Thermal-hydraulic characteristics and flow instability analysis of an HTGR helical tube steam generator

In High Temperature Gas-cooled Reactor (HTGR or HTR) steam generators, the water in helical tubes is heated unevenly along the tube length by high temperature helium which flows across the helical tube bundles. The flow and heat transfer of water and helium are coupled with each other. This paper presents the analysis of the thermal hydraulic characteristics under steady operating conditions, and two-phase flow instability behavior of HTGR steam generators using numerical simulation methods. Using the present code, the simulation results of the high pressure water forced convective boiling flow in single straight channel support the experimental results. This code can correctly predict the instability boundary. The results of steady state full power operating condition of HTGR steam generator are in agreement with its design value. The pressure drop versus the mass flux will not show a cubic curve after adding a throttle. The Density Wave Oscillation (DWO) instability in the starting process with low pressure, can also be eliminated by adding a throttle. Comparison of the effects of helium heated boundary condition and uniform heat flux boundary condition on flow stability shows that, helium heated boundary condition makes the system more unstable than uniform heat flux boundary condition when other conditions are the same. This is due to the fact that the helium heated boundary condition results in a smaller pressure drop in the preheater section, and a higher pressure drop in the two phase flow region compared with uniform heat flux boundary condition.