Bau-Shei Pei

An Improved Theoretical Critical Heat Flux Model for Low-Quality Flow

Based on the Helmholtz instability at the microlayer/vapor interface as a trigger condition for microlayer dryout, a previous study developed a mechanistic critical heat flux (CHF) model for subcooled flow boiling. An improved CHF model is implemented with more solid theoretical bases for subcooled and low-quality flow boiling under pressurized water reactor conditions. Comparisons between the predictions and experimental data show that the present model is more accurate than the well-known theoretical CHF model and empirical CHF correlations for water flowing through uniformly heated round tubes within the applicable ranges. The applicability of the present model to rod bundles is also under investigation. High satisfactory results are obtained from the comparisons of predicted to observed bundle critical power.

Two Simplified Methods to Determine Void Fractions for Two-Phase Flow

The void fraction is one of the most important quantities in experimental studies of two-phase flow. In this paper two simple and economical techniques to determine this quantity are developed and discussed. The improved impedance method, in which a high-frequency processing circuit is developed to measure and amplify the voltage changes between the electrodes, is the first method. The differential pressure (D/P) method, in which a commercial differential pressure transmitter is used to determine the static pressure of two-phase flow, is the second method. Experiments including tests in vertical and horizontal pipes for the impedance method and a vertical pipe test for the D/P method have been performed. Furthermore, theoretical models of these two techniques are developed. The test results show that most of the measured void fractions are within a 20% error band compared with the actual void fraction.

Verification of using a single void fraction sensor to identify two-phase flow patterns

This paper investigates methods using the signals detected by a single void fraction sensor to identify four kinds of typical vertical, cocurrent, upward, two-phase tube flow patterns. By analyzing 100 sets of time-varying void fraction signals acquired from an impedance device in an air-water two-phase loop, the results of the various methods are evaluated and demonstrated. With the high-frequency contribution fraction (HFCF) criteria, the success rate is 81%. An auxiliary criterion (the void fraction criterion) is proposed to increase the success rate to 92%. The results and the criteria from this study are compared with earlier studies. From the comparison, the applicability of the HFCF criterion to a system in which void fraction can be measured directly is verified.

A Model for Sound Velocity in a Two-Phase Air-Water Bubbly Flow

In this paper, wave propagation in a homogeneous, low void fraction, two-phase air-water bubbly flow is analyzed through the compressibility of a single bubble to derive a P({rho}) relation; the dispersion relation is then derived by a homogeneous model. The phase velocity and attenuation calculated from the model are compared with existing data and are in good agreement. The momentum transfer effect is considered through the virtual mass term and is significant at a higher void fraction. The interfacial heat transfer between phases is significant at low frequency, while bubble scattering effects are important at high frequency (near resonance). Bubble behavior at both low and high frequency is derived based on the isothermal and the adiabatic cases, respectively. The phase velocity occurs at the limiting condition in both cases. Furthermore, resonance is present in the model, and the resonant frequency is determined.

Identification of Two-Phase Flow Patterns by a Single Void Fraction Sensor

A wide range of combinations of gas and liquid flow rates that form various flow patterns are investigated. By analyzing the signal spectra detected by a single sensor using light techniques, the criteria for identifying two-phase flow patterns are proposed. By applying these criteria with only one parameter, the high-frequency contribution fraction (HFCF), the reasonable identifying performance is 76% when churn flow is counted and 88% when churn flow is not counted. When ..cap alpha..-bar is added as an auxiliary to HFCF, the identifying performance can be increased to 83 and 96%, depending on whether churn flow is counted. Both parameters can be acquired by signals from a single void fraction sensor. The criteria are expected to apply to other void fraction measurable systems for identifying two-phase flow patterns.

Sound attenuation and its relationship with interfacial area density in an air-water two-phase bubbly flow

Abstract A simple model is developed for predicting the dispersion and attenuation of sound in a homogeneous low void fraction air—water two-phase bubbly flow. The relationship between interfacial area density and sound attenuation is derived for dimensionless frequency, Ω (= ωR 0 2 /α l ) in the range 2000–8000. The interfacial area density is shown to be proportional to the square of the sound attenuation. Measurements of sound attenuation are performed for standing waves in a vertical waveguide containing an air—water bubbly mixture. The model of sound attenuation is compared with present and previous experimental data, and with previous theories. The interfacial area density predicted by the present model is compared with experimental data; the range of void fraction being 0.5–10% and the range of bubble radius being 0.6–2.2 mm.

Identification of Two-Phase Flow Regimes by Neutron Noise Analysis

The identification of two-phase flow patterns in a nuclear reactor core is important to the design and operation of a boiling water reactor (BWR). Basically, two-phase flow shows some fluctuating characteristics even at steady-state conditions. These fluctuating characteristics can be analyzed by statistical methods for obtaining flow signatures. There have been a number of experimental studies conducted that a concerned with the statistical properties of void fraction of neutron noise in experimental reactors or BWRs. In this study, the authors propose a new technique of identifying the patterns of air/water two-phase flow in the core of the zero power reactor of the Institute of Nuclear Energy Research. This technique is based on analyzing the statistical characteristics of the neutron noise signals from the in-pile test loop by time-series modeling. Although only limited two-phase flow conditions have been studied so far in this investigation. The results justify application of this technique to wider flow conditions and more complicated duct geometries.

A model for sound velocity and its relationship with interfacial area in a steam-water, two-phase bubbly flow

Wave propagation of low-void-fraction, two-phase bubbly flow is analyzed from the compressibility of a bubble and an assumption of homogeneity for steam bubbles. The phase velocity and attenuation calculated from the model are strongly dependent on frequency. The equilibrium sound velocity is derived for the limiting case as frequency approaches zero. The second resonance of a vapor bubble is also discussed. In addition, the void fraction, bubble size, and noncondensable gas effects are also analyzed in the model. In this paper the relationship between sound velocity and interfacial area density is derived, and a new method to measure interfacial area is proposed.

On-line measurement of low void fraction and mass flowrate with a side-tube and an orifice in two-phase flow

Abstract The development of practical and simple on-line measurement of two-phase mass flowrate is of prime interest to applied nuclear reactor safety research. Experiments were performed at a system pressure of 0.1 MPa. The highest value of the mass flowrate for air-water was 1233 kg m -2 s -1 , while the void fraction range was 0.1∼0.75, which was equivalent to 0.0004∼0.0023 for air quality X. For steam-water mixtures, where the mass flowrate was up to 1662 kg m -2 s -1 , the results showed good agreement, as did those for air-water two-phase flow. The measuring techniques are described briefly and a slip ratio associated with the drift-flux model is proposed to correlate the mass flowrate model.

Identification of two-phase flow patterns in a nuclear reactor by high-frequency contribution fraction

A method based on noise analysis techniques that can be applied to the identification of two-phase flow patterns in nuclear reactors is proposed. The identifying criterion, the high-frequency contribution fraction (HFCF), offers new potential to the in-core recognition of two-phase flow patterns. By analyzing 76 sets of signals acquired from a research nuclear reactor where two-phase flow patterns are generated in an in-core air/water loop, the typical signal, autocorrelogram, and spectrum of each flow pattern are demonstrated and evaluated. The identification success rate is 87 or 93%, depending on whether churn flow is counted. A method to improve the identification rate is also presented. This study demonstrates that the fluctuation characteristics above 10 Hz are induced by two-phase flow itself and are independent of the driving source; thus, it is adequate to apply the HFCF to the identification of two-phase flow patters. This study shows that it is possible to identify two-phase flow patterns by HFCF values.