J.S. Chang

Cross-sectional void fraction distribution measurements in a vertical annulus two-phase flow by high speed X-ray computed tomography and real-time neutron radiography techniques

Abstract A real-time neutron radiography (RTNR) system and a high speed X-ray computed tomography (X-CT) system are compared for measurement of two-phase flow. Each system is used to determine the flow regime, and the void fraction distribution in a vertical annulus flow channel with particular attention on the temporal resolution of the systems and the time behaviour of the two-phase flow. The annulus flow channel is operated as a bubble column and measurements obtained for gas flow rates from 0.0 to 30.0 l/min. Both the RTNR and the X-CT systems show that the two-dimensional void fraction distribution can be obtained. The X-CT system is shown to have a superior temporal resolution capable of resolving the void fraction distribution in an ( r , θ ) plane in 4.0 ms. The RTNR system is shown to obtain void fraction distribution in a ( r , z ) plane in 33.0 ms. Void fraction distribution for bubbly flow and slug flow is determined.

Ultrasonic system for the detection of transient liquid/gas interfaces using the pulse‐echo technique

An ultrasonic method has been chosen to examine liquid/gas interfacial areas in closed systems. Although commercial equipment was used for the transducer and ultrasonic analyzer, commercial transient memory devices were inadequate. Consequently, a real‐time data analysis and transient memory (DATM) unit was designed and constructed. A series of flight times for a succession of 1024 signals are calculated and recorded in the transient memory. The system clock operates at 10 MHz and increments a 12‐bit binary counter. The counter is capable of measuring return signal times as long as 212 μs (4.096 ms) with a 1‐μs resolution. The data are available as analog voltages for display on an oscilloscope or the digital data may be directly transferred to a microcomputer for analysis, display, or archival storage. The DATM device was successfully applied, collecting data for the analysis of rising bubbles in metallurgical systems and two‐phase flow in horizontal tubes.

Void Fraction Measurement by an Ultrasonic Transmission Technique in Bubbly Gas-Liquid Two-Phase Flow

Void fraction measurement by an ultrasonic transmission technique in bubbly gas-liquid two-phase flow is developed. Numerical modelling of the experiment is conducted by using an equivalent bubble method and also by Monte Carlo simulations. Comparison of experimental and numerical results shows agreement for small void fraction conditions (eg < 0.20).

Reflux condensation phenomena in single vertical tubes

Abstract A combined experimental and analytical study of reflux condensation phenomena in single vertical tubes with large 1/D ratios was conducted. From the experimental observations made, phenomenological modelling was conducted by combining an extension of the classical Nusselt theory with a linearized stability analysis of the condensate film flow. A comparison between the model and the corresponding experimental data is presented and discussed. The satisfactory agreement between the model and the data indicates that this model could be used in nuclear reactor accident analysis to estimate heat removal capabilities of U-tube steam generators when, on the tube side, the assumed heat rejection mechanism is total reflux condensation.

Numerical simulation of electric field distributions in electrohydrodynamic two-phase flow regimes

In electrohydrodynamic (EHD) flow boiling and condensation heat transfer applications an interdependence exists between two-phase flow patterns and the applied voltage, and subsequently the electric field distribution established. Unlike single-phase flow, in liquid/gas or phase change processes the electric field established is continuously changing as the flow pattern changes due to interfacial vaporization or condensation. To further complicate the variation in this dynamic field effect, the interaction between the electric field and the fluid introduce an electrical force that can also cause a redistribution of the phases. In an effort to understand and analyze this interaction, the electric field distribution must be determined. To contribute to this effort, the evaluation of the static electric field distribution is performed for various flow regimes to provide a qualitative assessment regarding the direction of phase migration and possible flow pattern transition and,to determine the net EHD force acting on the flow for an instant in time, i.e. for a given phase distribution.

Atomization Characteristics of Electrohydrodynamic Limestone/Water Slurry Spray

Abstract The electrohydrodynamic atomization of limestone-water slurry was experimentally investigated. A double emission atomization mode is observed, which is similar to the fat configuration mode. With increasing the applied voltage, the droplet size and column length decrease, the spread angle, the droplet velocity and the droplet charge-to-mass ratio increase. Experimental results show that the volume-to-surface mean diameter has a minimum value and the droplet size distribution characteristics have two peaks.

The effects of electrohydrodynamic flow on a corona torch

Abstract Experimental investigation has been conducted to study the effect of electrohydrodynamic (EHD) flow on the pressure drop and heat transfer in a corona torch. The results show that strong polarity effects on the EHD flow generated pressure drop in the system. However, the effect of EHD flow decreases with increasing gas flow rate.

An image processing approach for two-phase interfaces visualized by a real time neutron radiography technique

Abstract An image enhancement technique for a visualization of gas–liquid metal two-phase interfaces is developed for a real time neutron radiography (RTNR) technique, where the dynamic motion of bubbles inside liquid metal cannot be observed optically. The proposed image enhancement technique consisted of noise reduction, pipe–fluid interface determination, and image smoothing procedures. The results show that the RTNR technique is able to visualize the dynamics of gas–liquid metal interfaces, and also is able to determine more accurate two-phase flow parameters such as void fraction.

Separated two-phase flow regime parameter measurement by a high speed ultrasonic pulse-echo system

In this work, a high speed ultrasonic multitransducer pulse-echo system using a four transducer method was used for the dynamic characterization of gas-liquid two-phase separated flow regimes. The ultrasonic system consists of an ultrasonic pulse signal generator, multiplexer, 10 MHz (0.64 cm) ultrasonic transducers, and a data acquisition system. Four transducers are mounted on a horizontal 2.1 cm inner diameter circular pipe. The system uses a pulse-echo method sampled every 0.5 ms for a 1 s duration. A peak detection algorithm (the C-scan mode) is developed to extract the location of the gas-liquid interface after signal processing. Using the measured instantaneous location of the gas/liquid interface, two-phase flow interfacial parameters in separated flow regimes are determined such as liquid level and void fraction for stratified wavy and annular flow. The shape of the gas-liquid interface and, hence, the instantaneous and cross-sectional averaged void fraction is also determined. The results show that the high speed ultrasonic pulse-echo system provides accurate results for the determination of the liquid level within +/-1.5%, and the time averaged liquid level measurements performed in the present work agree within +/-10% with the theoretical models. The results also show that the time averaged void fraction measurements for a stratified smooth flow, stratified wavy flow, and annular flow qualitatively agree with the theoretical predictions.

Neutralization of static surface charges by an ac ionizer in a nitrogen and dry air environment

To elucidate the dynamics of static charge elimination, we measured charge decay and residual potential (balance or offset voltage) in gases with various ion mobilities. It was observed that surface charge decay, especially for positive charges, occurs much faster in nitrogen than in air. The residual potential on the probe is negative in pure nitrogen and increases toward positive values with the injection of small quantities of air in front of the ionizer. The fluctuations in the residual potential are generally less than 3 V peak-to-peak. For ionizer operations in nitrogen environments, the charge decay rate increases with superficial gas-flow rate. The results are consistent with a theory of the interaction between bipolar ions and a charged object.