Gongcheng Kexue Yu Jishu/Advanced Engineering Science

The DZ125 alloy has excellent mechanical, fatigue, and high-temperature behaviors. It has been widely used in high-temperature gas turbine components. Although a lot of researches have been carried out on the fatigue problem of the DZ125 alloy, there are still few systematical studies on the fatigue crack initiation mechanism and its failure mechanism during very high cycle fatigue (VHCF) by microscopic fracture analysis. It is found that the scatter of fatigue data increases as fatigue stress decreases, and the fatigue life does not increase significantly even at a low fatigue stress (<220 MPa). The above characteristics are directly related to the change of micro-crack initiation mechanism. Under a high fatigue stress, micro-crack tends to initiate from surface or sub-surface of specimen, and the large secondary crack is the main feature of its fracture morphology. At a low fatigue stress, micro-crack is likely to initiate from interior material defects, and the existence of these defects can seriously affect the fatigue life. Persistent slip bands (PSB) lead to the formation of rough surfaces, which is the main feature at the crack tip. The main crack competes with other secondary cracks and eventually causes fatigue fractures. The results of the Murakami equation show that the average stress intensity factor at micro-crack initiation stage is 3.15 MPa·m1/2, the average stress intensity factor at the onset of unstable crack propagation is 7.7 MPa·m1/2, and the average fracture toughness (KIC) is 15.70 MPa·m1/2.

With the increase of hydropower units and installed capacity in China,the fault problem during the transient process occurs frequently.The signal analysis method is widely used to deal with the problem.The variational mode decomposition (VMD) method,a relatively new self-adaptive method for processing non-linear and non-stationary signals,was adopted to investigate the vibration signals during the transient process of a hydropower unit in the Gezhouba power plant.The first step was to use VMD to analyze the simulated vibration signals of the transient process,and to verify the effectiveness of the VMD method in dealing with non-linear and unstable signals.The Hilbert-Huang transform (HHT) was then used to calculate the instantaneous frequency of each intrinsic mode functions (IMF) decomposed by VMD to obtain the corresponding Hilbert spectrum.These results were compared with those from empirical mode decomposition (EMD) method.From the final vibration signal analysis,the IMF decomposed by VMD could be divided into three parts.The first part was the trend of signal development.The second part was the basic decomposition of the signal with a strong regularity of "spindle" signal reflecting the change law of the transient process.The third part was high-frequency characteristic signals mainly caused by interference noise or unit failure.The corresponding Hilbert spectrum had the characteristics of small fluctuation and stable amplitude.The results showed good consistency between the frequency change of each component and the time-dependent rule of the rotational speed.This suggested that VMD was capable of extracting characteristic frequencies to facilitate the analysis of vibration characteristics.Compared with EMD,the VMD outperformed in self-adaptivity.It provided more accurate and effective analysis results and a better representation of the vibration rule during the transient process of the hydropower unit.

Rigid body guidance synthesis is a typical problem in mechanism design.In order to avoid the defects by traditional synthesis methods based on Burmester kinematic geometry theory,such as the limitation of mechanism structure type and the uncertainty of mechanism function,a generalized function oriented geometric synthesis method based on similarity transformation of the mechanism function modules was discussed.Since the standard mechanism modules were considered as basic design units for this method,the structure type of guidance mechanism modules need to be extended for a larger solution space,thereforethe geared linkage structure that can generate complex motion patterns was taken into account,aiming at the establishment of corresponding function module through kinematic analysis.At first,the transformation and mapping method between transmission function and guidance function were obtained through the configuration analysis of geared linkage,thus converting the transmission mechanism to the guidance mechanism through changing the rotation center of output gear.Secondly,the complex vector method was introduced for the kinematic analysis to obtain the guidance function,after that the geared linkage function module with full topology information was established,which included the transmission characteristics as well as the guidance characteristics,i.e. the relative dimensional parameters,the guidance angle performance, the guidance point trajectory and the motion geometric identifier.Then,in order to obtain the method for adjusting dimensional parameters of the preselected function module,a bidirectional mapping rule between relative dimensions and achievable guidance functions was revealed.At last,a practical guidance mechanism design task for powder production with partial parallel guidance function requirement was given, the similarity transformation of geometric identifier between geared linkage function module and the guidance task was presented, then the actual solution mechanism size and position parameters which meet the technical requirements were obtained. Finally the validity and practicability of the proposed generalized synthesis method were verified by the analysis of design results.

It has been showed directly or indirectly that the lubricant is possible to be sucked back into the mechanical seal with dimples or grooves in the existing studies.This phenomenon,termed "outlet suctio" in this paper,may decrease the leakage.Further studies are still required for this phenomenon and its mechanism.A three-dimensional fluid numerical simulation,based on the multiphase flow cavitation model in Fluent,was developed for the mechanical seal with inclined elliptical dimples.The causes of outlet suction and the influence of the outlet suction on leakage were studied by changing the differential pressure between the inner and outer diameter and the rotation rate.Firstly,in order to ensure the accuracy of calculation model,the fluid factor was calculated for the flow pattern check,the grid independence was tested for the grid size,and the simulation result was compared with the reference.Secondly,in the numerical simulation experiments,the phenomenon of outlet suction was observed directly in the streamline plots on the r-θ section of calculation unit beside the outlet section and the velocity distribution contours in radial direction on the outlet section.Thirdly,the pressure and radial velocity distribution on r-θ section was discussed.The results showed that the outlet suction occurs because the pressure close to the seal outlet falls below the external atmospheric pressure.This pressure decrease is caused by the hydrodynamic effect near the divergence gaps of dimples.The increase of hydrodynamic effect enhances the outlet suction.When the decreased pressure reaches the cavitation pressure,the cavitation occurs.The cavitation effect hinders the increase of outlet suction.Finally,the effect of Δp and n on outlet suction,and the effect of outlet suction on the leakage decrease were studied based on the analysis of the out flow rates and outlet suction flow rates.The leakage decreases because of the outlet suction,even to zero.The outlet suction caused by inclined elliptical dimples can be employed to decrease the leakage of mechanic seals.

The accurate calculation of boundary shear stress is very important to the deep understanding of sediment transportation and river evolution. The current research is limited to straight and mildly curved channels, while for a sharply-curved channel, the flow is complex accompanied by transverse circulation and large transverse slope which was affected by the dual effect of gravity and centrifugal force. As the boundary shear stress was affected by many factors, the applicability of differentcalculation methods requires further study. The distribution of three-dimensional flow velocity and dynamic-staticpressure difference were monitored by ADV as well as Preston tube in a 180° sharply-curved flume under the subcritical flow condition. The typical features of transverse flow circulation, redistribution of longitudinal flow velocity and turbulence kinetic energy were analyzed. Based on the analyses, four empirical formulae and k-ε model were selected to calculate the boundary shear stress on the control section of the flume. The computational results of turbulence kinetic energy method and Preston tube method agree well with the k-ε numerical simulation not only in distribution pattern but also in magnitude. Therefore, the above three methods are feasible for the calculation of boundary shear stress in a sharply-curved channel. The boundary shear stress in bed and bank slope of the whole flume was calculated by numerical simulation. The results show that the value of boundary shear stress is small and evenly distributed in thestraight section following the flow entrance, then gradually increases and exhibits non-uniformity in the bending section. Finally it reaches the maximum near the bank slope in the straight section in front of the outlet. On the transverse section, the boundary shear stress is well distributed along the bed while strongly fluctuated near the toe of slope in which the flow is complex with large circulation. Under the effect of the bend, the maximum boundary shear stress in the transverse section gradually shifted from convex bank to concave bank, in order to keep up with the mainstream. The maximal boundary shear stress of the flume is located in the convex bank of 110° cross-section of the bend and the concave bank of the cross-section 0.5 m downstream off the bend outlet. By changing the rate of flow but keeping the downstream water depth constent, the overall distribution of the boundary shear stress is similar and shows the characteristic of strong water flow along straight line while weak water flow along curve line. These researches provided support for flow shear transport mechanism, forecast of river evolution and safety management in the sharply-curved channel. © 2017, Editorial Department of Advanced Engineering Sciences. All right reserved.