Analysis of single phase flow instability in U-tubes of steam generator

Based on the one-dimensional thermal hydraulic model for U-tubes under natural circulation, the flow characteristic of the single phase fluid in the parallel U-tubes is studied. The critical pressure drop (CPD) and critical mass flow rate (CMFR) are also given and analyzed, which relate to the occurrence of reverse flow in U-tubes and are influenced by the inlet temperature. The U-tubes in marine and commercial steam generators are chosen to investigate the effect of inlet temperature on the flow instability. It is found that (1), the CMFR increases and the CPD decreases with the increase of the inlet temperature, (2) the inlet temperature has great influence on the space distribution of reverse flow in the marine steam generator with the small size, but it has little influence on that in the commercial steam generator with the large size.