超临界燃料喷射过程大涡模拟:状态方程的影响

针对Sandia实验室关于正庚烷喷雾的实验数据，基于CONVERGE软件采用大涡模拟方法对以正庚烷为燃料喷入超临界环境中的雾化过程进行了数值模拟。以实际气体状态方程Soave-Redlich-Kwong (SRK)和Peng-Robinson(PR)两个方程为基础，重点研究了两状态方程对超临界状态下燃料喷雾的发展过程、射流密度变化、燃料喷雾的质量分数随温度变化的影响，并用模拟所得与实验结果进行对比。结果表明，同一时间下PR方程模拟的燃料喷雾的贯穿度更大；两实际气体状态方程下射流表面都有大的密度梯度，并与实验所得的密度值相吻合；PR方程对于超临界工况的计算可能更优于SRK方程。燃料质量分数随温度的变化符合实际的情况，密度值的急剧变化验证了射流表面是一个介于液体与超临界流体之间的混合层，并可以通过密度梯度来推测混合层位置。     

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．     

内燃机跨临界/超临界燃料喷雾混合过程的机理与模型

高密度低温燃烧等内燃机燃烧新概念的提出,使得跨临界/超临界燃料喷射与喷雾混合成为内燃机研究者面临的一个重要课题.跨临界/超临界下的液体射流与喷雾形成具有与亚临界工况本质不同且极其复杂的机理.首先对跨临界/超临界液体射流喷雾的特征与机理做了简要介绍;然后,对迄今国内外在该领域的研究进展进行了比较系统全面的评述,同时指出了其局限与不足.在全面审视前人关于直喷式内燃机燃料喷雾混合过程的机理与模型研究的基础上,提出一个新的跨临界/超临界环境下液体燃料喷射与混合气形成的数理模型——"混合层回缩"(MLR)模型;并进而建议,采用微观、介观与宏观研究相结合的5步策略,构建直喷式发动机亚临界/跨临界/超临界燃料喷雾混合过程的全工况统一模型.     

环境压力对超临界流体喷雾特性影响的数值分析

基于大涡模拟方法对超临界喷射进行数值分析,使用真实气体状态方程SRK计算流体的热物性和输运特性.研究对象是定容弹内低温液氮超临界喷射,假定初始入口条件不变,重点考察了不同环境压力对超临界喷雾特性的影响.结果表明:由于射流流体与周围流体之间存在较大密度比,射流表面形成显著的密度分层,并起到限制射流流体向径向扩展的作用.随着环境压力的提高,由于射流和周围环境的密度比降低,射流表面不稳定增加,比较容易形成不稳定涡,从而更有利于射流与周围气体的混合,因而液核长度也比较短,表明随着压力的增大,射流与周围气体的混合效果更好.进一步分析发现,环境压力对热物性和输运特性的影响较大,而对湍动能和速度场的影响不大.     

基于多组分蒸发模型的柴油喷雾燃烧的LES研究

采用大涡模拟方法结合多组分蒸发模型对定容燃烧室内的柴油喷雾和燃烧过程进行了数值模拟研究.在大涡模拟中,采用动态结构模型模拟亚网格应力项,基于反褶积方法在湍动能的输运方程中增加了源项,表征燃油液滴与环境气体在亚网格尺度的相互作用;在多组分蒸发模型中,采用Gamma函数描述柴油液滴组分的摩尔分布,推导了基于连续热力学方法的气相组分控制方程的大涡模拟形式.将上述模型写入到多维CFD程序KIVA-4中,通过与无蒸发燃油喷雾和有蒸发喷雾燃烧过程相关实验数据的对比,计算得到了更为真实的喷雾形态和碳烟分布等结果,表明了该模型对提高柴油喷雾燃烧过程仿真性能具有巨大潜力.     

平面自由射流中燃料扩散的大涡模拟

对甲烷－空气预混气体的平面自由射流进行了大涡模拟,采用分步投影法求解动量方程,亚格子项采用标准Smagorinsky亚格子模式模拟,压力泊松方程采用修正的循环消去法快速求解,空间方向采用二阶精度的差分格式,在时间方向上采用二阶精度的显式差分格式。模拟结果给出了预混气体射流中拟序结构对燃料扩散的影响,表明促使燃料扩散的主要因素是射流中的拟序结构,由浓度梯度导致的组分扩散较弱。     

"Bump燃烧室"内新概念稀扩散燃烧混合气形成机理的研究

基于自行研制的实验装置,用片状激光诱导荧光法(PLIF)对普通商用柴油喷雾的撞壁混合过程进行了实验研究,并用CFD数值分析软件对其进行了模拟计算,二者结果基本吻合．平板和实际燃烧室的实验及计算结果均表明,撞壁射流在遇到限流沿(Bump)后会剥离壁面,形成二次空间射流,扩大撞壁射流与空气的空间混合体积及混合速率,出现与周围空气迅速混合的“闪混”现象,减少壁面燃油堆积量．计算结果还表明,Bump的存在改变了缸内气流运动的流场结构,Bump附近旋向相反的“双涡结构”极大地增强了二次空间射流对周围空气的卷吸,促进了燃油与空气的混合,是Bump燃烧室内稀混合气形成及稀扩散燃烧的关键所在．     

不同亚网格尺度应力模型在燃油喷雾大涡模拟中的应用

亚网格尺度应力模型是影响大涡模拟结果的一个关键因素.在K-方程亚网格湍动能模型的基础上,对其它比较常用的3种代数模型(Smagoringsky模型、动态Smagoringsky模型和WALE模型)在燃油喷雾中的适用性作了探讨.在定容弹中采用3个不同的燃油喷雾试验作为数值模拟的验证对象,分析了喷雾的演变过程.并将其应用于...     

直喷式柴油机燃烧的一个二维数学模型

本文建立了用于模拟和分析直喷式柴油机缸内主要过程的一个轴对称二维数学模型。缸内气体运动和燃油空气混合过程利用任意拉格朗日-欧拉法(ALE)进行数值分析。其网格单元为任意四边形,且可按规定连度运动。燃油喷雾用气体射流模拟。用简化的亚网格尺度(SGS)湍流粘性来模拟湍流输运。燃烧和排放计算采用部分平衡流方法:燃料的氧化和NO形成用化学动力学公式计算,而组分的离解用化学平衡方式处理。计算结果与实验及其他文献的计算结果相吻合,表明本方法对于内燃机的设计和研究是一种很有潜力的工具。     

燃油喷雾初始破碎及二次雾化机理的研究

基于大涡模拟LES(large eddy simulation)理论和VOF(Volume of Fluid)方法,考察了燃油喷雾初始时刻即时间尺度为微秒级的液柱破碎过程,分析了初始破碎的机理,给出了该时间尺度下液柱初始破碎过程的模型;通过对一特定条件下的柴油机喷油器的建模和喷雾过程的大涡数值模拟,获得了液柱初始时刻"伞状头部"的喷雾形态,所得计算结果与相应的试验数据符合较好;数值模拟还直观地展现了液滴背风RT破碎、哑铃型破碎以及液滴的聚集融合等液滴的二次雾化过程.同时,也说明了大涡模拟这种准直接数值方法较之DDM(discrete droplets model)方法所具有的优势和潜力.     

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%.     

Performance assessment of some ice TES systems

In this paper, a performance assessment of four main types of ice storage techniques for space cooling purposes, namely ice slurry systems, ice-on-coil systems (both internal and external melt), and encapsulated ice systems is conducted. A detailed analysis, coupled with a case study based on the literature data, follows. The ice making techniques are compared on the basis of energy and exergy performance criteria including charging, discharging and storage efficiencies, which make up the ice storage and retrieval process. Losses due to heat leakage and irreversibilities from entropy generation are included. A vapor-compression refrigeration cycle with R134a as the working fluid provides the cooling load, while the analysis is performed in both a full storage and partial storage process, with comparisons between these two. In the case of full storage, the energy efficiencies associated with the charging and discharging processes are well over 98% in all cases, while the exergy efficiencies ranged from 46% to 76% for the charging cycle and 18% to 24% for the discharging cycle. For the partial storage systems, all energy and exergy efficiencies were slightly less than that for full storage, due to the increasing effect wall heat leakage has on the decreased storage volume and load. The results show that energy analyses alone do not provide much useful insight into system behavior, since the vast majority of losses in all processes are a result of entropy generation which results from system irreversibilities.     

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.     

Effect of co-cultivation of Chlamydomonas reinhardtii with Azotobacter chroococcum on hydrogen production

Chlamydomonas reinhardtii cc124 and Azotobacter chroococcum bacteria were co-cultured with a series of volume ratios and under a variety of light densities to determine the optimal culture conditions and to investigate the mechanism by which co-cultivation improves H₂ yield. The results demonstrated that the optimal culture conditions for the highest H₂ production of the combined system were a 1:40 vol ratio of bacterial cultures to algal cultures under 200 μE m^?2 s^?1. Under these conditions, the maximal H₂ yield was 255 μmol mg^?1 Chl, which was approximately 15.9-fold of the control. The reasons for the improvement in H₂ yield included decreased O₂ content, enhanced algal growth, and increased H₂ase activity and starch content of the combined system.     

Natural-gas fueled spark-ignition (SI) and compression-ignition (CI) engine performance and emissions

Natural gas is a fossil fuel that has been used and investigated extensively for use in spark-ignition (SI) and compression-ignition (CI) engines. Compared with conventional gasoline engines, SI engines using natural gas can run at higher compression ratios, thus producing higher thermal efficiencies but also increased nitrogen oxide (NO_x) emissions, while producing lower emissions of carbon dioxide (CO₂), unburned hydrocarbons (HC) and carbon monoxide (CO). These engines also produce relatively less power than gasoline-fueled engines because of the convergence of one or more of three factors: a reduction in volumetric efficiency due to natural-gas injection in the intake manifold; the lower stoichiometric fuel/air ratio of natural gas compared to gasoline; and the lower equivalence ratio at which these engines may be run in order to reduce NO_x emissions. High NO_x emissions, especially at high loads, reduce with exhaust gas recirculation (EGR). However, EGR rates above a maximum value result in misfire and erratic engine operation. Hydrogen gas addition increases this EGR threshold significantly. In addition, hydrogen increases the flame speed of the natural gas-hydrogen mixture. Power levels can be increased with supercharging or turbocharging and intercooling. Natural gas is used to power CI engines via the dual-fuel mode, where a high-cetane fuel is injected along with the natural gas in order to provide a source of ignition for the charge. Thermal efficiency levels compared with normal diesel-fueled CI-engine operation are generally maintained with dual-fuel operation, and smoke levels are reduced significantly. At the same time, lower NO_x and CO₂ emissions, as well as higher HC and CO emissions compared with normal CI-engine operation at low and intermediate loads are recorded. These trends are caused by the low charge temperature and increased ignition delay, resulting in low combustion temperatures. Another factor is insufficient penetration and distribution of the pilot fuel in the charge, resulting in a lack of ignition centers. EGR admission at low and intermediate loads increases combustion temperatures, lowering unburned HC and CO emissions. Larger pilot fuel quantities at these load levels and hydrogen gas addition can also help increase combustion efficiency. Power output is lower at certain conditions than diesel-fueled engines, for reasons similar to those affecting power output of SI engines. In both cases the power output can be maintained with direct injection. Overall, natural gas can be used in both engine types; however further refinement and optimization of engines and fuel-injection systems is needed.     

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.     

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.     

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。