报告题目：Convection Driven by Internal Heating

报告人 Prof. Su Jian (苏健)
Universidade Federal do Rio de Janeiro, Brazil

主持人：李存标 教授
时间：9月20日（周五）下午15:00-17:00
地点：工学院1号楼210会议室

报告内容摘要：
Buoyancy driven flow is widely encountered in natural and engineering, and is as important as shear driven or pressure-gradient driven flows. Rayleigh-Bénard convection has intensively studied in more than a hundred years. Natural convection in differentially-heated cavities has also been widely studied. Meanwhile, natural convection driven by internal heat sources has received much less attention, although the problem is of great interests in geophysics, astrophysics, meteorology, and nuclear reactor design and safety analysis.  The physical phenomena in internal heating driven convection in a geometrically defined cavity are governed by two dimensionless numbers: the Prandtl number (Pr) and the Rayleigh number (Ra). For a given Prandtl number, different flow regimes can be identified depending on the Rayleigh number. At the lower end of Rayleigh number, the conductive regime exists, where the fluid velocity is zero everywhere. A first critical Rayleigh number may be determined by a linear stability analysis, above which a steady-state laminar convection regime is identified. With increasing Raleigh number, the left-right symmetry of flow and temperature fields is broken and an asymmetric steady-state laminar convection is established, which indicates the existence of multiple steady-state solutions and probably a Hopf-type bifurcation. For larger Rayleigh number, the steady-state laminar convection is no longer sustained. A second critical Rayleigh number may be determined for the onset of unsteady natural convection. Depending on the Rayleigh number, the unsteady laminar natural convection can be periodic, quasi-periodic, or chaotic. Eventually, for sufficiently large Rayleigh number, fully turbulent natural convection will be established. The mechanisms of transition from steady to unsteady, from periodic to chaotic, and from laminar to turbulent convection have not yet been well clarified. Accurate prediction of high Rayleigh number heat transfer for a wide range of Prandtl number is still to be achieved. In last few years, we have been study computationally the problem of internal heating driven natural convection in cavities. For steady-state laminar natural convection in a square cavity, we obtained a hybrid analytical-numerical solution using the generalized integral transform technique. The numerical results of the integral transform solution were in excellent agreement with that obtained by a commercial CFD code, ANSYS CFX, qualifying the CFD code for further computational studied of unsteady laminar convection. Oscillatory and chaotic regimes were identified numerically for higher Rayleigh numbers.   Turbulent natural convection in a two-dimensional semi-circular cavity with volumetric heat generation, representing a molten reactor core in the lower head of a light-water reactor during a severe accident, was simulated by using a shear stress transport (SST) turbulence model. Due to the limitation of commercial CFD codes, we moved to OpenFOAM,  an open-source CFD platform, in order to implement turbulence models not yet available in commercial and open-source packages. We implemented v2f model for Reynolds stresses and two models for turbulent heat fluxes, although the explicit algebraic heat flux models were shown to be unstable for high Rayleigh numbers. Currently, we are focused in conducting numerical simulations of high Rayleigh number turbulent natural convection in 2D square, 2D semi-circular, and 3D hemispherical cavities to establish Nusselt number correlations as a function of Rayleigh number for a given Prandtl number, which are of immediate importance to nuclear safety analysis, especially due to the severe accident in Fukushima in Japan, March 2011.

报告人简介：
Prof. Su Jian received BSc from University of Science and Technology of China, Hefei, MSc from Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing, and DSc from Universidade Federal do Rio de Janeiro, Brazil. He has been on the faculty of Nuclear Engineering Department, Graduate School of Engineering, Universidade Federal do Rio de Janeiro since 1998, and was Head of Department from Feb 2009 to Feb 2013. He is a Research Fellow (Level 1B) of Brazilian National Research Council (CNPq), and a Distinguished Scientist of the State of Rio de Janeiro (CNE/FAPERJ).  He had been academic visitor to Cambridge, Imperial College, McMaster, University of Hong Kong, and Peking University. His recent research interests include natural convection in cavities, multiphase and multicomponent flow, and nuclear reactor thermohydraulics.

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