同速对转涡轮盘腔内流动和换热特性

为研究对转盘旋转速度对盘腔内换热效果的影响,应用RNG k-ε湍流模型对中心进气、径向出流的同速对转涡轮盘腔内的流动和换热特性进行了数值模拟。揭示了盘腔内气体的流动结构、两盘壁面边界层内的径向速度、换热效率以及壁面温度分布特征,并进一步探究了转盘转速对上述流动和换热特征的影响规律。结果表明:对转盘腔内存在两个反向的回流涡胞,这两个涡胞的相对大小以及相遇的滞止点位置取决于进气惯性力和旋转力的相对大小;转盘近壁面流体径向流动速度与盘壁面的对流传热系数呈正相关变化;对于上游盘,低转速范围时转速对壁面换热影响取决于旋转径向力和进气惯性力对上游盘近壁面流体流动驱动作用的相对大小,高转速范围内增大转速,上游盘壁面传热系数增大,壁面温度降低;对于下游盘,壁面低半径区域的对流传热系数随转速增大而减小,而高半径区域的对流传热系数随转速增大而增大。     

带扰流片的矩形直通道内的流动与换热

建立了矩形直通道内三维可压缩流动与换热模型 ,对来流雷诺数Re在 1× 10 5～ 3× 10 5范围内 ,带顺排和错排扰流片的通道内部对流换热过程进行了模拟计算。计算结果表明 ,矩形扰流片具有明显的强化冷却效果 ,扰流片表面的对流换热系数明显高于光滑表面的值 ,有扰流片区域的壁面温度明显降低 ;错排扰流片与顺排扰流片相比 ,对流换热系数增大 4 %左右。计算结果归纳了传热和流动压降关系式。     

中心进气旋转盘的冷却效果实验研究

对涡轮发动机热部件冷却的主要指标目的是使热部件的总体温度尽可能低和热部件内部的温差尽可能小,因此对涡轮发动机涡轮盘的冷却除研究转盘表面的对流换热系数外,还应对以上两个指标进行研究。本文将发动机的涡轮盘简化成中心进气施转盘模型,用实验的方法研究了旋转雷诺数、进气雷诺数、盘罩间隙比和出气间隙比对转盘无量钢过余体平均温度和无量纲径向温差的影响,并给出了相应的准则关系式。     

采用扩展温度振荡法测量超临界CO_2管内对流换热特性

提出了考虑小通道内流体温度变化时测量管内对流换热系数的扩展温度振荡法模型，该法可用于测量各种光滑表面通道内流动的局部对流换热系数。实测了典型压力温度条件下2mm内径不锈钢圆管内超临界CO2对流换热系数。     

燃气轮机涡轮盘结构应力有限元分析

在某燃气轮机透平一级涡轮盘三维模型的温度和应力场计算基础上,为降低涡轮盘应力,用有限元通用程序对其作局部细网格计算研究并探讨影响因素,给出了改进设计建议,为涡轮盘的结构设计及疲劳寿命管理提供依据。     

双风道太阳墙内流动传热数值模拟分析

太阳墙的供暖效果与自身的太阳能利用效率紧密相关。为了提升太阳墙的太阳能利用效率,文章提出一种双风道太阳墙。利用CFD软件对不同风量下单、双风道太阳墙内空气的换热特性进行数值模拟,并给出了当太阳辐照度为311 W/m~2时,双风道进、出口温度差与太阳能利用效率的关系曲线。研究结果表明:随着风量的增加,太阳墙的太阳能利用效率逐渐升高,集热板温度逐渐降低,空气与集热板之间的对流换热系数逐渐升高;与单风道太阳墙相比,双风道太阳墙的瞬时集热效率显著升高,最高可达14.12%。     

三角扰流蛇形翅片管流动换热特性数值研究

蛇形翅片管作为空冷凝汽器基本换热面,其流动与换热性能对空冷电站经济性有着重要影响,基于此,对三角扰流蛇形翅片管的气侧换热及流动特性进行了数值研究,分析了流体温度、压力沿流程方向的变化规律以及迎面风速、扰流三角个数、扰流三角尺寸等因素对蛇形翅片管换热及流动性能的影响,结果表明三角扰流部位局部阻力是沿程阻力的25%～70%,扰流三角对换热有明显的促进作用,换热系数最大提高37%.本研究可为蛇形翅片管的设计及优化提供理论参考.     

涡轮叶片尾缘端部冲击流动与换热特性研究

针对涡轮叶片尾缘吸力面热应力集中,容易造成叶片尾部烧毁的现象,提出端部冲击扰流柱结构,采用Realizable k-ε湍流模型和增强壁面函数分析涡轮叶片尾缘内部流场和吸力面换热特性,研究不同冲击孔与扰流柱排列方式的影响,揭示端部冲击扰流柱结构的流场与换热机理。研究表明,端部冲击扰流柱结构对于改善吸力面的换热效果要优于中间冲击扰流柱结构,对端壁的换热有显著提高;各表面平均换热系数均随着压比的增大而增大,顺排结构时,冲击孔换热最强,扰流柱换热次之;叉排结构时,冲击孔换热最强,隔板迎风换热次之;近距离冲击,顺排的综合效果优于其它几种结构,而远距离冲击,叉排的综合效果最好,其吸力面温度分布较均匀。     

某压气机盘腔的流动换热数值仿真研究

为了获取进出口压力对压气机盘腔温度分布的影响规律,采用数值模拟的方法对一种真实的涡扇发动机压气机旋转盘腔进行了流动换热研究。结果表明,应用二维数值模拟能够较为准确的捕捉轴向通流盘腔的流动特征,数值方法及简化模型的可信度较高;随着盘腔进口压力的提高,盘腔进口至出口的温升增加,但温升对流量的变化相对不敏感。     

采用扩展温度振荡法测量超临界CO2管内对流换热特性

提出了考虑小通道内流体温度变化时测量管内对流换热系数的扩展温度振荡法模型,该法可用于测量各种光滑表面通道内流动的局部对流换热系数.实测了典型压力温度条件下2mm内径不锈钢圆管内超临界CO     

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.     

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.     

Decomposition of limestone in a solar reactor

The thermal decomposition of limestone has been selected as a model reaction for developing and testing an atmospheric open solar reactor. The reactor consists of a cyclone gas/particle separator which has been modified to let the concentrated solar energy enter through a windowless aperture. The reacting particles are directly exposed to the solar irradiation. Experimentation with a 60 kW reactor prototype was conducted at PSI's 90m² parabolic solar concentrator, in a continuous mode of operation. A counter-current flow heat exchanger was employed to preheat the reactants. Eighty five percent degree of calcination was obtained for cement raw material and 15% of the solar input was converted into chemical energy (enthalpy).The technical feasibility of the solar thermal decomposition of limestone was experimentally demonstrated. The use of solar energy as a source for high-temperature process heat offers the potential of reducing significantly the CO₂ emissions from lime producing plants. Such a solar thermochemical process can find application in sunny rural areas for avoiding deforestation.     

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.     

Biohydrogen production by Anabaena sp. PCC 7120 wild-type and mutants under different conditions: Light, nickel, propane, carbon dioxide and nitrogen

Increasing awareness of environmental problems caused by the current use of fossil fuel-based energy, has led to the search for alternatives. Hydrogen is a good alternative and the cyanobacterium Anabaena sp. PCC 7120 is naturally able to produce molecular hydrogen, photosynthetically from water and light. However, this H₂ is rapidly consumed by the uptake hydrogenase.This study evaluated the hydrogen production of Anabaena sp. PCC 7120 wild-type and mutants: hupL⁻ (deficient in the uptake hydrogenase), hoxH⁻ (deficient in the bidirectional hydrogenase) and hupL⁻/hoxH⁻ (deficient in both hydrogenases) on several experimental conditions, such as gas atmosphere (argon and propane with or without N₂ and/or CO₂ addition), light intensity (54 and 152 ??Em⁻²s⁻¹), light regime (continuous and light/dark cycles 16 h/8 h) and nickel concentrations in the culture medium.In every assay, the hupL⁻ and hupL⁻/hoxH⁻ mutants stood out over wild-type cells and the hoxH⁻ mutant. Nevertheless, the hupL⁻ mutant showed the best hydrogen production except in an argon atmosphere under 16 h light/8 h dark cycles at 54 ??Em⁻²s⁻¹ in the light period, with 1 ??M of NiCl₂ supplementation in the culture medium, and under a propane atmosphere.In all strains, higher light intensity leads to higher hydrogen production and if there is a daily 1% of CO₂ addition in the gas atmosphere, hydrogen production could increase 5.8 times, related to the great increase in heterocysts differentiation (5 times more, approximately), whereas nickel supplementation in the culture medium was not shown to increase hydrogen production. The daily incorporation of 1% of CO₂ plus 1% of N₂ did not affect positively hydrogen production rate.     

Investigation of the thermodynamic and kinetic properties of La–Fe–B system hydrogen-storage alloys

La–Fe–B hydrogen-storage alloys were prepared using a vacuum induction-quenching furnace with a rotating copper wheel. The thermodynamic and kinetic properties of the La–Fe–B hydrogen-storage alloys were investigated in this work. The P–C–I curves of the La–Fe–B alloys were measured over a H₂ pressure range of 10⁻³ MPa to 2.0 MPa at temperatures of 313, 328, 343 and 353 K. The P–C–I curves revealed that the maximum hydrogen-storage capacity of the alloys exceeded 1.23 wt% at a pressure of approximately 1.0 MPa and temperature of 313 K. The standard enthalpy of formation ΔH and standard entropy of formation ΔS for the alloys' hydrides, obtained according to the van't Hoff equation, were consistent with their application as anode materials in alkaline media. The alloys also exhibited good absorption/desorption kinetics at room temperature.     

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.     

The Solar Office in context

The goal of sustainability in buildings can only hope to be realised if buildings are designed to both conserve and generate energy. The Solar Office at Doxford International is designed to minimise the use of energy while its external fabric is designed to replace such energy that is used. The recently completed building is now subject of a comprehensive monitoring programme. The programme covers both the performance of the 73 kW_p photovoltaic installation and the environmental conditions within the building as a whole. Hour by hour findings are posted on a dedicated web site. Photovoltaics could have the same impact on building form and layout as the invention of the passenger lift at the end of the last century.     

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。     

Hydrogen recovery from the photovoltaic electroflocculation-flotation process for harvesting Chlorella pyrenoidosa microalgae

In this paper, an integrated process using photovoltaic power to harvest microalgae by electro-flocculation (EF) and hydrogen recovery is presented. It is mainly favorable in regions with high solar radiation. The electro-flocculation efficiency (EFE) of Chlorella pyrenoidosa microalgae was investigated using various types of electrodes (aluminum, iron, zinc, copper and a non-sacrificial electrode of carbon). The best results regarding the EFE, and biomass contamination were achieved with aluminum and carbon electrodes where the electrical energy demand of the process for harvesting 1 kg of algae biomass was 0.28 and 0.34 kWh, respectively, while the energy yield of harvested hydrogen was 0.052 and 0.005 kWh kg^?1, respectively. The highest harvesting efficiency of 95.83 ± 0.87% was obtained with the aluminum electrode.The experimental hydrogen yields obtained were comparable with those calculated from theory. With a low net energy demand, microalgae EF may be a useful and low-cost technology.