For nuclear power system, the supercritical CO2-based Brayton cycle is very promising for its potentially higher efficiency and compactness compared to steam-based Rankine cycle. Compressor is the critical component in the supercritical CO 2 -based cycle, which typically operates at an inlet fluid state close to the fluid critical point for optimal cycle efficiency. As the fluid parameters vary significantly near the critical point, the compressor is more vulnerable to flow instabilities and care must be taken in designing the compressor. The supercritical CO2 radial compressor features a compact design and the unsteady interactions between the impeller and the vaned diffuser are typically strong. A comprehensive understanding of the unsteady flow effects in the compressor is very helpful in guiding the aerodynamic design. However, little work has been performed on the flow analysis of the unsteady impeller-diffuser interactions in the supercritical CO2 radial compressor. In this work, the unsteady flow simulation of a supercritical CO2 radial compressor stage is carried out. Strong flow unsteadiness is observed and the isentropic efficiency shows a variation of over 20% within one revolution.

The flow rate can fluctuate in offshore nuclear power systems which are exposed to wind and waves, as well as in loops where flow instabilities occur, resulting in different thermal-hydraulic characteristics compared with that under steady flow. Among the thermal-hydraulic characteristics, onset of nucleate boiling (ONB) model determines whether the fluid is boiling, and boiling heat transfer is crucial to equipment performance and safety, both being key issues in subcooled flow boiling. Therefore, an experimental study was conducted to investigate how an imposed periodic flow oscillation affects the boiling inception and heat transfer of subcooled flow boiling of water in a vertical tube. The experiments were conducted under atmospheric pressure with the average flow rate ranging from 96kg/m2s to 287kg/m2s and heat flux ranging from 10kW/m2 to 197kW/m2. The relative pulsatile amplitude range is 0.1–0.3 and pulsatile period range is 10s-30s. Photographic images and thermal parameters such as temperatures and flow rate were recorded. The lack of nucleation site on the heated surface of the test section results in high wall superheat at ONB. The effects of pulsatile amplitude and period on superheat at boiling onset and average heat transfer were analyzed. The results show that the superheat at boiling inception is decreased when the average heat flux is lower than the heat flux at boiling inception of the corresponding steady flow, and the superheat at boiling onset is increased when the average heat flux is higher than the heat flux at boiling onset of the corresponding steady flow. The above effect of flow rate pulsation on superheat increases with increasing amplitude and decreasing period, and the mechanism can be explained by boiling nucleation theory. The lack of large active nucleation site also affects the boiling heat transfer. By comparing the contribution of nucleate boiling to heat transfer with the widely used Cooper’s pool boiling correlation, the subcooled flow boiling was found suppressed by convection. The average heat transfer of both the intermittent flow boiling and the single phase flow is influenced by flow oscillation.

An experimental investigation was conducted in a natural circulation (NC) loop to study the characteristics of two-phase flow instability under low pressure condition. A 3 × 3 rod bundle channel was used as the test section. The effects of heating power, inlet subcooling degree and system pressure on the two-phase NC flow instability types and stable boundaries were studied. The experimental results show that three typical flow conditions can occur in rod bundle channel under NC condition, which are single-phase NC flow, subcooled boiling NC flow oscillation and density wave oscillations (DWO). The oscillation amplitude and period of DWO can be enlarged by increasing the heat flux. Increasing the inlet subcooling degree can increase the marginal heating power of flow instability in NC system. The occurrence of DWO can be suppressed by increasing the system pressure. The flow instability boundary presented by the subcooling number and phase change number was also obtained in present work.

Due to the inherent safety characteristics of passive core injecting system, it is one of the most important mitigation strategies under loss of coolant accident (LOCA). However, flow instability during passive gravity-driven reflooding may occur, which can obstruct core reflooding process and reduce the heat removal rate from the reactor core. Therefore, exploring the characteristic and influencing mechanisms of flow instability during gravity-driven reflooding is necessary to analyze the effectiveness of passive core injecting system and mitigate the consequences of LOCA. In this work, small scale experiments are performed to investigate the flow instability during gravity-driven reflooding when the open ratio of steam outlet are in the ranges of 3.33–50%, gravity driving head are 16–20kPa and system pressure are 0.1–0.3MPa. The test facility consists of a cooling water storage tank, a test section and a condensate tank. The results show that flow instability could occur under specific conditions and have a strong regularity, which is divided into three stages: cooling water initial injection, cooling water expulsion and cooling water re-injection. In addition, the effects of open ratio of steam outlet, gravity driving head and system pressure on flow instability phenomenon are investigated. Increasing the open ratio of steam outlet accelerates the discharge rate of steam and prevents the accumulation of steam at the inner cylinder, which can contribute to cooling the heated rods and restrict the flow instability. And under low gravity driving head of 16kPa and low system pressure of 0.1MPa the temperature and pressure acutely change and flow instability is more likely to occur.

In order to study the effect of rolling motion on flow instability of parallel rectangular channels of natural circulation, the natural circulation reactor simulation system is used for physical prototype. And theory analysis model of parallel rectangular channels of natural circulation system under rolling motion is established and coded by Fortran. The results of the program are verified to the experiments, and the results are in good agreement. The flow instability boundaries of different pressure under static and rolling motion are calculated respectively. The results show that: 1) under static condition, with the increase of the pressure, the instability boundary line changes, and the system becomes more stable; 2) under rolling conditions, the heating power of instability boundary decreases comparing to the stable conditions. The instability occurs earlier; 3) the stability of the system decreases with the increasing of rolling amplitude and frequency.

Multiphase flow widely exists in the nature and engineering. The two-phase flow is the highlight of the studies about the flow in the vessel and steam explosion in nuclear severe accidents. The Moving Particle Semi-implicit (MPS) method is a fully-Lagrangian particle method without grid mesh which focuses on tracking the single particle and concerns with its movement. It has advantages in tracking complex multiphase flows compared with gird methods, and thus shows great potential in predicting multiphase flows. The objective of this thesis is to develop a general multiphase particle method based on the original MPS method and thus this work is of great significance for improving the numerical method for simulating the instability in reactor severe accident and two-phase flows in vessel. This research is intended to provide a study of the instability based on the MPS method. Latest achievements of mesh-free particle methods in instability are researched and a new multiphase MPS method, which is based on the original one, for simulating instability has been developed and validated. Based on referring to other researchers’ papers, the Pressure Poisson Equation (PPE), the viscosity term, the free surface particle determination part and the surface tension model are optimized or added. The numerical simulation on stratification behavior of two immiscible flows is carried out and results are analyzed after data processing. It is proved that the improved MPS method is more accurate than the original method in analysis of multiphase flows. In this paper, the main purposes are simulating and discussing Rayleigh-Taylor (R-T) instability and Kelvin-Helmholtz (K-H) instability. R-T and K-H instability play an important role in the mixing process of many layered flows. R-T instability occurs when a lower density fluid is supported by another density higher fluid or higher density fluid is accelerated by lower density fluid, and the resulting small perturbation increases and eventually forms turbulence. K-H instability is a small disturbance for two different densities, such as waves, at the interface of the two-phase fluid after giving a fixed acceleration in the fluid. Turbulence generated by R-T instability and K-H instability has an important effect in applications such as astrophysics, geophysics, and nuclear science.

A square array of cylinders subjected to axial flow is commonly encountered in nuclear reactors and other heat exchangers. Large-scale vortices have been observed in the gaps between the cylinders, both experimentally and numerically. These periodic flow instabilities occur in tightly-spaced cylinder arrays and originate from the velocity difference between the gap and the subchannel regions. The pressure fluctuations caused by the coherent vortex structures are possibly a source of fretting and fatigue in the aforementioned applications. In order to quantify and comprehend this phenomenon, Large-Eddy Simulations are performed on an incompressible, Newtonian fluid flowing adiabatically through a numerical domain containing a single rigid cylinder with periodic boundary conditions, representative for a cylinder in an infinite square array. Subsequently, the temporal frequency spectrum of the wall pressure profile is calculated. The spatial autocorrelation function of this Fourier spectrum, the so-called Cross Spectral Density function, contains information regarding the amplitude and convection speed of the pressure fluctuations. It is shown that the flow instability is strongest for a pitch-over-diameter ratio of 1.03. Also, the simulations indicate that the convection speed is monotonously increasing with the pitch-over-diameter ratio. An updated model for this convection speed is proposed. Finally, it is shown that the single-cylinder approximation has its limitations, but provides valuable information with minimal computational cost.

The Mini-Plate 2 (MP-2) irradiation test is a fueled experiment designed for irradiation in multiple test locations in the Advanced Test Reactor (ATR). MP-2 is considered a non-instrumented drop-in test where small aluminum-clad fuel plate samples are cooled directly by the ATR Primary Coolant System (PCS) water. The MP-2 fuel plate experiment will be irradiated in several different irradiation locations of the ATR. This fueled experiment contains aluminum-clad fuel mini plates consisting of monolithic U-Mo. Four different types of fuel plates were analyzed. A thermal analysis has been performed on the MP-2 experiment to be irradiated in the ATR at Idaho National Laboratory (INL). A new technique for calculating Departure from Nucleate Boiling Ratio (DNBR) and Flow Instability Ratio (FIR) using the commercial finite element and heat transfer code ABAQUS is demonstrated. This new technique calculates DNBR for the fuel plate surfaces and FIR for all water components for each finite element surface and node. Pressure drop data is fed into the calculations in order to geometrically calculate the water saturation temperature. Results from the DNBR and FIR calculations are displayed with the ABAQUS post processor named Viewer. By calculating these parameters at each location in the finite element model, conservatism is replaced with accuracy. This allows for a greater margin for the thermal hydraulic safety parameters.

A vortex diode is used as a highly reliable check-valve in nuclear applications, where it mainly benefits from the intrinsic properties of no moving parts and no leakage. Its basic principle is similar to the diode in an electric circuit. The typical structure of a vortex diode consists of a chamber with axial and tangential ports. When the fluid is injected through the axial port, a simple radial flow in the chamber leads to a relatively low flow resistance. On the other hand, in the reverse flow mode, a strongly swirling vortex can be set up in the chamber, resulting in a very high flow resistance. Several experimental studies found vortex-induced vibration of a vortex diode in the reverse flow mode, where it indicated that the flow was unstable in the vortex diode. This phenomenon may affect the reliability of the vortex diode. However, the mechanism has not been investigated systematically and profoundly. In this paper, 3-D simulations are carried out to help understand the related flow characteristics in the vortex diode. Standard k-ε model was selected for forward flow, while Reynolds stress model was selected for reverse flow. We have found that the results from transient simulations are in good agreement with experimental data. The transient simulations also capture the periodic pressure fluctuation in the vortex diode. Vortex diodes with different structures and geometrical parameters are simulated at different Reynolds number conditions. It is found that the characteristics of the pressure fluctuation are determined by the structure parameters and working conditions of the vortex diode. The flow instability is mainly caused by the asymmetry of the vortex diode. The work presented in this paper will be useful to give better understanding of flows in vortex diodes and to provide some guidance for optimizing the vortex diode.

Once-Through Steam Generator (OTSG) has the advantages of simple structure, good static characteristics, and can produce superheated steam. In recent years it has been widely used in nuclear power system for its good maneuverability. The working fluid flows once through the heat transfer tube forced by the pressure head of the feedwater pump. The water is heated by the coolant of primary side as it flows along the heat transfer tube. After preheating, evaporating and overheating, steam of required temperature is then produced. In addition, flow instability may occur in OTSG because of the present of two-phase flow. Once-through steam generation system is a typical nonlinear multi-variable coupling system, the water/heat storage capacity of secondary side is small, and the steam pressure is very sensitive to the load fluctuation. Various disturbances such as the change of feed water flow, the change of heat transfer rate from the primary side and the change of the secondary side load, will lead to the variety of the parameters in the heating channel. A more complex and precise water control system is therefore needed. An accurate system model is very important to optimize the control strategy and improve the control quality. Obtaining the dynamic mathematical model of the once-through steam generation system is the basis for its effective control. At present, the dynamic modeling of once-through steam generation system mainly adopts mechanism modeling method. By analyzing the inherent mechanism of steam generation process and using the basic conservation equation to derive the relationship between model variables. However, due to the complexity of OTSG and the feedwater system structure and the two-phase heat transfer mechanism, the mechanism modeling is very difficult and the model is not precise enough and very complex. Many nonlinear equations are included, which makes it difficult to determine an effective numerical method for real-time simulation. System identification is based on the actual measurement of input and output information of the process. The system model can be estimated without having to study its internal mechanism. In this paper, the dynamic model of once-through steam generation system in nuclear power plant is identified. Ensuring the stability of the steam outlet pressure during the operation of the system is very important to the safety of steam turbine. Therefore a three-input, two-output coupling system is obtained by analyzing the influence factor of the once-through steam generation system. The pseudo-random sequences are used as the input signal, and the Prediction-Error Minimization (PEM) method is used to identify the system. Dynamic state space models of the system are obtained. The multi-input and multi-output (MIMO) system are identified at different power levels, and the model verification are carried out by simulate the step response output. The results show that the state space model of the once-through steam generation system identified by PEM method is of high precision. The step response of the model and the output of the actual system are in good agreement with each other. The identification scheme proposed in this paper provides a new method and idea for the modeling of nuclear power plant system. The model can provide foundation and technical support for the research of high precision and high quality control system.

Natural circulation has been widely used in some evolutionary and innovative nuclear power plants. Natural circulation systems are susceptible to flow instabilities which are undesirable in the nuclear power devices. An experimentally investigation of two phase flow instability in up-flow boing channel under natural circulation is presented in this paper. Flow instability with and without flow reversal have been found. A pulse signal of water temperature at the inlet of the test section can be detected when the channel suffers from flow reversal. Single phase and two phase flow alternate in the channel regardless of the occurrence of flow reversal. Periodic oscillations with multiple high-order harmonic waves are confirmed by applying Fast Fourier Transform to the time traces of flow rates. Period of flow instability which is the reciprocal of the frequency with the largest amplitude in the amplitude-frequency plane are obtained. Period of flow oscillation presents a nonlinear change with the increase of mass flux. Period of flow instability increases rapidly with the increase of mass flux and decreases slowly when it reaches the maximum value.

An open natural circulation system has the characteristics of a simple structure, superior safety performance and strong heat removal capability. However, during long-term operation, the flow instability may occur due to the reduction of the driving force, which will have adverse effects on the heat removal capability and safe operation of the system. Thus, injecting air into the riser is designed in this paper to improve the driving force of the circulation flow, reduce the possibility of flow instability, and increase the heat removal capability. In order to investigate the influence of air injection on the evolution of flow pattern, resistance characteristics and circulation flow rate, the method of visual observation and data analysis is used based on different pore sizes porous media, air injection rate and submergence ratios. The ratio of the driving pressure head to the resistance pressure drop is proposed as the basis for assessing the effect of air injection on the ability of natural circulation. It is found that the driving force of natural circulation increases with the increase of air injection rate, and the circulation flow rate increases obviously when the bubbly flow appears in the riser. However, when the transition from bubbly flow to churn flow appears, the growth of the circulation flow rate slows down because the resistance increases faster than the driving force. Therefore, it can be known that the best performance is obtained when bubbly-churn flow appears in the top of the riser. What’s more, the capacity of lifting water will be reduced and churn flow will appear prematurely when the submergence ratio decreases. This means that in the process of open natural circulation system design, the submergence ratio of the system should be increased as much as possible. Finally, in this paper, it is found that the bubble pump with PS = 0.2 μm has better performance.

A narrow rectangular natural circulation experimental system is used to research flow pulsation occurred during the experiment. During the experiment, the flow pulsation phenomenon is found in narrow rectangular channels. When compared with density wave instability, it is found that the flow pulsation has certain characteristics of density wave instability, but at the same time, there will be a transient low flow phenomenon, which is a unique flow oscillation phenomenon in the narrow rectangular channel. The mechanism of flow oscillation phenomenon is analyzed, on this basis, a forecast equation of mass velocity is put forward. Forecast equation of mass velocity is used to calculate the natural circulation mass flow rate. Calculation results are in good agreement with the experimental results, which proves that the forecast equation of mass velocity is correct, provides control and prevention theory basis for natural circulation flow instability in narrow rectangular channel.

In thermohydraulic analysis, unsteady subcooled flow boiling is of vital importance to both natural circulation systems where flow instabilities are frequently encountered and offshore nuclear power systems which operate under the influence of ocean waves. An experimental study was conducted here to investigate how an imposed periodic flow oscillation affects the subcooled flow boiling heat transfer of water in a vertical tube. The average heat transfer characteristics and variations of the transient parameters are investigated. The result shows that there is a wall temperature overshoot as a consequence of boiling onset and the wall temperature downstream of boiling front could even drop below the saturation temperature under the high inlet subcooling of 75 °C. Under flow pulsation, intermittent flow boiling appears, when the imposed heat flux, q , is close to the boiling onset heat flux of steady flow, q s,onset . As a result of intermittent flow boiling, the average wall temperature of pulsating flow is lower than the wall temperature in steady flow when the q < q s,onset and the average wall temperature of pulsating flow is higher than the wall temperature in steady flow when the q > q s,onset . Moreover, during intermittent flow boiling, the boiling induced a decrease of the total pressure drop and can cause a large pressure fluctuation. In addition, two types variation of outlet fluid temperature fluctuations were observed and the wall temperatures present an extra local maximum and minimum values beside the extrema corresponding to the extremal mass flux.

Heat transfer, pressure drop, and instability phenomena in water-steam two-phase flows are important research areas with regard to thermal power plants. Here, demineralized water is vaporized to operate steam turbines that drive generators, converting the energy from fuel that is burned into electricity. To design the evaporation process, it is important to know the heat transfer coefficients of water-steam-mixtures and the pressure drop due to friction, elevation and momentum in terms of steam quality. Moreover, occurring instabilities need to be understood to prevent unstable and unsafe operation conditions. The aim of this research work is to design and construct a test rig to analyze these thermal-hydraulic phenomena. Furthermore, the obtained results will be applied to validate the heterogeneous flow model of the dynamic simulation software APROS (Advanced Process Simulation Software) from VTT (Technical Research Center of Finland). In contrast to previous studies in this field, the fluid used for the experiments is demineralized water — instead of usually used refrigerants — to eliminate possible side effects related to different fluid properties. Furthermore, the heated test section of the test rig is a thermal-hydraulic scale down from a thermal power plant’s low-pressure evaporator tube. The focus lies on a huge parameter variability in terms of operation conditions (sub-cooling, inlet pressure, mass flow, heating power, upstream compressible volume) and test rig layouts (vertical and horizontal test section, different lengths and inner diameters, single and parallel tubes) to analyze their impact on the different instability phenomena (density-wave oscillations, pressure-drop oscillations). Literature dealing with experimental investigation of two-phase flow in horizontal evaporator tubes exposed to natural circulation at low pressure is rare. This work aims to contribute especially to this field of research. Currently, the designed test rig is under construction. Once the experiments are performed, the results will be analyzed by means of stability maps and fast Fourier transformation (FFT) amongst others. The results will then be used to validate the heterogeneous six-equation model of the dynamic simulation software APROS with regard to heat transfer, adiabatic pressure drop and flow instabilities. The six-equation flow model (also known as Euler-Euler approach) implies a considerably higher accuracy than the usually regarded homogeneous three-equation model.

The U.S. High Performance Research Reactor Conversions fuel development team is focused on developing and qualifying the uranium-molybdenum (U-Mo) alloy monolithic fuel to support conversion of domestic research reactors to low enriched uranium. Several previous irradiations have demonstrated the favorable behavior of the monolithic fuel. The Full Scale Plate 1 (FSP-1) fuel plate experiment will be irradiated in the northeast (NE) flux trap of the Advanced Test Reactor (ATR). This fueled experiment contains six aluminum-clad fuel plates consisting of monolithic U-Mo fuel meat. Three different types of fuel plates with matching pairs for a total of six plates were analyzed. These three types of plates are: full burn, intermediate power, and thick meat. A thermal analysis has been performed on the FSP-1 experiment to be irradiated in the ATR at the Idaho National Laboratory (INL). A thermal safety evaluation was performed to demonstrate that the FSP-1 irradiation experiment complies with the thermal-hydraulic safety requirements of the ATR Safety Analysis Report (SAR). The ATR SAR requires that minimum safety margins to critical heat flux and flow instability be met in the case of a loss of commercial power with primary coolant pump coast-down to emergency flow. The thermal safety evaluation was performed at 26 MW NE lobe power to encompass the expected range of operating power during a standard cycle. Additional safety evaluations of reactivity insertion events, loss of coolant event, and free convection cooling in the reactor and in the canal are used to determine the response of the experiment to these events and conditions. This paper reports and shows that each safety evaluation complies with each safety requirement of the ATR SAR.

Small modular reactor (SMR) concept has been developed as one of the key solutions for the growing demand of safe and clean energy. SMR designs can be applied extensively in areas such as sea water desalination and small-scale power generation etc. Unlike conventional light water reactors, most SMRs greatly simplify the structure of reactor pressure vessel, usually eliminate pumps and use natural circulation to cool down the core and transfer energy. However, flow instability may easily occur and affect the entire two phase natural circulation, which is of great importance for the start-up and normal operation process of BWR-type SMRs. For PWR-type SMRs, two-phase natural circulation could exist during accidents such as small break loss of coolant accident (SBLOCA) and loss of heat sink. Current research aims to experimentally investigate potential flow instabilities related to natural circulation for a PWR-type SMR during the accidents. For current research, the NuScale reactor design is selected as the research prototype. In this paper, the design and scaling analysis of a scaled PWR-type experimental facility are provided. In order to experimentally study the natural circulation behavior of PWR-type SMR during accidental scenarios, detailed scaling analyses are necessary to ensure that the scaled phenomena could be obtained in a laboratory test facility. A three-level scaling method is used to get the scaling ratios derived from various non-dimensional numbers. An ideally scaled facility is first accomplished based on derived scaling ratios. RELAP5 simulations of both steady state and transient cases for the ideally scaled facility are performed and compared to the prototype to ensure the accuracy of the scaling analysis. Then the ideally scaled facility is modified under engineering considerations and an engineering scaled facility is designed. Similar RELAP5 analyses are performed on the engineering scaled facility and the results match well with those in the prototype and ideally scaled facility.

A theoretical model for flow instability in parallel rectangular channels under ocean conditions was established using lumped method based on the homogeneous model. The model consists of flow instability under static condition, the model of additional forces generated by ocean conditions, the constitutive and numerical models. The effect of ocean conditions was introduced through additional forces in the momentum equation generated by ocean conditions. The code was validated with the experimental data of flow instability under static conditions. Then the margin of flow instability, the threshold power and the periods of the system under static condition was calculated as a basis condition for comparison. The effect of the heaving condition on flow instability will be analyzed analytically. Besides these, the effect of asymmetric heating and throttling under heaving condition will be shown.

This paper presents some consideration and analysis on the flow instability inside the tubes of the moisture separator reheater (MSR), which is a combined equipment of the mist separator and the in-tube side condensation multi-tube type heat exchanger for superheating the exhaust from the high pressure turbine to the low pressure turbine. The condensed water in the tubes will be exhausted by the friction of excess steam vented through the tubes, and with such design, the flow in the tubes will be in an annular gas-liquid two-phase regime, ensuring the steady drainage of condensed water without subcool and the associated temperature oscillation at the tube end. The mechanism of the unstable two-phase flow dynamics in the tubes is discussed. The analysis and the results presented in this paper will be of interests to the related researchers and engineering applications.

3-D simulation of supercritical water flow instability in parallel channels and a natural circulation loop are presented. Results are obtained for various heating powers. The results show that, in the natural circulation loop the steady state mass flow will firstly increase with the heating power and then decrease. And mass flow grows with the growing of the inlet temperature, decreases with the growing of system pressure. Under a large heat flux, the parallel channels will experience the flow instability of out phase mass flow oscillation. And the oscillation amplitude will grow with the growing of heating power. At last, the numerical simulations are validated by B.T. Swapnalee’s experience formula.