Flow And Heat Transfer Characteristics Numerical Simulation On Once-through Steam Generator With Straight Tube

Compared with natural circulation steam generator,once-through steam generator(OTSG)directly generates superheated steam,which effectively improved the thermal efficiency of steam turbine and received extensive attention from researchers.The subcooled feed water of secondary loop flows upward around the tubes and it's heated by primary coolant to superheat steam,phase change,dryout and a large rise in wall temperature phenomenon that occur in this process,which make the study of OTSG become more difficult.Dryout is a phenomenon which boiling crisis occurs,subsequently a failure or burnout of heat exchangers.Accurately predict the flow and heat transfer characteristics is major important for assuring the safe operation of nuclear reactors.The straight pipe type once-through steam generator of B&W company was studied,a three-dimensional secondary side unit cell model and coupled heat transfer and flow between primary and secondary sides unit cell model are respectively established under the guidance of the similarity principle.Nucleate boiling region is computed by two-fluid model and two-fluid three field model is used to describe dryout region.Numerical simulation was implemented on two-phase flow and boiling heat transfer characteristics of the secondary side and coupled heat transfer and flow between primary and secondary sides in OTSG based on FLUENT software.The distribution of key parameters of OTSG are obtained,and the heat transfer characteristics of nucleate boiling and dryout region are also studied.The results suggest that heat transfer coefficient of every heat transfer region is in quantitatively good agreement with the empirical correlations of secondary side.The change tendency of internal and external wall temperature is similar,and the inner wall temperature is higher than the outer wall temperature because of the different fluid contact.The occurring of dryout phenomenon results in wall temperature experiences a sudden rise at around 7.3m downstream from inlet,the error of dryout location is 2.9% between numerical simulation and thermal calculation.At the dryout location,the liquid film disappears and no fluid is in contact with the wall and consequently the heat transfer coefficient drops dramatically which result in large rise in the wall temperature.The jump margin of inner and outer wall temperature is 12.5? and 20.5?,respectively.The increasing of average wall temperature is 16.5?,this value is reasonably good agreement with operating date.The simulation results also reveal that wall temperature oscillation phenomenon occurs at dryout position,It's the liquid film and vapor alternately contact with the wall and the two-phase flow pulsation phenomenon which cause wall temperature oscillation.The distribution of inner and outer wall heat flux is reasonable,and both of them get the maximum at dryout location,the heat flux at this point is critical heat flux.The numerical simulation results can provide technical support for structural design and safe operation of OTSG.