Single-phase density wave oscillation -A new phenomenon of flow instability in inverted U-Type Steam Generator

Hydrodynamic flow instability in Once Through Steam Generators (OTSG) is one of the important problems in the design and operation of Liquid Metal Fast Breeder Reactors (LMFBRs). Under certain operating conditions, water flow in OTSG is susceptible to instability due to the close coupling between the thermal and hydraulic processes. Sustained flow oscillations due to instability are undesirable since they result in flow mal-distribution among the tubes resulting in thermal stress, mechanical vibrations and system control problems. It is therefore, necessary to assess the operating conditions, under which instability occurs so that the system may be designed to operate always under stable conditions. The cause of the main type of instability, important for the design of SGs is the propagation of density waves. This type of low frequency instability is referred to in literature as parallel-channel, density wave, time delay or mass flow-void feedback instability. Dynamic instability (density wave oscillation DWO) occurs because of the phase mismatch between the primary perturbation (water flow) and the response to this perturbation (pressure drop). As many tubes are operating under essentially constant pressure heads, this mismatch can lead to sustained/diverging oscillations. Water flow oscillation in tubes manifests as oscillations in the steam temperature at the tube outlet/pressure fluctuations. However it is difficult to instrument individual tubes in SG for such measurement in an operating plant. If the flow oscillation in the tube manifests itself in the overall module flow, then fluctuation in the overall flow/flow noise could be utilized for on-line stability measurements. Towards this, experiments were conducted in the sodium heated once through steam generator in an OTSG model. To confirm the extent of oscillation in the steam temperature and in inlet water flow, 3 tubes out of 19, were monitored besides overall module flow. Main objective of the present study was to assess the occurrence of dynamic instability in SG through module inlet flow perturbations, measured by ΔP measurements across the orifice at entry to the tubes and steam temperature fluctuation measurement at the outlet of tube by bare thermocouples. This paper discusses the experiments carried out in the Steam Generator model of Prototype Fast Breeder Reactor to investigate the instability phenomenon, the instrumentation details, the results and its discussion.

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Density Wave Oscillation in a Vertical Type Natural Circulation Heat Recovery Steam Generator: A Numerical Study

Volume 7: Education; Industrial and Cogeneration; Marine; Oil and Gas Applications ◽

10.1115/gt2008-50035 ◽

2008 ◽

Author(s):

Heimo Walter ◽

Wladimir Linzer

Keyword(s):

Heat Flux ◽

Steam Generator ◽

Heat Recovery ◽

Natural Circulation ◽

Density Wave ◽

Heat Recovery Steam Generator ◽

Counter Flow ◽

Wave Oscillation ◽

Lower Amplitude ◽

Simulation Results

The dynamic flow instability, namely density wave oscillation (DWO), was investigated theoretically. The analysis was done for different design configurations of the evaporator of a vertical type natural circulation heat recovery steam generator (HRSG) at low operation pressure under hot start-up conditions. The study was done for co-current and counter flow designs of the HRSG evaporator, different drum heights and different heat flux distributions over the heating surface of the evaporator. The investigations for the HRSG show that the heat flux distribution to the evaporator tubes has an important influence on the flow stability. The simulation results indicate that a lower amplitude of the mass flow oscillation of the working medium is given by a more uniform heat flux to the single tubes of the evaporator. This leads the two-phase flow system to a more stable condition. This study has also shown that changes in the drum height of the boiler have no significant influence on the oscillation amplitude of the DWO. The simulation results have shown that the counter flow design is much more stable under the investigated conditions compared to the co-current design.

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Classification of Two-Phase Flow Instability by Density Wave Oscillation Model

Journal of Nuclear Science and Technology ◽

10.1080/18811248.1979.9730878 ◽

1979 ◽

Vol 16 (2) ◽

pp. 95-108 ◽

Cited By ~ 125

Author(s):

Kenji FUKUDA ◽

Tetsuo KOBORI

Keyword(s):

Two Phase Flow ◽

Flow Instability ◽

Density Wave ◽

Phase Flow ◽

Two Phase ◽

Wave Oscillation ◽

Oscillation Model

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Experimental Research of Flow Instability Onset Conditions in a Natural Circulation System With Subcooled Boiling

Volume 4: Thermal Hydraulics ◽

10.1115/icone21-15315 ◽

2013 ◽

Author(s):

Liang-ming Pan ◽

Jia Li ◽

De-qi Chen

Keyword(s):

Flow Instability ◽

Natural Circulation ◽

Stability Boundary ◽

Density Wave ◽

System Stability ◽

Circulation System ◽

Subcooled Boiling ◽

Boundary Area ◽

Wave Oscillation ◽

System Pressure

With water as the working medium, this paper has investigated into the factors which affect the onset points of flow instability in the natural circulation system with subcooled boiling. Study finds that the increase of entrance subcoolling and pressure in the system can improve system stability. Increasing threshold heat flux can make the subcooling boundary area of density wave oscillation bigger. Both entrance subcooling and system pressure have significant influence on the position where ONB appears at stability boundary. According to dimensionless analysis of two-phase flow differential equations, this paper has put forward an semi-empirical formulae that can forecast the onset conditions of density wave oscillation.

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Analysis of Density Wave Oscillations in Helically Coiled Tube Once-Through Steam Generator

Science and Technology of Nuclear Installations ◽

10.1155/2016/3071686 ◽

2016 ◽

Vol 2016 ◽

pp. 1-10

Author(s):

Junwei Hao ◽

Yaoli Zhang ◽

Jianxiang Zheng ◽

Zhiwei Zhou ◽

Xuanyu Sheng ◽

...

Keyword(s):

Steam Generator ◽

Flow Instability ◽

Density Wave ◽

System Stability ◽

Geometrical Parameters ◽

Coiled Tube ◽

Helically Coiled Tube ◽

Comparison Results ◽

The Impact ◽

Density Wave Oscillations

Helically coiled tube Once-Through Steam Generator (H-OTSG) is one of the key equipment types for small modular reactors. The flow instability of the secondary side of the H-OTSG is particularly serious, because the working condition is in the range of low and medium pressure. This paper presents research on density wave oscillations (DWO) in a typical countercurrent H-OTSG. Based on the steady-state calculation, the mathematical model of single-channel system was established, and the transfer function was derived. Using Nyquist stability criterion of the single variable, the stability cases were studied with an in-house computer program. According to the analyses, the impact law of the geometrical parameters to the system stability was obtained. RELAP5/MOD3.2 code was also used to simulate DWO in H-OTSG. The theoretical analyses of the in-house program were compared to the simulation results of RELAP5. A correction factor was introduced to reduce the error of RELAP5 when modeling helical geometry. The comparison results agreed well which showed that the correction is effective.

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Effect of Heaving Movement on Flow Instability in U-Tubes of Marine Steam Generator under Natural Circulation

Science and Technology of Nuclear Installations ◽

10.1155/2013/432817 ◽

2013 ◽

Vol 2013 ◽

pp. 1-7 ◽

Cited By ~ 4

Author(s):

Jianli Hao ◽

Wenzhen Chen ◽

De Zhang

Keyword(s):

Steam Generator ◽

Single Phase ◽

Flow Instability ◽

Reverse Flow ◽

Natural Circulation ◽

Critical Mass ◽

Space Distribution ◽

Tube Length ◽

One Dimensional ◽

Acceleration Amplitude

Under heaving movement conditions, the single phase flow instability in U-tubes is affected by the additional force, which will influence the marine reactor operation. In the present work, one-dimensional thermal-hydraulic model in U-tubes under heaving movement conditions is established, and the critical pressure drop (CPD) and critical mass flow rate (CMFR) which relate to the occurrence of reverse flow in U-tubes are proposed and analyzed. The effects of the heaving period and heaving acceleration amplitude on the flow instability in U-tubes with the different length are discussed. It is shown that (1) the CPD and CMFR are obviously affected by the heaving movement, which means that the reverse flow characteristic in U-tubes will be changed; (2) the fluctuation periods of the CPD and CMFR are the same as the heaving period, but the fluctuation magnitude of them is little affected by the heaving period; (3) the relative changes of CPD and CMFR are the linear function of heaving acceleration amplitude; and (4) the U-tube length has little influence on the relative changes of CPD and CMFR compared with the heaving acceleration amplitude, which means that the heaving movement has little influence on the space distribution of reverse flow in the U-tubes of marine steam generator.

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Flow Patterns During Flow Boiling Instability in Silicon-Based Pin-Fin Microchannels

Journal of Heat Transfer ◽

10.1115/1.4038194 ◽

2017 ◽

Vol 140 (3) ◽

Cited By ~ 8

Author(s):

Fayao Xu ◽

Huiying Wu ◽

Zhenyu Liu

Keyword(s):

Single Phase ◽

Flow Instability ◽

Flow Boiling ◽

Flow Patterns ◽

Phase Flow ◽

Single Phase Flow ◽

Pin Fin ◽

Short Period ◽

Pin Fins ◽

Amplitude Mode

In this paper, the flow patterns during water flow boiling instability in pin-fin microchannels were experimentally studied. Three types of pin-fin arrays (in-line/circular pin-fins, staggered/circular pin-fins, and staggered/square pin-fins) were used in the study. The flow instability started to occur as the outlet water reached the saturation temperature. Before the unstable boiling, a wider range of stable boiling existed in the pin-fin microchannels compared to that in the plain microchannels. Two flow instability modes for the temperature and pressure oscillations, which were long-period/large-amplitude mode and short-period/small-amplitude mode, were identified. The temperature variation during the oscillation period of the long-period/large-amplitude mode can be divided into two stages: increasing stage and decreasing stage. In the increasing stage, bubbly flow, vapor-slug flow, stratified flow, and wispy flow occurred sequentially with time for the in-line pin-fin microchannels; liquid single-phase flow, aforementioned four kinds of two-phase flow patterns, and vapor single-phase flow occurred sequentially with time for the staggered pin-fin microchannel. The flow pattern transitions in the decreasing stage were the inverse of those in the increasing stage for both in-line and staggered pin-fin microchannels. For the short-period/small-amplitude oscillation mode, only the wispy flow occurred. With the increase of heat flux, the wispy flow and the vapor single-phase flow occupied more and more time ratio during an oscillation period in the in-line and staggered pin-fin microchannels.

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