叶顶间隙对离心式压气机效率的影响研究

针对涡轮增压器压气机的叶顶间隙进行数值模拟计算,探究其对压气机效率损失的影响。研究结果表明:在均匀叶顶间隙下,随着叶顶间隙的增大,压气机的效率随之降低;在恒定转速下,随着压气机稳定流量的增加,叶顶间隙变化引起的效率衰减量逐渐增加;在恒定流量下,随着压气机转速的增加,叶顶间隙的变化引起的效率损失逐渐减小;在变叶顶间隙下,叶轮出口叶顶间隙的减小可以使压气机效率得到明显的改善。     

某高转速汽轮机末级流场分析及通流优化

采用数值计算的方法,对某高转速汽轮机末级通流进行了流场分析,得到以下结论:叶顶区域产生了二次流及集中涡系,尤其是12级静叶叶顶区域产生了明显的通道涡、壁角涡;叶根、叶顶区域产生了明显的激波;不同攻角特征造成叶片壁面出现贯穿叶根、叶顶的流动分离,尤其是压力面流动分离使叶片入口产生负压区。通过上述分析,得到如下优化方法:确定11级静叶的最优安装角为50.5651°;增加11级叶片中径、增加11级与12级级间的轴向距离相结合的方法,以减小涡系强度。     

电站锅炉小径管焊缝超声波检测技术

锅炉小径管焊缝超声波检测技术是一直无损检测中难题.通过小径管超声波检测工艺参数对裂纹检测结果的影响研究与分析,研究分析了回波波幅以及波形特征随超声波检测工艺参数影响变化关系.试验结果表明,随着探头k值减少,缺陷回波波幅增加;探头k值变化对缺陷回波波形产生较大影响.小k值探头检测小径管焊缝裂纹灵敏度较佳.     

单级冲动式汽轮机叶片气热耦合数值计算

本文针对水下特种动力用单级冲动式涡轮机叶片的温度场预测,采用气热耦合方法进行数值计算,并与非耦合方法计算进行了对比分析。结果表明,两种计算方法获得的主流场比较接近,而叶片表面的温度场有较大的区别;耦合计算表明沿轴向方向,从动叶进口到动叶出口,叶片内部温度温差逐渐缩小,在叶片入口处,从叶根到叶顶叶片的温度呈先降低后升高的趋势,在动叶出口处,叶片表面温度值趋于均匀,叶片温度梯度载荷不大。     

枞树型叶根超声相控阵检测技术研究

枞树型叶根结构复杂,运行中叶片根部应力较大,在不拆卸的状态下对其进行常规超声检测很难达到检测目的。专门针对枞树型叶根设计制作相应的调试和对比试块,并制订检测工艺。通过对枞树型叶根内外弧缺陷的模拟,提供不同深度、长度的缺陷对比并进行图像和数据分析,对实际检测具有较强的指导作用。     

叶顶间隙宽度变化对离心叶轮性能和内部流场的影响

为研究不同宽度叶顶间隙对亚音速半开式离心叶轮性能与内部流场的影响,对某工业用半开式离心叶轮选用不同形式和几何参数的叶顶间隙,通过数值模拟得出结论:随叶顶间隙宽度减小,叶顶附近湍动能减少,离心叶轮通道压比和效率更高,但是失速裕度也变小;在本研究范围内,综合考虑性能和失速裕度,叶顶间隙宽度为0.4 mm时最优;叶顶间隙宽度对泄漏流和主流相互作用有一定影响;随叶顶间隙宽度减少,叶顶间隙泄漏流量减少,本文的分析结果为离心压缩机的工业设计提供了一定参考。     

叶根几何参数和来流参数对叶片端壁传热影响的研究

针对典型燃气透平Pack-B叶片,建立了叶片端壁传热特性的数值模型。在验证了数值模型的基础上,研究了叶根几何参数和来流参数对叶片端壁传热特性的影响。结果表明:随着叶根倒角半径的增大,叶片端壁前缘区域的传热减弱,而端壁尾缘区域的传热增强;随着叶根倒角最小角度的增大,端壁前缘区域传热略有增强,而端壁尾缘区域的传热有所减弱;来流湍流度为1%时,由于端壁区域二次流结构发生显著变化,使得其端壁前缘区域和中部区域的传热明显高于湍流度为4%、6%和10%的情况,而其他区域的传热随着来流湍流度的提高而增强;随着来流雷诺数的增大,整个端壁的传热均增强。     

微小型燃气轮机燃烧室模化方法适用性的数值研究

为获得经济性好且准确性高的微小型燃气轮机燃烧室模化实验方法,采用简化的二维模型对不同模化准则、不同模化空气温度和不同燃料控制策略的模型分别进行数值模拟研究,并与设计状态进行对比。研究表明:对于燃料为甲烷的微小型燃气轮机燃烧室,随着压力指数n增大,混合气在燃烧反应区的停留时间增加,回流区面积减小;n选取1.75～2.0时能获得较为准确的污染物排放的模化效果;n=1,即采用等容积流率准则时,能获得较为准确的出口温度分布和流场相关的模化结果;采用等燃烧效率L准则,模化状态理论燃烧温度保持不变,空气温度从493 K降低至300 K时,反应区平均温度和停留时间变化不大,NO_x体积分数升高10×10~(-6),燃烧效率降低了0.5%,燃烧不充分,CO体积分数大幅升高。     

管式镀膜工艺对太阳电池并联电阻的影响

主要研究不同沉积方式下并联电阻的差异,同时对管式镀膜工艺在不同工艺条件下并联电阻的变化关系进行研究。研究表明,影响并联电阻的主要因素为温度、氮硅比,并联电阻随温度的降低而不断升高,随温度的升高而降低;且其随氮硅比的增加而上升,随氮硅比的减少而下降。     

不同热转化条件下秸秆灰的熔融特性研究

生物质灰熔融特性的影响因素众多,为了系统地研究生物质灰在不同热转化条件下的熔融特性,以小麦秸秆为例,系统研究了反应温度、热解气氛、O₂体积分数等变量对麦秆灰熔融特征温度的影响规律,探究了麦秆灰的熔融特性。结果表明：随着热解温度升高,灰熔融特征温度升高,这是因为温度升高,碱金属随之挥发,而碱金属含量越低,熔融温度越高;随着气化温度升高,软化温度、半球温度、流动温度变化都不明显,但变形温度明显升高。温度的改变会造成麦秆灰残余矿物质的变化,低温物质转变为高温物质,熔融特征温度进而发生变化。反应气氛改变,麦秆灰的熔融特征温度也会发生变化。在O₂体积分数为6%～18%时,灰熔融特征温度并无明显变化。     

枞树型叶根间隙漏气对级性能影响的试验研究

隔板汽封间隙、叶根径向汽封间隙及枞树型叶根与轮槽间隙三者之间匹配设计对透平级效率及转子推力的大小有显著影响。文章针对某1000 MW高压末三级空气透平试验进行相关研究,保持隔板汽封间隙、动叶根部径向汽封间隙不变的条件下,对枞树型叶根间隙改变前后级性能的变化进行试验分析。试验结果表明,叶根间隙封堵后,透平级效率降低,根部反动度升高,转子推力变大。合理的枞树型叶根间隙的存在有利于提高级性能,汽轮机通流设计时应注意叶根间隙与隔板汽封间隙的优化匹配。     

Modeling and simulation of heat transfer phenomena during investment casting

Determining the heat transfer phenomena during casting processes is an important parameter for measuring the overall performance of process. It gives information about the properties of the metal being casted and its possible behavior in the mold during casting process. Improper determination of heat transfer phenomena and use of improper molding materials and casting conditions leads to defects such as misruns, cold shuts, shrinkage, pin holes, air holes and porosity in final product. A mathematical model was developed using standard transport equations incorporating all heat transfer coefficients to calculate the time for solidification of metal in casting and computer simulation of the model was carried out in C++ to validate the model. The metal used was pure iron casted in investment molds of silica sand with zircon coating. It was shown that airflow near the mold surfaces was partially restricted due to geometry of the molds and arrangement of the pieces around a tree. So, the changes in heat transfer coefficient also contribute towards time of solidification. The time calculated was found to be in good agreement with experimental values.     

一种燃机叶片试板定向凝固过程的研究

明确铸件定向凝固过程中的温度变化规律．可以避免重型燃气轮机定向叶片铸造中出现的一些缺陷。文章选取了与某重型燃机第1级涡轮动叶尺寸相近的试板．采用定向凝固高温合金DZ445．研究了该试板在实际工业生产用定向结晶炉中的定向凝固过程,结果表明：在距离水冷铜盘位置〉50mm后固液界面形态将发生大的转变,温度梯度逐渐变小,〉100mm后温度梯度基本保持不变．这为后续制定叶片抽拉工艺给出了参考。该实验还通过热电偶获得的定向凝固过程中温度一时间曲线与ProCAST模拟结果基本一致．验证了模拟边界条件和设置参数的准确性．后续可以用于模拟结构复杂的叶片定向凝固过程．指导实际生产工艺.     

不同拉速条件下FTSC结晶器内钢液流动行为的物理模拟研究

通过物理模拟研究FTSC结晶器内的钢液流动行为,考察了拉速对结晶器内液面波动、液面裸露、卷渣及夹杂物去除的影响作用。实验结果表明:在本实验条件下,使用4孔水口浇注会在FTSC结晶器内水口一侧形成4个明显的回流区。随着拉速的增加,结晶器内液面波动逐渐加剧,出现液面裸露及卷渣的几率逐渐增大,夹杂物的去除率逐渐减小。     

Influence of Carbonization Conditions on the Properties of Coconut Shell Chars

The changes in chemical, physical, and mechanical properties of fully matured coconut shell chars in relation to carbonization temperature (range: 400–950°C) and time (range: zero–3 h) have been studied. These properties were found to be more susceptible to carbonization temperature than to time. The results indicated an increase in fixed carbon content and true specific gravity of shell chars with rise of carbonization temperature and soak time. The majority of volatilization occurred up to about 800°C. The calorific value of shell char increased sharply with rise of carbonization temperature up to 600°C, and thereafter it decreased to 800°C. The porosity of shell char increased with increase of carbonization temperature up to 600°C followed by a decrease with further rise of temperature up to the range studied. Prolonged soaking at carbonization temperatures of 600, 800, and 950°C, in general, led to slight increases in the porosity and calorific values of resulting shell chars. The results showed that the crushing strength of shell char decreased markedly on increasing the preparation temperature up to 600°C, followed by an increase thereafter. An increase in soaking time at carbonization temperatures of 600, 800, and 950°C also influenced the shell char strength.     

GROWTH INSTABILITY DURING PLANAR SOLIDIFICATION WITH A MOLD OF FINITE THERMAL CAPACITY

The temperature and the stress fields in the solidified layer and in the mold of finite thickness for a unidirectional casting process are investigated. Earlier solutions are extended to include the effect of the thermal capacity of the mold on the freezing front growth instability. A numerical solution is obtained for both the heat conduction and the residual stress problem. The results show that the perturbation in contact pressure tends asymptotically to a maximum value at larger times for the lower values of the thermal capacities of the mold materials. The magnitude of the contact pressure perturbation is decreased by the inclusion of the thermal capacity of the mold material, and this effect is enhanced for less distortive and thicker molds. The present article assumes that the thermal and mechanical problems are uncoupled along the casting mold interface. Despite this limitation, the results presented in this article indicate that a mold with a higher thermal capacity (or lower thermal diffusivity) might be less susceptible to thermoelastic instabilities associated with the contact pressure and its dependence on the thermal contact resistance at the casting mold interface.     

Spatial variation of heat flux at the metal–mold interface due to mold filling effects in gravity die-casting

Most of the research work pertaining to metal–mold heat transfer in casting solidification either assumes no spatial variation in the air gap formation or limits the study to only those experimental systems in which air gap formation is uniform. However, in gravity die-casting, filling effects induce variation in thermal field in the mold and casting regions. In this paper, we show that this thermal field variation greatly influences the time of air gap initiation along a vertical mold wall, which subsequently leads to the spatial variation of air gap and in turn, the heat flux at the metal–mold interface.In order to study the spatial variation of heat flux at the metal–mold interface, an experimental setup that involved mold filling was devised. A Serial-IHCP (inverse heat conduction problem) algorithm was used to estimate the multiple heat flux transients along the metal–mold interface. The analysis indicates that the fluxes at different mold segments (bottom, middle, and top) of the metal–mold interface reaches the peak value at different time steps, which shows that the initiation of air gap differs along the mold wall. The experimental and numerical results show that the heat transfer in the mold is two-dimensional during the entire period of phase change, which is initially caused by the filling effects and further enhanced by the spatial variation of the air gap at the metal–mold interface.     

THERMOMECHANICAL BEHAVIOR OF THE SOLIDIFYING SHELL WITHIN CONTINUOUS-CASTING BILLET MOLDS-A NUMERICAL APPROACH

A two-dimensional numerical model is given for the analysis of the coupled thermal and mechanical behavior of the solidifying shell within the mold during continuous casting of steel. The influence of different mold wall profiles on gap formation and heat flow during casting of billets is investigated. The calculated temperatures, stresses, and strains in the shell are used to estimate the risk for formation of longitudinal cracks. The effect of an initiated and growing macroscopic subsurface crack on the shell behavior is studied. The genesis of surface cracks is discussed. The calculated results are shown to be in reasonable agreement with experimental observations reported in the literature.     

SOLIDIFICATION OF A PURE METAL WITH FINITE THERMAL CAPACITANCE ON A SINUSOIDAL MOLD SURFACE

Previous models of mold microgeometry-induced gap nucleation during pure metal solidification neglected the thermal capacitance of the solidifying shell: this is equivalent to the assumption that the shell has a small Stefan number. Although this assumption leads to steady heat conduction in the shell, and hence simplifies the solution for the thermal field, the corresponding assumption of a small Stefan number material is generally not appropriate for metals. In the present work, we remove the small Stefan number restriction used in a previous model for solidification of a pure metal on a rigid, perfectly conducting mold. The mold has a sinusoidal surface microgeometry for which the ratio of the amplitude to the wavelength is much less than one. This makes the aspect ratio a convenient perturbation parameter. Molten metal initially at its fusion temperature is assumed to wet the mold surface perfectly, which is held a constant temperature below the fusion temperature. The temperature field in the growing metal shell is numerically evaluated, and the instantaneous temperature field is incorporated into an analytical solution for the stress field in the shell. The evolving thermomechanical distortion of the shell is modeled assuming that the shell material follows a thermohypoelastic constitutive law that is a rate formulation of thermoelasticity. The contact pressure profile at the shell/asperity interface, which is indicative of shell distortion due to the asperity geometry, is obtained from the stress field. The effects of the mold wavelength and shell thermal capacitance on the contact pressure, temporal and spatial evolution of gap nucleation at the shell/mold interface, and mean shell thickness are examined for pure aluminum and iron shells.     

The ignition of gases by rapidly heated surfaces

A surface heated by a frictional impact was simulated by the discharge of a bank of capacitors through a strip of tungsten foil. The resultant rise and fall of temperature were investigated by means of a two-color pyrometer. The heated foil was exposed to various concentrations of flammable gases, and the probability of ignition was investigated as a function of several variables, including gas concentration, temperature profile, and foil size. It was found that the peak temperature necessary for ignition decreased as the width of the foil was increased, and that 7% was the most easily ignitable concentration of methane in air. The size of the heated surface was of the order of 200 mm² and this necessitated a working range of peak temperatures of approximately 1500°C to 2200°C in the study of the ignition of methane-air. Some preliminary experiments indicated that propane-air could be ignited at much lower temperatures.