Study on two-phase flow instabilities in internally-ribbed tubes by using frequency domain method

A new mathematical model was proposed in this paper based on the frequency domain theory to investigate two-phase flow instabilities in internally-ribbed tubes. Two significant improvements were made to conventional models, one of which was that the heat storage of the tube wall metal, the internal wall heat flux and the external wall heat flux were considered as dynamic parameters, and another one was that a new feedback relationship was established. The new model was verified by comparing the prediction results to both the corresponding experimental data in literature and the results obtained with SIMULINK. Then the two-phase flow instabilities in internally-ribbed tubes were studied systematically. It was shown that the stability of two-phase flows increased with pressure, mass flux or inlet resistance coefficient. It was also found that there existed a worst point of the stability (WPS) of the two-phase flows when the inlet fluid subcooling increased. Two-phase flows in the 6-head ribs internally-ribbed tube (ϕ38.1 × 7.5 mm) was more stable than that in the 4-head ribs internally-ribbed tube (ϕ28 × 5.41 mm). The present model and results could provide guidance for the design and operation of boilers.