Browsing by Author "Zhen, Ni"
Now showing 1 - 2 of 2
Results Per Page
Sort Options
Item Prismatic modular reactor analysis with melcor(2009-05-15) Zhen, NiHydrogen, a more sustainable source of energy, is a potential substitute for hydrocarbon fuel for power generation. The Very High Temperature gas-cooled Reactor (VHTR) concept can produce hydrogen with high efficiency and in large quantities. The US Department of Energy plans to build a VHTR as a next-generation hydrogen/electricity production plant. This reactor concept is very different from that of commercial reactors in the US. In order to acquire licensing eligibility for VHTRs, analysis tools need to be validated and applied to design and evaluate VHTRs under operation conditions and accident scenarios. In this thesis, MELCOR, a severe accident code, was used to analyze one of the VHTR designs ? a prismatic core Next Generation Nuclear Plant (NGNP). The NGNP is based on General Atomics? (GA) Gas Turbine ? Modular Helium Reactor (GT-MHR) 600 MW design. According to the current literature survey, more data is available for the GT-MHR than for the NGNP. Therefore, for the purposes of extending MELCOR capabilities and code validation, a model of the GT-MHR reactor pressure vessel (RPV) was developed. Based on the currently available data, a model of the NGNP RPV was then developed through modifying the GT-MHR RPV model. For both RPV models, coolant outlet temperature under normal operating conditions corresponds well to the data from literature. The reactor cavity cooling systems (RCCS), which passively removes heat from the RPV wall to the outside atmosphere, was then added to this GT-MHR RPV model. With this model addition, the heat removal rate of the RCCS under normal operating conditions was calculated to correspond well to the data from references. Pressurized conduction cooldown (PCC), one of the important postulated accident scenarios for a prismatic core reactor, was simulated with the complete model. MELCOR has been demonstrated to have the ability of modeling a prismatic core VHTR. The calculated outlet temperature and mass flow rate under normal operation correspond well to references. However, the calculation for the heat distribution in the graphite and fuel is unsatisfactory which requires MELCOR modification for the PCC simulation. For future work, a complete model of the NGNP under normal operation conditions will be developed when additional data becomes available.Item Transition to Turbulence in Shear-Thinning Fluids(2014-04-23) Zhen, NiIn this dissertation, the effects of a shear-thinning fluid on the evolution of a hairpin vortex are investigated. The fluid viscosity is determined using a power law model and direct numerical simulations are performed using a pseudo-spectral code. The Reynolds number is defined using the initial maximum velocity and the initial viscosity at the wall. In the simulations, the Reynolds number and the initial strength of the hairpin vortex are fixed. We observe from 3D visualizations that the hairpin tends to lose its coherence more easily and breaks into small scale structures when the level of shear thinning is increased. The disintegration of the hairpin causes a decrease in the production of kinetic energy and an increase in dissipation. As a consequence, the transition to turbulence is delayed as the level of shear thinning is increased. In future work, we will investigate the effects of shear-thinning on fully developed turbulence, and we will study the effects of other rheological models such as the Carreau model.