Stability Of Natural Circulation Research Articles

Experimental investigation of thermal characteristics on a new loop pipe model for passive cooling system

The heat pipes have the potential to be used as a passive cooling system (PCS) in thermal installations including nuclear facilities. In that context, this experimental study is correlated with the research on the new type of Loop Heat Pipe (LHP) as the PCS. This work aimed to study the characteristics of LHP with capillary pipe wick. The experiments involved varying the pool water temperature as evaporator heat load at 35, 45, 55, and 65 °C, filling ratio charged at 20, 40, 60, 80, and 100 % of LHP evaporator volume, and the flow air velocity to the fins in the condenser at 0, 1, 1.5, 2, and 2.5 m/s. The LHP was operated in sub-atmospheric conditions with an initial pressure of 2666.4 Pa. The results show that LHP has a stable two-phase natural circulation under all variations of heat loads, filling ratios, and air velocities tested. The results also show that display excellent thermal performance with the lowest thermal resistance of 0.0524 ± 0.0004 °C/W operating at the highest water temperature source, the highest filling ratio, and the highest air velocity. It is concluded that the LHP model with capillary pipe wick has similar thermal characteristics to other types of heat pipes. This wick function effectively in preventing vapor flow from the evaporator toward the condenser, and otherwise it can facilitate as condensate path from the condenser to evaporator. The stable two-phase natural circulation stability analysis showed that the LHP with capillary wick has very good thermal performance as heat absorber and heat releaser.

Theoretical Analysis of Type-I Flow Instability of Natural Circulation

Fukuda and Kobori proposed three types of Leginegg instabilities and five types of density wave instabilities, according to different mechanisms of density wave instabilities. The instability dominated by gravity pressure drop is classified as Type-I flow instability. The instability dominated by friction pressure drop is classified as Type-II flow instability. And the instability dominated by accelerated pressure drop is classified as Type-III flow instability.In present work, the Type-I flow instability between rectangular parallel channels of natural circulation was studied theoretically. Models including two-phase flow instability, natural circulation system components were established in combination based on the homogenous model. A computational program was written. The program was validated with experiments, and the results matched well with the experiment data. The marginal stability boundary (MSB) maps under different parameters were obtained by using nondimensional numbers Nsub and Npch. The influence of different kinds of pressure drop, inlet subcooling temperature of heating channels of type-I flow instability were analyzed separately. And the phenomena were explained from mechanism and the change of void fraction. The results show that the induce of inlet subcooling temperature enhances the stability of system. With the increase in system pressure, the system stability of natural circulation is enhanced.

Stability analysis of a parallel channel-loop system with single-phase natural circulation under asymmetric conditions

Flow instability can cause mechanical oscillations in the equipment as well as periodic changes in the thermal stress, both of which have a great impact on the safety of the reactor. In addition, flow instability can interfere with the control system, making it difficult to capture stable parameters. In this study, to examine the change in the stability of a parallel channel-loop system with natural circulation under asymmetric conditions, the single-loop natural circulation system is taken as the starting point, and the control equations are obtained by using a dimensionless analysis method. The Fourier expansion of displacement term in the control equations is carried out to obtain the Jacobian matrix representing the single-loop natural circulation system. Based on this matrix, the Jacobian matrix model of parallel channel-loop system is constructed. According to the model, the natural circulation stability of parallel channel-loop under different load and resistance difference is analyzed, and the effects of geometric characteristics, coolant and power on the stability boundary under asymmetric condition are examined. The results show that there are two critical Reynolds numbers, and the system becomes unstable when the load difference introduced by the left and right loops is greater than the critical Reynolds numbers. The area of stable region can be increased by increasing the aspect ratio and heating zone length, and reducing the pipe diameter and cooling zone length. Furthermore, the stability boundary of natural circulation is sensitive to the aspect ratio and the length of heating zone. In addition, the stability can be improved within the allowable range of the natural circulation by choosing liquid metal coolant and increasing the pressure drop, and this model cannot accurately describe the changes in the stability boundary at low power. Overall, the above results can serve as useful reference for the design of the parallel channel-loop systems and can be used to improve the stability of natural circulation. The stability boundary can be used to preliminarily judge the stability distribution and the variation trend of the parallel channel-loop system with a certain degree of reliability.

Uncertainty analysis on natural circulation characteristics under ocean conditions

The stability of natural circulation is more susceptible to the uncertainties involved in the thermal-hydraulic process because the driving force is relatively small. Especially in the marine nuclear power plants, the complex ocean motions will introduce more uncertainties in the natural circulation operation. In this work, the joint probability distribution function of wave parameters was investigated, which provided a more realistic simulation of the ocean conditions. On that basis, it was applied to generate ocean motion parameters as uncertain input via the Metropolis-Hastings algorithm. A thermal-hydraulic code that is capable of simulating system behaviors under ocean conditions was developed to propagate the uncertainties. Additionally, the sensitivity analysis was carried out to identify the parameters that have a significant impact on natural circulation characteristics. Results show that the intense ocean motions will introduce large uncertainties in the natural circulation operation, especially at the startup stage. The rolling amplitude contributes the most on the natural circulation flow rate and the coolant temperature, while the heaving motions have less influence. Uncertainties introduced by the ocean motions will damage the natural circulation ability and stability, which need further considerations during the design optimization.

Theoretical investigation of two-phase flow instability between parallel channels of natural circulation in rolling motion

In present work, the two-phase flow instability between rectangular parallel channels of natural circulation under static and rolling conditions was coupled studied theoretically. Models including two-phase flow instability, natural circulation system components, and the additional force were established in combination based on the homogenous model. A computational program was written in FORTRAN language which was solved by Gear multi-value method by using control volume integrating method. The program was validated with experiments, and the results matched well with the experiment data. The marginal stability boundary (MSB) maps under different parameters were obtained by using nondimensional numbers Nsub and Npch. The influence of different kinds of pressure drop, inlet subcooling temperature of heating channels, system pressure, valve resistance, venturi flowmeters resistance, structure height, rolling condition, and the interaction effect of natural circulation and two-phase flow instability between rectangular parallel channels were analyzed. The results show that with the increase in system pressure and venturi flowmeters resistance, the system stability of natural circulation is enhanced. The influence of inlet subcooling temperature is nonlinear. The increase in valve resistance leads to the instability of system. The increase in structure height does not change system stability significantly. The rise in rolling angle and period both reduce the system stability.

The Development of Models to Predict the Stability of Natural Circulation of Heatoolant

Concept of safety of nuclear power plants involves in larger quantities the use of passive systems. One of the main passive systems in nuclear power plant – the system of cooling of the reactor core. This system is based on gravitational forces. In this regard, nuclear energy increases the significance of such physical process, as the natural circulation. In addition to the benefits of the system there are drawbacks. There is the instability of the two-phase coolant, pulsation temperature and pressure, rollover and stagnation of circulation.

Preliminary applications of CFD methodology on analysis of the single-phase laminar natural circulation under swing conditions

In nuclear power systems, the natural circulation loop will startup to remove the residual heat after losing the power for the forced circulation. The natural circulation will startup if the driving force caused by the density difference is large enough to overcome the resistance of the loop, however, the system will encounter additional forces, including the transport force and the Coriolis force, which may change the driving force and break the stability of natural circulation. In this work, the CFD model for a natural circulation loop was developed to study the dynamic startup performances and the pseudo steady-state operations under swing conditions. Using CFD methodology can avoid employing the correlations for wall friction and heat transfer using in system code, which may be not applicable for swing systems. Based on the CFD analysis, parametric studies were carried out, including swing period, maximum swing angle, heat power, swing center and the swing phase difference. The swing angle, swing period, swing phase difference and heated power can significantly affect the thermohydraulics and stability of the natural circulation systems, while the swing radius has little influence. This work proved that CFD methodology may be qualified for analyzing the natural circulation system under swing conditions.

Stability of Natural Circulation in a Vertical Boiler-Utilizer Loop with Horizontal Evaporator Pipes During Startup

Theoretical studies and field tests of the stability of natural circulation in the pipes of the horizontal evaporator of the PK-55 boiler-utilizer at the South-west Heat and Electric Power Plant (Yugo-zapadnaya TETs) are discussed. The experiments cover the stages of evaporator heating, onset of steam production, and stable natural circulation. During these studies the gas turbine was started and then held at a load of 5 MW. It is concluded that the evaporator with horizontal boiler-utilizer piping built for operation with repeated forced circulation can operate reliably for long periods in a natural circulation regime.

Experimental and numerical investigation of natural circulation stability of the SHUNCL thermal-hydraulic loop

Nowadays, the application of natural circulation due to safety margins that provide for cooling the nuclear reactor cores has drawn the attention of researchers and designers in this context. Employing the benefits of natural circulation demands a thorough study of various phenomena such as thermo-fluid dynamics of steam-liquid interaction with the associated challenges that the most significant one is instabilities due to the severe dependency of mass flow rate on the applied heat flux. For this purpose, SHUNCL1 thermal hydraulic loop has been designed and constructed under atmospheric conditions and an experimental scenario has been arranged to investigate the mass flow rate variation due to heat flux. It is observed that the system behavior particularly the mass flow rate shows a considerable sensitivity to the heat flux variation such that in specific limit of heat flux implementation, the system behavior undergoes severe fluctuations and never attains to a stable condition. In addition to the experimental study, a numerical simulation of the SHUNCL loop has been prepared using the RELAP5 code in order to more investigate the effect of thermal hydraulic parameters on the instability of the operating system. System pressure, inlet subcooling, chimney length, orificing diameter and the heat section length are analyzed. Also, stability boundary is obtained based on two dimensionless numbers, phase change (Npch) and subcooling (Nsub) and it was observed that the occurred instability in the SHUNCL thermal hydraulic loop is of the type-1 dynamic instability.

An approach to stability analysis and entropy generation minimization in the single-phase natural circulation loops

Single-phase natural circulation loops are frequently used in the systems which are related to the production of renewable energies. Natural circulation systems have low driving head and consequently, low heat removal capability. Thus, minimizing the entropy generation is a critical matter for designing an optimized natural circulation system performance. Moreover, the instability behavior of these systems is one of their most important disadvantages. In the present study, the entropy generation and non-linear stability analysis for a uniform and non-uniform diameter loop under single-phase natural circulation are investigated. In this work, a constant heat flux condition in the heater and constant wall temperature condition in the cooler are assumed. The effect of various parameters such as loop dimensions and heater power on the single-phase natural circulation stability and entropy generation in the loop are analyzed. Finally, the stability of the system in the form of a stability map is discussed, and entropy generation within the loop is calculated.

Thermohydraulic processes in a heat pipe with a central circulation insert at low pressure

Results are given of an experimental investigation of thermohydraulic processes occurring in a compact natural-circulation loop (heat pipe with a central circulation insert) at a low pressure of steam-water mixture, including low vacuum ptop = 22–176 kPa. Consideration is given to axial heat transfer in nonboiling water under conditions of bubbling mode of loop operation; to superheating of water, which rises up the adiabatic region, relative to the saturation temperature; and to true void fraction and hydrodynamic stability of natural circulation.

Experimental study on natural circulation and its stability in a heavy liquid metal loop

Motivated by the increasing interest in heavy liquid metal (HLM) cooled fast reactors and accelerator driven system (ADS), the TALL test facility was designed and constructed at KTH to investigate the thermal-hydraulic characteristics of HLM. In this paper, the HLM natural circulation characteristics in a HLM loop were investigated with experiments in the TALL test facility. The study includes measurements on (1) start-up of natural circulation from different initial conditions; (2) stability of natural circulation; (3) effects of influencing parameters and (4) capability of natural circulation. The experimental data are compared to predictions with a relevant code (RELAP5). Significant natural convection flow was observed in the experiments. It was found that the natural circulation was easily established and stabilized. It took only a few minutes to have a stable natural circulation prevailing from cold conditions. The natural circulation flowrate depends on the loop resistance, and the temperature difference between the hot leg and the cold leg, as determined by the power level and the heat sink capacity. The experiments show that the maximum flowrate for the natural circulation is ∼0.5 kg/s (corresponding to ∼0.5 m/s in the heat exchanger), resulting in heat removal of ∼15 kW from the core tank, which is comparable to the capacity of ∼100 W/cm of the electric heater elements. The preliminary analysis performed with the RELAP5 code is in reasonable agreement with the experimental data.

Study on the stability behaviour of two-phase natural circulation systems using a four-equation drift flux model

Theoretical investigations were carried out to study the influence of two-phase flow parameters such as friction factor multiplier, drift velocity and void distribution parameter on the stability of boiling two-phase natural circulation systems. The theoretical model considers a four-equation drift flux model which solves the linearised conservation equations of mass, momentum and energy applicable to boiling two-phase natural circulation systems. The model was applied to three boiling natural circulation loops wherein Type I and Type II instabilities were observed over a wide range of operating pressures. The two-phase friction loss was predicted using different friction factor multiplier models available in literature. It was found that these models influence the steady state and threshold powers for stability, especially the Type II instabilities in natural circulation significantly. Since the void fraction depends on the drift velocity and the void distribution parameter in two-phase flow, these parameters were varied and their effects on the natural circulation flow stability were investigated. It was found that an increase in either the drift velocity or the void distribution parameter reduces the unstable regions observed in the Type I or Type II flow instabilities in two-phase natural circulation systems. Further, investigations were carried out to study the effect of loop diameter on the Type I and Type II instabilities in natural circulation. This study is important to reveal the capability of the reduced diameter scaled facilities of the prototype systems to simulate natural circulation instabilities. The results indicate that with increase in the loop diameter, the threshold power of the Type I instability and the Type II instability increases. Moreover, the stability of natural circulation greatly enhances with increase in the diameter of the loop.

05/01336 The effect of wall friction in single-phase natural circulation stability at the transition between laminar and turbulent flow

05/01336 The effect of wall friction in single-phase natural circulation stability at the transition between laminar and turbulent flow

The effect of wall friction in single-phase natural circulation stability at the transition between laminar and turbulent flow

The present paper is focused on the prediction of stability of single-phase natural circulation in the range of Reynolds numbers characterizing the transition between laminar and turbulent flow. In particular, the predictions obtained by one-dimensional models making use of different assumptions for evaluating wall friction at this transition are discussed, also in front of experimental information from previous investigations. The starting point of the analysis is the discrepancy observed in the prediction of the linear stability behaviour of an unstable experimental loop obtained by thermal-hydraulic system codes adopting different friction laws. An in-depth investigation of the reasons for such discrepancy is made with the aid of computer programs developed for the one-dimensional linear and non-linear stability analysis of single-phase natural circulation loops. The programs allowed obtaining linear stability maps for the considered loop, which clearly show the effects of the assumptions made in dealing with friction at the transition between laminar and turbulent flow. The available information on the appropriate closure laws for friction in natural circulation, with particular emphasis on the transitional regime, is also discussed. Non-linear effects, coming into play when transient calculations are started far enough from the system fixed point, are shown to have a relevant role in the predicted stability behaviour. Finally, preliminary results obtained by the application of a computational fluid-dynamic code in the analysis of stability in the addressed loop are presented to point out an interesting field of future investigation.

Stabilising boiling water reactors by optimising the position of the feedwater sparger

The influence of changing the position of the feedwater sparger on the thermal-hydraulic stability of natural-circulation boiling water reactors (BWRs) is investigated. Frequency-domain, linear stability analysis is applied using a reduced-order dynamic model. It is found that the phase with which the temperature oscillations, created at the feedwater sparger, reach the core inlet has a significant influence on the stability. This phase is influenced by the position of the sparger. It is shown that moving the sparger to the core inlet has a stabilising influence. Similar result is found using the thermal-hydraulic code MONA.

The effect of truncation error on the numerical prediction of linear stability boundaries in a natural circulation single-phase loop

This paper discusses the effects of truncation error on the stability of natural circulation in a single-phase loop as predicted by finite difference numerical methods. A short description of the characteristics of the selected problem is firstly given, also showing results obtained by a modal solution. Different numerical schemes are applied and the related effect on predicted stability is shown. The results obtained are then discussed in relation to the applicability of thermal–hydraulic system codes in the analysis of fluid-dynamic instabilities in real systems.

Stability of Natural Circulation Boiling Water Reactors: Part II—Parametric Study of Coupled Neutronic-Thermohydraulic Stability

A parametric study of coupled neutronic-thermohydraulic stability of natural circulation boiling water reactors (BWRs) is performed. As an example, the stability characteristics of the Dutch Dodewaard reactor, which was cooled by natural circulation, are determined. The Dodewaard reactor can be considered as the prototype of next generation natural circulation BWRs. The stability issues that are identified for this prototype reactor are therefore important in the design of new natural circulation BWRs.Without a riser section installed, only one region of thermohydraulic instability exists in the stability plane. The significant gravitational pressure drop in a riser section, installed to enhance the natural circulation flow, gives rise to the emergence of an additional region of instability. The oscillations in this zone become especially important during low-power/low-pressure (reactor startup) conditions. Significant damping of these oscillations occurs in a reactor, due to the nuclear void reactivity feedback.A comparison between natural circulation in-phase and out-of-phase reactor stability is made, in particular important for large reactor cores but also yielding unexpected results for small reactors. The impact of downcomer inertia on the stability of the in-phase mode is investigated in detail. Typical trajectories in the dimensionless stability plane are calculated as a function of changing operating conditions, to investigate their influence on reactor dynamics.

The limits of conditional stability for single-phase natural circulation with throughflow in a figure-of-eight loop

This paper is based on the investigations on the stability of natural circulation with throughflow in a figure-of-eight loop, a configuration that is employed in the pressure tube type heavy water reactors. The investigations have been both experimental as well as theoretical. The latter include both the linear stability analysis as well as the nonlinear finite difference analysis. The finite difference analysis has been used not only to identify the limits of the stable and unstable regimes but also used to identify the limits of the conditionally stable regime. Good agreement is found between theory and experiment.

Study on natural circulation flow in a closed loop. 2nd report. Stability analysis of flow in a loop with an inverse U-shaped cooler.

An analysis of the stability of natural circulation in a closed loop with an inverse U-shaped cooler was performed. The loop consists of an inverse U-shaped tube cooler heating boxes and connecting bent pipes. The calculate flow at a steady state was obtained by one-dimensional analysis using a simplified loop model of the test loop. Stability of the natural circulation flow was studied by linear stability : the flow was investigated by linearized equations to determine whether perturbation of the temperature and the flow rate added in the steady state flow would grow or diappear. Nyquist criterion was used to predict the stability limit. The curves for neutral stability were obtained as a function of the two parameters, non-dimensional cooler heat flux and non-dimensional flow rate, for the various elvation differences between the cooler and the heater. The effect of experimental parameters on the flow stability was studied. The comparison between analysis and experiment showed favorable agreement.