Computational fluid dynamics were used to study the three-dimensional unsteady incompressible viscous flows in a centrifugal pump during rapid starting period (≈0.12 s). The rotational speed variation of the field around the impeller was realized by a dynamic slip region method, which combines the dynamic mesh method with nonconformal grid boundaries. In order to avoid introducing errors brought by the externally specified unsteady inlet and outlet boundary conditions, a physical model composed of a pipe system and pump was developed for numerical self-coupling computation. The proposed method makes the computation processes more close to the real conditions. Relations between the instantaneous flow evolutions and the corresponding transient flow-rate, head, efficiency and power were analyzed. Relative velocity comparisons between the transient and the corresponding quasisteady results were discussed. Observations of the formations and evolutions of the primary vortices filled between the startup blades illustrate the features of the transient internal flow. The computational transient performances qualitatively agree with published data, indicating that the present method is capable of solving unsteady flow in a centrifugal pump under transient operations.

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Modern pumps are designed to guarantee a sufficiently large operating range or to satisfy the performance requirements relative to more than one operating point. This study applies trailing-edge (TE) modification method based on TE rounding in the suction surface to widen the operating range of a mixed-flow pump. The effects of TE modification on the performance and internal flow of the mixed-flow pump are investigated through computational fluid dynamics (CFD) analysis. Local Euler head distribution is introduced to reveal the pattern of energy growth along the blade-aligned (BA) streamwise location. A pump model with TE modification is tested, and numerical results agree well with experimental data. The results show that TE modification significantly improves pump efficiency in the high flow rate region by more than 10%. The best pattern of normalized local Euler head distribution (NLEHD) is a convex curve of nearly constant growth rate. The overall heads are also improved, and the flow near the exit of the impeller exhibits better uniformity. This finding demonstrates that a small change in the TE of the impeller can influence flow structure in most areas of impeller channels and that the local Euler head distribution is closely related to pump efficiency. TE modification can effectively improve the performance of the mixed-flow pump with high flow rate.

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Transient characteristics and flows in a centrifugal pump during its starting period were experimentally and numerically investigated. The two-dimensional particle image velocimetry technique was used to capture transient flow evolutions in the pump's diffuser. A new dynamic slip region method that combines the dynamic mesh method with the non-conformal grid boundaries is proposed to resolve the transient flows caused by the started impeller. Numerical self-coupling was realized by establishing a circulation pipe system along with the pump model equivalent to the experimental pump system. Numerical and experimental results agree well in both explicit characteristics and internal transient flow structures, confirming the validity of the proposed method. Analysis of the instantaneous flow in the impellers indicates that for the early stage of the startup, the transient vortex evolution between blades is the main reason for the transient head coefficient being lower than the steady state value. The reversed flow at the blade inlet is a less important reason for this effect. In later stages, the weakening of the intensity of the spatial vortex visible on S 2m and the main flow stream are the main reasons for the explicit performance slowly rebounding to the steady value.

Reducibility is key for the use of bulk metal oxides in chemical transformations involving redox reactions, but probing microscopic processes of oxide reduction is challenging. This is because the insulating nature of bulk oxides restricts ion and electron spectroscopic measurements of oxide surfaces. Herein, using a combination of environmental transmission electron microscopy and atomistic modeling, we report direct in situ atomic-scale observations of the surface and subsurface dynamics and show that the hydrogen-induced CuO reduction occurs through the receding motion of Cu–O/Cu bilayer steps at the surface, the formation of the partially reduced CuO superstructure by the self-ordering of O vacancies in the subsurface, and the collapse of Cu–O layers in the bulk. All these substeps can be traced back to the progressively increased concentration and activity of O vacancies in the surface and subsurface of the oxide, thereby leading to the self-accelerated oxide reduction. These results demonstrate the microscopic details that may have a broader applicability in modulating various redox processes.

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Surface and subsurface are commonly considered as separate entities because of the difference in the bonding environment and are often investigated separately due to the experimental challenges in differentiating the surface and subsurface effects. Using in-situ atomic-scale transmission electron microscopy to resolve the surface and subsurface at the same time, we show that the hydrogen-CuO surface reaction results in structural oscillations in deeper atomic layers via the cycles of ordering and disordering of oxygen vacancies in the subsurface. Together with atomistic calculations, we show that the structural oscillations in the subsurface are induced by the hydrogen oxidation-induced cyclic loss of oxygen from the oxide surface. These results demonstrate the propagation of the surface reaction dynamics into the deeper layers in inducing nonstoichiometry in the subsurface and have significant implications in modulating various chemical processes involving surface-subsurface mass transport such as heterogeneous catalysis, oxidation, corrosion and carburization.