楔形仿螺旋肋片内冷通道的流动与换热

利用有限体积法对三维不可压缩的N-S方程进行离散,对上下表面带有错排间断性楔形肋片且对置的仿螺旋内冷通道进行了数值模拟。网格划分采用非结构化混合网格,湍流模型为kε-两方程模型,在近壁面处采用标准壁面函数法进行处理,速度和压力的耦合采用S IM PLE算法。计算获得了楔形仿螺旋肋片内冷通道在楔形肋片与主流方向夹角分别为0°、15°、30°时的三维流场分布。结果表明楔形仿螺旋肋片内冷通道的流场结构比较复杂,通道内流体流动达到了预期的仿螺旋流动效果。通道的平均努谢尔数随楔形肋片与主流夹角的增大而呈增大趋势,通道换热强度得到了明显的提高,但同时流动阻力也显著增加。     

旋转对涡轮工作叶片内冷通道换热影响的研究

为了获得涡轮工作叶片内冷通道的换热特性,采用非结构化网格及standard k-ε紊流模型,求解三维N-S方程,对带60°肋和气膜孔出流的旋转矩形通道内的三维流场进行了数值模拟,近壁面采用增强壁面函数处理.气动参数为:通道入口雷诺数60000,罗斯贝数0.22,气膜孔总出流比0.22.重点分析了静止通道和不同旋转方向的旋转通道的换热系数分布.计算结果表明,静止通道由于肋的存在使换热显著增强,不同的旋转方向对换热强化的效果不同,顺时针旋转时的增强效果要高于逆时针旋转时的增强效果.     

孔_肋相对位置对通道各壁面换热影响

数值模拟了肋和气膜孔的相对位置对矩形通道4个壁面换热特性的影响,重点分析了通道4个壁面换热系数差别以及3种气膜孔位置换热.计算结果和实验数据吻合较好.结果表明:气膜孔位置对同时带肋和气膜孔的下壁面影响最大,孔在肋间上游换热最好,孔在肋中间换热次之,孔在肋间下游换热最差,气膜孔位置对光滑的左右壁面换热影响较小,对只带肋的上壁面几乎没有影响.肋的扰流和气膜孔抽吸使通道下壁面换热系数增幅最大,左右壁面次之,上壁面最小.沿着流动方向,肋扰流和气膜孔出流共同作用导致带肋壁面换热增强因子先增大后减小,光滑壁面换热增强因子先保持不变后减小.     

带射流斜肋微通道的强化散热研究

在高热流密度(300 W/cm~2)条件下,对射流斜肋微通道的流动换热特性进行了数值研究,并与直通道、斜肋直通道和射流微通道进行了对比。结果表明:斜肋后方尾涡受射流抑制减小,受上游流体冲刷裹挟,下游射流发生明显漂移;斜肋有效拓展了通道换热面积,增强了流体扰动,射流使低温冷却水直接冲击通道底面换热,斜肋和射流两种强化换热方式的综合,不仅大幅降低了通道壁面温度,而且极大提高了壁温分布的均匀性,在增强通道换热的同时也没有带来过大阻力增加,和斜肋直通道和射流微通道相比,射流斜肋微通道的换热系数平均增加了21%和33%,综合换热性能和前两者相当。     

矩形肋片导流角度对内冷通道流动与换热特性的影响

利用数值模拟方法分析了矩形仿螺旋肋片内冷通道中肋片导流角度对内冷通道三维流场特性、换热特性以及流动阻力特性的影响。数值计算结果表明,肋片导流角度对内冷通道的流动与换热特性具有较大的影响。流场中冷却介质螺旋流动的强度随着肋片导流角增大而增强,肋片导流角度越大则内冷通道的换热强度越强,同时通道中流动阻力也明显增大。从内冷通道的综合换热效果来看,当肋片导流角度为7。时,矩形仿螺旋肋片内冷通道的综合换热效果最好。     

肋角度对矩形通道壁面换热影响的研究

采用非结构化网格和realizab lek-ε紊流模型,求解三维N-S方程,对带肋和气膜孔矩形通道在入口雷诺数9×104,气膜孔总出流比0.22时进行了换热特性的数值模拟。分析了肋角度对各个壁面换热特性的影响规律。结果表明,带肋和气膜孔的通道流场非常复杂,扰流肋的存在使各壁面的换热都得到了增强,但增强幅度的差别较大。     

仿螺旋肋片内冷通道流动与传热数值分析

利用数值分析方法研究了新型仿螺旋肋片内冷通道的传热与流动特性。采用横截面积为矩形、上下表面带有间断性倾斜矩形肋片叉排且对置的仿螺旋肋片内部冷却通道。分析了在通道宽高比AR=2.9、肋化比Ff/F=2.545、肋高与通道当量直径比e/Dh=0.336、肋间距与肋高比p/e=0.6、肋片与轴面的夹角β=15°及R e在1×104~2×105时的非旋转情况下,R e、肋片与主流方向夹角α等参数对内冷通道强化传热与流动阻力特性的影响。计算结果表明,该仿螺旋矩形肋片作为旋流形成装置起到了迫使流体旋转运动、提高流速和减小层流底层厚度的作用,通道内流体流动达到了预期的螺旋流动效果,通道平均换热系数得到了明显的提高,但同时流动阻力也显著增加。     

内冷通道特性对外表面气膜冷却换热影响研究

采用混合网格和realizable к-ε紊流模型,求解三维N-S方程,对气膜孔入口无横流、仅有横流和横流通道中有扰流肋的情况下,外表面气膜冷却换热特性进行了数值研究.具体分析了横流及肋的存在对气膜冷却外表面换热系数的影响.结果表明,横流及肋的存在使通道流场变得非常复杂,横流提高了外表面的换热系数,且使流动及换热呈现不对称特性分布;而扰流肋的存在减弱了横流对外表面的影响,换热系数相对于仅有横流时降低,且使外表面流动及换热的对称性有所改善.     

燃气轮机通道宽高比对换热特性的影响

采用结构化六面体网格和κ-ε湍流模型,求解三维N-S方程,对直冷却通道内置不同宽高比的W型扰流肋片在入口雷诺数为20 000时的空气流动与换热特性进行数值模拟。分析了不同肋片通道的宽高比对肋间壁面的换热效果和流动损失的影响,并对通道的宽高比进行全局寻优。结果表明,肋片通道的宽高比是影响带肋片直冷却通道的整体换热效果和综合冷却效率的主要因素,并存在一一对应的关系,整体换热效果最好时所对应的肋片宽高比为5,考虑热量损失的情况下换热效果最好时所对应的肋片宽高比为0. 48。     

气膜冷却平板通道的数值模拟

对无肋和带45°肋气膜冷却平板通道的三维对流换热与导热耦合传热问题进行了数值模拟。网格划分采用非结构化网格,湍流模型为SSTk-ω模型,近壁处采用壁面函数法,采用SIMPLEC算法求解速度和压力的耦合。计算获得了无肋和带45°肋气膜冷却平板的流场分布和平板内外表面的平均温度和平均换热系数。计算结果表明,带45°肋的气膜冷却平板表面平均温度较无肋气膜冷却平板表面平均温度低,而近气膜孔区域冷、热表面平均换热系数较无肋时高,而且肋的存在对增大冷空气出流比有利。     

Contents in Volume 23,2014

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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.     

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.     

Controls on the Karaha–Telaga Bodas geothermal reservoir,Indonesia

Karaha–Telaga Bodas is a partially vapor-dominated, fracture-controlled geothermal system located adjacent to Galunggung Volcano in western Java, Indonesia. The geothermal system consists of: (1) a caprock, ranging from several hundred to 1600 m in thickness, and characterized by a steep, conductive temperature gradient and low permeability; (2) an underlying vapor-dominated zone that extends below sea level; and (3) a deep liquid-dominated zone with measured temperatures up to 353 °C. Heat is provided by a tabular granodiorite stock encountered at about 3 km depth. A structural analysis of the geothermal system shows that the effective base of the reservoir is controlled either by the boundary between brittle and ductile deformational regimes or by the closure and collapse of fractures within volcanic rocks located above the brittle/ductile transition. The base of the caprock is determined by the distribution of initially low-permeability lithologies above the reservoir; the extent of pervasive clay alteration that has significantly reduced primary rock permeabilities; the distribution of secondary minerals deposited by descending waters; and, locally, by a downward change from a strike-slip to an extensional stress regime. Fluid-producing zones are controlled by both matrix and fracture permeabilities. High matrix permeabilities are associated with lacustrine, pyroclastic, and epiclastic deposits. Productive fractures are those showing the greatest tendency to slip and dilate under the present-day stress conditions. Although the reservoir appears to be in pressure communication across its length, fluid, and gas chemistries vary laterally, suggesting the presence of isolated convection cells.     

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.     

液压系统常见的故障诊断及处理

任何工程机械式液压设备使用时出现故障是不可避免的。但是怎样确定故障的原因及找到好的解决方法,这是使用者最关心的问题。讲述了液压系统常见的故障及其排除方法。     

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。