Technique for measurement of void fraction in stratified flows in horizontal tubes

Significant advances in modeling of intube evaporation and intube condensation have been made by the introduction of flow pattern regime based methods into prediction models. These methods provide much higher accuracy than older methods by using a heat transfer model specific to the particular flow pattern. Thus, a good representation of the two-phase interfacial flow structure, an accurate prediction of the void fraction, and a reliable flow pattern map are basic requirements for their continued development. In response, a new optical technique is being developed to observe and measure dynamic void fractions and two-phase flow structures in stratified types of two-phase flows, which tend to be the most difficult to model. The technique utilizes a laser sheet to illuminate the cross-section of the flow channel (a plain horizontal glass tube) and a CCD video camera to obtain the images through the glass tube wall. A specially developed image analysis subroutine (now nearly fully automatic) transforms and corrects the refraction distorted image to its true shape, thus obtaining the interfacial profile between the liquid and vapor. The technique has been successfully applied to round horizontal tubes of 10.0, 13.6 and 46.0 mm internal diameter. The most recent setup measures void fractions to within about +/-0.015 or less.