The spacer of Boiling Water Reactor (BWR) becomes the flow obstacle. Moreover the clearance between fuel rods of the innovative Water Reactor for Flexible fuel cycle (FLWR) becomes very narrow. Thus understanding of the thermal-fluid characteristics in the boiling channel equipped with the flow obstacle becomes more important. In this study, to clarify the flow obstacle effect, the experimental investigation was conducted with the forced convective boiling system. The test section was 8 mm in inner diameter, and the rod-type flow obstacle, which had 3.6 mm in diameter and 20 mm in length, was installed. The blockage ratio β = 20.3% which was the similar value of the present BWR reactor. The heating length LT was taken three different lengths, LT = 810, 840 and 900 mm. As the experimental results, the CHF was increased by the installing of the flow obstacle, and it was strongly influenced by the axial position of the flow obstacle. In the most of the case, the liquid film dryout was detected at the exit of the test section, while the CHF was observed at the upstream of the flow obstacle in the case of the LT = 810 mm. The calculation results of the liquid film flow model have shown a good agreement with these experimental results by using the presented influence length of the flow obstacle.

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