Evaluation of subcooled void fraction in downward flow under low mass flux condition by using neutron radiography

In downward flow, complex two-phase flow is caused by buoyancy of bubbles. It affects not only a void fraction profile but also a point of net vapor generation (PNVG). In order to understand the fundamental characteristics of the void fraction on subcooled boiling in downward flow, the void fraction was measured by using neutron radiography. The test sections were made of stainless steel tubes that had 400 mm in heated length with 10 and 15 mm in inner diameters. They were heated by direct Joule heating applying D.C. power. The void fraction characteristics were explained by dividing into three regions: i.e., a wall voidage region, a detached voidage region, and a bulk voidage region. PNVG was defined as the boundary between the detached voidage region and the bulk voidage region. In a downward flow, the thermal equilibrium quality at PNVG is smaller than in an upward flow. "Secondary PNVG" was also observed which was caused by the influence of a reverse flow of bubbles. A difference of thermal equilibrium quality between at primary PNVG and at secondary PNVG is lager in 15 mm in inner diameter than in 10 mm. The void fraction profile and PNVG characteristics in downward flow show dependence of inner diameter as well as mass flux. To demonstrate the void fraction profile involving with the influence of a reverse flow in a downward flow, an empirical void fraction correlation based on a drift-flux model with Lahey's flow quality model was developed. This correlation well agreed with the experimental results.