Self-compensating characteristic of steam–water mixture at low mass velocity in vertical upward parallel internally ribbed tubes

This paper presents an experimental investigation on Self-Compensating Characteristic (SCC) in vertical upward parallel tubes with low mass velocity of steam–water two-phase mixture. A physical model was built up using parallel internally ribbed tubes. A method called Differential Pressure Substitute was used to measure two-phase flow parameters. The results indicated that the SCC of vertical upward parallel tubes is caused by combined action of frictional pressure drop and gravitational pressure drop. The mass velocity in the tube with lower heat flux decreases first, and then increases with an increase in quality. The uneven heat fluxes among tubes are the main reasons that cause mass velocity differentials. Greater uneven heating ratio enhances the SCC in low quality region and weakens it in high quality region. The SCC has different variation rules in different pressure region. In the sub-critical pressure region, rising pressure weakens the SCC when quality is low and enhances it when quality is high. In near-critical pressure region, the mass velocity varies monotonically and slowly with the increase in quality because the difference between water and steam is minor in this pressure region. The results provide some instructive advices to improve the design and operation safety of once-through boiler.