Radiation effects in the transient hot-wire technique

The transient hot-wire technique is widely used for absolute measurements of the thermal conductivity and thermal diffusivity of fluids. It is well established that fluid radiation effects significantly influence these measurements, especially those for the thermal diffusivity. Corrections for radiation effects are based on the models developed and deviations of the measured data from the ideal line source model. In this paper, the effect of fluid radiation on the measurements of the thermal conductivity of n-pentane is presented. For comparison, the influence of thermal radiation effect on measurement of transparent fluids, such as argon is also shown. The difference between the influence of natural convection and thermal radiation is also demonstrated.     

热丝电流对HWCVD制备α-Si∶H膜结构及钝化效果的影响

采用热丝化学气相沉积(HWCVD)方法沉积本征非晶硅薄膜,研究了热丝电流对薄膜结构及其钝化单晶硅片效果的影响.采用光谱型椭偏仪分析了非晶硅薄膜的介电常数虚部ε2和薄膜空位浓度的变化,采用傅里叶红外光谱测试仪分析了膜中Si-HX键,使用硅片的少子寿命表征钝化效果.结果表明:在热丝电流(两根直径为0.5 mm的钽丝的总电流)为20.5～23.5 A时,随着热丝电流增大薄膜中空位浓度逐渐增大,薄膜中氢总含量在热丝电流约22.0 A时出现峰值,而此时薄膜微观结构参数R*最小,钝化效果在约21.5A处出现峰值,对应的表面复合速率低至2.9 cm/s.     

Traveling disturbance appearing in boundary layer transition in a Yawed cylinder

Boundary layers that develop over a body in fluid flow are in most cases three-dimensional owing to the spin, yaw, or surface curvature of the body. Therefore, the study of three-dimensional (3D) boundary-layer transition is essential to work in practical aerodynamics. The present investigation is concerned with the problem of 3D boundary layers over a yawed body. A yawed cylinder model that represents the leading edge portion of a swept wing and the mechanism of crossflow instability are investigated in detail using hot-wire velocimetry and a flow visualization technique. As a result, traveling disturbances having frequencies f₁ and f₂, which differ by about one order of magnitude, are detected in the transition region. The phase velocities and directions of travel of those disturbances are measured. Results for the low-frequency disturbance f₁ show qualitative coincidence with results numerically predicted for a crossflow unsteady disturbance. Nameley, F₁ travels nearly spanwise to the yawed cylinder and very close to the cylinder wall. The results for the high-frequency disturbance f₂ good agreement with the existing experimental results. The ₂ disturbance is found to be the high-frequency inflectional secondary instability that appears in 3D boundary layer transition in general. A two-stage transition process, where stationary crossflow vortices appear as the primary instability and a traveling inflectional disturbance is generated as a secondary instability, was observed. Secondary instability seems to play a major role in turbulent transition.     

Volume dependence of thermal conductivity and bulk modulus for poly(propylene glycol)

The thermal conductivity λ and heat capacity per unit volume of poly(propylene glycol) PPG (0.4 and 4.0 kg·mol⁻¹ in number-average molecular weight) have been measured in the temperature range 150–295 K at pressures up to 2 GPa using the transient hot-wire method. At 295 K and atmospheric pressure, λ = 0.147 W m⁻¹K⁻¹ for PPG (0.4 kg·mol⁻¹) and λ = 0.151 W m⁻¹K⁻¹ for PPG (4.0 kg·mol⁻¹). The temperature dependence of λ is less than 4 × 10⁻⁴ W m⁻¹K⁻² for both molecular weights. The bulk modulus has been measured in the temperature range 215–295 K up to 1.1 GPa. At atmospheric pressure, the room temperature bulk moduli are 1.97 GPa for PPG (0.4 kg·mol⁻¹) and 1.75 GPa for PPG (4.0 kg·mol⁻¹). These data were used to calculate the volume dependence of $ \lambda ,g\, = - \left( {\frac{{\partial \lambda /\lambda }}{{\partial V/V}}} \right)_T $. At room temperature and atmospheric pressure (liquid phase) we find g = 2.79 for PPG (0.4 kg·mol⁻¹) and g = 2.15 for PPG (4.0 kg·mol⁻¹). The volume dependence of g, (∂g/∂ log V)_T varies between −19 to −10 for both molecular weights. Under isochoric conditions, g is nearly independent of temperature. The difference in g between the glassy state and liquid phase is small and just outside the inaccuracy of g of about 8%. The theoretical model for λ by Horrocks and McLaughlin yields an overestimate of g by up to 120%. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36 : 345–355, 1998     

4:1椭球体分离流动湍流特性的试验研究

使用六线涡量探头,对α=30°条件下绕（4:1）椭球体分离流动的湍动能u’2,v’2,w’2,雷诺应力u’v’和涡量Ωx等进行了详尽的测量.应用频谱分析的方法揭示了绕（4:1）椭球体分离流内主附着涡外缘剪切层上的离散小涡的特性.     

热线法测定气体导热系数实验的探讨与改进

在准静态低气压下测量气体的热传导系数时，理论上在低压强范围内应得到Q^-1低-p^-1线性关系图象，但实验数据所绘出的图象出现明显的非线性．本文着重分析出现非线性的原因，并提出实验改进的方法。     

热丝法氢处理多晶硅锗薄膜

优化了热丝法氢处理多晶硅锗薄膜工艺条件.通过测试材料暗电导的温度特性得出多晶硅锗材料的电导激活能,从而考察氢处理效果.结果表明,采用此技术可有效减少多晶硅锗薄膜中的缺陷态.在优化氢处理时衬底和热丝的温度后,可以把处理时间缩短致30min之内,明显短于其他氢处理技术.     

Investigation of structure and properties of nanocrystalline silicon on various buffer layers

A comparative study of deposition nanocrystalline silicon (nc-Si) on various buffer layers is investigated. The nc-Si films were deposited in a hot-wire chemical vapor deposition (CVD) system. Through Hall measurement, scanning electron microscopy (SEM), atomic force microscope (AFM), Raman, and x-ray diffraction (XRD) analyses, it was found that the columnar grain (CG) size, mobility, and volume fraction of crystalline-deposited nc-Si films increase with an increase of the buffer layers’ surface roughness. The nc-Si film deposited on the nc-Si buffer layer possesses the highest Xc (volume fraction of crystalline) of 84.32%, Hall mobility of 45.9 (cm²/V s), and CG size of 200–220 nm, and it shows the strongest intensity of the XRD diffraction peak in (111).     

Determination of principal characteristics of turbulent swirling flow along annuli: Part 2: Measurement of turbulence components

Turbulence data for swirling flows along the annulus formed between two co-axial tubes are presented. The swirl was generated by a set of inlet guide vanes which produced, after settling and removal of wall boundary layers, a nominally free vortex flow over the complete entry plane of the test section. The work complements that described in Part 1 of the paper which considered the behaviour of time mean values under the same entry conditions¹. A hot wire anemometer was used to detect turbulence quantities and techniques have been developed for the measurement of these using the minimum number of simple probe geometries. The theory leading to the derivation of the separate turbulence parameters from hot wire measurements is described. A series of radial profiles are given as representative examples of the extensive data collected in addition to longitudinal variations of friction factor and shear stress. A brief discussion is given on the determination of eddy diffusivity as a function of radial and axial location.     

纳米流体强化传热研究

本文通过在液体中添加纳米级金属或金属氧化物粒子,研制了一种新型传热冷却工质—纳米流体,并对纳米流体的悬浮稳定性和均匀性进行了研究,给出的纳米流体电镜照片显示了悬浮液具有较高的分散性、稳定性;同时,介绍了纳米流体导热系数的理论分析方法,运用瞬态热线法测定了不同种类、不同体积份额配比的纳米流体的导热系数,分析了纳米粒子属性、份额、形状和尺度等因素对纳米流体导热系数的影响。