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The secondary loop (SL) of a Natural Circulation (NC) System serves as the coolant supply channel to the Heat Exchanger (HX) for effective passive heat removal by the NC system. Insufficient coolant or total loss of coolant along the cold leg of the SL will lead to a loss of heat sink accident (LOHS). This phenomenon negatively affects the heat removal capability of the HX. The experiment has now been carried out to investigate the loss of secondary loop coolant as it affects flow characteristics in the primary system of a natural circulation system under two-phase conditions carried out in a well-scaled down dimensioned NC loop having a single heated channel. Induced power of 9.0 KW and a 0.5 MPa system pressure were predetermined and maintained. Transient was activated by shutting down the inlet of the secondary loop (cold leg) and then NC flow behavior was observed in each test section along the flow centered on the predetermined conditions. When HX outlet temperature upsurges to around its inlet temperature, data recording is halted, and the experiment is brought to an end. The acquired data were analyzed at the inlet and outlet of both the heated sections and the HX in various flow regimes. While the transient scenario was activated, there exists transient stability in the NC loop with temperature increasing steadily and consequently descents into the instability region. Thereafter, NC flow drips into a stable two-phase flow, and subsequently, outlet temperature surges, and flow loses stability and purges into the two-phase instability region leading to CHF occurrence. This experiment is useful for more understanding of NC fluid behavior and further exploitation of the close NC rectangular loop in the event of LOHS accident. [ABSTRACT FROM AUTHOR]

The two-phase flow frictional pressure drop is a principal parameter of boiling heat transfer in narrow channels, and is affected by many factors due to the special structure and complex flow. In order to study relevant influence factors on two-phase flow frictional pressure drop in narrow channel from the aspect of quantitative analysis, Grey Theory is applied to analyze the experimental data in narrow channel by taking water as the medium. And then the intensity sequence of the factors that have effects on two-phase friction pressure drop are confirmed to be the outlet dryness, boiling length, heat flux, inlet pressure, Reynolds of two-phase, mass flow and inlet temperature. The results showed that applying the method of Grey Theory to analyze the principal parameters of boiling heat transfer in narrow channels is feasible. [ABSTRACT FROM AUTHOR]

The point of ONB (Onset of Nucleate Boiling) is a key point of boiling heat transfer in narrow channels. Due to the special structure and complex flow, the points of ONB in narrow channels are affected by many factors, their characteristics are not understood completely yet. In order to study relevant influence factors on ONB in narrow channel from the aspect of quantitative analysis, GRA (Grey Relational Analysis) is applied to analyze the experimental data of ONB in narrow channel by taking water as the working fluid. And then the intensity sequence of the factors that have effects on ONB are confirmed as the heat flux, outlet dryness, pressure, mass flow rate, inlet temperature and outlet temperature. Through analyzing the data of ONB of newly published literatures, the mechanisms for the main influence factors are suggested. [ABSTRACT FROM AUTHOR]

The influence of the important system parameters on Ledinegg instability was investigated in a narrow rectangular channel with deionized water. The experiment carried out under the condition of: P=0.47, 0.7 and 1 MPa, G=0∼2000 kg/(m2·s), ΔTsub,in=60, 75 and 92°C q=173.8, 255.9 and 294.6kW/m2. Based on the small disturbance theory, the negative slope region of the pressure drop-mass flux curve where two-phase mixture prevails was closely examined to quantify the susceptibility of the system to the Ledinegg instability under different flow conditions. The experimental results showed the effects of the heat flux, system pressure and inlet subcooling on the Ledinegg instability: reducing heat flux and inlet subcooling, increasing system pressure can enhance the flow stability in the narrow rectangular channel. [ABSTRACT FROM AUTHOR]

Bubble behavior under instable flow at atmospheric pressure were investigated in this paper. The visualization experiments were carried out on a vertical, narrow rectangular test section, with one side heating surface and upward flow. The section of the channel is 40×2mm. Mass flow ranged from 80 to 200kg/h, heat fluxes from 24 to 500kW/m2 and the subcooling from 5 to 35K. High-speed photography at rate of 5000 frames/s captured bubble behaviors. Bulk liquid velocities were found to fluctuat under instable flow, and backflow was observed. The bubbles were generated periodically. A few bubbles with smaller diameter and no sliding generated when the flow reversed. Many bubbles with larger diameter and long distance of sliding were generated as the flow velocity increased. [ABSTRACT FROM AUTHOR]

In a single-side heated rectangular channel, the bubble departure frequency in subcooled flow boiling under non-rolling and rolling motions was investigated by using a high speed camera. The deionized water was used as the working fluid. The experimental results showed that the bubble departure frequency under non-rolling condition had similar tendencies with general channel. The frequency increased with increasing heat flux and decreasing inlet subcooling and mass flux. When the heat flux was high enough, the frequency curve slowed down due to the increasing of bubble density. Under rolling condition, the bubble departure frequency was affected by the rolling motion. The frequency exhibited a sine wave with the same period as rolling motion. The fluctuation ofthat increased with increasing the rolling frequency and rolling angle. The reason for this phenomenon was the periodic affection of secondary flow due to temperature gradient. [ABSTRACT FROM AUTHOR]

In view of the character that wetness measurement for steam is difficult and inaccurate, a novel sensor of wetness measurement for flowing wet steam is developed by making use of capacitance method based on the fact that the water and steam have great different permittivities. The sensor can be installed in the steam pipe directly, so the steam will flow through it with full flow rate. Therefore the error due to drawing steam sample inaccurately is eliminated. By means of this sensor, on-line measurement of the wetness of wet steam can be realized. Based on the fact that air and steam have almost same permittivities, the flowing wet steam is simulated by spraying water into air flow in the experiment. Experiment with the novel sensor of wetness measurement in water/air flows shows that the frequency output decreases as the absolute humidity of flowing wet air increases. The relationship between the absolute humidity of air and the relative frequency deviation are obtained from the experimental data and this result accords with the theoretic conclusion. The sensor can endure high temperature and high pressure and has preferable precision, so it can be used to measure not only the wetness of flowing wet steam in nuclear power plants but also the water content of solid materials on-line after precise calibration. © 2007 American Institute of Physics [ABSTRACT FROM AUTHOR]