(A) study on the critical heat flux experiments and subchannel analysis model in a finned rod bundle

Abstract

CHF tests using several finned rod bundles were performed at low flow and low pressure conditions to widen the CHF data base from low to high flow conditions. Three types of test bundles were used; a hexagonal one with seven rods, a triangular one with three rods and a rectangular one with four rods. A total of 36 bundle CHF data were obtained from the three finned rod bundles. It was confirmed by the observation that the low flow CHF occurred in an annular flow region with a high heat flux of the liquid film dryout in the top end region of the test rod. The parametric trends of the CHF data for the mass flux and the inlet subcooling in the finned rod bundles agreed with the general understandings for the single finned rod or tube geometry. The bundle effects on the CHF were evaluated by a comparison with the single rod test results at low flow conditions. There are no differences between the bundle and single rod CHF data in the very low flow regions below a mass flux of 100kg/㎡/s. But there are clear distinctions over a mass flux of 100kg/㎡/s with an average difference of 21% and a maximum of 26%. The bundle CHF data and characteristics could be used importantly in the case of designing the research reactor with the finned rod bundle. In the experiment where the growth of the void inside the rod bundle is not visible, it may be difficult to detect the accurate ONB point. Hence, the ONB measurement in a rod bundle was tried by using a sound signal of a hydrophone. The hydrophone to measure the variation of the sound signal under water was inserted into the outlet region of the rectangular test section. Hydrophone signals were measured at slightly different power levels around the ONB point and then a frequency analyses were conducted. The three ONB measurements were performed in a range of a mass flux of 100 ~ 250kg/㎡/s. Frequency analyses of the hydrophone signals around the ONB point showed clear distinctions in the power spectral densities for two different...