Natural Circulation Loop Research Articles

Analysis of Unsteady Behaviour of Natural Circulation Helium Loop

This paper deals with the one-dimensional unsteady fluid flow model of the natural circulation loop. The presented model represents the experimental facility Helium Loop STU which is the physical model of an emergency coolant system of a nuclear reactor. The governing equations are solved according both Euler and Lagrange approaches on two parallel computational grids. Linearization of equations and semi-implicit discretization scheme are used to enhance the algorithm effectiveness. The simulation results were compared to experimental data. The model can be considered as high-accurate in comparison with experimental data (relative error 1.14÷1.15 % at specified time interval).

Scaling and distortion analysis using a simple natural circulation loop for FHR development

Integral effects tests (IETs) using simulant fluids are expected to play an important role in code validation for Fluoride-salt-cooled High-temperature Reactors (FHRs). In this work, a scaling analysis is conducted for a natural circulation loop to simulate expected behaviors in FHRs. Based on the dimensionless governing equations and the derived non-dimensional numbers, similarity criteria are developed for the natural circulation loop. A prototypical natural circulation loop is identified and several ideally scaled models, using the simulant fluid Dowtherm A and the prototypic fluid FLiBe, are designed based on the developed scaling criteria. Steady-state and transient analyses are performed using RELAP5-3D for both the prototypical and scaled models to evaluate whether the scaling criteria is maintained in the models. The model using Dowtherm A and reduced-height, reduced-area scaling replicates the dynamic response of the prototype highly consistently, with the maximum error less than 0.1%. The Dowtherm A simulant performs better than the FLiBe in the scaled models, especially in the reduction of scaling distortion caused by the parasitic heat loss.

An experimental study of two-phase flow instability in a multi-loop natural circulation system

In this research, an experiment was carried out in a multi-loop natural circulation system to study its response behaviors under single-phase and two-phase flow instability conditions. The experimental facility consists of primary and secondary natural circulation loops, as well as an external forced circulation loop, which serves as a final heat sink to the system. The secondary loop is meant to simulate passive residual heat removal system found in the advanced reactors and some small modular reactors. The experiment was carried out under low-pressure conditions and constant heating powers. Within single-phase region, the thermal hydraulic parameters were found to vary linearly with the heating powers. By gradually increasing the power beyond the single-phase region, oscillation of the thermal hydraulic parameters was observed in both natural circulation loops. Critical conditions for the occurrence of instabilities in both natural circulation loops were determined. Flow-induced instabilities causing vibrations and acoustical noises were observed in the shell and tube heat exchanger connecting the two natural circulation loops. At higher power, fully mature oscillations were observed and characterized. In addition, effect of temperature rise in the system heat sink was also investigated. The oscillation period of the observed oscillations is about twice the boiling channel residence period. The oscillation amplitudes increase with increasing heater’s inlet temperature and heat sink temperature for the primary and secondary loops respectively. However, the influence of the heater’s inlet temperature on the flow instability is observed to be non-linear. This paper can contribute in the identification and determination of threshold values of thermal hydraulic parameters that are capable of triggering flow instabilities, which are important in the design and safe operations of natural circulation systems. It can also provide some useful information about how temperature rise in the system heat sink can influence the occurrence and amplification of flow instability in a natural circulation system.

Transient thermo-hydraulics and performance characteristics of single-phase natural circulation loop using hybrid nanofluids

Transient analysis of vertical heating and horizontal cooling rectangular single-phase natural circulation loop with various water-based hybrid nanofluids (Al2O3 + Ag, Al2O3 + Cu, Al2O3 + TiO2, Al2O3 + CNT, Al2O3 + Graphene) with 1% volumetric concentration is studied numerically. Temporal fluctuation and time required to attain the steady-state, transient mass flow rate and energy-exergy performance parameters (effectiveness and total entropy generation) using hybrid nanofluids are compared with water. The effect of power input and geometry parameter (diameter and height) of the loop on transient performances is studied as well. The result reveals that the fluctuation of mass flow rate and time required to attain the steady-state are less for hybrid nanofluids as compared to water. However, platelet and cylindrical shaped nanoparticles yield a lower stability as compared to spherical shaped. The mass flow rate is enhanced with hybrid nanofluids, except Al2O3 + CNT and Al2O3 + Graphene, as compared to water. The energy-exergy performance of hybrid nanofluids is higher than water. The maximum increment in mass flow rate is shown by Al2O3 + Ag hybrid nanofluid (3%), whereas Al2O3 + Graphene shows the highest increment in effectiveness (25.4%) and highest decrement in total entropy generation (14.3%) as compared to water. Smaller diameter and lower height of loop are found preferable for the flow stability.

Experimental study of hydrodynamics and heat transfer in two-phase natural circulation loop with reference passive cooling systems of nuclear power plants

In spite of long year practical use of natural circulation systems and not less long period of study working characteristics of these systems the problem of reliable predicting calculations of natural circulation loops (NCL) with boiling coolant at low reduced pressures is not likely can be considered as being solved. At the same time the interest to solution of this problem arises today, because low pressure natural circulation loops are considered as main type of post accident passive cooling systems for nuclear power plants. The main difficulties for calculations are concerned with determining friction pressure losses, which are strongly depended on two-phase flow pattern. At low reduced pressures large difference between liquid and vapour (gas) specific volumes predetermines the strong change of flow pattern even at small change of mass flow quality. An attempts to calculate local two-phase flow parameters (void fraction, circulation velocity) in the wide range of mass flow qualities without considering the change of flow patterns often lead to large deviations of calculated values from the results of experimental measurements. Another specific feature of two-phase low pressure NCL is hydrodynamic flow instability with circulation velocity pulsations of high amplitude and the occurrence of reverse flows. This also presents problems in developing calculation method. In present paper a method for calculating a low-pressure NCL has been developed, in which local two-phase flow parameters (void fraction, the phases velocities, pressure) are calculated using a modified homogeneous model with taking into account the distribution factor and the phases slip and a model of an annular-dispersed flow with considering droplets entrainment and deposition. A modified homogeneous model was applied for description quasihomogeneous flow regimes. At high values of void fraction annular-dispersed flow model was used. Recommendations for change from one model to another in practical calculations have been formulated and verified. The proposed calculation method has been verified by the comparison the calculated thermo-hydraulic characteristics of laboratory natural circulation loop with the experimental data. The experiments have been carried out for boiling in the loop of three different liquids – water, ethanol and perfluorohexane (FC-72) at atmospheric pressure. The comparison of calculated and experimental results showed their good agreement.

Experimental and numerical study of heat and hydraulic characteristics of low head natural circulation loops with single-phase flow of working liquid

A 2D numerical simulation of velocity and temperature fields for laminar low flow regimes have been carried out for laboratory experimental natural circulation loop with vertical electrically heated circular tube as flow up section. Calculations have been done for the case of full-length heating of flow up section at constant heat flux density on the heated wall. The variant of loop design with negligibly small hydraulic losses due to local drag reduction and friction on down comer compared with hydraulic losses due to friction in flow up tube has been considered. Laminar flow regime as the regime of most complex friction factor behaviour in buoyancy driven flows was the subject of the analysis. On the basis of calculated velocity and temperature fields in heated zone the longitudinal change of friction factor and heat transfer coefficients have been determined. In general, according to 2D numerical simulation the wall shear stresses are mainly affected by the change of wall velocity gradient due to practically continuous velocity profiles deformation along the whole heated zone. The form of velocity profiles and the extent of their deformation in its turn depend upon the wall heat flux density and the hydraulic diameter. It is shown that in single-phase natural circulation loop where fluid flow is governed exclusively by buoyancy forces wall shear stresses change along the heated zone in a complex way and friction factor for use in 1D calculations can not be described by simple correlations in the form of ξ = a/Reb. In all calculated regimes including the lowest considered wall heat flux density the Nusselt numbers exceeds that for stabilized forced flow with constant thermophysical properties. After decreasing with the distance from the inlet to the heated section Nusselt numbers achieve minimum values and then start to increase.

Analysis of the Hydraulic Regimes of Vertical Waste-Heat Boilers of Combined-Cycle Gas-Turbine Units

An analysis of the hydraulic system of the natural circulation loop of the waste-heat boilers of combined-cycle gas turbine units is performed. It is demonstrated that the standard method of hydraulic calculation of steam boilers based on an evaluation of the reliability criteria of the hydraulic system and the thermal regime of the evaporator tubes is incorrect. The existence of loop-wide hydraulic persistence of flow in the low-pressure evaporator in natural circulation caused by elevated resistance of the steam-outlet tubes is demonstrated for the first time. Recommendations are given for increasing the reliability of the hydraulic system of the low-pressure evaporator loop and selecting an optimal combined water and chemical regime to prevent erosion wear of the metal of the inner surface of the evaporator tubes.

Prediction of dryout-type CHF for rod bundle in natural circulation loop under motion condition

Abstract In nuclear engineering, the occurrence of critical heat flux (CHF) is complicated for rod bundle, and it is much more difficult to predict the CHF when it is in natural circulation under motion condition. In this paper, the dryout-type CHF is investigated for the rod bundle in a natural circulation loop under rolling motion condition based on the coupled analysis of subchannel method, a one-dimensional system analysis method and a CHF mechanism model, namely the three-fluid model for annular flow. In order to consider the rolling effect of the natural circulation loop, the subchannel model is connected to the one-dimensional system code at the inlet and outlet of the rod bundle. The subchannel analysis provides the local thermal hydraulic parameters as input for the CHF mechanism model to calculate the occurrence of CHF. The rolling motion is modeled by additional motion forces in the momentum equation. First, the calculation methods of the natural circulation and CHF are validated by a published natural circulation experiment data and a CHF empirical correlation, respectively. Then, the CHF of the rod bundle in a natural circulation loop under both the stationary and rolling motion condition is predicted and analyzed. According to the calculation results, CHF under stationary condition is smaller than that under rolling motion condition. Besides, the CHF decreases with the increase of the rolling period and angular acceleration amplitude within the range of inlet subcooling and mass flux adopted in the current research. This paper can provide useful information for the prediction of CHF in natural circulation under motion condition, which is important for the nuclear reactor design improvement and safety analysis.

극저온 자연순환회로의 가속 및 저중력 구간 유량 분석

극저온 자연순환회로의 가속 및 저중력 구간 유량 분석

Unstable characteristics of the heat transfer pipe in the drain tank

A two-layer assembly comprised of a thimble and an inserted center tube was experimentally investigated. The assembly is a natural circulation loop, whose special features include highly integration and continuous heat transfer between the center tube and the thimble. In this paper, the assembly, operated as a mini autorun heat exchanger, is used in the drain tank of molten salt reactors. The unstably operation characteristics were analyzed compared with the previous natural circulation oscillations. Three kinds of oscillations, geysering, DWO and nearing zero oscillation, were found in sequence from the cold state to saturation. A new explanation to the generation of large bubbles was proposed according to the comparison of our experimental results and other references. The analyzed results also showed that stable boiling condition prefers to stay above the line of Nsub = 1.25 × Npch. The nearing zero oscillation is a new oscillation whose feature and mechanism were discussed.

Effect of operating pressure on the performance of a hybrid system of small modular boiling water reactor with external superheaters

With the increased role of intermittent renewables in the energy mix, it is important for the Nuclear Power Plants (NPPs) to develop flexible load-follow capabilities in order to be economically competitive. By combining NPP with external superheaters, there is possibility to increase power conversion cycle efficiency and reduce load to some extent while maintaining the nuclear reactor operation at 100% of its rated power and thus maximizing its economic value. In this paper, parametric study of a Small Modular Boiling Water Reactor (SMBWR) conceptual design combined with external superheaters is presented quantifying its performance at different system pressure. The first part of the work is focusing on how operating pressure will affect the SMBWR system as a whole, which includes neutronics, thermal hydraulics, and thermodynamics. WIMS-PANTHER package and COBRA-EN were used as the neutronic and thermal hydraulic tools. In addition, MATLAB models of the SMBWR natural circulation loop and balance of plant were developed for the study. The second part of the work examines the maneuvering capability of SMBWR. It is found that increasing the pressure from 6.5 to 10.0 MPa has no significant neutronic effects, while thermal efficiency is slightly improved. It is also found that SMBWR is able to reduce the load to 65% while maintaining the reactor power at 100% of its rated value.

Mitigation of Temperature Gradients in the Buffer Tank of a Natural Circulation Loop Simulating Passive Decay Heat Removal in a Pool Type Reactor

This study considers the decay heat removal through passive systems (i.e., natural convection) in the pool-type experimental reactor MYRRHA (SCK•CEN). The low velocities associated with natural convection may cause a degradation of the thermal mixing in the upper plenum of the reactor: buoyancy dominated phenomena, such as stratification, buoyant jets or convective cells may occur. The large temperature gradients associated with these phenomena may induce thermal loads on the structure, compromising its integrity. The objectives of this work are, in order, (1) achieving an understanding of the fluid mechanics leading to the formation of thermal gradients in the upper plenum and (2) mitigating the temperature gradients the upper plenum’s geometry. The methodology chosen to address the problem is experimental. A ''two-dimensional'' water facility modeling a slab of the reactor's primary loop was designed. Simultaneous measurements of velocity and temperature were performed in the model's buffer tank using thermocouples and innovative optical imaging techniques. It was observed that vertical thermal gradients develop in the tank because of the combined effect of stratification and advection. Important reductions of local gradients were observed when changing porosity and position of upper plenum’s buffer plates.

Analysis of natural circulation loop in MTRs using CONVEC code

Natural circulation is an important process in core cooling and decay heat removal of Material Testing Reactors (MTRs). Analysis of an open pool natural circulation loop (NCL) was performed using the CONVEC V3.4 code developed for steady state analysis of MTRs during natural convection. Verification and Validation of code calculations for similar natural circulation loops were provided. The code calculations were carried out to investigate the geometric parameters that enhance the natural loop cooling capacity or increase the margin to Onset of Nucleate Boiling (ONB) as a limiting condition. The results describe the operating envelop in terms of core power without ONB and pool temperature for various dimensions of loop components including the fuel element and pool depth. This paper is useful for the researchers working on the research reactor design.

Experimental and numerical study to optimize a design of passive moderator cooling system of an advanced nuclear reactor

The passive moderator cooling system (PMCS) for the Advanced Heavy Water Reactor is one of the passive systems incorporated to enhance safety of the reactor. The PMCS comprises of a coupled natural circulation loop, with a Gravity-driven-water pool as the ultimate sink and a shell and tube heat exchanger as the coupling component. Previously (Pal et al., 2016), a scaled experimental facility for PMCS was setup. In this work, validity of scaling theory was verified using 1D thermal-hydraulic code (RELAP5/MOD3.2) and simulations were compared with experimental data. Finally, PMCS was simulated for various geometrical parameters, e.g. elevation difference and pipe diameters, in an attempt to optimize the design. A good agreement with experimental data (within ± 8%), except for initial 2 h, was observed in RELAP/MOD3.2 code simulation to scaled experimental facility because of internal recirculations within Calandria being more prominent than overall circulatory flows in the passive loops.

A CFD-based simulation tool for the stability analysis of natural circulation systems

This paper presents a CFD-based simulation tool aimed at analysing the dynamics of natural circulation by means of the techniques provided by the linear stability theory. The tool can be adopted for a comprehensive stability analysis of free convection systems, giving information about the asymptotic behaviour, the spatial locations where perturbations have the largest impact on stability (i.e., a sensitivity analysis), and the so-called “transient energy growth”, as well.The CFD-based approach overcomes the simplified hypotheses and the constitutive relations usually adopted to study the dynamic behaviour of natural circulation systems (i.e., one-dimensional approximation, correlations for pressure drops and heat transfer, …), and allows extending the analysis to complex geometries. The developed tool, that is based on a MATLAB®-OpenFOAM® coupling, has been assessed in a classic buoyancy-driven benchmark (i.e., the “differentially heated 2D cavity” test case), comparing the results with a classic Arnoldi-iteration approach and literature results. Subsequently, the study of the linear stability of a natural circulation loop facility has been performed, considering for the first time also the 3D representation of the system. The results have turned out to be satisfactory, being able to give proper information about the stability of natural circulation systems.

Dynamic frequency response of a single-phase natural circulation loop under an imposed sinusoidal excitation

Natural circulation loops in industrial applications are always likely to experience both aperiodic and periodic variations in control parameters. Present work performs a comprehensive analyses to investigate the effect of periodic power supply on the nature of transient system response. Mathematical model of a rectangular single-phase loop is developed and a sinusoidal profile is imposed on the steady-state heater power. Cooling of the system is achieved in convective mode, whereas other sections are ideally insulated. Nature of temporal response is found to depend on the frequency of imposed sinusoid, showing both repetitive patterns and occasional irregular fluctuations. A natural frequency can be identified corresponding to each power level by performing Fast Fourier transform on the resultant transient dataset, which is found to be independent of the frequency of imposed profile. Magnitude of natural frequency increases continually with mean heater power within stable zone and corresponding critical time period exhibits a decreasing power-law profile. Both generally-stable and unstable system exhibits highly unstable oscillations and early flow reversal, when subjected to a profile with frequency analogous to the natural frequency of mean power level. Increase in amplitude of imposed sinusoid hardly affects the nature of the response, other than accelerating flow reversal and introducing phase lag in response. An active feedback control mechanism is also proposed by modulating additional pressure losses, which is able to suppress resonance behavior at both stable and unstable regime.

Experimental and Calculated Investigation of a Natural Circulation Loop’s Thermal-Hydraulic Characteristics

The results from experimental investigation into hydrodynamics and heat transfer in a two-phase natural circulation loop (NCL) under atmospheric pressure are presented. The experiments were carried out for liquids having essentially different properties: water, ethanol, and perfluorohexane C6F14 (the product trademark is FC-72). The circulation velocity in the NCL is not known in advance but is a complex function of the specified parameters (heat flux and liquid temperature at the heated section inlet) and of the two-phase flow internal characteristics. The liquid temperatures at the heated section inlet, the wall temperature over the section height, and also the circulation velocity were measured in the experiments at a specified heat flux; in addition, the two-phase flow at the loop riser leg outlet was filmed on video. The experiments and analysis have shown that flow hydrodynamic instability (circulation velocity pulsations) is really unavoidable in a two-phase NCL. Hydrodynamic instability with a high circulation velocity amplitude and with the occurrence of backward flows is typical for regimes involving significant liquid subcooling values at the heated section inlet and for NCLs containing an extended part with single-phase convection. This instability, which is characteristic for experiments with water, is due to the displacement of the boiling incipience section over the section height; the instability also persists at small subcooling values but with a low pulsation amplitude. Under the developed saturated liquid nucleate boiling conditions (at high heat flux values), the circulation velocity and wall temperature pulsations have small amplitudes, and the flow can be regarded as stable. In the experiments with perfluorohexane, the smallest wall temperature and circulation velocity pulsations were pointed out, which is attributed to a relatively high value of reduced pressure. In the experiments with ethanol, instability occurs in the developed nucleate boiling region (q > 35 kW/m2); this instability is caused by periodically alternating two-phase flow structure (regime). A procedure for calculating a low-pressure NCL is developed, in which the two-phase flow’s local parameters (void fraction, phase velocities, and pressure) are calculated according to a modified homogeneous model (taking into account the phase distribution factor and phases slip) and a dispersed-annular flow model taking into account the droplet entrainment and deposition phenomena. A comparison of the NCL calculation results with the experimental data obtained for three different liquids has shown that they are in good agreement with each other.

Study on the Movement and Deposition of Particles in Supercritical Water Natural Circulation Based on Grey Correlation Theory

The movement and deposition of particles that occur during their natural circulation in supercritical water exercise an important impact on the safe and stable operation of a supercritical water reactor (SCWR). When supercritical water flows in pipelines, a large number of corrosive particles may be generated due to pipeline corrosion or the purity of the fluid itself. The presence of particulate matter affects the heat transfer efficiency of the pipeline, increasing flow resistance and easily promoting heat transfer deterioration. ANSYS-CFX numerical analysis software was used to simulate the natural circulation loop of supercritical water, and micron particles were added in the initial flow field. The effects of heating power, particle concentration and particle diameter on particle deposition were obtained. Through this analysis, it can be concluded that the heating of the pipeline has a certain inhibitory effect on the deposition of particles. The rise in both initial particle concentration and particle diameter serve to reinforce the deposition of particles in the heating section. Depending on the degree of influence, the contributory parameters to particle deposition include particle diameter, particle concentration and heating power in turn.

Steady-State Energetic and Exergetic Performances of Single-Phase Natural Circulation Loop With Hybrid Nanofluids

Energy and exergy performances of natural circulation loop (NCL) with various water-based hybrid nanofluids (Al2O3 + TiO2, Al2O3 + CNT, Al2O3 + Ag, Al2O3 + Cu, Al2O3 + CuO, Al2O3 + graphene) with 1% volumetric concentration are compared in this study. New thermophysical property models have been proposed for hybrid nanofluids with different particle shapes and mixture ratio. Effects of power input, loop diameter, loop height, loop inclination and heater/cooler inclination on steady-state mass flow rate, effectiveness, and entropy generation are discussed as well. Results show that both the steady-state mass flow rate and energy–exergy performance are enhanced by using the hybrid nanofluids, except Al2O3 + graphene, which shows the performance decrement within the studied power range. Al2O3 + Ag hybrid nanofluid shows highest enhancement in mass flow rate of 4.8% compared to water. The shape of nanoparticle has shown a significant effect on steady-state performance; hybrid nanofluid having cylindrical and platelet shape nanoparticles yields lower mass flow rate than that of spherical shape. Mass flow rate increases with the increasing loop diameter and height, whereas decreases with the increasing loop and heater/cooler inclinations. Both effectiveness and entropy generation increase with the decreasing loop diameter and height, whereas increasing the loop and heater/cooler inclinations. This study reveals that the particle shape has a significant effect on the performance of hybrid nanofluids in NCL, and the use of hybrid nanofluid is more effective for higher power.

Impact of dissolved salts on two-phase flow and boiling heat transfer in a natural circulation loop

Two-phase natural circulation and boiling experiments are conducted in this study with artificial seawater as a heat transfer fluid or process coolant. This experimental work is focused on understanding the role of dissolved salts in water on the steady state two-phase natural circulation flow rates. Mass flow rates were measured at different heat flux levels with artificial seawater, deionized water and tap water in the experimental loop. It was found that the average quasi-steady state mass flow rates were the same at any particular heat flux level regardless of the fluid being used. Artificial seawater coolant exhibited more stable natural circulation at higher heat flux as compared to the other two coolants. Mass flow measurements recorded at 2 s intervals show that magnitude of flow oscillations are much higher in case of tap water and deionized water. Also, high speed camera images revealed that the bubble departure diameters are smaller in seawater compared to tap or deionized water. This can be attributed to the increased local wettability due to the presence of dissolved salts in the fluid. Visual observations of the riser section showed that bubble coalescence and formation of slugs is inhibited in the seawater. These observations also explain the reduced pressure drop across test section when seawater was used as a coolant.