Flow Regimes and Transition Criteria for Vertical Downward Two-Phase Flow in a Narrow Rectangular Channel

Abstract

Narrow rectangular channel is employed in nuclear research reactors using plate type nuclear fuels, high heat-flux compact heat exchangers, and high performance micro-electronics cooling system. It is important to study the hydrodynamics of co-current air-water two-phase flow in a vertical narrow rectangular channel since it is considerably different than in round tubes. Experiments have been carried out inside a narrow rectangular channel of 2.35 mm gap width at ambient temperature and pressure conditions and flow regime maps for vertical downward flow have been plotted based on measured data sets. Flow regimes were identified and classified based on visual observation and void fraction data. Flow visualization was done using a high speed camera, while void fraction analysis was done using electrical impedance method and digital image analysis. Seven different flow regimes were identified. Mechanistic models were developed for various flow regime transition criteria in co-current air-water two-phase flow for vertical downward flow inside a narrow rectangular channel. The newly developed criteria were compared to flow regime map of downward air-water two-phase flow inside a narrow rectangular channel. Overall, the present model showed good agreement with the experimental data.