紊流模型预测街道峡谷流场能力的定量指标评价

选取标准k-ε模型、realizable k-ε模型、重整化群(RNG)k-ε模型和剪切应力传输(SST)k-ω模型分别对街道峡谷水槽模型试验的时均速度和脉动强度进行了数值模拟,并依据文献给出的试验数据定性比较了各紊流模型的预测能力;按模型预测值应满足的基本和较高要求共给出6个定量评价指标(IFAC2、IFB、IFAC...     

进口来流条件对单体建筑绕流时均流场影响的数值模拟

数值模拟了6种进口条件下单体建筑物模型附近的时均流场,模拟值与风洞试验值基本吻合,进口时均流参数对速度模拟值的影响较大,进口紊流参数对速度模拟值的影响则较小;当进口时均速度按照试验情况设置但紊流参数不同时,速度模拟值在建筑物前方和顶部几乎无差别,但在建筑物后方,其差别开始显现,且进口紊流参数按近地面情况或试验值设置时的模拟结果相对较好。由于标准k-ε紊流模型对来流动能的耗散模拟过大,导致速度模拟值比试验值略偏小,人为增加近地面动能的进口条件能有效改善上述情况,在依据进口流量确定的进口速度条件下,建筑物附近的速度模拟值与试验值最吻合。     

三维紊流燃烧室流场的数值计算

运用圆柱从标系对单管回流燃烧室进行了数值模拟。紊流粘度模型采用k-ε双方程紊流模型来估算紊流粘度,燃烧模型采用EBU旋涡破碎燃烧模型来估算化学反应速度,热辐射模型采用比较简单的DTRM模型来计算热辐射量。计算结果能比较准确地反映燃烧室流场的流动状态,同时也为复杂形状的燃烧室造型提供了方法。     

发动机模型进气道内气体流场的模拟和实验研究

本文发展了一种可用于发动机进气门处流场计算的紊流模型。这是因为用标准的k-ε双方程紊流模型,发现其计算结果与作者的试验结果不够吻合。根据作者的实验发现,产生误差的一个重要原因是气门内气道处气体流动状态的各向异性。本文将紊动粘性系数μ_t 看作是一个张量,由此得到了文中修正的k-ε模型,并将标准的k-ε模型和修改了的k-ε模型的计算结果和实验数据进行了对比。结果指出,修改了的k-ε模型对气道内流场的预测精度又提高了一步。本文对模型气道内的流场利用热线风速仪进行了实验研究,考察了平均速度和紊动能的分布及生成规律,指出了设计气道时值得注意的一些问题。     

考虑湍流模型差异的高强化活塞振荡冷却效果研究

为研究不同湍流模型对高强化活塞振荡冷却效果的影响,分别采用RNGk-ε模型、Realizable k-ε模型和SSTk-ω模型对振荡冷却流体流动过程进行了模拟,对其传热效果进行了预测,并与试验结果进行比对。研究结果表明:RNGk-ε模型和Realizable k-ε模型的计算结果比较接近,但与试验值偏差较大;而采用SSTk-ω模型可以更为准确地模拟活塞振荡冷却流体运动规律,传热系数计算值与试验值一致性较高。随着雷诺数的升高,传热系数计算值的误差逐渐增大,说明SSTk-ω模型在低雷诺数条件下准确性更高。     

风力机翼型大攻角分离流的数值模拟

为了准确预测风力机翼型在大攻角下分离流动的气动性能,并且为风力机的设计与安全运行提供一种可靠的数值模拟手段,针对某风力机专用翼型,分别采用基于非定常不可压缩Navier-Stokes方程的大涡模拟(LES)模型、RNG k-ε模型和Standard k-ε模型对其气动性能进行数值模拟,并计算出翼型攻角为35～90°,雷诺数为2×106时的气动力参数。将不同湍流模型的计算结果与风洞试验数据进行比较,并分析流场结构。分析结果表明,LES模型能够准确地模拟出翼型表面的分离流动,计算结果与试验数据取得了很好的一致性,并且优于RNG k-ε模型和Standard k-ε模型的模拟结果。     

低水头大流量厂坝联合泄洪消能流场的二维数值模拟

在模型试验的基础上,应用k-ε紊流数值模型,采用VOF方法,对低水头大流量厂坝联合泄洪消能流场进行了二维数值模拟。研究对比分析了单独泄洪和联合泄洪时下游流场的流态、水面线、流速、压力等水力特性,结果表明,这种布置形式的电站下游,流速叠加后沿垂线出现均化,表孔水舌潜入点下移,底部回旋区减小,下游消能区水跃对电站正常发电的影响较小,采用的数值方法可行,计算结果与试验结果吻合较好,为进一步进行三维数值模拟打下了基础。     

抽水蓄能电站侧式双进/出水口三维数值模拟

采用Realizable k-ε紊流数学模型对抽水蓄能电站侧式双进/出水口进流水流特性进行数值模拟,并将计算结果与物理模型试验的测量结果进行比较,验证了三维数值模拟计算的准确可靠性.模拟结果表明,该方法计算流体力学方便快捷,并可达到物理模型试验的精度.     

直喷式柴油机气缸内各向异性紊流模拟和试验研究

本文发展了一种模拟直喷式柴油机气缸内各向异性紊流的方法。在建立雷诺应力表达式中，没有采用标准的ｋ－ε紊流模型所做的各向同性假设，从紊流的随机性质出发导出适合于各向异性紊流的雷诺应力表达式，对一个轴对称缩口形燃烧室在压缩过程中的流动情况进行了模拟和实际测量。计算与实测结果的对比表明，预测模型及方法有较好的精度。文中还对缸内紊流分布的特点进行讨论。     

三种紊流模型模拟溢流坝泄流精度的对比分析

为研究不同紊流模型对溢流坝泄洪数值模拟的适用性和准确度,采用标准κ-ε模型、RNGκ-ε模型及大涡模拟(LES)模型等3种经典紊流模型对某溢流坝泄流过程进行数值模拟;同时,将数值模拟出的溢流坝坝面的水流流态、流速及坝面压强等溢流坝泄流过程的重要水力指标结果与物理模型试验结果进行对比分析。结果表明,RNGκ-ε模型对溢流坝泄流的流态模拟效果最好,与物理模型试验结果最为一致;LES模型对坝面水流流速、坝面压强及空化数的模拟效果最好;采用标准κ-ε模型模拟出的各个指标精度最低。因此,针对溢流坝泄流过程数值模拟,应优先采用RNGκ-ε模型和LES模型。     

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

<正>~~     

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。     

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.     

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.