Critical heat flux and onset of nucleate boiling tests in a finned rod bundle

Abstract

Critical heat flux (CHF) tests using rod bundles were performed under low-flow conditions to supplement the CHF database for HANARO fuel. The test rod had the same geometric configuration as the HANARO fuel, and its aluminum cladding with fins was made by coextrusion on the stainless steel heating tube. Three types of test sections were used: hexagonal with seven rods, triangular with three rods, and rectangular with four rods. Each test bundle has three spacers axially, and a view window is located in the upper region of the test section. Flow patterns until the CHF condition are typically varied from bubbly flow to annular flow, and then CHF occurs through the long annular flow period. A total of 36 bundle CHF data were obtained from the 3 test sections. The results showed that the CHFs for bundles are larger than those for a single rod with the same geometrical dimension by a maximum 26% as the mass flux changes. It is considered that these results are induced by the enhancement of the turbulence and thermal mixing generated by the spacers. In addition, measurement of the onset of nucleate boiling (ONB) in the rectangular bundle was attempted using sound signals. A hydrophone was attached near the outlet wall of the test section. Hydrophone signals around the ONB point were measured and analyzed based on the frequency through the real fast Fourier transform. Frequency analysis showed clear differences in the power spectral densities for two different frequency ranges before and after ONB, which verifies the usefulness of sound signals for ONB detection.