Local Temperature Rise and Boiling Behavior Behind a Central Blockage in a Wire-Wrapped Pin Bundle

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Haga, K.*; M.Uotan*; Yamaguchi, K.*; M.hori*

Haga, K.*; M.Uotan*; Yamaguchi, K.*; M.hori*

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The succeeding series of central-type blockage experiments were completed by the present one with 61-pin bundle test section. The interpretations of the present test results and those of our previous bundle configurations were summarized in the present paper. Concerning the single-phase flow, an empirical formula was derived to estimate the temperature rises behind various central-type blockages: τU/d = 35.3 (D/p), where τU/d is the dimensionless residence time and D/p is the Ring Parameter newly introduced here. This formula was validated to be effective within the region up to x mockup scale (3.0<d<16.1 mm) and was also supported by the data of KfK and ORNL. It was guessed that the apparent peaking factor of temperature rises in the wake region that would be observed in case of bare bundle was about 1.5 and that the effect of three-dimensional distortion due to spacer wires was from 0.9 to 1.2. The estimation of peak temperature rise by the present formula and two kinds of constants led the conclusion that the central-type blockage will not cause local boiling without being detected by an in-core flow-meter. In case of sodium boiling two-phase flow, existence of a large boiling window could be identified through the experimental knowledge on oscillatory boiling and dry-out mechanism: the stable oscillatory behavior of a single bubble (or a cluster of bubbles) was never terminated untill the saturated region being widely built up enough to yeild the one-sided expansion of bubbly region and following clad melting. The oscillation improved the mass exchange between localized bubbly region and surrounding subcooled free stream region in compared with that under non-oscillatory conditions, and often relieved clads from melting.