Vapor fraction measurements in a steam–water tube at up to 15 bar using neutron radiography techniques

Real-time neutron radiography has been used to study the dynamic behavior of two-phase flow and measure the time averaged vapor fraction in a heated metal tube containing boiling steam–water operating at up to 15 bar pressure. The neutron radiographic technique is non-intrusive and requires no special transparent window region. This is the first time this technique has been used in an electrically heated pressurized flow loop. This unique experimental method offers the opportunity to observe and record on videotape, flow patterns and transient behavior of two-phase flow inside opaque containers without disturbing the environment. In this study the test sections consisted of stainless steel tubes with a 1.27 cm outer diameter and wall thicknesses of 0.084 and 0.124 cm. The experiments were carried out at the Pennsylvania State University 1 MW TRIGA reactor facility utilizing a Precise Optics neutron radiography camera. The inlet water temperature to the test section was varied between 120°C and 170°C and the flow rate set to 2.3 l/min. Tsat is 200°C at these conditions. The tube was resistively heated by passing high currents (∼1000 A) through the stainless steel wall. Scattering due to water in the ∼1 cm tube is significant and Monte Carlo calculations simulating the experiment were made to correct for this effect on the vapor fraction measurement. Details of the experimental technique, methods for analyzing the data and the results of the experiments are discussed.