粗糙圆管内超临界航空煤油湍流换热特性分析

给出了国产航空煤油RP-3的三组分热物性替代燃料模型。采用k-ε湍流模型结合增强壁面处理的方法对超临界压力下航空煤油RP-3在圆形粗糙冷却通道中的流动与换热过程进行数值研究。分析了粗糙元形状、高度以及间高比等因素对其超临界流动和传热特性的影响规律,探究了人为粗糙度强化超临界航空煤油换热的机理。结果表明,人为设置粗糙元能使壁面附近产生局部回流区和旋涡结构,强化煤油与受热壁面间的对流换热。通过合理布置粗糙元结构,能大幅降低圆形冷却通道的壁面温度,有效抑制航空煤油的超临界传热恶化现象的发生。     

超声速燃烧室再生冷却结构对传热的影响分析

采用有限容积法结合对流换热关联式与蒙特卡罗法,建立了超声速燃烧室再生冷却通道的耦合换热计算模型。冷却燃料为煤油,其密度、导热系数、动力粘度随温度和压力变化,煤油比热容与金属结构的热物性随温度变化。在考虑再生冷却面板尺寸与冷却燃料量保持不变的耦合性限制条件下,计算分析了非均匀热流密度下,冷却通道内壁厚度、高度、宽度及侧肋厚度对冷却性能的影响。研究结果表明,通道结构参数的变化引起结构传热热阻和冷却剂对流换热性能以及总换热面积、通道个数的变化,在传热分析中应综合考虑。     

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。     

基于辐射-对流耦合的中餐灶热能优化研究

基于对流-辐射耦合机理,采用理论计算和数值模拟相结合的方法对中餐一体化炉灶进行热能优化研究。研究结果表明:烟气物性对耦合换热影响较大,锅底物性对耦合换热影响较小,且辐射抑制对流传热过程,当锅底的对流传热量占总吸热量比例较大时,传热效率较高。     

辐射通道一维稳态温度场数值模拟

为得到辐射对流通道中的温度分布，依据能量守恒原理，建立了辐射、对流非线性边界条件下圆形管壁与管内空气的传热数学模型，提出了管壁温度、管内冷却空气温度一维稳态换热有限差分求解方法，其中辐射换热计算采用基于辐射传递系数的蒙特卡罗法。分析了相关参数对辐射通道温度分布的影响，所研究的参数包括辐射器表面温度、管道长度与半径比、管内冷却空气流速等。计算结果表明：辐射器表面温度是影响辐射通道最高温度的主要因素。此方法可为辐射通道精细的热工特性计算提供温度场数据。     

刚性壁面法向振动对流动换热的影响

采用理论推导与数值模拟结合的方法,通过求解流动方程及能量方程得到壁面法向振动下流体流动特性变化规律,并分析振动对换热的影响。结果表明:层流状态下,低强度振动使壁面附近流体质点产生法向速度,但影响范围很小,对换热影响有限;随着振动强度的增大,流场逐渐转变为湍流,导致换热系数提升。通过数值模拟计算壁面平均努塞尔数随振动强度及来流速度的变化规律。结果表明:对于低速流体,当振动达到某一临界值后能增强换热效果,努塞尔数随振动强度增大而增加,最佳换热相位角在200°左右,低雷诺数下振动强化换热效果较为明显,对流换热系数最大可提升300%。     

Sitrling发动机燃烧及换热分析

基于碳氢燃料燃烧化学平衡反应计算，得到燃烧产物温度及组分成份，在此基础上计算燃气物性，从而计算了热气机外燃系统加热管的对流换热系数，辐射换热系数及后排换热管的肋片换热对燃用空气和液氧两种燃烧情形进行了对比计算。     

水平单管外混合气体对流冷凝换热的理论研究

采用修正的膜模型与Nusselt凝结理论结合的方法,对含湿混合气体自上而下横掠水平管外时的对流冷凝换热机理进行研究,建立了液膜流动和传热模型,进行数值求解并分析了雷诺数、壁面温度及水蒸汽浓度等因素对混合气体冷凝换热的影响。计算结果表明:水管外壁液膜厚度分布很大程度上受气体边界层对液膜剪切力的影响。而局部努谢尔数不同于纯蒸气的的冷凝换热,它受气相热阻的影响很大,其分布状况类似于单相气体管外的对流换热。     

关于浮升力增强锅炉系统中湍流换热的研究

在锅炉系统中会遇到流体在一个表面加热,而另一个表面近似绝热的环形通道的湍流混合对流。这一问题中由于有浮升力的存在,使湍流换热被强化,从而使能源的利用率提高了。本文应用Fortran程序对锅炉内浮升力增强换热这一问题进行了数值模拟求解。     

填充床内对流换热的数值研究

为研究空气流入高温填充床时小球直径和空气流速变化对填充床内对流换热和压力损失等的影响,利用孔隙尺度介观方法对顺序排列多孔介质小球的三维填充床进行数值计算,数值计算与实验结果吻合较好。结果表明:填充床内固相和气相间存在热的非平衡性;当小球直径从2.8增大到5.6 mm时,在最高温度上游对流换热强度减小,在最高温度下游对流换热强度增大,同时,压力损失和最大无量纲速度减小;气体流速增大时,填充床内产生湍流运动。     

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.     

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.     

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.     

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.     

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.     

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。     

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.     

Economie d'énergie en trigénération

Trigeneration is defined as the production of three useful forms of energy—heat, cold and power—from a primary source of energy such as natural gas or oil. For instance, trigeneration systems typically produce electrical power via a reciprocating engine or gas turbine and recover a large percentage of the heat energy retained in the lubricating oil, exhaust gas and coolant water systems to maximize the utilization of the primary fuel. The heat produced can be totally or partially used to fuel absorption refrigerators. Therefore, trigeneration systems enjoy an inherently high efficiency and have the potential to significantly reduce the energy-related operation costs of facilities. In this paper, we describe a model of characterization of trigeneration systems trough the condition of primary energy saving and the quality index, compared to the separate production of heat, cold and power. The study highlights the importance of the choice of the separate production reference system on the level of primary energy saving and emissions reduction.     

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.     

Mineralogical characterization of the intraseam layers of Lofoi lignite deposits in Florina basin (western Makedonia,northwest Greece)

The mineralogical composition of intraseam layers from Lofoi lignite deposits (northwest Greece) is the subject of the present study. The samples were examined by means of X-ray diffraction (XRD), thermo-gravimetric (TG/DTG) and differential thermal analysis (DTA), and Fourier transform infrared (FT-IR) spectrometry. The clay minerals prevail in most samples, with illite-muscovite being the dominant phase, and kaolinite and chlorite being the other major clay components. No smectite was found. Quartz and feldspars, dominate in two cases. The studied materials are characterized as clays to clayey sands, showing significant similarities with the intraseam layers of the adjacent Achlada lignite deposits.