Experimental investigation of two phase turbulent mixing rate under bubbly flow regime in simulated subchannels of a natural circulation pressure tube type BWR

In this paper, experiments were carried out to determine the two phase turbulent mixing rate under bubbly flow regime in the simulated subchannels of the reactor. The size of rod and the pitch in the test was same as that of actual rod bundle in the prototype. Three subchannels are considered in 1/12th of the cross section of the rod bundle. Water and air was used as the working fluid and the turbulent mixing tests were carried out at atmospheric condition without heat addition. The void fraction was varied from 0 to 0.3 under various range of superficial liquid velocity. Turbulent mixing rate was experimentally determined by adding tracer fluid in one subchannel and measuring the concentration of that in other subchannels at the end of the flow path. The test data were compared with existing models in literature. It was found that existing models could predict the measured turbulent mixing rate in the rod bundle of reactor within range (average error) of ±66%.