Experimental and theoretical investigation on two types of density-wave oscillation in parallel twin narrow rectangular channels under forced circulation

Experimental and theoretical investigation on density-wave oscillation (DWO) was presented in this study. Experiment was performed on flow instability in parallel twin vertical rectangular channels under forced circulation. In the experiment, two types of DWOs were observed at different threshold condition. One type of DWOs occurred at low power with low exit quality, which was defined as Type I instability. The other type was observed at high power with high exit quality, which was defined as Type II instability. The effects of thermal parameters on two types of flow instability were investigated at pressure ranged from 3 to 8 MPa, mass velocity ranged from 200 to 500 kg/m2s, and inlet subcooling ranged from 40 to 140 °C. It was found that both types of DWOs were reduced by the increasing pressure and mass velocity, but the effects of inlet subcooling on Type I and Type II DWO were converse. Oscillation periods of Type I instability in this study were from 1.5 s to 2.5 s, and the period decreased with increasing mass velocity and decreasing inlet subcooling. Stability maps of Type I and Type II instabilities were also obtained, which showed that the unstable zones of Type I and Type II instabilities were discontinuous and the unstable zones of Type I instability were closer to liquid region. Finally, a new mathematical model of flow instability was developed, and the results of prediction for instability boundaries were obtained.