流化床密相区流动特性的数值模拟

流化床内气固两相流动一直是实验研究和数值模拟的热点。基于Eulerian双流体模型,本文建立了流化床内的气固两相流动模型,采用FLUENT软件对流化床密相区两相流动特性、床内气泡的产生运动和爆裂等特性进行了数值模拟。模型中,将颗粒相看作是连续介质,建立与气相相同形式的数学模型;采用了离散介质动力理论,引入颗粒温度来描述固相粘性应力,并用气固曳力进行气固两相耦合。模拟得到了气泡产生、运动和爆裂的变化过程,与实验结果相一致。采用不同的曳力模型对流化床稠密两相流动进行了模拟,与Kuipers实验对比,结果表明采用Gidaspow曳力模型描述流化床稠密两相流动特性更准确。     

循环流化床内颗粒运动的PIV测试

作为一种瞬时全场测速技术,PIV测试技术被尝试用于测量循环流化床(CFB)内颗粒运动,以进一步了解循环流化床内复杂的气固两相流动特性。初步实验已在一个截面为200mm×200mm高为4m的冷态循环流化床实验台上完成,并运用二值化互相关图像处理算法,获得了床内截面上的颗粒流动矢量图。初步测试结果较好地反映了循环流化床内颗粒流动的一些特性,表明PIV技术在循环流化床气固两相流体特性研究中具有较好的应用前景。     

颗粒轨道模型中相间耦合关系及曳力计算的研究

在气固两相流多尺度模拟的确定性颗粒轨道模型中，相间耦合关系及颗粒曳力计算对模拟结果具有举足轻重的影响。基于简化的硬球模型，运用不同的相间耦合关系及曳力公式对气固流化床进行了模拟。研究结果表明：在确定性颗粒轨道模型中，应该基于颗粒群的曳力计算进行相间耦合，且基于Wen和Yu以及Di Felice的曳力公式能够较好地模拟鼓泡流化床和循环流化床的基本流动特征；而基于双流体模型建立相间耦合关系及进行颗粒曳力计算的正确性尚需进一步研究。图4表2参12     

矩形平壁循环流化床冷态流动特性研究

在循环流化床冷态试验台架上,以空气为流化介质,砂子（０．１￣０．４５ｍｍ）为床料,采用反射式光导纤维探针,分别测定了铅轴向和径向的颗粒浓度分布,特别是考察了矩形截面循环床边角对气固两相流动的影响,对径向颗粒浓度分布的影响,考察了壁面附近的气固流动。引入了时序分析方法对采样数据建立了参数模型。实验结果对加深循环流化床内气固两相流动的理解具有重要意义。     

流化床提升管内气固两相流动压降的预测及试验验证

提出了提升管压降的计算公式,并考虑了两个模型用于计算提升管中颗粒相的速度.搭建了双循环流化床冷态试验装置台,利用差压变送器对气固提升管内轴向压降进行了系统测试,研究了颗粒循环流率对压降的影响.将利用提升管压降公式计算得到的压降与试验测得的压降进行了比较.结果表明:在提升管加速段,假定提升管气固两相的滑移速度等于颗粒相的终端速度,将计算得到的颗粒相速度代入压降计算式中,得到的预测值与试验值比较吻合;在提升管充分发展段,将利用滑移因子ψ计算得到的颗粒相速度代入压降计算式中,得到的预测值与试验值较吻合.     

燃煤循环流化床模型与试验研究

利用循环流化床内气－固两相流动等基础方面的研究成果,根据本文床内气固浓－淡流动模型,建立适用不同结构参数的循环流化床燃烧模型,考虑了床内气体、固体颗粒的返混、循环过程,以及煤燃烧、ＮＯ的生成和分解、颗粒磨损等因素。在循环流化床燃烧试验台上进行实验研究,模型仿真结果和实验数据吻合良好,表明气固两相浓－淡流动模型所建立的循环流化床燃烧系统模型可以正确地模拟循环流化床的燃烧过程。     

稠密气固两相流动过程模拟的改进模型与应用

提出了模拟稠密气固两相流动的改进模型。湍流流场采用改进的k-ε-εe模型，颗粒的聚合效应采用聚合力的当量直径折算模型计算。将颗粒团作为离散相，研究颗粒团的运动、碰撞、破碎与合并。应用上述模型数值模拟了循环流化床内的稠密气固两相流动。得到了床内气相速度、颗粒团分布、颗粒浓度分布及颗粒团大小分布等详细两相流场信息。计算结果合理，与前人实验结果相符。模拟结果详细揭示了循环流化床内稠密气固两相流动的基本特征。图8表2参8     

旋流风冷冷渣器结构试验研究

针对目前国内循环流化床锅炉冷渣器的使用现状,研究了一种新型CFB锅炉冷渣器,设计了冷渣器的主冷却段,采用CFD方法对其内部气固两相流动进行了数值模拟,得到新型冷渣器流场中速度与温度分布以及固体颗粒的运动轨迹,为CFB锅炉的优化设计提供了可靠依据。     

基于EMMS-1M曳力模型的空气反应器气固流动及化学反应特性数值分析

针对传统曳力模型难以准确预测气固相间曳力的问题,考虑到计算精度和模型通用性,选用经过介尺度修正的EMMS-1M曳力模型耦合双流体模型,模拟了颗粒直径和进口气速对空气反应器冷态流化和热态化学反应的影响。结果表明：颗粒直径减小和进口气速增加,使床层气体压力分布更加均匀,床层轴向颗粒体积分数降低,床层颗粒返混现象减弱,颗粒轴向速度增加,温度升高促进反应正向进行,氧气转化率升高;进口气速增加,使颗粒在床层分布均匀,气固混合彻底,促进了化学反应,但进口气速过高,会携带出大量的颗粒,且在床层的停留时间减少,不利于气固化学反应的发生。     

曳力模型对流态化两相流动数值计算影响的研究

颗粒间曳力模型对于流态化两相流动数值模拟有着重要的影响。本文利用FLUENT作为数值计算工具,采用欧拉双流体模型对微米级别(粒径5 um)的细颗粒在不同曳力模型下的气固流态化两相流动进行数值模拟对比分析,并采用优选的曳力模型对粒径500 um和50 um的颗粒进行流态化流动的数值模拟。模拟结果表明Gidaspow曳力模型在所研究的范围框架下有着更加良好的表现,更加符合实际情况。     

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.