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Institute: search.filters.UBSInstitut.Institut für Kernenergetik und EnergiesystemeSubject: search.filters.subject.530

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    Thermo-hydraulic analysis of wall bounded flows with supercritical carbon dioxide using direct numerical simulation
    (Stuttgart : Institute of Nuclear Technology and Energy Systems, 2018) Pandey, Sandeep; Laurien, Eckart (Prof. Dr.-Ing. habil.)
    The power cycle based on supercritical carbon dioxide technologies promises a higher thermal efficiency and a compact plant layout. However, heat transfer and hydraulic characteristics are peculiar in the near-critical region due to the sharp variation of thermophysical properties in a narrow temperature and pressure range. Therefore, this works presents the results of several direct numerical simulations (DNS) of turbulent wall-bounded flow at supercritical pressure. The spatially developing pipe flows are simulated with the low Mach number approximation to characterize the cooling process of supercritical carbon dioxide. The upward and downward flow of carbon dioxide in vertical orientation has been considered. Heat transfer deterioration followed by recovery is observed in the downward flow while enhancement occurs in the upward flow as compared to forced convection. During the heat transfer deterioration, sweep and ejection events are decreased greatly, triggering the reduction in turbulence. The recovery in turbulence is brought by the Q1 and Q3 (also known as outward and inward interaction) events, contrary to the conventional belief about turbulence generation. The turbulence anisotropy of the Reynolds stress tensor showed that the turbulence structure becomes rod-like during the deteriorated heat transfer regime in the downward flow and disc-like for the upward flow. In addition to low Mach number DNS, a framework for using fully-compressible discontinuous Galerkin spectral element method for DNS of supercritical carbon dioxide is presented. A turbulent channel flow is considered to demonstrate the ability of this framework and to observe the effects of Mach number in the supercritical fluid regime. The increase in the Mach number increases the turbulence in the flow for a given Reynolds number. Finally, a computationally light data-driven approach for heat transfer and hydraulic characteristics modeling of supercritical fluids is presented based on the deep neural network. This innovative approach has shown remarkable prediction capabilities.
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    Single-phase fluid flow and heat transfer in microtubes
    (2008) McPhail, Stephen John; Groll, Manfred (Prof.)
    The current experimental investigation focuses on the single-phase flow characteristics inside microtubes, both in adiabatic conditions as with heat input. In the literature, contrasting effects concerning pressure drop and heat transfer are reported that are tied to miniaturization of the flow passage. That is why a systematic variation of conditions has been carried out to evaluate the possible occurrence of anomalies tied to the increased viscous deformation, rarefaction and pressure drop in microchannels. It has been verified that miniaturization of the test section brings about considerable difficulties through altered peripheral conditions: measuring equipment, material thickness and total fluid volume do not scale to the same degree as the characteristic dimension of the system (i.e. the inner diameter of the studied microtube). This can have a substantial effect on the experimental output and accuracy. The findings of this investigation have led to the conclusion that no anomalous phenomena occur solely through reduction of the flow passage down to 30 μm inner diameter. The friction factor for laminar liquid flow in tubes is in agreement with the classic correlation of Hagen-Poiseuille. Up to shear rates of 106 s-1 and a relative roughness of 1%, no alterations could be measured, as also with degassed water flow over a hydrophobic surface. In compressible flow, even a ten-fold pressure drop between inlet and outlet did not evidence significant deviations from the incompressible flow relation for friction. This points to a near-reversible process of expansion under these conditions. No rarefaction effects could be detected in the experiments carried out. In microchannels, the liquid shear rate at the wall is very great, causing irreversible heat production which increases the bulk fluid temperature of the flow. This increase is known as viscous dissipation and is tied directly to the friction at the wall. A derivation is provided, with experimental validation, that shows how the friction factor can be expressed in terms of this temperature rise only. Adding heat input to the microtubes showed that the effect of incongruous scaling can be very disturbing. In laminar flow, a distinct reduction was observed in the heat transfer efficacy of thick-walled glass microtubes compared to thin walled stainless steel tubes of the same inner diameter. The large thermal resistance of the former compared to the small volume of flow causes the heat put in to select preferential paths (through peripheral attachments to the test section), thereby not reaching the fluid-wall interface completely, falsifying the assumed experimental conditions. Otherwise, a strong heat transfer improvement was observed in the thermal development region, recommending the use of short microchannels in single-phase heat exchanging applications, providing the added benefit of reduced head loss. In turbulent flow, no significant deviations were found with respect to the predictions of the - conventional - Gnielinski correlation for the heat transfer coefficient.
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    Numerical evaluation of criticality in debris beds formed during severe accidents in light water reactors
    (Stuttgart : Institute of Nuclear Technology and Energy Systems, 2021) Freiría López, María; Starflinger, Jörg (Prof. Dr.-Ing.)
    After the Fukushima accident, the interest of the scientific community in severe accident (SA) research has been renewed. Great efforts are being made internationally to reassess and strengthen the safety of nuclear power plants. The recriticality potential in debris beds formed after the core meltdown is one of the SA research issues that needs further attention, and it is also the focus of this work. An inadvertent criticality event may cause the release of nuclear radiation and have severe consequences. Thus, the criticality in debris beds must be evaluated to predict possible risks and establish the appropriate control measures if necessary. The available criticality data for debris beds are still very scarce. Thus, the Japan Atomic Energy Agency has begun the ambitious task of building a criticality map for debris beds. That is an arduous enterprise, which requires the investigation of appropriate debris bed models and numerous computations under a broad range of possible conditions. A global effort and international cooperation are essential. The present work aims to contribute to this common endeavor by improving debris bed models, extending the criticality database, and facilitating future analyses. Alternatives for modeling the debris bed characteristics with a potential influence on the criticality are discussed in this thesis, from the most conservative assumptions to more realistic approaches. Among other things, it was found that debris beds can be modeled with high accuracy as spheres regularly arranged in a water matrix if an adequate equivalent diameter is chosen. Besides, coupled neutronic-thermohydraulic calculations were proven to be not necessary for assessing the criticality of Fukushima debris beds. This work also investigates the criticality characteristics of UO2-concrete systems. The calculation results prove the good moderation capacities of concrete, which has a significant positive reactivity effect at very low porosities. Not only the bound water is capable of thermalizing neutrons but also the SiO2, a major component of concrete. Consequently, MCCI products should be treated carefully in the criticality analyses. A preliminary conservative criticality assessment of Fukushima debris beds has revealed safety parameter ranges, i.e., conditions for which recriticality can be excluded. On the one hand, dry debris beds cannot become critical under any conditions due to the lack of sufficient moderator. On the other hand, debris beds submerged in water will remain subcritical if the porosity is sufficiently low (< 0.24 for debris beds without concrete, < 0.1 if concrete is mixed with fuel), the mass is sufficiently small (< 124 kg), or the cooling water is sufficiently borated (> 2600 ppm B). Finally, a statistical method is proposed as an alternative and more realistic way to evaluate the criticality in debris beds. A first exploratory analysis of the debris bed at Fukushima Unit 1 reveals that the probability of recriticality is extremely low. Additionally, the sensitivity analysis has concluded that the amount of control rod material (B4C) mixed with fuel is by far the most relevant parameter. Other parameters with a strong correlation with keff are the percentage of fuel in the corium, the amount of debris in particulate form, and the debris bed spreading. Based on them, future areas of research and improvement are proposed.