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[en] Highlights: • The thermal performance of a single-row heat exchanger was investigated. • Grid generated turbulence (GGT) was experimentally characterized. • GGT increased heat transfer in the single-row heat exchanger. • Flow visualizations showed vast changes in flow field under elevated free-stream turbulence. - Abstract: The thermal performance of single-row heat exchangers is generally considered to be inferior to multi-row heat exchangers. It is therefore desirable to optimize the former; especially for those cases which cannot accommodate an alternative. In this study, the heat transfer performance of a single-row circular-finned tube heat exchanger was investigated experimentally under elevated disturbance levels. Specifically, grid-generated turbulence (GGT)—the characteristics of which had been measured beforehand with a hot-wire anemometer—was applied to a clean-flow benchmark case and measurements taken at the heat exchanger outlet. It was shown that when the grid is positioned appropriately, a mean enhancement (in global Nusselt number) of up to 11% can be achieved. Flow visualizations revealed the flow structures responsible for the increase in heat transfer.

[en] We present recent advances with the quantum Monte Carlo (QMC) method in its application to molecular systems. The QMC method is a procedure for solving the Schroedinger equation statistically, by the simulation of an appropriate random process. The formal similarity of the Schroedinger equation with a diffusion equation allows one to calculate quantum mechanical expectation values as Monte Carlo averages over an ensemble of random walks. We have previously obtained highly accurate correlation energies for a number of molecules, as well as the singlet-triplet splitting in methylene and the barrier height for the H + H₂ exchange reaction. Recently we have begun a program of extending the QMC approach to the calculation of analytic derivatives of the energy. A brief description of the approach is presented here, together with some preliminary results. In addition, we are now computing expectation values of properties other than the energy. We summarize how standard QMC must be modified, and present some results for H₂ and N₂. Finally, we describe preliminary work toward the goal of obtaining accurate molecular excited states through QMC. 24 refs., 5 tabs

[en] Excited states of ¹¹Be have been studied with several transfer reactions. Nine states between 3.96 MeV and 25.0 MeV excitation energy show the characteristic energy dependence of a rotational band. The deduced large moment-of-inertia of this band is consistent with a two-α structure with large deformation. For ¹²Be four high lying states at 7.30 MeV, 10.7 MeV, 14.6 MeV and 21.7 MeV, which were observed in the ⁹Be(¹⁵N,¹²N)¹²Be reaction, also form a rotational band with almost the same moment-of-inertia as for ¹¹Be, using the tentative spin assignments of 2⁺ - 8⁺

[en] A many-body potential model for the description of actinide oxide systems, which is robust at high temperatures, is reported for the first time. The embedded atom method is used to describe many-body interactions ensuring good reproduction of a range of thermophysical properties (lattice parameter, bulk modulus, enthalpy and specific heat) between 300 and 3000 K for AmO₂, CeO₂, CmO₂, NpO₂, ThO₂, PuO₂ and UO₂. Additionally, the model predicts a melting point for UO₂ between 3000 and 3100 K, in close agreement with experiment. Oxygen–oxygen interactions are fixed across the actinide oxide series because it facilitates the modelling of oxide solid solutions. The new potential is also used to predict the energies of Schottky and Frenkel pair disorder processes. (paper)

[en] Atomic scale molecular dynamics simulations have been used to predict the location of glass modifying Na, Li and Mg species in a borosilicate Magnox type waste glass adjacent to interfaces with the (100) and (110) surfaces of MgO, CaO and SrO crystals. These simulations show a considerable increase in alkali and alkali earth concentration adjacent to specific interfaces. In particular, there are significant, systematic changes in Na, Li and Mg position and concentration as a function of both the crystals terminating surface and composition. (authors)

[en] Point and small cluster defects in magnesium aluminate spinel have been studied from a first principles viewpoint. Typical point defects that occur during collision cascade simulations are cation anti-site defects, which have a small formation energy and are very stable, O and Mg split interstitials and vacancies. Isolated Al interstitials were found to be energetically unfavourable but could occur as part of a split Mg-Al pair or as a three atom-three vacancy Al 'ring' defect, previously observed in collision cascades using empirical potentials. The structure and energetics of the defects were investigated using density functional theory (DFT) and the results compared to simulations using empirical fixed charge potentials. Each point defect was studied in a variety of supercell sizes in order to ensure convergence. It was found that empirical potential simulations significantly overestimate formation energies, but that the type and relative stability of the defects are well predicted by the empirical potentials both for point defects and small defect clusters.

[en] Segregation of the cation substitution defects Ba²⁺, Sr²⁺, Ce⁴⁺ and Zr⁴⁺ to the stable low index surfaces of UO₂ has been predicted using atomistic simulation techniques. While Ce⁴⁺ and Zr⁴⁺ substitute simply for U⁴⁺, charge compensation in the form of oxygen vacancies is required in the case of Ba²⁺ and Sr²⁺. Three surfaces are considered: (110), (111) and (100). Although the (111) and (110) are perfect cleaved surfaces, the (100) necessarily incorporates a series of surface defects to neutralize the inherent dipole. The segregation energies of these cations depend strongly on the surface to which the segregation is proceeding. Furthermore, it is also a function of the orientation of the segregating defect cluster with respect to the surface and, in the case of the (100) dipolar surface, the configuration of the surface defects

[en] Molecular dynamics simulations have been performed to investigate oxygen transport in (U_xPu_x-1)_0.95Gd_0.05O_1.975, (U_xTh_x-1)_0.95Gd_0.05O_1.975 and (Pu_xTh_x-1)_0.95Gd_0.05O_1.975 between 1000 and 3200 K. Oxygen diffusivity and corresponding activation energies are examined and compared to values for the undoped (U_xPu_x-1)O₂, (U_xTh_x-1)O₂ and (Pu_xTh_x-1)O₂ systems where compositions between end members display enhanced diffusivity. Below the superionic transition oxygen diffusivity for the Gd doped systems is orders of magnitude greater compared to their undoped counterparts. But, enhanced diffusivity for doped mixed actinide cation compositions is not observed compared to doped end members. Furthermore, changes in activation energy suggest changes in diffusion regime, which correspond to the creation of thermally activated oxygen defects.

[en] Building upon work in which we examined defect production and stability in spinels, we now turn to defect kinetics. Using temperature accelerated dynamics (TAD), we characterize the kinetics of defects in three spinel oxides: magnesium aluminate MgAl₂O₄, magnesium gallate MgGa₂O₄, and magnesium indate MgIn₂O₄. These materials have varying tendencies to disorder on the cation sublattices. In order to understand chemical composition effects, we first examine defect kinetics in perfectly ordered, or normal, spinels, focusing on point defects on each sublattice. We then examine the role that cation disorder has on defect mobility. Using TAD, we find that disorder creates local environments which strongly trap point defects, effectively reducing their mobility. We explore the consequences of this trapping via kinetic Monte Carlo (KMC) simulations on the oxygen vacancy (V_O) in MgGa₂O₄, finding that V_O mobility is directly related to the degree of inversion in the system