Complex Three-dimensional Phenomena Research Articles

3D thermal hydraulic characteristics analysis of pool-type upper plenum for lead-cooled fast reactor with multi-scale coupling program

The large space plenum of the lead-cooled reactor has complex three-dimensional thermal hydraulic phenomena such as thermal stratification and coolant mixing, which affects the safety performance of the reactor. In this paper, based on the multi-scale coupling program ATHLET-OpenFOAM, three-dimensional simulation of the hot pool for lead-cooled fast reactor M2LFR-1000 is carried out. Considering the axis symmetry of the hot pool model and reducing the amount of calculation, 1/8 model of the hot pool is selected to build the model by CFD, and the rest is simulated by system program ATHLET. The steady-state operation condition is first calculated by coupling program. Based on the calculation of steady-state condition, the main thermal parameters of M2LFR-1000 primary coolant system and the complex three-dimensional phenomena of the upper plenum under unprotected loss of flow accident (ULOF) are studied, and the results of multi-scale coupling calculation are compared with those of ATHLET alone. The results show that there is an obvious thermal stratification in the inner region and the outer region of the upper plenum in the early period of the accident, which slows down the speed of establishing natural circulation. During ULOF accident, there are obvious vortex flow changes in the inlet outer side and the outlet area of the upper plenum.

Analysis of a full-scale integral test in PERSEO facility by using TRACE code

Over the last decades a lot of experimental researches have been done to increase the reliability of passive decay heat removal systems implementing in-pool immersed heat exchanger. In this framework, a domestic research program on innovative safety systems was carried out leading the design and the development of the PERSEO facility at the SIET laboratories. The configuration of the system consists of an heat exchanger contained in a small pool which is connected both at the bottom and at the top to a large water reservoir pool.Within the frame of a national research program funded by the Italian minister of economic development, the DEIM department of the University of Palermo in cooperation with ENEA has developed a computational model of the PERSEO facility in order to simulate its behaviour during an integrated test. The analysis here presented has been performed by using the best-estimate TRACE code and - in order to highlight the capabilities and limits of the TRACE model in reproducing qualitatively and quantitatively the experimental trends - the main results have been compared with the experimental data. The comparison shows that the model is able to predict the overall behaviour of the plant during the meaningful phases of the transient analysed. Nevertheless, some improvements in the modelling of certain components in which take place complex three-dimensional phenomena are suggested in order to reduce some discrepancies observed between code results and test measurements.

Towards Depth-Resolved Optical Imaging of Cardiac Electrical Activity.

The spatiotemporal dynamics of arrhythmias are likely to be complex three-dimensional phenomena. Yet, the lack of high-resolution three-dimensional imaging techniques, both in the clinic and the experimental lab, limits our ability to better understand the mechanisms of such arrhythmias. Optical mapping using voltage-sensitive dyes is a widely used tool in experimental electrophysiology. It has been known for decades that even in its most basic application, epi-fluorescence, the optical signal contains information from within a certain intramural volume. Understanding of this fundamental property of optical signals has paved the way towards novel three-dimensional optical imaging techniques. Here, we review our current understanding of the three-dimensional nature of optical signals; how penetration depths of cardiac optical imaging can be improved by using novel imaging modalities and finally, we highlight new techniques inspired from optical tomography and aiming at full depth-resolved optical mapping of cardiac electrical activity.

Well-Posed Models of Porous Buildings for Macroscopic Ventilation Analysis>

Macroscopic methods of building ventilation analysis developed in the past fifty years have proven to be accurate and thus useful for purposes of single-and multi-zone building infiltration, air quality, smoke spread, thermal comfort, and integrated HVAC/building ventilation system analysis. These methods fail, however, to provide the same level of accuracy when applied to the analysis of wind-driven airflow through porous buildings. While it may be unreasonable to rely solely on macroscopic methods to model the complex three-dimensional phenomena associated with airflow through porous buildings, the accuracy of these methods may be improved if the analyst poses the airflow problem in a physically consistent and complete manner with the numerical consequences of the formulation in mind. This paper considers a number of strategies to achieve these objectives including: a) the coupled use of mass and mechanical energy conservation principles, b) the application of total pressure boundary conditions, c) the selection of appropriate loss coefficients for flow-limiting openings, d) corrections for non-normal wind directions, and e) the impact of system model topology, nonlinearity, and initial solution estimate on solution numerics.