Initiation of water turbulence in concurrent two-phase flow

Abstract

Stratified horizontal two-phase flow with condensation attracted attention in attempting to avoid water hammer problem during reactor LOCA/LOOP (loss of coolant/offsite power) accidents. Less complicated cases of direct steam condensation to stationary water were studied before. However, two-phase flow problem becomes even more difficult due to transfer of steam momentum to interface and its positive feedback to condensation intensity. Despite numerous experimental and analytical studies, the understanding level and modelling capabilities of condensation in two-phase flow are still moderate. Steam side heat and mass delivery to interface are limited by speed of sound. Waterside heat transfer depends on convection intensity and can vary by several orders of magnitude. That is why it is important to perceive the processes, which impede and accelerate the heat removal from interphase to water bulk. Therefore, in order to understand physics of the direct steam condensation better, the efforts are being made to investigate different two-phase flow conditions using modern measurement techniques. Water temperature field was investigated by using infrared camera at different conditions of condensing two-phase flow inside the rectangular short and narrow horizontal channel (1.00 m long, 0.02 m width and 0.10 m height). The velocities of water were 0.014 and 0.056 m/s (1000<Re<2700), while steam flowed at 8 and 12 m/s. As it was expected, increasing steam velocity accelerated the condensation intensity; also, a localized intensification of turbulence was observed. The location of this entire water cross-section penetrating turbulence depends on flow conditions. The possible explanation of this phenomenon is that it may be a consequence of down-flow building-up velocity and temperature gradients in the water. That means less viscous and faster flow of near surface water layer. It facilitates the surface renewal and accelerates condensation, and the local steam velocity near the interface becomes higher. These effects have positive feedback on each other, and turbulence spreads to water bulk.