The energy-discharge characteristics of pump-turbines in pump mode with a hump region are significantly important for operating stability. To investigate the flow characteristics, 3D steady numerical simulations are conducted for a given guide vane opening of 32 mm by solving Reynolds-averaged Navier-Stokes (RANS) equations using the shear-stress transport (SST) k-ω turbulence model. Based on the validation of computational fluid dynamics (CFD) results using experimental benchmarks, the part-load (0.45φ_BEP), drooping zone load (0.65φ_BEP), near best efficiency point (BEP) (0.90φ_BEP), BEP (1.00φ_BEP), and overload (1.24φ_BEP) regions are chosen to analyze how and why the fluid properties change in the runner. The causes of flow separation and spatial characteristics of flow at different load points are obtained through the analysis of flow angle and hydraulic losses. The results show that flow angle at the leading and trailing edge from the crown to the band distributes differently among these five operating points. Then, the reasons for drooping are investigated based on the Euler theory. It is found that drooping behavior comes from both the incidence/deviation effect and frictional losses. In addition, the runner losses are more consequential to drooping as shown by hydraulic loss analysis.

Multi-actuated rigid-flexible dynamic system exists widely in precision machinery and electrical control fields. The performances, such as kinematic, dynamic, electrical, magnetic, and thermal performances, are correlated and difficult to trap precisely. Therefore, a multi-actuated mechanism design method considering structure flexibility using correlated performance reinforcing is put forward. A system containing flexible subparts with multi degrees of freedom (DOF) with physical coordinate is converted into modal coordinate using ‘DOF×modal order’ square matrix. The structure flexibility is described by modal superposition of the shape mode which is considered as additional generalized coordinates. A dynamic equation with large DOF is formulated and reduced based on Craig-Bampton modal truncation. Using analogical design methodology with and without structure flexibility of the low voltage circuit breaker (LVCB), the extent of the performance impact of each subpart is obtained by calculating correlated Holm force, Lorentz force, electrodynamic repulsion force, electromagnetic force, and cantilevered bimetallic strip force. Design of experiments method is employed to reveal the hard-measuring properties using correlated relatively easy-measuring parameters. The trip mechanism is validated by an electrical performance experiment. results show that the structure flexibility can decrease the tripping velocity, which is non-negligible, especially for high frequency tripping. The method provides a reference significance for similar multi-actuated mechanism design.

The interaction of the machine-process in grinding frequently brings unpredictable results to the quality of the products and processing stability. This paper presents a multi-grains grinding model to simulate the precision grinding process of cemented carbide inserts. The interaction between the grinding process and machine tool is then investigated based on the proposed grinding model. First, the real topography of the grinding wheel is simulated. Based on the assumption of spacing distribution of multi-grains and the virtual grid method, the hexahedron abrasive grains are randomly distributed on the surface of the virtual grinding wheel and the postures of abrasive grains are randomly allocated. Second, the grinding model is built by importing the virtual grinding wheel model into Deform-3D software, and the grinding force values are obtained by simulation. The validity of the proposed grinding model is verified by experiments. Then, the interaction coupling simulation of the machine tool structure and grinding process is built to investigate the interaction mechanism. The simulations reveal that remarkable interactive effects exist between the deformation of the grinding wheel of the machine tool and grinding force. The finite element method (FEM) coupling simulation method proposed in this paper can be used to predict the machine-process interaction.

It is well known that the mean stress or stress ratio of fatigue loadings has a strong effect on the shape of S-N curves. An understanding of the relationships among S-N curves corresponding to different mean stresses or stress ratios would be very useful in engineering applications. In this study, based on continuum damage mechanics, a mathematical expression of an S-N curve is deduced from a new damage evolution law. This mathematical expression can well represent the whole S-N curve, not only the linear part in bi-logarithmic diagrams, but also the transitional part near the fatigue limit. The effect of mean stress on an S-N curve is represented by two state parameters. The relationships between these state parameters and the mean stress are proposed and examined. By using these relationships, the concepts of equivalent symmetric amplitude and equivalent symmetric cycles are introduced. We have found that all S-N curves under non-symmetric states can be rearranged into the same curve as that of symmetric fatigue by adopting these equivalent parameters.

We performed a transient numerical investigation on a microchannel heat sink with multiple impinging jets (MHSMIJ) to explore the effects on the fluid flow and heat transfer characteristics of the MHSMIJ of an unsteady impinging jet and heat flux imposed upon the substrate surface by using a computational fluid dynamics method. The heat fluxes being imposed upon the substrate surface and the inlet velocities of the jet were all set as sinusoidal functions with different amplitudes and periods with time. The effects of the amplitudes and periods of the functions on the substrate properties were analyzed. Cooling performance was evaluated by calculating the periodic average surface heat transfer coefficient, average temperature uniformity, and temperature variation of the target surfaces over a period. The results indicated that the surface heat transfer coefficient and average temperature of the cooled surface oscillated with the periodic heat fluxes, accompanied by obvious phase lags. The phase lag has a significant dependence on the periods, but little dependence on the amplitudes. The material properties of the substrate have complex influences on the transient behavior of the MHSMIJ. The periodic heat flux and periodic jet velocity significantly affected the transient thermal performance of the MHSMIJ, but had less effect on its overall performance. Further, transient heat flux and jet velocity caused non-uniform and transient temperature distributions, which will cause thermal fatigue phenomenon, and thereby have effect on the longevity of the MHSMIJ.

High power pulse tube cryocoolers are expected to be a very promising candidate for high temperature superconductors (HTS) cooling. Unfortunately there are still some problems significantly deteriorating the performance of these cryocoolers, one of which is temperature inhomogeneity. Several different theories have been proposed to explain the mechanism and many factors have been indicated as contributors to the generation and development of temperature inhomogeneity. However, some relations between these factors are seldom noticed, nor classified. The underlying mechanisms are not yet clear. The paper classifies, as internal and external, factors leading to temperature inhomogeneity based on their location. We examine some apparently unreasonable assumptions that have been made and difficulties in simulation and measurement. Theoretical and experimental research on the driving mechanism and suppression of temperature inhomogeneity is reviewed, and potential analysis and measurement methods which could be used in future are identified.

Using Goodier’s thermo-elastic displacement potential and Laplace transform, a semi-analytical method is developed for calculating the displacement and stress induced by heat transfer in sparsely fractured granitic rocks with saturated water flow and distributed heat sources. An integral equation of the thermo-elastic displacement potential is formulated in the Laplace-transformed domain. The fractures are discretized into rectangular elements, and the elemental integrals that involve singularities are calculated analytically. The numerical solutions of the potential are calculated using numerical Laplace inversion, and the temperature-gradient-induced displacements and stresses are calculated using central differences. The method is employed to examine the characteristics of the temperature-gradient-induced displacement and stress for a hypothetical problem that is intended to mimic the near-field environment of deep geological repositories of high-level radioactive wastes. Among other things, the results reveal the following: (1) In early time of operation of the repository, the region of rock under thermal expansion and compressive is limited; (2) As the intensity of the heat source gets smaller with time, only a small portion of the rock expands whereas the remaining portion contracts; (3) Downstream peak temperatures may be higher due to the supply of thermal energy by the water-flow-facilitated heat transfer, and patterns of influences of the water velocities on the thermal stress and displacement are similar; (4) Sufficiently close heat sources would cause superposition of the heating effects and make the near-field temperature increase significantly.