Measurement of dynamic void fractions in stratified types of flow

Abstract

A new non-intrusive computerized image analysis and optical observation method for accurately measuring dynamic void fraction has recently been developed by Thome and coworkers [Dynamic void fractions in stratified types of flow, part I: new optical measurement technique, Int. J. Multiphase Flow 31 (2004); Dynamic void fractions in stratified types of flow part II: measurements for R-22 and R-410a, Int. J. Multiphase Flow 31 (2004)]. This technique is applied to circular horizontal sight glass tubes using a monochromatic laser sheet to illuminate the two-phase flow coupled with image processing to measure cross-sectional void fractions and dry angles in stratified types of flow. The refraction effects on the cross-sectional images are overcome by reconstructing the video images by computer. From these, the shape at the vapor–liquid interface is detected and the void fraction is accurately determined (to an estimated accuracy of about ±0.01) over a wide range of void fraction values (from 0.05 to 0.95). In addition, the dry angle around the upper perimeter of the tube is also obtained. The system has been coupled to a flow boiling test facility to obtain dynamic and time-averaged void fractions in a horizontal tube for two refrigerants: R-22 and R-410A. About 227,000 images were analysed so far in this study for a 13.6 mm tube to provide the same number of dynamic void fraction measurements and 238 time-averaged void fraction values. A summary of the technique is described here with additional analysis of the previous results and new results (based on processing 83,000 new images) are presented here for void fractions in an 8.0 mm sightglass tube with R-22 at mass velocities of 100 and 150 kg/(m 2 s). The new method also has the potential to measure interfacial wave contours and other pertinent geometrical characteristics of stratified two-phase flows.