亚/超临界环境下碳氢燃料液滴的蒸发与燃烧特性

采用挂滴法对正常重力下处于亚/超临界压力环境中的不同碳氢燃料液滴蒸发与燃烧现象做了详细的试验研究.采用嵌入液滴内部的热电偶和高速相机分别记录液滴温度变化和液滴发展图像.结果表明:在亚临界压力环境下,液滴燃烧过程具有平衡蒸发阶段,符合准定常假设,但在超临界压力环境下,液滴燃烧过程不再出现平衡蒸发阶段,准定常假设已不成立;液滴燃烧持续时间在亚临界状态下随着压力的增加而迅速减小,此时相平衡控制液滴燃烧速率的大小,但在超临界状态下,液滴与环境气体之间的界面变得模糊不清,燃烧持续时间随着环境压力的增加不再继续减小,而是趋于一稳定值,此时液滴已不存在相变过程,扩散系数开始影响燃烧速率;燃烧持续时间变化趋势在临界压力处的转变反映出临界压力点是判断液滴是否进入超临界燃烧的重要依据,液滴燃烧过程中液滴完全蒸发所占的时间比重在亚临界压力环境下变化不大,而在超临界压力环境下迅速减小,相对更早地完成液滴蒸发.     

亚临界和超临界压力下燃料液滴的蒸发特性

以实际气体状态方程为基础,建立了适用于高压下的导热系数、扩散系数等物性参数的计算方法,并将高压汽液相平衡、混合物临界点以及蒸发焓的概念引入到液滴表面的传热传质过程中,以此为基础建立了单个燃料液滴的高压蒸发模型.研究了亚临界和超临界压力下壬烷液滴在氮气中的蒸发过程及其物理控制因素,重点探讨了超临界压力下液滴蒸发过程中液滴表面自亚临界状态向超临界状态的迁移过程及迁移条件.结果表明,在亚临界压力下,液滴蒸发始终受相变控制.在超临界压力下,当液滴表面由燃料和环境气体组成的混合物达到其临界点时,液滴表面将发生自亚临界状态向超临界状态的迁移.在液滴表面迁移之后,液滴表面消失,燃料自高浓度的燃料核心向远方场的扩散过程不受相变控制.另外,随着环境温度的升高液滴表面发生迁移所需的最低环境压力逐渐降低.     

亚/超临界压力下碳氢燃料液滴的燃烧特性

建立了一个适用于超临界压力下包括能够正确描述跨临界迁移现象的液滴燃烧模型,提出了跨临界迁移时刻液滴表面处燃料质量流束有限的新观点.利用开发的计算模型,以碳氢燃料液滴自燃着火为研究对象,研究了亚临界和超临界压力下单液滴的燃烧特性,并从传热传质过程出发,阐明了跨临界迁移前、后液滴燃烧过程的热质输运机理及物理控制因素.结果表明:在亚临界压力下,液滴燃烧不会发生跨临界迁移现象,燃烧过程始终受燃料在液滴表面处相变的控制,液滴的燃烧速度取决于传热过程,并且液滴温度受该压力下燃料沸点的限制而增长缓慢.而在超临界压力下,液滴着火之后很快发生跨临界迁移现象,此后燃料向燃烧反应区域的扩散不存在相变,液滴的燃烧速度取决于传质过程,并且液滴温度不再受燃料沸点的限制而持续升高.     

超临界环境下燃料液滴蒸发过程的计算研究

建立了研究燃料液滴在超临界环境下蒸发的计算模型,并对液滴的蒸发过程进行了编程计算.模型基于气液两相的守恒方程,并详细考虑了液滴表面的气液相平衡.模型采用了Peng-Robinson(PR)状态方程.计算结果表明:超临界压力下,液滴周围气体在液相中的溶解很明显.燃料液滴只有在强超临界环境中蒸发时,液滴表面才能发生由亚临界状态到超临界状态的迁移;而在弱超临界环境中蒸发时,液滴表面不会发生迁移.随着环境压力的升高,液滴寿命先是下降然后升高;而当环境温度升高时,液滴寿命不断下降.     

二甲醚液滴高压蒸发的数值模拟

在分析燃油液滴高压蒸发规律的基础上,考虑液滴内部的热传导过程、内部环流和非理想气体效应,建立了高压蒸发模型,并利用该模型对二甲醚(DME)单液滴的蒸发过程进行了数值模拟分析。采用状态方程法计算了DME-N2体系的气液相平衡。结果表明:高压有利于燃料液滴蒸发;即使环境压力超过燃油的临界压力,其平衡蒸发温度也未必能达到临界温度。     

丁烷-空气超临界燃烧中温度对迁移过程的影响

建立了单一燃料液滴燃烧过程的计算模型和数值方法,并利用之计算了丁烷-空气系统的蒸发和燃烧过程,研究了环境温度对迁移过程的影响.计算结果表明,在环境压力为2倍丁烷临界压力下,当环境温度为丁烷临界温度3倍时,液滴表面状态几乎在着火的同时实现了亚临界向超临界状态的迁移.环境温度低于3倍丁烷临界温度时,液滴会在迁移前着火,并借助火焰产生的热量完成迁移过程;当环境温度高于3倍丁烷临界温度时迁移时间短于着火延迟,液滴不利用燃烧产生的热量而依靠自周围高温介质传来的热量提高自身温度,进而完成迁移过程.此外,随着环境温度的升高,着火延迟时间和迁移时刻均逐渐变短.     

亚/超临界环境下燃料的喷雾燃烧特性研究

采用高温高压定容燃烧弹系统对亚/超临界环境下燃料在纯氧中的喷射燃烧现象进行试验研究,通过高速相机和阴影法进行燃烧图像信息的采集,研究在相同的超临界温度下,正己烷和乙醚两种燃料喷射燃烧火焰特性随环境压力和喷油脉宽的改变而呈现的差异性。试验结果表明:在亚临界压力下,两种燃料在不同喷油脉宽下的燃烧时间均随着环境压力的升高而升高,火焰发展后期基本保持较为稳定的形态和亮度,符合传统的喷射燃烧过程。正己烷在临界压力点下火焰长度最短,形态较为规则;超临界压力下,火焰形态有稳定趋势但仍有轻微波动,且火焰长度略短于亚临界;喷油脉宽改变时,燃烧时间在临界压力附近呈现出不同的趋势。乙醚的燃烧时间随压力的升高而增大,在临界压力处达到最大值,后随着压力的继续升高而下降,不同压力条件下火焰形态略有差别。燃烧现象存在差异性主要是由于亚/超临界坏境下燃料的物性参数和破碎蒸发机理不同。     

不同湍流强度下液雾燃烧的直接数值模拟

采用欧拉方法求解气相场,采用拉格朗日方法跟踪液滴颗粒,对液雾燃烧进行了直接数值模拟研究.分析了不同湍流强度下液雾燃烧的液滴蒸发特性和燃烧特性.发现在低混合分数区域,液滴蒸发速率约等于零.当混合分数大于夹层内平均混合分数的时候,液滴蒸发速率随混合分数线性增加.液雾燃烧的着火时间与相同条件下气体混合物的着火时间相比会更长.湍流在混合分数空间中抑制了液体燃料的蒸发,但是能够加强燃料和氧化剂的混合,从而进一步促进化学反应的进行.     

基于分子动力学的燃油液滴超临界蒸发特性

基于分子动力学模拟的方法,对氮气环境中单个烷烃液滴的蒸发过程进行了模拟研究,揭示了液滴在亚临界和超临界条件下液滴蒸发特性的显著差异．对正十二烷液滴在氮气环境内的蒸发过程进行分子动力学模拟,结果表明：在超临界温度和压力条件下,液滴的温度持续上升,能够超过燃油组分的临界温度;此时,液滴与周围气相区的密度差异近乎消失,气-液相交界变得难以辨别,明显不同于亚临界条件下典型的气-液两相蒸发特征;蒸发速率随环境温度的升高而增大．在较低的压力范围内,升高环境压力能够提升液滴蒸发速率,但当压力达到一个特定值后,随着环境压力的升高蒸发速率反而会降低,同时液滴转变为超临界蒸发状态所需的最小压力随环境温度的升高而降低．对于双组分混合液滴,在亚临界环境条件下,液滴内的轻质组分优先蒸发;而在超临界环境条件下,液滴内各个组分近乎保持同步蒸发,两个燃油组分共同主导液滴的完整蒸发过程．     

生物乙醇喷雾蒸发和预混过程的数值模拟研究

基于生物乙醇燃料的贫燃预混、预蒸发燃烧技术(Lean Premixed Pervaporation,LPP),采用数值模拟方法,研究了预混室内生物乙醇雾化蒸发流场,分析了预热空气温度为500、600和1 000 K以及旋流数为0.47、0.8和1.41时的生物乙醇蒸发和气体混合特性的规律。研究表明:在LPP预混室旋流流场中,中心回流区宽度随预混室距离的增加先增大后减小,并且会受喷雾射流的影响拉伸变长,中心回流区随旋流强度的增大更贴近喷雾出口,角回流区的长度随旋流强度增大而缩短直至消失,旋流强度对液雾整体蒸发速率影响不大,但会影响液雾分布;进气温度增加会增大进气速度,提高液滴蒸发速率,缩短液雾炬长度;液滴蒸发过程存在一定程度上的压力振荡,会对LPP不稳定燃烧过程产生一定影响。     

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%.     

Contents in Volume 23,2014

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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.     

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.