Instantaneous Void Fraction Research Articles

A New Electrode-mesh Tomograph for Advanced Studies on Bubbly Flow Characteristics.

For studies on the characteristics of bubbly flow in a rectangular channel (20×100mm) a new electrode-mesh tomograph have been applied. The measuring principle is based on local conductivity measurement and a signal concerning. The applied sensor scans the local void fraction distribution in 2 parallel planes, separated 1.5mm in flow direction, with a resolution of 6.1×2.2mm and an overall sampling rate of 1200Hz (all 256 points). Algorithm for the calculation of the local instantaneous void fraction distribution and the true gas velocity are presented. Based on these values the approximate shapes of bubbles have been reconstructed and the gas volume flow through the sensor evaluated. The superficial gas velocity as well as the local distribution of the gas volume flux can be calculated. An extensive sensitivity study illustrating the applicability and accuracy is presented, based on experiment observations as well as theoretical considerations. The evaluated results are compared with high-speed video observations of the flow field as well as data comparing the reconstructed volume flow with measurements by a laminar flow meter. Good agreement can be stated.

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

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.

Void Fraction Measurement in Subcooled-Boiling Flow Using High-Frame-Rate Neutron Radiography

A high-frame-rate neutron radiography (NR) technique was applied to measure the void fraction distribution in forced-convective subcooled-boiling flow. The focus was experimental technique and error estimation of the high-frame-rate NR. The results of void fraction measurement in the boiling flow were described. Measurement errors on instantaneous and time-averaged void fractions were evaluated experimentally and analytically. Measurement errors were within 18 and 2% for instantaneous void fraction (measurement time is 0.89 ms), and time-averaged void fraction, respectively. The void fraction distribution of subcooled boiling was measured using atmospheric-pressure water in rectangular channels with channel width 30 mm, heated length 100 mm, channel gap 3 and 5 mm, inlet water subcooling from 10 to 30 K, and mass velocity ranging from 240 to 2000 kg/(m2·s). One side of the channel was heated homogeneously. Instantaneous void fraction and time-averaged void fraction distribution were measured parametrically. The effects of flow parameters on void fraction were investigated.

Measurement of Boiling Flow by High-Frame-Rate Neutron Radiography. 1st Report. Error Estimation and Void Fraction Measurement.

Void fraction measurement by high-frame-rate neutron radiography and the error estimation were conducted. Measurement error on instantaneous void fraction and time-averaged void fraction of forced-convective subcooled boiling was evaluated experimentally and analytically. Measurement error would be within 18% and 2% for instantaneous void fraction (measurement time is 0.89 ms), and time-averaged void fraction, respectively. Void fraction distribution of subcooled-boiling was measured using atmospheric-pressure water in rectangular channels with channel width 30 mm, heated length 100 mm, channel gap 3 mm and 5 mm, inlet water subcooling from 10 to 30 K and mass velocity ranging from 240 to 2000 kg/(m2·s) . One-side of the channel was heated homogeneously. Instantaneous void fraction and time-averaged void fraction distribution were measured par-

Characteristic Behavior of Bubbles and Slugs in Transient Two-Phase Flow Using Image-Processing Method.

Simulation of transient two-phase flow has been performed by solving transient hydrodynamic equations. However, constitution relations used in this simulation are primarily based on steady-state experimental results. Thus it is important to understand the transient behavior of bubbles and slugs, in particular, transient behavior of the void fraction, the interfacial area and the flow pattern, to confirm the applicabibity of the present simulation method and to advance two-phase flow simulation further. The present study deals with measurement of transient two-phase flow. We have measured local and instantaneous void fractions using imaging techniques, and compared the experimental data with simulation results.

Determination of Time-Dependent Void Fraction Distribution in Bubbly Two-Phase Flow by A Real-Time Neutron Radiography Technique

Radiographic images generated by the real-time neutron radiography system (RTNR) are analyzed by customized image processing software of the determination of instantaneous void fraction distribution. The cross-sectional averaged axial void fraction profiles and the two-dimensional void fraction profiles are determined simultaneously for each image. Various flow regimes are generated to determine if the RTNR system can accurately predict the void distribution in the radial, axial, and temporal coordinates. The results show the RTNR system correctly determines the void fraction distribution for each flow regime; however, accuracy decreases with decreasing void fraction. For net water thicknesses > 1.0 cm, the accuracy decreases with increasing water thickness due to the extreme sensitivity of thermal neutron interactions with light water. Heavy water is a more suitable fluid than light water for void fraction measurements in large-diameter flow systems.

Determination of gas-liquid bubble column instantaneous interfacial area and void fraction by a real-time neutron radiography method

In this work, the axial profiles of a gas-liquid bubble column instantanous interfacial area and void fraction are determined by a RTNR method. The void fraction was also determined independently from liquid level and an ultrasonic transmission method for comparisons. In the sieve plate apparatus, there is no net flow of liquid, and the gas phase is dispersed homogeneously by the plate into bubbles of approximately equal size across a 5.4 cm ID aluminum tube. The results show that the mean bubble diameter increases slightly with increasing superficial gas velocity. The interfacial area increases sharply at low gas flow rates ( U gs⩽0.007 m/s). As the mean bubble diameter becomes almost constant, the interfacial area increases slightly with superficial velocity. Detailed discussion will be given axial profiles and of void fraction and interfacial area.

Void fraction measurements in vertical slug flow: applications to slug characteristics and transition

A conductance probe is used to detect the instantaneous void fraction at the centerline of a vertical tube in upward gas-liquid flow for a wide range of gas flow rates, so that various flow patterns and the transitions between them could be studied. The information was digitized and recorded for further processing, which included short-time-averaging and histograms. The results obtained were used for flow pattern recognition and for the estimation of the liquid slug holdup and its length. Certain conclusions regarding the existing two-phase flow models were drawn.

Studies of gas→liquid (non-Newtonian) slug flow: void fraction meter, void fraction and slug characteristics

Measurements are reported of the void fraction, slug velocity and slug frequency for the cocurrent slug flow of highly viscous non-Newtonian fluids and air. The experimental work was performed in a horizontal tubular test section 2.54 cm in internal diameter and 10.67 m long. Three different polymer solutions were used over wide concentration ranges. The void fraction and slug characterization measurements were carried out with a specially constructed coil void fraction meter operating in a capacitive mode. This system allowed not only the measurement of the instantaneous void fraction but also a recording of the slug voidage trace. Correlations are presented for void fraction, slug velocity and slug frequency, noting in all cases the effects of fluid rheology and apparent viscosity. (1) An investigation of the effects of non-Newtonian liquids on the behaviour of cocurrent gas-liquid slug flow has been undertaken. (2) To allow experimentation with viscous fluids, a new electrically resonating void fraction meter was devised which worked satisfactorily for a wide variety of polymer solutions when it was carefully tuned. (3) Empirical rheological constants were obtained for a wide range of polymer solutions. (4) Slug frequencies, velocities and average void fractions were measured. (5) Liquid slugs travel at approximately the maximum gas velocity even when the liquid becomes quite viscous. (6) The method of Gregory and Scott 16 correlates slug frequency data for each individual polymer system. No generalized correlation has been obtained. (7) The void fraction data can be expressed according to the method of Zuber and Findlay. However, the straight line curves are peculiar to each individual polymer system. (8) Most importantly, the void fraction data are correlated according to the method of Lockhart and Martinelli provided that the liquid phase pressure drop in the parameter X is calculated by allowing for the non-Newtonian characteristics of the liquid phase. This is the most general and useful result of the work.