Numerical Simulation Of Dryout And Post-Dryout Heat Transfer In The Straight-pipe Once-through Steam Generator

With the increasing demands of PWR nuclear power system on the efficiency and volume of the steam generator.the once-through steam generator has been widely considered for its high efficiency,compactness and modularization.The secondary fluid of the once-through steam generator undergoes the different thermodynamic states,where the complex flow and heat transfer process is from subcooled water heat convection to superheated steam heat convection,inevitably causing a flow boiling crisis-dryout deterioration phenomenon.The occurrence of dryout causes the heat transfer coefficient to decrease sharply and the wall temperature to increase sharply,which may lead to stress corrosion and aging failure of the tube bundle.The present researches mainly focus on dryout in the straight tubes heated by constant heat fluxes,and there still lack the predictions for dryout in the actual once-through steam generator.Therefore,it is of great significance to study dryout and post-dryout heat transfer characteristics of the once-through steam generator.The straight-pipe once-through steam generator designed by Babcock&Wilcox company is used as the prototype,and the simplified three-dimensional unit-pipe physical model is established based on the approximated modular method.The mathematical model,which can be used to accurately describe the single-phase heat convection,heat conduction of the tube bundle,secondary liquid-steam flow and heat transfer behavior of the heat transfer regions including nucleate boiling region,liquid film forced heat convection region,and liquid deficient region,has been used,and the closure correlations of mass,momentum and energy transfers between the flow-fields and the interactions between the flow-field and the wall have been supplemented,to conduct the following research works:Aiming at dryout and post-dryout heat transfer characteristics of the actual straight-pipe once-through steam generator,the numerical simulations are conducted based on a fitting heat flux and a coupled heat transfer boundary between primary and secondary sides,respectively.It is found that although the liquid droplets still remain in a saturation state in the liquid deficient region,the steam has absorbed the heat through heat convection with the wall and begins to be in a superheated state,resulting in a deviation from thermodynamic equilibrium phenomenon.The actual steam quality rises with a smaller gradient compared with that in the liquid film forced heat convection region.The soaring amplitude of wall temperature of the secondary side under the coupled heat transfer boundary is small(about 26 K),and the wall temperature gradient is only several tens of K/m.Compared with these results,not only is dryout position under the constant heat flux boundary is in the upstream of that under the coupled boundary.but also the wall temperature gradient is several hundreds of K/m.which means that close attention should be paid to the high wall temperature gradient caused by dryout and the burnout problems that may arise from it for the once-through steam generators or heat exchangers involved in dryout.Aiming at the phenomenon of steam superheating in the liquid deficient region,a new parameter-deviation level(post-dl)for characterizing post-dryout deviation level from thermodynamic equilibrium is defined and a method for predicting the superheated steam temperature in the liquid deficient region is proposed.It is found that the deviation level is getting higher and higher with the development of heat transfer.The average deviation level is about 26%in the present condition.The parameter-deviation level can be used to predict the superheated steam temperature in the liquid deficient region,which provides some reference for the accurate heat transfer partition of once-through steam generators.Aiming at dryout characteristics under the different operating conditions and tube support plate structures of the once-through steam generator,the influences of the operating parameters and tube support plates on dryout and post-dryout heat transfer are numerically studied.It is found that the influence of mass flux on post-dryout wall temperature is opposite with the influence of mass flux on wall temperature in post-departure from nucleate boiling region.This is due to the difference in the occurrence mechanism of the two heat transfer deteriorations.The pressure affects the dryout position and the soaring amplitude of wall temperature.Dryout position moves to the downstream as the pressure increases,and the corresponding soaring amplitude of wall temperature decreases.The slip ratio at the case which considers tube support plate and gap is lower than that at the other two cases,and dryout position is at the upstream of that at the case which dose not consider tube support plate.The enhanced heat transfer effect generated by the vortexes formed at the downstream of the tube support plate can greatly alleviate the soaring phenomenon of the wall temperature to some extent.The soaring amplitude of wall temperature at constant heat flux boundary decreases from 300 K at the case which dose not consider tube support plate to 200 K.This shows that the supporting structure installed between the flow channels is beneficial to reduce the wall temperature to some extent.Aiming at the problem of high wall temperature in the liquid deficient region of the once-through steam generator,the Euler-Lagrange approach is used to numerically study the flow and heat transfer of continuous steam and discrete droplet in the liquid deficient region with spurt of droplets.And the influences of the droplet diameter,droplet axial velocity,droplet radial disturbance,and the critical steam quality at dryout position,on dryout and post-dryout heat transfer are revealed.It is found that the decreasing droplet diameter helps to mitigate the damages to some extent caused by dryout deterioration and post-dryout deviation from thermodynamic equilibrium.The droplet axial velocity directly determines the axial distribution of the wall temperature in the liquid-deficient region.But when it reaches or exceeds 5 m/s,the influence of droplet axial velocity on post-dryout heat transfer is negligible.The critical steam quality is another key factor which affects the distribution of the wall temperature.Post-dryout wall temperature gradually changes from a downward trend to an upward trend with the increasing critical steam quality.The appropriate amount of droplets can effectively decrease the soaring amplitude of wall temperature and post-dryout overall wall temperature,and the performance of heat transfer enhancement can reach 60%in the present operating range.