Experimental study on the resistance characteristics of the rod bundle channel with spacer grid under low-frequency pulsating flows

Abstract The spacer grid is widely used with the fuel rod buddle to stabilize the fuel assembly structure and reduce the vibration effect in the pressurized water reactor (PWR). To study the flow resistance characteristics of the working fluid under periodic fluctuation of the floating nuclear reactor (FNR), an experimental study of the pulsating flow resistance characteristics of a 5 × 5 fuel rod bundle (P/D = 1.326 and W/D = 1.268) was carried out. The ranges of the cycle-averaged Reynolds number Re ta  = 0.8 × 10 3 –9 × 10 3 , pulsating amplitude A u  = 0.2–0.8 and dimensionless frequency ω ′  = 1.26–2.81 were covered in this experimental study. The experimental results indicated that the flow rate lags behind the pressure drop in the fuel rod bundle channel, and the phase lags increase with the increase of ω ′ but change slightly with A u and Re ta . The instantaneous friction factor and local resistance coefficient are greater than the steady-state value at the end of deceleration and the whole acceleration stage in the pulsating flow cycle, and smaller than the steady-state value at the beginning of the deceleration stage. The present study compared the flow resistance of the accelerating and decelerating half-cycle with the steady-state value and found that the speed of the accelerating half-cycle increases gradually with the increase of A u , but the speed of the decelerating half-cycle flow resistance decreases gradually with the increase of A u . In addition, the obtained results have also indicated that the local resistance coefficient of the spacer grid is closer to the steady-state value than the friction factor in the accelerating and decelerating half-cycle. During the pulsating flow, the cycle-average friction factor and local resistance coefficient in the whole cycle are larger than the steady-state value under the corresponding Reynolds number. The normalization parameters of C f  =  λ  ta / λ st and C sg  = K ta / K st are proposed to evaluate the effect of pulsating flow on the cycle-averaged flow resistance. C f and C sg increase with the increase of A u and ω ′ , and decrease with the increase of Re ta . In addition, it is found that C sg is smaller than C f under the same pulsation parameters, which it means that the influence of fluid fluctuation on the resistance of the spacer grid is relatively small. Finally, the new correlations for predicting λ ta and K ta in the pulsating flow are established through dimensional analysis, and the prediction result is reasonably good.