An experimental investigation was conducted to study the cooldown behavior of the Poloidal Field (PF) magnets. A full-scale model of a section of the cooling passages was built for this purpose. LN2, was used as the coolant in the vertical passages at mass fluxes ranging between 42 to 140 Kg/m2-s and heat fluxes from 17000 to 75000 W/m2 with nearly saturated liquid at the inlet. The heat flux was controlled with resistive heaters. Two distinct boiling regimes were observed. The first, Type I consists of film-boiling throughout the passage. Type II has nucleate boiling at the inlet, and is followed, in a short distance, to film-boiling. The equilibrium quality at the burnout point was calculated and was found to increase slightly with decreasing heat flux. The Leidenfrost temperature at which liquid Nitrogen wets the wall was measured to be about 180–200 K. Heat transfer coefficients in the order of 200–500 W/m2-K for film boiling and 5000 to 10000 W/m2-k for nucleate boiling in the type II regime were attained. Higher heat flux and mass flow rates resulted in higher heat transfer coefficients. Empirical correlations were obtained for the nucleate region of the type II regime.

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