Experimental study on natural circulation flow resistance characteristics in a 3 × 3 rod bundle channel under rolling motion conditions

In order to study and analyze natural circulation flow resistance characteristics in a 3 × 3 rod bundle channel under rolling motion conditions, a natural circulation loop was set up and a series of experiments were conducted. The results show that the flow transition Reynolds number for this rod bundle channel in a steady condition is approximately 800. Correlations for rod bundle friction factor and spacer grid local resistance coefficient are fitted. Rolling motion increases the cycle-averaged friction factors when the steady Reynolds number is low. The cycle-averaged local resistance coefficients are generally larger than the steady values. No obvious phase shift is observed between the frictional pressure drop and flow rate and the total pressure drop fluctuation is dominated by the gravity pressure drop. With the inlet fluid subcooling increasing, the absolute pressure drop fluctuation amplitude keeps approximately invariable while the relative fluctuation amplitude increases. Additionally, the relative pressure drop fluctuation amplitude increases with the maximum rolling angular acceleration. The instantaneous friction factor variation rules are dominated by the flow regimes and explained by different variation gradients in different flow regimes. The steady correlation is proved applicative for the instantaneous friction factor calculation in a laminar regime. However, the instantaneous local resistance coefficient fluctuates more drastically than the steady calculation values.