Non-trivial effects of high-frequency excitation for strongly damped mechanical systems

Some non-trivial effects are investigated, which can occur if strongly damped mechanical systems are subjected to strong high-frequency (HF) excitation. The main result is a theoretical prediction, supported by numerical simulation, that for such systems the (quasi-) equilibrium states can change substantially with the level of damping. For example, a strongly damped pendulum, with a hinge vibrated at high frequency along an elliptical path with horizontal or vertical axis, will line up along a line offset from the vertical; the offset vanishes for very light or very strong damping, attaining a maximum that can be substantial (depending on the strength of the HF excitation) for finite values of the damping. The analysis is focused on the differences between the classic results for weakly damped systems, and new effects for which the strong damping terms are responsible. The analysis is based on a slightly modified averaging technique, and includes an elementary example of an elliptically excited pendulum for illustration, alongside with a generalization to a broader class of strongly damped dynamical systems with HF excitation. As an application example, the non-trivial behavior of a classical optimally controlled non-linear system is investigated, illustrating how HF excitation may cause the controller to leave the system in an unexpected equilibrium state, quite different from the setpoint. The effects can be interesting for specialists in control of mechanical systems and structures. However the obtained results are more general. Similar effects could be expected first of all for microsystems where damping forces are typically dominating over inertia forces.     

Effects of strong compressibility in natural convective flows through porous media with a near-critical fluid

The dynamics and heat transfer in a porous medium occupied by a liquid with parameters in the neighborhood of the critical point of “liquid-gas” transition are simulated numerically within the framework of the equations of dynamics of a porous medium with a compressible liquid phase and the Van-der-Waals equation of state. Adiabatic heating of the liquid phase in a porous layer initiated by a jump in temperature on one of the boundaries is investigated under microgravity conditions. Thermo-gravitational convection in the unsteady and steady-state regimes is simulated in rectangular domains and the effect of adiabatic heating on convection is studied. Calibration relations between the Rayleigh-Darcy and Prandtl numbers in the basic system of equations and their real analogs are obtained. A comparison is made with convection in a porous media occupied by a perfect gas.     

Thermo-electro-structural coupled analyses of crack arrest by Joule heating

Using the finite element method, thermo-electro-structural coupled analyses of the cracked conducting plate under high electric current have been solved. The crack contact condition and temperature-dependent material properties are considered in this analysis. The crack tip temperature, electric current density factor, stress intensity factor and strain energy density factor are obtained for discussions. Due to high electric current density and Joule heating at the crack tip, a circular melting area may exist around the tip. After cooling, a circular void or hole may occur at the crack tip and the crack arrest is achieved. The crack tip temperature decreases when the crack contact area increases. The proper tensile load is necessary for making the crack open enough and causing high current density at the crack tip and associated crack arrest. On the other hand, the crack tip temperature increases with time by the increasing external current and Joule heating. The values of mode-I stress intensity factor and strain energy density factor decrease with time due to the thermal deformation around the crack tip. Because of the temperature-dependent resistivity, the variation of the electric current density factor is complicated. In addition, it is not easy to create a crack-arrest condition when the crack length relative to the plate width is too small.     

钢中氧化物夹杂差热油取峰形分离研究

实验探讨了构成峰形重叠的各种因素,对分峰参数进一步优化组合,获得良好分峰效果;试验确定了谱碳钢中若干氧化物的释放峰位,并经Ｘ－射线结构分析予以佐证,为差热抽取法测定钢中氧化物奠定了可靠的实验基础。方法特点：快速（约５ｍｉｎ）、灵敏、低成本,时获得钢中总氧物及氧化物氧量。     

热离子、超热离子和离子加热

通过区分热离子和超热离子,讨论了ＨＬ－１Ｍ等离子体中性粒子束加热的」初步实验结果,分析表明,虽然电子对离子的磁撞加热机制仍然占主导地位,但中性粒子束也加热了离子。     

快波加热的天线设计

托卡马克中离子回旋频段的天线耦合效率主要取决于天线的体制和天线的尺寸。根据等离子体对不同谱分量的吸收性能差别,以及天线激励电流的谱应与等离子体对射频波吸收谱相匹配,对快波加热天线设计问题进行了细致地探讨。     

激光加热物体的三维模型计算

将物体的热传导系数、比热容、密度、物体对激光的吸收系数考虑成温度的函数,并考虑到物体向周围空间有热辐射和对流换热交换能量等情况下,用控制容积法对激光加热物体作三维真实边界的计算分析.这一结果在激光加热物体温度场的计算上有广泛的实用性.     

Frequency Dependence of the Dynamic Heat Capacity at the Melting Temperature of Polyethylene Crystals

The dynamic heat capacity of polyethylene was measured in the heating process over two decades of the modulating frequency using the light heating modulated temperature DSC. The dynamic heat capacity exhibited clear frequency dependence from 95°C to the end of the melting of the crystals. Frequency dependence of this work was compared with that of the quasi-isothermal measurement. The relaxation time estimated in this work was much shorter than that of the quasi-isothermal measurement. It was found that notable heat exchange between the sample and reference sides occurred between 120 and 135°C. Frequency dependence of the heat exchange was studied.This revised version was published online in November 2005 with corrections to the Cover Date.     

Effect of Heating Temperature on the Properties of Chars and Activated Carbons Prepared From Oil Palm Stones

The effects of heating temperature of pyrolysis and activation processes on the textural and chemical properties of the chars and activated carbons prepared from oil palm stones, an abundant palm-oil mill solid waste, were studied. For both pyrolysis and activation, relatively high temperature was essential to develop pore structures in the chars and activated carbons. However, too high temperature would cause pore narrowing and pore enlargement during pyrolysis and activation, respectively. The temperature had an insignificant effect on the inorganic components of the chars and activated carbons, but affected their surface organic functional groups significantly. This revised version was published online in August 2006 with corrections to the Cover Date.     

微波等离子CVD金刚石设备中基片加热材料的三维温度场型研究

在微波等离子CVD设备中,微波电磁场的不均匀分布将导致等离子球和基片的温度不均匀,从而降低CVD金刚石的质量,因而基片加热是需要的.材料的吸收微波能的能力同微波频率、电场强度、材料的介电常数和介电损耗及材料体积相关,材料的介电常数、介电损耗、导热率又同温度相关.基于热力学理论,本文用强吸收微波能的SiC材料作为了基片加热材料,放在微波等离子腔的基片下,研究三维轴对称温度场模型,该模型的基片加热材料的介电常数、介电损耗和体积随温度变化,获得了温度分布的解析式,计算结果显示该模型能得到直径76.2mm的均匀温度分布,温度变化少于10℃,因而可用于基片加热.