Experimental Study on Convective Boiling Heat Transfer in Narrow-Gap Annulus Tubes

Since convective boiling or highly subcooled single-phase forced convection in micro-channels is an effective cooling mechanism with a wide range of applications, more experimental and theoretical studies are required to explain and verify the forced convection heat transfer phenomenon in narrow channels. In this experimental study, we model the convective boiling behavior of water with low latent heat substance Freon 113 (R-113), with the purpose of saving power consumption and visualizing experiments. Both heat transfer and pressure drop characteristics were measured in subcooled and saturated concentric narrow gap forced convection boiling. Data were obtained to qualitatively identify the effects of gap size, pressure, flow rate and wall superheat on boiling regimes and the transition between various regimes. Some significant differences from unconfined forced convection boiling were found,and also, the flow patterns in narrow vertical annulus tubes have been studied quantitatively.

Numerical simulation of temperature field evolution of metallic alloy surface due to pulsed laser treatment

This paper performed a numerical simulation on temperature field evolution for the surface layer of a metallic alloy subjected to pulsed Nd:YAG laser treatment. The enthalpy method was adopted to solve the moving boundary problem, I.e. Stefan problem. Computational results were obtained to show the temperature field evolution. Effects of latent heat and mushy zone width on the temperature field were investigated. The results also show very high values of temperature gradient and cooling rate, which are typical characteristics during the solidification process.

Novel Fe70Zr10Ni6Al4Si6B4 thick metallic glass coating produced by laser cladding

A novel multicomponent thick metallic glass coating has been synthesised by laser cladding. The maximum coating thickness was I mm. The clad cooling rate restrained the epitaxial growth of dendrites in the metallic glass coating. The metallic glass had high glass forming ability with a wide supercooled liquid region ranging from 59 to 70 K. The metallic glass coating also revealed high hardness and good corrosion resistance.

Numerical Study Of Heat Transfer Mechanism In Turbulent Supercritical Co2 Channel Flow

Direct numerical simulation (DNS) of supercritical CO2 turbulent channel flow has been performed to investigate the heat transfer mechanism of supercritical fluid. In the present DNS, full compressible Navier-Stokes equations and Peng-Robison state equation are solved. Due to effects of the mean density variation in the wall normal direction, mean velocity in the cooling region becomes high compared with that in the heating region. The mean width between high-and low-speed streaks near the wall decreases in the cooling region, which means that turbulence in the cooling region is enhanced and lots of fine scale eddies are created due to the local high Reynolds number effects. From the turbulent kinetic energy budget, it is found that compressibility effects related with pressure fluctuation and dilatation of velocity fluctuation can be ignored even for supercritical condition. However, the effect of density fluctuation on turbulent kinetic energy cannot be ignored. In the cooling region, low kinematic viscosity and high thermal conductivity in the low speed streaks modify fine scale structure and turbulent transport of temperature, which results in high Nusselt number in the cooling condition of the supercritical CO2.

Internal Friction Of Bend-Deformed Nanocrystalline Nickel By Mechanical Spectroscopy

Internal friction of nanocrystalline nickel is investigated by mechanical spectroscopy from 360 K to 120 K. Two relaxation peaks are found when nanocrystalline nickel is bent up to 10% strain at room temperature and fast cooling. However, these two peaks disappear when the sample is annealed at room temperature in vacuum for ten days. The occurrence and disappearance of the two relaxation peaks can be explained by the interactions of partial dislocations and point defects in nanocrystalline materials.

Residual Stress And Deformation Analysis Of Anodic Bonded Multi-Layer Of Glass And Aluminum

The bonding of glass wafer to aluminum foils in multi-layer assemblies was made by the common anodic bonding process. The bonding was performed at temperatures in the range 350-450 degrees C and with an applied voltage in the range 400-700 V under a pressure of 0.05 MPa. Residual stress and deformation in samples of two-layer (aluminum/glass) and three-layer (glass/aluminum/glass) were analyzed by nonlinear finite element simulation software MARC. The stress and strain varying with cooling time were obtained. The analyzed results show that deformation of the three-layer sample is significantly smaller than that of the two-layer sample, because of the symmetric structure of the three-layer sample. This has an important advantage in MEMS fabrication. The maximum equivalent stresses locate in the transition layer in both samples, which will become weakness in bonded sample.

On The Thermally Induced Interfacial Delamination Of A Segmented Coating

The thermally induced interfacial delamination problem of a segmented coating is investigated using finite element method (FEM). The coating-substrate system, modeled as a coated semi-infinite medium with periodic segmentation cracks within coating, is assumed to be exposed to convective cooling from surface. The failure criterion based on the interfacial fracture toughness is adopted, in which the energy release rate for an interface crack is considered to be the driving force for interfacial delamination extension. The results confirm that a segmented coating has higher delamination resistance than an intact one under the same thermal transients, as the segmentation crack spacing is smaller than a critical value. Based on dimensional analysis, sensitivity analyses of the crack driving force are also obtained as a function of various dimensionless parameters such as time, convection severity and material constants. These results may provide some helpful references for the integrity of coating-substrate systems under thermal loading. (C) 2007 Elsevier B.V. All rights reserved.

Thermal fatigue on pistons induced by shaped high power laser. Part I: Experimental study of transient temperature field and temperature oscillation

Thermal fatigue behavior is one of the foremost considerations in the design and operation of diesel engines. It is found that thermal fatigue is closely related to the temperature field and temperature fluctuation in the structure. In this paper, spatially shaped high power laser was introduced to simulate thermal loadings on the piston. The incident Gaussian beam was transformed into concentric multi-circular beam of specific intensity distribution with the help of diffractive optical element (DOE), and the transient temperature fields in the piston similar to those under working conditions could be achieved by setting up appropriate loading cycles. Simulation tests for typical thermal loading conditions, i.e., thermal high cycle fatigue (HCF) and thermal shock (or thermal low cycle fatigue, LCF) were carried out. Several important parameters that affect the transient temperature fields and/or temperature oscillations, including controlling mode, intensity distribution of shaped laser, laser power, temporal profile of laser pulse, heating time and cooling time in one thermal cycle, etc., were investigated and discussed. The results show that as a novel method, the shaped high power laser can simulate thermal loadings on pistons efficiently, and it is helpful in the study of thermal fatigue behavior in pistons. (C) 2007 Elsevier Ltd. All rights reserved.

Effects Of The Length Of A Cylindrical Solid Shield On The Entrainment Of Ambient Air Into Turbulent And Laminar Impinging Argon Plasma Jets

When materials processing is conducted in air surroundings by use of an impinging plasma jet, the ambient air will be entrained into the materials processing region, resulting in unfavorable oxidation of the feedstock metal particles injected into the plasma jet and of metallic substrate material. Using a cylindrical solid shield may avoid the air entrainment if the shield length is suitably selected and this approach has the merit that expensive vacuum chamber and its pumping system are not needed. Modeling study is thus conducted to reveal how the length of the cylindrical solid shield affects the ambient air entrainment when materials processing (spraying, remelting, hardening, etc.) is conducted by use of a turbulent or laminar argon plasma jet impinging normally upon a flat substrate in atmospheric air. It is shown that the mass flow rate of the ambient air entrained into the impinging plasma jet cannot be appreciably reduced unless the cylindrical shield is long enough. In order to completely avoid the air entrainment, the gap between the downstream-end section of the cylindrical solid shield and the substrate surface must be carefully selected, and the suitable size of the gap for the turbulent plasma jet is appreciably larger than that for the laminar one. The overheating of the solid shield or the substrate could become a problem for the turbulent case, and thus additional cooling measure may be needed when the entrainment of ambient air into the turbulent impinging plasma jet is to be completely avoided.

Influence of thermal residual stresses on the composite macroscopic behavior

The influence of the thermal residual stress on the deformation behavior of a composite has been analyzed with a new micromechanical method. The method is based on secant moduli approximation and a new homogenized effective stress to characterize the plastic state of the matrix. It is found that the generated thermal residual stresses after cooling and their influence on the subsequent deformation behavior depends significantly on the aspect ratio of the inclusions. With prolate inclusions, the presence of thermal residual stresses generate a higher compressive hardening curves of the composite, but it is reversed with oblate inclusions. For particle reinforced composite, thermal residual stresses induce a tensile hardening curve higher than the compressive one and this is in agreement with experimental observations. (C) 1998 Elsevier Science Ltd.

热喷涂构件中残余应力的理论分析

本文发展一种新的分析涂层结构（平板、梁）热残余应力的模型，可以研究骤冷过程（Quenching）和冷却过程（Cooling）在涂层结构内引发的残余应力分布。与以往模型相比，其优势在于：它可以考虑源于喷涂过程的涂层孔隙率、温度梯度等因素对于涂层和基底内残余应力的影响。其中孔隙率和温度分布由计算机模拟涂层沉积过程得到。另外，当基底的材料和几何参数被固定时，我们分析了诸如涂层的理想模量、厚度、热膨胀系数等参数，对于涂层结构中最终残余应力分布的改变机理。

蒸发液层对流稳定性理论与实验研究进展

综述了蒸发液层稳定性理论分析和蒸发对流实验研究方面的最新进展．首先回顾历史上经典的单层流Marangoni不稳定性分析．利用经典的单层流模型可以很好解释无蒸发液层的稳定性特性，但是由于经典的单层流模型没有考虑蒸汽层与液层之间的动力学耦合关系，所以不能完全解释蒸发液层的Marangoni不稳定性特性．有的学者建立了考虑蒸汽层与液层的热耦合与动力学耦合关系的两层流模型，并采用了界面温度连续这样的假设．而在实验的观测中，蒸发界面处的温度是不连续的，特别是在蒸发量比较大的情况下，汽／液界面处温度跳跃很明显．由于界面温度连续假设在处于非平衡状态的系统中是不成立的，所以这些模型虽然能给出一些新的有关系统稳定性的特性，但还是不能完全解释蒸发液层的Marangoni不稳定性的特性，特别是为什么从底部冷却液层的时候，在实验中仍然能够观察到Marangoni对流涡胞的出现的原因．本文总结了前人的研究成果，同时给出了蒸发系统的动力学建模过程和实验研究方法，并对各种模型的稳定性特性进行了总结．最后，指出了现有理论中存在的问题和有待进一步研究的问题．

Thermal Residual Stresses in SiCw/Al Composite

<正> The SiCw/6061Al composites were fabricated by squeeze casting method. Varia-tions of thermal residual stresses with quenching temperature, cooling manner, aging time and thethermal-cold cycle process in thin specimens,and the distributions of thermal residual stresses alongthe distances from the surface and changes with heating temperatnres in thick specimens were stud-ied by means of X-ray diffraction (XRD). The effects of residual stresses on the mierostructure, di-mensional stability and age-hardening behavior were studied by SEM, TEM observations, and tensiletest. The results showed that there existed macrostress, microstress and thermal mismatch stress inSiCw/Al compo-site,and the presence of microstress and thermal mismatch stress has no influenceon the measurement of macrostress, but the macrostress can affect the measured value of thermalmismatch stress. Thermal res dual stress induced during the composite fabrication process, will be further in-creased when the composite were subjected to the e

连续冷却多阶段快速凝固过程起始形核温度的计算

基于快速凝固的瞬态形核理论和对连续冷却条件下整个温度区间的瞬态形核数和晶核体积积分，导出了连续冷却多阶段快速凝固条件下过冷熔体中各析出相起始形核温度的计算方法。假定界面传热处于牛顿冷却方式，对Ni－Al合金平面流铸条带形成过程中析出的化合物相起始形核温度做了计算。

Undercooling and Rapid Solidification of Nb-Si Eutectic Alloys Studied by Long Drop Tube

Niobium-silicide alloys have great potential for high temperature turbine applications. The two-phase Nb/Nb5Si3 in situ composites exhibit a good balance in mechanical properties. Using the 52 in drop tube, the effect of undercooling and rapid solidification on the solidification process and micro-structural characterization of Nb-Si eutectic alloy was studied. The microstructures of the Nb-Si composites were investigated by optics microscope (OM), X-ray diffraction (XRD) and scanning electron microscope (SEM) equipped with X-ray energy dispersive spectrometry (EDS). Up to 480 K, deep undercooling of the Nb-Si eutectic samples was successfully obtained, which corresponds to 25% of the liquidus temperature. Contrasting to the conventional microstructure usually found in the Nb-Si eutectic alloy, the microstructure of the undercooled sample is divided into the fine and coarse regions. The most commonly observed microstructure is Nb+Nb5Si3, and the Nb3Si phase is not be found. The change of coarseness of microstructure is due to different cooling rates during and after recalescence. The large undercooling is sufficient to completely bypass the high temperature phase field.