Analysis Of Density Wave Oscillations In Helically Coiled Tube Once-Through Steam Generator

Helically coiled tube Once-Through Steam Generator(H-OTSG)is widely adopted in advanced small modular reactors(SMR)and other industrial heat exchange equipment for its compactness,enhanced heat transfer efficiency and capability of accommodating thermal expansion.Under certain working conditions,two phase flow instability that endangers safety of plants can occur in the secondary side of H-OTSG.To ensure safety of reactors,it is essential to analyze flow instability phenomenon in H-OTSG and avoid its occurrence.The majority of current codes to investigate flow instabilities in steam generators focus on straight tubes or U-tubes.These codes are not applicable to H-OTSG.Uniform heat flux boundary is usually employed in the calculation of these codes and the non-uniform heat flux boundary of the real heat transfer process in steam generators tends to be ignored.To investigate flow instabilities in H-OTSG more precisely,code dedicated to analyze flow instabilities in helical tubes needs to be developed.A special Fortran code to analyze density wave oscillations(DWO)in H-OTSG was developed based on STEAMFREQ which adopts single variable linear frequency domain methods to predict DWO in straight tubes.The mathematic models in the code,including the heat transfer and pressure drop models were modified for helical geometry.In the development process,modules to analyze helical geometry were added,the heat transfer medium was replaced and the calculation methods were optimized.The capability of STEAMFREQ-H on steady state calculation were verified by comparing to the design data of SMART.Steady state calculation and flow instability analysis were carried out with STEAMFREQ-H,focusing on a typical H-OTSG in SMR under development in China at the present.Parametric effects on DWO in the H-OTSG were analyzed based on different Nyquist curves,main influence factors of DWO were classified and solutions to eliminate DWO in the H-OTSG were proposed.To verify the results of STEAMFREQ-H,RELAP5/MOD 3.2 code was employed to simulate DWO phenomenon of H-OTSG in time domain.By comparing results of STEAMFREQ-H and RELAP5/MOD 3.2,the frequency domain code was verified and the capability of RELAP5/MOD 3.2 to simulate helical tube was evaluated.