of the 200 MW nuclear heating reactor (NHR-200). In this paper, the static mathematical model of the capacitance control rod position measurement sensor (CCRMS) were established. The experimental research, finite element method (FEM) simulation, and model validation were carried out for the case of the ungrounded metal medium, grounded metal medium, dielectric medium control rod measured rod and eccentricity. The results show that the validity of the proposed model is verified and the CCRMS is basically realized the one-step display measurement requirement. Furthermore, the eccentricity inconsistency of the measured rod is solved. In addition, the end effect of the CCRMS can be reduced reasonably but the eccentricity of the measured rod still have some influences to a certain extent. Moreover, the static characteristics of the double-helix structure CCRMS were obtained based on the proposed model. [ABSTRACT FROM AUTHOR]

• The work concerns coarse particle conveying in the small absorber sphere system. • Present an overview of recent experiments and discrete element simulation at INET. • We present some CFD-DEM results of coarse particle conveying obtained so far. Pneumatic conveying of absorber spheres in the pebble-bed high-temperature gas-cooled reactor is a special application of pneumatic conveying technique in the nuclear engineering. It may be regarded as an intermittent circulation of absorber spheres between the side reflector boring and the storage bin in the reactor. The whole process consists of several sub-processes, e.g., the granular discharge from the sphere storage bin into the boring, granular discharge from the side reflector boring into the feeder, particle entrainment in the feeder, sphere conveying in the transport pipe, and gas-solid separation in the storage bin. We give a brief summary of the experimental and discrete element simulation work done recently at the Institute of Nuclear and New Energy Technology of Tsinghua University, mainly from the viewpoint of gas-solid flow and granular flow. CFD-DEM coupling simulation is recently conducted to investigate the coarse particle conveying in the small absorber sphere system. We present some CFD-DEM simulation results of coarse particle conveying in ambient air and high-pressure helium gas. Based on the needs of engineering practice and scientific research, some research requirements are suggested. Further work is still needed, to make a comprehensive understanding of the conveying process, and to optimize the design and operation of the conveying system. [ABSTRACT FROM AUTHOR]