水平多管式换热器内部相变石蜡储热特性实验研究

设计并搭建了水平多管式相变储热系统,以水为传热流体(HTF)、石蜡为相变材料(PCM),通过实验对储热系统的具体蓄热特性和不同操作条件下HTF与PCM之间的传热特性进行定量分析,评估了HTF体积流量和进口温度对紧凑型低温相变储热系统功率输入、吸热完成时间以及储存能量的影响。该系统主要由一个聚碳酸酯壳和水平定向的多管换热器以及石蜡组成,其中石蜡相变温度约为41℃。结果表明：随着HTF进口温度或体积流量的增加,吸热完成时间减少,平均吸热功率增大,且增加速率都随着进口温度的增大而变小;HTF体积流量分别为4.5,6.0和7.5 L/min时,吸热过程耗时300.7,252.9和226.7 min;在58,64和70℃的进口温度下,吸热完成时间分别为270.1,226.7和204.9 min;提高HTF进口温度,会导致换热结束时石蜡温度与HTF出口温度出现越来越靠近的趋势,而在提高HTF体积流量时,却呈现相反的趋势。     

相变蓄热型空气源热泵系统蓄、释热特性与供暖节能运行温度条件的研究

文章通过对典型城市最冷月的日逐时室外空气的平均温度进行分析,并结合空气源热泵的运行特性、建筑供暖负荷规律和人们的用能规律,提出了一种相变蓄热型空气源热泵系统,实现了通过日蓄、夜释的方式弥补低温时段空气源热泵供热能力不足的目的。文章设计了新型翅片管式相变蓄热器,开展了相变蓄热型空气源热泵系统蓄、释热特性实验。实验结果表明:翅片管式相变蓄热器蓄热时,相变蓄热材料温度分布均匀,空气源热泵的冷凝温度与相变蓄热材料之间的温度差为1.1℃,这有利于降低空气源热泵冷凝温度、提高空气源热泵性能系数;翅片管式相变蓄热器释热时,相变蓄热器入口水温对释热速度具有重要影响。同时,对相变蓄热型空气源热泵系统的蓄热能效进行分析,得到了相变蓄热型空气源热泵系统供暖节能运行温度条件。     

组合式高温相变蓄热器蓄热过程的数值模拟

通过对相同管径蓄热管组成的高温相变蓄热器蓄热过程进行模拟,得到了蓄热过程中相变材料(PCM)温度和液相率随时间的变化曲线以及不同时刻液相率的分布云图。针对温度曲线和云图所显示的问题,在保证蓄热量的前提下,提出了蓄热管组合式排布的设计方案,并对其蓄热性能进行了模拟。研究结果表明,采用所提出的组合式方案有效地减少了蓄热时间,降低了"死区"对蓄热器蓄热性能的影响,使PCM的液相率分布更加均匀,可为用于太阳能热发电的高温相变蓄热器的优化设计提供理论依据。     

固体蓄热器填充床容积蓄热率的分析与研究

以空气作为热媒介质、不规则耐高温蓄热体作为蓄热材料的固体蓄热器填充床为研究对象,建立填充床容积蓄热率的计算数学模型,通过数值计算的方法,分析蓄热材料的当量直径d_e、空气质量流速G、空气进口温度T_(1,0)、填充床长径比l/D以及蓄热材料的种类对填充床容积蓄热率的影响,得到相应的变化规律曲线并通过实验方法进行验证,为进一步分析填充床蓄热性能随时间与空间的变化规律提供依据。     

环腔型汽车相变蓄能器蓄能特性研究

通过对相变蓄能器的蓄能及相变过程进行了模拟分析,得到了相变材料在蓄能过程中液相分数及温度随时间的变化规律。模拟了不同相变材料初温、不同流体初温、不同工况下相变蓄能器的蓄能情况。得到了冬天和夏天相变蓄热器初始温度不同时,液相率的变化影响不明显,设计的相变蓄能器在40 min左右完成蓄能过程,符合城市生活的一个出行时间。在进口水温为368K时对蓄热最有利,能达到最好的蓄热效果。当进口流速0.113 m/s时,此后流速继续增大,蓄热时间已没有明显的变化,这时再通过减小流体与材料间对流换热热阻来加强换热意义不大。     

用于储存太阳能的相变蓄热器蓄热性能研究

利用计算流体力学软件Fluent的凝固/熔化模型,模拟了用于储存太阳能的两种相变蓄热器的蓄热过程,得到了蓄热过程中相变区的温度云图,分析了两种蓄热器的蓄热能力和传热方式。研究结果表明,合理增加内管数量可以提高相变过程中的对流换热强度和蓄热能力。     

基于圆柱形单元的储热装置放热实验研究

蓄热水箱作为太阳能供暖系统的重要核心设备,其性能直接影响着储能系统的整体运行效率。设计一种基于圆柱形相变单元的相变储热装置,并搭建相变蓄热水箱性能测试平台,通过单一控制变量法得到储热装置放热过程的温度变化曲线。研究表明：对于空间一定的储热装置,在等质量相变材料（PCM）时,相变单元的直径对装置放热速率的影响较大;相变单元之间的间距对装置放热速率的影响较小;当增大换热流体（HTF）的入口流量及降低HTF入口温度时,能大大减少储热装置的放热时间,提高储热装置的整体性能。     

添加物对石蜡相变螺旋盘管蓄热器蓄热和放热性能的影响

对以石蜡为相变材料的螺旋盘管蓄热器的蓄热和放热性能进行了实验研究,探讨了在石蜡中添加铜粉、硅粉和不锈钢丝带对石蜡螺旋盘管蓄热器蓄热和放热性能的影响。结果表明：在蓄热过程中,随着加热时间的增加,蓄热器内的温度分布不均匀性逐渐增大;纯石蜡蓄热器内温度分布不均匀性最为严重;插入不锈钢丝带的蓄热器内温度分布最均匀。在放热过程中,纯石蜡蓄热器的出口水温下降最快;而石蜡加不锈钢丝带的蓄热器出口水温最高。     

光热发电高温固体蓄热系统的优化设计

为了解决原有光热发电蓄热系统蓄热温度低和蓄热成本高等问题,设计出一种由取热、蓄热和用热三个子系统组成的新型高温固体蓄热系统。以空气作为热媒介质、安全耐高温蓄热体作为蓄热材料的高温固体蓄热系统为研究对象,分析计算了蓄热器的保温特性以及蓄热系统整体热效率和流动阻力。结果表明,蓄热系统运行稳定,蓄热器散热损失小于5%,系统整体热效率大于85%,可供实际工程设计参考。     

套管式相变蓄热器强化传热研究

蓄热技术可有效解决热能供给与需求在时间和强度上不匹配的矛盾,提高能源利用率。针对蓄热器换热效率低和蓄热器内部温度分布不均匀等问题,结合套管式和多管式蓄热器的换热特性,开发出高效蓄热换热器。研究结果表明,通过添加翅片使相变材料融化过程节省时间66.67%;增加翅片长度能够改善相变材料凝固过程中相变"死区"对整体放热时间的影响,使凝固过程节省时间73%。     

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.     

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.     

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.     

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.