J. L. H. Faccini

An Advanced Ultrasonic Technique for Flow and Void Fraction Measurements of Two-Phase Flow

In this paper, we present a hybrid type contrapropagating transmission ultrasonic technique (CPTU) for flow and time averaging ultrasonic transmission intensity void fraction measurements (TATIU) of air-water two-phase flow, which is tested in the new two-phase flow test section mounted recently onto an existing single phase flow rig at the Nuclear Engineering Institute (IEN)/CNEN, Brazil. The circular pipe test section is made of 51.2 mm stainless steel, followed by a transparent extruded acrylic pipe aimed at flow visualization. The two-phase flow rig operates in several flow regimes: bubbly, smooth stratified, wavy stratified and slug flow. The observed flow patterns are compared with the Mandhane et al.’s experimental and Lightstone et al.’s numerical flow regime map for horizontal two phase flows. These flow patterns will be identified by time averaging transmission intensity ultrasonic techniques which have been developed to meet this particular application. A contrapropagating transmission ultrasonic flowmeter is used to measure the flow rate of liquid phase. A pulse-echo TATIU ultrasonic technique used to measure the void fraction of the horizontal test section assembling at IEN is presented. Other flow parameters can be deduced by processing the signals obtained by the CPTU ultrasonic flowmeter and the pulse-echo generator-receiver (TATIU system).Copyright

Numerical Modelling of Stratified Gas-Liquid Flow in Inclined Circular Pipes

In this paper, a fully developed stratified gas-liquid flow in inclined circular pipes is numerically modeled. The model is applied on a stratified gas-liquid downward flow with smooth and horizontal interface, in pipes with inclination angles varying from 0 to −10 degrees. A system of non-linear differential equations, consisting of the Reynolds averaged Navier-Stokes equations with the κ – ω turbulence model, are solved by using an inner iteration loop based on the Newton-Raphson scheme and the finite element method. Numerical solutions are obtained for the liquid height and pressure gradient which were compared with experimental and numerical data. An excellent agreement with the experimental data was obtained, leading to conclusion that the present model is adequate to simulate the stratified gas-liquid downward flow, and it can be used to estimate the flow parameters such as the liquid height and pressure gradient.© 2009 ASME

Experimental and Numerical Investigation of Stratified Gas–Liquid Flow in Downward-Inclined Pipes

This paper reports an experimental and numerical investigation of stratified gas–liquid two-phase flow in downward-inclined circular pipes. Reynolds averaged Navier–Stokes equations with the κ–ω turbulence model were solved by using the least-square finite-element method to simulate the stratified gas–liquid flow. Experiments were carried out in an air–water two-phase flow loop with a test section of 7.8-m-long circular pipe with 1 inch inner diameter for 3 downward-inclined angles, −2.5°, −5.0°, and −10.0°. The height of the liquid layer was measured by using a pulse–echo ultrasonic technique with a single fast transducer and a visualization technique with a high-speed digital camera. Numerical results for the liquid height and hold-up as a function of inclination angles were compared favorably with experimental results of the present study and literature data.

Experimental Measurement of Interfacial Parameters of Elongated Bubbles in Two Phase Gas-Liquid Flow in Horizontal and Slightly Inclined Circular Pipes

This paper reports an experimental study of the measurement of elongated bubbles velocities and their longitudinal shapes using a high speed ultrasonic system in concurrent horizontal and at 5° and 10° inclined upward flow. The circular pipe test section is made of 25.6 mm stainless steel, followed by a transparent acrylic pipe with the same diameter. The high speed ultrasonic system consists of two transducers (10 MHz/6.35 mm diameter), a generator/multiplexer board that convert analog signals into digital data at a rate of 100 million frames per second, and a software that stores all the frames and the results of the time of flight of each signal. The results are compared with a visualization technique that consists of a high-speed digital camera recording images at rates of 125 and 250 frames per second. This range of liquid superficial velocity is from 0.2 to 1.1 m/s and that of the gas superficial velocity is from 0.35 to 1.0 m/s. The results obtained with the two experimental techniques show a good agreement among them for the elongated bubbles lengths and velocities, while having great statistics dispersion. The measured bubble shape is in agreement with literature data.Copyright

Measurement of Natural Circulation Two-Phase Flow by Hybrid Doppler Contrapropagating and Pulse Echo Ultrasonic Techniques

In this work, a hybrid ultrasonic technique for diagnostic natural circulation gas-liquid two-phase flow is experimentally investigated. The method is applied on a two-phase natural circulation loop using a contrapropagating transmission flow rate measurement technique in the single phase section and a Doppler velocity measurement and single transducer pulse echo technique in the two-phase region. The two-phase natural circulation loop used in this work consists of a two-phase vertical section where the liquid has an induced flow rate by an air-lift pumping technique. Gas is injected into the lower region of the two-phase test section, and rises to a separation tank causing the liquid to circulate naturally about the loop. The liquid slug velocities, the air flow rates, and the liquid flow rates are measured, respectively, in the two-phase and single-phase section of the loop as well as two-phase interfaces and time averaged void fraction. A comparison with available results from other investigators is given, in terms of liquid superficial velocity and bubble velocity for two flow patterns generated in the loop; bubbly flow and slug flow. The results show that the present hybrid system can be used to identify not only flow regime but also key two-phase flow parameters for natural circulation conditions.Copyright

Numerical and Experimental Investigation of Stratified Gas-Liquid Two-Phase Flow in Horizontal Circular Pipes

This paper reports numerical and experimental investigation of stratified gas-liquid two-phase flow in horizontal circular pipes. The Reynolds averaged Navier Stokes equations (RANS) with the k-{omega} model for a fully developed stratified gas-liquid two-phase flow are solved by using the finite element method. A smooth and horizontal interface surface is assumed without considering the interfacial waves. The continuity of the shear stress across the interface is enforced with the continuity of the velocity being automatically satisfied by the variational formulation. For each given interface position and longitudinal pressure gradient, an inner iteration loop runs to solve the nonlinear equations. The Newton-Raphson scheme is used to solve the transcendental equations by an outer iteration to determine the interface position and pressure gradient for a given pair of volumetric flow rates. The interface position in a 51.2 mm ID circular pipe was measured experimentally by the ultrasonic pulse-echo technique. The numerical results were also compared with experimental results in a 21 mm ID circular pipe reported by Masala [1]. The good agreement between the numerical and experimental results indicates that the k-{omega} model can be applied for the numerical simulation of stratified gas-liquid two-phase flow. (authors)

Development of a Nuclear Reactor Two-Phase Natural Circulation CFD Model

A circuit of natural convection similar to the passive reactor heat removal systems of the AP600 reactor and of the APEX experiment was constructed in the Nuclear Engineering Institute located in Rio de Janeiro. In an attempt to improve usual numeric methods, it was developed a new Computational Fluid Dynamics (CFD) scheme to solve one-dimensional (1D) two-phase natural circulation transport equations. To compare the new scheme with other numeric methods, it was used the results of the two-phase experiment conducted at the University of Sao Paulo (USP). It was compared the frequency and the amplitude of the temperature oscillations at several positions of the spatial domain during a two-phase heating transient. The results of the new method show a better agreement with the experiment than that obtained from the RELAP and the TRAC series of computer programs.Copyright