Transient flow and heat transfer in helical coil steam generator

The behaviors of flow and heat transfer in helical coil steam generator during the start-up processes were experimentally investigated. The coil diameter and the inner diameter of the tube are 256 mm and 0.011 mm respectively. The ranges of test parameters were as follows: the maximum heat flux was 600 kW/m², the maximum mass flux was 1200 kg/(m²·s), and the maximum pressure was 3.0 MPa. The transient processes, namely the synchro-start-up processes, were investigated in a closed cycle loop with deionized water, which was referred to because the pump load and the heat load were increased synchronously from zero. The influences of different factors on the start-ups were investigated to determine the optimized start-up operation for fast start-up and safety. The experimental results show that the transient single-phase heat transfer is less than that in the stable state because of the weakened enhancement of heat transfer resulting from a secondary flow, and that it can be estimated with the Dittus-Boelter correlation which is used for the straight tube flow. The transient boiling heat transfer can be predicted by correlation for the steady state case. The critical heat transfer phenomena during synchro-start-up processes were analyzed and a criterion for safety evaluations was proposed. It is found that the critical heat flux is much lower than that on the stable condition.