Explosive vaporization in microenclosures

The explosive vaporization of a liquid above planar microheaters induces a fast increase of pressure that is exploited in many thermally driven actuators in MEMS components such as ink jet printer cartridges, pumps, valves and optical switches. Some of these components need to enclose the working fluid as it is the case of valves in which the heated liquid is separated from the flow that it regulates by a flexible membrane. To achieve a better insight into the thermodynamic processes involved, the present work investigates experimentally an enclosed microsystem designed and fabricated for this purpose, composed of a small liquid volume (8 nL) heated by a electric pulse for 2 μs supplied to a planar microfabricated heater. During the heating, the temperature-induced change in resistance can be determined by imposing a defined current and measuring the voltage drop over the heater. While the chip is based on a silicon substrate with integrated platinum heaters and sensors, the structure enclosing the fluid (cavity and fluidic access to it) is made of a silicone elastomer, poly(dimethylsiloxane) (PDMS). This transparent material is widely used in microfluidics and allows for flexible and transparent walls that can be deflected by increasing the pressure inside the cavity. To seal the system the inlet and the outlet were closed by blocking them with a metallic stab. In the present work we visualize vaporization of isopropanol in contact with a suddenly heated planar resistor for two different cavity heights, 150 μm and 16 μm. The rate of temperature rise of the thin liquid layer in contact with the heater is of the order of 10⁷ K s⁻¹ for a pulse duration of 2 μs. We compare bubble growth and collapse for the open and closed systems. Compared to the open system, the bubble growth in the closed system is considerably damped.     

Experimental investigation of creep-crack-tip deformation using moiré interferometry

High-temperature moiré interferometry was applied to obtain full-field creep-crack-tip displacements of a three-point bend Al 2024-T4 specimen uner constant temperature of 200°C up to 720 hr.C ^* was evaluated by the moiré data obtained at discrete time intervals. Test results indicate that under steady-state creep condition, the creepcrack-tipv-displacement rate agrees with the asymptotic solution based on theC ^* integral. However, no creeping behavior was observed for the crack-tipu-displacement field aftert=276 hr. This discrepancy may be due to the initial large creep-crack-tip blunting and cavitation damage which alter the creep-crack-tip singular field such that theC ^* integral is no longer applicable to characterize the steady-state creep-crack-tip field. Postmortem studies of the tested specimen also revealed transgranular fracture path with the ordered grain boundaries perpendicular to the crack line, which may relate to the existence of the substantial crack-tip blunting and noncreeping behavior of theu-displacement field, i.e., the size and shape of material grain boundaries play an important role in the crack-tip-creeping behavior of the material.Paper was preseted at the 1991 SEM Spring Conference on Experimental Mechanics held in Milwaukee, WI on June 9–13.     

退火和调质42CrMo合金钢单轴循环塑性行为的实验研究

在室温下对退火和调质42CrMo合金钢进行了单轴应变控制和应力控制的系统循环实验,并对它们的应变循环和应力循环特性进行比较.揭示和分析了应变幅值、平均应变及其历史对材料应变循环特性的影响以及应力幅值、平均应力及其历史对棘轮行为的影响.讨论了应变循环和应力循环间的交互作用以及不同热处理工艺下材料循环变形行为间的区别.研究发现材料的热处理工艺、平均应力和应力幅值及其历史对材料的棘轮行为都有很大的影响.得到了一些有助于进行合理本构描述的结果.     

A theoretical model for electroelastic analysis in piezoelectric fibre push-out test

A theoretical model is presented to study the elastic deformation process and frictional sliding behavior in single piezoelectric fibre push-out tests. Based on the theoretical model and some necessary simplifications, stress and electric fields are obtained for push-out tests of a circular piezoelectric fibre embedded in an elastic matrix. Numerical results of a piezoelectric fibre/expoxy matrix system are presented to verify the proposed formulation. The study shows that there is a significant effect of the piezoelectric parameter and embedded fibre length on stress transfer, electric field distribution and load-displacement curve of the frictional sliding process. This study also indicates that the piezoelectric effect has a distinct influence on the mechanical behavior and properties of the interface in a fibre/matrix system.     

亚临界雷诺数下圆柱绕流的大涡模拟

本文应用Smagorinsky涡粘性模式和二阶精度的有限体积法对圆柱绕流的流场进行大涡模拟．求解了非正交曲线坐标系下的N-S方程,对雷诺数为100和20000的工况进行了计算．计算结果与实验及动力涡粘性模式的结果进行了比较,表明计算对于层流及高亚临界雷诺数的湍流流动是合理的     

Effect of vapor condensation on forced convection heat transfer of moistened gas

The forced convection heat transfer with water vapor condensation is studied both theoretically and experimentally when wet flue gas passes downwards through a bank of horizontal tubes. Extraordinarily, discussions are concentrated on the effect of water vapor condensation on forced convection heat transfer. In the experiments, the air–steam mixture is used to simulate the flue gas of a natural gas fired boiler, and the vapor mass fraction ranges from 3.2 to 12.8%. By theoretical analysis, a new dimensionless number defined as augmentation factor is derived to account for the effect of condensation of relatively small amount of water vapor on convection heat transfer, and a consequent correlation is proposed based on the experimental data to describe the combined convection–condensation heat transfer. Good agreement can be found between the values of the Nusselt number obtained from the experiments and calculated by the correlation. The maximum deviation is within ±6%. The experimental results also shows that the convection–condensation heat transfer coefficient increases with Reynolds number and bulk vapor mass fraction, and is 1∼3.5 times that of the forced convection without condensation.     

旋翼尾流与地面干扰时地面涡现象的研究

用Ｎ－Ｓ方程对近地飞行时旋翼尾流与地面干扰时产生的地面涡现象进行了数值计算旋翼对流场的作用由分布在特定区域内的动量源项模拟结果表明，旋翼尾流撞到地面后的卷起和轴向流动的拉伸作用是形成地面涡的原因；地面边界层形成的二次分离涡向地面涡内输入（与尾流所携带的涡量）相反的涡量，而使地面涡保持平衡；地面涡的存在和运动使旋翼附近流场大大改变     

含动脉分支的体循环模拟实验系统

本文首先建立了含分支动脉的血液体循环集中参数模型，在计算机仿真研究的基础上，设计、研制出含主要动脉分支的体循环模实验系统，系统由微机实施控制，数据采集与处理。实验结果表明它既可较好地模拟左心室、主动脉弓附近的血流动力学特性，同时又可较好地模拟左锁骨下动脉、挠动脉处的血流压力脉搏波基本特征。     

Si/Ge/Si异质横向SPiN二极管固态等离子体解析模型

采用横向表面PiN(SPiN)二极管构造的硅基可重构天线具有众多优于传统天线的独特优势, 是实现天线小型化和提升雷达与微波通信系统性能的有效技术途径. 本文提出一种Si/Ge/Si异质横向SPiN二极管, 并基于双极扩散模型与Fletcher型边界条件, 在大注入条件下建立了二极管结电压、电流密度与本征区固态等离子体浓度分布解析模型, 并数值模拟分析了本征区长度、P⁺与N⁺区掺杂浓度、外加电压对所建模型的影响. 结果表明, 固态等离子体浓度随本征区长度的增加下降, 随外加电压的增加而指数上升, 随P⁺与N⁺区掺杂浓度的提高而上升, 电流密度随外加电压的增加而指数上升. 同等条件下, 异质SPiN二极管的固态等离子体浓度相比同质二极管提高近7倍以上. 本文所建模型为硅基可重构天线的设计与应用提供有效的参考.