Multi-Scale CFD Simulations of Momentum and Buoyancy Driven Flows in Nuclear Reactor Systems

The present study focuses on the validation of the multi-scale CFD simulations for the nuclear systems. It is mainly divided into the three applications; First is the validation analysis of the large scale PANDA experiment for the containment safety, Second is mainly focused on the international benchmark project GEMIX in order to quantify the level of the involved uncertainty in CFD simulations, due to the turbulence modeling, and third is the validation study for the PWR 5x5 bundle with mixing vane to investigate and validate flow structures at the downstream of the mixing vane. The first part of the work being proposed is to conduct further analysis on the containment safety by using available scale resolved modeling such as LES. In addition to the scale-resolved simulation, available data reduction techniques such as Proper Orthogonal Decomposition is applied to extract coherent turbulent structures in the flow. The second part, in order to quantify the level of the involved uncertainty in CFD simulations, benchmark activities have been conducted by various groups at different scales. The GEMIX test facility was used to develop benchmark data. The facility involves the mixing of two fluids that are initially separated. One of the two streams is water with sugar dissolved and other stream is distilled water, which produces density differences between the two streams. Velocity fields and concentration measurements were acquired. The velocity fields were acquired using PIV and concentrations of each fluid were acquired using LIF. The activity provided the opportunity to quantify the validation study for CFD. The third part, the single-phase hydraulic problem presented involves the steadystate turbulent flow field modelling and resolution around a scaled PWR mixing vane grid tested by Texas A&M University. The most vital contribution of this part will be the contribution to the literature with high resolution direct 3-D experimental and computational results comparison. Additionally, validated numerical results is processed to create higher order turbulent statistics database to help turbulence modeling for the subchannel analysis and contribute to further verification and validation studies. POD and DMD techniques are used to extract coherent turbulent structures numerically

Hysteresis features of the transition-metal dichalcogenides VX2 (X = S, Se, and Te)

Very recently, it has been shown that vanadium dichalcogenides (VX2, X = S, Se and Te) monolayers show intrinsic ferromagnetism, and their critical temperatures are nearly to or beyond room temperature. Hence, they would have wide potential applications in next-generation nanoelectronic and spintronic devices. In this work, being inspired by a recent study we systematically perform Monte Carlo simulations based on single-site update Metropolis algorithm to investigate the hysteresis features of VX2 monolayers for a wide range of temperatures up to 600 K. Our simulation results indicate that, both remanence and coercivity values tend to decrease with increasing temperature. Furthermore, it is found that hysteresis curves start to evolve from rectangular at the lower temperature regions to nearly S-shaped with increasing temperature

CFD Simulations of Erosion of a Stratified Layer by a Buoyant Jet in a Large Vessel

One of the most important parameters in the analysis of containment safety of the light water reactors during a loss of coolant accident (LOCA) is the prediction of the hydrogen concentration. To ensure proper design of the containment to mitigate the fire/explosive risk created by the flammable hydrogen gas, this concentration build up must be analyzed. Lumped parameter (LP) codes are the main tools used in containment thermal-hydraulic analysis. However, they are limited when it comes to scenarios which require higher fidelity analysis of local phenomena. While the use of computational fluid dynamics (CFD) allows for higher fidelity analyses, CFD requires a comprehensive validation study due to turbulence and condensation modeling. During a LOCA accident, the leaked hydrogen from the primary circuit can form a stable stratified layer at the top of the containment building. The formation and erosion of a stratified layer is a challenging numerical problem due to the interaction mechanism of the jet flow with the stratified layer. The OECD-NEA conducted an experiment at the Paul Scherrer Institute (PSI) as a part of the third International Benchmark Study (IBE-3) to investigate the erosion of the stratified layer by a vertical air-helium jet from the bottom of the large vessel. During the experiment, CFD grade experimental data was generated that could be used for comparative studies. In the present study, the experiment is simulated by using the STAR-CCM+ CFD code with various turbulence models including Reynolds-Averaged Navier-Stokes (RANS) models and Large Eddy Simulation (LES). The Realizable k-ε and k-ω SST showed good agreement with the experimental when predicting the erosion of the stratified layer and global mixing of the gas components. The LES model also showed good agreement for velocity and faster erosion with experimental data, while the cost of the LES simulation was much higher than RANS simulations. The current validation study contributes to the sensitivity analysis of the turbulence models for erosion behavior in the stratified layer. In addition to that, the results of this study will provide a foundation to discuss the feasibility of the CFD code usage in containment level thermal hydraulic analysis

Electronic and magnetic properties of the monolayer RuCl3_3: A first-principles and Monte Carlo study

Recent experiments revealed that monolayer $\alpha$-RuCl$_3$ can be obtain by chemical exfoliation method and exfoliation or restacking of nanosheets can manipulate the magnetic properties of the materials. In this present paper, the electronic and magnetic properties of $\alpha$-RuCl$_3$ monolayer are investigated by combining first-principles calculations and Monte Carlo simulations. From first-principles calculations, we found that the spin configuration FM corresponds to the ground state for $\alpha$-RuCl$_3$, however, the other excited zigzag oriented spin configuration has energy of 5 meV/atom higher than the ground state. Energy band gap has been obtained as $3$ meV using PBE functionals. When spin-orbit coupling effect is taken into account, corresponding energy gap is determined to be as $57$ meV. We also investigate the effect of Hubbard U energy terms on the electronic band structure of $\alpha$-RuCl$_3$ monolayer and revealed band gap increases approximately linear with increasing U value. Moreover, spin-spin coupling terms ($J_1$, $J_2$, $J_3$) have been obtained using first principles calculations. By benefiting from these terms, Monte Carlo simulations with single site update Metropolis algorithm have been implemented to elucidate magnetic properties of the considered system. Thermal variations of magnetization, susceptibility and also specific heat curves indicate that monolayer $\alpha$-RuCl$_3$ exhibits a phase transition between ordered and disordered phases at the Curie temperature $14.21$ K. We believe that this study can be utilized to improve two-dimensional magnet materials

Dik manyetik alan altında Rashba ve Dresselhaus spin yörünge etkileşimli parabolik kuşatılmış kuantum telinin elektronik yapısı

Bu tezde, spin-yörünge çiftleniminin dik manyetik alan altındaki parabolik hapsetme potansiyeline sahip kuantum telinin enerji spektrumu ve spin dağılımları üzerine etkisini teorik olarak inceledik. Buna ek olarak, değiştokuş-korelasyon katkısını içeren spin-yörünge sistemlerini de inceledik. Sonlu elemanlar yöntemini kullanarak Schrödinger denkleminin nümerik çözümlerini yüksek hassasiyetle elde ettik. Elde etiğimiz sonuçlar, spin-yörünge çiftlenimi ile etkin manyetik alan arasındaki etkileşimlerin band yapısını önemli derecede değiştirdiğini, ek altband uçdeğerleri ve enerji aralıkları oluşturduğunu ortaya koymaktadır. Bu sonuçlara ek olarak enerji altbandları arasındaki spin ayrılmalarının büyüklüğünün manyetik alanın şiddetine bağlı olduğunu elde ettik. Ayrıca spin dağılım desenlerinin uygulanan manyetik alana ve spin-yörünge çiftleniminin şiddetine güçlü bir şekilde bağlı olduğu sonucunu elde ettik. Dış manyetik alan ve spin-yörünge çiftlenim terimleri arasındaki yarışmacı etkileşim enerji altbandlarındaki çiftlenimlerden dolayı spin dağılımında karmaşık özellikleri ortaya çıkarmaktadır. Kuantum telinin genişliği boyunca spin yoğunluğunun uzaysal dağılımının spin-yörünge çiftleniminin kuvveti, manyetik alan ve yük taşıyıcı yoğunluğu aracılığı ile önemli ölçüde değiştirilebildiğini gördük. Değiştokuş-korelasyon katkısının altbandların yerel maksimumları civarında bandın düzleşen bir davranışa sebep olduğunu ve bütün enerji altbandlarını daha düşük enerji değerlerine kaydırdığını gözlemledik. Ayrıca, değiştokuş-korelasyon ve spin-yörünge çiftleniminin enerji dağılımında asimetriye neden olduğu sonucunu elde ettik In this thesis, we have investigated theoretically the effect of spin-orbit coupling on the energy level spectrum and spin texturing of a quantum wire with parabolic confining potential subjected to perpendicular magnetic field. Additionally we have also taken into account exchange-correlation contribution. We have used finite element method to get numerical solutions of Schrödinger equation with high accuracy. Our results have been revealed that the interplay of the spin-orbit coupling with effective magnetic field considerably modifies the band structure, producing additional subband extrema and energy gaps. In addition to these, we have obtained that the magnitude of spin splitting between energy subbands depends on the strength of the magnetic field. We have also found that spin orientation strongly depends on the applied external magnetic field and the strengths of SO couplings. Competing effects between external magnetic field and spin-orbit coupling terms have introduced complex features in spin texturing owing to couplings in energy subbands. We have seen that spatial modulation of spin density along the wire width can be considerably modified by spin-orbit coupling strength, magnetic field and charge carrier concentration. We have observed that the presence of exchange-correlation contribution leads to a softening behavior in the local maxima at subbands and shifts all energy subbands to lower energy values. We have also obtained that the combined effect of exchange-correlation and spin-orbit coupling produces asymmetry in the dispersion relations

Monitoring the electronic, thermal and optical properties of two-dimensional MoO2 under strain via vibrational spectroscopies: a first-principles investigation

This study presents the electronic, mechanical, thermal, vibrational and optical properties of the MoO2 monolayer under the effect of biaxial and uniaxial compressive/tensile strain, using first-principles calculations based on density functional theory. It has been found that the mechanical strength of MoO2 is higher than other MoX2 (X = S, Se, Te) monolayers. Dynamical stability analysis shows that MoO2 is stable up to 6% compressive and at least 8% tensile strain. Strain dependent Raman modes are investigated along the biaxial directions. It was obtained that phonon softening and hardening occurred under tensile and compressive strain owing to the increase and decrease of the bond lengths of the MoO2 structure. Our results also imply that the electronic band structure of the MoO2 monolayer can be tuned with strain and the energy bandgap decreases with increasing biaxial tensile strain up to 4%. For larger values of strain, a semiconductor to semimetal transition is observed; however, this kind of transition is not observed for uniaxial tensile strain. Besides, we report that MoO2 has a negative thermal expansion coefficient (TEC) in the range of extremely low-temperatures (0 K to 33 K) similar to other 2D MoX2 monolayers. For temperatures above 600 K, it possesses a positive TEC with an approximate maximum value of 12 x 10(-6) K-1. We carried out optical property calculations by solving the Bethe-Salpeter equation and found that the MoO2 monolayer has two strongly bound excitons below the quasiparticle absorption edge. Overall, our results shed light on experimental studies and suggest that the MoO2 monolayer should be an excellent candidate for new design layered semiconductors, electronics, and optoelectronic devices

Controlling electronic structure of single-layered HfX3 (X=S, Se) trichalcogenides through systematic Zr doping

The electronic structures of Hf1-xZrxS3 and Hf1-xZrxSe3 trichalcogenides are investigated by first-principles calculation. In particular, step change of Zr concentration is intensively investigated. Our calculations reveal that doping of Zr atoms increase the strength of cohesion between the atoms in HfX3 (X = S, Se) monolayers, and results in occurring of energetically more stable alloys. In addition, doping of Zr atoms in HfS3 causes band gap bowing, which means the curve of band gap values shows quadratic nonlinearities while change from semimetal to semiconductor is observed in HfSe3 case. The examined band structures indicate that Hf1-xZrxS3 monolayers have very suitable band gap values for water splitting and also their band edge potentials have sufficiently higher or lower positions than the required potential values for the reduction or oxidation potentials

High-throughput computational screening of 2D materials for thermoelectrics

High-performance thermoelectric materials are critical in recuperating the thermal losses in various machinery and promising in renewable energy applications. In this respect, the search for novel thermoelectric materials has attracted considerable attention. In particular, low dimensional materials have been proposed as potential candidates due to their unique and controllable thermal and electronic transport properties. The considerable potential of several two-dimensional materials as thermoelectric devices has already been uncovered and many new candidates that merit further research have been suggested. In this regard, we comprehensively investigate the thermoelectric coefficients and electronic fitness function (EFF) of a large family of structurally isotropic and anisotropic two-dimensional layered materials using density functional theory combined with semi-classical Boltzmann transport theory. With this high-throughput screening, we bring to light additional 2D crystals that haven't been previously classified as favorable TE materials. We predict that Pb2Se2, GeS2, As-2, NiS2, Hf2O6, Zr2O6, AsBrS, ISbTe, ISbSe, AsISe, and AsITe are promising isotropic thermoelectric materials due to their considerably high EFF values. In addition to these materials, Hf2Br4, Zr2Br4, Hf2Cl4, Zr2Cl4, Hf2O6, Zr(2)O(6)and Os(2)O(4)exhibit strong anisotropy and possess prominently high EFF values

The influence of surface functionalization on thermal transport and thermoelectric properties of MXene monolayers

The newest members of a two-dimensional material family, involving transition metal carbides and nitrides (called MXenes), have garnered increasing attention due to their tunable electronic and thermal properties depending on the chemical composition and functionalization. This flexibility can be exploited to fabricate efficient electrochemical energy storage (batteries) and energy conversion (thermoelectric) devices. In this study, we calculated the Seebeck coefficients and lattice thermal conductivity values of oxygen terminated M2CO2 (where M = Ti, Zr, Hf, Sc) monolayer MXene crystals in two different functionalization configurations (model-II (MD-II) and model-III (MD-III)), using density functional theory and Boltzmann transport theory. We estimated the thermoelectric figure-of-merit, zT, of these materials by two different approaches, as well. First of all, we found that the structural model (i.e. adsorption site of oxygen atom on the surface of MXene) has a paramount impact on the electronic and thermoelectric properties of MXene crystals, which can be exploited to engineer the thermoelectric properties of these materials. The lattice thermal conductivity kappa(l), Seebeck coefficient and zT values may vary by 40% depending on the structural model. The MD-III configuration always has the larger band gap, Seebeck coefficient and zT, and smaller kappa(l) as compared to the MD-II structure due to a larger band gap, highly flat valence band and reduced crystal symmetry in the former. The MD-III configuration of Ti2CO2 and Zr2CO2 has the lowest kappa(l) as compared to the same configuration of Hf2CO2 and Sc2CO2. Among all the considered structures, the MD-II configuration of Hf2CO2 has the highest kappa(l), and Ti2CO2 and Zr2CO2 in the MD-III configuration have the lowest kappa(l). For instance, while the band gap of the MD-II configuration of Ti2CO2 is 0.26 eV, it becomes 0.69 eV in MD-III. The zT(max) value may reach up to 1.1 depending on the structural model of MXene