Numerical study on the formation of Taylor bubbles in capillary tubes

Numerical simulations have been carried out for the transient formation of Taylor bubbles in a nozzle/tube co-flow arrangement by solving the unsteady, incompressible Navier–Stokes equations. A level set method was used to track the two-phase interface. The calculated bubble size, shape, liquid film thickness, bubble length, drift velocity, pressure drop and flow fields of Taylor flow agree well with the literature data. For a given nozzle/tube configuration, the formation of Taylor bubbles is found to be mainly dependent on the relative magnitude of gas and liquid superficial velocity. However, even under the same liquid and gas superficial velocities, the change of nozzle geometry alone can drastically change the size of Taylor bubbles and the pressure drop behavior inside a given capillary. This indicates that the widely used flow pattern map presented in terms of liquid and gas superficial velocities is not unique.     

Suitability of three different two-equation turbulence models in predicting film cooling performance over a rotating blade

The suitability of three different two-equation turbulence models in predicting film cooling effectiveness on a rotating blade was investigated and they are the commonly used standard k-ε model, the k-ω model and the shear stress transport k-ω model. To fulfill this target, both numerical simulation and the experimental investigation were carried out for a rotating blade having a flat test surface with a 4 mm diameter straight circular cooling hole in 30° inclined injection. The blade rotated at five different speeds of 0, 300, 500, 800 and 1000 rpm. The momentum ratio was set to be 0.285 and the Reynolds (Re_D) number based on the mainstream velocity and hydraulic diameter of the mainstream channel is 1.45 × 10⁵. The averaged density ratio was chosen to be 1.026 with air as both the coolant and the mainstream. Comparison between the numerical work and the experimental results indicated that (1) the rotating speed is the most critical parameter influencing the film cooling effectiveness distributions and the pressure surface could be remarkably different from the suction surface, (2) as for the algebraic averaged film cooling effectiveness, numerical predictions of the three turbulence models all overshoot compared with the experimental results, (3) among the three turbulence models, the standard k-ε model gave the poorest prediction.     

弯曲载荷下机织复合材料T型接头渐进失效分析

根据树脂传递模塑(RTM)成型的缎纹机织复合材料T型接头的结构特征和纤维布局特点, 基于ANSYS有限元数值分析平台, 建立符合其真实结构的几何模型和有限元分析模型。基于渐进失效强度预测方法的基本思想, 使用有限元计算软件ANSYS的参数化设计语言(APDL)开发相应的程序, 实现改进形式的Hashin失效准则。采用合适的最终失效评价方法, 建立二维机织结构复合材料T型接头受弯曲载荷时的渐进失效预测方法, 能够有效地模拟从初始加载到最终失效过程中机织复合材料T型接头结构的力学响应及损伤的萌生与发展, 并预测结构的静强度。      

Computational study of a new scheme for a film-cooling hole on convex surface of turbine blades

An investigation on a heart-shaped film cooling hole is performed for one row, two staggered rows, and three staggered rows on a convex surface. The results are compared with those obtained from a simple cylinder hole as a baseline. Three-dimensional computational study for two heart-shaped holes of two crown angles, φ = 60° and φ = 90°, is conducted to ascertain adiabatic film cooling effectiveness to validate whether a heart-shaped cooling hole mitigates the vortexes responsible for the lift-off phenomenon in conventional simple cylinder hole. This paper also attempts to show that the staggered arrangement of a heart-shaped hole provides higher cooling performance. The result reveals that a heart-shaped hole highly mitigates the vortexes, thereby providing more coolant-surface attachment. The results reflect the tremendous increment in centerline and lateral adiabatic film cooling effectiveness for both crown angles, φ = 60° and φ = 90°. The heart-shaped hole of crown angle φ = 60° shows higher centerline effectiveness compared with that of the heart-shaped hole crown angle of φ = 90°. The latter provides higher lateral effectiveness. The relatively small volume and high effectiveness of the heart-shaped cooling hole is important and promising for the aero engine industry.     

Properties of hybrid fibre reinforced shell for investment casting

In order to improve the properties of silicon sol shell for investment casting process, a varying content of hybrid fibres (aluminium silicate and polypropylene) was introduced into slurry for preparation of fibre-reinforced shell in the present work. The bending strength, self-load deformation at elevated temperature, and the permeability of fibre-reinforced shell specimens were investigated and the fracture surfaces of shell specimens were observed by SEM. The results show that the bending strength of green shell increases with content of fibres in it. The maximum bending strength of 4.96 MPa was obtained in the fired shell with 0.6 wt% hybrid fibres addition. The high temperature self-loaded deformation of specimens of shell reinforced with a hybrid fibre addition above 0.6 wt% is higher than that of the unreinforced. However, the shell with a hybrid fibre addition up to 0.4 wt% exhibits the lower self-loaded deformation at high temperature compared to the unreinforced. It is also found that the permeability of shell specimens can be improved by hybrid fibres addition. Based on the fracture surfaces observation using a scanning electron microscope (SEM), the failure mode of the green shell reinforced with hybrid fibres is identified as fibre rupturing from the substrate of shell specimens, and/ or debonding from adhesive film surrounding it in shell. Even though the specimens of shell being fired at 900 °C for 2 h, the same failure features also exist in the fracture surfaces of specimens. This indicates that the specimens of shell can still be reinforced with aluminium silica fibres (residue of hybrid fibres) for their interpenetrating fibres network structure although go through firing.     

Opcode vector: An efficient scheme to detect soft errors in instructions

Bit flips on instructions may affect the execution of the processor depending on the Instruction Set Architecture (ISA) and the location of the flipped bits. Intrinsically, ISAs may detect bit upsets if the errors on the instructions produce exceptions that halt the execution. In this paper, we explore a dynamic checking of the instructions to detect errors before execution. The scheme is based on loading an approximate representation of the instructions based on a vector that identifies the opcodes used in the program in a special purpose register. During execution, instructions are first checked on the register and on a negative an error is detected as the instruction has an opcode that does not correspond to any of the ones used in the program. Since we use an approximate representation, a small number of false positives can occur for erroneous instructions which may still be detected if they lead to a system crash. The proposed opcode vector scheme is compared with the use of a Bloom filter (BF) that has been previously proposed to detect errors on instructions. In both cases, a check can produce false positives but not false negatives. The Bloom filter is built using all the bits in the instruction. On the other hand, the opcode vector uses only a few bits of the instruction. In both cases, the check is combined with a previous error propagation scheme. In the opcode case, this ensures that all errors corrupt the opcode bits while for the BF, the error propagation reduces the number of false positives. The proposed approach has two main benefits. The first one is an increase in the error detection rate as the set of valid instructions is restricted to those used in the program allowing the detection of invalid instructions even if they do not lead to a system crash. The second one is that errors are detected before the crash. This is done at the cost of adding a small register for the vector of opcodes and some control logic. This is significantly simpler than in the case of the BF that needs to compute several hash functions and access several bits on the register to perform the check. We evaluated this approach on binary files of the ARM Cortex M0 core. According to our findings, the proposed vector of opcodes is more effective to detect errors than the BF and its detection rate is less dependent on the program size. Based on those results, it seems that the proposed method can be an interesting option to detect errors in instructions for systems on which a small overhead can be introduced if it improves reliability.     

Comparison of corrosion behavior of Ti3SiC2 and Ti3AlC2 in NaCl solutions with Ti

Corrosion behavior of self-sintered, ternary-layered titanium silicon carbide (Ti₃SiC₂) and titanium aluminum carbide (Ti₃AlC₂) fabricated by an in-situ solid-liquid reaction/hot pressing process was investigated by potentiodynamic polarization, potentiostatic polarization and electrochemical impedance spectroscopy in a 3.5% NaCl solution. Commercially pure titanium (Ti) was selected for comparison through XRD, XPS, SEM and EDS examinations for elucidating both the passivation behavior and corrosion mechanism of the alloys. Both Ti₃SiC₂ and Ti₃AlC₂ exhibited significantly superior passivation characteristics compared to Ti; Ti₃SiC₂ also showed better corrosion resistance. The silicon/aluminum site is prone to attack, and the difference in the diffusion rate between the A-site atoms and titanium decreases the passivation ability of the MAX phase. CP titanium exhibited a lower passivation current density and did not undergo breakdown in the test potential region while two MAX phases are destroyed. Nevertheless, the corrosion resistances of Ti₃SiC₂ and Ti₃AlC₂ are comparable to that of CP titanium.     

A weighted hidden Markov model approach for continuous-state tool wear monitoring and tool life prediction

Tool wear is one of the important indicators to reflect the health status of a machining system. In order to obtain tool’s wear status, tool condition monitoring (TCM) utilizes advanced sensor techniques, hoping to find out the wear status through those sensor signals. In this paper, a novel weighted hidden Markov model (HMM)-based approach is proposed for tool wear monitoring and tool life prediction, using the signals provided by TCM techniques. To describe the dynamic nature of wear evolution, a weighted HMM is first developed, which takes wear rate as the hidden state and formulates multiple HMMs in a weighted manner to include sufficient historical information. Explicit formulas to estimate the model parameters are also provided. Then, a particular probabilistic approach using the weighted HMM is proposed to estimate tool wear and predict tool’s remaining useful life during tool operation. The proposed weighted HMM-based approach is tested on a real dataset of a high-speed CNC milling machine cutters. The experimental results show that this approach is effective in estimating tool wear and predicting tool life, and it outperforms the conventional HMM approach.     

Characterization of hot deformation behavior of Haynes230 by using processing maps

The hot deformation characteristics of Haynes230 has been investigated in the temperature range 1050–1250 °C and strain rate range 0.001–10 s^?1 using hot compression tests. Power dissipation map for hot working are developed on the basis of the Dynamic Materials Model. The map exhibits two domains of dynamic recrystallization (DRX): one occurring in the temperature range of 1200–1250 °C and in the strain rate range of 0.001–0.03 s^?1, which associated with grain coarsening; the other occurring in the temperature range of 1100–1200 °C and strain rate range of 0.001–0.01 s^?1, which are the optimum condition for hot working of this material. The average apparent activation energy for hot deformation is calculated to be 449 kJ/mol. The material undergoes flow instabilities at temperatures of 1050–1100 °C and at strain rates of 1–10 s^?1, as predicted by the continuum instability criterion. The manifestations of the instabilities have been observed to be adiabatic shear bands which are confirmed by optical observation.