STUDY ON DRY-OUT HEAT FLUX OF TWO-PHASE NATURAL CIRCULATION

The feasibility study in thermal-hydraulics for the future light water reactor concept is carried out. One of the essential studies is the two-phase flow instability during start-up in the natural circulation boiling water reactor (BWR) concept. It is anticipated that the occurrence of the two-phase flow instabilities during start-up significantly affects the feasibility concept, since it would cause the complexity in raising and maneuvering the power output. The purpose of the current study is to experimentally investigate the driving mechanism of the geysering and density wave oscillation in the natural circulation loop, induced by a range of system operating pressure and increasing heat flux in vertical parallel channels. The pressure range of atmospheric up to about 4 bars, and the input heat flux range of 0 up to 577 kW/m2 are applied in these experiments. An experimental apparatus of twin boiling upflow channels to simulate natural circulation flow loop has been designed, constructed and operated. The natural circulation in the loop occurs due to the density difference between two-phase region in the channels and the single-phase liquid in the downcomer. The objective of the study is to propose a rational start-up procedure in which the geysering and density wave oscillation can be prevented during startup, according to its system pressure and heat flux. Previous studies have clarified that three (3) kinds of thermo-hydraulics instabilities may occur during start-up in the natural circulation BWR depending on its procedure and reactor configuration, which are (1) geysering induced by condensation, (2) natural circulation induced by hydrostatic head fluctuation in steam separator, and (3) density wave oscillation.

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Two-phase natural circulation loop behaviour at atmospheric and subatmospheric conditions

Proceedings of the Institution of Mechanical Engineers Part E Journal of Process Mechanical Engineering ◽

10.1177/0954408918787401 ◽

2018 ◽

Vol 233 (4) ◽

pp. 687-700

Author(s):

S Venkata Sai Sudheer ◽

K Kiran Kumar ◽

Karthik Balasubramanian

Keyword(s):

Heat Flux ◽

Physical Properties ◽

Spatial Variation ◽

Mass Flow Rate ◽

Mass Flow ◽

Flow Rate ◽

Natural Circulation ◽

Two Phase ◽

Natural Circulation Loop ◽

Thermo Physical Properties

This paper aims to present the steady-state behaviour of two-phase natural circulation loop at atmospheric and sub-atmospheric conditions. One-dimensional numerical approach is adopted to evaluate various system parameters, with special emphasis on spatial variation of thermo-physical properties and flashing. Homogeneous equilibrium model is applied for two-phase flows. An in-house code is developed in MATLAB to solve numerical model iteratively. It is observed that consideration of spatial variation of thermo-physical properties can precisely predict the loop behaviour. The evaluated results are validated with the open literature and reasonably good agreement is observed. The heater inlet temperature, inlet pressure and heat flux are found to have significant influence on spatial variation of pressure, temperature and enthalpy. As system pressure decreases from atmospheric to sub-atmospheric (1–0.8 atm), it is observed that the sub-atmospheric loop gives a higher mass flow rate compared to atmospheric loop at lower heat fluxes. However, as the heat flux increases in the sub-atmospheric loop, the mass flow rate is reduced due to increased drag force in the loop.

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The Effect of the Location of Nucleation Sites on the Thermal-Hydraulic Stability of a Short-Tube Natural Circulation Loop

Journal of Engineering for Gas Turbines and Power ◽

10.1115/1.4005970 ◽

2012 ◽

Vol 134 (6) ◽

Author(s):

Marek Kapitz ◽

Stefan aus der Wiesche

Keyword(s):

Heat Flux ◽

Chaotic Behavior ◽

Natural Circulation ◽

Nucleation Site ◽

Operating Conditions ◽

Strong Impact ◽

Site Location ◽

Two Phase ◽

Nucleation Sites ◽

Hydraulic Stability

The use of small two-phase natural circulation loops is an attractive option for efficient cooling applications and innovative steam generators. An experimental study was conducted to examine how the thermal-hydraulic stability and operating conditions of these devices are affected by nucleation sites. A very smooth glass tube with artificial nucleation sites was used as a boiling channel. The mass flow rate was obtained as a function of heat flux and nucleation site location. Nucleation sites have a strong impact on stability behavior, particularly for low heat flux levels. The observed flow instabilities were analyzed with regard to nonlinear effects and chaotic behavior.

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Natural-Circulation Tests With Water at 800 to 2000 Psia Under Nonboiling, Local Boiling, and Bulk Boiling Conditions

Journal of Heat Transfer ◽

10.1115/1.3682253 ◽

1961 ◽

Vol 83 (3) ◽

pp. 261-273 ◽

Cited By ~ 8

Author(s):

O. J. Mendler ◽

A. S. Rathbun ◽

N. E. Van Huff ◽

A. Weiss

Keyword(s):

Heat Flux ◽

Natural Circulation ◽

Rectangular Channel ◽

Heat Fluxes ◽

Circulation Loop ◽

Flow Conditions ◽

Two Phase ◽

Forced Circulation ◽

Flow Fluctuations ◽

Natural Circulation Loop

Natural and forced-circulation test data for a closed-loop system are presented and analyzed. The data were obtained at pressures of 800, 1200, 1600, and 2000 psia from the natural-circulation loop at the Bettis Laboratory, using single rectangular channel test sections (0.100 in. × 1.0 in. × 27.0 in. long, 0.200 in. × 1.0 in. × 27.0 in. long, and 0.250 in. × 1.0 in. × 27.0 in. long). Heat fluxes ranged from 50,000 Btu/hr-sq ft to burnout with inlet subcoolings of 20, 70, and 100 deg F. The results showed that single and two-phase pressure drop, burnout heat flux, and riser density measured under natural-circulation operation are no different from those measured with forced circulation at the same thermal and fluid flow conditions. For the loop studied, it was shown that natural-circulation-loop flow rates can be predicted to within 10 per cent for both single and two-phase flow. Some data for slip ratios at liquid velocities less than 1/2 fps and for two-phase exit losses were obtained. Flow fluctuations were noted during some of the natural-circulation runs; these occurred before burnout heat flux was reached. In some instances these fluctuations were severe enough to cause a premature burnout.

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Analysis of Flow Instabilities in Two-Phase Natural Circulation

Volume 9: Heat Transfer, Fluid Flows, and Thermal Systems, Parts A, B and C ◽

10.1115/imece2009-10236 ◽

2009 ◽

Cited By ~ 1

Author(s):

Geping Wu

Keyword(s):

Heat Flux ◽

Pressure Drop ◽

Flow Rate ◽

Natural Circulation ◽

Flow Instabilities ◽

High Heat Flux ◽

Circulation Flow ◽

Two Phase ◽

Drift Flux Model ◽

Circulation Flow Rate

Safety concerns of nuclear reactors have attracted the attention of researchers on flow instabilities in natural circulation boiling loops. In this theoretical study, a drift flux model which solves the conservation equations of mass, momentum and energy applicable to boiling two-phase natural circulation systems is adopted. The influence of two-phase flow parameters such as drift velocity and void distribution parameter on the loop flow rate is weak. The model is used to analysis the effects of heat flux and inlet subcooling on steady circulation flow rate. High circulation flow rate is accompanied by high heat flux and low inlet subcooling. According to the region and number of meeting points which connects the resistance pressure drop curve and the driving pressure drop curve, flow excursion and density-wave instability sometimes may occur. Further, investigations are carried out to study the effect of heat flux and system pressure on the instabilities region in natural circulation.

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Effect of the Nucleation Site Location on the Thermalhydraulic Stability of a Short-Tube Natural Circulation Loop

ASME 2011 Power Conference, Volume 2 ◽

10.1115/power2011-55033 ◽

2011 ◽

Author(s):

Marek Kapitz ◽

Stefan aus der Wiesche

Keyword(s):

Heat Flux ◽

Power Plants ◽

Chaotic Behavior ◽

Natural Circulation ◽

Nucleation Site ◽

Strong Impact ◽

Site Location ◽

Two Phase ◽

Operation Conditions ◽

Nucleation Sites

For efficient cooling applications in power plants, the use of small two-phase natural circulation loops becomes attractive. An experimental study was carried out to examine how thermal hydraulic stability and operation conditions of these devices are affected by nucleation sites. A very smooth glass tube with artificial nucleation sites have been employed as boiling channel. The mass flow rate has been determined as function of heat flux and nucleation site location. Particular for low heat flux levels, the nucleation sites have a strong impact to the stability behavior. The observed flow instabilities have been analyzed with regard to non-linear effects and chaotic behavior.

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Analysis of RESCUE External Reactor Cooling Experiments Using RELAP5 Mod3.3 and ASTEC V2.0 Codes

Volume 6: Beyond Design Basis Events; Student Paper Competition ◽

10.1115/icone21-15537 ◽

2013 ◽

Author(s):

Martin Kubic ◽

Milan Bachraty ◽

Miroslav Barnak ◽

Peter Matejovic ◽

David Guenadou ◽

...

Keyword(s):

Heat Flux ◽

Natural Circulation ◽

Safety Margin ◽

Reactor Vessel ◽

Severe Accident ◽

Research Network ◽

Reactor Wall ◽

Two Phase ◽

Cooling Loop ◽

Reactor Surface

Safety margin of the in-vessel retention strategy is given by the difference between thermal load acting on inner reactor surface and coolability limit (in terms of critical heat flux) on outer reactor surface. In order to study the two-phase flow in external reactor vessel cooling loop and heat transfer from curved reactor wall, the RESCUE-2 (Representative loops for External System Cooling Understanding Experiments) experimental facility was erected in CEA Cadarache in France. The facility consists of electrically heated simulator of reactor vessel with an ellipsoidal lower head and cooling loop that enables natural circulation of coolant around the reactor wall. In the frame of SARNET 2 project (Severe Accident Research Network of Excellence, 7th EU Framework Programme) several experiments devoted to external reactor vessel cooling phenomena with certain relevance to VVER-440/V213 reactors, were performed on this facility. The heat flux profile generated by electrical heaters in these experiments was based on the results that were obtained by using ASTEC code for this reactor design. The results of two RESCUE-2 experiments are used in this paper for benchmarking of RELAP 5 Mod.3.3 code and ASTEC V2.0 code.

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THE MECHANISM OF RAISING AND QUANTIFICATION OF SPECIFIC HEAT FLUX AT BOILING OF NANOFLUIDS IN FREE CONVECTION CONDITIONS

Energy Technologies & Resource Saving ◽

10.33070/etars.3.2017.03 ◽

2017 ◽

pp. 25-34

Author(s):

V.N. Moraru

Keyword(s):

Heat Transfer ◽

Heat Flux ◽

Heat Capacity ◽

Specific Heat ◽

Chemical Properties ◽

Heating Surface ◽

Physical Models ◽

Superheated Liquid ◽

Two Phase ◽

Boiling Process

The results of our work and a number of foreign studies indicate that the sharp increase in the heat transfer parameters (specific heat flux q and heat transfer coefficient _) at the boiling of nanofluids as compared to the base liquid (water) is due not only and not so much to the increase of the thermal conductivity of the nanofluids, but an intensification of the boiling process caused by a change in the state of the heating surface, its topological and chemical properties (porosity, roughness, wettability). The latter leads to a change in the internal characteristics of the boiling process and the average temperature of the superheated liquid layer. This circumstance makes it possible, on the basis of physical models of the liquids boiling and taking into account the parameters of the surface state (temperature, pressure) and properties of the coolant (the density and heat capacity of the liquid, the specific heat of vaporization and the heat capacity of the vapor), and also the internal characteristics of the boiling of liquids, to calculate the value of specific heat flux q. In this paper, the difference in the mechanisms of heat transfer during the boiling of single-phase (water) and two-phase nanofluids has been studied and a quantitative estimate of the q values for the boiling of the nanofluid is carried out based on the internal characteristics of the boiling process. The satisfactory agreement of the calculated values with the experimental data is a confirmation that the key factor in the growth of the heat transfer intensity at the boiling of nanofluids is indeed a change in the nature and microrelief of the heating surface. Bibl. 20, Fig. 9, Tab. 2.

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