Masanori Aritomi

Geysering in parallel boiling channels

Abstract The authors have been investigating the fundamentals of thermo-hydraulic instabilities which may occur during the start-up in natural circulation BWRs in order to establish a rational start-up procedure and reactor configuration. In this paper, geysering is investigated experimentally in twin parallel channels in both natural and forced circulations under various conditions of heat input, inlet subcooling and non-heated riser length from the upper end of the heated section to the outlet plenum. The relationship between flow reversal and the onset of geysering is studied in particular. It makes clear that geysering in parallel boiling channels in natural circulation is fundamentally identical to that in forced circulation. Next, the effects of a large bubble covering the entire flow cross section and subcooling in the outlet plenum upon the occurrence of geysering are clarified. Finally, differences in behavior of geysering in a single channel as compared with that in parallel channels are discussed.

Approach towards spatial phase reconstruction in transient bubbly flow using a wire-mesh sensor

Abstract A wire-mesh sensor, which is based on local conductivity measurement, has been applied to studies on the characteristics of bubble flow in a rectangular channel ( 20×100 mm 2 ). Special design of the sensor allowed the measurement of the local instantaneous true gas velocity besides the measurement of the local instantaneous void fraction. A review of an already published method for true gas velocity measurement under consideration of the uncertainty caused by limitations in the sampling frequency is presented. A cluster-algorithm is proposed for the evaluation of bubble size distribution and volume flow reconstruction. The validity of this algorithm for spatial field reconstruction was benchmarked by theoretical considerations as well as comparison of the calculated with alternatively measured data. Good agreement was stated. The achieved information was used to obtain plots showing the bubble/slug velocity (up to the second statistical momentum) depending on the spherical-equivalent bubble diameter. This information was measured inside a transient bubble flow with void fraction of up to 20%. Occurring phenomena are explained by presented Fourier spectra of the cross-sectional averaged void fraction and the gas volume flow.

Fundamental Study on Thermo-Hydraulics during Start-Up in Natural Circulation Boiling Water Reactors, (II)

Abstract The authors have been investigating the fundamentals of thermo-hydraulic instabilities which may appear during the start-up in natural circulation BWRs in order to understand their driving mechanisms and to examine the methods preventing their occurrence with an aim of establishing a rational start-up procedure and reactor configuration. In this paper, ‘Natural Circulation Oscillation’ was investigated experimentally under various conditions of heat input, inlet subcooling and non-heated riser length to reveal the driving mechanism and its feature. As a result, it was made clear that the in-phase natural circulation oscillation was induced in parallel channels by hydrostatic head fluctuation in a long vertical non-heated channel such as steam separators and a divided chimney in natural circulation BWRs due to alternate flow of vapor and liquid therein while the vapor generation rate is insufficient.

Analytical study of nuclear-coupled density-wave instability in a natural circulation pressure tube type boiling water reactor

Abstract An analytical model has been developed to study the nuclear-coupled density-wave instability in the Indian advanced heavy water reactor (AHWR) which is a natural circulation pressure tube type boiling water reactor. The model considers a point kinetics model for the neutron dynamics and a lumped parameter model for the fuel thermal dynamics along with the conservation equations of mass, momentum and energy and equation of state for the coolant. In addition, to study the effect of neutron interactions between different parts of the core, the model considers a coupled multipoint kinetics equation in place of simple point kinetics equation. Linear stability theory was applied to reveal the instability of in-phase and out-of-phase modes in the boiling channels of the AHWR. The results indicate that the stability behavior of the reactor is greatly influenced by the void reactivity coefficient, fuel time constant, radial power distribution and channel inlet orificing. The delayed neutrons were found to have a strong influence on the Type I and Type II instabilities observed at low and high channel powers, respectively. Also, it was found that the coupled multipoint kinetics model and the modal point kinetics model predict the same threshold power for out-of-phase instability if the coupling coefficient in the former model is half the eigen value separation between the fundamental and the first harmonic mode in the latter model. Decay ratio maps were predicted considering various operating parameters of the reactor, which are useful for its design.

Instabilities in Parallel Channel of Forced-Convection Boiling Upflow System, (II)

The instabilities observed in a parallel-channel system carrying boiling fluid in forced upward flow have been studied theoretically and experimentally, using water as test fluid. The limits of the stable flow in such a parallel-channel system (the stable boundary) are sought, and the nature of inlet flow oscillation in the unstable region has been examined experimentally under various conditions of inlet velocity, heat flux, liquid temperature, cross section of channel and entrance throttling. The results obtained are compared with calculations based on the mathematical model reported in a companion paper, and good agreement is seen between analysis and experiment in respect of the stable boundary. The experimental results are further examined through phase analysis using the same model.

Development of Pulse Ultrasonic Doppler Method for Flow Rate Measurement in Power Plant Multilines Flow Rate Measurement on Metal Pipe

Ultrasonic Doppler method for a flow metering system has been developed. The method has the capability to obtain instantaneous velocity profiles along the ultrasonic beam. Our purpose is to apply the ultrasonic Doppler method to a flow rate measurement of feed- or recirculation- water in power plants. The principle of the flow measurement method is based on the integration of an instantaneous velocity profile over a pipe diameter. Hence, it is expected to eliminate installation problems such as entry length, also to follow transient flow rate precisely by increasing ultrasonic transducers. In this paper, we report that the errors are less than 1% just below a bend and sudden expansion pipe employing three measuring lines. And then, for constructing a basic system of a flow rate measurement in power plants, a transmission of ultrasound through a metallic wall is investigated, at first. Afterward, since there is no ultrasonic reflectors in the feedwater in power plants, cavitation bubbles are induced as ultrasonic reflectors and the results are appeared that cavitation bubbles are effective when the pipe material is metallic.

Instabilities in Parallel Channel of Forced-Convection Boiling Upflow System, (II): Experimental Results

The instabilities observed in a parallel-channel system carrying boiling fluid in forced upward flow have been studied theoretically and experimentally, using water as test fluid. The limits of the stable flow in such a parallel-channel system (the stable boundary) are sought, and the nature of inlet flow oscillation in the unstable region has been examined experimentally under various conditions of inlet velocity, heat flux, liquid temperature, cross section of channel and entrance throttling. The results obtained are compared with calculations based on the mathematical model reported in a companion paper, and good agreement is seen between analysis and experiment in respect of the stable boundary. The experimental results are further examined through phase analysis using the same model.

Effect of pressure on critical heat flux in uniformly heated vertical annulus under low flow conditions

Critical heat flux (CHF) experiments have been carried out in a wide range of pressure for an internally heated vertical annulus. The experimental conditions covered a range of pressure from 0.57 to 15.01 MPa, mass fluxes of 0 kg m−2 s−1 and from 200 to 650 kg m−2 s−1, and inlet subcoolings from 85 to 413 kJ kg−1. Most of the CHFs were identified to the dryout of the liquid film in the annular-mist flow. For the mass fluxes of 550 and 650 kg m−2 s−1, the CHFs had a maximum value at a pressure of 2–3 MPa, and the pressure at the maximum CHF values had a trend moving toward the pressure at the peak value of pool boiling CHF as the mass flux decreased. The CHF data under a zero mass flux condition indicate that both the effects of pressure and inlet subcooling on the CHF were smaller, compared with those for the CHF with a net water upflow. The Doerffer correlation using the 1995 CHF look-up table and the Bowring correlation show a good prediction capability for the present CHF data.

Experimental study on the boiling phenomena within a narrow gap

Abstract The experiments were carried out with annular narrow gaps having the gap widths 0.2,0.3,0.4,0.5, 1.0 and 1.5 mm for the following two cases: (a) for the “open bottom” case, the heat transfer coefficient was improved as the gap width decreases, but it was not affected by gap lengths in the range 40

Study on the stability behaviour of a natural circulation pressure tube type boiling water reactor

L . (b) for the “closed bottom” case, the heat transfer coefficient was not affected by gap width or length. The transition heat flux could be correlated by the equivalent gap length defined in terms of the cross-sectional area of the open end.