方柱和圆柱微结构表面上蒸汽的珠状凝结

为分析不同微结构表面上的凝结传热特性,通过飞秒激光技术在硅表面上加工出两种微结构-圆柱和方柱,其接触角均超过150°。在搭建可视化凝结实验平台的基础上,通过实验对两种不同微结构表面的凝结传热性能及液滴动态特性进行了研究。实验研究表明:在同一流量条件下,随着表面过冷度的增加,珠状凝结的热流量增加,而传热系数下降;当冷却水流量从40 g/min增大到446.9 g/min时,微结构表面的热通量从25.32 kW/m~2增加到137.19 kW/m~2;由于表面微结构形状和排列方式的不同,相同条件下圆柱微结构表面比方柱微结构表面具有更好的传热性能。     

Study of Marangoni Condensation Characteristics on Non-isothermal Surfaces

在带有温度梯度的凝结表面上进行了一系列质量分数酒精蒸气的Marangoni凝结试验,研究了酒精蒸气的传热系数.结果表明：与日本学者Utaka的试验数据相比,质量分数为99%的水和1%酒精混合蒸气在同样试验条件下（p=84.5 kPa、v=2 m/s）的传热系数提高了15%;在相似的试验条件下,带温度梯度试验块其传热系数的提升是由于液膜内部存在两种相反的运动,这两种相反运动扰乱了凝结液膜,从而减小了液膜的热阻;对传热系数随过冷度变化的规律进行了定性分析.     

带定位格架的5×5棒束通道内过冷沸腾流动传热数值研究

基于流体体积函数(VOF)两相流模型,通过在控制方程中加入适当的质量源项和能量源项,建立了过冷沸腾模型,利用该模型对5×5定位格架棒束通道内的过冷沸腾现象进行数值模拟,研究了有无过冷沸腾现象2种工况下的传热特性以及入口速度、入口过冷度等因素对燃料组件内传热特性的影响。结果表明:有过冷沸腾现象时流体的传热效果优于无过冷沸腾现象,在有过冷沸腾现象时,总体上通道内传热系数与入口速度呈正相关,与入口过冷度呈负相关;在局部区域,相变传热占据主导因素,过冷沸腾程度越强,流体的传热效果越好。     

近临界及超临界压力区垂直光管和内螺纹管传热特性的试验研究

阐述了眚上升布置的内螺纹管和光管在近临界及超临界压力区的传热牧场 生的试验结果。在近临界压力区。随着压力向临界压力造近,光管的传热特性变差,传热恶化的临界干度下降得很厉害,甚至在过冷区就会发生壁温飞升;内螺纹管在近临界压力区可以消除传热恶化,但是随着靠近临界压力其抑制传热恶化的能力下降。传热恶化后的光管和内螺纹管的最小传热系数分别在压力为21．0MPa和22．0MPa。超过临界压力后,光管和内螺纹管的传热特性得到改善,内螺纹管在高焓值区可以降低壁温。     

垂直布置B30波槽管管外膜状凝结的实验研究

在略高于大气压的凝结条件下,对垂直布置渡槽管管外膜状凝结换热进行了实验研究。实验结果表明：垂直布置时波槽管具有一定的强化传热效果,在实验范围内,最佳波槽管的总传热系数约比光管提高27％-43％,而阻力系数约为光管的2．94-3．48倍,垂直布置的强化传热效果不如水平布置的好。通过对实验数据的回归分析,得到了垂直管管内对流换热、管外凝结换热及阻力系数的实验关联式。     

R410A/润滑油混合物在5 mm铜管内流动冷凝传热特性与理论预测模型研究

针对润滑油冷凝性能影响分析不足以及理论预测模型对小管径冷凝传热系数预测精度低的问题,对R410A/润滑油混合物在5 mm水平强化管内流动冷凝传热特性进行了实验研究,探究了质量流率、干度和润滑油质量分数3个参数对冷凝传热性能的影响。实验工况：质量流率为200,300和400 kg/（m²·s）,干度为0.1～0.9,润滑油质量分数为0,1%,3%和5%。实验结果表明：对于混合物,冷凝传热系数随着质量流率的增大而增大,随着干度的增大先增大后减小,润滑油质量分数对冷凝传热特性具有重要影响;基于实验数据和理论分析对理论预测模型进行了修正,修正后的模型在±15%误差带内预测精度比达到94.6%。     

Marangoni凝结表面传热系数的影响因素分析

对水和酒精混合蒸气在竖直平板上的凝结传热进行了研究。利用液膜覆盖率,分别按考虑和忽略浓度边界层扩散热阻计算了凝结表面传热系数,并将计算结果与实验值进行了比较。结果发现同时考虑液膜导热热阻及浓度边界层扩散热阻的计算值在极小酒精质量分数时高于实验值,在较小酒精质量分数时和实验值接近,但是在高酒精质量分数时低于实验值。由此可见计算凝结表面传热系数时,在小酒精质量分数条件下扩散热阻可以忽略,但在高酒精质量分数条件下扩散热阻对整个热阻的贡献较大,必须考虑其对传热的影响。     

熔盐与过冷沸腾水传热特性实验研究

为研究熔盐蒸汽发生器的传热特性,以Solar Salt二元硝酸盐为工质,对熔盐与过冷沸腾水在蒸汽发生器中的传热规律进行实验研究.实验结果表明:熔盐传热系数随着水侧与熔盐的质量流速的增大而增大,随着水侧压力的升高而减小;当水侧在过冷水状态时,熔盐传热系数随着熔盐入口温度的升高而增大;当水侧在过冷沸腾状态时,熔盐传热系数随...     

微尺度流道相变传热特性的实验研究

为了探究微流道相变传热的特性,本文以去离子水为工质,对不同结构尺寸的铝制矩形流道中的相变传热特性进行了实验研究.结果表明:在本实验范围内,传热系数随流道尺寸减小而增大.传热系数随着热流密度的增加而增加.在同一热流密度下,随着干度的增大,传热系数变化不大.     

真空条件下珠状凝结传热特性的理论分析与试验研究

从理论分析和试验研究两个方面研究了真空状态下珠状凝结的传热特性.首先从蒸汽流场、气膜热阻和表面能3个角度进行理论分析,分析结果表明,珠状凝结的换热系数K值随着真空的提高而增大.然后进行真空条件下的珠状凝结试验,经过数据分析表明,试验结果与理论分析相吻合,即在真空条件下,凝结换热系数K值随着真空度的提高而增大,在较大Re数时(Re＝34755),真空度每提高0.01 MPa,换热系数可提高10%左右.     

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.     

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.     

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.     

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.