现代柴油机燃烧过程中微粒质量的变化规律

利用全气缸取样技术,对不同工况下的柴油机燃烧过程中微粒组分质量生成历程进行了研究.实验结果表明,燃烧形成的干碳烟质量曲线呈单峰状,峰值出现在上止点后10~15°CA之间,在燃烧后期,约有81%~92%的干碳烟被氧化.随着燃空当量比从=0.41增大到=0.53,缸内干碳烟质量峰值增加了4.57%~45.42%;喷油压力升高,虽然干碳烟质量峰值增大,但氧化比例也明显提高.此外,在燃烧初期,微粒中可溶有机物SOF的含量超过80%.     

柴油机燃烧形成微粒的形态特性及微量元素分析

基于电控燃油喷射柴油机的全气缸取样系统,利用场发射透射电子显微镜和图像处理技术,对柴油机燃烧过程中形成微粒的形态特性和微量元素进行了分析.结果表明,柴油机燃烧过程中形成的微粒主要呈现两种形态:一种是由基本碳粒子凝结而成的典型微粒;另一种足富含金属和非金属元素的无定形微粒.其中金属元素主要来源于润滑机油,贯穿整个燃烧过程,且独立存在.另外,典型微粒具有分形结构特性,分维数介于1.2～1.74,且在扩散燃烧初期有降低的趋势.     

柴油、生物柴油后喷燃烧过程中颗粒物粒数粒径及质量的变化规律

基于全气缸取样平台,在一台高压共轨柴油机上,对近后喷和远后喷策略下生物柴油和柴油后喷燃烧过程中颗粒物的粒数粒径分布、总粒子数密度及质量的变化规律进行了研究.实验结果表明,在后喷燃烧过程中各阶段,生物柴油和柴油所产生的颗粒物的粒数粒径分布、总粒子数密度及质量变化规律相似.颗粒物粒数粒径均呈类似对数正态分布,生物柴油的峰值粒径范围为39.2~69.8,nm,柴油的峰值粒径范围为60.4~93.1,nm.与柴油相比,在相同后喷燃烧阶段生物柴油的颗粒物的总粒子数密度较高,但质量浓度较低.     

二级增压对重型高压共轨柴油机燃烧及微粒排放特性的影响

针对某电控高压共轨重型柴油机匹配二级增压系统后的燃烧、排放及微粒粒度分布特征进行了试验研究,分析了二级增压系统对柴油机性能、燃烧及微粒排放的影响规律。研究结果表明:重型高压共轨柴油机微粒更趋于超细化,核态微粒数量浓度峰值在8~15nm,绝大多数微粒粒径200nm;中高负荷工况下核态微粒比例达93%以上。匹配二级增压系统后,进气量和空燃比明显增大,缸内最高燃烧压力及放热率峰值升高,消光烟度及微粒排放大幅度降低;13工况微粒加权平均比排放量比单级增压减少了75.6%;低负荷工况下核态微粒数量及其所占比例高于单级增压,积聚态微粒数量有所降低;中高负荷工况下50nm以下的核态微粒数量及其所占比例降低,且微粒总数及积聚态微粒数量减少,核态微粒平均粒径增大。     

风冷非道路柴油机排放特性研究

利用车载排放检测仪(SEMTECH)和电子低压冲击仪(ELPI)进行了某非道路风冷非增压柴油机的气态污染物的测量和排放颗粒物粒径及质量分布特征研究.试验按照非道路柴油机排放标准规定的工况进行测量.试验结果表明:试验用非道路柴油机气态污染物中NOx的排放较差;微粒数量排放均成单峰对数正态分布,积聚模态微粒数浓度随负荷增加而增大,随负荷的减小积聚模态的粒径分布向小粒径方向移动,变化规律与车用柴油机相似,但粒径整体要大于车用柴油机;不同粒径微粒的质量呈双峰分布,分别对应积聚模态和粗粒子模态,积聚模态微粒对总质量排放的贡献较大,但与车用柴油机相比分担率有所不同.     

后喷策略下喷油压力对柴油机缸内颗粒物粒径分布和形貌特征的影响

基于全气缸取样平台,运用称重法、发动机粒数粒径测试分析仪(EEPS)和透射电子显微镜(TEM)等手段对采集到的燃烧过程中的颗粒物进行分析处理,研究高压共轨柴油机在后喷策略下,喷油压力对于柴油机燃烧过程中颗粒物粒径分布和形貌特征的影响.实验结果表明:后喷的加入会延长燃烧持续期;当喷油压力由100,MPa上升到120,MPa时,缸内燃烧温度压力升高、燃烧始点提前,预混燃烧在整个燃烧过程中所占比例增大,生成颗粒物粒径分布特征由双峰变为单峰,粒子数密度和碳烟质量浓度均降低,除此之外增大喷油压力还会导致燃烧过程中生成的链状颗粒减少,分形维数增大,同时减小后喷对于燃烧持续期的影响.     

柴油机缸内团聚态颗粒氧化过程中的破碎

基于全气缸取样系统采集不同燃烧时刻的柴油机碳烟,使用粒数粒径测试分析仪和透射电子显微镜测量了碳烟的粒径分布、数密度、分形维数和团聚度,进而获得团聚态颗粒的破碎速率,在上述工作的基础上,分析了柴油机缸内碳烟氧化主导阶段团聚态颗粒物的破碎现象.结果表明:碳烟氧化主导阶段初期,团聚态颗粒破碎速率高,碳烟颗粒总粒数密度和核态颗粒数密度增加,同时分形维数和团聚度明显减小.随氧化主导阶段燃烧反应的进行,破碎速率逐渐降低,总颗粒数密度逐渐减小,核态颗粒数密度先增加后减小,而分形维数和团聚度呈现上升的趋势.     

电控高压共轨柴油机排气微粒的测量及数值模拟

利用改进后的全气缸取样系统,对不同工况下高压共轨电控柴油机燃烧过程中的微粒生成历程进行测量.同时,采用三维CFD数值模拟方法分析了微粒的生成机理.结果表明,微粒质量浓度曲线呈单峰状,微粒生成质量随负荷的增大而增加;发动机排出微粒是碳烟粒子生成和氧化这两个过程综合作用的结果.加强缸内的气流运动和合理组织燃烧过程,有助于减少微粒的生成.     

后喷间隔角对柴油机燃烧过程中颗粒物数密度、粒径分布及质量的影响

基于全气缸取样平台,在一台高压共轨直喷柴油机上,研究了不同后喷间隔角对柴油机燃烧过程中颗粒物的数密度、粒径分布以及质量的影响.结果表明:单次喷射工况,颗粒物数密度随曲轴转角呈单峰分布,峰值出现在11~14°,CA ATDC,排气颗粒物数密度比缸内颗粒物的峰值数密度减少75%,以上;引入后喷,随着后喷间隔角的增大,缸内颗粒物数密度呈先升高后降低的趋势.颗粒物粒径分布在单次喷射工况呈类似对数正态分布,主要分布范围在6.04~339,nm之间,其峰值粒径主要分布在69.8~93.1,nm之间;引入后喷,缸内颗粒物的峰值粒径先减小后增大,并在排气阶段峰值粒径达到最大,且随着后喷间隔角的增大,缸内颗粒物峰值粒径呈增大的趋势.单次喷射工况下缸内颗粒物的质量随着曲轴转角呈单峰分布;引入后喷,缸内颗粒物质量峰值随后喷间隔角的增大呈先增大后减小的趋势.后喷对碳烟质量的影响规律与其对颗粒物质量的影响规律基本相同.     

柴油机运行工况对排气管内碳烟尺寸的影响

改变柴油机运行工况,采集排气管内碳烟颗粒,通过透射电镜对碳烟颗粒进行观测,拍摄碳烟颗粒形貌图片,采用Image-ProPlus软件统计分析碳烟颗粒的平均投影面积(A)、平均回转半径(R_g)、包含基本碳粒子的平均数(N_p)以及基本碳粒子的平均粒径(d_p)等特征参数,得到碳烟颗粒的分形维数.研究发现:改变柴油机负荷对排气管内的碳烟颗粒尺寸的影响要远大于改变转速带来的影响.转速不变时,随着负荷的增大,A、Rg、N_p和d_p均呈"v"型变化趋势,分形维数呈上升趋势;负荷不变时,随着转速的增大,A、R_g、N_p、d_p小幅度上升,分形维数基本不变.     

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.     

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.     

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%.     

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.     

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.     

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.     

Exergetic evaluation of 5 biowastes-to-biofuels routes via gasification

This paper presents the exergy analysis results for the production of several biofuels, i.e., SNG (synthetic natural gas), methanol, Fischer–Tropsch fuels, hydrogen, as well as heat and electricity, from several biowastes generated in the Dutch province of Friesland, selected as one of the typical European regions. Biowastes have been classified in 5 virtual streams according to their ultimate and proximate analysis. All production chains have been modeled in Aspen Plus in order to analyze their technical performance. The common steps for all the production chains are: pre-treatment, gasification, gas cleaning, water–gas-shift reactions, catalytic reactors, final gas separation and upgrading. Optionally a gas turbine and steam turbines are used to produce heat and electricity from unconverted gas and heat removal, respectively. The results show that, in terms of mass conversion, methanol production seems to be the most efficient process for all the biowastes. SNG synthesis is preferred when exergetic efficiency is the objective parameter, but hydrogen process is more efficient when the performance is analyzed by means of the 1st Law of Thermodynamics. The main exergy losses account for the gasification section, except in the electricity and heat production chain, where the combined cycle is less efficient.     

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.