Hydrodynamics of a natural circulation loop in a scaled-down steam drum-riser-downcomer assembly

•Experimental investigation of loop hydrodynamics in a scaled-down simulated AHWR.•Identification of flow regimes and transition analyzing conductance probe signal.•Downcomer flow maximizes with fully developed churn flow and lowest for bubbly flow.•Highest downcomer flow rate is achieved with identical air supply to both risers.•Interaction of varying flow patterns reduces downcomer flow for unequal operation. Complex interactions of different phases, widely varying frictional characteristics of different flow regimes and the involvement of multiple scales of transport make the modelling of a two-phase natural circulation loop (NCL) exceedingly difficult. The knowledge base about the dependency of downcomer flow rate on riser-side flow patterns, particularly for systems with multiple parallel channels is barely developed, necessitating the need for detailed experimentation. The present study focuses on developing a scaled-down test facility relevant to the Advanced Heavy Water Reactor conceived in the atomic energy programme of India to study the hydrodynamics of the NCL using air and water as test fluids. An experimental facility with two risers, one downcomer and a phase-separating drum was fabricated. Conductivity probes and photographic techniques are used to characterize the two phase flow. Normalized voltage signals obtained from the amplified output of conductivity probes and their subsequent analysis through probability distribution function reveal the presence of different two-phase flow patterns in the riser tubes. With the increase in air supply per riser void fraction in the two-phase mixture increases and gradually flow patterns transform from bubbly to fully developed annular through slug, churn and dispersed annular flow regimes. Downcomer flow rate increases rapidly with air supply till a maximum and then starts decreasing due to enhanced frictional forces. However, the maximum value of downcomer water flow for any magnitude of total air flow can be obtained only for identical air supply through both risers. Any disparity in air flow through the risers has been found to cause as much as 2% reduction in downcomer flow rate owing to the interaction of varying flow patterns through the two riser tubes.