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The NHR-200-II nuclear heating reactor is a multi-purpose small integral pressurized water reactor (iPWR) developed by the Institute of Nuclear and New Energy Technology (INET) of Tsinghua University. The design of NHR-200-II features a reactor core with thermal power of 200MW, in-vessel hydraulically-driven control rods and passive residual heat removal (PRHR) systems, et.al. Passive residual heat removal experiments were conducted in a scaled integral test facility for NHR-200-II. The PRHR experiments in the scaled facility were simulated by a layered RELAP5 system model to study the flow characteristics of the PRHR system in different primary fluid temperatures and different valve states. The phenomenon of reversed flow occurred in some primary heat exchangers in the numerical simulations when the primary fluid temperature was higher than certain level, which was consistent to the experiments. The simulated uneven outlet temperature distribution of the primary heat exchangers was also consistent with the experimental data when the isolation valves for the steam generator was kept open. Thermal stratification effect in the headers of the PRHR system played an important role in the phenomenon of uneven outlet temperature distributions, and the layered RELAP5 model was proven to be an efficient method for preliminary estimation of thermal stratification effect in the headers.

NHR-200-II is a small integral pressurized water reactor with a core thermal power of 200MW. Two parallel columns of passive residual heat removal (PRHR) system are connected to the two intermediate circuits of NHR-200-II to remove core decay heat in postulated accidents. It was confirmed by scaled experiments that the decay heat removal capacity of the PRHR system was severely deteriorated once the isolation valves for steam generator on the intermediate circuits failed to close. Thermal stratification effects in the ring headers of PRHR system played a significant role in the deterioration of PRHR capacity. A RELAP5 system model and a CFD model were developed for the PRHR system of NHR-200-II, in both models, we focused on the prediction of thermal stratification phenomena in the ring headers of PRHR. Both the case of normal PRHR scenarios (i.e. the SG branches were successfully isolated) and the case of isolation failure of the SG branches during PRHR startup were numerically simulated with the developed CFD and RELAP5 models. The numerical results from both CFD and RELAP5 models showed that the thermal stratification effects in normal PRHR scenarios can be neglected, however, both models gave similar conclusions that the thermal stratification effect was strong in the ring headers in case of isolation failure of the SG branches. Possible physical mechanism of thermal stratification in PRHR of NHR-200-II was explained depending on results of numerical simulations.

To develop a novel passive valve with a spherical disc for reactors, it is necessary to find a suitable numerical model to study the flow characteristics of flow past a sphere in confined spaces. The problem of flow past a sphere arises in many fields, and the phenomenon of drag crisis of a sphere in external flow, in which the drag coefficient of the sphere drops off suddenly as the Reynolds number increases, still occurs in internal flows. With the rapid development of transitional turbulence models in the past decade, we try to improve the drag coefficient prediction of flow past a sphere in a pipe with a wide range of Reynolds numbers using transitional turbulence models. In this study, we numerically investigated the flow past a sphere in a pipe with a blockage ratio (the ratio of sphere diameter to pipe diameter) of 0.667 with three transitional turbulence models. The SST k-ω turbulence model without transition effects was also employed for comparison purposes. The studied Reynolds numbers ranged from subcritical region to supercritical region (5E+3 ⩽ Re ⩽ 3E+5). The skin friction coefficients and pressure coefficients of the sphere were calculated and compared for the four turbulence models. Compared to the measured drag coefficient data of a sphere in a pipe with a similar blockage ratio in the literature, the Intermittency transition model performs best in predicting the drag coefficient than the other turbulence models. None of the four turbulence models can adequately capture the drag crisis phenomenon, i.e., the calculated drag coefficients drop smoothly near the critical region of Reynolds numbers for all four turbulence models. The pipe wall effect is not neglectable in the estimation of the sphere drag coefficient by the pressure drop between the ends of the pipe.

The applicability of the extremum principles of entropy generation and entransy dissipation is studied for heat exchanger optimization. The extremum principle of entransy dissipation gives better optimization results when heat exchanger is only for the purpose of heating and cooling, while the extremum principle of entropy generation is better for the heat exchanger optimization when it works in the Brayton cycle. The two optimization principles are approximately equivalent when the temperature drops of the streams in a heat exchanger are small.

The large-scale upper circulations and meridional overturning in the upper layer of the South China Sea (SCS) with idealized bottom topography in winter and summer are investigated. Simulations with the GFDL general circulation model are carried out under the conditions of open or enclosed boundary regarding transport in the Luzon Strait. The intrusion area of Kuroshio, its impact on the meridional overturning in the upper layer of the SCS and seasonal characteristic of this impact are explored, respectively. The model is forced by climatological wind stress and relaxed to monthly mean climatological temperature and salinity. The resultant meridional overturning is non-enclosed, with transporting from north to south in the surface and returning to north at the depth of about 500 m in winter, about 200 m in summer, with amplitudes of 10(5) m(3)/s. It shows the transporting path of intermediate water of the SCS and offers an idealized reference for further study on dynamics of wind-driven and thermohaline circulation of the SCS.

[1] The temporal-spatial structure and the formation mechanisms of the South China Sea (SCS) circulation are studied using the Princeton Ocean Model (POM). The model well reproduces the observed sea surface height (SSH) annual cycle and some current system such as the SCS Western Boundary Current (SCSWBC) system and the Kuroshio Loop Current (KLC). Four sensitivity experiments are carried out to reveal the dynamic mechanisms of the SCS circulation. The results show that most of the seasonal variability of the SCS is controlled predominantly by wind forcing. The Kuroshio affects the mean SSH significantly but contribute little to the variability. The SCSWBC system consists of the SCS Warm Current (SCSWC) and the Vietnam Coastal Current (VCC). In winter the SCSWBC splits into two branches separated at similar to14degrees-18degreesN. The northern branch is the SCSWC flowing northward and the southern branch is the VCC flowing southward. In summer, however, the SCSWBC flows unidirectionally northward from the Karimata Strait to the Taiwan Strait. The temporal variation of the SCSWBC, especially the VCC, is determined by the wind forcing over the interior SCS. Buoyancy forcing can strengthen (weaken) the summer (winter) SCSWBC. The KLC is crucial to the SCS circulation north of 18degreesN. It ultimately determines the appearance of the SCSWC in winter.