The once-through steam generator which was designed by B & W company was simplified. The flow and heat transfer process in the secondary side under different conditions for the once-through steam generator was numerically simulated based on constant heat flux

and the numerical results were compared with the classic correlations of frictional pressure drop. The results show that Martinelli-Nelson correlation can be used to predict the two-phase frictional pressure drop along with the occurrence of dryout. The frictional pressure drop increases with the increasing steam quality

and the frictional pressure drop gradient obviously increases when the dryout happens. The two-phase frictional pressure drop within the tube increases with the increasing mass flow rate and heat flux

while decreases with the rising operating pressure. The effect of mass flow rate and operating pressure on friction pressure drop is obvious

while the heat flux has little effect on the friction pressure drop.