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Highlights • This study presents experimental studies of the single-phase resistance. • Study of resistance characteristics in rod bundles under low Reynolds number. • An improved correlation of the frictional resistance coefficient is obtained. • Introduce kinematic viscosity correction factor for correction. • This newly developed correlation predicts data with a relative error within ±5%. Abstract A considerable amount of research has gone into understanding the thermal-hydraulics characteristics of rod bundle channels which is of great importance for the nuclear industry. This paper will show that the development of the correlation of the frictional resistance coefficient in rod bundle channels under natural circulation conditions is well understood. This paper presents experimental studies of the single-phase resistance and obtains an improved correlation of the frictional resistance coefficient under natural circulation conditions based upon thermal-hydraulics characteristics which are well understood. It is also shown that this newly developed correlation closely predicts data. The experimental results show that the frictional resistance coefficient of a rod bundle channel under natural circulation is related to the physical properties of fluid and the size of the rod bundle channel. According to different flow regions, the relationship between the frictional resistance coefficient and Reynolds number is discussed separately, and the expressions of frictional resistance coefficient with Reynolds number in laminar flow region and transition region are obtained. After considering the influence of fluid properties on the frictional resistance coefficient, this paper introduces the kinematic viscosity correction factor to correct the frictional resistance coefficient. Finally, the complete expression of the frictional resistance coefficient was obtained. This newly developed predictive expression is in good agreement with experimental data in both the laminar and transition flow regions, with a relative error within ±5%. Thus we now have a predictor of the frictional resistance coefficient under natural circulation conditions based upon more complete thermal-hydraulic parameters in a static state. [ABSTRACT FROM AUTHOR]

• Single-phase flow behaviors in a natural circulation system have been studied. • An axially truncated heated channel installed in a modified closed rectangular natural circulation loop has been investigated. • Irregular heat distributions associated to the non-uniform channel were analyzed for different heating powers at low system pressure. • Transitions in heat peaking along the axially heated section of the non-uniform heat distribution were discussed. An experimental investigation has been carried out on heat distribution in a non-uniform heating channel of a natural circulation (NC) loop under single-phase condition. A heated channel with a truncated axial cosine heat flux distribution was used in the NC circuit and the non-uniformity of the channel is defined in its outer dimension which is convex in shape. The NC experimental loop similar to the primary loop of a Pressurized Water Reactor (PWR) was first reviewed and adjusted to determine the various critical positions for the thermocouples along the heated and flow channels. The experimental procedures involve setting the heating channel inlet temperature at different heating power, followed by observation of the thermal hydraulic responses. When a stable flow is achieved at each inlet parameters, data are recorded till the onset of flow instability. The findings through the results obtained, extend the knowledge on adoption of the NC systems and contribute to a better understanding of the impact of irregular heat distribution on heat transfer capability of the NC loop. [ABSTRACT FROM AUTHOR]

• Natural circulation PDO in rolling motion is established in experiment research. • Maximal Lyapunov exponent is calculated to analysis chaos characteristics of PDO. • Spectrum analysis method is used to reveal the effect of rolling motion on PDO. • Frequency-locking oscillation is discovered in composite oscillation of PDO. • Resonance oscillation occurs in co-frequency oscillation of PDO. A series of experimental study on natural circulation pressure drop oscillations (PDO) under rolling motion are carried out. PDO under rolling motion is a chaotic motion, the thermal–hydraulic mechanism and evolution route are analyzed by maximal Lyapunov exponent and spectrum analysis. When rolling motion has different periods with PDO, composite oscillations are formed. The evolution route can be divided into 3 parts: thermal–hydraulic stable region, frequency-locking oscillation region and rolling effect weakening region. When heat flux is low, PDO is forced to have the same frequency as rolling motion. Frequency-locking oscillation has increasing maximal Lyapunov exponents and a broadband spectrum. But while under high heat flux, rolling effect is weakened. Evolution route of co-frequency oscillations has 3 parts: thermal–hydraulic stable region, bimodal oscillation region and resonance oscillation region. Bimodal oscillation region shows strong chaotic characteristic and cannot form regular waveforms. Rolling motion reduces chaos characteristic when resonance oscillation occurs. [ABSTRACT FROM AUTHOR]

• Natural circulation experiments to study flow characteristics during loss of heat sink are performed. • Thermal hydraulic analysis at different boundary conditions of the resulting natural circulation flow phenomena were carried out. • Single and two-phase flow behavior in closed natural circulation test loop were studied. • Steady state and transient conditions at different stage of flow regime were critically observed and presented. • Effects of loss of heat sink on the heat source and heat sink of natural circulation system were determined. Flow characteristics during loss of coolant to the heat exchanger (HX) via the secondary loop cold leg of a natural circulation (NC) test facility referred here as loss of heat sink transient (LOHS) in a closed rectangular NC system having a single heated channel at two different heating powers and system pressures has been investigated experimentally. Two LOHS experiments (LOHS - 1 and LOHS - 2) were conducted in a well dimensioned closed loop NC test facility designed to represent the primary system of a nuclear reactor. Operating conditions were set and maintained at 6.0 kW and 9.0 kW with system pressures of 0.2 MPa and 0.5 MPa respectively for both single-phase and single-to-two-phase flow transition to determine their thermal hydraulic responses at these predetermined parameters. The transients were initiated by closing the cold leg of the secondary loop supplying coolant to the tube-in-shell heat exchanger while NC flow phenomena were monitored at each vital section along the flow map based on the pre-determined inlet operating conditions. LOHS experiment stops when critical heat flux (CHF) occurs or heat exchanger outlet (HX outlet) temperature rises too high to approximately equal its inlet temperature. Results were acquired using a system-design platform and development environment LabVIEW, programmed in a dedicated computer that was interfaced with the coupled sensors in the measuring devices. Common computational tools which include MATLAB, Ms Excel and Origin software were used for the analysis of the acquired data. Results at the inlet and outlet of heated sections as well as the inlet and outlet of condensers of both tests were analyzed and discussed in details at various flow regimes with the associated phenomena. It is concluded that while stable flow was maintained before transient with slug flow occurring in the heating channel, the system experienced transient stability resulting in steady temperature increase and subsequently drops into an instability region when the transient scenario was activated. In addition, the NC system transited into stable two-phase flow and further loses stability when HX outlet temperature surges to its inlet value with CHF occurrence. These results are presented to contribute to a more extensive utilization of NC systems and understanding of flow behavior during LOHS in advanced reactors. [ABSTRACT FROM AUTHOR]

• Experiment has been carried out to study flow phenomena in a natural circulation system. • A closed rectangular natural circulation loop with single heated channel has been investigated. • The Effects of Temperature and Heating Power variations at low system pressure on flow characteristics are presented. • Single-phase flow behaviors are observed and critically analyzed. Experimental investigation of single-phase, low pressure natural circulation flow phenomena has been carried out. The study aims to critically analyze the thermal hydraulic parameters (temperature and heating power) variations as it affects transfer of heat in a closed-loop rectangular natura l circulation facility having a single heated channel at predetermined inlet sub cooled temperature varied from 40 to 80 °C. Heating power varies between 7 kW−13 kW to portray several conditions of the reactor power in response to temperature changes while sustaining single-phase flow circulation through the hydraulic loop. Real-time signals of thermal hydraulic responses are observed via a graphical interface for data acquisition when stable flow or self-sustained flow oscillations are achieved. Results obtained were analyzed at several sections of the hydraulic circuit which are basically pressure drops and flow rate due to temperature differences and heating power increments. These results and observations made through them thus contribute to the database which is important for characterizing natural circulation phenomena and acquisition of dominants parameters for the scaling analysis of future designs. [ABSTRACT FROM AUTHOR]

The thermal-hydraulic characteristics of the rod bundle channel under ocean conditions have attracted much attention. In this paper, the three-dimensional CFD numerical simulation software STAR-CCM+ is used to study the frictional resistance characteristics of the rod bundle under non-vertical conditions at inclination angles of 0°, 15°, 30° and 45°, so as to provide a reference for future reactor core design of floating nuclear power plants. Using CFD, the relationship between the frictional resistance coefficient and the Reynolds Number (Re) is obtained in a wide range of Re, including the laminar regime, the transition regime and the turbulent regime. The predicted correlation results are good and have an estimated error within ±10%. The influence of the heating flux and the inlet temperature of the test bundle is considered, and a kinematic viscosity correction coefficient is introduced. Owing to the structural characteristics of the square rod bundle channel, this paper considers two different directions of tilt: rotations about an axis perpendicular to the channel faces and about an axis which passes through opposite corners of the channel. A difference in the frictional resistance coefficient between inclined and vertical conditions only occurs in the laminar flow regime, and there is no frictional resistance coefficient difference in the transition regime or the turbulent regime. Additionally, the frictional resistance coefficient is reduced considerably if the boundary layer is disturbed, due to the reversed fluid flow in the laminar regime. • The relationship of the frictional coefficient is obtained in a wide range of Re • Two different directions of tilt are considered. • The coefficient is reduced in the laminar regime under inclined condition. • The developed correlation predicts data with a relative error within ±10%. • The reversed flow will decrease the frictional resistance coefficient. [ABSTRACT FROM AUTHOR]