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京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第21753号 / 工博第4570号 / 新制||工||1712(附属図書館) / 京都大学大学院工学研究科機械理工学専攻 / (主査)教授 西脇 眞二, 教授 松原 厚, 教授 黒瀬 良一 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

A computer program based on the discrete-element method has been developed to compute friction factors and Nusselt Numbers for fully-developed turbulent flows with randomly-rough surfaces. Formulations of the discrete-element model for fully-developed turbulent flows inside circular pipes and between infinite parallel plates with the necessary adaptations for randomly-rough surfaces are provided. Utilizing the output of a three-dimensional profilometer, proper description of the randomly-rough surface is necessary for use within the discrete-element model. Proper description of the randomly-rough surface is achieved by the McClain (2002) method of characterization. Predictions from the discrete-element model computer program are compared with the classical, laminar and turbulent, smooth-wall results. In addition to the smooth-wall evaluations, predictions are compared with experimental results for turbulent internal flows with deterministic surface roughness. Predictions from the model demonstrated excellent agreement in all cases. Friction factor and Nusselt Number predictions for fully-developed flows over randomly-rough surfaces are also presented. With the friction factor and Nusselt Number data, velocity profiles for flows over randomly-rough, deterministically-rough and smooth surfaces are provided for comparison.

The effects of appendages on flow characteristics and scalar mixing in gap-connected twin-subchannel geometries has been assessed. The assessment considers a symmetric, rectangular compound channel geometry connected by a single rectangular gap using computational fluid dynamics (CFD). Detailed numerical models (geometry and turbulence), characterizing the full test section from a reference experimental study, are generated and validated against measurements. Time varying details of the gap induced periodic structures and appendage induced vortices are captured through calculations in an unsteady Reynolds averaged Navier-Stokes (RANS) framework coupled with the Spalart-Allmaras (SA) turbulence model closing the RANS equations. Companion simulations are performed at each of two Reynolds numbers (2690 and 7500), one with and one without a gap-centered appendage. The appendage size modelled is representative of CANDU endplates. The appendage effects on flow characteristics and mixing are isolated through comparison of the associated simulations. In the absence of appendages, fluid exchange between subchannels is dominated by quasi-periodic flow pulsations through the gap formed due to flow instability in the near gap region. Without a gap-centered appendage, the magnitude, frequency, and structure length of the gap flow pulsations are well predicted by the model at both Reynolds numbers. The total tracer transfer between subchannels is reasonably well predicted for Re = 2690 (within approximately 17% of the experimental value). The model fails to capture the measured increase in scalar transfer through the gap with increased Reynolds number, underpredicting scalar mixing by 55% at Re = 7500. An argument is presented that the use of an isotropic turbulence model in the channel (SA), which precludes the development of channel secondary flows, is the source of the discrepancy between modelled and measured mixing at Re = 7500. Appendages, such as those introduced by end plates or bearing pads in CANDU fuel bundles, augment the exchange process between subchannels. With an appendage representative of a CANDU fuel bundle endplate introduced into the gap region, crossflow velocity and frequency are predicted to increase immediately downstream of the appendage due to flow diversion and vortex shedding. The higher local frequency is shown to be consistent with the vortex shedding frequency calculated for a stationary rectangular cylinder at the gap conditions. Further downstream, gap induced instabilities begin to re-establish as the dominant contributor to crossflow pulsations although they are not fully recovered by the test section exit. Mixing is augmented more by the appendage with increasing Reynolds number for the range examined. / Thesis / Master of Applied Science (MASc) / The fuel bundle and pressure tube assembly in the core of a CANDU reactor forms an intricate web of subchannels of varying geometries with interconnecting gaps. Heat generated within the fuel bundles is removed by coolant flowing through the pressure tube and within the bundle subchannels. Although flow is nominally axial along the length of the rod bundles, coolant is free to move between subchannels through the gaps by a variety of mechanisms. Detailed fluid flow in these rod bundle geometries is a complex 3D phenomenon, strongly affected by fluid turbulence and flow instabilities associated with the subchannel geometry. This flow is investigated in the current study and extended to include the effect of appendages, which hold the fuel rods in place, to determine their impact on mixing along the length of the bundle. Particular applications of the results of this study are in the areas of nuclear reactor performance and safety. The extent of coolant exchange between subchannels affects the local subchannel flow and temperature and, as a result, local cooling at the fuel element surfaces. Fuel element cooling is a principal component of reactor analysis under both normal operating conditions and postulated accident scenarios.

Yes / Turbulence-induced secondary currents are commonly present in straight natural as well as artificial open channels without bed forms. Different structures of cellular secondary currents can be seen in open-channel flows due to various bed configurations. In our study, mathematical models of turbulence-induced secondary currents in the vertical and transverse directions within a straight open rectangular channel with alternate rough and smooth longitudinal bed strips are proposed. The proposed models are derived using appropriate theoretical and mathematical analysis. Most of the previous models of secondary currents in the literature are proposed empirically and without proper mathematical derivations. The effects of fluid viscosity and eddy diffusivity are included in the present study to make it more practical. Initially, the governing equation for vertical secondary flow velocity is derived from continuity and the Reynolds-Averaged Navier Stokes equations. Then, the proposed problem is divided into two sub-considerations, corresponding to the base flow and perturbed flow. Finally, these sub-problems are analytically solved using method of variables separation with suitable boundary conditions. Different models to consider two different types of bed-roughness configurations (i.e. equal and unequal lengths of smooth and rough longitudinal bed strips) are obtained. Apart from velocity formulations, models of the stream function are proposed for these two types of bed configurations. All proposed models are validated using existing experimental data for the various bed configurations in open-channel flows and satisfactory results have been obtained. These present models are also compared with empirical models from the literature and they are found to be more effective in representing both types of bed-roughness configurations. The effects of bed configuration on the streamlines of settling velocity are also investigated. Results show that laterally-skewed secondary cells (which occurs due to unequal smooth and rough bed strips), have significant effects on the closed ω-streamlines in terms of shape and location of the centre of these streamlines. More precisely, it is found that the area of the downflow zone proportionally increases with the length of rough-bed strips.

<p dir="ltr">The computational campaign reported in this thesis focuses on a series of experiments at Mach 2.85 carried out in the 1980s at NASA Ames Research Center on a set of cylinder / skewed flare configurations designed to produce highly three-dimensional shockwave / boundary-layer interactions in the absence of end-wall effects. Computations carried out in that era were unable to match the experimental results using the numerical techniques, turbulence models, and grid resolution available at the time. In the present work, newer Reynolds-averaged Navier-Stokes and detached eddy simulation methods have been applied to these flows, and relatively good agreement has been obtained with the experimental data. Difficulty in capturing the correct separation bubble size was encountered with initial detached eddy simulations, but the introduction of resolved turbulence via a boundary layer trip produced much better results. This thesis reports on results obtained for four inclination angles (0 deg, 5 deg, 10 deg, and 23 deg) of the skewed flare. Detached eddy simulation is seen to be an economical alternative to large eddy simulation for capturing many features of large-scale separation unsteadiness over long time intervals at true Reynolds number.</p>

The U.S. Evolutionary Power Reactor (EPR) is a new, large-scale pressurized water reactor made by AREVA NP Inc. Surrounding the core of this reactor is a steel wall structure sitting inside called the heavy reflector. The purpose of the heavy reflector is to reduce the neutron flux escaping the core and thus increase the efficiency of the reactor while reducing the damage to the structures surrounding the core as well. The heavy reflector is heated due to absorption of the gamma radiation, and this heat is removed by the water flowing through 832 cooling channels drilled through the heavy reflector. In this project, the temperature distribution in the heavy reflector was investigated to ascertain that the maximum temperature does not exceed the allowable temperature of 350 C, with the intent of modifying the flow distribution in the cooling channels to alleviate any hot spots. The analysis was conducted in two steps. First, the flow distribution in the cooling channels was calculated to test for any maldistribution. The temperature distribution in the heavy reflector was then calculated by simulating the conjugate heat transfer with this flow distribution as the coolant input. The turbulent nature of the flow through the cooling channels made the calculation of the flow distribution computationally expensive. In order to resolve this problem, a simplification method using the "equivalent flow resistance" was developed. The method was validated by conducting a few case studies. Using the simplified model, the flow distribution was calculated and was found to be fairly uniform. The conjugate heat transfer calculation was conducted. The same simplification method used in the flow distribution analysis could not be applied to this calculation; therefore, the computational cost of this model was reduced by lowering the grid density in the fluid region. The results showed that the maximum temperature in the heavy reflector is 347.7 C, which is below the maximum allowable temperature of 350 C. Additional studies were conducted to test the sensitivity of the maximum temperature with change in the flow distribution in the cooling channels. Through multiple calculations, the maximum temperature did not drop more than 3 C; therefore, it was concluded that the flow distribution in the cooling channels does not have significant effect on the maximum temperature in the heavy reflector. / Master of Science