Resistance characteristics of air–water two-phase flow in a rolling 3 × 3 rod bundle

•Pressure drop components in rolling rod bundle are investigated.•Effects of rolling on time-averaged frictional resistance are studied.•Transient fluctuations of frictional resistance in rolling motion are studied.•Effects of rolling on transient resistance are investigated.•New correlation for calculating transient frictional resistance is developed.

Experimental investigation on ambient air–water two-phase flow in a 3 × 3 rod bundle subjected to rolling motion was carried out. Time-averaged and transient frictional resistance in rolling motion were studied in detailed. The results indicate that although time-averaged flow rate and frictional pressure drop in rolling rod bundle are very close to that under steady vertical condition, rolling motion induces obvious transient periodical fluctuation of frictional pressure gradient at low gas and liquid flow rate. While the periodical fluctuations become unconspicuous at high gas and liquid flow rate. In addition, the fluctuation amplitude of relative frictional gradient increases with the increasing of rolling amplitude. However, it is nearly invariable with the rolling period. The traditional correlations which are developed for non-rolling condition give poor predictions for transient frictional gradient when it fluctuates significantly. By taking into account of the influences of rolling parameters, a new correlation applied to calculate the transient frictional resistance in rolling rod bundle is achieved, showing good agreement with experimental results.