剪切流中液滴变形及破裂的数值模拟

基于格子Boltzmann颜色梯度模型,针对二维剪切流场中液滴的变形及破裂进行了数值模拟,研究了剪切流动中毛细数Ca、雷诺数Re和流体黏度比λ对液滴变形的影响,并在Re-Ca相图中区分了三种不同的液滴变形或破裂方式。为了进一步掌握界面对液滴内部不同位置流体的作用,引入示踪粒子并分析其在液滴内的运动规律。结果表明：随着Ca的增大,液滴的变形效果和偏转角度更明显,示踪粒子运动受到的影响较大;随着Re的增加,液滴的变形程度加大,但偏转角度受其影响较弱,示踪粒子运动受其影响显著;在低黏度比（λ < 0.8）时,液滴会经历大幅度变形过程,当黏度比继续增大,其变形参数反而会减小;示踪粒子离初始圆心距离越近,运动路程越大,受到的剪切作用越大,且其在剪切作用下离界面越来越近,并最终达到稳定状态。     

液滴在连续气流作用下的变形与破碎

基于Open FOAM,采用简化耦合的多相流模型(S-CLSVOF)对液滴破碎进行了数值模拟研究.自适应网格加密(AMR)技术的应用提高了界面捕捉能力,同时减低了计算成本.基于该数值方法,首先考察了We数对液滴拽力系数和破碎形态的影响.结果表明:We数并不影响变形初期的拽力系数;随着We数增加,破碎液滴依次呈现帽状、帽状/液网及帽状/液丝结构.其次,研究了宽Ohnesorge数(0.001~2.000)范围内的袋式破碎临界We数,并基于模拟结果和Rayleigh-Taylor不稳定性,改进了袋式破碎临界We数的理论预测模型.最后,考察了液滴对相互作用下的破碎,结果表明,回流区的竞争/融合分别抑制/促进液滴对的变形与破碎.     

基于SPH方法水滴撞击恒温壁面流动与换热的研究

基于连续表面张力模型,提出了一种液滴和壁面表面张力的处理方法,结合光滑粒子流体动力学方法,建立了模拟大密度差表面张力问题的SPH算法。通过模拟水滴撞击壁面后的运动过程,与实验结果进行对比,验证了算法的有效性。应用该方法,先后模拟了单个低温大水滴,以不同直径和不同速度,撞击不同表面接触特性恒温壁面的运动过程。结果表明:当壁面的接触角在一定范围内,水滴的吸热率随接触角的增大而减小;而水滴的吸热率随着撞击速度和初始直径的增大均增大。     

两相格子Boltzmann方法模拟大密度比下双液滴冲击液膜的流动过程

基于两相格子Boltzmann模型,对大密度比下有一定水平间距的双液滴冲击液膜的流动过程进行了仿真,模拟了不同液滴初始间距和不同雷诺数下液滴冲击液膜的动态过程,重点分析了不同液滴初始间距和雷诺数下产生不同冲击和溅射现象的原因,以及不同参数对冲击和溅射行为的影响.总结了中心位置水花溅射高度随时间的变化规律,并论述了冲击和溅射过程中的内在作用机理.结果表明:在一定范围内,初始间距和雷诺数越大,中心位置水花溅射高度上升越快;随着雷诺数的增大,冲击、碰撞的程度越剧烈,中心位置水花顶端有液滴飞出.     

用格子Boltzmann方法模拟液滴合并过程

用格子Boltzmann方法中的伪势模型对两个液滴的合并过程进行了数值模拟。详细研究了两液滴能否合并临界尺寸和液滴合并过程中液桥的形成与演化的关系，并研究了表面张力对合并速度的影响。研究结果发现当两个液滴之间距离小于2倍界面厚度时，两个液滴在不受外力的作用下能够自动合并；液桥的宽度与演化时间有一定的指数关系；表面张力越大，合并速度越快，这个结果与前人的理论预测和实验结果一致。     

基于格子Boltzmann 方法的通道内液滴驱替研究

用多相多组分格子Boltzmann模型中的伪势模型对单通道中壁面上液滴驱替进行模拟。主要目的是研究液滴与其外部流体分子质量比对驱替过程及临界邦德数的影响。首先用伪势模型模拟静态气泡,模拟结果与Young-Laplace定律吻合验证了模型的正确性。然后考虑流体与壁面作用力,对壁面上静态液滴润湿性进行模拟,得到参数g0w与接触角的关系。最后取g0w=0.04即液滴为接触角110&#176;的非润湿性流体,加入质量力,考察液滴与其外部流体分子质量比不同时液滴驱替情况。模拟结果表明,当分子质量比增大时,液滴的界面张力减小,使液滴脱落的临界质量力减小,而临界邦德数增大。     

液滴在异形凹槽表面上运动特性的模拟

采用格子Boltzmann方法中的伪势模型对凹槽表面上液滴的运动情况进行了数值模拟。首先根据激光烧蚀金属表面后的痕迹提出了一种异形凹槽结构,建立了异形凹槽表面的二维模型;然后模拟了不同深度的异形凹槽表面上液滴的运动情况。模拟结果表明,异形凹槽深度越浅,液滴运动速度越快。最后在异形凹槽的基础上比较了前后沿不同结构对液滴运动特性的影响,发现将异形凹槽表面上凸起的前后沿改成直角结构会增大凹槽表面对液滴运动的阻力。     

液滴撞击低温超疏水表面运动状态的数值模拟研究

本文针对液滴撞击固体表面后出现的反弹和黏附两种运动状态,通过引入基于连续温度函数的能量方程,建立了用于计算液滴运动、传热和相变过程的数值模型,并与实验结果对比,验证了数值模型的准确性。进而模拟了具有不同韦伯数We和奥内佐格数Oh的液滴撞击低温超疏水表面的运动、传热和相变过程。结果表明：液滴撞击后的运动状态主要取决于奥内佐格数Oh,临界范围是0.022-0.026,而基本不受韦伯数We的影响。当液滴撞击后的运动状态为反弹时,韦伯数We越大的液滴,最大铺展直径越大。而当韦伯数相同时,随着奥内佐格数Oh的增大,液滴与表面的接触时间越大。     

高韦伯数液滴撞壁诱导的中间射流破碎现象

针对内燃机缸内附壁油膜问题,模拟液滴撞壁过程,设计开发了气体驱动单液滴高速撞击热壁面试验系统.研究了液滴高速撞壁对二次雾化的影响,同时考虑壁面温度对撞击效果的影响.结果表明,目前液滴撞壁研究的韦伯数范围多是20～1 000,而通过使用驱动气体(氮气)将小液滴加速至10.8 m/s,从而产生碰壁韦伯数高达4 057的液滴;液滴在韦伯数高达一定条件下发生射流破碎现象,热效应并不是唯一导致其产生的因素;随着韦伯数的增加,射流高度及产生的次级液滴数量也随之增加;随着壁面温度升高,发生中间射流现象时的韦伯数随之降低;在高韦伯数条件下,在远高于水的Leidenfrost临界温度时并没有明显的Leidenfrost现象产生.     

液滴撞击高温壁面的运动特性

利用相界面追踪复合Level Set-VOF方法,综合考虑液滴蒸发传热及接触热阻作用,对单液滴撞击高温壁面运动及影响因素进行了研究.研究表明,由于液滴蒸发传热作用,液滴撞击高温壁面会发生Leidenfrost现象;撞击速度较小时,液滴撞壁后发生铺展反弹,随着撞击速度的增大,液滴撞壁后会发生破碎现象;壁面温度的升高利于液滴破碎发生;液滴初始直径的减小有利于液滴撞壁后的反弹,而液滴直径的增大有利于液滴撞壁后破碎.通过数值模拟,给出了一定条件下柴油液滴撞击高温壁面反弹及破碎的临界条件.     

Ash-forming elements in four Scandinavian wood species. Part 1: Summer harvest

Woody biomass in Finland and Sweden comprises mainly four wood species: spruce, pine, birch and aspen. To study the ash, which may cause problems for the combustion device, one tree of each species were cut down and prepared for comparisons with fuel samples. Well-defined samples of wood, bark and foliage were analyzed on 11 ash-forming elements: Si, Al, Fe, Ca, Mg, Mn, Na, K, P, S and Cl. The ash content in the wood tissues (0.2–0.7%) was low compared to the ash content in the bark tissues (1.9–6.4%) and the foliage (2.4–7.7%). The woods’ content of ash-forming elements was consequently low; the highest contents were of Ca (410–1340 ppm) and K (200–1310), followed by Mg (70–290), Mn (15–240) and P (0–350). Present in the wood was also Si (50–190), S (50–200) and Cl (30–110). The bark tissues showed much higher element contents; Ca (4800–19,100 ppm) and K (1600–6400) were the dominating elements, followed by Mg (210–2400), P (210–1200), Mn (110–1100) and S (310–750), but the Cl contents (40–330) were only moderately higher in the bark than in the wood. The young foliage (shoots and deciduous leaves) had the highest K (7100–25,000 ppm), P (1600–5300) and S (1100–2600) contents of all tissues, while the shoots of spruce had the highest Cl contents (820–1360) and its needles the highest Si content (5000–11,300). This paper presented a new approach in fuel characterization: the method excludes the presence of impurities, and focus on different categories of plant tissues. This made it possible to discuss the contents of ash element in a wide spectrum of fuel-types, which are of large importance for the energy production in Finland and Sweden.     

NEXT GENERATION ENGINEERED MATERIALS FOR ULTRA SUPERCRITICAL STEAM TURBINES

正1 ABSTRACT To reduce the effect of global warming on our climate,the levels of CO2emissions should be reduced.One way to do this is to increase the efficiency of electricity production from fossil fuels.This will in turn reduce the amount of CO2emissions for a given power output.Using US practice for efficiency calculations,then a move from a typical US plant running at 37%efficiency to a 760℃/38.5 MPa(1 400/5 580 psi)plant running at 48%efficiency would reduce CO2emissions by 170kg/MW.hr or 25%.     

Aerodynamic performance effects of leading-edge geometry in gas-turbine blades

The purpose of this paper is to illustrate the advantages of the direct surface-curvature distribution blade-design method, originally proposed by Korakianitis, for the leading-edge design of turbine blades, and by extension for other types of airfoil shapes. The leading edge shape is critical in the blade design process, and it is quite difficult to completely control with inverse, semi-inverse or other direct-design methods. The blade-design method is briefly reviewed, and then the effort is concentrated on smoothly blending the leading edge shape (circle or ellipse, etc.) with the main part of the blade surface, in a manner that avoids leading-edge flow-disturbance and flow-separation regions. Specifically in the leading edge region we return to the second-order (parabolic) construction line coupled with a revised smoothing equation between the leading-edge shape and the main part of the blade. The Hodson–Dominy blade has been used as an example to show the ability of this blade-design method to remove leading-edge separation bubbles in gas turbine blades and other airfoil shapes that have very sharp changes in curvature near the leading edge. An additional gas turbine blade example has been used to illustrate the ability of this method to design leading edge shapes that avoid leading-edge separation bubbles at off-design conditions. This gas turbine blade example has inlet flow angle 0°, outlet flow angle −64.3°, and tangential lift coefficient 1.045, in a region of parameters where the leading edge shape is critical for the overall blade performance. Computed results at incidences of −10°,   −5°,   +5°,   +10° are used to illustrate the complete removal of leading edge flow-disturbance regions, thus minimizing the possibility of leading-edge separation bubbles, while concurrently minimizing the stagnation pressure drop from inlet to outlet. These results using two difficult example cases of leading edge geometries illustrate the superiority and utility of this blade-design method when compared with other direct or inverse blade-design methods.     

Hydrogen production by steam-gasification of carbonaceous materials using concentrated solar energy – V. Reactor modeling,optimization, and scale-up

A chemical reactor for the steam-gasification of carbonaceous particles (e.g. coal, coke) is considered for using concentrated solar radiation as the energy source of high-temperature process heat. A two-phase reactor model that couples radiative, convective, and conductive heat transfer to the chemical kinetics is applied to optimize the reactor geometrical configuration and operational parameters (feedstock's initial particle size, feeding rates, and solar power input) for maximum reaction extent and solar-to-chemical energy conversion efficiency of a 5 kW prototype reactor and its scale-up to 300 kW. For the 300 kW reactor, complete reaction extent is predicted for an initial feedstock particle size up to 35 μm at residence times of less than 10 s and peak temperatures of 1818 K, yielding high-quality syngas with a calorific content that has been solar-upgraded by 19% over that of the petcoke gasified.     

Contents in Volume 23,2014

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汽轮机数字电液调节系统挂闸异常的技术完善

分析了200MW汽轮机数字电液调节系统在运行中存在的挂闸异常问题,采取了相应的技术处理措施,且运行实践效果良好。     

新型高压共轨ECU升压模块的数字化开发与优化

为了提高喷油器电磁阀的响应速率,提出了一种基于CPLD(复杂可编程逻辑器件)应用于高压共轨ECU的数字升压模块。鉴于该升压电路结构参数多,其升压电压的恢复响应要求高等特征,基于Pspice建立了升压电路的仿真模型,研究了不同电路参数下升压模块的输出特性,全面优化了该升压模块的性能。结果显示,该升压模块的最大转换效率可以达90%以上。在柴油发动机上对ECU的试验表明,升压电压最大波动不超过10%,其恢复时间仅为1.3ms,功率管最大温升仅为41℃,满足整机运行范围内ECU的需求。     

Trace metal chemistry and silicification of microorganisms in geothermal sinter, Taupo Volcanic Zone, New Zealand

As part of a pilot study investigating the role of microorganisms in the immobilisation of As, Sb, B, Tl and Hg, the inorganic geochemistry of seven different active sinter deposits and their contact fluids were characterised. A comprehensive series of sequential extractions for a suite of trace elements was carried out on siliceous sinter and a mixed silica-carbonate sinter. The extractions showed whether metals were loosely exchangeable or bound to carbonate, oxide, organic or crystalline fractions. Hyperthermophilic microbial communities associated with sinters deposited from high temperature (92–94°C) fluids at a variety of geothermal sources were investigated using SEM. The rapidity and style of silicification of the hyperthermophiles can be correlated with the dissolved silica content of the fluid. Although high concentrations of Hg and Tl were found associated with the organic fraction of the sinters, there was no evidence to suggest that any of the heavy metals were associated preferentially with the hyperthermophiles at the high temperature (92–94°C) ends of the terrestrial thermal spring ecosystems studied.     

Assessment methods of material ageing using Sdobyrev''s creep rupture model

The physical aspects of the activation energy, in higher and high temperatures, of the metal creep process were examined. The research results of creep-rupture in a uniaxial stress state and the criterion of creep-rupture in biaxial stress states, at two temperatures, are then presented. For these studies creep-rupture, taking case iron as an example the energy and pseudoenergy activation was determined. For complex stress states the criterion of creep-rupture was taken to be Sdobyrev's, i.e. σ_red = σ₁ β + (1 − β)σ_i, where: σ₁-maximal principal stress, σ_i-stress intensity, β-material constant (at variable temperature β = β(T)). The methods of assessment of the material ageing grade are given in percentages of ageing of new material in the following mechanical properties: 1) creep strength in uniaxial stress state, 2) activation energy in uniaxial stress state, 3) criterion creep strength in complex stress states, 4) activation pseudoenergy in complex stress states. The methods 1) and 3) are the relatively simplest because they result from experimental investigations only at nominal temperature of the structure work, however, for methods 2) and 4) it is necessary to perform the experimental investigations at least at two temperatures.     

Production of hydrogen from sewage biosolids in a continuously fed bioreactor: Effect of hydraulic retention time and sparging

Hydrogen was produced from primary sewage biosolids via mesophilic anaerobic fermentation in a continuously fed bioreactor. Prior to fermentation the sewage biosolids were heated to 70 °C for 1 h to inactivate methanogens and during fermentation a cellulose degrading enzyme was added to improve substrate availability. Hydraulic retention times (HRT) of 18, 24, 36 and 48 h were evaluated for the duration of hydrogen production. Without sparging a hydraulic retention time of 24 h resulted in the longest period of hydrogen production (3 days), during which a hydrogen yield of 21.9 L H₂ kg⁻¹ VS added to the bioreactor was achieved. Methods of preventing the decline of hydrogen production during continuous fermentation were evaluated. Of the techniques evaluated using nitrogen gas to sparge the bioreactor contents proved to be more effective than flushing just the headspace of the bioreactor. Sparging at 0.06 L L min⁻¹ successfully prevented a decline in hydrogen production and resulted in a yield of 27.0  L H₂ kg⁻¹ VS added, over a period of greater than 12 days or 12 HRT. The use of sparging also delayed the build up of acetic acid in the bioreactor, suggesting that it serves to inhibit homoacetogenesis and thus maintain hydrogen production.