Study On Two-Phase Flow And Critical Heat Flux Mechanism In An Inclined Rectangular Channel Under Low Flow Rates

In order to ensure safety of the nuclear power plants,External Reactor Vessel Cooling(ERVC)as an In-Vessel Retention(IVR)strategy is one of the most important passive severe accident mitigation methods.Currently,some large-scale test facilities have been setup to verify the feasibility and reliability of the IVR-ERVC strategy for various nuclear reactor units.However,the boiling phenomena and CHF mechanism in the ERVC have not been well understood.In addition,due to the great uncertainty of the molten corium distribution in the lower head of the reactor vessel and the corresponding heat transfer characteristics,the experimental results obtained by the large-scale facitlities could not be used for accurate analyzing the severe accident and optimizing the ERVC design.So,in this study,a reatangular channel is used to perform flow boiling experiments with various mass flow rates and inlet qualities under different orientions,which helps to simulate the natural convective boiling on the downward outer surface of reactor vessel lower head and so as to better understand the corresponding flow boiling heat transfer charatersitics and CHF mechnism.Under the severe accident with the core meltdown,ERVC would act that the residue heat would be removed by the convective flow boiling between the downward reactor vessel outer surface and the baffler.The heat transfer characteristics and CHF at certain polar angle of the reactor lower head are greatly affected by the local orientation and upstream two phase flow conditions(such as flow rate,void distribution,etc.).In order to better understand such a complex physical process,adiabatic steam-water two-phase flow experiments with various mass flow rates and inlet qualities under different orientations were performed in the rectangular channel to investigate the characteristics of the void distrution and averaged void fraction in the cross section.At the same time,CFD model was also developed and the simulated results were compared with the expiermental data.The experimental results show that the averaged void fraction decreases with the increase of channel orientation at beginning,and then it continuously increases for both slug and churn flow.Based on the drift flux model,the distribution parameters and drift velocities were also analyzed.The drift velocity shows a nonmonotonic variation trend with the averaged void fraction due to the drift velocity first increases and then decreases with the increase of orientation angle.The CFD simulation results based on the inhomogeneous Euler-Euler two fluid model show the same conclusion.In addition,the flow boiling experiments were performed in the rectangular channel and various main parameters(such as orientation,mass flow rate,quality,etc.)were separated to investigate a single factor effect on the heat transfer characteristics and CHF value.The channel orientations vary from 15 to 90°,the mass flow rates vary from 110 to 288 kg/(m~2s)and the qualities vary from 0.003 to 0.036.The heat transfer experimental results show that the mass flow rate and quality effects on the heat transfer coefficient(HTC)are very weak in our tests.While the effect of orientation angle can be separated into two orientation regions by the value of 45°.When the orientation angle is bigger than 45°,the HTC is almost the same as 90°case while when the orientation angle is smaller than 45°,the HTC keeps constant which is lower than 90°case.Compared with different empirical formulas of saturated boiling HTC,it is found that the Liu&Winterton correlation can well predict the experimental HTC results in 90°orientation channel.Based on such correlation,a new model was developed by considering the orientation effect,which has an error of-18%~15%comparing with the experimental data.CHF experimental results show that the mass flow rate,quality and channel orientation all have positive effect on the CHF.Furthermore,the orientation effect on CHF is found to be a combination effect of mass flow rate and quality.The CHF increases with the increase of channel orientation.However,the orientation effect on CHF will be weaken as the mass flow rate or quality increases.Finally,based on experimental data,the CHF mechanism and corresponding physical model are investigated.Four existing flow boiling CHF models(boundary-layer separation model,bubble crowding model,sublayer dryout model and interfacial lift-off model)are studied while none of their original forms is suitable to the current experimental conditions:the flow boiling of a single side heater surface with different channel orientation under low mass flow rate and low quality.Thus,a new CHF model was developed based on the sublayer dryout theory and the separate flow theory.The predicted results by the developed model show a good agreement with the experimental data by an error range of-25% to 20%.