Thermo-hydraulic Instabilities Research Articles

Effect of inlet throttling on thermohydraulic instability in a large scale water-based RCCS: A system-level analysis with RELAP5-3D

This paper presents results from system-level modeling of a water-based reactor cavity cooling system using RELAP5-3D. The computational model is benchmarked with experimental data from a half-scale RCCS test facility at Argonne National Laboratory. The model prediction is first compared with a two-phase oscillatory baseline experimental case where mixed accuracy is obtained. The model shows reasonable prediction of mass flow rate, pressure, and temperature but significant overprediction of void fraction. The model prediction is then compared with a fault case where the inlet of the risers is gradually reduced using a throttling valve. As the valve is closed, the model is able to predict some major flow phenomena observed in the experiment such as the dampening of oscillations, the reintroduction of oscillations, as well as boiling, flashing, and geysering in the risers. However, the timeline of these events are not well captured by the model. The model is also used to investigate the evolution of flow regime in the chimney. This work highlights that the semi-empirical constitutive relations used in RELAP-3D could have a strong influence on the accuracy of the model in two-phase oscillatory flows.

Effect of inlet throttling on thermohydraulic instability in a large scale water-based RCCS: An experimental study

The objective of the present experimental study is to investigate the effect of inlet throttling on the thermohydraulic stability of a large scale water-based Reactor Cavity Cooling System (RCCS). The test was performed using the water-based Natural convection Shutdown heat removal Test Facility (NSTF) at Argonne, which represented a ½ axial scale and 12.5° sector slice of the full scale Framatome 625 MWt SC-HTGR RCCS concept. A two-phase steady state was first established through direct condensate refill, followed by increased inlet throttling over 10 stages, corresponding to a loss coefficient K over the range of 0.05–653. With the inlet throttling gradually increased, the system experienced a unique transition process between stabilization and destabilization. Through a stability analysis, three instability mechanisms were identified in the present test, including a compound mechanism due to both natural circulation oscillations (NCOs) and density wave oscillations (DWOs), Type-II DWOs, and geysering.

Flow boiling in geometrically modified microchannels

Microchannels are a promising solution for high-heat-flux thermal management scenarios, including high-performance microelectronics cooling and power electronics cooling. However, thermohydraulic instabilities result from the rapid vapor bubble formation. The prior literature has examined several methods, including constricted inlet microchannels, expanding microchannels, and auxiliary jetting microchannels, to mitigate the effect of these instabilities. Computational fluid dynamics and heat transfer (CFD/HT) modeling of the flow boiling phenomena in these microchannel configurations has seen limited examination, and one-to-one numerical comparisons of the different mitigation strategies have not been performed. In the present investigation, CFD/HT analyses using a three-dimensional (3D) volume of fluid model coupled with a phase-change model for the interfacial heat and mass transfer were performed for multiple microchannel configurations (constricted inlet, expanding, and auxiliary jetting microchannels). A benchmark case of a rectangular microchannel was examined to quantify baseline thermohydraulic performance. Results demonstrated slight to significant thermal performance improvements for all cases, and significant pressure benefits for the expanding and jetting cases, consistent with experimental results in the literature. Bubble dynamics and visualization for the baseline and alternative configurations are provided to give insight into their underlying physics, and the differences in performance were investigated and compared with available literature.

Resolving mode mixing in boiling water reactors instability analysis using variational mode decomposition

SummaryIn dynamic, complex and nonlinear systems, the analysis of transients, instabilities or unusual events is fundamental, as well as the knowledge of the system's behavior in the frequency space. What is usual in this type of study is to have discrete and digitally acquired data, due to a sampling process with a determined period or frequency. Extracting information from this data set is done by numerical methods, such as the Fourier Transform (FFT), the Hilbert‐Huang Transform (HHT) or techniques from chaos mathematics (Attractors). In this way we have scarce data, with some loss of information, analyzed with numerical techniques that introduce, also, some dilution of the information. In particular, the techniques of analysis in the frequency space, by their very nature, blur, frequencies and bandwidths. This leads in some cases to the identification of unsuitable or very wide frequencies or intervals, which make the extracted information unusable. To this effect, we have studied the VMD technique that provides more real and representative frequencies of the system as well as narrower bandwidths that make possible better identification of phenomena and influences. This technique has been applied to the analysis of thermohydraulic instabilities in nuclear boiling water reactors (BWR). The cases had already been analyzed with STFT and HHT techniques. The results of the application of the VMD shows that the mode mixing is solved, so modes obtained are monofrequency and these frequency bands are narrower than in previous methodologies. This improved performance allows using backbone plots for identifying which modes are excited in instabilities events.

Experimental studies of non-stationary thermo-hydraulic processes at freon R113 boiling

An experimental unit has been created to study the processes of boiling and condensation in a closed loop. Experimental studies of non-stationary thermo-hydraulic processes at freon R 113 boiling have been carried out inside tubes of low pressure natural circulation systems. Key factors have been identified affecting auto-oscillations characteristics: filling level of the loop, pressure, heat flux density, geometric characteristics of the evaporating elements. Four characteristic modes of operation of such units have been found out. Thermo-hydraulic auto-oscillations at high thermal loads have been studied. Process parameters and conditions under which there is no thermo-hydraulic instability have been determined. The phenomenon of thermo-hydraulic resonance has been detected. The influence of the heating method on the instability characteristics has been revealed.

ЭКСПЕРИМЕНТАЛЬНОЕ ИССЛЕДОВАНИЕ ТЕПЛОГИДРАВЛИКИ ВИТОГО ПАРОГЕНЕРИРУЮЩЕГО КАНАЛА, ОБОГРЕВАЕМОГО СВИНЦОВЫМ ТЕПЛОНОСИТЕЛЕМ ПРИ ПРОДОЛЬНОМ И ПОПЕРЕЧНОМ ТЕЧЕНИИ

The study of heat transfer in spiral coiled tubes is of great interest in view of the widespread use of such channels in engineering practice, in particular, in nuclear power engineering in the form of steam generators at research reactors and nuclear power plants. In the projected BREST-OD-300 reactor facility (RF), a configuration of helical coiled tubes is considered as a steam generator. Thermal hydraulic tests of the model steam generator RF BREST-OD-300 (version 2000) with helical coiled tubes with longitudinal coolant flow were carried out in SSC RF - IPPE at the SPRUT facility in 2011-2013 years. The test program was aimed to study heat transfer and thermal hydraulic stability of the steam generating tubes. Throughout the range of variation the regime parameters, regimes with a reversal circulation in the water loop have not been detected. Despite the fact that the results of conducted tests on the steam generator model gave extensive information on heat transfer in different zones of the steam generating tube, however, the insufficient number of heat transfer tubes in the module (only three) does not allow to conclude that the full hydrodynamic stability of BREST RF steam generator in all possible modes of operational parameters. On the other hand, in a real construction the motion of the heating coolant is omitted with flow around the bundle of tubes close to the transverse flow. Therefore, insufficient reasoning for the transferring results obtained on a three-tube model to a full-scale steam generator served as the basis for testing of a multi-tube full-height fragment model of a reduced diameter one row of the tube bundle actual steam generator. During the tests, there were no noises typical of the unstable operation of water loop. There were no oscillations of water and steam temperature, respectively, at the inlet to the collectors and out of the collectors. At high lead temperatures, the temperature of the superheated steam was always close to the inlet temperature of the lead. The tests showed the absence of the thermohydraulic instability, as in the case of longitudinal and transverse coolant flows in the investigated modes of lead and water parameters. Other parameters being equal, the steam temperature at the outlet of the steam generating tube in case of transverse flow was higher than in the case of longitudinal flow. The experimental data obtained during the testing are primarily necessary for the verification of codes that allow the correct calculation of the various operating modes of the BREST-OD-300 steam generator.

Thermal hydraulic studies on cold start-up in a multichannel natural circulation system

Abstract Two-phase natural circulation has been adopted as the heat removal mechanism in the primary loop of Advanced Heavy Water Reactor (AHWR). For safe and reliable operation, it is required that the system should be operated in stable regions where no thermo-hydraulic oscillations are present. For the experimental study of thermo-hydraulic instabilities, a facility called Parallel Channel Natural Circulation Test Facility (PCNCTF) was erected and commissioned at the Indian Institute of Technology Bombay (IITB). The loop in this facility is geometrically similar to the primary loop of AHWR. The present paper focuses on the thermal hydraulics of cold start-up process in a Natural Circulation Systems (NCS) in general and AHWR in particular. Experiments were conducted in the three channel loop of PCNCTF as per the prescribed philosophy to be adopted in AHWR. The response of the system was then simulated satisfactorily using RELAP/MOD3.2 after some tuning. Subsequently, parametric studies were carried out with the developed numerical model to accelerate the cold start-up process by increasing power levels. Finally, a rational start-up methodology is developed which could further accelerate the cold start-up.

Critical heat flux in a 0.38 mm microchannel and actions for suppression of flow boiling instabilities

This paper presents experimental results for critical heat flux in a 0.38mm internal diameter tube during saturated flow boiling. Experiments were performed for refrigerant R134a flowing inside a horizontal stainless steel circular channel of 70mm heated length, mass velocities ranging from 200 to 1400kg/m2s, saturation temperature of 31°C and critical heat fluxes up to 215kW/m2. A parametric study of the effect of mass velocity revealed the same trends observed in previous studies with 1.1 and 2.2mm internal diameter tubes. In contrast, prediction methods that performed well in previous works for internal diameters higher than 1.00mm failed to predict the data for the 0.38mm tube. An investigation into the reasons for the failure of these methods revealed thermo-hydraulic instabilities are more pronounced for the 0.38mm tube and actions to cancel these effects are required. Tests revealed that a saturated inlet vapor quality near 5% combined with a high inlet pressure drop could increase the critical heat flux up to 50% in comparison to the results without any control actions. Moreover, conventional CHF predictive methods from the literature provided a reasonable prediction of the results for the 0.38mm tube when instability effects were minimized.

C222 パラレルチャンネル装置における不安定沸騰挙動に対する流路抵抗の影響(OS9 熱・流動(5))

Thermo-hydraulic instabilities of both natural and forced circulation in parallel boiling channels have been investigated. Experimental loop is designed to simulate flow and boiling conditions in natural circulation boiling water reactor (pump can be used to examine forced circulation), heated parts of parallel channels are followed by non-heated parts with ball valves. For different combinations of flow resistance (set with different valve positions), instabilities have been measured. The effect of different combinations of pressure loss coefficient of each channel on instabilities has been analyzed, e g effect on instabilities' frequencies and amplitudes.

Analytical solution of the problem of supercritical fluid instability in a heated channel

The paper represents an investigation into thermohydraulic instability in flow of a supercritical fluid with respect to a “density wave”. An analytical solution was obtained for the stability boundary separating stable and unstable modes of the fluid flow. Effects of the thermophysical properties and wall thickness on the flow stability were studied. It was shown that an increase in the thermal conductivity and the thickness of the wall leads to the increase in the flow stability. The theoretically obtained stability boundary was compared with experimental data obtained for the cooling system of superconducting magnets. Taking into account the thermal conjugation “wall-coolant” lifts the problem to the new higher level: an additional parameter is involved into the mathematical description, which causes qualitative changes in the character of the solution.

Multi Parameters Effect on Thermohydraulic Instability in Natural Circulation Boiling Water Reactor during Startup

The purpose of the study is to experimentally investigate driving mechanism of major instabilities simulated in a natural circulation experimental loop, under a predetermined range of system operating pressure and inlet subcoolings. Pressure range of 0.1 up to 0.7 MPa, input heat flux range of 0 up to 577 kW/m 2 , and inlet subcoolings of 5, 10 and 15 K respectively, are applied in the experiments. The objective of the study is to formulate a rational startup procedure, in which major thermohydraulic instabilities can be detected and prevented. The study clarifies that four (4) kinds of thermohydraulic instability might occur even up to a higher pressure of 0.7 MPa. The instabilities' sequence is as follows: (1) geysering induced by condensation accompanied by flashing, (2) oscillation induced by hydrostatic head fluctuation, (3) density wave oscillations, and (4) flashing accompanying those instabilities. The experiments confirmed that the geysering region gets narrower and suppressed with the increased system pressure. With chimneys, natural circulation can be achieved reliably and more easily. However, the flashing in the chimney cannot be avoided at low system pressure. Stable two-phase natural circulation can be established if the system pressure is increased beyond 0.7 MPa, after the high frequency density eave oscillation thoroughly suppressed. The experiments were analyzed based on frequency domain of each instability phenomenon.

低減速自然循環沸騰水型炉における起動時の流動安定性に関する基礎研究(NP3 新型炉技術)

Reduced-moderation natural circulation BWR has been promoted by JAERI in Japan as one of advanced small and medium-sized reactors from the aspect of more effective utilization of uranium and introduction of passive safety systems in conformity with the natural law. However, various thermo-hydraulic instabilities might be induced in low pressure and power conditions due to an elimination of re-circulation pumps. Therefore, the purpose of this study is to experimentally investigate the characteristics of thermo-hydraulic instabilities with natural circulation loop consists of twin parallel boiling channels. Resulting characteristics has been compared with Aritomi and Chiang's to clarify the effect of length of non-heated length in particular, and then the rational reactor configuration has been proposed.

Transport Mechanism of Thermohydraulic Instability in Natural Circulation Boiling Water Reactors during Startup

This paper presents experimental study on transport mechanism of thermohydraulic instability, which may occur in natural circulation boiling water reactor during startup. The research was carried out using a natural circulation experimental loop featuring twin parallel boiling channels with chimney assembly. The experiments were performed with the pressure range of 0.1 to 0.7MPa and maximum heat flux of 577kW/m2. The objective of the study is to formulate thermohydraulic stability maps required for determining rational startup procedure of the reactor, in which the instability could be prevented. The study clarified that the flow modes during startup consist of the following sequence: (1) single-phase flow, (2) geysering, (3) oscillation due to hydrostatic head fluctuation, (4) density wave oscillation, (5) transition oscillation, and (6) stable two-phase flow. The main findings of the experiments are as follows: First, low amplitude geysering still occurs at 0.7 MPa under lower heat flux and high inlet subcooling. Second, stable two-phase natural circulation is achieved with system pressure as low as 0.2 MPa, under medium heat flux, and subcooling lower than 5 K. Third, oscillation due to hydrostatic head fluctuation only occurs under atmospheric condition. Finally, thermohydraulic stability maps and rational startup procedure are formulated.

ICONE11-36077 MULTI PARAMETERS EFFECT ON THERMOHYDRAULIC INSTABILITY IN NATURAL CIRCULATION BOILING WATER REACTOR DURING STARTUP

Feasibility study on the future light water reactor has been carried out. Small- and medium-sized natural circulation boiling water reactor (BWR) has been one of the concepts. One of the indispensable topics is thermohydraulic instability during startup. Occurrence of the instability during the startup negatively affects safety, reliability and operability, as it would cause complexity in raising and maneuvering reactor power. The purpose of the study is to experimentally investigate driving mechanism of major instabilities simulated in a natural circulation experimental loop, under a predetermined range of system operating pressure and inlet subcoolings. Pressure range of 0.1 up to 0.7 MPa, input heat flux range of 0 up to 577kW/m^2,and inlet subcoolings of 5,10 and 15K respectively, were applied in the experiments. The objective of this paper is to offer new experimental data as the basis to propose rational startup procedure, in which various thermohydraulic instabilities can be prevented. The study clarifies that four (4) kinds of thermohydraulic instability might occur even up to a higher pressure of 0.7 MPa. The clarified sequence instabilities are as follows : (1) geysering induced by condensation accompanied by flashing, (2) oscillation induced by hydrostatic head fluctuation, (3) density wave oscillations, and (4) flashing accompanying those instabilities. The experiments confirmed that the geysering region gets narrower and suppressed with the increased system pressure. With chimneys, natural circulation can be achieved reliably and more easily. However, the flashing in the chimney cannot be avoided at low system pressure. Stable two-phase natural circulation can be established if the system pressure is increased beyond 0.7 MPa, after the high frequency density eave oscillation thoroughly suppressed. The experiments were analyzed based on frequency domain of each instability phenomenon.

Transport Mechanism of Thermohydraulic Instability in Natural Circulation Boiling Water Reactors during Startup

Transport Mechanism of Thermohydraulic Instability in Natural Circulation Boiling Water Reactors during Startup

A Study on Thermo-Hydraulic Instability of Boiling Natural Circulation Loop with a Chimney. 4th Report. An Analytical Consideration of the Stability and Thermo-Hydraulic Characteristics in the Chimney in High Pressure.

Thermo-hydraulic instabilities of a boiling natural circulation loop with a chimney under high pressure were investigated using linear stability analysis. Drift-flux model was used for two-phase flow model. The instability regions as well as the thermo-hydraulic characteristics in the chimney such as wavy feature were examined, which were compared with the characteristics in low pressure. Instability could occur when exit quality was relatively low, which was the same manner as the characteristics in low pressure. In high-pressure, void was generated near channel exit, and void wave propagated in the chimney. In low pressure, steam was generated only near the chimney exit due to gravity induced flashing, and single-phase enthalpy wave, that is, temperature wave propagated in single-phase flow region. Though flow could be very stable in the high pressure and high power condition, the decay ratio of higher mode could be larger than that of lower mode.

Thermo-hydraulic instability of boiling natural circulation loop induced by flashing (analytical consideration)

An analytical study is presented on the thermo-hydraulic stability of a boiling natural circulation loop with a chimney at low pressure start-up. The effect of flashing induced by the pressure drop in the channel and the chimney due to gravity head on the instability is considered. A method to analyze linear stability is developed, in which a drift-flux model is used. The analytical result of a stability map agrees very well with the experimental one obtained in a previous report. Instability does not occur when the heater power is too low to generate voids in the chimney and only natural circulation of single phase can be induced. Instability tends to occur when boiling occurs only near the chimney exit due to flashing. This instability phenomenon has some similarities with density wave oscillation, such as the phase difference of temperature between the boiling region and non-boiling region, and the oscillation period which is near to the time required for fluid to pass through the chimney. However, there are also some differences from density wave oscillation, such as the boiling region is very short, and pressure fluctuation can affect void fraction fluctuation.

B110 チムニを有する沸騰自然循環ループの不安定流動に関する研究(軸方向高次モードの影響の検討)

Thermo-hydraulic instabilities of a boiling natural circulation loop with a chimney under low and high pressure were investigated using linear stability analysis. The effect of nuclear coupling was also considered. Both in low- and high-pressure conditions, instability could occur when exit quality was relatively low. In low-pressure condition, flashing only near the exit of the chimney could induce instability, and enthalpy wave of single-phase flow was generated in the chimney. In high-pressure condition, void was generated near channel exit, and void wave propagated in the chimney. In the high pressure and high power condition, though flow could be very stable, the decay ratio of higher mode could be larger than that of lower mode. The sensitivity on decay ratio to the thermal power, inlet subcooling, void reactivity feedback coefficient and so on could be very low when there was a long chimney.

A possible scenario for the development of thermohydraulic instability

A possible scenario for the development of thermohydraulic instability

Linear Analysis of Thermo-hydraulic Instabilities of the Advanced Heavy Water Reactor (AHWR)

Analysis was carried out to predict the threshold of instability for Ledinegg type and density wave oscillations for the Indian Advanced Heavy Water Reactor (AHWR) which is a Natural Circulation Pressure Tube Type Boiling Water Reactor. The mathematical model considers homogeneous two-phase flow and the conservation equations are solved analytically to obtain the steady state thermo-hydraulic characteristics and flow stability map. The model was applied for the AHWR concept after it had been validated with the test data obtained from a simple forced circulation loop with small parallel boiling channels and from the High Temperature Loop (PNC). The results indicate that the proposed design configuration of the AHWR may experience both Ledinegg type (static instability) and Type-I and Type-II density wave oscillations depending on the operating condition. The effects of various geometric and operating parameters on these types of instabilities were studied. It can be seen from the results that the Ledinegg type instability is suppressed with an increase in pressure and disappears when the operating pressure is higher than 0.7MPa. But the density wave instability may occur even at 7 MPa. In addition, it is found that for parallel multiple channels operating under natural circulation condition, the stability of density wave instability is not always enhanced by increasing the throttling coefficients at the inlet of channels.